Investigating the mechanism involved in regulating ultraviolet radiation induced p16 expression

Exposure to ultraviolet radiation (UVR) and the familial melanoma susceptibility gene p16INK4A are among the major risk factors which have been identified to contribute to the development of melanoma. UVR can act as a mitogen to stimulate cell proliferation and as a genotoxin by causing DNA damage in skin. UVR is also known to activate the MAP kinase signaling pathways, including the c-JNK kinase, p38 MAP kinase and ERK signaling pathways. These MAP kinase pathways have been reported to regulate a number of downstream effectors involved in controlling transcription, translation, transcript stability, and protein activity and stability. These in turn determine the UVR induced cellular responses such as proliferation, differentiation or apoptosis. The gene locus p16INK4A encodes the protein p16 which functions as a negative regulator of cell cycle progression and as a tumor suppressor is found deleted or mutated in 10-30% of primary melanomas and > 80% of melanoma cell lines. Previous work has shown that a suberythemal dose of UVR induces the expression of p16 in both the A2058 melanoma cell line and human skin. The regulatory mechanism controlling the expression of p16 in response to UVR is currently unclear. This study aims to define the mechanism involved in UVR induced p16 expression. A cell line model was established using serum starved quiescent A2058 cells to mimic the quiescent cells at the basal layer of the epidermis. The physiological relevance of this in vitro model was then verified in vivo in the human skin model. Data from this study has shown that UVR induced ERK signaling regulates p16 expression at both the transcriptional and translational level. Transcriptional control of p16 expression was observed in irradiated quiescent A2058 cells, where an increase in p16 mRNA corresponded to the increase in p16 protein. This observation was validated in human skin. Two transcription factors known to regulate expression of p16INK4A, MiTF and JunB, are potential downstream targets of UVR induced ERK signaling. Data also suggested the possible involvement of other components contributing to the UVR induced p16 expression observed in through ERK signaling, as MiTF is only expressed by the melanocytes, and the staining pattern of JunB did not correlate with the patchy p16 protein staining observed in human skin. Evidence indicating translational control of p16 expression was noted in the irradiated non serum starved A2058 cells, where an increase in p16 protein was detected despite the lack of elevated p16 mRNA. Additional evidence for translational contol was observed in irradiated human skin, where the uniform p16 mRNA expression in the skin did not correspond to the patchy p16 protein staining. Data from this study suggests a biological connection between melanoma risk factors, UVR and p16 with other melanoma genes, BRAF, MC1R and MiTF. BRAF plays a vital role in the ERK signaling pathway. ERK can regulate MiTF activity and stability, and BMSH (a ligand for MC1R receptor) has been shown to potentiate the increase of p16 expression irradiated skin, indicating p16 expression could be regulated via MC1R signaling. Taken together, these results demonstrate that the suberythemal UVR induced p16 expression is regulated by the ERK signaling pathway at both the transcriptional and translational levels. Further work is required to provide a better understanding of the mechanism by which UVR activated ERK regulates p16 expression