ACTIVATION OF THE TNF-ALPHA-P55 RECEPTOR INDUCES MYOCYTE PROLIFERATION AND MODULATES AGONIST-EVOKED CALCIUM TRANSIENTS IN CULTURED HUMAN TRACHEAL SMOOTH-MUSCLE CELLS

Evidence suggests that cytokines may modulate smooth muscle cell funct ion in a variety of inflammatory diseases. In the present study, we ch aracterized the specific receptor subtypes that mediate tumor necrosis factor alpha (TNF alpha) effects on myocyte proliferation and on agon ist-induced calcium transients in cultured human tracheal smooth muscl e cells (TSMC). Pretreatment of human TSMC with TNF alpha potentiated cytosolic calcium [(Ca2+)(i)] transients evoked by carbachol. In a sim ilar manner, selective TNF alpha-p55 receptor agonists such as htr-9, an activating monoclonal antibody, or a recombinant TNF-p55 (rTNF-p55) , which specifically activates the TNF alpha-p55 receptor but not the TNF alpha-p75 receptor, also augmented [Ca2+](i) transients evoked by carbachol. In parallel experiments, TNF alpha, rTNF alpha-p55, and htr -9 induced human TSMC proliferation as measured by the 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. Interestin gly, activation of the TNF alpha-p75 receptor with a selective agonist , recombinant TNF alpha-p75 (rTNF alpha-p75), or inhibition of the TNF alpha-p75 receptor with utr-1, an inhibitory anti-TNF alpha-p75 recep tor antibody, had no effect on TNF alpha-augmented calcium transients or on myocyte growth. To further confirm the receptor specificity of t hese findings, immunocytochemical studies were performed using recepto r-specific antibodies. These studies demonstrated marked cell-surface expression of the TNF alpha-p55 receptor compared with expression of t he TNF alpha-p75 receptor on human TSMC. Taken together, our results s uggest that TNF alpha modulates agonist-induced calcium transients and induces human TSMC proliferation by specific activation of the TNF al pha-p55 receptor. Further studies addressing the cellular and molecula r mechanisms regulating cytokine modulation of airway smooth muscle fu nction may provide new insight into mechanisms that induce airway hype rresponsiveness in asthma.