Interferon Gamma Binds to Extracellular Matrix Chondroitin-Sulfate Proteoglycans, Thus Enhancing Its Cellular Response

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Abstract
The amino acid sequence of interferon gamma (IFN-γ) has basic amino acid clusters similar to the heparin-binding consensus sequences found in other proteins that bind to proteoglycans (PGs). We investigated whether recombinant human IFN-γ could bind to extracellular matrix (ECM) PGs secreted by human arterial smooth muscle cells (HASMCs) in vitro and whether the interaction affected the cellular response to IFN-γ. As an in vitro model of ECM we used the basement membrane from HASMCs in culture. The binding of125I-IFN-γ to ECM was reduced significantly by pretreatment of ECM with chondroitinase ABC, an enzyme that degrades chondroitin-sulfate glycosaminoglycans. IFN-γ binding to ECM was reduced by increasing concentrations of chondroitin-6-sulfate.125I-IFN-γ (0.05 to 2 ng/mL) binding data indicated an apparentKdof 2×10−11mol/L and a maximum binding of 1.6×106IFN-γ molecules bound per square millimeter of ECM. Experiments with synthetic peptides suggested that residues 127 through 135 (AKTGKRKRS) are involved in the binding. The binding to chondroitin-sulfate PGs was confirmed by affinity chromatography of isolated [35S]chondroitin-sulfate PGs from ECM and cell-culture medium on immobilized IFN-γ. The binding was abolished by treatment with chondroitinase ABC. ECM-bound IFN-γ was more effective in inducing the expression of class II major histocompatibility antigens such as HLA-DR in HASMCs and human arterial endothelial cells than soluble IFN-γ. These results suggest a role for chondroitin-sulfate PGs in immobilizing IFN-γ in the ECM compartment and enhancing the cellular response to IFN-γ.