Catabolism of cholesterol to bile salts by rat hepatocytes maintained in monolayers

Abstract

The liver plays a central role in the metabolism of cholesterol being the major site at which lipoproteins are both assembled and degraded and the only organ where cholesterol can be degraded to bile salts. The synthesis of bile salts by the liver provides the major pathway for the removal of cholesterol from the body.

The results in this thesis describe the characterisation of a rat hepatocyte monolayer system, suitable for studying the synthesis of bile salts. The utilization of the cholesterol derived from a high density lipoprotein subfraction (HDL2) for the synthesis of bile salts was also investigated.

Following the isolation of a viable cell preparation, hepatocytes were maintained in monolayers for up to 24h. During this period hepatocytes were shown to maintain their viability and to synthesise and secrete bile salts, as determined by radioimmunoassay of conjugated cholic, chenodeoxycholic and ß-muricholic acids. The rate of synthesis of these bile salts by hepatocytes was increased by feeding rats cholestyramine for at least 5 days prior to the preparation of hepatocyte monolayers.

Incubation of hepatocyte monolayers with rat HDL2 had no effect on the synthesis of the three bile acid conjugates measured when the cells were obtained from rats fed the pellet diet. However, when the experiment was repeated using hepatocytes obtained from rats fed cholestyramine, HDL2 was found to increase the synthesis of the bile salts measured. This is the first report that a defined lipoprotein fraction can increase the synthesis of bile salts.

In an attempt to ascertain the reason for the increase in the synthesis of bile salts, hepatocytes isolated from cholestyramine- vi fed rats were incubated in the presence of HDL2 radiolabelled with either [4- 14Clcholesterol or [4- 14C]cholesteryl oleate. The degradation of the radiolabelled HDL2-cholesterol to bile salts was subsequently determined. The results indicated that the increase in the synthesis of bile salts was due to the utilization of HDL2-cholesteryl ester.

Finally, the effect of HDL2 on the synthesis of cholesterol and the utilization of newly synthesised cholesterol for the synthesis of bile salts in hepatocyte monolayers was determined. The results showed that HDL2 had no effect on either cholesterol synthesis or the utilization of newly synthesised cholesterol for the synthesis of bile salts.