Expression of translation initiation factors eIF-4E and eIF-2 alpha and a potential physiologic role of continuous protein synthesis in human platelets

It is generally believed that platelets do not have a functionally signific ant protein synthetic machinery. However, our analysis demonstrated that no rmal bone marrow megakaryocytes express high levels of translation initiati on factors eIF-4E and eIF-2 alpha and the expression of these protein synth esis initiation factors is continued in platelets (as determined by immunoh istochemistry and Western blot analysis). Both eIF-4E and eIF-2 alpha are k ey regulators of protein synthesis. The eIF-4E is a rate-limiting part of a multisubunit complex, eIF-4F, that binds to the 5' cap structure present i n virtually all eukaryotic mRNAs, and carries out transfer of mRNAs to ribo somes for translation. Translation initiation factor eIF-2 alpha is also a rate-limiting protein which associates with two other proteins to form an e IF-2 initiation factor complex responsible for the transfer of initiator me thionyl-tRNA to the 40S ribosomal subunit. We confirm that expression of eI F-4E and eIF-2 alpha is biologically relevant in that platelets continue pr otein synthesis, albeit at a 16 times lower rate than WBC (as determined by S-35-labeled amino acid incorporation, SDS-PAGE and scintillation counting ). Finally, we determined that protein synthesis inhibitors (puromycin and emetine) attenuate the platelet aggregation response to a combination of AD P and epinephrine, but potentiate the response to collagen. Our data are co nsistent with the existence of different signal transducing pathways mediat ing the response to ADP/epinephrine and collagen. We suggest that the ADP/e pinephrine response is positively affected by continuously synthesized prot eins, while the response to collagen is modulated by continuously produced inhibitory proteins. Taken together, our results suggest that continuous pr otein synthesis is important for platelet function and its role in platelet physiology and pathophysiology deserves further study.