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

The pseudokinase domain in receptor guanylyl cyclases

Cyclic GMP is produced by enzymes called guanylyl cyclases, of which the membrane-associated forms contain an intracellular pseudokinase domain that allosterically regulates the C-terminal guanylyl cyclase domain. Ligand binding to the extracellular domain of these single transmembrane-spanning domain receptors elicits an increase in cGMP levels in the cell. The pseudokinase domain (or kinase-homology domain) in these receptors appears to be critical for ligand-mediated activation. While the pseudokinase domain does not possess kinase activity, biochemical evidence indicates that the domain can bind ATP and thereby allosterically regulate the catalytic activity of these receptors. The pseudokinase domain also appears to be the site of interaction of regulatory proteins, as seen in the retinal guanylyl cyclases that are involved in visual signal transduction. In the absence of structural information on the pseudokinase-guanylyl cyclase domain organization of any member of this family of receptors, biochemical evidence has provided clues to the physical interaction of the pseudokinase and guanylyl cyclase domain. An α-helical linker region between the pseudokinase domain and the guanylyl cyclase domain regulates the basal activity of these receptors in the absence of a stimulatory ligand and is important for stabilizing the structure of the pseudokinase domain that can bind ATP. Here, we present an overview of salient features of ATP-mediated regulation of receptor guanylyl cyclases and describe biochemical approaches that allow a clearer understanding of the intricate interplay between the pseudokinase domain and catalytic domain in these proteins.

Hepatocyte nuclear factor 4-alpha involvement in liver and intestinal inflammatory networks

Hepatocyte nuclear factor 4-alpha (HNF4-α) is a nuclear receptor regulating metabolism, cell junctions, differentiation and proliferation in liver and intestinal epithelial cells. Mutations within the HNF4A gene are associated with human diseases such as maturity-onset diabetes of the young. Recently, HNF4A has also been described as a susceptibility gene for ulcerative colitis in genome-wide association studies. In addition, specific HNF4A genetic variants have been identified in pediatric cohorts of Crohn’s disease. Results obtained from knockout mice supported that HNF4-α can protect the intestinal mucosae against inflammation. However, the exact molecular links behind HNF4-α and inflammatory bowel diseases remains elusive. In this review, we will summarize the current knowledge about the role of HNF4-α and its isoforms in inflammation. Specific nature of HNF4-α P1 and P2 classes of isoforms will be summarized. HNF4-α role as a hepatocyte mediator for cytokines relays during liver inflammation will be integrated based on documented examples of the literature. Conclusions that can be made from these earlier liver studies will serve as a basis to extrapolate correlations and divergences applicable to intestinal inflammation. Finally, potential functional roles for HNF4-α isoforms in protecting the intestinal mucosae from chronic and pathological inflammation will be presented.

Current understanding of guanylin peptides actions

Guanylin peptides (GPs) family includes guanylin (GN), uroguanylin (UGN), lymphoguanylin, and recently discovered renoguanylin. This growing family is proposed to be intestinal natriuretic peptides. After ingestion of a salty meal, GN and UGN are secreted into the intestinal lumen, where they inhibit sodium absorption and induce anion and water secretion. At the same conditions, those hormones stimulate renal electrolyte excretion by inducing natriuresis, kaliuresis, and diuresis and therefore prevent hypernatremia and hypervolemia after salty meals. In the intestine, a well-known receptor for GPs is guanylate cyclase C (GC-C) whose activation increases intracellular concentration of cGMP. However, in the kidney of GC-C-deficient mice, effects of GPs are unaltered, which could be by new cGMP-independent signaling pathway (G-protein-coupled receptor). This is not unusual as atrial natriuretic peptide also activates two different types of receptors: guanylate cylcase A and clearance receptor which is also G-protein coupled receptor. Physiological role of GPs in other organs (liver, pancreas, lung, sweat glands, and male reproductive system) needs to be discovered. However, it is known that they are involved in pathological conditions like cystic fibrosis, asthma, intestinal tumors, kidney and heart failure, obesity, and metabolic syndrome.

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.

A Toxicogenomics Approach to Identify New Plausible Epigenetic Mechanisms of Ochratoxin A Carcinogenicity in Rat

Ochratoxin A (OTA) is a mycotoxin occurring naturally in a wide range of food commodities. In animals, it has been shown to cause a variety of adverse effects, nephrocarcinogenicity being the most prominent. Because of its high toxic potency and the continuous exposure of the human population, OTA has raised public health concerns. There is significant debate on how to use the rat carcinogenicity data to assess the potential risk to humans. In this context, the question of the mechanism of action of OTA appears of key importance and was studied through the application of a toxicogenomics approach. Male Fischer rats were fed OTA for up to 2 years. Renal tumors were discovered during the last 6 months of the study. The total tumor incidence reached 25% at the end of the study. Gene expression profile was analyzed in groups of animals taken in intervals from 7 days to 12 months. Tissue-specific responses were observed in kidney versus liver. For selected genes, microarray data were confirmed at both mRNA and protein levels. In kidney, several genes known as markers of kidney injury and cell regeneration were significantly modulated by OTA. The expression of genes known to be involved in DNA synthesis and repair, or genes induced as a result of DNA damage, was only marginally modulated. Very little or no effect was found amongst genes associated with apoptosis. Alterations of gene expression indicating effects on calcium homeostasis and a disruption of pathways regulated by the transcription factors hepatocyte nuclear factor 4 alpha (HNF4alpha) and nuclear factor-erythroid 2-related factor 2 (Nrf2) were observed in the kidney but not in the liver. Previous data have suggested that a reduction in HNF4alpha may be associated with nephrocarcinogenicity. Many Nrf2-regulated genes are involved in chemical detoxication and antioxidant defense. The depletion of these genes is likely to impair the defense potential of the cells, resulting in chronic elevation of oxidative stress in the kidney. The inhibition of defense mechanism appears as a highly plausible new mechanism, which could contribute to OTA carcinogenicity.

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.

Reciprocal regulation and integration of signaling by intracellular calcium and cyclic GMP

Calcium and guanosine-3',5'-cyclic monophosphate (cGMP) are second messenger molecules that regulate opposing physiological functions, reflected in the reciprocal regulation of their intracellular concentrations, in many systems. Indeed, cGMP and Ca2+ constitute discrete points of integration between multiple cell signaling cascades in both convergent and parallel pathways. This chapter describes the molecular mechanisms regulating intracellular Ca2+ and cGMP, and their integration in specific cellular responses.

Coordinate upregulation of guanylin and uroguanylin expression by hypertonicity in HT29-18-N2 cells

Guanylin and uroguanylin are particulate guanylate cyclase-activating peptides that are secreted from the epithelia of the intestine, kidney, pancreas, and salivary gland. These peptides elicit chloride and bicarbonate secretion via the cystic fibrosis transmembrane conductance regulator. To test the hypothesis that hypertonicity mediates an increase in guanylin and uroguanylin mRNA, we subjected HT29-18-N2 to osmotic stress. Guanylin and uroguanylin RNA were increased substantially in the presence of hypertonicity but only with solutes that were relatively impermeable to the cell membrane. This hypertonicity-mediated increase was transcriptional and did not require protein synthesis. Herbimycin A and mitogen-activated protein kinase inhibitors SB-203580 and PD-98059 had no effect on basal or induced levels of guanylin or uroguanylin. Both staurosporine and prolonged exposure to phorbol ester reduced basal levels and completely blocked hypertonicity-related increases in guanylin or uroguanylin RNA. These data suggest that serine/theonine protein kinases, possibly protein kinase C (PKC), mediate the hypertonicity-associated increase in guanylin and uroguanylin RNA. We conclude that guanylin and uroguanylin are released in response to hypertonic stress and that regulation of these genes may be mediated by PKC isoforms.