The differential role of phospholipase a₂ on HL-60 cell proliferation and differentiation processes

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.