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Superinduction of metallothionein I by inhibition of protein synthesis: role of a labile repressor in MTF-1 mediated gene transcription.

Authors
Bi-YY; Lin-GX; Millecchia-L; Ma-Q
Source
J Biochem Mol Toxicol 2006 Apr; 20(2):57-68
NIOSHTIC No.
20030071
Abstract
Induction of metallothioneins (MTs) through the metal-activated transcription factor-1 (MTF-1) provides a model response for analyzing transcriptional gene regulation by heavy metals. Here, we report inhibition of protein synthesis by cycloheximide (CHX) increases induction of Mt1 by approximately five-fold, a phenomenon designated as "superinduction." Characterization of superinduction revealed it is time- and concentration-dependent of CHX, requires the presence of an MTF-1 activator, and occurs at a transcriptional level, suggesting a labile repressor in the control of Mt1 induction. Genetic analyses using Mtf1 null cells and a metal response element (MRE)-driven reporter construct showed that superinduction of Mt1 is mediated through MTF-1 and MRE-dependent transcription. Analyses of intracellular zinc content by inductively coupled plasma emission spectroscopy and fluorescence imaging demonstrated that treatment with CHX alone or CHX plus an inducer does not increase the total zinc accumulation or the concentration of free zinc in cells under the conditions in which superinduction occurs. Moreover, superinduction was observed in cells cultured in a zinc-depleted medium, suggesting that superinduction does not involve elevation of intracellular zinc concentration. Northern blotting showed that Cd, CHX, or Cd + CHX does not affect the expression of the mRNA of MTF-1. Immunoblotting using antibodies specific for MTF-1 demonstrated that Cd induces a down-regulation of the MTF-1 protein, whereas cotreatment with Cd and CHX blocked the Cd-induced degradation of MTF-1. The findings reveal a new mechanistic aspect of the superinduction of Mt1, in which a labile repressor negatively controls agonist-induced turnover of the MTF-1 protein.
Keywords
Protein-synthesis; Heavy-metals; Metals; Genes; Cell-cultures; Zinc-compounds; Proteins
CODEN
JBMTFQ
CAS No.
7440-66-6; 66-81-9
Publication Date
20060401
Document Type
Journal Article
Email Address
qam1@cdc.gov
Fiscal Year
2006
NTIS Accession No.
NTIS Price
Issue of Publication
2
ISSN
1095-6670
NIOSH Division
HELD
Source Name
Journal of Biochemical and Molecular Toxicology
State
WV
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