RETRACTED: Thapsigargin triggers cardiac contractile dysfunction via NADPH oxidase-mediated mitochondrial dysfunction: Role of Akt dephosphorylation

Abstract

ER stress triggers myocardial contractile dysfunction although the underlying mechanism is still elusive. Given that NADPH oxidase was recently implicated in ER stress-induced tissue injury, this study was designed to examine the role of NADPH oxidase in the ER stress-induced cardiac mechanical defects and the impact of Akt activation on ER stress-induced cardiac anomalies. WT and transgenic mice with cardiac-specific overexpression of active mutant of Akt (MyAkt) were subjected to the ER stress inducer thapsigargin (1 and 3 mg/kg, i.p. for 48 hr). Thapsigargin compromised echocardiographic parameters including elevated LVESD and reduced fractional shortening, suppressed cardiomyocyte contractile function, intracellular Ca2+ handling and cell survival, along with enhanced carbonyl formation, apoptosis, superoxide production, NADPH oxidase expression and mitochondrial damage. Interestingly, these anomalies were attenuated or mitigated by chronic Akt activation. Treatment with thapsigargin also dephosphorylated Akt and its downstream signal GSK3β (leading to activation of GSK3β), the effect of which was abrogated in MyAkt hearts. Knockdown of cytosolic subunit of NADPH oxidase p47phox using siRNA abrogated thapsigargin-induced apoptosis and cell death in H9C2 myoblasts. In vitro exposure of thapsigargin induced murine cardiomyocyte dysfunction reminiscent of the in vivo setting, the effects of which were ablated by the NADPH oxidase inhibitor apocynin and the mitochondrial Ca2+ uptake inhibitor Ru360. In addition, apocynin abrogated thapsigargin-induced loss of mitochondrial membrane potential and permeation pore opening, similar to those induced by chronic Akt activation. In summary, these data suggest that ER stress interrupts cardiac contractile and intracellular Ca2+ homeostasis, cell survival and mitochondrial integrity through an Akt dephosphorylation and NADPH oxidase-dependent mechanism.

Aldehyde Dehydrogenase-2 Deficiency Aggravates Cardiac Dysfunction Elicited by Endoplasmic Reticulum Stress Induction

“…Previous studies also reported that 4-HNE triggered suppression of Akt activity (27), compromised cardiomyocytes mechanical function and protein damage (25). ER stress induction was found to promote carbonyl formation (14,17), whereas carbonyl formation was also upregulated in response to 4-HNE exposure, which was attenuated by ALDH2 overexpression (25). Taken together, the above evidence suggests that the role of ALDH2 against ER stress, at least in part, was attributed to the detoxification of 4-HNE with ALDH2.…”

“…However, the role of Akt signaling in regulation of ER stress is controversial. Activation of Akt was found to rescue the cardiac mechanical function defect induced by ER stress, whereas it did not affect the protein levels of ER stress markers (14,17). Recent studies demonstrated that inhibition of ER stress by intermedin was blocked by the PI3K inhibitor (29).…”

“…Given that NADPH oxidase was implicated in ER-induced loss of cell survival (17), the role of NADPH oxidase (p47 phox subunit) in the cytoprotection of ALDH2 after ER stress induction was evaluated. The protein levels of the p47 phox subunit was significantly increased by tunicamycin and augmented by ALDH2 knockdown ( Figure 3E).…”

“…subunit was involved in apoptosis induced by thapsigargin (17), it is plausible to speculate that p47 phox subunit may be the downstream of Akt in the cytoprotection of ALDH2 (Figure 7).…”

Nox NADPH Oxidases and the Endoplasmic Reticulum

“…Such results should be interpreted bearing in mind the complex pathways connecting Nox to ER stress-dependent target responses, which can be indirect and involve collateral signaling, such as inflammatory mediators (193), metabolic adaptations, cell adhesion processes, and many others. A few examples can be listed regarding these studies: Nox2 in atherosclerosis (104); Nox2 in neurodegenerative diseases (16); p47phox in impaired cardiac contractility (196); p47phox in cytokine-induced inflammatory angiogenesis and autophagy (141); Nox4 in glycation product-induced fibroblast apoptosis (108); Rac1 in saturated fat-induced JNK activation in hepatocytes (149); Rac1 in diabetes-induced cardiac hypertrophy, collagen deposition, and contractility (103); Rac1 in high glucose-induced ER stress in culture cardiomyocytes (103); Nox activity in vascular cell calcification (105). ER stress plays a prominent role in Toxoplasma gondii-induced trophoblast apoptosis, whereas an increase in Nox1 expression and oxidant generation preceded UPR signaling and downstream activation of caspase 12, and UPR-related apoptotic signaling was abolished by N-acetylcysteine (183).…”