Selective Activation of the Serotonin Receptor 2B Isoform Increases Cholangiocyte Growth and Induces the Expression of the Fibroblast Growth Factor 1 (FGF1)

Abstract

Serotonin (5HT) exerts its effects through 16 different receptor isoforms. The activation of 5-HT receptors (5HTR) 1A and 1B inhibits biliary hyperplasia in bile duct ligated (BDL) rats, however, the effects of other 5HTR subtypes on biliary growth are unknown. Cholangiocytes secrete several angiogenic factors including VEGF that regulates biliary mass in an autocrine fashion. FGF1 modulates the function of hepatocytes, although the effects on cholangiocyte growth are unknown. We aim to: (i) assess the effects of activation of other 5HTR subtypes on biliary growth; and (ii) identify a role for FGF1 in 5HTR2B-induced biliary growth. Methods: The expression of 5HTRs and FGF receptors were assessed in liver sections and cholangiocytes. Normal and BDL rats were treated with specific agonists for the 5HTR subtypes, 2A, 2B, 2C, 3, 4, 5, 6 and 7, for 1 wk and intrahepatic bile duct mass (IBDM) was assessed. Normal rat cholangiocyte (NRC) and large mouse cholangiocyte (LMC) lines were treated In Vitro with 5HTR 2A (TCB2), 2B (BW 723C86) and 2C (WAY 161503) agonists in the absence/presence of BAPTA/AM (Ca2+ chelator), H89 (PKA inhibitor) or Go6976 (PKC inhibitor) and proliferation was assessed by MTS assays and PCNA immunoblots. The expression of FGF1 was assessed by qPCR and immunoblots in: (i) cholangiocytes from normal and BDL rats; and (ii) NRC and LMC cells following treatments with 5HTR 2A, 2B and 2C agonists. The effects of FGF1 on biliary growth was studied by: i) measuring IBDM in normal and BDL rats treated In Vivo with FGF1 (2.5 nmoles/hr/Kg BW by osmotic minipumps) for 1 wk; and ii) PCNA immunoblots in NRC and LMC treated In Vitro with FGF1 for 48 hr. Results: Bile ducts (in liver sections), NRC and LMC express all serotonin receptors. Activation of 5HTR 2B increased cholangiocyte growth in normal and BDL rats In Vivo and biliary lines In Vitro. The In Vivo administration of SB 204741 (antagonist for 5HTR 2B) to BDL rats for 1 wk decreased biliary hyperplasia. There were no changes in cholangiocyte proliferation after the In Vivo administration of agonists for 5HTR 2A, 2C, 3, 4, 5, or 6. 5HTR7 agonist inhibited biliary growth and will be evaluated further in future experiments. The effects of 5HTR 2B agonists were blocked by H89 suggesting a cAMPdependent pathway. There was a concomitant increase in the expression of FGF1 in cholangiocytes treated with 5HTR 2B In Vivo and In Vitro. Cholangiocytes express all FGFR isoforms (I to IV) and treatment of cholangiocytes with FGF1 In Vivo and In Vitro increased biliary proliferation. Conclusions: Activation of 5HTR 2B increased cholangiocyte growth via a cAMP-dependent pathway. There was a parallel increase in FGF1 expression, which may in turn stimulate biliary growth in an autocrine manner. The activation/deactivation of 5HTR 2B may be important for the treatment of cholangiopathies.