Insulin and insulin-like growth factor (IGF) signaling systems regulate the malignant phenotype. However, targeting of the type I IGF receptor (IGF-IR) has shown little activity in clinical trials. One potential reason for these disappointing results is that activation of the closely related insulin receptor (InR) could compensate for IGF-IR blockade. Since both receptors phosphorylate the insulin receptor substrates (IRS), perhaps a better strategy would involving targeting of this key post-receptor protein. Two IRS proteins are expressed in breast cancer cells. IRS-1 is regulated by estradiol in breast cancer cell lines, while IRS-2 is the predominant IRS species in hormone receptor negative cells. NT157, a small-molecule tyrphostin, binds IGF-1R but does not affect receptor autophosphorylation. Instead, it downregulates IRS proteins in several model systems. In primary breast cancers, IRS-1 was positively correlated to ERα expression in the TCGA database. In ERα+ and basal-like breast cancer cell lines NT157 treatment suppressed IRS protein expression in a dose dependent manner. Short term exposure to NT157 treatment did not affect IGF-I, IGF-II, and insulin induced activation of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK), but longer exposure resulted in inhibition of these signaling pathways. NT157 resulted in serine phosphorylation of IRS proteins and was dependent on MAPK activation. Serine phosphorylation resulted in disassociation between IRS proteins and their receptors resulting in IRS degradation. NT157 decreased S phase fraction, monolayer, and anchorage independent growth after IGF/insulin treatment in ERα+ breast cancer cells. NT157 downregulation of IRS protein expression also sensitized ERα+ breast cancer cells to rapamycin. Moreover, NT157 inhibited the growth of tamoxifen resistant ERα+ breast cancer cells. In the basal-like breast cancer cells (MDA-MB-231), NT157 repressed the proliferation (G2/M abrogation) and migration through downregulation of IRS1/2 protein. Given that both IGF-IR and InR play a role in cancer biology, targeting of IRS adapter proteins could be a more effective inhibitory strategy compared to receptor-targeting approaches.

Citation Format: Yee D, Temiz NA, Levitzki A, Yang Y. Insulin receptor substrate (IRS) targeting by the tyrophostin NT157 inhibits breast cancer cell growth. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr PD2-07.