Drosophila mitochondrial topoisomerase III alpha affects the aging process via maintenance of mitochondrial function and genome integrity.

BACKGROUND: Mitochondria play important roles in providing metabolic energy and key metabolites for synthesis of cellular building blocks. Mitochondria have additional functions in other cellular processes, including programmed cell death and aging. A previous study revealed Drosophila mitochondrial topoisomerase III alpha (Top3α) contributes to the maintenance of the mitochondrial genome and male germ-line stem cells. However, the involvement of mitochondrial Top3α in the mitochondrion-mediated aging process remains unclear. In this study, the M1L flies, in which Top3α protein lacks the mitochondrial import sequence and is thus present in cell nuclei but not in mitochondria, is used as a model system to examine the role of mitochondrial Top3α in the aging of fruit flies. RESULTS: Here, we reported that M1L flies exhibit mitochondrial defects which affect the aging process. First, we observed that M1L flies have a shorter life span, which was correlated with a significant reduction in the mitochondrial DNA copy number, the mitochondrial membrane potential, and ATP content compared with those of both wildtype and transgene-rescued flies of the same age. Second, we performed a mobility assay and electron microscopic analysis to demonstrate that the locomotion defect and mitophagy of M1L flies were enhanced with age, as compared with the controls. Finally, we showed that the correlation between the mtDNA deletion level and aging in M1L flies resembles what was reported in mammalian systems. CONCLUSIONS: The results reported here demonstrate that mitochondrial Top3α ablation results in mitochondrial genome instability and its dysfunction, thereby accelerating the aging process.

Topoisomerase IIß deficiency enhances camptothecin-induced apoptosis.

Camptothecin (CPT), a topoisomerase (Top) I-targeting drug that stabilizes Top1-DNA covalent adducts, can induce S-phase-specific cytotoxicity due to the arrest of progressing replication forks. However, CPT-induced non-S-phase cytotoxicity is less well characterized. In this study, we have identified topoisomerase IIß (Top2ß) as a specific determinant for CPT sensitivity, but not for many other cytotoxic agents, in non-S-phase cells. First, quiescent mouse embryonic fibroblasts (MEFs) lacking Top2ß were shown to be hypersensitive to CPT with prominent induction of apoptosis. Second, ICRF-187, a Top2 catalytic inhibitor known to deplete Top2ß, specifically sensitized MEFs to CPT. To explore the molecular basis for CPT hypersensitivity in Top2ß-deficient cells, we found that upon CPT exposure, the RNA polymerase II large subunit (RNAP LS) became progressively depleted, followed by recovery to nearly the original level in wild-type MEFs, whereas RNAP LS remained depleted without recovery in Top2ß-deficient cells. Concomitant with the reduction of the RNAP LS level, the p53 protein level was greatly induced. Interestingly, RNAP LS depletion has been well documented to lead to p53-dependent apoptosis. Altogether, our findings support a model in which Top2ß deficiency promotes CPT-induced apoptosis in quiescent non-S-phase cells, possibly due to RNAP LS depletion and p53 accumulation.

Signaling mechanism of tumor cell-induced up-regulation of E3 ubiquitin ligase UBR2.

The N-end rule pathway contributes significantly to accelerated muscle proteolysis mediated by the ubiquitin-proteasome pathway in various catabolic conditions. UBR2 (aka E3α-II) is the only known E3 ubiquitin ligase of the N-end rule pathway that is up-regulated by cachectic stimuli including proinflammatory cytokines and tumors. However, the signaling mechanism through which UBR2 is up-regulated remains undetermined. Here we identify a signaling pathway that mediates tumor cell-induced up-regulation of UBR2. UBR2 expression in C2C12 myotubes was up-regulated by conditioned medium from Lewis lung carcinoma cells or C26 colon adenocarcinoma cells, which was blocked by a pharmacological inhibitor of p38α/ß mitogen-activated protein kinase (MAPK), SB202190. Similarly, SB202190 administration (i.p.) abolished UBR2 up-regulation in the tibialis anterior of LLC tumor-bearing mice. Genetic gain and loss of function assays in C2C12 myotubes indicated that tumor-induced activation of the p38ß isoform is sufficient and necessary for UBR2 up-regulation. In addition, UBR2 up-regulation required p38ß-mediated phosphorylation of CCAAT/enhancer binding protein (C/EBP)-ß Thr-188, which was critical to C/EBPß binding to the UBR2 promoter. Furthermore, luciferase reporter assay revealed that the C/EBPß binding motif in the UBR2 promoter is a functional C/EBPß-responsive cis-element that enhances the promoter activity on activation by p38ß. Finally, genetic ablation of C/EBPß blocked UBR2 up-regulation in LLC tumor-bearing mice. These results suggest that UBR2 up-regulation in cachectic muscle is mediated by the p38ß-C/EBPß signaling pathway responsible for the bulk of tumor-induced muscle proteolysis.

Dietary isothiocyanate-induced apoptosis via thiol modification of DNA topoisomerase IIα.

Studies in animal models have indicated that dietary isothiocyanates (ITCs) exhibit cancer preventive activities through carcinogen detoxification-dependent and -independent mechanisms. The carcinogen detoxification-independent mechanism of cancer prevention by ITCs has been attributed at least in part to their ability to induce apoptosis of transformed (initiated) cells (e.g. through suppression of IκB kinase and nuclear factor κB as well as other proposed mechanisms). In the current studies we show that ITC-induced apoptosis of oncogene-transformed cells involves thiol modification of DNA topoisomerase II (Top2) based on the following observations. 1) siRNA-mediated knockdown of Top2α in both SV40-transformed MEFs and Ras-transformed human mammary epithelial MCF-10A cells resulted in reduced ITC sensitivity. 2) ITCs, like some anticancer drugs and cancer-preventive dietary components, were shown to induce reversible Top2α cleavage complexes in vitro. 3) ITC-induced Top2α cleavage complexes were abolished by co-incubation with excess glutathione. In addition, proteomic analysis revealed that several cysteine residues on human Top2α were covalently modified by benzyl-ITC, suggesting that ITC-induced Top2α cleavage complexes may involve cysteine modification. Interestingly, consistent with the thiol modification mechanism for Top2α cleavage complex induction, the thiol-reactive selenocysteine, but not the non-thiol-reactive selenomethionine, was shown to induce Top2α cleavage complexes. In the aggregate, our results suggest that thiol modification of Top2α may contribute to apoptosis induction in transformed cells by ITCs.

A G-quadruplex stabilizer induces M-phase cell cycle arrest.

G-quadruplex stabilizers such as telomestatin and HXDV bind with exquisite specificity to G-quadruplexes, but not to triplex, duplex, or single-stranded DNAs. Studies have suggested that the antiproliferative and possibly anti-tumor activities of these compounds are linked to their inhibitory effect on telomerase and/or telomere function. In the current studies, we show that HXDV, a synthetic analog of telomestatin, exhibits antiproliferative activity against both telomerase-positive and -negative cells and induces robust apoptosis within 16 h of treatment, suggesting a mode of action independent of telomerase. HXDV was also shown to inhibit cell cycle progression causing M-phase cell cycle arrest, as evidenced by accumulation of cells with 4 n DNA content, increased mitotic index, separated centrosomes, elevated histone H3 phosphorylation at Ser-10 (an M-phase marker), and defective chromosome alignment and spindle fiber assembly (revealed by time-lapse microscopy). The M-phase arrest caused by HXDV paralleled with reduction in the expression level of the major M-phase checkpoint regulator Aurora A. All these cellular effects appear to depend on the G-quadruplex binding activity of HXDV as its non-G-quadruplex binding analog, TXTLeu, is completely devoid of all these effects. In the aggregate, our results suggest that HXDV, which exhibits anti-proliferative and apoptotic activities, is also a novel M-phase blocker, with a mode of action dependent on its G-quadruplex binding activity.

Genistein induces topoisomerase IIbeta- and proteasome-mediated DNA sequence rearrangements: Implications in infant leukemia.

Genistein is a bioflavonoid enriched in soy products. However, high levels of maternal soy consumption have been linked to the development of infant leukemia ALL and AML. The majority of infant leukemia is linked to mixed lineage leukemia gene (MLL) translocations. Previous studies have implicated topoisomerase II (Top2) in genistein-induced infant leukemia. In order to understand the roles of the two Top2 isozymes in and the molecular mechanism for genistein-induced infant leukemia, we carried out studies in vitro using purified recombinant human Top2 isozymes, as well as studies in cultured mouse myeloid progenitor cells (32Dc13) and Top2beta knockout mouse embryonic fibroblasts (MEFs). First, we showed that genistein efficiently induced both Top2alpha and Top2beta cleavage complexes in the purified system as well as in cultured mouse cells. Second, genistein induced proteasomal degradation of Top2beta in 32Dc13 cells. Third, the genistein-induced DNA double-strand break (DSB) signal, gamma-H2AX, was dependent on the Top2beta isozyme and proteasome activity. Fourth, the requirement for Top2beta and proteasome activity was mirrored in genistein-induced DNA sequence rearrangements, as monitored by a DNA integration assay. Together, our results suggest a model in which genistein-induced Top2beta cleavage complexes are processed by proteasome, leading to the exposure of otherwise Top2beta-concealed DSBs and subsequent chromosome rearrangements, and implicate a major role of Top2beta and proteasome in genistein-induced infant leukemia.

Activin A induces skeletal muscle catabolism via p38ß mitogen-activated protein kinase.

BACKGROUND: Activation of type IIB activin receptor (ActRIIB) in skeletal muscle leads to muscle atrophy because of increased muscle protein degradation. However, the intracellular signalling mechanism that mediates ActRIIB-activated muscle catabolism is poorly defined. METHODS: We investigated the role of p38ß mitogen-activated protein kinases (MAPK) in mediating ActRIIB ligand activin A-activated muscle catabolic pathways in C2C12 myotubes and in mice with perturbation of this kinase pharmacologically and genetically. RESULTS: Treatment of C2C12 myotubes with activin A or myostatin rapidly activated p38 MAPK and its effector C/EBPß within 1 h. Paradoxically, Akt was activated at the same time through a p38 MAPK-independent mechanism. These events were followed by up-regulation of ubiquitin ligases atrogin1 (MAFbx) and UBR2 (E3α-II), as well as increase in LC3-II, a marker of autophagosome formation, leading to myofibrillar protein loss and myotube atrophy. The catabolic effects of activin A were abolished by p38α/ß MAPK inhibitor SB202190. Using small interfering RNA-mediated gene knockdown, we found that the catabolic activity of activin A was dependent on p38ß MAPK specifically. Importantly, systemic administration of activin A to mice similarly activated the catabolic pathways in vivo, and this effect was blocked by SB202190. Further, activin A failed to activate the catabolic pathways in mice with muscle-specific knockout of p38ß MAPK. Interestingly, activin A up-regulated MuRF1 in a p38 MAPK-independent manner, and MuRF1 did not appear responsible for activin A-induced myosin heavy chain loss and muscle atrophy. CONCLUSIONS: ActRIIB-mediated activation of muscle catabolism is dependent on p38ß MAPK-activated signalling.

Topoisomerase IIbeta is required for proper retinal development and survival of postmitotic cells.

Topoisomerase IIbeta (Top2b) is an enzyme that modulates DNA supercoiling by catalyzing the passage of DNA duplexes through one another. It is ubiquitously expressed in postmitotic cells and known to function during the development of neuromuscular junctions in the diaphragm and the proper formation of laminar structure in the cerebral cortex. However, due to the perinatal death phenotype of the traditional constitutive and brain-specific Top2b knockout mice, the precise in vivo function of Top2b, especially during postnatal neural development, remains to be determined. Using both the constitutive and retina-specific knockout mouse models, we showed that Top2b deficiency resulted in delayed neuronal differentiation, degeneration of the plexiform layers and outer segment of photoreceptors, as well as dramatic reduction in cell number in the retina. Genome-wide transcriptome analysis by RNA sequencing revealed that genes involved in neuronal survival and neural system development were preferentially affected in Top2b-deficient retinas. Collectively, our findings have indicated an important function of Top2b in proper development and the maintenance/survival of postmitotic neurons in the retina.

Activation of a novel ubiquitin-independent proteasome pathway when RNA polymerase II encounters a protein roadblock.

Topoisomerase IIß (Top2ß)-DNA cleavage complexes are known to arrest elongating RNA polymerase II (RNAPII), triggering a proteasomal degradation of the RNAPII large subunit (RNAPII LS) and Top2ß itself as a prelude to DNA repair. Here, we demonstrate that the degradation of Top2ß occurs through a novel ubiquitin-independent mechanism that requires only 19S AAA ATPases and 20S proteasome. Our results suggest that 19S AAA ATPases play a dual role in sensing the Top2ß cleavage complex and coordinating its degradation by 20S proteasome when RNAPII is persistently stalled by the Top2ß protein roadblock. Clarification of this transcription-associated proteasome pathway could shed light on a general role of 19S AAA ATPases in processing tight protein-DNA complexes during transcription elongation.

Inhibition of hepatitis C virus replication by antimonial compounds.

Chronic hepatitis C virus (HCV) infection is a worldwide health problem causing serious complications, such as liver cirrhosis and hepatoma. Alpha interferon (IFN-alpha) or its polyethylene glycol-modified form combined with ribavirin is the only recommended therapy. However, an alternative therapy is needed due to the unsatisfactory cure rate of the IFN-based therapy. Using a modified reporter assay based on the HCV subgenomic-replicon system, we found that sodium stibogluconate (SSG), a compound used for leishmania treatment, suppressed HCV replication. We have previously reported that SSG is effective at inhibiting HCV replication in a cell line permissive for HCV infection/replication and in an ex vivo assay using fresh human liver slices obtained from patients infected with HCV (26). In this study, we show that the SSG 50% inhibitory dose for HCV replication is 0.2 to 0.3 mg/ml (equivalent to 345 to 517 microM of Sb) in the HCV subgenomic-replicon system. We also found that SSG and IFN-alpha exert a strong synergistic anti-HCV effect in both the traditional isobologram analysis and the median effect principle (CalcuSyn analysis). The combination of SSG and IFN-alpha could sustain the antiviral response better than SSG or IFN-alpha alone. The results suggest that SSG may be a good drug candidate for use in combination with other therapeutics, such as IFN-alpha and ribavirin, to treat HCV infection.

Inhibition of hepatitis C virus replication by arsenic trioxide.

Hepatitis C virus (HCV) is a serious global problem, and present therapeutics are inadequate to cure HCV infection. In the present study, various antiviral assays show that As2O3 at submicromolar concentrations is capable of inhibiting HCV replication. The 50% effective concentration (EC50) of As2O3 required to inhibit HCV replication was 0.35 microM when it was determined by a reporter-based HCV replication assay, and the EC50 was below 0.2 microM when it was determined by quantitative reverse transcription-PCR analysis. As2O3 did not cause cellular toxicity at this concentration, as revealed by an MTS [3-(4,5-dimethylthiozol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt] assay. A combination of As2O3 and alpha interferon exerted synergistic effects against HCV, as revealed by a multiple linear logistic model and isobologram analysis. Furthermore, in an alternative HCV antiviral system that may recapitulate additional steps involved in HCV infection and replication, As2O3 at 0.3 microM totally abolished the HCV signal, whereas alpha interferon at a high dose (5,000 IU/ml) only partially suppressed the HCV signal. The study highlights the indications for use of a novel class of anti-HCV agent. Further elucidation of the exact antiviral mechanism of As2O3 may lead to the development of agents with potent activities against HCV or related viruses.

Inhibition of severe acute respiratory syndrome coronavirus replication by niclosamide.

Antiviral agents are urgently needed to fight severe acute respiratory syndrome (SARS). We showed that niclosamide, an existing antihelminthic drug, was able to inhibit replication of a newly discovered coronavirus, SARS-CoV; viral antigen synthesis was totally abolished at a niclosamide concentration of 1.56 microM, as revealed by immunoblot analysis. Thus, niclosamide represents a promising drug candidate for the effective treatment of SARS-CoV infection.