The regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase in the isolated perfused rat liver.

Abstract

A B S T R A C T The effect of perfusion of an isolated rat liver on hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase was studied. In liver removed during the basal period of the diurnal cycle of enzyme activity, a 227±41% increase in enzyme activity occurred after 3 h of a l)lasna-free perfusion. This could be prevented by the addition of cycloheximide or pure cholesterol (dispersed with lecithin) to the perfusate. In contrast, the continuous addition of taurocholate or taurochenodeoxycholate, alone or in combination, at a variety of rates did not prevent the increase in enzyme activity. The added bile salts were efficiently extracted from the perfusate and excreted in the bile. The addition of these bile salts to a cholesterol-enriched perfusate did not alter the effect obtained with cholesterol alone.If the perfusate contained whole serum, the increase induced by perfusion in the basal period was smaller (88±27%) than with plasma-free perfusate. Again, the major bile salts of the rat failed to prevent the increase in enzyme activity induced by liver perfusion.If livers were removed and perfused at the height of the diurnal cycle of enzyme activity, the enzyme activity remained high (2+10% increase) rather than decreasing, as occurs in vivo. If cholesterol was added to these perfusions, a 52-+-4% decrease was induced. Bile salt addition induced no decrease. From these results it is concluded that the major bile salts are not direct regulators of hepatic cholesterol synthesis, but pure cholesterol, in the absence of bile salt or lipoprotein, is able to initiate the mechanism that represses hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase.

Role of lipoprotein-X in the pathogenesis of cholestatic hypercholesterolemia. Uptake of lipoprotein-X and its effect on 3-hydroxy-3-methylglutaryl coenzyme A reductase and chylomicron remnant removal in human fibroblasts, lymphocytes, and in the rat.

“…6, a twofold increase in the activity of the enzyme was noted at the end of the perfusion period in livers, which had been perfused in the absence of lipoproteins in the medium. This may be due to the loss of cholesterol by the liver during perfusion (38). Addition of LDL to the medium did not suppress this increase in enzyme activity.…”

Regulation of rat biliary cholesterol secretion by agents that alter intrahepatic cholesterol metabolism. Evidence for a distinct biliary precursor pool.

“…However, a thorough series of studies by Turley and Dietschy (6) in the rat suggested that the rate of hepatic cholesterol synthesis per se does not determine the rate of biliary cholesterol secretion. Moreover, it has been shown that chenodeoxycholate's mechanism of action is not through direct inhibition of HMG CoA reductase (15). Lastly, under normal circumstances, only 10-20% ofbiliary cholesterol is derived from new synthesis (16,17).…”

Inhibition of Cholesterol Synthesis in Rat Liver by Physiological Doses of Taurocholate

“…First, experiments performed in vitro [7,8] and in thc perfused liver [24,25] gave no evidence of a rapid inhibition of cholesterogenic enzymes by bile salts. Moreover, wc observed a coincidence of a greater sensitivity of inhibition by bile salts apparent at 22 h with the diurnal phase of most active synthesis of the 3-hydroxy-3-methylglutarylCoA reductase protein [26].…”

“…Although our results do not allow final conclusions as to the mechanisms mediating the inhibition it is tempting to speculate that it is caused by inhibition of enzyme synthesis rather than by inhibition ofpreexist-ing enzyme molecules. First, experiments performed in vitro [7,8] and in thc perfused liver [24,25] gave no evidence of a rapid inhibition of cholesterogenic enzymes by bile salts. Moreover, wc observed a coincidence of a greater sensitivity of inhibition by bile salts apparent at 22 h with the diurnal phase of most active synthesis of the 3-hydroxy-3-methylglutaryl-CoA reductase protein [26].…”