Regulation of zinc metallothionein II mRNA level in rat brain

Metallothionein isoforms I and II (MTI and MTII) have been identified in the rat brain, monkey brain, bovine retina, pineal gland and hippocampus, and in the neuroblastoma IMR 32. Since intraperitoneally administered zinc passes across the blood-brain barrier slowly, the rat brain metallothionein can be induced in a time- and dose-dependent fashion only following intracerebroventricularly (i.c.v.) administered zinc sulfate at a rate of 0.20 ?mol/?l/h for 24 h using an Alzet minipump. The zinc-induced proteins, incorporate large quantities of [(35)S]cysteine, bind (65)Zn and produce two isoforms which contain 17 and 18 cysteine residues, respectively, but lack aromatic amino acids or histidine. In this communication, we report that i.c.v.-administered zinc in a bolus of 0.1 and 0.5 ?mol increased the synthesis of poly A(+) RNA from 6.6 to 8.0 and 9.6 ?g/g brain tissue, respectively. Furthermore, we probed the poly A(+) RNA with (32)P-labeled 180 base pair BamH1/PvuII restriction fragment containing the cDNA for human MTII from the phMT-II(3) plasmid. Slot blot analysis of poly A(+) RNA revealed a dose-dependent increase in brain MTII hybridizable mRNA. Northern blot analysis of poly A(+) RNA extracted from the rat liver and brain using (32)P-labeled cDNA exhibited a major band of hybridization at 700 bases in both tissues. These data provide evidence that the zinc-induced MT synthesis in the brain is associated with an accumulation of mRNA which is analogous to the zinc-induced synthesis of hepatic MT mRNA. However, other evidence indicates that the factors regulating the synthesis of the brain and the hepatic metallothioneins are not identical.