Unraveling the functional role of the orphan solute carrier, SLC22A24 in the transport of steroid conjugates through metabolomic and genome-wide association studies

Abstract

Variation in steroid hormone levels has wide implications for health and disease. The genes encoding the proteins involved in steroid disposition represent key determinants of interindividual variation in steroid levels and ultimately, their effects. Beginning with metabolomic data from genome-wide association studies (GWAS), we observed that genetic variants in the orphan transporter, SLC22A24 were significantly associated with levels of androsterone glucuronide and etiocholanolone glucuronide (sentinel SNPs p-value <1x10-30). In cells over-expressing human or various mammalian orthologs of SLC22A24, we showed that steroid conjugates and bile acids were substrates of the transporter. Phylogenetic, genomic, and transcriptomic analyses suggested that SLC22A24 has a specialized role in the kidney and appears to function in the reabsorption of organic anions, and in particular, anionic steroids. Phenome-wide analysis showed that functional variants of SLC22A24 are associated with human disease such as cardiovascular diseases and acne, which have been linked to dysregulated steroid metabolism. Collectively, these functional genomic studies reveal a previously uncharacterized protein involved in steroid homeostasis, opening up new possibilities for SLC22A24 as a pharmacological target for regulating steroid levels.

Reclassification of SLC22 Transporters: Analysis of OAT, OCT, OCTN, and other Family Members Reveals 8 Functional Subgroups

“…This subgroup also includes a large rodent-specific expansion, consisting of Slc22a19 and Slc22a26-30. Although the rodentspecific expansion is greatly understudied, transport data for rat Slc22a9/a24 shows shared substrate specificity for estrone sulfate with SLC22A24, but not for bile acids or glucuronidated steroids, which is consistent with the lack of glucuronides in rat urine and serum (61). While sulfatases are extremely highly conserved amongst humans, rats, and mice, the separation of rodent-and nonrodent-specific OATS4 groups is likely due to the species differences in expression and function of glucuronidases (63).…”

“…Multi-specific transporters, like those in the OATS1 and OCT subgroups, handle a diverse set of drugs, toxins, endogenous metabolites, and signaling molecules (14,75). Conversely, the OATS4 subgroup appears to be a collection of relatively mono-specific transporters with an affinity for conjugated sex steroid hormones, specifically etiocholanolone glucuronide (61). Previous evolutionary studies have suggested that multispecific transporters arose before the mono-specific transporters (10).…”

“…While Rst and Oat-pg do not share substrate specificity, together, they combine to play the role of URAT1 and OAT4 by handling uric acid and prostaglandins(59).GWAS analyses support the association of all human members of this subgroup with one common metabolite, etiocholanolone glucuronide, a conjugated sex hormone, with a p-value of 4.12x10-27 or lower for all members(Table 3,Supplementary Table 1)(60). While this group shares at least one conjugated sex hormone, SLC22A24 and SLC22A9 appear to be more oligospecific transporters, with SLC22A9 linked to short chain fatty acids and SLC22A24 linked to bile acids(61,62). SLC22A10 and SLC22A25 are only linked to conjugated sex hormones, making them relatively mono-specific transporters (Supplementary…”

“…where it is predicted to reabsorb these conjugated steroids (61). This subgroup also includes a large rodent-specific expansion, consisting of Slc22a19 and Slc22a26-30.…”

“…This region is predicted to govern substrate specificity of transporters of the MFS, to which the SLC22 family belongs (87). Recent publications defining the substrate specificity of SLC22A24 point to a more narrow range of substrates and conservation of this specific region amongst OATS4 members may explain the association of conjugated steroid hormones with SLC22A9, SLC22A10, SLC22A24, and SLC22A25 in GWAS studies (60,61). Although further analysis is required to fully understand the relationship between structure and substrate specificity in SLC22 transporters, we provided a basis for investigation into specific regions that may determine functional patterns.…”