title: Inhibition of Na/K-ATPase promotes myocardial tumor necrosis factor-alpha protein expression and cardiac dysfunction via calcium/mTOR signaling in endotoxemia.
abstract: Tumor necrosis factor-α (TNF-α) is a major pro-inflammatory cytokine that causes cardiac dysfunction during sepsis. Na/K-ATPase regulates intracellular Ca(2+), which activates mammalian target of rapamycin (mTOR), a regulator of protein synthesis. The aim of this study was to investigate the role of Na/K-ATPase/mTOR signaling in myocardial TNF-α expression during endotoxemia. Results showed that treatment with LPS decreased Na/K-ATPase activity in the myocardium in vivo and in cultured neonatal cardiomyocytes. Inhibition of Na/K-ATPase by ouabain enhanced LPS-induced myocardial TNF-α protein production, but had no effect on TNF-α mRNA expression. More importantly, ouabain further decreased in vivo cardiac function in endotoxemic mice, which was blocked by etanercept, a TNF-α antagonist. LPS-induced reduction in Na/K-ATPase activity was prevented by inhibition of PI3K, Rac1 and NADPH oxidase using LY294002, a dominant-negative Rac1 adenovirus (Ad-Rac1N17) and apocynin, respectively. To assess the role of Rac1 in Ca(2+) handling, Ca(2+) transients in adult cardiomyocytes from cardiomyocyte-specific Rac1 knockout (Rac1(CKO)) and wild-type (WT) mice were determined. LPS increased intracellular Ca(2+) in WT but not in Rac1(CKO) cardiomyocytes. Furthermore, LPS rapidly increased mTOR phosphorylation in cardiomyocytes, which was blocked by Rac1N17 and an inhibitor of calmodulin-dependent protein kinases (CaMKs) KN93, but enhanced by ouabain. Rapamycin, an inhibitor of mTOR suppressed TNF-α protein levels without any significant effect on its mRNA expression or global protein synthesis. In conclusion, myocardial Na/K-ATPase activity is inhibited during endotoxemia via PI3K/Rac1/NADPH oxidase activation. Inhibition of Na/K-ATPase activates Ca(2+)/CaMK/mTOR signaling, which promotes myocardial TNF-α protein production and cardiac dysfunction during endotoxemia.
authors.affiliation: Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, University of Western Ontario, London, ON, N6A 5C1, Canada.
authors.forename: Ting
authors.initials: T
authors.lastname: Zhang
authors.forename: Xiangru
authors.initials: X
authors.lastname: Lu
authors.forename: Jenny
authors.initials: J
authors.lastname: Li
authors.forename: Peter
authors.initials: P
authors.lastname: Chidiac
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Sims
authors.forename: Qingping
authors.initials: Q
authors.lastname: Feng
mesh_terms: D000818:Animals; D015153:Blotting, Western; D020013:Calcium Signaling; D054729:Calcium-Calmodulin-Dependent Protein Kinase Type 1; D019446:Endotoxemia / Q000378:metabolism*; D004789:Enzyme Activation / Q000502:physiology; D004797:Enzyme-Linked Immunosorbent Assay; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009206:Myocardium / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism; D014409:Tumor Necrosis Factor-alpha / Q000096:biosynthesis*
chemical_list: D014409:Tumor Necrosis Factor-alpha; C546843:mTOR protein, mouse; D058570:TOR Serine-Threonine Kinases; D054729:Calcium-Calmodulin-Dependent Protein Kinase Type 1; D000254:Sodium-Potassium-Exchanging ATPase