ALTERATIONS IN THE EXPRESSION AND CELLULAR-LOCALIZATION OF PROTEIN-KINASE-C ISOZYME-EPSILON AND ISOZYME-THETA ARE ASSOCIATED WITH INSULIN-RESISTANCE IN SKELETAL-MUSCLE OF THE HIGH-FAT-FED RAT
C. Schmitzpeiffer et al., ALTERATIONS IN THE EXPRESSION AND CELLULAR-LOCALIZATION OF PROTEIN-KINASE-C ISOZYME-EPSILON AND ISOZYME-THETA ARE ASSOCIATED WITH INSULIN-RESISTANCE IN SKELETAL-MUSCLE OF THE HIGH-FAT-FED RAT, Diabetes, 46(2), 1997, pp. 169-178
We have tested the hypothesis that changes in the levels and cellular
location of protein kinase C (PKC) isozymes might be associated with t
he development of insulin resistance in skeletal muscles from the high
-fat-fed rat. Lipid measurements showed that triglyceride and diacylgl
ycerol, an activator of PKC, were elevated four- and twofold, respecti
vely. PKC activity assays indicated that the proportion of membrane-as
sociated calcium-independent PKC was also increased. As determined by
immunoblotting, total (particulate plus cytosolic) PKC alpha, epsilon,
and zeta levels were not different between control and fat-fed rats.
However, the ratio of particulate to cytosolic PKC epsilon in red musc
les from fat-fed rats was increased nearly sixfold, suggesting chronic
activation. In contrast, the amount of cytosolic PKC theta was downre
gulated to 45% of control, while the ratio of particulate to cytosolic
levels increased, suggesting a combination of chronic activation and
downregulation. Interestingly, while insulin infusion in glucose-clamp
ed rats increased the proportion of PKC theta in the particulate fract
ion of red muscle, this was potentiated by fat-feeding, suggesting tha
t the translocation is a consequence of altered lipid flux rather than
a proximal event in insulin signaling. PKC epsilon and theta measurem
ents from individual rats correlated with triglyceride content of red
gastrocnemius muscle; they did not correlate with plasma glucose, whic
h was not elevated in fat-fed rats, suggesting that they were not simp
ly a consequence of hyperglycemia. Our results suggest that these spec
ific alterations in PKC epsilon and PKC theta might contribute to the
link between increased lipid availability and muscle insulin resistanc
e previously described using high-fat-fed rats.