Inhibition of insulin signaling and glycogen synthesis by phorbol dibutyrate in rat skeletal muscle

Numerous studies have shown a correlation between changes in protein kinase C (PKC) distribution and/or activity and insulin resistance in skeletal muscle. To investigate which PKC isoforms might be involved and how they affect insulin action and signaling, studies were carried out in rat soleus muscle incubated with phorbol esters. Muscles preincubated for 1 h with 1 microM phorbol 12,13-dibutyrate (PDBu) showed an impaired ability of insulin to stimulate glucose incorporation into glycogen and a translocation of PKC-alpha, -betaI, -theta, and -epsilon, and probably -betaII, from the cytosol to membranes. Preincubation with 1 microM PDBu decreased activation of the insulin receptor tyrosine kinase by insulin and to an even greater extent the phosphorylation of Akt/protein kinase B and glycogen synthase kinase-3. However, it failed to diminish the activation of phosphatidylinositol 3'-kinase by insulin. Despite these changes in signaling, the stimulation by insulin of glucose transport (2-deoxyglucose uptake) and glucose incorporation into lipid and oxidation to CO2 was unaffected. The results indicate that preincubation of skeletal muscle with phorbol ester leads to a translocation of multiple conventional and novel PKC isoforms and to an impairment of several, but not all, events in the insulin-signaling cascade. They also demonstrate that these changes are associated with an inhibition of insulin-stimulated glycogen synthesis but that, at the concentration of PDBu used here, glucose transport, its incorporation into lipid, and its oxidation to CO2 are unaffected.

Increased protein kinase C theta in skeletal muscle of diabetic patients

In this study we have investigated whether protein kinase C (PKC) protein and activity are increased in skeletal muscle of human diabetic patients. The protein content of different PKC isoforms (beta, Theta, epsilon, delta, mu, and zeta) in the particulate fraction was measured, using Western analysis, in human rectus abdominus skeletal muscle from obese (hyperinsulinemic, normoglycemic) and obese diabetic (hyperinsulinemic, hyperglycemic) subjects. PKC Theta protein content was significantly higher in the particulate fraction of muscle from diabetic patients compared with the nondiabetic controls. PKC Theta was immunoprecipitated and its activity was measured in muscle from diabetic and nondiabetic controls. There was a significant increase in PKC Theta activity in muscle from diabetic patients compared with muscle from nondiabetic controls. Therefore, both PKC Theta protein content and activity were significantly increased in the particulate fraction in muscle from diabetic patients, suggesting the involvement of this isoform in diabetes. Most of the PKC Theta protein was found in the cytosol. There was no change in cytosolic PKC Theta protein content in muscle from diabetic patients compared with muscle from nondiabetic controls. Thus, the increase in particulate-associated PKC Theta was likely due to translocation and activation rather than an increase in protein mass.

Involvement of protein kinase C in human skeletal muscle insulin resistance and obesity

This study was conducted to investigate the possible involvement of protein kinase C (PKC) and serine/threonine phosphorylation of the insulin receptor in insulin resistance and/or obesity. Insulin receptor tyrosine kinase activity was depressed in muscle from obese insulin-resistant patients compared with lean insulin-responsive control subjects. Alkaline phosphatase treatment resulted in a significant 48% increase in in vitro insulin-stimulated receptor tyrosine kinase activity in obese but not lean muscle. To investigate the involvement of PKC in skeletal muscle insulin resistance and/or obesity, membrane-associated PKC activity and the protein content of various PKC isoforms were measured in human skeletal muscle from lean, insulin-responsive, and obese insulin-resistant patients. Membrane-associated PKC activity was not changed; however, PKC-beta protein content, assayed by Western blot analysis, was significantly higher, whereas PKC-theta, -eta, and -mu were significantly lower in muscle from obese patients compared with muscle from lean control subjects. Incubation of muscle strips with insulin significantly increased membrane-associated PKC activity in muscle from obese but not lean subjects. PKC-delta, -beta, and -theta were translocated from the cytosol to the membrane fraction in response to insulin treatment. These results suggest that in skeletal muscle from insulin-resistant obese patients, insulin receptor tyrosine kinase activity was reduced because of hyperphosphorylation on serine/threonine residues. Membrane-associated PKC-beta protein was elevated under basal conditions, and membrane-associated total PKC activity was increased under insulin-stimulated conditions in muscle from obese insulin-resistant patients. Thus, we postulate that the decreased tyrosine kinase activity of the insulin receptor may be caused by serine/threonine phosphorylation by PKC.

Protein kinase C modulates insulin action in human skeletal muscle

There is good evidence from cell lines and rodents that elevated protein kinase C (PKC) overexpression/activity causes insulin resistance. Therefore, the present study determined the effects of PKC activation/inhibition on insulin-mediated glucose transport in incubated human skeletal muscle and primary adipocytes to discern a potential role for PKC in insulin action. Rectus abdominus muscle strips or adipocytes from obese, insulin-resistant, and insulin-sensitive patients were incubated in vitro under basal and insulin (100 nM)-stimulated conditions in the presence of GF 109203X (GF), a PKC inhibitor, or 12-deoxyphorbol 13-phenylacetate 20-acetate (dPPA), a PKC activator. PKC inhibition had no effect on basal glucose transport. GF increased (P < 0.05) insulin-stimulated 2-deoxyglucose (2-DOG) transport approximately twofold above basal. GF plus insulin also increased (P < 0.05) insulin receptor tyrosine phosphorylation 48% and phosphatidylinositol 3-kinase (PI 3-kinase) activity approximately 50% (P < 0.05) vs. insulin treatment alone. Similar results for GF on glucose uptake were observed in human primary adipocytes. Further support for the hypothesis that elevated PKC activity is related to insulin resistance comes from the finding that PKC activation by dPPA was associated with a 40% decrease (P < 0.05) in insulin-stimulated 2-DOG transport. Incubation of insulin-sensitive muscles with GF also resulted in enhanced insulin action ( approximately 3-fold above basal). These data demonstrate that certain PKC inhibitors augment insulin-mediated glucose uptake and suggest that PKC may modulate insulin action in human skeletal muscle.