- Email: [email protected]
Endothelin and the kidney
ENDOTHELIN A N D THE KIDNEY
Ponnal Nambi
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin Expression in the Kidney . . . . . . . . . . . . . . . . . . . . . . . . . . Renal Endothelin Receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin Receptor Signal Transduction in the Kidney . . . . . . . . . . . . . . . . Endothelin and Mesangium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin and Tubules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin and Renal Vasculature . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin Receptor Antagonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin in Renal Pathophysiology . . . . . . . . . . . . . . . . . . . . . . . . . . Chronic Renal Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acute Renal Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
207 208 209 209 210 211 212 212 213 214 215 216 216 216
INTRODUCTION E n d o t h e l i n - 1 (ET-1), a 2 1 - a m i n o - a c i d peptide, initially i s o l a t e d f r o m the c u l t u r e m e d i u m o f p o r c i n e aortic e n d o t h e l i a l cells b y Y a n a g i s a w a et al. ( 1 9 8 8 ) is the m o s t p o t e n t v a s o c o n s t r i c t o r yet identified. In a d d i t i o n to e n d o t h e l i a l cells, ET-1 is also
Advances in Organ Biology Volume 9, pages 207-218. Copyright © 2000 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0617-3
207
208
PONNAL NAMBI
synthesized and released from a number of other ceil types such as mesangial cells, renal tubular cells, smooth muscle cells, and neuronal cells (Yanagisawa et al., 1988; Lemonnier Degouville et al., 1989; Simonson and Dunn, 1990). Endothelin has been shown to mediate its effects by binding to specific cell surface receptors that are located on many tissues and cells (Yanagisawa and Masaki, 1989; Sokolovsky, 1992; Huggins et al., 1993). It is proposed that ET-1 can act in an autocrine and paracrine fashion, indicating that it can be released at or near the site of action. The receptors for endothelin have been classified as ET a and ET B based on the differences in the binding profile of ET-1 and related agonists such as ET-3 and sarafotoxin 6c ($6c; a snake venom peptide that shares a high degree of homology with ET-1). Both ET A and ET B receptors have been cloned and expressed from a number of species including humans (Arai et al., 1990; Sakurai et al., 1990; Elshourbagy et al., 1993; Bergsma et al., 1995; Huggins, 1997). ET A receptors display high affinity for ET- 1 and ET-2, whereas ETBreceptors display equal affinity for ET- 1, ET-2, ET-3, and $6c. ET-3 and $6c are 100 and 1000 times, respectively, less potent than ET- 1 and ET-2 for ET A receptors. In addition, two more subtypes of ET receptors, ET c and ETAx, have been cloned and expressed fromXenopus melanophores (Karne et al., 1993) and heart (Kumar et al., 1994), respectively (Nambi, 1998).
ENDOTHELIN EXPRESSION IN THE KIDNEY A number of molecular biological techniques including Northern blot and ribonuclease protection analyses of RNA isolated from rat kidney displayed mRNAs for both prepro ET-1 and prepro ET-3 (Maccumber et al., 1989; Nunez et al., 1990). The expression of these two mRNAs appears to be species specific because prepro ET-1 expression was predominant in pig kidney, whereas prepro ET-3 expression was predominant in rat kidney (Nunez et al., 1990). Data from in situ hybridization studies indicate that prepro ET-1 was predominantly expressed in the vasa recta bundles of medulla (Maccumber et al., 1989) and vascular sites of cortex and medulla. Using reverse transcriptase and polymerase chain reaction, ET-1 mRNA has been localized to rat nephrons where it was present primarily in the glomerulus and inner medullary collecting duct. ET- 1 mRNA was also shown to be present in the proximal convoluted and straight tubules. ET-l-like immunoreactivity was observed in renal cortex, medulla, and papilla, although the vasa recta of distal nephron segments in the renal papilla showed the predominant localization of this peptide (Nambi and Brooks, 1997). The synthesis and/or release of ET-1 in the kidney has been shown to be influenced by a number of factors. Many renal epithelial cells in culture (such as MDCK, LLCPK, NRK-52E, BHK-21, and RK- 13) secrete ET- 1, and this secretion is potentiated by various factors such as thrombin, bradykinin, ATP, platelet activating factor, transforming growth factor 13, tumor necrosis factor, interleukin1[3, and cyclosporine A. In addition, human mesangial cells have also been shown to secrete ET in response to transforming growth factor-[3, arginine vasopressin,
Endothelin and the Kidney
209
and thrombin. On the other hand, atrial natriuretic peptide (ANP) and heparin have been shown to decrease ET-1 synthesis and release. The kidney also can degrade ET-1, probably by the action of enzymes such as neutral endopeptidase. The level of immunoreactive ET in urine is six times higher than in plasma of normal volunteers, indicating that the kidney plays a major role in ET synthesis as well as metabolism. The observation that plasma ET levels were significantly increased in patients undergoing hemodialysis, renal transplantation, or peritoneal dialysis further supports a role for ET in renal function (Kon and Badr, 1991).
RENAL ENDOTHELIN RECEPTORS Binding sites for ET-1 in kidneys from a number of species were described as early as 1989. Although these studies did not distinguish receptor subtypes, a very high density of ET binding sites was identified in mammalian kidney in the glomeruli, inner medulla, and inner strip of outer medulla. Proximal tubules displayed moderate binding sites, whereas no binding was observed in the outer stripe of the outer medulla. Later studies demonstrated the presence of subtypes of ET receptors based on the binding profile of the subtype-selective ligands ET-3 and $6c (Williams et al., 1991). In addition, discovery of BQ123 as an ETA-Selective antagonist (Ihara et al., 1992) enabled the quantitation of the subtypes of ET receptors in various regions of the kidney. The presence of ET receptors was also demonstrated in microdissected rat nephron segments. Molecular biological techniques such as reverse transcriptase polymerase chain reaction have been used to detect large signals for the ET B receptor subtype in the initial and terminal inner medullary collecting duct and glomerulus, whereas small signals were found in the cortical collecting duct, outer medullary collecting duct, vasa recta, and arcuate artery. Although ET A receptor mRNA was shown to be present in the glomerulus, binding studies using [ 125I]-ET-1 or [125I]-ET-3 indicated that the predominant subtype was ET B receptors. ET receptors are differentially expressed along the nephron. Cell culture studies indicate the presence of high levels of ET B receptors in glomerular, epithelial, and endothelial cells of the thin segments of Henle's loop. Studies reported on mesangial cell ET receptors are somewhat confusing. Mesangial cells have been shown to contain both ET A and ET B receptors (Simonson, 1993; Clavell et al., 1995; Nambi, 1996).
ENDOTHELIN RECEPTOR SIGNAL TRANSDUCTION IN THE KIDNEY ET receptors belong to the superfamily of seven transmembrane G protein-coupled receptors which mediate their biological actions through a number of GTP binding proteins and specific effector molecules. Binding of ET to its receptors results in the formation of a number of second messengers including inositol phosphates, diacylglycerol, intracellular calcium, cyclic adenosine monophosphate, cyclic
210
PONNAL NAMBI
guanosine monophosphate, and arachidonic acid (Marsen et al., 1994; Nambi et al., 1995). Depending on the coupling of the receptor to the effector system, different second messenger(s) are generated in different tissues. ET mediates both short-term (contraction) and long-term (mitogenesis) responses. ET-1 stimulates phospholipase C activity in mesangial cells through a pertussis toxin-sensitive G protein. Activation of protein kinase C through this pathway results in the release of arachidonic acid. In mesangial cells, this pathway appears to be very critical since arachidonic acid metabolism results in the synthesis of prostaglandins which regulate a number of functions in the kidney including natriuresis, diuresis, vascular tone, and renin secretion. ET has also been shown to stimulate phospholipase D in mesangial cells through a PKC-dependent pathway. In addition to phospholipase C and D activation, ET also activates phospholipase A 2 in mesangial cells leading to the formation of arachidonic acid which is metabolized to specific eicosanoids, depending on the species. In contrast to the observation that addition of ET to most cells and tissues resulted in the activation of phospholipase C, studies performed to evaluate the involvement of ET in the activation of cyclic nucleotide pathways have yielded different results. Studies in rat nephron segments have shown that ET-1 inhibits vasopressin-mediated cAMP accumulation by a PKC-dependent pathway, whereas in rat mesangial cells, ET-1 has been shown to stimulate cAMP accumulation through a PGE2dependent pathway. Exposure of NG 108-15 cells (mouse neuroblastoma/rat glioma hybrid cells) to ET-1 resulted in a calcium-dependent increase in the accumulation of cGMP that was inhibited by nitric oxide synthase inhibitors and hemoglobin. Similar observations have been reported using rat glomeruli and the increase in cGMP was shown to be mediated by ETB receptors. Thus, ET-induced increase in cGMP appears to be mediated by the nitric oxide synthase pathway. ET has also been shown to activate nuclear signal transduction pathways resulting in the expression of transcription factors such as c-fos, c-jun, and c-myc (Simonson et al., 1992). These transcription factors have been implicated in the mitogenic property of ET observed in many cells.
ENDOTHELIN A N D MESANGIUM Exposure of mesangial cells to ET has been shown to result in increased cell number and [3H]thymidine incorporation which are indicative of a proliferative response. El'-1 has also been shown to increase mesangial DNA topoisomerase I activity and this activation was inhibited by pretreatment with pertussis toxin. Although the mitogenic signals mediated by ET-1 are not well understood, it is possible that an ET-l-induced increase in platelet-derived growth factor A and B chains might be involved in this process. In addition, the induction of protooncogenes such as c-fos, c-jun, and c-myc might contribute to ET-l-mediated mitogenic response in mesangial cells (Simonson et al., 1992).
Endothelin and the Kidney
211
ET-mediated mesangial cell contraction has been shown as an increase in intensity and number of tension-generated wrinkles of silicon rubber substratum on which mesangial cells were grown as well as a decrease in mesangial cell planar surface area. ET-l-induced increases in intracellular calcium have been shown to be responsible for ET-mediated contraction in mesangial cells (Badr et al., 1989).
ENDOTHELIN A N D TUBULES ET has been shown to cause natriuresis at low concentrations and inhibit tubular sodium and bicarbonate transport as well as Na+/K+-ATPase activity. ET-mediated inhibition of Na+/K+-ATPase appears to be mediated by prostaglandins because this response was abolished by ibuprofen. Systemic infusions of ET-1 have had variable effects on sodium. In some studies, systemic infusion of ET- 1 caused a decrease in sodium excretion, whereas in other studies, ET caused a natriuresis, despite a decrease in renal blood flow and GFR. Studies performed with isolated perfused kidneys also demonstrate ET-mediated increases in Na + excretion, despite a decrease in GFR, which is different from whole animal studies where intrarenal infusion of low doses of ET-l was without effect on Na + excretion, whereas higher doses caused a decrease in Na + excretion. In addition, ETs have been shown to increase plasma levels of ANE which is another important regulator of renal sodium handling. Coinfusion of ANP along with ET-1 has resulted in inhibition of ET-mediated renal effects. ET has been shown to have some water diuretic activity, indicating that it has an effect on water reabsorption, despite its inhibitory effects on GFR and RBE The water diuretic effect of ET- 1 appears to be species specific. This diuretic effect of ET has been suggested to be mediated through its inhibitory effect on AVP-induced cAMP accumulation and independent of its effect on Na + reabsorption and renal hemodynamics. ET inhibited AVP-induced cAMP accumulation in cortical collecting duct, outer medullary collecting duct, and inner medullary collecting duct, whereas it did not affect AVP-mediated cAMP accumulation in other nephron segments. In addition to its effects on the kidney, ETs may act on the posterior pituitary to alter vasopressin release, and there are suggestions that ETs might interact with AVP at the hypothalamic-pituitary axis. ETs appear to have multiple effects on renin release and the renin-angiotensin system which is the key regulator of extracellular fluid volume. While systemic infusions of ET resulted in a dramatic increase in renin activity, possibly through the intrarenal baroreceptor and macula-densa-mediated pathways, in vitro experiments performed with cortical slices, glomeruli, or juxtaglomerular cells demonstrated an inhibition of renin release by ET-1.
212
PONNAL NAMBI
ENDOTHELIN A N D RENAL VASCULATURE Depending on the mode of infusion, ET- 1 has been shown to cause differential renal effects. Systemic infusion of ET-1 caused a significant increase in renal vascular resistance and a concomitant decrease in renal blood flow (King et al., 1989; Denton and Anderson, 1990; Kon and Badr, 1991). ET-3 was much weaker than ET-1 in increasing renal vascular resistance and, in addition, caused a transient vasorelaxation, although the magnitude of this vasodilation was less than that observed in other vascular beds (Yanagisawa et al., 1988; Lemonnier Degouville et al., 1989). The vasodilatory response mediated by ETs was attenuated by indomethacin, whereas the vasoconstrictor response was augmented, suggesting that the prostaglandins might serve to counteract the effect of ETs on renal vasoconstriction. Micropuncture studies with ET-1 suggest that ET-1 can contract both afferent and efferent arterioles (Badr et al., 1989). The signal transduction pathways involved in ET-mediated renal vasoconstriction involve both influx of extracellular calcium as well as mobilization of intracellular Ca 2÷. This conclusion is based on the observations that Ca 2÷ channel blockers such as verapamil and nicardipine attenuated ET-mediated afferent arteriolar response only, whereas the efferent arteriolar responses appear to be mediated by the release of intracellular Ca 2÷ (Simonson and Dunn, 1990). Infusion of ET- 1 directly into the renal artery of rats or dogs resulted in a transient increase followed by a sustained decrease of renal blood flow. Experiments performed with isolated kidneys of rat and rabbit also demonstrated ET-mediated increases in vascular resistance which was partially dependent on the presence of extracellular Ca 2+.
EN DOTH ELI N RECEPTOR A N T A G O N ISTS Enzyme inhibitors which will block the generation of agonists, or receptor antagonists which will block agonist-mediated activation of the receptor are excellent tools to determine the physiological role of an agonist. Invariably, the results obtained from in vivo and in vitro studies have been quite different for many antagonists. Endothelin is not an exception to this. Even though exogenously administered ET-1 is the most potent vasoconstrictor yet identified, studies with ET receptor antagonists failed to identify the vasoconstrictor response mediated by endogenous ET in normal animals (Gellai et al., 1994a). No significant changes were observed in systemic or renal hemodynamics of conscious or anesthetized rats and dogs against ETA-selective or ETa/ETB-selective antagonists. These findings suggest that the contribution of endogenous ET- 1 to the maintenance of systemic and renal vascular tone is minimal in normotensive animals (Gellai, 1997). An unexpected finding was that mice with the ET-1 gene deleted had elevated blood pressure, suggesting that the predominant role of ET-1 in normal animal is vasodilation (Kurihara et al., 1994). While an ET a receptor antagonist had no effect on blood pressure and renal
Endothelin and the Kidney
213
blood flow in normotensive animals, the ETB-selective antagonist, RES 701-1, potentiated systemic and renal vasoconstriction induced by bolus injection of ET-1 or $6c in conscious chronically instrumented rats (Gellai et al., 1997). These effects were attenuated by a mixed ETA/ETB receptor antagonist but not by the ET aselective antagonist, BQ123 (Gellai et al., 1997). These findings clearly suggest that ET plays an important role in the control of basal vascular tone by mediating vasodilation and vasoconstriction. These effects appear to be mediated by different ET B receptor subtypes. The ET B receptor that mediates the vasodilation is sensitive to RES 701-1, whereas the ET B receptor that is involved in vasoconstriction is insensitive to RES 701-1 (Gellai et al., 1996). Similar conclusions have been drawn from studies performed in anesthetized pigs. In conscious Sprague-Dawley (SD) rats, exogenous administration of ET-1 and $6c resulted in a dose-dependent decrease in renal blood flow which was unaffected by infusion of BQ123 (ETa-selective antagonist). Spontaneously hypertensive (SH) rats were more sensitive to the renal vasoconstrictor effects of ET-1 and $6c. While $6c and ET-1 were equipotent in SD rats, $6c was more potent than ET-1 in SH rats. These functional data agreed well with the radioligand binding data obtained from these rat kidneys. There was an increase in the proportion and affinity of ET B receptors present in the cortex of SH rats compared to SD rats (Gellai et al., 1994a). This further confirms the involvement of ET B receptors in the maintenance of vascular tone in rats. The only physiological role identified for ET A receptor in the rat kidney is sodium handling (Gellai et al., 1994a), whereas in the dog, sodium excretion is regulated by the ET Breceptor (Clavell et al., 1995; Nambi and Brooks, 1997). Systemic infusion of ET-1 (nonselective agonist) to healthy human volunteers resulted in an increase in arterial pressure and decreases in renal plasma flow, GFR, and sodium excretion, whereas infusion of ET-3 (ETB-selective agonist) had no effect on blood pressure, electrolyte excretion, and renal hemodynamics, suggesting that in humans, the systemic and renal vasoconstrictor effects of ET-1 are mediated by ETA receptors (Kaasjager et al., 1997).
ENDOTHELIN IN RENAL PATHOPHYSIOLOGY Involvement of ET in a number of renal diseases including chronic renal failure, ischemia-induced acute renal failure, cyclosporine- and radiocontrast-induced nephrotoxicity has been suggested by a number of investigators (Kon and Badr, 1991; Marsen et al., 1994; Kohan, 1997; Nambi and Brooks, 1997). The evidence is compelling because of the beneficial effects demonstrated by ET receptor antagonists or neutralizing antibodies to ET in these animal models (Kon and Badr, 1991; Gellai et al., 1994b).
214
PONNAL NAMBI
Chronic Renal Diseases Chronic renal insufficiency invariably progresses to end-stage renal failure, and a common histologic end point is glomerulosclerosis. Although the exact mechanisms that are involved in the development and progression of chronic renal failure are not known, the primary causes appear to be hypertension and type I and type II diabetes. Several pathophysiologic changes such as intraglomerular hypertension, intrinsic glomerular thrombosis, mesangial cell proliferation, and/or matrix production have been suggested to contribute to the pathogenesis of chronic renal failure. In addition, altered glomerular permeability to proteins and lipids leading to glomerulosclerosis, inflammation, and interstitial fibrosis has been suggested to play a critical role in the development and progression of chronic renal failure (Klahr et al., 1988). Although the role of ET in the development and/or progression of chronic renal disease is not clear, it is possible that ET could be involved in all of the above-mentioned processes. ET has been shown to act as a potent mitogen for mesangial cells, possibly through the upregulation of protooncogenes, and many of the proinflammatory mediators such as thrombin, transforming growth factor-[3, and platelet-derived growth factor have been shown to stimulate ET production in glomerular endothelial and mesangial cells (Simonson et al., 1992). In addition, ET increased intraglomerular pressure and decreased single nephron filtration rate (Badr et al., 1989). Increased levels of ET gene expression as well as urinary excretion have been reported in rats with 5/6 nephrectomy-induced chronic renal failure as well as diabetic rat models of chronic renal failure. There was a good correlation between the urinary excretion of ET, proteinuria, and hypertension. ET gene expression was also increased in the glomeruli of streptozotocin-induced diabetic rats. In cpk/cpk mice which develop polycystic kidney disease, there was a good correlation between the ET- 1 as well as ET receptor mRNA expression and disease progression. Clinical studies conducted in patients with chronic renal disease reveal increased plasma levels and urinary excretion of ET. Increase in plasma ET correlated well with the increase in plasma creatinine in these patients. A pathogenic role for ET-1 in chronic renal disease has been supported by the recent study performed in the rat remnant kidney model where the ETA receptor antagonist, FR 139367, was effective in limiting the glomerular injury, reducing urinary protein excretion, renal c-fos expression, mesangial hypercellularity, and the deterioration of renal function (Benigni et al., 1993). ET has also been suggested to be a contributing factor to the problems associated with the treatment of patients with end-stage renal disease. Plasma ET levels are elevated to a greater degree in chronic maintenance dialysis patients who require recombinant erythropoietin for treatment of renal diseaseinduced anemia than in nondialyzed patients with chronic renal failure. This is supported by the observation that erythropoietin leads to an increase in ET release from endothelial cells, and intravenous administration of erythropoietin to these
Endothelin and the Kidney
215
patients resulted in a significant increase in plasma ET levels that correlated well with the increase in blood pressure.
Acute Renal Failure Development of acute renal failure continues to be associated with high mortality rate, even though many advances have been made in the treatment of critically ill patients. Irrespective of whether the origin is ischemic or nephrotoxic, acute renal failure is always characterized by reduction in renal blood flow that invariably accompanies renal tubular dysfunction (Lieberthal and Levinsky, 1992). ET-1, the most potent vasoconstrictor yet identified, has been shown to decrease glomerular filtration rate as well as renal blood flow. A pathogenic role for ET in the development as well as maintenance of acute renal failure has been suggested based on the observations that plasma ET levels are elevated in patients with acute renal failure resulting from a variety of etiologies. Since hypoxia is a potent stimulus of ET secretion, it is possible that ET might play an important role in ischemia-induced acute renal failure. In fact, renal expression of prepro ET-1 mRNA was upregulated in rats 6 hours after renal ischemia. In addition to the increased expression of ET-1, there was an increase in ET binding following ischemia. More direct evidence for the involvement of ET-1 in acute renal failure comes from the data obtained with ET antibodies and ET receptor antagonists. Antibodies to ET-1 as well as ET receptor antagonists have been shown to protect against ischemia-induced acute renal failure. BQ123 (ETA-selective antagonist) as well as SB 209670 (nonselective antagonist) not only prevented but reversed ischemia-induced renal failure in rats. Based on these observations in rats, the receptor involved in this process might be ETA. In contrast, in the dog, BQ123 had no effect in preventing the decrease in glomerular filtration rate induced by aortic cross-clamping, whereas the nonselective antagonist, SB 209670, was beneficial in attenuating the decrease in glomerular filtration rate and the increase in fractional sodium excretion. These findings indicate that there are significant species differences between rat and dog. Also, based on the observations that in rat and dog, renal vasoconstriction is mediated by ETB and ETA receptors, respectively, and that BQ123 was beneficial in rat but not in dog, mechanisms other than ET-induced vasoconstriction such as tubular effects have been implicated by some investigators. Radiocontrast medium, which is of great use as a diagnostic tool, has been shown to be associated with the development of acute renal failure due to its intense renal vasoconstrictor effect. Radiocontrast nephropathy is a common cause of acute renal failure, accounting for approximately 10-12% of the hospitalized patients. Preexisting renal insufficiency in patients increases the risk of radiocontrast nephropathy by approximately fivefold compared to patients with normal kidney function. Renal vasoconstriction and renal medullary hypoxia are believed to play an important role in the pathogenesis of radiocontrast nephropathy. In rat and dog models of radio-
216
PONNAL NAMBI
contrast nephropathy, renal blood flow reduction was partially reversed by endothelin receptor antagonist. Major side effects of cyclosporine A, a potent immunosuppressive agent used primarily to prevent allograft rejection, include acute and chronic renal failure and hypertension. There is increasing evidence demonstrating the involvement of ET in cyclosporine A-induced nephrotoxicity. This is based on the observation that cyclosporine A treatment causes an increase in ET release from endothelial and mesangial cells, an increase in urinary ET excretion, an increase in plasma ET levels, an increase in ETB receptor mRNA in mesangial cells, and an increase in renal and cardiac ET receptors. In addition, treatment with nifedipine inhibited cyclosporine A-mediated renal dYsfunction as well as increased urinary ET excretion. Also, ET antibodies and ET receptor antagonists have been shown to attenuate or abolish cyclosporine A-induced renal dysfunction in animal models. Currently, there are no effective therapies for the treatment of established acute renal failure in humans. Based on the evidence obtained from animal studies, inhibition of the effects of ET may represent a novel therapeutic intervention.
SUMMARY ET has been shown to exert significant effects on glomerular, tubular, and vascular functions of the kidney. ET receptor subtypes mediating these effects of ET appear to be different in different species. ET production, metabolism as well as ET binding are altered in a number of renal diseases, and ET receptor antagonists as well as antibodies have been shown to be beneficial in these diseases. Although a number of very potent and selective ET receptor antagonists are available, the future challenge lies in our understanding of the subtype of ET receptor that is involved in different disease processes.
ACKNOWLEDGMENT I would like to thank Sue Tirri for expert secretarial assistance.
REFERENCES Arai, H., Hori, S., Aramori,I., Ohkubo,H., and Nakanishi, S. (1990). Cloningand expressionofa cDNA encoding an endothelin receptor. Nature 348, 730-732. Badr, K.F., Murray, J.J., Breyer, M.D., Takahashi, K., Inagami, T., and Harris, R.C. (1989). Mesangial cell, glomerularand renal vascularresponses to ET in the rat kidney. J. Clin. Invest. 83,336-342. Benigni, A., Zoja, C., Coma, D., Orisio,S., Longaretti,L., Bertani, T., and Remuzzi,G. (1993).A specific ET subtypeA receptor antagonistprotects against injury in renal disease progression. KidneyInt. 44, 440-444. Bergsma, D.J., Elshourbagy,N., and Kumar, C. (1995). Molecularbiology of endothelin receptors. In: Endothelin Receptors from Gene to the Human (Ruffolo, R.R., Jr., ed.). CRC Press, Boca Raton, pp. 37-57.
Endothelin and the Kidney
217
Clavell, A.L., Stingo, A.J., Margulies, K.B., Brandt, R.R., and Burnett, J.C., Jr. (1995) Role of endothelin receptor subtypes in the in vivo regulation of renal function. Am. J. Physiol. 268, F455-F460. Denton, K.M., and Anderson, W.P. (1990). Vascular actions of endothelin in the rabbit kidney. Clin. Exp. Pharmacol. Physiol. 17, 861-872. Elshourbagy, N.A., Korman, D.R., Wu, H.-L., S3/lvester, D.R., Lee, J.A., Nuthulaganti, P., Bergsma, D.J., Kumar, C.S., and Nambi, P. (1993). Molecular characterization and regulation of the human endothelin receptors. J. Biol. Chem. 268, 3873-3879. Gellai, M. (1997). Physiological role of endothelin in cardiovascular and renal hemodynamics: Studies in animals. Curr. Opin. Nephrol. Hypertens. 6, 64-68. Gellai, M., DeWolf, R., Pullen, M., and Nambi, P. (1994a). Distribution and functional role of renal ET receptor subtypes in normotensive and hypertensive rats. Kidney Int. 46, 1287-1294. Gellai, M., Jugus, M., Fletcher, T.A., DeWolf, R., and Nambi, P. (1994b). Reversal of post-ischemic acute renal failure with a selective ET A receptor antagonist in the rat. J. Clin. Invest. 93,900-904. Gellai, M., Fletcher, T., Pullen, M., and Nambi, P. (1996). Evidence for the existence of endothelin B receptor subtypes and their physiological roles in the rat. Am. J. Physiol. 271, R254-R261. Gellai, M., DeWolf, R., Fletcher, T., and Nambi, P. (1997). Contribution of endogenous endothelin-1 to the maintenance of vascular tone: Role of nitric oxide. Pharmacology 55,299-308. Huggins, J.P. (1997). Endothelin receptor molecular biology. In: Endothelins in Biology and Medicine (Huggins, J.P., and Pelton, J.T., (eds.). CRC Press, Boca Raton, pp. 27-43. Huggins, J.P., Pelton, J.T., and Miller, R.C. (1993). The structure and specificity of endothelin receptors: Their importance in physiology and medicine. Pharm. Ther. 59, 59-123. Ihara, M., Noguchi, K., Sachi, T., Fukuroda, T., Tsuchida, S., Kimura, S., Fukami, T., Ishikawa, K., Nishikibe, M., and Yano, M. (1992). Biological profiles of highly potent novel endothelin antagonists selective for the ET A receptor. Life Sci. 50, 247-255. Kaasjager, K.A., Shaw, S., Koomans, H.A., and Rabelink, T.J. (1997). Roles of endothelin receptor subtypes in systemic and renal responses to endothelin-1 in humans. J. Am. Soc. Nephrol. 8, 32-39. Karne, S., Jayawickreme, C.K., and Lerner, M.R. (1993). Cloning and characterization of an endothelin3 specific receptor (ETc receptor) from Xenopus laevis dermal melanophores. J. Biol. Chem. 268, 19126-19133. King, A.J., Brenner, B.M., and Anderson, S. (1989). Endothelin: A potent renal and systemic vasoconstrictor peptide. Am. J. Physiol. 256, F105 l-F1058. Klahr, S., Schreiner, G., and Ichikawa, I. (1988). The progression of renal disease. N. Engl. J. Med. 318, 1657-1666. Kohan, D.E. (1997). Endothelins in the normal and diseased kidney. Am. J. Kidney Dis. 29, 2-26. Kon, V., and Badr, K.E (1991). Biological actions and pathophysiologic significance of ET in the kidney. Kidney Int. 40, 1-12. Kumar, C., Mwangi, V., Nuthulaganti, P., Wu; H.-L., Pullen, M., Brun, K., Aiyar, H., Morris, R.A., Naughton, R., and Nambi, P. (1994). Cloning and characterization of a novel endothelin receptor from Xenopus heart. J. Biol. Chem. 269, 13414-13420. Kurihara, Y., Kurihara, H., Suzuki, H., Kodama, T., Maemura, K., Nagai, R., Oda, H., Kuwaki, T., Cao, W., and Kamada, N. (1994). Elevated blood pressure and craniofacial abnormalities in mice deficient in ET-1. Nature 368, 703-710. Lemonnier Degouville, A.-C., Lippton, H. L., Cavero, I., Summer, W.R., and Hyman, A.L. (1989). Endothelin--A new family of endothelium-derived peptides with widespread biological properties. Life Sci. 45, 1499-1513. Lieberthal, W., and Levinsky, N.G. (1992). Acute renal failure, In: Acute Renal Failure in the Kidney: Physiology and Pathophysiology, 2nd ed. (Seldin, D.W., and Giebisch, G., eds.). Raven Press, New York. Maccumber, M.W., Ross, C.A., Glaser, B.M., and Synder, S.H. (1989). Endothelin: Visualization of mRNAs by in situ hybridization provides evidence for local action. Proc. Natl. Acad. Sci. USA 86, 7285-7289.
218
PONNAL NAMBI
Marsen, T.A., Schramek, H., and Dunn, M.J. (1994). Renal actions of ET: Linking cellular signalling pathways to kidney disease. Kidney Int. 45,336-344. Nambi, P. (1996). Endothelin receptors in normal and diseased kidneys. Clin. Exp. Pharmacol. Physiol. 23,326-330. Nambi, P. (1998). Novel endothelin receptors. In: Endothelin Receptors and Signalling Mechanisms (Pollack, D., and Highsmith, R.R., eds.). Springer-Verlag, Berlin, pp. 17-21. Nambi, P., and Brooks, D.P. (1997). Effects of endothelin in the kidney. In: Endothelins in Biology and Medicine (Huggins, J.P., and Pelton, J.T., eds.). CRC Press, Boca Raton, pp. 171-190. Nambi, P., Kumar, C., and Ohlstein, E.H. (1995). Signal transduction processes involved in endothelinmediated responses. In: Endothelin Receptors: From the Gene to the Human (Ruffolo, R.R., Jr., ed.). CRC Press, Boca Raton, pp. 59-77. Nunez, D.J., Brown, M.J., Davenport, A.P., Neylon, C.B., Schofield, J.P., and Wyse, R.K. (1990). Endothelin-1 mRNA is widely expressed in porcine and human tissues. J. Clin. Invest. 85, 1537-1541. Sakurai, T., Yanagisawa, M., Takuma, Y., Miyazaki, H., Kimura, S., Goto, K., and Masaki, T. (1990). Cloning of a cDNA encoding a nonisopeptide-selective subtype of an endothelin receptor. Nature 348, 732-735. Simonson, M.S. (1993). Endothelins: Multifunctional renal peptides. Physiol. Rev. 73,375-411. Simonson, M.S., and Dunn, M.J. (1990). Cellular signalling by peptides of the endothelin gene family. FASEB J. 4, 2989-3000. Simonson, M.S., and Dunn, M.J. (1993). Endothelin peptides and the kidney. Annu. Rev. Physiol. 55, 249-265. Simonson, M.S., Jones, J.M., and Dunn, M.J. (1992). Cytosolic and nuclear signaling by endothelin peptides: Implications for the mesangial response to glomerular injury. Kidney Int. 41,542-545. Sokolovsky, M. (1992). Endothelins and sarafotoxins: Physiological regulation, receptor subtypes and transmembrane signalling. Pharm. Ther. 54, 129-149. Williams, D.L., Jr., Jones, K.L., Pettibone, D.J., Lis, E.V., and Clineschmidt, B.V. (1991). Sarafotoxin 6c: An agonist which distinguishes between endothelin receptor subtypes. Biochem. Biophys. Res. Commun. 185, 867-873. Yanagisawa, M., and Masaki, T. (1989). Molecular biology and biochemistry of the endothelins. Trends Pharmacol. Sci. 10, 374-378. Yanagisawa, M., Kurihara, H., Kimura, S., Tomobe, Y., Kobayashi, M., Mitsui, Y., Yazaki, Y., Goto, K., and Masaki, T. (1988). A novel potent vasoconstrictor peptide produced by vascular endothelial cells. Nature 332, 411-415.
Ponnal Nambi
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin Expression in the Kidney . . . . . . . . . . . . . . . . . . . . . . . . . . Renal Endothelin Receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin Receptor Signal Transduction in the Kidney . . . . . . . . . . . . . . . . Endothelin and Mesangium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin and Tubules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin and Renal Vasculature . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin Receptor Antagonists . . . . . . . . . . . . . . . . . . . . . . . . . . . . Endothelin in Renal Pathophysiology . . . . . . . . . . . . . . . . . . . . . . . . . . Chronic Renal Diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acute Renal Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
207 208 209 209 210 211 212 212 213 214 215 216 216 216
INTRODUCTION E n d o t h e l i n - 1 (ET-1), a 2 1 - a m i n o - a c i d peptide, initially i s o l a t e d f r o m the c u l t u r e m e d i u m o f p o r c i n e aortic e n d o t h e l i a l cells b y Y a n a g i s a w a et al. ( 1 9 8 8 ) is the m o s t p o t e n t v a s o c o n s t r i c t o r yet identified. In a d d i t i o n to e n d o t h e l i a l cells, ET-1 is also
Advances in Organ Biology Volume 9, pages 207-218. Copyright © 2000 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 0-7623-0617-3
207
208
PONNAL NAMBI
synthesized and released from a number of other ceil types such as mesangial cells, renal tubular cells, smooth muscle cells, and neuronal cells (Yanagisawa et al., 1988; Lemonnier Degouville et al., 1989; Simonson and Dunn, 1990). Endothelin has been shown to mediate its effects by binding to specific cell surface receptors that are located on many tissues and cells (Yanagisawa and Masaki, 1989; Sokolovsky, 1992; Huggins et al., 1993). It is proposed that ET-1 can act in an autocrine and paracrine fashion, indicating that it can be released at or near the site of action. The receptors for endothelin have been classified as ET a and ET B based on the differences in the binding profile of ET-1 and related agonists such as ET-3 and sarafotoxin 6c ($6c; a snake venom peptide that shares a high degree of homology with ET-1). Both ET A and ET B receptors have been cloned and expressed from a number of species including humans (Arai et al., 1990; Sakurai et al., 1990; Elshourbagy et al., 1993; Bergsma et al., 1995; Huggins, 1997). ET A receptors display high affinity for ET- 1 and ET-2, whereas ETBreceptors display equal affinity for ET- 1, ET-2, ET-3, and $6c. ET-3 and $6c are 100 and 1000 times, respectively, less potent than ET- 1 and ET-2 for ET A receptors. In addition, two more subtypes of ET receptors, ET c and ETAx, have been cloned and expressed fromXenopus melanophores (Karne et al., 1993) and heart (Kumar et al., 1994), respectively (Nambi, 1998).
ENDOTHELIN EXPRESSION IN THE KIDNEY A number of molecular biological techniques including Northern blot and ribonuclease protection analyses of RNA isolated from rat kidney displayed mRNAs for both prepro ET-1 and prepro ET-3 (Maccumber et al., 1989; Nunez et al., 1990). The expression of these two mRNAs appears to be species specific because prepro ET-1 expression was predominant in pig kidney, whereas prepro ET-3 expression was predominant in rat kidney (Nunez et al., 1990). Data from in situ hybridization studies indicate that prepro ET-1 was predominantly expressed in the vasa recta bundles of medulla (Maccumber et al., 1989) and vascular sites of cortex and medulla. Using reverse transcriptase and polymerase chain reaction, ET-1 mRNA has been localized to rat nephrons where it was present primarily in the glomerulus and inner medullary collecting duct. ET- 1 mRNA was also shown to be present in the proximal convoluted and straight tubules. ET-l-like immunoreactivity was observed in renal cortex, medulla, and papilla, although the vasa recta of distal nephron segments in the renal papilla showed the predominant localization of this peptide (Nambi and Brooks, 1997). The synthesis and/or release of ET-1 in the kidney has been shown to be influenced by a number of factors. Many renal epithelial cells in culture (such as MDCK, LLCPK, NRK-52E, BHK-21, and RK- 13) secrete ET- 1, and this secretion is potentiated by various factors such as thrombin, bradykinin, ATP, platelet activating factor, transforming growth factor 13, tumor necrosis factor, interleukin1[3, and cyclosporine A. In addition, human mesangial cells have also been shown to secrete ET in response to transforming growth factor-[3, arginine vasopressin,
Endothelin and the Kidney
209
and thrombin. On the other hand, atrial natriuretic peptide (ANP) and heparin have been shown to decrease ET-1 synthesis and release. The kidney also can degrade ET-1, probably by the action of enzymes such as neutral endopeptidase. The level of immunoreactive ET in urine is six times higher than in plasma of normal volunteers, indicating that the kidney plays a major role in ET synthesis as well as metabolism. The observation that plasma ET levels were significantly increased in patients undergoing hemodialysis, renal transplantation, or peritoneal dialysis further supports a role for ET in renal function (Kon and Badr, 1991).
RENAL ENDOTHELIN RECEPTORS Binding sites for ET-1 in kidneys from a number of species were described as early as 1989. Although these studies did not distinguish receptor subtypes, a very high density of ET binding sites was identified in mammalian kidney in the glomeruli, inner medulla, and inner strip of outer medulla. Proximal tubules displayed moderate binding sites, whereas no binding was observed in the outer stripe of the outer medulla. Later studies demonstrated the presence of subtypes of ET receptors based on the binding profile of the subtype-selective ligands ET-3 and $6c (Williams et al., 1991). In addition, discovery of BQ123 as an ETA-Selective antagonist (Ihara et al., 1992) enabled the quantitation of the subtypes of ET receptors in various regions of the kidney. The presence of ET receptors was also demonstrated in microdissected rat nephron segments. Molecular biological techniques such as reverse transcriptase polymerase chain reaction have been used to detect large signals for the ET B receptor subtype in the initial and terminal inner medullary collecting duct and glomerulus, whereas small signals were found in the cortical collecting duct, outer medullary collecting duct, vasa recta, and arcuate artery. Although ET A receptor mRNA was shown to be present in the glomerulus, binding studies using [ 125I]-ET-1 or [125I]-ET-3 indicated that the predominant subtype was ET B receptors. ET receptors are differentially expressed along the nephron. Cell culture studies indicate the presence of high levels of ET B receptors in glomerular, epithelial, and endothelial cells of the thin segments of Henle's loop. Studies reported on mesangial cell ET receptors are somewhat confusing. Mesangial cells have been shown to contain both ET A and ET B receptors (Simonson, 1993; Clavell et al., 1995; Nambi, 1996).
ENDOTHELIN RECEPTOR SIGNAL TRANSDUCTION IN THE KIDNEY ET receptors belong to the superfamily of seven transmembrane G protein-coupled receptors which mediate their biological actions through a number of GTP binding proteins and specific effector molecules. Binding of ET to its receptors results in the formation of a number of second messengers including inositol phosphates, diacylglycerol, intracellular calcium, cyclic adenosine monophosphate, cyclic
210
PONNAL NAMBI
guanosine monophosphate, and arachidonic acid (Marsen et al., 1994; Nambi et al., 1995). Depending on the coupling of the receptor to the effector system, different second messenger(s) are generated in different tissues. ET mediates both short-term (contraction) and long-term (mitogenesis) responses. ET-1 stimulates phospholipase C activity in mesangial cells through a pertussis toxin-sensitive G protein. Activation of protein kinase C through this pathway results in the release of arachidonic acid. In mesangial cells, this pathway appears to be very critical since arachidonic acid metabolism results in the synthesis of prostaglandins which regulate a number of functions in the kidney including natriuresis, diuresis, vascular tone, and renin secretion. ET has also been shown to stimulate phospholipase D in mesangial cells through a PKC-dependent pathway. In addition to phospholipase C and D activation, ET also activates phospholipase A 2 in mesangial cells leading to the formation of arachidonic acid which is metabolized to specific eicosanoids, depending on the species. In contrast to the observation that addition of ET to most cells and tissues resulted in the activation of phospholipase C, studies performed to evaluate the involvement of ET in the activation of cyclic nucleotide pathways have yielded different results. Studies in rat nephron segments have shown that ET-1 inhibits vasopressin-mediated cAMP accumulation by a PKC-dependent pathway, whereas in rat mesangial cells, ET-1 has been shown to stimulate cAMP accumulation through a PGE2dependent pathway. Exposure of NG 108-15 cells (mouse neuroblastoma/rat glioma hybrid cells) to ET-1 resulted in a calcium-dependent increase in the accumulation of cGMP that was inhibited by nitric oxide synthase inhibitors and hemoglobin. Similar observations have been reported using rat glomeruli and the increase in cGMP was shown to be mediated by ETB receptors. Thus, ET-induced increase in cGMP appears to be mediated by the nitric oxide synthase pathway. ET has also been shown to activate nuclear signal transduction pathways resulting in the expression of transcription factors such as c-fos, c-jun, and c-myc (Simonson et al., 1992). These transcription factors have been implicated in the mitogenic property of ET observed in many cells.
ENDOTHELIN A N D MESANGIUM Exposure of mesangial cells to ET has been shown to result in increased cell number and [3H]thymidine incorporation which are indicative of a proliferative response. El'-1 has also been shown to increase mesangial DNA topoisomerase I activity and this activation was inhibited by pretreatment with pertussis toxin. Although the mitogenic signals mediated by ET-1 are not well understood, it is possible that an ET-l-induced increase in platelet-derived growth factor A and B chains might be involved in this process. In addition, the induction of protooncogenes such as c-fos, c-jun, and c-myc might contribute to ET-l-mediated mitogenic response in mesangial cells (Simonson et al., 1992).
Endothelin and the Kidney
211
ET-mediated mesangial cell contraction has been shown as an increase in intensity and number of tension-generated wrinkles of silicon rubber substratum on which mesangial cells were grown as well as a decrease in mesangial cell planar surface area. ET-l-induced increases in intracellular calcium have been shown to be responsible for ET-mediated contraction in mesangial cells (Badr et al., 1989).
ENDOTHELIN A N D TUBULES ET has been shown to cause natriuresis at low concentrations and inhibit tubular sodium and bicarbonate transport as well as Na+/K+-ATPase activity. ET-mediated inhibition of Na+/K+-ATPase appears to be mediated by prostaglandins because this response was abolished by ibuprofen. Systemic infusions of ET-1 have had variable effects on sodium. In some studies, systemic infusion of ET- 1 caused a decrease in sodium excretion, whereas in other studies, ET caused a natriuresis, despite a decrease in renal blood flow and GFR. Studies performed with isolated perfused kidneys also demonstrate ET-mediated increases in Na + excretion, despite a decrease in GFR, which is different from whole animal studies where intrarenal infusion of low doses of ET-l was without effect on Na + excretion, whereas higher doses caused a decrease in Na + excretion. In addition, ETs have been shown to increase plasma levels of ANE which is another important regulator of renal sodium handling. Coinfusion of ANP along with ET-1 has resulted in inhibition of ET-mediated renal effects. ET has been shown to have some water diuretic activity, indicating that it has an effect on water reabsorption, despite its inhibitory effects on GFR and RBE The water diuretic effect of ET- 1 appears to be species specific. This diuretic effect of ET has been suggested to be mediated through its inhibitory effect on AVP-induced cAMP accumulation and independent of its effect on Na + reabsorption and renal hemodynamics. ET inhibited AVP-induced cAMP accumulation in cortical collecting duct, outer medullary collecting duct, and inner medullary collecting duct, whereas it did not affect AVP-mediated cAMP accumulation in other nephron segments. In addition to its effects on the kidney, ETs may act on the posterior pituitary to alter vasopressin release, and there are suggestions that ETs might interact with AVP at the hypothalamic-pituitary axis. ETs appear to have multiple effects on renin release and the renin-angiotensin system which is the key regulator of extracellular fluid volume. While systemic infusions of ET resulted in a dramatic increase in renin activity, possibly through the intrarenal baroreceptor and macula-densa-mediated pathways, in vitro experiments performed with cortical slices, glomeruli, or juxtaglomerular cells demonstrated an inhibition of renin release by ET-1.
212
PONNAL NAMBI
ENDOTHELIN A N D RENAL VASCULATURE Depending on the mode of infusion, ET- 1 has been shown to cause differential renal effects. Systemic infusion of ET-1 caused a significant increase in renal vascular resistance and a concomitant decrease in renal blood flow (King et al., 1989; Denton and Anderson, 1990; Kon and Badr, 1991). ET-3 was much weaker than ET-1 in increasing renal vascular resistance and, in addition, caused a transient vasorelaxation, although the magnitude of this vasodilation was less than that observed in other vascular beds (Yanagisawa et al., 1988; Lemonnier Degouville et al., 1989). The vasodilatory response mediated by ETs was attenuated by indomethacin, whereas the vasoconstrictor response was augmented, suggesting that the prostaglandins might serve to counteract the effect of ETs on renal vasoconstriction. Micropuncture studies with ET-1 suggest that ET-1 can contract both afferent and efferent arterioles (Badr et al., 1989). The signal transduction pathways involved in ET-mediated renal vasoconstriction involve both influx of extracellular calcium as well as mobilization of intracellular Ca 2÷. This conclusion is based on the observations that Ca 2÷ channel blockers such as verapamil and nicardipine attenuated ET-mediated afferent arteriolar response only, whereas the efferent arteriolar responses appear to be mediated by the release of intracellular Ca 2÷ (Simonson and Dunn, 1990). Infusion of ET- 1 directly into the renal artery of rats or dogs resulted in a transient increase followed by a sustained decrease of renal blood flow. Experiments performed with isolated kidneys of rat and rabbit also demonstrated ET-mediated increases in vascular resistance which was partially dependent on the presence of extracellular Ca 2+.
EN DOTH ELI N RECEPTOR A N T A G O N ISTS Enzyme inhibitors which will block the generation of agonists, or receptor antagonists which will block agonist-mediated activation of the receptor are excellent tools to determine the physiological role of an agonist. Invariably, the results obtained from in vivo and in vitro studies have been quite different for many antagonists. Endothelin is not an exception to this. Even though exogenously administered ET-1 is the most potent vasoconstrictor yet identified, studies with ET receptor antagonists failed to identify the vasoconstrictor response mediated by endogenous ET in normal animals (Gellai et al., 1994a). No significant changes were observed in systemic or renal hemodynamics of conscious or anesthetized rats and dogs against ETA-selective or ETa/ETB-selective antagonists. These findings suggest that the contribution of endogenous ET- 1 to the maintenance of systemic and renal vascular tone is minimal in normotensive animals (Gellai, 1997). An unexpected finding was that mice with the ET-1 gene deleted had elevated blood pressure, suggesting that the predominant role of ET-1 in normal animal is vasodilation (Kurihara et al., 1994). While an ET a receptor antagonist had no effect on blood pressure and renal
Endothelin and the Kidney
213
blood flow in normotensive animals, the ETB-selective antagonist, RES 701-1, potentiated systemic and renal vasoconstriction induced by bolus injection of ET-1 or $6c in conscious chronically instrumented rats (Gellai et al., 1997). These effects were attenuated by a mixed ETA/ETB receptor antagonist but not by the ET aselective antagonist, BQ123 (Gellai et al., 1997). These findings clearly suggest that ET plays an important role in the control of basal vascular tone by mediating vasodilation and vasoconstriction. These effects appear to be mediated by different ET B receptor subtypes. The ET B receptor that mediates the vasodilation is sensitive to RES 701-1, whereas the ET B receptor that is involved in vasoconstriction is insensitive to RES 701-1 (Gellai et al., 1996). Similar conclusions have been drawn from studies performed in anesthetized pigs. In conscious Sprague-Dawley (SD) rats, exogenous administration of ET-1 and $6c resulted in a dose-dependent decrease in renal blood flow which was unaffected by infusion of BQ123 (ETa-selective antagonist). Spontaneously hypertensive (SH) rats were more sensitive to the renal vasoconstrictor effects of ET-1 and $6c. While $6c and ET-1 were equipotent in SD rats, $6c was more potent than ET-1 in SH rats. These functional data agreed well with the radioligand binding data obtained from these rat kidneys. There was an increase in the proportion and affinity of ET B receptors present in the cortex of SH rats compared to SD rats (Gellai et al., 1994a). This further confirms the involvement of ET B receptors in the maintenance of vascular tone in rats. The only physiological role identified for ET A receptor in the rat kidney is sodium handling (Gellai et al., 1994a), whereas in the dog, sodium excretion is regulated by the ET Breceptor (Clavell et al., 1995; Nambi and Brooks, 1997). Systemic infusion of ET-1 (nonselective agonist) to healthy human volunteers resulted in an increase in arterial pressure and decreases in renal plasma flow, GFR, and sodium excretion, whereas infusion of ET-3 (ETB-selective agonist) had no effect on blood pressure, electrolyte excretion, and renal hemodynamics, suggesting that in humans, the systemic and renal vasoconstrictor effects of ET-1 are mediated by ETA receptors (Kaasjager et al., 1997).
ENDOTHELIN IN RENAL PATHOPHYSIOLOGY Involvement of ET in a number of renal diseases including chronic renal failure, ischemia-induced acute renal failure, cyclosporine- and radiocontrast-induced nephrotoxicity has been suggested by a number of investigators (Kon and Badr, 1991; Marsen et al., 1994; Kohan, 1997; Nambi and Brooks, 1997). The evidence is compelling because of the beneficial effects demonstrated by ET receptor antagonists or neutralizing antibodies to ET in these animal models (Kon and Badr, 1991; Gellai et al., 1994b).
214
PONNAL NAMBI
Chronic Renal Diseases Chronic renal insufficiency invariably progresses to end-stage renal failure, and a common histologic end point is glomerulosclerosis. Although the exact mechanisms that are involved in the development and progression of chronic renal failure are not known, the primary causes appear to be hypertension and type I and type II diabetes. Several pathophysiologic changes such as intraglomerular hypertension, intrinsic glomerular thrombosis, mesangial cell proliferation, and/or matrix production have been suggested to contribute to the pathogenesis of chronic renal failure. In addition, altered glomerular permeability to proteins and lipids leading to glomerulosclerosis, inflammation, and interstitial fibrosis has been suggested to play a critical role in the development and progression of chronic renal failure (Klahr et al., 1988). Although the role of ET in the development and/or progression of chronic renal disease is not clear, it is possible that ET could be involved in all of the above-mentioned processes. ET has been shown to act as a potent mitogen for mesangial cells, possibly through the upregulation of protooncogenes, and many of the proinflammatory mediators such as thrombin, transforming growth factor-[3, and platelet-derived growth factor have been shown to stimulate ET production in glomerular endothelial and mesangial cells (Simonson et al., 1992). In addition, ET increased intraglomerular pressure and decreased single nephron filtration rate (Badr et al., 1989). Increased levels of ET gene expression as well as urinary excretion have been reported in rats with 5/6 nephrectomy-induced chronic renal failure as well as diabetic rat models of chronic renal failure. There was a good correlation between the urinary excretion of ET, proteinuria, and hypertension. ET gene expression was also increased in the glomeruli of streptozotocin-induced diabetic rats. In cpk/cpk mice which develop polycystic kidney disease, there was a good correlation between the ET- 1 as well as ET receptor mRNA expression and disease progression. Clinical studies conducted in patients with chronic renal disease reveal increased plasma levels and urinary excretion of ET. Increase in plasma ET correlated well with the increase in plasma creatinine in these patients. A pathogenic role for ET-1 in chronic renal disease has been supported by the recent study performed in the rat remnant kidney model where the ETA receptor antagonist, FR 139367, was effective in limiting the glomerular injury, reducing urinary protein excretion, renal c-fos expression, mesangial hypercellularity, and the deterioration of renal function (Benigni et al., 1993). ET has also been suggested to be a contributing factor to the problems associated with the treatment of patients with end-stage renal disease. Plasma ET levels are elevated to a greater degree in chronic maintenance dialysis patients who require recombinant erythropoietin for treatment of renal diseaseinduced anemia than in nondialyzed patients with chronic renal failure. This is supported by the observation that erythropoietin leads to an increase in ET release from endothelial cells, and intravenous administration of erythropoietin to these
Endothelin and the Kidney
215
patients resulted in a significant increase in plasma ET levels that correlated well with the increase in blood pressure.
Acute Renal Failure Development of acute renal failure continues to be associated with high mortality rate, even though many advances have been made in the treatment of critically ill patients. Irrespective of whether the origin is ischemic or nephrotoxic, acute renal failure is always characterized by reduction in renal blood flow that invariably accompanies renal tubular dysfunction (Lieberthal and Levinsky, 1992). ET-1, the most potent vasoconstrictor yet identified, has been shown to decrease glomerular filtration rate as well as renal blood flow. A pathogenic role for ET in the development as well as maintenance of acute renal failure has been suggested based on the observations that plasma ET levels are elevated in patients with acute renal failure resulting from a variety of etiologies. Since hypoxia is a potent stimulus of ET secretion, it is possible that ET might play an important role in ischemia-induced acute renal failure. In fact, renal expression of prepro ET-1 mRNA was upregulated in rats 6 hours after renal ischemia. In addition to the increased expression of ET-1, there was an increase in ET binding following ischemia. More direct evidence for the involvement of ET-1 in acute renal failure comes from the data obtained with ET antibodies and ET receptor antagonists. Antibodies to ET-1 as well as ET receptor antagonists have been shown to protect against ischemia-induced acute renal failure. BQ123 (ETA-selective antagonist) as well as SB 209670 (nonselective antagonist) not only prevented but reversed ischemia-induced renal failure in rats. Based on these observations in rats, the receptor involved in this process might be ETA. In contrast, in the dog, BQ123 had no effect in preventing the decrease in glomerular filtration rate induced by aortic cross-clamping, whereas the nonselective antagonist, SB 209670, was beneficial in attenuating the decrease in glomerular filtration rate and the increase in fractional sodium excretion. These findings indicate that there are significant species differences between rat and dog. Also, based on the observations that in rat and dog, renal vasoconstriction is mediated by ETB and ETA receptors, respectively, and that BQ123 was beneficial in rat but not in dog, mechanisms other than ET-induced vasoconstriction such as tubular effects have been implicated by some investigators. Radiocontrast medium, which is of great use as a diagnostic tool, has been shown to be associated with the development of acute renal failure due to its intense renal vasoconstrictor effect. Radiocontrast nephropathy is a common cause of acute renal failure, accounting for approximately 10-12% of the hospitalized patients. Preexisting renal insufficiency in patients increases the risk of radiocontrast nephropathy by approximately fivefold compared to patients with normal kidney function. Renal vasoconstriction and renal medullary hypoxia are believed to play an important role in the pathogenesis of radiocontrast nephropathy. In rat and dog models of radio-
216
PONNAL NAMBI
contrast nephropathy, renal blood flow reduction was partially reversed by endothelin receptor antagonist. Major side effects of cyclosporine A, a potent immunosuppressive agent used primarily to prevent allograft rejection, include acute and chronic renal failure and hypertension. There is increasing evidence demonstrating the involvement of ET in cyclosporine A-induced nephrotoxicity. This is based on the observation that cyclosporine A treatment causes an increase in ET release from endothelial and mesangial cells, an increase in urinary ET excretion, an increase in plasma ET levels, an increase in ETB receptor mRNA in mesangial cells, and an increase in renal and cardiac ET receptors. In addition, treatment with nifedipine inhibited cyclosporine A-mediated renal dYsfunction as well as increased urinary ET excretion. Also, ET antibodies and ET receptor antagonists have been shown to attenuate or abolish cyclosporine A-induced renal dysfunction in animal models. Currently, there are no effective therapies for the treatment of established acute renal failure in humans. Based on the evidence obtained from animal studies, inhibition of the effects of ET may represent a novel therapeutic intervention.
SUMMARY ET has been shown to exert significant effects on glomerular, tubular, and vascular functions of the kidney. ET receptor subtypes mediating these effects of ET appear to be different in different species. ET production, metabolism as well as ET binding are altered in a number of renal diseases, and ET receptor antagonists as well as antibodies have been shown to be beneficial in these diseases. Although a number of very potent and selective ET receptor antagonists are available, the future challenge lies in our understanding of the subtype of ET receptor that is involved in different disease processes.
ACKNOWLEDGMENT I would like to thank Sue Tirri for expert secretarial assistance.
REFERENCES Arai, H., Hori, S., Aramori,I., Ohkubo,H., and Nakanishi, S. (1990). Cloningand expressionofa cDNA encoding an endothelin receptor. Nature 348, 730-732. Badr, K.F., Murray, J.J., Breyer, M.D., Takahashi, K., Inagami, T., and Harris, R.C. (1989). Mesangial cell, glomerularand renal vascularresponses to ET in the rat kidney. J. Clin. Invest. 83,336-342. Benigni, A., Zoja, C., Coma, D., Orisio,S., Longaretti,L., Bertani, T., and Remuzzi,G. (1993).A specific ET subtypeA receptor antagonistprotects against injury in renal disease progression. KidneyInt. 44, 440-444. Bergsma, D.J., Elshourbagy,N., and Kumar, C. (1995). Molecularbiology of endothelin receptors. In: Endothelin Receptors from Gene to the Human (Ruffolo, R.R., Jr., ed.). CRC Press, Boca Raton, pp. 37-57.
Endothelin and the Kidney
217
Clavell, A.L., Stingo, A.J., Margulies, K.B., Brandt, R.R., and Burnett, J.C., Jr. (1995) Role of endothelin receptor subtypes in the in vivo regulation of renal function. Am. J. Physiol. 268, F455-F460. Denton, K.M., and Anderson, W.P. (1990). Vascular actions of endothelin in the rabbit kidney. Clin. Exp. Pharmacol. Physiol. 17, 861-872. Elshourbagy, N.A., Korman, D.R., Wu, H.-L., S3/lvester, D.R., Lee, J.A., Nuthulaganti, P., Bergsma, D.J., Kumar, C.S., and Nambi, P. (1993). Molecular characterization and regulation of the human endothelin receptors. J. Biol. Chem. 268, 3873-3879. Gellai, M. (1997). Physiological role of endothelin in cardiovascular and renal hemodynamics: Studies in animals. Curr. Opin. Nephrol. Hypertens. 6, 64-68. Gellai, M., DeWolf, R., Pullen, M., and Nambi, P. (1994a). Distribution and functional role of renal ET receptor subtypes in normotensive and hypertensive rats. Kidney Int. 46, 1287-1294. Gellai, M., Jugus, M., Fletcher, T.A., DeWolf, R., and Nambi, P. (1994b). Reversal of post-ischemic acute renal failure with a selective ET A receptor antagonist in the rat. J. Clin. Invest. 93,900-904. Gellai, M., Fletcher, T., Pullen, M., and Nambi, P. (1996). Evidence for the existence of endothelin B receptor subtypes and their physiological roles in the rat. Am. J. Physiol. 271, R254-R261. Gellai, M., DeWolf, R., Fletcher, T., and Nambi, P. (1997). Contribution of endogenous endothelin-1 to the maintenance of vascular tone: Role of nitric oxide. Pharmacology 55,299-308. Huggins, J.P. (1997). Endothelin receptor molecular biology. In: Endothelins in Biology and Medicine (Huggins, J.P., and Pelton, J.T., (eds.). CRC Press, Boca Raton, pp. 27-43. Huggins, J.P., Pelton, J.T., and Miller, R.C. (1993). The structure and specificity of endothelin receptors: Their importance in physiology and medicine. Pharm. Ther. 59, 59-123. Ihara, M., Noguchi, K., Sachi, T., Fukuroda, T., Tsuchida, S., Kimura, S., Fukami, T., Ishikawa, K., Nishikibe, M., and Yano, M. (1992). Biological profiles of highly potent novel endothelin antagonists selective for the ET A receptor. Life Sci. 50, 247-255. Kaasjager, K.A., Shaw, S., Koomans, H.A., and Rabelink, T.J. (1997). Roles of endothelin receptor subtypes in systemic and renal responses to endothelin-1 in humans. J. Am. Soc. Nephrol. 8, 32-39. Karne, S., Jayawickreme, C.K., and Lerner, M.R. (1993). Cloning and characterization of an endothelin3 specific receptor (ETc receptor) from Xenopus laevis dermal melanophores. J. Biol. Chem. 268, 19126-19133. King, A.J., Brenner, B.M., and Anderson, S. (1989). Endothelin: A potent renal and systemic vasoconstrictor peptide. Am. J. Physiol. 256, F105 l-F1058. Klahr, S., Schreiner, G., and Ichikawa, I. (1988). The progression of renal disease. N. Engl. J. Med. 318, 1657-1666. Kohan, D.E. (1997). Endothelins in the normal and diseased kidney. Am. J. Kidney Dis. 29, 2-26. Kon, V., and Badr, K.E (1991). Biological actions and pathophysiologic significance of ET in the kidney. Kidney Int. 40, 1-12. Kumar, C., Mwangi, V., Nuthulaganti, P., Wu; H.-L., Pullen, M., Brun, K., Aiyar, H., Morris, R.A., Naughton, R., and Nambi, P. (1994). Cloning and characterization of a novel endothelin receptor from Xenopus heart. J. Biol. Chem. 269, 13414-13420. Kurihara, Y., Kurihara, H., Suzuki, H., Kodama, T., Maemura, K., Nagai, R., Oda, H., Kuwaki, T., Cao, W., and Kamada, N. (1994). Elevated blood pressure and craniofacial abnormalities in mice deficient in ET-1. Nature 368, 703-710. Lemonnier Degouville, A.-C., Lippton, H. L., Cavero, I., Summer, W.R., and Hyman, A.L. (1989). Endothelin--A new family of endothelium-derived peptides with widespread biological properties. Life Sci. 45, 1499-1513. Lieberthal, W., and Levinsky, N.G. (1992). Acute renal failure, In: Acute Renal Failure in the Kidney: Physiology and Pathophysiology, 2nd ed. (Seldin, D.W., and Giebisch, G., eds.). Raven Press, New York. Maccumber, M.W., Ross, C.A., Glaser, B.M., and Synder, S.H. (1989). Endothelin: Visualization of mRNAs by in situ hybridization provides evidence for local action. Proc. Natl. Acad. Sci. USA 86, 7285-7289.
218
PONNAL NAMBI
Marsen, T.A., Schramek, H., and Dunn, M.J. (1994). Renal actions of ET: Linking cellular signalling pathways to kidney disease. Kidney Int. 45,336-344. Nambi, P. (1996). Endothelin receptors in normal and diseased kidneys. Clin. Exp. Pharmacol. Physiol. 23,326-330. Nambi, P. (1998). Novel endothelin receptors. In: Endothelin Receptors and Signalling Mechanisms (Pollack, D., and Highsmith, R.R., eds.). Springer-Verlag, Berlin, pp. 17-21. Nambi, P., and Brooks, D.P. (1997). Effects of endothelin in the kidney. In: Endothelins in Biology and Medicine (Huggins, J.P., and Pelton, J.T., eds.). CRC Press, Boca Raton, pp. 171-190. Nambi, P., Kumar, C., and Ohlstein, E.H. (1995). Signal transduction processes involved in endothelinmediated responses. In: Endothelin Receptors: From the Gene to the Human (Ruffolo, R.R., Jr., ed.). CRC Press, Boca Raton, pp. 59-77. Nunez, D.J., Brown, M.J., Davenport, A.P., Neylon, C.B., Schofield, J.P., and Wyse, R.K. (1990). Endothelin-1 mRNA is widely expressed in porcine and human tissues. J. Clin. Invest. 85, 1537-1541. Sakurai, T., Yanagisawa, M., Takuma, Y., Miyazaki, H., Kimura, S., Goto, K., and Masaki, T. (1990). Cloning of a cDNA encoding a nonisopeptide-selective subtype of an endothelin receptor. Nature 348, 732-735. Simonson, M.S. (1993). Endothelins: Multifunctional renal peptides. Physiol. Rev. 73,375-411. Simonson, M.S., and Dunn, M.J. (1990). Cellular signalling by peptides of the endothelin gene family. FASEB J. 4, 2989-3000. Simonson, M.S., and Dunn, M.J. (1993). Endothelin peptides and the kidney. Annu. Rev. Physiol. 55, 249-265. Simonson, M.S., Jones, J.M., and Dunn, M.J. (1992). Cytosolic and nuclear signaling by endothelin peptides: Implications for the mesangial response to glomerular injury. Kidney Int. 41,542-545. Sokolovsky, M. (1992). Endothelins and sarafotoxins: Physiological regulation, receptor subtypes and transmembrane signalling. Pharm. Ther. 54, 129-149. Williams, D.L., Jr., Jones, K.L., Pettibone, D.J., Lis, E.V., and Clineschmidt, B.V. (1991). Sarafotoxin 6c: An agonist which distinguishes between endothelin receptor subtypes. Biochem. Biophys. Res. Commun. 185, 867-873. Yanagisawa, M., and Masaki, T. (1989). Molecular biology and biochemistry of the endothelins. Trends Pharmacol. Sci. 10, 374-378. Yanagisawa, M., Kurihara, H., Kimura, S., Tomobe, Y., Kobayashi, M., Mitsui, Y., Yazaki, Y., Goto, K., and Masaki, T. (1988). A novel potent vasoconstrictor peptide produced by vascular endothelial cells. Nature 332, 411-415.