JMJD2B Mediates Apoptosis In Prostate Cancer Cells And RAD21Participates In Senescence Of Human Breast Cancer Cells

This thesis is composed of two parts of research work.Part Ⅰ：TSA-induced JMJD2B downregulation is associated with Cyclin B1-dependentsurvivin degradation and apoptosis in LNCap cellsHistone deacetylase (HDAC) inhibitors are emerging as a novel class of anti-tumoragents and have manifested the ability to induce apoptosis of cancer cells, and a significantnumber of genes have been identified as potential effectors responsible for HDACinhibitor-induced apoptosis. However, the mechanistic actions of these HDAC inhibitors inthis process remain largely undefined. We here report that the treatment of LNCap prostatecancer cells with HDAC inhibitor trichostatin A (TSA) resulted in downregulation of theJumonji domain-containing protein2B (JMJD2B). We also found that the TSA-mediateddecrease in survivin expression in LNCap cells was partly attributable to downregulation ofJMJD2B expression. This effect was attributable to the promoted degradation of survivinprotein through inhibition of Cyclin B1/Cdc2complex-mediated survivin Thr34phosphorylation. Consequently, knockdown of JMJD2B enhanced TSA-induced apoptosis byregulating the Cyclin B1-dependent survivin degradation to potentiate the apoptosispathways.Part Ⅱ：Suppression of RAD21induces senescence of MDA-MB-231human breastcancer cells through activation of the pRb pathway by c-Myc downregulationCell senescence, an irreversible cell cycle arrest, reflects a safeguard program that limitsthe capacity of uncontrolled cell proliferation. To identify new genetic events controllingsenescence, we have performed a loss-of-function genetic screen on human cancer cells.Among the5positive candidate genes resulted from the screening, we focused on RAD21inthe following study. We found that knockdown of RAD21by RNA interference inMDA-MB-231human breast cancer cells results in the appearance of several senescentmarkers, including enhanced senescence-associated-β-galactosidase activity and formation ofheterochromatin foci, elevated levels of p21and engaging the pRb pathway. Intriguingly,RAD21knockdown led to a down-regulation of c-Myc and its targets, including CDK4, anegative regulator of pRb, blocking pRb phosphorylation and engage pRb-mediatedtranscriptional repression of E2F. The down-regulation of c-Myc was in partproteasome-dependent and correlated with its localization to promyelocytic leukemia(PML)bodies, which were found to be highly abundant during RAD21knockdown-inducedsenescence. Importantly, introduction of c-Myc allowed cells to bypass senescence induced by RAD21knockdown. Furthermore, we show that the pharmacologic inhibition of p38MAPKsignaling pathways implicated in RAD21knockdown-induced senescence. Together, theseresults reveal an unanticipated function of RAD21of its involvement in cellular senescencemainly through activation of the pRb pathway by c-Myc down-regulation.