Basement Membrane Derived Fibulin-1 and Fibulin-5 Function as Angiogenesis Inhibitors and Suppress Tumor Growth

Abstract

The fibulins are a family of secreted glycoproteins that are characterized by repeated epidermal-growth-factor–like domains and a unique C-terminus structure. Fibulins modulate cell morphology, growth, adhesion, and motility. Our initial basement membrane degradome screen using Cathepsin D, a tumor microenvironment–associated protease, contained fragments of fibulin-1 and full length fibulin-5. In this report, we evaluate the antiangiogenic activity of fibulin-1 and fibulin-5. Tumor studies demonstrate that both fibulin-1 and fibulin-5 suppress HT1080 tumor growth. CD31 labeling and TUNEL assay further reveal that fibulin-1 suppression of HT1080 tumor growth is associated with diminished angiogenesis and also enhanced apoptosis of endothelial cells and tumor cells. In contrast, fibulin-5 inhibits tumor angiogenesis with a minimal anti-apoptotic affect. Cathepsin D digestion of fibulin-1 produces a fragment with nearly the same molecular weight as fibulin-5, and this fragment (named Neostatin) inhibits endothelial cell proliferation. Additionally, degradation of basement membrane by cathepsin D liberates both fibulin-1 fragments and fibulin-5, which function to inhibit angiogenesis.