Suppression of Insulin Receptor Substrate 1 Inhibits Breast Cancer Growth In Vitro and in Female Athymic Mice

Targeting 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. IRS-1 gene deletion by CRISPR/Cas9 editing resulted in suppression of IGF-I, insulin, and estrogen-stimulated growth in hormone-dependent MCF-7L breast cancer cells. A doxycycline-inducible IRS-1 shRNA lentiviral construct was also used to infect MCF-7L 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 that IRS-1 is a critical regulator of endocrine responsive breast cancer. Efforts to target this adaptor protein could have broader growth inhibitory effects and receptor targeting.