Pharmacologic action of Escherichia coli heat-stable (STa) enterotoxin

Differences in virulence attributes between cytolethal distending toxin positive and negative Campylobacter jejuni strains

Campylobacter jejuni is a common gastrointestinal bacterial pathogen. Although cytolethal distending toxin (CDT) is proposed to be an important virulence determinant of this pathogen, how CDT(+) and CDT(-) strains differ in their biological properties remains largely unknown. The virulence properties of CDT(+) and CDT(-) strains were studied on HeLa cells and in the suckling mouse model. Presence of the cdtB gene in Campylobacter species was determined by PCR. Five each of CDT(+) and CDT(-) C. jejuni strains were subjected to adherence, invasion and cytotoxicity assay on the HeLa cell line. Bacterial culture supernatants with and without CDT activity were inoculated intragastrically into 2-day-old suckling mice. The mice were sacrificed within 48 h. Histopathological examination of stomach, jejunum, ileum and colon was performed by haematoxylin/eosin staining. cdtB was detected in 88 % and 14 % of C. jejuni and Campylobacter coli strains, respectively. CDT(+) C. jejuni strains adhered to and invaded HeLa cells in significantly higher numbers than CDT(-) strains [CDT(+) vs CDT(-), adherence 2.7 x 10(4)+/-3.5 x 10(4) vs 2.7 x 10(2)+/-1.9 x 10(2); invasion 1.0 x 10(3)+/-1.3 x 10(3) vs 1.4 x 10(1)+/-3.1 x 10(1); P<0.01]. Culture supernatants of all CDT(+) strains demonstrated CDT activity on HeLa cells. Mice inoculated with supernatant containing CDT activity had moderate to severe pathology in different parts of their gastrointestinal tract, with the colon being the major target. Mice inoculated with supernatant lacking CDT activity showed no significant pathology in the gastrointestinal tract. The results demonstrate that CDT(+) C. jejuni strains adhere to and invade epithelial cells more efficiently than CDT(-) strains. CDT is responsible for intestinal pathology and the colon is the major target.

Bacterial enterotoxins are associated with resistance to colon cancer

One half million patients suffer from colorectal cancer in industrialized nations, yet this disease exhibits a low incidence in under-developed countries. This geographic imbalance suggests an environmental contribution to the resistance of endemic populations to intestinal neoplasia. A common epidemiological characteristic of these colon cancer-spared regions is the prevalence of enterotoxigenic bacteria associated with diarrheal disease. Here, a bacterial heat-stable enterotoxin was demonstrated to suppress colon cancer cell proliferation by a guanylyl cyclase C-mediated signaling cascade. The heat-stable enterotoxin suppressed proliferation by increasing intracellular cGMP, an effect mimicked by the cell-permeant analog 8-br-cGMP. The antiproliferative effects of the enterotoxin and 8-br-cGMP were reversed by L-cis-diltiazem, a cyclic nucleotide-gated channel inhibitor, as well as by removal of extracellular Ca(2+), or chelation of intracellular Ca(2+). In fact, both the enterotoxin and 8-br-cGMP induced an L-cis-diltiazem-sensitive conductance, promoting Ca(2+) influx and inhibition of DNA synthesis in colon cancer cells. Induction of this previously unrecognized antiproliferative signaling pathway by bacterial enterotoxin could contribute to the resistance of endemic populations to intestinal neoplasia, and offers a paradigm for targeted prevention and therapy of primary and metastatic colorectal cancer.

Promoter analysis of a medaka fish intestinal guanylyl cyclase gene

We characterized the promoter activity of a medaka fish intestinal guanylyl cyclase gene, OlGC6, by assay of enzyme activity in response to various promoter-luciferase fusion gene constructs introduced into CACO-2 cells and medaka fish embryos. A transient transfection assay of the various fusion gene constructs showed that the nucleotides between -98 and -89 in the 5'-flanking region of the OlGC6 gene are essential for transcription of the OlGC6 gene in CACO-2, and that the OlGC6 gene fragment between -98 and +50 is sufficient to drive gene expression in the medaka fish intestine. An electrophoretic mobility shift assay and ultraviolet (UV) cross-linking experiments demonstrated that a nuclear protein from CACO-2 cells and the adult medaka fish intestinal cells binds specifically to the AGACCTTTGC nucleotides in the regulatory element.

Examination of diarrheagenicity of cytolethal distending toxin: suckling mouse response to the products of the cdtABC genes of Shigella dysenteriae

Some strains of Escherichia coli, Shigella spp., and Campylobacter spp. that have been implicated in diarrheal disease produce cytolethal distending toxin (CDT). CDT induces unique morphological changes in Chinese hamster ovary cells, but its association with diarrheal disease is unclear. We studied the diarrheagenicity of CDT using the cdt genes that we originally cloned from Shigella dysenteriae. The cdt genes were subcloned into a high-copy-number plasmid in E. coli JM109 to achieve high-level CDT production into the culture supernatant. An isogenic CDT- derivative was constructed by deletion of the 0.9-kb sequence internal to the cdt genes. A suckling mouse model was established, in which the intragastrically administered culture supernatant of the CDT+ E. coli strain induced excretion of loose and/or watery feces more often than did that of the CDT- strain in 24 h. The partially purified CDT preparation induced profuse watery diarrhea by 12 h in this model. High-level intestinal fluid accumulation in 4 h appeared to be related to the watery diarrhea. The results indicate that CDT is diarrheagenic to suckling mice and suggest that diarrheagenicity is dependent on CDT level. The preparations containing wild-type CDT induced tissue damage (necrosis and reparative hyperplasia) in the descending colon, whereas the tissues of the small intestines remained apparently intact. Association between the colonic damage and diarrhea is unclear and needs further investigation.

Role of the enteric nervous system in the maintained hypersecretion induced by enterotoxin STa in the nutritionally deprived intestine

Electrogenic (Cl-) secretion was measured as the short circuit current (Isc, microA/cm2) across muscle-stripped sheets of jejunum and ileum incubated in vitro after removal from fed rats, rats starved for three days, and chronically undernourished rats (50% of fed control intake for 21 days). Concentration and Isc response curves for serially-added mucosal Escherichia coli STa enterotoxin showed that the rats which had undergone dietary deprivation had a larger secretory Isc maximum but the ED50 values were unchanged compared with fed animals. In fed intestine the action of STa was transient, with an Isc peak and subsequent decay to the baseline over 60 minutes but in the undernourished intestine the response consisted of a significantly greater peak than that of the fed state (jejunum = 94%; ileum = 168%) and the Isc was maintained at or near the peak for at least 60 minutes. The starved intestine had a less well developed maintenance of its enhanced peak Isc. Serosal tetrodotoxin (1 microM) had no effect on the initial peak Isc values but caused a decay of the maintained Isc down to the basal or fed levels in the starved and, especially, in the undernourished intestines. Thus, dietary deprivation, especially chronic undernutrition, enhances the maximum electrogenic secretion due to STa and creates a new neural path in the submucosal plexus that, when activated by STa, maintains its enhanced secretory action. Its putative role in exacerbating secretory diarrhoea in malnourished human subjects could be an important component underlying the known relation between malnourishment and the increased severity of diarrhoea.

Enterotoxin Escherichia coli STa activates a nitric oxide-dependent myenteric plexus secretory reflex in the rat ileum

1. Mucosally added enterotoxin Escherichia coli STa increased the electrogenic Cl- secretion measured as the short-circuit current (Isc) across isolated muscle-stripped and muscle-unstripped rat mid-ilea incubated in vitro. 2. Pretreatment with serosal L-NAME (N omega-nitro-L-arginine methyl ester) or tetrodotoxin (TTX) significantly reduced the maximum Isc and the duration of action of STa in the unstripped but not stripped ilea. D-NAME (serosal), indomethacin or 5-hydroxy-tryptamine-desensitization was ineffective on STa-induced Isc in either stripped or unstripped ilea. 3. Serosal capsaicin reduced the maximum Isc of STa and its duration of action in unstripped ilea. 4. L-Arginine induced a significantly larger increase in the Isc across unstripped ilea than across stripped ilea; this could be significantly reduced by serosal L-NAME or TTX, although these were ineffective in stripped ilea. 5. Pretreatment of anaesthetized rats with I.P. L-NAME suppressed the fluid secretion induced by luminal STa in ilea in vivo but had no effect on that induced by luminal carbachol. 6. Mucosal STa increased electrogenic Cl- secretion across intact rat ileum in vitro by activating a capsaicin-sensitive, nitric oxide-dependent myenteric plexus-mediated secretory reflex. The suppression by L-NAME of STa induced ileal fluid secretion in vivo probably involves the inhibition of this reflex.