Aims: Stress-inducible heat shock protein 22 (Hsp22) confers protection against ischemia through induction of the inducible isoform of nitric oxide synthase (iNOS). Hsp22 over-expression in vivo significantly stimulates cardiac mitochondrial respiration, whereas Hsp22 deletion in vivo shows a reciprocal effect. It has also been shown in Drosophila that Hsp22 is expressed in the mitochondria that depends on its N-terminal domain. We hypothesized that Hsp22-mediated regulation of mitochondrial function is dependent upon its mitochondrial translocation together with iNOS.
Methods and Results: Adenoviruses harboring either the full coding sequence of Hsp22 (Ad-WT-Hsp22) or a mutant lacking a 20 amino acid putative N-terminal mitochondrial localization sequence (Ad-N20-Hsp22) were generated, and infected in rat neonatal cardiomyocytes. Compared to β-Gal control, Ad-WT-Hsp22 accumulated in mitochondria by 2.5 fold (P<0.05), reduced chelerythrine-induced apoptosis by 60% (P<0.01), and increased oxygen consumption rate by 2-fold (P<0.01). This latter effect was abolished upon addition of the specific iNOS inhibitor, 1400W. Ad-WT-Hsp22 significantly increased global iNOS expression by about 2-fold (P<0.01), and also increased its mitochondrial localization by 2.5 fold vs β-gal (P<0.05). Upon comparable over-expression, the Ad-N20-Hsp22 mutant did not show significant mitochondrial translocation, protection against apoptosis or stimulation of mitochondrial respiration. Although Ad-N20-Hsp22 did increase global iNOS expression by 6-fold it did not significantly promote iNOS mitochondrial translocation.
Conclusion: Translocation of both Hsp22 and iNOS to the mitochondria is necessary for the stimulation of oxidative metabolism and protection against apoptosis.