Abstract
The anti-inflammatory actions of endogenous glucocorticoids (GCs) are regulated by the activities of the GC-activating and -inactivating enzymes, 11β-hydroxysteroid dehydrogenase (11β-HSD)-1 and 11β-HSD2, respectively, that catalyze the interconversion of the inert GC, cortisone, and its bioactive derivative, cortisol. Proinflammatory cytokines regulate 11β-HSD1 expression in various cell types and thereby modulate the bioavailability of cortisol to the glucocorticoid receptor (GR). Since endogenous GCs reportedly attenuate the airway asthmatic response to allergen exposure, we investigated whether airway smooth muscle (ASM) exhibits cytokine-induced changes in 11β-HSD1 expression that enable the ASM to regulate its own bioavailability of GC and, accordingly, the protective effect of GR signaling on airway function under proasthmatic conditions. Human ASM cells exposed to the primary proasthmatic T helper type 2 (Th2) cytokine, IL-13, exhibited upregulated expression of 11β-HSD1, an effect that was attributed to activation of the transcription factor, AP-1, coupled to MAPK signaling via the ERK1/2 and JNK pathways. The induction of 11β-HSD1 expression and its oxoreductase activity by IL-13 (also IL-4) served to amplify the conversion of cortisone to cortisol by the cytokine-exposed ASM and, hence, heighten GR-mediated transcriptional activation. Extended studies demonstrated that this amplified 11β-HSD1-dependent GC activation enabled physiologically relevant concentrations of cortisone to exert enhanced protection of ASM tissues from the proasthmatic effects of IL-13 on ASM constrictor and relaxation responsiveness. Collectively, these novel findings identify a Th2 cytokine-driven homeostatic feedback mechanism in ASM that enhances its responsiveness to endogenous GCs by upregulating 11β-HSD1 activity, thereby curtailing the adverse effects of the proasthmatic cytokine on airway function.