Akt and ERK Control the Proliferative Response of Mammary Epithelial Cells to the Growth Factors IGF-1 and EGF Through the Cell Cycle Inhibitor p57Kip2

The cell cycle inhibitor p57 is induced by Akt but suppressed by ERK to control growth factor–induced proliferation and limit oncogenic behavior. Checks and Balances Growth factors stimulate various downstream pathways to engage the cell cycle and promote cell proliferation. Two of these pathways, the Akt pathway and the ERK pathway, are commonly used as indicators of proliferative or oncogenic signaling. Noting that the Akt and ERK pathways not only can sometimes act synergistically with each other but also can suppress each other’s signaling, Worster et al. explored their involvement in the proliferative response to insulin, insulin-like growth factor 1 (IGF-1), and epidermal growth factor (EGF). Insulin or IGF-1 failed to stimulate the proliferation of mammary epithelial cells independently of EGF but enhanced the response to EGF, whereas EGF stimulated proliferation in the absence of insulin or IGF-1. Activation of the Akt pathway in the absence of ERK activity by IGF-1 or insulin led to an increase in the abundance of the cell cycle inhibitor p57 and proliferative arrest. Concurrent ERK signaling suppressed this increase in p57, enabling proliferation to occur, and p57 depletion enabled IGF-1–induced proliferation in the absence of EGF. Thus, p57 acted as a network sensor capable of detecting and limiting the proliferative response to aberrant signaling states in which the Akt pathway was activated in isolation. Epithelial cells respond to growth factors including epidermal growth factor (EGF), insulin-like growth factor 1 (IGF-1), and insulin. Using high-content immunofluorescence microscopy, we quantitated differences in signaling networks downstream of EGF, which stimulated proliferation of mammary epithelial cells, and insulin or IGF-1, which enhanced the proliferative response to EGF but did not stimulate proliferation independently. We found that the abundance of the cyclin-dependent kinase inhibitors p21Cip1 and p57Kip2 increased in response to IGF-1 or insulin but decreased in response to EGF. Depletion of p57Kip2, but not p21Cip1, rendered IGF-1 or insulin sufficient to induce cellular proliferation in the absence of EGF. Signaling through the PI3K (phosphatidylinositol 3-kinase)–Akt–mTOR (mammalian target of rapamycin) pathway was necessary and sufficient for the increase in p57Kip2, whereas MEK [mitogen-activated or extracellular signal–regulated protein kinase (ERK) kinase]–ERK activity suppressed this increase, forming a regulatory circuit that limited proliferation in response to unaccompanied Akt activity. Knockdown of p57Kip2 enhanced the proliferative phenotype induced by tumor-associated PI3K mutant variants and released mammary epithelial acini from growth arrest during morphogenesis in three-dimensional culture. These results provide a potential explanation for the context-dependent proliferative activities of insulin and IGF-1 and for the finding that the CDKN1C locus encoding p57Kip2 is silenced in many breast cancers, which frequently show hyperactivation of the PI3K pathway. The status of p57Kip2 may thus be an important factor to assess when considering targeted therapy against the ERK or PI3K pathways.