Endoplasmic reticulum stress in diabetic hearts abolishes erythropoietin-induced myocardial protection by impairment of phospho-glycogen synthase kinase-3beta-mediated suppression of mitochondrial permeability transition.
OBJECTIVE
Alteration in endoplasmic reticulum (ER) stress in diabetic hearts and its effect on cytoprotective signaling are unclear. Here, we examine the hypothesis that ER stress in diabetic hearts impairs phospho-glycogen synthase kinase (GSK)-3beta-mediated suppression of mitochondrial permeability transition pore (mPTP) opening, compromising myocardial response to cytoprotective signaling.


METHODS
A rat model of type 2 diabetes (OLETF) and its control (LETO) were treated with tauroursodeoxycholic acid (TUDCA) (100 mg . kg(-1) . day(-1) for 7 days), an ER stress modulator. Infarction was induced by 20-min coronary occlusion and 2-h reperfusion.


RESULTS
Levels of ER chaperones (GRP78 and GRP94) in the myocardium and level of nonphoshopho-GSK-3beta in the mitochondria were significantly higher in OLETF than in LETO rats. TUDCA normalized levels of GRP78 and GRP94 and mitochondrial GSK-3beta in OLETF rats. Administration of erythropoietin (EPO) induced phosphorylation of Akt and GSK-3beta and reduced infarct size (% risk area) from 47.4 +/- 5.2% to 23.9 +/- 3.5% in LETO hearts. However, neither phosphorylation of Akt and GSK-3beta nor infarct size limitation was induced by EPO in OLETF rats. The threshold for mPTP opening was significantly lower in mitochondria from EPO-treated OLETF rats than in those from EPO-treated LETO rats. TUDCA restored responses of GSK-3beta, mPTP opening threshold, and infarct size to EPO receptor activation in OLETF rats. There was a significant correlation between mPTP opening threshold and phospho-GSK-3beta-to-total GSK-3beta ratio in the mitochondrial fraction.


CONCLUSIONS
Disruption of protective signals leading to GSK-3beta phosphorylation and increase in mitochondrial GSK-3beta are dual mechanisms by which increased ER stress inhibits EPO-induced suppression of mPTP opening and cardioprotection in diabetic hearts.
Miki
Takayuki
T
Second Department of Internal Medicine, Sapporo Medical University, School of Medicine, Sapporo, Japan.
Miura
Tetsuji
T
Hotta
Hiroyuki
H
Tanno
Masaya
M
Yano
Toshiyuki
T
Sato
Takahiro
T
Terashima
Yoshiaki
Y
Takada
Akifumi
A
Ishikawa
Satoko
S
Shimamoto
Kazuaki
K
D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D001835:Body Weight; D002118:Calcium / Q000378:metabolism; D003924:Diabetes Mellitus, Type 2 / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D004921:Erythropoietin / Q000378:metabolism / Q000494:pharmacology*; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism*; D000071679:Glycogen Synthase Kinase 3 beta; D015151:Immunoblotting; D007425:Intracellular Membranes / Q000378:metabolism*; D008297:Male; D053078:Membrane Potential, Mitochondrial / Q000187:drug effects*; D008929:Mitochondria, Heart / Q000378:metabolism*; D009203:Myocardial Infarction / Q000378:metabolism* / Q000473:pathology*; D015425:Myocardial Reperfusion; D009206:Myocardium / Q000378:metabolism / Q000473:pathology; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000378:metabolism*; D010539:Permeability; D010766:Phosphorylation; D051381:Rats; D020334:Rats, Inbred OLETF; D013655:Taurochenodeoxycholic Acid
D001786:Blood Glucose; D004921:Erythropoietin; D013655:Taurochenodeoxycholic Acid; C031655:ursodoxicoltaurine; D000071679:Glycogen Synthase Kinase 3 beta; C000605508:Gsk3b protein, rat; D038362:Glycogen Synthase Kinase 3; D002118:Calcium