George M. O'Brien Kidney and Urologic Diseases Center, and Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
Introduction
Prostaglandins are mediators of vascular tone and salt and water homeostasis in the mammalian kidney; their regulation of glomerular haemodynamics and distal nephron function has been well described. Renin production and release is also known to be mediated by prostaglandins generated by afferent arteriole and by prostaglandin-dependent signalling from the macula densa [1].
There are two separate gene products with cyclo-oxygenase activity, cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2). The gene for COX-1, the constitutive prostaglandin G2/H2, encodes a 2.72.9 kb transcript, while the gene for COX-2, the inducible prostaglandin G2/H2 synthase, encodes a 4.24.5 kb transcript, which increases in response to inflammatory or mitogenic stimuli. In the kidney, constitutive prostaglandin G2/H2 synthase (COX-1) has been localized to mesangial cells, arteriolar endothelial cells, parietal epithelial cells of Bowman's capsule and cortical and medullary collecting ducts [2,3].
What is the expression of COX-2 in the kidney?
There is also localized expression of COX-2 in the developing and adult kidney. Using in situ hybridization and immunohistochemical localization, we have documented that COX-2 expression is localized to two cell types in normal adult rat kidney: (i) occasional macula densa cells and surrounding cTALH cells; and (ii) a subset of medullary interstitial cells near the papillary tip [3]. In the cortex, the immunoreactivity of stained cells was intense, but only one (and rarely two) COX-positive cells were observed per site. No COX-2 immunoreactivity was detected in arterioles, glomeruli or cortical or medullary collecting ducts.
Is there a role for COX-2 in regulating renin expression and release?
Following chronic salt depletion, COX-2 expression in the macula densa and peri-macula densa region increased significantly [3]. This finding suggested that prostaglandins generated by macula densa COX-2 might be involved in mediating renin release in response to volume depletion.
In the mammalian kidney, the macula densa is involved in regulating renin release by sensing alterations in luminal chloride via changes in the rate of Na+/K+/2Cl- cotransport. Inhibition of Na+/K+/2Cl- cotransport with loop diuretics results in a decrease in chloride reabsorption by the macula densa and an increase in renin secretion [3]. It has long been recognized that non steroidal anti-inflammatory drug (NSAID) administration can elicit a hyporeninaemic state, and studies using an isolated perfused juxtaglomerular preparation indicated that NSAID administration prevented the increases in renin release mediated by macula densa sensing of decreases in luminal NaCl [4]. Immunoreactive COX-1 cannot be detected in cortical thick limb or macula densa.
Harding et al. [5] first demonstrated a direct role for macula densa COX-2 activity in mediating renin production and release by showing that NS398, a selective COX-2 inhibitor, inhibited increases in renal renin expression in response to a low-salt diet. Our group has subsequently demonstrated that increases in renin mRNA expression and renal renin activity in response to angiotensin-converting enzyme (ACE) inhibition were also blunted by the highly selective COX-2 inhibitor, SC59236 [6]. We have further shown that in experimental renovascular hypertension, in which macula densa COX-2 expression is increased [7,8], COX-2 inhibition blunted increases in renin expression and lowered blood pressure [8]. In addition, our preliminary results have indicated that in COX-2 knockout mice, renal renin activity did not increase in response to ACE inhibition [9]. Direct evidence for a role for COX-2 has recently been provided by Traynor et al. [10], who determined that in an isolated perfused juxtaglomerular preparation, increased renin release in response to lowering the perfusate NaCl concentration was blocked by NS398 [10].
Do components of the reninangiotensinaldosterone system modulate COX-2 expression?
Because COX-2 is involved in regulation of renin production and release, we hypothesized that components of the reninangiotensin system might be involved in mediating expression of macula densa/cTALH COX-2 expression. Administration of either an ACE inhibitor or an angiotensin (AT) type 1 receptor (AT1R) antagonist to rats led to increases in macula densa COX-2 expression. Furthermore, mice with genetic disruption of the genes for both angiotensin type 1A (AT1a) and AT1b receptors expressed high levels of COX-2 in the macula densa [9]. We also determined that adrenalectomy increased macula densa/cTALH expression, which was reversed by administration of either glucocorticoids or mineralocorticoids [11]. Not only the glucocorticoid receptor antagonist, RU486, but also the mineralocorticoid antagonist, spironolactone, increased macula densa/cTALH COX-2, suggesting that MR as well as GR may inhibit basal COX-2 expression [11].
It is known that renal renin production is modulated by angiotensin II [12,13]. Increased renal tubule Na reabsorption, mediated directly by angiotensin II and indirectly by aldosterone, will re-establish intravascular volume homeostasis and thereby decrease the stimulus for renin release. In addition, angiotensin II directly inhibits renal renin production and release by a so-called short loop feedback inhibition [13]. Administration of either ACE inhibitors or AT1 receptor antagonists results in increases in juxtaglomerular renin expression, even in the absence of any detectable alteration in intravascular volume or renal haemodynamics [14].
It has traditionally been assumed that angiotensin II inhibits renin production by direct action on the juxtaglomerular cells. However, a recent study by Matsusaka et al. [15] in chimeric mice carrying regional null mutation of the AT1a receptor, the AT1 receptor subtype exclusively present in mouse juxtaglomerular (jg) cells, has questioned this assumption. In these studies, the jg apparatus of AT1a receptor -/- mice was markedly enlarged, with intense expression of renin mRNA and protein. In the chimeric mice, the changes in the jg apparatus were proportional to the degree of chimerism, but the degree of jg apparatus hypertrophy/hyperplasia and the expression of renin mRNA and protein were not different in AT1a receptor +/+ and AT1a receptor -/- jg cells. Therefore, the presence or absence of AT1 receptors on jg cells did not appear to be the determining factor of whether angiotensin II could regulate jg renin synthesis.
The results of our studies of COX-2 expression suggest an alternative or additional mechanism by which angiotensin II may inhibit renin release. Angiotensin II may act to inhibit cTALH/macula densa COX-2 expression by direct action and indirectly by increasing aldosterone production, thereby limiting the relative increases in COX-2 expression in response to volume depletion, and thus the macula densa's ability to signal renin release.
What is the pattern of COX-2 expression in human kidney?
Although COX-2 expression in macula densa/cTALH has been noted in kidney of mouse, rat, rabbit, and dog, it was previously controversial whether human kidney demonstrated the same COX-2 localization. Initial studies of COX-2 localization in human kidney failed to detect COX-2 in the macula densa region and instead reported expression in podocytes and arteriolar smooth muscle cells [16]. However, a more recent study was able to detect COX-2 in the macula densa, with an increase in people >60 years of age [17]. Furthermore, a preliminary report has also detected increased macula densa COX-2 expression in hyperreninaemic states (Bartter syndrome and congestive heart failure [18]. Therefore, it is likely that COX-2 may well play similar physiologic roles in the human kidney as has been noted for other mammals.
Conclusions
In summary, COX-2 mRNA and immunoreactive protein localize to the macula densa and adjacent cortical thick ascending limb in renal cortex, and chronic NaCl restriction increases expression of this enzyme. These findings suggest an integral role for eicosanoids generated by macula densa-associated COX-2 in mediating renin release. Since COX-2 is an immediate-early gene, its rapid and transient regulation of expression may allow the macula densa to modulate its prostaglandin production to provide appropriate regulation of the reninangiotensin system.
Acknowledgments
This work was supported by the Vanderbilt George O'Brien Kidney and Urologic Diseases Center (National Institutes of Health Grant DK 39261) and by funds from the Department of Veterans Affairs.
Notes
Correspondence and offprint requests to: R. C. Harris MD, Division of Nephrology, Department of Medicine, S-3223 MCN, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
References