Paracrine modulation of cholangiocyte serotonin synthesis orchestrates biliary remodeling in adults

Abstract

Paracrine signaling between cholangiocytes and stromal cells regulates biliary remodeling. Cholangiocytes have neuroepithelial characteristics and serotonin receptor agonists inhibit their growth, but whether they are capable of serotonin biosynthesis is unknown. We hypothesized that cholangiocytes synthesize serotonin and that cross talk between liver myofibroblasts (MF) and cholangiocytes regulates this process to influence biliary remodeling. Transwell cultures of cholangiocytes ± MF, and tryptophan hydroxylase-2 knockin (TPH2KI) mice with an inactivating mutation of the neuronal tryptophan hydroxylase (TPH) isoform, TPH2, were evaluated. Results in the cell culture models confirm that cholangiocytes have serotonin receptors and demonstrate for the first time that these cells express TPH2 and produce serotonin, which autoinhibits their growth but stimulates MF production of TGF-β1. Increased TGF-β1, in turn, counteracts autocrine inhibition of cholangiocyte growth by repressing cholangiocyte TPH2 expression. Studies of TPH2KI mice confirm that TPH2-mediated production of serotonin plays an important role in remodeling damaged bile ducts because mice with decreased TPH2 function have reduced biliary serotonin levels and exhibit excessive cholangiocyte proliferation, accumulation of aberrant ductules and liver progenitors, and increased liver fibrosis after bile duct ligation. This new evidence that cholangiocytes express the so-called neuronal isoform of TPH, synthesize serotonin de novo, and deploy serotonin as an autocrine/paracrine signal to regulate regeneration of the biliary tree complements earlier work that revealed that passive release of serotonin from platelets stimulates hepatocyte proliferation. Given the prevalent use of serotonin-modulating drugs, these findings have potentially important implications for recovery from various types of liver damage.

Smoothened is a master regulator of adult liver repair

“…Immunohistochemical staining to detect GLI2, SMO, αSMA, elastin, Krt19, SOX9, AFP, desmin, Ki67, and cyclin D1 was performed using the DAKO Envision System (DAKO Corporation) according to the manufacturer's protocol. Immunostaining were performed as described previously (69). Briefly, formalin-fixed paraffin-embedded liver tissues were cut into 5-μm sections and placed on glass slides.…”

Cellular and molecular mechanisms in liver fibrogenesis

“…Available data suggest in any case that 5-HT can sustain hepatocyte proliferation following PHX, likely by acting through a restrict number of receptors (5-HT1A, 2A and 2B), as also unequivocally indicated by the significant impairment of regeneration observed in Tph−/− mice [147,148]. Recently, it has proposed that cholangiocytes may represent an additional source of serotonin as resident liver cells, with serotonin that once released can in turn represses cholangiocyte proliferation through a negative feedback mechanism [149]. Interestingly, these Authors provided evidence suggesting that under conditions of biliary injury a peculiar cross-talk between cholangiocytes and MFs may occur, with 5-HT triggering TGFβ1 production by MFs (likely portal MFs) which, in turn, can act back on cholangiocytes to repress expression of tryptophan-hydroxylase 2 (TPH2, the enzyme generating serotonin) and enable their proliferation.…”

Regulators of Cholangiocyte Proliferation

“…This suggests that a serotonin-mediated autocrine loop is capable of limiting biliary growth and subsequent ductular reaction during cholestasis. A recent study using tryptophan hydroxylase-2 knock-in (TPH2KI) mice demonstrated that serotonin is vital for remodeling injured bile ducts, and mice with reduced biliary serotonin levels exhibit excessive cholangiocyte proliferation, duct accumulation, and liver fibrosis following BDL 93 . The same study also demonstrated that serotonin stimulates myofibroblast production of TGF-β1, which decreases tryptophan hydroxylase-2 (TPH2) expression and subsequently decreases serotonin levels and its effects on cholangiocyte proliferation 93 .…”