Arsenic trioxide suppresses paclitaxel-induced mitotic arrest

MicroRNA-Gene Interactions Impacted by Toxic Metal(oid)s during EMT and Carcinogenesis

Chronic environmental exposure to toxic metal(loid)s significantly contributes to human cancer development and progression. It is estimated that approximately 90% of cancer deaths are a result of metastasis of malignant cells, which is initiated by epithelial-mesenchymal transition (EMT) during early carcinogenesis. EMT is regulated by many families of genes and microRNAs (miRNAs) that control signaling pathways for cell survival, death, and/or differentiation. Recent mechanistic studies have shown that toxic metal(loid)s alter the expression of miRNAs responsible for regulating the expression of genes involved in EMT. Altered miRNA expressions have the potential to be biomarkers for predicting survival and responses to treatment in cancers. Significantly, miRNAs can be developed as therapeutic targets for cancer patients in the clinic. In this mini review, we summarize key findings from recent studies that highlight chemical-miRNA-gene interactions leading to the perturbation of EMT after exposure to toxic metal(loid)s including arsenic, cadmium, nickel, and chromium.

CBX8 interacts with chromatin PTEN and is involved in regulating mitotic progression

OBJECTIVES

Besides its role in regulating phosphatidylinositol-3 kinase (PI3K) signalling in the cytosol, PTEN also has a nuclear function. In this study, we attempted to understand the mechanism of chromatin PTEN in suppressing chromosomal instability during cell division.

MATERIALS AND METHODS

Immunocoprecipitation, ectopic expression, and deletional analyses were used to identify the physical interaction between Chromobox Homolog protein 8 (CBX8) and PTEN, as well as the functional domain(s) of PTEN mediating the interaction. Cell synchronization followed by immunoblotting was employed to study cell cycle regulation of CBX8 and the functional interaction between chromatin PTEN and CBX8. Small interfering RNAs (siRNAs) were used to study the role of PTEN and CBX8 in modulating histone epigenetic markers during the cell cycle.

RESULTS

Polycomb group (PcG) proteins including CBXs function to repress gene expression in a wide range of organisms including mammals. We recently showed that PTEN interacted with CBX8, a component of Polycomb Repressing Complex 1 (PRC1), and that CBX8 co-localized with PTEN in the nucleus. CBX8 levels were high, coinciding with its phosphorylation in mitosis. Phosphorylation of CBX8 was associated with monoubiquitinated PTEN and phosphorylated-BubR1 on chromatin. Moreover, CBX8 played an important role in cell proliferation and mitotic progression. Significantly, downregulation of either PTEN or CBX8 induced H3K27Me3 epigenetic marker in mitotic cells.

CONCLUSION

CBX8 is a new component that physically interacts with chromatin PTEN, playing an important role in regulating mitotic progression.

Chronic low dose arsenic exposure preferentially perturbs mitotic phase of the cell cycle

Environmental pollution is a big challenge for human survival. Arsenic compounds are well-known biohazard, the exposure of which is closely linked to onsets of various human diseases, particularly cancers. Upon chronically exposing to arsenic compounds, genomic integrity is often disrupted, leading to tumor development. However, the underlying mechanisms by which chronic, low dose arsenic exposure targets genetic stability to initiate carcinogenesis still remain not fully understood. In this study, human lung epithelial BEAS-2B cells and keratinocytes were treated with 0.5 μM of sodium arsenite for one month (designated as BEAS-2B-SA cells or keratinocytes-SA), and its effect on cell cycle responses was analyzed. After being arrested in mitotic phase of the cell cycle by nocodazole treatment, BEAS-2B-SA cells or keratinocytes-SA were delayed to enter next cytokinesis. The lagging exit of the cells from mitosis was accompanied by a sustained Plk1 phosphorylation, which led to a persistent activation of the mitotic regulators BubR1 and Cdc27. As the result, cyclin B1 (clnB1) degradation was attenuated. BEAS-2B-SA cells or keratinocytes-SA also expressed a constitutively active Akt. The cytogenetic analysis showed an increased numbers of aneuploidy in these cells. The suppression of Akt reversed the aberrant expressions of the mitotic regulators, delay of mitotic exit as well as chromosomal aberrations. Our findings suggest that a long-term exposure to low dose sodium arsenite aberrantly retains the catenation of mitosis, which facilitates establishing genetic instability and predisposes the cells to tumorigenesis.

Rac1 and Cdc42 Play Important Roles in Arsenic Neurotoxicity in Primary Cultured Rat Cerebellar Astrocytes

This study aimed to explore whether Rac1 and Cdc42, representative members of Ras homologue guanosine triphosphatases (Rho GTPases), are involved in neurotoxicity induced by arsenic exposure in rat nervous system. Expressions of Rac1 and Cdc42 in rat cerebellum and cerebrum exposed to different doses of NaAsO2 (Wistar rats drank 0, 2, 10, and 50 mg/L NaAsO2 water for 3 months) were examined. Both Rac1 and Cdc42 expressions increased significantly in a dose-dependent manner in cerebellum (P < 0.01) by Western blot and immunohistochemistry assay, but in cerebrum, Rac1 and Cdc42 expressions only in 2 mg/L exposure groups were significantly higher than those in control groups (P < 0.01). Five to 50 μM NaAsO2 decreased cell viability in a dose-dependent manner in primary cultured rat astrocytes, whereas 1 μM NaAsO2 increased the cell viability in these cells. Rac1 inhibitor, NSC23766, decreased NaAsO2-induced apoptosis and increased the cell viability in primary cultured rat cerebellar astrocytes exposed to 30 μM NaAsO2. Cdc42 inhibitor, ZCL278, increased cell viability in the cells exposed to 30 μM NaAsO2. Taken together, our current studies in vivo and in vitro indicate that activations of Rac1 and Cdc42 play a very important role in arsenic neurotoxicity in rat cerebellum, providing a new insight into arsenic neurotoxicity.

Low-dose arsenic-mediated metabolic shift is associated with activation of Polo-like kinase 1 (Plk1)

Arsenic is a well-established human carcinogen associated with cancers of the skin, liver, lung, kidney, and bladder. Although numerous carcinogenic pathways have been proposed, the molecular mechanisms underlying arsenic-associated cancer etiology are still elusive. The cellular responses to arsenic exposure are dose dependent. It was recently shown that low-dose arsenic leads to a metabolic shift from mitochondrial respiration to aerobic glycolysis via inactivation of tumor suppressor p53 and activation of NF-κB. However, how inactivation of p53, activation of NF-κB, and metabolic change are coordinated in response to low-dose arsenic exposure is still not completely understood. Polo-like kinase 1 (Plk1) is a well- documented regulator in many cell cycle-related events. Herein, we showed that low-dose arsenic leads to elevation of Plk1 in an NF-κB-dependent manner and that elevation of Plk1 contributes to the metabolic change from oxidative phosphorylation to glycolysis via activation of the PI3K/AKT/mTOR pathway. Furthermore, we showed that inhibition/depletion of Plk1 reverses low-dose arsenic-associated phenotypes, including enhanced cell proliferation, activation of the PI3K/AKT/mTOR pathway, and increased glycolysis. Finally, inhibition of the PI3K/AKT/mTOR pathway also antagonizes the enhanced glycolytic influx due to low-dose arsenic exposure. Our studies support the notion that Plk1 likely plays a critical role in cellular responses to low-dose arsenic.

The telomere/telomerase binding factor PinX1 regulates paclitaxel sensitivity depending on spindle assembly checkpoint in human cervical squamous cell carcinomas

Paclitaxel is a main ingredient in the combination chemotherapy treatment of advanced human cervical squamous cell carcinomas. We investigated the roles and underlying molecular mechanisms of PinX1 in cervical squamous cell carcinomas (CSCC) cells response to paclitaxel and its clinical significances. The expression dynamics of PinX1 was first examined by immunohistochemistry in 122 advanced CSCC patients treated with cisplatin/paclitaxel chemotherapy. The expression of PinX1 was significantly associated with the effects of cisplatin/paclitaxel chemotherapy in advanced CSCCs (P<0.05). High expression of PinX1 correlated with CSCC's response to cisplatin/paclitaxel chemotherapy, and was an independent predictor of shortened survival (P<0.05). A series of in vivo and in vitro assays were performed to elucidate the function of PinX1 on CSCC cells chemosensitivity to paclitaxel and underlying mechanisms. In CSCC cells, the levels of PinX1 were only associated with the cytotoxicity and sensitivity of paclitaxel, in which knockdown of PinX1 dramatically enhanced paclitaxel cytotoxicity, whereas the reestablishment of PinX1 levels substantially reduced the paclitaxel-induced killing effect. In addition, we identified that the ability of PinX1 to stabilize the tension between sister kinetochores and maintain the spindle assembly checkpoint was the main reason CSCC cells undergo apoptosis when treated with paclitaxel, and further studies demonstrated that shortened distance between sisters kinetochores by nocodazole confers upon PinX1-replenished cells a sensitivity to the death inducing paclitaxel effects. Furthermore, our study of CSCC cells xenografts in nude mice confirmed the role of PinX1 in paclitaxel sensitivity in vivo. Our data reveal that PinX1 could be used as a novel predictor for CSCC patient response to paclitaxel, and the role of PinX1-mediated paclitaxel sensitivity might represent a new direction for the development of a new generation of microtubule drugs.

Cisplatin plus sodium arsenite and hyperthermia induces pseudo-G1 associated apoptotic cell death in ovarian cancer cells

Cisplatin is effective against solid tumors including ovarian cancer. However, inherent or acquired cisplatin resistance limits clinical success. We recently demonstrated that a combination of sodium arsenite (NaAsO2) and hyperthermia sensitizes p53-expressing ovarian cancer cells to cisplatin by modulating DNA repair pathway and enhancing platinum accumulation. However, it is not understood how this combination therapy modulates cell cycle following platinum-DNA damage. The goal of the present study was to determine if NaAsO2 and hyperthermia alter cisplatin-induced G2 arrest and cause mitotic arrest and mitotic catastrophe. Human epithelial ovarian cancer cells (A2780 and A2780/CP70) were treated with cisplatin ± 20 μM NaAsO2 at 37 or 39°C for 1 h. Cisplatin ± NaAsO2 at 37 or 39°C caused cells to accumulate in G2/M compartment at 36 h after treatment. Western blot analysis of cyclin A and cyclin B suggested that combined NaAsO2, hyperthermia, and cisplatin induced mitotic arrest. However, we observed < 3% mitotic index and phosphorylation of histone H3 on serine 10 was undetectable. These results did not confirm mitotic arrest. BUBR1 (BUB1B) also was not phosphorylated, suggesting disrupted mitotic checkpoint. Postmitotic cells accumulated in pseudo-G1 as demonstrated by cyclin E stabilization, CDKN1A induction, and hypophosphorylation of retinoblastoma protein. These cells also were positive for Annexin V binding indicating they were apoptotic. In summary, cisplatin plus NaAsO2 and hyperthermia induced pseudo-G1 associated apoptosis in ovarian cancer cells.

Arsenic trioxide induces abnormal mitotic spindles through a PIP4KIIγ/Rho pathway

Arsenite-induced spindle abnormalities result in mitotic cell apoptosis in several cancer cell lines, but how arsenite induces these effects is not known. Evidence to date has revealed that arsenite activates Rho guanosine triphosphatases (GTPases). Because Rho GTPases regulate spindle orientation, chromosome congression, and cytokinesis, we therefore examined the involvement of Rho GTPases and their modulators in arsenite-induced mitotic abnormalities. We demonstrated that arsenic trioxide (ATO) disrupted the positioning of bipolar mitotic spindles and induced centrosome and spindle abnormalities. ATO increased the level of the active guanosine triphosphate-bound form of Rho. Inhibition of Rho-associated protein kinases (ROCKs) by Y-27632 ameliorated ATO-induced spindle defects, mitotic arrest, and cell death. These results indicate that ATO may induce spindle abnormalities and mitotic cell death through a Rho/ROCK pathway. In addition, screening of a human kinase and phosphatase shRNA library to select genes that mediate ATO induction of spindle abnormalities resulted in the identification of phosphatidylinositol-5-phosphate 4-kinase type-2 gamma (PIP4KIIγ), a phosphatidylinositol 4,5-biphosphate (PIP2) synthesis enzyme that belongs to the phosphatidylinositol phosphate kinase (PIPK) family. Sequestration of PIP2 by ectopic overexpression of the pleckstrin homology domain of phospholipase C-δ1 protected cells from ATO-induced cell death. Furthermore, depletion of PIP4KIIγ, but not other isoforms of the PIPK family, not only reduced Rho GTPase activation in ATO-treated cells but also alleviated ATO-induced spindle defects, mitotic arrest, and mitotic cell apoptosis. Thus, our results imply that ATO induces abnormalities in mitotic spindles through a PIP4KIIγ/Rho pathway, leading to apoptosis of mitotic cells.

Chromium induces chromosomal instability, which is partly due to deregulation of BubR1 and Emi1, two APC/C inhibitors

Disruption of cell cycle checkpoints and interference with the normal cell cycle progression frequently result in cell death or malignant transformation. Hexavalent chromium [Cr(VI)] is a well-known carcinogen that has been implicated in the occurrence of many types of human malignancies, including lung cancer. However, the exact mechanism by which Cr(VI) causes malignant transformation in the lung remains unknown. We have demonstrated that chronic exposure to a non-cytotoxic concentration of Cr(VI) induced a variety of chromosomal abnormalities, including premature sister chromatid separation, chromosomal breakage and the presence of lagging/misaligned chromosomes. After treatment with nocodazole, both HeLa and normal lung bronchial epithelial cells were arrested at mitosis. However, Cr(VI) significantly compromised M-phase arrest induced by nocodazole. Cr(VI) suppressed BubR1 activation and reduced expression of Emi1, leading to an unscheduled activation of APC/C. Consistent with this observation, Cr(VI) treatment caused enhanced polyubiquitination of geminin during mitotic release, while it deregulated the activity of Cdt1, a DNA replication licensing factor. Combined, these results suggest that Cr(VI)-induced chromosomal instability is partly due to a perturbation of APC/C activities, leading to chromosomal instability.

Suppression of genomic instabilities caused by chromosome mis-segregation: a perspective from studying BubR1 and Sgo1

Aneuploidy is a major manifestation of chromosomal instability, which is defined as a numerical abnormality of chromosomes in diploid cells. It is highly prevalent in a variety of human malignancies. Increased chromosomal instability is the major driving force for tumor development and progression. To suppress genomic stability during cell division, eukaryotic cells have evolved important molecular mechanisms, commonly referred to as checkpoints. The spindle checkpoint ensures that cells with defective mitotic spindles or a defective interaction between the spindles and kinetochores do not initiate chromosomal segregation during mitosis. Extensive studies have identified and characterized more than a dozen genes that play important roles in the regulation of the spindle checkpoint in mammalian cells. During the past decade, we have carried out extensive investigation of the role of BubR1 (Bub1-related kinase) and Sgo1 (shugoshin 1), two important gene products that safeguard accurate chromosome segregation during mitosis. This mini-review summarizes our studies, as well as those by other researchers in the field, on the functions of these two checkpoint proteins and their molecular regulation during mitosis. Further elucidation of the molecular mechanisms of the spindle checkpoint regulation has the potential to identify important mitotic targets for rational anticancer drug design.