have overlapping expression in certain tissues (i.e., brain, whole blood, and bone marrow) suggesting complementary roles in the regulation of the levels of ergothioneine, which is a substrate for both transporters. Inhibition studies of SLC22A15‐mediated ergothioneine uptake have identified various inhibitors for SLC22A15, including the antibiotic levofloxacin that also inhibits OCT1. Other notable inhibitors include gabapentin, tryptophan, ondansetron, hypaphorine and hercynine (both containing a similar backbone to ergothioneine), acetylcarnitine, propionylcarnitine, quinidine, trazadone, fluoxetine, donepezil, verapamil, chloroquine, and primaquine [79]. Some of these compounds overlap with known inhibitors of OCTN1, including quinidine and chloroquine. SLC22A15 has also been shown to efflux carnitine and can mediate the transcellular influx of ergothioneine, carnosine, carnitine, and creatine in a sodium‐dependent manner.
2.10.3 Human Genetic Studies
Several polymorphisms in or near SLC22A15 have been associated with monoacylglycerol levels (e.g., 1‐palmitoleoylglycerol) and triacylglycerol levels (e.g., TAG 52:1, TAG 48:2) [100, 101], as well as with fat‐related traits (e.g., body mass index, trunk fat mass, whole body fat mass) [102] in genome‐wide association studies. These associations are consistent with the functional role of SLC22A15 in the influx and efflux of carnitine and its derivatives. GWAS have also shown an association of polymorphisms in the SLC22A15 locus with neurological disorders, including autism and child developmental disorders. This is particularly interesting, given that SLC22A15 transports the antioxidant carnosine, which has also been noted as therapeutically beneficial in individuals diagnosed with autism and other neurological diseases [103].
Gene regulation of SLC22A15 is largely understudied. The transcription factor Yin Yang 1 (YY1) enhances SLC22A15 expression in colorectal cancer [104]. However, the identification of other transcription factors or regulatory mechanisms involved in the expression of SLC22A15 have not been identified to date.
2.11 SLC22A16 (FLIPT2, CT2, OCT6)
2.11.1 Introduction
SLC22A16 (also referred to as CT2 and its splice variant Flipt2/OCT6) was identified alongside SLC22A15 in 2002 through phylogenetic analyses [76, 77]. As homologs of OCTN1 and OCTN2, both transporters were suggested to play a role in carnitine transport, but no in vitro evidence identified actual substrates. Since then, limited studies have investigated CT2.
2.11.2 Substrate and Inhibitor Selectivity
Limited information has been obtained on the transport function of CT2. In vitro, CT2 has been shown to be a bidirectional transporter of carnitine, its derivatives, and betaine; it appears to play a role in the transport of these compounds in the testes [76]. In addition, CT2 mediates the transport of the chemotherapeutic drug, doxorubicin [1], and the anticancer polyamine analogue bleomycin‐A5 in cancer cells [105].
2.11.3 Regulation
Little is known about the regulatory factors of CT2; however, high expression is observed in a number of cancer cell lines. SLC22A16 mRNA levels are increased in endometrial cancer cell lines in the presence of progesterone [106], and CT2 is highly expressed in acute myeloid leukemia cells [107].
2.11.4 Human Genetic Studies
Multiple polymorphisms have been identified in SLC22A16 (Table 2.3), and one (c.146A>G) may contribute to interindividual pharmacokinetic variation of doxorubicin and doxorubicinol in Asian breast cancer patients [108]. Additionally, a GWAS investigating metabolite levels in whole blood identified polymorphisms in SLC22A16 that were associated with levels of two long‐chain acylcarnitines [87].
2.12 OCTN3
2.12.1 Introduction
Octn3 (encoded by Slc22a21) is the third member of the OCTN family. Octn3 was identified and has been studied primarily in mice. Octn3 has been detected in human skin and sperm using antibody against the mouse homolog but is not yet annotated in the human genome. In mice, Octn3 is found in the kidney, small intestine, and testes. Octn3 has also been identified in brain astrocytes that undergo fatty acid oxidation, and microscopy has demonstrated localization to the peroxisomal membrane [109]. Octn3 has about 33% homology to the human CT2 transporter (encoded by SLC22A16) that is also expressed primarily in the testes. Out of the three OCTN transporters, Octn3 has the highest specificity for carnitine [1].
2.12.2 Substrate and Inhibitor Selectivity
Octn3 transports carnitine independent of sodium, with a K m of 3 μM in reconstituted proteoliposomes containing ATP [110]. Carnitine transport is stimulated by ATP and is largely pH independent. Octn3 also transports acetylcarnitine. Significant inhibition of Octn3‐mediated carnitine transport has been observed by acetylcholine, TEA, butyric acid, GABA, betaine, butyrobetaine, acetylcarnitine, and palmitoylcarnitine. The chicken ortholog of Octn3 exhibits similar characteristics, demonstrating sodium‐independent carnitine transport in enterocytes and basolateral membrane vesicles (BLMV) isolated from chicken intestinal epithelia [111].
2.12.3 Regulation
In mouse, rat, and chicken enterocytes, Octn3 is located at the basolateral membrane, contrary to the localization of OCTN2 to the apical membrane [1]. Octn3 transcription in mice is mediated by PPARα in the kidney and small intestine, but not the testes [82]. Octn3 expression is dependent on binding of the zinc finger transcription factor Sp1 in mice; the disruption of this site suppresses transcription [1].
2.13 CONCLUSION
To understand physiologic and pharmacologic systems, it is critical to identify all of the components or proteins involved in those systems. The last few decades have ushered in a new understanding of the physiologic and pharmacologic roles of important zwitterions and organic cations, as the transporters involved in their absorption and disposition have been identified. It is now clear that transporters in the SLC22 family along with a few other transporters play key roles as determinants of systemic and tissue levels of cationic and zwitterionic drugs. At all levels from molecular to physiologic and pathophysiologic, there are major gaps in our knowledge. First and foremost, transporters for organic cations and zwitterions need to be discovered. With the recent deorphaning of SLC22A15, a key zwitterion transporter was identified in the human genome; however, many transporters remain orphans in the SLC superfamily and, in particular, in the SLC22 family. These transporters need to be deorphaned. Further, no transporter in the SLC22 family has been crystallized; therefore, the precise molecular transport mechanism is not known. Moreover, though many associations have been reproducibly observed between genetic variants in organic cation and zwitterion transporters and various clinical phenotypes, the mechanisms by which the transporter contributes to the phenotypes remain poorly understood. Rare variants in the transporters such as in SLC22A5 (OCTN2) are associated with fatal diseases, yet the function of these variants remain unknown and therapies remain poor at best. Finally, the physiologic, pharmacologic, and pathophysiologic systems that include these transporters need to be fully understood in order to obtain a full understanding of human biology and pharmacology.
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