Lysyl oxidase-like protein-2 regulates sprouting angiogenesis and type IV collagen assembly in the endothelial basement membrane

Abstract

Sprouting angiogenesis is associated with extensive extracellular matrix (ECM) remodeling. The molecular mechanisms involved in building the vascular microenvironment and its impact on capillary formation remain elusive. We therefore performed a proteomic analysis of ECM from endothelial cells maintained in hypoxia, a major stimulator of angiogenesis. Here, we report the characterization of lysyl oxidase-like protein-2 (LOXL2) as a hypoxia-target expressed in neovessels and accumulated in the endothelial ECM. IntroductionAngiogenesis occurs during development and tissue remodeling, and in the pathologic context of cardiovascular ischemic diseases or tumor growth. Sprouting of new vessels is initiated by stimulation of endothelial cells (ECs) by a combination of signals from the microenvironment that includes oxygen tension and growth factors. Cells from not yet vascularized tissue and ECs that invade this microenvironment are both hypoxia targets through the Hypoxia Inducible Factor (HIF) pathway. HIF activates transcription of genes coding for autocrine/paracrine factors like vascular endothelial growth factor (VEGF) and extracellular matrix (ECM) components. 1 Synergy between these responses is assumed through local concentration of growth factors in the ECM where they function as attractant for ECs. Specialized ECs, called tip cells, lead vascular growth by sending out filopodia to explore the hypoxic microenvironment. 2 Tip cells are thus continuously exposed to low oxygen concentration. 3 Stalk cells, located behind the tip cells, serve to vessel growth by proliferation, lumen formation and junction establishment. 4 Vascular ECM undergoes major remodeling during angiogenesis, consisting in ECM/basement membrane degradation, provisional ECM generation and assembly of a new basement membrane. ECM-mediated mechanotransductive signaling regulates 3D multicellular organization, including lumen formation and tubulogenesis. Thus, in addition to storing angiogenic factors and providing structural features, ECM is a dynamic promoter of angiogenesis. 5 Subendothelial basement membrane is composed of nonfibrillar collagen IV, laminin, perlecan and nidogens. Other collagens (VIII, XV, and XVIII), fibronectin and matricellular proteins (thrombospondin-1, Cyr61) are associated with the basement membrane. There is only little data concerning the assembly of the vascular microenvironment and its impact on angiogenesis. Genes encoding ECM components and enzymes involved in their assembly and stabilization are targets of hypoxia in ECs. 1 To identify key regulators of the angiogenesis-associated ECM remodeling, we performed a proteomic analysis of endothelial ECM generated in vitro under hypoxic conditions. We found that the ECM cross-linking enzyme, lysyl oxidase-like protein-2 (LOXL2) is a major target of hypoxia. Lysyl oxidases, consisting in lysyl oxidase (LOX) and 4 lysyl oxidase-like proteins (LOXL 1 to 4), catalyze the deamination of lysines and hydroxylysines, generating aldehydes that spontaneously react to form co...

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