Carcinogen and dietary lipid regulate ras expression and localization in rat colon without affecting farnesylation kinetics

Epidemiological and experimental data suggest that dietary fiber and fat ar e major determinants of colorectal cancer. However, the mechanisms by which these dietary constituents alter the incidence of colon cancer have not be en elucidated, Evidence indicates that dominant gain-of-function mutations short-circuit protooncogenes and contribute to the pathogenesis of cancer. Therefore, we began to dissect the mechanisms whereby dietary fat and fiber , fed during the initiation, promotion and progression stages of colon tumo rigenesis, regulate ras p21 localization, expression and mutation frequency . Male Sprague-Dawley rats (140) were provided with corn oil or fish oil an d pectin or cellulose plus or minus the carcinogen azoxymethane (AOM) in a 2x2x2 factorial design and killed after 34 weeks. We have previously shown adenocarcinoma incidence in these animals to be 70.3% (52/74) for corn oil + AOM and 56.1% (37/66) for fish oil + AOM (P < 0.05). Total ras expression as well as ras membrane:cytosol ratio was 4- to 6-fold higher in colon tum ors than in mucosa from AOM- or saline-injected rats. Expression of ras in the mucosal membrane fraction was 13% higher for animals fed corn oil compa red with fish oil feeding (P < 0.05), which is noteworthy since ras must be localized at the plasma membrane to function. The elevated ras membrane:cy tosol ratio in tumors was not due to increased farnesyl protein transferase activity or prenylation state, as nearly all detectable ras was in the pre nylated form, Phosphorylated p42 and p44 mitogen activated protein kinase ( ERK) expression was two-fold higher in tumor extracts compared with uninvol ved mucosa from AOM- and saline-injected rats (P < 0.05). The frequency of K-ras mutations was not significantly different between the various groups, but there was a trend toward a greater incidence of mutations in tumors fr om corn oil fed rats (85%) compared with fish oil fed rats (58%), Our resul ts indicate that the carcinogen-induced changes in ras expression and membr ane localization are associated with the in vivo activation of the ERK path way, In addition, suppression of tumor development by dietary n-3 polyunsat urated fatty acids may be partly due to a combined effect on colonic ras ex pression, membrane localization, and mutation frequency.