A nuclear receptor-mediated choleretic action of fibrates is associated with enhanced canalicular membrane fluidity and transporter activity mediating bile acid-independent bile secretion

Roles of PPAR activation in cancer therapeutic resistance: Implications for combination therapy and drug development

Therapeutic resistance is a major obstacle to successful treatment or effective containment of cancer. Peroxisome proliferator-activated receptors (PPARs) play an essential role in regulating energy homeostasis and determining cell fate. Despite of the pleiotropic roles of PPARs in cancer, numerous studies have suggested their intricate relationship with therapeutic resistance in cancer. In this review, we provided an overview of the roles of excessively activated PPARs in promoting resistance to modern anti-cancer treatments, including chemotherapy, radiotherapy, targeted therapy, and immunotherapy. The mechanisms through which activated PPARs contribute to therapeutic resistance in most cases include metabolic reprogramming, anti-oxidant defense, anti-apoptosis signaling, proliferation-promoting pathways, and induction of an immunosuppressive tumor microenvironment. In addition, we discussed the mechanisms through which activated PPARs lead to multidrug resistance in cancer, including drug efflux, epithelial-to-mesenchymal transition, and acquisition and maintenance of the cancer stem cell phenotype. Preliminary studies investigating the effect of combination therapies with PPAR antagonists have suggested the potential of these antagonists in reversing resistance and facilitating sustained cancer management. These findings will provide a valuable reference for further research on and clinical translation of PPAR-targeting treatment strategies.

Structure and Function of Hepatobiliary ATP Binding Cassette Transporters

The liver is beyond any doubt the most important metabolic organ of the human body. This function requires an intensive crosstalk within liver cellular structures, but also with other organs. Membrane transport proteins are therefore of upmost importance as they represent the sensors and mediators that shuttle signals from outside to the inside of liver cells and/or vice versa. In this review, we summarize the known literature of liver transport proteins with a clear emphasis on functional and structural information on ATP binding cassette (ABC) transporters, which are expressed in the human liver. These primary active membrane transporters form one of the largest families of membrane proteins. In the liver, they play an essential role in for example bile formation or xenobiotic export. Our review provides a state of the art and comprehensive summary of the current knowledge of hepatobiliary ABC transporters. Clearly, our knowledge has improved with a breath-taking speed over the last few years and will expand further. Thus, this review will provide the status quo and will lay the foundation for new and exciting avenues in liver membrane transporter research.

Alteration of MRP2 expression and the graft outcome after liver transplantation

Purpose

Multidrug resistance-associated protein (MRP) 2 is a glutathione conjugate in the canalicular membrane of hepatocytes. Early graft damage after liver transplantation (LT) can result in alteration of MRP2 expression. The purpose of this study was to evaluate the relationship between the pattern of MRP2 alteration and graft outcome.

Methods

Forty-one paraffin-embedded liver graft tissues obtained by protocol biopsy within 2 months after LT; these were stained using monoclonal antibodies of MRP2. We selected 15 live donor biopsy samples as a control, that showed homogenous canalicular staining for MRP2. The pattern of canalicular MRP2 staining of graft was classified into 3 types: homogenous (type C0), focal (type C1), and no (type C2,) staining of the canaliculi.

Results

In total, 17.1% graft tissues were type C0, 36.6% were type C1, and 46.3% were type C2. The median operation time was longer in patients with type C2 (562.6 minutes) than in patients with type C0 (393.8 minutes) (P = 0.038). The rates of posttransplant complications were higher in patients with type C2 (100%) than in patients with type C0 (42.9%) and C1 (73.3%) (P < 0.001).

Conclusion

MRP2 expression pattern was altered in 82.9% after LT. The pattern of MRP2 alteration was associated with longer operation time and higher rates of post-LT complications.

Thyroid hormone receptor β1 stimulates ABCB4 to increase biliary phosphatidylcholine excretion in mice

The ATP-binding cassette transporter ABCB4/MDR3 is critical for biliary phosphatidylcholine (PC) excretion at the canalicular membrane of hepatocytes. Defective ABCB4 gene expression and protein function result in various cholestatic liver and bile duct injuries. Thyroid hormone receptor (THR) is a major regulator of hepatic lipid metabolism; we explored its potential role in ABCB4 regulation. Thyroid hormone T3 stimulation to human hepatocyte models showed direct transcriptional activation of ABCB4 in a dose- and time-dependent manner. To determine whether THRβ1 (the main THR isoform of the liver) is involved in regulation, we tested THRβ1-specific agonists (e.g., GC-1, KB-141); these agonists resulted in greater stimulation than the native hormone. KB-141 activated hepatic ABCB44 expression in mice, which enhanced biliary PC secretion in vivo. We also identified THR response elements 6 kb upstream of the ABCB4 locus that were conserved in humans and mice. Thus, T3-via THRβ1 as a novel transcriptional activator regulates ABCB4 to increase ABCB4 protein levels at the canalicular membrane and promote PC secretion into bile. These findings may have important implications for understanding thyroid hormone function as a potential modifier of bile duct homeostasis and provide pharmacologic opportunities to improve liver function in hepatobiliary diseases caused by low ABCB4 expression.

Combination therapy of fenofibrate and ursodeoxycholic acid in patients with primary biliary cirrhosis who respond incompletely to UDCA monotherapy: a meta-analysis

Background

Although the effectiveness of treatment with ursodeoxycholic acid (UDCA) and fenofibrate for primary biliary cirrhosis (PBC) has been suggested by small trials, a systematic review to summarize the evidence has not yet been carried out.

Methods

A meta-analysis of all long-term randomized controlled trials comparing the combination of UDCA and fenofibrate with UDCA monotherapy was performed via electronic searches.

Results

Six trials, which included 84 patients, were assessed. Combination therapy with UDCA and fenofibrate was more effective than UDCA monotherapy in improving alkaline phosphatase (mean difference [MD]: −90.44 IU/L; 95% confidence interval [CI]: −119.95 to −60.92; P<0.00001), gamma-glutamyl transferase (MD: −61.58 IU/L; 95% CI: −122.80 to −0.35; P=0.05), immunoglobulin M (MD: −38.45 mg/dL; 95% CI: −64.38 to −12.51; P=0.004), and triglycerides (MD: −0.41 mg/dL; 95% CI: −0.82 to −0.01; P=0.05). However, their effects on pruritus (odds ratio [OR]: 0.39; 95% CI: 0.09–1.78; P=0.23), total bilirubin (MD: −0.05 mg/dL; 95% CI: −0.21 to 0.12; P=0.58), and alanine aminotransferase (MD: −3.31 IU/L; 95% CI: −14.60 to 7.97; P=0.56) did not differ significantly. This meta-analysis revealed no significant differences in the incidence of adverse events (OR: 0.21; 95% CI: 0.03–1.25; P=0.09) between patients treated with combination therapy and those treated with monotherapy.

Conclusion

In this meta-analysis, combination therapy with UDCA and fenofibrate was more effective in reducing alkaline phosphatase than UDCA monotherapy, but it did not improve clinical symptoms. There did not appear to be an increase in adverse events with combination therapy.

PPARα: A Master Regulator of Bilirubin Homeostasis

Hypolipidemic fibrates activate the peroxisome proliferator-activated receptor (PPAR) α to modulate lipid oxidation and metabolism. The present study aimed at evaluating how 3 PPARα agonists, namely, fenofibrate, gemfibrozil, and Wy14,643, affect bilirubin synthesis and metabolism. Human umbilical vein epithelial cells (HUVEC) and coronary artery smooth muscle cells (CASMC) were cultured in the absence or presence of the 3 activators, and mRNA, protein, and/or activity levels of the bilirubin synthesizing heme oxygenase- (HO-) 1 and biliverdin reductase (BVR) enzymes were determined. Human hepatocytes (HH) and HepG2 cells sustained similar treatments, except that the expression of the bilirubin conjugating UDP-glucuronosyltransferase (UGT) 1A1 enzyme and multidrug resistance-associated protein (MRP) 2 transporter was analyzed. In HUVECs, gemfibrozil, fenofibrate, and Wy14,643 upregulated HO-1 mRNA expression without affecting BVR. Wy14,643 and fenofibrate also caused HO-1 protein accumulation, while gemfibrozil and fenofibrate favored the secretion of bilirubin in cell media. Similar positive regulations were also observed with the 3 PPARα ligands in CASMCs where HO-1 mRNA and protein levels were increased. In HH and HepG2 cells, both UGT1A1 and MRP2 transcripts were also accumulating. These observations indicate that PPARα ligands activate bilirubin synthesis in vascular cells and metabolism in liver cells. The clinical implications of these regulatory events are discussed.

Nutritional factors (nutritional aspects) in biliary disorders: bile acid and lipid metabolism in gallstone diseases and pancreaticobiliary maljunction

Nutritional factors play a key role in the pathogenesis of biliary diseases such as gallstones and pancreaticobiliary maljunction. Gallstones are primarily classified into cholesterol stone and pigment stone according to the major composition. Cholesterol gallstone formation is very likely based upon supersaturated bile formation, and pigment stones are formed in bile rich in bilirubin. Thus, defects of hepatic metabolism of lipids and organic anions lead to biliary stones. Here, the recent understanding of cholesterol gallstone pathogenesis is elaborated. On the other hand, there is another important link of biliary lipid degradation to serious biliary disease, namely pancreaticobiliary maljunction. Lysophosphatidylcholine (lysoPC), a derivative of phosphatidylcholine hydrolysis by phospholipase A2, is a highly abundant bioactive lipid mediator present in circulation as well as in bile. Increases in bile of lysoPC and phospholipase A2 have been reported in pancreaticobiliary maljunction and considered to be the major risk factor for biliary tract cancers. Further, oxidized fatty acids have been established as a potent ligand for G2A, a member of G protein-coupled receptor family that mediates a diverse array of biological processes including cell growth and apoptosis. Thus, both of lysoPC and free fatty acids are supposed to play an important role through G2A in biliary inflammation and carcinogenesis of pancreaticobiliary maljunction. Taken together, nutritional factors, especially lipid compounds, are seemingly crucial in the pathogenesis of biliary diseases, and such a causal relationship is reviewed by mainly authors' previous publications.

Effects of three different fibrates on intrahepatic cholestasis experimentally induced in rats

Background. Activation of PPAR α modulates cholesterol metabolism and suppresses bile acid synthesis. This study aims to evaluate the effect of PPAR α agonists, fenofibrate, bezafibrate, and gemfibrozil, on acute cholestasis induced by ethinylestradiol (EE) plus chlorpromazine (CPZ) in rats. Method. 100 male albino rats (150-200 gm) were divided randomly into 10 equal groups. Control group received 1% methylcellulose vehicle; disease group received CPZ plus EE for 5 consecutive days; four groups received either ursodeoxycholic acid, fenofibrate, bezafibrate, or gemfibrozil for 7 days; 2 days before EE + CPZ, three other groups received one of the three fibrates after GW6471, a selective PPAR α antagonist in addition to EE + CPZ. The final group received GW6471 alone. Results. The three fibrates showed marked reduction (P < 0.05) in serum levels of ALP, GGT, ALT, AST, total bile acids, bilirubin, TNF α , and IL-1 β and in hepatic malondialdehyde level as well as a significant increase in bile flow rate (P < 0.05) in addition to improvements in histopathological parameters compared to diseased group. In groups which received GW6471, these effects were completely abolished with fenofibrate and partially blocked with bezafibrate and gemfibrozil. Conclusion. Short-term administration of fibrates to EE/CPZ-induced intrahepatic cholestatic rats exerted beneficial effects on hepatocellular damage and apoptosis. Fenofibrate anticholestatic effect was solely PPAR α dependent while other mechanisms played part in bezafibrate and gemfibrozil actions.

Effects of bezafibrate in nonalcoholic steatohepatitis model mice with monosodium glutamate-induced metabolic syndrome

Recently, we reported that monosodium glutamate-treated mice (MSG mice) developed severe hyperlipidemia and diabetes mellitus and several complications of obesity. MSG mice acquired fatty livers and subsequently underwent changes that are characteristic of nonalcoholic fatty liver disease (NAFLD)/nonalcoholic steatohepatitis (NASH). In the present study, the effects of bezafibrate on obesity, diabetes mellitus, and NAFLD/NASH were examined in MSG mice. A single dose of MSG (4 mg/g) was administered subcutaneously to neonatal male mice within 24h of birth. Bezafibrate was mixed into the normal feed for 8 weeks. The weight and body mass index of MSG mice increased significantly despite the unchanged intake of food. Triglyceride and total cholesterol levels in blood, visceral adipose tissue, and interscapular adipose tissue rose significantly. In the livers of MSG mice, moderate centrilobular microvesicular steatosis, ballooning degeneration with Mallory bodies, and scattered infiltration of neutrophils and lymphocytes were observed. Centrilobular hepatocytes were 4-hydroxynonenal-positive in MSG mice. Bezafibrate ameliorated the severity of diabetes mellitus, hyperinsulinemia, and hyperlipidemia. Adiponectin and leptin concentrations in blood improved, and the accumulation of visceral fat was inhibited. The expression of acyl-CoA oxidase, a beta-oxidation gene, and carnitine palmitoyl transferase, which regulates lipid metabolism, increased markedly on administration of bezafibrate. The liver pathology in MSG mice also improved with bezafibrate; specifically, macro- and microvesicles in hepatocytes nearly disappeared, and NAFLD activity score improved. It is concluded that bezafibrate inhibits the accumulation of visceral fat, following amelioration of hyperlipidemia, in MSG-induced obese mice, due to improvements in diabetes mellitus, fatty liver, and NAFLD.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

Bezafibrate treatment of primary biliary cirrhosis following incomplete response to ursodeoxycholic acid

BACKGROUND AND GOALS

Ursodeoxycholic acid (UDCA) is the only current pharmacologic treatment for primary biliary cirrhosis (PBC). However, some patients show persistent liver biochemical abnormalities even after 6 to 12 months treatment. Bezafibrate retard is a commonly used medication for hyperlipidemia. In Japanese studies, it was found to lower liver enzyme levels, apparently through its action on multiple drug resistance gene 3, a transport element of the ATP-dependent bile secretion system, and on peroxisome proliferator-activated receptor-alpha. The aim of this study was to evaluate the effect of adding bezafibrate to the treatment regimen in patients with PBC and a partial response to UDCA.

STUDY

The study group included 8 White patients, 7 women and 1 man, aged 52 to 76 years with PBC who had been treated at our Liver Institute with UDCA (900 mg/d to 1500 mg/d) for 2 to 11 years (mean, 5.7 y) with only a partial response (19% to 56% reduction in alkaline phosphatase level). Bezafibrate (400 mg/d) was added to UDCA and the patients were followed for 4 to 12 months.

RESULTS

Alkaline phosphatase levels (normal range, 35 to 104 U/L) decreased in all patients, from 140 to 360 U/L (mean, 201.2) to 68 to 158 U/L (mean, 98.4), and normalized in 6 patients. In addition, levels of gamma-glutamyl transferase (normal range, 6 to 42 U/L) decreased from 70 to 192 U/L (mean, 130) to 41 to 122 U/L (mean, 71.8). These findings were maintained throughout follow-up.

CONCLUSIONS

Combination therapy with bezafibrate and UDCA improves the biochemical profile of patients with PBC who respond only partially to UDCA. A larger controlled study is needed to evaluate the clinical implications of these findings.

Alteration of canalicular transporters in a mouse model of total parenteral nutrition

OBJECTIVES

Parenteral nutrition-associated liver disease (PNALD) is a major problem with prolonged total parenteral nutrition (TPN) administration. Our laboratory previously demonstrated significant changes in the expression of multidrug resistance genes (MDRs) 1 and 2, hepatocyte transporters, in a TPN mouse model. The present study hypothesized that these changes would lead to functional changes in the liver, and would contribute to the development of liver dysfunction.

MATERIALS AND METHODS

Mice received either intravenous saline and standard chow or TPN with or without intravenous lipids. Functional assays were performed after 7 days of infusion.

RESULTS

TPN with lipids led to a significant increase in serum bile acid levels, consistent with an early state of PNALD. Use of TPN without lipids prevented an elevation in bile acid levels. In both TPN groups, MDR2 expression was significantly (68%) lower than controls and bile phosphatidylcholine content, a functional measure of MDR2, was 40% less than controls. MDR1 expression in the TPN with lipid group was 31% higher than controls, whereas in the TPN without lipids mice there was no significant change. Hepatocyte extrusion of rhodamine dye, a measure of MDR1 function, declined only in the TPN with lipid group. Peroxisome proliferator-activated receptor-alpha expression decreased in both TPN groups. Fenofibrate given with TPN resulted in an increased expression of MDR1 and MDR2, and functionally increased hepatocyte rhodamine extrusion and presence of bile phosphatidylcholine in the TPN with lipid group.

CONCLUSIONS

The study shows that TPN led to alterations in the function of MDR1- and MDR2-expressed proteins. The changes help in the understanding of the mechanisms leading to PNALD, and suggest that fibrate administration may palliate these changes.

Function and pathophysiological importance of ABCB4 (MDR3 P-glycoprotein)

Like several other ATP-binding cassette (ABC) transporters, ABCB4 is a lipid translocator. It translocates phosphatidylcholine (PC) from the inner to the outer leaflet of the canalicular membrane of the hepatocyte. Its function is quite crucial as evidenced by a severe liver disease, progressive familial intrahepatic cholestasis type 3, which develops in persons with ABCB4 deficiency. Translocation of PC makes the phospholipid available for extraction into the canalicular lumen by bile salts. The primary function of biliary phospholipid excretion is to protect the membranes of cells facing the biliary tree against these bile salts: the uptake of PC in bile salt micelles reduces the detergent activity of these micelles. In this review, we will discuss the functional aspects of ABCB4 and the regulation of its expression. Furthermore, we will describe the clinical and biochemical consequences of complete and partial deficiency of ABCB4 function.