PI3K-FRAP/mTOR pathway is critical for hepatocyte proliferation whereas MEK/ERK supports both proliferation and survival

Abstract

Growth factors are known to favor both proliferation and survival of hepatocytes. In this work, we investigated the role of 2 main signaling pathways, phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK), in these processes. First, evidence was provided that the PI3K cascade as well as the MEK/ERK cascade is a key transduction pathway controlling hepatocyte proliferation, as ascertained by arrest of DNA synthesis in the presence of LY294002, a specific PI3K inhibitor. Inhibition of FRAP/mTOR by rapamycin also abrogated DNA replication and protein synthesis induced by growth factor. We showed that expression of cyclin D1 at messenger RNA (mRNA) and protein levels was regulated by this pathway. We highlighted that 4E-BP1 phosphorylation was not activated by epidermal growth factor (EGF) but was under an insulin-regulation mechanism through a PI3K-FRAP/mTOR activation that could account for the permissive role of insulin on hepatocyte proliferation. No interference between the MEK/ERK pathway and 4E-BP1 phosphorylation was detected, whereas p70S6K phosphorylation induced by EGF was under a U0126-sensitive regulation. Last, we established that the antiapoptotic function of EGF was dependent on MEK, whereas LY294002 and rapamycin had no direct effect on cell survival. Taken together, these data highlight the regulation and the role of 2 pathways that mediate growth-related response by acting onto distinct steps. In conclusion, hepatocyte progression in late G1 phase induced by EGF generates survival signals depending on MEK activation, whereas PI3K and MEK/ERK cascades are both necessary for hepatocyte replication. (HEPATOLOGY 2002;36:1079-1088 C ell cycle progression through G1 phase requires the integration of signals from the extracellular environment such as growth factors, cytokines, and extracellular matrix proteins mediated by different transduction cascades. The ability to induce cellular proliferation is often correlated with the ability to promote cell survival. Two signaling cascades have emerged as major players in the mitogenic and antiapoptotic response in many cells: the mitogen-activated protein kinase (MEK)/ extracellular signal-regulated kinase (ERK) and the phosphoinositide 3-kinase (PI3K) pathways.Growth factors as survival factors bind to cell surface receptors and trigger the activation of several kinases, including PI3K, which seems to be implicated in the control of growth and apoptosis of a wide range of cell types. 1 This kinase activates PDK1 that in turn leads to the activation of a serine/threonine kinase termed AKT or PKB, which plays a central role in promoting the survival of many cell types. PDK1 also phosphorylates and activates FRAP/mTOR (target of rapamycin). p70s6k, which regulates the ribosomal S6 subunit phosphorylation in response to mitogens, is controlled by FRAP/mTOR and plays an essential role in the translation machinery. 2-4 FRAP/mTOR also controls the phosphorylation of 4E-BP1, a key protein involved in t...

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