Most of the mitochondrial proteome originates from nuclear genes and is transported into the mitochondria after synthesis in the cytosol. Complex machineries which maintain the specificity of protein import and sorting include the TIM23 translocase responsible for the transfer of precursor proteins into the matrix, and the mitochondrial intermembrane space import and assembly (MIA) machinery required for the biogenesis of intermembrane space proteins. Dysfunction of mitochondrial protein sorting pathways results in diminishing specific substrate proteins, followed by systemic pathology of the organelle and organismal death. The cellular responses caused by accumulation of mitochondrial precursor proteins in the cytosol are mainly unknown. Here we present a comprehensive picture of the changes in the cellular transcriptome and proteome in response to a mitochondrial import defect and precursor over-accumulation stress. Pathways were identified that protect the cell against mitochondrial biogenesis defects by inhibiting protein synthesis and by activation of the proteasome, a major machine for cellular protein clearance. Proteasomal activity is modulated in proportion to the quantity of mislocalized mitochondrial precursor proteins in the cytosol. We propose that this type of unfolded protein response activated by mistargeting of proteins (UPRam) is beneficial for the cells. UPRam provides a means for buffering the consequences of physiological slowdown in mitochondrial protein import and for counteracting pathologies that are caused or contributed by mitochondrial dysfunction.
Background: Endurance training induces physiological cardiac hypertrophy and elevates adipose tissue lipolysis. Results: Adipose-specific adipose triglyceride lipase (Atgl)-knock-out mice exhibit attenuated exercise-induced cardiac hypertrophy likely mediated by the lack of C16:1n7 palmitoleate actions on the heart. Conclusion: Atgl-mediated adipose lipolysis regulates physiological cardiac hypertrophy. Significance: Adipose-derived lipokines may serve as important molecular mediators of cardiac physiology and pathology.
Carnitine β‐hydroxy‐γ‐(trimethylammonio)butyrate – a compound necessary in the peripheral tissues for a transfer of fatty acids for their oxidation within the cell, accumulates in the brain despite low β‐oxidation in this organ. In order to enter the brain, carnitine has to cross the blood–brain barrier formed by capillary endothelial cells which are in close interaction with astrocytes. Previous studies, demonstrating expression of mRNA coding two carnitine transporters – organic cation/carnitine transporter 2 (OCTN2) and B0,+ in endothelial cells, did not give any information on carnitine transporters polarity in endothelium. Therefore more detailed experiments were performed on expression and localization of a high affinity carnitine transporter OCTN2 in an in vitro model of the blood–brain barrier by real‐time PCR, western blot analysis, and immunocytochemistry. The amount of mRNA was comparable in endothelial cells and kidney, when referred to house‐keeping genes, it was, however, significantly lower in astrocytes. Polarity of OCTN2 localization was further studied in an in vitro model of the blood–brain barrier with use of anti‐OCTN2 antibodies. Z‐axis analysis of the confocal microscope pictures of endothelial cells, with anti‐P‐glycoprotein antibodies as the marker of apical membrane, showed OCTN2 localization at the basolateral membrane and in the cytoplasmic region in the vicinity of nuclei. Localization of OCTN2 suggest that carnitine can be also transported from the brain, playing an important role in removal of certain acyl esters.
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