RNA synthesis block by 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) triggers p53-dependent apoptosis in human colon carcinoma cells

5,6-Dichloro-1-β-D-ribofuranosylbenzimidazole (DRB) induces apoptosis in breast cancer cells through inhibiting of Mcl-1 expression

The effective treatment of breast cancer remains a profound clinical challenge, especially due to drug resistance and metastasis which unfortunately arise in many patients. The transcription inhibitor 5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole (DRB), as a selective inhibitor of cyclin-dependent kinase 9, was shown to be effective in inducing apoptosis in various hematopoietic malignancies. However, the anticancer efficacy of DRB against breast cancer is still unclear. Herein, we demonstrated that administration of DRB to the breast cancer cell line led to the inhibition of cellular proliferation and induction of the typical signs of apoptotic cells, including the increases in Annexin V-positive cells, DNA fragmentation, and activation of caspase-7, caspase-9, and poly (ADP ribose) polymerase (PARP). Treatment of DRB resulted in a rapid decline in the myeloid cell leukemia 1 (Mcl-1) protein, whereas levels of other antiapoptotic proteins did not change. Overexpression of Mcl-1 decreased the DRB-induced PARP cleavage, whereas knockdown of Mcl-1 enhanced the effects of DRB on PARP activation, indicating that loss of Mcl-1 accounts for the DRB-mediated apoptosis in MCF-7 cells, but not in T-47D. Furthermore, we found that co-treatment of MCF-7 cells with an inhibitor of AKT (LY294002) or an inhibitor of the proteasome (MG-132) significantly augmented the DRB-induced apoptosis. These data suggested that DRB in combination with LY294002 or MG-132 may have a greater therapeutic potency against breast cancer cells.

Distinct Responses to IL4 in Macrophages Mediated by JNK

IL(Interleukin)-4 is the main macrophage M2-type activator and induces an anti-inflammatory phenotype called alternative activation. The IL-4 signaling pathway involves the activation of STAT (Signal Transducer and Activator of Transcription)-6 and members of the MAPK (Mitogen-activated protein kinase) family. In primary-bone-marrow-derived macrophages, we observed a strong activation of JNK (Jun N-terminal kinase)-1 at early time points of IL-4 stimulation. Using selective inhibitors and a knockout model, we explored the contribution of JNK-1 activation to macrophages' response to IL-4. Our findings indicate that JNK-1 regulates the IL-4-mediated expression of genes typically involved in alternative activation, such as Arginase 1 or Mannose receptor, but not others, such as SOCS (suppressor of cytokine signaling) 1 or p21^Waf-1 (cyclin dependent kinase inhibitor 1A). Interestingly, we have observed that after macrophages are stimulated with IL-4, JNK-1 has the capacity to phosphorylate STAT-6 on serine but not on tyrosine. Chromatin immunoprecipitation assays revealed that functional JNK-1 is required for the recruitment of co-activators such as CBP (CREB-binding protein)/p300 on the promoter of Arginase 1 but not on p21^Waf-1. Taken together, these data demonstrate the critical role of STAT-6 serine phosphorylation by JNK-1 in distinct macrophage responses to IL-4.

Posttranscriptional actions of triiodothyronine on Tshb expression in TαT1 cells: New insights into molecular mechanisms of negative feedback

Rapid actions of triiodothyronine (T3) on thyrotropin (TSH) synthesis and secretion have been described in hypothyroid male rats. However, the molecular mechanisms remain unknown. TαT1 cells, a thyrotroph cell line, was used herein to characterize the possible non-genomic actions of T3 on the expression of alpha (Cga) and Tshb genes, and the posttranscriptional processing and translation of both transcripts. The involvement of αVβ3 integrin was also assessed. T3 quickly reduced Tshb mRNA content, poly(A) tail length and its association with ribosomes. The effect of T3 on Tshb gene expression was detected even in the presence of a transcription inhibitor. The decrease in Tshb mRNA content and polyadenylation depend on T3 interaction with αVβ3 integrin, while T3 reduced Cga mRNA content by transcriptional action. The translational rate of both transcripts was reduced by a mechanism, which does not depend on T3-αVβ3 integrin interaction. Results indicate that, in parallel with the inhibitory transcriptional action in Cga and Tshb gene expression, T3 rapidly triggers additional posttranscriptional mechanisms, reducing the TSH synthesis. These non-genomic actions partially depend on T3-αVβ3 integrin interaction at the plasma membrane of thyrotrophs and add new insights to the molecular mechanisms involved in T3 negative feedback loop.

Exosomal Metastasis‑Associated Lung Adenocarcinoma Transcript 1 Promotes Angiogenesis and Predicts Poor Prognosis in Epithelial Ovarian Cancer

Exosomes mediate cell-cell crosstalk in cancer progression by transferring their molecular cargos, including long noncoding RNAs (lncRNAs). Metastasis‑associated lung adenocarcinoma transcript 1 (MALAT1) is a well-known lncRNA associated with cancer angiogenesis and metastasis. However, the presence of MALAT1 in exosomes and the roles and clinical values of exosomal MALAT1 in epithelial ovarian cancer (EOC) remain unknown. The present study focused on the crosstalk between EOC cells and endothelial cells mediated by exosomal MALAT1 and aimed to explore the roles of exosomes and exosomal MALAT1 in EOC angiogenesis and to reveal the clinical relevance and prognostic predictive value of serum exosomal MALAT1 in EOC. We observed that MALAT1 was increased in both metastatic EOC cells and their secreted exosomes. Exosomal MALAT1 derived from EOC cells was transferred to recipient human umbilical vein endothelial cells (HUVECs) via exosomes. In vitro and in vivo experiments demonstrated that MALAT1 knockdown impaired the exosome-mediated proangiogenic activity of HUVECs through certain key angiogenesis-related genes. Clinically, elevated serum exosomal MALAT1 was highly correlated with an advanced and metastatic phenotype of EOC and was an independent predictive factor for EOC overall survival (OS). Moreover, a prognostic nomogram model we constructed showed a good prediction of the probability of 3-year OS of EOC patients according to the c-index (0.751, 95% confidence interval [CI]=0.691-0.811) and calibration curve. Collectively, our data provide a novel mechanism by which EOC cells transfer MALAT1 via exosomes to recipient HUVECs and influence HUVECs by stimulating angiogenesis-related gene expression, eventually promoting angiogenesis. Additionally, circulating exosomal MALAT1 can serve as a promising serum-based, noninvasive predictive biomarker for EOC prognosis.

Site-Specific Acetyl Lysine Antibodies Reveal Differential Regulation of Histone Acetylation upon Kinase Inhibition

Lysine acetylation regulates diverse biological functions for the modified proteins. Mass spectrometry-based proteomic approaches have identified thousands of lysine acetylation sites in cells and tissues. However, functional studies of these acetylation sites were limited by the lack of antibodies recognizing the specific modification sites. Here, we generated 55 site-specific acetyl lysine antibodies for the detection of this modification in cell lysates and evaluated the quality of these antibodies. Based on the immunoblotting analyses, we found that the nature of amino acid sequences adjacent to the modification sites affected the specificity of the site-specific acetyl lysine antibodies. Amino acids with charged, hydrophilic, small, or flexible side chains adjacent to the modification sites increase the likelihood of obtaining high quality site-specific acetyl lysine antibodies. This result may provide valuable insights in fine-tuning the amino acid sequences of the epitopes for the generation of site-specific acetyl lysine antibodies. Using the site-specific acetyl lysine antibodies, we further discovered that acetylation of histone 3 at four lysine residues was differentially regulated by kinase inhibitors. This result demonstrates the potential application of these antibodies in the study of new signaling pathways that lysine acetylation may participate in.

Cyclin Dependent Kinase 9 Inhibitors for Cancer Therapy

Cyclin dependent kinase (CDK) inhibitors have been the topic of intense research for nearly 2 decades due to their widely varied and critical functions within the cell. Recently CDK9 has emerged as a druggable target for the development of cancer therapeutics. CDK9 plays a crucial role in transcription regulation; specifically, CDK9 mediated transcriptional regulation of short-lived antiapoptotic proteins is critical for the survival of transformed cells. Focused chemical libraries based on a plethora of scaffolds have resulted in mixed success with regard to the development of selective CDK9 inhibitors. Here we review the regulation of CDK9, its cellular functions, and common core structures used to target CDK9, along with their selectivity profile and efficacy in vitro and in vivo.

Detrimental effect of CO2-driven seawater acidification on a crustacean brine shrimp, Artemia sinica

The effects of the decline in ocean pH, termed as ocean acidification due to the elevated carbon dioxide in the atmosphere, on calcifying organisms such as marine crustacean are unclear. To understand the possible effects of ocean acidification on the physiological responses of a marine model crustacean brine shrimp, Artemia sinica, three groups of the cysts or animals were raised at different pH levels (8.2 as control; 7.8 and 7.6 as acidification stress according to the predictions for the end of this century and next century accordingly) for 24 h or two weeks, respectively, followed by examination of their hatching success, morphological appearance such as deformity and microstructure of animal body, growth (i.e. body length), survival rate, expression of selected genes (involved in development, immunity and cellular activity etc), and biological activity of several key enzymes (participated in antioxidant responses and physiological reactions etc). Our results clearly demonstrated that the cysts hatching rate, growth at late stage of acidification stress, and animal survival rate of brine shrimp were all reduced due to lower pH level (7.6 & 7.8) on comparison to the control group (pH 8.2), but no obvious change in deformity or microstructure of brine shrimp was present under these acidification stress by microscopy observation and section analysis. In addition, the animals subjected to a lower pH level of seawater underwent changes on their gene expressions, including Spätzle, MyD88, Notch, Gram-negative bacteria binding protein, prophenoloxidase, Apoptosis inhibitor 5, Trachealess, Caveolin-1 and Cyclin K. Meanwhile, several key enzyme activities, including superoxide dismutase, catalase, peroxidase, alkaline phosphatase and acid phosphatase, were also affected by acidified seawater stress. Taken together, our findings supports the idea that CO2-driven seawater acidification indeed has a detrimental effect, in case of hatching success, growth and survival, on a model crustacean brine shrimp, which will increase the risk of juvenile brine shrimp and possibly also other crustaceans, as important live feeds for aquaculture being introduced in the ecosystem especially the marine food webs.

Apoptosis signal-regulating kinase 1 promotes Ochratoxin A-induced renal cytotoxicity

Oxidative stress and apoptosis are involved in Ochratoxin A (OTA)-induced renal cytotoxicity. Apoptosis signal-regulating kinase 1 (ASK1) is a Mitogen-Activated Protein Kinase Kinase Kinase (MAPKKK, MAP3K) family member that plays an important role in oxidative stress-induced cell apoptosis. In this study, we performed RNA interference of ASK1 in HEK293 cells and employed an iTRAQ-based quantitative proteomics approach to globally investigate the regulatory mechanism of ASK1 in OTA-induced renal cytotoxicity. Our results showed that ASK1 knockdown alleviated OTA-induced ROS generation and Δψm loss and thus desensitized the cells to OTA-induced apoptosis. We identified 33 and 24 differentially expressed proteins upon OTA treatment in scrambled and ASK1 knockdown cells, respectively. Pathway classification and analysis revealed that ASK1 participated in OTA-induced inhibition of mRNA splicing, nucleotide metabolism, the cell cycle, DNA repair, and the activation of lipid metabolism. We concluded that ASK1 plays an essential role in promoting OTA-induced renal cytotoxicity.

Characterization of molecular and cellular functions of the cyclin-dependent kinase CDK9 using a novel specific inhibitor

BACKGROUND AND PURPOSE

The cyclin-dependent kinase CDK9 is an important therapeutic target but currently available inhibitors exhibit low specificity and/or narrow therapeutic windows. Here we have used a new highly specific CDK9 inhibitor, LDC000067 to interrogate gene control mechanisms mediated by CDK9.

EXPERIMENTAL APPROACH

The selectivity of LDC000067 was established in functional kinase assays. Functions of CDK9 in gene expression were assessed with in vitro transcription experiments, single gene analyses and genome-wide expression profiling. Cultures of mouse embryonic stem cells, HeLa cells, several cancer cell lines, along with cells from patients with acute myelogenous leukaemia were also used to investigate cellular responses to LDC000067.

KEY RESULTS

The selectivity of LDC000067 for CDK9 over other CDKs exceeded that of the known inhibitors flavopiridol and DRB. LDC000067 inhibited in vitro transcription in an ATP-competitive and dose-dependent manner. Gene expression profiling of cells treated with LDC000067 demonstrated a selective reduction of short-lived mRNAs, including important regulators of proliferation and apoptosis. Analysis of de novo RNA synthesis suggested a wide ranging positive role of CDK9. At the molecular and cellular level, LDC000067 reproduced effects characteristic of CDK9 inhibition such as enhanced pausing of RNA polymerase II on genes and, most importantly, induction of apoptosis in cancer cells.

CONCLUSIONS AND IMPLICATIONS

Our study provides a framework for the mechanistic understanding of cellular responses to CDK9 inhibition. LDC000067 represents a promising lead for the development of clinically useful, highly specific CDK9 inhibitors.

The basal transcription machinery as a target for cancer therapy

General transcription is required for the growth and survival of all living cells. However, tumor cells require extraordinary levels of transcription, including the transcription of ribosomal RNA genes by RNA polymerase I (RNPI) and mRNA by RNA polymerase II (RNPII). In fact, cancer cells have mutations that directly enhance transcription and are frequently required for cancer transformation. For example, the recent discovery that MYC enhances the transcription of the majority genes in the genome correlates with the fact that several transcription interfering drugs preferentially kill cancer cells. In recent years, advances in the mechanistic studies of the basal transcription machinery and the discovery of drugs that interfere with multiple components of transcription are being used to combat cancer. For example, drugs such as triptolide that targets the general transcription factors TFIIH and JQ1 to inhibit BRD4 are administered to target the high proliferative rate of cancer cells. Given the importance of finding new strategies to preferentially sensitize tumor cells, this review primarily focuses on several transcription inhibitory drugs to demonstrate that the basal transcription machinery constitutes a potential target for the design of novel cancer drugs. We highlight the drugs’ mechanisms for interfering with tumor cell survival, their importance in cancer treatment and the challenges of clinical application.

Implication of transcriptional repression in compound C-induced apoptosis in cancer cells

Compound C, a well-known inhibitor of AMP-activated protein kinase (AMPK), has been reported to induce apoptosis in some types of cells. However, the underlying mechanisms remain largely unclear. Using a DNA microarray analysis, we found that the expression of many genes was downregulated upon treatment with compound C. Importantly, compound C caused transcriptional repression with the induction of p53, a well-known marker of transcriptional stress response, in several cancer cell lines. Compound C did not induce the phosphorylation of p53 but dramatically increased the protein level of p53 similar to some other transcriptional inhibitors, including 5,6-dichloro-1-β-D-ribobenzimidazole (DRB). Consistent with previous reports, we found that compound C initiated apoptotic death of cancer cells in an AMPK-independent manner. Similar to DRB and actinomycin D (ActD), two classic transcription inhibitors, compound C not only resulted in the loss of Bcl-2 and Bcl-xl protein but also induced the phosphorylation of eukaryotic initiation factor-alpha (eIF2α) on Ser51. Hence, the phosphorylation of eIF2α might be a novel marker of transcriptional inhibition. It is noteworthy that compound C-mediated apoptosis of cancer cells is correlated with decreased expression of Bcl-2 and Bcl-xl and the phosphorylation of eIF2α on Ser51. Remarkably, compound C exhibits potent anticancer activities in vivo. Taken together, our data suggest that compound C may be an attractive candidate for anticancer drug development.

Low expression of cyclinH and cyclin-dependent kinase 7 can decrease the proliferation of human esophageal squamous cell carcinoma

BACKGROUND

Increased expression of cyclinH (CCNH) and cyclin-dependent kinase 7 (CDK7) has a relationship with poor prognosis in most human cancers.

AIM

Investigate the expression of CCNH and CDK7 in human esophageal squamous cell carcinoma (ESCC) and the effect of chemotherapy on their expression.

METHODS

Western blotting and immunohistochemistry were used to measure the expression of CCNH and CDK7 proteins in ESCC and adjacent normal tissue in 98 patients. We use Cell Counting Kit-8 and cell flow to analyze the effects of cisplatin and interference of CCNH and CDK7 in cell cycle process.

RESULTS

Immunohistochemical analysis showed that CCNH and CDK7 expression were significantly associated with unfavorable clinicopathologic variables. CCNH and CDK7 protein levels were elevated in ESCC tissues in comparison with adjacent normal tissues. Survival analysis revealed that CCNH and CDK7 overexpression were significantly associated with overall survival (P < 0.001). Cisplatin or interference of CCNH or CDK7 led cells to grow slowly. Overexpression of CCNH and CDK7 in TE1 cells can lead to resistance to cisplatin.

CONCLUSIONS

We can conclude that CCNH and CDK7 may play an important role in the tumorigenesis and development of ESCC. CCNH and CDK7 expression affected the chemotherapy of tumor.

Cholesterol side-chain cleavage gene expression in theca cells: augmented transcriptional regulation and mRNA stability in polycystic ovary syndrome

Hyperandrogenism is characteristic of women with polycystic ovary syndrome (PCOS). Ovarian theca cells isolated from PCOS follicles and maintained in long-term culture produce elevated levels of progestins and androgens compared to normal theca cells. Augmented steroid production in PCOS theca cells is associated with changes in the expression of genes for several steroidogenic enzymes, including CYP11A1, which encodes cytochrome P450 cholesterol side-chain cleavage. Here, we further examined CYP11A1 gene expression, at both the transcriptional and post-transcriptional level in normal and PCOS theca cells propagated in long-term culture utilizing quantitative RT-PCR, functional promoter analyses, and mRNA degradation studies. The minimal element(s) that conferred increased basal and cAMP-dependent CYP11A1 promoter function were determined. CYP11A1 mRNA half-life in normal and PCOS theca cells was compared. Results of these cumulative studies showed that basal and forskolin stimulated steady state CYP11A1 mRNA abundance and CYP11A1 promoter activity were increased in PCOS theca cells. Deletion analysis of the CYP11A1 promoter demonstrated that augmented promoter function in PCOS theca cells results from increased basal regulation conferred by a minimal sequence between -160 and -90 bp of the transcriptional start site. The transcription factor, nuclear factor 1C2, was observed to regulate basal activity of this minimal CYP11A1 element. Examination of mRNA stability in normal and PCOS theca cells demonstrated that CYP11A1 mRNA half-life increased >2-fold, from approximately 9.22+/-1.62 h in normal cells, to 22.38+/-0.92 h in PCOS cells. Forskolin treatment did not prolong CYP11A1 mRNA stability in either normal or PCOS theca cells. The 5'-UTR of CYP11A1 mRNA confers increased basal mRNA stability in PCOS cells. In conclusion, these studies show that elevated steady state CYP11A1 mRNA abundance in PCOS cells results from increased transactivation of the CYP11A1 promoter and increased CYP11A1 mRNA stability.

Deacetylation of C/EBPβ is required for IL-4-induced arginase-1 expression in murine macrophages

The amount of arginine available at inflammatory loci is a limiting factor for the growth of several cells of the immune system. IL-4-induced activation of macrophages produced arginase-1, which converts arginine into ornithine, a precursor of polyamines and proline. Trichostatin A (TSA), a pan-inhibitor of histone deacetylases (HDACs), inhibited IL-4-induced arginase-1 expression. TSA showed promoter-specific effects on the IL-4-responsive genes. While TSA inhibited the expression of arginase-1, fizz1, and mrc1, other genes, such as ym,1 mgl1, and mgl2, were not affected. The inhibition of arginase-1 occurred at the transcriptional level with the inhibition of polymerase II binding to the promoter. IL-4 induced STAT6 phosphorylation and binding to DNA. These activities were not affected by TSA treatment. However, TSA inhibited C/EBPβ DNA binding. This inhibitor induced acetylation on lysine residues 215-216, which are critical for DNA binding. Finally, using macrophages from STAT6 KO mice we showed that STAT6 is required for the DNA binding of C/EBPβ. These results demonstrate that the acetylation/deacetylation balance strongly influences the expression of arginase-1, a gene of alternative activation of macrophages. These findings also provide a molecular mechanism to explain the control of gene expression through deacetylase activity.

Involvement of cyclin K posttranscriptional regulation in the formation of Artemia diapause cysts

Background

Artemia eggs tend to develop ovoviviparously to yield nauplius larvae in good rearing conditions; while under adverse situations, they tend to develop oviparously and encysted diapause embryos are formed instead. However, the intrinsic mechanisms regulating this process are not well understood.

Principal Finding

This study has characterized the function of cyclin K, a regulatory subunit of the positive transcription elongation factor b (P-TEFb) in the two different developmental pathways of Artemia. In the diapause-destined embryo, Western blots showed that the cyclin K protein was down-regulated as the embryo entered dormancy and reverted to relatively high levels of expression once development resumed, consistent with the fluctuations in phosphorylation of position 2 serines (Ser2) in the C-terminal domain (CTD) of the largest subunit (Rpb1) of RNA polymerase II (RNAP II). Interestingly, the cyclin K transcript levels remained constant during this process. In vitro translation data indicated that the template activity of cyclin K mRNA stored in the postdiapause cyst was repressed. In addition, in vivo knockdown of cyclin K in developing embryos by RNA interference eliminated phosphorylation of the CTD Ser2 of RNAP II and induced apoptosis by inhibiting the extracellular signal-regulated kinase (ERK) survival signaling pathway.

Conclusions/Significance

Taken together, these findings reveal a role for cyclin K in regulating RNAP II activity during diapause embryo development, which involves the post-transcriptional regulation of cyclin K. In addition, a further role was identified for cyclin K in regulating the control of cell survival during embryogenesis through ERK signaling pathways.

Interference with ER-α enhances the therapeutic efficacy of the selective CDK inhibitor roscovitine towards ER-positive breast cancer cells

In recent years many risk factors for the development of breast cancer that are linked to estrogens have been identified, and roscovitine (ROSC), a selective cyclin-dependent kinase (CDK) inhibitor, has been shown to be an efficient inhibitor of the proliferation of human breast cancer cells. Therefore, we have examined the possibility that interference with estrogen signaling pathways, using tamoxifen (TAM), a selective estrogen receptor modulator (SERM), could modulate the efficacy of treatment with ROSC. In conjunction with TAM, ROSC exhibited enhanced anti-proliferative activity and CDK inhibition, particularly in estrogen-dependent MCF-7 cells. The interaction between both drugs was synergistic. However, in ER-α-negative cells the interaction was antagonistic. Exposure of MCF-7 cells to ROSC abolished the activating phosphorylation of CDK2 and CDK7 at Ser(164/170). This in turn prevented the phosphorylation of the carboxyl-terminal repeat domain of RNA Polymerase II and ER-α at Ser(118), resulting in the down-regulation of the latter. Concomitantly, wt p53 was strongly activated by phosphorylation at Ser(46). Our results demonstrate that ROSC negatively affects the functional status of ER-α, making it potentially useful in the treatment of estrogen-dependent breast cancer cells.

Reconstitution of human MCF-7 breast cancer cells with caspase-3 does not sensitize them to action of CDK inhibitors

Human MCF-7 breast cancer cells are resistant to pro-apoptotic stimuli due to caspase-3 inactivation. On the other hand, they should be sensitive to agents like selective pharmacological inhibitors of cyclin-dependent kinases (CDKs) that (re)activate p53 tumor suppressor protein because they harbor intact p53 pathways. In this study we examined whether reconstitution of caspase-3 in MCF-7 cells sensitizes them to inhibitors of CDKs, by analyzing the effects of roscovitine (ROSC) and olomoucine (OLO), two closely related selective pharmacological CDK inhibitors, on both mother MCF-7 cells and a secondary mutant line, MCF-7.3.28 that stably expresses human caspase-3. The results show that ROSC is, as expected, much more potent than OLO. Surprisingly; however, ROSC and OLO reduced proliferation of parental MCF-7 cells more strongly than caspase-3-proficient counterparts. Both inhibitors arrest human breast cancer cells at the G(2)-phase of the cell cycle. Analysis of cell-cycle regulators by immunoblotting revealed that ROSC strongly induces p53 protein activity by inducing its phosphorylation at Ser46 in the MCF-7 cells lacking caspase-3, but not in caspase-3-proficient cells. Furthermore, reconstitution of caspase-3 in MCF-7 cells neither elevates the mitochondrial apoptosis rate nor significantly increases caspases activity upon ROSC treatment. However, the stabilization of p53 in response to DNA damaging agents is the same in both caspase negative and positive MCF-7 cells. Cytotoxic agents induce caspase-3-dependent apoptosis in caspase-3-proficient cells. These results indicate that reconstitution of MCF-7 cancer cells with caspase-3 sensitize them to the action of DNA damaging agents but not to ATP-like pharmacological inhibitors of CDKs.

Cell death induction in resting lymphocytes by pan-Cdk inhibitor, but not by Cdk4/6 selective inhibitor

Immunosuppression is one of the common side effects of many anti-tumor agents targeting proliferating cells. We previously reported the development of a new class of pan-cyclin-dependent kinase (Cdk) inhibitor compounds that induce immunosuppression in rodents. Here, we demonstrated that a pan-Cdk inhibitor, Compound 1 very rapidly reduced white blood cells in mice, only 8 h after administration. Compound 1 induced death of peripheral blood cells or purified resting (non-stimulated) lymphocytes ex vivo. Cell death was induced very rapidly, after 4 h of incubation, suggesting that acute immunosuppression observed in rodents might be, at least in part, due to direct cytotoxic effects of Compound 1 on resting lymphocytes. While cell cycle-related Cdks were not activated, the carboxyl terminal domain (CTD) of the largest subunit of RNA polymerase II was phosphorylated, indicating activation of Cdk7 or Cdk9, which phosphorylates this domain, in resting lymphocytes. Indeed, the pan-Cdk inhibitor suppressed CTD phosphorylation in resting cells at the dose required for cell death induction. Inhibition of Cdk7 or Cdk9 by Compound 1 was also confirmed by suppression of nuclear factor-kappa B (NF-κB)-dependent transcription activity in the human cancer cell line U2OS. Interestingly, a Cdk4/6 inhibitor with selectivity against Cdk7 and Cdk9 did not induce cell death in resting lymphocytes. These results suggest that CTD phosphorylation possibly by Cdk7 or Cdk9 might be important for survival of resting lymphocytes and that Cdk inhibitors without inhibitory activity on these kinases might be an attractive agent for cancer chemotherapy.

An RNA-directed nucleoside anti-metabolite, 1-(3-C-ethynyl-beta-d-ribo-pentofuranosyl)cytosine (ECyd), elicits antitumor effect via TP53-induced Glycolysis and Apoptosis Regulator (TIGAR) downregulation

1-(3-C-ethynyl-beta-d-ribo-pentofuranosyl)cytosine (ECyd) is a ribose-modified nucleoside analog of cytidine with potent anticancer activity in several cancers. The main antitumor mechanism of this promising RNA-directed nucleoside anti-metabolite is efficient blockade of RNA synthesis in cancer cells. Here, we examined the therapeutic potential of this RNA-directed anti-metabolite in in vitro models of nasopharyngeal cancer (NPC). In a panel of 6 NPC cell lines, ECyd effectively inhibited cellular proliferation at nM concentrations (IC(50): approximately 13-44nM). Moreover, cisplatin-resistant NPC cells were highly sensitive to ECyd (at nM concentration). The ECyd-mediated growth inhibition was associated with G(2)/M cell cycle arrest, PARP cleavage (a hallmark of apoptosis) and Bcl-2 downregulation, indicating induction of apoptosis by ECyd in NPC cells. Unexpectedly, ECyd-induced significant downregulation of TIGAR, a newly described dual regulator of apoptosis and glycolysis. More importantly, this novel action of ECyd on TIGAR was accompanied by marked depletion of NADPH, the major reducing power critically required for cell proliferation and survival. We hypothesized that ECyd-induced TIGAR downregulation was crucially involved in the antitumor activity of ECyd. Indeed, overexpression of TIGAR was able to rescue NPC cells from ECyd-induced growth inhibition, demonstrating a novel mechanistic action of ECyd on TIGAR. We demonstrated for the first time that an RNA-directed nucleoside analog, ECyd, exerts its antitumor activity via downregulation of a novel regulator of apoptosis, TIGAR. Moreover, ECyd may represent a novel therapy for NPC.

Awakening guardian angels: drugging the p53 pathway

Currently, around 11 million people are living with a tumour that contains an inactivating mutation of TP53 (the human gene that encodes p53) and another 11 million have tumours in which the p53 pathway is partially abrogated through the inactivation of other signalling or effector components. The p53 pathway is therefore a prime target for new cancer drug development, and several original approaches to drug discovery that could have wide applications to drug development are being used. In one approach, molecules that activate p53 by blocking protein-protein interactions with MDM2 are in early clinical development. Remarkable progress has also been made in the development of p53-binding molecules that can rescue the function of certain p53 mutants. Finally, cell-based assays are being used to discover compounds that exploit the p53 pathway by either seeking targets and compounds that show synthetic lethality with TP53 mutations or by looking for non-genotoxic activators of the p53 response.

The cyclin-dependent kinase inhibitor 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole induces nongenotoxic, DNA replication-independent apoptosis of normal and leukemic cells, regardless of their p53 status

Background

Current chemotherapy of human cancers focuses on the DNA damage pathway to induce a p53-mediated cellular response leading to either G1 arrest or apoptosis. However, genotoxic treatments may induce mutations and translocations that result in secondary malignancies or recurrent disease. In addition, about 50% of human cancers are associated with mutations in the p53 gene. Nongenotoxic activation of apoptosis by targeting specific molecular pathways thus provides an attractive therapeutic approach.

Methods

Normal and leukemic cells were evaluated for their sensitivity to 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) through cell viability and caspase activation tests. The apoptotic pathway induced by DRB was analysed by immunfluorescence and immunoblot analysis. H2AX phosphorylation and cell cycle analysis were performed to study the dependance of apoptosis on DNA damage and DNA replication, respectively. To investigate the role of p53 in DRB-induced apoptosis, specific p53 inhibitors were used. Statistical analysis on cell survival was performed with the test of independence.

Results

Here we report that DRB, an inhibitor of the transcriptional cyclin-dependent kinases (CDKs) 7 and 9, triggers DNA replication-independent apoptosis in normal and leukemic human cells regardless of their p53 status and without inducing DNA damage. Our data indicate that (i) in p53-competent cells, apoptosis induced by DRB relies on a cytosolic accumulation of p53 and subsequent Bax activation, (ii) in the absence of p53, it may rely on p73, and (iii) it is independent of ATM and NBS1 proteins. Notably, even apoptosis-resistant leukemic cells such as Raji were sensitive to DRB.

Conclusion

Our results indicate that DRB represents a potentially useful cancer chemotherapeutic strategy that employs both the p53-dependent and -independent apoptotic pathways without inducing genotoxic stress, thereby decreasing the risk of secondary malignancies.

Purvalanol A induces apoptosis and downregulation of antiapoptotic proteins through abrogation of phosphorylation of JAK2/STAT3 and RNA polymerase II

To clarify the mechanisms of purvalanol A in the induction of apoptosis, we investigated whether purvalanol A influenced the RNA synthesis and expression of RNA polymerase II and signal transducer and activator of transcription 3 (STAT3). When MKN45 cells were treated with 30 micromol/l purvalanol A, mitochondrial dysfunction occurred before the induction of the apoptosis and the expression of antiapoptotic proteins survivin, Bcl-XL, and Bcl-2 was reduced. The treatment with parvalanol A was also shown to reduce not only mRNA for these proteins but also global RNA synthesis. The phosphorylation of the carboxy-terminal domain of RNA polymerase II, which was involved in transcriptional regulation, was strongly inhibited by purvalanol A, followed by the partial inhibition of the expression of RNA polymerase II. Furthermore, the phosphorylation at Tyr705 of STAT3, which is known to be a phosphorylation site for Janus kinase 2 (JAK2), was completely inhibited by purvalanol A early (3 h) after drug treatment, although the phosphorylation of STAT3 at Ser727, which is a phosphorylation site for Ras/Raf/MEK and extracellular signal-regulated protein kinase 1/2, was still detectable until late (12 h) after treatment. In addition, the tyrosine phosphorylation of JAK2 was efficiently inhibited by purvalanol A. These results suggest that the inhibition of JAK2/STAT3 and RNA polymerase II is crucial in the downregulation of antiapoptotic proteins leading to the apoptotic cell death induced by parvalanol A.

Knockdown of TFIIS by RNA silencing inhibits cancer cell proliferation and induces apoptosis

Background

A common element among cancer cells is the presence of improperly controlled transcription. In these cells, the degree of specific activation of some genes is abnormal, and altering the aberrant transcription may therefore directly target cancer. TFIIS is a transcription elongation factor, which directly binds the transcription motor, RNA Polymerase II and allows it to read through various transcription arrest sites. We report on RNA interference of TFIIS, a transcription elongation factor, and its affect on proliferation of cancer cells in culture.

Methods

RNA interference was performed by transfecting siRNA to specifically knock down TFIIS expression in MCF7, MCF10A, PL45 and A549 cells. Levels of TFIIS expression were determined by the Quantigene method, and relative protein levels of TFIIS, c-myc and p53 were determined by C-ELISA. Induction of apoptosis was determined by an enzymatic Caspase 3/7 assay, as well as a non-enzymatic assay detecting cytoplasmic mono- and oligonucleosomes. A gene array analysis was conducted for effects of TFIIS siRNA on MCF7 and MCF10A cell lines.

Results

Knockdown of TFIIS reduced cancer cell proliferation in breast, lung and pancreatic cancer cell lines. More specifically, TFIIS knockdown in the MCF7 breast cancer cell line induced cancer cell death and increased c-myc and p53 expression whereas TFIIS knockdown in the non-cancerous breast cell line MCF10A was less affected. Differential effects of TFIIS knockdown in MCF7 and MCF10A cells included the estrogenic, c-myc and p53 pathways, as observed by C-ELISA and gene array, and were likely involved in MCF7 cell-death.

Conclusion

Although transcription is a fundamental process, targeting select core transcription factors may provide for a new and potent avenue for cancer therapeutics. In the present study, knockdown of TFIIS inhibited cancer cell proliferation, suggesting that TFIIS could be studied as a potential cancer target within the transcription machinery.

Transcriptional inhibitors, p53 and apoptoss

Transcriptional inhibitors (TI) repress global transcription and induce apoptosis. It has been suggested that induction of p53 is one of the hallmarks of global transcriptional repression. Two recent papers suggested that treatment of human cancer cells with TIs, leads to p53-dependent, transcription-independent or p53-dependent, transcription-dependent apoptosis. The latter mechanism is linked to the fact that TIs can be selective in their inhibitory effects thereby permitting transcription of some genes. However, the majority of other published data suggest that these drugs induce p53-independent apoptosis. In this article I discuss the mechanisms of TI-dependent cell death and the potential role of p53 in this process.

Differential sensitivity of human colon cancer cell lines to the nucleoside analogs ARC and DRB

Recently, we identified a nucleoside analog named ARC (4-amino-6-hydrazino-7-beta-D-ribofuranosyl-7H-Pyrrolo[2,3-d]pyrimidine-5-carboxamide), which has the properties of a general transcriptional inhibitor. Here, we report the characterization of ARC on a panel of colorectal cancer (CRC) cell lines. Cell death induced by ARC in CRC cells was accompanied by caspase-3 cleavage and correlated with the downregulation of antiapoptotic proteins, survivin and Mcl-1 and with the inhibition of Akt phosphorylation. At the same time, colon cancer cell lines were resistant to the well-known nucleoside analog DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole), which failed to downregulate Mcl-1 or survivin. Overall, ARC could represent an attractive candidate for anti-cancer drug development that targets multiple survival pathways in colon cancer cells.

Lipopolysaccharide up-regulates MHC class II expression on dendritic cells through an AP-1 enhancer without affecting the levels of CIITA

The expression of MHC class II genes is strictly tissue specific. In a limited number of cells, the expression of these genes is inducible by cytokines and only in dendritic and B cells is expression constitutive. LPS blocks the cytokine-dependent induction of these genes, but enhances their expression in dendritic and the B cell line A20. We have observed that LPS increased surface expression by raising I-A protein and mRNA levels. LPS does not enhance the expression of the transactivator CIITA. In transient transfection experiments, LPS induced the expression of the I-Abeta promoter, which contains an AP-1 box located between 1722 and 1729 bp upstream of the transcriptional start site. Mutation of this box abrogated the effect of LPS. The AP-1 box still responded to LPS when we moved it to -611 bp or even when it was in the opposite direction. LPS induced a complex that bound to the AP-1 box. However, in dendritic cells, the complex comprised c-jun and c-fos while in A20 cells only c-jun. This was confirmed by chromatin immune precipitation assays and the distinct induction of c-jun and c-fos mRNAs. Therefore, our results indicate that LPS exerts a novel regulatory mechanism in the control of MHC class II gene expression.

A novel transcriptional inhibitor induces apoptosis in tumor cells and exhibits antiangiogenic activity

Using a high-throughput cell-based assay, we identified a nucleoside analogue 4-amino-6-hydrazino-7-beta-D-ribofuranosyl-7H-pyrrolo(2,3-d)-pyrimidine-5-carboxamide (ARC), which has the properties of a general transcriptional inhibitor. Specifically, ARC inhibits the phosphorylation of RNA polymerase II by positive transcription elongation factor-b, leading to a block in transcriptional elongation. ARC was able to potently repress p53 targets p21 and hdm2 (human homologue of mdm2) protein levels, but dramatically increased p53 levels similar to other transcriptional inhibitors, including flavopiridol. This increase in p53 corresponded to the down-regulation of short-lived protein hdm2, which is a well-established negative regulator of p53. Remarkably, ARC induced potent apoptosis in human tumor and transformed, but not in normal cells, and possessed strong antiangiogenic activity in vitro. Although ARC promoted the accumulation of p53, ARC-induced apoptosis in tumor cells was p53-independent, suggesting that it may be useful for the treatment of tumors with functionally inactive p53. Furthermore, cell death induced by ARC had a strong correlation with down-regulation of the antiapoptotic gene survivin, which is often overexpressed in human tumors. Taken together, our data suggests that ARC may be an attractive candidate for anticancer drug development.

Targeting the cell cycle: a new approach to cancer therapy

The cell cycle represents a series of tightly integrated events that allow the cell to grow and proliferate. Critical parts of the cell cycle machinery are the cyclin-dependent kinases (CDKs), which, when activated, provide a means for the cell to move from one phase of the cell cycle to the next. The CDKs are regulated positively by cyclins and regulated negatively by naturally occurring CDK inhibitors (CDKIs). Cancer represents a dysregulation of the cell cycle such that cells that overexpress cyclins or do not express the CDKIs continue to undergo unregulated cell growth. The cell cycle also serves to protect the cell from DNA damage. Thus, cell cycle arrest, in fact, represents a survival mechanism that provides the tumor cell the opportunity to repair its own damaged DNA. Thus, abrogation of cell cycle checkpoints, before DNA repair is complete, can activate the apoptotic cascade, leading to cell death. Now in clinical trials are a series of targeted agents that directly inhibit the CDKs, inhibit unrestricted cell growth, and induce growth arrest. Recent attention has also focused on these drugs as inhibitors of transcription. In addition, there are now agents that abrogate the cell cycle checkpoints at critical time points that make the tumor cell susceptible to apoptosis. An understanding of the cell cycle is critical to understanding how best to clinically develop these agents, both as single agents and in combination with chemotherapy.

Dual action of the inhibitors of cyclin-dependent kinases: targeting of the cell-cycle progression and activation of wild-type p53 protein

The inhibition of cyclin-dependent kinases (CDKs) represents a novel approach to the therapy of human malignancies. Already in clinical trials, recently developed CDK inhibitors very efficiently target the rapidly proliferating cancer cells and inhibit their cell-cycle progression. Interestingly, some CDK inhibitors additionally affect the stability and activity of the tumour-suppressor protein p53, thereby enhancing their antiproliferative action towards cancer cells. Considering the fact that the p53 protein is mutated or inactivated in approximately 50% of all human cancers, the efficacy of CDK inhibitor therapy could differ between cancer cells depending on their p53 status. Moreover, recent reports demonstrating that some cancer cells can proliferate despite CDK2 inhibition questioned the central role of CDK2 in the cell-cycle control and suitability of CDK2 as a therapeutic target; however, the p53 activation that is mediated by CDK inhibitors could be essential for the efficacy of CDK inhibitors in therapy of CDK2-independent cancers. Furthermore, there is also reason to believe that CDK2 inhibitors could be used for another purpose, to protect normal cells from the effects of chemotherapy.

Transcription inhibition: a potential strategy for cancer therapeutics

Interference with transcription triggers a stress response leading to the induction of the tumour suppressor p53. If transcription is not restored within a certain time frame cells may undergo apoptosis in a p53-dependent and independent manner. The mechanisms by which blockage of transcription induces apoptosis may involve diminished levels of anti-apoptotic factors, inappropriate accumulation of proteins in the nucleus, accumulation of p53 at mitochondria or complications during replication. Many chemotherapeutic agents currently used in the clinic interfere with transcription and this interference may contribute to their anti-cancer activities. Future efforts should be directed towards exploring whether interference of transcription could be used as an anti-cancer therapeutic strategy.

Recent progress in the discovery and development of cyclin-dependent kinase inhibitors

Cyclin-dependent kinases (CDKs) have long been known to be the main facilitators of the cell proliferation cycle. However, they also play important roles in the regulation of the RNA polymerase II transcription cycle. Cancer cells display aberrant cell cycle regulation to gain proliferative advantages and they also appear to have an exaggerated dependence on RNA polymerase II transcriptional activity to sustain pro-survival and antiapoptotic signalling. A picture is now starting to emerge that both the cell-cycle and transcriptional functions of CDKs can be exploited pharmacologically with CDK inhibitors that possess appropriate selectivity profiles. In this article, recent advances into these mechanistic insights and how they can guide clinical development in terms of choice of indication are reviewed, as well as combinations with existing chemotherapies. An overview is also given of recent clinical trial results with the lead CDK inhibitor drug candidates seliciclib (CYC202, (R)-roscovitine; Cyclacel) and alvocidib (flavopiridol; Aventis-NCI), as well as the development of other clinical entries and advanced preclinical compounds. The discussion focuses on oncology, but we point out recent results with CDK inhibitors in virology and nephrology.

Transcription - guarding the genome by sensing DNA damage

Cells induce the expression of DNA-repair enzymes, activate cell-cycle checkpoints and, under some circumstances, undergo apoptosis in response to DNA-damaging agents. The mechanisms by which these cellular responses are triggered are not well understood, but there is recent evidence that the transcription machinery might be used in DNA-damage surveillance and in triggering DNA-damage responses to suppress mutagenesis. Transcription might also act as a DNA-damage dosimeter where the severity of blockage determines whether or not to induce cell death. Could transcription therefore be a potential therapeutic target for anticancer strategies?

Flavopiridol Induces p53 via Initial Inhibition of Mdm2 and p21 and, Independently of p53, Sensitizes Apoptosis-Reluctant Cells to Tumor Necrosis Factor

Flavopiridol (FP) inhibits gene expression and causes apoptosis, and these effects cannot be explained by inhibition of cyclin-dependent kinases that govern cell cycle. The simple and established notion that FP is an inhibitor of transcription predicts its effects. Because Mdm-2 targets p53 for degradation, FP, as predicted, dramatically induced p53 by inhibiting Mdm-2. Once p53 was induced, restoration of transcription (by removal of FP) resulted in superinduction of p21 and Mdm-2. Similarly, low concentrations of FP (50 nm) induced p21 and Mdm-2 because of their initial down-regulation. A sustained decrease of Mdm-2/p21 expression and accumulation of p53 coincided with near-maximal cytotoxicity of FP at concentrations >100 nm. Induction of p53 was a marker, not a cause, of cytotoxicity. FP caused rapid apoptosis (caspase-dependent cell death) in p53-null leukemia cells. In these cells, FP-induced apoptosis was converted to growth arrest by inhibitors of caspases. In apoptosis-reluctant A549 and PC3M cancer cells, FP inhibited cell proliferation but did not cause apoptosis. Like typical inhibitors of transcription, FP sensitized cells to apoptotic stimuli, allowing tumor necrosis factor to cause rapid and massive apoptosis in otherwise apoptosis-reluctant cells. We discuss that, as a reversible inhibitor of transcription, FP can be used clinically in novel rational drug combinations.

The topoisomerase IIbeta circular clamp arrests transcription and signals a 26S proteasome pathway

It has been proposed that the topoisomerase II (TOP2)beta-DNA covalent complex arrests transcription and triggers 26S proteasome-mediated degradation of TOP2beta. It is unclear whether the initial trigger for proteasomal degradation is due to DNA damage or transcriptional arrest. In the current study we show that the TOP2 catalytic inhibitor 4,4-(2,3-butanediyl)-bis(2,6-piperazinedione) (ICRF-193), which traps TOP2 into a circular clamp rather than the TOP2-DNA covalent complex, can also arrest transcription. Arrest of transcription, which is TOP2beta-dependent, is accompanied by proteasomal degradation of TOP2beta. Different from TOP2 poisons and other DNA-damaging agents, ICRF-193 did not induce proteasomal degradation of the large subunit of RNA polymerase II. These results suggest that proteasomal degradation of TOP2beta induced by the TOP2-DNA covalent complex or the TOP2 circular clamp is due to transcriptional arrest but not DNA damage. By contrast, degradation of the large subunit of RNA polymerase II is due to a DNA-damage signal.

Protein kinase CK2 and protein kinase D are associated with the COP9 signalosome

The COP9 signalosome (CSN) purified from human erythrocytes possesses kinase activity that phosphoryl ates proteins such as c-Jun and p53 with consequence for their ubiquitin (Ub)-dependent degradation. Here we show that protein kinase CK2 (CK2) and protein kinase D (PKD) co-purify with CSN. Immunoprecipitation and far-western blots reveal that CK2 and PKD are in fact associated with CSN. As indicated by electron microscopy with gold-labeled ATP, at least 10% of CSN particles are associated with kinases. Kinase activity, most likely due to CK2 and PKD, co-immuno precipitates with CSN from HeLa cells. CK2 binds to DeltaCSN3(111-403) and CSN7, whereas PKD interacts with full-length CSN3. CK2 phosphorylates CSN2 and CSN7, and PKD modifies CSN7. Both CK2 and PKD phosphorylate c-Jun as well as p53. CK2 phosphoryl ates Thr155, which targets p53 to degradation by the Ub system. Curcumin, emodin, DRB and resveratrol block CSN-associated kinases and induce degradation of c-Jun in HeLa cells. Curcumin treatment results in elevated amounts of c-Jun-Ub conjugates. We conclude that CK2 and PKD are recruited by CSN in order to regulate Ub conjugate formation.

P53: an ubiquitous target of anticancer drugs

The p53 tumor suppressor can induce growth arrest, apoptosis and cell senescence. Not surprisingly, p53 is an appealing target for therapeutic intervention. Although current anticancer agents do not directly interact with p53, these agents (including DNA damaging drugs, antimetabolites, microtubule-active drugs and inhibitors of the proteasome) cause accumulation of wt p53. Depending on the p53 status of cancer cells, diverse therapeutic strategies are under development. These include pharmacological rescue of mutant p53 function and reactivation of wt p53 in E6-expressing cells. For protection of normal cells, strategies range from abrogation of wt p53 induction, thereby decreasing the toxicity of DNA damaging agents, to activation of wt p53-dependent checkpoints, thereby protecting cells against cell cycle-dependent therapeutics.

Sensitivity of the origin decision point to specific inhibitors of cellular signaling and metabolism

Chinese hamster ovary (CHO) cells become committed to initiate DNA replication at specific sites within the dihydrofolate reductase (DHFR) locus at a discrete point during G1 phase, the origin decision point (ODP). To better understand the requirements for passage through the ODP, we evaluated the ability of various inhibitors of G1-phase progression to prevent passage through the ODP. Of several protein kinase inhibitors tested, only inhibitors of cyclin-dependent kinase (cdk) activity (roscovitine, olomoucine) prevented passage through the ODP. Inhibitors of MAP kinase (PD98059), PKA (KT5720), PKG (KT5823), as well as inhibition of integrin-mediated signaling by preventing cell adhesion, all arrested cells in the post-ODP stages of G1 phase. Intriguingly, inhibitors of proteasome-dependent proteolysis (MG132, ALLN, lactacystin) and transcription (DRB, alpha-amanitin, actinomycin D) also inhibited passage through the ODP, whereas inhibition of protein synthesis (cycloheximide) had no effect on the ODP. Cross-checking each inhibitor for its affect on transcription revealed that the ODP could be uncoupled from transcription; MG132 and lactacystin did not inhibit transcription, and KT5720 was a potent inhibitor of transcription. Importantly, cells that were arrested upstream of the ODP with either roscovitine or lactacystin contained functional prereplication complexes (pre-RCs), supporting previous findings that pre-RC formation is not sufficient for origin specification. These results demonstrate that specification of the DHFR origin is independent of growth signaling mechanisms and does not require G1-phase synthesis of a protein regulator such as a cyclin or Dbf4/ASK1, positioning the ODP after pre-RC formation but prior to the activation of the known S-phase promoting kinases.

Induction of ser15 and lys382 modifications of p53 by blockage of transcription elongation

Blockage of transcription has been shown to induce the tumor suppressor p53 in human cells. We here show that RNA synthesis inhibitors blocking the phosphorylation of the carboxyl terminal domain (CTD) of RNA polymerase II, such as DRB and H7, induced rapid nuclear accumulation of p53 proteins that were not phosphorylated at ser15 or acetylated at lys382. In contrast, agents that inhibit the elongation phase of transcription, such as UV light, camptothecin or actinomycin D, induced the accumulation of nuclear p53 proteins that were modified at both of these sites. Furthermore, using a panel of DNA repair-deficient cells we show that persistent DNA lesions in the transcribed strand of active genes are responsible for the induction of the ser15 and lys382 modifications following UV-irradiation. We conclude that inhibition of transcription is sufficient for the accumulation of p53 in the nucleus regardless of whether the ser15 site of p53 is phosphorylated or not. Importantly, blockage of the elongation phase of transcription triggers a distinct signaling pathway leading to p53 modifications on ser15 and lys382. We propose that the elongating RNA polymerase complex may act as a sensor of DNA damage and as an integrator of cellular stress signals.

Mitogen-induced up-regulation of non-smooth muscle isoform of alpha-tropomyosin in rat aortic smooth muscle cells

Correlation between the expression of the alpha-tropomyosin isoforms and cell growth was investigated in rat aortic smooth muscle cells. The levels of exon 1a, exons 1a+2a (smooth muscle type) and exons 1a+2b (non-smooth muscle type) were determined by reverse transcription-polymerase chain reaction (RT-PCR). When the cells were cultured, the level of exons 1a+2b transiently increased while reaching a maximum at 3-5 days. When the serum-deprived confluent cells were stimulated with 3-20% serum for 1.5 h, the level of exons 1a+2b increased by about twofold. The 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) but not 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimi de (GF 109203X) inhibited this up-regulation. Phorbol-12, 13-dibutyrate (PDB) mimicked the effect of serum. The DNA synthesis as determined by the incorporation of 5-bromo-2'-deoxy-uridine (BrdU) was not enhanced by the 1.5 h stimulation with serum or phorbol ester. The up-regulation of non-smooth muscle isoform of alpha-tropomyosin occurred during G(0)/G(1) transition before entering S phase. Protein phosphorylation is suggested to be involved in the up-regulation. However, the responsible kinase(s) remain to be elucidated.

Synergistic activation of p53-dependent transcription by two cooperating damage recognition pathways

High level activation of p53-dependent transcription occurs following cellular exposure to genotoxic damaging agents such as UV-C, while ionizing radiation damage does not induce a similarly potent induction of p53-dependent gene expression. Reasoning that one of the major differences between UV-C and ionizing radiation damage is that the latter does not inhibit general transcription, we attempted to reconstitute p53-dependent gene expression in ionizing irradiated cells by co-treatment with selected transcription inhibitors that alone do not activate p53. p53-dependent transcription can be dramatically enhanced by the treatment of ionizing irradiated cells with low doses of DRB, which on its own does not induce p53 activity. The mechanism of ionizing radiation-dependent activation of p53-dependent transcription using DRB is more likely due to inhibition of gene transcription rather than prolonged DNA damage, as the non-genotoxic and general transcription inhibitor Roscovitine also synergistically activates p53 function in ionizing irradiated cells. These results identify two distinct signal transduction pathways that cooperate to fully activate p53-dependent gene expression: one responding to lesions induced by ionizing radiation and the second being a kinase pathway that regulates general RNA Polymerase II activity.