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. Author manuscript; available in PMC: 2013 Sep 1.
Published in final edited form as: Photochem Photobiol. 2011 Nov 17;88(5):1066–1074. doi: 10.1111/j.1751-1097.2011.01020.x

Epigenetic Alterations in Ultraviolet Radiation-Induced Skin Carcinogenesis: Interaction of Bioactive Dietary Components on Epigenetic Targets†

Santosh K Katiyar 1,2,3,4,*, Tripti Singh 2, Ram Prasad 2, Qian Sun 2, Mudit Vaid 2
PMCID: PMC3288155  NIHMSID: NIHMS333706  PMID: 22017262

Abstract

The importance of epigenetic alterations in the development of various diseases including the cancers has been realized. As epigenetic changes are reversible heritable changes, these can be utilized as an effective strategy for the prevention of cancers. DNA methylation is the most characterized epigenetic mechanism that can be inherited without changing the DNA sequence. Although limited, but available data suggest that silencing of tumor suppressor genes in ultraviolet (UV) radiation-exposed epidermis leads to photocarcinogenesis and is associated with a network of epigenetic modifications including alterations in DNA methylation, DNA methyltransferases and histone acetylations. Various bioactive dietary components have been shown to protect skin from UV radiation-induced skin tumors in animal models. The role of bioactive dietary components, such as, (−)-epicatechins from green tea and proanthocyanidins from grape seeds, has been assessed in chemoprevention of UV-induced skin carcinogenesis and underlying epigenetic mechanism in vitro and in vivo animal models. These bioactive components have the ability to block UV-induced DNA hypermethylation and histone modifications in the skin required for the silencing of tumor suppressor genes (e.g., Cip1/p21, p16INK4a). These information are of importance for understanding the role of epigenetic modulation in UV-induced skin tumor and the chemopreventive mechanism of bioactive dietary components.

INTRODUCTION

Cutaneous malignancies are most common in fair skin individuals particularly in Caucasians. The common malignancies include the non-melanoma, which comprises of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), melanoma and cutaneous lymphoma. Chronic exposure to ultraviolet (UV) radiation is a well-recognized etiological factor for these skin cancers, which accounts for the ~1.3 million new cases of skin cancer each year in the USA (1). The incidence of skin cancer is equivalent to the incidence of malignancies in all other organs combined (2), and thus represents a major public health problem. Thus it is a major burden on the health-care system as it has been estimated that the cost of treating non-melanoma and melanoma skin cancers in the United States cost approximately $3.0 billion annually (www.cancer.org/statistics). Cancer is a manifestation of both abnormal genetic and epigenetic events. DNA methylation is the most characterized epigenetic mechanism that can be inherited without changing the DNA sequence (3). Recent studies have revealed that both global DNA hypomethylation and regional hypermethylation occur in tumorigenesis (4, 5), suggesting that improper methylation processes contribute to cancer initiation or progression. Both DNA hypermethylation and hypomethylation can contribute to carcinogenesis by silencing of tumor suppressor genes, upregulation of oncogenes, and/or decreased genomic stability (6, 7). It has been established that alterations in both DNA methylation and histone modifications play a critical role in the silencing of key tumor suppressors (3, 5) and that leads to the initiation and progression of skin cancers.

EPIGENETIC EVENTS IN SKIN CARCINOGENESIS: A HYPOTHETICAL MODEL

It is expected that human skin cells start life with a relatively uniform epigenetic code. Aging and the chronic UV exposure can alter epigenetic information in epidermal cells such that, over time, epigenetic mosaicism develops in patches of cells that have subtly altered gene expression, as shown in Figure 1. The sum total of changes in some of these epidermal patches favors increased proliferation, reduced apoptosis, and/or altered differentiation, promoting the acquisition of further molecular changes (epigenetic and genetic) that lead to neoplastic transformation and skin tumor development. If this is the case, epigenetic alterations in UV-exposed skin should have great potential as targets for very early interventions to prevent skin cancer caused by UV radiation in humans. Bioactive food components have been shown to affect the progression of epigenetic alterations. As epigenetic changes are reversible, it is suggested that topical application or dietary supplementation of bioactive food components may block or inhibit the progression of epigenetic changes in cells exposed to UV radiation and that will result in prevention of skin cancer.

Figure 1.

Figure 1

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This hypothetical model depicts the: (i) progression of a normal skin cell into a cancer cell following epigenetic alterations under the influence of UV radiation, and (ii) the chemopreventive mechanism or targets of bioactive dietary components. Left Panel, cells containing a relatively uniform epigenetic code. Middle Panel, aging and the associated exposures to UV radiation alter epigenetic information in epidermal cell subsets such that, epigenetic mosaicism develops in patches of epidermal cells that have subtly altered gene expression. Right Panel, the sum total of changes in some of these patches result in neoplastic transformation. Topical application or dietary supplementation of bioactive food components may block, inhibit or slow down the progressing epigenetic alterations in cells exposed to UV radiation.

MECHANISMS ASSOCIATED WITH EPIGENETIC MODIFICATION

Three distinct mechanisms appear intricately related in initiating and sustaining epigenetic modifications: DNA methylation, histone modifications and RNA-associated silencing. These mechanisms are critical components in normal development and cell growth. DNA methylation is involved in transcriptional silencing of genes, regulation of expression of imprinted genes, regulation of a number of tumor suppressor genes in cancer and silencing of genes located on the inactive X chromosomes (7, 8). The epigenetic modulations that occur around the DNA play a significant role in the regulation of gene expression. DNA methylation is a most characterized epigenetic mechanism and about 70–80% of the CpGs (cytosine and guanosine) in the mammalian DNA are methylated in the 5′-regulatory region, although 5-methylcytosine can occasionally be found in CpN dinucleotides in mammals (3–6). In many disease processes associated with DNA damage, such as cancer and aging, hypermethylation dominates resulting in gene silencing without a change in the coding region of the gene, and thus they are potentially reversible. The mechanisms involved in suppression of transcription of genes via hypermethylation at CpG islands and histone modifications are an area of active research (9). Also, hypermethylation of CpG dinucleotides near the transcriptional regulatory region may initiate the recruitment of the methyl-CpG binding domains like MeCp2 and MBD1 family proteins that mediate silencing of genes via facilitation of a repressive chromatin environment (10, 11). Although these methyl-CpG binding domains can all recruit histone deacetylase (HDAC)-containing repressor complexes, distinctive features of each of these proteins vary in a tissue- and gene-specific manner (12).

DNA METHYLATION

It is now clear that the genome contains information in two forms, genetic and epigenetic. The genetic information provides the blueprint for the manufacture of all the proteins necessary to create a living thing while the epigenetic information provides instructions on how, where, and when the genetic information should be used (3–6). Epigenetic refers to heritable changes in gene expression that occur without a change in DNA sequence (3). DNA methylation, primarily at the C5 position of cytosine, affects gene expression in many biological processes such as differentiation, genomic imprinting, DNA mutation, and DNA repair (8, 13–15). DNA methylation is controlled at several different levels in normal and cancer state. The addition of methyl groups is carried out by a family of enzymes, DNA methyltransferases (Dnmts). Three Dnmts (Dnmt1, Dnmt3a and Dnmt3b) are required for establishment and maintenance of DNA methylation patters, as shown in Figure 2. Dnmt1 appears to be responsible for the maintenance of established patterns of DNA methylation, while Dnmt3a and Dnmt3b seem to mediate establishment of new or de novo DNA methylation patterns. Diseased cells such as cancer cells may be different in that Dnmt1 alone is not responsible for maintaining abnormal gene hypermethylation and both Dnmt1 and 3b may cooperate for this function. DNA hypermethylation, usually occurring at promoter CpG islands, is a major epigenetic mechanism in silencing the expression of tumor suppressor genes (16–19). The importance of promoter hypermethylation as well as global hypomethylation in carcinogenesis has been discussed extensively (17–20). It has been shown that approximately half of tumor-suppressor genes are inactivated in sporadic cancers more often by epigenetic, than by genetic, mechanisms. Several genes with tumor-suppressor properties in mouse models are inactivated exclusively by epigenetic mechanisms in human neoplasia (21, 22). Overall, there is strong evidence that the neoplastic phenotype in many cases is due to epigenetic-based pathway alterations. Every study that has examined epigenetic timing has concluded that epigenetic changes occur very early in neoplasia and precede epithelial malignancy (23–25).

Figure 2.

Figure 2

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This schematic diagram depicts the conversion of cytosine into 5-methyl cytosine by the action of methyltransferase (Mtase). In this process different DNA methyltransferase enzymes play roles, such as Dnmt1, which has a role in normal maintenance of DNA methylation process while Dnmt3a and Dnmt3b are called De novo methylation enzymes. The inhibitory effect of bioactive component on Dnmts activity will block or reduce the hypermethylation of DNA and thus will lead to the reversal of epigenetic alterations.

HISTONE MODIFICATIONS

Histone modifications have also been defined as epigenetic modifiers, and are catalyzed by many enzymes, such as acetylation on specific lysine residues by histone acetyltransferases (HATs), deacetylation by histone deacetylases (HDACs), methylation of lysine and arginine by histone methyltransferases (HMTs), demethylation of lysine residues by histone demethylases (DMTs), and phosphorylation of specific serine groups by histone kinases (HKs) (26). These histone modifications induce chromatin alterations that allow access to the various transcriptional activators and/or repressors at gene promoters, and therefore they play an important role in gene regulation and tumorigenesis (27–29). Specific histone modification appear to act as programmed ‘codes’ which can be identified by specific proteins to bring about distinct downstream events such as transcriptional activation or repression. Histone deacetylation at a promoter region, induced by either lack of HAT function or increased HDAC activity, results in silencing of the tumor suppressor genes, such as p21WAF1/Cip1, in tumorigenesis (30, 31). Conversely, histone hyperacetylation at certain promoters through either increased HAT activity or decreased HDAC activity, results in an activation of normally repressed genes (32, 33). Collectively, it has been suggested that the aberrant enrichment of HDAC and HAT activities may trigger carcinogenesis (34). It has been recognized that next to DNA methylation, histone acetylation and histone methylation are the most well characterized epigenetic marks. Generally, tri-methylation at H3-K4, H3-K36, or H3-K79 results in an open chromatin configuration and is also characterized by a high level of histone acetylation, which is mediated by histone acetyltransferase. In contrast, histone deacetylases have the ability to remove this epigenetic mark, leading to transcriptional repression. Histone demethylases have the opposite effect on transcription, e.g., the histone demethylase LSD1 is responsible for H3-K4 demethylation, which leads to transcriptional inactivation.

UV RADIATION AND EPIGENETIC MODIFICATIONS

Although many environmental and genetic factors contribute to the development of various skin diseases, the most important factor is the chronic exposure of the skin to solar UV radiation. Exposure of the skin to UV radiation induces oxidative stress, inflammatory responses, DNA damage and suppression of immune reactions (1, 35). Chronic and sustained inflammation, oxidative stress and DNA damage significantly alter epigenetic regulators. Notably, it has been observed that chronic inflammation markedly accelerates acquisition of DNA methylation changes (36). Sathyanarayana et al. have investigated the aberrant promoter methylation status of genes in basal cell carcinoma and squamous cell carcinoma lesions of the skin (37). They found high frequencies of methylation of several known tumor suppressor genes in skin cancers, such as CDH1, CDH3, LAMA3, LAMC2, RASSF1A, etc. (37). The major goal of this study was to find out whether there was sun exposure related methylation in various skin lesions and whether some kind of molecular biomarkers can be developed which can distinguish between sun-exposed and sun-protected skin lesions. This study concluded that sun-exposed skin was positively associated with gene methylation pattern. Murao et al. have found epigenetic abnormalities in cutaneous squamous cell carcinomas and frequent inactivation of the RB1/p16 and p53 pathways. However, the epigenetic abnormalities associated with p53 expression were not clear (38). It has been suggested that the major mechanism by which UV radiation induces skin tumor development is via mutations of the p53 gene. The mutations are predominantly the UV-signature mutation, i.e., a C→T transitions at dipyrimidine sites, which result from UV radiation-induced cyclobutane pyrimidine dimer formation (39). Interestingly, methylation of cytosine has been shown to enhance the pyrimidine dimer formation resulting from exposure of cells to solar UV radiation by 5 to 15-fold due to the higher energy absorption of the 5-methyl cytosine (5-mC) as compared to the cytosine in the DNA. Many of the signature mutations occur at CpG sites (39); thus, the presence of 5-mC in mammalian DNA adds to the UVB-associated mutational burden through several different mechanisms (3, 5, 15).

The existing information on DNA methylation patterns in UV-exposed skin and tumors is scant and only poorly understood. However, Nandakumar et al has tried to characterize the effect of UV radiation on the skin and skin tumors for epigenetic modifications, specifically DNA methylation and histone modifications (40). This systematic analysis of UV-exposed skin and UV-induced skin tumor samples indicate that UV exposure induces a DNA hypermethylation pattern in the epidermis of the skin and UV-induced skin tumors of mice. Moreover, the DNA hypermethylation was consistent with the finding of enhanced levels of Dnmts in the UV-exposed mouse skin and UV-induced skin tumors. Intriguingly, the distribution patterns of the Dnmts (Dnmt1, Dnmt3a and Dnmt3b) within the UVB-exposed skin and the preferential distribution of the intensely staining Dnmt3a and Dnmt3b cells in the basal layer of the UVB-exposed skin suggests that the de novo synthesis of Dnmt3a and Dnmt3b may contribute to a major chromatin mark for cellular proliferation on chronic UVB-exposure of the skin and within UVB-induced skin tumors. The findings of this study indicate that the increase in the levels of global DNA methylation and the levels of Dnmt activity occur relatively quickly in UV-exposed skin and prior to the onset of recognizable pre-neoplastic changes or the appearance of skin tumor lesions. It has been shown that UV radiation induces p53 mutations and methylation in keratinocytes at a relatively early age in chronically sun-exposed areas of human skin (41). Mittal et al. (42) have reported the presence of DNA hypomethylation in sporadic patches in skin of UV exposed mice, however these patches of DNA hypomethylation were significantly less frequent whereas DNA hypermethylation was predominant over the UVB radiation-exposed skin and in UVB-induced skin tumors.

Studies have shown an association between DNA hypermethylation and histone hypoacetylation with downregulation of the tumor suppressor genes (43, 44). The studies conducted in Dr. Katiyar’s laboratory have found hypoacetylation of H3 and H4 histones in the transcriptionally-silenced p16INK4a and RASSF1A regions on UVB-exposure (40). These “hotspots” were selected based on the functional analysis reports, which indicated that the increased density of the promoter methylation of the p16INK4a and RASSF1A was associated with altered chromatin rearrangements, marked by a depletion of acetylated histones H3 and H4 (45–48). This study also found an increase in the recruitment of MeCP2 and methyl-CpG binding domain 1 to the p16INK4a and RASSF1A methylated heterochromatin on chronic UV-exposure of the skin. These new investigations suggest the involvement of MeCP2 and methyl-binding domain 1 in the chromatin-remodeling and transcriptional silencing of the tumor suppressor genes p16INK4a and RASSF1A on chronic UV exposure. The study by Nandakumar et al (40) suggests a strong link between DNA methylation and histone acetylation on chronic exposure of the skin to UV irradiation in mouse model; however, to fully understand the epigenetic pathways that are triggered by chronic UV exposures and their roles in the development of skin cancer, further studies of other tumor suppressor genes (like Cip1/p21, Kip1/p27 and p53, etc.), oncogenes and other chromatin remodeling factors are needed. This report for the first time demonstrated that chronic exposure of the mouse skin to UV radiation stimulates the expression and activity of Dnmts that may lead to the aberrant hypermethylation of DNA. This, in turn, stimulates other DNA methylation-associated epigenetic mechanisms, such as the recruitment of methyl binding proteins and histone hypoacetylation. Together, these events are associated with the down-regulation of tumor suppressor genes, most likely through the formation of closed chromatin structure. The analysis of squamous cell carcinoma from human skin also showed the higher levels of DNA hypermethylation, global DNA methylation, and increased DNMT activity compared to normal human skin samples. These information support the clinical significance of the role of epigenetic modulations in skin diseases associated with UVB-irradiation of the skin and that these epigenetic modulations in skin contribute to the tumor development in UV-exposed skin. Importantly the epigenetic modifications are reversible; therefore, targeting of these events may lead to the development of novel therapeutic strategies for the prevention of UV radiation-induced skin cancers in humans. Such epigenetic targeting could be achieved through the use of novel demethylating agents or inhibitors of histone deacetylation, which would correct aberrant DNA hypermethylation patterns in UV-exposed skin and restore normal growth control in skin cells. Some inhibitors related to DNA hypermethylation and histone deacetylation have been developed for treatment of cancers, but long-term safety, resistance and toxicity concerns have called into question their long-term use as clinical chemopreventive agents. Under these circumstances, bioactive dietary components from plant origin offer promising new options for the development of effective chemopreventive strategies especially when the agents are nontoxic at effective doses.

EFFECT OF BIOACTIVE DIETARY COMPONENTS ON UV-INDUCED EPIGENETIC MODULATIONS

The information on the effect of phytochemicals and particularly the effect of dietary or food components on UV radiation-induced epigenetic modifications in the skin and UV induced skin cancer are very limited; however, it is considered as a promising area of investigation. Although, Hippocrates proclaimed 25 centuries ago, “Let food be thy medicine and medicine be thy food”, the investigations on the importance of medicinal values of food components in the treatment or prevention of diseases are limited. For the last two decades, there has been increasing awareness and considerable interest in the use of phytochemicals including dietary components for their medicinal values against multiple diseases including UV radiation-induced skin cancers. It has been noted that out of 121 prescription drugs in use for cancer treatment, 90 are derived from natural plant sources and ~74% of these chemotherapeutic drugs were discovered by investigating a folklore claim (49, 50).

Phytochemicals, particularly those which can be administered as dietary supplements, offer promising new options for the development of more effective chemopreventive and chemotherapeutic strategies for UV radiation-induced melanoma and non-melanoma skin cancers. It is notable that epigenetic alterations are reversible and dietary food or bioactive components may have the ability to reverse, inhibit or delay the epigenetic modifications and that may have an impact on the prevention of the disease. It is not necessary that every bioactive dietary component has this ability. As is evident from the available literature, DNA hypermethylation leads to the epigenetic modulation and histone modifications in the genome, and that lead to the initiation of disease including the risk of cancers (3–5). Any bioactive component if has the ability to block DNA hypermethylation or able to reverse DNA methylation, then it will be considered as a change in right direction and beneficial. In this context, proanthocyanidins are naturally occurring compounds that are widely found in fruits, vegetables, nuts, seeds, flowers and barks of some plants. They are a class of compounds that take the form of oligomers or polymers of polyhydroxy flavan-3-ol units, such as (+)-catechin and (−)-epicatechin (51). Grape seeds are rich source of proanthocyanidins with 60–70% of seeds content. The supplementation of the AIN76A control diet with grape seed proanthocyanidins (GSPs) at the concentrations of 0.2% and 0.5% (w/w) inhibits the growth and development of UV radiation-induced skin tumors in mice (52). This inhibitory effect of GSPs on UV-induced skin tumor growth was associated with the reduction in UV-induced inflammatory responses and oxidative stress (53, 54). To determine whether anti-skin carcinogenic effect of GSPs is associated with the modifications in the epigenetic targets in UV-exposed skin of the mice, the UVB-exposed skin and skin tumors were subjected to the analysis of epigenetic regulators. Dietary GSPs inhibited the levels of UVB-induced global DNA methylation in the skin as well as in skin tumor samples. The levels of 5-mC and Dnmt activity were reduced and the levels of tumor suppressor genes, such as Cip1/p21 and p16INK4a, were enhanced or restored, in both chronic UVB-exposed skin and UVB-induced skin tumors of those mice which were given GSPs in diet (0.5%, w/w) compared with non-GSPs-treated control animals (S. K. Katiyar, unpublished data). These limited but promising results suggest that the prevention of UVB-induced skin tumor development by dietary GSPs is associated with the decreased DNA methylation and the reactivation of tumor suppressor genes in UVB-exposed skin and UVB-induced skin tumors.

The effects of other bioactive components, such as from green tea were also examined on the epigenetic regulators in UVB-exposed skin and UVB-induced skin tumors using in vivo SKH-1 hairless mouse model. The anti-photocarcinogenic effects of green tea catechins or polyphenols (GTPs) from the plant Camellia sinensis have been studied extensively in vitro and in vivo models. The experimental studies conducted in author’s laboratory and from others have demonstrated that GTPs are effective cancer chemopreventive agents (reviewed in 55, 56). Oral administration of GTPs in drinking water (0.2%, w/v) of mice significantly inhibited UVB-induced skin tumorigenesis in terms of tumor incidence and tumor multiplicity (57). Experimental studies also have demonstrated that oral administration of water extract of green tea, either caffeinated or decaffeinated, prevents UV-induced skin tumorigenesis in mouse skin (58, 59). Topical application of (−)-epigallocatechin-3-gallate (EGCG, 1mg/cm2 skin area), a major and most active constituent of GTPs, blocks UVB-induced inflammatory responses, inhibits oxidative stress and photocarcinogenesis in mice (42, 60). Prevention of UV-induced skin tumor development in mouse skin by the administration of GTPs or EGCG is associated with the control of cell cycle regulatory check points, cell proliferation and inflammatory mediators (60–63), which are also regulated by epigenetic mechanisms. The inhibition of photocarcinogenesis by EGCG was associated with DNA hypomethylation, though sporadically, in UVB-exposed skin (42). Recently, the studies from the research laboratory of Dr. Katiyar suggest that oral administration of GTPs (0.2%, w/v) in drinking water of mice for approximately 24 weeks inhibits chronic UVB exposure-induced DNA hypermethylation and the activity of Dnmts in mouse skin. Administration of GTPs also reactivated tumor suppressor genes, Cip1/p21 and p16INK4a, while decreased HDAC activity in UVB-induced skin tumors compared with non-GTPs-treated UVB-induced skin tumors (V. Nandakumar, unpublished data).

To further verify the epigenetic targets and mechanism of action of green tea polyphenols against skin carcinogenesis, in vitro model of human skin cancer cells was used. Nandakumar et al (64) have investigated epigenetic regulation by tea catechins and elucidated the underlying mechanisms using the A431 and SCC13 human skin carcinoma cell lines as an in vitro model system. Also, 5-aza-2′-deoxycytidine (5-Aza-dc), a well-characterized DNA methylation inhibitor, and trichostatin A (TSA), a well known HDAC inhibitor, were used as controls. The characterization of tea catechins revealed that (−)-epicatechin, (−)-gallocatechin and (−)-epigallocatechin are the weaker epigenetic modulators while (−)-epicatechin-3-gallate and EGCG are comparatively better epigenetic modulators than other tea epicatechins. Treatment of cells with EGCG (5, 10, and 20μg/ml concentration) for 6 days resulted in significant decrease in the levels of global DNA methylation, 5-mC in DNA and the Dnmt activity of A431 and SCC13 cells (64). The 5-Aza-dc also significantly decreased the levels of global DNA methylation, and reduced the activity as well as mRNA and protein expressions of DNMT1, DNMT3a and DNMT3b in A431 cells. These results indicate that the action of EGCG is similar to 5-Aza-dc, which is a well characterized DNA methylation inhibitor (64). The effects of EGCG (5–20 μg/ml concentration) on epigenetic modifications in melanoma cell lines (A375 and Hs294t) were also studied under identical conditions. EGCG also showed promising effect on DNA demethylation and HDAC activity in melanoma cancer cell lines (T. Singh, unpublished data).

Studies have shown that histone modifications are linked to transcriptional activation and repression of genes (65). Normally histone acetylation results in an open chromatin structure associated with transcriptional activation, whereas histone deacetylation leads to a closed chromatin structure with transcriptional repression (65). Post-transcriptional modifications of histone methylation and acetylation may contribute to cancer development by modulation of the expression of tumor suppressor genes and oncogenes. HDACs play a key role in regulation of histone deacetylation. Deacetylation and methylation of H3-Lys 9 are the most common histone modifications involved in epigenetic repression of genes (66). In vitro in cancer cell model, EGCG significantly decreases HDAC activity after the treatment of skin cancer cells. These epigenetic modulations are found to be slow and time-dependent. EGCG decreased levels of methylated H3-Lys 9, but increased levels of acetylated H3-Lys 9 and H3-Lys 14. Similarly, TSA also decreases HDAC activity and increases acetylated H3-Lys 9 and H3-Lys 14 (64). These observations suggest that EGCG decreases HDAC activity to maintain H3-Lys 9 at a high level of acetylation and a low level of methylation which may favor transcriptional activation of genes. It appears that EGCG and 5-Aza-dc may inhibit methyltransferase to prevent histone methylation. Treatment of cells with EGCG also increases acetylation of H4-Lys 5, H4-Lys 12 and H4-Lys 16 which may also favor transcriptional activation of tumor suppressor genes. These studies revealed that unlike 5-Aza-dc and TSA, EGCG has dual actions involving both DNA demethylation and modifications of histone acetylation and methylation. Similar to EGCG, other dietary phytochemicals, such as dially disulfide and genistein, have showed epigenetic alterations on DNA methylation and histone modifications in colon and prostate cancer cells (30, 67). Lycopene also demethylated the GSTP1 promoter and reactivated GSTP1 expression in human breast cancer cells (68). EGCG also has been shown to induce epigenetic modifications in human prostate, colon and esophageal cancer cell lines (69). EGCG reactivates or re-expresses mRNA expression of tumor suppressor genes and proteins of p16INK4a and Cip1/p21 in human skin cancer cells, and this re-activation of tumor suppressor genes will contribute to the suppression of cancer cell proliferation and induction of apoptosis. The role of p53 tumor suppressor gene in UV-induced skin carcinogenesis is less clearly identified (37). Similar to EGCG, 5-Aza-dc also reactivated the mRNA expression of both tumor suppressor genes in A431 cells. The reactivation of silenced tumor suppressor genes by EGCG is associated with DNA demethylation and histone modifications in cancer cells.

In summary, these limited but promising investigations suggest that dietary phytochemicals have the ability to restore or reactivate the expression of the DNA hypermethylation-silenced genes, p16INK4a and Cip1/p21, in human skin cancer cells by downregulation of DNMT and HDAC activities. These findings are of importance for understanding the anticancer mechanisms and clinical applications of dietary bioactive components. Further, in vivo animal studies with UV radiation and dietary components are required to develop better understandings on the epigenetic regulation and molecular targets by phytochemicals in the chemoprevention of UV radiation-induced skin cancers. Since, changes in DNA methylation and/or histone modifications are reversible; it may also lead to the development of novel chemopreventive and therapeutic strategies to reverse the transformed phenotype through the use of known or novel demethylating agents to correct aberrant methylation patterns and restore growth control in tumor cells, and thus lead to the prevention or treatment of UV radiation-induced skin cancers.

Acknowledgments

The work reported from Dr. Katiyar’s laboratory was supported by the funds from National Cancer Institute/NCCAM/NIH (CA140197, AT2536, CA140832,) and Veterans Affairs Merit Review Award (S.K.K.). The content of this article does not necessarily reflect the views or policies of the funding sources.

Biographies

Santosh K. Katiyar, Ph.D., is a Professor of Dermatology at the University of Alabama at Birmingham, Birmingham, AL in USA. He has made significant contribution towards the understanding of natural plant products, particularly dietary bioactive components, on the prevention of ultraviolet radiation-induced skin cancer. He focused on studying the molecular targets, particularly immunological alterations induced by UV radiation, oxidative stress, inflammation, DNA damage and DNA repair, and development of newer and more effective agents for skin cancer chemoprevention. He also has interest to develop new botanical agents for the prevention of the invasive or metastatic potential of melanoma cells.

Tripti Singh was born in India and obtained her MS degree in Nutrition Science in 2007 from Punjab Agricultural University, Ludhiana, Punjab, India. Because of her interest in nutrition and cancer, she moved to the University of Alabama at Birmingham, AL, USA, in 2007 to expand her knowledge in this field. She joined the research group of Dr. Katiyar and has been involved in studying the role of dietary bioactive components in prevention of photocarcinogenesis and lung cancer. She has interest in cell cycle regulation, apoptosis, cancer cell metastasis and epigenetics and cancer, and using both in vitro and in vivo animal models.

Ram Prasad obtained his PhD degree in 2009 in Biosciences (Specialization in Liver Cirrhosis and Carcinoma) at the Jamia Millia Islamia (A Central University), New Delhi, in India. After completion of his doctoral research in India, he moved to USA in 2010 and joined the research laboratory of Prof. S. K. Katiyar at the University of Alabama at Birmingham, AL, USA, and gaining training and experience in chemoprevention of cancer, including the cancers of skin, pancreatic and head and neck. He has particular interest in photocarcinogenesis and its prevention by dietary phytochemicals using in vitro and in vivo animal models.

Mudit Vaid was born and educated in India. He obtained his PhD in Bio-Chemistry in 2008 at the Institute of Genomics and Integrative Biology, Delhi, India. He is presently working with Dr. Santosh K Katiyar as a Research Associate at the Department of Dermatology, University of Alabama at Birmingham, AL, USA. His research activity is focused on investigation of chemopreventive mechanisms of plant-derived phytochemicals against melanoma and non-melanoma skin cancer. He also has interest in studying the effect of epigenetic modulations in cancers.

Qian Sun was born and educated in China. He obtained his Ph.D. degree in Food Science in 2010 at the China Agricultural University, Beijing, China. He is presently working with Dr. Santosh K Katiyar, Professor of Dermatology, as a postdoctoral fellow at the Department of Dermatology, University of Alabama at Birmingham, AL, USA. He is receiving training and experience in causes, mechanisms and chemopreventive approaches of skin carcinogenesis and cutaneous photoaging using various phytochemicals. Dr. Sun also has interest in cancer cell metastasis and its mechansim.

Footnotes

†

This paper is part of the Special Issue in Commemoration of the 70th birthday of Dr. David R. Bickers

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