Cellular Effects of Guanylin and Uroguanylin

JASN Express. Published on December 28, 2005 as doi: 10.1681/ASN.2005080818 Review

Aleksandra Sinic´and Eberhard Schlatter Universita¨tsklinikum Mu¨nster, Medizinische Klinik und Poliklinik D, Experimentelle Nephrologie, Mu¨nster, Germany

Ingestion of a salty meal induces secretion of guanylin (GN) and uroguanylin (UGN) into the intestinal lumen, where they ؉ ؊ ؊ inhibit Na absorption and induce Cl , HCO3 , and water secretion. Simultaneously, these hormones stimulate renal electrolyte excretion by inducing natriuresis, kaliuresis, and diuresis. GN and UGN therefore participate in the prevention of hypernatremia and hypervolemia after salty meals. The signaling pathway of GN and UGN in the intestine is well known. They activate enterocytes via guanylate cyclase C (GC-C), which leads to cGMP-dependent inhibition of Na؉/H؉ exchange and activation of the cystic fibrosis transmembrane regulator. In GC-C–deficient mice, GN and UGN still produce renal natriuresis, kaliuresis, and diuresis, suggesting different signaling pathways in the kidney compared with the intestine. Signaling pathways for GN and UGN in the kidney differ along the various nephron segments. In proximal tubule cells, a cGMP- and GC-C–dependent signaling was demonstrated for both peptides. In addition, UGN activates a pertussis toxin– sensitive G-protein–coupled receptor. A similar dual signaling pathway is also known for atrial natriuretic peptide. Recently, a cGMP-independent signaling pathway for GN and UGN was also shown in principal cells of the human and mouse cortical collecting duct. Because GN and UGN activate different signaling pathways in specific organs and even within the kidney, this review focuses on more recent findings on cellular effects and signaling mechanisms of these peptides and their pathophysiologic implications in the intestine and the kidney. J Am Soc Nephrol ●●: ●●●–●●●, ●●●●. doi: 10.1681/ASN.2005080818

uanylin peptides, guanylin (GN) and uroguanylin endogenous peptides GN and UGN, STa contains three di- (UGN), are small, heat-stable peptides with 15 to 19 sulfide bonds. It is presumed that this is the reason for its G amino acids. Human GN consists of 15 amino acids stronger and uncontrolled activation of GC-C that leads to and possesses two disulfide bonds between the cysteins in marked intestinal secretion of electrolytes and water and is positions 4 to 12 and 7 to 15 (1). Human UGN consists of 16 manifested as secretory diarrhea (9). The identification of a amino acids and also has two disulfide bonds at the same receptor for the exogenous enterotoxin in the intestine and in positions (2). These disulfide bonds are essential for the activity other tissues (kidney, reproductive system, and lung) led to of the peptides. The genes for guanylin peptides are located on the search for endogenous ligands for GC-C and their phys- the human chromosome 1 (p33 to p36) and the mouse chromo- iologic function. In the early 1990s, GN was isolated from rat some 4 (3). Human GN and UGN are coded by different but intestine (5) and UGN from opossum urine (6). Other re- similar genes that consist of three exons and two introns. Both cently discovered new members of this peptide family are peptides are synthesized as prepropeptide. Guanylin peptides renoguanylin (10) and lymphoguanylin (11). Like GN and are produced in the intestine after ingestion of a salty meal and UGN, renoguanylin has two disulfide bonds, whereas lym- secreted into the intestinal lumen (4). In the intestine, GN and phoguanylin possesses only one. Lymphoguanylin is less UGN activate enterocytes via guanylate cyclase C (GC-C), with potent than GN and UGN in the intestine (11). As guanylin cGMP as second messenger (5,6). Activation of this pathway Ϫ Ϫ peptides regulate electrolyte excretion by the intestine and leads to the secretion of Cl and HCO3 and to the inhibition ϩ of Na absorption, which drives water secretion. In addition, the kidney, they complement other well-known hormonal these hormones induce an increased salt and water excretion in systems that are involved in regulation of water and electro- the kidney (7). The decrease of salt absorption in the intestine lyte homeostasis, such as the renin-angiotensin-aldosterone together with the increase of renal salt excretion prevents the system, adiuretin (vasopressin) or atrial natriuretic peptide development of hypernatremia after ingestion of high amounts (ANP), brain natriuretic peptide (BNP), and C type natri- of salt. uretic peptide (CNP). GC-C was first described as a receptor for the exogenous A number of excellent reviews on various aspects of guanylin heat-stable enterotoxin of Escherichia coli (STa) (8). Unlike the peptides exist (12,13); however, recent growing understanding of cellular signaling mechanisms also in humans is not covered in these reviews. Therefore, the aim of this review is to sum- Published online ahead of print. Publication date available at www.jasn.org. marize the current knowledge on cellular effects of guanylin Address correspondence to: Dr. Eberhard Schlatter, Universita¨tsklinikum Mu¨n- peptides and their signaling pathways and possible pathophys- ster, Medizinische Klinik und Poliklinik D, Experimentelle Nephrologie, iologic implications in different organs, mainly the intestine Domagkstrasse 3a, Mu¨nster 48149, Germany. Phone: ϩ49-251-83-56991; Fax: ϩ49- 251-83-56973; E-mail: [email protected] and the kidney of different species including man.

Guanylate Cyclases The big family of guanylate cyclases consists of one cytoplasmic (soluble) guanylate cyclase, which is the receptor for nitric oxide and carbonic monoxide, and six isoforms of membrane guanylate cyclases. GC-A, GC-B, and GC-C are receptors for natriuretic peptides that regulate electrolyte and water homeostasis and control BP (14). GC-A (or NPR-A) is the receptor for ANP and BNP; GC-B (or NPR-B) is the receptor for CNP. A third receptor that binds these three peptides, the so-called clearance receptor (or NPR-C), has no guanylate cyclase activity, and hormone binding to this receptor leads to endocytosis of the hormone-receptor complex and activation of a pertussis toxin–sensitive G-protein. The latter mediates an inhibition of adenylate cyclase (15,16). Guanylate cyclases D, E, and F (GC-D, GC-E, and GC-F) are expressed in sensory organs, and, until now, their agonists and functions are not known (17–20). Figure 1. Guanylate cyclase C (GC-C) forming a dimer. The GC-G is also an orphan receptor without known agonist, and it binding of guanylin (GN), uroguanylin (UGN), or heat-stable is present in skeletal muscles, lungs, and intestine. Hirsch et al. enterotoxin of Escherichia coli (STa) to the extracellular domain (21) showed for the first time the presence of GC-G also in the of GC-C leads to activation of the catalytic domain with the kidney and identified this guanylate cyclase as the only mem- production of cGMP and the dephosphorylation of the kinase brane-bound isoform present in principal cells of rat cortical homology domain with receptor desensitization. For details, collecting duct (CCD). Table 1 summarizes the tissue distribu- see text. L, ligand. tion and known agonists for the eight guanylate cyclases identified (22,23). terminal tail increased STa-mediated cGMP generation by 70% GC-C when compared with the nonphosphorylated receptor (31,32). GC-C is the main receptor for guanylin peptides in the intes- The existence of additional receptors for guanylin peptides tine (Figure 1). GC-C forms dimers, trimers, or tetramers when became evident when, for example, intestinal and renal effects inserted into the plasma membrane (24). mRNA for GC-C is of these peptides were examined in GC-C–deficient mice. present also in adrenal glands, brain, the embryonic or regen- These mice are resistant to intestinal secretion produced by STa erating but not adult liver, placenta, testis, airways, spleen, (33–35), but approximately 10% of the intestinal binding sites thymus, and lymphatic nodes (25–27). Human and rat GC-C for these peptides are still present when compared with wild- have 71% homology in the extracellular domain and 91% in the type mice. In the kidney, guanylin peptides still induce salure- intracellular domain. The extracellular domain must be glyco- sis and diuresis in these GC-C–deficient mice (36), strongly sylated for complete receptor activity (28,29), and dephosphor- suggesting an additional receptor and possibly signaling cas- ylation caused by binding of an agonist to the extracellular cade for guanylin peptides at least in the kidney and to some domain leads to loss of receptor sensitivity (30). extent also in the intestine. That renal effects of guanylin pep- The most studied regulator of GC-C activity is protein kinase tides are maintained when GC-C is absent also indicates that C (PKC). GC-C phosphorylation by PKC leads to an increase in GC-C plays no or only a minor role for these peptides in the cGMP accumulation. PKC-induced phosphorylation at the C- kidney.

Table 1. Expression of cytoplasmic and membrane-bound guanylate cyclasesa

Cyclases Tissue Distribution Agonists

Cytoplasmic Skeletal muscle, platelets, lung, liver, kidney, heart, CNS NO, CO GC-A Smooth muscle, kidney, adrenal gland, heart, CNS ANP, BNP GC-B Fibroblasts, heart, CNS CNP GC-C Intestine, adrenal gland, CNS, lung, reproductive glands, kidney STa, GN, UGN GC-D Olfactory cells Unknown GC-E Retina Unknown GC-F Retina Unknown GC-G Skeletal muscle, lung, intestine, kidney Unknown

aANP, atrial natriuretic peptide; BNP, brain natriuretic peptide; CNP, C-type natriuretic peptide; CNS, central nervous system; CO, carbon monoxide; NO, nitric oxide; STa, heat-stable enterotoxin of Escherichia coli; GN, guanylin; UGN, uroguanylin. J Am Soc Nephrol ●●: ●●●–●●●, ●●●● Cellular Effects of Guanylin and Uroguanylin 3

Signaling Pathways of Guanylin Peptides in the Intestine Guanylin peptides secreted into the intestinal lumen in response to high oral salt intake bind to GC-C localized in the luminal membrane of enterocytes and induce electrolyte and water excretion by a complex signaling cascade (Figure 2): 1. Increase of the intracellular concentration of cGMP (5,6,8) ϩ ϩ 2. Inhibition of Na /H exchange, which results in decreased ϩ Na reabsorption (37) 3. Activation of PKG II (38,39) 4. Activation of PKA directly (40,41) or indirectly via inhibition of phosphodiesterase III (PDE III), which leads to an increase of cellular cAMP and activation of PKA (39) 5. PKG II and PKA activate cystic fibrosis transmembrane reg- Ϫ ulator (CFTR) in the luminal membrane (42), leading to Cl secretion into the intestinal lumen Ϫ Ϫ 6. CFTR activates the Cl /HCO3 exchanger, which leads to bicarbonate secretion into the intestinal lumen (43) Figure 3. Relative expression of GN (f) and UGN (Ⅺ) mRNA along the intestinal tract. Adapted from reference (46). GN and UGN are expressed along the intestinal tract (Figure 3) together with GC-C (44–46), which is the major intestinal receptor for GN and UGN. However, the expression pattern of tinct from GC-C are also located on the basolateral membrane ϩ these two hormones is differential, i.e., highest expression of of colonocytes (47). STa also activates the Ca2 signaling path- UGN in the jejunum and highest expression of GN in ileum to way with activation of PKC (48,49), which could be indepen- proximal colon. The distal colon expresses only small amounts dent of GC-C activation. of both hormones. The effects of GN and UGN on cellular cGMP via GC-C As indicated above, a few studies show the existence of a depend on extracellular pH. UGN is more potent at pH 5 signaling pathway at least for STa distinct of GC-C (33–35). compared with pH 8. Conversely, GN is more potent at pH 8 Furthermore, binding sites for guanylin peptides that are dis- compared with pH 5 (Figure 4). As this pH dependence affects the ligand/receptor interaction, it was suggested that the N- terminal ends of the UGN and GN molecules are responsible for this pH dependence (50). The duodenum secretes bicarbonate to neutralize the acidic pH of the stomach contents that enter the duodenum. This acid-stimulated duodenal mucosal bicarbonate secretion is cGMP dependent. UGN regulates the luminal pH in the duodenum by activation of bicarbonate ϩ secretion and inhibition of H secretion because unlike GN,

Figure 2. Signaling pathway of guanylin peptides in the intestine. GN, UGN, and STa activate membrane-bound GC-C, which increases the intracellular concentration of cGMP. cGMP ϩ ϩ inhibits the Na /H exchanger (NHE) and activates protein kinase G type II (PKG II) and protein kinase A (PKA). Both Ϫ Ϫ protein kinases activate Cl and HCO3 secretion via activation of cystic fibrosis transmembrane regulator (CFTR) followed by Ϫ Ϫ an activation of the Cl /HCO3 exchanger. Depicted is a sche- matic drawing of an enterocyte including the major compo- Figure 4. GN and UGN increase the cellular concentration of nents necessary for electrolyte transport. PDE III, phosphodi- cGMP pH-dependently. GN is more potent at pH 8.0, and UGN esterase type III (inhibited by cGMP). is more potent at pH 5.0. Adapted from reference (50). 4 Journal of the American Society of Nephrology J Am Soc Nephrol ●●: ●●●–●●●, ●●●●

UGN is localized predominantly in the proximal part of intestinal tract (Figure 3). This effect is enhanced as UGN has higher Ϫ activity at acidic pH and leads to stronger HCO3 secretion into duodenal lumen.

Signaling Mechanisms of Guanylin Peptides in the Kidney ϩ ϩ Guanylin peptides increase the secretion of Na ,K , and water in the kidney without changes in GFR or renal blood flow (7,36). UGN is proposed to be an intestinal natriuretic peptide, as natriuresis produced by oral salt load is decreased in UGN- deficient mice (51). Thus, the action of UGN in the kidney can be endocrine (produced in the intestine with actions in the kidney), paracrine (produced by the kidney), or both. Kinoshita et al. (52) showed higher UGN concentration in blood and urine of individuals who were on a high-salt diet compared with those who were on a low-salt diet, which speaks in favor of an endocrine function of UGN. Recently, Fukae et al. (53) showed that rats that are fed a high-salt diet have higher UGN and cGMP concentrations in the urine but show no changes in the Figure 5. Relative mRNA expression of GN (A) and UGN (B) in concentration of UGN in the plasma compared with control various mouse nephron segments. GL, glomerulus; PT, proxi- animals. UGN is expressed in the kidney, and its expression is mal tubule; TAL, thick ascending limb of Henle’s loop; CD, higher in animals that are on a high-salt diet (53,54). It is still collecting duct; GAPDH, glyceraldehyde-3-phosphate dehy- not clear whether the increase in the urinary concentrations of drogenase. Mean values Ϯ SEM, n ϭ 4. Adapted from reference UGN and cGMP (52,53) are due to local production and secre- (54). tion of UGN within the kidney or to filtration in the glomerulus. Both GN and UGN circulate in the plasma and can be filtered (54) localized GC-C in the mouse kidney only in glomeruli and in the glomerulus into the primary urine. UGN is excreted via proximal tubules. In the human kidney, we detected GC-C the final urine, whereas hardly any GN can be found in the again only in the kidney cortex and specifically in proximal urine (6,55). A possible reason that GN is not present in the final tubules (Figure 6) (60,61). In contrary to these findings in mouse urine is its sensitivity to chymotrypsin, which quickly degrades and human, in rat kidney, Carrithers et al. (58) found mRNA for GN after glomerular filtration or secretion of GN into the GC-C in all nephron segments, suggesting differences in tissue tubular lumen (9,56). These differences in the degradation and distribution of GC-C in different species. A few years ago, it expression pattern of GN and UGN suggest that GN probably became evident that proximal tubules are targets of guanylin acts strictly autocrine, whereas UGN has paracrine activity in peptides. Guanylin peptides increase the intracellular cGMP the kidney. The concentration of UGN in the urine correlates concentration in a proximal tubule cell line of opossum kidney with that of urinary cGMP, which again supports the hypoth- (OK cells) via the opossum kidney GC-C (OK-GC), which is the esis that cGMP and the GC-C signaling pathways are activated analog to the human GC-C (62). Recently, we demonstrated an by UGN at least in the cells of the proximal tubule. However, increase in the cellular cGMP concentration induced by guany- another possible source of cGMP in the urine could be the lin peptides also in a human proximal tubule cell line (IHKE1) activation of other members of the guanylate cyclase family, (61). In this cell line, GN and UGN also activated GC-C. such as the orphan receptor GC-G, which is present in the The intestinal pH dependence for the binding of GN and luminal membrane of principal cells of rat and mouse CCD UGN to GC-C (see above) probably also is relevant in the (21,57). The source of guanylin peptides in the tubular lumen kidney. In the human proximal tubule cell line, UGN activated besides glomerular filtration from the circulation could also be tubular cells because mRNA for GN and UGN is also expressed in kidney epithelial cells (54,58,59). Similar to the difference in the axial expression along the intestine, GN and UGN are not expressed equally along the different nephron segments (Fig- ure 5). UGN is present mostly in the proximal tubule, again comparable to the proximal intestine, and GN is more pronounced in the collecting duct, in analogy to GN expression in the colon. Figure 6. Expression of mRNA for GC-C in human nephron Markedly different from the expression along the entire in- segments. G, glomeruli; CCD, cortical collecting ducts; M, testinal tract, GC-C, the first receptor for guanylin peptides marker; (Ϫ), negative control (no cDNA); (ϩ), positive control. identified, is not present in all nephron segments. Potthast et al. Adapted from reference (60). J Am Soc Nephrol ●●: ●●●–●●●, ●●●● Cellular Effects of Guanylin and Uroguanylin 5

GC-C at pH 5 more pronounced than at pH 8 (Figure 4). At alkaline pH, when UGN effects on GC-C in these proximal tubular cells were lower, an additional signaling pathway via pertussis toxin–sensitive G-proteins was revealed (61). There- fore, tubular pH can play an important role in activating GN versus UGN signaling pathways and activating different signaling pathways for UGN (Figure 7). This might be even more relevant along the medullary collecting duct, where tubular pH can be significantly acidic. Data on effects of guanylin peptides along this nephron segment, however, are still completely miss- ing. Data from various proximal tubule cells in culture suggest ϩ that guanylin peptides inhibit Na reabsorption in this ϩ nephron segment. UGN decreases the expression of the Na / ϩ K -ATPase, which would lower the concentration gradient for ϩ ϩ Na and therefore reduce Na -coupled electrolyte and sub- strate reabsorption from the tubular lumen (63). According to our own observations in the human proximal tubule cell line, ϩ GN and UGN decrease the electrical driving force for Na ϩ reabsorption by inhibiting K channels via cGMP and depolar- izing cells (61). Finally, the increase in cellular cGMP by GN and UGN should, like in the intestine (37), inhibit luminal ϩ ϩ ϩ Na /H exchange and thereby reduce Na and volume reabsorption. Indeed, in the human proximal tubule cell line, UGN ϩ ϩ inhibited Na /H exchange activity (Schlatter E., unpublished results). As mentioned above, guanylin peptides still cause Figure 8. Signaling pathways of guanylin peptides in renal natriuresis, kaliuresis, and diuresis in GC-C–deficient mice, proximal tubule cells. Guanylin peptides activate two signaling suggesting a second receptor in the kidney for these peptides pathways. One is GC-C and cGMP dependent. cGMP decreases ϩ (36). In addition to this GC-C– and cGMP-dependent signaling Na reabsorption by inhibiting different proteins in the mem- ϩ ϩ ϩ ϩ ϩ pathway, UGN can activate a pertussis toxin–sensitive G-pro- brane: K channels, Na /H exchanger, and Na /K -AT- ϩ tein that leads to activation of a K conductance (61). Which Pase. The other signaling pathway involves activation of a signaling pathway will be activated by UGN could depend on pertussis toxin–sensitive G-protein. AQP1, aquaporin 1; NBC, ϩ Ϫ ϩ ϩ luminal pH (Figure 7). Figure 8 shows a summary of actions of Na /HCO3 co-transporter; NHE3, Na /H exchanger; OK- guanylin peptides on proximal tubular cells on ion and water GC, guanylate cyclase from opossum kidney. transport. The molecular identity of this second receptor, the physiologic role of this cGMP-independent signaling pathway, and the relative contributions of these two signaling pathways remains to be elucidated. The presence of two receptors for one peptide even at the same cell is known for other hormones that also act in the kidney. ANP activates two different types of receptors (64); one is GC-A, and, like GC-C, its activation leads to the production of cGMP (65). The other receptor is the natriuretic peptide receptor type C, or clearance receptor (NPR-C), which is a pertussis toxin–sensitive G-protein–coupled receptor (15,16). It remains to be clarified whether the G-protein–coupled receptors for UGN and ANP are the same or similar. Fine regulation of renal electrolyte and water excretion takes place in the collecting duct. Principal cells of the CCD are ϩ ϩ ϩ responsible for K secretion (via K channels [ROMK]), Na ϩ reabsorption (via epithelial Na channels [ENaC]), and water Figure 7. pH dependence of effects of guanylin peptides on reabsorption (via aquaporins 2, 3, and 4). In line with the membrane voltages of human proximal tubule cells. At pH 5.5, UGN activated GC-C and depolarized cells; at pH 8.0, it acti- observation that GC-C–deficient mice still exert natriuresis, vated a G-protein–coupled receptor and hyperpolarized the kaliuresis, and diuresis upon infusion of guanylin peptides (36) cells. GN and STa depolarize cells only via activation of the is the absence of mRNA for GC-C in CCD of mouse and man GC-C–dependent signaling pathway. Mean values Ϯ SEM, n (54,60). However, guanylin peptides induce changes in mem- given in brackets. Adapted from reference (61). brane voltages of principal cells of mouse and human CCD 6 Journal of the American Society of Nephrology J Am Soc Nephrol ●●: ●●●–●●●, ●●●● both in wild-type and in GC-C–deficient mice, suggesting mod- ification of electrogenic ion transport also in this nephron segment (57,60). Similar to the situation in the human proximal tubule cell line, in principal cells of isolated mouse and human CCD segments, guanylin peptides activate a signaling pathway distinct from GC-C and independent of cGMP. Guanylin peptides depolarize these cells, whereas the membrane-permeable analog of cGMP, 8Br-cGMP, hyperpolarizes the same cells, excluding cGMP as a possible second messenger in this signaling pathway for guanylin peptides in these cells (60). This pathway of GN and UGN in the CCD involves again a G- protein–coupled receptor, like in the proximal tubule cell line. Activation of this receptor leads to stimulation of phospholipase A2 (PLA2) and liberation of arachidonic acid. It has been reported before that arachidonic acid inhibits the secretory ROMK channels located at the luminal membrane of principal ϩ cells (66). Inhibition of the secretory K channels by guanylin ϩ peptides therefore decreases K secretion and lowers the elec- ϩ trical driving force for Na reabsorption that results in natriuresis. In mouse principal cells of the CCD, apparently in addition to this cGMP-independent signaling pathway, another pathway for guanylin peptides exists. This pathway is cGMP dependent but GC-C independent (57). The receptor that is activated by guanylin peptides in the mouse CCD and leads to Figure 9. Signaling pathways of GN and UGN in a principal cell cGMP and PKG activation is still not identified. A possible of the CCD. Guanylin peptides activate phospholipase A2 candidate for this receptor is the orphan receptor guanylate (PLA2), which leads to an increase in the intracellular concentration of arachidonic acid and an inhibition of luminal ROMK cyclase G (21,57). Activation of this pathway results in activa- ϩ ϩ K channels. ENaC, epithelial sodium channels; AQP 2, 3, 4, tion of cGMP-dependent K channels in the basolateral mem- ϩ ϩ aquaporins 2, 3, and 4; NKA, Na /K ATPase. brane of principal cells and cell hyperpolarization. Figure 9 shows a model of a principal cell of the CCD indicating all mechanisms of actions of guanylin peptides identified so far. It tides are also involved in the regulation of electrolyte and water is evident from this scheme that the guanylin peptides are the first secretion in those organs via CFTR. Guanylin peptides and peptides identified that act in this nephron segment primarily on parts of their signaling pathway are present in the pancreas. ϩ ϩ K channels without directly affecting Na channels. In the hu- John et al. (69) showed that GN and STa increase intracellular man CCD, the depolarization of principal cells via inhibition of the cGMP via GC-C in a human pancreatic cell line. Kulaksiz et al. ϩ secretory K channels results in natriuresis as the predominant (70,71) detected mRNA for GC-C and UGN in cells of the ϩ effect observed in vivo with these peptides (7,36,67). Reduced Na exocrine part of the pancreas, and guanylin peptides increase reabsorption also decreases water reabsorption in this nephron cGMP, which leads to activation of CFTR in these cells. segment and thus causes diuresis. GN was shown to decrease the Guanylin peptides and parts of the GC-C–dependent signal- cell volume and increase the luminal space in a concentration- and ing pathway are also found in airways (26,27), sweat glands, time-dependent manner, which suggests secretion of water and the adrenal medulla, and the male reproductive system (72–74). consequently diuresis (68). However, the kaliuresis also caused by According to this limited information on effects of guanylin guanylin peptides cannot be explained from the data obtained in peptides in various organs, the GC-C–dependent signaling the CCD and may be due to effects of these peptides in other parts pathway seems to be the predominant signaling pathway in of the nephron, e.g., the thick ascending limb or the medullary most organs and only in the kidney does the signaling involve collecting duct. Again, the physiologic role of the second cGMP- different receptors and signaling mechanisms. dependent signaling pathway and the relative contributions of these two signaling pathways in the CCD of the mouse remains to Guanylin Peptides in Pathophysiology be elucidated. This second pathway involves an activation of Exiting observations are the involvement of guanylin pep- ϩ ϩ basolateral K channels and thereby should increase Na reab- tides in the development and possible treatment of intestinal ϩ sorption without an increase in K secretion. tumors. GN and UGN are less expressed or they are not present at all in colon adenocarcinoma, adenoma, and intestine polyps Cellular Effects of Guanylin Peptides in in human and mouse (75–77). The gene for GN is located at 1 Other Organs p34–35, which is a tumor-modifying region. One can presume Patients with cystic fibrosis undergo changes in water and that loss of GN activity leads to or is the result of tumor electrolyte secretion in the liver, the pancreas, the lung, and the development. Pitari et al. (78) hypothesized that the reason for intestine. Thus, it might be speculated whether guanylin pep- the lower incidence of intestinal tumors in third-world coun- J Am Soc Nephrol ●●: ●●●–●●●, ●●●● Cellular Effects of Guanylin and Uroguanylin 7 tries is the higher incidence of intestinal infections that stimu- ular identity of these renal receptors for guanylin peptides late intestinal GC-C and lead to an increase in cGMP. Guanylin remains to be determined. First indications of an involvement peptides and also STa in addition regulate proliferation and of these peptides in various diseases indicate also a pathophys- differentiation, prolonging the cell cycle, and induce apoptosis iologic role and possibly a therapeutic value of these peptides. of T84, CaCo2 cells, and mouse intestinal cells (78–81). Shailubahai et al. (77) showed that oral application of UGN Acknowledgments leads to an decrease in number and size of polyps in mice that The data of the authors laboratory cited and presented in this review develop intestinal polyposis, which allows us to speculate on were funded by a grant of the Medical Faculty of the University of the possible usage of UGN in the treatment of intestinal tumors. Mu¨nster (IMF, KU 21 98 09) and by grants of the Deutsche Forschungs- The involvement of guanylin peptides was suggested also in gemeinschaft (Schl 277/11-1 to 11-3). different kidney diseases. More than 11 yr ago, Nakazato et al. (82) showed that human GN increases in patients who have chronic renal failure and undergo hemodialysis. 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