Stimulation of glycerol production in fat cells by cholera toxin

Sequence of events mediating the effect of cholera toxin on rat thymocytes

We have found that in rat thymocytes binding of [(125)I]choleragen is followed by cellular accumulation of cyclic 3',5'-AMP which, in turn, is followed by stimulation of amino acid transport. Binding of cholera toxin was complete by 30 min and remained constant for the subsequent 150 min. After stimulation by choleragen, cellular cyclic 3',5'-AMP became maximal by 30 min, after which it declined steadily so that by 90 min of incubation, cellular cyclic nucleotide levels were only 20% of those seen at 30 min. Stimulation of amino acid transport, although detectable by 15 min, did not become maximal until 120 min (by which time cellular cyclic 3',5'-AMP had decreased by more than 80%). We have also used this system to delineate the step at which various pharmacologic agents and hormones act to alter the sequence of events mediating the response of rat thymocytes to cholera toxin. The ability of cycloheximide to abolish choleragen-stimulated amino acid influx without reducing [(125)I]choleragen binding or cellular cyclic 3',5'-AMP suggests that cyclic nucleotide stimulation of amino acid transport includes a step involving protein synthesis.

Interaction of cholera toxin and toxin derivatives with lymphocytes. I. Binding properties and interference with lectin-induced cellular stimulation

The interaction of cholera toxin and a number of toxin derivatives, containing different proportions of light and heavy toxin-composing subunits (L and H), with mouse lymphocytes was studied. Experiments with [(125)I]toxin showed that a single cell can rapidly, within minutes, bind up to 40,000 molecules of toxin, the association constant was estimated to 7 +/- 4 x 10(8) liters/mol, and binding was found to be very similar at 37 degrees C and 5 degrees C. Immunofluorescence studies revealed that the toxin attachment is located on the cell surface, and that purified L subunit but not H subunit binds to the cells. A natural cholera toxoid, built up by aggregated L subunits, showed almost identical binding properties as toxin to the cells. Pure G(M1) ganglioside, the proposed membrane receptor structure for toxin, prevented entirely the cellular binding of both toxin and toxoid. Cholera toxin in concentrations down to approximately 5 x 10(-11) mol/liter (corresponding to 10 bound molecules/cell) inhibited thymus cells from being stimulated to DNA synthesis by concanavalin A (con A), and spleen cells from such stimulation by phytohemagglutinin. The G(M1) ganglioside but not a series of other pure structurally related gangliosides and neutral glycosphingolipids neutralized this toxin activity. Toxin derivatives which, in similarity with toxin, possessed H as well as L subunits but in other proportions, were potent inhibitors of con A-induced thymocyte stimulation, whereas the natural toxoid (aggregated L subunits), purified toxin L subunit and purified toxin H subunit were up to 300-fold less active on a weight basis. The capacity of cholera proteins to inhibit con A-induced thymocyte stimulation correlated well with their activity in the rabbit intradermal toxicity assay. The inhibitory action of cholera toxin on con A-induced thymocyte stimulation did not depend on decreased cell viability from the toxin treatment, nor was it caused by a reaction between toxin and con A. [(125)I]con A bound equally well to the cells when toxin was present as when it was absent, which proves that the toxin did not compete for cellular con A receptors. Nor did the toxin seem to disturb the general mobility of membrane receptors or their ability to accumulate in caps. It is concluded that the L type of subunit confers rapid and firm binding of cholera toxin to lymphocyte membranes, probably to G(M1) ganglioside receptors. For biologic activity the additional presence of H subunit is important. One manifestation of toxin action on lymphocytes is inhibition of lectin-induced DNA synthesis; probably this effect relates to the ability of cholera toxin to raise the levels of intracellular cyclic 3'5'-adenosine monophosphate.

Development of a purified cholera toxoid. II. Preparation of a stable, antigenic toxoid by reaction of purified toxin with glutaraldehyde

Evidence is presented which confirms that cholera toxoids obtained by reaction of purified toxin with Formalin possess the ability to partially reactivate both in vivo and in vitro. At the same time, conditions are presented for the preparation of stable, antigenic cholera toxoids by reaction of purified toxin with glutaraldehyde. Treatment of purified cholera toxin with approximately 200 mol of glutaraldehyde per mol of toxin at pH 7.8 reproducibly resulted in the preparation of toxoids which: (i) possessed less than 20 bluing doses per 100 mug; (ii) did not reactivate in vivo or in vitro; (iii) precipitated with, and neutralized antitoxin; (iv) elevated prolonged serum antitoxin in immunized rabbits; (v) protected immunized guinea pigs against toxin skin challenge; and (vi) lent themselves to enhanced antigenicity by means of an in situ adjuvant system which may be suitable for man. Acrylamide gel electrophoresis and molecular sieve chromatography of a series of glutaraldehyde-derived toxoids suggested that the reaction products consisted of monomeric and polymeric species and that the proportion of higher-molecular-weight species was determined by the relative concentrations of toxin and glutaraldehyde. The results suggested a relationship between complete and irreversible elimination of toxicity and the formation of higher-molecular-weight toxoids.

Alkaline phosphatase. Possible induction by cyclic AMP after cholera enterotoxin administration

The present studies were undertaken to determine the role, if any, of cyclic 3',5'-adenosine monophosphate (cyclic AMP) as a chemical inducer of rat liver alkaline phosphatase. Cholera enterotoxin, given intravenously to rats, led to a rapid rise in the activity of hepatic adenyl cyclase that was 7(1/2) times greater than control values in 6 h. Cyclic AMP levels were also significantly increased above control values while the activity of cyclic nucleotide phosphodiesterase was unchanged. Hepatic alkaline phosphatase activity was increased 5(1/2) times above control in 12 h, but its rise followed that of adenyl cyclase and cyclic AMP by several hours. Cycloheximide inhibited the rise of hepatic alkaline phosphatase but not that of adenyl cyclase. The administration of glucagon, a known stimulator of hepatic adenyl cyclase, and of dibutyryl cyclic AMP, led to similar striking increases in hepatic alkaline phosphatase activity. This alkaline phosphatase increase was blocked by the prior administration of cycloheximide. Bile duct ligation, a known stimulator of hepatic alkaline phosphatase activity, failed to produce any significant changes in adenyl cyclase or cyclic AMP. Concomitant treatment of rats with bile duct ligation and cholera enterotoxin or bile duct ligation and glucagon, had no additive effect on the increase in hepatic alkaline phosphatase activity, although the increase occurred earlier. These results suggest that: (a) cyclic AMP may act as an inducer of hepatic alkaline phosphatase: (b) the stimulation of hepatic alkaline phosphatase by cholera enterotoxin is mediated by cyclic AMP; (c) the rise in hepatic alkaline phosphatase following bile duct ligation is not mediated by cyclic AMP; (d) the same alkaline phosphatase in rat liver may be induced by two (or more) mechanisms, only one of which requires cyclic AMP.

Stimulation of steroid secretion in adrenal tumor cells by choleragen

Choleragen, the pure protein from cholera toxin, stimulates steroid secretion by Y-1 adrenal tumor cells in culture. The secreted steroids are the same as seen after addition of adrenocorticotropic hormone. Half-maximal stimulation occurs at 15 pM; stimulation is essentially irreversible by washing and partially reversible (for about 1 hr) by antibody, and there is a latent period of about 60 min before stimulation is seen. Stimulation of adenylate cyclase occurs at about 30-fold higher choleragen concentrations. Gangliosides inhibit choleragen stimulation when added before but not after the toxin. Lipopolysaccharides from Escherichia coli, Salmonella typhosa, and Serratia marcescens also stimulate steroid secretion, but are less potent than choleragen.

Effects of cycloheximide on the response of intestinal mucosa to cholera enterotoxin

Prior studies have indicated that effects of cholera enterotoxin (CT) on the small intestine are delayed in onset and involve an interaction with adenyl cyclase in the mucosa. It has also been shown that the administration of cycloheximide to rabbits in doses which inhibit crypt cell mitoses (20 mg/kg), diminishes CT-induced fluid production in jejunal loops. These latter studies have been interpreted as indications that CT-related intestinal secretion is a crypt cell function and that it is mediated by a CT-induced protein. The present study was undertaken to delineate more precisely the nature of the interaction in the intestine between cycloheximide and cholera toxin. Pretreatment of rabbits with cycloheximide reduced by 60% the secretory response to CT in isolated ileal loops with intact blood supply. Sodium and chloride flux measurements on mucosa isolated from these and control loops indicated that this antisecretory effect of cycloheximide persists in vitro. Measurements of radioactive leucine incorporation into mucosal protein indicated that the dose of cycloheximide employed inhibited protein synthesis by 90%. This inhibitory effect was shown to be independent of any effect of cycloheximide on amino acid uptake across the brush border. Measurements of adenyl cyclase activity and cyclic AMP levels in ileal mucosa of cycloheximide pretreated and control animals indicated that cycloheximide did not diminish the CT-induced increases in these parameters. These observations demonstrate that cycloheximide reduces CT-induced intestinal fluid production without interfering with the CT-induced augmentation of adenyl cyclase activity or the consequent rise in cyclic. AMP concentration. Since the antisecretory effect of cycloheximide persists in vitro, it probably involves a direct interaction of the antibiotic with mucosal cell ion transport mechanisms rather than an indirect effect mediated by other humoral or neurogenic factors. The present observations also suggest that the secretory response of the intestine to CT involves neither the synthesis of new adenyl cyclase nor that of a protein modifying its activity.

Differential inhibitory effects of cholera toxoids and ganglioside on the enterotoxins of Vibrio cholerae and Escherichia coli

Natural cholera toxoid appears to act as a competitive inhibitor of cholera enterotoxin and is thus a useful tool for studying the interaction of cholera enterotoxin with cell membranes. Cholera enterotoxin binds to gut mucosa more rapidly than does its natural toxoid. Once binding occurs, however, it appears to be prolonged for both materials. Formalinized cholera toxoid has no inhibitory effect upon cholera enterotoxin. Enterotoxic activity, ability to bind to gut mucosa, and antitoxigenicity appear to be independent properties of cholera enterotoxin. Natural cholera toxoid does not inhibit Escherichia coli enterotoxin, indicating that although the two enterotoxins activate the same mucosal secretory mechanism they occupy different binding sites in the mucosa. Ganglioside, which may be the mucosal receptor of cholera enterotoxin, is highly efficient in deactivating cholera enterotoxin. By contrast, ganglioside is relatively inefficient in deactivating heat-labile E. coli enterotoxin and is without effect upon the heat-stable component of E. coli enterotoxin. These findings suggest that ganglioside is not likely to be the mucosal receptor for E. coli enterotoxin. Differences in cellular binding of E. coli and cholera enterotoxins may explain, at least in part, the marked differences in the time of onset and duration of their effects upon gut secretion.

Effects of cholera toxin on in vitro models of immediate and delayed hypersensitivity. Further evidence for the role of cyclic adenosine 3',5'-monophosphate

Cholera enterotoxin inhibits the antigen-induced. IgE-mediated release of histamine from human leukocytes and the lysis of allogeneic mastocytoma cells by splenic lymphocytes from specifically immunized mice. This effect requires a prolonged preincubation time of the toxin with the lymphocyte/leukocyte preparations: a demonstrable inhibition requires about 30 min of pre-incubation and the toxin activity is still increasing at 90-180 min. Cholera enterotoxin also stimulates adenyl cyclase and leads to increased levels of cyclic AMP in the lymphocyte/leukocyte preparations. The concentration of toxin required for both cyclic AMP accumulation and inhibition of the biologic responses is about the same (ca. 1 ng/ml), and the time course of cyclic AMP accumulation parallels the development of inhibitory activity. Both activities, inhibition of the in vitro hypersensitivity reactions and cyclic AMP accumulation, are blocked by cholera antitoxin and by a toxoid prepared from the toxin (choleragenoid). These are specific antagonists in that they do not block the inhibiting activity or rise in cyclic AMP levels caused by other adenyl cyclase stimulators. Because cholera enterotoxin has no known activity other than the stimulation of adenyl cyclase and because of its unusual time course and the availability of specific antagonists, this data considerably strengthens the hypothesis that the cyclic AMP system influences the expression of these two forms of hypersensitivity phenomena.

Stimulation of intestinal mucosal adenyl cyclase by cholera enterotoxin and prostaglandins

The effects of several prostaglandins (PG) and a highly purified preparation of cholera enterotoxin (CT) on intestinal mucosal adenyl cyclase activity and the effect of CT on intestinal mucosal cyclic 3',5'-adenosine monophosphate concentration were determined in guinea pig and rabbit small intestine and were correlated with the effects of the same agents on ion transport. Adenyl cyclase activity, measured in a crude membrane fraction of the mucosa, was found at all levels of the small intestine with the highest activity per milligram protein in the duodenum. The prostaglandins, when added directly to the assay, increased adenyl cyclase activity; the greatest effect (2-fold increase) was obtained with PGE(1) (maximal effect at 0.03 mM) and PGE(2). The prostaglandins also increased short-circuit current (SCC) in isolated guinea pig ileal mucosa, with PGE(1) and PGE(2) again giving the greatest effects. The prior addition of theophylline (10 mM) reduced the subsequent SCC response to PGE(1) and vice versa. It was concluded, therefore, that the SCC response to PGE(1), like the response to theophylline, represented active Cl secretion. CT increased adenyl cyclase activity in guinea pig and rabbit ileal mucosa when preincubated with the mucosa from 1 to 2.5 hr in vitro or for 2.5 hr in vivo but not when added directly to the assay. The increments in activity caused by PGE(1) and NaF were the same in CT-treated and control mucosa. Cyclic 3',5'-AMP concentration in rabbit ileal mucosa was increased 3.5-fold after a 2 hr preincubation with CT in vitro. Phosphodiesterase activity in the crude membrane fraction of the mucosa was unaffected by either CT or PGE(1). A variety of other agents including insulin, glucagon, parathormone, thyroid-stimulating hormone, L-thyroxine, thyrocalcitonin, vasopressin, and epinephrine all failed to change adenyl cyclase activity. It is concluded that CT and certain prostaglandins produce small intestinal fluid secretion by increasing mucosal adenyl cyclase activity, thereby stimulating an active secretory process.

Elevated concentration of adenosine 3':5'-cyclic monophosphate in intestinal mucosa after treatment with cholera toxin

A heat-labile factor in cell-free filtrate of a Vibrio cholerae culture induces a marked rise in the wet-weight concentration of adenosine 3':5'-cyclic monophosphate (cyclic AMP) in the intestinal mucosa of the dog. The increase becomes appreciable 1-1.5 hr after intraluminal administration of the filtrate, about the same time as the onset of intestinal secretion in response to a heat-labile enterotoxin in the filtrate. These results are consistent with the hypothesis that cyclic AMP may be an intermediary in the intestinal secretory response to cholera toxin.