Multispecific Drug TransporterSlc22a8(Oat3) Regulates Multiple Metabolic and Signaling Pathways

Abstract

Multispecific drug transporters of the solute carrier and ATPbinding cassette families are highly conserved through evolution, but their true physiologic role remains unclear. Analyses of the organic anion transporter 3 (OAT3; encoded by Slc22a8/Oat3, originally Roct) knockout mouse have confirmed its critical role in the renal handling of common drugs (e.g., antibiotics, antivirals, diuretics) and toxins. Previous targeted metabolomics of the knockout of the closely related Oat1 have demonstrated a central metabolic role, but the same approach with Oat3 failed to reveal a similar set of endogenous substrates. Nevertheless, the Oat3 knockout is the only Oat described so far with a physiologically significant phenotype, suggesting the disturbance of metabolic or signaling pathways. Here we analyzed global gene expression in Oat3 knockout tissue, which implicated OAT3 in phase I and phase II metabolism (drug metabolizing enzymes or DMEs), as well as signaling pathways. Metabolic reconstruction with the recently developed "mouse Recon1" supported the involvement of Oat3 in the aforementioned pathways. Untargeted metabolomics were used to determine whether the predicted metabolic alterations could be confirmed. Many significant changes were observed; several metabolites were tested for direct interaction with mOAT3, whereas others were supported by published data. Oat3 thus appears critical for the handling of phase I (hydroxylation) and phase II (glucuronidation) metabolites. Oat3 also plays a role in bioenergetic pathways (e.g., the tricarboxylic acid cycle), as well as those involving vitamins (e.g., folate), steroids, prostaglandins, gut microbiome products, uremic toxins, cyclic nucleotides, amino acids, glycans, and possibly hyaluronic acid. The data seemingly consistent with the Remote Sensing and Signaling Hypothesis (Ahn and Nigam, 2009;Wu et al., 2011), also suggests that Oat3 is essential for the handling of dietary flavonoids and antioxidants.

Handling of Drugs, Metabolites, and Uremic Toxins by Kidney Proximal Tubule Drug Transporters

“…Signaling molecules, such as cyclic nucleotides, prostaglandins, odorants, and conjugated steroids, are also eliminated via the OATs and other SLC and ABC drug transporters. Recent systems biology and omics integration of metabolomics and transcriptomics data from Oat knockout mice suggests that OATs and possibly all multispecific drug transporters play a role in the regulation of systemic and tissue metabolic and signaling processes (1,7,8,65,67,69). This type of information has led to the remote sensing and signaling hypothesis discussed below (65,69).…”

“…Metabolomic studies in knockout animals, particularly Oat1 and Oat3 knockout mice, have confirmed a central role for drug transporters in the transport of many important metabolites and signaling molecules (8,14,61,67,68). These include a-ketoglutarate, which plays a central role in the Krebs cycle (tricarboxylic acid cycle); vitamins; molecules with antioxidant properties (e.g., urate and flavonoids); and the gut microbial derivatives already described.…”

“…In Oat1 knockout mice, some of these organic anions accumulate (61), although the mice do not appear ill and have normal life spans (14). Putative uremic toxins are also affected to varying degrees in Oat3 knockout mice and OATP4C1 transgenic rats (8,62,63). Uremic toxins interact with other SLC and ABC transporters as well.…”

“…One of the primary renal transporters involved appears to be OAT1, which transports several gut microbial products or their metabolites (61,67). OAT1, OAT3, and other SLC and ABC transporters are also necessary for the elimination of dietary natural products, including a wide range of flavonoids, as well as vitamins, and thus might indirectly regulate vitamindependent metabolic pathways (8,67).…”

“…These include drugs (e.g., antibiotics, antivirals, diuretics, nonsteroidal anti-inflammatory drugs, and antidiabetic agents), physiologically important metabolites (e.g., folate, a-ketoglutarate, urate, and carnitine), nutrients (e.g., vitamins and flavonoids), signaling molecules (e.g., odorants, cyclic nucleotides, and prostaglandins), exogenous toxins (e.g., mercurial conjugates and aristolochic acid), gut microbiome products (e.g., kynurenine), and endogenous toxins (socalled uremic toxins, such as indoxyl sulfate) (1)(2)(3)(4)(5)(6)(7). Apart from excreting unmodified small molecule drugs, the kidney handles many conjugated metabolites, most of which are produced by phase 1 and phase 2 metabolism in the liver (e.g., products of hydroxylation, sulfation, and glucuronidation reactions) (8). Genes for phase 1 and 2 reactions are also expressed in the kidney and are likely to be very important in metabolic functions of the proximal tubule cells of kidney as well (9), although this area of research is underexplored.…”