for hMATE1 and hMATE2‐K [41, 42]. Notably, transport of these drugs tends to be higher capacity, lower affinity, and with moderate stereoselectivity compared with OCT‐type transporters [41]. Dual expression of hOCT2 and hMATE1 in MDCK cells enabled the transepithelial transport of atenolol compared to control cells [43]. These data suggest that this transporter system is responsible for the renal secretion of atenolol.
3.3.2.4 Alkaloids
Alkaloid drugs are used clinically for their anticholinergic properties for diseases such as chronic obstructive pulmonary disease, asthma, ocular disorders, and gastrointestinal illness. In particular, the quaternary amine derivatives of alkaloids, namely ipratropium and trospium, are transported by MATE transporters. The transcellular transport of ipratropium and trospium is accomplished by hMATE1 in combination with either hOCT1 or hOCT2 in MDCKII cells [44, 45]. These studies may provide insights into the hepatic and/or renal clearance of alkaloid‐type drugs.
3.3.2.5 Paraquat
Paraquat is a divalent cation and herbicide widely used across the globe. It is highly toxic to the kidneys, as well as the liver and lungs. Paraquat is excreted via glomerular filtration and tubular secretion [46]. Renal uptake of paraquat is presumed to occur via OCT2/Oct2 on the basolateral membrane [47]. hMATE1 and rMate1 demonstrate time‐dependent in vitro uptake of paraquat [47]. When expressed in proteoliposomes, hMATE1 is capable of transporting paraquat, but interestingly paraquat/proton exchange is stimulated by inside‐negative membrane potential and inhibited by inside‐positive membrane potential, which contrasts the electroneutral transport observed with prototypical substrates such as TEA [48]. Overexpression of hMATE1 not only enhances paraquat uptake but also heightens cytotoxicity by over 5‐fold compared with control cells [47]. In mice lacking Mate1, paraquat accumulates in plasma, lung, and kidneys to a greater extent than in wild‐type mice [33]. As a result, Mate1‐knockout mice exhibit enhanced susceptibility to paraquat‐induced nephrotoxicity [33]. While paraquat is a unique pesticide whose disposition and toxicity are dependent upon Mate activity, other classes of pesticides appear to have minimal interaction [49].
3.3.2.6 Endogenous Molecules
Endogenous molecules have been identified as substrates of MATE/Mate transporters. For example, the initial development of Mate1 knockout mice confirmed the in vitro evidence that this transporter can efflux creatinine. Baseline levels of creatinine within the blood of Mate1 knockout mice are elevated [32]. N 1‐methyladenosine, which is a metabolite of niacin or nicotinamide, is a substrate of MATE2‐K in vitro [50]. Treatment of mice with a Mate inhibitor increased plasma concentrations of N 1‐methyladenosine in a time‐dependent manner and reduced its overall renal clearance [50]. The cationic neurotransmitter dopamine is also a substrate of hMATE1, hMATE2‐K, and mMate1 in vitro. in vivo, mice lacking mMate1 have reduced urinary excretion of dopamine. Moreover, the ability of renally synthesized dopamine to cause natriuresis and diuresis is absent in Mate1 knockout mice [51]. These data may be relevant for identifying endogenous biomarkers of OCT/MATE function in drug interaction and pharmacogenetic studies (see below).
3.3.3 Inhibitors
MATE transport can be inhibited by a number of chemicals. Structural examples are shown in Fig. 3.2. The antiparasitic drug, pyrimethamine, is considered one of the most potent and specific inhibitors. Inhibition of mMate1/hMATE1 is achieved at nanomolar concentrations of pyrimethamine, whereas micromolar concentrations are required to block mOct1/hOCT2 and mOct2/hOCT2 [52]. Cis‐inhibition of TEA transport by hMATE1 is observed with cimetidine, quinidine, procainamide, and verapamil and to a lesser degree nicotine, serotonin, and choline [5, 6]. Likewise, a number of hormones can inhibit TEA transport in hMATE1‐ and mMate1‐expressing cells including corticosterone, testosterone, progesterone, and androstenedione [15]. Notably, no effects on hMATE1 or hMATE2‐K activity by organic anions such as p‐aminohippurate, uric acid, and beta‐lactam antibiotics were observed [5, 6].
3.4 STRUCTURE
3.4.1 Modeling of Ligand Interactions
A variety of experimental approaches, including Bayesian machine learning, binary classification modeling, molecular docking and pharmacophore modeling, have been employed to define physicochemical and structural properties of MATE ligands. Inhibitors of MATE1 have been defined by their cationic charge, high molecular weight, and lipophilicity [53]. Using a combination of in vitro and computational approaches, a pharmacophore for hMATE1 and hMATE2‐K inhibitors was defined that favors shared features of high‐affinity inhibitors (such as pyrimethamine and quinidine) and avoids structures observed in low‐affinity inhibitors (such as histamine, caffeine, and chloramphenicol) [54]. Refinement of the model resulted in a pharmacophore for MATE inhibition that included two hydrophobes, a hydrogen‐bond acceptor, and an ionizable feature [54]. A subsequent combinatorial pharmacophore model for hMATE1 inhibition predicted multiple sites for ligand interaction. These included two regions for competitive inhibition by smaller molecules, as well as accommodation of large inhibitors within the central cavity of the transporter where they act noncompetitively to prevent the conformational changes needed for organic cation transport [55].
Transporter‐based drug–drug interactions can often be dependent upon the identity of the victim substrate being evaluated, as has been demonstrated for OCT [56, 57]. Interestingly, while some early studies with a small number of substrates suggested this may be the case for MATE transporters [58], a more robust subsequent analysis has suggested otherwise. Using four structurally distinct organic cation substrates (metformin, cimetidine, MPP, and N,N,N‐trimethyl‐2‐[methyl(7‐nitrobenzo[c][1,2,5]oxadiazol‐4‐yl)amino]ethanaminium iodide) and over 400 drugs, it was determined that the identity of the substrate has little influence on ligand interaction and inhibition of hMATE1‐mediated uptake of substrates in overexpressing cells [59]. Similar results were observed in a separate study using metformin and atenolol as substrates [60]. Notably, there were no significant differences in the apparent Michaelis constant of the transported substrate and the 50% inhibitor concentration calculated from the inhibition of transport across the four substrates. These data would suggest that there is a shared binding site for interaction of substrates and inhibitors on the external surface of hMATE1.
3.4.2 Secondary Structure
With the original cloning of hMATE1, it was predicted that the secondary structure contained 12 transmembrane helices with internal‐facing NH2‐ and COOH‐termini [5]. Subsequent predictions pointed to a 13th transmembrane domain for hMATE and rbMate1 that would result in extracellular localization of the carboxy‐terminus. This additional domain was not anticipated for mMate1 protein and instead the additional amino acids were thought to form a long cytoplasmic COOH terminus [20]. These predictions were supported by experimental evidence that showed extracellular accessibility of the carboxy‐terminus of rbMate1, but not mMate1 [20]. Epitope tagging and cysteine accessibility scanning affirmed that rbMate1 protein includes 13 transmembrane domains with an intracellular NH2‐terminus and extracellular COOH‐terminus (Fig. 3.3) [61]. Subsequently, a functionally active variant of mMate1, mMate1b, was identified and revealed to contain a long hydrophobic tail, similar to other MATE transporters. This region of mMate1b encodes a 13th transmembrane domain that results in an extracellular carboxy terminus [62]. The exact purpose of the 13th transmembrane domain has remained unclear as the first 12 domains form the “functional core” of the rbMate1, hMATE1, and mMate1b orthologs [63]. Truncated mutant forms of MATE1/Mate1 proteins retain functional activity, ligand binding, and multi‐selectivity of substrates, leading researchers to posit other potential roles for this domain in substrate translocation, stabilization in the membrane bilayer, oligomerization, or protein–protein interactions [61, 62].
Using