Studies on long-term effects of two peroxisome proliferators inducing high and low carcinogenic activities respectively, indicated that factors other than oxidative injury may be important in the carcinogeneity of such chemicals. Speculation on the existence of specific receptors for mediating the action of peroxisome proliferators has recently been supported by the isolation of new members of the steroid hormone receptor superfamily which are activated by peroxisome proliferators: the mouse peroxisome proliferator-activated receptor (PPAR), the Xenopus laevis PPAR and the rat PPAR. Moreover, it has been shown that PPARs are capable of activating the promoter of the rat acyl coenzyme A oxidase gene, the key enzyme of peroxisomal fatty acid beta-oxidation, and mediates the induction of rabbit CYP4A6, a cytochrome P450 fatty acid omega- hydroxylase, by clofibric acid. This activation is carried out through specific response elements that have recently been identified. Our approach has been to examine whether differences between human and mouse PPAR could account for the marked differences in responsiveness to peroxisome proliferators on human hepatocarcinogenesis. To achieve this goal, we isolated the human PPAR from a human liver cDNA library and compared its structure and ability to activate the ACO response element with that of mouse PPAR. B-lymphoblastoid cells were used to construct a system which can be easily used to determine whether a chemical will be a peroxisome proliferator. The system uses an episomally-replicating plasmid containing the human PPAR cDNA under control of the thymidine kinase gene promoter, and a reporter gene having a PPAR response element driving expression of the CYP2A6 cDNA. A variety of constructs and promoters were tested to achieve maximal differences between untreated and peroxisome proliferator-treated cells.
