Dimethylnitrosamine-induced liver injury in rats: the early deposition of collagen

Abstract

Dimethylnitrosamine (DMN) is a potent hepatotoxin that can cause fibrosis of the liver. It's ability to provide a suitable rapid experimental murine model for early human cirrhosis was examined. The drug was administered to adult male albino rats in order to document sequential pathological and biochemical alterations. Injury was produced by intraperitoneal injections of DMN on three consecutive days of each week over a 3-week period. A rapid increase in collagen content was documented, with linear increases occurring from days 7 to 21. Livers were examined for histopathological changes on days 7, 14 and 21 following the beginning of exposure. Severe centrilobular congestion and haemorrhagic necrosis could be observed on day 7. Centrilobular necrosis and intense neutrophilic infiltration were observed on day 14. By day 21, collagen fiber deposition could be observed, together with severe centrilobular necrosis, with focal fatty changes, bile duct proliferation, bridging necrosis and fibrosis surrounding the central veins. A decrease in total protein and increase in DNA were also documented. DMN-induced liver injury in rats appears to be a potential animal model for early human cirrhosis and the rapid deposition of collagen, and may serve as a convenient procedure for screening antifibrotic agents.

Introduction

Dimethylnitrosamine (DMN) is a potent hepatotoxin, carcinogen and mutagen (Haggerty and Holsapple, 1990). Its hepatotoxicity was first reported by Barnes and Magee (1954) following an industrial accident. The toxicity produced by DMN is mediated by its reactive metabolites and not by the parent compound. Its metabolic half-life is <10 min in rodents and about 20 min in non-human primates (Anderson et al., 1992). DMN targets primarily the liver, which contains the necessary enzymes for its metabolic activation. Metabolism in the liver is by a microsomal membrane-bound enzyme, cytochrome P-450IIE1 (Yang et al., 1985, Yang et al., 1990, Yoo et al., 1988). Activation and degradation of DMN produces formaldehyde and methanol, and the alkylating intermediate reacts with nucleic acids and proteins to form methylated macromolecules.

In the rat model, DMN administration causes severe necrosis, but also deposition of extracellular matrix proteins in the liver, particularly collagen (Ala-Kokko et al., 1987, Savolainen et al., 1988). A detailed study on the role of intracellular enzymes in collagen biosynthesis induced by DMN has been reported Risteli and Kivirikko, 1976a, Risteli and Kivirikko, 1976b. Glycoprotein metabolism has also been investigated following DMN treatment (George and Chandrakasan, 1996a). But a comprehensive histopathological investigation of the temporal pathophysiological changes in the liver following sequential administration of DMN has not been carried out previously. The present investigation is aimed at studying such changes during DMN administration using the male albino rat model. We were particularly interested in developing a model for the very early deposition of collagen that might be useful as a rapid tool for screening antifibrotic agents.