The skin provides one of the body's major interfaces with the environment and functions importantly in homeostasis by protecting against an array of potentially noxious physical and chemical agents. One component of this barrier function is the cytochrome P-450-dependent monooxygenase, a membrane-bound complex that generally converts lipophilic biologically active substrates into hydrophilic inactive metabolites to facilitate detoxification. This genetically controlled enzyme system has been highly conserved and consists of at least 10 gene families of which that designated as P-450 1A1 has been a major focus of this laboratory for the past 15 years. Under certain conditions, P-450 1A1 can convert biologically inert precursor substances such as polyaromatic hydrocarbon (PAH) carcinogens into highly reactive electrophilic metabolites that can bind to macromolecules such as DNA and elicit structural changes that can initiate mutagenesis. This double-edged metabolic sword may be a major determinant of cancer risk in target tissues exposed to environmental carcinogens since there is evidence that P-450 mediated DNA modification results in the expression of proto-oncogenes and their activation/mutation into oncogenic forms that are crucial for the development of cancer. One such activated oncogene that has been identified in squamous cell carcinomas induced in animals and occurring in some humans is known as Ha-ras. Despite clearcut evidence for a chronological relationship between carcinogen metabolism and Ha-ras gene activation in skin carcinogenesis, no direct evidence exists to link these phenomena in the pathogenesis of cutaneous cancer. To test the hypothesis that P-450 and Ha-ras gene expression are linked in the development of malignant neoplasms in the skin, it is planned to evaluate the effect of cutaneous exposure of tumor-susceptible and tumor-resistant inbred strains of mice to the PAH carcinogens 7,12 dimethylbenz[a]anthracene (DMBA) and benzo[a]pyrene (BP) on the expression and activity of P-450 1A1, the P-450 isozyme we have previously isolated and characterized in rodent epidermis and on the expression and point mutation of the Ha-ras oncogene. Tumor induction studies using initiation-promotion and complete carcinogenic protocols will be conducted in the same strains of animals and the expression of P-450 1A1 and Ha-ras assessed throughout the course of tumor induction. Comparative studies are also planned in a transgenic mouse expressing the v-Ha-ras gene to further clarify these relationships. Finally, it is planned to employ primary cultures of keratinocytes obtained from these animals and from human subjects for comparative studies of the effect of carcinogen exposure on P-450 1A1 and Ha-ras gene expression. the use of these molecular biological approaches for the study of P-450 and Ha-ras gene expression in the skin will help to unravel the relative contributions of carcinogen exposure and genotype on susceptibility to cancer in this major organ interface with the environment.
Showing the most recent 10 out of 105 publications
