Many structurally unrelated nonmutagenic
peroxisome proliferators induce altered areas, neoplastic nodules, and
hepatocellular carcinomas in rats. Unlike the lesions induced by genotoxic hepatocarcinogens, these lesions do not
stain positively for the phenotypic markers
gamma-glutamyl transpeptidase (GGT) and
glutathione-S-transferase P (GST-P). To ascertain whether the absence of immunocytochemically detectable GST-P and GGT
proteins in peroxisome proliferator-induced neoplastic lesions is due to the absence of specific mRNAs, we analyzed the total
RNA isolated from
hepatocellular carcinomas induced by three different
peroxisome proliferators (
ciprofibrate,
Wy-14643, and
BR-931) and the genotoxic
carcinogens,
2-acetylaminofluorene and
aflatoxin B1 (AFB), for the presence of GST-P, GGT, and
alpha-fetoprotein (AFP) mRNAs. Northern and dot blot analysis of total
RNA isolated from liver
tumors induced by three different
peroxisome proliferators revealed no detectable GST-P, GGT, and AFP mRNAs. GST-P
mRNA was also not detected in a transplantable
hepatocellular carcinoma established from a liver
tumor induced by
ciprofibrate. In contrast, GST-P
mRNA levels were high in primary liver
tumors induced by both
2-acetylaminofluorene and AFB and the two transplantable
hepatocellular carcinomas established from such
tumors. By immunoblot method, GST-P
protein was found to be abundant in both primary and transplantable liver
tumors induced by genotoxic
carcinogens but not in those derived from peroxisome proliferator treatment. The GGT and AFP mRNAs were also not found in all 18 liver
tumors induced by
peroxisome proliferators that were analyzed and also in the
ciprofibrate-derived transplantable liver
tumor. The expression of GGT and AFP genes in liver
tumors induced by
2-acetylaminofluorene and AFB was variable. These studies with
peroxisome proliferators show that the GST-P and GGT gene derepression is not essential for the hepatocarcinogenesis or successful
tumor transplantation. Further characterization of the molecular basis for the differential expression, particularly of the GST-P gene in liver
tumors, may help identification of the critical event(s) in hepatocarcinogenesis by genotoxic
carcinogens and nongenotoxic
peroxisome proliferators.