Differentiation induced increase of platelet-activating factor acetylhydrolase in HL-60 cells.

Platelet-activating factor (PAF) acetylhydrolase catalyzes the conversion of PAF to lyso-PAF and acetate. In this study we show that induced cellular differentiation of HL-60 cells grown in chemically defined media by dimethylsulfoxide (DMSO) to granulocytic cells increases the acetylhydrolase activity with a concomitant increased secretion of the enzyme into the media. This increase in acetylhydrolase activity is blocked by the presence of actinomycin D (1 microM) or cycloheximide (1-2 microM) in the culture media. Acetylhydrolase is located both in the cytosolic and particulate fractions; the relative distribution of acetylhydrolase activity in the particulate fraction and cytosol increases and decreases respectively, as the differentiation progresses. The addition of an intracellular protein transport inhibitor, monensin, causes further accumulation of acetylhydrolase activity in the particulate fraction and a decrease in the media, with no effect on the acetylhydrolase activity in the cytosol. Acetylhydrolase in differentiated HL-60 cells acquires properties similar to those of the plasma acetylhydrolase in that it becomes less sensitive to 5,5'-dithiobis-2-nitrobenzoic acid and p-bromophenacylbromide inhibition than the acetylhydrolase in undifferentiated cells. The acetylhydrolase secreted into the media by the differentiated cells was almost totally insensitive to these inhibitors, whereas the acetylhydrolase from the particulate fraction gave an intermediate response; the cytosolic acetylhydrolase was sensitive to both inhibitors. However, the acetylhydrolase secreted by differentiated HL-60 cells has a different electrophoretic mobility, temperature sensitivity, and association with lipoproteins when compared to that of human plasma acetylhydrolase. Collectively, these results indicate cellular differentiation induces intracellular acetylhydrolase activity through a mechanism involving both transcriptional and translational events. Furthermore, the acetylhydrolase synthesized during the DMSO-induced differentiation of HL-60 cells is then secreted into the media via the intracellular membrane transport system for proteins. Based on results obtained with HL-60 cells as a cell model, it is likely that more than one isoform of acetylhydrolase exists in the extracellular milieu.