Lipoprotein regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase in cultured intestinal mucosa.

In vitro regulation of the key enzyme of cholesterol synthesis, 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34), was studied in 24-h organ culture of rabbit ileum. Preincubation of mucosal homogenate in phosphate buffer increased apparent reductase activity more than 5-fold. The activation was blocked in the presence of 150 mM NaF, suggesting interconversion of latent reductase by dephosphorylation. No significant further activation was achieved by potato phosphatase treatment. During culture total reductase activity was stimulated by 125% in lipoprotein-free medium. Addition of low density lipoprotein to the medium resulted in a dose-dependent suppression of reductase activity between 0.2 and 1.0 mM lipoprotein cholesterol. At these concentrations LDL inhibition reached 51 and 75% of control, respectively. The reductase response to very low density and high density lipoproteins was biphasic, with a 2- to 3-fold stimulation at 0.1-0.3 mM and a suppression at higher levels. No such stimulation was observed with high density lipoproteins obtained from cholesterol fed animals. Similarly, 'hypercholesterolemic' very low density lipoproteins enhanced reductase activity only slightly. It is concluded that lipoproteins are potent regulators of cholesterol synthesis in cultured intestine.