Targeting of the type I insulin-like growth factor receptor (IGF-IR) has not been successful in breast cancer. Data suggest the highly homologous insulin receptor (IR) may be an alternate growth stimulatory pathway used by cancer cells. Since both receptors phosphorylate the insulin receptor substrate-1 (IRS-1) protein as an immediate consequence of ligand binding, disruption of both receptors could be accomplished by suppression of IRS-1. To test this hypothesis, we created a doxycycline-inducible IRS-1 shRNA lentiviral construct to infect hormone dependent MCF-7 breast cancer cells. IRS-1 shRNA downregulation resulted in decreased responses to IGF-I, insulin, and estradiol in monolayer and anchorage independent growth assays. Decreased IRS-1 levels also suppressed estradiol stimulated gene expression and estrogen receptor binding to DNA. Xenograft growth was also inhibited by induction of IRS-1 shRNA. These data show IRS-1 is a critical regulator of endocrine responsive breast cancer. To further study the critical effects of IRS1 in ER positive breast cancer biology, CRISPR technology was used to delete IRS1 from MCF-7L cells. We analyzed 5 CRISPR IRS1 gene edited clones. Two different commercial available IRS1 antibodies were used to examine IRS1 protein expression. Clones #16, #20 did not show any IRS1 expression, while clones #22, #33 and #38 showed truncated IRS1 expression. IGFIRβ and Insulin R levels were comparable to the parent MCF-7L cells in all the clones, while IRS2 and ERα levels were significantly lower in clone #16. IRS phosphorylation, detected by PY20 antibody, was significantly lower in clone #16. The downstream signaling measured by pAKT and pErk1,2 did not change in all clones compared to those in MCF-7L. In all the clones, ERα ser118 phosphorylation by estradiol stimulation (10 nM) and ERα downregulation by fulvestrant treatment (100 nM) were not affected by IRS1 knockout. IGF-I (5 nM), insulin (10 nM) and estradiol (10 nM) stimulated monolayer cell growth in clone #16 was completely inhibited, while other clones were partially inhibited. In contrast, growth in clone #22 was not inhibited due to extremely high IRS2 expression and compensation. In conclusion, when the IRS1 gene was deleted, multiple growth stimulatory ligands (IGF-I, insulin and estradiol) were inhibited in breast cancer cells. However, IRS-2 compensated for IRS-1 gene deletion in one of the clones we studied. Thus, targeting the IRS proteins may inhibit multiple ligands that cause growth stimulation in ER positive breast cancer therapy.

Citation Format: Xihong Zhang, Douglas Yee. Gene deletion of IRS1 inhibited IGF-I, insulin and estradiol stimulated MCF-7L breast cancer cell growth [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-10-09.