INSULIN-RECEPTOR SUBSTRATE-1 IS THE PREDOMINANT SIGNALING MOLECULE ACTIVATED BY INSULIN-LIKE GROWTH-FACTOR-I, INSULIN, AND INTERLEUKIN-4 INESTROGEN RECEPTOR-POSITIVE HUMAN BREAST-CANCER CELLS

Because insulin-like growth factor-I (IGF-I), insulin, and interleukin -4 (IL-4) have known biological effects in breast cancer cells and sig nal through insulin receptor substrate (IRS) adaptor proteins, we exam ined the expression and function of IRS-1 and IRS-2 in breast tumors a nd cell lines. IRS-1 and IRS-2 were expressed by cell lines and primar y breast tumor specimens. IGF-I, insulin, and IL-4 treatment of MCF-7 and ZR-75, and IGF-I treatment of T47-D breast cancer cells, resulted in much greater tyrosine phosphorylation of IRS-1 compared with IRS-2. Furthermore, IGF-I stimulated greater tyrosine phosphorylation of IRS -1 than either insulin or IL-4. IGF-I treatment also enhanced associat ion of the p85 regulatory subunit of phosphatidylinositol S-kinase wit h IRS-1 and stimulated increased enzymatic activity compared with IL-4 and insulin in all three cell lines. Similarly, mitogen-activated pro tein kinase activity was greater in IGF-I-stimulated cells. To determi ne the functional significance of the activation of these pathways, we inhibited activation of phosphatidylinositol 3-kinase with wortmannin and mitogen-activated protein kinase with PD098059. Both compounds in hibited IGF-stimulated growth, suggesting that both pathways contribut ed to the mitogenic response to IGF-I. We conclude that IRS-1, and not IRS-2, is the predominant signaling molecule activated by IGF-I, insu lin, and IL-4. Furthermore, enhanced tyrosine phosphorylation of IRS-1 by IGF-I, compared with either insulin or IL-4, is associated with gr eater activation of mitogenic downstream signaling pathways resulting in enhanced cell growth.