VEGF-dependent tumor angiogenesis requires inverse and reciprocal regulation of VEGFR1 and VEGFR2

Vascular endothelial growth factor (VEGF) signaling is critical for tumor angiogenesis. However, therapies based on inhibition of VEGF receptors (VEGFRs) have shown modest results for patients with Cancer. Surprisingly little is known about mechanisms underlying the regulation of VEGFR1/Flt-1 and VEGFR2/KDR/Flk-1 expression, the main targets of these drugs. Here, analysis of tissue microarrays revealed an inversely reciprocal pattern of VEGFR regulation in the endothelium of human squamous-cell carcinomas (high VEGFR1/Flt-1, low VEGFR2/KDR/Flk-1), as compared with the endothelium of control tissues (low VEGFR1/Flt-1, high VEGFR2/KDR/Flk-1). Mechanistic studies demonstrated that VEGF signals through the Akt/ERK pathway to inhibit constitutive ubiquitination and induce rapid VEGFR1/Flt-1 accumulation in endothelial cells. Surprisingly, VEGFR1/Flt-1 is primarily localized in the nucleus of endothelial cells. In contrast, VEGF signals through the JNK/c-Jun pathway to induce endocytosis, nuclear translocation, and downregulation of VEGFR2/KDR/Flk-1 via ubiquitination. VEGFR1/Flt-1 signaling is required for endothelial-cell survival, while VEGFR2/KDR/Flk-1 regulates capillary tube formation. Notably, the antiangiogenic effect of bevacizumab (anti-VEGF antibody) requires normalization of VEGFR1/Flt-1 and VEGFR2/KDR/Flk-1 levels in human squamous-cell carcinomas vascularized with human blood vessels in immunodeficient mice. Collectively, this work demonstrates that VEGF-induced angiogenesis requires inverse regulation of VEGFR1/Flt-1 and VEGFR2/KDR/Flk-1 in tumor-associated endothelial cells.