Low density lipoprotein receptor-related protein (LRP) is a heparin-dependent adhesion receptor for connective tissue growth factor (CTGF) in rat activated hepatic stellate cells

Abstract

Connective tissue growth factor (CTGF) is a cysteine-rich, extracellular matrix-associated heparin-binding protein implicated in a variety of fibrotic disorders. CTGF is initially synthesized as a mosaic protein containing four discrete structural modules (CTGF 1–4) but this is susceptible to proteolytic cleavage yielding isoforms comprising modules 3 and 4 (CTGF 3–4) or module 4 alone (CTGF 4). In this study, we show that cultured rat hepatic stellate cells (HSCs) produce CTGF 1–4 and CTGF 3–4 following treatment with transforming growth factor-β and that CTGF is a cell adhesion factor for activated HSCs. Low density lipoprotein receptor-associated protein (LRP) is a receptor for CTGF 1–4 or CTGF 3–4, but not CTGF 4, whereas cell surface heparan sulfate proteoglycans (HSPGs) are binding sites for all CTGF isoforms. Prior occupancy of LRP with other LRP ligands, receptor associated protein, anti-LRP, or a thrombospondin type I peptide (TEWSACSKTCG) resulted in a 50% decrease in the adhesion of activated HSCs to CTGF 1–4 or CTGF 3–4 whereas there was no effect on CTGF 4-mediated adhesion. Co-incubation of CTGF with heparin or perturbation of cell surface HSPGs with heparinase or sodium chlorate completely blocked adhesion of activated HSCs to all CTGF isoforms. Freshly isolated HSCs demonstrated only weak binding to CTGF but strong binding to fibronectin. Thus HSC adhesion is at least partially promoted by CTGF through its binding to LRP, a process that is heparin-dependent. CTGF–LRP interactions are likely mediated by module 3 and CTGF–heparin interactions occur principally in module 4, although additional motifs may account for the heparin-dependency of LRP binding. These data show that LRP and HSPGs are utilized by HSCs for binding to CTGF and suggest that these cell surface molecules may be involved in mediating CTGF activity or adhesive signaling during the activation process.