Peroxisome proliferator-activated receptor α activates human multidrug resistance transporter 3/ATP-binding cassette protein subfamily B4 transcription and increases rat biliary phosphatidylcholine secretion.

The multidrug resistance transporter 3 (MDR3, Mdr2/Abcb4 in rodent), encoded by the adenosine triphosphate-binding cassette superfamily (ATP)-binding cassette proteins subtype B4 (ABCB4), is predominantly expressed in the liver1 and localized to the canalicular membrane of hepatocytes, where it is the major determinant of biliary phosphatidylcholine (PC) secretion.2 Specifically, MDR3/ABCB4 functions as a floppase and translocates PC from the inner to the outer leaflet of the canalicular membrane for bile salt extraction.3 Together with cholesterol and bile acids, PC forms biliary micelles, which inactivate the toxic detergent action of bile salts and prevent damage to epithelial cells lining the bile duct as well as inhibiting the formation of cholesterol gallstones. The functional importance of MDR3 was first demonstrated in Mdr2 knockout mice which lack biliary phospholipids and develop bile duct injury and progressive liver disease, closely resembling that of primary sclerosing cholangitis (PSC) in humans.4,5 Clinically, mutations and polymorphisms of MDR3 contribute to cholestatic liver injury, including a subtype of progressive familial intrahepatic cholestasis type 3, intrahepatic cholestasis of pregnancy, low phospholipid cholelithiasis, drug-induced and idiopathic chronic cholestasis.6 Thus, MDR3 represents an important pharmacological target. Chronic cholestasis, including primary biliary cirrhosis (PBC) and PSC, leads to liver fibrosis and cirrhosis, which eventually results in liver failure and the need for liver transplantation. The only therapeutic option available for these patients is ursodeoxycholic acid (UDCA), which slows the progression of PBC, particularly in stage I and II of the disease. However, some patients only partially respond to UDCA therapy, while more advanced cases usually do not respond. Furthermore, UDCA does not improve survival in patients with PSC, emphasizing the need for alternative therapies. Peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors that belong to the superfamily of nuclear receptors. There are three PPAR isoforms: α, β/δ, and γ, and they regulate gene expression by forming a heterodimer with the retinoic acid receptor (RXR) and bind to peroxisome proliferator response elements (PPRE) containing direct repeats (DR-1) of the consensus sequence AGGTCA separated by a single nucleotide.7 Fibrates are synthetic PPAR ligands used clinically for the treatment of dyslipidemia,8 although each fibrate differs slightly in its specificity for the different PPAR subtypes.9 PPARα is predominantly expressed in the liver and regulates the transcription of several genes involved in lipid metabolism. Interestingly, PPARα ligands, including fenofibrate, ciprofibrate, and clofibrate, reportedly up-regulate Mdr2 messenger RNA (mRNA) and protein expression10,11 in a PPARα-dependent manner.10 Additionally, fenofibrate improves biomarkers of cholestasis following bile-duct ligation in mice.12 PPARα has also been shown to inhibit the expression of genes involved in inflammation by negatively interfering with nuclear factor kappa B (NF-κB) signaling,13 and fenofibrate up-regulates the human bile acid metabolizing enzymes of the UDP-glucuronosyltransferase family.14–16 These actions have important and direct benefits for patients with chronic cholestatic liver disease. Clinical data shows that fenofibrate reduces symptoms and improves liver function abnormalities in some patients with PBC who do not fully respond to UDCA.17 These data suggest that fenofibrate may have an important role as therapy for cholestatic liver disease, although the mechanism(s) responsible for its actions remain unknown. This study examined the ability of fenofibrate to induce the expression of human MDR3 and the mechanisms that underlie this phenomenon. Our findings show that fenofibrate up-regulates the expression of MDR3 by activating PPARα. We further demonstrate that PPARα binds to specific PPREs in the human MDR3 promoter. We also show that fenofibrate increases canalicular excretion of fluorescently labeled PC. We present direct evidence for the first time that the human MDR3 gene is transactivated by PPARα and strongly suggest that PPARα up-regulates the expression of MDR3 to facilitate hepatic export of phospholipids. These findings provide a molecular mechanism by which fenofibrate may improve symptoms and liver function in patients with chronic cholestatic liver disease.