Corticosteroid inhibition of growth-related oncogene protein-α via mitogen-activated kinase phosphatase-1 in airway smooth muscle cells

Expression of the inflammatory chemokine, growth-related oncogene protein-alpha (GRO-alpha), from airway smooth muscle cells (ASMC) is regulated by pathways involving NF-kappa B and MAPK activation. We determined the effects of dexamethasone on GRO-alpha induced by IL-1 beta or TNF-alpha with respect to the role of MAPK pathways and of MAPK phosphatase-1 (MKP-1). Human ASMC were studied in primary culture at confluence. Dexamethasone (10(-8)-10(-5) M) partially inhibited GRO-alpha expression and release induced by IL-1 beta and TNF-alpha; this was associated with an inhibition of JNK, but not of p38 or ERK phosphorylation. Together with IL-1 beta or TNF-alpha, dexamethasone rapidly induced mRNA and protein expression of MKP-1, which dephosphorylates MAPKs. Using MKP-1 small interfering RNA (siRNA) to block the expression of IL-1 beta- and dexamethasone-induced MKP-1 by 50%, JNK phosphorylation was doubled. The inhibitory effect of dexamethasone on GRO-a release was partially reversed in ASMC treated with MKP-1 siRNA compared with those treated with scrambled siRNA. In contrast, overexpression of MKP-1 led to a reduction in IL-1 beta-induced release of GRO-alpha, but the inhibitory effects of dexamethasone were preserved. Nuclear translocation of the glucocorticoid receptor was increased in ASMC exposed to dexamethasone and IL-1 beta. Using chromatin immunoprecipitation assay, glucocorticoid receptor binding to the MKP-1 promoter was increased by IL-1 beta and dexamethasone compared with either alone. Glucocorticoids and IL-1 beta or TNF-alpha modulate GRO-alpha release partly through the inhibition of JNK pathway, resulting from an up-regulation of MKP-1 expression.