Functional KCa3.1 Channels Regulate Steroid Insensitivity in Bronchial Smooth Muscle Cells

Identifying the factors responsible for relative glucocorticosteroid (GC) resistance present in patients with severe asthma and finding tools to reverse it are of paramount importance. In asthma we see in vivo evidence of GC-resistant pathways in airway smooth muscle (ASM) bundles that can be modeled in vitro by exposing cultured ASM cells to TNF-alpha/IFN-gamma. This action drives GC insensitivity via protein phosphatase 5-dependent impairment of GC receptor phosphorylation. In this study, we investigated whether K(Ca)3.1 ion channels modulate the activity of GC-resistant pathways using our ASM model of GC insensitivity. Immunohistochemical staining of endobronchial biopsies revealed that K(Ca)3.1 channels are localized to the plasma membrane and nucleus of ASM in both healthy controls and asthmatic patients, irrespective of disease severity. Western blot assays and immunofluorescence staining confirmed the nuclear localization of K(Ca)3.1 channels in ASM cells. The functional importance of K(Ca)3.1 channels in the regulation of GC-resistant chemokines induced by TNF-alpha/IFN-gamma was assessed using complementary inhibitory strategies, including K(Ca)3.1 blockers (TRAM-34 and ICA-17043) or K(Ca)3.1-specific small hairpin RNA delivered by adenoviruses. K(Ca)3.1 channel blockade led to a significant reduction of fluticasone-resistant CX3CL1, CCL5, and CCL11 gene and protein expression. K(Ca)3.1 channel blockade also restored fluticasone-induced GC receptor-a phosphorylation at Ser(211) and transactivation properties via the suppression of cytokine-induced protein phosphatase 5 expression. The effect of K(Ca)3.1 blockade was evident in ASM cells from both healthy controls and asthmatic subjects. In summary, K(Ca)3.1 channels contribute to the regulation of GC-resistant inflammatory pathways in ASM cells: blocking K(Ca)3.1 channels may enhance corticosteroid activity in severe asthma.