As a specific microvascular complication of diabetes, diabetic retinopathy (DR) causes severe visual impairment in patients with diabetes. The expression of microRNA-126 (miRNA/miR-126) has previously been found to be significantly decreased in the serum of patients with DR. In the present study, the functions of miR-126 and its mechanisms of action in experimental diabetic retinopathy were examined in rats with streptozotocin (STZ)-induced diabetes and in high glucose (HG)-induced human retinal capillary endothelial cells (HRCECs). In vivo , diabetic rat models were established and the rats were intravitreally injected with lentivirus expressing rno-miR-126 (lenti-miR-126) or negative control (lenti-NC). RT-qPCR was used to determine the miR-126 level in the serum and retina. Paraffin sections and retinal vasculature were used to determine the extent of retinopathy. The protein content of vascular endothelial growth factor (VEGF) and pigment epithelium-derived factor (PEDF) in the retina was used as an auxiliary measurement of retinopathy. Western blot analysis and immunofluorescence staining were used to measure the expression of polo-like kinase 4 (PLK4) in rat retinal tissue. In vitro , the cells were transfected with miR-126 inhibitor or mimic and treated with the PLK4 inhibitor, CFI-400945 fumarate. RT-qPCR and western blot analysis were used to detect the miR-126 level and PLK4 expression. Cell proliferation and migration were measured by EdU and Transwell assays. The diabetic rats were found to exhibit downregulated serum and retinal miR-126 levels compared with the non-diabetic rats. The intravitreal delivery of miR-126 alleviated retinopathy and reduced the diabetes-induced upregulation of PLK4 in retinal tissues. Luciferase reporter assays confirmed that PLK4 mRNA was the target of miR-126. In HG-induced HRCECs, transfection with miR-126 mimic increased the miR-126 level, whereas it downregulated that of its downstream target, PLK4, which was opposite to the effects exerted by the miR-126 inhibitor. Furthermore, miR-126 mimic and CFI-400945 fumarate reduced the HG-induced upregulation of PLK4 expression, as well as cell proliferation and migration. On the whole, the findings of the present study demonstrate that miR-126 reduces experimental diabetic retinopathy and suppresses endothelial cell proliferation and migration by targeting PLK4. Thus, miR-126 and CFI-400945 fumarate may be therapeutic targets for DR.

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