Elucidation of the mechanism of vitamin A potentiation of carbon tetrachloride-induced liver injury

Abstract

The liver is a target tissue for Vitamin A toxicity. High doses of Vitamin A have been shown to produce hepatomegaly, portal hypertension, fatty liver, and hepatic degeneration and fibrosis. Of concern to us was the potential interactions of Vitamin A with other known or potential hepatotoxicants. Male SD rats (18o-200 gm) were given Vitamin A (retinol, 250,000 IU/Kg) daily for 7 days by oral gavage. 24 hr after the last dose of Vitamin A, they were then challenged with CCl₄ (0.15 ml/Kg, ip). Ethane, as a marker of lipid peroxidation, was measured during the first 2 hr and hepatic injury was assessed at 24 hr after CCl₄. There was approximately 5-fold increase in ethane exhalation and 17-fold increase in Plasma GPT activity in Vitamin A/CCl₄ group. There was also increase in the incidence of hepatocellular necrosis. Vitamin A pretreatment did not increase the metabolism of 14CCl₄ as examined by the amount of exhaled ¹⁴CO₂, and the covalent binding of ¹⁴C-equivalents to liver lipids and proteins. In addition, liver levels of Vitamin E or GSH were not changed by Vitamin A. Electron microscopic analysis of livers from Vitamin A treated rats revealed activated Kupffer cells. To determine if the Kupffer cells were functionally more active, the clearance of intravenously administered colloidal carbon from the blood of Vitamin A treated rats was compared to that of control rats. It was found that Vitamin A treated rats cleared carbon particles three times faster than that of controls. Therefore, we hypothesized that Vitamin A activation of Kupffer cells may be the mechanism of potentiation of liver injury. When stimulated, these activated cells could release active oxygen species which could promote peroxidation of hepatocyte membranes and potentiate CCl₄-induced liver injury. To test this hypothesis animals were treated with superoxide dismutase or catalase to catalyze the degradation of active oxygen species; or with methyl palmitate to deactivate Kupffer cells. Superoxide dismutase, catalase or methyl palmitate completely inhibited the Vitamin A potentiation of CCl₄ induced liver injury, but did not alter the toxicity of CCl₄ in non-Vitamin A treated rats. In addition, these agents inhibited the enhanced lipid peroxidation observed in Vitamin A treated rats administered CCl₄. Therefore, we conclude that the Vitamin A potentiation of CCl₄ induced liver injury results from Vitamin A's ability to activate Kupffer cells. Upon minimal hepatocellular injury produced by CCl₄, these activated Kupffer cells are stimulated to release activated oxygen species. It is these reactive intermediates that induce the enhanced lipid peroxidation that results in the potentiated response.