Vascular endothelial growth factor (VEGF) stimulates angiogenesis by activating VEGF receptor-2 (VEGFR-2). The role of its homolog, placental growth factor (PlGF), remains unknown. Both VEGF and PlGF bind to VEGF receptor-1 (VEGFR-1), but it is unknown whether VEGFR-1, which exists as a soluble or a membrane-bound type, is an inert decoy or a signaling receptor for PlGF during angiogenesis. Here, we report that embryonic angiogenesis in mice was not affected by deficiency of PlGF (Pgf-/-). VEGF-B, another ligand of VEGFR-1, did not rescue development in Pgf-/- mice. However, loss of PlGF impaired angiogenesis, plasma extravasation and collateral growth during ischemia, inflammation, wound healing and cancer. Transplantation of wild-type bone marrow rescued the impaired angiogenesis and collateral growth in Pgf-/- mice, indicating that PlGF might have contributed to vessel growth in the adult by mobilizing bone-marrow-derived cells. The synergism between PlGF and VEGF was specific, as PlGF deficiency impaired the response to VEGF, but not to bFGF or histamine. VEGFR-1 was activated by PlGF, given that anti-VEGFR-1 antibodies and a Src-kinase inhibitor blocked the endothelial response to PlGF or VEGF/PlGF. By upregulating PlGF and the signaling subtype of VEGFR-1, endothelial cells amplify their responsiveness to VEGF during the 'angiogenic switch' in many pathological disorders.

We have previously shown that monocytes adhere to the vascular wall during collateral vessel growth (arteriogenesis) and capillary sprouting (angiogenesis). In this study we investigated the association of monocyte accumulation with both the production of the cytokines-basic fibroblast growth factor (bFGF) and TNF-alpha-and vessel proliferation in the rabbit after femoral artery occlusion. In particular, we studied the effects of an increase in monocyte recruitment by LPS on capillary density as well as collateral and peripheral conductance after 7 d of occlusion. Monocytes accumulated around day 3 in collateral arteries when maximal proliferation was observed, and stained strongly for bFGF and TNF-alpha. In the lower limb where angiogenesis was shown to be predominant, macrophage accumulation was also closely associated with maximal proliferation (around day 7). LPS treatment significantly increased capillary density (424+/-26.1 n/mm2 vs. 312+/-20.7 n/mm2; P < 0.05) and peripheral conductance (109+/-33.8 ml/min/100 mmHg vs. 45+/-6.8 ml/min/100 mmHg; P < 0.05) as compared with untreated animals after 7 d of occlusion. These results indicate that monocyte activation plays a major role in angiogenesis and collateral artery growth.

No abstract

Previous studies in the canine heart had shown that the growth of collateral arteries occurs via proliferative enlargement of pre-existing arteriolar connections (arteriogenesis). In the present study, we investigated the ultrastructure and molecular histology of growing and remodeling collateral arteries that develop after femoral artery occlusion in rabbits as a function of time from 2 h to 240 days after occlusion. Pre-existent arteriolar collaterals had a diameter of about 50 microm. They consisted of one to two layers of smooth muscle cells (SMCs) and were morphologically indistinguishable from normal arterioles. The stages of arteriogenesis consisted of arteriolar thinning, followed by transformation of SMCs from the contractile- into the proliferative- and synthetic phenotype. Endothelial cells (ECs) and SMCs proliferated, and SMCs migrated and formed a neo-intima. Intercellular adhesion molecule (ICAM-1) and vascular cell adhesion molecule (VCAM-1) showed early upregulation in ECs, which was accompanied by accumulation of blood-derived macrophages. Mitosis of ECs and SMCs started about 24 h after occlusion, whereas adhesion molecule expression and monocyte adhesion occurred as early as 12 h after occlusion, suggesting a role of monocytes in vascular cell proliferation. Treatment of rabbits with the pro-inflammatory cytokine MCP-1 increased monocyte adhesion and accelerated vascular remodeling. In vitro shear-stress experiments in cultured ECs revealed an increased phosphorylation of the focal contacts after 30 min and induction of ICAM-1 and VCAM-1 expression between 2 h and 6 h after shear onset, suggesting that shear stress may be the initiating event. We conclude that the process of arteriogenesis, which leads to the positive remodeling of an arteriole into an artery up to 12 times its original size, can be modified by modulators of inflammation.

Monocytes are activated during collateral artery growth in vivo, and monocyte chemotactic protein-1 (MCP-1) has been shown to be upregulated by shear stress in vitro. In order to investigate whether MCP-1 enhances collateral growth after femoral artery occlusion, 12 rabbits were randomly assigned to receive either MCP-1, PBS, or no local infusion via osmotic minipump. Seven days after occlusion, isolated hindlimbs were perfused with autologous blood at different pressures, measuring flows at maximal vasodilation via flow probe and radioactive microspheres, as well as peripheral pressures. This allowed the calculation of collateral (thigh) and peripheral (lower limb) conductances from pressure-flow tracings (slope of the curve). Collateral growth on postmortem angiograms was restricted to the thigh and was markedly enhanced with MCP-1 treatment. Both collateral and peripheral conductances were significantly elevated in animals with MCP-1 treatment compared with the control group, reaching values of nonoccluded hindlimbs after only 1 week of occlusion (collateral conductance, 70.6 +/- 19.23 versus 25.1 +/- 2.59 mL/min per 100 mm Hg; P < .01; peripheral conductance, 119.3 +/- 22.37 versus 45.4 +/- 6.80 mL/min per 100 mm Hg; P < .05). These results suggest that activation of monocytes plays an important role in collateral growth as well as in capillary sprouting.

Abstract-Growth of collateral vessels is potentially able to preserve structure and a variable degree of function in subtended tissues in the presence of arterial occlusions. The process of transformation of a small arteriole into much larger conductance artery is called arteriogenesis. Small arterioles that interconnect side branches proximal from the arterial occlusion with distal ones experience increased fluid shear stress because of the increased blood flow velocity attributable to the pressure gradient along the bridging collaterals. This activates the endothelium and leads to monocyte adhesion and infiltration with the subsequent production of growth factors and proteases. Preexistent arterioles are essential. Their presence is genetically determined. Arteriogenesis is not organ-or species-specific; coronary or peripheral collateral vessels develop following the same design principles in mice, rats, rabbits, or dogs. In contrast to angiogenesis, arteriogenesis is not dependent on the presence of hypoxia/ischemia.

It remains unclear whether capillary sprouting (angiogenesis) and in situ growth of muscular collateral arteries share the same or different molecular mechanisms. To study the role of ischemia in these two forms of vascular proliferation, we measured tissue flows and maximum collateral conductances in hindlimbs of 22 rabbits previously subjected to either acute, 7-day, 21-day, or no femoral artery occlusion. After 1 wk of femoral artery occlusion, corkscrew collaterals were observed radiographically in the thigh. These collaterals showed histochemical evidence for active proliferation of endothelial and smooth muscle cells. Maximum collateral conductance increased sixfold in the 1st wk. Perfusion deficits, however, were only observed in the distal adductor muscles (region of collateral reentry). In the lower leg, which suffered from a profound perfusion deficit, conductance increased in the absence of any visible collateral arteries but with evidence for capillary proliferation. This study therefore demonstrates that upon femoral artery occlusion angiogenesis occurs in regions of profound ischemia, whereas no direct correlation can be drawn between ischemia and collateral artery development.

Independent of the cause of heart failure, chronic low grade inflammation is present in failing human myocardium. This may significantly contribute to the structural deterioration that is the basis of reduced cardiac function in congestive heart failure.