Peroxisome Proliferator-Activated Receptor as a Therapeutic Target in Human Breast Cancer

Objective: Peroxisome proliferator-activated receptor γ (PPARγ) is aberrantly expressed in most breast tumors, suggesting the potential of agents that target this receptor in treatment and chemoprevention of breast cancer. However, whether PPARγ leads to the promotion or reduction of tumor formation has remained controversial and is further complicated by the ability of its available thiazolidinedione (TZD) ligands to activate another PPAR subtype, PPARδ. Method: To examine the role of each receptor in breast cancer biology, we performed a systematic evaluation of PPARγ and PPARδ agonists on the growth of human estrogen receptor (ER)-positive and -negative breast cancer cells. Results: In this manner we found that TZD-activated PPARγ was highly effective in suppressing the proliferation of estrogen-dependent cancer cells. Activated PPARδ, however, displayed growth-enhancing effects independent of estradiol and regardless of the ER status of the cells. Strikingly, in ER-negative cancer cells expressing a favorable PPARδ/γ ratio, TZDs promoted growth in a PPAR γ-independent manner by direct activation of PPARδ. A screen for ligands with increased receptor selectivity compared to TZDs revealed that GW7845 functioned as a full agonist of PPARγ, yet this agent lacked the ability to activate PPARδ and elicit its associated mitogenic effects. Conclusion: These studies indicate that selective PPAR γ modulators (SPPARγMs) that lack agonist activity on PPARδ may be clinically useful in future cancer treatment and chemoprevention.