Low-Dose Irradiation Promotes Tissue Revascularization through Matrix Metalloproteinase-9 Mediated VEGF Release from Mast Cells.

Irradiation is one of the pillars in the treatment of malignancies. The combination of radiotherapy and anti-angiogenic strategies has been shown to increase the tumor response in various tumor models. The more than additive effect of irradiation and anti-angiogenic treatment suggested that irradiation might have a pro-angiogenic effect. However, the mechanism remained unclear.

Bone marrow(BM)-derived progenitor cells contribute to tissue regeneration by promoting angiogenesis/vasculogenesis. We demonstrated that chemokine/cytokine mediated progenitor mobilization is dependent on the activation of matrix metalloproteinase-9 (MMP-9). Here, we show that following irradiation hematopoietic and endothelial progenitors are released into circulation in MMP-9 wild-type, but not MMP-9 deficient (−/−) mice. We have observed that low-dose irradiation fosters vascular regeneration in a limb ischemia model. Vascular regeneration was driven by the upregulation of MMP-9 mediating the release of soluble Kit-ligand (KitL) and increasing plasma vascular endothelial growth factor (VEGF), followed by mobilization of BM-derived hematopoietic and endothelial progenitor cells. Release of sKitL and production of VEGF were impaired in MMP-9-/- mice resulting in failure of mobilization of hematopoietic and endothelial progenitors, and delayed vessel formation in the ischemic limb. The blood vessels forming in the ischemic tissue of MMP-9−/− mice lacked smooth muscle cell coverage, whereas stable vessels were formed in MMP-9 wild-type animals. But which cell type might be responsible for the observed VEGF increase following irradiation? Mast cells are known to harbor a variety of growth factors and angiogenic factors, including VEGF and have been shown to have pro-angiogenic effects. We analyzed various tissues of irradiated and non-irradiated controls. The number of mast cells was increased in the irradiated muscle tissue in MMP-9 wild-type, but not MMP-9−/− mice. VEGF was mainly produced by mast cells in a MMP-9 dependent manner as determined by in situ hybridization. Likewise, the number of mast cells was increased in the ischemic tissue of MMP-9 wild-type, but not in MMP-9 deficient mice. We could show that both VEGF and KitL can promote mast cell migration and that irradiation-induced soluble KitL in collaboration with VEGF promoted migration of mast cells in vitro.

Taken together, low-dose irradiation promoted hematopoietic and endothelial progenitor cell mobilization and activation of mast cells thereby promoting vasculogenesis/angiogenesis in a hind limb model. These data not only show a novel mechanism of neovascularization and tissue regeneration but suggest that low-dose irradiation can be used for therapeutic angiogenesis augmenting collateral vessel growth in ischemic tissues.