The survival pathways phosphatidylinositol‐3 kinase (PI3‐K)/phosphoinositide‐dependent protein kinase 1 (PDK1)/Akt modulate liver regeneration through hepatocyte size rather than proliferation†

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Abstract
Liver regeneration comprises a series of complicated processes. The current study was designed to investigate the roles of phosphoinositide-dependent protein kinase 1 (PDK1)-associated pathways in liver regeneration after partial hepatectomy (PH) using liver-specific Pdk1-knockout (L-Pdk1KO) and Pdk1/STAT3 double KO (L-DKO) mice. There was no liver regeneration, and 70% PH was lethal in L-Pdk1KO mice. Liver regeneration was severely impaired equally in L-Pdk1KO and L-DKO mice, even after nonlethal 30% PH. There was no cell growth (measured as increase of cell size) after hepatectomy in L-Pdk1KO mice, although the post-PH mitotic response was the same as in controls. As expected, hepatectomy did not induce hepatic Akt-phosphorylation (Thr308) in L-Pdk1KO mice, and post-PH phosphorylation of Akt, mammalian target of rapamycin (mTOR), p70 ribosomal S6 kinase (p70S6K), and S6 were also reduced. To examine the specific role of PDK1-associated signals, a “pif-pocket” mutant of PDK1, which allows PDK1 only to phosphorylate Akt, was used. Liver regeneration was recovered in L-Pdk1KO mice with a “pif-pocket” mutant of PDK1. This re-activated Akt in L-Pdk1KO mice liver and induced post-PH cell growth, without affecting cell proliferation. Further deletion of STAT3 (L-DKO mice) did not further deteriorate liver regeneration, although this certainly reduced post-PH mitotic response. These findings indicate that PDK1/Akt contribute to liver regeneration by regulating cell size. Regarding phosphatidylinositol-3 kinase (PI3-K), immediate upstream signal of PDK1, activation of PI3-K induced cell proliferation via STAT3 activation in the liver of L-Pdk1KO mice but did not improve impaired liver regeneration. This confirmed the pivotal role of PDK1 in liver regeneration and cell growth. Conclusion: PDK1/Akt-mediated responsive cell growth is essential for normal liver regeneration after PH, especially when cell proliferation is impaired. (HEPATOLOGY 2009;49:204-214.)