Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, C56
Potential selective role for CXCR7/CXCL12 signalling in the lung in response to hypoxic stress.
C. Costello1, K. Howell1, S. Doherty2, F. Martin3, J. Belperio4, M. Keane1, S. Gaine2, P. McLoughlin1
1. School of Medicine and Medical Science, Conway Institute, UCD, Dublin, Ireland. 2. Department of Respiratory Medicine, Mater Misericordiae University Hospital, UCD, Dublin, Ireland. 3. School of Biomolecular and Biomedical Science, Conway Institute, UCD, Dublin, Ireland. 4. Division of Pulmonary and Critical Care Medicine, David Geffen School of Medicine, UCLA, Los Angeles, California, USA.
Pulmonary hypoxia is a common complication of chronic lung diseases leading to the development of pulmonary hypertension. The underlying sustained increase in vascular resistance in hypoxia is a response unique to the lung. Thus, we hypothesized that there must be genes whose expression is altered selectively in the lung in response to alveolar hypoxia. Previous experiments using a subtractive array-based strategy to compare gene responses of primary human microvascular endothelial cells from the lung and systemic circulations under normoxic (21% O2) and hypoxic conditions (1% O2) showed that a recently identified CXCL12 receptor, namely CXCR7, was selectively upregulated in response to hypoxia in the lung. This finding was of interest given that in many chronic lung diseases, vascular loss and damage is a key step associated with disease progression and CXCL12 is a potent secreted pro-angiogenic chemokine. To investigate this finding further, adult male C57Bl6 mice were housed in normoxic (inspired oxygen 21%; n=8) or hypoxic conditions (inspired oxygen 10% for 48hrs; n=8). For TaqMan studies, mice were killed by cervical dislocation and organs snap-frozen in liquid nitrogen. For immunohistochemical studies, mice were housed as above (n=10 per group), euthanised (sodium pentobarbitone, 60mg/kg i.p.) and the heart and lungs removed en-bloc. Lungs were fully inflated via the trachea with 4% paraformaldehyde overnight and then embedded in paraffin. CXCL12 levels in plasma from patients with chronic hypertensive lung disease or normal controls were determined by ELISA and immunohistochemistry was carried out on explanted human hypertensive lungs. In vitro experiments examined the ability of CXCL12 to induce wound healing in human microvascular endothelial cells. TaqMan analysis of a panel of murine tissues showed a >2-fold increase in CXCR7 mRNA only in lung tissue. A second CXCL12 receptor, CXCR4, was down-regulated in the lung in the same animal model. Immunohistochemistry demonstrated that CXCR7 protein was significantly increased and CXCR4 was significantly decreased in the hypoxic lung in vivo. In vitro experiments showed that CXCL12 induced wound healing in microvascular endothelial cells. Finally, CXCL12 was significantly elevated in the plasma of patients with chronic hypertensive lung disease and CXCR7 was also highly expressed in explanted human hypertensive lungs. These novel results suggest that signalling via CXCR7/CXCL12 pathway is selectively up-regulated in the lung in response to hypoxia and may play a role in pulmonary vascular disease. Future studies will now decipher the exact role of CXCR4 and CXCR7 receptors in mediating angiogenic responses.
Where applicable, experiments conform with Society ethical requirements