Developmental coordinate expression of triacylglycerol and small molecular weight apoB synthesis and secretion by rat hepatocytes.

Glycerolipid and apoB synthesis and secretion were examined in hepatocytes obtained from fetal, suckling (day 6), and adult rats in order to examine the developmental regulation of lipoprotein production. The capacity to synthesize [3H]triacylglycerol (from [3H]glycerol) followed the order: adult greater than day 6 greater than fetal. Addition of 1 mM oleic acid to the incubation media stimulated the incorporation of [3H]glycerol into triacylglycerol 6.7- and 3.6-fold by fetal and adult hepatocytes, respectively. After maximal stimulation by 1 mM oleic acid, triacylglycerol secretion by fetal cells was still only 39% of the amount secreted by adult cells that had been treated similarly. Fetal cells stimulated with 1 mM oleic acid synthesized the same amount of triacylglycerol as adult cells that had been treated with 0.1 mM oleic acid. However, the fetal cells secreted only one-third as much triacylglycerol, further demonstrating relatively impaired secretion of triacylglycerol. In order to determine whether low triacylglycerol secretion was associated with differences in apoB metabolism, cells were incubated with [35S]methionine and apoB was quantified after immunoprecipitation. Fetal cells synthesized and secreted nearly equal amounts of large molecular weight and small molecular weight apoB. In contrast, adult cells synthesized and secreted nearly twice as much small molecular weight apoB as large molecular weight apoB. Moreover, although fetal and adult cells secreted large molecular weight apoB at similar rates, adult cells synthesized and secreted small molecular weight apoB at rates that were nearly two times higher than fetal cells. These data suggest that the ability to assemble and secrete VLDL varies in parallel with the developmental expression of small molecular weight apoB. These studies also show the usefulness of the cultured rat hepatocyte model for examining the ontogeny and regulation of lipoprotein assembly/secretion.

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)