Aldehyde dehydrogenase 3 expression is decreased by clofibrate via PPAR gamma induction in JM2 rat hepatoma cell line

In normal liver aldehyde dehydrogenase 3 (ALDH3) is poorly expressed. In hepatoma cells, its expression increases in direct correlation with the degree of deviation and increased ALDH3 activity is one cause of resistance to the toxicity of drugs and lipid peroxidation aldehydes. Hepatoma cells with high ALDH3 content are more resistant to the cytotoxic effect of aldehydes than those with low ALDH3, and inhibition of the enzyme with aldehydes, specific inhibitors or antisense oligonucleotides (AS-ODN), decreases cell growth. It remains open how ALDH3 influences cell growth or cell phenotype. Recently, we have shown that enrichment of a highly deviated rat hepatoma cell line, JM2, with arachidonic acid, a natural ligand of peroxisome proliferator activated receptors (PPARs), inhibits growth, partially restores ALDH2 and ALDH3 to their normal levels and induces PPAR expression. In the present study we address the effect of clofibrate, a hypolipidemic drug and synthetic PPAR ligand on ALDH gene expression. We show that treatment of JM2 cells with clofibrate inhibits cell growth, induces PPARgamma and decreases ALDH3 expression. To determine the relationship between PPARgamma and ALDH3 expression, we exposed JM2 cells to AS-ODN against PPARgamma. AS-ODN reduced PPARgamma content and prevented the inhibitory effect of clofibrate on cell proliferation and ALDH3 expression. Since these results indicate that ALDH3 expression is under PPAR control, we examined the 5' flanking sequence of the ALDH3 gene, but were unable to find any sequence similar to any known peroxisome proliferator response element. We thus believe that the effect of PPARgamma on ALDH3 occurs via other transcription factors, whose identity remain to be determined. The results indicate that PPARgamma plays a key role in regulation of growth and differentiation of hepatoma cells, and that ALDH3 collaborates in modulating cell proliferation and in determining some aspects of the hepatoma phenotype, i.e. resistance to drugs and to lipid peroxidation products.