Clinical manifestations of asthma result in part from chronic inflammation that leads to mucous cell metaplasia (MCM), the appearance of mucous cells in peripheral airways that are normally devoid of these cells. In a mouse model of asthma, systemic immunization with ovalbumin (OVA) followed by repeated exposures to OVA aerosols initially induces inflammation and MCM, while prolonged exposures cause increase of IFNgamma levels and resolution of MCM. Instillation of IFNgamma in allergen-exposed mice induces expression of Bax, a pro-apoptotic protein, and accelerates the resolution of MCM by causing apoptosis. Mice deficient in Bax or Stat 1, an obligatory signaling molecule for IFNgamma, do not resolve MCM during prolonged exposures to allergen. IL-13 inhibits IFNgamma-induced Bax expression and apoptosis in mucous cells of mice and in normal human bronchial epithelial cells (HBEs). Our guiding hypothesis is that IL-13 induces MCM and counteracts the role of IFNgamma to activate Stat 1 and induce apoptosis through a Bax-mediated pathway in metaplastic mucous cells.
In Aim 1, we will determine the pathway by which IL-13 inhibits IFNgamma signaling and Bax mediated resolution of MCM. We will determine whether IL-13 directly inhibits Bax expression or activates Stat 3, which is known to cause expression of anti-apoptotic proteins, Bcl-2 and BcI-lxL. Furthermore, we will determine whether IL-13 requires signaling through IL-4Ralpha and Stat 6 to inhibit IFNgamma-induced Bax expression.
In Aim 2, we will determine the pathway by which IFNgamma induces Bax expression in allergen-induced MCM and whether the Bax-mediated pathway is essential to resolve MCM. We will investigate whether IFNgamma signals through IFN(R and Stat 1 to induce Bax in allergen-induced MCM and whether IFN( induces surface expression of IL-13Ralpha2 to inhibit IL-13 signaling through Stat 6. We will determine the requirement of Bax in decreasing MCM by instilling IFNgamma in Bax-deficient mice that have allergen-induced MCM. Our preliminary results with human autopsy tissues and bronchial brushings show that Bax is expressed in mucous cells from non-asthmatics and is absent in asthmatics. Therefore, in Aim 3, we will determine whether inflammatory mediators from asthmatics suppress expression of Bax and enhance mucous cell survival. The difference in the percentages of Bax-expressing cells among subjects with asthma, chronic bronchitis, and controls without respiratory diseases will be investigated using autopsy tissues and bronchial brushings. Furthermore, we will determine the effect of bronchoalveolar lavage fluid in inducing MCM and Bax expression in HBEs. These studies will provide new strategies to reduce mucous cell numbers in asthmatic patients.
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