Control of mitochondrial metabolism by metabolic stress and hypoxia

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An increased work load of an organism requires adjustments of metabolism. At the cellular Level this is achieved by inducing mitochondrial biogenesis to increase the capacity to produce ATP. However, the increased oxygen consumption might result in hypoxia when the oxygen demand is not matched by an appropriate supply. This is likely to happen in patients with ischemic diseases. Cellular hypoxia might increase the formation of oxygen radicals (ROS) leading to cell damage. Thus, a major adaptation to hypoxia is a decrease in mitochondrial activity in order to prevent the damage. Cells with increased metabolic demand but inadequate oxygen supply (typical clinical situations are e.g. an ischemic heart that has to pump against an increased vascular resistance; an edematous lung trying to remove alveolar edema fluid by ion pumping) are therefore exposed to opposing stimuli, one that increases mitochondrial activity, and a second trying to prevent that. Clinically (and shown by our preliminary results), hypoxia seems to dominate over increased metabolic capacity as indicated by the hibernating heart and decreased myocardial contractility. However, the mechanisms overriding the signals increasing mitochondrial biogenesis in hypoxia are poorly understood. Knowledge of these mechanisms provides a better understanding of a clinical situation of a patient and might result in an improved treatment. Therefore, in this project we want to test the hypothesis that hypoxia prevents an increase in mitochondrial activity in order to protect cells from potential damage by ROS, and we want to elucidate the involved signaling pathways.Experiments will be performed on H10 cells, a model cell used for studying cardiomyocyte signaling pathways. Cells will be exposed to AICAR, which inside the cell mimics an increased concentration of AMP, which is indicative of an increased ATP-turnover, i.e. an increased ATP demand. Cells will also be exposed to hypoxia simulating the impairment of oxygen supply as one significant aspect of ischemia. Major readouts will be mitochondrial oxygen consumption, mitochondrial membrane potential, and ROS production. Measurement of the AMP-kinase - PGC1 - axis and major proteins of the electron transfer chain by RT-PCR and Western blot of will indicate altered mitochondrial biogenesis. Hypoxia-inducible factor (HIF) induced decrease mechanisms studied will include BNIP3 dependent mitophagy, pyruvate dehydrogenase (PDH)-kinase (PDK) dependent inactivation of PDH. HIF-independent mechanisms will be studied after silencing HIF-1alpha with shRNA introduced with an available adenoviral transfection system. All required techniques are established in our laboratory.