Retinoid potentiation of chemically-induced hepatic damage: The involvement of tumor necrosis factor and endotoxin

Abstract

Administration of large doses of retinol (75 mg/kg/day) results in potentiation of carbon tetrachloride-induced hepatic damage in rats. Further studies from our laboratory suggest that one of the primary mechanisms responsible for potentiation may involve the increased release of tumor necrosis factor (TNF) from activated Kupffer cells (KC). The exact mechanism of KC activation in this model is unknown. However, recent studies suggest that retinol pretreatment elevates plasma endotoxin levels and increases the proliferation of gram negative bacteria in the small intestine (SI). Therefore this dissertation project was designed to investigate the ability of other retinoids to potentiate chemically-induced hepatic damage and to investigate the involvement of TNF and endotoxin in retinol potentiation of CCl₄-induced hepatic damage. In order to conduct these studies, male Sprague Dawley rats (250 g) received retinol for 7 days by oral gavage. On day 8, either CCl₄ (200 ul/kg) or BrCCl₃ (20 ul/kg) was administered i.p. On day 9, the animals were killed via CO₂ asphyxiation. Plasma samples were collected for analysis of TNF levels (ELISA), endotoxin levels (Limulus Ameobocyte Assay) and alanine aminotransferase (ALT) activity. Inhibition of KC activity was achieved by the administration of GdCl₃ (10 mg/kg). Neutralization of TNF was achieved through the administration of a monoclona/polyclonal TNF antibody (250 ul/275 gram rat). Neomycin/polymyxin B (250 mg/20 mg) were used for endotoxin inhibition (EI). Retinol, retinoic acid or retinyl palmitate pretreatment potentiated significantly (p < 0.05) CCl₄ or BrCCl₃-induced hepatic damage. Retinol pretreatment elevated significantly (p < 0.05) plasma levels of TNF and endotoxin. Retinol pretreatment also increased the growth of SI gram negative bacteria. Administration of KC inhibitors, TNF antibody or EI reduced significantly (p < 0.05) the degree of potentiation (98%, 58%, 55% reductions, respectively). These studies suggest that retinol-induced proliferation of SI-derived gram negative bacteria and subsequent elevation of plasma endotoxin may activate Kupffer cells to release high levels of TNF. These high levels of TNF play an important role in retinol potentiation of CCl₄-induced hepatic damage.