Peroxisome proliferator-activated receptor gamma activates fas ligand gene promoter inducing apoptosis in human breast cancer cells

In just over a decade, apart from established metabolic actions, peroxisome proliferator-activated receptor gamma (PPAR gamma) has evolved as key therapeutic target in cancer disease. Fas ligand (FasL), a trans-membrane protein, induces apoptosis by crosslinking with the Fas receptor. Despite the FasL relevance, little is available on the regulation of its expression. In the current study, we explored for the first time the potential role of PPAR gamma in triggering apoptotic events through the Fas/FasL pathway in breast cancer cells. In MCF7 cells, by reverse transcription-polymerase chain reaction and Western blotting, we showed that the synthetic PPAR gamma ligand rosiglitazone (BRL) enhanced FasL expression, that was abrogated by a specific PPAR gamma antagonist GW9662. Transient transfection assays demonstrated that BRL transactivated human FasL promoter gene in a PPAR gamma-dependent manner. Progressive 5' deletion analysis has identified a minimal promoter fragment spanning nucleotides from -318 to -237 bp, which is still sensitive to BRL treatment. FasL promoter activity was abrogated by mithramycin, suggesting an involvement of Sp1 transcription factor in PPAR gamma action. Electrophoretic mobility shift and chromatin immuno-precipitation assays demonstrated that BRL increased the binding of PPAR gamma and Sp1 to the Sp1 sequence located within the FasL gene promoter. The role of PPAR gamma and Fas/FasL pathways in BRL-induced apoptotic events was assessed by caspase 8 cleavage in the presence of GW as well as PPAR gamma and FasL RNA interferences. Our results indicate that PPAR gamma positively regulates the FasL gene expression also in MDA-MB231 and in BT20, revealing a new molecular mechanism by which BRL induces apoptosis in breast cancer cells.