Inhibition of Vascular Endothelial Growth Factor (VEGF) Signaling in Cancer Causes Loss of Endothelial Fenestrations, Regression of Tumor Vessels, and Appearance of Basement Membrane Ghosts

Abstract

Angiogenesis inhibitors are receiving increased attention as cancer therapeutics, but little is known of the cellular effects of these inhibitors on tumor vessels. We sought to determine whether two agents, AG013736 and VEGF-Trap, that inhibit vascular endothelial growth factor (VEGF) signaling, merely stop angiogenesis or cause regression of existing tumor vessels. Here, we report that treatment with these inhibitors caused robust and early changes in endothelial cells, pericytes, and basement membrane of vessels in spontaneous islet-cell tumors of RIP-Tag2 transgenic mice and in subcutaneously implanted Lewis lung carcinomas. Strikingly, within 24 hours, endothelial fenestrations in RIP-Tag2 tumors disappeared, vascular sprouting was suppressed, and patency and blood flow ceased in some vessels. By 7 days, vascular density decreased more than 70%, and VEGFR-2 and VEGFR-3 expression was reduced in surviving endothelial cells.

Mechanisms of adverse effects of anti-VEGF therapy for cancer

“…The amount of regression was dose-dependent and varied from organ to organ, with a maximum of 68% in thyroid. But two tumours examined under the same conditions had even greater vascular regression (Inai et al, 2004;Kamba et al, 2006). Little or no capillary regression was detected in brain, retina, skeletal muscle, cardiac muscle, or lung under these conditions.…”

“…Reductions in endothelial fenestrations were evident within 24 h of inhibition of VEGF signalling (Inai et al, 2004). In the thyroid, which has heavily fenestrated capillaries, fenestrations were reduced by as much as 88% on capillaries that survived 7 days of treatment.…”

“…Examination of the simple vascular network of the mouse trachea ( Figure 1A), treated systemically for 1 -28 days with an inhibitor of VEGF signalling, revealed rapid regression of some normal mucosal capillaries (Baffert et al, , 2006aInai et al, 2004). After only 1 day of treatment, fibrin accumulated and patency was lost in some capillaries (Figure 1B -D;Baffert et al, 2004Baffert et al, , 2006aInai et al, 2004).…”

“…The magnitude of capillary loss after 10-day treatment depended on the age of the mice: 39% at 4 weeks of age, 28% at 8 weeks, and 14% at 16 weeks . Empty sleeves of vascular basement membrane persisted for several weeks after endothelial cells regressed ( Figure 1E and F), and not only marked the location of capillary regression, but also served as a scaffold for vascular regrowth (Figure 1G and H;Inai et al, 2004;Baffert et al, 2006a).…”

“…Vascular endothelial growth factor (VEGF) not only drives angiogenesis, but also serves as a survival factor for endothelial cells and promotes the abnormal phenotype of blood vessels in tumours (Inai et al, 2004). Unlike tumour vessels that have VEGF as survival factor, the normal adult vasculature is regarded as largely independent of VEGF for survival, stability, and normal function (Longo et al, 2002).…”