Functional Consequences of Phospholipase A₂ Activation in Human Monocytes

Abstract

Human monocytes release arachidonic acid upon stimulation with a variety of soluble or particulate agents. These include: phorbol esters (i.e., 12-0-tetradecanoate phorbol-13-acetate, TPA), calcium ionophores (ionomycin), serum-treated zymosan (STZ) concanavalin A (Con A), and, to a minor degree, lipopolysaccharides (LPS). Protein Kinase C activation or increased intracellular Ca²⁺ are common features of the actions of most, if not all, of these stimuli. Prevention of PKC activation by the use of staurosporine or chelation of extracellular calcium by EGTA selectively impaired AA release, indicating that PLA₂ may be regulated by either pathway concurrently. The generation of inositol phosphates and diacylglycerol by the action of phospholipase C, notably upon interaction with opsonized particles during phagocytosis, apparently constitutes the physiological correlate of stimulation via these agents.

Release of arachidonic acid by the action of PLA₂ or other phospholipid hydrolyzing enzymes leads directly to the formation of cyclooxygenase products. In the presence of markedly elevated calcium concentrations, 5-lipoxygenase (LO) is activated as well, leading to the formation and release of leukotrienes.

Agents which stimulate AA release also initiate other monocyte functions, including generation of reactive oxygen intermediates and lymphokine release. This observation makes it tempting to implicate PLA₂ activation in many aspects of monocyte physiology. However, no correlation with PLA₂ activation and either superoxide or lymphokine release was found when multiple stimuli, including TPA, ionomycin, serum-treated zymosan, concanavalin A, or LPS, were compared simultaneously. Instead, our results indicate that PLA₂ activation is regulated by the same mechanisms, including PKC activation and increased Ca²⁺, as are other enzymes which determine expression of monocyte function.