Up-regulation of ICAM-1 by cytokines in human tracheal smooth muscle cellsinvolves an NF-kappa B-dependent signaling pathway that is only partially sensitive to dexamethasone

Although the precise mechanisms by which steroids mediate their therapeutic effects remain unknown, steroids have been reported to abrogate cytokine-i nduced activation of the transcription factor NF-kappa B. In some cell type s, NF-kappa R activation is necessary to regulate cytokine-mediated cellula r functions. However, compelling evidence suggests that the steroid inhibit ion of NF-kappa B is complex and cell specific. Using EMSA, we show that st imulation with TNF-alpha or IL-1 beta induces NF-kappa B DNA-binding activi ty in human airway smooth muscle cells, TNF-alpha and IL-1 beta also increa sed luciferase activity in airway smooth muscle cells transfected with a re porter plasmid containing kappa B enhancer elements. Cytokines activated NF -kappa B by rapidly degrading its cytosolic inhibitor I kappa B alpha, whic h was then regenerated after 60 min. Cytokine-mediated I kappa B alpha reap pearance was completely blocked by the protein synthesis inhibitor cyclohex imide, Inhibition of cytokine-mediated I kappa B alpha proteolysis using th e protease inhibitors N-tosyl-L-phenylalanine chloromethyl ketone and N-ace tyl-L-leucinyl-L-leucinyl-norleucinal also inhibited cytokine-mediated earl y expression of ICAM-1. Although dexamethasone partially inhibited IL-1 bet a- and TNF-alpha-induced up-regulation of ICAM-1 at 4 h, dexamethasone had no effect on cytokine-induced ICAM-1 expression at 18-24 h, In addition, ne ither cytokine-induced degradation or resynthesis of I kappa B alpha nor NF -kappa B DNA-binding activity were affected by desamethasone. In cells tran sfected with the luciferase reporter, dexamethasone did not affect TNF-alph a-induced NF-kappa B-dependent transcription. Interestingly, cytokine-media ted expression of cyclooxggenase-2 was completely abrogated by dexamethason e at 6 h, Together, these data demonstrate that cytokine-mediated NF-kappa B activation and ICAM-1 expression involve activation of a steroid-insensit ive pathway.