Thioredoxin Reductase 1 in relation to cancer
The thioredoxin system and cancer
The number of published papers on thioredoxin systems has increased tremendously in recent years, with the papers on its relation to cancer increasing slightly faster than the field in general (Fig. 1). As of the end of 2006, about 4300 papers could be found in PubMed that dealt with thioredoxin in one way or the other, while about 400 articles were retrieved using the more defined combination of keywords “thioredoxin and cancer”. It is currently clear that the thioredoxin system is of importance for cancer through many separate cellular pathways and this is a rapidly expanding research field.
Research on thioredoxin in general is a rapidly increasing field, with more than 3500 papers published before 2005 whereof about half of them were published in the last five years of that period (bar graph, left Y-axis). The sub-class of papers retrieved from using a keyword combination of “Thioredoxin” and “Cancer” shows that more than 300 papers were published before 2005, with half of those papers written after year 2000. Today, when this summary was made (September, 2006), 4285 papers are retrieved with the keyword “Thioredoxin” (23% increase in number of papers within the last two years) while “Thioredoxin and cancer” retrieves 409 papers (32% increase in the last two years).
Further reading in "The thioredoxin system in cancer".
This figure illustrates the complex interplay of thioredoxin-regulated cellular functions linked to DNA synthesis, Antioxidant defense, Gene regulation and Intracellular signaling pathways that are modulated by or dependent upon reduction by thioredoxin. The redox state of thioredoxin is, in turn, dependent upon the activity of thioredoxin reductase, as well as other factors such as VDUP-1 (TBP2), oxidative stress as well as intra- or extracellular localization. It should be noted that most, if not all, of the functions of the thioredoxin system are dependent upon thioredoxin reductase(s), which are selenium dependent enzymes also reducing a number of other substrates in addition to thioredoxin. Targeting of thioredoxin reductase is also a potentially important mechanism for anticancer efficacy of electrophilic drugs.
Selenium compromised TrxR1 directly induces apoptosis
We discovered that TrxR1 either deficient in selenocysteine or with the selenocysteine residue derivatized by electrophilic compounds, such as cisplatin, could directly induce apoptosis when introduced into cells. This phenomenon may explain some of the mechanism for therapeutic effects of electrophilic drugs used in cancer therapy and potentially also some of the symptoms seen in selenium deficiency. The molecular mechanism for this apoptosis induction is currently investigated.
The first report was described in the following paper "Rapid induction of cell death by selenium-compromised thioredoxin reductase 1 but not by the fully active enzyme containing selenocysteine".
HeLa cells were treated with BioPORTER, TrxR1/BioPORTER or SecTRAPs/BioPORTER for 3 h as indicated in the figure, whereupon the images of the cells stained with the ROS-sensitive marker DCFH were taken, as described in the text. The cell nuclei were counterstained with Hoechst 33342. Ascorbic acid (Vit C) and/or ŚÁ-tocopherol (Vit E) was added 1 h in advance to the cells before treatment. Two distinct experiments with two samples in each treatment were performed with similar results and representative images of the observed staining patterns are shown. The three right-most pictures displaying the intracellular patterns of DCF fluorescence are shown at higher magnification than the three panels to the left displaying the overview of DCF fluorescence in control cells or in cells treated with either TrxR1 or SecTRAPs together with BioPORTER.
This result was described in "Cell death by SecTRAPs: thioredoxin reductase as a prooxidant killer of cells".
In the same paper, we have also concluded the following model for the formation as well as the function of the SecTRAP.
We propose that during most conditions of normal cell growth the Sec residue in TrxR1 is intact and the role of this enzyme is thus to sustain the many cellular functions of the thioredoxin system. This activity is dependent upon the redox active C-terminal Sec-containing active site in TrxR1. SecTRAPs may however be formed from TrxR1 if its Sec residue becomes compromised, either by removal or by derivatization with electrophilic compounds, while the FAD and N-terminal redox active CVNVGC motif of the enzyme are kept intact.
SecTRAPs lack the Trx reducing activity of native TrxR1 but become potent inducers of cell death by a gain of function. This cell death is, as shown in the present study, rapid, does not require induction of protein synthesis, involves production of reactive oxygen species and is prevented by caspase inhibitors. If, on the other hand, both the CVNVGC motif and the C-terminal selenolthiol motif become inactivated, e.g. by certain types of TrxR1 inhibitors or if the overall expression of TrxR1 is diminished in cells, impaired cell function or cell death may still occur. In such cases the cellular consequences would however not be due to a gain of function in derivatives of TrxR1, but rather to hampered functions of the complete cellular thioredoxin system.
Splice variants of TrxR1 protein isoforms, different 5'-UTR cDNA's and TXNRD1 variants
We have found evidence of a number of different splice variants of TXNRD1-derived transcripts, resulting in both different TrxR1 protein isoforms and a number ofl cDNA variants encoding the same protein product. The significance of these variants is not known and they are currently further studied in our laboratory.
Our initial findings are presented and further discussed in "Prominent expression of the selenoprotein thioredoxin reductase in the medullary rays of the rat kidney and thioredoxin reductase mRNA variants differing at the 5' untranslated region" and "Evidence for intriguingly complex transcription of human thioredoxin reductase 1".
In the same study where the different murine 5'-UTR domains were first found and discussed (previous figure), in situ hybridization using adult rat kidney was also performed, inspired by surprisingly high levels of TrxR1 mRNA in rat kidney as judged by Northern blots. The expression pattern was highly organized, as seen in this figure, with a prominent expression in the medullary rays. The pattern of expression exactly follows the proximal tubules, which is interesting as these cells are known to produce another selenoprotein, plasma glutathione peroxidase, which is secreted to plasma from these cells.
We identified the core promoter for human TrxR1 and found it to have typical characteristics of a housekeeping gene, which has interest in view of the mRNA being regulated by AU-rich elements. Further reading in "The core promoter of human thioredoxin reductase 1: cloning, transcriptional activity, and Oct-1, Sp1, and Sp3 binding reveal a housekeeping-type promoter for the AU-rich element-regulated gene".
TXNRD1_v3's unique expression pattern and it's role in the cell membrane protrusions
The TXNRD1_v3 isoform of human TrxR1 (short form notation "v3") has an atypical glutaredoxin domain in fusion with the common TrxR1 core module. It is encoded by the β1 transcript, which is transcribed from a promoter upstream of the more commonly used core promoter. We found that v3 is highly expressed in Leydig cells of the testis, but it is also found in ovary, some additional tissues and a number of cancer cell lines, it is responsive to sex hormone treatment, and it localizes to cell membranes in transfected cells. The glutaredoxin domain of this protein was also found to have the capacity to induce a formation of cell membrane protrusions, with actin and tubulin polymerisation following v3 into the protrusions. The biological importance of v3 is still unknown.
This was a collaborative project with Anastasios Damdimopoulos, Antonio Miranda-Vizuete and Alberto Jimenez.