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Insulin‐stimulated cell growth in insulin receptor substrate‐1–deficient ZR‐75‐1 cells is mediated by a phosphatidylinositol‐3‐kinase–independent pathway

In many human breast cancers and cultured cell lines, insulin receptor expression is elevated, and insulin, via its own insulin receptor, can stimulate cell growth. It has recently been demonstrated that the enzyme phosphatidylinositol‐3‐kinase (PI3‐K) mediates various aspects of insulin receptor signaling including cell growth. In order to understand the mechanisms for insulin‐stimulated cell growth in human breast cancer, we measured insulin‐stimulable PI3‐K activity in a non‐transformed breast epithelial cell line, MCF‐10A, and in two malignantly transformed cell lines, ZR‐75‐1 and MDA‐MB157. All three cell lines express comparable amounts of insulin receptors whose tyrosine autophosphorylation is increased by insulin, and in these cell lines insulin stimulates growth. In MDA‐MB157 and MCF‐10A cells, insulin stimulated PI3‐K activity three‐ to fourfold. In ZR‐75‐1 cells, however, insulin did not stimulate PI3‐K activity. In ZR‐75‐1 cells PI3‐K protein was present, and its activity was stimulated by epidermal growth factor, suggesting that there might be a defect in insulin receptor signaling upstream of PI3‐K and downstream of the insulin receptor. Next, we studied insulin receptor substrate‐1 (IRS‐1), a major endogenous substrate for the insulin receptor which, when tyrosine is phosphorylated by the insulin receptor, interacts with and activates PI3‐K. In ZR‐75‐1 cells, there were reduced levels of protein for IRS‐1. In these cells, both Shc tyrosine phosphorylation and mitogen‐activated protein kinase (MAP‐K) activity were increased by the insulin receptor (indicating that the p21ras pathway may account for insulin‐stimulated cell growth in ZR‐75‐1 cells).