Abstract
Purpose: :
Excessive apoptosis of retinal pericytes, a key initiator of diabetic retinopathy (DR), is observed in diabetic state with hyperglycemia and not in insulin resistant states although both conditions exhibit increases in inflammatory and oxidative markers. We have reported that inhibition of anti-apoptotic activity of survival factors such as platelet derived growth factor (PDGF) by tyrosine phosphatase SHP-1 is a key mechanism by which hyperglycemia is causing pericyte apoptosis in DR. Since insulin deficiency or insulin resistance is commonly present in DR, we determined whether hyperglycemia can inhibit insulin signaling and anti-apoptotic actions in pericytes.
Methods: :
Primary cultures of bovine retinal pericytes (BRP) were isolated and cultured in DMEM plus 15% serum with either normal (5.6mM) or high (22.6mM) glucose levels for 3 days with and without the presence of insulin or PDGF. Pericyte apoptosis was assessed by DNA fragmentation by quantify cytosolic oligonucleosome-bound DNA. Zuker lean, fatty and diabetic fatty rats were used for in vivo studies.
Results: :
Exposure of BRPs to high vs. normal glucose levels increased apoptosis by 2 folds (p<0.01). Insulin (100nM) decreased apoptosis by 40% and 70% in normal and high glucose media, respectively (p<0.05). In contrast, PDGF decreased apoptosis only in normal glucose but not in high glucose media. Insulin signaling as measured by activation of Akt (p-Akt) and MAPK (p-MAPK) in BRPs showed increase of 3 to 4 fold in normal and high glucose media equally. In contrast, PDGF’s induction of Akt activation was significantly inhibited by high glucose condition. In vivo studies showed that intravitreous injection of insulin activated pAkt and p-MAPK by 2 fold, significantly in wild type, insulin resistant and diabetic rodents equally whereas PGDF’s action on p-Akt activation was significantly blunted in retina of diabetic rodents.
Conclusions: :
These data show, unlike other survival factors such as PDGF, insulin’s signaling and action to prevent BRP apoptosis induced by oxidant or hyperglycemia is not inhibited by diabetes or hyperglycemia. Selective preservation of insulin’s anti-apoptotic actions on retinal pericytes could provide a mechanism for the lack of retinal pathology in obesity and insulin resistant states. Activation of insulin receptor in the retinal pericytes could be a new therapeutic target to prevent DR.
Keywords: diabetic retinopathy • diabetes • retina