Physiological and pathophysiological implications of PGE2 and the PGE2 synthases in the kidney

Accepted Manuscript Title: Physiological and Pathophysiological Implications of PGE2 and the PGE2 Synthases in the Kidney Authors: Jing Wang, Min Liu, Xiaoyan Zhang, Guangrui Yang, Lihong Chen PII: DOI: Reference:

S1098-8823(17)30046-1 https://doi.org/10.1016/j.prostaglandins.2017.10.006 PRO 6260

To appear in:

Prostaglandins and Other Lipid Mediators

Received date: Revised date: Accepted date:

31-3-2017 9-10-2017 31-10-2017

Please cite this article as: Wang Jing, Liu Min, Zhang Xiaoyan, Yang Guangrui, Chen Lihong.Physiological and Pathophysiological Implications of PGE2 and the PGE2 Synthases in the Kidney.Prostaglandins and Other Lipid Mediators https://doi.org/10.1016/j.prostaglandins.2017.10.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Physiological and Pathophysiological Implications of PGE2 and the PGE2 Synthases in the Kidney Jing Wang1, Min Liu1, Xiaoyan Zhang1, Guangrui Yang2*, Lihong Chen1*

1

Advanced Institute for Medical Sciences, Dalian Medical University, Dalian,

116044, China; 2

School of Life Science and Biotechnology, Dalian University of Technology,

Dalian, 116023, China. *Corresponding author: Dr. Lihong Chen; Email: [email protected] or Dr. Guangrui Yang; Email: [email protected]

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Highlights

PGE2 is the most abundant prostanoid synthesized in the kidney.

The three PGE2 synthases, mPGES-1, mPGES-2 and cPGES, are constitutively expressed in the kidney.

Genetic or pharmacological studies suggest mPGES-1 as a key player in renal function maintenance.

Due to the cardiovascular side effects of COX-2 selective inhibitors, mPGES-1 serves a promising drug target for inflammatory diseases, while the effects of mPGES-1 inhibition in the kidney require further comprehensive investigation.

Abstract Prostaglandin E2 (PGE2) is the most abundant prostanoid synthesized in the kidney and plays an important role in renal function. Physiologically, PGE2 regulates renal hemodynamics, water and sodium metabolism, blood pressure, and so on. As a wellknown proinflammatory lipid mediator, PGE2 also substantially mediates renal injury under many pathophysiological conditions. Multiple enzymes are involved in

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renal PGE2 biosynthesis, including the three main PGE2 terminal synthases, i.e. microsomal PGE2 synthase-1 (mPGES-1), mPGES-2 and cytosolic PGE2 synthase (cPGES). In the kidney, mPGES-1 is highly expressed in the collecting duct where it is the dominant contributor of PGE2 biosynthesis and participates in blood pressure regulation and renal hemodynamic maintenance. mPGES-2 protein is mainly expressed in the renal cortex and the outer stripe of the outer medulla. cPGES is diffusely expressed in all nephron segments. Roles of mPGES-2 and cPGES in renal function have not been clearly characterized. Here we summarize the role of PGE 2 in the kidney, highlight the contribution of the three PGE2 synthases, particularly mPGES-1, in blood pressure regulation and renal hemodynamics, and outline the contribution of mPGES-1 to kidney diseases. A clearer understanding of the role of PGE2 in the kidney could pave the way for development of new therapeutic approaches.

Key words: PGE2; mPGES-1; blood pressure; renal hemodynamics; kidney disease.

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Introduction Prostaglandin (PG) E2 is a key product of arachidonic acid (AA) metabolism in the kidney, where it plays critical roles in mediating kidney development [1-3] and maintaining renal function, including body fluid homeostasis, sodium excretion, and blood pressure regulation [4, 5]. PGE2 is produced via three sequential enzymatic reactions [6]. Firstly, AA is released from membrane glycerophospholipids by cytosolic phospholipase A2 (cPLA2). Then AA is converted into PGH2, a common prostanoid precursor, by cyclooxygenase (COX) -1 or COX-2. PGH2 is further catalyzed into bioactive PGE2 through terminal PGE2 synthases [7]. So far, at least three distinct types of PGE2 synthase (PGES) have been identified, including two microsomal and one cytosolic synthases, designated mPGES-1, mPGES-2 and cPGES, or PTGES1-3, respectively [6, 8-10]. Among the three PGE2 synthases, mPGES-1 expression is relatively low in most tissues with obvious exception in some urogenital organs [8]. Compared to the other two enzymes, mPGES-1 displays a high catalytic activity [8, 11] and is highly inducible, coupling with the induction of COX-2, in states of injury and inflammation [12-15]. mPGES-2 is constitutively expressed in various cells and functionally couples with both COX-1 and COX-2 for PGE2 production in vitro [15]. cPGES is expressed ubiquitously in a constitutive manner and was considered to functionally couple with COX-1 for the physiological PGE2 production [9].

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However, neither mPGES-2 nor cPGES displayed a convincing capability for PGE2 production in vivo. Jania et al. failed to detect reduction of PGE2 content in any examined tissues including kidney of mPGES-2 KO mice [16]., which It’s reported that may be explained by the formation of a complex of mPGES-2 could form a complex with glutathione and heme in vivo, ultimately leadingwhich may contribute to the loss of a capacity to convert PGH2 to PGE2 [17]. Although mice lacking cPGES had a reduction in PGE2 content in embryonic lung (not in the heart or liver), this reduction was accompanied by a global decrease of prostanoid-generating enzymes, including cPLA2, COX-1, and COX-2, and other prostanoids, including PGI2 and TXB2. This suggests that the reduction of PGE2 in the lung is secondary to a developmental defect, but not a direct deficiency of PGE2 synthase activity [18]. In contrast to mPGES-2 and cPGES, growing evidence suggests that mPGES-1 serves as the key enzyme in controlling both baseline and inducible PGE2 production and participates in various physiological and pathological processes, including inflammation, pain, fever, blood pressure regulation, atherogenesis, and cancer [19]. In the mouse kidney, PGE2 is the most abundant prostanoid at baseline in both cortex and medulla [20, 21], where it plays critical roles in maintaining blood pressure and renal function in volume-contracted states and exerts antihypertensive effect upon high-salt consumption, respectively [4]. In addition, PGE2 mediates renal injury in various kidney diseases, including acute or chronic kidney injuries,

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diabetic nephropathy, renal cell carcinoma, and so on. [5, 22]. Until now, the role of PGE2 and its synthetic and action pathways in the kidney have been well summarized [23, 24], however, most of researchers have focused on COX-2 and the four PGE2 receptors, i.e. EP1-4. Here, we discuss and update our understanding regarding the role of PGE2 synthases in renal physiology and diseases.

Renal expression of PGE2 synthases The distribution of three PGESs within the kidney has been investigated. As aforementioned, mPGES-1 attracts more attention. It contributes to half amount of basal PGE2 production and more than eighty percent upon LPS stimulation [25], although the overall expression of mPGES-1 in mouse kidney is lower than mPGES2 and cPGES at basal level [26]. In situ hybridization studies of mouse [27, 28] and rabbit kidney [28], RT-PCR [29], and RNA-Seq analyses (Figure 1) [30] on microdissected rat nephron segments all demonstrate that mPGES-1 mRNA is predominantly expressed in the whole collecting duct (CD) with some variance within the CD sub-segments. More specifically, Schneider et al. found that mPGES1 mRNA was highly expressed in renal medullary collecting ducts (MCD), and to a lesser extent in cortical collecting ducts (CCD) of both mouse and rabbit kidney [28]. Through RNA-Seq analysis, Lee et al. found that rat inner medullary collecting duct (IMCD) displayed the lowest mPGES-1 mRNA expression along the entire length

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of the CD [30]. This discrepancy may reflect divergence between species and/or differences in specificity or sensitivity that are inherent to the methods employed for analysis. Regardless, in situ hybridization in rabbit kidney also showed low, but detectable, expression of mPGES-1 in macula densa (MD) and renal medullary interstitial cells (RMICs) [28]. Similarly to mRNA, clear immunostaining of mPGES-1 protein was found in CD, followed by MD and RMICs of both mouse [28] and rabbit kidney [31, 32], and the descending thin limbs (DTL) in rabbit kidney(Figure 2) [31]. Within the CD, the immunoreactive labeling was restricted to the principle cells but not intercalated cells [31]. In the CD, mPGES-1 parallels the constitutive expression of COX-1 [29, 33], suggesting a functional coupling of these proteins in the distal nephron for basal PGE2 biosynthesis. This may also explain why the collecting duct has the highest capacity for PGE2 generation among all nephron segments [21]. In contrast, relatively low mPGES-1 protein expression was observed in MD and RMICs, where it is colocalized with COX-2 [31, 33]. Interestingly, mPGES-1 and COX-2 in MD is highly inducible under low salt condition [31]. The localization of the other two isoforms of PGESs (mPGES-2 and cPGES) in the kidney was less studied. Immunohistochemistry on mouse kidney showed that mPGES-2 is mainly expressed in the renal cortex and the outer stripe of the outer medulla (Figure 2). Abundant signals were observed in the distal convoluted tubule

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(DCT), followed by CD, the proximal convoluted tubule (PCT), and the thick limbs of the loops of Henle. Only weak signals were observed in the glomeruli [34]. This expression pattern is largely consistent with the findings in the RNA-Seq database using rat microdissected nephron segments (Figure 1) [30]. In contrast to mPGES-1 in the collecting ducts, expression of mPGES-2 appeared to selectively present in intercalated cells, but not in the principal cells [34], therefy thereby implying a nonredundant and complementary role of these two enzymes in this region. The renal expression of cPGES was studied using in situ hybridization, which indicated diffuse signal in all nephron segments [35].

PGE2, mPGES-1 and blood pressure regulation Among its wide range of physiological actions, PGE2 has a number of effects that are related to affect blood pressure (BP) homeostasis. It is a potent vasodilator in both peripheral [36] and renal vasculature [37], thus it increases renal blood flow. Additionally, PGE2 can directly act on salt and water transport in the epithelial cells and consequently potentiate natriuresis [38, 39]. These effects explain the reason why NSAID and COX-2 inhibitors may cause edema and hypertension [40, 41]. However, the role of mPGES-1 in BP control is still controversial. Use the pure C57BL/6 or mixed DBA/1lacJ×C57BL/6 mice, Jia et al. and Zhang et al. found that,

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mPGES-1 null mice exhibited an impaired ability to excrete Na +, and developed more severe hypertension upon chronic salt loading and AngII infusion when compared to the wild type controls in both acute and chronic settings [42-45]. Notably, high salt increases mPGES-1 expression in the CD [42, 46], which implies that in the distal nephron, mPGES-1 derived PGE2 acts in an autocrine manner to control sodium transport, and/or modulates the sodium excretion in the thick ascending limb in a paracrine manner. Inconsistently, however, some other studies showed that there’s no obvious difference in the blood pressure between mPGES-1 knockout (KO) and wild type (WT) mice under a range of dietary sodium intakes or AngII infusion [47-49]. Ang II induced hypertension was also unaltered by mPGES1 deletion in hyperlipidemic mice [50]. Interestingly, Facemire et al. found that the genetic background significantly impacts the blood pressure response to mPGES-1 deficiency, thus no difference between the WT and KO groups of the DBA/1 mice, while for the 129 background, WT mice showed significantly higher BP than the KOs [51]. This might partly explain the above discrepancies. Furthermore, distinct experimental conditions might be another possibility that contributes to the inconsistency, for example, the treatment duration of the evocative hypertensive stimuli (short term vs. long term), mode of administration (DOCA salt vs. regular high salt, bolus vs. continued AngII infusion), or the BP reading approaches (tail cuff vs. telemetry), and so on. Nevertheless, these seemingly conflicting

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physiological patterns necessitate a more thorough exploration. PGE2 is also a potent secretagogue for renin, which contributes to blood pressure regulation via a different physiological avenue. Renovascular hypertension is a consequence of renin-angiotensin system (RAS) activation by PGE2 and is in opposition to the direct effects of PGE2 on renal vasculature that lead to vasodilation and natriuresis [52, 53]. PGE2 directly stimulates renin production in isolated juxtaglomerular apparatus (JGA) cells [53-55] and, in conscious dogs, chronic intrarenal PGE2 infusion increases renal renin secretion, resulting in hypertension [52]. Animal studies show that aorta coarctation increases COX-2 expression in the macula densa and cortical thick ascending limb [56], while COX-2 inhibition reduces renin activity and lowers blood pressure in this renovascular hypertensive animal models [56], indicating that COX-2-derived prostaglandins are involved in renin synthesis and are released during renal vascular hypertension. Although the role of mPGES-1 in renovascularhypertension has not been reported thus far, considering the coupling of inducible COX-2 and mPGES-1 in various abnormal states as well as their co-localization in the macula densa, we presume that mPGES1 makes a significant contribution to PGE2 generation and renin secretion in the pathogenesis of renal vascular hypertension. This is somewhat puzzling; however, this contradiction suggests that PGE2 derived from COX-2 and mPGES-1 plays a diverse role in blood pressure regulation – the exact nature of which is dependent on

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the site of action. The mechanism of PGE2 to bidirectionally modulate vascular tone and blood pressure has been partially illuminated by studies on its four G-protein coupled Eprostanoid receptors, EP1, EP2, EP3, and EP4 [57, 58]. Endogenous EP1 or EP3 receptor activation contributes to hypertension in spontaneously hypertensive rat and in salt loading and chronic Ang II induced hypertensive mice [59, 60], while mice that lack the EP2 or EP4 receptor in myeloid cells develop profound salt-sensitive hypertension[61, 62] ， and the EP4 activation induces vascular relaxation and deletion of EP4 results in diminished vasodepressor response to PGE2 [36, 63]. Generally, this reflects the predominate vasodilatory effect of PGE2 on the activation of the EP2 and EP4 receptors, however, within the kidney, PGE2 also acts on EP2 or EP4 receptors in juxtaglomerular cells, regulates glomerular tone, induces renin release, and eventually leads to hypertension [64-66].

PGE2, mPGES-1 and renal hemodynamic Endogenous PGE2 can maintain glomerular filtration rate by dilating the afferent arteriole [67, 68]. COX-2 derived PGE2 in the macula dense densa was believed to mediate this action [69]. In both humans and rats under volume contracted conditions, COX-2 expression dramatically increases in the macula densa and cortical thick ascending limb [69, 70]. Co-expression and induction of COX-2

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and mPGES-1 in the macula densa implies a possible contribution of mPGES-1 in mediating the inducible PGE2 production [28, 31, 32] in the maintenance of normal renal function under certain pathophysiological conditions, such as congestive heart failure, nephrotic syndrome, and cirrhosis with ascites. Furthermore, both COX-2 and mPGES-1 are localized in the renal medullary interstitial cells [31, 71] that are in direct contact with the vasa recta [72]. Such an association supports the notion that the PGE2 system in these cells may play an important role in maintaining blood supply in the renal medulla. Previous studies showed that COX-2 inhibition directly reduced renal medullary blood flow in angiotensin II-infused mice [73]. HoweverWhile, Salazar et al. found that the selective inhibitor of mPGES-1, PF-458, did not elicit significant changes in renal blood flow or glomerular filtration rate with normal or low Na+ intake. This absence of change might be explained by a compensatory elevation in PGI2 production [74]. Moreover, PF-458 did not induce significant changes in renal sodium excretion, plasma renin activity, or plasma aldosterone concentrations. Taken together,T these results suggest that mPGES-1 inhibition might represent an alternative pharmacological target approach to COX2 inhibition for avoiding the undesirable renal side effects observed with NSAIDs or COX-2 selective inhibitors when Na+ intake is normal or low. Although promising, howbeit, if pharmacological mPGES-1 inhibition, such as PF-458, will display equivalence in analgesic and anti-inflammatory efficacy compared to COX-2

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inhibition needs to be clearly evaluated in both animal models and in human. In addition, if the inhibitors will have effect on pathological renal conditions warrants deeper investigation. In total, the impact of prolonged mPGES-1 inhibition on BP regulation remains a big concern, and its effects on renal hemodynamics require further illustration.

PGE2, mPGES-1 and water and sodium metabolism Evidence from many studies indicates a contribution of PGE2 to renal water and sodium metabolism [75-77]. PGE2 possesses a natriuretic property due to its ability to inhibit Na+ transport in the distal nephron. It is demonstrated that, in response to dehydration, urinary PGE2 production is clearly increased and is accompanied by the induction of COX-2 in the renal medulla [78]. In addition, COX-2 is a primary source of furosemide-stimulated PGE2 production. Blockage of the Na-K-Cl cotransporter, NKCC2, by furosemide partly stimulates prostanoid generation via up-regulation of COX-2 [79]. A series of studies also revealed an important role of mPGES-1 in mediating PGE2 production, promoting urinary Na+ excretion and reducing urine concentrating under various conditions. For example, by employing mPGES-1 KO mice, researchers have identified a physiological role of this enzyme in mediating the natriuretic and diuretic responses to both chronic salt and acute water loading [42, 80]. Subsequently, Jia et al found that mPGES-1 KO mice were

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largely resistant to lithium-induced polyuria [81], water deprivation induced urine concentrating defect, and natriuresis [82, 83]. mPGES-1 deletion could also impair aldosterone escape and enhance sodium appetite [84]. Everything considered, these data suggest that mPGES-1 serves as the dominate enzyme for the production of renal PGE2 that contributes to renal fluid metabolism. There are also some counterexamples. Francois et al. failed to observe altered urinary osmolality in mPGES-1 KO mice at either baseline or after twelve hours of water deprivation [48]. Likewise, although urinary PGE2 excretion was attenuated in mPGES-1-deficient mice, the furosemide-associated diuresis was reduced only in male, but not female KO mice. Stimulation of renin by furosemide was not affected by mPGES-1 deficiency as well [48]. There is a possibility that the residual level of PGE2 generated in the kidneys or the product rediversion to other prostanoids in mPGES1 null mice is compensatory to fulfill the urine concentrating function in this case.

PGE2, mPGES-1 and renal diseases Accumulating evidence from in vivo and in vitro experiments as well as from clinical studies has shown that PGE2 in the kidney is involved in the pathophysiological

processes

of

various

renal

diseases,

including

glomerulonephritis, renal cell carcinoma, diabetic nephropathy, and acute or chronic kidney injuries [22, 70, 85-90].

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As the key terminal enzyme for PGE2 biosynthesis, the role of mPGES-1 in multiple renal diseases was also investigated. For example, mPGES-1 deletion attenuates cisplatin-induced renal dysfunction and tubular damage, accompanied by suppressed cytokine expression and oxidative stress [91]. Moreover, both mPGES1 KO mice and knockdown podocytes by siRNA showed significant attenuation of adriamycin-induced podocyte apoptosis, improved phenotypic alteration, blunted inflammatory responses, and decreased adriamycin-induced albuminuria [92]. Due to the negligible expression of mPGES-1 in podocyte and proximal convoluted tubule, these data seem somewhat puzzling, a paracrine regulation of the PGE2 from the macula densa or distal nephron might be an explanation, however, more evidence is clearly required to be explored. In the 5/6 nephrectomy model, comparing to the WT mice, the increased blood urea, nitrogen, and creatinine, reduced creatinine clearance, robust proteinuria, and glomerular morphological lesions were all significantly attenuated in KO mice. In addition, the renal morphology and function and the pathologic inflammatory response were all significantly improved in KO mice after subtotal resection of the kidney [93]. Thus, mPGES-1 seems to be of importance in both acute kidney injury and chronic renal failure and may serve as a promising target for the development of novel anti-inflammatory and renal protective drugs. However, this is not always the case. For instance, deficiency of mPGES-1 does not protect the kidney from endotoxin or renal ischemia/reperfusion

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induced injury [91]. In the setting of STZ-induced type 1 diabetes, kidney injury indices including urinary albumin excretion, kidney weight and the kidney histology, did not show obvious differences between the WT and KO mice [94]. Although mPGES-1 was significantly induced in glomeruli in obese db/db mice, there is no direct evidence supporting the involvement of this enzyme in type 2 diabetic kidney injury [95]. Conversely, Luo et al.’s most recent study showed that deficiency of mPGES-1 even exacerbates renal fibrosis and inflammation in the unilateral ureteral obstruction (UUO) mouse model [96], pointing to a possibility that mPGES-1 may also exert beneficial renal effects under given conditions. Nevertheless, these discrepancies are not very surprising and pretty consistent with the protective effects of COX-2 deletion in various kidney injury models, but not in the UUO model [97]. Our current understanding of the renal effects of COX-2 and mPGES-1 inhibition is still in a state of convolution owing to the fact that these effects vary with the pathogenic mechanisms of different kidney diseases and their accompanying pathological insults. Given the existence of the cell selective effects of COX-2 [98, 99] and mPGES-1 [100] in the cardiovascular system, it is worth considering the possibility of the existence of a similar cell type-specific renal effect for both COX2 and mPGES-1. In fact, although genetic evidence shows that global COX-2 deficiency exaggerates apoptosis and renal damage in UUO model [97], Yang et al. evidenced that, in the same model, the macrophage selectively COX-2 depletion

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seems to be protective to tissue injury after obstruction [101]. Clearly, it is necessary to put more effort in clarifying these controversies and gaining more insight into the precise contribution of PGE2 in renal intrinsic cells and/or infiltrated immune cells as they relate to the pathology of kidney diseases. It is critical to realize, however, that all current endeavors that examine the role of mPGES-1 in the kidney were from animal models. The extent to which these models simulate responses in humans remains unknown. As it stands, there are even still controversies in regard to the function of mPGES-1 in mouse kidney alone. More evidence in human subjects and other animal species is needed to further clarify the renal function of mPGES-1 and other PGE2 synthases.

Conclusion In this mini-review, we have summarized the research advances in the renal expression, regulation, physiological and pathophysiological roles of the three prostaglandin E2 synthases, especially mPGES-1. PGE2 derived from these PGESs plays multiple physiological functions in the kidney, including maintenance of glomerular filtration, modulation of water and sodium excretion, participation in blood pressure regulation, and mediation of kidney development. In addition, the inducible mPGES-1 is involved in the pathophysiology of renal vascular hypertension, acute kidney injuries and chronic kidney diseases.

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Due to the severe side effects of nonsteroidal anti-inflammatory drugs, especially COX-2 selective inhibitors, mPGES-1 has become a promising pharmacological target for the intervention of inflammatory diseases. The role of mPGES-1 and its derived PGE2 in the kidney also attracts intensive attention. Although it sounds encouraging in some cases, growing data have demonstrated a quite diverse, disease- and pathogen- specific contribution of mPGES-1 to renal health and diseases. Two challenging questions remain: (1), would mPGES-1 inhibition curtail the renal side effects of COX-2 inhibitors, such as peripheral edema, hypertension, or a compromised glomerular filtration rate; and (2), would targeting mPGES-1 globally or through cell-selective inhibition serve as new therapeutic approaches for kidney injuries, especially inflammatory kidney diseases? Further investigation is still needed to fully appreciate the rich spectrum of cascading effects that arise from kidney-specific mPGES-1 inhibition. Only then will we be able to ascertain the potential implications of pharmacological modulation.

Abbreviations AA: arachidonic acid; CCD: cortex collecting duct; CD: collecting duct; COX: cyclooxygenase; DCT: distal convoluted tubule; DTL: descending thin limb; EP: Eprostanoid receptor (PGE2 receptor); IMCD: inner medullary collecting duct; KO: knockout; MCD: medullary collecting duct; MD: macula densa; OMCD: outer

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medullary collecting duct; PGE2: Prostaglandin E2; PGES: prostaglandin E synthase; PLA2: phospholipase A2; PT: proximal tubule; RNA-Seq: RNA-sequencing; RMIC: renal medullary interstitial cell; RPKM: reads per kilobase of transcript per million mapped reads; RT-PCR: reverse transcriptase-polymerase chain reaction; TAL: thick ascending limb; WT: wild type.

Conflict of interest The authors have declared that no conflict of interest exists.

Acknowledgements The authors are grateful to Damien Lekkas for reading and commenting on this paper. This work was supported by a grant from the Natural Science Foundation of China 81670242 and 81570643.

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Figure 1. Heat map showing mRNA expression of PGE2 synthetic enzymes and receptors along the nephron segments (size not to scale) based on RNA-Seq database, https://helixweb.nih.gov/ESBL/Database/NephronRNAseq/index.html.

G:

glomerulus; PCT: proximal convoluted tubule; PST: proximal straight tubule; OMDTL: outer medullary descending thin limb; IMDTL: inner medullary descending thin limb; ATL: ascending thin limb; mTAL; medullary thick ascending limb; cTAL: cortical thick ascending limb; DCT: distal convoluted tubule; CNT: connecting tubule; CCD: cortical collecting duct; OMCD: outer medullary collecting duct; IMCD: inner medullary collecting duct.

Figure 2. Schematic diagram of mPGES-1 and mPGES-2 protein localization in the kidney. G: glomerulus; PT: proximal tubule; DTL: descending thin limb; TAL: thick ascending limb; MD: macula densa; DCT: distal convoluted tubule; CCD: cortical 24

collecting duct (dots represent intercalated cells); MCD: medullary collecting duct; RMIC: renal medullary interstitial cell.

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