Ultraviolet A protective potential of plant extracts and phytochemicals

Chronic exposure to solar radiation is related to an increased incidence of various skin disorders, including premature skin aging and melanoma and non-melanoma skin cancers. Ultraviolet (UV) photons in particular are responsible for skin damage. Solar UV photons mainly belong to UVA wavebands, however UVA radiation has been mostly ignored for a long time. At the cellular level, UVA photons mainly provoke indirect oxidative damage to biomolecules via the massive generation of unstable and highly reactive compounds. Human skin has several effective mechanisms that forestall, repair and eliminate damage caused by solar radiation. Regardless, some damage persists and can accumulate with chronic exposure. Therefore, conscious protection against solar radiation (UVB+UVA) is necessary. Besides traditional types of photoprotection such as sunscreen use, new strategies are being searched for and developed. One very popular protective strategy is the application of phytochemicals as active ingredients of photoprotection preparations instead of synthetic chemicals. Phytochemicals usually possess additional biological activities besides absorbing the energy of photons, and those properties (e.g. antioxidant, anti-inflammatory) magnify the protective potential of phytochemicals and extracts. Therefore, compounds of natural origin are in the interest of researchers as well as developers. In this review, only studies on UVA protection with well-documented experimental conditions are summarized. This article includes 17 well standardized plant extracts (Camellia sinensis (L.) Kuntze, Silybum marianum L. Gaertn., Punica granatum L., Polypodium aureum L., Vaccinium myrtillus L., Lonicera caerulea L., Thymus vulgaris L., Opuntia ficus-indica (L.) Mill., Morinda citrifolia L., Aloe vera (L.) Burm.f., Oenothera paradoxa Hudziok, Galinsoga parviflora Cav., Galinsoga quadriradiata Ruiz et Pavón, Hippophae rhamnoides L., Cola acuminata Schott & Endl., Theobroma cacao L. and Amaranthus cruentus L.) and 26 phytochemicals.


INTRODUCTION
Solar radiation is electromagnetic radiation produced by the Sun, ranging from about 0.25 to 4.5 μm in wavelength including infrared, visible, and ultraviolet (UV) light. Sunlight is an inseparable part of human life. It is a key factor in plant photosynthesis as well as in vitamin D 3 synthesis in human skin. However, it also has several negative effects on human health. The harmful effects are mainly associated with the UV waveband. The amount of UV radiation reaching the earth's surface has been increasing over the last few decades. Human lifestyle has changed as well, especially time spent in outdoor activities (sports, sunbathing, or sunbed tanning). As a result, our body, particularly our skin, receives higher doses of UV radiation than before, which is linked with a more frequent incidence of various acute and chronic detrimental cutaneous effects, including skin cancer. Originally, it was thought that only the high energetic photons of the UVB waveband (280-315 nm) are responsible for the adverse effects of solar light. As a result, the photoprotection of human skin was focused on protection against UVB rays for many years. More recently, an increasing number of independent studies indicated that UVA radiation (315-400 nm) also induces damage to skin cells. It is therefore essential to have effective protection against UVA radiation as well 1 . This review summarises studies on pure compounds or extracts derived from natural sources that have demonstrated protective potential against UVA radiation in skin cells and/or tissue.

UVA RADIATION
The sun is primarily a UVA source, as UVA amounts to over 90% of the UV radiation reaching the earth's surface. UVA radiation is further subdivided into UVA1 (340-400 nm) and UVA2 (315-340 nm). UVA photons penetrate deep into the epidermis and dermis of the skin. About 80% of UVA radiation reaches the dermo-epidermal junction and penetrates into the papillary dermis. In this way UVA may affect most of the skin cells, especially keratinocytes, melanocytes, fibroblasts and endothelial cells in blood vessels. UVA-induced responses in cells occur predominantly via the activation of oxidative processes initiated by endogenous photosensitization. UVA photons primarily initialize the production of reactive oxygen and nitrogen species (ROS, RNS) through interacting with endogenous chromophores (photosensitizers). In particular superoxide and singlet oxygen is produced. The latter can be dismutated to hydrogen peroxide, which in turn can produce hydroxyl radicals after reacting with metal ions. RNS include nitric oxide and peroxynitrite. The cellular photosensitizers have not been fully characterized, but candidates include bases of nucleic acids, aromatic amino acids, NAD(P)H, heme, quinones, flavins, porphyrins, 7-dehydrocholesterol, eumelanin, urocanic acid etc. ROS/ RNS can oxidize cellular proteins, lipids, and saccharides. The oxidized products of lipids include alkoxyl radicals, aldehydes, alkanes, lipid (hydro)peroxides and epoxides. These highly reactive compounds may further provoke oxidative damage to biomolecules. As for proteins, all amino acid side chains can be oxidized to generate protein carbonyl groups. The sulphydryl groups of methionine and cysteine are particularly susceptible to oxidation. Several DNA replication and repair proteins have been shown to be targets of reactive compounds that arise upon UVA exposure. ROS/RNS can also induce various types of oxidative DNA lesions such as single-strand breaks and DNA-protein crosslinks, but mainly altered DNA bases. Due to their lowest ionisation potential, guanine bases are the most susceptible to oxidation, and 8-hydroxydeoxyguanine (8-OH-dG) is a characteristic oxidative product. UVA was also found to produce a significant yield of cyclobutane-pyrimidine dimers (CPD). ROS/RNS, unstable oxidised products as well as DNA lesions can affect various cellular pathways and the expression of numerous genes such as inflammatory cytokines, transcription factors, matrix metalloproteinases (MMP), mitogen-activated protein kinases (MAPK), or pro-and anti-apoptotic genes. These signalling molecules initiate the development of pathological changes in skin tissue such as altered epidermal cell proliferation and differentiation, decrease in collagen synthesis, upregulation of extracellular matrix-degrading enzymes such as collagenase (MMP-1), stromelysin (MMP-3), gelatinase (MMP-9) or elastase. On the other hand, increased levels of oxidatively modified molecules stimulate the activity, activation or synthesis of protective molecules such as nuclear factor erythroid-2 related factor 2 (Nrf2), phase 2 detoxifying enzymes (e.g., glutathione S-transferase (GST), NADPH quinone oxidoreductase-1 (NQO1), γ-glutamate-L-cysteine ligase (γ-GCL) or heme oxygenase-1 (HO-1)), DNA reparation enzymes and others that aim to suppress the adverse biological effects of UVA radiation. However, intensive exposure to UVA light can exceed protective mechanisms, and chronic exposure to UVA radiation leads to the step-by-step accumulation of oxidatively modified molecules and loss of the vascular network, all of which may result in skin inflammation, immunosuppression, photodermatosis, premature skin aging (photoaging) and/or carcinogenesis 1-4 ( Fig. 1).

PLANT EXTRACTS AND PHYTOCHEMICALS WITH UVA PROTECTIVE POTENTIAL
In response to the environment, many living organisms produce specific secondary metabolites that help them to grow successfully and continue their lineage. Some of these secondary metabolites have been found to provide UV protective properties, even though they probably have other physiological roles. These compounds with UV protective activities are widely distributed across the microbial, plant and animal kingdoms, and share some common chemical features 5 . They act as UV absorbers (blockers) including phenolic acids, flavonoids, non-flavonoids and terpenoids. These compounds can prevent or diminish the penetration of UV photons into the skin tissue, resulting in a reduction in the interaction of UV photons with endogenous chromophores, oxidative stress, DNA damage and inflammation. Besides these abilities, some phytochemicals were also found to modulate multiple signalling pathways 6 . Therefore, these naturally occurring compounds have gained considerable attention for their potential use as effective agents for preventing or reducing the UV-induced oxidative modification of biomolecules and subsequent events involved in skin pathology such as photoaging and skin cancer. This review makes an attempt to summarize the results of research on the protective properties of various phytochemicals or extracts, with the focus on UVA radiation. Only papers with well characterized phytochemicals or extracts and experimental conditions were selected for the review.

Camellia sinensis
Tea (Camellia sinensis (L.) Kuntze) is a plant of the Theaceae family. A hot water infusion of tea leaves is one of the most consumed beverages in the world. Tea is divided into subtypes based on its processing. White tea consists of minimally processed young leaves. Green tea originates from minimally processed mature leaves, while oolong tea is semi-fermented and black tea is fully fermented. Green tea is an abundant source of polyphenols known as catechins. They generally account for 30-42% of the dry solids in brewed green tea. The major catechins found in green tea include (-)-epicatechin (EC), (-)-epicatechin-3-gallate (ECG), (-)-epigallocatechin (EGC), and (-)-epigallocatechin-3-gallate (EGCG). EGCG is the most abundant and forms 50-80% of the total amount of catechins. The polyphenol content of black tea is different from that of green tea due to the degree of oxidation during processing. It mainly contains the following polyphenols: thearubigins, theaflavins, flavonols, and catechins. Among the tea polyphenols, EGCG has been shown to be the most effective chemoprotective agent against cutaneous inflammatory or carcinogenic responses. The potential of tea polyphenols against UVBinduced DNA damage, inflammation, oxidative stress, alterations in cell signalling pathways, and epigenetics changes that play a pivotal role in photocarcinogenesis are summarized in a recent review 7 . EGCG is also the most commonly evaluated green tea component in terms of UVA protection. The EGCG pre-treatment of a spontaneously transformed aneuploid immortal human skin keratinocyte cell line (HaCaT) reduced UVA-induced ROS generation, DNA single-strand breaks and alkali-labile site formation, hypoxanthine-guanine phosphoribosyl transferase (HPRT) mutant frequency 8 as well as nuclear factor kappa B (NF-κB) nuclear translocation and interleukin-6 (IL-6) secretion 9 . EGCG protected normal human dermal fibroblasts (NHDF) against UVA damage by downregulating the transcription activity of Jun protein and the mRNA and protein level of MMP-1 (ref. 10 ). The treatment of human infant skin fibroblasts with EGCG decreased beta-galactosidase positive cell number and the frequency of the HPRT gene mutation in UVA-irradiated cells 11 . EGCG inhibited the UVA-induced cell death of ARPE19 adult human retinal pigment epithelial cells. In addition, EGCG suppressed intracellular hydrogen peroxide generation, extracellular signal-regulated kinases (ERK), c-Jun N-terminal kinase 1/2 (JNK) and p38 kinase activation as well as cyclooxygenase 2 (COX-2) protein expression in the irradiated cells 12 . The pre-treatment of male Wistar albino rats with EGCG in a hydrophilic ointment resulted in a significant reduction in sunburn cell count and degeneration and disintegration in the dermo-epidermal junction in UVA-exposed animals 13 . Pre-treatment with EGCG reduced wrinkles in the dorsal trunk and also prevented the reduction of collagen synthesis in the skin of UVAirradiated hairless mice 14 .
The UVA protection of less abundant green tea polyphenols was also evaluated in several in vitro studies. The pre-treatment of a FEK4 human skin fibroblast cell line with EGC caused a strong reduction in UVA-induced HO-1, but MMP-1 and COX-2 mRNA expression was stimulated. On the other hand, in a KB human oral carcinoma cell line, UVA-stimulated COX-2 mRNA was strongly reduced with EGC (ref. 15 ). In UVA-irradiated HaCaT, EGC pretreatment increased viability, and reduced hydrogen peroxide production and ERK activation 16 . The pre-treatment of FEK4 fibroblasts with EC and its metabolite 3'-O-methyl epicatechin (MeOEC) increased cell viability and reduced the number of necrotic cells. EC further prevented the suppression of HO-1 transcription 17 . EC and MeOEC also suppressed the UVA-mediated release of chelatable iron and the mRNA expression of MMP-1 in FEK4 fibroblasts. EC treatment also protected the lysosomal membrane against UVA-induced damage 18 . In both studies 17,18 , EC was more effective than its metabolite. The application of a nano-gel formulation of catechin to the skin of male Wistar rats reduced the oxidative stress induced by UVA, most significantly improving the level of cutaneous antioxidant enzymes (superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase (CAT)), and reducing the level of thiobarbituric acid reactive substances (TBARS) (ref. 19 ).

Silybum marianum
Silymarin (SM) is a standardized polyphenol fraction from the seeds of Silybum marianum L. Gaertner (syn. Carduus marianus L. or milk thistle) from the Asteraceae family. S. marianum is one of the oldest known plants, and has been widely used in traditional European medicine since ancient times. The plant as well as SM is rich in polyphenols. SM contains approximately 70-80% flavonolignans, and 20-30% is a chemically undefined fraction, mostly comprised of polymeric and oxidised polyphenolic compounds. The main polyphenolic component of SM is the flavonolignan silybin (SB). Other less abundant components are the flavonolignans isosilybin (ISB), silychristin (SC), silydianin (SD) and the flavonoid taxifolin. A minor component but with a significant biological potential is the flavonolignan 2,3-dehydrosilybin (DHSB) (ref. 20 ). SM has been primarily used in the treatment of liver disorders including hepatitis, alcoholic liver diseases, and cirrhosis, and is also useful against toxin-induced liver toxicity. Laboratory studies suggest that there is no significant difference between SM and its major component SB in terms of biological activities. Research results showed antioxidant activity, modulation of the immune system and various signalling pathways for both SM and SB (ref. 21 ). Several reports documented the ability of SM and SB to reduce chemically-and UVB-stimulated skin damage, including carcinogenesis 21,22 .
The pre-treatment of NHDF with SM and SB resulted in a reduction in UVA-stimulated ROS and the production of DNA single-strand breaks, as well as in the prevention of glutathione (GSH) depletion, a decrease in the activation of caspase-3 and MMP-1 protein. SM also moderately increased HO-1 and reduced the level of heat shock protein 70 (HSP 70) (ref. 23 ). Also, less abundant flavonolignans in silymarin, ISB, SD, SC and DHSB demonstrated the ability to reduce UVA-induced damage in NHDF (ref. 24 ). The post-treatment of UVA-irradiated HaCaT with SM (ref. 25 ) and SB (ref. 26 ) resulted in the diminution of UVA-caused ROS production, caspase-3 activation, GSH and ATP depletion as well as lipid peroxidation and the formation of DNA single-strand breaks.
However, besides many reports on the photoprotection of SM and its main congener SB, there are also controversial studies demonstrating an in vitro phototoxic activity of SM (ref. 27 ) and SB (ref. 28 ). The phototoxic potential of the minor flavonolignan DHSB, a dihydroxy derivative of SB, was recently also described in skin cells 24,29 . DHSB is very potent antioxidant with a moiety that is structurally similar to quercetin, which has also been shown to be a phototoxic compound. The phototoxic potential of DHSB and perhaps SM needs be confirmed in vivo or in clinical trials.

Punica granatum
Pomegranate also known as grenade, granats, and punica apple, is a fruit of the Punica granatum L. tree from the family Lythraceae. It is indigenous to the Himalayas in northern India through to Iran, parts of Southeast Asia, the East Indies, and tropical Africa, and grows in almost all parts of the Mediterranean region. Pomegranate is rich in phenolic acids, flavanols, flavones, flavonones, anthocyanidins, and anthocyanins 30 . The fruit and its pericarp contain two main polyphenol groups, anthocyanins (cyanidin and delphinidin) and hydrolysable tannins (punicalin, pedunculagin, punicalagin, and gallagic and el-lagic esters of glucose). Pomegranate has been recognized since antiquity for its healing properties. Pomegranate fruit extract exhibits antioxidant and anti-inflammatory properties. Many in vitro and in vivo studies demonstrated a photoprotective effect of pomegranate against UVB radiation 22 .
UVA-induced ROS generation and the cell death of SKU-1064 human skin fibroblasts were reduced by pomegranate extract pre-treatment in a dose-dependent fashion 31 . A concentrated pomegranate solution decreased UVA-induced MMP-1 activity in neonatal NHDF (ref. 32 ). The pre-treatment of normal human epidermal keratinocytes (NHEK) with pomegranate fruit extract resulted in a dose-dependent inhibition of the UVA-mediated phosphorylation of signal transducer and activator of transcription 3 at Tyr705, AKT at Ser473 and ERK1/2. The extract also inhibited the UVA-mediated phosphorylation of mTOR (a serine-threonine kinase involved in cell growth), an increase in Ki-67 (a protein associated with cellular proliferation and ribosomal RNA transcription) and proliferating cell nuclear antigen (PCNA), an essential factor in DNA replication. Pre-treatment with the extract also increased UVA-induced cell-cycle arrest in the G1 phase and the expression of pro-apoptotic proteins Bax and Bad, with downregulation of the expression of the anti-apoptotic protein Bcl-xL (ref. 33 ).

Polypodium leucotomos
The tropical fern Polypodium leucotomos (syn. Phlebodium aureum L. or Polypodium aureum L.) from the Polypodiaceae family is native to Central and South America. P. leucotomos is rich in phenolic compounds, especially benzoate and cinnamate derivatives. Chlorogenic, p-coumaric, vanillic, cinnamic, caffeic and ferulic acid are the most abundant of them, and these compounds are known for their photoprotective potential (see part Phytochemicals with UVA protective potential). P. leucotomos has been traditionally used for treating various skin disorders (e.g., psoriasis and atopic dermatitis) in its areas of origin 34 . Clinical scientific evidence suggests that P. leucotomos is also beneficial for the treatment of vitiligo and the prevention of polymorphic light eruption. Photoprotective activity has been assessed in animals, healthy volunteers as well as in patients suffering from several cutaneous diseases such as vitiligo, psoriasis, idiopathic photodermatosis or melasma 35,36 .
A hydrophilic extract of P. leucotomos efficiently protected NHDF and HaCaT survival and restored their proliferative capability when the cells were exposed to UVA radiation. The extract also prevented UVA-induced morphological changes in NHDF, especially the disorganisation of F-actin-based cytoskeletal structures, coalescence of the tubulin cytoskeleton and mislocalization of adhesion molecules such as cadherins and integrins 37 . P. leucotomos extract inhibited the protein expression of MMP-1 and MMP-2 and stimulated the levels of the tissue inhibitors of MMP-1 and -2 (TIMP-1 and TIMP-2) in UVA-irradiated NHDF and CRL-1619 melanoma cells. The extract pre-treatment also stimulated the protein levels of collagen I and V in UVA-radiated fibroblasts 38 .
Oral pre-treatment of male hairless rats with P. leucotomos extract significantly reduced GSH oxidation and the formation of oxidized GSH (GSSG) in both erythrocytes and plasma stimulated with UVB/UVA radiation (9:10). The extract also significantly increased CAT activity and the GSH⁄ GSSG ratio, and reduced the depletion of Langerhans cells in the epidermis of irradiated animals 39 . The oral administration of P. leucotomos extract to human volunteers (phototypes II to III) reduced negative clinical signs of PUVA therapy, in particular phototoxic reaction and pigmentation. In the skin of individuals treated with the extract, a significant reduction in sunburn cells, preservation of Langerhans cells, decrease in mast cells infiltration, and decrease in vasodilation were found 40 .

Vaccinium myrtillus
Bilberry (Vaccinium myrtillus L.), is a well-known small shrub, also called blueberry, huckleberry or whortleberry that grows wildly in Northern Europe, North America, and Asia. Bilberry belongs to the Ericaceae family. The fruit is a blue-coloured, fleshy berry, which contains several bioactive secondary metabolites, including flavonoids, vitamins, sugars, and pectin. In terms of flavonoids, anthocyanins are the most widely studied class of bioactive compounds of this plant. Multiple pharmacological activities (related to the anthocyanidin fraction) include antioxidant, anti-inflammatory, anti-atherosclerosis, or wound healing 41 . Some studies showed the beneficial effects of bilberry on skin. For example anthocyanins from bilberry have been found to alleviate pruritus in a mouse model of chronic allergic contact dermatitis 42 or to protect HaCaT against UVB-induced damage 43 .
The pre-treatment of HaCaT with V. myrtillus fruit extract resulted in the attenuation of UVA-stimulated ROS formation, lipid peroxidation and the depletion of intracellular GSH (ref. 44 ). In another study, V. myrtillus extract increased the cell viability of HaCaT and reduced UVA-provoked ROS generation, malondialdehyde (MDA) production, DNA damage and the number of apoptotic cells 45 . Anthocyanins that are rich in the ethanolic extract from bilberry (BE) as well as BE-loaded ultra-deformable liposomes increased the viability of UVA-irradiated HaCaT (ref. 46 ).

Lonicera caerulea
Lonicera caerulea L., also called blue honeysuckle, honeyberry, edible honeysuckle or sweet berry honeysuckle, is a shrub from the Caprifoliaceae family. L. caerulea can be found mainly in northern Russia, China, and Japan. Its berries are oval to long in shape and dark navy blue to purple in colour. The fruits contain several groups of phenolics including anthocyanins, flavonoids and low-molecular-weight phenolic acids. These compounds have been reported to have multiple biological activities, including antioxidant, anti-inflammatory and cytoprotective 47 . The potential of L. caerulea polyphenols and berries to reduce UVB-induced damage was demonstrated in vitro 43 and in vivo 48 .
The pre-and post-treatment of HaCaT with the polyphenolic fraction of L. caerulea significantly increased cell viability and suppressed UVA-induced ROS production, lipid peroxidation and the depletion of reduced GSH (ref. 49 ). Consuming L. caerulea berries reduced MDA production and increased CAT activity and GSH level in the skin and erythrocytes of hairless mice. The protein levels of NQO-1 and γ-GCL were reduced, while HO-1 level was increased in the skin of UVA-exposed mice fed L. caerulea berries. The plasma level of IL-17 was increased and of IL-12 was reduced in the mice consuming the berries exposed to UVA light. Histological changes in the nuclei of basal cells induced by UVA exposure were reduced in the animals consuming L. caerulea berries 50 .

Thymus vulgaris
A flowering, aromatic perennial herb from the Lamiaceae family, Thymus vulgaris L. is commonly known as thyme. T. vulgaris is native to southern Europe from the western Mediterranean to southern Italy. Abundant constituents of T. vulgaris include the terpenes thymol (2-isopropyl-5-methylphenol), carvacrol and borneol. Essential oils extracted from fresh leaves and flowers are used as aroma additives in food, pharmaceuticals and cosmetics. Thyme possesses various beneficial effects such as antioxidant, antiseptic, antimicrobial, carminative, antitussive, or spasmolytic 51 . T. vulgaris extracts were reported to be immunomodulatory, anti-inflammatory or hepatoprotective agents 52 . An ethanolic extract of T. vulgaris seeds reduced UVB-caused damage to NHDF and hairless mice skin 53 . T. vulgaris leaf water extract was demonstrated to reduce UVB-induced damage in ex vivo human skin 54 .
A leaf extract from T. vulgaris and thymol protected the low differentiated keratinocyte cell line NCTC 2544 against UVA-induced damage, and especially reduced ROS level, lipid peroxidation and DNA damage 55 .

Opuntia ficus-indica
Opuntia ficus-indica L. Mill., commonly called prickly pear or nopal cactus, belongs to the dicotyledonous angiosperm Cactaceae family. O. ficus-indica is a tropical and subtropical plant that grows in arid and semiarid areas in Mexico, Latin America, South Africa and Mediterranean countries. The cactus cladodes contain vitamins and various phenolic acids and flavonoids, especially quercetin 3-methyl ether, a highly efficient radical scavenger. Extracts of O. ficus-indica have antioxidant and anti-inflammatory activity and remarkably improve wound healing 56 .
Water extract from O. ficus-indica cladodes was shown to protect HaCaT against UVA radiation, particularly against UVA-induced ROS production, lipid peroxidation and GSH depletion. Moreover, the cleavage of caspase-3 and caspase-7 and the phosphorylation of p38 and MAPK-activated protein kinase 2 (proteins directly involved in stress signalling pathways induced by UVA radiation) were reduced in irradiated cells pre-treated with the extract 57 .

Morinda citrifolia
Morinda citrifolia L. is a tropical tree with a distinctive, ovoid, "grenade-like" yellow fruit. M. citrifolia, commonly known as noni (other names include Pain bush, Pain killer tree, Cheese fruit, Forbidden fruit, Headache tree, Nino, Pinuela, Hog apple or Wild pine), belongs to the Rubiaceae family. M. citrifolia is widely distributed in areas of Micronesia, Hawaii, Tahiti, Australia, and Southeast Asia. Various compounds were identified in its leaves, fruits and roots, such as vitamin C and A, carotene terpenoids, alkaloids, anthraquinones, flavonoids and their glycosides. The fruits, roots, bark and leaves of M. citrifolia have been used throughout Polynesia as a folk medicine for the treatment of many diseases including various cancers, burns, skin inflammation and wounds 58 . M. citrifolia leaf extract was also shown to reduce UVBinduced erythema in human volunteers 59 .
An ethanolic extract of M. citrifolia seeds exhibited an inhibitory effect on MMP-1 secretion in UVA-irradiated NHDF. A constituent of the extract, 3,3-bisdemethylpinoresinol, inhibited this MMP-1 secretion as well. The compound also reduced the phosphorylation of p38 and JNK (ref. 60 ).
A whole-leaf extract of A. vera reduced UVA-induced photodamage to HaCaT. The extract increased cell viability, membrane integrity and lysosomal stability and reduced ROS generation and morphological changes (cell size, granularity) (ref. 66 ).

Oenothera paradoxa
Evening primrose (Oenothera paradoxa Hudziok) is a biennial herb originating from Mexico and Central America, belonging to the Oenotheraceae family. Today, it is also cultivated in Europe and parts of Asia for the production of seeds that are a great source of γ-linolenic acid. Defatted seeds represent a prominent source of polyphenolic compounds (flavonoids, phenolic acids, and hydrolyzable tannins). In traditional medicine, the whole plant or leaf juice is used for its analgesic and wound healing properties, especially as topical remedies to alleviate cutaneous inflammation. A number of studies demonstrated the antioxidant activity of O. paradoxa defatted seed extract 67,68 . O. paradoxa defatted seed extract exhibited an antimigratory, anti-invasive and antimetastatic potential towards prostate and breast cancer cells 69 . An anticancer activity of O. paradoxa defatted seeds extract was also found on skin melanoma cells 70 .
The pre-treatment of NHDF with an aqueous extract of O. paradoxa defatted seeds increased the number of viable cells, decreased the release of lactate dehydrogenase (LDH), ROS production, lipid peroxidation and the number of cells in late apoptosis after UVA exposure 71 .

Hippophae rhamnoides
Sea buckthorn (Hippophae rhamnoides L.) is a spiny deciduous flowering shrub in the Elaeagnaceae family. It is native to the cold-temperate regions of Europe and Asia. It is used in the food and cosmetics industries. The oil prepared from soft parts and from the seeds is rich in vitamins (C, E and K), polyphenols, carotenoids, sterols, minerals, amino acids, saturated and unsaturated fatty acids. The H. rhamnoides seed oil protected a UVA-and UVB-treated CDD 1102 KERTr human keratinocyte cell line and CCD 1112Sk human fibroblasts. The oil pretreatment prevents the UVA-stimulated production of ROS and depletion of non-enzymatic antioxidants (thioredoxin (Trx), GSH and vitamins A and E) and enzymatic antioxidants (SOD, glutathione reductase (GSR), GPx, Trx reductase (TrxR)) and stimulated the Nrf2 dependent enzymatic antioxidant HO-1. Sea buckthorn oil also inhibited UVA-or UVB-stimulated phospholipase A2 activity and the production of lipid peroxidation products, 4-hydroxynonenal (4-HNE) and 8-isoprostaglandin F2α. The modulation of endocannabinoid receptors CB1 and CB2 was specific for the different types of UV radiation and skin cells 74 .

Cola acuminata
Cola nut (Cola acuminata Schott & Endl. or Sterculia acuminata) also called "kola nut" is a popular edible plant native to West Africa. The seed of this large tree has been used in African folk medicine namely for aiding digestion and coughs. The main constituents of cola nut are caffeine, theobromine, and polyphenols, including D-catechin, L-epicatechin, procyanidin B1 and B2 and tannins [75][76][77] . The benefitial health effects of substances mentioned above are so notable as chocolade is explored as functional food 76 . Topically applied cola nut extract, and pure alkaloids theobromine, theophylline and caffeine markedly reduced UVA-induced wrinkle formation and histological alterations in dorsal skin of hairless mice, including changes in extracellular matrix proteins and infiltration of leukocytes 77 .

Theobroma cacao
Theobroma cacao L., also called cocoa tree and cacao tree is an evergreen tree in the family Malvaceae, native to tropical regions Central and South America. Cacao beans have been traditionally used to treat the pain of pregnancy, fever, and cough in Central America. Cacao beans contain flavonoids, catechins, epicatechins, procyanidins and xanthine derivatives (caffeine and theobromine). Cacao has been reported to have anti-inflammatory effect, modulates NF-κB and redox sensitive signalling pathways, stimulates immune response, influence insulin resistance and has positive effect on cardiovascular system 78,79 . Topical application of cacao beans extract suppressed wrinkle formation, changes in dermal connective tissue and neutrophil infiltration caused by UVA-irradiation in vivo 77 .

Amaranthus cruentus
Amaranthus cruentus L. is an annual plant with dark pink flowers. Its grains known as amaranth seeds are used as cereal, rich to flavonoids, phenolic acids, essential amino acids, bioactive peptides, micro-and macronutrients including minerals and vitamins. Amaranth seeds contain hydrophilic and hydrophobic antioxidants that contribute anti-inflammatory activity, in lowering risk of the oxidative stress related diseases (diabetes, obesity, cardiovascular disease) and improving gut health. Oil obtained by cold-pressing the grain is valuable due to the presence of unsaturated fatty acids, tocopherols, tocotrienols, phytosterols, and squalene 80,81 . A recent study evaluated effect of pre-and post-treatment with amaranth oil on UVAirradiated human skin fibroblasts. The amaranth oil activity was rather poor and its use in combination with other sunscreens is recommended 82 .

Ferulic acid
Ferulic acid (FA, 4-hydroxy-3-methoxycinnamic acid) is a ubiquitous phenolic acid found in Commelinidae plants (rice, wheat, oats, and pineapple), grasses, grains, vegetables, flowers, fruits, leaves, beans, coffee beans, artichoke, peanut and nuts, in its free or conjugated (e.g., polysaccharides, glycoproteins, polyamines) form. FA is more easily absorbed into the body and stays in the blood longer than any other phenolic acids. FA exhibits a wide range of biological effects including antioxidant, anti-inflammatory, anti-allergic, anticarcinogenic, antimicrobial, antiviral, hepatoprotective, vasodilatory or antithrombotic 83 . Its structure (Fig. 2) is similar to tyrosine and thus FA inhibits melanin formation through competitive inhibition with tyrosine 84 . FA is a strong UV absorber. FA alone 85 or in combination with vitamin C and E (ref. 86,87 ) exhibited a considerable protection against UVB-induced skin damage, including UVB-induced carcinogenesis.
Relative to UVA light, the FA pre-treatment of the B16F10 mouse melanoma cell line led to the inhibition of UVA-induced melanin synthesis as well as tyrosinase activity and its protein expression. FA also reduced UVA-induced ROS and 8-OH-dG formation and GSH depletion 88 . In HaCaT, FA pre-treatment reduced the UVA-induced decrease in cell viability, ROS formation and the induction of MMP-1 activity and mRNA expression. Moreover, FA was able to upregulate GSH content, γ-GCL mRNA level as well as the activities and mRNA expression of CAT and GPx in UVA-irradiated cells 89 . The pre-treatment of NHDF with FA increased cell viability after exposure to various UVA doses. UVA-stimulated G1phase arrest in NHDF was also reduced by FA in a dosedependent manner. Cells pre-treated with FA exhibited its effect on nucleotide excision repair with a significant increase in the expression of the genes of xeroderma pigmentosum complementation group A and C. FA reduced UVA-induced ROS formation and increased the levels of SOD-1 and CAT mRNA. FA also inhibits cellular senescence by reducing senescence-related markers, especially β-galactosidase activity and the mRNA expression of tumour suppressor protein p16, MMP-1 and MMP-3 (ref. 90 ). The in vivo application of FA in the form of a nanogel significantly increased the level of SOD, GPx and CAT and reduced the MDA level in UVA-irradiated rat skin 91 .

Caffeic acid
Another abundant phenolic acid, caffeic acid (CA, 3,4-dihydroxycinnamic acid, Fig. 2) occurs in free and/or various combined forms in a large number of fruits (e.g., blueberries, gooseberries, blackcurrant, orange, lemon, pears), vegetables (e.g., kale, radishes, cabbage, Brussels sprouts, carrots, celery, lettuce, potatoes, eggplant), and herbs (e.g., thyme, sage, basil, oregano) as well as in bee propolis, olive oil and beverages (e.g., apple juice, wine, tea and coffee) (ref. 92,93 ). Studies have demonstrated antioxidant, anti-inflammatory, antitumour and antimeta-  static effects of CA (ref. 93 ). Several papers documented a UVB protective potential of CA (ref. [93][94][95][96]. In a B16F10 mouse melanoma cell line, CA pretreatment reduced UVA-induced melanin synthesis and tyrosinase activity and the protein's expression. CA also reduced UVA-induced ROS and 8-OH-dG production and GSH depletion. CA further inhibited the UVA-mediated downregulation of the nuclear level of Nrf-2, the mRNA level of the γ-GCL catalytic and modifier subunit, GST and NQO1, and the protein level and activity of γ-GCL, GST and NQO1 (ref. 88 ). In HaCaT, CA reduced UVAinduced cytotoxicity, ROS formation and the induction of MMP-1 activity and mRNA expression. Moreover, CA upregulated GSH content, γ-GCL mRNA level as well as the activities and mRNA level of CAT and GPx in UVAexposed cells 89 . The topical and oral treatment of hairless mice suppressed intrinsic and UVA-induced ROS generation in skin tissue 97 .

Cinnamic acid
Cinnamic acid (CIN, 3-phenylacrylic acid) is a phenolic acid generally obtained from cinnamon (Cinnamomum cassia (L.) J. Presl). Other sources include citrus fruits, grapes (Vitis vinifera L.), tea (Camellia sinensis (L.) Kuntze), cocoa (Theobroma cacao L.), spinach, celery and brassica vegetables 98 . CIN processes several pharmacological activities, including antioxidant, antimicrobial, anticancer and anti-inflammatory. CIN exists in trans-or cis-form. The trans-CIN (t-CIN, Fig. 2) is the predominant form in nature due to its high stability 99 . CIN was demonstrated to be a potent inhibitor of tyrosinase activity, it also inhibited tyrosinase expression and melanin production in vitro, and exhibited a depigmenting activity on a UVB-induced hyper-pigmentation model in vivo 100 .
The pre-treatment of Hs68 human foreskin fibroblast-derived cells with t-CIN reduced UVA-induced cyto toxicity, ROS generation, MMP-1 and MMP-3 overexpression and procollagen type I degradation. t-CIN also inhibited UVA-induced photoaging via the suppression of activator protein 1 activation. t-CIN enhanced the nuclear translocation of Nrf2 as well as inducing HO-1 and γ-GCL protein expression. t-CIN-induced Nrf2 translocation was mediated through protein kinase C (PKC), AMP-activated protein kinase, casein kinase II or ROS signalling cascades. Topically applied t-CIN significantly suppressed MMP-1 and MMP-3 activation and maintained sufficient type I procollagen levels in the skin of repeatedly UVAirradiated female athymic nude mice (BALB/c-nu) (ref. 99 ).

Rosmarinic acid
Rosmarinic acid (RA), a phenolic acid, is an ester of caffeic acid and 3,4-dihydroxyphenyllactic acid (Fig. 2). It got its name according to its initial isolation from rosemary (Rosmarinus officinalis). RA is commonly found in species of the Lamiaceae family. However, it is also found in species of other higher plant families and in some fern and hornwort species. RA occurs in species used commonly as culinary and/or medicinal plants such as Ocimum basilicum (basil), Melissa officinalis (lemon balm), Origanum majorana (marjoram), Salvia officinalis (sage), Thymus vulgaris (thyme), Mentha piperita (peppermint) or Prunella vulgaris (self-heal) (ref. 101,102 ). RA has a number of interesting biological activities, e.g., antioxidant, anti-inflammatory, antiviral or antibacterial 101 . Several studies found RA to possess the ability to reduce UVB-induced damage to HaCaT (ref. [103][104][105]. Topically applied RA exhibited a concentration-dependent suppressive effect on intrinsic and UVA-induced ROS generation in the skin of hairless mice 97 . The oral administration of RA to female albino Swiss mice led to a reduction in cutaneous dysplasia provoked by chronic UVA exposure 106 . The post-treatment of UVA-irradiated HaCaT with RA increased cell viability and reduced ROS production, intracellular lipid peroxidation, ATP and GSH depletion as well as DNA damage and the activation of caspase-3 (ref. 107 ).

Curcumin
Curcumin (CUR, diferuloylmethane, Fig. 2) is a yellow-coloured compound extracted from the rhizome of Curcuma longa L. (turmeric). CUR has been employed as a culinary spice and for the treatment of various diseases, including skin disorders and wound healing, dating back to 4000 years ago 108 . CUR decreased UVB-induced oxidative stress, inflammation and DNA damage in vitro 109 and in vivo 110 .
In UVA-exposed NHDF, CUR reduced the formation of ROS, endoplasmic reticulum stress, inflammation and apoptosis, and increased the activity of antioxidant defence enzymes (SOD and CAT). CUR also influenced collagen metabolism by decreasing MMP-1 and MMP-3 expression and promoted the reparation of cells by increasing the expression of transforming growth factor-β (TGF-β) and the protein Smad2/3, and decreased the expression of Smad7, a TGF-β inhibitor 111 .
QE pre-treatment was found to reduce UVA-induced ROS and 8-OH-dG production, GSH depletion, melanin synthesis as well as tyrosinase activity and the protein's expression in B16F10 melanoma cells. QE also inhibited the UVA-mediated downregulation of nuclear Nrf2 level and Nrf2-antioxidant responsible element (ARE) transcriptional activity in cells. QE prevented the UVA-mediated mRNA downregulation of GST, NQO1 and γ-GCL catalytic and modifier subunit as well as the decrease in protein expression and activity of the Nrf2 target antioxidant enzymes (particularly GST, NQO1 and γ-GCL) (ref. 88 ). Further, QE pre-treatment suppressed UVA-induced ROS production, GSH depletion and apoptosis in HaCaT (ref. 117 ). QE pre-treatment significantly decreased UVA-induced DNA damage to C3H10T1/2 mouse embryo fibroblasts) (ref. 118 ) and inhibited UVAstimulated collagenase protein and mRNA level in NHDF (ref. 119 ). In the EpiDerm™ model, QE significantly decreased UVA-induced MMP-1 and tumour necrosis factor alpha (TNF-α) secretion 120 . The intraperitoneal application of QE before repeated UVA exposure resulted in a reduced MDA level and increased activities of GPx, GSR, CAT and SOD in the skin 121 , erythrocytes 122 and liver tissue 123 of female Spraque-Dawley rats. QE was also found to enhance the photostability (UVA+UVB) of the two UV filters, butyl methoxydibenzoylmethane and octyl methoxycinnamate in oil-in-water emulsions 124 .
On the other hand, QE was shown to undergo slow decomposition to a mixture of C-ring-opened products. The presence of a triplet sensitizer greatly increases UV radiation-mediated QE decomposition. Thus the presence of endogenous photosensitizers in the skin could potentially affect the UV stability of QE, suggesting that the further study of QE for both its photoprotective properties and photostability in skin is warranted 125 . Besides a dosedependent photodegradation in aqueous and organic environments, QE exhibited a phototoxic effect on fibroblasts (Balb/c 3T3 cells and NHDF) and keratinocytes (HaCaT and NHEK). QE pre-treatment and subsequent UVA ex-posure further resulted in increased ROS production and intracellular GSH level depletion in NHDF (ref. 126 ).
The AP pre-treatment of NHDF reduced MMP-1 protein and mRNA level in UVA-exposed cells 119 . AP further increased cell viability and inhibited ROS production in UVA-irradiated HaCaT. AP also decreased UVAstimulated mRNA expression and activity of MMP-1 as well as the protein and mRNA expression of transcription factors c-Jun and c-Fos and the phosphorylation of MAPK, particularly ERK1/2, JNK1/2 and p38 MAPK. AP further decreased the UVA-induced influx of Ca 2+ into HaCaT and the phosphorylation of Ca 2+ /calmodulin-dependent kinases 130 . Pre-treatment with AP also protected NHDF against UVA-induced senescence. The AP posttreatment of UVA-exposed NHDF increased cell viability and decreased the expression of MMP-1 (ref. 128 ). A photosafety study found AP photostability in the UVA and VIS range and no phototoxic potential in the 3T3 neutral red uptake phototoxicity test after UVA/VIS exposure 131 .
LU exhibited an inhibitory effect on the MMP-1 protein and mRNA level in NHDF (ref. 119 ). Pre-treatment of HaCaT with LU increased cell viability and reduced ROS generation after UVA exposure. LU also inhibited UVAinduced mRNA expression and the activity of MMP-1 as well as the protein and mRNA expression of transcription factors c-Jun and c-Fos and the phosphorylation of kinases (ERK1/2, JNK1/2 and p38 MAPK). LU further decreased the UVA-induced influx of Ca 2+ into HaCaT and the phosphorylation of Ca 2+ /calmodulin-dependent kinases 130 . The application of LU to UVA-exposed NHDF decreased ROS production and non-apoptotic programmed cell death (autophagy). In addition, the protein expression of hypoxia inducible factor-1α and the classical autophagy-associated proteins, microtubule-associated protein light chain 3 and beclin 1 were decreased in the UVA-irradiated NHDF (ref. 133 ).

Rutin
Rutin (RU, 3,3′,4′,5,7-pentahydroxyflavone-3-rhamnoglucoside) also known as rutoside, quercetin 3-rutinoside, and sophorin, is a flavonol glycoside composed of quercetin and the disaccharide rutinose (Fig. 2). RU is abundantly found in plants, such as passion flower, buckwheat, tea, apples, lemons, and onions. RU exhibits several pharmacological properties including antioxidant, anticarcinogenic, cytoprotective, antiplatelet, antithrombotic, vasoprotective, cardioprotective and neuroprotective 134 . As for the skin, RU was shown to increase skin elasticity and decrease signs of skin aging, especially the length, area and number of wrinkles in human volunteers. RU also increased the mRNA expression of collagen I and decreased the mRNA expression of collagenase in NHDF (ref. 135 ). The UVB protection of RU is also quite well documented in vitro and in vivo 134,[136][137][138][139] .
The RU pre-treatment of NHDF strongly protected against the UVA-induced increase in expression of proteins involved in antioxidant (such as SOD, TrxR, and peroxiredoxins 1/2) and inflammatory (e.g., IL-17, serine/ threonine-protein kinase 2, and tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein zeta) response 136 . RU also reduced the UVA-induced oxidative modification of the membrane of CCD 1112Sk human fibroblasts, specifically the negative charge on the membrane surface, sialic acid concentration and level of lipid peroxidation 137 . In another study on CCD 1112Sk human fibroblasts, RU application also counteracted ROS generation and prevented UVA-and UVB-induced changes in the composition of membrane phospholipids and free fatty acids content, the oxidation of phospholipids and biomembrane destruction, depletion of the GSH/GSSG ratio, GPx activity and levels of vitamins E and C (ref. 140 ). RU further reduced UVA-induced pro-inflammatory response and ROS generation, and enhanced the activity/ level of antioxidants (SOD, GPx, vitamin E, GSH, and Trx). RU also normalized UVA-induced Nrf2 expression and prevented changes in the levels of the lipid peroxidation markers MDA and 4-HNE. RU pre-treatment prevented the protein modifications (particularly tyrosine derivatives formation) and decreased the levels of the proapoptotic markers caspase-3 and cytochrome c (ref. 138 ). The pre-treatment of B16F10 cells with RU lead to the inhibition of UVA-induced melanin synthesis as well as the activity and protein expression of tyrosinase, a reduction in UVA-induced ROS and 8-OH-dG formation and GSH depletion 88 . The pre-treatment of C3H10T1/2 fibroblasts with RU significantly decreased UVA-induced DNA damage 118 . RU also reduced UVA-stimulated damage to a CDD 1102 KERTr immortalized keratinocyte cell line and CCD 1112Sk human fibroblasts. RU especially stimu-lated antioxidant enzyme activity (CAT, SOD, TrxR) and reduced ROS generation. The effect of RU was increased by its combination with ascorbic acid, which besides other effects stimulated the UV-induced activity of the transmembrane transporter bilitranslocase, responsible for the transport of RU into cells 139 .
The DHM pre-treatment of HaCaT attenuated UVAinduced ROS generation, mitochondrial membrane potential decrease, lipid peroxidation, GPx protein decrease and the phosphorylation of histone γ-H2AX, a sensitive biomarker for DNA damage. DHM also reduced the mRNA level of IL-1β, IL-6, COX-2 and Bax, increased the expression of the anti-apoptotic proteins Bcl-2 and Bcl-xL, and decreased the expression of the pro-apoptotic protein Bax as well as the activation of caspase-3. Additionally, DHM treatment also prevented the nuclear translocation of NF-κB/p65 and phosphorylation of c-Jun (ref. 146 ).

Genistein
The isoflavone genistein (GE, 5,7,4'-trihydroxyisoflavone; 5,7-dihydroxy-3-(4-hydroxyphenyl)chromen-4-one, Fig. 2) is abundantly found in soy and other legumes. GE also exists in the glycosylated form genistin. GE acts as a selective estrogen receptor modulator. Several in vitro and in vivo studies demonstrated the beneficial effects of GE on skin, e.g., for the treatment of keloid scars, stimulation of collagen biosynthesis, acceleration of wound healing or protection of UVB-induced photodamage 147 .
As for UVA light, GE suppressed 8-OH-dG formation in purified UVA-irradiated DNA (ref. 148 ). The application of GE significantly decreased psoralen and UVA-induced (PUVA) photodamage in hairless mice, especially skin thickening, cutaneous erythema and ulceration, inflammatory changes throughout the epidermis and PCNApositive cells, and completely inhibited the cleavage of poly (ADP-ribose) polymerase and caspase-3 (ref. 149 ).

Hesperidin
Hesperidin (HE, (3′,5,7-trihydroxy-4′-methoxyflavanone-7-rutinoside) is a flavanone glycoside, comprised of the aglycone hesperetin and the disaccharide rutinose (Fig. 2). HE is isolated in large amounts from the rinds of the ordinary orange Citrus aurantium L., Citrus sinensis, Citrus unshiu and other species of the genus Citrus (family Rutaceae). It has been reported to occur in many plants other than Citrus, such as in the families Fabaceae, Betulaceae, Lamiaceae and Papilionaceae. HE has been reported to possess significant antioxidant, anti-inflammatory, analgesic, immunomodulatory and wound healing effects 147 . Several reports also demonstrated a UVB-protective potential of HE in vitro 151,152 and in vivo 153,154 .
HE significantly prevented UVA-induced oxidative damage, in particular increased cell viability and SOD activity and reduced MDA level in HaCaT. Also a decrease in TNF-α, IL1-β and IL-6 protein and mRNA was observed 155 .

Chrysanthemin
Chrysanthemin is one of the most common anthocyanins, also known as kuromanin or cyanidin-3-O-glucoside and cyanidin-3-O-β-glucopyranoside (CG; Fig. 2) CG is found in a variety of fruits and vegetables, including strawberries, blackberries, blueberries, raspberries, cherries, pomegranate, pigmented oranges, red cabbage, black rice, black beans or purple sweet potatoes 156 . CG is a potent antioxidant and protects against oxidative damage 157 . CG was also reported to reduce UVB-induced damage in vitro [158][159][160] and in vivo 161,162 .
The CG pretreatment of HaCaT cells inhibited the UVA-induced formation of apoptotic cells and DNA fragmentation as well as hydrogen peroxide production 157 . CG increased cell viability, Bcl-2 expression, Bcl-2/Bax ratio and ERK phosphorylation and decreased ROS production and the levels of caspase-3 and phosphorylated p38 in UVA-irradiated NHDF (ref. 163 ).

Resveratrol
Resveratrol (RE, trans-3,5,4'-trihydroxystilbene, Fig. 2) is a polyphenol belonging to the stilbenoid class. It is a phytoalexin synthesized mainly in grapes, with higher quantities in the red varieties. Other plant sources include peanuts, hops, cacao and berries (blueberries, bilberries, and cranberries) (ref. 164 ). RE is an effective antioxidant and anti-inflammatory agent. In vitro studies demonstrated its antiproliferative activity in various skin cancer cells.
Several in vitro and in vivo studies have also demonstrated the UVB photoprotective activity of RE (ref. 164,165 ).
In retinal pigment epithelial (RPE) cells, RE reduced the UVA-provoked decrease in cell viability, the generation of intracellular hydrogen peroxide and COX-2 expression. RE also lowered the UVA-induced activation of ERK, JNK and p38 kinase in RPE cells 166 . The pre-and posttreatment of HaCaT cells with RE resulted in an increase in cell viability, and a reduction in lipid peroxidation and ROS production. Moreover, RE facilitated Nrf2 accumulation in the nucleus; as a result, the activities of the antioxidant enzymes GST and SOD were also enhanced. Kelch-like ECH-associated protein 1 (Keap1), a repressor of Nrf2 in the cytoplasm, was clearly reduced by RE treatment while the level of Keap1 mRNA was still increased 167 . A derivative of RE, oxyresveratrol (oxyRE), increased cell viability and suppressed intracellular ROS when cells were pre-treated before UVA exposure. Posttreatment with oxyRE reduced nitrotyrosine, 8-OH-dG and CPDs formation and stimulated p53 expression 168 .
On the other hand, the pre-treatment of HaCaT and NHEK with RE caused a massive increase in oxidative stress in mitochondria, leading to a decrease in the mitochondrial membrane potential and consequently to apoptosis of the keratinocytes 169 . Another study showed that RE potentiates the production of 8-OH-dG in UVAirradiated genomic DNA. Moreover, the combination of RE with UVA significantly enhanced the production of DNA strand breaks and the cell death of HaCaT (ref. 170 ). In NHEK, RE enhanced the response stimulated by UVA+UVB (10:1), as it had a synergistic effect on the activation of the aryl hydrocarbon receptor pathway, cytochrome P450 1A1 transcription and IL-8 expression, as well as on NFκB activation and the nuclear translocation of the epidermal growth factor receptor 171 . These suggest that RE might have a potentially hazardous effect when topically applied to the skin, especially on regions exposed to sunlight.
EA pre-treatment markedly increased HaCaT cell viability and suppressed UVA-induced ROS generation and lipid peroxidation. Moreover, EA pre-treatment reduced single-strand break formation and apoptosis by reducing DNA fragmentation, mitochondria dysfunction, endoplasmic reticulum stress, caspase-3 activation, and Bcl-2/Bax deregulation in irradiated cells. The antioxidant potential of EA was demonstrated as the downregulation of Keap1 and the augmented nuclear translocation and transcriptional activation of Nrf2 both with and without UVA irradiation, and increased the expression of HO-1 and SOD (ref. 173 ).
The pre-treatment of NHDF with CAR suppressed the MMP-1 mRNA elevation stimulated by UVA irradiation 176 . In UVA-exposed Hs68 human skin fibroblasts, CAR not only reduced the mRNA expression of MMP-1, but also of MMP-3 and MMP-9 (ref. 175 ).

Asiatic acid
Asiatic acid (AA, 2α,23-dihydroxyursolic acid), also known as dammarolic acid, is a pentacyclic triterpenoid (Fig. 2). It is abundantly present in many edible and medicinal plants including Centella asiatica, which is a renowned herb in many traditional medicine formulations 175 . The herb grows in the tropical regions of Asia, Oceania, Africa and America. In traditional Asian medicine, the herb has been used for hundreds of years to improve the healing of small wounds, scratches, burns, and hypertrophic wounds. Previous studies demonstrated that AA could be also used for wound healing and various dermal applications 180,181 . AA exhibited numerous pharmacological activities such as antioxidant, anti-inflammatory, hepatoprotective, cardioprotective, neuroprotective, and anticancer properties 179 . The pre-treatment of HaCaT with AA decreased UVA-induced ROS production, lipid peroxidation, MMP-2 activity and protein level and p53 protein expression 178 .
ZER pre-treatment substantially suppressed UVAinduced HaCaT cell death and LDH release, ROS production, DNA single-strand breaks, DNA fragmentation and a dysregulated Bax/Bcl-2 ratio. ZER cytoprotective properties were associated with an increased nuclear translocation of Nrf2 and elevated ARE luciferase activity. The activation of Nrf2/ARE signalling was accompanied by the induction of HO-1 and γ-GCL catalytic subunit genes. ZER-induced Nrf2 transcriptional activation was mediated by the p38 MAPK, phosphoinositide 3-kinase/protein kinase B (AKT) and PKC signalling cascades. In the UVA-treated skin of nude mice, ZER pre-treatment significantly ameliorated UVA cytotoxicity via increased nuclear Nrf2 translocation and Nrf2-dependent antioxidant gene expression (HO-1 and γ-GCL catalytic subunit) (ref. 184 ).

beta-Carotene
β-Carotene (β-CAR) is a tetraterpenoid that consists of eight isoprene units (40 carbon atoms) in a core structure of conjugated double bonds substituted with two β-ionone rings (Fig. 2). β-CAR is a strongly red-orange pigment that occurs in many different fruits and vegetables such as mango, papaya, orange, apricot, watermelon, cantaloupe, pumpkin, carrot, red pepper, tomato and sweet potato. The colour of β-CAR in some bright green leafy vegetables is masked by chlorophyll, e.g. in spinach, kale, lettuce, broccoli or cabbage 185,186 . A number of studies investigated the effect of β-CAR on the prevention of solar erythema formation with contradictory results. The literature analysis implies that doses of about 10 mg/day are required to provide UVB photoprotection 186 .
As for UVA radiation, β-CAR inhibits JNK activation and the mRNA and protein expression of TNF-α, IL-1β, and IL-10 in UVA-exposed HaCaT (ref. 187 ). In the same cells, β-CAR suppressed the UVA-induced mRNA level of MMP-1, MMP-3, and MMP-10, three major MMPs involved in photoaging 188 . Microarray analysis in irradiated HaCaT showed that β-CAR inhibited UVA-induced extracellular matrix degradation and enhanced UVA-stimulated tanning-associated protease-activated receptor 2 (ref. 189 ). The application of β-CAR to NHDF reduced the level of mtDNA mutagenesis provoked by repeated UVA expo-sure 190 . β-CAR pre-treatment resulted in suppression of the UVA-induced transcriptional activation of HO-1 in normal human FEK4 fibroblasts 191 and induced the mRNA and protein level of pro-inflammatory IL-6 as well as the protein level of HO-1 in human skin HFP-1 fibroblasts 192 . The application of -CAR to rat kidney fibroblasts prior to UVA exposure significantly increased the activities of CAT and SOD, while the level of TBARS was reduced 193 .
On the other hand, the pre-treatment of C3H10T1/2 fibroblasts with β-CAR increased UVA-induced DNA damage. The combination of β-CAR with flavonoids (naringin > RU > QE) significantly suppressed the photo-oxidative effect of β-CAR (ref. 118 ). In another study, β-CAR increased membrane damage, stimulated HO-1 expression and induced caspase-3 activity in NHDF exposed to UVA light 194 . Similarly, in UVA-irradiated HFP-1 fibroblasts, β-CAR pre-treatment strongly enhanced HO-1 mRNA and protein expression. The level of this oxidative stress marker was suppressed by the concomitant addition of vitamin E, but only moderately by vitamin C (ref. 195 ). These effects may be associated with the photodecomposition of β-CAR stimulated by UVA radiation that was shown in vitro. The rapid decomposition of β-CAR was further accelerated by sulphide and reduced by radical scavengers (dithiothreitol, thiourea) (ref. 196 ).

Astaxanthin
Astaxanthin (AST, 3'3-dihydroxy-β,β-carotene-4,4'dione) is a red-orange lipophilic xanthophyll carotenoid, which is structurally similar to β-CAR but possesses an additional hydroxyl and ketone group on each β-ionone ring (Fig. 2). AST is a ubiquitous secondary metabolite naturally synthesized by a number of plants as well as bacteria, yeasts, microalgae crustaceans and salmonids. The commercial production of AST has traditionally been by chemical synthesis, but the microalga Haematococcus pluvialis seems to be the most promising source for its industrial biological production. AST has several essential biological functions in marine animals, including pigmentation, stress tolerance, reproductive capacity, protection against UV light effects, immune response and protection against the oxidation of macromolecules. From several studies including clinical trials, beneficial effects of AST on skin physiology and pathology are well documented including antioxidant, anti-inflammatory and UVB photoprotective action 197 .
The pre-treatment of NHDF with AST reduced the disruption of cell membrane integrity, level of intracellular ROS, amount of TBARS, number of apoptotic cells as well as the activity of caspase-3. AST also counteracted the decrease in CAT and SOD activity and reduced the mRNA and protein level of HO-1. The application of AST to rat kidney fibroblasts before UVA irradiation increased the activities of CAT and SOD and reduced the level of TBARS. AST was much more effective than the other carotenoids (β-CAR and lutein) (ref. 193 ). AST also reduced oxidative DNA damage to human neuroblastoma and rat trachea epithelial cells exposed to UVA radiation 198 . Synthetic AST and an algal extract (14% of AST) de-creased UVA-induced DNA alterations to 1BR-3 human skin fibroblasts, a human melanocyte cell line and human intestinal CaCo-2 cells. The algal extract ex hibited a comparable UVA protection to 10 μM AST on all three of the above cell types. Further, in 1BR-3 cells, the synthetic AST and algal extract prevented UVA-induced alterations to SOD activity and GSH content. Similarly, AST and the extract prevented the depletion of GSH in CaCo-2 cells 199 . The addition of AST to NHDF immediately after UVA exposure significantly attenuated the induction of MMP-1 and neutral endopeptidase expression at the gene and protein level as well as reducing MMP-1 and skin fibroblast elastase activity and IL-6 secretion 200 . Dietary AST reduced features of chronic UVA exposure such as TEWL and wrinkle formation in female hairless mice. AST treatment also reduced the mRNA level of epidermal (lympho-epithelial Kazal-type related inhibitor, steroid sulfatase and aquaporin 3) and dermal (MMP-13) markers increased by chronic UVA irradiation 201 .

Lycopene
Lycopene ( (Fig. 2). The major sources for LY are tomatoes and tomato products, although red carrot, apricot, papaya, pink grapefruit, guava and watermelon also contain LY in considerable amounts 202 . The pharmacological properties of LY are linked mainly to its antioxidant activity that was demonstrated in many in vitro, ex vivo and in vivo studies 186 .
On the other hand, LY enrichment enhanced UVAinduced HO-1 expression and strongly induced DNA damage in UVA-irradiated C3H10T1/2 mouse embryo fibroblasts 204 . Similarly, LY pre-treatment of NHDF potentiated the UVA-induced elevation of MMP-1 and HO-1 mRNA levels 176 .
The application of LT to rat kidney fibroblasts prior to UVA exposure protected the cells against UVA-induced decrease in CAT and SOD and increase in TBARS level 193 . LT also reduced oxidative DNA damage in human neuroblastoma and rat trachea epithelial cells exposed to UVA radiation 198 . In a placebo-controlled, double-blind, randomized crossover study, the oral treatment of healthy volunteers with LT completely inhibited UVA1 (340-400 nm) and UVA/B induced the mRNA upregulation of HO-1, ICAM-1 and MMP-1 (ref. 203 ).
The application of ZEA reduced oxidative DNA damage in human neuroblastoma and rat trachea epithelial cells exposed to UVA radiation 198 .
On the other hand, the rapid decomposition of ZEA stimulated by UVA radiation was demonstrated in vitro. The photodecomposition of ZEA was more rapid than that of β-CAR due to the presence of hydroxyl groups. The decomposition was accelerated by sulphide and reduced in the presence of dithiothreitol or thiourea 196 .

L-Ergothioneine
The low-molecular-weight thiol-based antioxidant L-ergothioneine (EGT, 2-mercaptohistidinetrimethylbetaine) is a derivative of the amino acid histidine. EGT has two functional groups: an L-thiol (SH) group and a quaternary ammonium group N + -(CH 3 ) 3 (Fig. 2). Despite apparently being exclusively synthesized in fungi, cyanobacteria and mycobacteria, EGT is ubiquitously present in most cells and the tissues of plants and mammals. Some foods were found to possess particularly high levels of EGT, for example certain species of mushrooms, black and red beans, red meat, liver, kidney and grains. Numerous in vitro assays have demonstrated the antioxi-dant and cytoprotective capabilities of EGT against a wide range of cellular stressors, including UV and gamma radiation 210 .
In UVA-exposed NHDF, EGT reduced the protein and mRNA level of MMP-1 (ref. 211 ). The EGT treatment of HaCaT prior to UVA exposure significantly prevented LDH leakage into the medium. UVA-induced ROS formation and DNA oxidative damage was remarkably suppressed by EGT, with a parallel inhibition of apoptosis. Furthermore, EGT alleviated UVA-induced mitochondrial dysfunction. A dose-dependent increase in the antioxidant proteins HO-1, NQO-1 and γ-GCL, and reduced GSH produced by EGT was associated with an upregulated Nrf2 and downregulated Keap-1 level 212 . EGT enhanced the level of reduced GSH and protected NHDF from the induction of photoaging-associated mtDNA damage (mutation and deletion) (ref. 213 ).

CONCLUSION
The exposure of human skin to solar radiation intensifies skin aging commonly known as photoaging and escalates the probability of developing skin cancer. Although skin protection against solar radiation has been studied for decades, research was focused on UVB, and UVA was mostly ignored. The plant kingdom represents the main conventional source of new candidates with photoprotective activity. In this review, phytochemicals and plant extracts with UVA-protective potency are summarized. As documented, there is a range of phytochemicals that possess UVA protective properties, many of them also protect against UVB photons and thus may be useful in the protection and treatment of UV light-caused damage; alone or in combination with traditional UV filters. However, most of the compounds and extracts listed in this review were only tested in very simple in vitro or possibly ex vivo systems. This fact results from the prohibition of animal use for this purpose in many countries around the world. The safety of new photoprotective phytochemicals may be based on their long-term use in traditional medicine. However, clinical trials are needed to validate the effectiveness of new candidates for photoprotective preparations. The demanding nature and ethical limits of studies on human volunteers is most likely the most restrictive step in the rapid introduction of new photoprotective agents for practical use.

Search strategy and selection criteria
The aim of our research strategy was to evaluate well documented studies on the UVA photoprotective properties of phytochemicals and plant extracts. The articles from 1998 to 2019 were searched using the PubMed and Web of Science databases. All searches were up-to-date since January 2020. The search terms used included "UVA photoprotection", "UVA protection", "plant and UVA protection", "polyphenols and UVA protection". Only articles in English were reviewed.