Changes in hepatic gene expression in response to hepatoprotective levels of zinc

Zinc (Zn) is an essential trace element with many biological roles as, for example, in hundreds of Zn-containing enzymes and thousands of Zn-finger proteins (1). Zn also acts as a growth cofactor, immunoregulator and antioxidant with anti-inflammatory and anti-apoptotic effects (2–4). The liver is an important tissue for regulation of Zn storage and homeostasis, and Zn is clearly necessary for appropriate liver function (4). Zinc administration at non-toxic levels protects against chemically induced acute and chronic liver injury in experimental animals (Table 1). Zn treatment reduces the hepatotoxicity produced by a wide variety of diverse agents such as carbon tetrachloride (5), bromobenzene (6), thioacetamide (7), ethanol (8), D-galactosamine (9), endotoxin (10), D-galactosamine plus lipopolysaccharides or tumour necrosis factor (TNF)-α (11). Zn pretreatment also decreases liver injury produced by drug overdose, such as acetaminophen (12, 13), salicylate (14) and cisplatin (15). Zn is also well known to protect against hepatotoxicity of metals, such as cadmium (16), mercury (17), arsenic (18), nickel (19) and copper (20). However, the precise mechanisms through which Zn produces this generalised hepatoprotection to such a widely divergent group of hepatotoxicants are not fully defined. Table 1 Hepatoprotective levels of zinc protects against liver injury from various chemicals and metals Microarray analysis of gene expression has been recently used to profile genetic response patterns associated with Zn deficiency (21–23). The most consistent change in the livers of Zn-deficient rats is a dramatic (−13 to −100 fold) downregulation of metallothionein (MT), which is encoded by the Mt1 and Mt2 genes, along with a variety of aberrant expression of genes encoding lipid metabolism, xenobiotic metabolism, stress response and signal transduction (21–23). MT is an important Zn-regulated small metal-binding protein, often playing an important role in the magnitude and progression of toxic insult in the liver (24). This is probably through maintenance of cellular Zn homeostasis, as well as through its antioxidant properties, as MT is rich in sulphydryl groups (1, 3, 25). However, little is known about the gene expression profiles following Zn supplementation, or after treatment with a hepatoprotective level of Zn. Thus, it is clearly important to profile the gene expression patterns following a hepatoprotective low dose of Zn exposure in experimental animals for a variety of reasons, including both toxicological and potentially nutritional. This study was designed to examine hepatic gene expression response following treatment with known hepatoprotective levels of Zn in rats and mice. The dose (100 μmol/kg, s.c.) and duration (once daily for 4 days) of Zn treatment used in this study is well documented to achieve chemical- and self-hepatoprotection without itself producing any liver damage, and thus the gene response patterns involved can be evaluated in the absence of any toxic response. In addition, hepatic Zn and several protective proteins, particularly MT, were determined. The results clearly demonstrate that Zn-induced gene expression patterns are similar in hepato-protected rats and mice, and that such Zn-induced gene expression profiles, particularly the dramatic increases in Mt expression, are important in the generalised hepato-protective effects of Zn against diverse toxic insults to the liver.