Selective Inhibition of Cholera Toxin- and Catecholamine- Stimulated Lipolysis by Blocking Agents

Transcription

1 INFECTION AND IMMUNITY, Nov. 1975, P Copyright ( 1975 American Society for Microbiology Vol. 12, No. 5 Printed in U.S.A. Selective Inhibition of Cholera Toxin- and Catecholamine- Stimulated Lipolysis by Blocking Agents MICHAEL S. KATZ AND WILLIAM B. GREENOUGH III* Departments ofmedicine, The Johns Hopkins University School ofmedicine, * The Baltimore City Hospitals, and The Johns Hopkins Hospital, Baltimore; Maryland 2125 Received for publication 12 June 1975 Vibrio cholerae enterotoxin stimulates lipolysis in rat epididymal fat cell suspensions. Like hormones this toxin increases adenylate cyclase activity, raising levels of cyclic adenosine 3',5'-monophosphate (camp), which activates a cellular lipase. Using specific blocking agents, we studied the responses to the adrenergic lipolytic hormones epinephrine, norepinephrine, and isoproterenol, and to cholera toxin. All stimulators were used at 1 x threshold dose. Propranolol (34,M), a 8 blocking agent, inhibited epinephrine stimulation (P <.1) but not that of toxin (P >.2). Choleragenoid (25 u.g/ml), a natural toxoid of cholera toxin, blocked stimulation by toxin (P <.1) but not that of the adrenergic agents (P >.2). A,8 blocker, practolol (3 mm), inhibited stimulation by the catecholamines tested (P <.5) but not that of toxin (P >.5). Higher concentrations of propranolol (34,uM) and the a blocking agents phenoxybenzamine (3 mm) and phentolamine (1.6 mm) inhibited all agonists (P <.1). The response to theophylline was inhibited by all blockers (P <.5) except propranolol at the lower concentration (34 um). A combined 8 and a blockade using propranolol and epinephrine together did not inhibit toxin-mediated lipolysis. It appears that stimulation by cholera toxin is independent of adrenergic receptors. A major inhibition of theophylline-mediated lipolysis by a blocking drugs indicated a nonspecific effect of these agents at the concentrations used. The uninhibited response to toxin in the presence of propranolol and epinephrine suggests a lack of relationship of the toxin receptor to either a or,b receptors. An enterotoxin produced by Vibrio cholerae increases adenylate cyclase activity in all tissues tested that possess this enzyme. The consequent increased level of cyclic adenosine 3',5'- monophosphate (camp) produces responses characteristic of the tissue stimulated (1, 19). Enhanced lipolysis in response to cholera toxin has been demonstrated in suspensions of fat cells from the epididymal fat pads of young rats (24). In this system the effect was specific to the purified toxin which is also responsible for electrolyte secretion from the intestine (8, 12). The conversion of triglyceride into free fatty acids and glycerol in this cell system is correlated with increased activity of adenylate cyclase (16). The receptors for cholera toxin are probably a specific monosialosylganglioside (GM,) (4, 14, 17), and the binding of toxin to fat cell membrane receptors has been investigated (5). In the present studies we sought functional relationships between receptors for cholera toxin and those for catecholamines by using specific blocking agents. A methylxanthine, theophylline, was used as a stimulus for lipolysis which could detect major nonspecific effects by blocking agents on camp-induced lipolysis. The blockade of cholera toxin was accomplished by a natural product of the separation of the pure toxin designated "choleragenoid" (6, 11, 18). It has subsequently been shown that choleragenoid is made up of smaller subunits which are characterized by their binding affinity for monosialosylganglioside (GM,) (7, 22) and is lacking only the adenylate cyclase-stimulating fragment of cholera toxin. Our results suggest that cholera toxin acts through receptors distinct from adrenergic sites. (This work was presented in part at the meetings of the American Federation for Clinical Research in New Orleans, January 1972 [15].) MATERIALS AND METHODS Hormones and chemical and biological materials. Highly purified V. cholerae toxin, choleragen, and its naturally occurring toxoid, choleragenoid (11), were kindly supplied by Richard Finkelstein, Department of Microbiology, The University of Texas Southwestern Medical School. Practolol [4-(2- OH-3-isopropylamine-propoxy)acetanilide] was a gift from Ayerst Laboratories, Montreal, Canada. Mate- 964

2 VOL. 12, 1975 CHOLERAGEN AND CATECHOLAMINE EFFECTS ON FAT CELLS 965 rials obtained commercially were epinephrine hydrochloride, norepinephrine bitartrate, isoproterenol hydrochloride, propranolol hydrochloride, phentolamine mesylate, phenoxybenzamine hydrochloride, collagenase (B grade from Worthington, contaminated with peptidase), fraction V bovine albumin (Armour Co., Kankakee, Minn.), glycerokinase (Calbiochem), and glycerol phosphate dehydrogenase (Calbiochem). Preparation of cell suspensions, incubation procedure, glycerol determination. Young male Sprague-Dawley rats weighing 13 to 18 g were stunned with a blow on the head. The epididymal fat pads were removed, placed in plastic counting vials containing 4 mg of collagenase per ml in Krebs- Ringer phosphate buffer with 3% albumin at a ph of 7.2, and minced. Approximately 2 g of minced fat was incubated in 5 ml of collagenase for 45 min in a shaking water bath at 37.5 C. The resulting slurry of cells and digested stroma was passed through a silk screen, and the effluent cell suspension was washed three times with fresh Krebs-Ringer albumin buffer. After the final wash the infranatant buffer was entirely removed, the cells were weighed, and a final suspension containing.25 g of fat cells per ml was prepared (2). This cell concentration was used for all experiments. The conversion of triglycerides to free fatty acids and glycerol was measured by assay of glycerol released to the medium during each incubation. An enzymatic method employing the spectrophotometric measurement of the conversion of nicotinamide adenine dinucleotide to reduced nicotinamide adenine dinucleotide by the enzymatic removal of hydrogen from glycerol was used, omitting deproteinization (25). Experimental design. Fat cell suspensions were aliquotted.25 ml into each vial. Other reactants or buffer made a final volume of.75 ml. The concentra- tions of all reagents are expressed as a function of the.5 ml of buffer to which the cell suspension was added. The duration of incubation determined from the time course (Fig. 1) of the several stimulating agents used was selected to insure an equivalent response for each agent in a range of glycerol production adequately measurable by the assay. For catecholamines 3 min was used and for cholera toxin 3 h was used. Theophylline was used at 3 min. Each stimulator was added in a final concentration that was 1 times the threshold dose. Such concentrations consistently elicited a maximum glycerol release in the incubation time selected and were expressed as 1% stimulation. When irnhibitors were employed, the specific inhibitor was preincubated with the fat cell suspension for 5 min prior to the addition of stimulating agent. The reduction of lipolysis in the presence of blocking agents was expressed as the percentage of inhibition and compared to the paired suspension with only the stimulator present. Theophylline, which increases camp levels by inhibition of the phosphodiesterase enzyme that converts camp to adenosine 5-monophosphate, was used as a control for major nonspecific effects blocking agents might have on the ability of increased camp to produce maximal lipolysis. All data are expressed + 1 standard error of the mean, and probabilities were computed using the Student's t test. RESULTS Two blocking agents were selected for both a and,3 adrenergic receptor sites. One agent (choleragenoid), which binds to the monosialosylganglioside receptor of cell membranes but does not exert an effect on adenylate cyclase, was used to block specifically the action of cholera toxin..9' 4. E.7 -J o Epinephrine 9AM - Isoproterenol.8uM &-- Theophylline lomm -C Norepinephrine.6uM r-u Cholerogen.2MLg/ml 2 TIME hours FIG. 1. Time course of stimulation of lipolysis by agonists.

3 966 KATZ AND GREENOUGH The effect of blocking agents was observed on each of five stimulating agents (cholera toxin, epinephrine, norepinephrine, isoproterenol, and theophylline). The results are summarized in Table 1. Inhibition by choleragenoid. Choleragenoid (25,ug/ml) reduced by 87.5% the stimulatory effects of cholera toxin (.2,ug/ml). The effects of catecholamines were inhibited to a minimal degree, as was lipolysis due to theophylline. The degree of inhibition of the response to toxin was in all instances markedly greater than any effects on the other agonists. Inhibition by adrenergic blocking agents. Propranolol, a 8 adrenergic blocking agent, was used at two concentrations, 34 and 34,uM. At the lower concentration significant inhibition was observed only for epinephrine (91.%; P <.1). However, norepinephrine and isoproterenol were not tested. There was no appreciable effect on the response to either cholera toxin or theophylline. At the higher concentration major effects were seen on those agonists mediated by surface receptors, but the theophylline response was only minimally affected (Table 1). Another blocking agent, practolol (3 mm), reduced responses to epinephrine, norepinephrine, and isoproterenol (Table 1). However, it was less specific, also somewhat affecting responses to cholera toxin and theophylline. Inhibition by a adrenergic blocking agents. Phentolamine (1.6,uM) reduced the response by fat cells to cholera toxin (.2 ug/ml) by 75.4% and responses to the catecholamines tested by 74.3 to 95.%. Response to theophylline was also reduced by 53.5%, indicating a major nonspecific effect on the camp system. INFECT. IMMUN. Phenoxybenzamine (3 mm) similarly exerted a significant nonspecific inhibitory effect by inhibiting theophylline stimulation by 34%. It also inhibited response to cholera toxin by 85%, in addition to inhibition of response to catecholamines by 62 to 66%. Inhibition of both a and adrenergic sites by epinephrine with propranolol. Since the lipolytic response to epinephrine is dependent on its interaction with the /8 receptors, this response can be completely blocked by propranolol. Epinephrine also interacts with a receptors and presumably occupies the a surface receptor sites (3). An experiment was carried out in which fat cells were preincubated for 5 min with propranolol (34 um); then epinephrine (9,uM) was added for an additional 5 min before the addition of cholera toxin. The results of a 3- h incubation with several concentrations of toxin are shown in Fig. 2. There was no inhibition by the combined propranolol-epinephrine blockade of the response to cholera toxin. DISCUSSION The present studies employed a design in which the end result, lipolysis, of the interaction of an agonist and the fat cell was used as an index of an effective exposure of cell surface receptors to the agent being tested. Interpretation of data from these studies rests on the following assumptions. (i) In the intact cell a receptor interacting with an agonist will be reflected by increased lipolysis. (ii) Specific blocking agents acting at the cell surface will alter the lipolytic response to an agonist by blocking a particular receptor. (iii) Agents that in addition to blocking surface receptors also affect camp-mediated lipolysis within the cell TABLE 1. Relative effect of blocking agents on stimulation of lipolysis Blocking agent, (% inhibition)" Agonist Choleragenoid Propranolol Propranolol Practolol (25 tgiml)b (34 1.M) (34 IAM) (3 mm) Cholera toxin 87.5 ± 6.5* 1.4 ± * 2.9 ± 12.1 (.2,ug/ml) n =15 n =6 n = 11 n= 6 Epinephrine 14.1 ± ± 4.4* * 64.6 ± 9.1* (9,AM) n =9 n =5 n = 11 n= 6 Norepinephrine 3.6 ± * 93.2 ± 2.4* (.6,uM) n =4 n = 5 n= 5 Isoproterenol 2.8 ± * 82.8 ± 6.6* (.8,uM) n= 4 n= 6 n= 6 Theophylline, 3' 7.7 ± 2.5* 15. ± * 1.3 ± 4.5 Incubation (1 mm) n = 11 n =5 n = 13 n = 8 a Inhibition by blocking agents of stimulated lipolysis. Inhibition is expressed as percentage of reduction of stimulated lipolysis in the absence of blocking agent + 1 standard error. Asterisk indicates inhibition with P value less than.1 from Student's t test. b Concentration of agent.

4 VOL. 12, 1975 CHOLERAGEN AND CATECHOLAMINE EFFECTS ON FAT CELLS 967 In S E -J w C.) *-* Cholera Toxin -O Cholera Toxin with Propronolol and Epinephrine CHOLERA TOXIN nanogroms / m FIG. 2. Dose-response curve in fat cells exposed to propranolol 34 M) and epinephrine (9 pm). cholera toxin for 3 h in the presence or absence of will have an effect on lipolysis due to methylxanthines. Minor effects may be more difficult to interpret, since theophylline is known to have some effect on adenylate cyclase (21) in addition to its primary effect on phosphodiesterase (1). Lower concentrations of propranolol selectively inhibited epinephrine-mediated lipolysis but not that produced by cholera toxin (Table 1). Choleragenoid quite specifically inhibited lipolysis due to cholera toxin. Practolol, another agent that blocks f3 adrenergic receptors, was also quite selective in its action. Higher concentrations of propranolol and even the lowest effective concentrations of two drugs which interfere with a receptors, phentolamine and phenoxybenzamine, were poorly selective, interfering with theophylline-stimulated lipolysis as well. The problem of assessing the relationship of a receptors to those for cholera toxin was circumvented by using epinephrine and propranolol together. The lipolytic action of epinephrine mediated by (8 receptors was obliterated by a low concentration of propranolol, and the a sites were blocked by epinephrine itself. Such preparations responded fully to toxin. The results (Fig. 2) indicated that there is little if any overlap between the receptors for catecholamines and those for toxin. Recently an interaction has been reported between epinephrine and cholera toxin in which fat cells previously exposed to toxin are more responsive to epinephrine (2, 9, 13). The experiments reported here would not measure such an effect, since in no case were cells preincubated with toxin prior to the application of catecholamines or their inhibitors. It is likely that this "sensitizing" effect is not associated with the addition of extra receptors but rather that there is an effect by cholera toxin through its ganglioside receptor on a common pathway, which regulates the responsiveness of adenylate cyclase to its several hormonal signals (U. Ganguly and W. B. Greenough III, Proc. Natl. Acad. Sci. U.S.A., in press). Additional hormones need to be studied to determine whether the toxin receptor is truly an aberrant receptor accessible only to toxin (23) or whether this receptor with its powerful regulation of adenylate cyclase is shared with any normal humoral integrating substance. ACKNOWLEDGMENTS This investigation was supported by the United States- Japan Cooperative Medical Science Program administered by The National Institute of Allergy and Infectious Diseases, Public Health Service grants Al 829 and Al We are also grateful to the Gerontology Research Center of The National Institutes of Child Health and Human Development for the use of facilities provided under its Guest Scientist Program. LITERATURE CITED 1. Beavo, J. A., N. L. Rogers,. B. Crofford, J. G. Hardman, G. W. Sutherland, and E. V. Newman Effects of xanthine derivatives on lipolysis and on adenosine 3',5'-monophosphate phosphodiesterase activity. Mol. Pharmacol. 6: Beckman, B., J. Flores, P. A. Witkum, and G. W. G. Sharp Studies on the mode of action of cholera toxin: effects on solubilized adenylate cyclase. J. Clin. Invest. 53:

5 968 KATZ AND GREENOUGH 3. Burns, T. W., P. E. Langley, and G. A. Robison Adrenergic receptors and cyclic AMP in the regulation of human adipose tissue lipolysis. Ann. N.Y. Acad. Sci. 185: Cuatrecasas, P Gangliosides and membrane receptors for cholera toxin. Biochemistry 12: Cuatrecasas, P Cholera toxin-fat cell interaction and the mechanism of activation of the lipolytic response. Biochemistry 12: Cuatrecasas, P Vibrio cholerae choleragenoid. Mechanism of inhibition of cholera toxin action. Biochemistry 12: Cuatrecasas, P., I. Parikh, and M. D. Hollenberg Affinity chromatography and structural analysis of Vibrio cholerae enterotoxin-ganglioside agarose and the biological effects of ganglioside-containing soluble polymers. Biochemistry 12: Curlin, G. T., W. H. Mosley, and W. B. Greenough III Cholera antitoxin titrations: a comparative study of fat cells, ileal loop, and rabbit skin assays. J. Infect. Dis. 127: Field, M Mode of action ofcholera toxin: stabilization of catecholamine-sensitive adenylate cyclase in turkey erythrocytes. Proc. Natl. Acad. Sci. U.S.A. 71: Finkelstein, R. A Cholera, p In A. Laskin and H. Lechevalier (ed.), CRC critical reviews in microbiology, vol. 2. Chemical Rubber Co., Cleveland. 11. Finkelstein, R. A., and J. J. LoSpalluto Production of highly purified choleragen and choleragenoid. J. Infect. Dis. 121:S63-S Greenough, W. B., III, N. F. Pierce, and M. Vaughan Titration of cholera enterotoxin and antitoxin in isolated fat cells. J. Infect. Dis. 121:S111-S Hewlett, E. L., R. L. Guerrant, D. J. Evans, Jr., and W. B. Greenough Ifl Toxins of Vibrio cholerae and Escherichia coli stimulate adenyl cyclase in rat fat INFECT. IMMUN. cells. Nature (London) 249: Holmgren, J., I. Lonnroth, and L. Svennerholm Tissue receptor for cholera exotoxin: postulated structure from studies with GM, ganglioside and related glycolipids. Infect. Immun. 8: Katz, M. S., and W. B. Greenough III Selective inhibition of fat cell response to cholera toxin and epinephrine by blocking agents. Clin. Res. 2: Khoo, J. C., A. A. Aquino, and D. Steinberg The mechanism of activation of hormone-sensitive lipase in human adipose tissue. J. Clin. Invest. 53: King, C. A., and W. E. van Heyningen Deactivation of cholera toxin by a sialidase-resistant monosialosylganglioside. J. IrLfect. Dis. 123: Pierce, N. F Differential inhibitory effects of cholera toxoids and ganglioside on the enterotoxins of Vibrio cholerae and Escherichia coli. J. Exp. Med. 137: Pierce, N. F., C. C. J. Carpenter, and W. B. Greenough III Cholera toxin and its mode of action. Bacteriol. Rev. 35: Rodbell, M Metabolism of isolated fat cells. J. Biol. Chem. 239: Sheppard, H Inhibition of norepinephrine stimulated adenyl cyclase by theophylline. Nature (London) 228: van Heyningen, S Cholera toxin: interaction of subunits with ganglioside GM,. Science 183: van Heyningen, W. E Gangliosides as membrane receptors for tetanus toxin, cholera toxin and serotonin. Nature (London) 249: Vaughan, M., N. F. Pierce, and W. B. Greenough III Stimulation of glycerol production in fat cells by cholera toxin. Nature (London) 226: Wieland, Glycerol, p In H. U. Bergmeyer (ed.), Method of enzymatic analysis, 2nd ed. Academic Press Irc., New York.