The Effect Of Death Domain-associated Protein DAXX In Ovarian Cancer And Ovarian Epithelium Cells

Ovarian cancer is the fifth most common cause of death from allcancers among women in developed countries and is the primary cause ofdeath among gynecological malignancies. In mature woman, the OSE is asingle layer of cuboidal epithelium that covers the ovary surface and hassecretory and transport functions. The integrated DNA of OSE cells at anovarian rupture site is compromised during the ovulatory process. DNAdamage induced by ovulation in OSE cells at the periphery of the ovulatorysite has been reported in sheep. Chemical agents like methoxychlor can alsoinduce oxidative stress and DNA damage in OSE cells. Valko et al reportedoxidative DNA damage in various neoplasms and suggested that suchdamage may be an early genetic abnormality associated with malignancy.Small molecules that cause DNA damage, such as cisplatin (CDDP),bleomycin (BLM), etoposide (VP-16), and doxorubicin, are first-lineanticancer agents for human ovarian cancer whose effects result, in part,from apoptosis induction. Chemoresistance remains a major hurdle tosuccessful treatment. Recent evidence indicates that the inability of cancer cells to undergo apoptosis is a key determinant of chemotherapy resistance.Dysregulation of pro-apoptotic mediators, such as FAS, caspases, and p53,and anti-apoptotic pathways, such as AKT, X-linked IAP, andFAS-associated death domain-like interleukin-1b-converting enzyme(FLICE)-like inhibitory protein (FLIP) pathways, has been demonstrated inchemoresistant cells.DAXX was initially identified as a pro-apoptotic protein that boundto the death domain of the CD95death receptor. By activating the c-JunNH2-terminal kinase-pathway, DAXX was shown to enhance bothCD95-mediated and transforming growth factor-β-dependent apoptosis.Interestingly, Daxx down-regulation by RNA interference was alsoassociated with increased apoptosis. Moreover, targeted disruption of themurine Daxx gene resulted in embryonic lethality due to extensive globalapoptosis, which suggested that DAXX could also have anti-apoptoticeffects.Aside from its controversial role in apoptosis, DAXX is a wellestablished transcription regulator. DAXX can interact with several crucialproteins involved in transcriptional silencing, such as histone deacetylasesHDAC1and HDAC2and DNA methyltransferase DNMT1. DAXXrepresses the activity of several transcriptional factors, including C/EBPβ,c-Met, Pax3, Ets1, p53and its family members p73and p63,glucocorticoid receptor, androgen receptor, and SMAD4. Consistent with its involvement in transcriptional regulation, DAXXis predominantly a nuclear protein. In the nucleus, it primarily localizes tosub-nuclear structures, so-called PML-oncogenic domains (PODs), bybinding to SUMO-modified PML. Promyelocytic leukemia (PML) proteinnuclear bodies (NBs) are macromolecular nuclear domains that are found invirtually all mammalian cells. PML nuclear bodies (PML-NBs) have beenfunctionally linked to various fundamental cellular processes, includingtranscriptional control, tumor suppression, and apoptosis regulation. Insupport of the important role of PML and its associated NBs in apoptosisregulation, several apoptotic regulators localize to PML-NBs and cells fromPML-deficient mice display severe apoptotic defects. However, thepossible involvement of DAXX and PML in the proliferation and apoptosisof ovarian cancer cells has not been examined.Part I, we both over-expressed Daxx and depleted Daxx in primarymOSE cells. We found that Daxx deletion accelerated senescence in ap53/p21-dependent manner and promoted DNA damage by interacting withPML bodies without affecting cell cycle progression. These results suggestthat DAXX may transform mOSE cells to an ovarian oncogenic phenotypeand may be an anti-cancer target.Part II, to Understanding the genes involved in apoptosis and DNAdamage responses may improve therapeutic strategies for ovarian cancer，we found that DAXX was highly expressed in human ovarian surface epithelial tumors, but not in granulosa cell tumors. In cultured ovariancancer cells, DAXX interacted with promyelocytic leukemia protein PMLand localized to sub-nuclear domains (so-called PML nuclear bodies). Arole for DAXX in ovarian cancer cell proliferation, metastasis, andradio/chemoresistance was examined. Over-expression of DAXXenhanced multiple ovarian cancer cell lines’ proliferation, colonyformation, and migration, whereas Daxx depletion by RNA interferencehad the opposite effects. When transplanted into nude mice, ovariancancer cells that over-expressed DAXX displayed enhanced tumorigenesiscapability in vivo, whereas Daxx depletion inhibited tumor development.Importantly, Daxx induced tumorigenic transformation of normal ovariansurface epithelial cells. Daxx also protected ovarian cancer cells againstX-irradiation-and chemotherapy-induced DNA damage by interacting withPML. Taken together, our results suggest that DAXX is a novel ovariancancer oncogene that promotes ovarian cancer cell proliferation andchemoresistance in ovarian cancer cells. Thus, modulating DAXX-PMLnuclear body activity may be an effective strategy for preventing therecurrence and chemoresistance of ovarian cancers.