Sorting of the specific granule protein, NGAL, during granulocytic maturation of HL-60 cells

The different types of human neutrophil granules (azurophil, specific, and gelatinase granules) are formed sequentially during maturation of neutrophils from the promyelocyte stage to the band cell stage. The promyelocytic HL-60 cells can maturate to segmented granulocytes but are incapable of activating the transcription of any known intragranular protein, normally located in specific or gelatinase granules. To study the sorting of granule proteins during maturation, we transfected HL-60 cells with the specific granule protein NGAL, inserted under control of a cytomegalovirus promoter. We previously showed that NGAL is sorted to azurophil granules and colocalizes with myeloperoxidase in undifferentiated HL-60 cells. We show here that, when such transfected HL-60 cells differentiate into granulocytes, newly synthesized NGAL is not retained in granules but is constitutively secreted. This indicates that highly specific mechanisms must exist that are responsible for diverting transport vesicles into storage granules, and that HL-60 cells not only lack the ability to activate transcription of specific granule proteins, but also lose the ability to form storage granules during maturation.