All tied up

Blood vessels that feed developing tumours do not show the highly organized vascular pattern that is seen in normal tissues. Sunyoung Lee and colleagues show that this might, in part, be explained by the differing actions of matrix-tethered and untethered vascular endothelial growth factor (VEGF).

Once secreted from the cell, VEGF becomes associated with the extracellular matrix (ECM) through the heparin-binding or ECM-binding domain in its carboxyl terminus, and is thought to act in a paracrine fashion. VEGF is released from the ECM during matrix breakdown, which is mediated by enzymes such as the heparinases and plasmin. Matrix metalloproteinases (MMPs) have also been implicated in ECM breakdown and the liberation of VEGF, but by an unclear mechanism. Therefore, Lee and colleagues asked if MMPs function like plasmin and release VEGF by degrading matrix proteins or if, in fact, they cleave VEGF itself.

The authors found that the matrix-bound form of VEGF is efficiently cleaved by MMPs 3, 7, 9 and 19, into a stable, soluble cleavage product, VEGF₁₁₃. Antibody studies on ascites isolated from patients with ovarian cancer verified that VEGF₁₁₃ is present in vivo. Moreover, using matrix-assisted laser desorption time-of-flight mass spectrometry, they demonstrated that VEGF₁₁₃ contains the VEGF receptor (VEGFR)-binding domain, which is released from the ECM-binding domain by MMPs.

VEGF₁₁₃ is able to interact with VEGFR and to induce its phosphorylation and activation like wild-type VEGF. So, to analyse the biological function of VEGF₁₁₃, the researchers tested wild-type VEGF, VEGF₁₁₃ and a non-MMP cleavable form — VEGF(Δ108–118) — in angiogenesis assays in vitro. All these forms of VEGF could induce angiogenesis, but, notably, they induced different patterns of blood-vessel growth. The wild-type form of VEGF induced tortuous blood vessels similar to those seen normally in tumours in which VEGF acts without other angiogenic factors. However, VEGF₁₁₃ produced very wide and leaky blood vessels (see accompanying picture), and VEGF(Δ108–118) produced thin, highly interconnected blood vessels.

In vivo, xenotransplants of human cancer cell lines expressing these different forms of VEGF resulted in different rates of tumour growth. The tumours expressing VEGF₁₁₃ did not grow well and remained pale, whereas those expressing VEGF(Δ108–118) grew faster than those expressing wild-type VEGF. Further characterization of these proteins indicates that the disorganized patterning of tumour blood vessels might reflect the different actions of VEGF — soluble VEGF (VEGF₁₁₃) that is present in discrete patches where MMPs are active induces proliferation of endothelial cells, resulting in dilated blood vessels, whereas ECM-bound VEGF induces the active sprouting and branching of new blood vessels from existing ones.

These findings illustrate that the effect of VEGF during angiogenesis is complex and the availability and action of MMPs in tumours might go some way to explain the heterogeneous pattern of tumour blood vessels. Importantly, however, as the VEGF₁₁₃ tumours did not grow well, it is unlikely that circulating levels of VEGF will correlate with tumour progression. Therefore, VEGF levels might not be a suitable prognostic factor.