Rapid Induction of Cell Death by Selenium-compromised Thioredoxin Reductase 1 but Not by the Fully Active Enzyme Containing Selenocysteine

Abstract

Mammalian thioredoxin reductases are selenoproteins. For native catalytic activity, these enzymes utilize a C-terminal -Gly-Cys-Sec-Gly-COOH sequence (where Sec is selenocysteine) forming a redox active selenenylsulfide/selenolthiol motif. A range of cellular systems depend upon or are regulated by thioredoxin reductase and its major protein substrate thioredoxin, including apoptosis signal-regulating kinase 1, peroxiredoxins, methionine sulfoxide reductase, and several transcription factors. Cytosolic thioredoxin reductase 1 (TrxR1) is moreover inhibited by various electrophilic anticancer compounds. TrxR1 is hence generally considered to promote cell viability. However, several recent studies have suggested that TrxR1 may promote apoptosis, and the enzyme was identified as GRIM-12 (gene associated with retinoid interferon-induced mortality 12). Transient transfection with GRIM-12/TrxR1 was also shown to directly induce cell death. To further analyze such effects, we have here employed lipid-mediated delivery of recombinant TrxR1 preparations into human A549 cells, thereby bypassing selenoprotein translation to facilitate assessment of the protein-related effects on cell viability. We found that selenium-deficient TrxR1, having a two-amino acid-truncated C-terminal -Gly-Cys-COOH motif, rapidly induced cell death (38 +/- 29% apoptotic cells after 4 h; p < 0.005 compared with controls). Cell death induction was also promoted by selenium-compromised TrxR1 derivatized with either cis-diamminedichloroplatinum (II) (cisplatin) or dinitrophenyl moieties but not by the structurally related non-selenoprotein glutathione reductase. In contrast, TrxR1 with intact selenocysteine could not promote cell death. The direct cellular effects of selenium-compromised forms of TrxR1 may be important for the pathophysiology of selenium deficiency as well as for the efficacy of antiproliferative drugs targeting the selenocysteine moiety of this enzyme.

Crystal Structure and Catalysis of the Selenoprotein Thioredoxin Reductase 1

“…Hence, without resonance effects of Tyr-116 and therefore a lack of an efficient reductive half-reaction, the result would easily be the observed increased propensity for juglone reduction but diminished activity with Trx as substrate. Similarly, the higher resistance to cisplatin derivatization upon Tyr-116 mutation also becomes understandable, because the selenolate of Sec-498 is the prime target for cisplatin targeting (57,69), whereas the selenenylsulfide is essentially inert to reaction with electrophilic compounds (70). Again, if Tyr-116 mutations would lead to an attenuated reduction of the selenenylsulfide, according to the model put forward here, the observed effects on the kinetics of the Tyr-116 mutations become explicable.…”

Oxidation of thioredoxin reductase in HeLa cells stimulated with tumor necrosis factor‐α

“…Recently, it was shown that introduction of the SeCys-deficient inactive TrxR1 into A549 human lung carcinoma cells induces rapid cell death [31]. To determine whether or not the TrxR2DN induction also affects cell death, TrxR2DN-induced and -uninduced cells were treated with TNF-a, etoposide, or UV irradiation and then cytoplasmic DNA fragmentation was examined.…”

“…Therefore, the SeCys residue can act as a cellular redox sensor sensing H 2 O 2 produced in cells which are exposed to the ROS-inducing factors such as epidermal growth factor [13,30]. Recently, it was shown that incorporation of SeCys residue deficient TrxR1 into human cells rapidly induced cell death [31]. From these observations, it is expected that the selenolate anion of SeCys residue of TrxRs is the primary target of ROS produced by TNF-a and that oxidation of the SeCys residue subsequently accelerates ROS mediated apoptosis.…”

Identification of thioredoxin reductase 1-regulated genes using small interference RNA and cDNA microarray

“…TrxR1, a component of the TrxR1/Trx redox system, is a selenoprotein involved in several biological functions, and is known to have either pro-or anti-apoptotic activities, 3,4 probably by regulating genes with opposing functions. Because TrxR1 is target for anti-cancer drugs, 10 the identification of the genes involved in the different responses is extremely important for therapeutic reasons.…”

“…While the selenoprotein has proliferative effects, the truncated form exhibits strong pro-apoptotic activity. 3,4 It has been suggested that the TrxR1/Trx system affects cellular processes by regulating the activity of transcription factors, leading to changes in gene expression. Some examples of this are shown in the modulation of p53, 5 NF-kB, 6 Sp-1, 7 and HIF1-α 8 .…”