Increased protein kinase C activity is linked to reduced insulin receptor autophosphorylation in liver of starved rats.
Phosphorylation of the insulin receptor beta-subunit on serine/threonine residues by protein kinase C reduces both receptor kinase activity and insulin action in cultured cells. Whether this mechanism regulates insulin action in intact animals was investigated in rats rendered insulin-resistant by 3 days of starvation. Insulin-stimulated autophosphorylation of the partially purified hepatic insulin receptor beta-subunit was decreased by 45% in starved animals compared to fed controls. This autophosphorylation defect was entirely reversed by removal of pre-existing phosphate from the receptor with alkaline phosphatase, suggesting that increased basal phosphorylation on serine/threonine residues may cause the decreased receptor tyrosine kinase activity. Tryptic removal of a C-terminal region of the receptor beta-subunit containing the Ser/Thr phosphorylation sites similarly normalized receptor autophosphorylation. To investigate which kinase(s) may be responsible for such increased Ser/Thr phosphorylation in vivo, protein kinase C and cAMP-dependent protein kinase A in liver were studied. A 2-fold increase in protein kinase C activity was found in both cytosol and membrane extracts from starved rats as compared to controls, while protein kinase A activity was diminished in the cytosol of starved rats. A parallel increase in protein kinase C was demonstrated by immunoblotting with a polyclonal antibody which recognizes several protein kinase C isoforms. These findings suggest that in starved, insulin-resistant animals, an increase in hepatic protein kinase C activity is associated with increased Ser/Thr phosphorylation which in turn decreases autophosphorylation and function of the insulin receptor kinase.
Karasik
A
A
Joslin Diabetes Center, Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts.
Rothenberg
P L
PL
Yamada
K
K
White
M F
MF
Kahn
C R
CR
D000818:Animals; D002462:Cell Membrane / Q000201:enzymology; D003600:Cytosol / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D011494:Protein Kinases / Q000378:metabolism; D051381:Rats; D011972:Receptor, Insulin / Q000302:isolation & purification / Q000378:metabolism*; D012016:Reference Values; D013217:Starvation
D011494:Protein Kinases; D011972:Receptor, Insulin; D011493:Protein Kinase C