inhibitors. More details will be provided in the next section.
2.3 OCT1
2.3.1 Substrate and Inhibitor Selectivity
Compounds that are commonly transported by hOCT1 include the model cations MPP+, TEA, tetrapropylammonium (TPrA), tetrabutylammonium (TBuA), N‐methylquinine, and N‐(4.4‐azo‐n‐pentyl)‐21‐deoxyajmalinium, the endogenous compounds choline, acetylcholine and agmatine, and the drugs quinidine, quinine, acyclovir, ganciclovir, metformin, sumatriptan, ondansetron, morphine, and several anticancer agents, e.g., anthracyclines (Table 2.2) [16]. Both human and mouse OCT1 are high‐capacity thiamine (vitamin B1) transporters that respond to the uptake of dietary thiamine to liver [18]. The series of n‐tetraalkylammonium (nTAA) compounds has been shown to have different affinity among human, rabbit, rat, and mouse OCT1. While the larger nTAAs are transported at greater rate in hOCT, the smaller nTAAs are transported at greater rate in rOCT1 or mOCT1 [19]. It is suggested that molecular mass or hydrophobicity may affect differences in recognition of OCT substrates across species. In terms of inhibition, some inhibitors of OCTs show differences in potency among the individual subtypes. For example, the inhibition potency of phencyclidine, diphenhydramine, prazosin, citalopram, and atropine is greater for hOCT1 compared with hOCT2 and hOCT3. In contrast, corticosterone shows stronger inhibition on hOCT3 than OCT1 [1]. Besides influx transport, OCT1 has been demonstrated as an efflux transporter of acylcarnitine from liver to plasma [20].
2.3.2 Regulation
In human, OCT1 is predominantly expressed in hepatocytes. Thus, the SLC22A1 gene is suggested to be regulated by the liver‐enriched transcription factors, such as hepatocyte nuclear factor 4a (HNF4a), CCAAT/enhancer binding proteins α and β, hepatocyte nuclear factor 1a (HNF1a) and 3c (HNF3c). Two co‐operating HNF4a response elements have been identified between nucleotides ‐1479 and ‐1441 of the 5′‐flanking regions of the SLC22A1 gene, which are upstream of the transcription initiation sites. In electrophoretic mobility shift assay (EMSA), recombinant HNF4a directly interacts with both sites. Mutation of these sites in SLC22A1 promoter luciferase reporter constructs abolish transactivation [21]. These motifs are conserved in primates, but not rodents, indicating different patterns of SLC22A1/Slc22a1 gene regulation in these species [22]. In addition, upstream stimulating factors USF1 and USF2 have been identified to regulate basal hepatic expression of OCT1 via a cognate E‐box [22]. OCT1 expression can be modulated by ligand‐dependent nuclear receptors such as pregnane X receptor, farnesoid X receptor, constitutive androstane receptor, glucocorticoid receptor or peroxisome proliferator‐activated receptor α and γ. In addition, the transcription expression level of OCT1 can be regulated by epigenetic methylation [2]. Moreover, OCT1 can be subject to post‐translational modulation. Stimulation of either protein kinase A(PKA) or PKC increases uptake of the fluorescent compound ASP+ in HEK‐293 cells stably transfected with rat OCT1 [1]. However, this effect may be species‐dependent [23]. As with rOCT1, hOCT1 appears to be positively regulated by the p56lck tyrosine kinase, as evidenced by reduced hOCT1 activity after treatment with aminogenistein. Human OCT1 has further been shown to be regulated by the Ca2+/calmodulin complex, which appears to affect the affinity of the tested substrates [23], possibly due to phosphorylation of the OCT1 protein.
FIGURE 2.2 (a) Alternating‐access transport model for translocation of substrates by organic cation and zwitterion transporters. (b) Types of transport mechanisms for organic cation and zwitterion transporters. Created with BioRender.com.
2.3.3 Animal Models
In vivo studies in mice in which individual transporters have been removed genetically (knockout (KO) mice) provide valuable insights in potential physiologic and biomedical functions of OCTs. However, species differences between OCTs of humans and rodents impose limitations on the ability to apply conclusions obtained from the mouse experiments to humans. Oct1 knockout mice are viable and fertile, with no obvious physiological abnormalities when compared with their wild‐type littermates, suggesting that Oct1 has minimal impact on normal physiology [24]. However, at the biochemical level, disruption of OCT1 in mice affects both lipid and glucose metabolism by reducing the hepatic uptake of thiamine [2]. Additionally, disruption of Oct1 in mice has significant impact on the disposition of organic cations. For example, when administered the prototypical organic cation, TEA, Oct1 −/− mice show significantly reduced uptake of TEA into the liver. In accordance with reduced hepatic uptake of TEA, biliary excretion is lower in Oct1 −/− mice [24]. Additionally, direct intestinal excretion of TEA is reduced by approximately 50%. In addition to TEA, Oct1 −/− mice have similar decreases in hepatic uptake of other OCT1 substrates (i.e., MPP+ and meta‐iodobenzylguanidine (MIBG)) [24].
In addition to pharmacokinetic effects, knockout of Oct1 in mice can have implications for prescription drugs, exemplified by metformin, an anti‐hyperglycemic prescription medication used as a first‐line treatment for Type 2 diabetes. Despite similar pharmacokinetic profiles between Oct1 −/− and wild‐type mice, Oct1 −/− mice showed greater than 30‐fold decrease in metformin uptake into liver, the site of action for metformin, compared with wild‐type littermates [25]. Further studies investigated the role of Oct1 in the development of metformin‐induced lactic acidosis, a leading toxicity from this drug. A significant increase in serum lactic acid concentration was observed after administration of metformin to wild‐type mice, but only slight elevations in serum lactate were seen in Oct1 −/− mice [26]. Taken together, these results suggest that Oct1‐mediated metformin transport is a limiting step in metformin uptake into liver, and that the lactic acidosis induced by metformin is related to the availability of the drug to its target organ. Recent studies have demonstrated large effects of knocking out Oct1 on the hepatic uptake and clearance of sumatriptan and fenoterol, and lesser effects on ondansetron [27].
TABLE 2.2 Selected substrates and inhibitors of the major organic cation and zwitterion transporters, OCT1‐3 and OCTN1‐2 [6, 16, 17]
| Transporter | Model substrates | Substrates | Model inhibitors | Inhibitors |
|---|---|---|---|---|
| OCT1 | MPP+, TEA, ASP+, metformin | Endogenous: serotonin, acylcarnitines, choline, acetylcholine, creatinine, agmatine, thiamine Exogenous: acyclovir, quinidine, quinine, thiamine, sumatriptan, ondansetron, morphine | Quinidine, verapamil | Exogenous: atropine, abacavir, tenofovir, zidovudine, spironolactone, ondansetron, quinine, midazolam |
| OCT2 | TEA, ASP+, MPP+, NBD‐MTMA, metformin | Endogenous: creatinine, choline, serotonin, dopamine, histamine Exogenous: amphetamine, cisplatin, cimetidine, phenformin | Quinidine, cimetidine | Endogenous: testosterone Exogenous: doxepin, zolpidem, ritonavir, imipramine, tramadol, tacrine, olanzapine |
| OCT3 | MPP+, ASP+, metformin |
|
