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

Abstract

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.

Copper induces Cu-ATPase ATP7A mRNA in a fish cell line, SAF1

“…It is hypothesised that MTF1 activation occurs through mobilization of free pools of Zn from other proteins by metals such as Cd or oxidative stress (Li et al, 2006). However, there is also evidence for induction of MT by silver and arsenic through mechanisms other then Zn mobilization (Mayer et al, 2003;He and Ma 2009). Importantly, since neither Cd nor Zn resulted in changes in ATP7A mRNA, it is likely that the increase in ATP7A expression caused by Cu treatment is not by an MTF1-dependent mechanism.…”

CHAPTER 17. Arsenic, Antimony and Bismuth

“…107 Arsenic is not a cumulative toxin in the human body but has been shown to induce and bind metallothioneins (MT) through transcription factor 1. 105,108,109 Arsenic trioxide induces MT-II, MT-II and MT-III in human glioblastoma cells and this may function as a cytoprotective mechanism against arsenicinduced brain tumours, but a lot more research is still needed here. Variations in the sensitivity of different people to arsenic-related toxicity are in part attributable to genetic polymorphisms influencing the expression of intracellular MT synthesis and arsenic binding or complexing.…”

The legless lizard Anguis fragilis (slow worm) has a potent metal-responsive transcription factor 1 (MTF-1)

“…Moreover, the overall length of slow worm MTF-1 (738 aa) is similar to Anolis (754 aa) and human (753 aa) and thus clearly different from MTF-1 in the mouse (675 aa) and another rodent, the South American capybara Hydrochoerus hydrochaeris (638 aa) (Lindert et al, 2008). The 'cysteine cluster' CQCQCAC, which is indispensable for MTF-1 activity (Chen et al, 2004;He and Ma, 2009;Günther et al, 2012a) (Figure 1), is also perfectly conserved in Anguis.…”