Mfge8 diminishes the severity of tissue fibrosis in mice by binding and targeting collagen for uptake by macrophages

Abstract

Milk fat globule epidermal growth factor 8 (Mfge8) is a soluble glycoprotein known to regulate inflammation and immunity by mediating apoptotic cell clearance. Since fibrosis can occur as a result of exaggerated apoptosis and inflammation, we set out to investigate the hypothesis that Mfge8 might negatively regulate tissue fibrosis. We report here that Mfge8 does decrease the severity of tissue fibrosis in a mouse model of pulmonary fibrosis; however, it does so not through effects on inflammation and apoptotic cell clearance, but by binding and targeting collagen for cellular uptake through its discoidin domains. Initial analysis revealed that Mfge8 -/-mice exhibited enhanced pulmonary fibrosis after bleomycin-induced lung injury. However, they did not have increased inflammation or impaired apoptotic cell clearance after lung injury compared with Mfge8 +/+ mice; rather, they had a defect in collagen turnover. Further experiments indicated that Mfge8 directly bound collagen and that Mfge8 -/-macrophages exhibited defective collagen uptake that could be rescued by recombinant Mfge8 containing at least one discoidin domain. These data demonstrate a critical role for Mfge8 in decreasing the severity of murine tissue fibrosis by facilitating the removal of accumulated collagen.

Control of lung vascular permeability and endotoxin-induced pulmonary oedema by changes in extracellular matrix mechanics

“…However, BAPN treatment did not significantly alter messenger RNA (mRNA) expression of these collagens, and the proportion of each collagen type was similar to that exhibited by control lungs (P40.05, unpaired T-test). We confirmed these results by quantitating 4-hydroxyproline levels as a measure of the amount of collagen present in the tissue 27 . BAPN treatment did not significantly alter lung hydroxyproline levels, and there was no change in the hydroxyproline levels in serum (P40.05, unpaired T-test), which are commonly elevated with increased collagen turnover 28 .…”

“…LOX activity in the serum and lung homogenate was measured using LOX activity assay kit (ABD Bioquest, CA) in which LOX substrate that releases hydrogen peroxide was detected 13,37,55 . Hydroxyproline in lung tissue or serum was measured using Hydroxyproline assay kit (Biovision, Milpitas, CA) 27 . The levels of IL-1 and TNF-a in the lung homogenate were measured by enzyme-linked immunosorbent assay (ELISA) (R&D systems).…”

Post-weaning increases in the milk-fat globule EGF-factor VIII on fat globules in mouse milk and in the uptake of the fat globules by HC11 mammary epithelial cells

“…On the other hand, no effect of the RGD-peptide indicates that MFG uptake by HC11 cells is not simply enhanced by binding of MFG-E8-coated MFGs to RGD-containing integrins on the surface of HC11 cells. Similar results, that is, MFG-E8 dependent but RGD-peptide independent uptake, have been reported previously in a study on collagen uptake by macrophage cells in a mouse fibrosis model (30). However, MFGs resemble apoptotic bodies in their structure and components rather than collagen matrix.…”

Mechanisms of Lung Fibrosis Resolution

“…Mfge8 knockout macrophages exhibited defective collagen uptake that could be rescued by recombinant Mfge8. 37 Mfge8 also negatively regulates inflammation and autoimmunity by facilitating the clearance of apoptotic cells. 38 This dual role suggests that the failure to resolve may be imprecise coordination of effectors, such as Mfge8, that bridge apoptosis and enzymatic ECM breakdown.…”