Peroxisome proliferator-activated receptors (PPARs) compose a subfamily of nuclear hormone receptors functioning as transcriptional regulators. Originally, the PPARgamma ligand known as thiazolidinedione (TZD) was used for the treatment of diabetic patients. However, recent studies have shown that TZD also has an antitumor effect that inhibits cell growth in several types of human malignant neoplasms, including leukemia cell lines. Since pioglitazone is the only TZD currently available in clinics in Japan and the role of TZD in normal human hematopoietic cells or primary leukemia cells has not been previously reported, we investigated the effect of pioglitazone on human normal hematopoietic progenitor cells, primary leukemia cells, and leukemia cell lines (HL60, K562, U937, HEL, CEM, Jurkat, and NALM1). Pioglitazone inhibited the proliferation of leukemia cells in a dose-dependent manner. The viable cell numbers of HL60, K562, and Jurkat leukemia cell lines were profoundly reduced when the cells were cocultured with pioglitazone. Colony formation in the leukemia cell lines as well as the primary leukemia cells was significantly inhibited to 20-71% and 1-25% of that in control cultures by the addition of 100 and 300 microM of pioglitazone, respectively. However, the CFU-E and CFU-GM colonies of cells obtained from healthy volunteers were not altered in the presence of 100 microM of pioglitazone. Pioglitazone (300 microM) induced slight decrease of CFU-E and CFU-GM. BFU-E was more sensitive to pioglitazone than CFU-E and CFU-GM. Pioglitazone-induced growth inhibition in HL60 cells was associated with cell cycle arrest at the G1 phase, as has been reported for another TZD, troglitazone. Similar levels of PPARgamma protein were observed in both leukemia and normal bone marrow cells by Western blotting, suggesting that the expression of PPARgamma protein was not associated with the inhibitory potency of pioglitazone. In conclusion, our results suggest that pioglitazone may offer a new therapeutic approach to aid in the treatment of leukemia.