Anti-angiogenic activity of rPAI-1(23) and vasa vasorum regression

Abstract

The vasa vasorum are unique networks of vessels that become angiogenic in response to changes in the vessel wall. Structural studies, using various imaging modalities, show that the vasa vasorum form a plexus of microvessels during the atherosclerotic disease process. The events that stimulate vasa vasorum neovascularization remain unclear. Anti-angiogenic molecules have been shown to inhibit/regress the neovascularization; they provide significant insight into vasa vasorum function, structure, and specific requirements for growth and stability. This review discusses evidence for and against potential stimulators of vasa vasorum neovascularization. Anti-angiogenic rPAI-123, a truncated isoform of plasminogen activator inhibitor-1 (PAI-1) stimulates a novel pathway for regulating plasmin activity. This mechanism contributes significantly to vasa vasorum regression/collapse and is discussed as a model of regression.

Figure 1. Vasa vasorum form a plexus in hypercholesterolemic mice

LDLR^−/−/ApoB^100/100 (DKO), were fed normal chow diet (CH) or high fat, high cholesterol Paigen's diet without cholate (PD). The PD-fed mice develop plaque and angiogenic adventitial vasa vasorum. The PD mice were treated with saline or anti-angiogenic rPAI-1₂₃ for the last 6 weeks of PD. Mice were perfused with FITC-labeled lectin (white). Descending aorta whole mounts were imaged by confocal microscopy at 20× magnification (A) CH; (B) PD, saline; (C) PD, rPAI-1₂₃ and at 63× magnification (D) CH; (E) PD, saline; (F) PD, rPAI-1₂₃. Arrows indicate position of plexus. N=8 per group. Scale bar = 20µm. Courtesy of J Mollmark, A Park et al., Arteriogenesis, Thrombosis and Vascular Biology, 9/2012, in press.

Figure 2. rPAI-1₂₃ and PAI-1 act synergistically to modulate plasmin activity

(A) Biochemical reactions containing plasminogen, tPA, rPAI-1₂₃ and PAI-1 were performed. The concentrations of rPAI-1₂₃ and PAI-1 were varied. The reactants were combined simultaneously in a reaction mixture and incubated at 37° C. for 1h.Plasmin activity was measured in a Chromozym PL assay. Data shown as mean ± standard deviation and probability values were determined by ANOVA. *p≤0.05, **p≤0.001vs. 90 nM rPAI-1₂₃. S = reactants added simultaneously; L = plasminogen incubated for 1h at 37° C before adding tPA. Note that PAI-1 anti-proteolytic activity is blocked by rPAI-1₂₃ in a dose dependent manner. (B) Schematic illustrating the effects of rPAI-1₂₃ and PAI-1interactions with plasminogen. Courtesy of J Mollmark, S Ravi et al., Circ. Res., 2011.

Figure 3. Plasmin remodels the angiogenic vasa vasorum

Atherosclerosis mouse model LDLR^−/−ApoB^100/100 (DKO) and PAI-1 deficient LDLR^−/−ApoB^100/100/ PAI-1^−/− (DKO/PAI-1^−/−, plasmin in excess) mice were CH-fed for 32 weeks or CH for 12 weeks followed by PD from 12–32 weeks of age and treated with saline or rPAI-1₂₃ for the last 6 weeks of PD. Mice were perfused with FITC-labeled lectin to detect plasmin effects on adventitial vessel structure. Descending aorta whole mounts were imaged by confocal microscopy at 20× magnification. (A) DKO, CH (B) DKO, PD, saline; (C) DKO, PD, rPAI-1₂₃; (D) DKO/ PAI-1^−/−, PD, saline; (E) DKO/ PAI-1^−/−, PD, rPAI-1₂₃. Arrows indicate position of plexus. Scale bar =20µm. Courtesy of J Mollmark, A Park et al., Arteriogenesis, Thrombosis and Vascular Biology, 2012.