Stimulation of tumour necrosis factor release by cytotoxic analogues of platelet-activating factor

The capacity of cytotoxic analogues of platelet-activating factor (PAF) to stimulate tumour necrosis factor-alpha (TNF-alpha) synthesis and release by human monocytes was determined. Cell-associated TNF-alpha was quantified by protein immunoblotting and released TNF-alpha was quantified by cytotoxicity bioassay. Picomolar concentrations of methoxyPAF, SDZ 62-759, SDZ 68-826, SDZ 62-434 and SRI 62-834 induced a two- to fivefold increase in cell-associated and released TNF-alpha. These compounds were as potent as PAF for stimulating monocytes. In contrast, they lacked direct platelet-activating activity and inhibited platelet aggregation induced by PAF selectivity. The analogues inhibited PAF binding to platelets but not to monocytes. The PAF binding antagonists kadsurenone, BN52021 and WEB2086 inhibited TNF-alpha release induced by 10(-11) M PAF or methoxyPAF by a maximum of only 30-60% whereas they inhibited platelet aggregation by 10(-8) M PAF completely. Monocyte receptors for methoxyPAF were evaluated. Scatchard analysis of [3H]methoxyPAF binding to monocytes revealed large numbers of relatively low affinity receptors (Kd = 5.9 +/- 0.5 x 10(-7) M; 9.1 +/- 4.2 x 10(7) sites/monocyte). These values do not correspond to binding constants of monocyte receptors for PAF and do not account for monocyte activation by picomolar concentrations of methoxyPAF. Cytotoxic analogues of PAF stimulate TNF-alpha synthesis and release but they do not stimulate monocytes by interacting with PAF receptors.