Rapid decrease in the expression of 3-hydroxy-3-methylglutaryl-CoA reductase protein owing to inhibition of its rate of synthesis after Ca2+ mobilization in rat hepatocytes. Inability of taurolithocholate to mimic the effect.
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.
Zammit
V A
VA
Hannah Research Institute, Ayr, Scotland, U.K.
Caldwell
A M
AM
Kolodziej
M P
MP
D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D005934:Glucagon / Q000494:pharmacology; D006873:Hydroquinones / Q000494:pharmacology; D006903:Hydroxymethylglutaryl CoA Reductases / Q000096:biosynthesis / Q000378:metabolism*; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D010656:Phenylephrine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013658:Taurolithocholic Acid / Q000494:pharmacology*; D014667:Vasopressins / Q000008:administration & dosage / Q000494:pharmacology
D006873:Hydroquinones; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors; D014667:Vasopressins; D010656:Phenylephrine; C019359:2,5-di-tert-butylhydroquinone; D000001:Calcimycin; D013658:Taurolithocholic Acid; D005934:Glucagon; D006903:Hydroxymethylglutaryl CoA Reductases; D002118:Calcium