Regulation of bile acid synthesis in cultured rat hepatocytes: stimulation by apoE-rich high density lipoproteins.

Cultured rat hepatocytes obtained by liver perfusion with collagenase in the presence of soybean trypsin inhibitor were used to examine the role of high density lipoproteins (HDL) in supplying cholesterol to the hepatocyte for bile acid synthesis. Within 6 hr of adding HDL (d 1.07-1.21 g/ml) obtained from rat serum there was a significant stimulation of bile acid synthesis and secretion that reached 2-fold after 24 hr. The stimulation by HDL occurred at normal plasma concentrations (i.e., 500 micrograms/ml) and showed further stimulation in a dose-dependent manner reaching a maximum stimulation of 2- to 2.5-fold. The stimulation of bile acid synthesis was dependent on the cholesteryl ester content of the HDL. Several lines of evidence show that the HDL is taken up by a receptor-mediated process dependent on apoE. These include: 1) at the same concentration (500 micrograms/ml) apoE-poor HDL (not retained by heparin affinity chromatography of HDL isolated from the plasma of rats fasted for 72 hr stimulated bile acid synthesis by 48%, whereas apoE-rich HDL stimulated bile acid synthesis by 110%; 2) reductive methylation totally blocked the stimulation of bile acid synthesis by HDL; 3) HDLC, which contained apoE as its major protein component, also maximally stimulated bile acid synthesis; and 4) human HDL, which contained no detectable apoE, failed to stimulate bile acid synthesis. Additional studies showed that apoE-enriched HDL and HDLC both inhibited cholesterol synthesis (determined by the incorporation of 3H2O) and caused a net accumulation of cholesteryl esters in hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)