Proliferation of fibroblasts is vital for adequate wound healing but is probably also involved in different hyperproliferative disorders such as atherosclerosis and cancer. The regeneration of tissue usually starts with coagulation, involving release of mitogenic and inflammatory factors from activated platelets. This study focuses on the role of eicosanoids in the proliferative effects of platelets on human fibroblasts. We show that the phospholipase A (2) inhibitor 7,7-dimethyl-5,8-eicosadienoic acid (DMDA), the combined cyclooxygenase (COX) and lipoxygenase (LOX) inhibitor 5,8,11,14-eicosatetraynoic acid (ETYA) and the LOX inhibitor 5,8,11-eicosatriynoic acid (ETI) block the platelet-induced proliferation of serum starved subconfluent human fibroblasts. Anti-proliferative effects were also obtained by specific inhibition of 5-LOX with 5,6-dehydro arachidonic acid (5,6-dAA), whereas the 12-LOX inhibitor cinnamyl-3,4-dihydroxy- a -cyanocinnamate (CDC) did not affect the platelet-stimulated growth of fibroblasts. The expression of 5-LOX was analyzed by reverse-transcriptase-mediated PCR (RT-PCR), Western blotting and HPLC. 5-LOX message and protein was detected in fibroblasts but not in platelets. Incubation with platelets markedly increased, already after one hour, the expression of 5-LOX in the fibroblast culture. The increased 5-LOX activity was associated with an elevated level of the 5-LOX metabolite 5-hydroxyeicosatetraenoic acid (5-HETE) reaching its maximum after 1 - 2 hours of co-incubation of fibroblasts and platelets. The 5-HETE production was reduced by the inhibitors DMDA, ETYA and ETI. In conclusion, this study suggests that platelet-stimulated proliferation of fibroblasts is mediated by an increased 5-LOX activity, which supports recent findings indicating a crucial role for this enzyme in proliferative disorders such as atherosclerosis.

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In healthy persons, a daily dose of 2,000 IU of vitamin E given for 10 days does not prolong the bleeding time and has no influence on collagen- and ADP-induced platelet aggregation nor on platelet prostaglandin synthesis. Therefore, even high doses of vitamin E have no place in platelet function suppressing therapy.

3 commonly used parameters for platelet aggregability were tested for their dependence on citrate concentrations. Relatively minor changes in citrate concentrations markedly influenced ADP threshold values for secondary aggregation. Collagen induced platelet malondialdehyde (MDA) production was reduced by increasing the citrate level. This effect was still present in plasma which was also anticoagulated with EDTA. Thrombin induced MDA production was considerably higher in EDTA than in citrate plasma. Citrate appears to inhibit aggregation as well as MDA synthesis. The latter effect does not seem to be solely caused by the effect of citrate on ionized calcium levels.

Cyclooxygenase activity in human platelets reappears after ingestion of aspirin as a function of the platelet production rate. In different studies the activity reappeared without delay or with an interval of at least 48 h after stopping the drug. Because inhibition of megakaryocyte cyclooxygenase is the sole likely explanation for a delay we determined thromboxane B2 in human megakaryocytes obtained under local anaesthesia. We found that aspirin does not inhibit human megakaryocytes in vivo but does so in vitro. Therefore, it does not seem likely that there is actually a delay in platelet cyclooxygenase resurgence after aspirin intake. In order to suppress platelet cyclooxygenase constantly, aspirin should be given once or twice a day and not once or twice a week.