Role of Mouse Organic Anion Transporter 3 (mOat3) as a Basolateral Prostaglandin E2 Transport Pathway

Functional analysis of mutant SLCO2A1 transporters found in patients with chronic enteropathy associated with SLCO2A1

BACKGROUND AND AIM

Chronic enteropathy associated with the solute carrier organic anion transporter family member 2A1 (SLCO2A1), or CEAS, causes anemia and hypoalbuminemia in young people. Dysfunction of the SLCO2A1 transporter protein is thought to involve genetic mutation, but mutant proteins have not been functionally characterized. We examined the prostaglandin E₂ (PGE₂ ) transport ability of recombinant SLCO2A1 proteins containing 11 SLCO2A1 mutations found in CEAS patients.

METHODS

Wild-type and mutant SLCO2A1 proteins were forcibly expressed in Xenopus laevis oocytes, and measurements of PGE₂ uptake and transport capacity were compared. The membrane protein topology and functionality of the eight SLCO2A1 mutations involving single-nucleotide substitutions were predicted using computer analysis.

RESULTS

The extent of functional disruption of the 11 SLCO2A1 mutations identified in CEAS patients was variable, with 10 mutations (421GT, 547GA, 664GA, 770GA, 830dupT, 830delT, 940 + 1GA, 1372GT, 1647GT, and 1807CT) resulting in loss or reduction of PGE₂ transport, excluding 97GC.

CONCLUSION

PGE₂ transport ability of recombinant SLCO2A1 in X. laevis oocytes was hindered in 10/11 SLCO2A1 mutations identified in patients with CEAS. Further studies on the relationships between the different mutations and PGE₂ transport and clinical features, such as severity, are needed.

Prostaglandin E2 induces chloride secretion through crosstalk between cAMP and calcium signaling in mouse inner medullary collecting duct cells

Under conditions of high dietary salt intake, prostaglandin E2 (PGE2) production is increased in the collecting duct and promotes urinary sodium chloride (NaCl) excretion; however, the molecular mechanisms by which PGE2 increases NaCl excretion in this context have not been clearly defined. We used the mouse inner medullary collecting duct (mIMCD)-K2 cell line to characterize mechanisms underlying PGE2-regulated NaCl transport. When epithelial Na(+) channels were inhibited, PGE2 exclusively stimulated basolateral EP4 receptors to increase short-circuit current (Isc(PGE2)). We found that Isc(PGE2) was sensitive to inhibition by H-89 and CFTR-172, indicating that EP4 receptors signal through protein kinase A to induce Cl(-) secretion via cystic fibrosis transmembrane conductance regulator (CFTR). Unexpectedly, we also found that Isc(PGE2) was sensitive to inhibition by BAPTA-AM (Ca(2+) chelator), 2-aminoethoxydiphenyl borate (2-APB) (inositol triphosphate receptor blocker), and flufenamic acid (FFA) [Ca(2+)-activated Cl(-) channel (CACC) inhibitor], suggesting that EP4 receptors also signal through Ca(2+) to induce Cl(-) secretion via CACC. Additionally, we observed that PGE2 stimulated an increase in Isc through crosstalk between cAMP and Ca(2+) signaling; BAPTA-AM or 2-APB inhibited a component of Isc(PGE2) that was sensitive to CFTR-172 inhibition; H-89 inhibited a component of Isc(PGE2) that was sensitive to FFA inhibition. Together, our findings indicate that PGE2 activates basolateral EP4 receptors and signals through both cAMP and Ca(2+) to stimulate Cl(-) secretion in IMCD-K2 cells. We propose that these signaling pathways, and the crosstalk between them, may provide a concerted mechanism for enhancing urinary NaCl excretion under conditions of high dietary NaCl intake.

Sex-dependent expression of Oat3 (Slc22a8) and Oat1 (Slc22a6) proteins in murine kidneys

In the mouse kidney, organic anion transporter 3 (mOat3, Slc22a8) was previously localized to the basolateral membrane (BLM) of proximal tubule (PT), thick ascending limb of Henle, macula densa, distal tubule, and cortical collecting duct. However, the specificity of anti-Oat3 antibodies (Oat3-Ab) used in these studies was not properly verified. Moreover, the sex-dependent expression of mOat3, and of the functionally similar transporter mOat1 (Slc22a6), in the mouse kidney has been studied at mRNA level, whereas their protein expression is poorly documented. Here we investigated 1) specificity of Oat3-Abs by using Oat3 knockout (KO) mice, 2) cell localization of renal mOat3 with a specific mOat3-Ab, 3) sex-dependent expression of renal mOat3 and mOat1 proteins, and 4) hormone(s) responsible for observed sex differences. As previously shown, an Oat3-Ab against the rat protein stained the BLM of various nephron segments in wild-type (WT) mice, but the same staining pattern was noted along the nephron of Oat3 KO mice. However, the mOat3-Ab exclusively stained the BLM of PT in WT mice, where it colocalized with the mOat1 protein, whereas no staining of Oat3 protein was noted in the kidney of Oat3 KO mice. The expression of mOat3 protein was lower in male mice, upregulated by castration, and downregulated by testosterone treatment. The expression of mOat1 protein was stronger in males, downregulated by castration, and upregulated by testosterone treatment. Thus, at the protein level, mOat3 and mOat1 exhibit sex-dependent expression with an opposite pattern; mOat3 is female dominant due to androgen inhibition, while mOat1 is male dominant due to androgen stimulation.

Role of the blood-cerebrospinal fluid barrier transporter as a cerebral clearance system for prostaglandin E₂ produced in the brain

An increasing level of prostaglandin (PG) E(2) is involved in the progression of neuroinflammation induced by ischemia and bacterial infection. Although an imbalance in the rates of production and clearance of PGE(2) under these pathological conditions appears to affect the concentration of PGE(2) in the cerebrospinal fluid (CSF), the regulatory system remains incompletely understood. The purpose of this study was to investigate the cellular system of PGE(2) production via microsomal PGE synthetase-1 (mPGES-1), the inducible PGE(2) -generating enzyme, and PGE(2) elimination from the CSF via the blood-CSF barrier (BCSFB). Immunohistochemical analysis revealed that mPGES-1 was expressed in the soma and perivascular sheets of astrocytes, pia mater, and brain blood vessel endothelial cells, suggesting that these cells are local production sites of PGE(2) in the CSF. The in vivo PGE(2) elimination clearance from the CSF was eightfold greater than that of d-mannitol, which is considered to reflect CSF bulk flow. This process was inhibited by the simultaneous injection of unlabeled PGE(2) and β-lactam antibiotics, such as benzylpenicillin, cefazolin, and ceftriaxone, which are substrates and/or inhibitors of organic anion transporter 3 (OAT3). The characteristics of PGE(2) uptake by the isolated choroid plexus were at least partially consistent with those of OAT3. OAT3 was able to mediate PGE(2) transport with a Michaelis-Menten constant of 4.24 μM. These findings indicate that a system regulating the PGE(2) level in the CSF involves OAT3-mediated PGE(2) uptake by choroid plexus epithelial cells, acting as a cerebral clearance pathway via the BCSFB of locally produced PGE(2) .

A role for the organic anion transporter OAT3 in renal creatinine secretion in mice

Tubular secretion of the organic cation, creatinine, limits its value as a marker of glomerular filtration rate (GFR) but the molecular determinants of this pathway are unclear. The organic anion transporters, OAT1 and OAT3, are expressed on the basolateral membrane of the proximal tubule and transport organic anions but also neutral compounds and cations. Here, we demonstrate specific uptake of creatinine into mouse mOat1- and mOat3-microinjected Xenopus laevis oocytes at a concentration of 10 μM (i.e., similar to physiological plasma levels), which was inhibited by both probenecid and cimetidine, prototypical competitive inhibitors of organic anion and cation transporters, respectively. Renal creatinine clearance was consistently greater than inulin clearance (as a measure of GFR) in wild-type (WT) mice but not in mice lacking OAT1 (Oat1-/-) and OAT3 (Oat3-/-). WT mice presented renal creatinine net secretion (0.23 ± 0.03 μg/min) which represented 45 ± 6% of total renal creatinine excretion. Mean values for renal creatinine net secretion and renal creatinine secretion fraction were not different from zero in Oat1-/- (-0.03 ± 0.10 μg/min; -3 ± 18%) and Oat3-/- (0.01 ± 0.06 μg/min; -6 ± 19%), with greater variability in Oat1-/-. Expression of OAT3 protein in the renal membranes of Oat1-/- mice was reduced to ∼6% of WT levels, and that of OAT1 in Oat3-/- mice to ∼60%, possibly as a consequence of the genes for Oat1 and Oat3 having adjacent chromosomal locations. Plasma creatinine concentrations of Oat3-/- were elevated in clearance studies under anesthesia but not following brief isoflurane anesthesia, indicating that the former condition enhanced the quantitative contribution of OAT3 for renal creatinine secretion. The results are consistent with a contribution of OAT3 and possibly OAT1 to renal creatinine secretion in mice.

Attenuation of prostaglandin E2 elimination across the mouse blood-brain barrier in lipopolysaccharide-induced inflammation and additive inhibitory effect of cefmetazole

Background

Peripheral administration of lipopolysaccharide (LPS) induces inflammation and increases cerebral prostaglandin E₂ (PGE₂) concentration. PGE₂ is eliminated from brain across the blood-brain barrier (BBB) in mice, and this process is inhibited by intracerebral or intravenous pre-administration of anti-inflammatory drugs and antibiotics such as cefmetazole and cefazolin that inhibit multidrug resistance-associated protein 4 (Mrp4/Abcc4)-mediated PGE₂ transport. The purpose of this study was to examine the effect of LPS-induced inflammation on PGE₂ elimination from brain, and whether antibiotics further inhibit PGE₂ elimination in LPS-treated mice.

Methods

[³H]PGE₂ elimination across the BBB of intraperitoneally LPS-treated mice was assessed by the brain efflux index (BEI) method. Transporter protein amounts in brain capillaries were quantified by liquid chromatography-tandem mass spectrometry.

Results

The apparent elimination rate of [³H]PGE₂ from brain was lower by 87%, in LPS-treated mice compared with saline-treated mice. The Mrp4 protein amount was unchanged in brain capillaries of LPS-treated mice compared with saline-treated mice, while the protein amounts of organic anion transporter 3 (Oat3/Slc22a8) and organic anion transporting polypeptide 1a4 (Oatp1a4/Slco1a4) were decreased by 26% and 39%, respectively. Either intracerebral or intravenous pre-administration of cefmetazole further inhibited PGE₂ elimination in LPS-treated mice. However, intracerebral or intravenous pre-administration of cefazolin had little effect on PGE₂ elimination in LPS-treated mice, or in LPS-untreated mice given Oat3 and Oatp1a4 inhibitors. These results indicate that peripheral administration of cefmetazole inhibits PGE₂ elimination across the BBB in LPS-treated mice.

Conclusion

PGE₂ elimination across the BBB is attenuated in an LPS-induced mouse model of inflammation. Peripheral administration of cefmetazole further inhibits PGE₂ elimination in LPS-treated mice.

In vitro and in vivo evidence of the importance of organic anion transporters (OATs) in drug therapy

Organic anion transporters 1-10 (OAT1-10) and the urate transporter 1 (URAT1) belong to the SLC22A gene family and accept a huge variety of chemically unrelated endogenous and exogenous organic anions including many frequently described drugs. OAT1 and OAT3 are located in the basolateral membrane of renal proximal tubule cells and are responsible for drug uptake from the blood into the cells. OAT4 in the apical membrane of human proximal tubule cells is related to drug exit into the lumen and to uptake of estrone sulfate and urate from the lumen into the cell. URAT1 is the major urate-absorbing transporter in the apical membrane and is a target for uricosuric drugs. OAT10, also located in the luminal membrane, transports nicotinate with high affinity and interacts with drugs. Major extrarenal locations of OATs include the blood-brain barrier for OAT3, the placenta for OAT4, the nasal epithelium for OAT6, and the liver for OAT2 and OAT7. For all transporters we provide information on cloning, tissue distribution, factors influencing OAT abundance, interaction with endogenous compounds and different drug classes, drug/drug interactions and, if known, single nucleotide polymorphisms.

Organic anion transporters: discovery, pharmacology, regulation and roles in pathophysiology

Our understanding of the mechanisms behind inter- and intra-patient variability in drug response is inadequate. Advances in the cytochrome P450 drug metabolizing enzyme field have been remarkable, but those in the drug transporter field have trailed behind. Currently, however, interest in carrier-mediated disposition of pharmacotherapeutics is on a substantial uprise. This is exemplified by the 2006 FDA guidance statement directed to the pharmaceutical industry. The guidance recommended that industry ascertain whether novel drug entities interact with transporters. This suggestion likely stems from the observation that several novel cloned transporters contribute significantly to the disposition of various approved drugs. Many drugs bear anionic functional groups, and thus interact with organic anion transporters (OATs). Collectively, these transporters are nearly ubiquitously expressed in barrier epithelia. Moreover, several reports indicate that OATs are subject to diverse forms of regulation, much like drug metabolizing enzymes and receptors. Thus, critical to furthering our understanding of patient- and condition-specific responses to pharmacotherapy is the complete characterization of OAT interactions with drugs and regulatory factors. This review provides the reader with a comprehensive account of the function and substrate profile of cloned OATs. In addition, a major focus of this review is on the regulation of OATs including the impact of transcriptional and epigenetic factors, phosphorylation, hormones and gender.

Update on the molecular physiology of organic anion transporters

PURPOSE OF REVIEW

Organic anion transporters (OATs) mediate the renal absorption and excretion of a wide range of metabolites and xenobiotics. We discuss the recent advances that have been made in elucidating the binding and transport characteristics of OATs, new insights into their physiological role and regulation by various factors, and pharmacogenetics.

RECENT FINDINGS

Overlapping substrate specificity among the OATs is well established. However, recent findings have suggested distinct differences in the structural binding determinants among the OATs, which have important implications for understanding drug interactions and drug design. A potential role for OATs in blood pressure regulation and remote sensing has been reported. Meanwhile, factors regulating the expression of OATs continue to be identified and characterized. The effect of renal ischemia on OAT expression and function is currently being explored. Finally, recent studies identifying various OAT polymorphisms may facilitate prediction of individual drug response and toxicity.

SUMMARY

As progress is made in unveiling the many functional aspects of the OATs, it is becoming clear that their significance is not only limited to a role in drug elimination from the body, but also extends to other vital physiological roles. Further delineation of the function and regulation of the OATs will uncover enormous potential clinical and pharmacological applications.

Organic anion transporter 3 inhibitors as potential novel antihypertensives

Hypertension is an exceedingly common disease with potentially devastating complications. Unfortunately, existing treatments are often only partially effective, indicating the utility of the development of novel therapeutics. It has recently been discovered that loss of renal organic anion transporter 3 (Oat3) results in decreased blood pressure. This suggests (regardless of the underlying mechanisms, which are, as yet, unclear) that inhibition of Oat3 function might also decrease blood pressure, and, therefore, that inhibitors of Oat3 might serve as novel drugs in the treatment of hypertension. In support of this hypothesis, it has been found that at least some potent Oat3 inhibitors can lower blood pressure. These recent findings are discussed in the context of the alternating access model that has been proposed for organic anion transporter function, with particular attention to the little-appreciated phenomenon of trans-inhibition.

Organic anion transporter 3 contributes to the regulation of blood pressure

Renal organic anion transporters (OAT) are known to mediate the excretion of many drugs, but their function in normal physiology is not well understood. In this study, mice lacking organic anion transporter 3 (Oat3) had a 10 to 15% lower BP than wild-type mice, raising the possibility that Oat3 transports an endogenous regulator of BP. The aldosterone response to a low-salt diet was blunted in Oat3-null mice, but baseline aldosterone concentration was higher in these mice, suggesting that aldosterone dysregulation does not fully explain the lower BP in the basal state; therefore, both targeted and global metabolomic analyses of plasma and urine were performed, and several potential endogenous substrates of Oat3 were found to accumulate in the plasma of Oat3-null mice. One of these substrates, thymidine, was transported by Oat3 expressed in vitro. In vivo, thymidine, as well as two of the most potent Oat3 inhibitors that were characterized, reduced BP by 10 to 15%; therefore, Oat3 seems to regulate BP, and Oat3 inhibitors might be therapeutically useful antihypertensive agents. Moreover, polymorphisms in human OAT3 might contribute to the genetic variation in susceptibility to hypertension.

Elimination of organic anions in response to an early stage of renal ischemia-reperfusion in the rat: role of basolateral plasma membrane transporters and cortical renal blood flow

BACKGROUND/AIMS

The knowledge of molecular mechanisms determining drug pharmacokinetics in pathological states is relevant for the development of new therapeutic approaches. This study was undertaken to evaluate the cortical renal blood flow (cRBF) and the renal protein expression of the organic anion transporters (OAT1 and OAT3) in association with the elimination of organic anions in an early stage of renal ischemia-reperfusion.

METHODS

Ischemic acute renal failure (ARF) was induced in adult male Wistar rats by occlusion of both renal pedicles during 60 min, followed by 60 min of reperfusion (ARF group). Pair-fed sham-operated rats served as controls. The renal protein expression of OAT1 and OAT3 was evaluated by immunohistochemistry techniques and by Western blotting in renal cortex homogenates and in basolateral plasma membranes. A pharmacokinetic study of p-aminohippurate (PAH, a prototypical organic anion) was performed. cRBF was determined using fluorescent microspheres.

RESULTS

ARF rats displayed a significant decrease in systemic clearance and in renal excretion of PAH. OAT1 and OAT3 protein abundance showed a statistically significant reduction both in homogenates and in basolateral plasma membranes from ARF rats. Immunohistochemical studies confirmed the changes in the cortical renal expression of these transporters. ARF animals also showed a decrease in cRBF.

CONCLUSIONS

The decrease in PAH elimination observed in an early stage of renal ischemia-reperfusion in male Wistar rats might be explained by the sum of the lower OAT1 and OAT3 expression in renal basolateral plasma membranes plus the decrease in cRBF. These findings might have significant implications in the development of novel pharmacological strategies to be applied in the initial stages of ischemic ARF.