The Macrophage-Stimulating Protein (MSP) and RON Receptor Are Potential Novel Angiogenic Biomarkers in Proliferative Diabetic Retinopathy.

PURPOSE: Macrophage-stimulating protein (MSP) as a crucial cytokine of the Hippo pathway mediates its activity via the tyrosine kinase receptor RON and functions in activating macrophages. We investigated the expression of the MSP/RON protein couple in the intraocular microenvironment of patients with proliferative diabetic retinopathy (PDR).

METHODS

Epiretinal fibrovascular membranes from 12 patients, vitreous fluid samples from 32 PDR and 25 nondiabetic patients, rat retinas, human retinal microvascular endothelial cells (HRMECs) and human retinal Müller glial cells were studied by ELISA, Western blot analysis and immunohistochemistry.

RESULTS

MSP and vascular endothelial growth factor (VEGF) levels were significantly higher in vitreous fluid samples from patients with PDR than in nondiabetic control patients (p < 0.001 for both comparisons). MSP levels in PDR patients with active neovascularization were significantly higher than those in involuted PDR patients (p < 0.001). Levels of MSP correlated significantly with those of VEGF. In epiretinal membranes, immunohistochemical analysis revealed MSP expression in CD68-expressing monocytes/ macrophages, vascular endothelial cells and myofibroblasts. RON expression was limited to monocytes/ macrophages. Significant positive correlations were detected between the numbers of blood vessels expressing CD31, reflecting angiogenic activity of PDR, and the numbers of blood vessels expressing MSP and stromal cells expressing MSP and RON. Western blot analysis demonstrated MSP expression in rat retinas, HRMECs and Müller cells.

CONCLUSIONS

MSP and RON represent potential new biomarkers of angiogenesis in PDR.