Abstract
The Ins(1,4,5)P3 receptor was examined in human promyelocytic leukaemic cells (HL-60) and in HL-60 cells differentiated towards granulocytes with either retinoic acid (RA) or dimethyl sulphoxide (Me2SO). HL-60 cell membranes enriched in marker enzyme activities of the endoplasmic reticulum and the plasma membrane possess a high-affinity binding site for [3H]Ins(1,4,5)P3 (KD = 22 nM). Electrotransfer studies indicate that Ins(1,4,[32P]5)P3 binds specifically to a 260 kDa protein of HL-60 cell membranes. This Ins(1,4,5)P3-binding protein selectively binds Ca(2+)-mobilizing inositol phosphates and other inositol phosphates which also bind to the purified InsP3 receptor, suggesting that the Ins(1,4,5)P3-binding protein of HL-60 cell membranes is the InsP3 receptor. When HL-60 cells are incubated with 1 microM-RA or with 1.25% Me2SO the cells differentiate within 5-7 days into cells resembling neutrophils in both structure and function. Treated cells cease to proliferate, acquire the ability to reduce Nitro Blue Tetrazolium dye, and undergo morphological changes typical of differentiated granulocytes. Concomitant with HL-60 cell differentiation, the maximal [3H]Ins(1,4,5)P3 binding in membranes increases 3-4-fold, with no change in KD. The results suggest that there is an absolute increase in the level of the InsP3 receptor during HL-60 cell differentiation and that the expression of this signal-transducing protein may be specifically regulated by differentiation factors.