The SDF-1-CXCR4 signaling pathway: a molecular hub modulating neo-angiogenesis

Abstract

Pro-angiogenic bone marrow (BM) cells include subsets of hematopoietic cells that provide vascular support and endothelial progenitor cells (EPCs), which under certain permissive conditions could differentiate into functional vascular cells. Recent evidence demonstrates that the chemokine stromal-cell derived factor-1 (SDF-1, also known as CXCL12) has a major role in the recruitment and retention of CXCR4(+) BM cells to the neo-angiogenic niches supporting revascularization of ischemic tissue and tumor growth. However, the precise mechanism by which activation of CXCR4 modulates neo-angiogenesis is not clear. SDF-1 not only promotes revascularization by engaging with CXCR4 expressed on the vascular cells but also supports mobilization of pro-angiogenic CXCR4(+)VEGFR1(+) hematopoietic cells, thereby accelerating revascularization of ischemic organs. Here, we attempt to define the multiple functions of the SDF-1-CXCR4 signaling pathway in the regulation of neo-vascularization during acute ischemia and tumor growth. In particular, we introduce the concept that, by modulating plasma SDF-1 levels, the CXCR4 antagonist AMD3100 acutely promotes, while chronic AMD3100 treatment inhibits, mobilization of pro-angiogenic cells. We will also discuss strategies to modulate the mobilization of essential subsets of BM cells that participate in neo-angiogenesis, setting up the stage for enhancing revascularization or targeting tumor vessels by exploiting CXCR4 agonists and antagonists, respectively.

Figure 1

Regulation of neo-vascularization by SDF-1. The chemokine SDF-1 is produced by hypoxic conditions, vascular injury or tumors, and is released in the circulation. SDF-1 signaling induces a complex remodeling of the BM microenvironment involving proteases, Kit-ligand (KitL) and NO production, leading to mobilization of CXCR4⁺ angiogenic cells. Ultimately, SDF-1 expression in the neo-angiogenic niche recruits CXCR4⁺ cells and mediates their proper retention, differentiation and pro-angiogenic activities in coordination with other angiogenic factors such as VEGF-A.

Figure 2

Reconciling the SDF-1-mediated mobilization paradox: a matter of stem cell niches? (a) Localization of HSCs to the osteoblastic and vascular niches might provide an explanation as to how both SDF-1 signaling and CXCR4 inhibition lead to stem cell mobilization. We propose that CXCR4 inhibition by antagonists, such as AMD3100, rapidly mobilizes readily accessible progenitor cells located within the vicinity of the vascular niche. Alternatively, AMD3100 induces a rapid surge in intravascular release of SDF-1 that recruits readily available perivascular stem cells. SDF-1-dependent mobilization requires remodeling of the constituents within the BM, where HSCs are tightly bound to the osteoblastic niche, and as such are not readily available to enter the peripheral circulation. (b) AMD3100 induces rapid elevation of plasma SDF-1. C57bl/6J mice were injected with either phosphate buffered saline or AMD3100 at 1.25 mg/kg [intraperitoneally (i.p.)] twice daily and levels of SDF-1 in the plasma were measured at different time points by ELISA. Plasma levels of SDF-1 peaked at 6 h with AMD3100 (2.8-fold increase as compared with the PBS-treated controls, n = 6, *P < 0.05, where * represents statistical significance compared with PBS-treated controls) and then rapidly declined within the first 24 h of treatment. Plasma levels of SDF-1 were below baseline after 72 h of treatment with AMD3100 as compared with the PBS-treated controls. Arrows indicate time points of AMD3100 administration. (c) Number of circulating CXCR4⁺VEGFR1⁺ cells in the peripheral blood was measured and analyzed by two-color flow cytometry at the indicated time points after treatment with AMD3100 (1.25 mg/kg i.p. twice daily) in C57Bl/6J mice (n = 5, *P < 0.05, where * represents statistical significance compared with time 0). Consistent with the trend observed in SDF-1 plasma levels, the number of mobilized CXCR4⁺VEGFR1⁺ cells peaked at 6 h after AMD3100 treatment, then progressively declined in the ensuing 5-day period. Arrows indicate time points of AMD3100 administration.