Modulating steroid insensitive pathways in airway smooth muscle cells

Approximately 5 to 10% of patients with asthma have difficult-to-control or severe disease that is refractory or poorly responsive to glucocorticoids (GC). This group represents a significant unmet clinical need and is responsible for a disproportionate share of health care costs and morbidity associated with the disease. This thesis investigated the underlying molecular mechanisms of steroid insensitivity in airway smooth muscle (ASM) in patients with severe asthma. We used a natural compound isolated from an African plant called Compound A (CpdA) in an in vitro model of steroid insensitivity of ASM cells treated with TNFα and IFNγ. This is the first report to show that CpdA, independently of the GRα, differentially suppressed the expression of GC-insensitive chemokines in ASM cells. We elucidated one mechanism responsible for the inhibition of fluticasone-resistant CXCL10 by CpdA via up-regulation of the MAPK deactivator, MKP-1. This thesis described the modulation of fluticasone-resistant chemokines (CCL5, CX3CL1) via inhibition of the MAPK pathway (p38MAPK, JNK), raising the possibility that targeting these kinases in ASM could be beneficial in treating steroid insensitivity. The potential synergistic effect of using MAPK inhibitors (p38, ERK) in combination with fluticasone showed a beneficial effect in reducing the production of GC-resistant chemokines (CCL5 and CXCL10). Finally, this study identified another potential mechanism involved in CpdA action via inhibition of the transcription factor IRF-1 which we showed was important in driving cytokine-induced CCL5 and CX3CL1 expression. More importantly, IRF-1 was activated within the ASM in biopsies from severe asthmatics and its expression was unaffected by GC treatment. IRF-1 expression within the ASM from severe asthmatic biopsies correlated with serum IgE, sputum eosinophils and ENo levels, with some differences depending on treatment. This thesis provided evidence for the effectiveness of CpdA in the suppression of GC-insensitive genes in our in vitro model. This work supports the involvement of IRF-1 in driving steroid insensitive pathways in ASM cells and suggests that targeting IRF-1 axis could offer an alternative therapy for asthma. Inhibition of MAPK kinases and up-regulation of endogenous MAPK deactivators could also be potential strategies for treatment.