EXPRESSION AND SECRETION OF GLYCOSYLPHOSPHATIDYLINOSITOL-SPECIFIC PHOSPHOLIPASE-D BY MYELOID CELL-LINES

Glycosylphosphatidylinositol (GPI)-specific phospholipase D (GPI-PLD) is abundant in mammalian plasma. It could potentially regulate the sur face expression of GPI-anchored proteins, but it remains to be establi shed which tissue(s) or cell type(s) are the principal sources of the circulating enzyme. Here we report that all the myeloid cell lines tes ted, including K562 (multipotential blast), KG-1 (human myeloblast), H L-60, NB4, PLB-985 (human promyelocyte), U937 (human promonocyte), THP -1 (human monocyte) and J774, RAW264.7 (mouse monocyte/macrophage), co ntained GPI-degrading activity. T.l.c. analysis of reaction products c onfirmed the activity as a phospholipase D. These cells also exhibited positive immunofluorescent staining with an anti-GPI-PLD monoclonal a ntibody. The expression of GPI-PLD activity was not substantially redu ced when the cells were cultured in either serum-free medium or GPI-PL D-depleted regular medium. Both granulocytic and monocytic differentia tion of myelomonoblastic lines (e.g. HL-60) induced by dimethyl sulpho xide or phorbol diester respectively was accompanied by a 2-3-fold inc rease in GPI-PLD activity. J774 and HL-60 cells secreted GPI-PLD into the medium constitutively. Taken together, these data suggest that mye loid cells are a potential contributor to the circulating GPI-PLD pool . As leucocytes express many important GPI-anchored surface antigens, these cells may prove to be a valuable model system for studying the p hysiological functions of GPI-PLD.