BILE-ACID SYNTHESIS IN HAMSTER HEPATOCYTES IN PRIMARY CULTURE - SOURCES OF CHOLESTEROL AND COMPARISON WITH OTHER SPECIES

The synthesis of bile acids by primary hamster hepatocytes in culture has been studied. Measurable rates of bile acid synthesis were obtaine d from cells prepared from livers of animals fed 2% w/w cholestyramine to induce the synthesis of bile acids through the rate-limiting enzym e cholesterol 7 alpha-hydroxylase. The effects of various sources of s ubstrate for bile acid synthesis in these cultured cells were examined over a period of 24 h and the results compared with published or para llel studies in primary rat hepatocytes or in the human hepatoma cell line, HepG2. In all the cells, bile acid synthesis was stimulated by t he addition of 7 alpha-hydroxycholesterol, indicating the rate-limitin g role of the cholesterol 7 alpha-hydroxylase. Bile acid synthesis in the hamster hepatocytes was also stimulated by a variety of sources of cholesterol as substrate, mevalonic acid (increasing the production o f newly-synthesised cholesterol in the cell), and as an exogenous sour ce, hamster LDL. Similarly, if cholesterol was diverted from intracell ular esterification using the ACAT inhibitor Dup128, a further increas e in bile acid synthesis could be demonstrated. These results show tha t hepatocytes obtained from cholestyramine-treated hamsters are defici ent in substrate cholesterol for bile acid synthesis. A similar conclu sion can be drawn from the published work with rat hepatocytes and is further supported by experiments on the regulation of cholesterol 7 al pha-hydroxylase activity at the mRNA and the protein level, although s ome in vivo studies in animals and studies in man have led authors to suggest that cholesterol 7 alpha-hydroxylase is saturated with substra te.