Regulation of Metallothionein Transcription by the Metal-responsive Transcription Factor MTF-1

The robust induction of metallothionein-I and II (MT-I and MT-II) genes by several heavy metals such as zinc and cadmium requires the specific transcription factor metal-responsive transcription factor 1 (MTF1). Chromium (VI), a major environmental carcinogen, not only failed to activate these genes but also inhibited their induction by Zn 2؉ or Cd 2؉ . The heavy metal-induced expression of another MTF1 target gene, zinc transporter 1 (ZnT-1), was also down-regulated by Cr 6؉ . By contrast, the expression of two MTF1-independent Cd 2؉ -inducible genes, heme oxygenase 1 (HO-1) that was augmented in presence Cr 6؉ , whereas the onset of apoptosis was significantly delayed in cells overexpressing MTF1.Chromium, a potential human mutagen and carcinogen (1, 2), exists in many different oxidation states in the environment, Cr 6ϩ and Cr 3ϩ being the most stable forms. In contrast to Cr 6ϩ , Cr 3ϩ is generally benign and is considered an essential nutrient required for normal sugar and fat metabolism (3, 4). Although the predominant natural source of Cr 3ϩ is present in the environment, the majority of Cr 6ϩ originates from industrial activities. Hexavalent chromium, a powerful oxidant, is actively internalized by the anion transporter located on the plasma membrane (5). Subsequently, Cr 6ϩ is reduced through the reactive Cr 5ϩ and Cr 4ϩ intermediates to trivalent chromium, (6, 7), which are highly impermeable to cell membrane.The molecular mechanism(s) for the Cr 6ϩ -mediated toxicity has not been fully elucidated. The chromium intermediates appear to interact directly with cellular constituents that lead to generation of reactive oxygen species (8, 9). Chromium treatment results in DNA strand breakage (10, 11) and DNA-protein cross-linking in vivo (12, 13). Reactive oxygen species generated during intracellular reduction of Cr 6ϩ affects almost every aspect of cellular function. Damage to cellular macromolecules and aberrations in gene expression ultimately lead to apoptosis or necrosis in the majority of the cells and uncontrolled cell proliferation in a few, causing cancer. Apoptosis induced by Cr 6ϩ treatment in lung epithelial cells involves both p53-dependent and p53-independent pathways (14, 15). Cr 6ϩ also activates a stress response protein (NF-B), which in turn activates anti-apoptotic proteins (16), thus protecting cells from apoptotic cell death.Our laboratory has been studying the molecular mechanisms of heavy metal-induced expression of metallothionein genes (17). Metallothionein I and II (MT-I 1 and MT-II) belong to a family of low molecular weight, cysteine-rich, and high metalcontaining (as metal-thiolate clusters) stress response proteins that protect cells not only from heavy metals but also from reactive oxygen species (18,19). Metallothioneins induced in response to heavy metals like zinc, cadmium, copper, mercury, gold, silver, cobalt, nickel, and bismuth act as scavengers of these toxic metals and help maintain zinc and copper homeostasis (17,20). It was of interest to examine the induction of ...

Section: Discussionmentioning

confidence: 39%

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confidence: 99%

Chromium(VI) Down-regulates Heavy Metal-induced Metallothionein Gene Transcription by Modifying Transactivation Potential of the Key Transcription Factor, Metal-responsive Transcription Factor 1

Majumder¹,

Ghoshal²,

Summers

et al. 2003

“…Other inducers, such as glucocorticoids and alkylating agents, may also interact with MTF1-associated proteins to activate MTF1, because these inducers are unlikely to interact with the zinc fingers directly. Phosphorylation of MTF1 was observed and may contribute to nuclear translocation of activated MTF1 (33)(34)(35). Arsenic (As 3ϩ ) is a human carcinogen and a pleiotropic toxic metalloid that causes cancer (particularly of the skin, lung, bladder, and liver), neuropathy, cardiovascular lesions, ovarian dysfunction, aberrant embryonic development, and postnatal growth retardation in humans (36 -42).…”

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confidence: 99%

Induction of Metallothionein I by Arsenic via Metal-activated Transcription Factor 1

2009

Metal-activated transcription factor 1 (MTF1) mediates the induction of metallothioneins I and II by zinc and stress signals. The mechanism of MTF1 activation has not been well understood. We analyzed the interaction between arsenic (As 3؉ ) and MTF1 for Mt1 induction. As 3؉ potently induces Mt1 mRNA expression in mouse hepa1c1c7 cells. Induction is dependent upon functional MTF1 as induction is lost in Mtf1 knockout cells but is restored upon reconstitution with Mtf1; moreover, As 3؉ induces the binding of MTF1 to the metal response elements of endogenous Mt1. Induction is not affected by modulating zinc concentrations but is markedly enhanced by cycloheximide. Phenylarsine oxide (PAO), which covalently binds to vicinal protein cysteine thiol groups, induces Mt1 with a magnitude of higher potency than that of As 3؉ . PAO affinity beads effectively pulls down the carboxyl half of MTF1 (MTF1 321-675 ) by binding to a cluster of five cysteine residues near the terminus. Preincubation with As 3؉ , Cd 2؉ , Co 2؉ , Ni 2؉ , Ag ؉ , Hg 2؉ , and Bi 3؉ blocks pulldown of MTF1 321-675 by PAO beads in vitro and in vivo, indicating that binding of the metal inducers to the same C-terminal cysteine cluster as PAO occurs. Deletion of the C-terminal cysteine cluster or mutation of the cysteine residues abolishes or markedly reduces the transcription activation activity of MTF1 and the ability of MTF1 to restore Mt1 induction in Mtf1 knockout cells. The findings demonstrate a critical role of the C-terminal cysteine cluster of MTF1 in arsenic sensing and gene transcription via arsenic-cysteine thiol interaction.Humans are constantly exposed to a wide variety of metals from dietary, environmental, and occupational sources. Trace elements, such as copper, iron, and zinc, are required for certain biological functions under physiological concentrations. However, most metals are toxic and cause a wide range of pathological conditions, including cancer, toxicity, and chronic diseases in humans (1, 2). Induction of metallothioneins (MT I and II), 2 metal transporters, and antioxidative proteins represent a major means of metal detoxification in the body to maintain trace elements within a physiological range or to protect the body from the damage by metal overload. MTs are small, cysteine-rich, metal-binding proteins expressed in all eukaryotes and some prokaryotes. (3)(4)(5). MTs regulate metal homeostasis by sequestering metals in protein-bound forms, providing a zinc reserve, and serving as a scavenger to quench ROS and other free radicals. On the other hand, metal transporters export metals out of the cells, and antioxidant proteins/enzymes antagonize metalinduced oxidative stress.

“…Phosphorylation is the only modification that has been reported but the biological significance and the site(s) of modification remains unknown (21,22). Additionally, whether zinc treatment alters the phosphorylation level of MTF-1 remains controversial (22,42).…”

Section: Discussionmentioning

confidence: 99%

“…Alternatively, zinc and cadmium activate MTF-1 by phosphorylating MTF-1 through different kinase cascades. The exact mechanism of the modifications remains unknown (21)(22)(23).…”

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confidence: 99%

The Role of Small Ubiquitin-like Modifier-interacting Motif in the Assembly and Regulation of Metal-responsive Transcription Factor 1

Liu

Lin

Chen

et al. 2011

Metal-responsive transcription factor 1 (MTF-1) 2 is a zinc finger protein that regulates gene expressions in response to various stresses, such as heavy metal exposure (1), oxidative stress (2), and hypoxia (3). MTF-1 genes have been identified in many species from insects to mammals, and encode proteins with high amino acid identities (4 -6). The N-terminal region of MTF-1 has six Cys 2 -His 2 -type zinc finger motifs that specifically bind to metal responsive elements (MREs) at the promoters of target genes. Three other distinct domains (an acidic, a proline-rich, and a serine/threonine-rich) on MTF-1 are responsible for the transactivation activity of the protein (7,8).Metallothioneins (MTs) are the most extensively studied target genes of MTF-1 (7). MTs are low molecular weight, cysteine-rich proteins that play important roles in zinc homeostasis and heavy metal detoxification (9). MTF-1 is responsible for the basal and metal-induced expression of MT genes (10). In addition, MTF-1 regulates the expression of zinc transporter-1 (11), ␥-glutamyl-cysteine synthetase heavy chain (12), and placental growth factor (13). It also plays a critical role in embryonic development because disruption of the Mtf-1 gene in mouse leads to oxidative damage-derived impairment of hepatocytic development and fetal lethality (12).MTF-1 resides mainly in the cytoplasm. Upon metal exposure or stress induction, MTF-1 translocates into the nucleus and binds to MREs (14,15). Several factors, such as USF-1, USF-2, NF-B, and HIF-1␣, can modulate the transcriptional activity of . Histone acetyltransferase p300/ CBP and transcription factor Sp1 can rapidly form a complex with MTF-1 upon zinc treatment and stimulate gene transcription (20). Alternatively, zinc and cadmium activate MTF-1 by phosphorylating MTF-1 through different kinase cascades. The exact mechanism of the modifications remains unknown (21-23).Post-translational modification of proteins by small ubiquitin-like modifier (SUMO) has emerged as an important regulatory mechanism of cellular processes (24). Among the three major SUMO isoforms identified in mammalian cells, SUMO-1 is the best characterized isoform. SUMO-1 and ubiquitin have only 18% sequence identity but share similar structural folds (25). Comparable with ubiquitination, SUMO conjugates covalently to substrates via a specific enzymatic cascade, including the heterodimeric SUMO-activating enzyme SAE1/SAE2 (E1), the SUMO-conjugating enzyme Ubc9 (E2), and in some cases, the SUMO E3 ligases (26). Sumoylation of proteins occurs mostly on lysine residues with a ⌿KXE (where ⌿ is a bulky hydrophobic residue and X is any amino acid) sequence and the modification can be dynamically reversed by the sentrin/SUMO-specific proteases (SENPs) (27). In contrast to the ubiquitination of proteins for degradation, the functional consequences of protein sumoylation are diverse. Sumoylation * This work was supported by Grant NSC98-2311-B-007-015-MY3 from the National Science Council, Taiwan, Republic of China. □ S The on-line version...