Mechanisms of regulation of rat hepatic metallothionein-I and metallothionein-II levels following administration of zinc

Abstract

The purpose of this study was to determine the role of transcription, translation, and protein degradation on the accumulation of metallothionein-I (MT-I) and metallothionein-II (MT-II) in rat liver following induction of these proteins by Zn. The time course of MT induction indicated that concentrations of MT-I and MT-II, quantitated by high-performance liquid chromatography, were similar at 6 hr after administration of 1000 μmol Zn/kg (sc), but thereafter the concentration of MT-II was always higher than that of MT-I. By 24 hr after Zn administration, the concentration of MT-II in liver was more than two times that of MT-I. This difference increased with time such that by 96 hr the concentration of MT-II was more than five times that of MT-I. MT-I and MT-II mRNA levels, measured by Northern blot hybridization with mouse cRNA probes, increased coordinately following afministration of Zn. MT mRNAs increased to maximum levels 6–9 hr after Zn administration, at which times MT-II mRNA was about two times more abundant than MT-I mRNA. MT mRNA levels remained elevated above control for as long as 36 hr after Zn administration. The relative rates of synthesis for MT-I and MT-II were determined by quantitating incorporation of [ 35S]cysteine into MTs during a 2-hr period. For both proteins, the maximum relative rates of synthesis were observed 6–9 hr after administration of Zn, in parallel with the increase in mRNA levels. When MT synthesis was at maximal levels, there was approximately two times more [ 35S]cysteine incorporated into MT-II than MT-I, but at no other times were differences observed. In contrast to MT mRNA levels, MT synthesis returned to control levels by 24 hr after administration of Zn. Half-lives of the isometallothioneins, determined by pulse-labeling experiments, were calculated to be 12.2 ± 0.8 and 21.9 ± 3.0 hr for MT-I and MT-II, respectively. Thus, Zn treatment increases transcription of both MT-I and MT-II genes and the synthesis of MT-I and MT-II. However, Zn-induced MT-II is more stable than MT-I. These results suggest that differences in the rate of synthesis and degradation of MT-I and MT-II lead to a greater and more prolonged induction of MT-II following administration of Zn.