Expression and secretion of glycosylphosphatidylinositol-specific phospholipase D by myeloid cell lines

Abstract

Glycosylphosphatidylinositol (GPI)-specific phospholipase D (GPI-PLD) is abundant in mammalian plasma. It could potentially regulate the surface expression of GPI-anchored proteins, but it remains to be established which tissue(s) or cell type(s) are the principal sources of the circulating enzyme. Here we report that all the myeloid cell lines tested, including K562 (multipotential blast), KG-1 (human myeloblast), HL-60, NB4, PLB-985 (human promyelocyte), U937 (human promonocyte), THP-1 (human monocyte) and J774, RAW264.7 (mouse monocyte/macrophage), contained GPI-degrading activity. T.l.c. analysis of reaction products confirmed the activity as a phospholipase D. These cells also exhibited positive immunofluorescent staining with an anti-GPI-PLD monoclonal antibody. The expression of GPI-PLD activity was not substantially reduced when the cells were cultured in either serum-free medium or GPI-PLD-depleted regular medium. Both granulocytic and monocytic differentiation of myelomonoblastic lines (e.g. HL-60) induced by dimethyl sulphoxide or phorbol diester respectively was accompanied by a 2-3-fold increase in GPI-PLD activity. J774 and HL-60 cells secreted GPI-PLD into the medium constitutively. Taken together, these data suggest that myeloid 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 physiological functions of GPI-PLD.