DECREASED EXPRESSION OF PROTEIN-KINASE-C ALPHA,BETA, AND EPSILON IN SOLEUS MUSCLE OF ZUCKER OBESE (FA/FA) RATS

Skeletal muscle is one of the first tissues to become insulin resistant in genetically obese rodents. The activation of protein kinase-C (PKC) in rat skeletal muscle is mediated by insulin stimulation of diacylglycerol (DAG) levels. Defects in the activation of PKC in the heart and liver of obese Zucker rats indicate that an abnormality in either stimulation of DAG or PKC occurs in obese tissues. DAG levels were significantly increased in soleus muscle from 15- to 19-week-old obese (falfa) Zucker rats. PKC activity was diminished in soleus muscle from falfa rats. Decreased levels of PKCalpha and -beta activity were noted after resolution of common PKC isozymes (Ca2+ and phospholipid dependent) by hydroxyapatite chromatography. Immunoreactivity of PKC-alpha, -beta, and -epsilon also indicated that their levels are diminished in fa/fa soleus muscle by 70-90%. To determine at which level down-regulation occurs (i.e. gene expression or protein turnover), mRNA levels were examined by Northern blot analysis of total RNA. PKCalpha and -beta levels were diminished in Zucker obese soleus muscle compared to soleus from Zucker lean control (fa/-) animals, and PKCepsilon mRNA was not detected on the same blots. The transcript size for PKCbeta mRNA in Zucker soleus muscle was unique. Both lean and obese Zucker muscle tissues expressed three transcripts that hybridized with the full-length PKCbeta cDNA probes, with sizes ranging between 2.5-1.7 kilobases. Levels of all PKCbeta transcripts were significantly decreased in obese Zucker tissues. Thus, levels of common PKC isozyme mRNA, protein, and enzyme activity in soleus muscle of the obese Zucker rat are decreased even though levels of the endogenous PKC activator DAG are elevated. The decreased levels of PKC may be related to the etiology of insulin resistance in skeletal muscle.