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.
Department of Biochemistry, School of Medicine, East Carolina University, Greenville, North Carolina 27858, USA.
S I
SI
Itani
Q
Q
Zhou
W J
WJ
Pories
K G
KG
MacDonald
G L
GL
Dohm
D000328:Adult; D000469:Alkaline Phosphatase / Q000378:metabolism; D001786:Blood Glucose / Q000378:metabolism; D015992:Body Mass Index; D002462:Cell Membrane / Q000201:enzymology; D005230:Fatty Acids, Nonesterified / Q000097:blood; D005260:Female; D006801:Humans; D007328:Insulin / Q000097:blood; D007333:Insulin Resistance / Q000502:physiology*; D007527:Isoenzymes / Q000378:metabolism; D008297:Male; D018482:Muscle, Skeletal / Q000201:enzymology / Q000503:physiopathology*; D009765:Obesity / Q000503:physiopathology*; D011493:Protein Kinase C / Q000378:metabolism*; D011972:Receptor, Insulin / Q000378:metabolism*; D012016:Reference Values; D013851:Thinness
D001786:Blood Glucose; D005230:Fatty Acids, Nonesterified; D007328:Insulin; D007527:Isoenzymes; D011972:Receptor, Insulin; D011493:Protein Kinase C; D000469:Alkaline Phosphatase