Peroxisome proliferators are a diverse group of chemicals that include hypolipidemic drugs, plasticizers and solvents used in the chemical industry, herbicides and pesticides. In rats and mice, these chemicals cause an increase in the number and size of peroxisomes in the liver, kidney and heart tissue and following chronic administration, cause hepatocarcinogenesis. However, species differences in peroxisome proliferator responses are very significant. For example, peroxisome proliferators such as clofibrate and gemfibrozil, are highly effective lipid and cholesterol-lowering drugs in humans, but do not cause peroxisome proliferation. The peroxisome proliferator-activated receptor alpha (PPARa), was found to mediate target gene promoter trans-activation response to peroxisome proliferators. PPARa-null mice have been produced which were shown to be totally resistant to peroxisome proliferation after administration of the potent peroxisome proliferator WY-14,643, suggesting that PPARa is necessary for the carcinogenic action of peroxisome proliferators. Elucidation of the mechanism by which peroxisome proliferators induce carcinogenesis is a prerequisite for assessing the health risk to humans in the pharmaceutical use of hypolipidemic drugs, other drug candidates and chemicals that exhibit peroxisome proliferation in rodent model systems. Therefore, to make sensible judgments concerning the safety of these drugs, it is crucial to completely understand the mechanism of action of peroxisome proliferators and the species differences in biological activity and carcinogenicity mediated by peroxisome proliferators. It is hypothesized that humans are resistant to the adverse effects of peroxisome proliferators due to low level expression of PPARa in the liver. This is in contrast to mice and rats which have a high level expression of hepatic PPARa. Therefore, the aim of this proposal is to investigate whether expression of the human PPARa gene, expressed at different levels in the liver, will yield clues to the mechanism of species differences in response to peroxisome proliferators. Transgenic mice will be made expressing low and high hepatic PPARa and then it will be determined whether hepatic levels of this receptor are correlated with peroxisome proliferator induced peroxisome proliferation, lipid lowering and cancer. Mice containing the complete human PPARa gene or a human PPARa cDNa under control of a heterologous liver-specific promoter, will be prepared by standard transgenic procedures. The transgenic mice will be bred with the PPARa-null mice to introduce the human gene or cDNA into the null background. Two lines of transgenic mice will be made using the following DNAs: 1) the human cDNA under control of a constitutively-active mouse liver-specific transthyretin (TTR) promoter and 2) an expression plasmid containing the tetracycline-positive inducible (Tet-on) promoter controlling expression of the TTR promoter in the liver. While the TTR- PPARa construct results in the continuous production of PPARa specifically in the liver, the Tet- PPARa transgene allows one to increase PPARa synthesis as necessary by means of induction of the promoter with doxycycline. In addition, levels of expression can be controlled by concentrations of the antibiotic. Among these two possibilities, it is hopeful that a mouse can be generated that will be valuable for further studies. It will also be extremely important to identify founder transgenics that express the low levels of PPARa as found in humans and to compare their responses to peroxisome proliferators to mice expressing high levels of receptor in the liver. These studies will therefore, contribute to understanding the mechanism of action of peroxisome proliferators and the species differences in biological activity and carcinogenicity mediated by peroxisome proliferators.