Angiopoietins in tumours: the angiogenic switch.

On first view, the literature pertaining to the expression of the angiopoietins in tumours is confusing and does not readily offer a consensus pattern. Apparently conflicting publications report increased, decreased or unchanged expression levels of both angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) in a wide range of tumours. However, closer scrutiny of the literature, taking into account relative increases or decreases of each factor, reveals a consensus pattern, seen in almost all instances of expression profiling of the angiopoietins in tumours. What becomes apparent is that although absolute levels of either angiopoietin may increase or decrease, the ratio of Ang-1:Ang-2 shifts in favour of Ang-2. Given that Ang-2 is a destabilization factor, rendering vasculature in a more plastic state amenable to sprouting (under the influence of vascular endothelial growth factor, VEGF) or regression, this analysis suggests that tumours shift the angiogenic balance towards a pro-angiogenic state through altering the balance between the angiopoietins. This in turn implicates Ang-2 as a candidate for the angiogenic switch and also as an important potential therapeutic target.