DIFFERENTIATION-INDUCED INCREASE OF PLATELET-ACTIVATING-FACTOR ACETYLHYDROLASE IN HL-60 CELLS

Platelet-activating factor (PAF) acetylhydrolase catalyzes the convers ion of PAF to lyso-PAF and acetate. In this study we show that induced cellular differentiation of HL-60 cells grown in chemically defined m edia by dimethylsulfoxide (DMSO) to granulocytic cells increases the a cetylhydrolase activity with a concomitant increased secretion of the enzyme into the media. This increase in acetylhydrolase activity is bl ocked by the presence of actinomycin D (1 mu M) or cycloheximide (1-2 mu M) in the culture media. Acetylhydrolase is located both in the cyt osolic and particulate fractions; the relative distribution of acetylh ydrolase activity in the particulate fraction and cytosol increases an d decreases respectively, as the differentiation progresses. The addit ion of an intracellular protein transport inhibitor, monensin, causes furl,her accumulation of acetylhydrolase activity in the particulate f raction and a decrease in the media, with no effect on the acetylhydro lase activity in the cytosol. Acetylhydrolase in differentiated HL-60 cells acquires properties similar to those of the plasma acetylhydrola se 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 b y the differentiated cells was almost totally insensitive to these inh ibitors, whereas the acetylhydrolase from the particulate fraction gav e an intermediate response; the cytosolic acetylhydrolase was sensitiv e to bath inhibitors. However, the acetylhydrolase secreted by differe ntiated HL-60 cells has a different electrophoretic mobility, temperat ure sensitivity, and association with lipoproteins when compared to th at of human plasma acetylhydrolase. Collectively, these results indica te cellular differentiation induces intracellular acetylhydrolase acti vity through a mechanism involving both transcriptional and translatio nal events. Furthermore, the acetylhydrolase synthesized during the DM SO-induced differentiation of HL-60 cells is then secreted into the me dia via the intracellular membrane transport system for proteins. Base d on results obtained with HL-60 cells as a cell model, it is likely t hat more than one isoform of acetylhydrolase exists in the extracellul ar milieu.