Retinal plasminogen and plasmin content in experimental diabetic retinopathy and the effect of pharmacological blockade of cellular protein kinases

Background. Diabetic retinopathy (DR) is accompanied by dysregulation of proteolysis, in particular activation of the plasmin system, which contributes to neuroinflammation and neurodegeneration of the retina. The study of the retinal content of plasminogen and plasmin and the effect of pharmacological blockade of cellular protein kinases allows us to clarify the role of these components in the pathogenesis of DR and to substantiate new approaches to its pathogenetic treatment. Objective: to determine the content of plasminogen and plasmin in the retinal tissue and the effect of sorafenib, a cellular protein kinase inhibitor, on it in experimental DR. Materials and methods. DR was modeled in male Wistar rats by a single injection of streptozotocin (50 mg/kg; Sigma-Aldrich Co, China). Rats were divided into 4 groups: control; with the administration of insulin (30 U; Novo Nordisk A/S, Bagsvaerd, Denmark), a protein kinase inhibitor sorafenib (Cipla, India) at a dose of 50 mg/kg, and with the administration of insulin and sorafenib. Immunoblotting was performed using monoclonal antibodies against plasminogen and plasmin (Invitrogen, USA). Results. With the development of experimental DR in the absence of treatment, there was a significant accumulation of plasminogen and plasmin in the retina: after 28 days and 2 months, by 18.9 and 33.1 and by 10.5 and 12.7 times, respectively (p < 0.001). The administration of insulin and sorafenib both separately and together did not affect an increase in plasminogen content. An increase in plasmin level did not change with the separate action of the sorafenib and was inhibited by 1.4–1.5 times (p < 0.05) with insulin and the combination of insulin with sorafenib. The expression of caspase-3 in the control group significantly increased in ganglion cells, processes of astrocytes and Müller cells; ganglion cell loss and signs of their degeneration were noted. This was prevented by the administration of insulin and the combination of insulin with sorafenib. Conclusions. In DR, there was a significant accumulation of plasminogen and plasmin in the retina, which indicates the activation of the plasmin system. Insulin and its combination with sorafenib partially reduce plasmin level and caspase-3 expression, preventing neurodegene­ration. This confirms the promising potential of pharmacological modulation of cellular protein kinases for retinal protection in DR.