MRP1

MRP1 (multidrug resistance-associated protein 1)

Aliases: ABC29, ABCC, GS-X, MRP
Gene name: ATP binding cassette subfamily C member 1 (ABCC1)

Summary

ABCC1 or multidrug resistance-associated protein 1 (MRP1) is a unidirectional efflux transporter with ubiquitous tissue distribution and wide substrate specificity including important therapeutics. The main roles of this transporter are (i) efflux of xenobiotics and endogenous metabolites, (ii) transport of inflammatory mediators (e.g. LTC4), and (iii) defense against oxidative stress. MRP1 plays a role in the development of drug resistance of various types of cancer, and contributes to inflammatory responses [1, 2]. While it does not appear to play a significant role in the absorption or elimination of drugs, it is an important modulator of drug tissue exposure and the cellular elimination of metabolites. Due to lack of clinical evidence for a notable role in drug absorption, elimination, or DDI, MRP1 is not recommended for investigation by the FDA and EMA. Nevertheless, its role in chemotherapeutic pharmacology and drug distribution may prove important for some NCEs.

Localization

MRP1 is ubiquitously expressed in humans. The highest mRNA levels were reported for testis, cardiomyocytes, placenta, prostate, lung, thymus, and kidney, with lower expression in the small intestine, colon, brain, and mononuclear cells. MRP1 is highly expressed in epithelial and endothelial cells with a barrier function but is located on the basolateral membrane of these polarized cells. Although apical localization in brain capillary endothelial cells has been suggested [3], this has not been confirmed by other studies [4, 5]. It is often more highly expressed in rapidly dividing cells, unlike other ABC transporters such as MRP2 and MDR1 [1, 2]. 
In addition to the plasma membrane, MRP1 also localizes to the mitochondria and intracellular organelles such as the endoplasmic reticulum (ER) and endocytic vesicles. Mitochondrial MRP protects mitochondrial DNA from damage, while ER/vesicular MRP1 may serve as a sequestering mechanism. Intracellular MRP1 rapidly translocates to the plasma membrane upon induction [6].

Function, physiology, and clinically significant polymorphisms

The 190-kDa MRP1 has a core structure consisting of two transmembrane domains (TMD), each followed by a nucleotide binding domain (NBD). In common with MRP2, 3, 6, and 7, MRP1 contains a third TMD (TMD0) with five predicted transmembrane segments and an extracytosolic NH2 terminus connected to the core structure by a linker region (L0) [7]. The TMD0 appears to be important for MRP1 trafficking to the plasma membrane [8], and the precise roles, mechanisms and dependencies of TMD0 and L0 are the subject of intensive research [8-11]. 
MRP1 has broad substrate specificity, transporting hydrophobic and anionic molecules, glucuronide and glutathione conjugates, as well as endogenous glutathione [12]. Although many MRP1 substrates are conjugated to glutathione, co-transport of free glutathione is often observed, and appears to stimulate transport of substrates such as vincristine and daunorubicin [13]. Glutathione itself is a low affinity substrate of MRP1 (KM = 1-5 mM) [14]. The inflammatory cytokine LTC4 and its main metabolite LTD4 are among the highest-affinity MRP1 substrates, suggesting a key role for MRP1 in cytokine release from LTC4-producing cells. Intracellular LTC4 accumulation was observed in Mrp1 (-/-) mice [15]. 
Cryo-EM reconstruction of the LTC4-bound form of bovine MRP1 confirmed a bipartite substrate binding site, with a positively charged ‘P-pocket’ coordinating the GSH moiety and a hydrophobic ‘H-pocket’ accommodating the lipid tail of the molecule [16]. The dual nature of the binding site explains the ability of MRP1 to recognize a spectrum of substrates with different chemical structures including large, amphipathic molecules.
Whereas the role of glutathione in preventing oxidative stress is well understood, the precise dynamics of MRP1 in regulating cellular glutathione levels requires clarification [13]. Cellular exposure to reactive oxygen species (ROS) rapidly depletes GSH whilst increasing GSSG. GSSG is transported more efficiently by MRP1 than GSH, therefore it may help to maintain a healthy GSSG/GSH ratio [14]. Selective inhibition of MRP1 by MK571 also promotes 4-hydroxy-2-nonenal (HNE)-induced oxidative stress and cell death. HNE is a chemically reactive aldehyde produced during lipid peroxidation; thus, a protective role for MRP1 is postulated, possibly by MRP1-mediated transport of the HNE-SG complex.
Numerous chemotherapeutic agents, including doxorubicin and vinblastine, have been reported to induce MRP1 expression, and a role for nuclear hormone regulation via CAR has been reported [1]. Notch1 signaling has also been shown to regulate MRP1 expression in cultured cancer cells [17] as well as bronchial epithelial cells [18].

Clinical significance and polymorphisms

Despite its broad substrate specificity, a clear demonstration of MRP1 involvement in the absorption or elimination of drugs is lacking. However, a role in modulating drug tissue distribution and metabolite efflux is evident, which grants a place for MRP1 in pharmacology and toxicity. Because tissue levels are challenging to measure in humans in vivo, most information has been derived from preclinical species, in vitro studies, and clinical observations.
MRP1 is implicated in the lack of chemotherapeutic response to several clinically important drugs. High expression of MRP1 confers resistance to a variety of natural-product anticancer drugs such as vinca alkaloids, anthracyclines, and epipodophyllotoxins; conversely, Mrp1 -/- mice, although viable, healthy, and fertile with normal phenotype, are hypersensitive to cytotoxic drugs [19]. Being a key player in the blood-brain barrier, MRP1 in fact seems to be more accountable than P-gp for the chemoresistance of glial-derived brain tumors [20]. The topoisomerase I inhibitor irinotecan (CPT-11), along with its major active metabolite SN-38 and its glucuronide, are actively effluxed through MRP1. Lymphocytes from HIV-positive patients with lower MRP1 expression showed a significantly higher accumulation of both ritonavir and saquinavir compared to those with higher MRP1 expression. Multiple chemical toxicants and their metabolites are also known substrates of MRP1, e.g. aflatoxin and the GSH conjugates of herbicide metolachlor [14]. 
In studies using triple-knockout Mrp1 -/- mice, Mrp1 did not significantly influence grepafloxacin systemic exposure or elimination but did modulate distribution to the heart, trachea, kidney, and spleen [21], implying that Mrp1 may play a more fundamental role in drug tissue distribution than in absorption or elimination. However, as MRP1 is highly expressed at the lung epithelial barrier it may significantly influence the pulmonary disposition of inhaled drugs, as it was demonstrated in Mrp1 -/- rats where the lung AUC of an intratracheally administered prodrug was 352% higher compared to wild-type [22].
There are at least 15 naturally occurring mutations identified in MRP1, and many of them have been found to affect its in vitro transport activity. Polymorphisms and mutagenesis studies have been reviewed in [14]. Although many MRP1 SNPs are known, their incidence in populations is reported to be relatively low. In mainland Chinese population the MRP1 polymorphism allelic frequencies of Cys43Ser (128G>C), Thr73Ile (218C>T), Arg723Gln (2168G>A) and Arg1058Gln (3173G>A) were 0.5%, 1.4%, 5.8% and 0.5%, respectively [23]. MRP1 is vital to protecting the heart against cardiotoxic chemotherapeutics such as anthracyclines, and some polymorphisms including rs3743527 and rs246221 influence the efficiency of cardioprotection [24]. Germline genetic variation in ABBC1 was associated with the severity of hematological adverse events upon 5-FU/epirubicin/cyclophosphamide treatment [25] and predicted failure of cyclophosphamide and doxorubicin therapy [26] in breast cancer patients, and explained varying response rates to methotrexate in rheumatoid arthritis [27].

Regulatory requirements

As there is no strong association of MRP1 with the absorption and elimination of drugs, and a thorough understanding of its role in drug distribution is lacking, neither the FDA nor the EMA guidance makes any specific reference to this transporter. Nonetheless, given its broad substrate specificity and role in modulating cellular exposure to drugs, its evaluation may be appropriate for some NCEs [28].

References

1.    Bakos, E. and L. Homolya, Portrait of multifaceted transporter, the multidrug resistance-associated protein 1 (MRP1/ABCC1). Pflugers Arch, 2007. 453(5): p. 621-41.
2.    Deeley, R.G., C. Westlake, and S.P. Cole, Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins. Physiol Rev, 2006. 86(3): p. 849-99.
3.    Zhang, Y., et al., Plasma membrane localization of multidrug resistance-associated protein homologs in brain capillary endothelial cells. J Pharmacol Exp Ther, 2004. 311(2): p. 449-55.
4.    Roberts, L.M., et al., Subcellular localization of transporters along the rat blood-brain barrier and blood-cerebral-spinal fluid barrier by in vivo biotinylation. Neuroscience, 2008. 155(2): p. 423-38.
5.    Uchida, Y., et al., Quantitative targeted absolute proteomics of human blood-brain barrier transporters and receptors. J Neurochem, 2011. 117(2): p. 333-45.
6.    Lu, J.F., D. Pokharel, and M. Bebawy, MRP1 and its role in anticancer drug resistance. Drug Metab Rev, 2015. 47(4): p. 406-19.
7.    Rosenberg, M.F., et al., The structure of the multidrug resistance protein 1 (MRP1/ABCC1). crystallization and single-particle analysis. J Biol Chem, 2001. 276(19): p. 16076-82.
8.    Bakos, E., et al., Characterization of the amino-terminal regions in the human multidrug resistance protein (MRP1). J Cell Sci, 2000. 113 Pt 24: p. 4451-61.
9.    Bakos, E., et al., Functional multidrug resistance protein (MRP1) lacking the N-terminal transmembrane domain. J Biol Chem, 1998. 273(48): p. 32167-75.
10.    Bakos, E., et al., Membrane topology and glycosylation of the human multidrug resistance-associated protein. J Biol Chem, 1996. 271(21): p. 12322-6.
11.    Westlake, C.J., S.P. Cole, and R.G. Deeley, Role of the NH2-terminal membrane spanning domain of multidrug resistance protein 1/ABCC1 in protein processing and trafficking. Mol Biol Cell, 2005. 16(5): p. 2483-92.
12.    Cole, S.P., Multidrug resistance protein 1 (MRP1, ABCC1), a "multitasking" ATP-binding cassette (ABC) transporter. J Biol Chem, 2014. 289(45): p. 30880-8.
13.    Hooijberg, J.H., et al., The effect of glutathione on the ATPase activity of MRP1 in its natural membranes. FEBS Lett, 2000. 469(1): p. 47-51.
14.    He, S.M., et al., Structural and functional properties of human multidrug resistance protein 1 (MRP1/ABCC1). Curr Med Chem, 2011. 18(3): p. 439-81.
15.    Robbiani, D.F., et al., The leukotriene C(4) transporter MRP1 regulates CCL19 (MIP-3beta, ELC)-dependent mobilization of dendritic cells to lymph nodes. Cell, 2000. 103(5): p. 757-68.
16.    Johnson, Z.L. and J. Chen, Structural Basis of Substrate Recognition by the Multidrug Resistance Protein MRP1. Cell, 2017. 168(6): p. 1075-1085 e9.
17.    Cho, S., et al., Notch1 regulates the expression of the multidrug resistance gene ABCC1/MRP1 in cultured cancer cells. Proc Natl Acad Sci U S A, 2011. 108(51): p. 20778-83.
18.    Wu, J., et al., Allyl isothiocyanate may reverse the expression of MRP1 in COPD rats via the Notch1 signaling pathway. Arch Pharm Res, 2019. 42(11): p. 1000-1011.
19.    Wijnholds, J., et al., Increased sensitivity to anticancer drugs and decreased inflammatory response in mice lacking the multidrug resistance-associated protein. Nat Med, 1997. 3(11): p. 1275-9.
20.    Spiegl-Kreinecker, S., et al., Expression and functional activity of the ABC-transporter proteins P-glycoprotein and multidrug-resistance protein 1 in human brain tumor cells and astrocytes. J Neurooncol, 2002. 57(1): p. 27-36.
21.    Sasabe, H., et al., Differential involvement of multidrug resistance-associated protein 1 and P-glycoprotein in tissue distribution and excretion of grepafloxacin in mice. J Pharmacol Exp Ther, 2004. 310(2): p. 648-55.
22.    Mairinger, S., et al., Assessing the Activity of Multidrug Resistance-Associated Protein 1 at the Lung Epithelial Barrier. J Nucl Med, 2020.
23.    Yin, J.Y., et al., Characterization and analyses of multidrug resistance-associated protein 1 (MRP1/ABCC1) polymorphisms in Chinese population. Pharmacogenet Genomics, 2009. 19(3): p. 206-16.
24.    Semsei, A.F., et al., ABCC1 polymorphisms in anthracycline-induced cardiotoxicity in childhood acute lymphoblastic leukaemia. Cell Biol Int, 2012. 36(1): p. 79-86.
25.    Vulsteke, C., et al., Genetic variability in the multidrug resistance associated protein-1 (ABCC1/MRP1) predicts hematological toxicity in breast cancer patients receiving (neo-)adjuvant chemotherapy with 5-fluorouracil, epirubicin and cyclophosphamide (FEC). Ann Oncol, 2013. 24(6): p. 1513-25.
26.    Xiao, Q., et al., Germline variant burden in multidrug resistance transporters is a therapy-specific predictor of survival in breast cancer patients. Int J Cancer, 2020. 146(9): p. 2475-2487.
27.    Lima, A., et al., Pharmacogenomics of Methotrexate Membrane Transport Pathway: Can Clinical Response to Methotrexate in Rheumatoid Arthritis Be Predicted? Int J Mol Sci, 2015. 16(6): p. 13760-80.
28.    Drug Interaction Studies - Study Design, Data Analysis, Implications for Dosing, and Labeling Recommendations. U.S. Department of Health and Human Services, Food and Drug Administration, Center for Drug Evaluation and Research (CDER), 2012.