EXPRESSION OF AZUROPHIL AND SPECIFIC GRANULE PROTEINS DURING DIFFERENTIATION OF NB4 CELLS IN NEUTROPHILS

Neutrophils contain several populations of secretory granules with cha racteristic sets of proteins. Granule proteins are sorted into their r espective granule types by temporal regulation of their expression dur ing cell differentiation and/or by specific targeting signals. We inve stigated the expression of some granule proteins in human promyelocyti c NB4 cells. Like other myeloid cell lines which can be differentiated into neutrophils, NB4 cells lack the specific-granule population. We report here that, nevertheless, they express the specific-granule matr ix protein lactoferrin, when differentiated with retinoic acid. Lactof errin and the azurophil-granule protein beta-glucuronidase were simult aneously expressed, whereas myeloperoxidase expression had stopped, sh owing that azurophil-granule proteins are not all produced concomitant ly. Cell fractionation by Percoll gradient revealed that while beta-gl ucuronidase co-fractionated with myeloperoxidase, lactoferrin was most ly contained in a vesicular compartment free of markers for azurophil granules, plasma membrane, and Golgi. This vesicular compartment was n ot implicated in regulated exocytosis since it was not mobilized by se cretagogues, which, in parallel, induced the release of myeloperoxidas e. Furthermore, the specific granule-membrane protein cytochrome b558 also became expressed during NB4-cell differentiation. However, it did not co-localize with lactoferrin but was present in the plasma-membra ne fraction. Therefore, differentiation of NB4 cells with retinoic aci d leads to the expression of specific-and azurophil-granule proteins a nd provides a unique cell line model to study the mechanisms involved in the sorting of azurophil- and specific-granule proteins. (C) 1998 W iley-Liss, Inc.