Abstract P3-07-01: IRS1 expression is required for estrogen stimulated growth in breast cancer cells

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Abstract Insulin receptor substrate (IRS) proteins are adaptor proteins phosphorylated by activated type I insulin-like growth factor receptor (IGF-1R) and insulin receptor (IR). In addition to their roles in normal cell physiology, we have shown IRS expression is required for breast cancer cell growth and motility. IRS1 plays a more important role in mitogenesis and survival while IRS2 in cancer cell metastasis. Moreover, this family of adaptor proteins is also involved in the signal transduction of many other transmembrane receptors. Thus, IRS proteins could be potential cancer therapeutic targets. We have previously shown that reduced IRS1 impairs IGF and insuliln stimulated cancer cell growth and tumorigenicity even when signaling is not affected due to possible compensation of IRS2 and other adaptor proteins. To further study the role of IRS-1 function in breast cancer, we created doxycycline inducible IRS-1 shRNA cells. We also found that IRS1 knockdown significantly reduced estradiol stimulated growth. To investigate if reduced IRS1 regulated E2 stimulated binding of ERa ChIP assay was performed using the pS2 promoter. Compared to parental MCF-7L cells, inducible IRS1 knock down clone 3G5 demonstrated significantly reduced promoter binding. Degradation of IRS-1 with a pharmacologic antagonist (NT-157) also diminished ERa binding to pS2 promoter. Further qRT-PCR analysis for mRNA levels of estrogen regulated genes, such as PGR, TFF1, etc. showed those genes were significantly down regulated. To evaluate effects of IRS-1 levels on estrogen stimulated growth, we evaluated 3G5-B12 (a subclone of 3G5) cell growth in a xenograft model. 3G5-B12 cells have similar levels of IGFIR and ERa expression compared to their parent cells. Estrogen dependent xenograft growth was similar to those of parent MCF-7L cells. After doxycycline was administered in the diet, 3G5-B12 tumor growth was significantly inhibited compared to those fed with normal mouse diet, and this inhibition was prolonged at least 2 and a half months. Parental MCF-7L tumor growth was not affected by the doxycycline diet. Doxycycline inducible IRS1 knock down significantly prolonged the time before tumors reached 1000mm3 compared with those tumors on a normal diet. In conclusion, IRS-1 is required for optimal estrogen receptor function as measured by promoter binding and xenograft growth. Our data suggested suppression of IRS-1 function may target several growth pathways in breast cancer cells and represents a new drug target. Citation Format: Zhang X, Varma S, Yee D. IRS1 expression is required for estrogen stimulated growth in breast cancer cells [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-07-01.