Metallothionein induction attenuates the effects of glutathione depletors in rat hepatocytes.

Abstract

Metallothionein (MT) is a protein involved in heavy metal homeostasis and detoxification. According to several studies, MT could be involved in the antioxidant defense system, in which glutathione (GSH) is an essential component. The aim of this study was to verify the implication of MT in the antioxidant defense system in isolated rat hepatocytes. For this purpose, hepatocyte cultures were exposed to treatments known to modify MT or GSH levels. Zinc (Zn) was used as an inducer of MT while diethyl maleate (DEM) and buthionine sulfoximine (BSO) were used as GSH depletors. GSH, MT, and antioxidant enzyme activities were measured under conditions of MT induction and GSH depletion. Induction of MT synthesis through an 18-hour exposure to Zn (20 microM), did not result in any significant change in GSH levels or in activities of the antioxidant enzymes, glutathione-peroxidase (GSH-Px), catalase, and superoxide dismutase (SOD). DEM caused GSH depletion in cells, whether they were exposed to Zn or not, that lasted one h; after that time, GSH rose back to basal levels. BSO also caused GSH-depletion in cells exposed or unexposed to Zn, and no recovery in GSH levels was detectable during the entire period of exposure (12 h). However, GSH depletion induced by both DEM or BSO was attenuated in Zn-treated hepatocytes. Moreover, DEM and BSO exposures led to a depletion of MT levels in Zn-treated hepatocytes, indicating a link between GSH and MT metabolism. In cells unexposed to either Zn, DEM or BSO, there was an increase in GSH-Px and SOD activities after 6 and 12 h of incubation, respectively. Under the same conditions, catalase activity was inhibited after 6 h of incubation and returned to the activity found at t = 0 after 12 h of incubation. DEM and BSO treatments had no significant effect on GSH-Px or SOD activities although they led to inhibition of catalase activity. Taken together, our data indicate that MT induction, which creates a new pool of thiol groups in the cell cytosol, can attenuate GSH depletion induced by DEM or BSO. It appears that catalase is most sensitive to oxidative stress and that MT induction can antagonize the deleterious effects of such stress on the enzyme. This study supports the view that MT is part of the hepatocyte antioxidant-defense-system.