Fibulin-5, an integrin-binding matricellular protein: its function in development and disease

Abstract

Interactions between the extracellular matrix (ECM) and cells are critical in embryonic development, tissue homeostasis, physiological remodeling, and tumorigenesis. Matricellular proteins, a group of ECM components, mediate cell-ECM interactions. One such molecule, Fibulin-5 is a 66-kDa glycoprotein secreted by various cell types, including vascular smooth muscle cells (SMCs), fibroblasts, and endothelial cells. Fibulin-5 contributes to the formation of elastic fibers by binding to structural components including tropoelastin and fibrillin-1, and to cross-linking enzymes, aiding elastic fiber assembly. Mice deficient in the fibulin-5 gene (Fbln5) exhibit systemic elastic fiber defects with manifestations of loose skin, tortuous aorta, emphysematous lung and genital prolapse. Although Fbln5 expression is down-regulated after birth, following the completion of elastic fiber formation, expression is reactivated upon tissue injury, affecting diverse cellular functions independent of its elastogenic function. Fibulin-5 contains an evolutionally conserved arginine-glycine-aspartic acid (RGD) motif in the N-terminal region, which mediates binding to a subset of integrins, including alpha5beta1, alphavbeta3, and alphavbeta5. Fibulin-5 enhances substrate attachment of endothelial cells, while inhibiting migration and proliferation in a cell type- and context-dependent manner. The antagonistic function of fibulin-5 in angiogenesis has been demonstrated in vitro and in vivo; fibulin-5 may block angiogenesis by inducing the anti-angiogenic molecule thrompospondin-1, by antagonizing VEGF(165)-mediated signaling, and/or by antagonizing fibronectin-mediated signaling through directly binding and blocking the alpha5beta1 fibronectin receptor. The overall effect of fibulin-5 on tumor growth depends on the balance between the inhibitory property of fibulin-5 on angiogenesis and the direct effect of fibulin-5 on proliferation and migration of tumor cells. However, the effect of tumor-derived versus host microenvironment-derived fibulin-5 remains to be evaluated.

Fig. 1

Schematic presentation of fibulin family proteins. Class I fibulins include long fibulins (fibulin-1, -2, and -6) and Class II fibulins include short fibulins (fibulin-3, -4, -5, and -7). Fibulin-5 contains an evolutionally conserved RGD (arginine-glycine-asparatic acid) sequence in the first cbEGF motif, whereas the RGD sequence in fibulin-2 is not conserved among the species. Human fibulin-6 with 44 repeats of immunoglobulin domain is shown. Parentheses indicate synonym(s) of the corresponding fibulin shown in the figure

Fig. 2

Alignment of fibulin-5 from Human, Orangutan, Bovine, Mouse, Rat, Chicken, Xenopus, and Danio in the first calcium binding EGF-like domain with insertion a and a C-terminal fibulin module b. Green shade represents a consensus of calcium binding motifs, yellow shade represents the RGD sequence, and orange shade represents a conserved sequence across the species. Red dot indicates arginine at position 77 shown to undergo proteolytic cleavage. The amino acid sequence of the C-terminal elastin-binding domain is indicated in b. GenBank accession numbers used to obtain primary sequences are Human (CAB38568), Orangutan (NP_001125375), Bovine (NP_001014946), Mouse (NP_035942), Rat (NP_062026), Chicken (XP_421423), Xenopus (NP_001025619) and Danio (NP_001005979)

Fig. 3

A model of elastic fiber assembly. a Secretion of tropoelastin (TE) from elastogenic cells (green). Tropoelastin undergoes self-aggregation and fibulin-5 (purple) mediates this process. Tropoealstin binds both N-terminal (N) and C-terminal tropoelastin-binding domains (square) of fibulin-5. b Fibulin-5 binds tropoelastin, microfibrils (blue), and lysyl oxidase-like enzyme (yellow) to aid in elastic fiber assembly. c Cross-linked insoluble elastin (grey) is polymerized and organized into functional elastic fibers