Changes in the levels of inositol lipids and phosphates during the differentiation of HL60 promyelocytic cells towards neutrophils or monocytes

HL60 cells were adapted to grow in a serum-free medium containing 1 mg l$^{-1}$ inositol, in which they differentiated normally towards neutrophils (in 0.9% by volume dimethylsulphoxide) and towards monocytes (in 10 nM phorbol myristate acetate). Cells that had been equilibrium-labelled with [2-$^{3}$H]myo-inositol contained a complex pattern of inositol metabolites, several of which were at relatively high concentrations. These included InsP$_{5}$ and InsP$_{6}$, which were present at concentrations of about 25 $\mu $M and 60 $\mu $M, respectively. Striking and different changes occurred in the levels of some of the inositol polyphosphates as the cells differentiated towards either neutrophils or monocytes. Most notable were a large but gradual accumulation of Ins(1,3,4,5,6)P$_{5}$ as HL60 cells decreased in size and acquired neutrophil characteristics, and much more rapid and sequential declines in InsP$_{4}$, InsP$_{5}$ and InsP$_{6}$ as the cells started to take on monocyte character. There was a marked accumulation of free inositol and of phosphatidylinositol in the cells during neutrophil differentiation, probably caused at least in part by an increased rate of inositol uptake providing an increased intracellular inositol supply. The same accumulation of Ins(1,3,4,5,6)P$_{5}$ occurred during neutrophil differentiation, whether it was induced by dimethylsulphoxide or by a combination of retinoic acid and a T-lymphocyte cell line-derived differentiation factor. Ins(1,4,5)P$_{3}$, a physiological intracellular mediator of Ca$^{2+}$ release from membrane stores, did not change in concentration during these differentiation processes. These observations suggest that some of the more abundant cellular inositol polyphosphates play some important, but not yet understood, role either in the processes of haemopoietic differentiation or in the expression of differentiated cell character in myeloid cells.