[Regulatory properties of adenylyl and guanylyl cyclases in human spermatozoa]

Completion of the process of maturation of spermatozoa (capacitation) occurs in the female genital tract. As a result, spermatozoa acquire hypermobility and the capability for the acrosomal reaction, which determine their fertility. There exist data that participating in these processes are the adenylyl cyclase and guanylyl cyclase signaling systems that are present in human and mammalian spermatozoa. The goal of the present work to characterize these systems in the human ejaculate spermatozoa (ES) and in the human fertile spermatozoa (FS) isolated by the density gradient centrifugation. In the FS homogenate the basal activity of adenylyl cyclase (AC) activity was significantly higher that in ES: 47 +/- 5 vs 28 +/- 3 pmol cAMP/min per mg of protein, respectively. At the same time, the AC-stimulating effects of non-hormonal activators of AC forms (NaHCO3, Mn2+, forskolin, and non-hydrolysable GTP analogue - GppNHp) were lower as compared with ES. The AC activity in ES were stimulated by isoproterenol, serotonin, PACAP-38, and, to the letter degree, by noradrenalin and adenosine. Among the AC-inhibiting hormones, adenosine alone decreased the enzyme activity. In FS, on the contrary, the AC-inhibiting effects of adenosine, noradrenaline, and serotonin were clearly expressed, whereas the stimulatory effects of these hormones were attenuated or absent. The basal activity of guanylyl cyclase (GC) in the ES and FS homogenates amounted to 27 +/- 3 and to 21 +/- 2 pmol cGMP for 1 min per 1 mg protein, respectively, and was significantly increased in the presence of 10 mM Mn2+. The GC-stimulating effects of natriuretic peptides - atrial natriuretic peptide (ANP) and the C-type natriuretic peptide (CNP)--activators of the GC receptor forms, were in ES significantly higher than in FS, the ANP effect being more pronounced as compared with CNP. The obtained data indicate multiplicity of the cAMP- and cGMP-dependent signal cascades regulating fertility of human spermatozoa. It has been established that sensitivity of AC and GC to hormones in the total ES pool and in the supermobile FS fraction isolated by centrifugation differs essentially, which is to be taken into account when using FS for auxiliary reproductive technologies.

Distribution and characterization of the Escherichia coli heat-stable enterotoxin (STa) receptor throughout the intestinal tract of newborn camels (Camelus dromedaries)

Heat-stable enterotoxin (STa) secretion from Enterotoxigenic Escherichia coli (ETEC) is crucial for the pathogenesis of diarrhea in both animal and human. The goal of this study was to investigate the distribution of the STa-specific receptors in the newborn camel's enterocytes and brush border membrane vesicles (BBMVs). Flow cytometric analysis was used to investigate the density of STa-receptors on enterocytes and BBMVs prepared from anterior jejunum, posterior jejunum, ileum, and colon. Strong density and affinity of STa-receptors was present on enterocytes and BBMVs of the ileum compared to that in the other intestinal segments. It was concluded that the ileum is the major target for STa action in newborn camels.

Molecular staging estimates occult tumor burden in colorectal cancer

Tumor cells in regional lymph nodes are a key prognostic marker of survival and predictive marker of response to adjuvant chemotherapy in colorectal cancer. However, clinicopathologic techniques to detect lymph node metastases remain imperfect, and approximately 30% of patients with lymph nodes negative by histology (pN0) develop recurrent disease, reflecting occult metastases that escape detection. These observations underscore an unmet clinical need for accurate approaches to identify occult nodal metastases in colorectal cancer patients. GUCY2C is a receptor whose expression normally is restricted to intestinal epithelial cells, but is universally overexpressed by colorectal cancer cells. A prospective, multicenter, blinded clinical trial established the prognostic utility of GUCY2C qRT-PCR to detect occult nodal metastases in pN0 colorectal cancer patients. Molecular staging revealed that approximately 13% of pN0 patients were free of cancer cells, while approximately 87% had GUCY2C results that suggested occult metastases. The presence of occult nodal metastases was the most powerful independent predictor of time to recurrence and disease-free survival. These observations establish the utility of molecular detection of occult nodal metastases for assessing prognostic risk in pN0 colorectal cancer patients. Advancing GUCY2C into staging paradigms in clinical laboratories will require validation in independent patient populations, definition of the relationship between the quantity of occult tumor metastases and risk, and determination of the utility of GUCY2C qRT-PCR to identify pN0 patients who might benefit from adjuvant chemotherapy.

GUCY2C reverse transcriptase PCR to stage pN0 colorectal cancer patients

The most important prognostic marker of survival and predictive marker of response to adjuvant chemotherapy in colon cancer patients is tumor cells in regional lymph nodes. Despite their importance, standard techniques to assess nodal metastases remain imperfect, as approximately 30% of patients with histology-negative lymph nodes (pN0) die of recurrent disease, reflecting occult metastases that escape detection. These observations highlight the clinical need for novel, accurate approaches to detect occult lymph node metastases in patients with colon cancer. GUCY2C is a biomarker whose expression normally is restricted to intestinal cells, but is near universally overexpressed by colorectal cancer cells. Recently, a prospective, multicenter, blinded clinical trial demonstrated for the first time that the prognostic utility of GUCY2C quantitative reverse transcriptase (qRT)-PCR to detect occult lymph node metastases in pN0 colorectal cancer patients. Molecular staging revealed that approximately 13% of pN0 patients were free of tumor cells, while approximately 87% had GUCY2C results that suggested occult metastases. The presence of occult lymph node metastases was the strongest independent predictor of time to recurrence and disease-free survival. These observations establish the utility of molecular detection of occult lymph node metastases for estimating prognostic risk in pN0 colorectal cancer patients. Advancing this molecular diagnostic into staging paradigms in clinical laboratories will require validation in independent patient populations, definition of the relationship between the quantity of occult tumor metastases and risk, and determination of the utility of GUCY2C qRT-PCR to identify pN0 patients who might benefit from adjuvant chemotherapy.

Guanylyl cyclase C prevents colon cancer metastasis by regulating tumor epithelial cell matrix metalloproteinase-9

Matrix metalloproteinase-9 (MMP-9) produced by colorectal cancer cells is a critical determinant of metastatic disease progression and an attractive target for antimetastatic strategies to reduce colon cancer mortality. Cellular signaling by cyclic GMP (cGMP) regulates MMP-9 dynamics in various cell systems, and the bacterial enterotoxin receptor guanylyl cyclase C (GCC), the principle source of cGMP in colonocytes, which is overexpressed in colorectal cancers, inhibits tumor initiation and progression in the intestine. Here, we show that ligand-dependent GCC signaling through cGMP induces functional remodeling of cancer cell MMP-9 reflected by a compartmental redistribution of this gelatinase, in which intracellular retention resulted in reciprocal extracellular depletion. Functional remodeling of MMP-9 by GCC signaling reduced the ability of colon cancer cells to degrade matrix components, organize the actin cytoskeleton to form locomotory organelles and spread, and hematogenously seed distant organs. Of significance, GCC effects on cancer cell MMP-9 prevented establishment of metastatic colonies by colorectal cancer cells in the mouse peritoneum in vivo. Because endogenous hormones for GCC are uniformly deficient in intestinal tumors, reactivation of dormant GCC signaling with exogenous administration of GCC agonists may represent a specific intervention to target MMP-9 functions in colon cancer cells. The notion that GCC-mediated regulation of cancer cell MMP-9 disrupts metastasis, in turn, underscores the unexplored utility of GCC hormone replacement therapy in the chemoprevention of colorectal cancer progression.

Association of GUCY2C expression in lymph nodes with time to recurrence and disease-free survival in pN0 colorectal cancer

CONTEXT

The established relationship between lymph node metastasis and prognosis in colorectal cancer suggests that recurrence in 25% of patients with lymph nodes free of tumor cells by histopathology (pN0) reflects the presence of occult metastases. Guanylyl cyclase 2C (GUCY2C) is a marker expressed by colorectal tumors that could reveal occult metastases in lymph nodes and better estimate recurrence risk.

OBJECTIVE

To examine the association of occult lymph node metastases detected by quantifying GUCY2C messenger RNA, using the reverse transcriptase-polymerase chain reaction, with recurrence and survival in patients with colorectal cancer.

DESIGN, SETTING, AND PARTICIPANTS

Prospective study of 257 patients with pN0 colorectal cancer enrolled between March 2002 and June 2007 at 9 US and Canadian centers (7 academic medical centers and 2 community hospitals) provided 2570 fresh lymph nodes measuring 5 mm or larger for histopathology and GUCY2C messenger RNA analysis. Patients were followed up for a median of 24 months (range, 2-63 months) for disease recurrence or death.

MAIN OUTCOME MEASURES

Time to recurrence (primary outcome) and disease-free survival (secondary outcome) relative to expression of GUCY2C in lymph nodes.

RESULTS

Thirty-two patients (12.5%) had lymph nodes negative for GUCY2C (pN0 [mol-]), and all but 2 remained free of disease during follow-up (recurrence rate, 6.3%; 95% confidence interval [CI], 0.8%-20.8%). Conversely, 225 patients (87.5%) had lymph nodes positive for GUCY2C (pN0 [mol+]), and 47 developed recurrent disease (20.9%; 95% CI, 15.8%-26.8%) (P = .006). Multivariate analyses revealed that GUCY2C in lymph nodes was an independent marker of prognosis. Patients who were pN0 (mol+) exhibited earlier time to recurrence (adjusted hazard ratio, 4.66; 95% CI, 1.11-19.57; P = .04) and reduced disease-free survival (adjusted hazard ratio, 3.27; 95% CI, 1.15-9.29; P = .03).

CONCLUSION

Expression of GUCY2C in histologically negative lymph nodes appears to be independently associated with time to recurrence and disease-free survival in patients with pN0 colorectal cancer.

Inferring maps of forces inside cell membrane microdomains

Mapping of the forces on biomolecules in cell membranes has spurred the development of effective labels, e.g., organic fluorophores and nanoparticles, to track trajectories of single biomolecules. Standard methods use particular statistics, namely the mean square displacement, to analyze the underlying dynamics. Here, we introduce general inference methods to fully exploit information in the experimental trajectories, providing sharp estimates of the forces and the diffusion coefficients in membrane microdomains. Rapid and reliable convergence of the inference scheme is demonstrated on trajectories generated numerically. The method is then applied to infer forces and potentials acting on the receptor of the toxin labeled by lanthanide-ion nanoparticles. Our scheme is applicable to any labeled biomolecule and results show its general relevance for membrane compartmentation.

[Expression of guanylyl cyclase-c and caudal type homeobox transcription factor 2 in human gastric cancer and precursor lesions]

OBJECTIVE

To investigate the expression of guanylyl cyclase-C (GC-C) and caudal type homeobox transcription factor 2 (CDX2) in human gastric mucosa at different stages and the significance thereof.

METHODS

An Immunofluorescence method was used to detect the expression of GC-C and CDX-2 in 23 specimens of gastric carcinoma and matching noncancerous tissues. Western blotting was used to detect the protein expression of GC-C and CDX2 in the gastric carcinoma tissues and matching noncancerous tissues too.

RESULTS

The GC-C and CDX2 expression rates were 39.1% and 39.1% respectively in the intestinal metaplasia specimens, 55.6% and 55.6% respectively in the dysplasia specimens, and 56.7 % and 60.0% in the gastric carcinoma specimens, all significantly higher than those in the normal mucosa specimens (all P = 0.000) without significant differences in the expression of GC-C and CDX-2 among the 3 pathological groups. The GC-C and CDX-2 expression was positively correlated with Lauren classification, The expression levels of GC-C and CDX-2 were significantly higher in the intestinal-type than in the diffuse-type gastric carcinoma (P < 0.05). The GC-C expression was positively correlated with the expression of CDX-2 in intestinal metaplasia and gastric carcinoma.

CONCLUSION

Ectopic expression of GC-C and CDX2 in human gastric mucosa may play an important role in the carcinogenesis of intestinal-type gastric carcinoma. Detection of GC-C and CDX2 helps diagnose gastric carcinoma and precursor lesions.

Regulation of steroidogenesis by atrial natriuretic peptide (ANP) in the rat testis: differential involvement of GC-A and C receptors

Previous studies have established a stimulatory effect of natriuretic peptides (NP) on testosterone production in mouse Leydig cells as intense as that of LH. Chronic administration of ANP in mice, on the other side, reduced testosterone levels. So, the understanding of the role of ANP on testicular steroidogenesis has been impaired by discrepant findings. The aim of the present study was to clarify the physiological role of ANP in the rat testis steroidogenesis using a model that preserves the interactions between testis cells and a medium devoid of any circulating factors that could interfere with testosterone production. First, ANP was immunolocalized in the interstitial compartment of the rat testis, mainly in Leydig cells. We also determined the presence of ANP and both GC-A (guanylyl cyclase A) and C receptors by real-time PCR in testis. Perfusion in vitro of testis with ANP (1 and 3x10(-7)M) stimulated testosterone production in a time- and dose-dependent manner. On the other side, testosterone secretion induced by LH was blunted by ANP. Similar effect was obtained using the specific C receptor ligand, cANF, indicating the involvement of C receptor in such response. In conclusion, ANP stimulated testosterone production in the rat testis perfused in vitro but decreased testosterone production LH-induced, effect that seems to involve C receptor. To this extent, our results suggest the existence of a local and complex peptidergic system in the rat testis, involving ANP and its receptors that could importantly modulate the androgen biosynthesis.

Toxins which activate guanylate cyclase: heat-stable enterotoxins

Certain enteropathogenic bacteria, including strains of Escherichia coli, Klebsiella pneumoniae and Yersinia enterocolitica, elicit their diarrhoeagenic effects by elaborating small molecular weight, heat-stable enterotoxins (STs). Structural and functional characteristics indicate that ST peptides are heterogeneous and two major subtypes, STa and STb, have been identified. Molecules of STa, unlike those of STb, are methanol-soluble and elicit their pathogenic effects by activating host cell guanylate cyclase activity and thereby increasing tissue cyclic GMP content: this increase in cyclic GMP causes fluid secretion. STa binds to specific proteinaceous receptors on intestinal cells but the nature of STa-receptor coupling to guanylate cyclase is poorly understood. The actions of STa, including binding to its receptor, activation of guanylate cyclase and stimulation of electrolyte transport, are rapid, reversible and tissue-specific. STa activates only particulate and not soluble guanylate cyclase. It alters the Vmax but not the apparent Km of this enzyme for Mg-GTP or Mn-GTP. At concentrations above 0.5-1 mM, calcium inhibits the STa activation of guanylate cyclase. The effects of calmodulin antagonists such as chlorpromazine on the activation of guanylate cyclase by STa are less clear. Inhibitors of phospholipid and arachidonic acid cascade pathways interfere with both basal and STa-stimulated guanylate cyclase. Membrane integrity is essential for STa activation of guanylate cyclase and the STa-receptor complex may activate the enzyme by intramembrane protein-protein interactions and/or perturbations. Interference with membrane phospholipid could alter such coupling.

Progress in the development of prognostic and predictive markers for gastrointestinal malignancies

Gastrointestinal cancers remain a significant cause of morbidity and mortality. While increasing therapeutic options have improved outcomes for many patients, they have also complicated treatment decision-making. Unfortunately, most patients with advanced gastrointestinal malignancies die from their disease. Prognostic and predictive markers could improve treatment significantly by identifying patients who may or may not require a given therapy, and determining those most likely to benefit from a therapy. Candidates for such markers include blood antigens and circulating tumor cells, tumor enzyme and gene expression, and pharmacodynamic endpoints. In this review, we summarize reported and ongoing research to define and validate prognostic and predictive markers in gastrointestinal malignancies, with an emphasis on colorectal cancer and brief overview of pancreatic and neuroendocrine tumors.

A validated quantitative assay to detect occult micrometastases by reverse transcriptase-polymerase chain reaction of guanylyl cyclase C in patients with colorectal cancer

PURPOSE

Guanylyl cyclase C (GCC), a receptor for bacterial diarrheagenic enterotoxins, may be a prognostic and predictive marker to detect occult micrometastases in patients undergoing staging for colorectal cancer. However, quantification of GCC expression in tissues by the quantitative reverse transcription-PCR (qRT-PCR) has not undergone analytic and clinicopathologic validation.

EXPERIMENTAL DESIGN

A technique to quantify GCC mRNA in tissues employing RT-PCR was developed and validated employing external calibration standards of RNA complementary to GCC.

RESULTS

GCC qRT-PCR exhibited reaction efficiencies >92%, coefficients of variations <5%, linearity >6 orders of magnitude, and a limit of quantification of >25 copies of GCC cRNA. This assay confirmed that GCC mRNA was overexpressed by colorectal tumors from 41 patients, which correlated with increased GCC protein quantified by immunohistochemistry. Analyses obtained with 164 lymph nodes from patients free of cancer and 15 nodes harboring metastases established a threshold for metastatic disease of approximately 200 GCC mRNA copies/mug total RNA, with a sensitivity of 93% and specificity of 97%. GCC mRNA above that threshold was detected in 76 of 367 (approximately 21%) nodes free of disease by histopathology from 6 of 23 (26%) patients, suggesting the presence of occult micrometastases.

CONCLUSIONS

Quantifying GCC mRNA in tissues by RT-PCR employing external calibration standards is analytically robust and reproducible, with high clinicopathologic sensitivity and specificity. This validated assay is being applied to approximately 10,000 lymph nodes in a prospective trial to define the sensitivity of GCC qRT-PCR for staging patients with colorectal cancer.

Cancer mucosa antigens as a novel immunotherapeutic class of tumor-associated antigen

Colorectal cancer is a leading cause of cancer-related mortality worldwide. Surgery and chemoradiation exhibit incomplete efficacy and, ultimately, 50% of patients die of metastatic disease. In the context of that unmet clinical need, immunotherapeutic approaches have enjoyed limited success, partly because of a paucity of suitable antigen targets. However, exploitation of immune compartmentalization, employing antigens with expression restricted to normal intestinal mucosa and derivative colorectal tumors--cancer mucosa antigens (CMAs)--may represent a previously unrecognized class of immune targets supporting efficacious antitumor immunotherapy. Guanylyl cyclase C (GCC) is an intestine/colorectal cancer-restricted protein ideally suited as the first CMA for clinical evaluation.

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.

Guanylin peptides and colorectal cancer (CRC)

Agonists of guanylyl-C receptor, such as guanylin/uroguanylin, are correlated not only with the intestinal cell epithelial physiology but also with the colorectal cancer tumorigenesis. Activation of the second intracellular messenger cyclic guanosine monophosphate by guanylyl cyclase-C receptor results in a complex intracellular signalling cascade involving the phosphodiesterase, the ion channels and the protein kinase. After an analytical review of relevant new knowledge, new diagnostic and therapeutic approaches for colorectal cancer are discussed.

Opportunities for near-infrared thermal ablation of colorectal metastases by guanylyl cyclase C-targeted gold nanoshells

Colorectal cancer is the third most common malignancy and the second most common cause of cancer-related mortality worldwide. While surgery remains the mainstay of therapy, approximately 50% of patients who undergo resection develop parenchymal metastatic disease. Unfortunately, current therapeutic regimens offer little improvement to the survival of patients with parenchymal metastases in the liver and lung. In that context, there is a significant unrealized opportunity at the intersection of engineering and biology for the development of novel targeted therapeutic approaches to colorectal cancer metastases. This opportunity exploits the discovery that an intestinal receptor, guanylyl cyclase C, which mediates diarrhea induced by bacterial heat-stable enterotoxins (STs), is over-expressed by metastatic colorectal tumors only. Moreover, it leverages recent advances in the fabrication of metal nanoshells with defined thicknesses absorb near-infrared (NIR) light, resulting in resonance and transfer of thermal energies of more than 40 degrees C. Thus, the conjugation of ST to gold nanoshells, which can undergo resonance excitation by NIR light and emit heat, represents a previously unrecognized approach for the targeted therapy of parenchymal colorectal cancer metastases, specifically to the liver and lung. This article discusses the potential of ST-targeted nanoshells for NIR thermal ablation of metastatic colorectal tumors and highlights the significant challenges and solutions linked to the translation of this emerging technology to patient care.

Guanylyl cyclase C: a molecular marker for staging and postoperative surveillance of patients with colorectal cancer

Staging patients with colorectal cancer defines their prognosis and therapeutic management. Unfortunately, histopathology, the current standard for staging, is relatively insensitive for detecting occult micrometastases and a significant fraction of patients are understaged and, consequently, undertreated. Similarly, current approaches to postoperative surveillance of patients with colorectal cancer detect disease recurrence at a point when interventions have little impact on survival. The detection of rare cells in tissue, for accurately staging patients, and in blood, for detecting disease recurrence, could be facilitated by employing sensitive and specific markers of disease. Guanylyl cyclase C (GCC), the receptor for the diarrheagenic bacterial heat-stable enterotoxin, is expressed selectively by cells derived from intestinal mucosa, including normal intestinal cells and colorectal tumor cells, but not by extragastrointestinal tissues and tumors. The nearly uniform expression of relatively high levels by metastatic colorectal tumors suggests that GCC may be a sensitive and specific molecular marker for metastatic colorectal cancer cells. Employing GCC reverse transcriptase PCR, occult colorectal cancer micrometastases were detected in lymph nodes that escaped detection by histopathology. Moreover, marker expression correlated with the risk of disease recurrence. Similarly, GCC reverse transcriptase PCR revealed the presence of tumor cells in blood of all patients examined with metastatic colorectal cancer and, in some studies, was associated with an increased risk of disease recurrence and mortality. These observations suggest that GCC reverse transcriptase PCR is a sensitive and specific technique for identifying tumor cells in extraintestinal sites and may be useful for staging and postoperative surveillance of patients with colorectal cancer.

Interaction of Escherichia coli heat-stable enterotoxin (STa) with its putative receptor on the intestinal tract of newborn kids

The elaboration of heat stable enterotoxin (STa) is an important step in the pathogenesis of enterotoxigenic Escherichia coli (ETEC), which causes severe diarrhea in newborn animals. In this study, the distribution of the STa-specific receptors on enterocytes and brush border membrane vesicles (BBMVs) prepared from the anterior jejunum, posterior jejunum, ileum and colon of newborn kids was investigated. The density of STa-receptors on enterocytes and BBMVs was higher in the posterior jejunum than that in other segments of the kids' intestines. Additionally, the affinity of the posterior jejunum STa-receptors was higher than the affinity of receptors present on the epithelium of other intestinal segments. Our findings suggest that the posterior jejunum is a major target for STa within the intestinal tract of newborn kids.

Exisulind and guanylyl cyclase C induce distinct antineoplastic signaling mechanisms in human colon cancer cells

The nonsteroidal anti-inflammatory drug sulindac is metabolized to sulindac sulfone (exisulind), an antineoplastic agent that inhibits growth and induces apoptosis in solid tumors. In colon cancer cells, the antineoplastic effects of exisulind have been attributed, in part, to induction of cyclic guanosine 3',5'-monophosphate (cGMP) signaling through inhibition of cGMP-specific phosphodiesterases, which elevates intracellular cGMP, and novel expression of cGMP-dependent protein kinase (PKG) Ibeta, the presumed downstream effector mediating apoptosis. Here, inhibition of proliferation and induction of cell death by exisulind was dissociated from cGMP signaling in human colon cancer cells. Accumulation of intracellular cGMP produced by an exogenous cell-permeant analogue of cGMP or a potent agonist of guanylyl cyclase C yielded cytostasis without cell death. Surprisingly, the antiproliferative effects of induced cGMP accumulation were paradoxically less than additive, rather than synergistic, when combined with exisulind. Further, although exisulind induced expression of PKG Ibeta, it did not elevate intracellular cGMP and its efficacy was not altered by inhibition or activation of PKG I. Rather, PKG I induced by exisulind may mediate desensitization of cytostasis induced by cGMP. Thus, cytotoxic effects of exisulind are independent of cGMP signaling in human colon cancer cells. Moreover, combination therapies, including exisulind and agents that induce cGMP signaling, may require careful evaluation in patients with colon cancer.

Polarized apical sorting of guanylyl cyclase C is specified by a cytosolic signal

Receptor guanylyl cyclases respond to ligand stimulation by increasing intracellular cGMP, thereby initiating a variety of cell-signaling pathways. Furthermore, these proteins are differentially localized at the apical and basolateral membranes of epithelial cells. We have identified a region of 11 amino acids in the cytosolic COOH terminus of guanylyl cyclase C (GCC) required for normal apical localization in Madin-Darby canine kidney (MDCK) cells. These amino acids share no significant sequence homology with previously identified cytosolic apical sorting determinants. However, these amino acids are highly conserved and are sufficient to confer apical polarity to the interleukin-2 receptor alpha-chain (Tac). Additionally, we find two molecular weight species of GCC in lysates prepared from MDCK cells over-expressing GCC but observe only the fully mature species on the cell surface. Using pulse-chase analysis in polarized MDCK cells, we followed the generation of this mature species over time finding it to be detectable only at the apical cell surface. These data support the hypothesis that selective apical sorting can be determined using short, cytosolic amino acid motifs and argue for the existence of apical sorting machinery comparable with the machinery identified for basolateral protein traffic.

The proximal convoluted tubule is a target for the uroguanylin-regulated natriuretic response

OBJECTIVES AND METHODS

Guanylin and uroguanylin are peptides synthesized in the intestine and kidney that are postulated to have both paracrine and endocrine functions, forming a potential enteric-renal link to coordinate salt ingestion with natriuresis. To explore the in vivo role of guanylin and uroguanylin in the regulation of sodium excretion, we used gene-targeted mice in which the uroguanylin, guanylin or the peptide receptor guanylate cyclase C gene expression had been ablated.

RESULTS

Metabolic balance studies demonstrated that there was impaired excretion of a sodium load in uroguanylin (but not in guanylin or guanylate cyclase C) knockout mice. Uroguanylin-dependent natriuresis occurred without an increase in circulating prouroguanylin. A distinct morphological phenotype was present in the proximal convoluted tubules of uroguanylin knockout animals after an enteral salt loading. Marked vacuolization of the proximal convoluted tubule epithelial cells was observed by using light and electron microscopy. There was also a change in the distribution of the sodium hydrogen exchanger 3 (NHE3) after an enteral salt loading. In wild-type animals, there was a partial redistribution of NHE3 from the villus fraction to the less accessible submicrovillus membrane compartment, but this effect was less apparent in uroguanylin knockout animals, presumably resulting in greater Na/H exchange.

CONCLUSIONS

Together, these findings further establish a role for uroguanylin in fluid homeostasis and support a role for uroguanylin as an integral component of a signaling mechanism that mediates changes in Na excretion in response to an enteral salt loading. Proximal tubular NHE3 activity is a possible target for uroguanylin-mediated changes in Na excretion.

Bile acids induce ectopic expression of intestinal guanylyl cyclase C Through nuclear factor-kappaB and Cdx2 in human esophageal cells

BACKGROUND & AIMS

Although progression to adenocarcinoma at the gastroesophageal junction reflects exposure to acid and bile acids associated with reflux, mechanisms mediating this transformation remain undefined. Guanylyl cyclase C (GC-C), an intestine-specific tumor suppressor, may represent a mechanism-based marker and target of transformation at the gastroesophageal junction. The present studies examine the expression of GC-C in normal tissues and tumors from esophagus and stomach and mechanisms regulating its expression by acid and bile acids.

METHODS

Gene expression was examined by reverse-transcription polymerase chain reaction, promoter analysis, immunohistochemistry, immunoblotting, and functional analysis. Promoter transactivation was quantified by using luciferase constructs and mutational analysis. DNA binding of transcription factors was examined by electromobility shift analysis.

RESULTS

GC-C mRNA and protein were ectopically expressed in approximately 80% of adenocarcinomas arising in, but not in normal, esophagus and stomach. Similarly, in OE19 human esophageal cancer cells, deoxycholate and acid induced expression of GC-C. This was associated with the induction of expression of Cdx2, a transcription factor required for GC-C expression. In turn, induction of Cdx2 expression by deoxycholate was mediated by binding sites in the proximal promoter for nuclear factor kappaB (NF-kappaB). Furthermore, deoxycholate increased NF-kappaB activity, associated with nuclear translocation and Cdx2 promoter binding of the NF-kappaB subunit p50. Moreover, a dominant negative construct for NF-kappaB prevented deoxycholate-induced p50 nuclear translocation and activation of the Cdx2 promoter.

CONCLUSIONS

Transformation associated with reflux at the gastroesophageal junction reflects activation by bile acid and acid of a transcriptional program involving NF-kappaB and Cdx2, which mediate intestinal metaplasia and ectopic expression of GC-C.

Disulfide linkages and a three-dimensional structure model of the extracellular ligand-binding domain of guanylyl cyclase C

Guanylyl cyclase C (GC-C) is a single-transmembrane receptor that is specifically activated by endogenous ligands, including guanylin, and the exogenous ligand, heat-stable enterotoxin. Using combined HPLC separation and MS analysis techniques the positions of the disulfide linkages in the extracellular ligand-binding domain (ECD) of GC-C were determined to be between Cys7-Cys94, Cys72-Cys77, Cys101-Cys128 and Cys179-Cys226. Furthermore, a three-dimensional structural model of the ECD was constructed by homology modeling, using the structure of the ECD of GC-A as a template (van den Akker et al., 2000, Nature, 406: 101-104) and the information of the disulfide linkages. Although the GC-C model was similar to the known structure of GC-A, importantly its ligand-binding site appears to be located on the quite different region from that in GC-A.

A potential role for the endothelin ETA receptor in salt-sensitive hypertension of the proANP gene-disrupted mouse

We have previously shown that the partial disruption of the gene for atrial natriuretic peptide (ANP) results in a salt-sensitive phenotype. The present study examined the possibility that alterations in either the ANP natriuretic pathway or endothelin (ET) system in the kidney of the salt-challenged ANP +/- mouse was responsible for its salt-sensitive phenotype. Plasma ANP levels and renal cGMP activity were increased in response to a salt load in both ANP +/+ and +/- mice. However, the mRNA expression of proANP was found to be increased only in the ANP +/- kidney along with its guanylyl cyclase-linked receptor, NPRA; the upregulation of NPRA mRNA was limited to the renal medulla. This suggests that the renal ANP pathway remains capable of responding to a salt load in the ANP +/- animal, but may be compensating for other dysfunctional pathways. We also report a significant increase in renal ET-1 mRNA and ETA receptor protein expression in medulla and cortex of the salt-treated, ANP +/- mouse, but not its wild-type counterpart. In fact, ETA expression decreased in the renal cortex of the ANP +/+ salt-treated animal. The ETB receptor expression was not affected by diet in either genotype. We hypothesize that the salt-sensitive hypertension in the ANP +/- mouse is exacerbated, and possibly driven by the vasoconstrictive effects resulting from an upregulated ET-1/ETA pathway.

Interruption of homologous desensitization in cyclic guanosine 3',5'-monophosphate signaling restores colon cancer cytostasis by bacterial enterotoxins

Bacterial diarrheagenic heat-stable enterotoxins induce colon cancer cell cytostasis by targeting guanylyl cyclase C (GCC) signaling. Anticancer actions of these toxins are mediated by cyclic guanosine 3',5'-monophosphate (cGMP)-dependent influx of Ca2+ through cyclic nucleotide-gated channels. However, prolonged stimulation of GCC produces resistance in tumor cells to heat-stable enterotoxin-induced cytostasis. Resistance reflects rapid (tachyphylaxis) and slow (bradyphylaxis) mechanisms of desensitization induced by cGMP. Tachyphylaxis is mediated by cGMP-dependent protein kinase, which limits the conductance of cyclic nucleotide-gated channels, reducing the influx of Ca2+ propagating the antiproliferative signal from the membrane to the nucleus. In contrast, bradyphylaxis is mediated by cGMP-dependent allosteric activation of phosphodiesterase 5, which shapes the amplitude and duration of heat-stable enterotoxin-dependent cyclic nucleotide accumulation required for cytostasis. Importantly, interruption of tachyphylaxis and bradyphylaxis restores cancer cell cytostasis induced by heat-stable enterotoxins. Thus, regimens that incorporate cytostatic bacterial enterotoxins and inhibitors of cGMP-mediated desensitization offer a previously unrecognized therapeutic paradigm for treatment and prevention of colorectal cancer.

The putative tumor suppressor Cdx2 is overexpressed by human colorectal adenocarcinomas

PURPOSE

The current paradigm suggests that the homeodomain transcription factor Cdx2, which directs the development and maintenance of the intestinal epithelium, is a tumor suppressor in the colon and rectum. Although a cardinal property of tumor suppressors is their inactivation during carcinogenesis, the expression of Cdx2 in colorectal tumors has not been compared with that in normal mucosa. Here, Cdx2 expression and function was quantified in tumors and matched normal mucosa from patients with colorectal cancer.

EXPERIMENTAL DESIGN

Cdx2 expression was quantified by reverse transcription-PCR, immunoblot analysis, and immunohistochemistry. Transcriptional activity was explored by quantifying expression of an endogenous downstream target of Cdx2, guanylyl cyclase C (GCC), in tissues by quantitative reverse transcription-PCR and expression of exogenous Cdx2-specific luciferase promoter constructs in epithelial cells isolated from tumors and normal mucosa.

RESULTS

Most (>80%) colorectal tumors overexpressed Cdx2 mRNA and protein compared with normal mucosa, with median fold increases of 3.6 and 1.4, respectively (P<0.002). Concomitantly, immunohistochemistry revealed elevated levels of Cdx2 in nuclei of tumor cells compared with normal epithelial cells. Further, tumors exhibited increased expression of GCC compared with normal mucosa. Moreover, cells isolated from tumors overexpressed a Cdx2-specific luciferase promoter construct compared with normal mucosal cells.

CONCLUSION

These observations show, for the first time, the structural and functional overexpression of Cdx2 by human colorectal tumors compared with matched normal mucosa. They suggest that loss of Cdx2 expression or transcriptional activity is an infrequent event during tumorigenesis, which does not contribute to molecular mechanisms underlying initiation and progression of most colorectal tumors.

Uroguanylin and guanylin regulate transport of mouse cortical collecting duct independent of guanylate cyclase C

BACKGROUND

Electrolyte and water homeostasis mostly depend on differentially regulated intestinal and renal transport. Guanylin and uroguanylin were proposed as first hormones linking intestinal with renal electrolyte and water transport, which is disturbed in pathophysiology. Guanylate cyclase C is the intestinal receptor for these peptides, but in guanylate cyclase C-deficient mice renal effects are retained. Unlike for the intestine the sites of renal actions and cellular mechanisms of guanylin peptides are still unclear.

METHODS

After first data on proximal tubular effects in this study their effects are examined in detail in mouse cortical collecting duct (CCD). Effects of guanylin peptides on principal cells of isolated mouse CCD were studied by slow whole-cell patch-clamp analysis, reverse transcription-polymerase chain reaction (RT-PCR), and microfluorimetric measurements of intracellular Ca2+.

RESULTS

Guanylin peptides depolarized or hyperpolarized principal cells. Whereas 8-Br-cyclic guanosine monophosphate (8-Br-cGMP) hyperpolarized, 8-Br-cyclic adenosine monophosphate (8-Br-cAMP) depolarized principal cells. All effects of guanylin peptides were inhibited by Ba2+. Hyperpolarizations were blocked by clotrimazole or protein kinase G (PKG) inhibition, suggesting an involvement of basolateral Ca2+- and cGMP-dependent K+ channels. Effects remained in CCD isolated from guanylate cyclase C-deficient mice. Depolarizations were inhibited by arachidonic acid or inhibition of phospholipase A2 (PLA2), but not by protein kinase A (PKA) inhibition. Conclusion. These results suggest the existence of two signaling pathways for guanylin peptides in principal cells of mouse CCD. One pathway is cGMP- and PKG-dependent but not mediated by guanylate cyclase C, the second involves PLA2 and arachidonic acid. The first pathway most likely leads to an activation of the basolateral K+-conductance while the latter probably results in decreased activity of ROMK channels in the luminal membrane.

Statistical algorithm for assuring similar efficiency in standards and samples for absolute quantification by real-time reverse transcription polymerase chain reaction

Reverse transcription (RT) followed by the polymerase chain reaction (PCR) is the method of choice for quantifying rare transcripts in biological samples. A key assumption underlying the absolute quantification of transcripts is similar amplification efficiencies of all external standards and samples. However, efficiencies can vary between individual reactions, a problem that can be magnified when quantifying transcripts of low abundance. Here, an algorithm to assure that calibration standards and samples meet the assumption of similar amplification efficiencies underlying absolute quantification is presented. Individual reaction efficiency is estimated by fitting an exponential growth model to the fluorescence data in the exponential phase of the reaction. Next, reactions of standards with outlying estimates of amplification rates are eliminated using the boxplot outlier detection rule. Then, estimates of amplification rates of outlier-free standards are employed to define exact tolerance intervals, which are used to eliminate kinetic outliers from test samples. This algorithm was employed to eliminate kinetic outliers prior to defining the baseline expression of guanylyl cyclase C mRNA, a marker for colorectal cancer, in blood of healthy volunteers. These studies demonstrate that elimination of kinetic outliers from calibration standards and test samples improves the accuracy of absolute transcript quantification by RT-PCR.

Guanylyl cyclase C as a reliable immunohistochemical marker and its ligand Escherichia coli heat-stable enterotoxin as a potential protein-delivering vehicle for colorectal cancer cells

mRNA-based technologies and preclinical research in a variety of animal models have shown that guanylyl cyclase C (GCC) is a highly sensitive and specific molecular marker for the diagnosis of colorectal cancer (CRC). GCC is also a receptor for Escherichia coli (E. coli) heat-stable enterotoxin (STa) and can be used for STa-directed delivery of small-sized imaging agents to human CRC tumours. In this study, we have evaluated GCC as a new immunohistochemical (IHC) marker for CRC tissues and STa as a suitable vector for delivering high-sized protein molecules to CRC cells. Firstly, we have developed a highly sensitive EnVision(+)-based IHC staining method for detecting GCC in serial paraffin-embedded sections of primary and metastatic CRC (38 cases) or non-CRC (14 cases) adenocarcinomas. Carcinoembryonic antigen (CEA) and cytokeratin 20 (CK20) were chosen as controls. Our results indicate that GCC staining was positive in 100% of CRC tumours and was comparable to CEA (95%) or CK20 (92%). In contrast to CEA and CK20, GCC was negative in all of the extra-intestinal non-CRC tumours examined. GCC appears to display higher specificity than either CEA or CK20 while retaining high sensitivity, suggesting that it is a better CRC marker than CEA or CK20. Secondly, STa was genetically coupled to green fluorescent protein (GFP) and the resulting GFP-tagged STa was characterized for expression in E. coli and enterotoxicity in mouse. The binding characteristics of GFP-STa in CRC Caco-2 cells were followed by immunofluorescence microscopy. In this work we show that GFP-tagged STa is biologically active and has retained its ability to internalise into Caco-2 cells making it a potential vehicle for the delivery of anticancer therapeutic protein agents.

Lack of guanylyl cyclase C, the receptor for Escherichia coli heat-stable enterotoxin, results in reduced polyp formation and increased apoptosis in the multiple intestinal neoplasia (Min) mouse model

Guanylyl cyclase C (GC-C), a transmembrane receptor for bacterial heat-stable enterotoxin and the mammalian peptides guanylin and uroguanylin, mediates intestinal ion secretion and affects intestinal cell growth via cyclic GMP signaling. In intestinal tumors, GC-C expression is maintained while guanylin and uroguanylin expression is lost, suggesting a role for GC-C activation in tumor formation or growth. We show by in situ hybridization that GC-C expression is retained in adenomas from multiple intestinal neoplasia (Apc(Min/+)) mice. In order to determine the in vivo role of GC-C in intestinal tumorigenesis, we generated Apc(Min/+) mice homozygous for a targeted deletion of the gene encoding GC-C and hypothesized that these mice would have increased tumor multiplicity and size compared to wild-type Apc(Min/+) mice on the same genetic background. In contrast, the absence of GC-C resulted in a reduction of median polyp number by 55%. There was no change in the median diameter of polyps, suggesting no effect on tumor growth. Somatic loss of the wild-type Apc allele, an initiating event in intestinal tumorigenesis, also occurred in polyps from GC-C-deficient Apc(Min/+) mice. We have found increased levels of apoptosis as well as increased caspase-3 and caspase-7 gene expression in the intestines of GC-C-deficient Apc(Min/+) mice compared with Apc(Min/+) mice. We propose that these alterations are a possible compensatory mechanism by which loss of GC-C signaling also affects tumorigenesis.

Recognition and signal transduction mechanism of Escherichia coli heat-stable enterotoxin and its receptor, guanylate cyclase C

Guanylate cyclase C (GC-C), a member of the membrane-bound GC family, consists of an extracellular domain (ECD) and an intracellular domain, which are connected by a single-transmembrane region. GC-C is a receptor protein, i.e. specifically stimulated by the endogenous peptides guanylin, uroguanylin, lymphoguanylin, and the exogenous peptide heat-stable enterotoxin (ST(a)), secreted by pathogenic Escherichia coli and acting on the intestinal brush border membranes. The binding of these peptide ligands to the ECD of GC-C results in the synthesis of cyclic GMP in cells, which, in turn, regulates a variety of intracellular physiologic processes. As the cloning of GC-C, its physiologic functions of each domain have been vigorously investigated. The structural characterization of the ligand-binding domain of the receptor promises to provide important clues for better understanding of the mechanisms of receptor recognition and activation. Recently, structural data for each domain of membrane-bound GCs and related proteins has become available. Coupling information obtained from such work and validation of structure-function relationships of GC-C and its ligands should allow for three-dimensional mapping of their interaction site in detail. Our approach to this issue involved designing photoaffinity-labeling ST(a) analogs, capable of binding covalently to the ligand-binding region of the ECD of GC-C. The photoaffinity-labeling ligand was used to covalently label a soluble form of the recombinant ECD protein. Mass spectrometric analyses of an endoproteinase digest of the ECD revealed that the ligand specifically bound to a narrow region contained in the membrane-proximal subdomain of the ECD of GC-C. These results will enable us to identify the possible binding motifs within the ligand-binding domain by computer modeling. In this review, we summarize the available data on the recognition mechanism between ST(a) and GC-C at the molecular level.

Mechanisms of actions of guanylin peptides in the kidney

After a salty meal, stimulation of salt excretion via the kidney is a possible mechanism to prevent hypernatremia and hypervolemia. Besides the well known hormonal regulators of salt and water excretion in the distal nephron, arginine vasopressin and aldosterone, guanylin (GN) peptides produced in the intestine were proposed to be intestinal natriuretic peptides. These peptides inhibit Na+ absorption in the intestine and induce natriuresis, kaliuresis and diuresis in the kidney. The signaling pathway of GN peptides in the intestine is well known. They activate enterocytes via guanylate cyclase C (GC-C) and increase the cellular concentration of cGMP which leads to secretion of Cl-, HCO3- and water into the intestinal lumen and to inhibition of Na+ absorption. Guanylin peptides are filtered in the glomerulus, and additionally synthesized and excreted by tubular cells. They activate receptors located in the luminal membrane of the tubular cells along the nephron. In GC-C deficient mice renal effects of GN peptides are retained. In human, rat, and opossum proximal tubule cells, a cGMP-dependent signaling was demonstrated, but in addition GN peptides apparently also activate a PT-sensitive G-protein coupled receptor. A similar dual signaling pathway is also known for other natriuretic peptides like atrial natriuretic peptide. A cGMP-independent signaling pathway of GN peptides is also shown for principal cells of the human cortical collecting duct where the final hormonal regulation of electrolyte homeostasis takes place. This review will focus on the current knowledge on renal actions of GN peptides and specifically address novel GC-C- and cGMP-independent signaling mechanisms.

Enterotoxin/Guanylin Receptor Type Guanylyl Cyclases in Non-Mammalian Vertebrates

Cyclic GMP is a ubiquitous intracellular second messenger produced by guanylyl cyclases (GCs). The enterotoxin/guanylin receptor type membrane GC (designated as GC-C in mammals) is activated by exogenous ligands such as heat-stable enterotoxins (STa), small peptides secreted by some pathogenic strains of Escherichia coli which cause severe secretory diarrhea and also activated by endogenous ligands such as guanylin and uroguanylin. The STa/guanylin receptor type membrane GC, as well as other type membrane GCs, is composed of an extracellular domain, a single transmembrane domain, and an intracellular region comprising a kinase-like domain and a catalytic domain. The STa/guanylin receptor type membrane GC is identified in various vertebrates including fishes, amphibians, reptiles, and birds, implying that it serves some important and undefined physiological roles in the intestine of non-mammalian vertebrates, e.g. the regulation of water and salt absorption. In mammals, only a single membrane GC (GC-C) is known to be the STa/guanylin receptor. On the contrary, two membrane GC cDNAs are cloned from the intestine of the European eel Anguilla anguilla (GC-C1 and GC-C2) and the medaka fish Oryzias latipes (OlGC6 and OlGC9). OlGC6 and OlGC9 are structurally distinct and show different ligand responsibility. Accumulated evidences indicate that the transcriptional regulatory mechanism of the human GC-C gene is different from that of the corresponding medaka fish GC gene; the human GC-C gene is regulated by Cdx2 and/or HNF-4, and the medaka fish OlGC6 gene is regulated by OlPC4, which is a medaka fish homologue of the mammalian transcriptional positive co-factor 4 (PC4). Furthermore, the transcriptional regulatory mechanism of the OlGC9 gene is different from those of both the OlGC6 and human GC-C genes, indicating that the study on these two medaka fish GCs will be useful for further understanding of the STa/guanylin receptor type membrane GC in the vertebrates.

Guanylyl cyclase C is a marker of intestinal metaplasia, dysplasia, and adenocarcinoma of the gastrointestinal tract

Gastrointestinal (GI) tumors continue to be major causes of cancer-related mortality, in part, reflecting metastases that escape detection by histopathology. Moreover, although approximately 10% of carcinomas arise from unknown locations, these tumors frequently originate in the GI tract. Guanylyl cyclase C (GC-C) is a receptor selectively expressed by intestinal epithelial cells whose persistent expression by colorectal carcinomas and ectopic expression by adenocarcinomas of the upper GI tract suggest its use as a marker for GI malignancies. Here, expression of GC-C protein, identified by immunohistochemistry, was examined in tissues and tumors arising from the human GI tract. Guanylyl cyclase C protein was expressed by epithelial cells from the duodenum to the rectum, but not by those in normal esophagus and stomach. Expression was retained in tubular adenomas, inflammatory bowel disease, premalignant lesions, and in primary and metastatic adenocarcinomas from the colon, including metastases to lymph nodes and liver. Moreover, GC-C was ectopically expressed in all cases of dysplasia and adenocarcinomas arising from intestinal metaplasia in esophagus and stomach. Thus, GC-C appears to be an immunohistochemical marker for identifying adenocarcinomas of unknown origin, metastases in patients undergoing staging for GI adenocarcinomas, and intestinal metaplasia, dysplasia, and tumors arising therein in the upper GI tract.

The inhibitory mechanism of YC-1, a benzyl indazole, on smooth muscle cell proliferation: an in vitro and in vivo study

The pharmacological mechanisms of a synthetic compound 1-benzyl-3-(5'-hydroxymethyl-2'-furyl) indazole (YC-1) in preventing smooth muscle cell proliferation remains to be elucidated. The present study was aimed to explore the effects of YC-1 on certain molecules responsible for cell proliferation, including transforming growth factor (TGF)-beta1, soluble guanylyl cyclase (sGC) and focal adhesion kinase (FAK). The in vivo assay was correlated to the in vitro results of YC-1 on vascular stenosis. YC-1 was applied topically via a pluronic gel onto the balloon-injured rat carotid arteries, which were then harvested two weeks later for histological analysis. Our in vitro results showed that TGF-beta1 was suppressed by YC-1 by 50%. The translational level of sGC was threefold activated by YC-1 while the transcription level of sGC was increased up to 24-fold. FAK, the molecule responsible for cell proliferation and migration, was suppressed by YC-1 on the translational levels for 72%. These in vitro results were in consistent with the in vivo observation that the area ratio of neointima to media was reduced by YC-1. This study provides insights into the pharmacological mechanisms of YC-1 in preventing abnormal smooth muscle cell proliferation and thus supports the use of YC-1 as an adjuvant therapy for balloon injury-induced restenosis.

Molecular detection of clinical colorectal cancer metastasis: how should multiple markers be put to use?

BACKGROUND AND AIMS

Up to 45% of colorectal cancer (CRC) patients will develop local recurrence or metastasis following curative resection. The latter is due to cells shed from the primary carcinoma prior to or during surgery. The aim of this study was to contribute toward a "rational"-approach for detecting these disseminated tumor cells (DTC) using a combination of independent markers and detection methods.

PATIENTS/METHODS

Liver, lymph node, and bone marrow samples from 246 CRC patients were screened for DTC using three markers: mutated K-ras was detected by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP), and cytokeratin 20 (CK20) and guanylylcyclase C (GCC), indicating circulating epithelial cells, were tracked by nested reverse-transcription (RT) PCR.

RESULTS

The rate of positive findings of the individual markers (CK20: 88%; GCC: 88%; K-ras: 67%) and their combinations (88-50%) was significantly higher in biopsies from liver metastases than in liver samples from patients without evident distant metastasis (M0; p<0.03). The detection rate of individual markers (except GCC) was also significantly elevated in inconspicuous liver tissue adjacent to metastasis compared with specimens from M0 patients. When using the concomitant detection of all three markers as criterion for DTC in the liver of M0 patients, however, no patient was DTC-positive. Therefore, the concomitant presence of the two CEC markers (CK20 plus GCC) and/or the presence of mutated K-ras were preferred for a combined evaluation, which resulted in a 24% detection rate for biopsies from both liver lobes. This translates into 39% of M0 patients with at least one positive liver biopsy.

CONCLUSION

Our results suggest that the concomitant detection of CK20 plus GCC and/or the presence of mutated K-ras are a rational approach for tracking CEC/DTC in CRC patients.

Structural insights into the regulation and the activation mechanism of mammalian guanylyl cyclases

Guanylyl cyclases (GC) are proteins that are essential for the production of the second messenger cyclic guanosine monophosphate (cGMP). Mammalian GC have attracted considerable interest due to their roles in important physiological processes such as vasodilation, vision, and bone growth. In addition, their link to disease and concomitant pharmaceutical potential have made these cyclases a long standing target for probing their intriguing mechanism of activation with the aim of drug development. The vasodilatory drugs nitroglycerin and nesiritide act through (different) GC pathways and have both been shown to provide beneficial relief for congestive heart failure patients. New structural insights are recently emerging on the activation mechanism and regulation of these receptors. The aim of this review is to discuss the interesting differences and similarities between members of the soluble and membrane bound GC in detail and put these in context with the structural knowledge that is available to date. These efforts contribute to an enhanced understanding of the GC and will likely lead to an increased success in structure-based therapeutic intervention.

Quantitative analysis of the cardiac fibroblast transcriptome-implications for NO/cGMP signaling

Cardiac fibroblasts regulate tissue repair and remodeling in the heart. To quantify transcript levels in these cells we performed a comprehensive gene expression study using serial analysis of gene expression (SAGE). Among 110,169 sequenced tags we could identify 30,507 unique transcripts. A comparison of SAGE data from cardiac fibroblasts with data derived from total mouse heart revealed a number of fibroblast-specific genes. Cardiac fibroblasts expressed a specific collection of collagens, matrix proteins and metalloproteinases, growth factors, and components of signaling pathways. The NO/cGMP signaling pathway was represented by the mRNAs for alpha(1) and beta(1) subunits of guanylyl cyclase, cGMP-dependent protein kinase type I (cGK I), and, interestingly, the G-kinase-anchoring protein GKAP42. The expression of cGK I was verified by RT-PCR and Western blot. To establish a functional role for cGK I in cardiac fibroblasts we studied its effect on cell proliferation. Selective activation of cGK I with a cGMP analog inhibited the proliferation of serum-stimulated cardiac fibroblasts, which express cGK I, but not higher passage fibroblasts, which contain no detectable cGK I. Currently, our data suggest that cGK I mediates the inhibitory effects of the NO/cGMP pathway on cardiac fibroblast growth. Furthermore the SAGE library of transcripts expressed in cardiac fibroblasts provides a basis for future investigations into the pathological regulatory mechanisms underlying cardiac fibrosis.

Guanylin and uroguanylin induce natriuresis in mice lacking guanylyl cyclase-C receptor

BACKGROUND

Guanylin (GN) and uroguanylin (UGN) are intestinally derived peptide hormones that are similar in structure and activity to the diarrhea-causing Escherichia coli heat-stable enterotoxins (STa). These secretagogues have been shown to affect fluid, Na+, K+, and Cl- transport in both the intestine and kidney, presumably by intracellular cyclic guanosine monophosphate (cGMP)-dependent signal transduction. However, the in vivo consequences of GN, UGN, and STa on renal function and their mechanism of action have yet to be rigorously tested.

METHODS

We hypothesized that intravenous administration of GN, UGN, or STa would cause an increase in natriuresis in wild-type mice via cGMP and guanylyl cyclase-C (GC-C, Gucy2c), the only known receptor for these peptide-hormones, and that the peptide-induced natriuresis would be blunted in genetically altered mice devoid of GC-C receptors (GC-C(-/-) null).

RESULTS

In wild-type mice using a modified renal clearance model, GN, UGN, and STa elicited significant natriuresis, kaliuresis, and diuresis as well as increased urinary cGMP levels in a time- and dose-dependent fashion. Absolute and fractional urinary sodium excretion levels were greatest approximately 40 minutes following a bolus infusion with pharmacologic doses of these peptides. Unexpectedly, GC-C(-/-) null mice also responded to the GN peptides similarly to that observed in wild-type mice. Glomerular filtration rate (GFR), blood pressure, and plasma cGMP in the mice (wild-type or GC-C(-/-) null) did not significantly vary between the vehicle- and peptide-treatment groups. The effects of UGN may also influence long-term renal function due to down-regulation of the Na+/K+ ATPase gamma-subunit and the Cl- channel ClC-K2 by 60% and 75%, respectively, as assessed by differential display polymerase chain reaction (PCR) (DD-PCR) and Northern blot analysis of kidney mRNA from mice treated with UGN.

CONCLUSION

GN, UGN, and STa act on the mouse kidney, in part, through a cGMP-dependent, GC-C-independent mechanism, causing significant natriuresis by renal tubular processes. UGN may have further long-term effects on the kidney by altering the expression of such transport-associated proteins as Na+/K+ ATPase and ClC-K2.

Glycosylation of the receptor guanylate cyclase C: role in ligand binding and catalytic activity

GC-C (guanylate cyclase C) is the receptor for heat-stable enterotoxins, guanylin and uroguanylin peptides. Ligand binding to the extracellular domain of GC-C activates the guanylate cyclase domain leading to accumulation of cGMP. GC-C is expressed as differentially glycosylated forms in HEK-293 cells (human embryonic kidney-293 cells). In the present study, we show that the 145 kDa form of GC-C contains sialic acid and galactose residues and is present on the PM (plasma membrane) of cells, whereas the 130 kDa form is a high mannose form that is resident in the endoplasmic reticulum and serves as the precursor for the PM-associated form. Ligand-binding affinities of the differentially glycosylated forms are similar, indicating that glycosylation of GC-C does not play a role in direct ligand interaction. However, ligand-stimulated guanylate cyclase activity was observed only for the fully mature form of the receptor present on the PM, suggesting that glycosylation had a role to play in imparting a conformation to the receptor that allows ligand stimulation. Treatment of cells at 20 degrees C led to intracellular accumulation of a mature glycosylated form of GC-C that now showed ligand-stimulated guanylate cyclase activity, indicating that localization of GC-C was not critical for its catalytic activity. To determine if complex glycosylation was required for ligand-stimulated activation of GC-C, the receptor was expressed in HEK-293 cells that were deficient in N -acetylglucosaminyltransferase 1. This minimally glycosylated form of the receptor was expressed on the cell surface and could bind a ligand with an affinity comparable with the 145 kDa form of the receptor. However, this form of the receptor was poorly activated by the ligand. Therefore our studies indicate a novel role for glycosidic modification of GC-C during its biosynthesis, in imparting subtle conformational changes in the receptor that allow for ligand-mediated activation and perhaps regulation of basal activity.

Tyrphostins are inhibitors of guanylyl and adenylyl cyclases

Guanylyl cyclase C (GC-C), the receptor for guanylin, uroguanylin, and the heat-stable enterotoxin, regulates fluid balance in the intestine and extraintestinal tissues. The receptor has an extracellular domain, a single transmembrane spanning domain, and an intracellular domain that harbors a region homologous to protein kinases, followed by the C-terminal guanylyl cyclase domain. Adenine nucleotides can regulate the guanylyl cyclase activity of GC-C by binding to the intracellular kinase homology domain (KHD). In this study, we have tested the effect of several protein kinase inhibitors on GC-C activity and find that the tyrphostins, known to be tyrosine kinase inhibitors, could inhibit GC-C activity in vitro. Tyrphostin A25 (AG82) was the most potent inhibitor with an IC(50) of approximately 15 microM. The mechanism of inhibition was found to be noncompetitive with respect to both the substrate MnGTP and the metal cofactor. Interestingly, the activity of the catalytic domain of GC-C (lacking the KHD) expressed in insect cells was also inhibited by tyrphostin A25 with an IC(50) of approximately 5 microM. As with the full-length receptor, inhibition was found to be noncompetitive with respect to MnGTP. Inhibition was reversible, ruling out a covalent modification of the receptor. Structurally similar proteins such as the soluble guanylyl cyclase and the adenylyl cyclases were also inhibited by tyrphostin A25. Evaluation of a number of tyrphostins allowed us to identify the requirement of two vicinal hydroxyl groups in the tyrphostin for effective inhibition of cyclase activity. Therefore, our studies are the first to report that nucleotide cyclases are inhibited by tyrphostins and suggest that novel inhibitors based on the tyrphostin scaffold can be developed, which could aid in a greater understanding of nucleotide cyclase structure and function.

Glycosphingolipids as toxin receptors

A number of proteins produced by certain bacteria and plants are potently toxic to mammalian cells. This toxicity results from their ability to catalytically modify macromolecules that are required for essential cellular functions such as vesicular trafficking, cytoskeletal assembly, signalling or protein synthesis. To reach their targets, these proteins bind specific surface receptors before endocytosis and translocation across an internal membrane. The surface receptors exploited by different toxins include a range of proteins and lipids. Here we focus on specific glycosphingolipid receptors and two well-characterised subsets of toxins that exploit them for surface binding, intracellular trafficking, and signalling events.

Autosomal dominant cone-rod dystrophy with R838H and R838C mutations in the GUCY2D gene in Japanese patients

PURPOSE

To describe the clinical phenotypes of two Japanese families with autosomal dominant cone-rod dystrophy (CORD) caused by an R838H or R838C mutation.

METHODS

Complete ophthalmological examinations were performed on three affected individuals from two Japanese families with autosomal dominant CORD. One family had an R838H mutation, and the other family had an R838C mutation in the GUCY2D gene. The tests included best-corrected visual acuity, slit-lamp and fundus examinations, fundus photography, electroretinography, Goldmann kinetic perimetry, and automated light- and dark-adapted static perimetry.

RESULTS

The three patients showed essentially normal fundus or little pigmentary change in the maculae by indirect ophthalmoscopy, and only fluorescein angiography revealed clear atrophy of the retinal pigmented epithelium around the fovea. Central or paracentral scotoma was detected by the Goldmann kinetic visual field test. Electroretinography as well as light-adapted and dark-adapted two-color perimetry showed more severe impairment of cone than of rod function. The clinical features in our patients resembled those in Caucasian families with R838H or R838C mutations.

CONCLUSIONS

The R838H and R838C mutations in GUCY2D cause CORD in the Japanese population. These mutations can cause a similar clinical phenotype in other races.

Ability of blood group A-active glycosphingolipids to act as Escherichia coli heat-labile enterotoxin receptors in HT-29 cells

We examined the ability of blood group A-active glycoconjugates to act as receptors for Escherichia coli heat-labile type I enterotoxin (LT-I) in HT-29 cells. These cells contained ~4 times more specific binding sites for LT-I than for cholera toxin (CT). Binding of LT-I could not be blocked by the B subunit of CT (CT-B), indicating the existence of LT-I receptors in addition to the glycosphingolipid GM1. LT-I was able to increase levels of cyclic adenosine monophosphate (AMP), even in the presence of CT-B. Helix pomatia and anti-blood group A antibody caused a dose-dependent inhibition of binding of LT-I to cells and production of cyclic AMP. LT-I recognized several complex blood group A-active glycosphingolipids from cells, and this interaction was also interfered with by H. pomatia. Treatment of cells with D,L-threo-1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol diminished surface expression of blood group A-active glycosphingolipids and binding of LT-I to non-GM1 receptors. These observations suggest that blood group A-active glycosphingolipids can function as alternative receptors for LT-I in HT-29 cells.

Detection of occult metastasis in lymph nodes from colorectal cancer patients: a multiple-marker reverse transcriptase-polymerase chain reaction study

INTRODUCTION

Lymph node status is a key factor for disease staging and is the main determinant for adjuvant therapy of colorectal cancer. The current staging procedure is unable to identify occult metastasis in lymph nodes, which is likely to be an important cause of treatment failure in some early-stage patients. The detection of occult metastasis could identify a patient subgroup at risk for disease relapse that would benefit from adjuvant therapy. The purpose of this study was to establish and test a multimarker reverse transcriptase-polymerase chain reaction assay for the molecular detection of occult metastases in lymph nodes.

METHODS

Forty-four patients with colorectal cancer and 14 patients with benign bowel diseases undergoing colonic resection were enrolled in the study. Reverse transcriptase-polymerase chain reaction was used to detect expression of three epithelial markers, carcinoembryonic antigen, cytokeratin 20, and guanylyl cyclase C, in fresh colorectal lymph node tissue.

RESULTS

Forty-six of 47 (97.9 percent) histologically positive lymph nodes were also positive by reverse transcriptase-polymerase chain reaction. Of 221 histologically negative nodes, 97 (43.9 percent) were positive for at least one of the three markers by reverse transcriptase-polymerase chain reaction: 24.9 percent for carcinoembryonic antigen, 16.7 percent for cytokeratin 20, and 24.9 percent for guanylyl cyclase C. Among these were 13 of 20 stage I and II cases, implying a staging shift to stage III by molecular diagnosis of occult metastasis. Fifty-nine additional nodes were found to be positive for occult metastases in 22 of 24 stage III and IV patients.

CONCLUSIONS

These results indicate that occult metastases are detectable by reverse transcriptase-polymerase chain reaction in histologically negative lymph nodes from colorectal cancer. The use of a panel of three markers improves the specificity of the method.