Excessive accumulation of autofluorescent lipofuscin in the liver during hepatocarcinogenesis by methyl clofenapate and other hypolipidemic peroxisome proliterators

Several hypolipidemic drugs and certain industrial plasticizers induce proliferation of peroxisomes and enhance the activities of peroxisome-associated enzymes in the livers of rodents. Evidence now suggests that potent hepatic peroxisome proliferators as a class are carcinogenic, although they do not appear to be mutagenic in the Salmonella microsome assay system. We now report that long-term administration of methyl-2-[4-(ρ-chlorophenyl)phenoxy]-2-methylpropionate, a potent hepatic peroxisome proliferator, at a dietary concentration of 0.1% (w/w), induced hepatocellular carcinomas in 14 of 14 male F344 rats between 65 and 75 weeks. The tumor cells contained several peroxisomes. Consistent with this observation was the finding of increased levels of carnitine acetyltransferase, heat-labile peroxisomal enoyl coenzyme hydratase, and peroxisomal β-oxidation system in the tumors. As expected, methyl clofenapate was not mutagenic in the Salmonella microsome assay using Salmonella typhimurium strains TA98 and TA100. Abundant accumulation of autofluorescent lipofuscin in the nontumorous portions of liver in rats bearing methyl-2-[4-(ρ-chlorophenyl)phenoxy]-2-methylpropionate-induced hepatocellular carcinomas was observed. The examination in this study of livers of rats bearing hepatocellular carcinomas induced previously by five other hypolipidemic peroxisome proliferators provided retrospective evidence for increased accumulation of autofluorescent pigment in the liver cells. It is suggested that accumulation of lipofuscin in the livers of rats fed peroxisome proliferators serves as evidence for the increased production of biologically damaging free radicals as a result of H₂O₂ generated by sustained proliferation of peroxisomes. The hypothesis that persistent proliferation of peroxisomes and increase in peroxisomal β-oxidation system serves as endogenous initiator of the neoplastic transformation of liver cells by increasing the intracellular production of DNA-damaging H₂O₂ and other reactive oxygen intermediates (OH^., O₂^T, ¹O₂) remains to be tested.