This study was conducted to investigate the possible involvement of protein
kinase C (PKC) and serine/threonine phosphorylation of the insulin recepto
r in insulin resistance and/or obesity, Insulin receptor tyrosine kinase ac
tivity was depressed in muscle from obese insulin-resistant patients compar
ed with lean insulin-responsive control subjects. Alkaline phosphatase trea
tment resulted in a significant 48% increase in in vitro insulin-stimulated
receptor tyrosine kinase activity in obese but not lean muscle. To investi
gate the involvement of PKC in skeletal muscle insulin resistance and/or ob
esity, membrane-associated PKC activity and the protein content of various
PKC isoforms were measured in human skeletal muscle from lean, insulin-resp
onsive, and obese insulin-resistant patients. Membrane-associated PKC activ
ity was not changed; however, PKC-beta protein content, assayed by Western
blot analysis, was significantly higher, whereas PKC-theta, -eta, and -mu w
ere significantly lower in muscle from obese patients compared with muscle
from lean control subjects. Incubation of muscle strips with insulin signif
icantly 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 r
esults suggest that in skeletal muscle from insulin-resistant obese patient
s, insulin receptor tyrosine kinase activity was reduced because of hyperph
osphorylation on serine/threonine residues. Membrane-associated PKC-beta pr
otein was elevated under basal conditions, and membrane-associated total PK
C activity was increased under insulin-stimulated conditions in muscle from
obese insulin-resistant patients. Thus, we postulate that the decreased ty
rosine kinase activity of the insulin receptor may be caused by serine/thre
onine phosphorylation by PKC.