Farnesoid X Receptor Activates Transcription of the Phospholipid Pump MDR3

Abstract

The human multidrug resistance gene MDR3 encodes a P-glycoprotein that belongs to the ATP-binding cassette transporter family (ABCB4). MDR3 is a critical trans-locator for phospholipids across canalicular membranes of hepatocytes, evidenced by the fact that human MDR3 deficiencies result in progressive familial intrahepatic cholestasis type III. It has been reported previously that MDR3 expression is modulated by hormones, cellular stress, and xenobiotics. Here we show that the MDR3 gene is trans-activated by the farnesoid X receptor (FXR) via a direct binding of FXR/retinoid X receptor ␣ heterodimers to a highly conserved inverted repeat element (a FXR response element) at the distal promoter (؊1970 to ؊1958). In FXR trans-activation assays, both the endogenous FXR agonist chenodeoxycholate and the synthetic agonist GW4064 activated the MDR3 promoter. Deletion or mutation of this inverted repeat element abolished FXR-mediated MDR3 promoter activation. Consistent with these data, MDR3 mRNA was significantly induced by both chenodeoxycholate and GW4064 in primary human hepatocytes in time-and dose-dependent fashions.In conclusion, we demonstrate that MDR3 expression is directly up-regulated by FXR. These results, together with the previous report that the bile salt export pump is a direct FXR target, suggest that FXR coordinately controls secretion of bile salts and phospholipids. Results of this study further support the notion that FXR is a master regulator of lipid metabolism.ATP-binding cassette (ABC) 1 transporters constitute a large family of proteins, and many have been shown to be involved in lipid transport. MDR3 (ABCB4), a P-glycoprotein, is predominantly expressed in the liver (1) and is the critical trans-locator for phospholipids across canalicular membranes of hepatocytes (2). The MDR3 function is essential for the liver as evidenced by the fact that MDR3 deficiencies in humans result in progressive familial intrahepatic cholestasis type III (3,4). A number of factors, such as hormones, cellular stress, and xenobiotics have been shown to modulate MDR3 expression (5-7). However, the underlying molecular mechanisms for MDR3 gene regulation are unclear. In this study, we demonstrate that the bile acid receptor FXR directly regulates expression of MDR3.FXR is a nuclear receptor for bile acids and regulates expression of a number of genes in which products are critically important for bile acid and cholesterol homeostasis (8 -11). FXR functions as a heterodimer with the 9-cis-retinoic acid receptor (RXR␣) (12, 13), and the FXR/RXR␣ heterodimer activates gene transcription via binding to a specific DNA sequence comprised of two inverted hexamer repeats separated by one nucleotide (IR-1) in the target promoter. To date, there is only one reported case in which FXR down-regulates apolipoprotein A-I expression via a FXR monomer or homodimer binding to an .Previous studies (15, 16) have shown that agonist-bound FXR directly regulates expression of the bile salt export pump (BSEP), an ABC transporter (ABCB11) ...

Benefit of farnesoid X receptor inhibition in obstructive cholestasis

“…Multidrug resistance proteins 1 and 2 (Mdr1͞ABCB1; Mdr2͞ ABCB4) were significantly induced in WT and PXRKO mice but not in FXR-null mice after BDL (Table 1). Mdr2 codes for the phospholipid export pump at the canalicular membrane and hMDR3 (ABCB4) is known to be regulated by FXR (18). Mdr1 can be induced by xenobiotics via PXR (19), but our results show that in cholestasis, FXR regulates induction of both Mdr1 and 2 as well as BSEP and, therefore, is a major regulator of these important canalicular transporters.…”

Hepatobiliary transporters and drug-induced cholestasis

“…Transcriptional regulation of BSEP and MDR3 is mediated by FXR. [39][40][41] FXR-mediated activation of BSEP and MDR3 leads to increased bile salt efflux and the formation of mixed micelles in the biliary tree during cholestatic episodes, thereby preventing toxic effects of bile salts on hepatocytes and cholangiocytes. In addition, FXR has been shown to induce MRP2 expression in human hepatocytes, which might constitute another compensatory mechanism during cholestasis.…”

Single-nucleotide polymorphism analysis of the multidrug resistance protein 3 gene for the detection of clinical progression in Japanese patients with primary biliary cirrhosis

“…[28][29][30] In addition, UDCA facilitates the expression of BSEP and MDR3 in the bile ducts through activation of nuclear receptors, such as farnesoid X and retinoid X receptors. [31][32][33] Therefore, MDR3 probably plays an important role in the response to UDCA treatment, as well as both the progression and prognosis of PBC.…”