Zinc-Metallothionein Levels Are Correlated with Enhanced Glucocorticoid Responsiveness in Mouse Cells Exposed to ZnCl2, HgCl2, and Heat Shock

Abstract

Metallothioneins (MTs) are the major low molecular weight, zinc-binding proteins in mammalian cells. It has been hypothesized that they play a role in the function of zinc-dependent signal transduction proteins and transcription factors. We investigated the capacity of zinc and other metal ions and conditions to increase both Zn-associated MT levels and the receptiveness of cells to transcriptional activation mediated by the zinc-dependent glucocorticoid receptor (GR). We studied, in a GR-responsive mouse mammary-tumor cell line, the ability of dexamethasone (DEX) to stimulate transcription of a chloramphenicol acetyltransferase (CAT) gene controlled by a mouse mammary-tumor virus promoter. In cells pretreated with 20 to 100 microM ZnCl(2), DEX-induced CAT activity correlated with zinc-induced MT levels. However, 0.05 to 0.5 microM CdCl(2) had no effect on CAT activity, despite an increase in Cd-associated MT. Copper-associated MT was detected in cells treated with 20 microM CuCl(2,) but there was no change in the level of Zn-MT, nor was CAT activity altered in cells exposed to 5 to 20 microM CuCl(2). These results may reflect a functional difference between zinc-associated MT, and MT associated with other metals. Significantly more CAT activity was observed in both heat-shocked cells and in cells exposed to 40 or 50 nM HgCl(2). Although absolute amounts of MT were unchanged by these two treatments, a higher percentage of total cellular zinc was associated with the MT protein fractions after treatment. Changes in GR levels could not account for variations in CAT activity. These data indicate that hormonal signalling can be altered by exposure to metal salts and heat shock, and the effect is correlated with the level of Zn-MT.

Metallothionein Mediates the Level and Activity of Nuclear Factor ΚB in Murine Fibroblasts

“…Although the mechanism by which MTs may mediate NF-B activity is not known, there is a wealth of circumstantial evidence that they regulate zinc availability to proteins jpet.aspetjournals.org (Koropatnick and Zalups, 2000;DeMoor et al, 2001). However, MT is capable of either donating or sequestering zinc, depending on the affinity of MT for zinc compared with other proteins, overall zinc status within cells, and cellular location of MTs and putatively dependent proteins (Koropatnick and Zalups, 2000).…”

“…MTs bind a variety of metal ions (Ag, Hg, Cu, Cd, and others) but are associated primarily with zinc in most mammalian cells (reviewed in DeMoor et al, 2001). MTs are highly homologous and of similar structure, implying common biological function.…”

The Functions of Metamorphic Metallothioneins in Zinc and Copper Metabolism

“…For example, cadmium-induced MT expression proceeds through Cd 2+ displacing Zn 2+ in MT and the released Zn 2+ then activating MTF-1 [46,47]. MT also modulates glucocorticoid responsiveness of cells [48]. Thus MT participates in controlling the occupancy of Zn 2+ -dependent transcription factor with Zn 2+ either through its role in buffering Zn 2+ or in delivering Zn 2+ to the transcription factors directly.…”

The Glucocorticoid Receptor: A Revisited Target for Toxins

“…As with cadmium, this could be inhibited by the reducing agent DTT, suggesting the involvement of dithiols in the ligand binding region [136]. In 2305 cells, zinc increases dexamethasone induction of the GR-responsive MMTV promoter possibly through a metallothionein‑mediated pathway [134]. Although not well described, zinc may have effects on GR function.…”