Ras p21 farnesylation in ultraviolet B radiation-induced tumors in the skin of SKH-1 hairless mice.

Cutaneous exposure to solar ultraviolet B (UVB) radiation is well recognized as the major cause of skin cancer in humans; however, the precise molecular mechanisms whereby UVB mediates carcinogenesis remains unclear. The involvement of activated ras oncogenes has been demonstrated extensively in both animal and human skin cancers. Activated ras oncogenes encode mutated ras p21 that exist in the guanosine triphosphate-bound active state and, following localization to the inner side of the plasma membrane, cause cellular transformation. This membrane association requires three post-translational modifications occurring at the C-terminus of the ras p21. The farnesylation of p21 by a cytosolic enzyme known as farnesyltransferase (FTase) is the critical step that triggers biologic functions of the ras p21. In this study, FTase activity was found to be substantially higher (approximately threefold) in UVB radiation-induced tumors in SKH-1 hairless mice compared to epidermis from controls. Western blot analysis showed significantly higher levels of Ha-ras p21 in both cytosolic and membrane fractions prepared from tumors compared to epidermis. Pan ras antibody against mutated p21 at codon 12 showed very strong reactivity for ras val-12p21 in tumors but not in normal epidermis, suggesting a gly to val substitution at 12th position in ras p21 in UVB-induced tumors. Our data indicate that enhanced FTase activity and the processing of overexpressed p21 in UVB-induced tumors are correlated, and predict the role of point mutation at the 12th codon of the ras oncogene during photocarcinogenesis in mice.