Induction of PPAR gamma signaling and autophagy as a mechanism mediating acquired rapamycin resistance in breast cancer models

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Rapamycin is a product of the soil bacterium S. hygroscopicus most widely used as an immunosuppressive agent and is an emerging cancer therapeutic. It functions by binding to FK506-binding protein 12 (FKBP12), which in turn complexes with and inhibits mTOR. mTOR is a critical regulator of cellular proliferation by controlling protein synthesis via sensing growth factor and nutrient availability. In mouse models that mimic the progression of human breast cancer from DCIS to metastatic disease, we have shown rapamycin can significantly decrease the growth of premalignant mammary intraepithelial neoplastic outgrowth (MIN-O) lesions and Met-1 tumors. However, these lesions were not abolished and rapamycin-resistant tumors emerged. Gene expression profiling implicated the activation of peroxisome proliferator-activated receptor gamma (PPARg) signaling as the molecular basis underlying rapamycin resistance. PPARg is a member of the nuclear receptor superfamily of proteins with transcription factor activity, controlling diverse cellular functions including fat and lipid metabolism, cellular proliferation and apoptosis. Both in vitro and in mouse models we observe an induction of PPARg expression and its downstream targets after rapamycin treatment. Furthermore, this is preceded by marked nuclear translocation of PPARg. The exact role of PPARg in breast cancer survival is not clear. PPARg agonists have shown antiproliferative effects in breast cancer cells. However, mice overexpressing PPARg show increased breast cancer development when mated with transgenic MMTV-PyV-mT mice. In both the MIN-O and Met-1 mouse models, PPARg promoted tumor growth. Conversely, the addition of GW9662, a PPARg antagonist, enhanced the antiproliferative effect of rapamycin. Rapamycin-treated MIN-O lesions and cell lines also exhibited characteristic features of autophagy, including accumulation of autophagic vesicles. This occurred with rapid kinetics in vitro and was supported biochemically by increased conversion of light chain 3 (LC3B) to the cleaved, type II form. Rapamycin is a well-known inducer of autophagy, but depending upon the cellular context, autophagy can function in survival or as an intermediate in cell death. The exact role of autophagy in breast cancer is still not fully understood. In these mouse models, it is unclear whether autophagy is aiding rapamycin resistance or is responsible for the antiproliferative effect of rapamycin. In summary, our results suggest that PPARg antagonists, already widely used experimentally, may have clinical utility in the treatment of breast cancer and to combat rapamycin resistance. Further investigation into the function of autophagy in these models may clarify its role in breast tumor development and in overcoming resistance to chemotherapy regimens through the adjuvant use of inhibitors of autophagy.