Abstract P2-06-03: Insulin receptor isoform signaling in breast cancer
Authors: Zhang X , Chan JY , Pan Y , Dong C , Yee D 
Published: May 2019

Abstract
Recent data shows that insulin receptor (IR) and the type I insulin-like growth factor receptor (IGF1R) play important roles in breast cancer cell biology. Targeting only IGF1R has not been successful perhaps due to compensation by IR. IR exists in two isoforms, fetal isoform IR-A is a splice variant of IR which excludes exon 11. The adult/metabolic isoform IR-B is the predominant species expressed in normal tissues, while the fetal form IR-A is more highly expressed in breast cancer. IR-A mRNA in endocrine resistant cells is expressed at levels 24-fold higher than IGF1R expression (Gradishar, et al. Clin Cancer Res 22:301 2016 PMID: 26324738). In addition to its homodimer, IR-A can also dimerize with IGF1R to form a hybrid. Homodimer IR-A responds to IGF-II and insulin, while IGF1R/IR-A hybrid can also respond to IGF-I. Previous studies in our lab showed down regulation of IGFIR increases the sensitivity of breast cancer cells to insulin. To further investigate the roles of IR-A and IR-B in breast cancer biology, we cloned IR-A and IR-B isoforms into pLV-mCherry and pLJM1EGFP lentiviral expression vectors. Then MCF-7L breast cancer cells were infected with IR-A, IR-A vector control, IR-B and IR-B vector control. Both pooled and single clones were studied. Our results showed that both IR-A and IR-B were highly expressed in MCF-7L cells with the introduced IR tagged species present at a higher molecular weight confirmed by immunoprecipitation and immunoblot. The lower migrating species was endogenous IR. IR-A-pool/Clones had strong basal IR tyrosine phosphorylation, while IR-B-pool/clones did not. This basal phosphorylation did not activate downstream signaling as measured by pAKT and pErk1,2. IR-A basal phosphorylation was completely inhibited by BMS754807 (0.3μM), a dual IGFIR/IR tyrosine kinase inhibitor. Total levels of IRS1, IRS2, IGF1R, ERa, AKT and Erk1,2 levels were not changed in the over-expressing cells. Ligands-IGF-I (5nM), IGF-II (10nM) and Insulin (10nM) stimulated IGF1R/IR, IRS, AKT and Erk1,2 phosphorylation in IR-A-pool and high expression clone IR-A-G5. Cells expressing IR-A activated downstream signaling (IRS, AKT and Erk1,2) at 0.1nM insulin, while IR-A vector control cells required 1nM insulin. IR-B-pool overexpressing cells were not more sensitive to insulin compared to parental cells. Insulin significantly increased Erk1,2 phosphorylation in IR-A-G5 cell while IGF-I stimulation was minimal. In contrast, Erk1,2 phosphorylation in parental cells and vector control cells was primarily mediated by IGF-I, not insulin. IR-A overexpressing cells were stimulated in monolayer growth by insulin. These data show that IR-A expression, as seen in endocrine resistant breast cancer cells, sensitizes breast cancers to low concentrations of insulin. Thus, IR-A expression could serve as a target in breast cancer.
Citation Format: Zhang X, Chan JY, Pan Y, Dong C, Yee D. Insulin receptor isoform signaling in breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-06-03.