Complementary actions of VEGF and angiopoietin-1 on blood vessel growth and leakage

Abstract

Vascular endothelial growth factor (VEGF) and Angiopoietins are families of vascular-specific growth factors that regulate blood vessel growth, maturation and function. To learn more about the effects of these factors in vivo, we have overexpressed VEGF-A or Angiopoietin-1 (Ang1) in two systems in mice, and examined the effects on blood vessel growth and function. In one set of studies, VEGF, Ang1, or both factors, were transgenically overexpressed in the skin under the keratin-14 (K14) promoter. The skin of mice overexpressing VEGF (K14-VEGF) had numerous tortuous, capillary-sized vessels which were leaky to the plasma tracer Evans blue under baseline conditions. In contrast, the skin of mice overexpressing Ang1 (K14-Ang1) had enlarged dermal vessels without a significant increase in vessel number. These enlarged vessels were less leaky than those of wild-type mice in response to inflammatory stimuli. In double transgenic mice overexpressing VEGF and Ang1, the size and number of skin vessels were both increased; however, the vessels were not leaky. In a second set of studies, VEGF or Ang1 was systemically delivered using an adenoviral approach. Intravenous injection of adenovirus encoding VEGF (Adeno-VEGF) resulted in widespread tissue oedema within 1-2 days after administration, whereas injection of Adeno-Ang1 resulted in the skin vessels becoming less leaky in response to topical inflammatory stimuli or local injection of VEGF. The decreased leakage was not accompanied by morphological changes. Thus, overexpressing VEGF appears to promote growth of new vessels accompanied by plasma leakage, whereas overexpressing Ang1 promotes the enlargement of existing vessels and a resistance to leakage. Further understanding of the interrelationship of these factors during normal development could lead to their application in the treatment of ischaemic diseases.

Fig. 1

Outline of Evans blue leakage experiments in (A) K14 transgenic mice and (B) adenovirus treated mice. (A) In transgenic mice, secreted factor (VEGF or Ang1 – green) is overexpressed in embryonic skin and throughout the lifetime of the mouse. Evans blue dye (blue) was injected into adult mice, and then inflammatory stimuli (red) were applied topically to ear skin on one side (other ear served as control). (B) In adenovirus experiments, adenovirus encoding factor (VEGF or Ang1 – green hexagons) was injected intravenously into normal adult mice (white). Factor was overexpressed in liver and secreted into circulation. Inflammatory stimuli were applied topically to the distal site: the ear skin on one side.

Fig. 2

Morphology of blood vessels in ear skin of control, K14-VEGF, K14-Ang1, and Adeno-Ang1 mice. Vessels were stained by intravascular perfusion of biotinylated Lycopersicon esculentum lectin and DAB-peroxidase reaction, and examined in whole mounts (Thurston et al. 1999, 2000). Lectin binds to the luminal surface of endothelium and reveals blood vessels