Asthma is associated with airway obstruction, airway inflammation, and bronchoconstriction. Immune cell infiltration and their production of inflammatory cytokines, such as interleukin (IL)–13, IL-17A, and tumor necrosis factor–α (TNF-α), increase airway smooth muscle contractility by increasing the calcium sensitivity of a pathway involving the guanosine triphosphatase RhoA, the kinase ROCK2, myosin light chain phosphatase, and myosin light chain. Mfge8 is an integrin-binding and phosphatidylserine-binding protein that functions in the clearance of apoptotic cells. Comparison of wild-type and mfge8-deficient mice in a mouse model of asthma (induced by ovalbumin) revealed that the mfge8-deficient mice had increased pulmonary resistance, a measure of airway hyperresponsiveness, but the abundance of infiltrated immune cells, the types of cells present, and the abundance of the inflammatory cytokines were the same between the two genotypes of mice. Isolated tracheal rings from the ovalbumin-exposed, mfge8-deficient mice exhibited increased contractility when exposed to two different calcium-increasing stimuli. In the absence of ovalbumin exposure, tracheal rings from the mfge8-deficient mice also exhibited increased contractility in response to IL-13, TNF-α, or IL-17. Addition of recombinant Mfge8 (rMfge8) rescued the enhanced contraction exhibited by the mfge8-deficient mouse tracheal rings exposed to IL-13, and application of rMfge8 to wild-type mouse tracheal rings prevented the IL-13–mediated increase in contractility. Experiments with various deletion or mutation constructs indicated that the integrin-binding domain was necessary for Mfge8 to inhibit tracheal ring contraction. Western blotting analysis of tracheal rings from mfge8-deficient mice exposed to IL-13 showed that, compared with wild-type mouse tracheal rings, there was increased phosphorylation of myosin light chain and myosin light chain phosphatase, increased nuclear accumulation of nuclear factor κB (NF-κB), and increased abundance of RhoA and its target kinase ROCK2. RhoA activity assays revealed increased IL-13–induced RhoA activity in the tracheal rings from the mfge8-deficient mice, which was blocked by rMfge8. Bronchial rings from human lungs also exhibited reduced IL-13–mediated contraction when the human homolog of Mfge8 (lactadherin) was added. Endobronchial biopsy samples from a small number of asthma patients and normal donors showed that lactadherin was less abundant in the samples from the asthmatic patients, suggesting that this pathway is relevant for regulating airway hyperresponsiveness and contractility in patients.
M. Kudo, S. M. A. K. Soltani, S. A. Sakuma, W. McKleroy, T.-H. Lee, P. G. Woodruff, J. W. Lee, K. Huang, C. Chen, M. Arjomandi, X. Huang, K. Atabai, Mgfe8 suppresses airway hyperresponsiveness in asthma by regulating smooth muscle contraction. Proc. Natl. Acad. Sci. U.S.A. 110, 660–665 (2013).[Abstract] [Full Text]