Effect of bacterial toxins on human B cell activation. II. Mitogenic activity of the B subunit of cholera toxin.
The B subunit of cholera toxin (CT) but not the entire CT was found to induce the proliferation of resting human B lymphocytes. A significant mitogenic effect was observed for B subunit concentrations greater than 1 microgram/ml and reached a maximum of stimulation at 10 micrograms/ml. As already described for B lymphocytes preactivated with Staphylococcus aureus Cowan Strain I (SAC). B lymphocytes preactivated with the B subunit of CT, but not with the entire CT, were able to proliferate in response to exogenous interleukin 2 (IL 2) and to the low-molecular weight B cell growth factor (BCGF). To determine the transmembrane signaling system used by the B subunit of CT to mediate its biological effects, we compared the transmembrane signals used by the entire CT, its B subunit and SAC. In comparison to the entire CT, which directly activates adenylate cyclase and increases intracellular cAMP levels, neither the B subunit nor SAC modified the cAMP content. In contrast, although SAC induced inositol phosphate generation neither CT nor the separate subunits were able to induce such a production. Moreover, changes in the fluorescence of indo-1-loaded B lymphocytes revealed that mitogenic doses of either the B subunit or SAC induced a rapid and sustained increase in cytoplasmic free Ca2+ concentration ([Ca2+]i). The effect of the B subunit appeared to be largely dependent on the presence of extracellular Ca2+, because in Ca2(+)-free medium no [Ca2+]i uptake was observed. In contrast, the SAC-induced [Ca2+]i uptake is substantially, but not totally, inhibited in Ca2(+)-free medium, suggesting that part of the rise in [Ca2+]i was due to the release from internal stores. Moreover, fluorimetric measurements on loaded cells with 2',7'-bis(carboxyethyl)-5(6')-carboxyfluorescein revealed that SAC induced a rapid cytoplasmic alkalinization via activation of Na+/H+ exchange, whereas the entire CT and its B subunit had no effect on intracellular pH. Taken together, these data suggest that, in comparison to SAC, the mitogenic effect of the B subunit of CT was mediated through different intracellular biochemical pathways.
Dugas
B
B
Institut Henri Beaufour, Les Ulis, France.
Paul-Eugène
N
N
Génot
E
E
Mencia-Huerta
J M
JM
Braquet
P
P
Kolb
J P
JP
D001402:B-Lymphocytes / Q000187:drug effects*; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002772:Cholera Toxin / Q000378:metabolism / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000096:biosynthesis; D007376:Interleukin-2 / Q000502:physiology; D015847:Interleukin-4 / Q000502:physiology; D008213:Lymphocyte Activation / Q000187:drug effects; D046911:Macromolecular Substances; D008934:Mitogens; D015398:Signal Transduction / Q000276:immunology; D013211:Staphylococcus aureus / Q000276:immunology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D007295:Inositol Phosphates; D007376:Interleukin-2; D046911:Macromolecular Substances; D008934:Mitogens; D015847:Interleukin-4; D002772:Cholera Toxin; D000242:Cyclic AMP; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium