Elevated expression of Ha-ras is an early event in two-stage skin carcinogenesis in SENCAR mice

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Abstract
Alterations in the expression of the Ha- ras oncogene were investigated in SENCAR mice epidermis at various stages of initiation and promotion during two-stage skin carcinogenesis in SENCAR mice. Adult SENCAR mice were treated with 200 nmol of the (+) enantiomer of benzo[a]pyrene 7, 8-diol 9, 10-epoxide- anti (BPDE- anti ), a potent initiating agent, followed by repetitive treatments with the tumor promoter 12– 0 -tetradecanoylphorbol-13-acetate (TPA). Other mice were ‘sham’-initiated with the (−) enantiomer of BPDE- anti , which is inactive as a tumor initiator. Polyadenylated RNA was isolated from pre-tumor epidermis and tumors at eight different stages of tumorigenesis and analyzed for changes in Ha- ras expression using Northern blot hybridization. Significantly enhanced levels of Ha- ras RNA were observed in TPA-pro-moted papillomas as early as 7 weeks after initiation. Only trace amounts of Ha- ras RNA were present in untreated epidermis or epidermis treated with the (+) or (−) enantiomer followed by 2-12 treatments with TPA (pre-papilloma stage). Southern blot hybridization of tumor DNA indicated that the increased expression of the Ha- ras oncogene was not due to gene amplification. We conclude that elevated levels of Ha- ras expression can occur at an early stage of tumor development in mouse epidermis in vivo and may play a role in tumorigenesis.

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