Glutaredoxin 2 and oxidative stress identification of two testis/cancer specific isoforms of human Grx2 in neuro-and medulloblastomas.

AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA LB-266 Oxidative stress constitutes a constant threat to the integrity of proteins, lipids, and DNA. Once repair- and turnover processes fail to remove oxidative damage, crucial cellular processes like apoptosis or growth are being disturbed and can lead to cancer etc. Protein thiols in form of cysteine residues are key players in redox sensing and regulation. The thioredoxin family of proteins consists, among others, of Trxs (thioredoxins) and Grxs (glutaredoxins). Those small (9-16 kDa) glutathione dependent thiol-disulphide oxidoreductases share a dicysteine active site motif (Cys-X-X-Cys) in a common structure called the Trx-fold enabling them to specifically reduce oxidized proteins. So far 3 different human Grxs have been identified in human, the well characterized cytosolic hGrx1, the mainly mitochondrial hGrx2 and the monothiol hGrx5. We have shown that transient knock down of hGrx2 sensitizes HeLa cells towards cell death induced by doxorubicin and phenyl arsene oxide, two commonly used anti-cancer compounds. Immunohistochemistry on patient samples revealed an highly elevated expression of hGrx2 in neuro- and medullablastomas. We identified 3 transcript variants of hGrx2. The mitochondrial hGrx2a is ubiquitously expressed, emphasizing the general importance for mitochondrial redox homeostasis. The new cytosolic variant hGrx2c and the cytosolic / nuclear hGrx2b are expressed exclusively in testis and various cancer cell lines. Our data presents the first indications for hGrx2 outside the mitochondria. Furthermore, in vitro data revealed that hGrx2a and hGrx2c form an inactive holo-dimer coordinated by a GSH-mixed 2-Iron-2-Sulfur cluster. During oxidative stress conditions, the GSSG/GSH ratio increases, the dimer gets disassembled and hGrx2a/c get activated. This 2Fe/2S cluster therefore represents a GSH-dependent redox sensor at the crossroad between cell death and survival. hGrx2b is unable to form a holo-complex, in contrary to previous results we found that this testis/cancer specific isoform is present in both the cytosol and the nucleus. Our results provide evidence for the presence of three hGrx2 protein isoforms in vivo and suggest additional roles of this oxidative stress-induced and apoptosis inhibiting protein in cellular differentiation and tumor progression.