Characterization of the murine SIRT3 mitochondrial localization sequence and comparison of mitochondrial enrichment and deacetylase activity of long and short SIRT3 isoforms.

SIRT3 is identified as the major mitochondrial deacetylase. Two distinct isoforms of the murine SIRT3 have been identified with the short isoform having no recognizable mitochondrial localization sequence (MLS) and the long isoform having a putative MLS. A recent study questions the mitochondrial deacetylase activity of this short isoform. In contrast, the long isoform has been shown to be predominantly mitochondrial with robust deacetylase activity. In this study, we investigate whether the amino-terminus of the long SIRT3 isoform is a legitimate MLS and evaluate in-situ mitochondrial deacetylase activity of both isoforms. We confirm the presence of long and short isoforms in murine liver and kidney. The long isoform is generated via intra-exon splicing creating a frame-shift to expose a novel upstream translation start site. Mitochondrial localization is significantly more robust following transfection of the long compared with the short isoform. Insertion of this alternatively spliced novel 5' sequence upstream of a GFP-reporter plasmid shows greater than 80% enrichment in mitochondria, confirming this region as a legitimate mitochondrial localization sequence. Despite lower mitochondrial expression of the short isoform, the capacity to deacetylate mitochondrial proteins and to restore mitochondrial respiration is equally robust following transient transfection of either isoform into SIRT3 knockout embryonic fibroblasts. How these alternative transcripts are regulated and whether they modulate distinct targets is unknown. Furthermore, in contrast to exclusive mitochondrial enrichment of endogenous SIRT3, overexpression of both isoforms shows nuclear localization. This overexpression effect, may partially account for previously observed divergent phenotypes attributed to SIRT3.

Associations of perceived stress and state anger with symptom burden and functional status in patients with heart failure.

Background: Psychosocial stress and anger trigger cardiovascular events, but their relationship to heart failure (HF) exacerbations is unclear. We investigated perceived stress and anger associations with HF functional status and symptoms. Methods and Results: In a prospective cohort study (BETRHEART), 144 patients with HF (77% male; 57.5 ± 11.5 years) were evaluated for perceived stress (Perceived Stress Scale; PSS) and state anger (STAXI) at baseline and every 2 weeks for 3 months. Objective functional status (6-min walk test; 6MWT) and health status (Kansas City Cardiomyopathy Questionnaire; KCCQ) were also measured biweekly. Linear mixed model analyses indicated that average PSS and greater than usual increases in PSS were associated with worsened KCCQ scores. Greater than usual increases in PSS were associated with worsened 6MWT. Average anger levels were associated with worsened KCCQ, and increases in anger were associated with worsened 6MWT. Adjusting for PSS, anger associations were no longer statistically significant. Adjusting for anger, PSS associations with KCCQ and 6MWT remained significant. Conclusion: In patients with HF, both perceived stress and anger are associated with poorer functional and health status, but perceived stress is a stronger predictor. Negative effects of anger on HF functional status and health status may partly operate through psychological stress.

SIRT3 is regulated by nutrient excess and modulates hepatic susceptibility to lipotoxicity.

SIRT3 is the primary mitochondrial deacetylase that modulates mitochondrial metabolic and oxidative stress regulatory pathways. However, its role in response to nutrient excess remains unknown. Thus, we investigated SIRT3 regulation of the electron transfer chain and evaluated the role of SIRT3 in hepatic lipotoxic stress. SIRT3-depleted HepG2 cells show diffuse disruption in mitochondrial electron transfer chain functioning, a concurrent reduction in the mitochondrial membrane potential, and excess basal reactive oxygen species levels. As this phenotype may predispose to increased lipotoxic hepatic susceptibility we evaluated the expression of SIRT3 in murine liver after chronic high-fat feeding. In this nutrient-excess model SIRT3 transcript and protein levels are downregulated in parallel with increased hepatic fat storage and oxidative stress. Palmitate was used to investigate lipotoxic susceptibility in SIRT3 knockout mouse primary hepatocytes and SIRT3-siRNA-transfected HepG2 cells. Under SIRT3-deficient conditions palmitate enhances reactive oxygen species and increases hepatocyte death. Reconstitution of SIRT3 levels and/or treatment with N-acetylcysteine ameliorates these adverse effects. In conclusion SIRT3 functions to ameliorate hepatic lipotoxicity, although paradoxically, exposure to high fat downregulates this adaptive program in the liver. This SIRT3-dependent lipotoxic susceptibility is possibly modulated, in part, by SIRT3-mediated control of electron transfer chain flux.