INDUCTION OF METALLOTHIONEIN BY ALPHA-HEDERIN

α-Hederin (α-Hed) is a triterpenoid saponin that has been shown to protect against some hepatotoxicants. This study examined the protective effect of α-Hed against cadmium (Cd) hepatotoxicity and the mechanism of protection. α-Hed pretreatment (100 μmol/kg, sc) dramatically decreased Cd (3.7 mg/ kg, iv) hepatotoxicity as indicated by a reduction of serum alanine aminotransferase and sorbitol dehydrogenase, as well as by histopathological examination. α-Hed did not produce protection by decreasing the distribution of Cd to the liver, as higher amounts of Cd were found in the liver of α-Hed-pretreated mice. However, there was a marked alteration in subcellular distribution of Cd in the α-Hed-pretreated mice, with much less Cd distributing to nuclei, mitochondria, and microsomes and more in the cytosol. The increased cytosolic Cd was found primarily bound to a low-molecular-weight protein, metallothionein (MT). α-Hed (10-300 μmol/kg, sc) produced a dose-dependent increase in hepatic MT with a 100-fold increase over controls 24 hr after a single injection of 100 μmol/kg, as determined by the Cd/hemoglobin assay. The hepatic MT increase produced by α-Hed is relatively long lasting, in that it is still eight times control values 6 days after a single administration. The induction of MT was also relatively specific for the liver, as little or no increase in MT was observed in other tissues. Furthermore, α-Hed increased both hepatic MT-I and MT-II levels. Northern blot analysis revealed that α-Hed rapidly increased MT mRNA levels. In conclusion, α-Hed decreases the hepatotoxicity of Cd by inducing MT, which binds Cd in the cytosol, and thus reduces the amount of Cd in the critical cellular organelles. α-Hed is an effective inducer of both MT-I and MT-II in liver, and this effect is associated with an increase in MT mRNA. © 1993 Academic Press. All rights reserved.