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Gq/11 signaling maintains healthy lungs
Loss of signaling by the cytokine transforming growth factor–β (TGFβ) in mice results in emphysema-like symptoms, whereas excessive TGFβ signaling results in pulmonary fibrosis and ventilator-associated lung injury. G proteins of the Gq/11 and G12/13 families mediate the integrin-dependent activation and release of latent TGFβ from the epithelial cells. John et al. found that mice deficient in Gq/11, but not those deficient in G12/13, in lung epithelial cells had defective TGFβ activation and emphysema-like symptoms. In addition, the Gq/11-deficient mice had lung inflammation associated with increased amounts of the cytokine IL-33. However, the mice were protected from ventilator-induced injury. Together, these data suggest that Gq/11 signaling is required for optimal TGFβ activation in the lung and the prevention of inflammation.
Abstract
Heterotrimeric guanine nucleotide–binding protein (G protein) signaling links hundreds of G protein–coupled receptors with four G protein signaling pathways. Two of these, one mediated by Gq and G11 (Gq/11) and the other by G12 and G13 (G12/13), are implicated in the force-dependent activation of transforming growth factor–β (TGFβ) in lung epithelial cells. Reduced TGFβ activation in alveolar cells leads to emphysema, whereas enhanced TGFβ activation promotes acute lung injury and idiopathic pulmonary fibrosis. Therefore, precise control of alveolar TGFβ activation is essential for alveolar homeostasis. We investigated the involvement of the Gq/11 and G12/13 pathways in epithelial cells in generating active TGFβ and regulating alveolar inflammation. Mice deficient in both Gαq and Gα11 developed inflammation that was primarily caused by alternatively activated (M2-polarized) macrophages, enhanced matrix metalloproteinase 12 (MMP12) production, and age-related alveolar airspace enlargement consistent with emphysema. Mice with impaired Gq/11 signaling had reduced stretch-mediated generation of TGFβ by epithelial cells and enhanced macrophage MMP12 synthesis but were protected from the effects of ventilator-induced lung injury. Furthermore, synthesis of the cytokine interleukin-33 (IL-33) was increased in these alveolar epithelial cells, resulting in the M2-type polarization of alveolar macrophages independently of the effect on TGFβ. Our results suggest that alveolar Gq/11 signaling maintains alveolar homeostasis and likely independently increases TGFβ activation in response to the mechanical stress of the epithelium and decreases epithelial IL-33 synthesis. Together, these findings suggest that disruption of Gq/11 signaling promotes inflammatory emphysema but protects against mechanically induced lung injury.
