Klinik und Poliklinik für Innere Medizin II, Nephrology, University of Regensburg, Regensburg, Germany
Introduction
At a conference jointly organized by Professor B. Krämer, and Dr M. Kammerl (all of Regensburg, Germany), leading scientists met to discuss research developments on the topic of cyclo-oxygenase-2 (COX-2) with regard to renal function. Main topics were pharmacology and clinical use of COX-2 inhibition, renal COX-2 expression, interaction of COX-2 with other regulatory systems, COX-2 and renin regulation, and finally COX-2 inhibition in human disease.
Pharmacology and clinical use of COX-2 inhibition
H. Kellner (München, Germany) discussed COX-2 inhibition from a rheumatologist's point of view. Non-steroidal anti-inflammatory drugs play a major role in the treatment of rheumatic diseases. However, their use is limited by the development of side-effects, primarily in the gastrointestinal tract. Compared to unselective NSAIDs, the highly selective COX-2 inhibitors celecoxib and rofecoxib have proved to be efficacious in the treatment of patients with rheumatoid arthritis and osteoarthritis. The rate of severe gastrointestinal side-effects was markedly lower; however, side-effects, with an incidence of more than 3%, including peripheral oedema and dizziness have been reported. A major drawback is the cost of therapy, which is up to 10 times more expensive than conventional NSAIDs.
G. Geisslinger (Frankfurt, Germany) discussed the pharmacology of COX-2 inhibition, concentrating on the aspect of selectivity of cyclo-oxygenase-2 compared to cyclo-oxygenase-1 (COX-1). In addition to highly selective inhibitors of COX-2 such as rofecoxib (>800-fold selectivity) and celecoxib (about 400-fold selectivity), compounds like meloxicam and nimesulide are classified as preferential inhibitors of COX-2 with published selectivities for COX-2 from 3- to 70-fold and from 5- to 16-fold respectively. Despite having a high homology, COX-2 forms a side-pocket due to substitution of an isoleucine 523 for a valine 523 not present in COX-1, enabling the synthesis of highly selective COX-2 inhibitors.
Renal COX-2 expression
R.C. Harris (Nashville, TN, USA) discussed the role of COX-2 in kidney development. Immunoreactive COX-2 is first observed in kidneys at the embryonic stage, notably in cells undergoing induction or morphogenesis. In postnatal kidneys, COX-2 expression is relatively low initially. It increases in the first 2 postnatal weeks, but gradually declines thereafter to low levels in normal adult rats. In line with these observations, chronic use of COX inhibitors during human pregnancy has been related to fetal maldevelopment, with evidence of poorly differentiated glomeruli in the outer cortex, undifferentiated tubular epithelia, and tubular dilatation. Furthermore, targeted disruption of murine COX-2 has indicated an important role for COX-2 in renal development. Similar to observations in humans, homozygous COX-2 null mice present small kidneys with fewer developed nephrons, i.e. immature glomeruli and dysplastic tubules in the outer cortex and hypoplasia or atrophy of the medulla. In contrast, mice with targeted disruption of COX-1 have no apparent renal abnormalities.
Confirming the major role of COX-2 for renal development in mice and rats, a study carried out by Dr Kömhoff in the laboratory of Professor Harris used a maternally administered selective COX-2 inhibitor, which caused renal lesions similar to those observed in the homozygous COX-2 null mice and in humans after maternal use of NSAIDs.
S. Bachmann (Berlin, Germany) reported the expression and localization of COX-2 in rodent kidneys, in macula densa cells, cTAL cells, intercalated cells of the CNT, CCD, OMCD, IMCD, and in papillary interstitial cells, in both rats and mice. COX-2 immunoreactivity was up-regulated in clipped kidneys and down-regulated in contralateral kidneys at the juxtaglomerular apparatus (JGA). Up-regulation of COX-2 was paralleled by enhanced NOS1 expression in the clipped kidney. NOS1 knockout mice showed a parallel down-regulation of COX-2 and renin mRNA expression at the JGA. When stimulated, COX-2 was expressed in more numerous cells of the cTAL, including the macula densa itself, whereas no changes were seen in collecting duct COX-2 levels. A functional role of COX-2 in cTAL, JGA, collecting duct and papillary interstitium is suggested. It is suggested that COX-2 mediates local vascular tone as well as renin stimulation at the JGA. Furthermore, the results are in agreement with a stimulatory role of macula densa-generated nitric oxide (NO) on the synthesis of COX-2.
R. M. Nüsing (Marburg, Germany) discussed the expression of COX-1 and COX-2 in human kidneys. COX-1 expression in humans was observed in collecting duct cells, interstitial cells, endothelial cells, and in smooth-muscle cells of pre- and post-glomerular vessels. Expression of COX-2 was limited to endothelial and smooth muscles of vessels, and to intraglomerular podocytes. These observations were confirmed by in situ mRNA analysis. In hyperprostaglandin E syndrome (HPS), a sub-form of Bartter's syndrome with significant renal salt and water loss, expression of COX-2 immunoreactive protein was detected in cells of the macula densa in eight of 11 patients. Nimesulide, a preferential inhibitor of COX-2, was as effective as indomethacin in the treatment of patients with HPS, decreasing urine excretion of prostaglandin (PG) E2 by about 90%. These data suggest that in man, COX-2 participates in renal PG synthesis and that COX-2 activity regulates renin release.
M. Goppelt-Struebe (Erlangen-Nürnberg, Germany) discussed the regulation of cyclo-oxygenases in mesangial cells in vitro and in vivo. In vitro COX-2 mRNA and protein are up-regulated by various types of mediators such as serotonin, platelet-derived growth factor, interleukin 1, and endothelin. However, immunohistochemical investigations of rat kidneys did not reveal significant expression of either cyclo-oxygenase isoform in glomerular mesangial cells. Furthermore, in the rat model of anti Thy-1 nephritis, COX-2 was not up-regulated within the glomeruli at various time points of the disease in contrast to a marked up-regulation of glomerular COX-1 at day 6. The cause for this marked difference between in vitro up-regulation of COX-2 in mesangial cells and no such response in animals in vivo may be explained in different ways, including down-regulation of mitogen-activated kinases or other kinases when mesangial cells are grown within an intact extracellular matrix.
J. Schnermann (Bethesda, MD, USA) discussed the regulation of COX-2 expression by salt intake, inflammation, and tonicity. In states of low extracellular fluid volume, renal cortical COX-2 expression is increased and likely to play a causal role in the up-regulation of the reninangiotensin system and thereby in blood pressure preservation. The renal cortical prostaglandin system counteracts the effect of angiotensin II and other vasoconstrictors on renal vasomotor tone. COX-2 is found at much greater concentrations in the renal medulla, where it may be expressed by both interstitial and collecting duct cells. Renal medullary COX-2 is up-regulated by high salt intake, suggesting that it plays a role in reducing salt reabsorption and elevating renal medullary blood flow. Therefore, renal medullary prostaglandins appear to activate salt excretory and antihypertensive mechanisms. Medullary COX-2 expression may also increase after dehydration, possibly exerting a cytoprotective effect during increased medullary tonicity. The renal medulla is also the site where lipopolysaccharide and other proinflammatory agents cause an induction of COX-2. The effect of hypertonicity on COX-2 expression in inner medullary collecting duct cells is mediated by activation of p38, erk, and jnk2 MAP kinases.
Interaction of COX-2 with other regulatory systems
J. D. Imig (New Orleans, LA, USA) discussed the role of cyclo-oxygenases for tubuloglomerular feedback (TGF). COX-2 inhibition significantly enhances the vasoconstrictor response to increasing renal perfusion pressure and TGF-mediated preglomerular vasoconstriction during acetazolamide treatment that increases flow past the macula densa. The latter effect of TGF-mediated preglomerular vasoconstriction was limited by a NOS inhibitor. This suggested that during an increased activation of TGF-dependent vasoconstrictor signals, increased nNOS-derived nitric oxide dilates the preglomerular vasculature directly and stimulates COX-2 vasodilatory metabolites. In kidneys from bradykinin B2 receptor (-/-) mice, COX-2 protein was significantly decreased, suggesting a role of bradykinin in the control of kidney COX-2 levels.
J. Pfeilschifter (Frankfurt, Germany), discussed cross-communication between NO and eicosanoid-producing systems. Inflammatory cytokines such as interleukin 1 or tumour necrosis factor- induce the expression of the inducible NO synthase and a secretory PLA2 type IIA. Most importantly, NO itself modulates gene transcription of iNOS and sPLA2 which may form the basis for acute and chronic inflammatory processes in the kidney. This cross-talk between NO and the eicosanoid-producing PLA2 system is complemented by regulatory feedback mechanisms triggered by lipid mediators generated in mesangial cells and modulating iNOS gene transcription.
B. L. Jensen (Odense, Denmark) discussed the regulation of renal prostaglandin receptors. There exist at least four different PGE2 receptor isoforms, designated EP1 to EP4, and a single PGI2 receptor, IP. The EP3 receptor is the most abundant sub-type expressed in the rat kidney, and is followed by EP1>EP4>IP>EP2. EP1 and EP3 receptors are predominantly expressed, and EP2 receptors exclusively expressed, in the rat kidney medulla. The EP4 receptor is expressed in all kidney regions. In cortex, EP4 receptors can be detected in glomeruli, in preglomerular vessels, in renin-secreting juxtaglomerular granular cells, and in distal convoluted tubules and cortical collecting ducts. In the medulla, EP4 receptors are found in vasa recta. IP-receptor transcripts are found mainly in kidney cortex. The EP2 receptor can be detected in vasa recta of the outer medulla. Maintaining rats on a low- or high-salt diet has no effect on mRNA levels of EP1, EP2, or IP receptors in kidney regions. In contrast, EP4-transcripts in glomeruli are increased twofold by salt deprivation. The EP3 mRNA level is increased twofold by a high salt intake in the outer medulla. In isolated juxtaglomerular (JG) cells from salt-deprived animals, PGE2-evoked cyclic adenosine monophosphate production and renin secretion are significantly higher compared with JG cells obtained from salt-loaded animals. Taken together, the data suggest that EP4 receptors mediate effects of PGE2 on JG cells and that both EP2 and EP4 receptors could be involved in the vasodilatory responses to PGE2 in renal resistance vessels. Moreover, subtype-specific, regional changes in PG receptor expression are involved in the renal adaptation to changes in salt intake.
F. X. Salazar (Murcia, Spain) presented possible interactions between NO and prostaglandins for acute and long-term control of renal function. The vasoconstrictor and antinatriuretic effects induced by the prolonged infusion of a non-selective COX inhibitor were significantly enhanced when NO synthesis was reduced, probably secondary to the unrestrained vasoconstriction produced by endogenous angiotensin II. When NO synthesis is reduced, COX-1 appears to be more important than COX-2 for producing the prostaglandins involved in renal haemodynamic and excretory functions. COX-2-derived prostaglandins participate in the regulation of renal excretory function when sodium intake is normal or low, and are involved in the regulation of the renal haemodynamic when sodium intake is reduced. COX-2-derived prostaglandins are of importance to counteract renal vasoconstrictive effects of noradrenaline. The renal vasodilatory effects of bradykinin are mediated by COX-2 and NO, and the increase in COX-2 seems to be NO mediated.
COX-2 and renin regulation
K. F. Hilgers (Erlangen-Nürnberg, Germany) discussed COX-2 and renal renin expression in the clipped kidney. COX-2 and renin expression are both up-regulated in the clipped kidney of rats with unilateral renal artery stenosis. However, when using NS3 93, a selective COX-2 inhibitor, the renin stimulatory role of renal artery stenosis could not be blocked.
A. Kurtz (Regensburg, Germany) discussed the role of prostaglandins in the control of the renin system. Prostacyclin and PGE2 stimulate renin secretion and gene expression in isolated juxtaglomerular cells. It has long been known that non-selective COX inhibitors block renin secretion and renin gene expression both in animals and in man. The availability of selective COX-2 inhibitors allowed studies in the involvement of cyclo-oxygenase isoforms for the regulation of the renin system. The available experimental data suggest that cyclo-oxygenase-2 is involved in the stimulation of the renin system due to a low-sodium diet or blocking the macula densa with frusemide. However, with regard to stimulation of renal renin secretion and renin gene expression after blockade of the renin angiotensin system, and in the two-kidney, one-clip model of renal vascular hypertension, the role of COX-2-derived prostaglandins is less clear. Very recent data obtained in this experimental model suggest that the parallel up-regulation of COX-2 and renin may not be causally related.
COX-2 inhibition in human disease
J. C. Frölich (Hannover, Germany), discussed the renal effects of COX inhibition in man. Dehydration, congestive heart failure, nephrotic syndrome, and liver cirrhosis with ascites, as well as impaired renal function, predispose patients to marked reduction of GFR and sodium retention. NSAIDs may aggravate hypertension or heart failure, especially when combined with ACE inhibitors and potassium-sparing diuretics. Decreased water elimination may cause life-threatening water intoxication, at least partially due to COX inhibitory actions. Renin release following frusemide was inhibited by the preferential COX-2 inhibitor meloxicam as effectively as by indomethacin. The new, highly selective COX-2 inhibitors celecoxib and rofecoxib have already been shown to cause sodium retention. Interestingly, salicylate has no effect either on renal PGE2 production or on platelet aggregation, in contrast to salicylic acid. The non-acetylated salicylates have to be considered as a separate group of COX inhibitors.
B. K. Krämer (Regensburg, Germany) demonstrated results in humans on a low-sodium diet for 8 days with or without 50 mg daily of the highly selective COX-2 inhibitor rofecoxib. Plasma renin activity increased two- to threefold from baseline on a low-sodium diet, and fivefold 30 min after frusemide i.v. on day 8; it was still increased nearly fivefold at day 9. All these effects were completely blocked by rofecoxib. In addition, a similar increase of plasma aldosterone concentrations was also blocked by rofecoxib. These data suggest that in man, stimulation of renin secretion due to a low-sodium diet or frusemide is largely dependent on an intact COX-2.
C. Blume (Düsseldorf, Germany) investigated the effect of selective COX-2 inhibition with flosulide on the course of passive Heymann nephritis (PHN) in rats. Flosulide decreased proteinuria dose-independently, decreased GFR slightly, and suppressed markedly both COX-1 and COX-2 protein production at higher doses. Flosulide was considered to be possibly beneficial in PHN, although it may have compound-specific nephrotoxic side-effects.
R. A. K. Stahl (Hamburg, Germany) discussed the role of prostaglandins in glomerulonephritis (GN), employing different experimental models (Thy-1 and anti-glomerular basement membrane (GBM) models for mesangioproliferative GN or anti-GBM disease). Although COX-2 was up-regulated in these animal models, treatment with a selective COX-2 inhibitor enhanced the inflammatory response in these models. However, the pro-inflammatory effect of non-selective COX inhibitors was much more marked. These data suggest that COX-1-related and COX-2-related prostaglandins are important immune modulators in inflammatory kidney diseases.
A. Luchner (Regensburg, Germany) discussed the role of prostaglandins for renal function in heart failure, emphasizing the importance of PGE2 and prostacyclin in preserving renal excretory function. Furthermore, prostaglandins in part mediate vasodilatory and natriuretic effects of other vasoactive factors, e.g. adrenomedullin or bradykinin, which are activated in heart failure.
Conclusions
Highly selective COX-2 inhibitors have become clinically available, conferring important benefits with regard to gastrointestinal side-effects in susceptible patients with, for example, rheumatic diseases. Renal safety of these compounds has not been the focus of interest in the available large clinical trials [1].
Renal COX-2 is the dominant isoform during renal development, and is also markedly expressed in adult rodent and human kidneys, both in cortical and medullary structures, implicating important roles for normal renal function, for example for up-regulation of the renin system in hypovolaemia or for activation of salt excretion in response to high salt intake. Prostaglandin EP4 and EP3 receptors are up-regulated in parallel with COX-2 in the cortex and medulla respectively, thus enhancing the action of COX-2-derived prostaglandins.
Furthermore, COX-2 is an important modulator of tubuloglomerular feedback, interacts with and may be induced by NO and bradykinin, and counteracts renal vasoconstrictors.
In both rodents and man, renin secretion and renin gene expression induced by a low-salt-diet and frusemide treatment are mediated partially or completely by COX-2-derived renal prostaglandins. The role of COX-2-derived prostaglandins in other states of a stimulated renin system is less clear.
Renal COX-2 is up-regulated in a variety of experimental nephritis and ablation models and in human lupus nephritis, and COX-2 inhibitors were beneficial in some (but not all) studies. This issue clearly needs more study.
Finally, as with unselective NSAIDs, COX-2 inhibitors reduce sodium excretion, and may cause acute renal failure and cardiac decompensation [1,2]. Therefore, COX-2 inhibitors have to be handled cautiously in susceptible patients such as elderly subjects with pre-existing renal failure.
References