Transcriptional regulation of the rat Mrp3 gene promoter by the specificity protein (Sp) family members and CCAAT/enhancer binding proteins

Bile formation and secretion

Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions.

Elevated hepatic multidrug resistance-associated protein 3/ATP-binding cassette subfamily C 3 expression in human obstructive cholestasis is mediated through tumor necrosis factor alpha and c-Jun NH2-terminal kinase/stress-activated protein kinase-signaling pathway

Multidrug resistance-associated protein 3 (MRP3, ABC subfamily C [ABCC]3) plays an important role in protecting hepatocytes and other tissues by excreting an array of toxic organic anion conjugates, including bile salts. MRP3/ABCC3 expression is increased in the liver of some cholestatic patients, but the molecular mechanism of this up-regulation remains elusive. In this report, we assessed liver MRP3/ABCC3 expression in patients (n = 22) with obstructive cholestasis caused by gallstone blockage of bile ducts and noncholestatic patient controls (n = 22). MRP3/ABCC3 messenger RNA (mRNA) and protein expression were significantly increased by 3.4- and 4.6-fold, respectively, in these cholestatic patients where elevated plasma tumor necrosis factor alpha (TNFα) (4.7-fold; P < 0.01) and hepatic specificity protein 1 transcription factor (SP1) and liver receptor homolog 1 expression (3.1- and 2.1-fold at mRNA level, 3.5- and 2.5-fold at protein level, respectively) were also observed. The induction of hepatic MRP3/ABCC3 mRNA expression is significantly positively correlated with the level of plasma TNFα in these patients. In HepG2 cells, TNFα treatment induced SP1 and MRP3/ABCC3 expression in a dose- and time-dependent manner, where increased phosphorylation of c-Jun NH2-terminal kinase/stress-activated protein kinase (JNK/SAPK) was also detected. These inductions were significantly reduced in the presence of the JNK inhibitor, SP600125. TNFα treatment enhanced HepG2 cell nuclear extract-binding activity to the MRP3/ABCC3 promoter, but was abolished by SP600125, as demonstrated by electrophoretic mobility shift assay (EMSA). An increase in nuclear protein-binding activity to the MRP3/ABCC3 promoter, consisting primarily of SP1, was also observed in liver samples from cholestatic patients, as assessed by supershift EMSA assays.

CONCLUSIONS

Our findings indicate that up-regulation of hepatic MRP3/ABCC3 expression in human obstructive cholestasis is likely triggered by TNFα, mediated by activation of JNK/SAPK and SP1.

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.

Association of pregnane X receptor with multidrug resistance-related protein 3 and its role in human colon cancer chemoresistance

BACKGROUND

Pregnane xenobiotic receptor (PXR), a ligand-activated transcription factor, regulates the drug metabolism and transport. Its activation can reduce the efficacy of antineoplastic agents. The aim of this study was to investigate the role of PXR and the relationship between PXR and multidrug resistance-related protein 3 (MRP3) in human colon cancer chemoresistance.

RESULTS

The results showed that both the mitochondrial RNA (mRNA) and protein levels of PXR and MRP3 were much higher in colon cancer tissues than that in nonneoplastic tissues by reverse transcriptase polymerase chain reaction and Western blot analysis. MRP3 mRNA was significantly correlated with PXR mRNA in cancerous (P = 0.001) and nonneoplastic (P < 0.001) colon tissues with Pearson correlation test. The expressions of PXR, SP1, and MRP3 were markedly enhanced after rifampicin treatment. On the other hand, the protein level of MRP3 decreased after stable RNA interference of PXR. It also observed that PXR, activated by rifampicin or knocked down via short hairpin RNAs, could enhance or reduce cells resistance to the chemotherapeutic agents through 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay.

CONCLUSIONS

The results suggested that PXR, associated with MRP3, may play an important role in human colon cancer resistance to chemotherapeutics and SP1 may be involved in the induction of MRP3 by PXR activation.

Cooperative regulation of Fc receptor gamma-chain gene expression by multiple transcription factors, including Sp1, GABP, and Elf-1

The Fc receptor gamma-chain (FcRgamma), which was first identified as a constituent of the high affinity IgE receptor, associates with various cell surface receptors to mediate intracellular signals. We identified three transcriptional enhancer elements in the 5' region of the human FcRgamma gene; one of the cis-elements was recognized by the transcription factor Sp-1 and another was recognized by GABP or Elf-1. The sequence of the other element was similar to a binding motif of the C/EBP family. Overexpression experiments showed that these transcription factors cooperatively activated the FcRgamma promoter. Furthermore, inactivation of the GABP-binding site by nucleotide substitutions as well as repression of GABPalpha expression by RNA interference reduced Sp1-mediated transactivation of the FcRgamma promoter, demonstrating that Sp1 and GABP synergistically activated the FcRgamma promoter. This synergistic activation was suggested to require physical interaction between the two transcription factors, because the Ets domain of GABPalpha was demonstrated to directly bind Sp1. On the other hand, GABP and Elf-1, whose recognition sequences overlapped, were shown to bind the FcRgamma gene with similar affinity in the context of chromatin, although Elf-1 exerted weaker enhancer activity for FcRgamma gene expression than did GABP. Both were thought to compete for binding to the element, because additional expression of Elf-1 in combination with Sp1 and GABP reduced FcRgamma promoter activity. Such functional and physical interactions among transcription factors involved in the cooperative regulation of FcRgamma gene expression as revealed in this study will become promising targets for medical applications against various immune diseases involving FcRgamma.

Multidrug resistance-associated protein 3 (Mrp3/Abcc3/Moat-D) is expressed in the SAE Squalus acanthias shark embryo-derived cell line

The multidrug resistance-associated protein 3 (MRP3/Mrp3) is a member of the ATP-binding cassette (ABC) protein family of membrane transporters and related proteins that act on a variety of xenobiotic and anionic molecules to transfer these substrates in an ATP-dependent manner. In recent years, useful comparative information regarding evolutionarily conserved structure and transport functions of these proteins has accrued through the use of primitive marine animals such as cartilaginous fish. Until recently, one missing tool in comparative studies with cartilaginous fish was cell culture. We have derived from the embryo of Squalus acanthias, the spiny dogfish shark, the S. acanthias embryo (SAE) mesenchymal stem cell line. This is the first continuously proliferating cell line from a cartilaginous fish. We identified expression of Mrp3 in this cell line, cloned the molecule, and examined molecular and cellular physiological aspects of the protein. Shark Mrp3 is characterized by three membrane-spanning domains and two nucleotide-binding domains. Multiple alignments with other species showed that the shark Mrp3 amino acid sequence was well conserved. The shark sequence was overall 64% identical to human MRP3, 72% identical to chicken Mrp3, and 71% identical to frog and stickleback Mrp3. Highest identity between shark and human amino acid sequence (82%) was seen in the carboxyl-terminal nucleotide-binding domain of the proteins. Cell culture experiments showed that mRNA for the protein was induced as much as 25-fold by peptide growth factors, fetal bovine serum, and lipid nutritional components, with the largest effect mediated by a combination of lipids including unsaturated and saturated fatty acids, cholesterol, and vitamin E.

Beneficial Effect of Spironolactone Administration on Ethynylestradiol-Induced Cholestasis in the Rat: Involvement of Up-Regulation of Multidrug Resistance-Associated Protein 2

The effect of spironolactone (SL) administration on 17alpha-ethynylestradiol (EE)-induced cholestasis was studied, with emphasis on expression and activity of Mrps. Adult male Wistar rats were divided into the following groups: EE (5 mg/kg daily for 5 days, s.c.), SL (200 micromol/kg daily for 3 days, i.p.), EE+SL (same doses, SL administered the last 3 days of EE treatment), and controls. SL prevented the decrease in bile salt-independent fraction of bile flow induced by EE, in association with normalization of biliary excretion of glutathione. Western blot studies indicate that EE decreased the expression of multidrug resistance-associated protein 2 (Mrp2) by 41% and increased that of Mrp3 by 200%, whereas SL only affected Mrp2 expression (+60%) with respect to controls. The EE+SL group showed increased levels of Mrp2 and Mrp3 to the same extent as that registered for the individual treatments. Real-time polymerase chain reaction studies indicated that up-regulation of Mrp2 and Mrp3 by SL and EE, respectively, was at the transcriptional level. To estimate Mrp2 and Mrp3 activities, apical and basolateral excretion of acetaminophen glucuronide (APAP-glu), a common substrate for both transporters, was measured in the recirculating isolated perfused liver model. Biliary/perfusate excretion ratio was decreased in EE (-88%) and increased in SL (+36%) with respect to controls. Coadministration of rats with SL partially prevented (-53%) impairment induced by EE in this ratio. In conclusion, SL administration to EE-induced cholestatic rats counteracted the decrease in bile flow and biliary excretion of glutathione and APAP-glu, a model Mrp substrate, findings associated with up-regulation of Mrp2 expression.

Nuclear receptors RXRalpha:RARalpha are repressors for human MRP3 expression

Multidrug resistance-associated protein MRP3/Mrp3 (ABCC3) is upregulated in cholestasis, an adaptive response that may protect the liver from accumulation of toxic compounds, such as bile salts and bilirubin conjugates. However, the mechanism of this upregulation is poorly understood. We and others have previously reported that fetoprotein transcription factor/liver receptor homolog-1 is an activator of MRP3/Mrp3 expression. In searching for additional regulatory elements in the human MRP3 promoter, we have now identified nuclear receptor retinoic X receptor-alpha:retinoic acid receptor-alpha (RXRalpha:RARalpha) as a repressor of MRP3 activation by transcription factor Sp1. A luciferase reporter assay demonstrated that cotransfection of transcription factor Sp1 stimulates the MRP3 promoter activity and that additions of RXRalpha:RARalpha abrogated this activation in a dose-dependent manner. Site mutations and gel shift assays have identified a Sp1 binding GC box motif at -113 to -108 nts upstream from the MRP3 translation start site, where RXRalpha:RARalpha specifically reduced Sp1 binding to this site. Mutation of the GC box also reduced MRP3 promoter activity. The functional role of RXRalpha:RARalpha as a repressor of MRP3 expression was further confirmed by RARalpha small-interfering RNA knockdown in HepG2 cells, which upregulated endogenous MRP3 expression. In summary, our results indicate that activator Sp1 and repressor RXRalpha:RARalpha act in concert to regulate MRP3 expression. Since RXRalpha:RARalpha expression is diminished by cholestatic liver injury, loss of RXRalpha:RARalpha may lead to upregulation of MRP3/Mrp3 expression in these disorders.

KLF6 and HSF4 transcriptionally regulate multidrug resistance transporters during inflammation

Endotoxin-induced inflammation alters the hepatic expression of the drug efflux transporter genes mdr1b (Abcb1b) and mrp3 (Abcc3) in rats. In this study, we identified a novel kruppel-like zinc finger protein 6 (KLF6) cis-element on the rat mdr1b promoter which is important for basal activity and IL-1beta and endotoxin-mediated induction in gene transcription. Interestingly, KLF6 also functioned as a negative transcriptional regulator, inhibiting TNF-alpha-mediated induction of mdr1b. Furthermore, novel CCAAT/enhancer binding protein beta (C/EBPbeta) and heat shock factor 4 (HSF4) transcription binding sites were identified on the rat mrp3 promoter. Deletion of the HSF4 element significantly increased transcriptional activity of the mrp3 gene when exposed to TNF-alpha. Endotoxin treatment significantly affected transcriptional activity only in C/EBPbeta and HSF4 double deletion mrp3 promoter constructs. In summary, KLF6 and HSF4 are stimuli-specific regulatory elements which may be important in the control of the rat mdr1b and mrp3 genes during health and disease.

Direct interaction of C/EBPdelta and Sp1 at the GC-enriched promoter region synergizes the IL-10 gene transcription in mouse macrophage

We previously reported that LPS activates the transcription of the IL-10 gene through the Sp1 and C/EBP binding sites and indicated that Sp1, C/EBPbeta and C/EBPdelta can coactivate the IL-10 gene expression in mouse macrophage cells [Liu Y.-W., Tseng H.-P., Chen L.-C., Chen B.-K., Chang W.-C. J. Immunol. 171: 821-828, 2003]. In the present report, we demonstrated the direct physical interaction between C/EBPdelta and Sp1, and also mapped the interaction domains of these two proteins. C/EBPdelta binds to Sp1 via its basic region leucine zipper domain. The C-terminus of Sp1 was also the major region interacting with C/EBPdelta. However, both glutamine- and serine/threonine-rich homologus regions of Sp1 also interacted with C/EBPdelta. The binding of Sp1 and C/EBPdelta as a complex to the Sp1 binding site on the promoter of IL-10 was further confirmed by using the DNA affinity precipitation assay. By using Sp1-deficient SL2 cells, we also found that the overexpressions of C/EBPdelta and Sp1 synergically activate the transcriptional activity of IL-10 gene. Taken together, our present results revealed a novel mechanism of a superactivation of Sp1 by C/EBPdelta via a direct interaction between these two transcription factors leading to the activation of the IL-10 gene in mouse macrophage cells.