A Novel View of the Role of Prostaglandin E2 (PGE2) in Facilitating Engraftment of HSPCs By Activating the NOX2–ROS–Nlrp3 Inflammasome Axis to Incorporate the CXCR4 Receptor into Membrane Lipid Rafts

Background. It is known that prostaglandin E2 (PGE2) increases the homing and engraftment of hematopoietic stem/progenitor cells (HSPCs). However, aside from its role in upregulation of CXCR4 receptor expression on the surface of these cells, the exact mechanism has not been proposed. We have demonstrated in the past that an important step enabling the migration of HSPCs is the incorporation of CXCR4 into membrane lipid rafts on the leading surface (leading edge, in two dimensions) of migrating cells, which facilitates its interaction with cell migration signaling pathways (Wysoczynski M et al. Incorporation of CXCR4 into membrane lipid rafts primes homing-related responses of hematopoietic stem/progenitor cells to an SDF-1 gradient. Blood. 2005;105(1):40-48). Recently, we reported that Nlrp3 inflammasome-deficient HSPCs show a defect in lipid raft formation that results in defective migration of these cells in response to an SDF-1 gradient and their defective homing and engraftment after transplantation (Adamiak, M et al. Nlrp3 Inflammasome Signaling Regulates the Homing and Engraftment of Hematopoietic Stem Cells (HSPCs) by Enhancing Incorporation of CXCR4 Receptor into Membrane Lipid Rafts. Stem Cell Rev and Rep (2020). https://doi.org/10.1007/s12015-020-10005-w). An important activator of Nlrp3 inflammasomes is reactive oxygen species (ROS). Importantly, the enzyme that generates ROS, known as NADPH oxidase 2 (NOX2), is also associated with cell membrane lipid rafts. Hypothesis. Given the known roles of PGE2, membrane lipid rafts, and the Nlrp3 inflammasome in migration, homing, and engraftment of HSPCs, we hypothesized that PGE2 signaling promotes Nlrp3 inflammasome activation in a Nox2–ROS-dependent manner that results in incorporation of CXCR4 into membrane lipid rafts, which better explains the role of PGE2 in these phenomena. Materials and Methods. To test this hypothesis, murine SKL and human CD34⁺ cells enriched for HSPCs were stimulated with PGE2 to evaluate activation of genes of the Nlrp3 inflammasome complex at the mRNA and protein levels. Next, HSPCs from Nox2-KO mice were tested for membrane lipid raft formation in functional chemotaxis assays in response to SDF-1 gradients under conditions promoting membrane lipid raft formation. Formation of membrane lipid rafts in Nox2-KO cells was also evaluated by confocal analysis in the presence or absence of PGE2. Finally, the effect of the PGE2–Nox2–Nlrp3 inflammasome axis on the formation of membrane lipid rafts was evaluated in the presence of the ROS scavenger N-acethyl-cysteine (NAC). Results. We provide for the first time evidence that PGE2 activates Nlrp3 inflammasomes in HSPCs in a Nox2–ROS-dependent manner. This Nlrp3 inflammasome activation increases at the leading surface of migrating HSPCs with incorporation of the CXCR4 receptor into membrane lipid rafts. Formation of membrane lipid rafts was absent in Nox2-KO and Nlrp3-KO mouse HSPCs and in normal wild type cells after their exposure to NAC. Moreover, we also observed that Nox2-KO and Nlrp3-KO mice had a lower basal level of CXCR4 expression. Conclusions. Our results for the first time explain the role of PGE2 in promoting homing and migration of HSPCs, which occurs in response to PGE2 by activation of the Nox2–ROS–Nlrp3 inflammasome axis and thereby promotes incorporation of the CXCR4 receptor into membrane lipid rafts. Moreover, basal expression of the CXCR4 receptor was at a low level on the surface of HSPCs from Nlrp3-KO mice. Thus, our results provide evidence for the importance of the Nox2–ROS–Nlrp3 inflammasome axis in PGE2-mediated homing and engraftment of HSPCs and the role of PGE2-mediated lipid raft formation for optimal responsiveness of CXCR4 to SDF-1 in the BM microenvironment.