Activation of human monocytes with lipopolysaccharide (LPS) results in the production of matrix metalloproteinases (MMPs) through a prostaglandin E2 (PGE2)-cAMP-dependent pathway. In this study, the early signaling events involved in this signal transduction pathway were evaluated. Pretreatment of human peripheral blood monocytes with herbimycin A, a tyrosine kinase inhibitor, or arachidonyl trifluoromethyl ketone (AACOCF3), a specific inhibitor of cytosolic phospholipase A2 (cPLA2) inhibited the induction of PGE2 by LPS. This resulted in the inhibition of protein expression of gelatinase B (MMP-9) and interstitial collagenase (MMP-1), two major MMPs secreted by activated monocytes. Addition of arachidonic acid (AA) reversed the inhibitory effect of herbimycin A or AACOCF3 on monocyte MMP production, indicating the importance of tyrosine phosphorylation and the involvement of cPLA2 at an early stage in the signal transduction pathway of MMPs. This finding was further supported by LPS-induced shift in cPLA2 migration and tyrosine phosphorylation based on immunoblotting and immunoprecipitation studies. These results provide evidence that tyrosine phosphorylation of cPLA2 is one of the initial steps needed for the LPS induced MMP production in human monocytes.
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