CHARACTERISTICS AND REGULATION OF BILE-SALT SYNTHESIS AND SECRETION BY HUMAN HEPATOMA HEPG2 CELLS

Bile salt uptake, synthesis and secretion by the human hepatoma-derive d cell line HepG2 were studied. The cells transported and secreted bil e salts largely by means of passive mechanisms. The cells synthesized and secreted the normal human primary bile salts. The ratio of cholate to chenodeoxycholate was 1.5:1. The degree of conjugation, about 35%, was lower than normal, and the glycine-to-taurine ratio was abnormal (4.5:1). This was not due to amino acid deficiency in the medium. Cont rary to the report of others, little 3 alpha,7 alpha,12 alpha-trihydro xy-5 beta-cholestan-26-oic acid was secreted. This was confirmed by ga s chromatography-mass spectrometry. The total rate of synthesis was ab out 33% that of normal Liver. The specific activity of bile salts synt hesized from [H-3]mevalonate was about 20 times higher than that of th e cellular cholesterol derived from the same precursor. The regulation of bile salt synthesis by two compounds that could alter the precurso r pool of cholesterol was studied. After a 24-hr incubation in serum-f ree medium, the compound 25(OH)cholesterol inhibited the rate of bile salt synthesis compared with control values, possibly by depleting the intracellular free cholesterol pool. Surprisingly, however, progester one, which inhibits cholesterol esterification and should have expande d this pool, also inhibited bile salt synthesis under those conditions . The effect of these compounds on the level of mRNA for cholesterol 7 alpha-hydroxylase was also determined by Northern-blot analysis. The cholesterol 7 alpha-hydrosylase mRNA was 3.7 kb, similar to that in th e rat. The incubation of cells in 25(OH)cholesterol or progesterone, a s above, resulted in a decreased level of mRNA. The reduction was prop ortional to the reduction in bile salt synthesis, suggesting that thes e compounds act at a pretranslational level. Taken together, these res ults suggest that our particular subclone of HepG2 cells will be usefu l for studies of the regulation of bile salt synthesis, but not of tra nsport, by human liver-derived tissue.