Anchorage-independent activation of mitogen-activated protein kinase through phosphatidylinositol-3 kinase by insulin-like growth factor I

Insulin-like growth factor I (IGF-I) is a well-established mitogen in human breast cancer cells. We show here that human breast cancer MCF-7 cells, which were prevented from attaching to the substratum and were floating in medium, responded to IGF-I and initiated DNA synthesis. The addition of IGF-I to floating cells induced activation of protein kinase B (PKB)/Akt, as to cells attached to the substratum. In addition, mitogen-activated protein kinase (MAPK)/extracellular response kinase (ERK) and its upstream kinases, ERK kinase (MEK) and Raf-1, were activated by IGF-I in floating cells. While the IGF-I-induced activation of PKB/Akt was inhibited by PI3-K inhibitor LY294002 but not by MEK inhibitor PD98059, the activation of both MEK and ERK by IGF-I was inhibited by both. These findings suggest that the IGF-I signal that leads to stimulation of DNA synthesis of MCF-7 cells is transduced to ERK through PI3-K, only when they are anchorage-deficient.