Expression of peroxisome proliferator-activated receptor gamma and the growth inhibitory effect of its synthetic ligands in human salivary gland cancer cell lines.

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor superfamily of ligand-activated transcription factors. It is expressed in several tissue types, including adipose tissue in which it stimulates adipogenesis. Recent studies have demonstrated that PPARgamma ligands induce cellular differentiation and inhibit cell growth in carcinomas of various organs including the breast, prostate, lung, colon, stomach, bladder, and pancreas. However, whether PPARgamma is expressed in human salivary gland tumors and its function in this tissue is unknown. In the present study, we examined PPARgamma gene expression in human salivary gland cancer cells and tested its ligands for any antitumor effect. PPARgamma mRNA was detected by RT-PCR in both benign and malignant salivary gland tumor tissues. The effect of PPARgamma on cell growth was investigated using four human salivary gland cancer cell lines; HSG, AZA3, HSY and TYS, which were confirmed to express PPARgamma1 mRNA and protein. Retinoid X receptor alpha protein, which forms heterodimers with PPARgamma, was also detected in all the cells tested. Data obtained by luciferase assay indicated that the intrinsic PPARgamma protein was activated by the synthetic ligands, troglitazone and pioglitazone, but not by the natural ligand, 15-deoxy-delta12, 14-prostaglandin J2. The synthetic PPARgamma ligands, particularly troglitazone, caused significant dose-dependent inhibition of cancer cell growth. Furthermore, the overexpression of PPARgamma1 or PPARgamma2 in cancer cells also reduced significantly their growth rate. These results suggest that PPARgamma and its synthetic ligands suppress the growth of human salivary gland cancer cells and it may be a useful molecular target for cancer treatment.