ROLE FOR P38 MITOGEN-ACTIVATED PROTEIN-KINASE IN PLATELET-AGGREGATIONCAUSED BY COLLAGEN OR A THROMBOXANE ANALOG

p38 mitogen-activated protein kinase (MAPK) was identified in platelet s on the basis of (a) its reactivity with antibodies to C-terminal and N-terminal peptides, and (b) its ability to activate MAPK-activated p rotein kinase-2, which phosphorylates the small heat shock protein, hs p27. p38 MAPK was activated in platelets by collagen fibers, a collage n-related cross-linked peptide, thrombin, or the thromboxane analogue U46619. A highly specific inhibitor of p38 MAPK, a pyridinyl imidazole known as SB203580, inhibited the platelet enzyme in vitro (IC50 simil ar to 0.5 mu M). At similar concentrations it also inhibited agonist-s timulated phosphorylation of hsp27 in platelets, and platelet aggregat ion and secretion induced by minimal aggregatory concentrations of col lagen or U46619, but not thrombin. Inhibition of aggregation was overc ome by increasing agonist dose. SB203580 might act by inhibiting throm boxane generation, but this was only inhibited by 10-20% at low agonis t concentrations. p38 MAPK provides a crucial signal, which is necessa ry for aggregation caused by minimal concentrations of collagen fibers or U46619. Thrombin or high doses of these agonists generate signals that bypass the enzyme, or render the enzyme no longer rate-limiting.