Allergic asthma is a chronic inflammatory disease that directly affects large American population with an estimated total annual healthcare cost of 20 billion dollars. The heterogeneous nature driven by different underlying disease processes makes it challenging to treat and control the asthma symptoms. Therefore, in addition to having the effective disease prevention and management strategies, the development of novel and phenotype-specific therapeutic approaches is of great interest. Interleukin (IL)-13 has been recognized as a central mediator of TH2-dominant asthmatic phenotypes including excess mucus production and airway hyperresponsiveness. Consequently, therapeutics targeting IL-13 are very attractive with studies already in phase II clinical trials. But initial studies reported some adverse events associated with the use of IL-13 targeting therapeutics because IL-13 also elicits other immunoregulatory effects. For example, IL-13 suppresses TH17 cytokine production in an IL-10-dependent manner and thereby may play an important role in TH17-associated autoimmune diseases. Accordingly, patients receiving IL-13?targeted therapeutics had significantly increased adverse events involving the musculoskeletal and autoimmune-related diseases. Therefore, identifying targets that specifically affect the proliferative and metaplastic activity of IL-13 is crucial. The current study proposes a novel approach that switches the proliferative IL-13 to one that causes cell death in airway epithelial cells. IL-13 exerts proliferative effect by modulating various programmed cell death pathways. The anti-apoptotic protein, Bcl-2 inhibits apoptosis directly by interacting with various pro-apoptotic BH3-domain effector proteins. Indeed, our preliminary studies showed that IL-13 induced the proliferation and sustained the hyperplastic mucous cells by upregulating Bcl-2 expression. Blocking Bcl-2 by shRNA-based approach and by a small molecule Bcl-2 antagonist, ABT-263 reduced the mucus expression and caused mucous cell death. Screening for the pro-apoptotic factors showed that IL-13 induced a BH3-domain only protein, Bik, an epithelial cell death regulator in a STAT1-dependent manner. Thus, IL-13 modulates both Bcl-2 and Bik, the anti- and pro- cell death proteins, respectively to alter the cell fate, and blocking Bcl-2 results in a Bik-mediated cell death. Accordingly, the lung tissue sections from ABT-263 treated mice showed apically elevated apoptotic epithelial cells suggesting that epithelial extrusion or a programmed elimination might be involved in the removal of dying cells. In addition, the in-vitro studies with induced expression of Bik selectively removed the dying cells by extrusion with a characteristic actinomyosin ring formation. Therefore, we propose to investigate the cell death and extrusion process of airway epithelial cells by fixed- and live-cell imaging, and to interrogate the signaling pathways involved. We propose to test our findings in-vitro in both monolayer and differentiated culture settings, and in animal models. The efficacy of inhaled ABT-263 in regulating hyperplastic mucous cells will be tested in relevant mouse models of allergic asthma. More importantly, therapeutics for controlling mucous hypersecretion are limited with only few potential drugs currently in preclinical and clinical trials, therefore targeting hyperplastic mucous cells could represent a new class of mucolytic therapies. The use of ABT-263, an orally bioavailable Bcl-2 inhibitor that is currently being tested for human cancer treatment could help expedite its efficacy testing in asthmatics with debilitating mucous phenotype. In addition, the proposed studies will help develop a comprehensive understanding about the molecular mechanisms by which blocking Bcl-2 facilitates the IL-13?mediated mucous cell death. The long term objective of these studies is to help understand the pathogenesis of mucus hypersecretion observed in chronic airway diseases.
The cytokine IL-13 is a central mediator of the asthma phenotype. Allergic asthmatics have elevated levels of IL-13 which increases the number of mucous cells and mucus secretion that blocks the airways, resulting in either reduced lung function or when massive causes fatality. The present studies are based on a novel finding that this IL-13 signaling can be switched to cause cell death in proliferating airway epithelial cells and help reduce the excess mucous cells. The outlined approach will provide a rationale for a small molecule therapeutics that can be used to reduce mucus overproduction, and alleviate airway obstruction and asthmatic exacerbations.
