Enhancement of ultraviolet B-induced skin tumor development in phospholipase Cε-knockout mice is associated with decreased cell death.

Phospholipase C (PLC) ε is a phosphoinositide-specific PLC regulated by small guanosine triphosphatases including Ras and Rap. Our previous studies revealed that PLCε gene-knockout (PLCε(-/-)) mice exhibit marked resistance to tumor formation in two-stage skin chemical carcinogenesis using 7,12-dimethylbenz(a)anthracene as an initiator and 12-O-tetradecanoylphorbol-13-acetate as a promoter. In this model, PLCε functions in tumor promotion through augmentation of 12-O-tetradecanoylphorbol-13-acetate-induced inflammation. In this study, we have further assessed the role of PLCε in tumorigenesis using a mouse model of ultraviolet (UV) B-induced skin tumor development. We irradiated PLCε(+/+), PLCε(+/-) or PLCε(-/-) mice with doses of UVB increasing from 1 to 10 kJ/m(2) three times a week for a total of 25 weeks and observed tumor formation for up to 50 weeks. In sharp contrast to the results from the two-stage chemical carcinogenesis study, PLCε(-/-) mice developed a large number of neoplasms including malignant tumors, whereas PLCε(+/+) and PLCε(+/-) mice developed a relatively small number of benign tumors. However, UVB-induced skin inflammation was greatly suppressed in PLCε(-/-) mice, as observed with 12-O-tetradecanoylphorbol-13-acetate-induced inflammation, implying that PLCε's role in the suppression of UVB-induced tumorigenesis is not mediated by inflammation. Studies of the tumor initiation stage revealed that UVB-induced cell death in the skin was markedly suppressed in PLCε(-/-)mice. Our findings identify a novel function for PLCε as a critical molecule regulating UVB-induced cell death and suggest that resistance to UVB-induced cell death conferred by the absence of PLCε is closely related to the higher incidence of skin tumor formation.