Selective estrogen receptor modulators (SERMs) such as tamoxifen have been a conventional therapy for breast cancer patients expressing estrogen receptor, representing the most common breast cancer type. The development of secondary resistance to SERM is an unsolved clinical dilemma, leading to the exploration of more targeted therapies. The insulin-like growth factor (IGF) system is well documented and is implicated as a contributor to endocrine resistance. Unfortunately, antibodies directed against type I IGF receptor (IGF1R) failed to demonstrate efficacy in endocrine resistant breast cancer. We have previously shown that tamoxifen resistant (TamR) breast cancer cells lose IGF1R yet retain expression of insulin receptor (InsR), a closely related receptor to IGF1R. Thus, we hypothesized that InsR may serve as a compensatory pathway to the loss of IGF1R in TamR cells and InsR may be a target in the therapy of endocrine resistant breast cancer. Indeed, our preliminary results show that TamR breast cancer cells were more sensitive to insulin stimulation. As compared to the parental cells, TamR cells showed stronger PI3K/AKT and MAPK/ERK activation, greater MTT proliferation growth and anchorage-independent growth upon insulin treatment. Knocking down InsR in TamR cells with shRNA was able to attenuate their sensitivity towards insulin-mediated PI3K/MAPK activation and growth, suggesting InsR targeting may be necessary in endocrine resistant breast cancer. To develop new agents to target InsR, we used yeast surface display to develop an InsR-selective protein scaffold – the 10th type III domain of human fibronectin (Fn3). This technique has arisen as an alternative method for the development of specific binders of cell surface proteins. Compared to antibodies, the Fn3 proteins are smaller (∼10kDa), thermally more stable, lack disulfide bonds and have the ability to bind a large variety of proteins. Using error-prone polymerase mutagenesis, we identified improved Fn3 proteins with increased InsR affinity, specificity, and stability. Our current results show that InsR is an important target in endocrine resistant breast cancer cells. Development of engineered Fn3 peptides with increased InsR binding specificity and affinity for InsR compared to IGF1R could be used to disrupt signaling through this pathway. Engineered Fn3 will further be evaluated as a potential imaging and therapeutic tool.

Citation Format: Jie Ying Chan, Benjamin Hackel, Douglas Yee. Insulin receptor targeting in breast cancer through yeast surface display [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P4-06-02.