Abstract

Tumor necrosis factor-α (TNF), an inflammatory cytokine, has a potentially important role in the pathogenesis of bronchial asthma and may contribute to airway hyper-responsiveness. Recent evidence has revealed that TNF can increase the Ca²⁺ sensitivity of agonist-stimulated myosin light chain₂₀ (MLC₂₀) phosphorylation and contractility in guinea pig airway smooth muscle (ASM). In the present study, the potential intracellular pathways responsible for this TNF-induced Ca²⁺ sensitization were investigated. In permeabilized cultured guinea pig ASM cells, recombinant human TNF stimulated an increase in Ca²⁺-activated MLC₂₀ phosphorylation under Ca²⁺ “clamp” conditions. This increased MLC₂₀ phosphorylation was inhibited by preincubation with the Rho-kinase inhibitor Y27632. TNF also increased the proportion of GTP-bound RhoA, as measured using rhotekin Rho-binding domain, in a time course compatible with a role in the TNF-induced Ca²⁺sensitization. In cultured human ASM cells, recombinant human TNF also activated RhoA with a similar time course. In addition, TNF stimulated phosphorylation of the regulatory subunit of the myosin phosphatase, which was inhibited by Y27632. Although human ASM cells expressed both receptor subtypes, TNF-R1 and TNF-R2, the activation of RhoA was predominantly via stimulation of the TNF-R1, although RhoA did not immunoprecipitate with the TNF-R1. In conclusion, the TNF-induced increase in the Ca²⁺ sensitivity of MLC₂₀phosphorylation is through stimulation of the TNF-R1 receptor and via a RhoA/Rho-kinase pathway leading to inhibition of the myosin light chain phosphatase. This intracellular mechanism may contribute to TNF-induced airway hyper-responsiveness.