Mechanisms of endothelial cell-dependent leukocyte adhesion stimulated by platelet-activating factor

Platelet-activating factor (PAF) stimulates leukocyte-endothelial cell (EC) adhesion through its effects either on leukocytes or on ECs. ECs may be injured, synthesize, or express new adhesive proteins to increase leukocyte adhesion. Intermediary mediators produced by activated ECs are also likely involved in promoting leukocyte adhesion. Our experiments demonstrated that PAF induced no obvious damage to bovine pulmonary artery ECs evaluated by lactic dehydrogenase release rate, angiotensin-converting enzyme activity, and cellular malondialdehyde content. Treatment of EC monolayers with 10(-9) M PAF increased polymorphonuclear leukocyte (PMN) adhesion. Increasing PAF concentration did not induce more PMN adherence. PAF elicited both a rapid and prolonged increment of PMN adherence to EC monolayers. The rapid adherence was greatly attenuated by pretreatment of ECs with PAF receptor antagonist SRI 63-441 but was not affected by pretreatment of PMNs with SRI 63-441, suggesting that PAF increases PMN adherence rapidly through its effects on specific receptors on ECs. Increased PMN adherence lasted if PAF treatment of ECs was sustained for 3 or 6 h. Pretreatment of ECs with actinomycin D, a protein synthesis inhibitor, significantly decreased PAF-induced sustained PMN adherence, but the inhibition is incomplete, suggesting that other mechanisms than protein synthesis also participated in the prolonged PMN adherence. We concluded from the results that PAF may induce both rapid and prolonged PMN adhesion to ECs. The effects are receptor mediated. The prolonged PMN adhesion is partly the result of protein synthesis.