The Tumor-Promoter Phorbol Ester (12-0-Tetradecanoyl-Phorbol-13-Acetate), a Potent Aggregating Agent for Blood Platelets

Abstract

The phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate, a potent tumor-promoting agent, caused irreversible platelet aggregation when more than 0.02 µM was stirred with human citrated or heparinized platelet-rich plasma (PRP). With washed platelets, 1 nM was effective. The alcohol phorbol, which has little tumor-promoting activity, failed to cause platelet aggregation. With all but low concentrations of phorbol ester, aggregation was succeeded by a rapid phase. The latter was prevented or reduced by enzymes which destroy ADP and by aspirin, was associated with a change in platelet shape, and was presumably due to released ADP. At higher concentrations, only a rapid phase was seen, and these inhibitors were not effective. Low concentrations did not aggregate platelets in PRP containing sufficient EDTA or EGTA to chelate ionized calcium or in PRP from thrombasthenic patients; higher concentrations caused slight aggregation. Both the primary, non-ADP-dependent aggregation and the rapid ADP-dependent aggregation were markedly inhibited by substances which increase cyclic AMP, metabolic inhibitors, and the sulfhydryl inhibitor N-ethylmaleimide. Phorbol ester reduced platelet cyclic AMP only when it had been previously elevated by prostaglandin E1. 1 µM did not release ß-glucuronidase, lactic dehydrogenase, or inflammatory material from platelets in 4–5 min despite marked aggregation, but liberated all three in 30 min. The possibility is discussed that low phorbol ester concentrations cause primary aggregation by a direct action on platelet actomyosin.

Adducin in platelets: activation-induced phosphorylation by PKC and proteolysis by calpain

“…[28][29][30] To verify that the phosphorylation of adducin detected with the antiphosphoadducin antibody was due to the action of PKC, platelets were stimulated with PMA that directly activates PKC. Figure 2 shows a comparison of thrombin activation (1 U/mL) versus activation with PMA (100 nmol/L).…”

Effects of Phorbol Myristate Acetate on the Metabolism and Ultrastructure of Neutrophils in Chronic Granulomatous Disease

“…In some cell systems PMA causes a rapid fall in the intracellular level of cyclic 3',5'-AMP (30) and in others stimulates increased levels of cyclic 3',5' GMP (31). High concentrations of PMA have been noted to damage cell membranes, labilize lysosomes, and disrupt mitochondria (32), while small amounts can stimulate platelet aggregation and secretion without causing the cells to lose cytoplasmic enzymes that would suggest injury or increased permeability (33,34). Several other effects of PMA on cells and subcellular organelles have been observed (35)(36)(37), and the multiplicity of actions has made it difficult to determine the specific mechanism by which it influences carcinogenesis.…”

D3 Phosphoinositides and Outside-in integrin Signaling by Glycoprotein IIb-IIIa Mediate Platelet Actin Assembly and Filopodial Extension Induced by Phorbol 12-Myristate 13-Acetate

“…The actin assembly induced by PMA is slower and more sustained that the actin polymerization response to chemoattractants and growth factors, and PMA, unlike these other stimuli, does not induce cell translocation (26,28,29). In platelets, PMA generates an irreversible shape change (18,30), centripetal contractions (31, 32) and cell-cell aggregation (30). We have now defined a signal transduction pathway by which PMA mediates a slow actin assembly that extends filopods from the surface of blood platelets.…”