Prevention of photocarcinogenesis by (-)-epigallocatechin-3-gallate is mediated through interleukin-12-dependent DNA repair

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Earlier we have shown that epigallocatechin-3-gallate (EGCG), a polyphenol from green tea, prevents solar ultraviolet (UV) radiation-induced immunosuppression and photocarcinogenesis in mice. UV radiation-induced immunosuppression has been implicated as a risk factor for skin cancer. We have demonstrated that prevention of UV radiation-induced immunosuppression by EGCG is associated with the induction of immunoregulatory cytokine interleukin (IL)-12. Since, IL-12 has been shown to have anti-tumor activity and DNA repair ability we extended our studies to determine whether chemopreventive effect of EGCG against photocarcinogenesis is mediated through IL-12. For this purpose, we used genetically modified mouse model (IL-12 knockout (KO) mice on C3H/HeN background) and xeroderma pigmentosum (XP)-A deficient and XPA-proficient human fibroblasts. In preliminary studies, we found that inhibition of photocarcinogenesis by EGCG was associated with the increased presence of IL-12 in tumors compared with non-EGCG treated animals. As UVB-induced DNA damage, predominantly in the form of cyclobutane pyrimidine dimers (CPDs), has been recognized as an important molecular trigger for UVB-induced immunosuppression and initiation of photocarcinogenesis, we determined the effect of EGCG on UVB-induced CPD in the mouse skin. Immunostaining and dot blot analysis indicated that the number of UVB-induced CPDs were reduced or repaired faster in EGCG treated C3H/HeN mouse skin than in non-EGCG treated mouse. IL-12 has been shown to have the ability to repair UVB-induced CPDs through induction of NER enzymes. Therefore, we determined whether EGCG stimulates NER enzymes through IL-12 induction. The XPA-deficient and XPA-proficient fibroblasts were UVB irradiated with and without treatment of EGCG. Immunostaining and dot blot analysis were performed to check the status of CPD. We observed that UVB-induced DNA damage in the form of CPD was significantly reduced in EGCG-treated XPA-proficient fibroblasts whereas this effect was not observed in XPA-deficient fibroblasts. These data indicated that repair of UVB-induced CPD by EGCG may be mediated through induction of NER enzymes. To further confirm whether anti-photocarcinogenic effect of EGCG is mediated through the induction of IL-12, we conducted photocarcinogenesis experiment using IL-12 KO and their wild-type counterparts. Mice were exposed to UVB with and without topical treatment of EGCG for 30 weeks. We found that treatment of EGCG did not prevent UVB-induced skin cancer in IL-12 KO mice but prevented it in their wild-type counterparts in terms of tumor incidence and tumor multiplicity. These data identify the new mechanism of skin cancer chemopreventive effect of EGCG, and also demonstrate a link between EGCG, IL-12 and DNA repair mechanism.