Impediment to calcium influx and reactive oxygen production accounts for the inhibition of neutrophil mac-1 up-regulation and adhesion by tetrandrine

We studied the mechanisms by which the plant alkaloid tetrandrine (TTD) inhibits Mac-1-dependent neutrophil adhesion to fibrinogen. TTD (0.1-10 mu M) significantly inhibited Mac-1 upregulation and neutrophil adhesion, as induced by N-formylmethionyl-leucyl-phenylalanine (fMLP) or phorbol-myristate acetate (PMA). Treatment of neutrophils with fMLP or PMA caused a rapid influx of Ca++ and accumulation of reactive oxygen species (ROS), both of which have been shown to enhance neutrophil adhesion via Mac-1 up-regulation. Because TTD antagonizes Ca++ influx and abrogates ROS, we examined the relationship between Ca++ influx, ROS formation, and Mac-1 expression in TTD-inhibited neutrophil adhesion. TID alone caused a slight but statistically significant increase in [Ca++](i) with no effect on adhesion. In contrast, TTD as well as two Ca++ channel antagonists, verapamil and nifedipine, markedly diminished fMLP- and PMA-induced Ca++ influx, Mac-1 up-regulation, and adhesion. TTD also inhibited increases in [Ca++](i) and adhesion induced by the ionophore A23187 but failed to inhibit those induced by thapsigargin, an agent mobilizing Ca++ from intracellular stores. Thus, TTD impeded Ca++ influx from outward to avert: neutrophil adhesion. Similarly, TTD and two ROS scavengers, superoxide dismutase and catalase, abolished ROS production, Mac-1 up-regulation, and neutrophil adhesion. Ca++ and ROS, therefore, represent two essential signals for Mac-1 up-regulation upon fMLP or PMA stimulation. Our data suggest that the antiadherent effect of TTD is mediated, in part, by the inhibition of Ca++ influx and ROS formation, resulting in suppressed up-regulation of Mac-1 and, in turn, neutrophil adhesion to fibrinogen.