Pioglitazone, a synthetic ligand of peroxisome proliferator-activated receptor (PPAR)gamma, causes preadipocyte proliferation through a mechanism which still remains elusive. Here, to address the mechanism, we investigated the effects of PPAR gamma and pioglitazone on the kinetics of cyclin-dependent kinase inhibitors, especially with p16(Ink4a) (p16) centered, by employing 3T3-L1 preadipocytes. Pioglitazone promoted preadipocyte proliferation by increasing S and G(2)/M cell-cycle entry, which was accompanied by decreased p16 mRNA expression. PPAR gamma overexpression along with the luciferase reporter assay confirmed that PPAR gamma was crucial for the downregulation of p16 mRNA transcription, and that the action was augmented by pioglitazone. Thus, pioglitazone exerted cell-cycle dependent promoting effect on preadipocyte proliferation, of which mechanisms include p16-downregulation through PPAR gamma. (C) 2011 Elsevier Inc. All rights reserved.