Store-operated Ca 2+ channels and Stromal Interaction Molecule 1 (STIM1) are targets for the actions of bile acids on liver cells

Abstract

Cholestasis is a significant contributor to liver pathology and can lead to primary sclerosis and liver failure. Cholestatic bile acids induce apoptosis and necrosis in hepatocytes but these effects can be partially alleviated by the pharmacological application of choleretic bile acids. These actions of bile acids on hepatocytes require changes in the release of Ca 2+ from intracellular stores and in Ca 2+ entry. However, the nature of the Ca 2+ entry pathway affected is not known. We show here using whole cell patch clamp experiments with H4-IIE liver cells that taurodeoxycholic acid (TDCA) and other choleretic bile acids reversibly activate an inwardly-rectifying current with characteristics similar to those of store-operated Ca 2+ channels (SOCs), while lithocholic acid (LCA) and other cholestatic bile acids inhibit SOCs. The activation of Ca 2+ entry was observed upon direct addition of the bile acid to the incubation medium, whereas the inhibition of SOCs required a 12 h pre-incubation. In cells loaded with fura-2, choleretic bile acids activated a Gd 3+-inhibitable Ca 2+ entry, while cholestatic bile acids inhibited the release of Ca 2+ from intracellular stores and Ca 2+ entry induced by 2,5-di-( tert-butyl)-1,4-benzohydro-quinone (DBHQ). TDCA and LCA each caused a reversible redistribution of stromal interaction molecule 1 (STIM1, the endoplasmic reticulum Ca 2+ sensor required for the activation of Ca 2+ release-activated Ca 2+ channels and some other SOCs) to puncta, similar to that induced by thapsigargin. Knockdown of Stim1 using siRNA caused substantial inhibition of Ca 2+-entry activated by choleretic bile acids. It is concluded that choleretic and cholestatic bile acids activate and inhibit, respectively, the previously well-characterised Ca 2+-selective hepatocyte SOCs through mechanisms which involve the bile acid-induced redistribution of STIM1.