Purpose: Selective proteolytic activation of protease-activated receptor-1 (PAR1) by thrombin and matrix metalloproteinase-1 (MMP-1) plays a central role in enhancing angiogenesis. We investigated the expression levels of thrombin, MMP-1, and PAR1 and correlated these levels with vascular endothelial growth factor (VEGF) in proliferative diabetic retinopathy (PDR). In addition, we examined the expression of PAR1 and thrombin in the retinas of diabetic rats and PAR1 in human retinal microvascular endothelial cells (HRMEC) following exposure to high-glucose, the proinflammatory cytokines interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and the hypoxia mimetic agent cobalt chloride (CoCl2).
Methods: Vitreous samples from 32 PDR and 23 nondiabetic patients, epiretinal membranes from 10 patients with PDR, retinas of rats, and HRMEC were studied by enzyme-linked immunosorbent assay (ELISA), immunohistochemistry, and Western blot analysis. An assay for in vitro cell migration angiogenesis was performed in HRMEC.
Results: In epiretinal membranes, PAR1 was expressed in vascular endothelial cells, CD45-expressing leukocytes, and myofibroblasts. ELISA and Western blot assays revealed significant increases in the expression levels of thrombin, MMP-1, and VEGF in vitreous samples from PDR patients compared to nondiabetic controls. Significant positive correlations were found between the levels of VEGF and the levels of thrombin (r = 0.41; p = 0.006) and MMP-1 (r = 0.66; p < 0.0001). Significant increases of cleaved PAR1 (approximately 50 kDa) and the proteolytically active thrombin (approximately 50 kDa) were detected in rat retinas after induction of diabetes. The proinflammatory cytokines IL-1β and TNF-α, but not high-glucose and CoCl2, induced upregulation of cleaved PAR1 (approximately 30 kDa) in HRMEC. In addition, thrombin and MMP-1 induced VEGF in HRMEC and vorapaxar, a PAR1 inhibitor, inhibited thrombin-induced migration in HRMEC.
Conclusions: Interactions among thrombin, MMP-1, PAR1, and VEGF might facilitate angiogenesis in PDR.
Keywords: Angiogenesis; matrix metalloproteinase-1; proliferative diabetic retinopathy; protease-activated receptor-1; thrombin.