Examination of molecular changes in acquired tamoxifen resistance and subsequent response to anti-IGF1R therapy.

Abstract

The type-I insulin like growth factor (IGF1R) contributes to the proliferation, survival, and metastasis of breast cancer cells. Disruption of IGF1R signaling alone or in combination with established therapies has emerged as an important strategy in cancer therapy. Several anti-IGF1R antibodies and tyrosine kinase inhibitors (TKI's) are being evaluated in phase I, II, or III clinical trials, often in endocrine resistant populations. Thus far, clinical trials have provided less than promising results. Although preclinical studies showed promising results, these studies were performed using endocrine sensitive cell models. Here, we sought to determine the efficacy of IGF1R inhibitors using an endocrine resistant human breast cancer cell model in vitro and in vivo. The first section of this work highlights the creation and characterization of a tamoxifen-resistant (TamR) cell line. We demonstrate in two estrogen receptor positive breast cancer cell lines that TamR cells maintain estrogen receptor expression. Levels of IGF1R, a known estrogen regulated gene, were greatly reduced in TamR cells. Further, signaling, proliferation, and anchorage-independent growth through the receptor were abolished in TamR cells. Interestingly, signaling and growth through the closely related insulin receptor (a non-estrogen regulated gene) remained intact in TamR cells. The second part of this work utilizes TamR cells to examine the efficacy of IGF1R inhibitors in endocrine resistant and sensitive breast cancer cells. We show that the signaling, proliferation, and anchorage-independent growth of endocrine sensitive MCF-7 cells can be inhibited using a variety of IGF1R antibodies. TamR cells, which lack IGF1R expression, are not affected by IGF1R antibody treatment in vitro or in vivo. In contrast, tyrosine kinase inhibitors which can inhibit both IGF1R and IR were able to inhibit the signaling, proliferation, and anchorage-independent growth of both TamR and parental cells. Taken together, our data demonstrate that tamoxifen resistant cells and tamoxifen treated xenografts have reduced levels of IGF1R, making IGF1R antibody treatment ineffective. Our work highlights the importance of evaluating new therapies using a preclinical model that matches the patient population the therapy will be used in. Finally, our data suggest that inhibition of IR may be necessary to manage tamoxifen resistant breast cancer.