Angiogenetic Signaling through Hypoxia

Abstract

Cell death mediated by hypoxia is a frequent event during the proliferation of tumor cell populations.¹ Hypoxia may induce apoptosis of areas of the growing tumor but it can also lead to necrotic death of the corresponding cells.^2,3 Tumor propagation and progression depends on the induction of tumor vascularization, ie, the angiogenetic switch.⁴ Although it is now well documented that tumor cells have the ability to induce angiogenesis by secretion of extracellular molecules promoting the outgrowth of small vessels, the stimulation of angiogenesis mediated by the necrotic cells themselves may be a very efficient mechanism by which tumors can escape growth limiting because of hypoxia.⁵ A group of molecules that may act as mediators of angiogenesis released by necrotic cells are members of the so-called high-mobility group protein family. High-mobility group proteins are small DNA-binding proteins playing an important role in transcriptional regulation.⁶ In addition, there is now increasing evidence that besides their role as regulators of transcription at least some members of that group of proteins can also exert extracellular functions. Of these proteins, HMGB1 currently has been investigated most intensively.^7,8 It can be secreted by certain cells and plays an important role in inflammation, cell migration, differentiation, and tumorigenesis and has been identified as one of the ligands binding to the receptor for advanced glycation end products (RAGE).^9,10 HMGB1 binding to RAGE activates key cell-signaling pathways such as MAP kinases and nuclear factor-κB.¹¹In vivo, two potential sources of circulating HMGB1 exist: HMGB1 released from damaged or necrotic cells and HMGB1 secreted from activated macrophages in response to, eg, oxygen stress, endotoxin, tumor necrosis factor-α, or interleukin-1β.^12–14 Scaffidi and colleagues¹⁵ showed that HMGB1 rapidly leaked out from permeabilized necrotic cells, but not from permeabilized apoptotic cells. Through its secretion by activated macrophages HMGB1 again activates macrophages, resulting in secretion of angiogenetic factors, eg, vascular endothelial growth factor (VEGF), tumor necrosis factor-α, and interleukin-8.^16,17

It is well documented that advanced glycation end products (AGEs) can promote angiogenesis.^18,19 Okamoto and colleagues¹⁹ have cultured skin microvascular endothelial cells with AGE resulting in a stimulated growth and tube formation. This effect was enlarged with increased expression of RAGE. Similar results were obtained by Yonekura and colleagues²⁰ who were able to show that the formation of the cord-like structure of endothelial cells induced by AGE was completely abolished by soluble RAGE. As for the molecular mechanism of AGE-induced angiogenesis the up-regulation of VEGF because of signaling via nuclear factor-κB seems to be the key effect.¹⁹

Nevertheless, Taguchi and colleagues¹¹ have attempted to exclude an angiogenetic effect of that protein by placing basic fibroblast growth factor-laden pellets into a corneal pocket. In these experiments, no differences in capillary outgrowth from the corneal limbus compared to that after treatment with the competitive inhibitor sRAGE were observed. sRAGE is a truncated variant of the RAGE receptor that binds HMGB1 and blocks the interaction with its receptor. Based on their results Taguchi and colleagues¹¹ have concluded that RAGE blockade does not impair the process of neovascularization at all. However, the latter experimental design does not allow drawing a general conclusion about possible angiogenetic effects of HMGB1. Therefore we used a spheroid model to re-examine the effects of HMGB1 on human endothelial cells.

Footnotes