This study was undertaken to investigate the influence of the peroxisome proliferator-activated receptor gamma (PPARgamma) agonists on the proliferation, apoptosis and tumorigenesis of breast cancer cells. PPARgamma investigation has been largely restricted to adipose tissue, where it plays a key role in differentiation, but recent data reveal that PPARgamma is expressed in several transformed cells. However, the function of PPARgamma activation in neoplastic cells is unclear. Activation of PPARgamma with the known prostanoid agonist 15-deoxy-Delta12,14-prostaglandin J(2) (15dPGJ(2)) or the thiazolidinedione (TZD) agonist troglitazone (TGZ) attenuated cellular proliferation of the estrogen receptor-negative breast cancer cell line MDA-MB-231, as well as the estrogen receptor-positive breast cancer cell line MCF-7. This was marked by a decrease in total cell number and by an inhibition of cell cycle progression. Addition of 15dPGJ(2) was not associated with an increase in cellular differentiation, as has been seen in other neoplastic cells, but rather induction of cellular events associated with programmed cell death, apoptosis. Video time-lapse microscopy revealed that 15dPGJ(2) induced morphological changes associated with apoptosis, including cellular rounding, blebbing, the production of echinoid spikes, blistering and cell lysis. In contrast, TGZ caused only a modest induction of apoptosis. These results were verified by histochemistry using the specific DNA stain DAPI to observe nuclear condensation, a marker of apoptosis. Finally, a brief exposure of MDA-MB-231 cells to 15dPGJ(2) initiated an irreversible apoptotic pathway that inhibited the growth of tumors in a nude mouse model. These findings illustrate that induction of apoptosis may be the primary biological response resulting from PPARgamma activation in some breast cancer cells and further suggests a potential role for PPARgamma ligands for the treatment of breast cancer.