Organic cation transporter 3: expression in failing and nonfailing human heart and functional characterization.

The organic cation transporter 3 (OCT3, SLC22A3) contributes to the control of cardiac catecholamine concentrations and is important for the disposition and action of cationic drugs, such as metformin, in the myocardium. We sought to characterize the regulation of OCT3 in failing human hearts and to study commonly prescribed drugs for their potential to interact with OCT3-dependent uptake of metformin. SLC22A3 was expressed high in the human heart with strongest OCT3 immunoreactivity in vascular endothelial cells. SLC22A3/OCT3 expression was not changed in failing human left ventricular myocardium compared with nonfailing control tissues and thus is not involved in altered catecholamine homeostasis generally observed in failing hearts. Michaelis-Menten kinetics of OCT3-mediated uptake of prototypical OCT substrates 1-methyl-4-phenylpyridinium and metformin were studied in human embryonic kidney 293 cells stably overexpressing OCT3. The affinity of 1-methyl-4-phenylpyridinium for OCT3 was much higher (Km 157 ± 16 µM) than the affinity of metformin (Km 2.46 ± 0.36 mM; P < 0.01), whereas maximum transport rate of 1-methyl-4-phenylpyridinium was significantly lower than that of metformin. Verapamil, carvedilol, imipramine, and cimetidine were competitive inhibitors of OCT3-mediated metformin uptake (Ki 3.6-15.8 µM). Altogether, OCT3 might be important for the cardiac disposition of cationic drugs, and OCT3-dependent interaction with concomitantly administered compounds may limit their disposition and effect.

Functional characterization of the human organic cation transporter 2 variant p.270Ala>Ser.

The organic cation transporter 2 (OCT2, SLC22A2) plays an important role for renal drug elimination. Recent clinical studies indicate an impact of the frequent nonsynonymous c.808G>T (p.270Ala>Ser) polymorphism on renal clearance of metformin and the extent of the metformin-cimetidine interaction. The role of this polymorphism for renal disposition of endogenous compounds and drugs other than metformin has not been investigated. In addition, it is unclear whether the observed genotype dependence of an OCT2-mediated drug-drug interaction might occur also with other OCT inhibitors. To address these issues, we generated human embryonic kidney cells stably expressing wild-type OCT2 or the p.270Ala>Ser variant. No differences in protein expression levels and membrane incorporation pattern were observed between the two cell lines. The p.270Ala>Ser variant significantly impaired uptake kinetics of 1-methyl-4-phenylpyridinium, dopamine, norepinephrine, and propranolol. V(max) values were significantly reduced for uptake of all four compounds mediated by the p.270Ala>Ser variant compared with wild-type OCT2. In addition, a significant difference in the affinity to wild-type and mutant OCT2 was observed for dopamine (K(m) dopamine: 932 +/- 77 versus 1285 +/- 132 microM). Moreover, out of a set of 27 compounds p.270Ala>Ser OCT2 was significantly less sensitive to inhibition by cimetidine, flurazepam, metformin, mexiletine, propranolol, and verapamil than wild-type OCT2 (e.g., for propranolol: IC(50) wild type versus p.270Ala>Ser 189 versus 895 microM, P < 0.001). Our results indicate that the common OCT2 c.808G>T single nucleotide polymorphism significantly alters uptake of endogenous compounds and drugs. Moreover, for selected compounds the extent of OCT2-mediated drug interactions could depend on OCT2 c.808G>T genotype.

Structural determinants of inhibitor interaction with the human organic cation transporter OCT2 (SLC22A2).

The organic cation transporter 2 (OCT2) provides an important pathway for the uptake of cationic compounds in the kidney, which is the essential step in their elimination from the organism. Although many drugs have been identified which interact with human OCT2, structural elements required for an interaction with OCT2 are not well defined. To address this issue, HEK293 cells stably expressing human OCT2 were generated. IC(50) values of commonly used drugs for inhibition of [(3)H]MPP(+) uptake were determined and correlated with physicochemical descriptors. We found only a significant correlation between the topological polar surface area (TPSA) and IC(50) values (r = 0.71, p < 0.0001). Structural alignment of most potent inhibitor drugs of OCT2-mediated MPP(+) uptake was used to construct a two-point pharmacophore consisting of an ion-pair interaction site and a hydrophobic aromatic site separated by 5.0 A. Taken together, our data identify structural determinants for inhibitor interactions with OCT2.

Activation of negative regulators of the hypoxia-inducible factor (HIF) pathway in human end-stage heart failure.

The hypoxia-inducible transcription factor HIF is induced early in acute myocardial ischemia in humans, but it is unknown whether this activation of HIF persists during chronic heart failure. The HIF system was characterized in left ventricular myocardia from 18 explanted failing hearts and 11 non-failing donor hearts by quantitative RT-PCR and Western analysis. HIF-1alpha mRNA levels were significantly decreased while its natural antisense transcript aHIF was nearly twofold higher (p<0.01) in failing myocardia than in control hearts. Moreover, compared to donor hearts a significantly increased expression of HIF-3alpha, which may act as a competitive inhibitor of HIF-1/2alpha activity, and PHD3, which upon hydroxylation of prolyl residues directs HIF-alpha subunits towards proteasomal degradation, was observed in the failing myocardium. Although negative regulators of HIF were induced, the HIF pathway obviously remains activated in chronic human heart failure, because prototype HIF target genes, such as ABCG2, VEGF, and BNIP3, were significantly induced.

ATP-binding cassette transporters in human heart failure.

Adenosine triphosphate-binding cassette (ABC) transporters are involved in energy-dependent transport of substrates across biological membranes. We hypothesized that their expression is altered during human heart failure, suggesting a pathophysiologic basis. Messenger ribonucleic acid quantification of all known ABC transporters revealed multiple alterations in ABC transporter expression in failing human hearts (New York Heart Association classification III-IV) compared to nonfailing controls. These include a loss of ABCC7 chloride channels and an increased expression of the K(ATP) channel regulatory subunits ABCC8. Moreover, ABCG2, an efflux pump for xenobiotics/drugs, was expressed at much higher levels in failing hearts compared to nonfailing control hearts. ABCG2 was found in cardiac capillary endothelial cells and cardiomyocytes. Experiments in cells stably transfected with human ABCG2 revealed that the peroxisome proliferator-activated receptor-gamma agonist rosiglitazone was transported by ABCG2 but also inhibited the export of the prototypical ABCG2 substrate pheophorbide A (IC(50) 25 microM). These results suggest that altered ABC transporter expression in failing hearts might contribute to impaired channel conductance or might affect the cardiac disposition of drugs.

ATP-binding cassette transporters in the heart.

Members of the ATP-binding cassette (ABC) protein superfamily are integral membrane proteins involved in energy-dependent transport of a wide variety of substrates across biologic membranes. ATP-binding cassette transporters serve as functional barriers against the entry of xenobiotics, for example, in the intestine or at the blood-brain barrier, or contribute to drug excretion, for example, in the kidney or the liver. Many human ABC transporters, such as ABCB1 (P-glycoprotein), ABCC5 (MRP5), or ABCC9 (SUR2), are expressed in the heart, suggesting an important role of these transporters in cardiac drug effects or physiology. Interestingly, mutations in ABCC9, a constituent of cardiac K(ATP) channels, can cause dilated cardiomyopathy in humans, providing evidence that dysfunction of cardiac ABC transporters might have clinical implications. This review aims to give insights into the possible functions of ABC transporters in the heart, their role in drug disposition, as well as control of intracellular cyclic nucleotide levels or regulation of K(ATP) channel conductivity.