Mammary cancer promotion and MAPK activation associated with consumption of a corn oil-based high-fat diet

Our earlier work has shown a selective promotional effect on the genesis of mammary carcinomas bearing a wild-type, but not mutant, Ha-ras codon 12 in a 1-methyl-1-nitrosourea (MNU)-induced carcinogenesis model by high-fat diets (Nutr Cancer 23, 283-290, 1995). To test the hypothesis that activation of the mitogen-activated protein kinase (MAPK) pathway is associated with this promotional effect, we compared the in vivo MAPK phosphorylation state of carcinomas from rats consuming a low-fat (5% corn oil, modified AIN-93G) with that from rats consuming a high-fat (25% corn oil) diet. Specifically, 21- day-old female Sprague-Dawley rats were given an intraperitoneal injection of MNU and one week later were randomized to the two diets for six weeks. The number of mammary carcinomas per rat was 68% greater in the high-fat group, and Ha-ras mutation was rare in this short-term model. The levels of the phosphorylated MAPK2 (active) and of proliferating cell nuclear antigen (PCNA) were significantly higher in carcinomas from the high-fat group, and the two parameters were substantially correlated (r² = 0.43, p < 0.01). The expression level of c-Raf was fourfold higher in the high-fat group but was only modestly associated with MAPK activation (r² = 0.35, p < 0.05). The levels of the total MAPK1 and MAPK2, guanosine triphosphatase-activating protein, Ha-Ras, and MAPK kinase did not change. These results suggest that an upregulation of c-Raf expression by high fat may in part account for the in vive MAPK activation, which in turn may enhance cell proliferation and mammary carcinogenesis.