Our lab is interested in the molecular mechanisms of Oxygen Sensing, and the importance of this process for Embryonic Development, Tissue Responses to Hypoxia, and Tumor Angiogenesis. We are testing the hypothesis that the mitochondria play a central role in detecting cellular oxygenation, and signal the onset of hypoxia by releasing reactive oxygen species (ROS). These intracellular oxidant signals trigger downstream signal transduction pathways responsible for the transcriptional and post-translational responses of the cell. Transcriptional activation of genes by Hypoxia-Inducible Factor-1 confers protection against more severe hypoxia by augmenting the expression of glycolytic enzymes and membrane glucose transporters. Other genes activated by HIF tend to augment tissue oxygen supply by increasing the release of vascular growth factors such as VEGF, erythropoietin, and vasoactive molecules that augment local blood flow. Current experiments are aimed at improving our understanding of how oxygen interacts with the mitochondrial electron transport chain to amplify ROS production, and at clarifying the targets that these reactive oxygen molecules act on to stabilize HIF and activate transcription.
In specific tissues, oxygen sensing is essential for normal function, but it can also contribute to disease pathogenesis. For example, during mammalian development, the lung tissue is hypoxic and blood flow is restricted in the pulmonary circulation in order to prevent escape of oxygen from the pulmonary capillaries to amniotic fluid. At birth, inflation of the lung with air causes an increase in lung oxygen levels, which triggers relaxation of pulmonary arteries. In Persistent Pulmonary Hypertension of the Newborn, failure of the pulmonary circulation to dilate results in elevated pulmonary arterial pressures and significant lung gas exchange dysfunction. Studies in our laboratory are testing the hypothesis that pulmonary vascular cells sense O2 at the mitochondria, and that ROS released from those organelles trigger an increase in cytosolic calcium, which causes smooth muscle cell contraction. In adult patients with hypoxic lung disease, similar activation of hypoxic vasoconstriction can lead to chronic pulmonary hypertension, which can progress to right heart failure. A fuller understanding of the mechanisms of oxygen sensing in health and disease may lead to insights into therapeutic inhibition of this response in disease states.
In solid tumors, consumption of oxygen by highly metabolic tumor cells leads to hypoxia and threatens glucose supplies. Hypoxic tumor cells retain their oxygen sensing capacity, and turn on expression of HIF-dependent genes, leading to tumor angiogenesis and increased blood supply, which permits further growth. We are currently exploring the hypothesis that the mitochondrial oxygen sensor is required for this response, and are pursuing genetic models to test this idea. A better understanding of how tumor cells detect hypoxia could lead to the discovery of therapeutic approaches that would prevent detection of hypoxia, and thereby prevent tumor progression.
Publications:
Guzy, R.D., Hoyos, B., Robin, E. Chen, H., Liu, L., Mansfield, K.D., Simon, M.C., Hammerling, U. and Schumacker, P.T. Mitochondrial Complex III is required for hypoxia-induced ROS production and HIF-1á stabilization. Cell Metabolism 1:401-408, 2005.
Mansfield, K.D., Guzy, R.D., Pan, Y., Young, R.M., Schumacker, P.T. and Simon, M.C. Cytochrome C is required for cellular oxygen sensing and hypoxic HIF activation. Cell Metabolism 1:393-400, 2005.
Waypa, G.B. and Schumacker, P.T. Hypoxic pulmonary vasoconstriction: redox events in oxygen sensing. J. Appl. Physiol. 98:404-414, 2005.
