Despite the constant exposure to genomic insults that may lead to malignancy, cancer is surprisingly a relatively rare occurrence, and this is largely credited to an elaborate network of endogenous tumor suppression. Many effectors of tumor suppression have been identified, and their functions when activated in damaged cells have in large part been elucidated. What is less clear is whether there are common target gene(s) of tumor suppression, whose expression must be ablated in order to block transformation and preserve cellular homeostasis. Fresh experimental evidence suggests that silencing of the mitotic regulator and cell death inhibitor, survivin, is a universal requirement for successful tumor suppression in humans.

Cancer is the second leading cause of death among men and women after heart disease. Though our knowledge associated with the complexities of the cancer network has significantly improved over the past several decades, we have only recently started to get a more complete molecular understanding of the disease. To better comprehend signaling pathways that prevent disease development, we focused our efforts on investigating endogenous tumor suppression networks in controlling effectors of cancer cell survival and proliferation. Survivin is one such effector molecule that controls both cell proliferation and survival. In order to identify how this protein is overexpressed in cancer cells as opposed to normal cells, we looked at signaling molecules that negatively regulate this inhibitor of apoptosis protein. PTEN and caspase 2 are two of the identified proteins that utilize their enzymatic activity to suppress tumor growth by inhibiting downstream cell survival effectors, namely survivin. PTEN uses its phosphatase activity to suppress the PI3K/AKT pathway and maintain cellular homeostasis. In the absence of AKT activity, FOXO transcription factors are able to target downstream gene expression and regulate cell proliferation and survival. Here we have identified survivin as a novel gene target of FOXO, which binds to a specific promoter region of survivin and suppresses its transcription. Alternatively, caspase 2 uses its catalytic activity to suppress survivin gene expression by targeting the NFκB pathway. Caspase 2 acts by cleaving a novel substrate known as RIP1 that prevents NFκB from entering the nucleus, thus inhibiting target gene transcription. Interestingly, survivin is known to be a direct gene target of NFκB that controls cancer cell survival. In our investigation, we found that survivin is downregulated upon caspase 2 activation via the NFκB pathway, resulting in decreased cell cycle kinetics, increased apoptotic threshold and suppressed tumor growth in mice. These studies conclude that survivin is a common effector molecule that is regulated by tumor suppressors to maintain cellular homeostasis. However, upon deactivation of the tumor suppressor pathway, survivin is deregulated and contributes significantly to disease progression. These observations may lead to potential therapeutic implications and novel targeting strategies that will help eradicate harmful cancer cells and spare surrounding healthy cells; often the most persistent problem of most conventional chemotherapy.

INTRODUCTION: Basal-type, or triple-negative, breast cancer (lacking estrogen receptor, progesterone receptor, and human epidermal growth factor receptor-2 expression) is a high-risk disease for which no molecular therapies are currently available. We studied genetic signatures of basal breast cancer potentially suitable for therapeutic intervention. METHODS: We analyzed protein expression of the Notch-1 intracellular domain and survivin by immunohistochemistry in a series of basal breast cancer patients. A hierarchical clustering and overall survival analysis was carried out on a microarray mRNA database of 232 breast cancer patients. Fifteen published mRNA datasets containing estrogen receptor-negative or estrogen receptor-positive samples were subjected to meta-analysis for co-segregated gene expression. Experiments of plasmid transfection and gene silencing were carried out in estrogen receptor-negative MDA-MB-231 breast cancer cells. RESULTS: The developmental signaling regulator Notch-1 was highly expressed in breast cancer, compared with normal tissue, and was segregated with basal disease. Higher Notch-1 levels correlated with progressively abbreviated overall survival, and with increased expression of survivin, a tumor-associated cell death and mitotic regulator implicated in stem cell viability. Analysis of Pearson's correlation coefficient indicated that Notch-1 and survivin co-segregated in basal breast cancer. Notch-1 stimulation in MDA-MB-231 cells increased survivin expression, whereas silencing Notch reduced survivin levels. CONCLUSIONS: A Notch-1-survivin functional gene signature is a hallmark of basal breast cancer, and may contribute to disease pathogenesis. Antagonists of Notch and survivin currently in the clinic may be tested as novel molecular therapy for these recurrence-prone patients.

The terminal step in cytokinesis, called abscission, requires resolution of the membrane connection between two prospective daughter cells. Our previous studies demonstrated that the coiled-coil protein centriolin localized to the midbody during cytokinesis and was required for abscission. Here we show that centriolin interacts with proteins of vesicle-targeting exocyst complexes and vesicle-fusion SNARE complexes. These complexes require centriolin for localization to a unique midbody-ring structure, and disruption of either complex inhibits abscission. Exocyst disruption induces accumulation of v-SNARE-containing vesicles at the midbody ring. In control cells, these v-SNARE vesicles colocalize with a GFP-tagged secreted polypeptide. The vesicles move to the midbody ring asymmetrically from one prospective daughter cell; the GFP signal is rapidly lost, suggesting membrane fusion; and subsequently the cell cleaves at the site of vesicle delivery/fusion. We propose that centriolin anchors protein complexes required for vesicle targeting and fusion and integrates membrane-vesicle fusion with abscission.

The involvement of myosin II in cytokinesis has been demonstrated with microinjection, genetic, and pharmacological approaches; however, the exact role of myosin II in cell division remains poorly understood. To address this question, we treated dividing normal rat kidney (NRK) cells with blebbistatin, a potent inhibitor of the nonmuscle myosin II ATPase. Blebbistatin caused a strong inhibition of cytokinesis but no detectable effect on the equatorial localization of actin or myosin. However, whereas these filaments dissociated from the equator in control cells during late cytokinesis, they persisted in blebbistatin-treated cells over an extended period of time. The accumulation of equatorial actin was caused by the inhibition of actin filament turnover, as suggested by a 2-fold increase in recovery half-time after fluorescence photobleaching. Local release of blebbistatin at the equator caused localized accumulation of equatorial actin and inhibition of cytokinesis, consistent with the function of myosin II along the furrow. However, treatment of the polar region also caused a high frequency of abnormal cytokinesis, suggesting that myosin II may play a second, global role. Our observations indicate that myosin II ATPase is not required for the assembly of equatorial cortex during cytokinesis but is essential for its subsequent turnover and remodeling.