Ligands for peroxisome proliferator-activated receptor γ (PPARγ) induce apoptosis and exert antiproliferative effects on several carcinoma cell lines. The present study investigates the expression of PPARγ and the possibility that agonists for PPARγ also inhibit the growth of human thyroid carcinoma cells. We examined this hypothesis using six cell lines, designated BHP thyroid carcinoma cells, which originated from patients with papillary thyroid carcinoma. RT-PCR analysis revealed that the thyroid carcinoma cell lines BHP2–7, 7–13, 10–3, and 18–21 express PPARγ. More PPARγ was expressed in carcinoma than in adjacent normal thyroid tissue in three of six samples of human papillary carcinoma of the thyroid.

PPARγ-positive thyroid carcinoma cells were treated with agonists of PPARγ, troglitazone, BRL 49653, and 15-deoxy-Δ12,14-prostaglandin J2. Troglitazone (10μ mol/L), BRL 49653 (10 μmol/L), and 15-deoxy-Δ12,14-prostaglandin J2 (1 μg/mL) decreased[ ³H]thymidine incorporation and reduced cell number, respectively, in BHP carcinoma cell lines that expressed PPARγ. Under low serum conditions, ligands for PPARγ induced condensation of the nucleus and fragmentation of chromatin into nucleosome ladders. These findings indicate that the death of thyroid carcinoma cells is a form of apoptosis.

To investigate the molecular mechanism of the apoptosis, we assessed expression of the apoptosis-regulatory genes bcl-2, bax, and c-myc. Troglitazone significantly increased the expression of c-myc messenger RNA but had no effect on the expression of bcl-2 and bax in thyroid carcinoma cells. These results suggest that, at least in part, the induction of apoptosis in human papillary thyroid carcinoma cells may be due to an increase of c-myc.

Troglitazone (500 mg/kg·day) significantly inhibited tumor growth and prevented distant metastasis of BHP18–21 tumors in nude mice in vivo.

Taken together, these results suggest that PPARγ agonist inhibit cell growth of some types of human thyroid cancer.