SUN-131 The Roles of Two Insulin Receptor Isoforms in Triple Negative Breast Cancer Growth

Abstract

Systemic hyperinsulinemia is believed to be an important factor in the progression of a number of cancers, including breast cancer by activating the insulin receptor (IR) signaling cascade in the tumor cells. The IR is expressed in two isoforms, IR-A and IR-B. IR-B is the full-length isoform, while IR-A is lacking 12 amino acids in the α-subunit due to exon 11 alternative splicing. IR-A is predominantly expressed in cancer tissues, while IR-B is mostly expressed in metabolic tissues. The IR and closely related insulin-like growth factor 1 receptor (IGF-1R) are expressed in different ratios in cancer cells. Compared with estrogen receptor positive breast cancers, triple negative breast cancers (TNBC) frequently have higher ratios of IR to IGF-1R. Hyperinsulinemia is associated with increased prevalence of TNBC in pre-menopausal women. Although new targeted therapies are emerging, among breast cancer subtypes TNBC continues to carry the worst prognosis and therefore developing a greater understanding of the links between IR signaling and TNBC progression is critical. The aim of this study is to understand the role of IR-A and IR-B on proliferation, metastasis and metabolism in breast cancer cells. We stably overexpressed human IR-A (IR-A OE) and IR-B (IR-B OE) in TNBC MDA-MB-231 (231) and murine c-myc/vegf overexpressing Mvt1 cells with lentiviral transduction using pLVX-IRES-puro HIV-1-based expression vectors with cDNA encoding the human IR-A,IR-B and control cDNA sequences. Native murine IR was silenced using lentiviral transduction of shRNA in the Mvt1 cells. Overexpression of IR was confirmed at a protein level by western blot, and RNA isoform expression was confirmed using real time PCR. Cell proliferation assays were performed in DMEM/10% FBS and revealed that MDA-MB-231 cells with IR-A OE cells had 15% higher proliferation rates than 231 IR-B OE cells. We then examined the IR signaling pathways by western blot in DMEM/10% FBS. No differences in phosphorylated or total ERK1/2 were observed between control, 231 IR-A OE and 231 IR-B OE cells. 231 IR-A OE cells were found to have 15-fold greater Akt phosphorylation (Ser473) than 231 control cells (p=0.0008) and 4 fold higher pAkt(Ser473) compared with 231 IR-B OE cells (p=0.0016). Further, we found that 231 IR-A OE cells had approximately 2 fold greater expression of c-myc protein compared with both 231 control (p=0.047) and 231 IR-B OE cells (p=0.026). No differences in c-myc expression were observed between 231 IRB OE and 231 control cells. In our previous studies we found that insulin stimulates c-myc expression and silencing the IR reduces c-myc expression in cancer cells. Our current studies show that IR-A, rather than IR-B is the insulin receptor isoform that regulates c-myc expression in human TNBC. Reference: (1) Belfiore et al., Endocr Relat Cancer. 2011; 18(4):R125-R147. (2) Ferguson et al., Breast Cancer Res. 2012; 14(1): R8.