Abstract 960: Endoplasmic Reticulum Stress Impairs Murine Cardiomyocyte Contractile Function via an Akt-Dependent Mechanism
Endoplasmic reticulum (ER) stress that reduces ER protein folding activates the unfolded protein response (UPR) and cell death. Recent studies have indicated that ER stress contributes to several diseases such as neurodegenerative disorders, diabetes and ischemia reperfusion-induced heart damage. However, whether ER stress directly affects cardiac contractile function has not been elucidated. The aim of the present study was to examine the effect of ER stress on cardiomyocyte contractile function and intracellular calcium homeostasis, and the signaling pathway(s) involved. Murine cardiomyocytes from adult male mice were exposed to ER stress inducers tunicamycin (an inhibitor of N-glycosylation in the ER, 3 mu gram/mL) or thapsigargin (an inhibitor of ER-specific calcium ATPase, 3 mu mol/L) for 4 – 6 hrs. ER stress was confirmed by IRE-1 phosphorylation. ER stress-activated unfolded protein response (UPR) was detected by the induction of Grp78 and phosphorylation of eukaryotic initiation factor 2-alpha (eIF2alpha). Cardiomyocyte contractile function and intracellular calcium homeostasis were evaluated including peak shortening (PS), time-to-PS, time-to-relengthening (TR90), maximal velocity of shortening /relengthening (+/− dL/dt), intracellular Ca2+ rise (rise in fura-2 fluorescent intensity, FFI), sarcoplasmic reticulum (SR) Ca2+ release and intracellular Ca2+ decay rate. Both ER stress inducers depressed PS, FFI, SR Ca2+ release, prolonged intracellular Ca2+ decay and TR90, without affecting other parameters. Treatment with tunicamycin or thapsigargin also caused dephosphorylation of Akt in cardiomyocytes. To explore if Akt dephosphorylation is a permissive step in altered cardiomyocyte mechanical function, murine cardiomyocytes from adult mice with cardiac-specific overexpression of active mutant of Akt (Myr-Akt) were used to repeat the tunicamycin or thapsigargin exposure study. Interestingly, cardiomyocytes from Myr-Akt mice were resistant to ER stress-induced mechanical defects, despite the presence of ER stress. These data suggest that ER stress may directly compromise cardiomyocyte contractile function and intracellular calcium homeostasis, possibly through an Akt-dependent mechanism.