Aliases: MOAT-B, MOATB
Gene name: ATP binding cassette subfamily C member 4 (ABCC4)
MRP4 is an ATP-dependent, unidirectional efflux transporter belonging to the C subfamily of the ABC protein superfamily. It is expressed in the kidney, blood-brain barrier (BBB), liver, and other tissues, and its localization to the basolateral or apical membranes is tissue-dependent. It plays an important role in alleviating the impact of cholestasis on hepatocytes by efflux of bile acids into the blood, and can be up-regulated in the liver in this instance. It may also be important in limiting CNS and hematopoietic cell exposure to xenobiotics, and as a transporter of cell signaling molecules. MRP4 has wide substrate specificity, including nucleoside analogues and antiviral drugs; however, reports of clinical DDIs due to MRP4 modulation are rare. MRP4 is highly polymorphic, and some of these polymorphisms may be clinically relevant. Current FDA and EMA guidances make no specific recommendations for MRP4. However, this transporter may be important where active renal secretion is suspected, or where changes in circulating levels of bile salts are observed.
MRP4 is found in a variety of human tissues, with high levels of mRNA expression in the kidney and prostate, and lower levels in the liver, testis, ovary, lung, adrenal gland, and in various neurons and blood cells. MRP4 localizes to different membranes in different polarized cells. In the liver, choroid plexus, prostatic acinar cells, and the GIT, MRP4 is expressed at the basolateral membrane, while in the brain capillary endothelium and renal proximal tubule cells MRP4 is expressed at the apical membrane [1-4]. This means that in the liver, for instance, it effluxes its substrates into the bloodstream, whilst in the kidney, it effluxes into the urine.
Function, physiology, and clinically significant polymorphisms
MRP4 with its 170-kDa molecular weight is the smallest among the MRP proteins. It comprises 12 putative membrane spanning helices and lacks the additional N-terminal helices found in MRP1, MRP2, MRP3, and MRP6 . MRP4 mediates the transport of endogenous substrates including signaling molecules (cAMP/, cGMP, eicosanoids), bile acids, urate, and conjugated steroid hormones. Clinical drug substrates include antivirals (adefovir, tenofovir), antibiotics (cephalosporins), diuretics (furosemide, hydrochlorothiazide), the antihypertensive olmesartan, and cytotoxic agents (methotrexate, 6-thioguanine, 6-mercaptopurine, topotecan) [2, 6]. Inhibitors of MRP4 include non-steroidal inflammatory drugs, phosphodiesterase inhibitors, cardiovascular drugs, and flavonoids, among others [6, 7]. MRP4 plays a role in the renal excretion of organic anions and drugs , in the modulation of signaling pathways , and in the protection of the brain through the blood-brain barrier and blood-cerebrospinal fluid barrier . Basolaterally expressed MRP4 in gastric and intestinal epithelia is thought to be involved in the oral absorption of cephalosporin antibiotics, dasatinib, and potentially other orally administered drugs [4, 11]. As with some other members of the MRP family, MRP4 transport is somewhat dependent on cotransport of glutathione, and this is reported to expand its substrate selectivity.
Given its ability to transport important intra- and intercellular mediators such as cyclic nucleotides and eicosanoids, the physiological repertoire of MRP4 is thought to cover platelet aggregation, cell migration and proliferation, angiogenesis, and cardiomyocyte contraction. For the same reason, it is also implicated in cancer progression .
Albeit there is no evidence of diseases linked to single nucleotide polymorphisms in the ABCC4 gene, and an in vivo study showed no obvious abnormalities in Mrp4-null mice [12, 13], polymorphisms can alter the expression level and the transport rate of the protein, and may influence therapeutic outcomes and adverse events.
There are no specific citations of clinically relevant DDIs ascribed to this transporter, and there is limited information on its role in the clinical ADME of drugs. However, there is evidence of its role in the renal elimination of tenofovir, as the renal clearance of tenofovir was 15% lower and its AUC was 32% higher in ABCC4 3463G carriers compared with wild types in one study . Since MRP4 also transports drugs which are used in HIV therapy, the efficacy of these drugs may be somewhat dependent on the expression of this transporter in T-cells [1, 5, 13, 15].
MRP4 transports bile acids in the presence of glutathione (GSH), and functions as a backup system for eliminating bile acids from hepatocytes. Although MRP4 expression in the liver is low, it can be induced by bile acids in cholestatic conditions [1, 8]; thus it plays an important compensatory role in protecting the liver from over-exposure to bile acids. The induction of MRP4 is FXR-independent and may instead be due to post-transcriptional regulation , as mRNA levels have been shown to remain unchanged .
MRP4 overexpression confers cellular resistance to nucleotide-base, nucleoside, and nucleotide analogues, as well as to certain cytotoxic agents, therefore adversely affecting some anticancer therapies. Overexpression of MRP4 confers doxorubicin resistance to osteosarcoma cells , and high MRP4 activity may be a determinant of resistance to arsenic-based chemotherapy regimens applied in leukaemia patients . Since MRP4 transports arsenic metabolites such as dimethylarsenic acid, and polymorphic variants differ in their ability to do so, MRP4 SNPs potentially contribute to the risk of arsenic-induced toxicity and tumorigenesis . SNPs in ABCC4 also affect dose tolerance of 6-mercaptopurine in pediatric acute lymphoblastic leukemia .
MRP4 protects hematopoietic cells, both healthy and leukemic, against cytarabine, and counters the myelosuppressive effect of prolonged beta-lactam use . Conversely, the 3348A>G SNP in homozygous form predisposes to beta-lactam-induced neutropenia.
Currently, neither the FDA nor the EMA guidance specifically recommends the study of MRP4 interactions for NCEs. This is probably due to the lack of clear clinical citations of MRP4-mediated DDIs. However, this transporter may be relevant to NCEs where active renal secretion is suspected, or where changes in circulating levels of bile salts are observed.
|Location||Endogenous substrates||In vitro substrates used experimentally||Substrate drugs||Inhibitors|
|kidney, prostate, placenta, liver, BBB||taurocholic acid, cAMP, cGMP, urate, DHEAS, E2-17ßG, p-aminohippurate, PGE1 and PGE2||DHEAS, E2-17ßG||
antivirals: acyclovir, ritonavir, adefovir, tenofovir;
|NSAIDs: dipyridamole, sulindac, tolmetin, indomethacin, piroxicam, naproxen, celecoxib, flurbiprofen, ibuprofen, ketoprofen, diclofenac;
phosphodiesterase inhibitors: sildenafil, zaprinast, trequinsin;
cardiovascular drugs: verapamil, losartan, telmisartan, candesartan;
flavonoids: quercetin, silymarin;
probenecid, dilazep, dantrolene, ceefourin
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2. Russel, F.G., J.B. Koenderink, and R. Masereeuw, Multidrug resistance protein 4 (MRP4/ABCC4): a versatile efflux transporter for drugs and signalling molecules. Trends Pharmacol Sci., 2008. 29(4): p. 200-7. Epub 2008 Mar 18.
3. Ritter, C.A., et al., Cellular export of drugs and signaling molecules by the ATP-binding cassette transporters MRP4 (ABCC4) and MRP5 (ABCC5). Drug Metab Rev., 2005. 37(1): p. 253-78.
4. de Waart, D.R., et al., Oral availability of cefadroxil depends on ABCC3 and ABCC4. Drug Metab Dispos, 2012. 40(3): p. 515-21.
5. Borst, P., et al., A family of drug transporters: the multidrug resistance-associated proteins. J Natl Cancer Inst., 2000. 92(16): p. 1295-302.
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9. Sassi, Y., et al., Multidrug resistance-associated protein 4 regulates cAMP-dependent signaling pathways and controls human and rat SMC proliferation. J Clin Invest., 2008. 118(8): p. 2747-57.
10. Leggas, M., et al., Mrp4 confers resistance to topotecan and protects the brain from chemotherapy. Mol Cell Biol., 2004. 24(17): p. 7612-21.
11. Furmanski, B.D., et al., Contribution of ABCC4-mediated gastric transport to the absorption and efficacy of dasatinib. Clin Cancer Res, 2013. 19(16): p. 4359-70.
12. Abla, N., et al., The human multidrug resistance protein 4 (MRP4, ABCC4): functional analysis of a highly polymorphic gene. J Pharmacol Exp Ther., 2008. 325(3): p. 859-68. Epub 2008 Mar 25.
13. Belinsky, M.G., et al., Multidrug resistance protein 4 protects bone marrow, thymus, spleen, and intestine from nucleotide analogue-induced damage. Cancer Res., 2007. 67(1): p. 262-8.
14. Kiser, J.J., et al., The effect of lopinavir/ritonavir on the renal clearance of tenofovir in HIV-infected patients. Clin Pharmacol Ther, 2008. 83(2): p. 265-72.
15. Reid, G., et al., Characterization of the transport of nucleoside analog drugs by the human multidrug resistance proteins MRP4 and MRP5. Mol Pharmacol., 2003. 63(5): p. 1094-103.
16. Wagner, M., et al., Role of farnesoid X receptor in determining hepatic ABC transporter expression and liver injury in bile duct-ligated mice. Gastroenterology., 2003. 125(3): p. 825-38.
17. Denk, G.U., et al., Multidrug resistance-associated protein 4 is up-regulated in liver but down-regulated in kidney in obstructive cholestasis in the rat. J Hepatol., 2004. 40(4): p. 585-91.
18. He, Z., et al., The overexpression of MRP4 is related to multidrug resistance in osteosarcoma cells. J Cancer Res Ther, 2015. 11(1): p. 18-23.
19. Yuan, B., et al., Multidrug resistance-associated protein 4 is a determinant of arsenite resistance. Oncol Rep, 2016. 35(1): p. 147-54.
20. Banerjee, M., et al., Polymorphic variants of MRP4/ABCC4 differentially modulate the transport of methylated arsenic metabolites and physiological organic anions. Biochem Pharmacol, 2016. 120: p. 72-82.
21. Tanaka, Y., et al., Multidrug resistance protein 4 (MRP4) polymorphisms impact the 6-mercaptopurine dose tolerance during maintenance therapy in Japanese childhood acute lymphoblastic leukemia. Pharmacogenomics J, 2015. 15(4): p. 380-4.
22. Drenberg, C.D., et al., ABCC4 Is a Determinant of Cytarabine-Induced Cytotoxicity and Myelosuppression. Clin Transl Sci, 2016. 9(1): p. 51-9.