Role of reversible phosphorylation in mediating changes in the activity of hepatic 3-hydroxy-3-methylglutaryl-CoA reductase during pregnancy and lactation in the rat

Decreased serum lecithin:cholesterol acyltransferase activity in spontaneous cases of fatty liver in cows

Lecithin:cholesterol acyltransferase (LCAT) activity in serum was evaluated in spontaneous cases of fatty liver in cows. The enzyme activity of 631 +/- 62 U (mean +/- SEM, decrease in nmol of free cholesterol per h per ml of serum) in cows with fatty liver (n = 16) was significantly (p < 0.01) lower than that in cows without fatty liver (979 +/- 22 U; n = 16). In addition to the decrease in LCAT activity, the concentrations of phosphatidylcholine (a fatty aryl donor for esterification of free cholesterol) and of cholesteryl esters (products of the LCAT reaction) were reduced in the high-density lipoprotein fractions from cows with fatty livers. The concentrations in the serum of apolipoprotein A-I, an activator of LCAT, was also reduced in cows with fatty livers. These results suggest that the decreased LCAT activity, which may be attributable to impaired hepatic secretion or to the suppression of the activity in the plasma by reduced concentrations of phosphatidylcholine and apolipoprotein A-I, resulted in the lower concentrations of cholesteryl esters. Because cholesteryl esters are utilized in steroidogenic tissues for the synthesis of steroid hormones such as progesterone and glucocorticoids, and insufficient supply of the cholesterols may be of relevance to the reduced fertility and immune competence observed in cows with fatty livers.

The activity of lecithin:cholesterol acyltransferase in the serum of cows at parturition or with fatty liver

The activity of lecithin:cholesterol acyltransferase (LCAT), which is responsible for esterification of plasma cholesterol, was evaluated in bovine serum. It was associated with the high-density lipoprotein fraction that contains apolipoprotein A-I, an activator of LCAT. In lactating cows, the activity was around 1000 U (decrease in nmol of free cholesterol per h per ml of serum), slightly higher than in 1-month-old calves. LCAT activity decreased around parturition, at which the time the serum concentrations of cholesteryl esters and free cholesterol were concomitantly decreased. A reduced LCAT activity was also found in cows with fatty livers induced by the administration of ethionine. In the cows with fatty livers, the serum concentration of cholesteryl esters was markedly decreased, whereas that of free cholesterol was only slightly decreased, thereby increasing the free- to esterified-cholesterol ratio. These results suggest that the decrease in LCAT activity may be involved in the reduction in fertility associated with fatty liver because esterification of cholesterol by LCAT is essential for its transport from the liver to peripheral tissues, such as the corpus luteum, and because cholesterol serves as the source of progesterone synthesis in the latter organ.

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.

Conditions that result in the mobilization and influx of Ca2+ into rat hepatocytes induce the rapid loss of 3-hydroxy-3-methylglutaryl-CoA reductase activity that is not reversed by phosphatase treatment

We investigated the effects of conditions that induce Ca2+ mobilization from intracellular stores and Ca2+ influx into hepatocytes on the expressed and total (fully dephosphorylated) activities of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase. Vasopressin and phenylephrine when added alone had small or negligible effects on the phosphorylation state of the enzyme, as judged from the expressed/total activity ratio. However, when added in combination with glucagon, they elicited appreciable increases in the phosphorylation of the enzyme. Glucagon on its own had no effect either on phosphorylation state or on total HMG-CoA reductase activity during 40 min of incubation. Under conditions of sustained Ca2+ influx (i.e. vasopressin or phenylephrine plus glucagon), there was a marked loss of total HMG-CoA reductase activity. This effect was more pronounced when vasopressin was used; 50% of the enzyme activity was lost within 40 min. The involvement of Ca2+ in these effects was verified directly by the use of ionophore A23187. Its addition to hepatocytes resulted both in a very pronounced increase in the phosphorylation state of the enzyme and in the loss of 50% of the total activity within 30 min. There was no correlation between the ability of any set of conditions to increase the phosphorylation of the enzyme and the subsequent loss of total HMG-CoA reductase activity. The latter parameter appeared to be directly related, however, to the maintenance of prolonged Ca2+ influx, as indicated by the continued activation of glycogen phosphorylase, measured in the same cells. The lack of a causal relationship between increased phosphorylation and loss of total activity was demonstrated directly by studies in which okadaic acid was used to induce phosphorylation of HMG-CoA reductase in hepatocytes by inhibition of phosphatase 1 and 2A activities. This was not accompanied by any loss of total enzyme activity. Neither did okadaic acid enhance the loss of reductase induced by A23187 when the two agents were added together. It is concluded that altered Ca2+ fluxes in hepatocytes in vivo, under conditions of acute or chronic stress (such as may be associated with trauma or diabetes respectively), may be involved in the regulation of the expression of HMG-CoA reductase activity through alteration of enzyme concentration in the liver.