Angiogenic factors, typified by VEGF and FGF basic (also known as FGF2), promote vascular endothelial cell (EC) proliferation and organization into vascular tubule networks, while angiogenesis inhibitors restrain the process. A novel EC-derived protein, vasohibin, has recently been described that operates in a direct negative feedback loop to limit VEGF and FGF basic-induced angiogenesis (Figure 1).1
Whereas vasohibin mRNA and protein are induced by VEGF in a time and concentration dependent manner, the secreted protein antagonizes the angiogenic effects of VEGF.2 In vitro, vasohibin blocks VEGF or FGF basic-induced HUVEC migration and microtubule formation.2 The inhibitory effect is selective for EC and is not observed with vascular smooth muscle cells or fibroblasts.2 Vasohibin is anti-angiogenic in the chorioallantoic membrane and corneal micropocket assays.2 Vasohibin-transfected tumor cells show decreased tumorigenicity, and the resulting tumors have less well developed vasculature than nontransfected cell tumors.2 Vasohibin also blocks neovascularization in pathological conditions such as retinal ischemia and adventitial angiogenesis following vascular injury.3,4
Several reports hint at how vasohibin is induced and exerts its effects. VEGF induction of vasohibin expression is partially mediated by VEGF R2 but is independent of VEGF R1.5 The induction requires PKCd participation, with either VEGF or FGF basic as the stimulus.5 Vasohibin upregulation is achieved by an increase in transcription rate2,3,5 and is attenuated by hypoxia, TNF-alpha, IL-1 beta, or IFN-gamma.2,5 The dampening effect of these conditions may permit increased angiogenesis in tumors and at inflammatory sites. Vasohibin does not alter VEGF-induced tyrosine phosphorylation of VEGF R2 or the activation of ERK1/2.2 Rather, a preliminary report suggests that it selectively suppresses the expression of VEGF R2 mRNA.3
|Figure 1. VEGF and FGF2 (FGF basic) induce vasohibin transcription by a PKCd-dependent mechanism. VEGF effects on the induction are partially dependent on VEGF R2. Vasohibin downregulates VEGF R2 transcription but does not alter VEGF R2 or ERK1/2 activation.|
There is a considerable array of predicted human vasohibin molecules. An alternately spliced isoform has a large deletion comprising the C-terminal half of the molecule. Transfected cells produce dominant species of vasohibin migrating at 42 kDa and 36 kDa.2,6 It is not known which of these forms might be relevant in vivo or precisely how they are generated. Based on mutagenesis studies of potential protease-sensitive sites, Sonoda et al. propose that cleavage at Arg76 gives rise to the 36 kD species, liberating an N-terminal region which is not required for blocking FGF basic-induced angiogenesis.6 The C-terminal basic region is required for blocking FGF basic-induced angiogenesis and also for mediating the heparin binding activity of vasohibin.6 Vasohibin-2 is encoded by a distinct gene for which three splice variants have been described.7 The full-length forms of vasohibin-2 and vasohibin have comparable inhibitory activity in in vitro angiogenesis assays and are expressed in a similar pattern. In contrast to vasohibin, vasohibin-2 is not inducible by FGF basic or VEGF in vitro, although both are upregulated in in vivo models of angiogenesis.7 Both vasohibin and vasohibin-2 are >95% conserved between human and mouse.
The details of vasohibin’s mechanisms of induction and action have yet to be elucidated. The range of splice variants, potential cleavage products, and multiple genes suggests a complex role for vasohibin in angiogenesis regulation.