National Institute of Diabetes, Digestive and Kidney Disease, NIH, Bethesda, Maryland, USA
Keywords: angiotensin II; cyclooxygenase-2
Prostaglandins and the secretion of renin
The tubular and cardiovascular effects of angiotensin II combine to make it one of the most important effectors of body Na conservation and blood pressure maintenance. Control of renin secretion, the rate-limiting step in the formation of angiotensin II, is therefore of critical importance for extracellular fluid volume and blood pressure homeostasis. Among the many factors that can alter renin secretion, special attention has been paid to the role of prostaglandins (PGs) as paracrine mediators because plasma and secretion levels of renin correlate directly with PG production in many clinical and experimental conditions. Covariation of renin and PG production is the result of a complex interaction that has the structure of a positive feedback relationship.
Initially it was found that angiotensin II as well as other Ca-liberating agents can augment the release of PGs into renal venous blood [1]. Numerous subsequent studies have confirmed that angiotensin II enhances PG production in vascular smooth muscle, endothelial, and mesangial cells. It has also been shown that this is the result of activation of phospholipases C and A2, and augmented arachidonate release [2]. In the intact kidney, angiotensin II-stimulation of PG production occurs mostly in the vascular space and is probably mediated by endothelial cyclooxygenase-1 (COX-1) as the isoform most prominently expressed in the cortical renal vasculature [3,4]. However, it has recently been shown that angiotensin II also augments the expression of the second cyclooxygenase isoform cyclooxygenase-2 (COX-2) in vascular smooth muscle cells [5]. Vascular production of vasodilator PGs acts as an important buffer against the renal constrictor action of angiotensin II and other vasoconstrictor agents.
While angiotensin II can stimulate PG production, the converse relation was shown to hold true as well: the intrarenal infusion of the PG precursor arachidonic acid increased plasma renin activity and inhibition of PG-synthase prevented renin stimulation [6]. Stimulation of renin secretion by PGs, most clearly by PGI2 and PGE2, was confirmed in renal cortical tissue slices, isolated glomeruli, and isolated juxtaglomerular granular cells [79]. The effect of PGE2 on renin release and expression is likely to be mediated by EP4 receptors on granular cells and signalling through an increase in cytosolic cAMP [10]. The macula densa mechanism for control of renin release was established as the physiological context in which prostaglandins were demonstrated to interact with renin-producing cells [11]. Direct evidence for PG-dependence of low NaCl-stimulation of renin secretion was obtained in the isolated perfused JGA preparation [12]. Participation of PGs in other renin control pathways such as sympathetic and baroreceptor mechanisms is less well established or unlikely. The mechanisms responsible for PG-dependent renin release, and the reasons why the macula densa mechanism was affected predominantly were unclear.
Cyclooxygenases in juxtaglomerular cells
Since PGs typically act as paracrine agents, the generation of PGs involved in control of renin secretion is expected to occur in cells located in the vicinity of the renin-producing juxtaglomerular granular cells. COX protein was shown immunohistochemically to be present in vascular endothelial cells, epithelial cells of Bowman's capsule, and mesangial cells, but not in the macula densa [4]. It was unclear how a lowering of luminal NaCl concentration and a reduction in macula densa NaCl uptake was coupled to increased COX activity in cells distant from the macula densa. The demonstration of COX-2 in macula densa and surrounding thick ascending limb (TAL) cells by Harris et al. [13] was a breakthrough finding since it immediately suggested a pathway along which PGs produced in the NaCl-sensing epithelium could interact with the renin-producing granular cells. Expression of COX-2 in the macula densa and in cells of the cortical TAL has been confirmed in different species even though the expression pattern is typically patchy and irregular, perhaps reflecting limitations in the sensitivity of the histochemical assay [1416]. RT-PCR and in situ hybridization has confirmed COX-2 mRNA-expression in the macula densa and cortical TAL [13,17,18]. Thus, COX-2, a cyclooxygenase isoform that is typically induced by cytokines and growth factors, is constitutively expressed in both the renal cortex and the renal medulla. Like COX-2 in general, COX-2 in the kidney has been found to be a highly inducible gene. Furthermore, renal COX-2 expression is differentially regulated in renal cortex and renal medulla. For example, renal cortical COX-2 expression increases in rats on a low NaCl diet, while renal medullary COX-2 increases in rats on a high NaCl diet [18,19]. A functional connection between COX-2 in the renal cortex and renin was suggested by observations showing that the expression of these proteins correlates directly in conditions such as renal artery stenosis, adrenalectomy, diuretic treatment, and Bartter syndrome [2022].
Cyclooxygenase-2 and macula densa control of renin release and expression
A causal relation between juxtaglomerular epithelial COX-2 and renin release has been demonstrated in the isolated perfused JGA preparation in which the COX-2 specific blocker NS-398 prevented the acute stimulation of renin secretion by low NaCl, whereas the COX-1 specific blocker valeryl salicylate was without effect [23]. Pharmacological inhibition of COX-2 with NS-398 or SC 58236 also blunted the increase in plasma renin activity caused by: a low NaCl diet [24]; a reduction in renal perfusion pressure [22], or by the administration of furosemide [25,26]. Furthermore, in a recent case report it was observed that the administration of a COX-2 inhibitor significantly reduced plasma renin and plasma aldosterone levels in a 2-year-old Bartter patient with a genetically established ROMK mutation [29]. Since in all of these conditions the macula densa mechanism is likely to contribute to renin secretion, it appears that PGs generated by macula densa COX-2 are a required component of the pathway through which a reduction in NaCl concentration and NaCl transport across the tubular epithelium near the vascular pole affects renin secretion. In addition to renin secretion there is also evidence that COX-2 products determine renin gene expression in juxtaglomerular granular cells. COX-2 inhibition is associated with a reduction in renal renin content and renin mRNA expression resulting from treatment with a low-salt diet or from a reduction in renal perfusion pressure [22,27]. In COX-2 knockout mice, renal renin content, renin mRNA, and renin activity was found to be reduced and renin activation by a low-salt diet was markedly attenuated [28].
In view of the direct correlation between angiotensin II and PG production in a number of cell types, it is surprising that angiotensin II appears to inhibit COX-2 expression in macula densa cells. COX-2 mRNA and protein levels were found to be upregulated by converting enzyme inhibitors and AT1A blockade [14,30]. Furthermore, COX-2 levels were elevated in angiotensin receptor knockout mice [14,28]. The administration of a COX-2 specific inhibitor reduced the increase in renin gene expression and renin release caused by captopril suggesting that COX-2 upregulation is in part responsible for renin activation by angiotensin II inhibition [14]. In primary cultures of cTAL cells phorbolester-induced COX-2 expression was inhibited by angiotensin II [14]. The mechanisms by which angiotensin II inhibits COX-2 expression are obscure.
Mechanisms of COX-2 regulation by low NaCl transport
Considerable progress has recently been made in elucidating the mechanism by which a reduction in luminal NaCl causes stimulation of PGE2 release and COX-2 expression [31,32]. In a macula densa derived cell line, a reduction in medium NaCl concentration caused a prompt and time-dependent increase in PGE2 release that was almost completely inhibited by NS-398 and was therefore largely mediated by COX-2. The increase in PGE2 release preceded the stimulation in COX-2 expression, suggesting that it resulted either from an increase in COX-2 enzyme activity or from an activation of PLA2 and enhanced liberation of arachidonate substrate. The presence of PLA2 and its regulation in macula densa cells has recently been demonstrated [15]. In both TAL and MD cells in culture, a reduction in medium NaCl also augmented the expression of COX-2 at both the mRNA and protein level. Ion substitution experiments indicate that the extracellular signal for COX-2 stimulation appears to be a reduction in Cl rather than in Na concentration. Cl dependency of COX-2 expression may be related to an involvement of the NKCC2 transporter whose inhibition also elevates COX-2 expression. The relative affinities of NKCC2 for Na and Cl suggest that a reduction in Cl concentration, but not in Na concentration, would reduce NaCl transport. Cl dependency of COX-2 expression is reminiscent of the earlier demonstration of Cl-dependent renin secretion [33]. The intracellular signalling events leading to COX-2 overexpression in response to low NaCl are initiated by rapid phosphorylation of p38 and Erk1/2 kinases and stimulation of MAP kinase pathways. Participation of MAP kinases is attested to by the inhibitory effect of SB 203580 and PD 98059, inhibitors of p38 and Erk1/2 mediated signalling events [31,32]. In other cell types, MAP kinases have been demonstrated to regulate COX-2 expression in response to cytokine or growth factor activation or hypertonic stimulation [3436].
Conclusion
There is firm evidence that activation of COX-2 in macula densa and adjacent TAL cells by low NaCl transport is a critical component of macula densa control of renin secretion and synthesis. Since renin secretion and synthesis are under the control of other factors than the macula densa, the overall effect of COX-2 inhibition on renin release would be a defining measure of the extent to which renin release and synthesis is MD/TAL-mediated. Conversely, one may surmise that lack of an effect of COX-2 blockade on the renin system would suggest dominance of other renin control mechanisms. Thus, while the use of COX-2 inhibitors has the potential side-effect of reducing levels of angiotensin II and aldosterone, this is only to be expected in conditions in which low NaCl transport by the macula densa is a major contributor to renin activation. The use of loop diuretics and the loss of function mutations of NKCC2 and ROMK in Bartter syndrome represent perhaps the best examples of hyperreninaemic states in which renin production is mainly driven by TAL/MD COX-2 and is therefore sensitive to suppression by COX-2 inhibitors.
Notes
Correspondence and offprint requests to: Jurgen Schnermann MD, NIDDK, NIH, Building 10, Room 4 D51, 10 Center Drive MSC 1370, Bethesda, MD 20892, USA.
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