Pertussis toxin can activate human platelets. Comparative effects of holotoxin and its ADP-ribosylating S1 subunit

The activation of phospholipase C in human platelets is coupled to agonist receptors via guanine nucleotide-binding protein(s), and prior treatment of permeabilized platelets with GTP gamma S, GDP beta S, or pertussis toxin modifies platelet responses to agonists. Pertussis toxin is thought to act primarily as an uncoupler of Gi from cell receptors due to its ADP-ribosylating activity. However, we have found that pertussis toxin by itself can act as an agonist for intact or permeabilized platelets. Though believed to lack receptors for pertussis toxin, intact platelets, when incubated with the toxin (5-20 micrograms/ml), undergo aggregation and accumulate inositol trisphosphate and phosphatidic acid. Treatment of platelets with aspirin, incubation in the presence of creatine phosphate/creatine phosphokinase, or omission of Ca2+ and fibrinogen do not affect toxin-mediated phospholipase C activation. These effects are not observed with the ADP-ribosylating S1 monomer of toxin in intact or permeabilized platelets. Further, modification of the holotoxin with N-ethylmaleimide eliminates the toxin's ADP-ribosylating activity but does not affect its promotion of platelet aggregation and phospholipase C activation. Therefore, the activating effect of holotoxin is separable from its ADP-ribosylating activity and does not depend either upon cyclooxygenase or the ADP that may be released during platelet activation. Given the combined potentially stimulatory and inhibitory effects of pertussis holotoxin, we suggest caution in interpretation of results with this material.