Nuclear Zinc Uptake and Interactions and Metallothionein Gene Expression are Influenced by Dietary Zinc in Rats

Abstract

Regulation of metallothionein gene expression by dietary zinc and the relationship of dietary zinc to nuclear zinc uptake was examined in growing rats. Zinc was fed at 5, 30 or 180 mg/kg, either in pelleted form for a 2-wk period (ad libitum) or for 2 h as a liquefied preparation (1 g in 4 mL). Two hours after the oral dose, the intestine and liver took up more zinc than other tissues. Nuclei purified from liver, kidney and spleen accumulated substantial amounts of zinc and directly reflected the dietary zinc level within the 2-h feeding period. Nuclei from kidney accumulated the largest amount of dietary zinc within 2 h, accounting for up to 6.2% of the total nuclear zinc concentration. Northern analysis demonstrated that metallothionein expression was proportional to dietary zinc intake in some tissues. It was greatest in kidney, followed in descending order by liver, intestine, spleen and heart. Thymus and lung metallothionein mRNA levels were not changed appreciably by dietary zinc intake. Chromatography of extracts from liver nuclei shows that 65Zn introduced into the portal supply is bound to discrete fractions of nuclear proteins. One of these fractions binds both 65Zn and a 32P-labeled oligonucleotide corresponding to the metal regulatory element of the metallothionein promoter. These results demonstrate that significant amounts of zinc from the diet are rapidly taken up by cell nuclei. Furthermore, they suggest that transcriptional regulation of the metallothionein gene and other genes with metal regulatory elements involves a direct interaction between the dietary supply and intranuclear factors that bind zinc.