Continuous Endothelial Cell Activation Increases Angiogenesis: Evidence for the Direct Role of Endothelium Linking Angiogenesis and Inflammation

Abstract

There is increasing evidence that chronic inflammation is tightly linked to diseases associated with endothelial dysfunction, including the induction of aberrant angiogenesis. While leukocytes have been described as mediators of inflammation-associated angiogenesis, the effects of direct chronic endothelial activation have not been addressed in this context. Using an uncleavable mutant of the transmembrane form of tumor necrosis factor-α (TNF-α), we have established models of stable TNF-α expression in endothelial cells in vitro and in transgenic mice in vivo. In the in vitro model, continuous endothelial activation leads to increased leukocyte cellular adhesion molecule expression and intracellular reactive oxygen species, hallmarks of a proinflammatory and dysfunctional endothelium. In addition, stable expression of TNF-α in endothelial cells increased angiogenic sprout formation in the presence but also in the absence of angiogenic growth factors. The partial neutralization of this effect by TNF-α antibodies and the inability of conditioned media from stable TNF-α-expressing endothelial cells to induce angiogenic activities in control endothelial cells suggest that this effect does not require expression of additional autocrine factors, but is an autonomous effect of the transmembrane TNF on the endothelial cells. Furthermore, using the Matrigel plug assay in vivo, increased angiogenesis was observed in endothelial TNF-α-expressing transgenic versus control mice. In conclusion, chronic inflammatory changes mediated by TNF-α can induce angiogenesis in vitro and in vivo, suggesting endothelial cell activation as a direct link between inflammation and angiogenesis.

Transmembrane TNF-α Promotes Suppressive Activities of Myeloid-Derived Suppressor Cells via TNFR2

“…Consistent with this observation, our data showed an association of angiogenesis with MDSC accumulation in tmTNFa-or sTNF-a-expressing tumors. Although tmTNF-a expression has been shown to be upregulated in the tumor endothelium (54) and play an angiogenic role (55), it is possible that tmTNF-a may be a factor in tumor microenvironment that would facilitate MDSCdependent tumor angiogenesis. Briefly, our in vivo data indicated again that tmTNF-a plays an important role in the activation of MDSCs, and thus promotes tumor immune escape, one of the mechanisms involved in tumor progression.…”

Differential Functions of Tumor Necrosis Factor Receptor 1 and 2 Signaling in Ischemia-Mediated Arteriogenesis and Angiogenesis

“…A recent study showed that EC-specific transgenic mice expressing a membrane-bound TNF (mtTNF, a strong activator of TNFR2 in vivo) induced constitutive EC activation and inflammatory angiogenesis. 41 The role of TNFR2 activation in these mice has not been determined. Our results showed that TNFR2 signaling is specifically activated in vascular ECs in ischemic tissue.…”

Tumor necrosis factor-α and endothelial cells modulate Notch signaling in the bone marrow microenvironment during inflammation

“…This notion is further supported by analysis of the Tie2-tmTNFα mice. In these mice, endothelial cells exhibit a chronic activated phenotype as result of endothelial-specific constitutive expression of the TNFα membranebound form [29,30] and not in response to exogenous TNFα. As a result of endothelial cell activation, we found that J2 expression was upregulated in BM endothelial cells and that it correlated with increased Notch activation on Lin − Sca + Kit + hematopoietic progenitors.…”

“…Hes5-GFP mice were BAC transgenics from the NIDS GENSAT project (Rockefeller University, www.gensat.org). Tie2-tmTNFα transgenic mice and WT non transgenic littermates were described in [29,30]. Mice were matched for gender and analyzed at 4-5 weeks of age.…”

“…In Tie2-tmTNFα transgenic mouse, a transmembrane, noncleavable form [mTNFD1-9,K(11)E] of TNFα is expressed under the control of the Tie2 promoter, in conjunction with an intronic enhancer element used to drive endothelial-specific expression [30]. In these mice, endothelial cells exhibit a chronic activated phenotype as result of constitutive expression of the TNFα membrane-bound form [29,30]. Analysis of J2 expression in Tie2-tmTNFα mice showed increased levels of J2 on BM cells with endothelial immunophenotype CD45 − CD31 + FLK1 + , compared to nontransgenic (WT) controls (Fig.…”