Targeting the anthrax receptors, TEM-8 and CMG-2, for anti-angiogenic therapy

Abstract

The anthrax toxin receptors tumor endothelial marker-8 (TEM-8) and capillary morphogenesis gene-2 (CMG-2) are responsible for allowing entry of anthrax toxin into host cells. These receptors were first discovered due to their enhanced expression on endothelial cells undergoing blood vessel growth or angiogenesis in model systems. Inhibition of angiogenesis is an important strategy for current anti-cancer therapies and treatment of retinal diseases. Functional roles for TEM-8 and CMG-2 in angiogenesis have recently emerged. TEM-8 appears to regulate endothelial cell migration and tubule formation whereas a role for CMG-2 in endothelial proliferation has been documented. TEM-8 and CMG-2 bind differentially to extracellular matrix proteins including collagen I, collagen IV and laminin and these properties may be responsible for their apparent roles in regulating endothelial cell behavior during angiogenesis. TEM-8-binding moieties have also been suggested to be useful in selectively targeting anti-angiogenic and anti-tumorigenic therapies to tumor endothelium. Additionally, studies of modified forms of lethal toxin (LeTx) have demonstrated that targeted inhibition of MAPKs within tumor vessels may represent an efficacious anti-angiogenic strategy.

Figure 1

Protein structure of the TEM-8 and CMG-2 splice variants. There are three known splice variants of TEM-8 and four splice variants of CMG-2. Putative signal peptides (SP), Von Willebrand factor type A (VWA) domains, transmembrane domains (TM) and cytoplasmic (CYT) domains are denoted. Tyrosine residues within the cytoplasmic tails of TEM-8 and CMG-2 are also shown (Y).

Figure 2

Facilitation of cellular anthrax toxin entry (A) and angiogenesis (B) by the anthrax toxin receptors. A. Entry of anthax toxin into macrophages. 1. PA binds to the palmitoylated and LRP6 bound macrophage cell anthrax receptors (TEM-8 or CMG-2). 2. PA is cleaved by furin like proteases with PA63 remaining bound to the receptor. 3. Clustering of anthrax toxin receptors and forming of a heptameric prepore induces src like kinase (SLK) signaling and binding of lethal toxin (LT) and edema toxin (ET) to the receptors. 4. SLK activation leads to phosphorylation of anthrax receptors. 5. Phosphorylation of anthrax receptors leads to ubiquitination of the receptors. 6. Endocytosis of LT and ET along with the receptors. 7. LT and ET are released into the cytoplasm. B. Putative functions of the anthrax receptors in angiogenesis. TEM-8 is known to bind to collagen I and collagen VI while CMG-2 binds to collagen IV and laminin. TEM-8 is thought to be involved in the regulation of endothelial cell migration and CMG-2 is proposed to regulate both endothelial cell proliferation and tubule formation. The influence of LRP6, SLK signaling, and anthrax receptor phosphorylation, palmitoylation, ubiquitination, and clustering, on these angiogenic functions in endothelial cells remains to be determined.