Characterization and partial purification of the human receptor for the heat-stable enterotoxin

The evolutionary divergence of receptor guanylyl cyclase C has implications for preclinical models for receptor-directed therapeutics

Mutations in receptor guanylyl cyclase C (GC-C) cause severe gastrointestinal disease, including meconium ileus, early onset acute diarrhea, and pediatric inflammatory bowel disease that continues into adulthood. Agonists of GC-C are US Food and Drug Administration-approved drugs for the treatment of constipation and irritable bowel syndrome. Therapeutic strategies targeting GC-C are tested in preclinical mouse models, assuming that murine GC-C mimics human GC-C in its biochemical properties and downstream signaling events. Here, we reveal important differences in ligand-binding affinity and GC activity between mouse GC-C and human GC-C. We generated a series of chimeric constructs of various domains of human and mouse GC-C to show that the extracellular domain of mouse GC-C contributed to log-orders lower affinity of mouse GC-C for ligands than human GC-C. Further, the Vmax of the murine GC domain was lower than that of human GC-C, and allosteric regulation of the receptor by ATP binding to the intracellular kinase-homology domain also differed. These altered properties are reflected in the high concentrations of ligands required to elicit signaling responses in the mouse gut in preclinical models and the specificity of a GC inhibitor towards human GC-C. Therefore, our studies identify considerations in using the murine model to test molecules for therapeutic purposes that work as either agonists or antagonists of GC-C, and vaccines for the bacterial heat-stable enterotoxin that causes watery diarrhea in humans.

The Molecular Basis of Heat-Stable Enterotoxin for Vaccine Development and Cancer Cell Detection

Heat-stable enterotoxin (ST_a) produced by Enterotoxigenic E. coli is responsible for causing acute diarrhea in infants in developing countries. However, the chemical synthesis of ST_a peptides with the native conformation and the correct intra-molecular disulfide bonds is a major hurdle for vaccine development. To address this issue, we herein report on the design and preparation of ST_a analogs and a convenient chemical method for obtaining ST_a molecules with the correct conformation. To develop an ST_a vaccine, we focused on a structure in a type II β-turn in the ST_a molecule and introduced a D-Lys residue as a conjugation site for carrier proteins. In addition, the -Glu-Leu- sequence in the ST_a molecule was replaced with a -Asp-Val- sequence to decrease the toxic activity of the peptide to make it more amenable for use in vaccinations. To solve several issues associated with the synthesis of ST_a, such as the formation of non-native disulfide isomers, the native disulfide pairings were regioselectively formed in a stepwise manner. A native form or topological isomer of the designed ST_a peptide, which possesses a right-handed or a left-handed spiral structure, respectively, were synthesized in high synthetic yields. The conformation of the synthetic ST_a peptide was also confirmed by CD and NMR spectroscopy. To further utilize the designed ST_a peptide, it was labeled with fluorescein for fluorescent detection, since recent studies have also focused on the use of ST_a for detecting cancer cells, such as Caco-2 and T84. The labeled ST_a peptide was able to specifically and efficiently detect 293T cells expressing the recombinant ST_a receptor (GC-C) protein and Caco-2 cells. The findings reported here provide an outline of the molecular basis for using ST_a for vaccine development and in the detection of cancer cells.

Mutational landscape of receptor guanylyl cyclase C: Functional analysis and disease-related mutations

Guanylyl cyclase C (GC-C) is the receptor for the heat-stable enterotoxin, which causes diarrhea, and the endogenous ligands, guanylin and uroguanylin. GC-C is predominantly expressed in the intestinal epithelium and regulates fluid and ion secretion in the gut. The receptor has a complex domain organization, and in the absence of structural information, mutational analysis provides clues to mechanisms of regulation of this protein. Here, we review the mutational landscape of this receptor that reveals regulatory features critical for its activity. We also summarize the available information on mutations in GC-C that have been reported in humans and contribute to severe gastrointestinal abnormalities. Since GC-C is also expressed in extra-intestinal tissues, it is likely that mutations thus far reported in humans may also affect other organ systems, warranting a close observation of these patients in future.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

From Escherichia coli heat-stable enterotoxin to mammalian endogenous guanylin hormones

The isolation of heat-stable enterotoxin (STa) from Escherichia coli and cholera toxin from Vibrio cholerae has increased our knowledge of specific mechanisms of action that could be used as pharmacological tools to understand the guanylyl cyclase-C and the adenylyl cyclase enzymatic systems. These discoveries have also been instrumental in increasing our understanding of the basic mechanisms that control the electrolyte and water balance in the gut, kidney, and urinary tracts under normal conditions and in disease. Herein, we review the evolution of genes of the guanylin family and STa genes from bacteria to fish and mammals. We also describe new developments and perspectives regarding these novel bacterial compounds and peptide hormones that act in electrolyte and water balance. The available data point toward new therapeutic perspectives for pathological features such as functional gastrointestinal disorders associated with constipation, colorectal cancer, cystic fibrosis, asthma, hypertension, gastrointestinal barrier function damage associated with enteropathy, enteric infection, malnutrition, satiety, food preferences, obesity, metabolic syndrome, and effects on behavior and brain disorders such as attention deficit, hyperactivity disorder, and schizophrenia.

Site-specific N-linked glycosylation of receptor guanylyl cyclase C regulates ligand binding, ligand-mediated activation and interaction with vesicular integral membrane protein 36, VIP36

Guanylyl cyclase C (GC-C) is a multidomain, membrane-associated receptor guanylyl cyclase. GC-C is primarily expressed in the gastrointestinal tract, where it mediates fluid-ion homeostasis, intestinal inflammation, and cell proliferation in a cGMP-dependent manner, following activation by its ligands guanylin, uroguanylin, or the heat-stable enterotoxin peptide (ST). GC-C is also expressed in neurons, where it plays a role in satiation and attention deficiency/hyperactive behavior. GC-C is glycosylated in the extracellular domain, and differentially glycosylated forms that are resident in the endoplasmic reticulum (130 kDa) and the plasma membrane (145 kDa) bind the ST peptide with equal affinity. When glycosylation of human GC-C was prevented, either by pharmacological intervention or by mutation of all of the 10 predicted glycosylation sites, ST binding and surface localization was abolished. Systematic mutagenesis of each of the 10 sites of glycosylation in GC-C, either singly or in combination, identified two sites that were critical for ligand binding and two that regulated ST-mediated activation. We also show that GC-C is the first identified receptor client of the lectin chaperone vesicular integral membrane protein, VIP36. Interaction with VIP36 is dependent on glycosylation at the same sites that allow GC-C to fold and bind ligand. Because glycosylation of proteins is altered in many diseases and in a tissue-dependent manner, the activity and/or glycan-mediated interactions of GC-C may have a crucial role to play in its functions in different cell types.

WITHDRAWN: Expression and functional characterization of guanylyl cyclase C receptor in HepG2 cells: Two-step regulation by dexamethasone and hepatocyte nuclear factor 4 (HNF4)

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Receptor guanylyl cyclase C (GC-C): regulation and signal transduction

Receptor guanylyl cyclase C (GC-C) is the target for the gastrointestinal hormones, guanylin, and uroguanylin as well as the bacterial heat-stable enterotoxins. The major site of expression of GC-C is in the gastrointestinal tract, although this receptor and its ligands play a role in ion secretion in other tissues as well. GC-C shares the domain organization seen in other members of the family of receptor guanylyl cyclases, though subtle differences highlight some of the unique features of GC-C. Gene knock outs in mice for GC-C or its ligands do not lead to embryonic lethality, but modulate responses of these mice to stable toxin peptides, dietary intake of salts, and development and differentiation of intestinal cells. It is clear that there is much to learn in future about the role of this evolutionarily conserved receptor, and its properties in intestinal and extra-intestinal tissues.

The linker region in receptor guanylyl cyclases is a key regulatory module: mutational analysis of guanylyl cyclase C

Receptor guanylyl cyclases are multidomain proteins, and ligand binding to the extracellular domain increases the levels of intracellular cGMP. The intracellular domain of these receptors is composed of a kinase homology domain (KHD), a linker of approximately 70 amino acids, followed by the C-terminal guanylyl cyclase domain. Mechanisms by which these receptors are allosterically regulated by ligand binding to the extracellular domain and ATP binding to the KHD are not completely understood. Here we examine the role of the linker region in receptor guanylyl cyclases by a series of point mutations in receptor guanylyl cyclase C. The linker region is predicted to adopt a coiled coil structure and aid in dimerization, but we find that the effects of mutations neither follow a pattern predicted for a coiled coil peptide nor abrogate dimerization. Importantly, this region is critical for repressing the guanylyl cyclase activity of the receptor in the absence of ligand and permitting ligand-mediated activation of the cyclase domain. Mutant receptors with high basal guanylyl cyclase activity show no further activation in the presence of non-ionic detergents, suggesting that hydrophobic interactions in the basal and inactive conformation of the guanylyl cyclase domain are disrupted by mutation. Equivalent mutations in the linker region of guanylyl cyclase A also elevated the basal activity and abolished ligand- and detergent-mediated activation. We, therefore, have defined a key regulatory role for the linker region of receptor guanylyl cyclases which serves as a transducer of information from the extracellular domain via the KHD to the catalytic domain.

Downregulation of human colon carcinoma cell (COLO-205) proliferation through PKG-MAP kinase mediated signaling cascade by E. coli heat stable enterotoxin (STa), a potent anti-angiogenic and anti-metastatic molecule

It was reported earlier that Escherichia coli heat stable enterotoxin (STa), a major causative agent of secretory diarrhea, can also inhibit the proliferation of colon carcinoma cells with the involvement of cGMP mediated calcium influx. In the present study it is shown that E. coli STa inhibits cell proliferation in the colonic carcinoma cell line COLO-205 by the PKG-ERK44/42 mediated signaling pathway. This enterotoxin negatively regulates cell proliferation by downregulating the activity of ERK44/42(MAPK) and subsequently the activity of a transcription regulatory protein cMyc. The antiproliferative effect of STa was reversed by LY83583, a guanylate cyclase (GC) inhibitor and KT5823, a PKG inhibitor. Thus suggesting the involvement of cGMP dependent protein kinase (PKG) in the downregulation of ERK44/42 and subsequent inactivation of cMyc activity. Moreover, it has been shown that a specific ERK44/42 inhibitor, PD98059, also inhibits cMyc activation and cell proliferation, which further confirms the involvement of ERK44/42 in the activation of cMyc. It is also shown that E. coli STa significantly inhibits the vascular endothelial growth factor (VEGF, a potent angiogenic factor) expression in COLO-205 cells and also downregulates vascular cell adhesion molecule-1 (VCAM-1, a potent metastatic factor) expression on the COLO-205 cell surface. So it is reported for the first time that E. coli STa inhibits the proliferation of the colonic carcinoma cell line COLO-205 by the PKG-ERK44/42 mediated pathway and it may have a role against the development of colon carcinoma.

The cGMP-binding, cGMP-specific phosphodiesterase (PDE5): intestinal cell expression, regulation and role in fluid secretion

The expression and regulation of the cGMP-binding, cGMP-specific phosphodiesterase, PDE5, was studied in intestinal cells. Both PDE5A1 and PDE5A2 splice forms were cloned from the cDNA prepared from human colonic T84 cells, and PDE5 activity was dependent on increases in intracellular cGMP levels which correlated with increased phosphorylation of the enzyme. PDE5 expression was monitored in different regions of the gastrointestinal tract and nearly 50% of the phosphodiesterase activity in the duodenum, jejunum, ileum and colon was inhibited by sildenafil citrate. Administration of the stable toxin to intestinal loops resulted in activation of PDE5. Inhibition of PDE5 by sildenafil citrate led to fluid accumulation in loops, suggesting a possible explanation for the side effect of diarrhoea observed in individuals administered sildenafil citrate. Our results therefore represent the first study on the expression and regulation of PDE5 in intestinal tissue, and indicate that mechanisms to control its activity may have important consequences in intestinal physiology.

Involvement of protein kinase C in the mechanism of action of Escherichia coli heat-stable enterotoxin (STa) in a human colonic carcinoma cell line, COLO-205

The present study was undertaken to determine the involvement of calcium-protein kinase C pathway in the mechanism of action of Escherichia coli heat stable enterotoxin (STa) apart from STa-induced activation of guanylate cyclase in human colonic carcinoma cell line COLO-205, which was used as a model cultured cell line to study the mechanism of action of E. coli STa. In response to E. coli STa, protein kinase C (PKC) activity was increased in a time-dependent manner with its physical translocation from cytosol to membrane. Inhibition of the PKC activity in membrane fraction and inhibition of its physical translocation in response to IP3-mediated calcium release inhibitor dantrolene suggested the involvement of intracellular store depletion in the regulation of PKC activity. Among different PKC isoforms, predominant involvement of calcium-dependent protein kinase C (PKC(alpha)) was specified using isotype-specific pseudosubstrate, which showed pronounce enzyme activity. Inhibition of enzyme activity by PKC(alpha)-specific inhibitor Gö6976 and immunoblott study employing isotype-specific antibody further demonstrated the involvement of calcium-dependent isoform of PKC in the mechanism of action of E. coli STa. Moreover, inhibition of guanylate cyclase activity by PKC(alpha)-specific inhibitor Gö6976 suggested the involvement of PKC(alpha) in the regulation of guanylate cyclase activity.

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.

Bimolecular complex between rolling and firm adhesion receptors required for cell arrest; CD44 association with VLA-4 in T cell extravasation

CD44 on activated T cells can initiate contact and mediate rolling on hyaluronan on endothelial cells. We have shown that the integrin VLA-4 is used preferentially over LFA-1 in conjunction with this rolling interaction for firm adhesion. Here, we show by coimmunoprecipitation and transfection studies that CD44 associates with VLA-4 but not LFA-1 on the plasma membrane of immune cells. Absence of the cytoplasmic portion of CD44 abrogates this coassociation and attendant firm adhesion. Moreover, in an in vivo model of lymphocyte homing, cells expressing only the truncated form of CD44 together with VLA-4 fail to traffic to an inflamed site, thereby defining a discrete biological role for the cytoplasmic domain. These studies demonstrate a molecular mechanism whereby coanchoring within a single bimolecular complex between a primary and secondary adhesion molecule regulates a cell's ability to firmly adhere, providing a fundamental alteration to the paradigm of leukocyte extravasation.

Cellular refractoriness to the heat-stable enterotoxin peptide is associated with alterations in levels of the differentially glycosylated forms of guanylyl cyclase C

The heat-stable enterotoxin peptides (ST) produced by enterotoxigenic Escherichia coli are one of the major causes of transitory diarrhea in the developing world. Toxin binding to its receptor, guanylyl cyclase C (GC-C), results in receptor activation and the production of high intracellular levels of cGMP. GC-C is expressed in two differentially glycosylated forms in intestinal epithelial cells. Prolonged exposure of human colonic cell lines to ST peptides induces cellular refractoriness to the ST peptide, in terms of intracellular cGMP accumulation. We have investigated the mechanism of cellular desensitization in human colonic Caco2 cells, and observe that exposure of cells to ST leads to a time and dose-dependent inability of cells to respond to the peptide in terms of GC-C stimulation, both in whole cells and membranes prepared from desensitized cells. This is concomitant with a 50% reduction in ST-binding activity in desensitized cells. Desensitization was correlated with a loss of the plasma membrane-associated, hyperglycosylated 145 kDa form of GC-C, while the predominant 130 kDa form, localized both on the plasma membrane and the endoplasmic reticulum, continued to be present in ST-treated cells. Desensitized cells recovered ST-responsiveness on removal of the ST peptide, which was correlated with a reappearance of the 145 kDa form on the cell surface, following processing of the endoplasmic reticulum-associated pool of the 130 kDa form. Selective internalization of the 145 kDa form of the receptor was required for cellular desensitization, as ST-treatment of cells at 4 degrees C did not lead to refractoriness. We therefore show a novel means of regulation of cellular responsiveness to the ST peptide, whereby altering cellular levels of the differentially glycosylated forms of GC-C can lead to differential ligand-mediated activation of the receptor.

Interaction of guanylyl cyclase C with SH3 domain of Src tyrosine kinase. Yet another mechanism for desensitization

Protein-protein interactions mediated by the Src homology 3 (SH3) domain have been implicated in the regulation of receptor functions for subcellular localization of proteins and the reorganization of cytoskeleton. The experiments described in this article begin to identify the interaction of the SH3 domain of Src tyrosine kinase with the guanylyl cyclase C receptor after activation with Escherichia coli heat-stable enterotoxin (ST). Only one of two post-translationally modified forms of guanylyl cyclase C from T84 colonic carcinoma cells bind to GST-SH3 fusion protein of Src and Hck tyrosine kinases. Interestingly, the GST-Src-SH3 fusion protein showed 2-fold more affinity to native guanylyl cyclase C in solution than the GST-Hck-SH3 fusion protein. The affinity of the GST-Src-SH3 fusion protein to guanylyl cyclase C increased on desensitization of receptor in vivo. An in vitro cyclase assay in the presence of GST-Src-SH3 fusion protein indicated inhibition of the catalytic activity of guanylyl cyclase C. The catalytic domain recombinant protein (GST-GCD) of guanylyl cyclase C could pull-down a 60-kDa protein that reacted with Src tyrosine antibody and also showed autophosphorylation. These data suggest that SH3 domain-mediated protein-protein interaction with the catalytic domain of guanylyl cyclase C inhibited the cyclase activity and that such an interaction, possibly mediated by Src tyrosine kinase or additional proteins, might be pivotal for the desensitization phenomenon of the guanylyl cyclase C receptor.

Functional inactivation of the human guanylyl cyclase C receptor: modeling and mutation of the protein kinase-like domain

Receptor guanylyl cyclases possess an extracellular ligand-binding domain, a single transmembrane region, a region with sequence similar to that of protein kinases, and a C-terminal guanylyl cyclase domain. ATP regulates the activity of guanylyl cyclase C (GC-C), the receptor for the guanylin and stable toxin family of peptides, presumably as a result of binding to the kinase homology domain (KHD). Modeling of the KHD of GC-C indicated that it could adopt a structure similar to that of tyrosine kinases, and sequence comparison with other protein kinases suggested that lysine(516) was positioned in the KHD to interact with ATP. A monoclonal antibody GCC:4D7, raised to the KHD of GC-C, did not recognize ATP-bound GC-C, and its epitope mapped to a region in the KHD of residues 491--568 of GC-C. Mutation of lysine(516) to an alanine in full-length GC-C (GC-C(K516A)) dramatically reduced the ligand-stimulated activity of mutant GC-C, altered the ATP-mediated effects observed with wild-type GC-C, and failed to react with the GCC:4D7 monoclonal antibody. ATP interaction with wild-type GC-C converted a high-molecular weight oligomer of GC-C to a smaller sized oligomer. In contrast, GC-C(K516A) did not exhibit an alteration in its oligomeric status on incubation with ATP. We therefore suggest that the KHD in receptor guanylyl cyclases provides a critical structural link between the extracellular domain and the catalytic domain in regulation of activity in this family of receptors, and the presence of K(516) is critical for the possible proper orientation of ATP in this domain.

Protein kinase C regulates transcription of the human guanylate cyclase C gene

Guanylate cyclase C is the receptor for the bacterial heat-stable enterotoxins and guanylin family of peptides, and mediates its action by elevating intracellular cGMP levels. Potentiation of ligand-stimulated activity of guanylate cyclase C in human colonic T84 cells is observed following activation of protein kinase C as a result of direct phosphorylation of guanylate cyclase C. Here, we show that prolonged exposure of cells to phorbol esters results in a decrease in guanylate cyclase C content in 4beta-phorbol 12-myristate 13-acetate-treated cells, as a consequence of a decrease in guanylate cyclase C mRNA levels. The reduction in guanylate cyclase C mRNA was inhibited when cells were treated with 4beta-phorbol 12-myristate 13-acetate (PMA) in the presence of staurosporine, indicating that a primary phosphorylation event by protein kinase C triggered the reduction in RNA levels. The reduction in guanylate cyclase C mRNA levels was not due to alterations in the half-life of guanylate cyclase C mRNA, but regulation occurred at the level of transcription of guanylate cyclase C mRNA. Expression in T84 cells of a guanylate cyclase C promoter-luciferase reporter plasmid, containing 1973 bp of promoter sequence of the guanylate cyclase C gene, indicated that luciferase activity was reduced markedly on PMA treatment of cells, and the protein kinase C-responsive element was present in a 129-bp region of the promoter, containing a HNF4 binding element. Electrophoretic mobility shift assays using an oligonucleotide corresponding to the HNF4 binding site, indicated a decrease in binding of the factor to its cognate sequence in nuclear extracts prepared from PMA-treated cells. We therefore show for the first time that regulation of guanylate cyclase C activity can be controlled at the transcriptional level by cross-talk with signaling pathways that modulate protein kinase C activity. We also suggest a novel regulation of the HNF4 transcription factor by protein kinase C.

Biochemical characterization of the intracellular domain of the human guanylyl cyclase C receptor provides evidence for a catalytically active homotrimer

Guanylyl cyclase C (GCC) is the receptor for the family of guanylin peptides and bacterial heat-stable enterotoxins (ST). The receptor is composed of an extracellular, ligand-binding domain and an intracellular domain with a region of homology to protein kinases and a guanylyl cyclase catalytic domain. We have expressed the entire intracellular domain of GCC in insect cells and purified the recombinant protein, GCC-IDbac, to study its catalytic activity and regulation. Kinetic properties of the purified protein were similar to that of full-length GCC, and high activity was observed when MnGTP was used as the substrate. Nonionic detergents, which stimulate the guanylyl cyclase activity of membrane-associated GCC, did not appreciably increase the activity of GCC-IDbac, indicating that activation of the receptor by Lubrol involved conformational changes that required the transmembrane and/or the extracellular domain. The guanylyl cyclase activity of GCC-IDbac was inhibited by Zn(2+), at concentrations shown to inhibit adenylyl cyclase, suggesting a structural homology between the two enzymes. Covalent cross-linking of GCC-IDbac indicated that the protein could associate as a dimer, but a large fraction was present as a trimer. Gel filtration analysis also showed that the major fraction of the protein eluted at a molecular size of a trimer, suggesting that the dimer detected by cross-linking represented subtle differences in the juxtaposition of the individual polypeptide chains. We therefore provide evidence that the trimeric state of GCC is catalytically active, and sequences required to generate the trimer are present in the intracellular domain of GCC.

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.

Hyaluronan anchoring and regulation on the surface of vascular endothelial cells is mediated through the functionally active form of CD44

CD44 on lymphocytes binding to its carbohydrate ligand hyaluronan can mediate primary adhesion (rolling interactions) of lymphocytes on vascular endothelial cells. This adhesion pathway is utilized in the extravasation of activated T cells from the blood into sites of inflammation and therefore influences patterns of lymphocyte homing and inflammation. Hyaluronan is a glycosaminoglycan found in the extracellular matrix and is involved in a number of biological processes. We have shown that the expression of hyaluronan on the surface of endothelial cells is inducible by proinflammatory cytokines. However, the manner through which hyaluronan is anchored to the endothelial cell surface so that it can resist shear forces and the mechanism of the regulation of the level of hyaluronan on the cell surface has not been investigated. In order to characterize potential hyaluronan receptors on endothelial cells, we performed analyses of cell surface staining by flow cytometry on intact endothelial cells and ligand blotting assays using membrane fractions. Hyaluronan binding activity was detected as a major species corresponding to the size of CD44, and this was confirmed to be the same by Western blotting and immunoprecipitation. Moreover, alterations in the surface level of hyaluronan after tumor necrosis factor-alpha stimulation is regulated primarily by changes in the cell surface levels of the hyaluronan-binding form of CD44. In laminar flow assays, lymphoid cells specifically roll on hyaluronan anchored by purified CD44 coated on glass tubes, indicating that the avidity of the endothelial CD44/hyaluronan interaction is sufficient to support rolling adhesions under conditions mimicking physiologic shear forces. Together these studies show that CD44 serves to anchor hyaluronan on endothelial cell surfaces, that activation of CD44 is a major regulator of endothelial surface hyaluronan expression, and that the non-covalent interaction between CD44 and hyaluronan is sufficient to provide resistance to shear under physiologic conditions and thereby support the initial steps of lymphocyte extravasation.

Homologous desensitization of the human guanylate cyclase C receptor. Cell-specific regulation of catalytic activity

Guanylate Cyclase C (GCC) serves as a receptor for the endogenous ligands, guanylin and uroguanylin, as well as the family of bacterial heat-stable enterotoxins (ST), which are one of the major causes of diarrhoea the world over. We had earlier provided evidence that GCC, present in the human colonic T84 cell line, is desensitized on prolonged exposure to ST, and this desensitization was reflected in a reduced ST-stimulated guanylate cyclase activity of GCC [Bakre, M.M. & Visweswariah, S.S. (1997) FEBS Lett. 408, 345-349]. In this study, we have investigated the mechanisms that underlie this cellular desensitization process. Desensitization of T84 cells was not a result of reduction in GCC present in membranes prepared from desensitized T84 cells, nor due to increased cGMP-phosphodiesterase activity associated with the membrane fraction. The decrease in ST-stimulatable guanylate cyclase activity of GCC was due to a dramatic reduction in the Vmax of the cyclase, which was also seen when MnGTP was used as the substrate. GCC undergoes ligand-induced inactivation in vitro, which is alleviated in the presence of ATP. In vivo desensitized GCC could be further inactivated in vitro when preincubated with ST, indicating that the two mechanisms of GCC inactivation are distinct. Cellular refractoriness as reflected by a reduced responsiveness to further ST-stimulation following prior exposure to IST, coupled with GCC desensitization was also observed in another colonic cell line, Caco2. However, HEK293 cells, stably transfected with GCC cDNA, when exposed to ST for prolonged periods, did not result in GCC desensitization, indicating that desensitization of GCC appeared to be a cell specific phenomenon. GCC expressed in HEK293-GCC cells, however, showed in vitro ligand induced inactivation, suggesting that there are two independent means of ligand-induced desensitization of GCC, perhaps distinct from the mechanisms that have been described earlier for other members of the guanylate cyclase receptor family.

Rise of intracellular free calcium levels with activation of inositol triphosphate in a human colonic carcinoma cell line (COLO 205) by heat-stable enterotoxin of Escherichia coli

The heat-stable enterotoxin (STa) produced by Escherichia coli has been found to increase rapidly two potential intracellular signals, inositol triphosphate and cytosolic free calcium in a human colonic cell line, COLO 205. Addition of STa to COLO 205 cells prelabelled with myo-[2-3H]inositol resulted in a rapid rise of [3H]inositol triphosphate. Using fluorescent indicator, Fura-2AM, intracellular free Ca2+ has been found to increase 5.12-fold compared to control. Suspension of cells in calcium-free buffer demonstrated STa-induced rapid rise of cytosolic Ca2+. The same result was found when extracellular calcium was chelated with EGTA. This effect was not observed with cells that were pretreated with dantrolene which suggest that the intracellular calcium rise might be due to mobilization from intracellular stores. This study demonstrated for the first time a change in cytosolic calcium in cultured human colonic cells by STa, which is accompanied by inositol triphosphate activation.

Rise of cytosolic Ca2+ and activation of membrane-bound guanylyl cyclase activity in rat enterocytes by heat-stable enterotoxin of Vibrio cholerae non-01

The cytosolic calcium level ([Ca2+]i) and the membrane-bound guanylyl cyclase activity in the isolated rat intestinal epithelial cells were investigated. Heat-stable enterotoxin of Vibrio cholerae non-01 (NAG-ST) was found to increase both the [Ca2+]i and the enzyme activity. These changes occur similarly until 5 min of incubation with NAG-ST, indicating that these changes might be involved in NAG-ST induced signal transduction in rat enterocytes.

Binding of Escherichia coli heat-stable enterotoxin and rise of cyclic GMP in COLO 205 human colonic carcinoma cells

Escherichia coli heat-stable enterotoxin (STa) was found to bind on the surface of human colonic (COLO 205) cells. The binding of [125I]STa to cell membranes was found to be specific, reversible and saturable. Scatchard analysis of the equilibrium binding demonstrated a single class of binding sites with a Kd of 0.5 x 10(-10) M. Autoradiographic analysis of polyacrylamide gel electrophoresis revealed the specific incorporation of [125I]STa into a single STa binding protein with a molecular mass of 95 kDa. Following incubation of COLO 205 cells with STa, a rise of intracellular cGMP was also evident.

Epitope conservation and immunohistochemical localization of the guanylin/stable toxin peptide receptor, guanylyl cyclase C

The heat-stable enterotoxins (ST) are a family of cysteine-rich low-molecular weight peptides produced by pathogenic bacteria, and are one of the major causes of watery diarrhea all over the world. These toxins mediate their action by binding to an intestinal cell surface receptor that is a membrane-associated guanylyl cyclase (GCC). This receptor also serves as the receptor for the recently characterised endogenous ligand, guanylin. We have expressed various domains of the receptor in Escherichia coli and used purified proteins for the generation of both polyclonal and monoclonal antibodies. While polyclonal antibodies were able to partially inhibit ST binding to the native receptor present in the T84 human colonic cell line, GCC:B10 monoclonal antibody did not interfere with ligand binding. Western blot analysis, using membranes prepared from human colonic T84 cells, detected two bands of size 160 and 140 kDa, representing alternately glycosylated forms of the receptor. Using the recombinant proteins, we could map the epitope of GCC:B10 monoclonal antibody to the intracellular domain of the receptor. We used the antibody to localize the receptor throughout the rat intestine, and in the porcine and bonnet monkey colon. We could detect receptor expression in the villus and the crypts of the duodenum, jejunum, ileum, and caecum, and in the crypts of the colon. Receptor expression was observed in cells that had earlier been shown to express cGMP-dependent kinase, but not the cystic fibrosis transmembrane regulator, a known downstream target of cGMP/G-kinase, which suggests that GCC/ cGMP could regulate additional cellular signal transduction machinery.

Dual regulation of heat-stable enterotoxin-mediated cGMP accumulation in T84 cells by receptor desensitization and increased phosphodiesterase activity

We report the regulation of cGMP accumulation induced by the heat-stable enterotoxin, STh, in the T84 human colonic cell line. STh binding to its receptor, guanylyl cyclase C (GCC), leads to elevated intracellular levels of cGMP. Prolonged exposure of T84 cells to STh induced refractoriness to further cGMP accumulation, without significant receptor internalization, but with reduced STh-induced cGMP synthesis by the receptor. Significantly, increased degradation of cGMP by a cGMP-specific phosphodiesterase was observed in desensitized cells. This is the first report on the desensitization of GCC, as well as the role of the Type V phosphodiesterase in inducing cellular refractoriness.

Phosphorylation and activation of the intestinal guanylyl cyclase receptor for Escherichia coli heat-stable toxin by protein kinase C

The heat-stable enterotoxin STa of E. coli causes diarrhea by binding to and stimulating intestinal membrane-bound guanylyl cyclase, triggering production of cyclic GMP. Agents which stimulate protein kinase C (PKC), including phorbol esters, synergistically enhance STa effects on cGMP and secretion. We investigated whether PKC causes phosphorylation of the STa receptor in vivo and in vitro. Immunoprecipitation of the STa receptor-guanylyl cyclase was carried out from extracts of T84 colon cells metabolically labelled with [32P]-phosphate using polyclonal anti-STa receptor antibody. The STa receptor was phosphorylated in its basal state, and 32P content in the 150 kDa holoreceptor band increased 2-fold in cells exposed to phorbol ester for 1 h. In vitro, immunopurified STa receptor was readily phosphorylated by purified rat brain PKC. Phosphorylation was inhibited 40% by 5 microM of a synthetic peptide corresponding to the sequence around Ser1029 of the STa receptor, a site previously proposed as a potential PKC phosphorylation site. Treatment of the immunopurified STaR/GC with purified PKC increased STa-stimulated guanylyl cyclase activity 2-fold. We conclude that PKC phosphorylates and activates the STa receptor/guanylyl cyclase in vitro and in vivo; Ser1029 of the STaR/GC remains a candidate phosphorylation site by PKC.

A conformational epitope in the N-terminus of the Escherichia coli heat-stable enterotoxins is involved in receptor-ligand interactions

The heat-stable enterotoxins are a family of low molecular weight, cysteine rich peptide toxins which are one of the major causes of watery diarrhea in children and adults. These toxins bind to a cell surface receptor in intestinal cells and mediate their action through elevation of intracellular cyclic GMP. We have generated a monoclonal antibody to these peptide toxins which is able to neutralise the activity of the peptides in a human colonic cell line, the T84 cell line. The monoclonal antibody, ST:G8, appears to be directed to an epitope distinct from antibodies previously generated, and prior incubation of this antibody, or Fab generated from this antibody, with full length STh and STp peptides prevents cGMP accumulation in T84 cells. This inhibition is a direct result of the antibody preventing binding of the peptides to the receptor. ST:G8 Mab does not recognize a 13-mer biologically active analog of STp, comprising the core sequence of STp peptide, suggesting that it is directed to a region in the N-terminus of the peptides, which may modulate receptor interaction/activation. The antibody recognizes a conformational epitope in the ST peptides, since reduction and carboxyamidation of ST abolishes antibody cross-reactivity. Differential cross-reactivity of the Mab with STh and STp peptides which differ markedly only in their N-termini, suggests that this antibody recognizes a distinct conformation in the two peptides, which is essential for receptor interaction.