TGFβ induces BIGH3 expression and human retinal pericyte apoptosis: a novel pathway of diabetic retinopathy

Purpose One of the earliest hallmarks of diabetic retinopathy is the loss of retinal pericytes. However, the mechanisms that promote pericyte dropout are unknown. In the present study, we propose a novel pathway in which pericyte apoptosis is mediated by macrophages, TGF beta and proapoptotic BIGH3 (TGF beta-induced Gene Human Clone 3) protein. Patients and methods To elucidate this pathway, we assayed human retinal pericyte (HRP) apoptosis by TUNEL assay, BIGH3 mRNA expression by qPCR, and BIGH3 protein expression by western blot analysis. HRP were treated with BIGH3 protein, TGF beta 1 and TGF beta 2 and inhibition assays were carried out by blocking with antibodies against BIGH3. The distribution of BIGH3 and CD68(+) macrophages were compared in a post-mortem donor eye with 7-year history of Type II diabetes and histopathogically confirmed non-proliferative diabetic retinopathy (NPDR). Results TGF beta induced a significant increase in BIGH3 mRNA and protein expression, and HRP apoptosis. BIGH3 treatment showed HRP undergo apoptosis in a dose-dependent manner. At 5 mu g/ ml, BIGH3 induced 3.5-times more apoptosis in HRP than in retinal endothelial cells. TGF beta induced apoptosis was inhibited by blocking with antibodies against BIGH3. In an example of NPDR, BIGH3 accumulated within the walls of the inner retina arterioles. Macrophage infiltrates were frequently associated with these vessels and the inner nuclear layer. Conclusion Together with our previously published results on macrophage-induced retinal endothelial cell apoptosis, the present study supports a novel inflammatory pathway mediated by macrophages and the BIGH3 protein leading to HRP apoptosis. As shown in human post-mortem globes, these observations are clinically relevant, suggesting a new mechanism underlying pericyte dropout during NPDR.