Down-regulation of telomerase activity via protein phosphatase 2A activation in salvicine-induced human leukemia HL-60 cell apoptosis

Design, synthesis and cytotoxic activity of sulfonylated derivatives of camptothecin

With the intention of advancing our research on diverse C-20 derivatives of camptothecin (CPT), 38 CPT derivatives bearing sulphonamide and sulfonylurea chemical scaffolds and different substituent groups have been designed, synthesised and evaluated in vitro for cytotoxicity against four tumour cell lines, A-549 (lung carcinoma), KB (nasopharyngeal carcinoma), MDA-MB-231 (triple-negative breast cancer) and KBvin (an MDR KB subiline). As a result, all the synthesised compounds showed promising in vitro cytotoxic activity against the four cancer cell lines tested, and were more potent than irinotecan. Importantly, compounds 12b, 12f, 12j and 13 l possessed better antiproliferative activity against all tested tumour cell lines with IC50 values of 0.0118 - 0.5478 μM, and resulted approximately 3 to 4 times more cytotoxic than topotecan against multidrug-resistant KBvin subline. Convincing evidences are achieved that incorporation of sulphonamide and sulfonylurea motifs into position-20 of camptothecin confers markedly enhanced cytotoxic activity against cancer cell lines.

Telomerase Inhibition in the Treatment of Leukemia: A Comprehensive Review

Leukemia, characterized by the uncontrolled proliferation and differentiation blockage of myeloid or lymphoid precursor cells, presents significant therapeutic challenges despite current treatment modalities like chemotherapy and stem cell transplantation. Pursuing novel therapeutic strategies that selectively target leukemic cells is critical for improving patient outcomes. Natural products offer a promising avenue for developing effective chemotherapy and preventive measures against leukemia, providing a rich source of biologically active compounds. Telomerase, a key enzyme involved in chromosome stabilization and mainly active in cancer cells, presents an attractive target for intervention. In this review article, we focus on the anti-leukemic potential of natural substances, emphasizing vitamins (such as A, D, and E) and polyphenols (including curcumin and indole-3-carbinol), which, in combination with telomerase inhibition, demonstrate reduced cytotoxicity compared to conventional chemotherapies. We discuss the role of human telomerase reverse transcriptase (hTERT), particularly its mRNA expression, as a potential therapeutic target, highlighting the promise of natural compounds in leukemia treatment and prevention.

Investigating the Anticancer Potential of Salvicine as a Modulator of Topoisomerase II and ROS Signaling Cascade

Salvicine is a new diterpenoid quinone substance from a natural source, specifically in a Chinese herb. It has powerful growth-controlling abilities against a broad range of human cancer cells in both in vitro and in vivo environments. A significant inhibitory effect of salvicine on multidrug-resistant (MDR) cells has also been discovered. Several research studies have examined the activities of salvicine on topoisomerase II (Topo II) by inducing reactive oxygen species (ROS) signaling. As opposed to the well-known Topo II toxin etoposide, salvicine mostly decreases the catalytic activity with a negligible DNA breakage effect, as revealed by several enzymatic experiments. Interestingly, salvicine dramatically reduces lung metastatic formation in the MDA-MB-435 orthotopic lung cancer cell line. Recent investigations have established that salvicine is a new non-intercalative Topo II toxin by interacting with the ATPase domains, increasing DNA-Topo II interaction, and suppressing DNA relegation and ATP hydrolysis. In addition, investigations have revealed that salvicine-induced ROS play a critical role in the anticancer-mediated signaling pathway, involving Topo II suppression, DNA damage, overcoming multidrug resistance, and tumor cell adhesion suppression, among other things. In the current study, we demonstrate the role of salvicine in regulating the ROS signaling pathway and the DNA damage response (DDR) in suppressing the progression of cancer cells. We depict the mechanism of action of salvicine in suppressing the DNA-Topo II complex through ROS induction along with a brief discussion of the anticancer perspective of salvicine.

Two New Lanostanoid Triterpenes from the Fruit Body of Ganoderma lucidum-the Major Component of SunRecome®

Two new lanostanoid triterpenes named 3β-hydroxy-4,4,14-trimethyl-7,11,15-trioxochol-8-en-24-oic acid (1) and 3β-hydroxy-7,11,12,15,23-pentaoxo-lanost-8-en-26-oic acid (2) were isolated from the fruit bodies of Ganoderma lucidum, which is the major component of the antitumor product SunRecome®. Their cytotoxic activities were assayed in vitro against HeLa, p388, SGC-7901, BEL-7402 cell lines, and the results showed that 1 had IC50 values of 18.00 μM against p388 cell, 12.70 μM against Hela cell, 22.00 μM against BEL-7402 cell, 1.50 μM against SGC-7901 cell and 2 had IC50 values of 9.85 μM against p388 cell, 17.10 μM against Hela cell, 51.00 μM against BEL-7402 cell, 42.00 μM against SGC-7901 cell.

Telomerase Inhibitors from Natural Products and Their Anticancer Potential

Telomeres and telomerase are nowadays exploring traits on targets for anticancer therapy. Telomerase is a unique reverse transcriptase enzyme, considered as a primary factor in almost all cancer cells, which is mainly responsible to regulate the telomere length. Hence, telomerase ensures the indefinite cell proliferation during malignancy—a hallmark of cancer—and this distinctive feature has provided telomerase as the preferred target for drug development in cancer therapy. Deactivation of telomerase and telomere destabilization by natural products provides an opening to succeed new targets for cancer therapy. This review aims to provide a fundamental knowledge for research on telomere, working regulation of telomerase and its various binding proteins to inhibit the telomere/telomerase complex. In addition, the review summarizes the inhibitors of the enzyme catalytic subunit and RNA component, natural products that target telomeres, and suppression of transcriptional and post-transcriptional levels. This extensive understanding of telomerase biology will provide indispensable information for enhancing the efficiency of rational anti-cancer drug design.

Role of diallyl disulfide-mediated cleavage of c-Myc and Sp-1 in the regulation of telomerase activity in human lymphoma cell line U937

OBJECTIVE

Garlic (Allium sativum) has been considered a wonder herb for years with a reputation of disease prevention. Telomerase, a ribonucleoprotein enzyme responsible for telomere integrity, is strongly up-regulated in different types of cancers. The aim of this study was to reveal the role of diallyl disulfide (DADS), an organosulfur component of garlic, on telomerase activity in human lymphoma with an emphasis on key transcription factors c-Myc and Sp-1.

METHODS

Human lymphoma cell line U937 was used as model cell line. Telomerase activity was measured by telomerase repeat amplification protocol assay, levels of related proteins and mRNAs were measured by Western blot and reverse transcriptase polymerase chain reaction, respectively. Moreover, in vitro binding assay was performed using radiolabeled double-stranded DNA having specific sequences to detect involvement of transcription factors in DADS-dependent modulation of telomerase activity.

RESULTS

The present study demonstrated DADS-mediated decrease in telomerase activity in U937 cells with concomitant transcriptional down-regulation of human telomerase reverse transcriptase (hTERT) that is caused by reduced binding of c-Myc and Sp-1 to their respective binding sites on hTERT promoter. Lowering of DNA-binding activity of c-Myc and Sp-1 due to DADS treatment is caused by the deactivation of these transcription factors due to cleavage. Additionally, Mad1-the repressor protein of hTERT expression-is also overexpressed in DADS-treated U937 cells.

CONCLUSIONS

These findings strongly suggest that DADS down-regulate telomerase activity through c-Myc-, Sp-1-, and Mad1-dependent transcriptional down-regulation of hTERT.

Expression and mechanism of regulation of PP2A/Pr65 in ameloblastoma

OBJECTIVE

This study aimed to investigate the expression of PP2A/PR65 protein in ameloblastoma and the molecular mechanisms underlying the regulation of PP2A/PR65. The association between PP2A/PR65 and the clinicopathological characteristics of tumor specimens in ameloblastoma were to provide a theoretical basis for the diagnosis, therapy and prognosis of ameloblastoma.

STUDY DESIGN

Streptavidin-peroxidase (S-P) immunohistochemical staining was used to detect PP2A/Pr65 expression changes in a total of 68 cases of ameloblastoma, six ameloblastic carcinomas, 21 squamous cell carcinomas and seven normal oral mucosas. Western blot was used to analyze PP2A/PR65 protein expression in 15 cases of ameloblastoma and three cases of normal oral mucosa.

RESULTS

Of the 68 cases analyzed, four cases were negative, 25 cases were weakly positive, 20 cases were moderately positive and 19 cases were strongly positive. In six cases of ameloblastic carcinoma, three cases were weak positive, one case was positive, two cases were strongly positive and none were negative. In 21 cases of squamous cell carcinomas, three cases were negative, 17 cases were weakly positive, one case was moderately positive and none were strongly positive. Western blot analysis showed that, PP2A/Pr65 protein expression was lower in ameloblastoma tissue compared with normal oral mucosa.

CONCLUSIONS

Reduced expression of PP2A/PR65 in ameloblastoma compared with normal oral mucosa indicates that PP2A/PR65 is involved in the occurrence and development of ameloblastoma.

Protein phosphatase 2A (PP2A) has a potential role in CAPE-induced apoptosis of CCRF-CEM cells via effecting human telomerase reverse transcriptase activity

Caffeic acid phenethyl ester (CAPE) is one of the most effective components of propolis which is collected by honey bees. The aim of this study was to investigate the cytotoxic and apoptotic effects of CAPE in the CCRF-CEM cell line and to clarify the role of serine/threonine protein phosphatase 2A (PP2A) and human telomerase reverse transcriptase (hTERT) activity as an underlining mechanism of CAPE-induced apoptosis. Trypan blue dye exclusion test and XTT methods were used to evaluate the cytotoxicity and ELISA based oligonucleotide detection, which can be seen during apoptosis, was used to determine apoptosis. Acridine orange/ethidium bromide dye technique was also used to evaluate apoptosis. The cytotoxic effect of CAPE was detected in a dose and time dependent manner with the IC(50) of 1 muM. ELISA and acridine orange/ethidium bromide methods have shown remarkable apoptosis at 48th hour in CAPE treated cells. To investigate the role of PP2A in CAPE-induced apoptosis of CCRF-CEM cells, we performed combination studies with CAPE and, Calyculin A and Okadaic acid, which are very well known inhibitors of PP2A, in IC(20) of inhibitors and IC(50) of CAPE. Combination studies revealed synergistic effect of both drugs by concomitant use. Western blot analyses of PP2A catalytic and regulatory subunits showed down-regulation of expression of PP2A catalytic subunit in CAPE treated cells at 48th hour. Since, PP2A is important in hTERT (telomerase catalytic subunit) activation and deactivation, we also performed hTERT activity in CAPE treated cells simultaneously. Treating cells with IC(50) of CAPE for 96 h with the intervals of 24 h showed marked reduction of hTERT activity. The reduction of hTERT activity in CAPE treated CCRF-CEM cells was more prominent in the initial 48 h. The variation of hTERT activity in CAPE treated CCRF-CEM cells may be the reason for the protein phosphatase interaction that occurred after treatment with CAPE.

Salvicine triggers DNA double-strand breaks and apoptosis by GSH-depletion-driven H2O2 generation and topoisomerase II inhibition

Glutathione (GSH), as the major small-molecule antioxidant in cells, has been implicated in the regulation of cell proliferation and apoptosis. Salvicine (SAL), a novel diterpenoid quinone compound, exhibits potent antitumor activities both in vitro and in vivo by poisoning topoisomerase II (Topo II) and has entered Phase II clinical trials for cancer therapy. Herein, we provide further evidence that SAL-induced DNA double-strand breaks (DSBs) and apoptosis by GSH depletion drives H2O2 generation and Topo II inhibition. Our data reveal that treatment with SAL results in a pronounced increase in intracellular H2O2 and is accompanied by the occurrence of DNA DSBs and apoptosis in epithelial HeLa cells. Furthermore, SAL was also noted to trigger a dramatic depletion of intracellular GSH via its direct reaction with GSH. Importantly, the introduction of GSH and overexpression of catalase antagonized SAL-mediated DNA DSBs and apoptosis, and the GSH synthesis inhibitor dl-buthionine-[S,R]-sulfoximine reduced SAL-mediated H2O2 generation, indicating that SAL-mediated H2O2 generation is derived from intracellular GSH depletion. Notably, SAL-mediated Topo II inhibition was also concentration-dependently reversed by GSH. Furthermore, we found that Topo II-defective HL-60/MX2 cells were almost completely resistant to SAL-induced DNA DSBs, suggesting that, in addition to its direct inhibitory effect on Topo II, SAL-mediated H2O2 generation may also trigger DNA DSBs via poisoning of Topo II. All these findings together suggest that GSH-depletion-driven H2O2 generation and Topo II inhibition are both critical for SAL-induced DNA DSBs and apoptosis.

Cytotoxic lanostanoid triterpenes from Ganoderma lucidum

Two new lanostanoid triterpenes, 23S-hydroxy-3,7,11,15-tetraoxo-lanost-8,24E-diene-26-oic acid (1), and 12beta-acetoxy-3beta-hydroxy-7,11,15,23-tetraoxo-lanost-8,20E-diene-26-oic acid (16), together with 17 known compounds, were isolated from the fruit bodies of Ganoderma lucidum. Their structures were established by spectroscopic methods, especially 2D-NMR and MS analyses and by comparison with literature data. The cytotoxic assay of the above compounds against p388, Hela, BEL-7402, and SGC-7901 human cancer cell lines showed their cytotoxicity with the IC50 values in the range of 8-25 microm.

Posttranscriptional regulation of the telomerase hTERT by gambogic acid in human gastric carcinoma 823 cells

We previously reported that gambogic acid (GA), a natural product, was an effective telomerase inhibitor by repressing hTERT promoter. In this study, posttranscriptional regulation of the telomerase hTERT by GA was investigated in BGC-823 human gastric carcinoma cells. The telomerase activity was detected by PCR-TRAP assay. RT-PCR assay and Western blot were performed to examine the repression of telomerase hTERT and c-Myc after GA or c-Myc-specific siRNA treatment. The results indicated that GA repressed telomerase activity and hTERT transcriptional activity via down-regulation of c-Myc expression in BGC-823 human gastric carcinoma cells. We further observed that hTERT transcriptional activity was not completely blocked by c-Myc-specific siRNA, suggesting that additional factors are involved in the repression of telomerase activity. The results of Western blot and immunoprecipitation assay revealed that GA inhibits the phosphorylation of Akt. The further results also confirmed that celecoxib, an inhibitor of Akt phosphorylation, could significantly repressed telomerase activity alone and enhance the repression of telomerase activity combined with GA. These data suggested that GA inhibits the posttranslational modification of hTERT by inhibiting the phosphorylation of Akt. Collectively, we suggest that GA represses telomerase activity not only by repressing hTERT transcriptional activity via c-Myc but also by posttranslational modification of hTERT via Akt.

Effects of isocamptothecin, a novel camptothecin analogue, on proliferation, apoptosis and telomerase activity in HaCaT cells

Camptothecin is a topoisomerase I inhibitor with definite anti-psoriatic effect. As it is limited in clinical application because of serious side effects and toxicity, many researchers are striving hard to develop derivatives or analogues of camptothecin with higher effects and less toxicity. To explore the anti-psoriatic potential of isocamptothecin, a novel camptothecin analogue, its effects on proliferation, apoptosis and telomerase activity were investigated in the human keratinocyte cell line HaCaT. Incubation with isocamptothecin resulted in a time- and concentration-dependent inhibition of HaCaT cell proliferation. However, isocamptothecin showed larger inhibitory concentration at 50% than camptothecin, suggesting far less cytotoxicity. In addition, isocamptothecin induced apoptosis in a concentration-dependent manner and induced typical morphologic features of apoptosis in HaCaT cells. Moreover, isocamptothecin downregulated the telomerase activity of HaCaT cells not only at concentrations of apoptosis induction but also at concentration insufficient to induce apoptosis, providing additional mechanisms that further account for its ability to inhibit keratinocytes proliferation and induce apoptosis. These results indicate that isocamptothecin possesses similar effects on keratinocytes with camptothecin, but shows far less cytotoxicity, it may probably become a promising agent for psoriasis therapy.

The telomeric protein TRF2 is critical for the protection of A549 cells from both telomere erosion and DNA double-strand breaks driven by salvicine

Telomere repeat binding factor 2 (TRF2) has been increasingly recognized to be involved in DNA damage response and telomere maintenance. Our previous report found that salvicine (SAL), a novel topoisomerase II poison, elicited DNA double-strand breaks and telomere erosion in separate experimental systems. However, it remains to be clarified whether they share a common response to these two events and in particular whether TRF2 is involved in this process. In this study, we found that SAL concurrently induced DNA double-strand breaks, telomeric DNA damage, and telomere erosion in lung carcinoma A549 cells. It was unexpected to find that SAL led to disruption of TRF2, independently of either its transcription or proteasome-mediated degradation. By overexpressing the full-length trf2 gene and transfecting TRF2 small interfering RNAs, we showed that TRF2 protein protected both telomeric and genomic DNA from the SAL-elicited events. It is noteworthy that although both the Ataxia-telangiectasia-mutated (ATM) and the ATM- and Rad3-related (ATR) kinases responded to the SAL-induced DNA damages, only ATR was essential for the telomere erosion. The study also showed that the activated ATR augmented the SAL-triggered TRF2 disruption, whereas TRF2 reduction in turn enhanced ATR function. All of these findings suggest the emerging significance of TRF2 protecting both telomeric DNA and genomic DNA on the one hand and reveal the mutual modulation between ATR and TRF2 in sensing DNA damage signaling during cancer development on the other hand.

Salvicine, a novel topoisomerase II inhibitor, exerts its potent anticancer activity by ROS generation

Salvicine is a novel diterpenoid quinone compound obtained by structural modification of a natural product lead isolated from a Chinese herb with potent growth inhibitory activity against a wide spectrum of human tumor cells in vitro and in mice bearing human tumor xenografts. Salvicine has also been found to have a profound cytotoxic effect on multidrug-resisitant (MDR) cells. Moreover, Salvicine significantly reduced the lung metastatic foci of MDA-MB-435 orthotopic xenograft. Recent studies demonstrated that salvicine is a novel non-intercalative topoisomerase II (Topo II) poison by binding to the ATPase domain, promoting DNA-Topo II binding and inhibiting Topo II-mediated DNA relegation and ATP hydrolysis. Further studies have indicated that salcivine-elicited ROS plays a central role in salvicine-induced cellular response including Topo II inhibition, DNA damage, circumventing MDR and tumor cell adhesion inhibition.

Biological screening of natural products and drug innovation in China

Natural products have been applied to human healthcare for thousands of years. Drug discovery in ancient times was largely by chance and based on clinical practices. As understanding of therapeutic benefits deepens and demands for natural products increase, previously serendipitous discoveries evolve into active searches for new medicines. Many drugs presently prescribed by physicians are either directly isolated from plants or are artificially modified versions of natural products. Scientists are looking for lead compounds with specific structures and pharmacological effects often from natural sources. Experiences and successes of Chinese scientists in this specialized area have resulted in a number of widely used drugs. The tremendous progress made in life sciences has not only revealed many pathological processes of diseases, but also led to the establishment of various molecular and cellular bioassays in conjunction with high-throughput technologies. This is advantageous and permits certain natural compounds that are difficult to isolate and purify, and compounds that are difficult to synthesize, to be assayed. The transition from traditional to empirical and to molecular screening will certainly increase the probability of discovering new leads and drug candidates from natural products.

Transcriptionally mediated inhibition of telomerase of fungal immunomodulatory protein from Ganoderma tsugae in A549 human lung adenocarcinoma cell line

Telomerase expression is the hallmark of tumor cells, and activation of this ribonucleoprotein complex may be a rate-limiting or critical step in cellular immortalization and oncogenesis. Fungal immunomodulatory protein, FIP-gts, has been isolated from Ganoderma tsugae. In the present study, we expressed and purified the recombinant fungal immunomodulatory protein reFIP-gts in E. coli. We found that reFIP-gts significantly and selectively inhibits the growth of A549 cancer cells while not affecting the growth of normal MRC-5 fibroblasts. The reFIP-gts suppression of telomerase activity is concentration-dependent, due to the downregulation of the telomerase catalytic subunit (hTERT). It also happens at the mRNA level. These results were confirmed by transient transfections of A549 cells with pGL3-Basic plasmid constructs containing the functional hTERT promoter and its E-box-deleted sequences cloned upstream of a luciferase reporter gene. With electrophoretic mobility shift assays and Western blotting, we demonstrated that in response to reFIP-gts, binding of c-myc transcriptional factor to the E-box sequence on the hTERT promoter is inhibited. These results show that reFIP-gts suppresses telomerase activity and inhibits transcriptional regulation of hTERT via a c-myc-responsive element-dependent mechanism. Our findings provide new insight into both the anticancer function of reFIP-gts and the regulation of hTERT/telomerase expression, which may be valuable in the development of a promising chemopreventive agent.

Telomeres and telomerase as targets for anticancer drug development

In most human cancers, the telomere erosion problem has been bypassed through the activation of a telomere maintenance system (usually activation of telomerase). Therefore, telomere and telomerase are attractive targets for anti-cancer therapeutic interventions. Here, we review a large panel of strategies that have been explored to date, from small inhibitors of the catalytic sub-unit of telomerase to anti-telomerase immunotherapy and gene therapy. The many positive results that are reported from anti-telomere/telomerase assays suggest a prudent optimism for a possible clinical application in a close future. However, we discuss some of the main limits for these approaches of antitumour drug development and why significant work remains before a clinically useful drug can be proposed to patients.

Repression of telomerase reverse transcriptase mRNA and hTERT promoter by gambogic acid in human gastric carcinoma cells

OBJECTIVES

To investigate the effects and potential mechanisms of gambogic acid (GA), a naturally occurring anticancer agent, on the expression and regulation of telomerase in human gastric carcinoma cells.

METHODS

GA-induced inhibition of cell proliferation was evaluated by the commonly employed MTT assay on two human gastric carcinoma cell lines, MGC-803 and SGC-7901. Telomerase activity and hTERT mRNA expression were determined by telomeric repeat amplication protocol-polymerase chain reaction and reverse transcription-polymerase chain reaction, respectively. The hTERT promoter activity was measured by luciferase assay. The expression of c-MYC, an apoptotic gene that modulates the expression of hTERT promoter, was quantified by Western blotting.

RESULTS

The proliferation of human gastric carcinoma cell lines, MGC-803 and SGC-7901, was significantly inhibited with GA treatment. Both telomerase activity and hTERT mRNA expression were notably decreased in cells treated with GA. The activity of hTERT promoter and the expression of c-MYC were also remarkably decreased in GA-treated cells.

CONCLUSION

This study demonstrated that GA treatment of human gastric carcinoma cell lines, MGC-803 and SGC-7901, significantly reduced the expression of c-MYC in a time- and concentration-dependent manner accompanied with the down-regulation of the hTERT transcription and the ultimate reduction in telomerase activity. Our results indicate that the hTERT is a target of c-MYC activity and identify a feasible mechanism of GA's potent anticancer activity.

Inhibition of human telomerase reverse transcriptase gene expression by gambogic acid in human hepatoma SMMC-7721 cells

The activation of human telomerase, a process regulated by the human telomerase reverse transcriptase (hTERT), is a crucial step during cellular immortalization and malignant transformation. We have reported that gambogic acid (GA), a natural product isolated from the gamboge resin of Garcinia hanburyi tree, is an effective telomerase inhibitor and thus displays potent anticancer activity both in vitro and in vivo. Here we present the direct interaction of GA with oncogene c-MYC, a ubiquitous transcription factor involved in the control of cell proliferation and differentiation, as the molecular mechanism of GA's inhibitory effect on telomerase activity. Consistent with the recently reported association between c-MYC overexpression and induction of telomerase activity, we find here that GA treatment of a human hepatoma cell line SMMC-7721 significantly reduced the expression of c-MYC in a time- and concentration-dependent manner accompanied with the down-regulation of the hTERT transcription and the ultimate reduction in telomerase activity. Our results indicate that the hTERT is a target of c-MYC activity and identify a feasible mechanism of GA's potent anticancer activity.

Induction of apoptosis and inhibition of telomerase activity in human bone marrow and HL-60 p53 null cells treated with anti-cancer drugs

Telomerase plays a key role in the maintenance of chromosomal stability in tumours, and the ability of anti-cancer agents to inhibit telomerase activity is under investigation. In this study, we evaluated the effect of etoposide and taxol, on the telomerase activity and telomere length in human leukaemia p53 null cells and human bone marrow cells, as well as apoptosis and cell cycle modulation. Results showed that after exposure to the drugs, HL-60 cells as well as the human progenitors underwent a block in G2 and subsequently apoptosis, whereas stromal cells from bone marrow did not undergo a block in G2 or enter apoptosis after etoposide exposure. Telomere length increased in stromal cells after treatment with both etoposide and taxol whereas in HL-60 cells only after etoposide treatment with. Bax, bcl-2 and bcl-x change their expression in stromal cells, whereas bcl-x was induced after drug treatment and bcl-2 down regulated in progenitor cells. Our data suggest that telomerase activity and apoptosis are correlated and they seem to be modulated by a common gene, bcl-2.

Reactive Oxygen Species Elicit Apoptosis by Concurrently Disrupting Topoisomerase II and DNA-Dependent Protein Kinase

Reactive oxygen species (ROS) are produced by all aerobic cells and have been implicated in the regulation of diverse cellular functions, including intracellular signaling, transcription activation, proliferation, and apoptosis. Salvicine, a novel diterpenoid quinone compound, demonstrates a broad spectrum of antitumor activities. Although salvicine is known to trap the DNA-topoisomerase II (Topo II) complex and induce DNA double-strand breaks (DSBs), its precise antitumor mechanisms remain to be clarified. In this study, we investigated whether salvicine altered the levels of ROS in breast cancer MCF-7 cells and whether these ROS contributed to the observed antitumoral activity. Our data revealed that salvicine stimulated intracellular ROS production and subsequently elicited notable DSBs. The addition of N-acetyl cysteine (NAC), an antioxidant, effectively attenuated the salvicine-induced ROS enhancement and subsequent DNA DSBs. Heat treatment reversed the accumulation of DNA DSBs, and the addition of NAC attenuated the Topo II-DNA cleavable complexes formation and the growth inhibition of salvicine-treated JN394top2-4 yeast cells, collectively indicating that Topo II is a target of the salvicine-induced ROS. On the other hand, when examining the impact of salvicine on DNA repair pathways, we unexpectedly observed that salvicine selectively down-regulated the catalytic subunit of DNA-dependent protein kinase (DNA-PK(CS)) protein levels and repressed DNA-PK kinase activity; both of these effects were attenuated by NAC pretreatment of MCF-7 cells. Finally and most importantly, NAC attenuated salvicine-induced apoptosis and cytotoxicity in MCF-7 cells. These results indicate that apart from its direct actions, salvicine generates ROS that modulate DNA damage and repair, contributing to the comprehensive biological consequences of salvicine treatment, such as DNA DSBs, apoptosis, and cytotoxicity in tumor cells. The finding of salvicine-induced ROS provides new evidence for the molecular mechanisms of this compound. Moreover, the effects of salvicine-induced ROS on Topo II and DNA-PK give new insights into the diverse biological activities of ROS.

The SIT4 gene, which encodes protein phosphatase 2A, is required for telomere function in Saccharomyces cerevisiae

Life span and number of cell divisions in eukaryotes are limited. The accumulation of stress-associated damage due to ageing may cause irreversible cell cycle arrest, so-called "cellular senescence". Although many genes have been implicated in determining life span, regulatory systems that counteract age-related stress have not yet been clarified. We examined senescence during a stress of Saccharomyces cerevisiae strains carrying disruptions in protein phosphatase (PPase)-encoding genes in order to identify the system counteracting senescence. Among these strains, short telomeres were found in the sit4 disruptant that lacks one form of protein phosphatase 2A (PP2A). Silencing ability in the subtelomeric region was impaired and hyperphosphorylation of Sir3 was also observed in this mutant. The sit4 mutant was found to have altered nucleoli and a life span as short as an sgs1 mutant. These observations suggest that the PP2A pathway regulates life span in yeast.

Telomerase inhibition is a specific early event in salvicine-treated human lung adenocarcinoma A549 cells

The telomere and telomerase have been suggested as targets for anticancer drug discovery. However, the mechanisms by which conventional anticancer drugs affect these targets are currently unclear. The novel topoisomerase II inhibitor, salvicine, suppresses telomerase activity in leukemia HL-60 cells. To further determine whether this activity of salvicine is specific to the hematological tumor and distinct from those of other conventional anticancer agents, we studied its effects on telomere and telomerase in a solid lung carcinoma cell line, A549. Differences in telomerase inhibition and telomere erosion were observed between salvcine and other anticancer agents. All anticancer agents (except adriamycin) induced shortening of the telomere, which was identified independent of replication, but only salvicine inhibited telomerase activity in A549 cells under conditions of high concentration and short-term exposure. At the low concentration and long-term exposure mode, all the tested anticancer agents shortened the telomere and inhibited telomerase activity in the same cell line. Notably, salvicine inhibited telomerase activity more severely than the other agents examined. Moreover, the compound inhibited telomerase activity in A549 cells indirectly in a concentration- and time-dependent manner. Salvicine did not affect the expression of hTERT, hTP1, and hTR mRNA in A549 cells following 4 h of exposure. Okadaic acid protected telomerase from inhibition by salvicine. These results indicate specificity of salvicine and diversity of anticancer agents in the mechanism of interference with telomerase and the telomere system. Our data should be helpful for designing the study in the development of agents acting on telomere and/or telomerase.

DNA damage, c-myc suppression and apoptosis induced by the novel topoisomerase II inhibitor, salvicine, in human breast cancer MCF-7 cells

Salvicine, a diterpenoid quinone compound, possesses potent in vitro and in vivo antitumor activity. Salvicine is a novel non-intercalative topoisomerase II poison. In this study salvicine induced evident DNA damage, which was further characterized as double-strand breaks mainly in MCF-7 human breast cancer cells. The degree of damage was highly correlated with growth inhibition of MCF-7. Using a PCR-stop assay we demonstrated that this damage was selective. Preferential damage occurred in the p2 promoter region, but not the 3'-end of the protooncogene c-myc. The expression of oncogenes, such as c-myc and c-jun, was additionally investigated. Salvicine induced a dose-dependent decrease in c-myc gene transcription, concomitant with an increase in c-jun expression. Furthermore, reverse-transcription PCR and Western blotting data revealed that salvicine failed to stimulate the mRNA and protein levels of p53 and its downstream targets p21 and bax. The phosphorylation degree of serine 15 of p53, which is thought to be an active form of p53 in response to cellular DNA damage, remained in a steady state. In view of these results, we propose that the downregulation of c-myc resulting from selective damage plays a role in apoptosis signaling. Moreover, salvicine-induced apoptosis in MCF-7 subsequent to DNA damage seems to be mediated through a p53-independent pathway.

Alternative splicing of human telomerase reverse transcriptase may not be involved in telomerase regulation during all-trans-retinoic acid-induced HL-60 cell differentiation

Alternative splicing of the human telomerase reverse transcriptase subunit (hTERT) suppresses telomerase activity during the development of human fetal kidney cells into mature cells. Tumor cell differentiation is the process of turning abnormal tumor cells into 'normal' cells accompanied by down-regulation of telomerase activity. However, the precise mechanism of the regulation of telomerase activity in differentiated cells is not fully understood. In this study, we observed the role of alternative splicing of hTERT in the regulation of telomerase activity in all-trans-retinoic acid (ATRA)-induced, differentiated HL-60 cells. ATRA-induced down-regulation of telomerase activity in differentiated HL-60 cells was associated with a decrease in hTERT and an increase in human telomerase-associated protein-1 (hTP1) transcription. Expression of full length variant hTERT alpha+ beta+ mRNA decreased in a dose- and time-dependent manner. The drop of hTERT beta- mRNA was time-dependent. hTERT alpha- and hTERT alpha- beta- mRNA were reduced dramatically after ATRA treatment. In the dose-effect study, hTERT alpha+ beta+ and hTERT beta- maintained a relatively stable ratio when telomerase activity decreased largely from treatment with 1 to 5 microM ATRA. Although the splicing pattern of hTERT mRNA was altered in time-effect research, the change was not related to the ATRA-treated decline of telomerase activity. The expression of alternative splicing variants of hTERT also decreased at the protein level. All these results suggested that alternative splicing of hTERT mRNA may not contribute to the suppression of telomerase activity during ATRA-induced HL-60 leukemia cell differentiation.