Protein kinase C-mediated insulin receptor phosphorylation in diabetic rat retina

Diabetic retinopathy (DR) involves a proliferation of vascular endothelial cells and loss of pericytes. There is a link among the action of protein kinase C (PKC) and insulin signaling. Thus, we investigated the differences between these cells in insulin receptor (IR) phosphorylation in DR.

Retinas were removed from streptozotocin-induced diabetic or healthy rats, and IR expression levels were compared by immunoblot and immunohistochemistry. In vitro assays also were performed in order to determine the expressions of phosphorylated IR in both cells cultured under 5.5 or 25 mM glucose by immunoblot. Cell viability was determined in both cells cultured under different concentrations of phorbol myristate acetate (PMA), a PKC activator. To determine the involvement of the PI3 kinase pathway of IR, PMA with or without wortmannin-induced changes in Akt was also analyzed.

Immunoreactivity to the IR was decreased in diabetic retina. High glucose (25 mM) increased phosphorylated IR levels in endothelial cells but not in pericytes. PMA (1 nM or higher) induced death of pericytes, while endothelial cells were increased. PMA increased phosphorylated Akt in endothelial cells and decreased in pericytes. Wortmannin suppressed the PMA-induced phosphorylation of Akt in endothelial cells.

The different responses to 25 mM glucose and PMA were observed between retinal endothelial cells and pericytes. Thus, IR phosphorylation is likely important for retinal vascular cells to survive in diabetic retina.