GWAS, genetic polymorphisms in the OCTN2 locus have strong associations with blood metabolite levels (carnitine, acetylcarnitine, LDL cholesterol, and fatty acids), fat‐free mass, and autoimmune disease (e.g., Crohn’s disease). Forming a haplotype with OCTN1 at the IBD5 locus, OCTN2 is associated with Type 1 diabetes and Crohn’s disease [2]. See Section 2.8.4 for details.
2.10 SLC22A15
2.10.1 Introduction
SLC22A15 was identified in 2002 through phylogenetic analyses and the use of basic alignment for peptides at a protein sequence similarity threshold of 35–40% [77]. This gene is homologous to the drosophila transporter, Orct, and was dubbed “Fly‐like putative transporter” (FLIPT1). To date, SLC22A15 has only been studied in vitro and no mouse models exist.
2.10.2 Substrate and Inhibitor Selectivity
Efforts to de‐orphan SLC22A15 have elucidated its function as a transporter. SLC22A15 preferentially transports the zwitterions ergothioneine and carnosine, two potent antioxidants found in the brain and other tissues [79]. Compared with the other zwitterion transporters in the SLC22A family, SLC22A15 exhibits a higher K m (lower affinity) for its substrates, including ergothioneine, carnosine, and creatine. The trend also holds for carnitine, which is transported by SLC22A15 with a much higher K m (99 μM) compared with other OCTN subclade family members. This transporter shows a weak interaction with organic cations TEA, MPP+, and cimetidine. SLC22A15 is therefore thought to be a determinant of its substrate levels at high substrate concentrations and may be co‐expressed with other SLC transporters that have higher affinity for the same substrates. For example, OCTN1 and SLC22A15 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].
