Angiopoietin-like 7 Secretion Is Induced by Glaucoma Stimuli and Its Concentration Is Elevated in Glaucomatous Aqueous Humor

Glaucoma is a complex set of diseases that cause visual impairment and blindness due to death of retinal ganglion cells, with elevated intraocular pressure (IOP) as an important risk factor. There are probably multiple pathways leading to glaucoma involving interactions between genetic and environmental factors1,2 and changes in gene expression patterns. Many studies have used microarray technology to search for genes that may have altered expression in glaucoma and therefore may be pathogenic.3–11 A challenge now is to choose specific candidates from these lists for further evaluation. We chose to investigate the possible involvement in glaucoma of angiopoietin-like 7 (ANGPTL7), also known as cornea-derived transcript 6 (CDT6), in part because it has been identified by microarray to be a highly induced mRNA in response to either dexamethasone (DEX) or transforming growth factor β (TGFβ) treatment of trabecular meshwork (TM) cells.8,9,11 ANGPTL7 was also identified as possibly being associated with primary open-angle glaucoma (POAG) in a proteomics study of TM tissue.12 The chromosomal location of the ANGPTL7 gene (1p36.22)13 is within the GLC3B locus (1p36.2-1p36.1) of primary congenital glaucoma,14 further supporting ANGPTL7 as a candidate glaucoma gene. ANGPTL7 is a member of the angiopoietin-like (ANGPTL) family of proteins that have high sequence and structural homology to the angiopoietins, which are important regulators of angiogenesis.13 Currently, seven members of the ANGPTL family have been identified (ANGPTL1-7), all of which are secreted proteins that form multimers via amino-terminal coiled-coil domains and contain carboxyl-terminal fibrinogen-like domains. All ANGPTL proteins studied to date have been shown to be involved in blood vessel formation or neovascularization in several models,15–17 including corneal angiogenesis assays.17,18 In addition, ANGPTL proteins have been demonstrated to play an important role in lipid metabolism by inhibition of phospholipid lipase.19–22 The lipid and vascular functions are mediated by distinct protein domains, with the amino-terminal coiled-coil domain mediating lipid metabolism22–24 and the carboxyl-terminal fibrinogen-like domain mediating vascular effects.18 ANGPTL proteins appear to be bi-functional, with both functions demonstrated for most of the family members studied. ANGPTL7 was first identified as highly expressed in a human corneal cDNA library,25 and the protein has been shown to be secreted as disulfide-linked multimers.26 Because of its corneal location, it was initially speculated that ANGPTL7 may have antiangiogenic properties. In a mouse tumor model, vascularization of tumors overexpressing ANGPTL7 was distorted, with deposits of collagen types I and V and proteoglycans near vascular endothelia.26 Melanoma cell lines transfected with ANGPTL7 also produced more collagen types I and V than did the vector-transfected control cells.26 The potential to alter collagen homeostasis makes ANGPTL7 an attractive candidate for involvement in glaucoma’s pathogenesis. To investigate this possibility, we extended previous microarray findings to the protein level. Consistent with previous studies, ANGPTL7 protein was increased in response to either DEX or TGFβ treatment of TM cells, and TM cell lines transfected with ANGPTL7 expressed more type I collagen. ANGPTL7 protein concentration was elevated in AH from patients with glaucoma and correlated with disease progression in the beagle model of POAG, supporting the possibility that ANGPTL7 is involved in the pathogenesis of glaucoma.