Expression of GC-C, a receptor-guanylate cyclase, and its endogenous ligands uroguanylin and guanylin along the rostrocaudal axis of the intestine

Members of the receptor-guanylate cyclase (rGC) family possess an intracellular catalytic domain that is regulated by an extracellular receptor domain. GC-C, an intestinally expressed rGC, was initially cloned by homology as an orphan receptor. The search for its ligands has yielded three candidates: STa (a bacterial toxin that causes traveler's diarrhea) and the endogenous peptides uroguanylin and guanylin. Here, by performing Northern and Western blots, and by measuring [125I]STa binding and STa-dependent elevation of cGMP levels, we investigate whether the distribution of GC-C matches that of its endogenous ligands in the rat intestine. We establish that 1) uroguanylin is essentially restricted to small bowel; 2) guanylin is very low in proximal small bowel, increasing to prominent levels in distal small bowel and throughout colon; 3) GC-C messenger RNA and STa-binding sites are uniformly expressed throughout the intestine; and 4) GC-C-mediated cGMP synthesis peaks at the proximal and distal extremes of the intestine (duodenum and colon), but is nearly absent in the middle (ileum). These observations suggest that GC-C's activity may be posttranslationally regulated, demonstrate that the distribution of GC-C is appropriate to mediate the actions of both uroguanylin and guanylin, and help to refine current hypotheses about the physiological role(s) of these peptides.     

Identification of two functional guanylin receptors in eel: multiple hormone-receptor system for osmoregulation in fish intestine and kidney

Guanylyl cyclase C (GC-C) is a single transmembrane receptor for a family of intestinal hormones, guanylins. In the eel, we previously identified three guanylins, whose gene expression was enhanced in the intestine after transfer from fresh water to seawater. However, only limited information is available about the structure and function of their receptor(s). In the present study, we cloned full-length cDNAs encoding two isoforms of GC-C, named GC-C1 and GC-C2, from eel intestine. The predicted GC-C proteins consisted of extracellular ligand-binding domain, membrane-spanning domain, kinase-like domain and cyclase catalytic domain, in which GC-C-specific sequences were largely conserved. Phylogenetic analyses showed that the cloned membrane GCs are grouped with the GC-C of other vertebrates but not with GC-A and GC-B. However, eel GC-Cs appear to have undergone unique structural evolution compared with other GC-Cs. The three eel guanylins (guanylin, uroguanylin and renoguanylin), but not eel atrial natriuretic peptide, stimulated cGMP production dose-dependently in COS cells expressing either of the cloned cDNAs, providing functional support for assignment as eel guanylin receptors. The potency order for cGMP production was uroguanylin > guanylin > or = renoguanylin for GC-C1; guanylin > or = renoguanylin > uroguanylin for GC-C2. The distinctive ligand selectivity was consistent with the low homology (53%) of the extracellular domain of the two GC-Cs compared with that observed for other domains (74-90%). Both GC-C genes were expressed in the alimentary tract (esophagus, stomach and intestine) and kidney, and their expression was higher in the intestine of seawater-adapted eels than that of freshwater eels just as observed with the guanylin genes. However, the expression of the receptor genes was unchanged for 24h after transfer of eels from fresh water to seawater or vice versa, showing slower response of the receptors to salinity changes than their ligands. Collectively, the multiple guanylin-GC-C system may be involved as a paracrine factor in seawater adaptation at the intestine and kidney of the eel.     

Guanylin-like peptides, guanylate cyclase and osmoregulation in the European eel (Anguilla anguilla)

Three guanylin-like peptides, guanylin, uroguanylin and renoguanylin and two guanylate cyclase type C (GC-C) receptor isoforms were cloned and sequenced from the European eel (Anguilla anguilla). All peptides and both receptors (GC-C1 and GC-C2) were predominantly expressed within the intestine and kidney of both sexually immature yellow, and sexually maturing, migratory silver eels. The derived amino acid sequences for the pre-prohormones and guanylate cyclase isoforms had structural features in common with sequences previously reported for guanylin-like peptides and guanylate cyclases from teleost fish and other species in general. The highest sequence homologies for the prohormones were found within the active, 15-16 amino acid C-terminal peptide domain, whereas the guanylate cyclase receptors exhibited highest homology throughout the transmembrane domain and intracellular region of the protein comprising the kinase homology, oligomerisation/coiled-coil and catalytic domains. In both yellow and silver eels, seawater (SW) acclimation induced sustained increases in the expression of uroguanylin and GC-C1 mRNAs within the intestine but no significant changes were found in the abundance of mRNAs for guanylin, renoguanylin or GC-C2. Likewise there were no significant changes in expression of any of the prohormone or receptor mRNAs within the renal kidney following transfer to SW. The results suggest that uroguanylin and GC-C1 are key components of a cGMP signalling system that may play an important role within intestinal enterocytes for the regulation of salt and water absorption in the SW-acclimated eel.     

Increases in guanylin and uroguanylin in a mouse model of osmotic diarrhea are guanylate cyclase C-independent.

BACKGROUND & AIMS: Guanylin and uroguanylin are peptide hormones that are homologous to the diarrhea-causing Escherichia coli enterotoxins. These secretagogues are released from the intestinal epithelia into the intestinal lumen and systemic circulation and bind to the receptor guanylate cyclase C (GC-C). We hypothesized that a hypertonic diet would result in osmotic diarrhea and cause a compensatory down-regulation of guanylin/uroguanylin. METHODS: Gut-to-carcass weights were used to measure fluid accumulation in the intestine. Northern and/or Western analysis was used to determine the levels of guanylin, uroguanylin, and GC-C in mice with osmotic diarrhea. RESULTS: Wild-type mice fed a polyethylene glycol or lactose-based diet developed weight loss, diarrhea, and an increased gut-to-carcass ratio. Unexpectedly, 2 days on either diet resulted in increased guanylin/uroguanylin RNA and prohormone throughout the intestine, elevated uroguanylin RNA, and prohormone levels in the kidney and increased levels of circulating prouroguanylin. GC-C-deficient mice given the lactose diet reacted with higher gut-to-carcass ratios. Although they did not develop diarrhea, GC-C-sufficient and -deficient mice on the lactose diet responded with elevated levels of guanylin and uroguanylin RNA and protein. A polyethylene glycol drinking water solution resulted in diarrhea, higher gut-to-carcass ratios, and induction of guanylin and uroguanylin in both GC-C heterozygous and null animals. CONCLUSIONS: We conclude that this model of osmotic diarrhea results in a GC-C-independent increase in intestinal fluid accumulation, in levels of these peptide ligands in the epithelia of the intestine, and in prouroguanylin in the kidney and blood.     

The intestinal guanylin system and seawater adaptation in eels

Guanylin and uroguanylin are principal intestinal hormones secreted into the lumen to regulate ion and water absorption via a specific receptor, guanylyl cyclase-C (GC-C). As the intestine is an essential organ for seawater (SW) adaptation in teleost fishes, the intestinal guanylin system may play a critical role in SW adaptation. Molecular biological studies identified multiple guanylins (guanylin, uroguanylin and renoguanylin) and their receptors (GC-C1 and GC-C2) in eels. The relative potency of the three ligands on cGMP production in transiently expressed receptors was uroguanylin > guanylin >or= renoguanylin for CG-C1 and guanylin >or= renoguanylin > uroguanylin for GC-C2. Eel guanylin and GC-C genes are expressed exclusively in the intestine and kidney, and the level of expression is greater in SW eels than in freshwater (FW) eels except for renoguanylin. Physiological studies using Ussing chambers showed that the middle and posterior intestine are major sites of action of guanylins, where they act on the mucosal side to decrease short circuit current (I(sc)) in a dose-dependent manner. The ID(50) of guanylins for transport inhibition was 50-fold greater than that of atrial natriuretic peptide that acts from the serosal side as an endocrine hormone. However, only guanylins reversed I(sc) to levels below zero. Pharmacological analyses using various blockers showed that among transporters and channels localized on the intestinal cells of SW teleost fish, the cystic fibrosis transmembrane conductance regulator Cl(-) channel (CFTR) on the apical membrane is the major target of guanylins. Collectively, guanylins are synthesized locally in the intestine and secreted into the lumen to act on the GC-Cs in the apical membrane of eel intestinal cells. Then, intracellular cGMP production after ligand-receptor interaction activates CFTR and probably induces Cl(-) and/or HCO3- secretion into the lumen as suggested in mammals. The physiological significance of the anion secretion induced by the luminal guanylin/GC-C system on SW adaptation may rival or exceed that of the serosally derived natriuretic peptides in the euryhaline eel.     

Uroguanylin: structure and activity of a second endogenous peptide that stimulates intestinal guanylate cyclase.

The intestinal hormone guanylin and bacterial heat-stable enterotoxins (STs) are members of a peptide family that activates intestinal membrane guanylate cyclase. Two different peptides that activate the human intestinal T84 cell guanylate cyclase have been purified from urine and intestinal mucosa of opossums (Didelphis virginiana). The highly acidic peptide, QEDCELCINVACTGC, was named uroguanylin because it was isolated from urine and shares 53% identity with guanylin. A second peptide, SHTCEICAFAACAGC, was purified from urine and intestinal mucosa. This alanine-rich peptide was 47% identical to uroguanylin and 73% identical to human guanylin, suggesting that it may be an opossum homologue of guanylin. Synthetic uroguanylin-(2-15) (i.e., EDCELCINVACTGC) was 10-fold more potent than synthetic rat guanylin, but both peptides were less potent than Escherichia coli ST in the T84 cell cGMP bioassay. Uroguanylin-(2-15) and guanylin inhibited 125I-ST binding to T84 cell receptors in competitive radioligand binding assays. Transepithelial Cl- secretion was stimulated by 1 microM uroguanylin, indicated by an increase in the short circuit current of T84 cells. Thus, uroguanylin is another paracrine hormone in the emerging peptide family that activates intestinal membrane guanylate cyclase. The second peptide may be the opossum form of guanylin, or perhaps, it is still another member of this peptide family. The presence of uroguanylin and guanylin in urine and receptors in proximal tubules suggests that these peptides may also originate from renal tissue and may regulate kidney function.     

Uroguanylin and guanylate cyclase C in the human pancreas: expression and mutuality of ligand/receptor localization as indicators of intercellular paracrine signaling pathways.

The intestinal peptide hormone uroguanylin regulates electrolyte/fluid transport in the gastrointestinal epithelium by binding to its receptor, guanylate cyclase C (GC-C), and thus specifically coupling to activation of cystic fibrosis transmembrane conductance regulator (CFTR). Since CFTR is crucially involved in pancreatic electrolyte secretion, we investigated the human pancreas for expression and cell-specific localization of uroguanylin and guanylate cyclase C as potential regulatory components of pancreatic electrolyte secretion. RT-PCR analyses with specific primers revealed that uroguanylin and GC-C are expressed in the human pancreas (and in the duodenum, used as positive control); at the translational level, western blotting analyses with peptide- and region-specific antibodies identified the presence of 12.5 kDa uroguanylin and 130 kDa GC-C in both human pancreatic and intestinal extracts. At the cellular level, uroguanylin and GC-C immunoreactivities were absent from the islets of Langerhans but were exclusively confined to the exocrine parenchyma. Hence, uroguanylin was localized to the centroacinar cells typical of the pancreas, and also to epithelial cells of the intercalated, intralobular and interlobular ducts where the peptide was primarily concentrated adluminally to the apical portion of the respective cells. Coincidently, correlative studies localized the GC-C receptor to the epithelial cells of the ductal network, where it was confined exclusively to the apical cell membrane that evidently represents the functionally relevant target membrane domain for the regulatory peptide. In view of the fact that CFTR is highly expressed in pancreatic ductal cells where uroguanylin and its receptor are also localized, we assume that uroguanylin, an intrinsic pancreatic peptide, is involved in the regulation of electrolyte/water secretion in the ductal system via GC-C and CFTR. The particular cellular expression of uroguanylin in duct cells and the localization of GC-C to the duct cell apical membrane domain predict a novel route of intercellular signaling and luminal activation of GC-C via the pancreatic juice.     

Effects of uroguanylin, an intestinal natriuretic peptide, on tubuloglomerular feedback.

Uroguanylin is an endogenous peptide that stimulates cyclic guanosine monophosphate (cGMP) production via the activation of guanylate cyclase C (GC-C) in the intestine and kidney. A high salt diet, but not intravenous salt load, enhances the secretion of biologically active uroguanylin from the intestine and increases its concentration in plasma and urine. Our purpose is to clarify the effect of uroguanylin on renal microcirculation and the tubuloglomerular feedback (TGF) mechanism. Clearance and micropuncture experiments were performed in anesthetized rats. TGF responsiveness was assessed in superficial nephrons by measuring the changes of early proximal flow rate (EPFR) in response to orthograde loop perfusion at 40 nl/min with artificial tubular fluid (ATF). Reductions in EPFR induced by loop perfusion during intravenous infusion of uroguanylin at the rate of 10 and 50 nmol/kg/h were similar yet significantly less than that during the control period (33+/-3% and 35+/-3% vs. 47+/-3%, p<0.05). Intraluminal application of uroguanylin at 10(-7) and 10(-5) mol/l in ATF decreased EPFR by 40+/-3% and 33+/-7%, respectively, with the latter value being significantly less than the control (p<0.05). Intravenous infusion of uroguanylin did not significantly change whole kidney function. Administration of atrial natriuretic peptide (ANP), which activates GC-A and B, significantly suppressed TGF-mediated EPFR reduction either intravenously (10 nmol/kg/h) or intraluminally (10(-5) mol/l in ATF) (9+/-3% and 13+/-2% vs. 47+/-3% of the control, p<0.05). In conclusion, uroguanylin clearly suppresses TGF both through intravenous and intraluminal routes, although the effects on glomerular microcirculation and whole kidney function are far less than those of ANP.     

Regulation of intestinal uroguanylin/guanylin receptor-mediated responses by mucosal acidity

Guanylin and uroguanylin are intestinal peptides that stimulate chloride secretion by activating a common set of receptor–guanylate cyclase signaling molecules located on the mucosal surface of enterocytes. High mucosal acidity, similar to the pH occurring within the fluid microclimate domain at the mucosal surface of the intestine, markedly enhances the cGMP accumulation responses of T84 human intestinal cells to uroguanylin. In contrast, a mucosal acidity of pH 5.0 renders guanylin essentially inactive. T84 cells were used as a model epithelium to further explore the concept that mucosal acidity imposes agonist selectivity for activation of the intestinal receptors for uroguanylin and guanylin, thus providing a rationale for the evolution of these related peptides. At an acidic mucosal pH of 5.0, uroguanylin is 100-fold more potent than guanylin, but at an alkaline pH of 8.0 guanylin is more potent than uroguanylin in stimulating intracellular cGMP accumulation and transepithelial chloride secretion. The relative affinities of uroguanylin and guanylin for binding to receptors on the mucosal surface of T84 cells is influenced dramatically by mucosal acidity, which explains the strong pH dependency of the cGMP and chloride secretion responses to these peptides. The guanylin-binding affinities for peptide–receptor interaction were reduced by 100-fold at pH 5 versus pH 8, whereas the affinities of uroguanylin for these receptors were increased 10-fold by acidic pH conditions. Deletion of the N-terminal acidic amino acids in uroguanylin demonstrated that these residues are responsible for the increase in binding affinities that are observed for uroguanylin at acidic pH. We conclude that guanylin and uroguanylin evolved distinctly different structures, which enables both peptides to regulate, in a pH-dependent fashion, the activity of receptors that control intestinal salt and water transport via cGMP.     

Structure, regulation, and function of mammalian membrane guanylyl cyclase receptors, with a focus on guanylyl cyclase-A

Besides soluble guanylyl cyclase (GC), the receptor for NO, there are at least seven plasma membrane enzymes that synthesize the second-messenger cGMP. All membrane GCs (GC-A through GC-G) share a basic topology, which consists of an extracellular ligand binding domain, a short transmembrane region, and an intracellular domain that contains the catalytic (GC) region. Although the presence of the extracellular domain suggests that all these enzymes function as receptors, specific ligands have been identified for only three of them (GC-A through GC-C). GC-A mediates the endocrine effects of atrial and B-type natriuretic peptides regulating arterial blood pressure and volume homeostasis and also local antihypertrophic actions in the heart. GC-B is a specific receptor for C-type natriuretic peptide, having more of a paracrine function in vascular regeneration and endochondral ossification. GC-C mediates the effects of guanylin and uroguanylin on intestinal electrolyte and water transport and on epithelial cell growth and differentiation. GC-E and GC-F are colocalized within the same photoreceptor cells of the retina and have an important role in phototransduction. Finally, the functions of GC-D (located in the olfactory neuroepithelium) and GC-G (expressed in highest amounts in lung, intestine, and skeletal muscle) are completely unknown. This review discusses the structure and functions of membrane GCs, with special emphasis on the physiological endocrine and cardiac functions of GC-A, the regulation of hormone-dependent GC-A activity, and the relevance of alterations of the atrial natriuretic peptide/GC-A system to cardiovascular diseases.     

洛哌丁胺对腹泻模型大鼠结肠水通道蛋白4表达的影响

目的检测腹泻模型大鼠在给予洛哌丁胺处理后结肠黏膜水通道蛋白4（AQP4）mRNA表达的变化,探讨洛哌丁胺在结肠水代谢巾的分子作用机制。方法采用逆转录聚合酶链反应（RT—PCR）分别对洛哌丁胺治疗组、模型对照组及正常对照组大鼠升、降结肠黏膜细胞AQP4mRNA表达进行半定量分析。结果①洛哌丁胺治疗组升、降结肠AQP4mRNA表达量均高于模型对照组及正常对照组（P〈0．01）。②模型对照组升结肠AQP4mRNA表达量较正常对照组大鼠减少（P〈0．05）,而降结肠表达差异无统计学意义（P〉0．05）。结论洛哌丁胺可以在转录水平上调腹泻大鼠结肠黏膜AQP4表达,使结肠对肠腔内水分的吸收增加。     

Guanylyl cyclase C agonists regulate progression through the cell cycle of human colon carcinoma cells

The effects of Escherichia coli heat-stable enterotoxin (ST) and uroguanylin were examined on the proliferation of T84 and Caco2 human colon carcinoma cells that express guanylyl cyclase C (GC-C) and SW480 human colon carcinoma cells that do not express this receptor. ST or uroguanylin inhibited proliferation of T84 and Caco2 cells, but not SW480 cells, in a concentration-dependent fashion, assessed by quantifying cell number, cell protein, and [(3)H]thymidine incorporation into DNA. These agonists did not inhibit proliferation by induction of apoptosis, assessed by TUNEL (terminal deoxynucleotidyl transferase-mediated dNTP-biotin nick end labeling of DNA fragments) assay and DNA laddering, or necrosis, assessed by trypan blue exclusion and lactate dehydrogenase release. Rather, ST prolonged the cell cycle, assessed by flow cytometry and [(3)H]thymidine incorporation into DNA. The cytostatic effects of GC-C agonists were associated with accumulation of intracellular cGMP, mimicked by the cell-permeant analog 8-Br-cGMP, and reproduced and potentiated by the cGMP-specific phosphodiesterase inhibitor zaprinast but not the inactive ST analog TJU 1-103. Thus, GC-C agonists regulate the proliferation of intestinal cells through cGMP-dependent mechanisms by delaying progression of the cell cycle. These data suggest that endogenous agonists of GC-C, such as uroguanylin, may play a role in regulating the balance between epithelial proliferation and differentiation in normal intestinal physiology. Therefore, GC-C ligands may be novel therapeutic agents for the treatment of patients with colorectal cancer.     

Cloning, comparative sequence analysis and mRNA expression of calcium-transporting genes in horses

Epithelial calcium transport occurs by paracellular and transcellular mechanisms. Transcellular transport in intestinal and renal epithelia involves several transport proteins, including transient receptor potential vanilloid member 5 (TRPV5), member 6 (TRPV6), calbindin D9k (CB9), calbindin D28k (CB28), sodium calcium exchanger 1 (NCX1), plasma membrane calcium ATPase 1 (PMCA1), and the vitamin D receptor (VDR). We are interested in the horse because of its unique calcium physiology (high blood calcium, high intestinal calcium absorption, high renal excretion of calcium, low vitamin D concentrations), and because horses often have dysregulated calcium balance with various diseases. We cloned the mRNA for equine TRPV5, TRPV6, CB9, CB28, NCX1, PMCA1, and VDR, performed comparative mRNA and protein sequence analysis, and quantified their mRNA expression in the kidney and gastrointestinal tract. Sequence homology for the mRNAs and proteins was high among mammals (>75%), with fish having the lowest homology (<75%). TRPV5, TRPV6, and CB9 expression was higher in the duodenum and proximal jejunum and followed a similar expression pattern. CB28 expression was greatest in the kidney. PMCA1 and NCX1 expression was similar throughout the intestine, but in the kidney PMCA1 expression was higher. Based on our findings, the proximal small intestine is the main site for transcellular calcium transport, with TRPV6 and CB9 serving as the main transport proteins. In the kidney, TRPV6, CB28, and PMCA1 are likely more important. The low VDR expression in the equine small intestine and kidney relative to the large intestine, together with the reported high intestinal absorption and renal excretion of calcium, and low vitamin D concentrations suggests that epithelial calcium transport in horses is not as dependent on vitamin D as in other species.     

肠易激综合征模型大鼠5-羟色胺7受体的表达及作用

目的 观察5-羟色胺7受体在肠易激综合征(IBS)不同亚型模型大鼠大脑和消化道组织中表达和分布的差异,探讨其在IBS发病中的作用.方法 45只成年SD大鼠分均为对照组、腹泻型IBS(IBS-D)组和便秘型IBS(IB-C)组.乙酸加束缚应激法制备IBS-D模型,冰水灌胃法制备IBS-C模型.免疫组化法和实时定量PCR法检测各组大鼠大脑、空肠、回肠、近端结肠、远端结肠中5-HT7受体的分布及表达差异.放射免疫法测定以上各组织中环磷酸腺苷(cAMP)含量.结果 免疫组化结果 显示,IBS-C组和IBS-D组海马及下丘脑、IBS-C组回肠、近端结肠、远端结肠5-HT7受体表达强于对照组(P＜0.05).实时定量PCR结果 显示,IBS-C和IBS-D组海马和下丘脑、IBS-C组回肠、近端结肠及远端结肠5-HT7受体mRNA明显高于对照组(P＜0.05).IBS-C和IBS-D组海马和下丘脑、IBS-C组近端结肠和远端结肠cAMP含量显著高于对照组(P＜0.05).结论 两组大鼠脑组织及IBS-C大鼠结肠5-HT7受体表达及cAMP水平明显增高,可能与IBS-C胃肠动力障碍和内脏感觉异常有关.     

Expression of iron absorption genes in mouse large intestine

OBJECTIVE: The large intestine has been reported to have a capacity for iron absorption and expresses genes for iron absorption normally found in the duodenum. The importance and function of these genes in the large intestine are not understood. We therefore investigated the cellular localization and regulation of expression of these genes in mouse caecum and colon. MATERIAL AND METHODS: Gene expression was measured by real-time PCR using RNA extracted from iron-deficient and hypoxic mouse large intestine, compared to controls. Protein localization and regulation were measured by immunohistochemistry using frozen sections of the large intestine from the same mice. RESULTS: Dcytb (duodenal ferric reductase) was expressed at very low levels in the large intestine, compared to the duodenum, while Ireg1 and DMT1 were expressed at significant levels in the large intestine and were increased in iron-deficient caecum, proximal and distal colon, with the most significant increases seen in the distal colon. Hypoxia increased Ireg1 expression in the proximal colon. Immunohistochemistry detected significant levels of only IREG1, which was localized to the basolateral membrane of colonic epithelial cells. CONCLUSIONS: Iron absorption genes were expressed at lower levels in mouse caecum and colon than in the duodenum. They are regulated by body iron requirements. Colonic epithelial cells express basolateral IREG1in the same fashion as in the duodenum and this protein could regulate colonic epithelial cell iron levels.     

Guanylyl cyclase C suppresses intestinal tumorigenesis by restricting proliferation and maintaining genomic integrity

BACKGROUND AND AIMS: The most commonly lost gene products in colorectal carcinogenesis include guanylin and uroguanylin, endogenous ligands for guanylyl cyclase C (GCC). Beyond intestinal fluid balance, GCC mediates diarrhea induced by bacterial enterotoxins, and an inverse relationship exists between enterotoxigenic Escherichia coli infections producing the exogenous GCC ligand ST and colorectal cancer worldwide. However, the role of GCC in neoplasia remains obscure. METHODS: Intestinal tumorigenesis was examined in wild-type (Gcc(+/+)) and GCC-deficient (Gcc(-/-)) mice carrying mutations in Apc (Apc(Min/+)) or exposed to the carcinogen azoxymethane. Markers of DNA damage, loss of Apc heterozygosity, and beta-catenin mutations were used to assess genomic integrity. Hyperproliferation was explored using Ki67 and cell cycle markers. Apoptosis was quantified by transferase biotin-dUTP nick end labeling analysis. RESULTS: In colons of Apc(Min/+) mice, deletion of Gcc increased tumor incidence and multiplicity, reflecting uncoupling of loss of genomic integrity and compensatory apoptosis. Conversely, in the small intestine, elimination of Gcc increased tumorigenesis by enhancing proliferation without altering genomic integrity. Moreover, these distinct but mutually reinforcing mechanisms collaborate in azoxymethane-exposed mice, and deletion of Gcc increased tumor initiation and growth associated with hypermutation and hyperproliferation, respectively, in conjunction with attenuated apoptosis. CONCLUSIONS: GCC suppresses tumor initiation and growth by maintaining genomic integrity and restricting proliferation. This previously unrecognized role of GCC in inhibiting tumorigenesis, together with the invariant disruption in guanylin and uroguanylin expression early in carcinogenesis, and the uniform over-expression of GCC by tumors, underscores the potential of oral administration of GCC ligands for targeted prevention and therapy of colorectal cancer.