Background & Aims
Non‐alcoholic fatty liver disease (NAFLD) is a major health problem and occurs frequently in the context of metabolic syndrome and type 2 diabetes mellitus. Hepatocyte‐specific ...Pten‐deficiency in mice was shown previously to result in hepatic steatosis due to hyperactivated AKT2. However, the role of peripheral insulin‐sensitive tissues on PTEN‐ and AKT2‐dependent accumulation of hepatic lipids has not been addressed.
Methods
Effects of systemically perturbed PTEN/AKT2 signalling on hepatic lipid content were studied in Pten‐haplodeficient (Pten+/−/Akt2+/+) mice and Pten‐haplodeficient mice lacking Akt2 (Pten+/−/Akt2−/−). The liver and skeletal muscle were characterized by histology and/or analysis of insulin signalling. To assess the effects of AKT2 activity in skeletal muscle on hepatic lipid content, AKT2 mutants were expressed in skeletal muscle of Pten+/+/Akt2+/+ and Pten+/−/Akt2+/+ mice using adeno‐associated virus 8.
Results
Pten+/−/Akt2+/+ mice were found to have a more than 2‐fold reduction in hepatic lipid content, at a level similar to that observed in Pten+/−/Akt2−/− mice. Insulin signalling in the livers of Pten+/−/Akt2+/+ mice was enhanced, indicating that extrahepatic factors prevent lipid accumulation. The skeletal muscle of Pten+/−/Akt2+/+ mice also showed enhanced insulin signalling. Skeletal muscle‐specific expression of constitutively active AKT2 reduced hepatic lipid content in Pten+/+/Akt2+/+ mice, and dominant negative AKT2 led to an increase in accumulation of hepatic lipids in both Pten+/+/Akt2+/+ and Pten+/−/Akt2+/+ mice.
Conclusion
Our results demonstrate that AKT2 activity in skeletal muscle critically affects lipid accumulation in the livers of Pten+/+/Akt2+/+ and Pten+/−/Akt2+/+ mice, and emphasize the role of skeletal muscle in the pathology of NAFLD.
Insulin resistance, a major contributor to the pathogenesis of type 2 diabetes, leads to increased hepatic glucose production (HGP) owing to an impaired ability of insulin to suppress hepatic ...gluconeogenesis. Nuclear receptor oestrogen-related receptor γ (ERRγ) is a major transcriptional regulator of hepatic gluconeogenesis. In this study, we investigated insulin-dependent post-translational modifications (PTMs) altering the transcriptional activity of ERRγ for the regulation of hepatic gluconeogenesis. We examined insulin-dependent phosphorylation and subcellular localisation of ERRγ in cultured cells and in the liver of C57/BL6, leptin receptor-deficient (db/db), liver-specific insulin receptor knockout (LIRKO) and protein kinase B (PKB) beta-deficient (Pkbbeta ^sup -/-^) mice. To demonstrate the role of ERRγ in the inhibitory action of insulin on hepatic gluconeogenesis, we carried out an insulin tolerance test in C57/BL6 mice expressing wild-type or phosphorylation-deficient mutant ERRγ. We demonstrated that insulin suppressed the transcriptional activity of ERRγ by promoting PKB/Akt-mediated phosphorylation of ERRγ at S179 and by eliciting translocation of ERRγ from the nucleus to the cytoplasm through interaction with 14-3-3, impairing its ability to promote hepatic gluconeogenesis. In addition, db/db, LIRKO and Pkbbeta ^sup -/-^ mice displayed enhanced ERRγ transcriptional activity due to a block in PKBbeta-mediated ERRγ phosphorylation during refeeding. Finally, the phosphorylation-deficient mutant ERRγ S179A was resistant to the inhibitory action of insulin on HGP. These results suggest that ERRγ is a major contributor to insulin action in maintaining hepatic glucose homeostasis.PUBLICATION ABSTRACT
Background The thymus constitutes the primary lymphoid organ for the majority of T cells. The phosphatidyl-inositol 3 kinase (PI3K) signaling pathway is involved in lymphoid development. Defects in ...single components of this pathway prevent thymocytes from progressing beyond early T cell developmental stages. Protein kinase B (PKB) is the main effector of the PI3K pathway. Methodology/Principal Findings To determine whether PKB mediates PI3K signaling in the thymus, we characterized PKB knockout thymi. Our results reveal a significant thymic hypocellularity in PKBα−/− neonates and an accumulation of early thymocyte subsets in PKBα−/− adult mice. Using thymic grafting and fetal liver cell transfer experiments, the latter finding was specifically attributed to the lack of PKBα within the lymphoid component of the thymus. Microarray analyses show that the absence of PKBα in early thymocyte subsets modifies the expression of genes known to be involved in pre-TCR signaling, in T cell activation, and in the transduction of interferon-mediated signals. Conclusions/Significance This report highlights the specific requirements of PKBα for thymic development and opens up new prospects as to the mechanism downstream of PKBα in early thymocytes.
Full activation of protein kinase B (PKB/Akt) requires phosphorylation on
Thr-308 and Ser-473. It is well established that Thr-308 is phosphorylated by
3-phosphoinositide-dependent kinase-1 (PDK1). ...Ser-473 phosphorylation is
mediated by both mammalian target of rapamycin-rictor complex (mTORC2) and
DNA-dependent protein kinase (DNA-PK) depending on type of stimulus. However,
the physiological role of DNA-PK in the regulation of PKB phosphorylation
remains to be established. To address this, we analyzed basal,
insulin-induced, and DNA damage-induced PKB Ser-473 phosphorylation in DNA-PK
catalytic subunit-null DNA-PKcs
-/-
mice. Our results revealed that
DNA-PK is required for DNA damage-induced phosphorylation but dispensable for
insulin- and growth factor-induced PKB Ser-473 phosphorylation. Moreover,
DNA-PKcs
-/-
mice showed a tissue-specific increase in basal PKB
phosphorylation. In particular, persistent PKB hyperactivity in the thymus
apparently contributed to spontaneous lymphomagenesis in
DNA-PKcs
-/-
mice. Significantly, these tumors could be prevented by
deletion of PKBα. These findings reveal stimulus-specific regulation of
PKB activation by specific upstream kinases and provide genetic evidence of
PKB deregulation in DNA-PKcs
-/-
mice.
The thymus constitutes the primary lymphoid organ for the majority of T cells. The phosphatidyl-inositol 3 kinase (PI3K) signaling pathway is involved in lymphoid development. Defects in single ...components of this pathway prevent thymocytes from progressing beyond early T cell developmental stages. Protein kinase B (PKB) is the main effector of the PI3K pathway. To determine whether PKB mediates PI3K signaling in the thymus, we characterized PKB knockout thymi. Our results reveal a significant thymic hypocellularity in PKBalpha.sup.-/- neonates and an accumulation of early thymocyte subsets in PKBalpha.sup.-/- adult mice. Using thymic grafting and fetal liver cell transfer experiments, the latter finding was specifically attributed to the lack of PKBalpha within the lymphoid component of the thymus. Microarray analyses show that the absence of PKBalpha in early thymocyte subsets modifies the expression of genes known to be involved in pre-TCR signaling, in T cell activation, and in the transduction of interferon-mediated signals. This report highlights the specific requirements of PKBalpha for thymic development and opens up new prospects as to the mechanism downstream of PKBalpha in early thymocytes.
The Carboxy-Terminal Modulator Protein Parcellier, Arnaud; Tintignac, Lionel A; Zhuravleva, Elena ...
PloS one,
05/2009, Letnik:
4, Številka:
5
Journal Article
Recenzirano
Mitochondria are central to the metabolism of cells and participate in many regulatory and signaling events. They are looked upon as dynamic tubular networks. We showed recently that the ...Carboxy-Terminal Modulator Protein (CTMP) is a mitochondrial protein that may be released into the cytosol under apoptotic conditions. Here we report an unexpected function of CTMP in mitochondrial homeostasis. In this study, both full length CTMP, and a CTMP mutant refractory to N-terminal cleavage and leading to an immature protein promote clustering of spherical mitochondria, suggesting a role for CTMP in the fission process. Indeed, cellular depletion of CTMP led to accumulation of swollen and interconnected mitochondria, without affecting the mitochondrial fusion process. Importantly, in vivo results support the relevance of these findings, as mitochondria from livers of adult CTMP knockout mice had a similar phenotype to cells depleted of CTMP. Together, these results lead us to propose that CTMP has a major function in mitochondrial dynamics and could be involved in the regulation of mitochondrial functions.
Protein kinase B (PKB/Akt) is a well-established regulator of several essential cellular processes. Here, we report a route by which activated PKB promotes survival in response to DNA insults in ...vivo. PKB activation following DNA damage requires 3-phosphoinositide-dependent kinase 1 (PDK1) and DNA-dependent protein kinase (DNA-PK). Active PKB localizes in the nucleus of gamma-irradiated cells adjacent to DNA double-strand breaks, where it colocalizes and interacts with DNA-PK. Levels of active PKB inversely correlate with DNA damage-induced apoptosis. A significant portion of p53- and DNA damage-regulated genes are misregulated in cells lacking PKBalpha. PKBalpha knockout mice show impaired DNA damage-dependent induction of p21 and increased tissue apoptosis after single-dose whole-body irradiation. Our findings place PKB downstream of DNA-PK in the DNA damage response signaling cascade, where it provides a prosurvival signal, in particular by affecting transcriptional p21 regulation. Furthermore, this function is apparently restricted to the PKBalpha isoform.
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