Inositol 1,3,4,5-Tetrakisphosphate Negatively Regulates Phosphatidylinositol-3,4,5- Trisphosphate Signaling in Neutrophils

Abstract

Many neutrophil functions are regulated by phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3) that mediates protein membrane translocation via binding to pleckstrin homolog (PH) domains within target proteins. Here we show that inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), a cytosolic small molecule, bound the same PH domain of target proteins and competed for binding to PtdIns(3,4,5)P3. In neutrophils, chemoattractant stimulation triggered rapid elevation in Ins(1,3,4,5)P4 concentration. Depletion of Ins(1,3,4,5)P4 by deleting the gene encoding InsP3KB, which converts Ins(1,4,5)P3 to Ins(1,3,4,5)P4, enhanced membrane translocation of the PtdIns(3,4,5)P3-specific PH domain. This led to enhanced sensitivity to chemoattractant stimulation, elevated superoxide production, and enhanced neutrophil recruitment to inflamed peritoneal cavity. On the contrary, augmentation of intracellular Ins(1,3,4,5)P4 concentration blocked PH domain-mediated membrane translocation of target proteins and dramatically decreased the sensitivity of neutrophils to chemoattractant stimulation. These findings establish a role for Ins(1,3,4,5)P4 in cellular signal transduction pathways and provide another mechanism for modulating PtdIns(3,4,5)P3 signaling in neutrophils.

Regulation of innate immunity by inositol 1,3,4,5-tetrakisphosphate

“…9,10 In neutrophils, disruption of InsP3KB does not affect the overall calcium signaling in the presence of extracellular calcium. 16 However, more detailed investigation revealed a much decreased calcium release from intracellular store and an enhanced calcium influx through store-operated calcium channels in InsP3KB null neutrophils stimulated with chemokines. The reduction of calcium release from intracellular store appears to be a result of calcium depletion from the store (Jia Y and Luo HR, unpublished data) (Fig.…”

“…14,15 Recently, we demonstrated that two inositol phosphates, InsP7 and Ins(1,3,4,5)P4, compete for Akt-PH domain binding with PtdIns(3,4,5)P3 both in vitro and in vivo, providing another level of regulation for Akt membrane translocation and activation. 16,17 In hematopoietic progenitor cells, depletion of Ins(1,3,4,5)P4 by disrupting InsP3KB gene enhances membrane translocation of PtdIns(3,4,5)P3 specific PH domain, thus augments the PtdIns(3,4,5)P3 downstream signals such as Akt. As a result, the phosphorylation of cell cycle inhibitory protein p21 cip1 , a target of Akt, is enhanced, leading to diminished cell cycle inhibitory effect of p21 cip1 .…”

“…1,2,16 Ins(1,4,5)P3 is formed from the hydrolysis of PtdIns(4,5)P2 by phospholipase C (PLC or PI-PLC). We examined fMLP-elicited Ins(1,4,5)P3 production in neutrophils.…”

“…26,32 However, unexpectedly, we observed accelerated apoptotic death in InsP3KB null neutrophils, in which Akt phosphorylation and activation was significantly enhanced. 16 After 24 hours of culturing, about 20% of wild-type neutrophils manifest clear morphological signs of apoptosis (Annexin V positive and/or 7-AAD positive). Disruption of InsP3KB results in a much enhanced apoptosis, with about 50% cells undergoing apoptotic death in 24 hours.…”

“…It is dramatically upregulated upon chemoattractant stimulation. 16 The low basal InsP3 kinase activity might be necessary for chemoattractant-stimulated neutrophils to generate/maintain the initial high level of Ins(1,4,5)P3, which is essential for down-stream calcium signaling. Alternatively, the regulation of InsP3 kinase by chemoattractant might simply be another mechanism for finely tuning intracellular Ins (1,3,4,5)P4 level.…”