Erythrocyte Catabolism by Macrophages In Vitro THE EFFECT OF HYDROCORTISONE ON ERYTHROPHAGOCYTOSIS AND ON THE INDUCTION OF HEME OXYGENASE

Abstract

Phagocytosis of erythrocytes was studied in vitro in an incubation system consisting of rat peritoneal macrophages and antibody-coated ⁵⁹Fe-labeled erythrocytes. The system was characterized in terms of the rate and magnitude of erythrophagocytosis, determined by the interiorization of the ⁵⁹Fe label. On incubation of 150 × 10⁶ macrophages with 75 × 10⁶ antibodycoated erythrocytes, erythrophagocytosis began within a few minutes and was essentially completed after 2 h when 50% of the offered red cells had been ingested by the macrophages. Heme oxygenase (HO) activity, which is very low in native macrophages, increased 4- to 10- fold in response to the ingested erythrocytes; this enzyme stimulation occurred with a delay of 3 h in relation to erythrophagocytosis. Actinomycin D or puromycin prevented the increase of HO activity without affecting erythrophagocytosis, which suggests that the enzyme stimulation was due to substrate-mediated enzyme induction. Hydrocortisone (HC) (0.1 mg/ml medium) dissociated erythrophagocytosis from HO induction, leaving the former unimpaired but completely suppressing the latter. The suppressive effect of HC on the enzyme induction was completely prevented by 5 mg glucose and 0.02 U insulin/ml of the medium. In macrophages engaged in erythrophagocytosis. HC also lowered glucose removal from the medium and reduced formation of ¹⁴CO₂ from [1-¹⁴C]glucose. These results suggest that induction of HO in macrophages by the hemoglobin of ingested erythrocytes requires intact transport or metabolism of glucose. Glucose utilization appears to be impaired by HC, but is restored by additional glucose and insulin. The findings suggest that plasma steroid concentrations in the pharmacological range could reduce bilirubin formation in phagocytic cells in vivo without affecting the sequestration and degradation of erythrocytes. This provides a possible explanation for the observation that in patients with hepatogenous jaundice, steroids often lower the serum bilirubin level.