GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR STIMULATES THE EXPRESSION OF THE 5-LIPOXYGENASE-ACTIVATING PROTEIN (FLAP) IN HUMAN NEUTROPHILS

The synthesis of leukotrienes in human blood neutrophils chiefly relie s on the activity of two enzymes, phospholipase A2 and 5-lipoxygenase (5-LO). In turn, the activation of the 5-LO requires the participation of a recently characterized membrane-bound protein, the 5-LO-activati ng protein (FLAP). In this study, we have investigated conditions unde r which FLAP expression in neutrophils may be modulated. Of several cy tokines tested, only granulocyte/macrophage colony-stimulating factor (GM-CSF) (and to a lesser extent tumor necrosis fact alpha) significan tly increased expression of FLAP. GM-CSF increased FLAP mRNA steady-st ate levels in a time- and dose-dependent manner. The stimulatory effec t of GM-CSF on FLAP mRNA was inhibited by prior treatment of the cells with the transcription inhibitor, actinomycin D, and pretreatment of the cells with the protein synthesis inhibitor, cycloheximide, failed to prevent the increase in FLAP mRNA induced by GM-CSF. The accumulati on of newly synthesized FLAP, as determined by immunoprecipitation aft er incorporation of S-35-labeled amino acids, was also increased after incubation of neutrophils with GM-CSF. In addition, the total level o f FLAP protein was increased in GM-CSF-treated neutrophils, as determi ned by two-dimensional gel electrophoresis, followed by Western blot. GM-CSF did not alter the stability of the FLAP protein, indicating tha t the effect of GM-CSF on FLAP accumulation was the consequence of inc reased de novo synthesis as opposed to decreased degradation of FLAP. Finally, incubation of neutrophils with the synthetic glucocorticoid d examethasone directly stimulated the upregulation of FLAP mRNA and pro tein, and enhanced the effect of GM-CSF. Taken together, these data de monstrate that FLAP expression may be upmodulated after appropriate st imulation of neutrophils. The increase in FLAP expression induced by G M-CSF in inflammatory conditions could confer upon neutrophils a prolo nged capacity to synthesize leukotrienes.