Direct demonstration that increased phosphorylation of 3-hydroxy-3-methylglutaryl-CoA reductase does not increase its rate of degradation in isolated rat hepatocytes.

Increased phosphorylation of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase has been suggested to target the protein towards an increased rate of degradation. Our previous observations [Zammit & Caldwell (1990) Biochem. J. 269, 373-379] suggested that, although Ca(2+)-mobilizing hormones and other effectors can alter both the phosphorylation state of the enzyme and its total activity in isolated rat hepatocytes, there appears to be no causal correlation between the two parameters. In the present paper we set out to make direct measurements of the specific rate of degradation of 35S-labelled HMG-CoA reductase in hepatocytes treated with agents that produced very marked and prolonged increases in the degree of phosphorylation of the protein, through different mechanisms. Okadaic acid (which inhibits phosphatases 1 and 2A), fructose (which increases cellular AMP through its metabolism to fructose 1-phosphate) and the Ca2+ ionophore A23187 (which also raises cellular AMP through an unknown mechanism) were all unable to alter the rate of HMG-CoA reductase degradation. We conclude that the basal rate of degradation of HMG-CoA reductase is unaffected by its phosphorylation state and that a transiently increased degree of phosphorylation cannot be the mechanism through which mevalonate increases the rate of degradation of the enzyme in rat hepatocytes and other cell types.