The mechanisms through which Ca2+ mobilization in rat hepatocytes results in the loss of total activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase [Zammit & Caldwell (1990) Biochem. J. 269, 373-379] were investigated. The loss of total activity was shown to be paralleled by an equal loss of immunoreactive HMG-CoA reductase protein after exposure of hepatocytes to optimal concentrations of vasopressin plus glucagon for 40 min. This loss of enzyme protein was due to an inhibition of enzyme synthesis; the rate of degradation was unaffected. Other Ca(2+)-mobilizing conditions (phenylephrine, glucagon, vasopressin added singly and A23187) also resulted in graded inhibition of synthesis of HMG-CoA reductase. These effects were accentuated by omission of Ca2+ from the cell incubation medium, suggesting that it is the depletion of an intracellular InsP3-sensitive pool of Ca2+ to which synthesis of HMG-CoA reductase is sensitive. In agreement with this we found that t-butylhydroxybenzoquinone, which inhibits the activity of the Ca(2+)-ATPase of the endoplasmic-reticular membrane, mimicked the action of Ca(2+)-mobilizing hormones. However, taurolithocholate, which transiently mobilizes Ca2+ from the same pool, was ineffective. All these effects on HMG-CoA reductase were accompanied by parallel inhibition of 35S incorporation from [35S]methionine into total protein, suggesting that inhibition of reductase synthesis formed part of a generalized response of the hepatocyte to Ca2+ mobilization. Inhibition of the rate of synthesis of HMG-CoA reductase was, however, more responsive to Ca2+ mobilization in the absence of added Ca2+ from the extracellular medium. The concentrations of vasopressin required to elicit the inhibition of synthesis of HMG-CoA reductase were of the same order as those that elicited activation of glycogen phosphorylase in hepatocytes.

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