2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a carcinogen in cooked meat, was determined to be a mammary gland carcinogen in female Sprague-Dawley rats on a high fat diet. With our protocol, tumors are induced following a 10-dose regimen of PhIP after a short latency period (25 weeks after treatment). The PhIP-induced mammary tumor incidence and mass was shown to be affected by the level of dietary fat given to the rats. A high-fat diet (23.5% corn oil) resulted in a two- to threefold higher incidence and weight of mammary gland cancer than a low fat diet (5% corn oil). To elucidate possible mechanisms of PhIP-induced mammary gland cancer and effect of dietary fat, tumors are being examined for genetic alterations by single strand conformation polymorphisms-polymerase chain reaction (SSCP-PCR) methods, differential display and gene cloning, and DNA sequencing. Tumors are being cultured to establish cell lines for cytogenetic analysis. Two epithelial cell lines have been established so far, and G-banding and chromosomal painting are underway. To date, approximately 50% of tumors have been found to carry mutations codons 12 and 13 of H-ras gene. Dietary fat does not appear to significantly alter the percentage of tumors carrying the H-ras mutations. By differential display, several genes have been shown to be differentially expressed between tumor and normal mammary gland, and gene cloning is currently underway. P53 mutations do not appear to be a feature in PhIP-induced rat mammary gland tumorigenesis, but studies are ongoing to examine p53 expression in mammary gland tumors and normal mammary gland. RT-PCR demonstrated a higher expression of neu in approximately 75% of tumors. The overexpression of neu is not due gene amplification, as shown by southern blot analysis. To date, no mutations in the neu oncogene have been found in PhIP-induced rat mammary tumors.
