Farnesoid X receptor is essential for the survival of renal medullary collecting duct cells under hypertonic stress

Significance

As a ligand-activated transcription factor, farnesoid X receptor (FXR) is abundantly expressed in the liver and small intestine, where it plays an important role in the maintenance of bile acid, lipid, and glucose homeostasis. Increasing evidence is emerging that FXR may be a critical regulatory factor in renal physiology and pathophysiology. The present study demonstrates an essential role of FXR in promoting cell survival of medullary collecting duct cells under hypertonic stress and identifies FXR-induced TonEBP expression and activation as an underlying mechanism. This work provides an insight into the role of FXR in renal urine concentration.

Abstract

Hypertonicity in renal medulla is critical for the kidney to produce concentrated urine. Renal medullary cells have to survive high medullary osmolarity during antidiuresis. Previous study reported that farnesoid X receptor (FXR), a nuclear receptor transcription factor activated by endogenous bile acids, increases urine concentrating ability by up-regulating aquaporin 2 expression in medullary collecting duct cells (MCDs). However, whether FXR is also involved in the maintenance of cell survival of MCDs under dehydration condition and hypertonic stress remains largely unknown. In the present study, we demonstrate that 24-hours water restriction selectively up-regulated renal medullary expression of FXR with little MCD apoptosis in wild-type mice. In contrast, water deprivation caused a massive apoptosis of MCDs in both global FXR gene-deficient mice and collecting duct-specific FXR knockout mice. In vitro studies showed that hypertonicity significantly increased FXR and tonicity response enhancer binding protein (TonEBP) expression in mIMCD3 cell line and primary cultured MCDs. Activation and overexpression of FXR markedly increased cell viability and decreased cell apoptosis under hyperosmotic conditions. In addition, FXR can increase gene expression and nuclear translocation of TonEBP. We conclude that FXR protects MCDs from hypertonicity-induced cell injury very likely via increasing TonEBP expression and nuclear translocation. This study provides insights into the molecular mechanism by which FXR enhances urine concentration via maintaining cell viability of MCDs under hyperosmotic condition.