Expression level of inositol trisphosphate and inositol tetrakisphosphate receptors and their influence on Ca2+ release in permeabilized HL-60 and T15 cells

To try to further define the mechanism of action of the putative second mes senger inositol 1,3,4,5-tetrakisphosphate (InsP(4)), we have studied its ef fects in permeabilized cells expressing different levels of inositol trisph osphate receptor (InsP(3)R) types I and III and of the GTPase-activating pr otein GAP1(IP4BP). During the growth curve of human HL-60 cells and mouse T 15 cells there was an increase in these proteins, which was further increas ed by differentiation (HL-60) and, marginally, by transformation (T15). T15 cells entering the stationary phase showed much lower concentrations of th ese proteins and expression was below detection in apoptotic HL-60 cells. R as(p21) showed a different pattern of expression. The ratios of InsP(3)R su btypes seem to affect the dose-response curve for inositol 2,4,5-trisphosph ate Ins(2,4,5)P-3. In permeabilized T15 cells the curve was approximately 5 -fold to the right of that obtained using HL-60 cells. However, permeabiliz ed untreated and differentiated HL-60 cells and T15 cells all showed a comp arable synergistic effect of InsP(4) on Ca2+ release stimulated by a concen tration of Ins(2,4,5)P-3, releasing approximately 20% of the Ins(1,4,5)P-3 sensitive Ca2+ pool. The data indicate that under these conditions InsP(4) is acting independently of cell type, of the ratio of inositol trisphosphat e receptor subtypes, and of the concentration of GAP1(IP4BP). (C) Harcourt Publishers Ltd 2000.