Evidence that a lipolytic enzyme—hematopoietic-specific phospholipase C-β2—promotes mobilization of hematopoietic stem cells by decreasing their lipid raft-mediated bone marrow retention and increasing the promobilizing effects of granulocytes

Abstract

Hematopoietic stem/progenitor cells (HSPCs) reside in the bone marrow (BM) microenvironment and are retained there by the interaction of membrane lipid raft-associated receptors, such as the α-chemokine receptor CXCR4 and the α4β1-integrin (VLA-4, very late antigen 4 receptor) receptor, with their respective specific ligands, stromal-derived factor 1 and vascular cell adhesion molecule 1, expressed in BM stem cell niches. The integrity of the lipid rafts containing these receptors is maintained by the glycolipid glycosylphosphatidylinositol anchor (GPI-A). It has been reported that a cleavage fragment of the fifth component of the activated complement cascade, C5a, has an important role in mobilizing HSPCs into the peripheral blood (PB) by (i) inducing degranulation of BM-residing granulocytes and (ii) promoting their egress from the BM into the PB so that they permeabilize the endothelial barrier for subsequent egress of HSPCs. We report here that hematopoietic cell-specific phospholipase C-β2 (PLC-β2) has a crucial role in pharmacological mobilization of HSPCs. On the one hand, when released during degranulation of granulocytes, it digests GPI-A, thereby disrupting membrane lipid rafts and impairing retention of HSPCs in BM niches. On the other hand, it is an intracellular enzyme required for degranulation of granulocytes and their egress from BM. In support of this dual role, we demonstrate that PLC-β2-knockout mice are poor mobilizers and provide, for the first time, evidence for the involvement of this lipolytic enzyme in the mobilization of HSPCs.

The Involvment of Hematopoietic-Specific PLC -β2 in Homing and Engraftment of Hematopoietic Stem/Progenitor Cells

“…Taking into consideration the important role of PLC-β2 in promoting detachment of HSPCs from BM niches, it is not surprising that PLC-β2-KO mice are poor mobilizers [5]. Nevertheless, while performing mobilization studies, we found that BM cells from these animals show somewhat reduced chemotaxis in response to several chemottractants involved in cell trafficking.…”

“…Animal studies were approved by the Animal Care and Use Committee of the University of Louisville (Louisville, KY, USA) [5, 20]. …”

“…PLC enzymes are associated with cell surface receptors that convert phosphatidyloinositol-4,5-biphosphate into two important second messengers, diacylglycerol (DAG) and inositol-1,4.5-triphosphate (IP 3 ) [3–5]. Among these isoforms, PLC-β2 is somewhat unique in being a hematopoietic-specific enzyme [1, 2].…”

“…Recently, we identified PLC-β2 as the first known lipolytic enzyme involved in the mobilization of hematopoietic stem/progenitor cells (HSPCs) from bone marrow (BM) into peripheral blood (PB) [5, 6]. These pro-mobilizing effects depend on two important mechanisms.…”

“…Their respective ligands, the α-chemokine stromal cell-derived factor 1 (SDF-1) and vascular adhesion molecule 1 (VCAM-1, also known as CD106), are expressed by cells in the BM microenvironment (e.g., osteoblasts and fibroblasts) [1, 6–11]. Secondly, PLC-β2, when released extracellularly from granulocytes and HSPCs upon stimulation, cleaves the glycolipid glycosylphosphatidylinositol anchor (GPI-A) in cell membranes and thus disrupts the structure of membrane lipid rafts, which are important in the retention of HSPCs in BM niches [5, 6, 12]. It is well known that both BM-retention receptors for HSPCs, CXCR4, and VLA-4, are membrane lipid raft receptors [6, 13–17].…”