Prostate cancer is the most prevalent cancer in males in developed countries. Nevertheless, the molecular pathogenesis of this disease remains largely elusive. In this thesis, I characterized and described the function of a putative tumor suppressor gene, TUSC3, on the short arm of chromosome 8. In our previous work, we defined loss of TUSC3 to be significantly associated with poor overall survival in ovarian cancer by systematically screening for tumor suppressor candidates on chromosome 8p22. Given that TUSC3 has been identified as putative tumor suppressor gene in prostate cancer, we focused on this cancer entity for further molecular and functional characterization of this gene. In silico prediction shows a high homology of TUSC3 to the Ost3p subunit of the oligosaccharyltransferase (OST) complex in yeast, postulating its possible function in posttranslational protein modification. It also predicts a putative thioredoxin domain within the TUSC3 protein, adding to the possible function of TUSC3 as a redox regulator within the endoplasmic reticulum.
In order to characterize TUSC3 in vitro and in vivo, we used loss and gain of function experiments in prostate cancer cell line models. We analyzed the role of TUSC3 in N-glycosylation and ER-stress as well as growth factor signaling in prostate cancer. We employed a varied spectrum of methods to achieve these aims, including standard biochemical assays as well as tissue microarray analyses, lectin based assays and transmission electron microscopy. Furthermore, we analyzed the effects of TUSC3 on carcinogenic properties of cancer cell lines, such as proliferation, migration, invasion, apoptosis and in vivo growth. We tried to elucidate the downstream effectors and mediators of increased carcinogenesis in TUSC3 knockdown cell lines. We find that knockdown of TUSC3 in prostate cancer uniformly leads to increased proliferation, invasion, and xenograft formation, although the individual effects depend on the genetic background of the cell line model. We uniformly see decreased ER stress in TUSC3 knockdown cell lines, thus suggesting a protective, antiapoptotic effect of TUSC3 loss.
We define a novel function of N-glycosylation in regulating ER stress and carcinogenesis of PTEN negative prostate cancer in particular. Our findings confirm the tumor suppressor function of TUSC3 and add to our understanding of posttranslational modifications in carcinogenesis.