The role of Agrin in tumor angiogenesis

Abstract

Angiogenesis is the process by which new blood vessels are formed from pre-existing capillaries. It is vital for tissue perfusion during fetal growth and organ development. In adults, angiogenesis is mostly quiescent and tightly regulated by a balance of pro/anti-angiogenic factors that modulate capillary formation and density. Solid tumors via a series of epigenetic alterations termed the angiogenic switch, hijacks this balance in favor of angiogenesis and tumor vascularization. Hence, the need for effective anti-angiogenic therapies is imperative in addressing tumor malignancies. However, the current lines of therapy, targeting single ligand-receptor(s) complexes such as vascular endothelial growth factor (VEGF) and its receptors (VEGFR1 and VEGFR2) have shown clinical limitations. Therefore, an effort to broaden the identification of key receptors and their underlying regulators as a therapeutic target(s) may offer improved clinical benefits. Though several proteoglycans have been implicated in cancer and angiogenesis, their roles in endothelial cell (EC) recruitment and vascularization during tumorigenesis remain poorly understood. Here we reveal that Agrin which is overexpressed and secreted in hepatocellular carcinoma (HCC) promotes vascularization during the early stages of tumor growth by recruiting ECs within localized and metastatic lesions. Moreover, Agrin facilitates adhesion of infiltrating cancer cells to ECs which is critical during the induction of tumor angiogenesis and metastatic dissemination. In ECs, Agrin stimulates endothelial proliferation, invasion, migration, and angiogenesis in-vitro and in-vivo. Agrin-induced angiogenesis and adherence to cancer cells are mediated via Integrinβ1, Lrp4-MuSK signaling that also requires focal adhesion kinase (FAK) activity. Mechanistically, we uncovered that Agrin regulates VEGFR2 levels that sustain the angiogenic property of ECs and adherence to cancer cells. Agrin attributes an ECM stiffness-based stabilization of VEGFR2 by enhancing interactions with Integrin-β1-Lrp4 and additionally stimulates endothelial Nitric-oxide synthase (e-NOS) signaling. Therefore, we propose that cross-talk between Agrin expressing cancer and endothelial cells favor angiogenesis via sustaining the VEGFR2 pathway. Therefore, we suggest that targeting Agrin in combination with existing therapies aimed at VEGF-VEGFR signaling may present an attractive modality for reducing tumor angiogenesis via down-regulation of VEGFR2.