Phospholipase activation during monocyte adherence and spreading

Phospholipase activation is an important element in cellular signal transduction. In our study we investigated the role and regulation of phospholipase activation during human monocyte adherence and spreading. In human monocytes, phospholipase inhibition (with bromophenacyl bromide (BPB) or manoalide) impaired cell adherence and spreading. In contrast, neither cyclooxygenase/lipoxygenase inhibition nor platelet activating factor receptor blockade affected these responses. The impaired adherence and spreading induced by phospholipase inhibition with BPB could be partially reversed by the addition of nM levels of arachidonate (20:4(n-6)). Dihomogammalinolenic acid (20:3(n-6)) could substitute for arachidonate, but other polyunsaturated fatty acids were ineffective in this regard. The phospholipase inhibitor, BPB was selective in its effects on cellular phospholipase activities. BPB inhibited adherence/spreading-related and PMA-stimulated phospholipase activities, but not Ca²⁺ ionophorestimulated phospholipase activity. To further probe for the role of Ca²⁺ in monocyte adherence and spreading, monocytes were loaded with MAPTAM (bis-(2-amino-5-methylphenoxy)-ethane-N,N,N′,N′, tetraacetic acid tetraacetoxymethyl ester), an EGTA analog. In contrast to phospholipase inhibition, intracellular Ca²⁺ chelation with MAPTAM did not affect monocyte adherence but did inhibit monocyte spreading. MAPTAM partially inhibited adherence/ spreading-stimulated phospholipase activity, but did not inhibit PMA-stimulated phospholipase activity. These data suggest that human monocyte adherence and spreading may sequentially activate Ca²⁺-independent and then Ca²⁺-dependent phospholipases to release arachidonate. The activation of phospholipase and the release of arachidonate appear to be integral parts of the adhesion process.