Mechanism for IL-1 beta-mediated neovascularization unmasked by IL-1 beta knock-out mice

We have reported that interleukin-1 beta (IL-1 beta) upregulates cardiac expression of vascular endothelial growth factor (VEGF) and VEGF receptor-2 (VEGFR-2), raising the possibility that IL-1 beta plays an important role in VEGF-mediated neovascularization. In this study, we examined the cellular mechanism for ischemia-induced neovascularization using IL-1 beta knock-out (-/-) mice. Recovery of blood perfusion in ischemic hindlimb in IL-1 beta-/- mice was markedly (43% decrease) impaired as compared with the wild-type mice. CD31(+) vessel numbers and Ki-67(+) neo-capillaries were significantly (P < 0.01) decreased 44% and 68%, respectively. IL-1 beta expression was localized in the capillary vessels in ischemic limb muscles. Ischemia-induced expressions of hypoxia-inducible factor 1 alpha (HIF-1 alpha), VEGF, its receptor VEGFR-2 and vascular cell adhesion molecule-1 (VCAM-1) were markedly inhibited in the IL-1 beta-/- mice. Hindlimb ischemia-induced an increase (1.22% out of total nuclear cell) in CD34(-)/B220(-)/CD3(-)/Flk-1(+) hematopoietic stem cell population in peripheral blood in the wild-type mice, whereas in the IL-1 beta-/- mice such increase was only 0.09%. Injection of IL-1 beta protein into the wild-type mice markedly increased the ratio of the CD34(-)/B220(-)/CD3(-)/Flk-1(+) cell population (from 0.03% to 0.7%) in the peripheral blood associated with an increase in the number of endothelial cells. Such IL-1 beta-mediated increases in cell numbers were blocked by co-injection of anti-VEGF antibody. CD34(-)/B220(-)CD3(-)Flk-1(+) cells trans-differentiated into eNOS- and CD31-expressing endothelial cells in vivo and in vitro. This study demonstrates that IL-1 beta plays a key role in ischemia-induced neovascularization by mobilizing CD34(-)/B220(-)CD3(-)Flk-1(+) endothelial precursor cells in a VEGF-dependent manner as well as by upregulating expressions of VEGF, VEGFR-2 and adhesion molecules on endothelial cells.