GP88 (PC-Cell Derived Growth Factor, progranulin) stimulates proliferation and confers letrozole resistance to aromatase overexpressing breast cancer cells

<p>Abstract</p> <p>Background</p> <p>Aromatase inhibitors (AI) that inhibit breast cancer cell growth by blocking estrogen synthesis have become the treatment of choice for post-menopausal women with estrogen receptor positive (ER⁺) breast cancer. However, some patients display de novo or acquired resistance to AI. Interactions between estrogen and growth factor signaling pathways have been identified in estrogen-responsive cells as one possible reason for acquisition of resistance. Our laboratory has characterized an autocrine growth factor overexpressed in invasive ductal carcinoma named PC-Cell Derived Growth Factor (GP88), also known as progranulin. In the present study, we investigated the role GP88 on the acquisition of resistance to letrozole in ER⁺breast cancer cells</p> <p>Methods</p> <p>We used two aromatase overexpressing human breast cancer cell lines MCF-7-CA cells and AC1 cells and their letrozole resistant counterparts as study models. Effect of stimulating or inhibiting GP88 expression on proliferation, anchorage-independent growth, survival and letrozole responsiveness was examined.</p> <p>Results</p> <p>GP88 induced cell proliferation and conferred letrozole resistance in a time- and dose-dependent fashion. Conversely, naturally letrozole resistant breast cancer cells displayed a 10-fold increase in GP88 expression when compared to letrozole sensitive cells. GP88 overexpression, or exogenous addition blocked the inhibitory effect of letrozole on proliferation, and stimulated survival and soft agar colony formation. In letrozole resistant cells, silencing GP88 by siRNA inhibited cell proliferation and restored their sensitivity to letrozole.</p> <p>Conclusion</p> <p>Our findings provide information on the role of an alternate growth and survival factor on the acquisition of aromatase inhibitor resistance in ER⁺breast cancer.</p

Combined Cancer Therapy: Strategies to Overcome Acquired Aromatase Inhibitor Resistance

Aromatase inhibitors (AIs) have become one of the mainstays of treatment of postmenopausal women with hormone receptor-positive breast cancer. However, acquired resistance to treatment continues to be a significant clinical challenge. There is increasing evidence from preclinical studies that activation of growth factor signaling pathways, as well as cross-talk between these pathways and estrogen receptor-alpha signaling pathways are important mechanisms that contribute to AI resistance. These preclinical studies have been the foundation for multiple randomized clinical trials that have evaluated combination targeted therapy in patients with advanced breast cancer. While the clinical benefit observed in these trials has been variable, the preclinical studies were successful in predicting clinical outcomes. This review focuses on mechanisms of acquired AI resistance and describes preclinical studies that have evaluated combination targeted therapy to overcome AI resistance, as well as clinical trials that have translated this information to the clinical setting