RNAi-mediated silencing of insulin receptor substrate 1 (IRS-1) enhances tamoxifen-induced cell death in MCF-7 breast cancer cells

Insulin receptor substrate 1 (IRS-1) is a major downstream signaling protein for insulin and insulin-like growth factor I (IGF-I) receptors, conveying signals to PI-3K/Akt and ERK1/2 pathways. In breast cancer, IRS-1 overexpression has been associated with tumor development, hormone-independence and antiestrogen-resistance. In part, these effects are related to potentiation of IRS-1/PI-3K/Akt signaling. In estrogen sensitive breast cancer cell lines, tamoxifen treatment reduces IRS-1 expression and function; consequently, inhibiting IRS-1/PI-3K signaling. We tested whether anti-IRS1 siRNA could inhibit growth and survival of estrogen-sensitive MCF-7 breast cancer cells, when used alone or in combination with TAM. Our results indicated: (a) out of four tested anti-IRS1 siRNAs, two siRNAs reduced IRS-1 protein by approximately three-fold in both growing and IGF-I-stimulated cells without affecting a closely related protein, IRS-2; (b) these effects paralleled IRS1 mRNA downregulation by approximately three-fold, measured by quantitative real time-polymerase chain reaction; (c) action of anti-IRS1 siRNAs induced the apoptotic response, observed by altered mitochondrial membrane potential coupled with downregulation of NF-kappaB target Bcl-xL and reduced cell viability; (d) anti-IRS1 siRNA treatment enhanced the cytotoxic effects of TAM by approximately 20%. In summary, anti-IRS1 RNAi strategy could become a potent tool to induce breast cancer cell death, especially if combined with standard TAM therapy.