Background: Modification of human airway smooth muscle (ASM) function by proinflammatory cytokines has been regarded as a potential mechanism underlying bronchial hyperresponsiveness in asthma. Human ASM cells express intercellular adhesion molecule (ICAM)-1 in response to cytokines. Synthetic ligands for peroxisome proliferator-activated receptor (PPAR)γ reportedly possess anti-inflammatory and immunomodulatory properties. In this study, we examined whether ciglitazone, a synthetic PPARγ ligand, can modulate the basal and tumor necrosis factor (TNF)α-induced ICAM1 gene expression in human ASM cells.
Methods: Human ASM cells were treated with TNFα. ICAM-1 expression was assessed by flow cytometry and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. PPARγ activity was inhibited by target-specific small interfering (si) RNA targeting PPARγ and GW9662, a PPARγ antagonist. Activity of nuclear factor (NF)-κB was assessed by using immunoblot analysis, immune-confocal images, and electrophoretic mobility shift assay (EMSA).
Results: By flow cytometry, ciglitazone alone had no effect on ICAM-1 expression in ASM cells, but inhibited ICAM-1 expression in response to TNFα (10 ng/ml) in a dose-dependent manner (1-10 μM). It also inhibited TNFα-induced ICAM1 gene expression by RT-PCR analysis. Knockdown of PPARγ gene by target-specific siRNA targeting PPARγ enhanced ICAM-1 expression and the inhibitory effect of ciglitazone on TNFα-induced ICAM-1 expression was reversed by PPARγ siRNA and GW9662. SN-50 (10 μg/ml), an inhibitor for nuclear translocation of NF-κB, inhibited TNFα-induced ICAM-1 expression. Ciglitazone did not prevent TNFα-induced degradation of the cytosolic inhibitor of NF-κB (IκB), but inhibited the nuclear translocation of p65 induced by TNFα and suppressed the NF-κB/DNA binding activity.
Conclusion: These findings suggest that ciglitazone inhibits TNFα-induced ICAM1 gene expression in human ASM cells through the ligand-dependent PPARγ activation and NF-κB-dependent pathway.