Purpose: Peroxisome proliferator-activated receptor gamma (PPARgamma) plays a important role in various physiological functions. We examined whether PPARgamma is expressed in primary squamous cell carcinoma and lymph node metastasis and whether PPARgamma is a potential target for tumor therapy.
Experimental design and results: A high-level expression of PPARgamma was observed in tumor cells of human primary squamous cell carcinoma, lymph node metastasis, and squamous cell carcinoma cell lines. Treatment with PPARgamma-specific antagonists, but not agonists, caused apoptotic cell death on squamous cell carcinoma cell lines in a concentration-dependent manner. Small interfering RNA for PPARgamma also inhibited cell adhesion and growth of squamous cell carcinomas. The phosphorylation of focal adhesion kinase (FAK) was decreased by treatment with PPARgamma antagonists, and resulted in decreases in phosphorylation of Erk and mitogen-activated protein kinase. Furthermore, PPARgamma antagonists decreased the adhesion of squamous cell carcinomas into fibronectin-coated plates, indicating the inhibition of interaction between squamous cell carcinomas and fibronectin. Expression of integrin alpha5, a counter adhesion molecule for fibronectin, was inhibited by the treatment with PPARgamma antagonists. These results indicate that the decrease in integrin alpha5 and following inhibition of cell adhesion may cause the inhibition of FAK signaling pathways. PPARgamma antagonists also strongly inhibited invasion of squamous cell carcinoma via down-regulation of CD151 expression.
Conclusions: The cell death caused by the PPARgamma antagonists was a result of direct interference with cell adhesion "anoikis" involving intracellular FAK signaling pathways. These results imply a potentially important and novel role for the inhibition of PPARgamma function via the use of specific antagonists in the treatment of squamous cell carcinoma and the prevention of tumor invasion and metastasis.