A relationship between the activities of hepatic lanosterol 14α-demethylase and 3-hydroxy-3-methylglutaryl-CoA reductase

Abstract

At 1-2 h after intragastric administration of ketoconazole, a cytochrome P-450 inhibitor, to rats, there was a 50-60% decrease in the activity of hepatic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase. Inhibition reached a maximum at 6-12 h after the drug was given, but after 24 h enzyme activity was stimulated by 60%. The rates of synthesis of hepatic non-saponifiable lipids in vivo showed a similar time-dependent pattern of change. During the first few hours after drug administration, the hepatic cytochrome P-450-dependent metabolism of lanosterol was suppressed in vivo. However, 24 h after treatment, this activity was stimulated, an effect which was also observed by pre-treatment of the rats with the drug for several days. Suppression of hepatic HMG-CoA reductase and lanosterol 14 alpha-demethylase activities was accompanied by a relative increase in the accumulation of labelled polar sterols in the liver in vivo. In the intestine, ketoconazole also resulted in a rapid decline in the rate of synthesis of non-saponifiable lipids and an inhibition of lanosterol 14 alpha-demethylation in vivo. However, in contrast with the liver, there was no stimulation of non-saponifiable lipid synthesis after 24 h.

The role of cytochrome P450 in the regulation of cholesterol biosynthesis

“…Measurement of HMG-CoA reductase activity in cell cultures was carried out using the method described by Gibbons et al (23). Isolation of the microsomal fraction from rat liver was carried out as described by Marco de la Calle et al (27). Measurement of HMG-CoA reductase activity in rat liver microsomes was carried out using the method of Brown et al (31).…”

“…of 3 H 2 O (0.2 mL, 4.0 mCi), and the animals were killed 1 h later. The livers were removed and a portion was used for determination of tritium incorporation into sterols (27). The remainder was used for preparation of microsomes (see above).…”

“…This paper revisits some of our previous work in this area (23)(24)(25)(26), which suggests that the oxygenated, naturally occurring cholesterol precursors II and III (Scheme 1) may have a regulatory role, either as inhibitors of SREBP maturation or as activating ligands for LXRα. This paper will also reassess the results of some of our original studies on the regulation of cholesterol biosynthesis (27,28) in terms of factors that affect the steady-state concentration and turnover of natural regulatory oxysterols.…”

“…Inhibition of cytochrome P450 in vivo and in vitro suppresses hepatic HMG-CoA reductase activity and sterologenic carbon flux. In view of the above inhibitory effects of CO, suggestive of the participation of cytochrome P450 in the oxidative metabolism of oxysterol regulators of HMG-CoA reductase, we determined the effects of administering the potent cytochrome P450 inhibitor ketoconazole on hepatic HMG-CoA reductase activity and carbon flux through the sterol pathway in intact animals (27) and in hepatocyte culture (28). In the experiments with intact animals, a solution of ketoconazole was administered intragastrically to groups of animals, and the animals were killed at various times thereafter.…”

“…At each time point, 1 h before killing, 3 a portion was used for preparation of microsomes for the assay of the expressed (i.e., dephosphorylated) and total (phosphorylated plus dephosphorylated) forms of HMG-CoA reductase (31). The remainder was used for isolation of the 3 H-labeled nonsaponifiable lipid and cholesterol-containing fractions (27). The 3 H content of these fractions was determined, and the values were used as a measure of carbon flux through the sterol pathway (33).…”