Hypoxia induces pulmonary fibroblast proliferation through NFAT signaling

Abstract Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive and typically fatal lung disease with a very low survival rate. Excess accumulation of fibroblasts, myofibroblasts and extracellular matrix creates hypoxic conditions within the lungs, causing asphyxiation. Hypoxia is, therefore, one of the prominent features of IPF. However, there have been few studies concerning the effects of hypoxia on pulmonary fibroblasts. In this study, we investigated the molecular mechanisms of hypoxia-induced lung fibroblast proliferation. Hypoxia increased the proliferation of normal human pulmonary fibroblasts and IPF fibroblasts after exposure for 3–6 days. Cell cycle analysis demonstrated that hypoxia promoted the G1/S phase transition. Hypoxia downregulated cyclin D1 and A2 levels, while it upregulated cyclin E1 protein levels. However, hypoxia had no effect on the protein expression levels of cyclin-dependent kinase 2, 4, and 6. Chemical inhibition of hypoxia-inducible factor (HIF)-2 reduced hypoxia-induced fibroblast proliferation. Moreover, silencing of Nuclear Factor Activated T cell (NFAT) c2 attenuated the hypoxia-mediated fibroblasts proliferation. Hypoxia also induced the nuclear translocation of NFATc2, as determined by immunofluorescence staining. NFAT reporter assays showed that hypoxia-induced NFAT signaling activation is dependent on HIF-2, but not HIF-1. Furthermore, the inhibition or silencing of HIF-2, but not HIF-1, reduced the hypoxia-mediated NFATc2 nuclear translocation. Our studies suggest that hypoxia induces the proliferation of human pulmonary fibroblasts through NFAT signaling and HIF-2

Mechanotransduction and Vascular Resistance

International audienceMechanotransduction is the process by which any cell transduces (converts) a mechanical signal into chemical cues. The vessel wall is permanently sheared by the moving blood particles as well as stretched and compressed by the pressure applied by the blood. Multiple types of mechanical stress fields are associated with flow patterns and unsteadiness.Mechanosensing occurs locally at the plasma membrane. It relies on detection of local changes in protein conformation that lead to ion channel opening, protein unfolding, modified enzyme kinetics, and variations in molecular interactions following exposure of buried binding site or, conversely, hiding them.Mechanotransduction initiates several signaling pathways. Multiple mediators include: At the cell surface, G-protein-coupled and protein tyrosine kinase receptors, ion channels, enzymes, adhesion molecules, and specialized plasmalemmal nanodomains At the cell cortex, the cortical actin network that regulates the cell-surface mechanics and signaling adaptors and effectors (e.g., small monomeric guanosine triphosphatases and heterotrimeric guanine nucleotide-binding proteins, kinases, phosphatases, and ubiquitins, among others) In the cytosol, enzymes, scaffolds, carriers such as endosomes, calcium concentration, and transcription factors In the nucleus, nuclear pore carriers, enzymes, and the transcriptional and translational machineryMechanotransduction by vascular cells regulate the contraction–relaxation state of vascular smooth myocytes, thereby regulating locally and quickly the size of the vascular lumen, that is, the local vascular resistance to blood flow. Once experiencing an unusual mechanical stress, vascular smooth myocytes react by contracting or relaxing according to the magnitude of the mechanical stress, the value of which rises above or falls below the range in which it fluctuates in normal conditions. Moreover, they receive chemical and electrochemical signals from endotheliocytes, themselves sensing the wall shear stress at their wetted (luminal) surface.Mechanotransduction thus regulates locally blood flow more rapidly than the endocrine regulation by remote tissues and even than that of the nervous system, which transmits signals very rapidly via afferent nerves and, after processing in the centers of the spinal cord and brain, efferent nerves