Synthesis of prostacyclin from platelet-derived endoperoxides by cultured human endothelial cells

Abstract

The authors have previously shown that aspirin-treated endothelial cells synthesize prostacyclin (PGI2) from the purified prostaglandin endoperoxide PGH2 (1978 J. Biol. Chem. 253: 7138). To ascertain whether aspirin-treated endothelial cells produce PGI2 from endoperoxides released by stimulated platelets, [3H]arachidonic acid-prelabeled platelets were reacted in aggregometer cuvettes with the calcium ionophore A 23187, thrombin, or collagen in the presence of aspirin-treated endothelial cell suspensions. This procedure permitted thin-layer radiochromatographic quantitation of [3H]PGI2 as [3H]6-keto-PGF(1α) and [3H]thromboxane A2 (TXA2) as [3H]TXB2, as well as analysis of platelet aggregation responses in the same sample. In the presence of aspirin-treated endothelial cells, platelet aggregation in response to all three agents was inhibited. [3H]6-keto-PGF(1α) was recovered from the supernates of the combined cell suspensions after stimulation by all three agents. The order of PGI2 production initiated by the stimuli was ionophore > thrombin > collagen. The amounts of platelet [3H]TXB2 recovered were markedly reduced by the addition of aspirin-treated endothelial cells. In separate experiments, 6-keto-PGF(1α) and TXB2 were quantitated by radioimmunoassay; the results paralleled those obtained with the use of radiolabeling. The quantity of 6-keto-PGF(1α) measured by radioimmunoassay represented amounts of PGI2 sufficient to inhibit platelet aggregation. These results were obtained when 200,000 platelets/μl were combined with 3,000-6,000 aspirin-treated endothelial cells/μl. At higher platelet levels the proportion of 6-keto-PGF(1α) to TXB2 decreased and platelet aggregation occurred. Control studies indicated that aspirin-treated endothelial cells could not synthesize PGI2 from exogenous radioactive or endogenous arachidonate when stimulated with thrombin. Therefore the endothelial cell suspensions could only have used endoperoxides from stimulated platelets. Thus, under the authors experimental conditions, production by endothelial cells of PGI2 from endoperoxides derived from activated platelets could be demonstrated by two independent methods. These experimental conditions included: (a) enhanced platelet-endothelial cell proximity, as attainable in stirred cell suspensions; (b) use of increased endothelial cell/platelet ratios; and (c) utilization of arachidonate of high specific activity in radiolabeling experiments. Furthermore, when a mixture of platelets and endothelial cells that were not treated with aspirin was stimulated with thrombin, more than twice as much 6-keto-PGF(1α) was formed than when endothelial cells were stimulated alone. These results indicate that endothelial cells can utilize platelet endoperoxides for PGI2 formation to a significant extent.