Effects of rat strain, diet composition, and phenobarbital on hepatic gamma-glutamyl transpeptidase histochemistry and on the induction of altered hepatocyte foci and hepatic tumors by diethylnitrosamine

To extend our ongoing characterization of modulatory influences on hepatic tumorigenesis, we examined effects of rat strain (Sprague-Dawley versus Fischer), diet composition (semipurified diet versus standard nonpurified laboratory chow), and dietary phenobarbital on the production of gamma-glutamyl transpeptidase (GGT)-positive hepatocyte foci and hepatic tumors initiated by diethylnitrosamine. In addition to GGT-positive foci, we observed, under certain conditions, the appearance of extensive hepatic GGT staining not associated with focal lesions. This elevated nonfocal GGT was found in rats of both strains fed the nonpurified rather than the purified diet, but the level of staining was higher in Fischer than in Sprague-Dawley rats. Enhancement of this nonfocal staining by dietary phenobarbital appeared insignificant. By comparison, frequencies of GGT-positive foci were generally higher in rats fed the semipurified rather than the nonpurified diet, and the frequencies of GGT-positive foci were invariably higher in Sprague-Dawley than in Fischer rats. Moreover, dietary phenobarbital generally enhanced focus production. Assessments of focus and tumor yields among these experimental groups showed that differences in focus frequencies did not correspond closely to differences in subsequent tumor formation. These results document the need to consider the influences of diet and rat strain on experimental end points in designing protocols for hepatocarcinogenesis studies, especially those involving GGT histochemistry. The data also raise questions about the mechanistic relevance of GGT induction to hepatocarcinogenesis and support our prior evidence against the putative lineal relationship between foci and tumors.