Selected Contribution: Synergism between TNF-α and IL-1β in airway smooth muscle cells: implications for β-adrenergic responsiveness

In human cultured airway smooth muscle cells, interleukin (IL)-1β increases cyclooxygenase (COX)-2 expression and PGE2 release, ultimately resulting in decreased β-adrenergic responsiveness. In this study, we aimed to determine whether tumor necrosis factor-α (TNF-α) synergizes with IL-1β in the induction of these events. TNF-α alone, at concentrations up to 10 ng/ml, had no effect on COX-2 protein expression; at concentrations as low as 0.1 ng/ml, it significantly enhanced the ability of IL-1β (0.2 ng/ml) to induce COX-2 and to increase PGE2 release. IL-1β and TNF-α in combination also significantly enhanced COX-2 promoter activity, indicating that synergism between the cytokines is mediated at the level of gene transcription. Although IL-1β and TNF-α each increased nuclear factor-κB activation and induced extracellular regulated kinase and p38 phosphorylation, combined administration of the cytokines did not enhance either nuclear factor-κB or mitogen-activated protein kinase activation. Combined administration of IL-1β (0.2 ng/ml) and TNF-α (0.1 or 1.0 ng/ml) reduced the ability of isoproterenol to decrease human airway smooth muscle cell stiffness, as measured by magnetic twisting cytometry, even though individually these cytokines, at these concentrations, had no effect on isoproterenol responses. Treatment with the selective COX-2 inhibitor NS-398 abolished the synergistic effects of TNF-α and IL-1β on β-adrenergic responsiveness. Our results indicate that low concentrations of IL-1β and TNF-α synergize to promote β-adrenergic hyporesponsiveness and that effects on COX-2 expression and PGE2 are responsible for these events. The data suggest that the simultaneous release in the airway, of even very small amounts of cytokines, can have important functional consequences.