Endotoxin inhibition of dexamethasone induction of tryptophan oxygenase in suspension culture of isolated rat parenchymal cells: Involvement of the hepatic nonparenchymal cell fraction

Abstract

The effect of endotoxin on the induction of tryptophan oxygenase activity by dexamethasone has been studied in suspension cultures of isolated rat hepatic parenchymal cells incubated alone or mixed with hepatic nonparenchymal cells. Hepatic cellular fractions were isolated from untreated rats and from animals injected 2 hr prior to killing with 2 mg/kg of endotoxin. Endotoxin (400 μg/ml) added to suspensions of parenchymal cells from untreated animals did not decrease the induction of tryptophan oxygenase by dexamethasone. Endotoxin addition to cell suspension containing parenchymal and nonparenchymal cells from untreated rats caused a significant diminution in the induction of enzyme activity elicited by 0.1 μM, but not by 20 μM, dexamethasone. The nonparenchymal cell fraction from untreated animals had no effect on enzyme induction in the absence of added endotoxin. However, nonparenchymal cells isolated from rats pretreated in vivo with endotoxin diminished the induction of tryptophan oxygenase activity elicited by 0.1 and 20 μM dexamethasone even in the absence of added endotoxin. The inhibitory effect of nonparenchymal cells from endotoxin-pretreated rats was related to cell concentration and was accentuated by the in vitro addition of endotoxin. Parenchymal cells from the endotoxin-pretreated animals had an altered sensitivity to dexamethasone. The ec 50 concentration for dexamethasone induction of tryptophan oxygenase was 40 and 500 μm for parenchymal cells from untreated and endotoxin-pretreated animals respectively. Parenchymal cells from the pretreated animals were more sensitive to the inhibitory effect of the nonparenchymal cells from the pretreated animals. These results are interpreted as indicating that endotoxin does not directly antagonize the induction of tryptophan oxygenase. The results suggest that this inhibition of tryptophan oxygenase induction is a mediated event involving an interaction of endotoxin with cellular constituents of the hepatic nonparenchymal cell fraction.