Regulation of alkaline phosphatase gene expression in human hepatoma cells by bile acids.

Bile acid-dependent secretion and the translationally regulated synthesis of alkaline phosphatase (ALP) in rat liver cell culture and by bile duct ligation has already been demonstrated. With the advent of ALP cDNA cloned sequences, the mechanism of the effect of bile acids on ALP activity and the expression of the ALP gene in different hepatoma cells was investigated. The HuH7 and HepG2 cells were treated with taurine-conjugated cholic acid (CA), chenodeoxycholic acid (CDCA) and ursodeoxycholic acid (UDCA) at 0-1 mmol/L and the numbers of viable cells were counted at 24, 48 and 72 h after treatment. A [3H]-thymidine incorporation study was performed with different bile acids at a concentration of 1 mmol/L for 48 h. The cellular activity of ALP in HuH7 and HepG2 cells by bile acids was measured by a substrate-specific enzymatic assay. To elucidate the effects of bile acids on ALP gene expression, a northern blotting experiment using hybridization with mouse placental ALP cDNA was performed. Cellular ALP activity was time- and dose-dependently increased in both HuH7 and HepG2 cells treated by CA and CDCA; however, no change in ALP activity was observed following treatment with UDCA compared with controls. Induction of ALP activity was dominant in HepG2 cells and independent of cell growth and proliferation. The addition of UDCA synergistically reduced the increased activity of ALP produced by CA and CDCA in both HuH7 and HepG2 cells. By northern blot analysis, the level of ALP mRNA was elevated by CA and CDCA; however, levels of ALP mRNA were suppressed by UDCA. In conclusion, CA and CDCA cause up-regulation of ALP mRNA and UDCA leads to down-regulation of ALP mRNA by its interaction with either CA or CDCA. We assume that increased ALP synthesis in hepatoma cells after bile acid treatment results from an enhanced rate of transcription rather than translation of mRNA.