Paracetamol hepatotoxicity in metallothionein-null mice

Abstract

The role of metallothionein (MT) in protecting the liver against paracetamol (PCT) toxicity was investigated in vivo and in vitro in mice lacking expression of MT−1 and MT−2 genes (MT−/−). In the fed, glycogen replete state, hepatotoxicity (PCT 300 mg/kg i.p.) at 6 h was significantly greater in MT−/− than MT+/+ mice. Plasma lactate dehydrogenase (LD) and alanine aminotransferase (ALT) were 5- and 13-fold greater respectively than in MT+/+ mice. Liver glycogen, glucose and zinc levels were significantly lower in MT−/− mice at this time. In contrast, hepatotoxicity (PCT 135 mg/kg i.p.) at 6 h was similar in both MT+/+ and MT−/− mice fasted 24 h, despite a doubling in liver MT in MT+/+ mice. No differences were found between MT−/− and MT+/+ mice in cytochrome P450 activity. Liver glutathione levels were the same in both groups of mice prior to fasting and were decreased to a similar extent (55–65%) following PCT treatment. Investigation of lower PCT doses (≤120 mg/kg) in fasted mice over 24 h demonstrated a greater susceptibility in female MT−/− mice with plasma LD, 2.4-fold and ALT, 7.5-fold greater than in MT +/+ mice at 120 mg/kg PCT. In male MT−/− mice, there was only a trend towards greater susceptibility at 110 mg/kg PCT compared to male MT+/+ mice, and at 120 mg/kg, both male genotypes were equally affected. Investigations with cultured hepatocytes supported the in vivo findings in that there was a trend towards greater toxicity (PCT at 1 and 5 mM for 24 h) in hepatocytes from fed MT−/− mice, with the difference diminished in association with greater hepatotoxicity in hepatocytes from fasted mice. Use of dexamethasone (Dex) to increase MT in the MT+/+ mouse hepatocytes protected from PCT toxicity. Zn alone was not protective. Zn plus Dex offered no protection despite higher MT levels. Generation of apo-MT with Dex may offer more protection than Zn-MT. In conclusion, MT−/− mice were more susceptible than MT+/+ mice to PCT toxicity in the fed state, but the increased susceptibility was much smaller, but still significant, when the effects of glycogen were minimised by fasting.