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state through their regulation of the movement and concentrations of key endogenous metabolites (e.g., nutrients, vitamins, antioxidants, gut microbiome products, signaling molecules, and neurotransmitters). Indeed, a wide variety of metabolites (e.g., α‐ketoglutarate), signaling molecules (e.g., cGMP, odorants, conjugated sex steroids, prostaglandins, bile acids), dietary compounds (e.g., flavonoids, folate, pantothenate), antioxidants (e.g., urate), and molecules originating in the gut microbiome (e.g., indoxyl sulfate, hippurate) bind Oats, and many of these same molecules accumulate in the Oat1 and Oat3 knockouts [37, 120, 124].

      4.5.2 Metabolic Pathways Regulated by OATs

      As described above, metabolomics analyses of the plasma and urine of Oat knockout animals have been performed. These studies provide important information on the various endogenous metabolites potentially handled in vivo by the renal Oats and begin to link the Oats to important metabolic pathways, including carbohydrate, fatty acid, and amino acid metabolism [120, 124, 125]. Focusing on Oat1, the potential role of this transporter in metabolic and signaling pathways was investigated using computational approaches to contextualize in vivo metabolomics, as well as transcriptomic data from wild‐type and Oat1 knockout animals [37]. Recon 1, a genome‐scale reconstruction of human metabolism [147], was used to integrate these high‐throughput Oat1‐specific data sets. Multiple metabolic pathways were linked to Oat1 function. Among these were the citric acid cycle, pentose phosphate, cholate, polyamine, and fatty acid metabolism. In vitro and ex vivo analysis demonstrated interactions between Oat1 and some key intermediates in these metabolic pathways, including polyamines (e.g., spermine and spermidine). These metabolic reconstructions were later constrained with Oat1 knockout serum metabolomics data and supported with pharmacophore models, as well as in vitro transport assays. These improved studies revealed a role for Oat1 in fatty acid metabolism, as well as folate biosynthesis [148]. Similar analyses have also been conducted with Oat3, which linked the transporter to Phase I and Phase II drug metabolism, as well as flavonoids and antioxidants [142].

      Early studies with knockout mice were limited by the identification of a small number of metabolites. In recent years, global metabolic profiling covering a wide range of biochemical pathways, including but not limited to amino acid metabolism, lipid metabolism, and carbohydrate metabolism, has further revealed the role of Oats in general physiology. Tryptophan metabolism is regulated by both Oat1 and Oat3, and metabolic task analysis revealed that loss of Oat1 leads to increased biosynthetic pathways for tryptophan intermediates. Humans treated with probenecid, a prototypical OAT inhibitor, also had multiple tryptophan derivatives elevated in their serum shortly after treatment, suggesting that drug–metabolite interactions predicted by knockout mice can occur in humans [123].

      In addition to tryptophan metabolism, Oat1 has also been shown to mediate circulating levels of several lipid pathways, including elevated polyunsaturated fatty acids and diacylglycerols and decreased bile acids and ceramides [129]. These results were supported by a metabolic reconstruction using Recon3D, the latest genome‐scale metabolic reconstruction that has a greater representation of reactions related to lipid molecules [149]. The knockout mice metabolomics were also supported with metabolomics from probenecid‐treated animals, which demonstrated that drug–metabolite interactions can impact circulating levels of prostaglandins and fatty acids. Traditionally, Oat1 is not associated with lipid‐like molecules, yet in vitro studies by other groups have shown that OAT1 interacts with dicarboxylates, suggesting that this understudied aspect may be a pivotal function of OATs [150].

      Serum metabolomics of the Oat3 knockout mice have revealed that Oat3 is a key contributor to the gut–liver–kidney axis through the regulation of important signaling molecules, such as bile acids [151]. The gut–liver–kidney axis is mainly understood as an avenue for drug clearance, where the intestine aids in the absorption, the liver contributes to detoxification, and the kidney is mainly for excretion. Analyzing this multi‐organ process from an endogenous perspective reveals that several compounds, like bile acids and tryptophan derivatives, can also be modified by enzymatic processes in the liver for improved entry into the kidney. Once in the kidney, these metabolites can exert their signaling effects. The data support the idea that the OATs, known to be expressed in many tissues and primarily known for drug and toxin clearance, are integral to a number of endogenous metabolic pathways [37].

      4.5.3 Chronic Kidney Disease and Uremic Toxins

In addition to their clear pharmacological importance as mediators of drug disposition and pharmacokinetics, environmental toxins are also transported by the Oats, including ochratoxin A, aristolochic acid, perfluoroctanoic acid, and mercury conjugates [23, 24]. Furthermore, the OATs, particularly OAT1 and OAT3, appear to play a major role in the disposition of uremic toxins. Uremic toxins are small molecules that are elevated in the blood of patients with chronic kidney disease (CKD). Metabolomics data from Oat1 and Oat3 null mice demonstrate the accumulation of uremic toxins in the blood of mice lacking these transporters [120, 121,152–154] These studies have also noted the accumulation of uremic toxins derived from the gut microbiome, which is particularly relevant as the importance of mammalian–microbiome interactions in conditions including cancer, diabetes, obesity, inflammatory bowel disease, and autoimmune diseases becomes increasingly clear [155]. The inability to clear these compounds can also lead to pathophysiological states in remote organs, as the accumulation of indoxyl sulfate, a high‐affinity OAT1 and OAT3 substrate, has been associated with cardiovascular risk [156]. Thus, it is important to understand how disease states, as well as the administration of drugs cleared through OATs impact the circulating levels of uremic toxins.

      Interestingly, CKD not only leads to a progressive loss in the ability of the kidney to handle and eliminate drugs and metabolites, but it also alters the disposition of drugs and metabolites handled by non‐renal tissues, particularly the liver [157]. Indeed, renal and non‐renal Oat expression and function in CKD animal models has been shown to be altered [76]. Impairment of liver function, such as that caused by cholestasis, also alters drug handling and the expression of transporters in the kidney [158]. In support of this notion, studies using Oat knockout animals suggest a possible role for these transporters in blood pressure regulation, diabetic ketoacidosis, and hepatic steatosis [4, 48, 51, 129].

      4.5.4 Pathophysiology

      As described above, one disease thought to be a result of altered organic anion clearance is gout, which is associated with uric acid crystal deposition in the kidney (resulting in nephropathy) or within joints (resulting in an acutely painful inflammatory arthritis) caused by hyperuricemia [159]. The organic anion transporter URAT1 (SLC22A12), the human homolog of the gene first identified as Rst, is thought to transport uric acid across the apical proximal tubular cell membrane from the tubule lumen back into the cell [23, 24]. Accordingly, case‐control and cohort studies have suggested that loss of function polymorphisms on SLC22A12 are associated with hypouricemia, due to inefficient tubular reabsorption of uric acid [160]. Nevertheless, the molecular basis of renal urate handling in vivo remains poorly understood, with at least 10 genes, mostly transporters, suggested to be associated with hyperuricemia [160]. In addition, analysis of the Rst knockout animal found that multiple transporters, including Rst, Oat1, Oat3, and others, contribute to overall urate handling perhaps as a larger transporter network [51, 122, 161]. Finally, mutations in SLC22A12 have also been linked to hypouricemic hyperuricosuria, which can lead to exercise‐induced uric acid stones [162]. Interestingly, as renal function declines and transport by OATs and URAT1 is compromised, it seems that intestinal ABCG2 is upregulated to serve as another route for urate transport [163].

      Acute kidney injury (AKI) is a common and complex condition, especially for patients in intensive care units. Drug/toxicant‐induced renal toxicity and renal ischemia/reperfusion are the well‐recognized causes of AKI [46]. Renal ischemia/reperfusion often reduces glomerular filtration rate (GFR) and impairs tubular functions, such as secretion and reabsorption [164, 165]. In the kidneys of the ischemic rats, the expression levels of Oat1 and Oat3 mRNA and protein were both decreased [164, 166]. Anti‐inflammatory drugs meclofenamate, quercetin, and resveratrol

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