Mitochondrial mRNA translation initiation contributes to oxidative metabolism in the myocardia of aged, obese mice

Abstract Being both advanced in age and obese each contribute to cardiac hypertrophy in a unique manner. Electron transport complexes I and IV are implicated in deficient electron transport during cardiomyopathies and contain the majority of protein subunits that are transcribed and translated by machinery localized within the mitochondria. Purpose To assess myocardial mt-mRNA translation factors in relation to mitochondrial content and mtDNA-encoded protein using a mouse model of aged obesity and to test the relationship of mt-mRNA translation initiation factor 2 (mtIF2) to oxidative capacity and the cellular oxidation-reduction (redox) state in cardiomyocytes. Methods Male C56BL/6 J mice fed lean or high fat diet were aged to either ~3 months or ~22 months, the heart was excised and analyzed using immunoblot and qPCR to assess differences in mitochondrial mRNA translation machinery. Using H9c2 cardiomyocytes, mtIF2 was knocked-down and oxidative metabolic characteristics assessed including oxidation/reduction state, bioenergetic flux, and hypoxic resistance was tested. Results Aged, obese mouse hearts were ~40% larger than young, lean controls and contained ~50% less mtIF2 protein alongside ~25–50% lower content of Cytb, a protein encoded by mtDNA. Reducing the level of mtIF2 by shRNA is associated with ~15–20% lower content of OXPHOS complex I and IV, ~30% lower optical redox ratio, ~40% oxygen reserve capacity, and ~20% less cell survival following hypoxia. Conclusion We present evidence of altered mt-mRNA translation during cardiac hypertrophy in aged obesity. We build on these results by demonstrating the necessity of mtIF2 in maintaining oxidative characteristics of cardiac muscle cells.