PI3K-FRAP/mTOR pathway is critical for hepatocyte proliferation whereas MEK/ERK supports both proliferation and survival.
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.
Coutant
Alexandre
A
INSERM U522, Unité de Recherches Hépatologiques, IFR 97, Hôpital Pontchaillou, Rennes, France.
Rescan
Claude
C
Gilot
David
D
Loyer
Pascal
P
Guguen-Guillouzo
Christiane
C
Baffet
Georges
G
D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002455:Cell Division / Q000502:physiology; D002470:Cell Survival / Q000502:physiology; D002478:Cells, Cultured; D019938:Cyclin D1 / Q000378:metabolism; D004247:DNA / Q000096:biosynthesis; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004815:Epidermal Growth Factor / Q000494:pharmacology; D016193:G1 Phase / Q000502:physiology; D022781:Hepatocytes / Q000166:cytology* / Q000201:enzymology*; D007328:Insulin / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins; D048728:MAP Kinase Kinase Kinase 1; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000502:physiology*; D008297:Male; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D014176:Protein Biosynthesis; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D017207:Rats, Sprague-Dawley; D016196:S Phase / Q000502:physiology; D058570:TOR Serine-Threonine Kinases
D002352:Carrier Proteins; D019938:Cyclin D1; D004247:DNA; D004791:Enzyme Inhibitors; D004815:Epidermal Growth Factor; D007328:Insulin; D047908:Intracellular Signaling Peptides and Proteins; D048728:MAP Kinase Kinase Kinase 1; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010750:Phosphoproteins; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; C499526:Eif4ebp1 protein, rat; C546842:MTOR protein, human; C546845:mTOR protein, rat; C482576:MAP3K1 protein, human