- The aim of this study is to explore the biological importance of phospholipase A2 (PLA2) activity and its metabolite arachidonic acid (AA) in the proliferation process and in the transduction pathway which initiates the induction of differentiation and regulates the differentiation of HL-60 cells toward granulocyte or monocyte phenotypes. The inhibitory effect of l,25(OH)2D3 and of RA on [3H]AA release from HL-60 cells prompted us to examine the role of PLA2 activity in HL-60 proliferation and differentiation processes. p-Bromophenacyl bromine (BPB), the specific inhibitor of PLA2 activity, significantly inhibited the proliferation of HL-60 cells and of fibroblasts (L929 and Swiss 3T3) in a dose-dependent manner, suggesting that PLA2 activity is important for HL-60 and fibroblast proliferation. The effect of BPB on proliferation is probably through its inhibitory effect on PLA2 activity, since the same doses of BPB which inhibited proliferation also inhibited PLA2 activity. In addition, BPB, in higher concentrations, caused an immediate inhibition of [3H]AA release from HL-60 cells and fibroblasts. The inhibitory effect of BPB (0.5 μM) on cell proliferation could be reversed by exogenous addition of free AA in HL-60 cells. Although BPB inhibited HL-60 proliferation, it did not cause any effect of differentiation. Likewise, BPB did not increase the differentiation when it was added together with either 1,25(OH)2D3 or RA. The addition of free AA alone did not induce HL-60 cell differentiation but potentiated HL-60 cell differentiation process in the presence of suboptimal concentration of either 1,25(OH)2D3 or RA. The differences in the effect of the inhibitor of PLA2 activity, BPB, and the addition of exogenous free AA on the proliferation and differentiation of HL-60 cells suggest that these two processes are under different regulatory mechanism.