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

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 mumol 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 administration 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.