Abstract 844: Modification Of Akt/GSK-3β Signaling And Mitochondrial GSK-3β By ER Stress Underlies Failure Of Erythropoietin To Protect Diabetic Hearts.
Background Recent studies using healthy animals have shown that erythropoietin (Epo) limits infarct size and suggest that suppression of glycogen synthase kinase (GSK)-3β activity by its Ser-9-phosphorylation increases myocardial tolerance against infarction. Here, we examined the hypothesis that protection by Epo is impaired in diabetic hearts by increased endoplasmic reticulum (ER) stress.
Methods A rat model of type 2 diabetes (25–28-week-old OLETF) and its control (LETO) underwent 20-min coronary occlusion/2-hr reperfusion. Infarction was expressed as % of risk area (%IS/RA). Sodium tauroursodeoxycholic acid (TUDCA, 100 mg/kg/day) was administered for 7 days to reduce ER stress. Using separate groups of rats, tissues were sampled before and after Epo treatment or at 5 min after reperfusion for immunoblotting.
Results In LETO, Epo (5,000 U/kg, i.v.) increased phosphorylation levels of Akt, ERK and GSK-3β and reduced %IS/RA from 47.4±5.2% to 23.9±3.5%. However, in OLETF, Epo neither induced phosphorylation of Akt and GSK-3β nor reduced %IS/RA (50.7±5.5% vs. 54.0±4.4%), whereas ERK was phosphorylated by EPO. In OLETF, GSK-3β protein and the ratio of mitochondrial GSK-3β to cytosolic GSK-3β were increased by 10% and 80%, respectively, compared with those in LETO. Furthermore, administration of SB216763 (1.2 mg/kg, i.v.), a GSK-3β inhibitor, to mimic the effect of GSK-3β phosphorylation reduced %IS/RA both in LETO and OLETF (28.3±3.7% and 27.9±4.9%). Levels of ER chaperones (GRP78 and GRP94) in the myocardium were higher in OLETF than in LETO, and TUDCA significantly suppressed this change in levels of ER chaperones. TUDCA normalized the ratio of mitochondrial/cytosolic GSK-3β and restored both response of GSK-3β phosphorylation to EPO receptor activation and myocardial protection by EPO (%IS/RA = 17.6±6.1%) in OLETF.
Conclusion Disruption of cellular signaling upstream of GSK-3β and increased constitutively active mitochondrial GSK-3β are induced by increased ER stress in the diabetic myocardium and are responsible for failure of EPO to protect diabetic hearts against infarction.