Purpose - The angiopoietin (Ang) system plays an important role in vascular stabilization and pathologic neovascularization. The hypothesis for the study was that, in addition to modulating endothelial cell behavior, the angiopoietin/Tie-2 system also regulates the pericyte apoptosis and/or the vessel maturation associated with diabetic retinopathy.
Methods - Tie-2 expression in cultured retinal pericytes was analyzed by using ELISA, Western Blot analysis, and flow cytometry. CD13 (aminopeptidase N) expression in pericytes was determined by Western blot analysis and Ang effects verified with Tie-2 antisense treatment. Cell proliferation was monitored by crystal violet uptake, and pericyte migration was assessed in a scrape wound. Annexin V-FITC flow cytometry was used to quantify pericyte apoptosis.
Results - Pericytes expressed a functionally active Tie-2 receptor upregulated by both Ang-1 and -2 (P < 0.05). In pericytes undergoing apoptosis induced by either TNF-alpha or high glucose Ang-1 increased survival (P < 0.05 for TNF-alpha; P < 0.01 for high glucose), whereas Ang-2 increased apoptosis. Ang-1 enhanced CD13 expression in a dose-dependent manner (P < 0.05) and stimulated pericyte migration in a synthetic matrix wound-healing assay that was associated with a change in F-actin organization. Addition of Tie-2 antisense confirmed that angiopoietins act through Tie-2.
Conclusions - These findings demonstrate that Tie-2 is functional in pericytes and may play an important role in the progression of diabetic retinopathy, by regulating pericyte loss and influencing the activation state and recruitment of pericytes.