Photoprotection of natural flavonoids

Article history: Received on: 28/07/2013 Revised on: 07/08/2013 Accepted on: 25/08/2013 Available online: 30/09/2013 Overexposure to ultraviolet (UV) radiation can cause a number of skin disorders such as erythema, edema, hyperpigmentation, immunosuppression, photoaging, and skin cancer. Since the level of UV radiation is increasing as a result of depletion of the stratospheric ozone and climate change, the protection of human skin against the harmful effects of UV radiation is an urgent need. Topical application of sunscreens is a strategy to protect the deleterious effect of UV radiation on the skin. Sunscreens today contain one or several synthetic UV filter molecules which protect the UV radiation exposed on the epidermis. These molecules are broadly divided into physical and chemical agents. Physical sunscreens reflect and scatter UVB, UVA and visible radiation. Chemical sunscreens act by absorbing ultraviolet radiation and re-emitting chemical energy as heat or light. Several synthetic UV filter molecules are available in the market but they have limited use because these active molecules may create adverse effects on human skin. Some information on possible photon induced reaction such as photoirritation, photosensitization and contact dermatitis by sunscreen products containing synthetic organic sunscreen agents. To overcome these side effects, naturally occurring compounds have gained considerable attention as photoprotective agents. Flavonoids, a group of natural occurring compounds, act as catalysts in the light phase of photosynthesis and as stress protectants in plant cells by scavenging reactive oxygen species (ROS). Natural flavonoids have the potential photoprotection because of their UV absorbing, their ability to act as direct and indirect antioxidants as well as anti-inflammatory and immunomodulatory agents which provide exciting platforms for the development of photoprotection. This review summarizes the structure and potential photoprotection activity of several natural flavonoids.


INTRODUCTION
Sun light composed of various wavelengths ranging from ultraviolet light through infrared to visible light.Among all, ultraviolet light is the most harmful to the skin (Svobodová et al., 2003).The amount of ultraviolet (UV) radiation reaching the Earth's surface has markedly increased in recent years (McKenzie et al., 2003).These phenomena related to the continuously growing of human skin disorders due to overexposure to UV radiation (Svobodová et al., 2003).The ultraviolet radiation (UVR) is divided into three categories dependent on wavelengths: short wavelengths UVC (200-280 nm), medium wavelengths UVB (280-320 nm), and long wavelengths UVA (320-400 nm) (Afaq et al., 2002;Duthie et al., 1999).

Effects of UV radiation on the skin
UVC is extremely damaging to the skin, even very short exposure.Fortunately, UVC radiation is completely absorbed by molecular oxygen and ozone in the Earth's atmosphere (Afaq et al., 2002).
. UVB radiation, known as burning rays, makes up 4-5% of UV light.UVB can penetrate the epidermis layer of the skin (160-180 nm) and considerate as the most active constituent of solar light.
UVB is 1000 times more capable of causing sunburn than UVA.It induces direct and indirect adverse biological effects, including the formation of pyrimidine photoproducts (Katiyar 2005), isomerization of trans-to cis-urocanic acid (Capote et al., 2006), induction of ornithine decarboxylase activity (Ahmad and Gilliam, 2001), stimulation of DNA synthesis (Andley et al., 1996), free radical production (Aitken et al., 2007), photoaging (Park et al., 2010) and photocarcinogenesis (Gruijl, 2000).It is considered to be responsible for inducing skin cancer (squamous and basal cell carcinoma) and immunosuppression (Adhami et al., 2008;Afaq and Mukhtar, 2001).UVA radiation, known as aging rays, represents more than ninety percent of UV radiation that reaches the Earth's surface (Svobodová et al., 2003).It can penetrate deeper into the epidermis and dermis of the skin (around 1 mm) and advance the generation of reactive oxygen species (ROS) (Wondrak et al., 2006).Compared to UVB, UVA is about 1000 times more .effective in the production of an immediate tanning effect which is caused by darkening of the melanin in the epidermis (Brenner and Hearing, 2008).Chronic UVA exposure can damage underlying structures in the dermis and cause premature photoaging of the skin (Ichihashi et al., 2009).It mainly causes skin sagging rather than wrinkle (Krutmann, 2001).UVA can produce structural damage to the DNA, impair the immune system, and lead to cancer.It has been linked to 67 % of malignant melanoma (Afaq et al., 2002;Adhami et al., 2008;Afaq and Mukhtar, 2002).
To exert biological effects, cellular chromophores must be absorbed UVR and transform the energy into a biochemical signal.Subsequent photobiochemical reactions induce biological responses resulting in harmful effects in the skin occurrence.The harmful effects of UVR in the skin can be divided into acute (sunburn or erythema, phototoxic reactions, photoallergy and photosensitivity) and chronic (photoaging, skin cancer and immunesuppression) (Adhami et al., 2008).The cellular chromophores for UVB radiations are nucleic acids, amino acids, such as tryptophan and tyrosine, quinines, flavins, porphyrins, and urocanic acid whereas cellular chromophore absorbing UVA radiations is only trans-urocanic acid (Goihman-Yahr, 1996).The main lesions induced by UVB are cyclobutane-pyrimidine dimers (CPDs) and 6-4 pyrimidine-pyrimidone (6-4PP) photoproducts.The DNA photoproduct blocks the RNA transcription leading to the activation of p53 gene that induces apoptosis of irradiated keratinocytes.Repair of photolesions is the primary response to DNA photodamage in surviving cells.However, if the damage persists into the S phase of the cell cycle, other repair mechanisms might lead to mutagenesis resulting mainly in characteristic cytosine (C) to thymine (T) substitution.When such mutations occur in the p53 gene, keratinocytes lose their ability to undergo the apoptotic process following high-dose UV exposure that results in the development of skin cancer (Clydesdale et al., 2001;Trautinger, 2001).
The exposure of skin to UV radiation results in the generation of ROS and reactive nitrogen species (RNS).The predominant ROS produced upon UV radiation are hydroxyl radical, superoxide anion and peroxyl radical and their active precursors namely singlet oxygen, hydrogenperoxide and ozone (Inal et al., 2001;Thiele and Elsner, 2001).Nitric oxide and nitric dioxide are RNS produced by UV radiation.ROS are constantly generated in keratinocytes and fibroblasts, and are rapidly removed by nonenzymic and enzymic antioxidant substances to prevent the living system from the harmful effects of free radicals.These maintain a proxidant/antioxidant balance results in the stabilization of cell structure.Excess of free radicals results in a cascade of events mediating progressive deterioration of cellular structure and function lead to a loss of cellular integrity by modification of DNA and also to abnormal expression of cellular genes.UV-generated ROS affect mitogen-activated protein kinase (MAPK) pathway.The nuclear factor kappa B (NF-κB) and activator protein 1 (AP1) families of transcription factor are then activated.Both may contribute to the induction of heme oxygenase-1 (HO-1) and matrix metalloproteinases (MMPs) in the skin.Increased level of HO-1 may elevate cellular levels of iron that can promote further ROS generation.MMPs induction leads to enhanced degradation of extracellular matrix proteins that favor wrinkle formation and metastases.ROS damage cell membranes by peroxidation of fatty acids within the phospholipid structure of the membrane.During this process, lipid peroxide radicals, lipid hydroperoxides and other fragmentation products, that are themselves active oxidizing agents, are formed.The lipid peroxides are comparatively longer-lived species and can initiate the chain reactions causing disruption of cell membrane functions (Thiele and Elsner 2001).
Moreover, exposure to UV radiation causes inflammation as well as local and systemic immunosuppression in both animals and humans ( Kripke, 1984;Halliday, 2005;Katiyar and Elmets 2001).Alterations in the morphology and function of antigenpresenting Langerhans cells, release of immunosuppressive cytokines, and enhanced prostaglandin synthesis have all been reported (Simon et al., 1992;Vermeer and Streilein 1990;Chung et al., 1986).Upregulation of TNF-alpha is a key early response to UVB by keratinocytes, and represents an important component of the inflammatory cascade in skin.UVB irradiation induces TNFalpha expression in both keratinocytes and fibroblasts, with TNFalpha mRNA induction seen as early as 1.5 h after UVB (Bashir et al., 2009).The immediate reaction also includes epidermal keratinocyte release of pro-inflammatory cytokines, interleukin-1 (IL-1) (Chung et al., 1996;Kondo et al., 1994;Feldmeyer et al., 2007) and then trigger the synthesis of other proinflammatory cytokines (Schwarz et al., 1986;Chung et al., 1996) which has numerous targets, such as other secreted cytokines, chemokines, growth factors, and their receptors, with broad implications to inflammatory responses (Yano et al., 2008).Thus, release of IL-6 and IL-8 induces further inflammation (Kondo et al., 1993).This release is believed to be due to DNA lesions characteristic to UVR, cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (Setlow and Carrier, 1966;Mitchell and Nairn, 1989;Mitchell, 1988).This cytokine UVB radiation induces not only IL-1 and IL-6, and TNF-alpha, but also IL-10 and IL-12, UVA radiation, however, induces only IL-10, mainly produced by dermal CD11b + macrophages and neutrophils that infiltrate epidermis after intense UV (Ichihashi et al., 2009).UV radiation exerts its immunosuppressive effects in several different ways by (i) inhibiting the function of antigen-presenting cells, (ii) inducing T cells with suppressor activity, and (iii) causing the release of immunosuppressive cytokines (Schwarz et al., 2002).IL-10 appears to be a key mediator of UV radiation-induced immunosuppression (Loser et al., 2007).In contrast to the effects of  prevents UVB radiation-induced local immunosuppression (Schwarz et al., 1996).Importantly, IL-12 was found to accelerate the removal of UV-induced DNA lesions in keratinocytes by inducing nucleotide-excision repair, strongly suggesting that it plays a protective role in photocarcinogenesis of the skin (Schwarz et al., 2002).Erythema is one of the clinically detectable consequences of UV radiation damage to the skin in humans.It occurs within 24 h after exposure to UV radiation (Andersen et al., 1991;Andersen et al., 1992) and can be visually assessed and further objectively quantified using reflectance spectroscopy (Andersen et al., 1990;Wagner et al., 2002).Furthermore, erythema is considered to be a biomarker of skin photodamage that can ultimately lead to skin cancer (O'Donovan et al., 2005;Young et al., 2000).

Photoprotection
The first defense against the sun is production of melanin which absorbs dangerous UV rays and thus protects skin cells from the detrimental effects of UV exposure (Pathak, 1995).In certain circumstances, the amount of melanin produced is not sufficient enough to protect the skin.Several strategies are applicable for protecting of skin against adverse effects of UV exposure.Sun avoidance during the peak UV radiation hours and the use of photoprotective clothing, wide-brimmed hats, and sunglasses, complemented with the use of broadspectrum sunscreens are principles of photoprotection (González, 2008).Sunscreen products contain sunscreen agents that can absorb or reflect UV radiation at the skin surface and thus protect the skin from the harmful effects of solar UV radiation.Based on their mechanism of action, sunscreen agents can be classified into two broad categories: physical and chemical sunscreens (Lowe et al., 1997).Physical sunscreens contain inert particles such as zinc oxide or titanium dioxide that reflect photons of both UVA and UVB away from the skin (Lowe et al., 1997).These particles also reflect visible photons therefore they are often visible on the skin surface lead to cosmetically undesirable for customers.Their advantage is that they are chemically inert and hence do not cause allergic sensitization (More, 2007).Chemical sunscreens are generally organic aromatic compounds conjugated with carbonyl group (Rai, 2012).This chromosphere allows the molecule to absorb ultraviolet rays and release lower energy rays thereby preventing the skin from damaging effects of UVR (Rai, 2012).Typical example for chemical sunscreen includes oxybenzone, sulisobenzone, octyl methoxy cinnamate, etc.Chemical sunscreens are more cosmetic appeal since they are invisible on the skin surface (Rai, 2012).However, UV absorption may activate organic sunscreens and they may consequently interact with cutaneous molecules, causing adverse skin reactions.Some of chemical sunscreen namely aminobenzoic acid and its esters, cinnamates and oxybenzone can cause contact dermatitis or photosensitivity reactions (Dromgoole and Maibach, 1990).Thus, the photoprotection from naturally occurring substances has gained considerable attention in recent years due to their wide range of biological activities.

Natural Flavonoids as Photoprotection
Flavonoids are a class of secondary plant phenolics with significant antioxidant and chelating properties.These substances were known for their beneficial effects on health long before these natural products were isolated.More than 4000 varieties of flavonoids have been identified, many of which are responsible for the attractive colors of flowers, fruit, and leaves (Brouillard, 1988).Apart from various vegetables and fruits, flavonoids are found in seeds, nuts, grains, spices, and different medicinal plants as well in beverages, such as wine (particularly red wine), tea, and (at lower levels) beer.In plant, flavonoids play different roles in the ecology.The protective effects of flavonoids in biological systems are ascribed to their capacity to transfer electrons free radicals, chelate metal catalysts (Ferrali et al., 1997), activate antioxidant enzymes (Elliott et al., 1992), reduce alpha-tocopherol radicals (Hirano et al., 2001), and inhibit oxidases (Cos et al., 1998).Furthermore, flavonoids protect plants from solar UV radiation and scavenge UV generated ROS (Shirley, 1996).Therefore, flavonoids have 3 different photoprotection effects including UV absorption, direct and indirect antioxidant properties, and modulate several signaling pathways.
The basic flavonoid structure is comprised of two benzene rings (A and B) linked through a heterocyclic pyran or pyrone (with a double bond) ring (C) in the middle as shown in Figure 1   The subclasses of flavonoids are primarily based on the presence (or absence) of a double bond on position 4 of the C (middle) ring, the presence (or absence) of a double bond between carbon atoms 2 and 3 of the C ring, and the presence of hydroxyl groups in the B ring.In the flavonoid structure, a phenyl group (B ring) is usually substituted at the 2-position of the pyrone ring except for isoflavones the substitution is at the 3-position.Within each subclass, individual compounds differ in the pattern of substitution of the A and B rings (Beecher, 2003).

Flavonols
The structure of flavonols is similar to flavones with addition of a hydroxyl at the 3-position of pyrone ring (C).Flavonols are found mainly in vagetables and fruits (Peterson and Dwyer, 1998).Onion, kale, broccoli, lettuce, tomato, apple, grape, berries, and tea are good sources of flavonols.The greener the leaf is, the more it contains flavonols (Herrmann 1976).The major flavonols found in nature are quercetin, keampferol, myricetin, and isorhamnetin (Survay, 2011).
Quercetin (3,5,7,3',4'-pentahydroxyflavone, Figure 3) is one of the most potent antioxidant compounds (Kim et al., 2006;Survay et al., 2011).It is present in various common fruit and vegetables (apples, grapes, lemons, tomatoes, onions, lettuce, broccoli, kale, cottonseed etc.), beverages (tea, red wine), herbs (Gingko biloba, Apocynum venetum, Poacynum hendersonii, Opuntia ficusindica), olive oil, and propolis from bee hives (Inal et al., 2001;Sestili et al., 1998).Among the flavonols, quercetin is one of the most effective antioxidants due to the o-hydroxy structure in the B ring, the 2, 3 double bond in conjugation with the 4-oxo function in the C-ring and the 3-and 5-OH groups with the 4-oxo function in the A and C rings (Rice-Evans et al., 1996).Quercetin has been demonstrated to protect skin antioxidant systems (glutathione peroxidase, glutathione reductase, catalase and superoxide dismutase activities) and against UVA irradiating damage in rats (Inal et al., 2001;Sestili et al., 1998).The photoprotective properties of quercetin could contribute with its antioxidant properties (Rice-Evans et al., 1996).Quercetin is believed to prevent UV radiation-induced damage to plants due to the increasing of quercetin biosynthesis after exposure to UVB radiation (Wilson et al., 1998;Solovchenko and Schmitz-Eiberger, 2003).This flavonol absorbs UV radiation with absorbance maxima in the UVA ( max = 365 nm) and UVC ( max = 256 nm) suggesting that one plausible photoprotective mechanism would be direct absorption of UV radiation, thereby preventing the formation of ROS and direct DNA damage (Russo et al., 2000).The UV energy absorbed by quercetin may be dissipated as heat, light (Falkovskaia et al., 1998) or through decomposition of quercetin (Fahlman and Krol 2009;Smith et al., 2000).The major photoproducts from either UVA or UVB radiation are 2,4,6trihydroxybenzaldehyde, 2-(3',4'-dihydroxybenzoyloxy)-4,6dihydroxybenzoic acid and 3,4-dihydroxyphenyl-ethanol as shown in Figure 3 (Zvezdanović et al., 2012).Topical application of the quercetin has been shown to be effective in the prevention of UVC radiation-induced liposome peroxidation (Saija et al., 2003).In animal, topical formulations containing quercetin successfully inhibit UVB-induced skin damage (Casagrande et al., 2006;Widyarini, 2006).The photoprotection properties of quercetin and its glucoside, rutin (quercetin-3-O-rutinoside), were evaluated for sun protection factor (SPF) in sunscreen creams (Choquenet et al., 2008).The results showed that 10% of quercetin and rutin cream gave SPF values similar to that of homosalate, a synthetic sunscreen agent.In association with titanium dioxide, these two flavonoids also provided a non-negligible level of photoprotection in the UVA range (Choquenet et al., 2008).

Flavanonols
Flavanonols are also referred to as dihydroflavonols which are considered as minor flavonoids subgroup (Tsao, 2010).The structure features of flavanonols are the same as flavonols except that there is no double bond between positions 2 and 3 of pyrone ring (C) (Tsao, 2010).A well-known flavanonol found in nature is silymarins (Flora et al, 1998).
Silymarin is a standardized extract of flavonolignans from the seeds of the milk thistle (DerMarderosian, 2001;Barceloux, 2008).The principal components of silymarin include the diastereoisomers silibinin A and silibinin B in a roughly 1:1 ratio, the diastereoisomers isosilibinin A and isosilibinin B, silicristin, and silidianin (Figure 4).These terms are those used in the European Pharmacopoeia, following the World Health Organization International Non-Proprietary Names of Pharmaceutical Substances.The United States Pharmacopoeia and several publications use the following terms for the same compounds, respectively: silybin A, silybin B, isosilybin A, isosilybin B, silychristin, and silydianin (Simanek et al., 2000).The main chemical constituents of silymarin are silibinin A and B represent 50-60% of the components of silymarin which are the most active components (Gazak et al., 2007;Kroll et al., 2007).Silymarin exhibits a high antioxidant capability owing to the polyphenol hydroxyls and the ability to form complexes with transition and other metal ions in the 3,4-or 4,5-positions (Soto et al., 2003).The antioxidant properties of silymarin are responsible for its high hepatoprotective, antitumor, and other effects (Soto et al., 2003;Kiruthiga et al., 2007;Pascual et al., 1993).Topical application of silymarin protects mouse skin against UVB induced suppression of contact hypersensitivity (CHS) response to contact sensitizer dinitrofluorobenzene and infiltration of inflammatory leukocytes which are responsible for the generation of oxidative stress (Katiyar et al., 1997;Katiyar, 2002).The treatment also resulted in significant reduction of UVB-induced immunosuppressive cytokine interleukin-10 producing cells and its production (Katiyar et al., 1997;Katiyar, 2002).Silymarin inhibits photocarcinogenesis through inhibition of UVB-induced oxidative stress, inflammation and suppression of immune system (Katiyar, 2005;Katiyar et al., 2008;Meeran et al., 2006).It has the ability to decrease UV radiation-induced apoptotic cell death of epidermal cells through repair of damaged DNA and thus lead to prevention of photocarcinogenesis.In normal human epidermal keratinocytes (NHEK), treatment with silymarin was significantly reduced or repaired the amount of UVB radiation-induced DNA damage.The repair mechanism of DNA damage by silymarin is mediated through the nucleotide excision repair (NER) mechanism, which was verified by using NER-proficient and NER-deficient human fibroblasts, and NERdeficient mouse model (Katiyar, 2011).

Isoflavones
The structure of isoflavones differs from flavones in location of phenyl group (B ring) as it substituted at the C3position of the pyrone ring (C ring) (Middleton, 2000).Isoflavones derived from many edible plants have been reported to possess significant antioxidant, estrogenic and tyrosine kinase inhibitory activity (Widyarini et al., 2001).Isoflavones are found most often in legumes, including soybeans, black beans, green beans, and chick peas (Messina, 1999).Soya isoflavones are also known to have potential role in the prevention against chronic diseases, such as cardiovascular diseases and cancers (Watanabe et al., 2002;Murkies et al., 1999).
Equol (4′,7-isoflavandiol) is an isoflavone metabolized from dietary isoflavone daidzein by the gut microflora in the intestines of mammals (Widyarini et al., 2001).In animal model, topical application of equol before solar simulated ultraviolet irradiation offers protection against the formation of UV induced cyclobutane pyrimidine dimers, formation of sunburn cell and erythema (Widyarini, 2006;Lin et al., 2008).However, the protection was less than that of other flavonoids, such as its precursor daidzein or genistein.Equol protected immunosuppression induced by the putative epidermal mediator, cis-urocanic acid, indicating a potential mechanism of action involving inactivation of this UV photoproduct (Widyarini et al., 2001).
Red clover (Trifolium pratense) is a source of isoflavones and some metabolically related compounds.Widyarini et al. found the protection from UV induced inflammation and immune suppression by topical application of isoflavonoid compounds from red clover after UV exposure in hairless mice.The results showed that 20 μM lotions of genistein and the metabolites equol, isoequol and the related derivative dehydroequol had powerful potential to reduce the inflammatory edema reaction and the suppression of contact hypersensitivity induced by moderate doses of solar simulated UV radiation (Widyarini et al., 2001).
Soybean cake, a byproduct of soybean oil processing, contains a high amount of isoflavones (Kao, 2002;Kao et al., 2005).Isoflavone extracts prepared from soybean cake showed better antioxidant activities than genistein, daidzein, genistin, and daidzin (Figure 5) (Kao et al., 2005).These extracts inhibited UVB induced keratinocyte death, release of hydrogen peroxide (H 2 O 2 ), and UVB induced MAPK phosphorylation (Chiang et al., 2007).Soy isoflavone extracted from soybean cake was further purified and evaluated for the protective effects on UVB induced damage (Huang et al., 2010).The results demonstrated that soy isoflavone extracted from soybean cake prevented human keratinocyte apoptosis, attenuated the level of erythema and transepidermal water loss (TEWL), reduced the epidermal thickness and increased the catalase activity and inhibit COX-2 and proliferating cellnuclear antigen (PCNA) expression in response to UVB exposure (Huang et al., 2010).
Several studies have demonstrated the photoprotection of green tea polyphenols (Yusuf et al., 2007;Rutter et al., 2003;Afaq et al., 2002;Camouse et al., 2009).In animal model, topical treatment and oral feeding of green tea polyphenolic, or EGCG, prior UV irradiation was found to process the protection against UVB-induced immune suppression and photocarcinogenesis (Yusuf et al., 2007;Rutter et al., 2003).In human model, topical treatment with the extract before UV exposure resulted in reduced DNA damage and erythema formation due to protection of DNA repair enzymes from inactivation by ROS and due to UVB absorption ability of green tea polyphenolic (Afaq et al., 2002).Camouse et al. have established that topical application of green and white tea extracts before exposure to UV irradiation significantly reduced oxidative damage to keratinocyte DNA and radiation-induced depletion of Langerhans cells (Camouse et al., 2009).

Anthocyanins
Anthocyanins are the principal pigments responsible for the colors of many fruits, vegetables, cereal grains, and flowers.The colors of anthocyanins range from yellow to purple (except green) (Yoshida et al., 2006;Takeda, 2006).The anthocyaninhealth properties are due to their peculiar chemical structure, as they are very reactive towards reactive oxygen species because of their electron deficiency (Galvano et al., 2004).Since anthocyanins absorb visible as well as UV radiation and are effective antioxidants and scavengers of active oxygen species (Gould et al., 2002), a leading hypothesis is that they protect the photosynthetic apparatus from the effects of excess incident visible or UVB radiation (Steyn et al., 2002;Gould 2004) and photooxidative stress (Gould et al., 2002;Hoch et al., 2003;Keskitalo et al., 2005;Field et al., 2001;Close and Beadle, 2003).
Cyanidin-3-glucoside (Figure 7) is the most common anthocyanin found in nature.The photopretection of cyanidin-3glucoside against UVA and UVB radiation was established in human keratinocytes (HaCaT).Pretreating the cells with cyanidin-3-glucoside clearly inhibited the adverse effects of UVB exposure including translocation of transcription factors NF-kB and AP-1, over expression of the proinflammatory cytokine IL-8, cleavage of procaspase-3 (a key step in apoptotic pathway), and DNA fragmentation (Cimino et al., 2006).
A red orange complex, mixture of three red orange varieties (Citrus sinensis varieties: Moro, Tarocco, Sanguinello), contains high amount of cyanidin 3-glycosides (Bonina et al., 1996;Bonina et al., 2002;Bonina et al., 2005).The complex provided efficient protection against photooxidative skin damage when topically applied immediately after skin exposure to UV radiations.The extract showed a protective effect on DNA cleavage, free radical scavenging capacity, inhibition of xanthine oxidase activity and antilipoperoxidative capacity (Russo et al., 2002).
Strawberry (Fragaria x ananassa) is one of the most commonly consumed berries worldwide as an important dietary source of bioactive compounds, most of which are natural antioxidants that contribute to the high nutritional quality of the fruit.The high variety and content of polyphenolic constituent of strawberries are mainly represented by anthocyanins and hydrolyzable tannins.Anthocyanins are the quantitatively most important in strawberry (Tulipani et al., 2008;Giampieri et al., 2012a;Aaby et al., 2007).Pelargonidin is the major representative aglycone, although also some cyanidin derivatives are generally observed (Figure 8).The strawberry extract showed strong antiinflammatory (Heim et al., 2012), antioxidant (Wang andLin, 2000), antimutagenic (Okuda et al., 1989), anticarcinogenic (Wedge et al., 2001).The photoprotective properties of strawberry have been demonstrated in human dermal fibroblasts (Giampieri et al., 2012b).A photoprotective activity was demonstrated by increasing cellular viability, and diminishing DNA damage on UVA-induced skin damage.Its photoprotective potential may be based on the high level of anthocyanins which is potentially able to block collagen destruction and inflammatory responses via transcriptional mechanisms of NF-κB and MAPK signaling (Giampieri et al., 2012b).

CONCLUSION
Exposure to ultraviolet radiation is associated with a variety of harmful effects ranging from photoaging to skin cancer.Many synthetic sunscreen agents are available in the market with certain limitation according to their potential for generating photoreaction adducts and may consequently create adverse effects on human skin.Natural compounds may work in various ways such as stimulate the immune response, induce gene suppression, block oxidative damage to DNA, etc. Natural flavonoids are one candidate for prevention of the adverse effects of UV radiation due to their UV absorbing property, antioxidant properties, and modulate several signaling pathways.However, the development of new substances must demonstrate for their safety in addition to their photoprotective effect.
(Beecher, 2003).The flavonoids are further divided into six subclasses (flavones, flavonols, flavanonols, isoflavones, flavanols, and anthocyanidins) based on the connection of the B ring to the C ring, as well as the oxidation state and functional groups of the C ring.

Fig. 4 :
Fig. 4: Chemical structure of the flavanonols found in milk thistle.

Fig. 7 :
Fig. 7: Chemical structures of the common anthocyanins found in nature.