Bile acid synthesis in primary cultures of rat and human hepatocytes

The regulation of hepatic bile acid formation is incompletely understood. Primary cultures of mammalian hepatocytes offer an opportunity to examine putative regulatory factors in relative isolation. Using rat and human hepatocytes in primary culture, we examined bile acid composition and the expression of the rate-limiting enzyme of formation, cholesterol 7α-hydroxylase. Control rat hepatocytes showed a declining bile acid production over 4 days, from 156 ± 24 ng/mL (67% cholic acid) on day 1 to 55 ± 11 ng/mL (55% cholic acid) on day 4. In addition to cholic acid, chenodeoxycholic acid, α-muricholic acid, and β-muricholic acid were formed. Treatment with triidothyronine (T₃) or dexamethasone alone had no significant effect on bile acid production. A combination of T₃ and dexamethasone significantly increased the total bile acid production on day 4 (224 ± 54 ng/mL) and resulted in a marked change in composition to 23% cholic acid and 77% non-12α-hydroxylated bile acids. Control rat hepatocytes had a cholesterol 7α-hydroxylase activity of 3.3 ± 0.6 pmol/mg protein/min after 4 days in culture. Cells treated with the combination of dexamethasone and T₃ had an activity of 16.4 ± 3.6 pmol/mg protein/min. The cholesterol 7α-hydroxylase messenger RNA (mRNA) levels, determined by solution hybridization after 4 days of culture, showed results similar to those for the activity data; control cells had 5.3 ± 0.9 cpm/μg total nucleic acids (tNAs). T_3- or dexamethasone-treated cells did not differ from control cells, whereas the combination of T₃ and dexamethasone increased the mRNA levels to 20.6 ± 2.8 cpm/μg tNAs. In human hepatocytes, isolated from donor liver, bile acid formation increased from 206 ± 79 ng/mL on day 2 to 1490 ± 594 ng/mL on day 6 and then declined slightly. Cholic acid and chenodeoxycholic acid were formed, constituting about 80% and 20%, respectively. The combined addition of T₃ and dexamethasone had a tendency to decrease rather than increase bile acid formation. Also, mRNA levels of the cholesterol 7α-hydroxylase increased severalfold in the human hepatocytes from day 2 to day 4 and then declined. The addition of T₃or dexamethasone did not effect the mRNA levels in any consistent way. It is noteworthy that the capacity of the cultured human hepatocytes to produce bile acids was higher than that of cultured rat hepatocytes, in spite of the fact that the production of bile acids in rat liver is 3- to 5-fold higher than that in human liver in vivo. It is also evident that while hormonal factors appear to regulate bile acid synthesis in the rat, no evidence for this was found in human hepatocytes. As the composition of bile acids secreted by human hepatocytes in primary culture closely resembles that found in vivo, this represents a useful model for further studies of the synthesis and regulation of bile acids.