Cooperation of Ha-ras and Bcl-2 during multistep skin carcinogenesis

Nonmelanoma skin cancer (NMSC) is the most frequently diagnosed cancer in the United States. Deregulation of bcl-2 and ras family members is commonly observed in NMSC. It has been previously demonstrated that simultaneous bcl-2 and Ha-ras gene expression in keratinocytes results in disordered differentiation and resistance to cell death induced by ultraviolet (UV) radiation. It was, therefore, interest to assess the extent of cooperation between bcl-2 and Ha-ras during multistep skin carcinogenesis. The keratin 1 promoter was used to generate HK1.ras and HK1.bcl-2 transgenic mice, which were subsequently crossed to generate HK1.ras/bcl-2 double transgenic mice. The apoptotic index (AI) following UV-irradiation was significantly lower in HK1.bcl-2 and HK1.ras/bcl-2 epidermis compared to control littermates. Interestingly, the AI of HK1.ras/bcl-2 mice was significantly lower than even HK1.bcl-2 mice following UV-irradiation. To investigate the interaction of these oncogenes in skin tumorigenesis, a two-stage chemical carcinogenesis protocol was used to induce tumors. The individual contributions of Ha-ras and bcl-2 to papilloma latency, incidence, and growth rate in HK1.ras/bcl-2 double transgenic mice was marginally additive. Papillomas arising in HK1.ras transgenic mice exhibited the highest rate of apoptosis whereas papillomas arising in the HK1.ras/bcl-2 double transgenic mice exhibited rates of apoptosis that were significantly lower than papillomas arising in either control littermate or HK1.ras mice. Constitutive expression of either Ha-ras or bcl-2 exhibited similar rates of malignant tumor progression and they were not significantly different than control littermates. Importantly, when these two oncoproteins were coexpressed, a significant, and synergistic, increase in malignant transformation was observed.