Expression and Function of Bile Acid Receptor TGR5 in Gastrointestinal Smooth Muscle

Abstract

Bile acids, the main active constituents of bile, act, in addition to their role in nutrient absorption, as signaling molecules to exert genomic and non-genomic effects via nuclear receptors and the plasma membrane G protein-coupled receptor TGR5. TGR5 is expressed in epithelial cells and enteric neurons of the gastrointestinal tract and may mediate the effects of bile acids on motility and secretion. However, the mechanisms by which bile acids regulate smooth muscle function are not known. AIM To determine the expression of TGR5 in gastrointestinal smooth muscle and identify their signaling pathways. METHODS TGR5 expression was determined by RT-PCR and western blot. G protein activation in response to TGR5 selective ligand, oleanolic acid (OA) was measured as increase in Gα binding to [35S]GTPγS. OA-induced increase in cAMP was measured by ELISA and muscle relaxation by scanning micrometry. RESULTS TGR5 expression was detected by RT-PCR and western blot in rabbit gastric muscle and human intestinal muscle. OA activated Gαs, but not Gαq, Gαi1, Gαi2, or Gαi3. Consistent with activation of Gαs, OA increased cAMP levels. OA did not elicit contraction, but caused relaxation of carbachol-induced muscle contraction. CONCLUSION Smooth muscle cells express membrane receptors (TGR5) for bile acids preferentially coupled to Gs. The receptors mediate stimulation of adenylyl cyclase activity and muscle relaxation.