Hepatoprotective levels of Zn treatment in rats or mice induced a pattern of hepatic gene expression changes, the most consistent change of which was increased MT expression at both the transcript and the protein levels. Modest induction of some genes encoding for acute-phase protein genes, antioxidant enzymes and cell proliferation-related genes were also evident. Some, but not all, metabolic enzyme genes were suppressed, perhaps in an attempt to switch cellular metabolic energy to acute-phase responses. The gene expression changes were similar between mice and rats, but mice were generally more responsive to the Zn treatments. The treatment levels of Zn used in this study were non-toxic, and no Zn-induced overt liver injury was evident. In general, most of the gene expression changes are subtle, or modest. These gene expression changes, particularly the dramatic increase in MT production, are probably important in the hepatoprotective effects of Zn against hepatotoxicant insult.
Metallothionein is a low-molecular-weight, metal-binding protein that plays important roles in the detoxication of heavy metals, in the homeostasis of essential metals and in the scavenging of free radicals (25
). Induction of MT is an important adaptive mechanism affecting the magnitude and progression of hepatotoxic lesions (24
). Induction of MT by Zn has been proposed to be a primary mechanism of protection against the hepato-toxicity produced by carbon tetrachloride (5
) and cadmium (16
). MT-null mice are unable to synthesize MT protein in response to hepatotoxicant insults and are clearly more sensitive to hepatotoxicity of cadmium (25
), carbon tetrachloride (27
), acetaminophen (28
), cisplatin (29
) and thioacetamide (30
). Thus, it can be concluded that the dramatic induction of MT is critical for Zn-induced generalised hepatoprotection.
However, MT induction alone is insufficient to completely explain Zn-induced generalised hepatoprotection against all toxic agents. For example, Zn protection against alcohol-induced liver injury is independent of MT (31
). Other mechanisms in addition to MT induction must also be involved in Zn-induced hepatoprotection, at least with some hepatotoxicants. The Keap1-Nrf2-ARE signalling pathways have emerged as important regulators of the mammalian defense system for detoxication of toxic agents such as carbon tetrachloride, acetaminophen, bromobenzene and furosemide (32
), and are often mediated through the upregulation of Nqo1
and other cellular antioxidant enzymes (32
), Thus, the induction of Nrf2
, although modest, may contribute to Zn-induced generalised hepatoprotection.
Other Zn-induced gene expression changes, although small, could also contribute to Zn-induced adaptive responses. For example, the transcription factor Egr1 and heat-shock proteins could mediate hepatic response to inflammatory stimuli (35
), and Hsp70
null mice are more susceptible to carbon tetrachloride hepatotoxicity (36
) and acetaminophen-induced liver injury (37
). Zn-induced increases in the expression of Egr1
, although modest, could be an important part of Zn-induced adaptive responses. Zn-treatment-induced increases in liver proliferation-related genes may also contribute to adaptation, as the proliferating livers are more resistant to liver injury (38
), and timely liver regeneration can prevent the progression of injury by upregulation of calpastatin (39
). Liver cell regeneration may also involve stem cell proliferation, and the regenerating cells require a large amount of Zn and Zn-binding MT protein during the priming step, soon after toxic insults (40
), indicating the important roles of Zn and Zn-induced MT in the liver.
It should also be kept in mind with Zn that the range between a safe level and toxic levels is relatively narrow. An overdose of Zn can be toxic to the liver or elsewhere (41
). Zn has a dual function in host defense, and can act as either a pro-oxidant or a pro-antioxidant, depending on cellular Zn metabolism and homeostasis (42
). In this regard, the redox biology of MT protein plays an important role in modulating the cytoprotective or cytotoxic capacity of Zn (42
). In the livers of Zn-deficient animals, marked downregulation of MT could weaken the host defense against various toxic stimuli and compromise the normal function of the body (21
). Although proper MT expression protects against hepatotoxicants (i.e. carbon tetrachloride), overexpression of MT or excess dietary Zn supplementation provide no further protection (27
). Similar to the dual functions of Zn, MT is also a double-edged sword and elevated extracellular MT could have adverse effects (25
). Thus, to maintain a constant state of cellular Zn homeosatsis is essential for host defense and normal cellular function, at least in part through MT regulation and adaptive response machineries (44
In summary, the current study demonstrated that non-toxic, hepatoprotective levels of Zn in experimental animals evoked a consistent pattern of gene expressions, including dramatic upregulation of MT, modest activation of Nrf2- and acute-response-related genes, and modest suppression of metabolic enzymes. These gene expression changes could play an integrated role in Zn-induced protection against various hepatotoxicants.