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Objective: PPARγ is a ligand-regulated transcription factor of the nuclear hormone receptor superfamily. It is expressed in certain normal tissues such as adipose tissue, as well as certain epithelial cancer types. Agonists like troglitazone have been shown to inhibit the in vitro and in vivo growth of selected epithelial cancer cell lines. Methods: We evaluated the in vitro cytotoxicity of a newer PPARγ agonist with a more favorable toxicity profile (pioglitazone [Actos®]) and two PPARγ antagonists (T0070907 and GW9662) in a panel of solid tumor and hematopoietic cancer cell lines by the MTT proliferation assay. Effects of the PPARγ agonist in combination with the antagonists or with chemotherapy drugs were also evaluated, and these results were correlated with PPARγ expression as assessed by RT-PCR and immunoblotting. Results: The IC50 for pioglitazone in solid tumor lines ranged from 11.1 μM to >120 μM (mean±SD=60.1±28.1). Hematopoietic (NHL and MM) cell lines were much less sensitive to this drug (IC50=82.0- 124.7; mean±SD=101.4±17.7). Surprisingly, both of the PPARγ antagonists were growth inhibitory for both solid and hematopoietic lines, and both were more potent than pioglitazone with IC50 values of 7.8 to 28.7 μM (T007 was more potent than GW9662). Since all three PPARγ ligands were cytotoxic, they were combined in pilot studies, in which pioglitazone and either T007 or GW9662 showed additive effects. Pioglitazone was sub-additive with standard chemotherapeutic agents (L-OHP, 5-FU or CPT-11) and was additive with radioimmunotherapy using 131I-MN-14 (anti-CEA) antibody. All epithelial cancer lines tested expressed PPARγ by RT-PCR and the majority expressed this protein. Amongst hematopoietic lines, only the myeloid lines, U937 and K562, were positive by RT-PCR, and only K562 expressed PPARγ protein. Preliminary PI-cell cycle analysis together with Annexin-V staining indicate G1 arrest to be the predominant inhibitory mechanism for pioglitazone in both epithelial and hematopoietic lines, with only a minor apoptotic effect. Conclusions: These data demonstrate novel and potent growth inhibitory effects of PPARγ antagonist drugs for both epithelial and hematopoietic neoplastic cells, with greater overall potency than the agonist drug. Both the PPARγ agonist and the antagonists are growth inhibitory for cancer cells independent of PPARγ expression levels. Also, the additive effects of combinations of agonist plus antagonist suggest non-overlapping mechanisms of action. These results confirm the effect of agonist drugs and reveal the potent inhibitory effect of PPARγ antagonists in cancer, suggesting the presence of novel targets of action.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]