Focal adhesion kinase (FAK) regulates insulin-stimulated glycogen synthesis in hepatocytes.
Experimental data support a role for FAK, an important component of the integrin signaling pathway, in insulin action. To test the hypothesis that FAK plays a regulatory role in hepatic insulin action, we overexpressed wild type (WT), a kinase inactive (KR), or a COOH-terminal focal adhesion targeting (FAT) sequence-truncated mutant of FAK in HepG2 hepatoma cells. In control untransfected (NON) and vector (CMV2)- and WT-transfected cells, insulin stimulated an expected 54 +/- 13, 37 +/- 4, and 47 +/- 12 increase in [U-(14)C]glucose incorporation into glycogen, respectively. This was entirely abolished in the presence of either KR (-1 +/- 7%) or FAT mutants (0 +/- 8%, n = 5, p < 0.05 for KR or FAT versus other groups), and this was associated with a significant attenuation of incremental insulin-stimulated glycogen synthase (GS) activity. Insulin-stimulated serine phosphorylation of Akt/protein kinase B was significantly impaired in mutant-transfected cells. Moreover, the ability of insulin to inactivate GS kinase-3beta (GSK-3beta), the regulatory enzyme immediately upstream of GS, by serine phosphorylation (308 +/- 16, 321 +/- 41, and 458 +/- 34 optical densitometric units (odu) in NON, CMV2, and WT, respectively, p < 0.02 for WT versus CMV2) was attenuated in the presence of either FAT (205 +/- 14, p < 0.01) or KR (189 +/- 4, p < 0.005) mutants. FAK co-immunoprecipitated with GSK-3beta, but only in cells overexpressing the KR (374 +/- 254 odu) and FAT (555 +/- 308) mutants was this association stimulated by insulin compared with NON (-209 +/- 92), CMV2 (-47 +/- 70), and WT (-39 +/- 31 odu). This suggests that FAK and GSK-3beta form both a constitutive association and a transient complex upon insulin stimulation, the dissociation of which requires normal function and localization of FAK. We conclude that FAK regulates the activity of Akt/protein kinase B and GSK-3beta and the association of GSK-3beta with FAK to influence insulin-stimulated glycogen synthesis in hepatocytes. Insulin action may be subject to regulation by the integrin signaling pathway, ensuring that these growth and differentiation-promoting pathways act in a coordinated and/or complementary manner.
Huang
Danshan
D
UCLA Gonda (Goldschmied) Diabetes Center and the Research Service, West Los Angeles Veterans Administration Medical Center, Los Angeles, California 90095, USA.
Cheung
Anthony T
AT
Parsons
J Thomas
JT
Bryer-Ash
Michael
M
D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; D038362:Glycogen Synthase Kinase 3; D006801:Humans; D007328:Insulin / Q000502:physiology*; D055504:Insulin Receptor Substrate Proteins; D008099:Liver / Q000502:physiology*; D008112:Liver Glycogen / Q000096:biosynthesis*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases; D011505:Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D014407:Tumor Cells, Cultured
D017871:Calcium-Calmodulin-Dependent Protein Kinases; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; D038362:Glycogen Synthase Kinase 3; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D008112:Liver Glycogen; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins; D017346:Protein Serine-Threonine Kinases; D011505:Protein-Tyrosine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; C526292:IRS1 protein, human; C496050:PTK2 protein, human; C494918:AKT1 protein, human