Characterizationof dibutyryl cyclic amp treated u937 cells: role of phosphorylation in cpla2 activation
The mechanism(s) responsible for receptor mediated activation of cytosolic phospholipase A2 (cPLA2) has not been fully elucidated, but there is evidence that both protein phosphorylation and calcium-dependent translocation are involved in the regulation of cPLA2. Treatment of cells with agents that induce the mobilization of arachidonic acid (AA) from phospholipid stores cause phosphorylation and activation of cPLA2 Treatment of U937 cells with dibutyryl cyclic AMP (cAMP) is known to induce cell surface expression of fMLP receptors. Stimulation of untreated U937 cells fail to release AA in response to ionomycin or fMLP, wheras cAMP-treated cells respond to both stimuli. Furthermore, immunoblot analysis reveals at least two phosphorylation states for cPLA2 in both cAMP treated and untreated cells, as judged by electrophoretic mobility. Interestingly, if cAMP treated cells are stimulated with fMLP, a complete electrophoretic shift to the higher molecular weight species in cPLA2 occurs. In contrast, ionomycin failed to demonstrate this shift in phosphorylation states in cAMP treated cells even though these cells are able to release AA following stimulation. We have determined that the inability of untreated cells to mobilize AA is not due to insufficient calcium levels as shown using Fura-2 loaded cells. Lack of differences in cPLA2 phosphorylation and Ca2' transients between untreated and cAMP treated cells suggest that there may be other regulatory processes that facilitate activation of cPLA2 and the mobilization of AA in treated cells.
Greoor, KR; Davem, LB; Lee, D; Burke, JR
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