Distinct inflammatory Th17 subsets emerge in autoimmunity and infection

Th17 cells play a critical role in both tissue homeostasis and inflammation during clearance of infections as well as autoimmune and inflammatory disorders. Despite numerous efforts to distinguish the homeostatic and inflammatory roles of Th17 cells, the mechanism underlying the divergent functions of inflammatory Th17 cells remains poorly understood. In this study, we demonstrate that the inflammatory Th17 cells involved in autoimmune colitis and those activated during colitogenic infection are distinguishable populations characterized by their differential responses to the pharmacological molecule, clofazimine (CLF). Unlike existing Th17 inhibitors, CLF selectively inhibits proautoimmune Th17 cells while preserving the functional state of infection-elicited Th17 cells partially by reducing the enzyme ALDH1L2. Overall, our study identifies two distinct subsets within the inflammatory Th17 compartment with distinct regulatory mechanisms. Furthermore, we highlight the feasibility to develop disease-promoting Th17 selective inhibitor for treating autoimmune diseases.

Compartmentalization and regulation of GTP in control of cellular phenotypes

Genetic or pharmacological inhibition of enzymes involved in GTP biosynthesis has substantial biological effects, underlining the need to better understand the function of GTP levels in regulation of cellular processes and the significance of targeting GTP biosynthesis enzymes for therapeutic intervention. Our current understanding of spatiotemporal regulation of GTP metabolism and its role in physiological and pathological cellular processes is far from complete. Novel methodologies such as genetically encoded sensors of free GTP offered insights into intracellular distribution and function of GTP molecules. In the current Review, we provide analysis of recent discoveries in the field of GTP metabolism and evaluate the key enzymes as molecular targets.

Phosphorylation of guanosine monophosphate reductase triggers a GTP-dependent switch from pro- to anti-oncogenic function of EPHA4

Signal transduction pathways post-translationally regulating nucleotide metabolism remain largely unknown. Guanosine monophosphate reductase (GMPR) is a nucleotide metabolism enzyme that decreases GTP pools by converting GMP to IMP. We observed that phosphorylation of GMPR at Tyr267 is critical for its activity and found that this phosphorylation by ephrin receptor tyrosine kinase EPHA4 decreases GTP pools in cell protrusions and levels of GTP-bound RAC1. EPHs possess oncogenic and tumor-suppressor activities, although the mechanisms underlying switches between these two modes are poorly understood. We demonstrated that GMPR plays a key role in EPHA4-mediated RAC1 suppression. This supersedes GMPR-independent activation of RAC1 by EPHA4, resulting in a negative overall effect on melanoma cell invasion and tumorigenicity. Accordingly, EPHA4 levels increase during melanoma progression and inversely correlate with GMPR levels in individual melanoma tumors. Therefore, phosphorylation of GMPR at Tyr267 is a metabolic signal transduction switch controlling GTP biosynthesis and transformed phenotypes.

Assessment of Intracellular GTP Levels Using Genetically Encoded Fluorescent Sensors

Changes in intracellular GTP levels, even incremental ones, profoundly affect the activity of several GTP-binding proteins ultimately resulting in alteration of several basal cellular phenotypes including cell motility, invasion, and tumorigenesis. However, until recently, no tools were available for GTP quantification in live cells. Therefore, in the current chapter, we describe the methodology for the quantitative assessment of spatiotemporal changes in GTP levels in the cells using genetically encoded fluorescent ratiometric GTP sensors termed GEVALs for GTP evaluators.

Cell-Based Methods for the Identification of Myc-Inhibitory Small Molecules

Oncoproteins encoded by dominant oncogenes have long been considered as targets for chemotherapeutic intervention. However, oncogenic transcription factors have often been dismissed as "undruggable." Members of the Myc family of transcription factors have been identified as promising targets for cancer chemotherapy in multiple publications reporting the requirement of Myc proteins for maintenance of almost every type of tumor. Here, we describe cell-based approaches to identify c-Myc small molecule inhibitors by screening complex libraries of diverse small molecules based on Myc functionality and specificity.

Abstract B32: A novel combination therapy for metastatic melanoma potentiates a gap junction positive feedback mechanism

We have developed a novel approach for treating melanoma by selectively targeting cancer cells with reactive oxygen species (ROS). This combination therapy incorporates ROS generated by 1) nonthermal plasma (NTP) that emits a multitude of ROS and 2) SR-4233 (TPZ) that undergoes a chemical change to become a ROS only in conditions of hypoxia. The results show an additive to synergistic effect of the combination therapy as compared to each agent individually. Since gap junctions promote the intercellular communication of small molecules up to 1 kDa, we assessed the effect of this combination therapy on metastatic melanoma with functional and nonfunctional gap junctions. We utilized 1205Lu metastatic melanoma cells expressing an empty vector, overexpressing a wild-type Cx43, or a dominant negative variant. The results demonstrated that NTP induced a highly localized cell death in the target area whereas TPZ partially reduced viability over the total surface area. However, when the combination of NTP/TPZ therapy was delivered, complete cell death was observed across the entire plate, specifically in the cells expressing high gap junctions. Similarly, in vivo studies of human metastatic melanoma in a mouse tumor model demonstrated a 90% reduction in tumor volume when treated with the combination of NTP/TPZ in high gap junction expressing tumors. Treatment with NTP/TPZ increased expression of genes activated by apoptosis and oxidative stress while decreasing genes related to cell migration. Microarray analysis also demonstrated a high induction of interleukins and cytokines, indicative of the potential for an immune response. In addition, the transcription factor, HIF1α, known to promote tumor progression, was decreased only under the conditions of NTP/TPZ with high gap junctions. Interestingly, the connexin protein, Cx26, was upregulated following treatment with NTP/TPZ, and the associated gap junctions were shown to maintain functionality during the onset of treatment. Therefore, we conclude that gap junctions both increase the efficacy of NTP/TPZ and are induced by the therapy, thus promoting a positive feedback mechanism of tumoricidal activity. Our unique approach to ROS induction in melanoma cells with NTP/TPZ shows preclinical efficacy, suggesting potential as a novel cancer treatment.

Citation Format: Archis Bagati, Zethan Koch, Timothy C. Hutcherson, Joseph Pechette, Hossein Dianat, Cory Higley, Lisa Chiu, Yesul Song, Jay Shah, Elana Chazen, Andrew Nicolais, Peter Casey, Kyle Thompson, Mikhail A. Nikiforov, Jennifer Zirnheld, Shoshanna N. Zucker. A novel combination therapy for metastatic melanoma potentiates a gap junction positive feedback mechanism [abstract]. In: Proceedings of the AACR Special Conference on Melanoma: From Biology to Target; 2019 Jan 15-18; Houston, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(19 Suppl):Abstract nr B32.

The role of polo-like kinase 3 in the response of BRAF-mutant cells to targeted anticancer therapies

The activation of oncogenic mitogen-activated protein kinase cascade via mutations in BRAF is often observed in human melanomas. Targeted inhibitors of BRAF (BRAFi), alone or as a part of a combination therapy, offer a significant benefit to such patients. Unfortunately, some cases are initially nonresponsive to these drugs, while others become refractory in the course of treatment, underscoring the need to understand and mitigate the underlying resistance mechanisms. We report that interference with polo-like kinase 3 (PLK3) reduces the tolerance of BRAF-mutant melanoma cells to BRAFi, while increased PLK3 expression has the opposite effect. Accordingly, PLK3 expression correlates with tolerance to BRAFi in a panel of BRAF-mutant cell lines and is elevated in a subset of recurring BRAFi-resistant melanomas. In PLK3-expressing cells, R406, a kinase inhibitor whose targets include PLK3, recapitulates the sensitizing effects of genetic PLK3 inhibitors. The findings support a role for PLK3 as a predictor of BRAFi efficacy and suggest suppression of PLK3 as a way to improve the efficacy of targeted therapy.

KLF9-dependent ROS regulate melanoma progression in stage-specific manner

Although antioxidants promote melanoma metastasis, the role of reactive oxygen species (ROS) in other stages of melanoma progression is controversial. Moreover, genes regulating ROS have not been functionally characterized throughout the entire tumor progression in mouse models of cancer. To address this question, we crossed mice-bearing knock-out of Klf9, an ubiquitous transcriptional regulator of oxidative stress, with two conditional melanocytic mouse models: Braf^CA mice, where Braf^V600E causes premalignant melanocytic hyperplasia, and Braf^CA/Pten^-/- mice, where Braf^V600E and loss of Pten induce primary melanomas and metastases. Klf9 deficiency inhibited premalignant melanocytic hyperplasia in Braf^CA mice but did not affect formation and growth of Braf^CA/Pten^-/- primary melanomas. It also, as expected, promoted Braf^CA/Pten^-/- metastasis. Treatment with antioxidant N-acetyl cysteine phenocopied loss of Klf9 including suppression of melanocytic hyperplasia. We were interested in a different role of Klf9 in regulation of cell proliferation in Braf^CA and Braf^CA/Pten^-/- melanocytic cells. Mechanistically, we demonstrated that BRAF^V600E signaling transcriptionally upregulated KLF9 and that KLF9-dependent ROS were required for full-scale activation of ERK1/2 and induction of cell proliferation by BRAF^V600E. PTEN depletion in BRAF^V600E-melanocytes did not further activate ERK1/2 and cell proliferation, but rendered these phenotypes insensitive to KLF9 and ROS. Our data identified an essential role of KLF9-dependent ROS in BRAF^V600E signaling in premalignant melanocytes, offered an explanation to variable role of ROS in premalignant and transformed melanocytic cells and suggested a novel mechanism for suppression of premalignant growth by topical antioxidants.

XBP1-KLF9 Axis Acts as a Molecular Rheostat to Control the Transition from Adaptive to Cytotoxic Unfolded Protein Response

Transcription factor XBP1s, activated by endoplasmic reticulum (ER) stress in a dose-dependent manner, plays a central role in adaptive unfolded protein response (UPR) via direct activation of multiple genes controlling protein refolding. Here, we report that elevation of ER stress above a critical threshold causes accumulation of XBP1s protein sufficient for binding to the promoter and activation of a gene encoding a transcription factor KLF9. In comparison to other XBP1s targets, KLF9 promoter contains an evolutionary conserved lower-affinity binding site that requires higher amounts of XBP1s for activation. In turn, KLF9 induces expression of two regulators of ER calcium storage, TMEM38B and ITPR1, facilitating additional calcium release from ER, exacerbation of ER stress, and cell death. Accordingly, Klf9 deficiency attenuates tunicamycin-induced ER stress in mouse liver. These data reveal a role for XBP1s in cytotoxic UPR and provide insights into mechanisms of life-or-death decisions in cells under ER stress.

Inhibition of the aryl hydrocarbon receptor/polyamine biosynthesis axis suppresses multiple myeloma

Polyamine inhibition for cancer therapy is, conceptually, an attractive approach but has yet to meet success in the clinical setting. The aryl hydrocarbon receptor (AHR) is the central transcriptional regulator of the xenobiotic response. Our study revealed that AHR also positively regulates intracellular polyamine production via direct transcriptional activation of 2 genes, ODC1 and AZIN1, which are involved in polyamine biosynthesis and control, respectively. In patients with multiple myeloma (MM), AHR levels were inversely correlated with survival, suggesting that AHR inhibition may be beneficial for the treatment of this disease. We identified clofazimine (CLF), an FDA-approved anti-leprosy drug, as a potent AHR antagonist and a suppressor of polyamine biosynthesis. Experiments in a transgenic model of MM (Vk*Myc mice) and in immunocompromised mice bearing MM cell xenografts revealed high efficacy of CLF comparable to that of bortezomib, a first-in-class proteasome inhibitor used for the treatment of MM. This study identifies a previously unrecognized regulatory axis between AHR and polyamine metabolism and reveals CLF as an inhibitor of AHR and a potentially clinically relevant anti-MM agent.

Abstract B052: Leveraging the metabolic stress of polyamine biosynthesis in prostate cancer towards a novel therapeutic approach

Prostatic epithelial cells secrete high levels of acetylated polyamines into the prostatic lumen. This distinctive characteristic places added strain on connected pathways, forcing increased metabolite production from both one-carbon metabolism and the methionine cycle to maintain nucleotide and S-adenosylmethionine (SAM) pools, respectively. More importantly, this stress is increased in prostate cancer (CaP) due to increased polyamine biosynthesis, DNA synthesis, and proliferation. The metabolic flux is driven by the activity of spermidine/spermine N1-acetyltransferase (SSAT), which acetylates the polyamines leading to their secretion into the lumen, which drives demand for biosynthesis of polyamines. To overcome this stress, the methionine salvage pathway (MSP) recycles the one-carbon unit lost to polyamine biosynthesis back to the methionine cycle, allowing for replenishment of SAM pools. The rate-limiting enzyme involved in this process is methylthioadenosine phosphorylase (MTAP). We have found that CaP relies on the MSP to relieve the strain caused by high polyamine biosynthesis and that both genetic and pharmacologic inhibition of MTAP blocks CaP xenograft growth. We hypothesized that this dependence can be enhanced by increasing the activity of SSAT. Recent results from our lab have shown that pharmacologic inhibition of MTAP alongside SSAT upregulation is synergistic in multiple androgen-sensitive and castration-recurrent CaP cell lines. We are currently exploring the effects of our combination treatment in castration-recurrent xenografts in vivo as well as in human CaP treatment-naïve tumors from radical prostatectomies grown as ex vivo explants. Preliminary results reveal that our treatments are effective at slowing growth in a subset of xenografts in vivo as well as impacting our target enzymes, polyamine levels, and increasing apoptosis in our ex vivo system. We expect that simultaneous targeting of multiple, converging pathways that are exceptionally important for prostate will lead to significant delay or prevention of disease recurrence.

Citation Format: Hayley C. Affronti, Anthony J. Pellerite, Aryn Rowsam, Spencer R. Rosario, Robert A. Casero, Mikhail A. Nikiforov, James Phillips, Dominic J. Smiraglia. Leveraging the metabolic stress of polyamine biosynthesis in prostate cancer towards a novel therapeutic approach [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr B052.

Melanoma Suppressor Functions of the Carcinoma Oncogene FOXQ1

Lineage-specific regulation of tumor progression by the same transcription factor is understudied. We find that levels of the FOXQ1 transcription factor, an oncogene in carcinomas, are decreased during melanoma progression. Moreover, in contrast to carcinomas, FOXQ1 suppresses epithelial-to-mesenchymal transition, invasion, and metastasis in melanoma cells. We find that these lineage-specific functions of FOXQ1 largely depend on its ability to activate (in carcinomas) or repress (in melanoma) transcription of the N-cadherin gene (CDH2). We demonstrate that FOXQ1 interacts with nuclear β-catenin and TLE proteins, and the β-catenin/TLE ratio, which is higher in carcinoma than melanoma cells, determines the effect of FOXQ1 on CDH2 transcription. Accordingly, other FOXQ1-dependent phenotypes can be manipulated by altering nuclear β-catenin or TLE proteins levels. Our data identify FOXQ1 as a melanoma suppressor and establish a mechanism underlying its inverse lineage-specific transcriptional regulation of transformed phenotypes.

FOXQ1 controls the induced differentiation of melanocytic cells

Oncogenic transcription factor FOXQ1 has been implicated in promotion of multiple transformed phenotypes in carcinoma cells. Recently, we have characterized FOXQ1 as a melanoma tumor suppressor that acts via repression of N-cadherin gene, and invasion and metastasis. Here we report that FOXQ1 induces differentiation in normal and transformed melanocytic cells at least partially via direct transcriptional activation of MITF gene, melanocytic lineage-specific regulator of differentiation. Importantly, we demonstrate that pigmentation induced in cultured melanocytic cells and in mice by activation of cAMP/CREB1 pathway depends in large part on FOXQ1. Moreover, our data reveal that FOXQ1 acts as a critical mediator of BRAF^V600E-dependent regulation of MITF levels, thus providing a novel link between two major signal transduction pathways controlling MITF and differentiation in melanocytic cells.

Detection of Nucleotide Disbalance in Cells Undergoing Oncogene-Induced Senescence

DNA damage response has been characterized as an important mediator of senescence phenotypes induced by activated oncogenes in normal human cells. Depletion of intracellular deoxyribonucleotide pools has been recently recognized as one of the major causes for DNA damage in these cells. Cells undergoing oncogene-induced senescence display decreased expression of several rate-limiting enzymes involved in the biosynthesis of deoxyribonucleotides, including thymidylate synthase (TS) and ribonucleotide reductase (RR). Individual depletion of these enzymes leads to premature senescence. Reciprocally, ectopic expression of TS and RR or addition of deoxyribonucleosides resulted in suppression of senescence phenotypes in normal or tumor cells caused by overexpression of activated HRAS or depletion of C-MYC, respectively. Therefore, in the current chapter, we will describe a methodology for the quantitative measurement of nucleotide pools in senescent cells.

Oxidative stress and proteasome inhibitors in multiple myeloma

Multiple myeloma is a form of plasma cell neoplasm that accounts for approximately 10% of all hematological malignancies. Recently, several novel drugs have been discovered that almost doubled the overall survival of multiple myeloma patients. One of these drugs, the first-in-class proteasome inhibitor bortezomib (Velcade) has demonstrated remarkable response rates in multiple myeloma patients, and yet, currently this disease remains incurable. The major factor undermining the success of multiple myeloma treatment is a rapidly emerging resistance to the available therapy. Thus, the development of stand-alone or adjuvant anti-myeloma agents becomes of paramount importance. Overproduction of intracellular reactive oxygen species (ROS) often accompanies malignant transformation due to oncogene activation and/or enhanced metabolism in tumor cells. As a result, these cells possess higher levels of ROS and lower levels of antioxidant molecules compared to their normal counterparts. Unbalanced production of ROS leads to oxidative stress which, if left unchecked, could be toxic for the cell. In multiple myeloma cells where high rates of immunoglobulin synthesis is an additional factor contributing to overproduction of ROS, further induction of oxidative stress can be an effective strategy to cope with this disease. Here we will review the available data on the role of oxidative stress in the cytotoxicity of proteasome inhibitors and the use of ROS-inducing compounds as anti-myeloma agents.

Slowing down the Grand Touring Prototype speed of cancer cells

Enzymes involved in de novo production of guanosine triphosphate (GTP) have been recently revealed as integral components of melanoma progression through modulation of the activity of small GTPases. Here, we discuss the biology and therapeutic implications of these findings.

Pharmacological targeting of guanosine monophosphate synthase suppresses melanoma cell invasion and tumorigenicity

Malignant melanoma possesses one of the highest metastatic potentials among human cancers. Acquisition of invasive phenotypes is a prerequisite for melanoma metastases. Elucidation of the molecular mechanisms underlying melanoma invasion will greatly enhance the design of novel agents for melanoma therapeutic intervention. Here, we report that guanosine monophosphate synthase (GMPS), an enzyme required for the de novo biosynthesis of GMP, has a major role in invasion and tumorigenicity of cells derived from either BRAF(V600E) or NRAS(Q61R) human metastatic melanomas. Moreover, GMPS levels are increased in metastatic human melanoma specimens compared with primary melanomas arguing that GMPS is an attractive candidate for anti-melanoma therapy. Accordingly, for the first time we demonstrate that angustmycin A, a nucleoside-analog inhibitor of GMPS produced by Streptomyces hygroscopius efficiently suppresses melanoma cell invasion in vitro and tumorigenicity in immunocompromised mice. Our data identify GMPS as a powerful driver of melanoma cell invasion and warrant further investigation of angustmycin A as a novel anti-melanoma agent.

Abstract 847: Suppression of p53R2 but not R2 radiosensitizes mutant p53 tumor cells

Gemcitabine (2’,2’-difluroro-2’-deoxycytidine;dFdCyd) is a potent radiosensitizer in tumor cells in vitro and in vivo. dFdCyd elicits cytotoxicity primarily via incorporation of its triphosphate, dFdCTP, into DNA, whereas inhibition of ribonucleotide reductase (RR) by dFdCDP produces a profound depletion of dATP which correlates to radiosensitization. We have demonstrated that dNTP imbalances generated by dFdCyd produce mismatches in DNA, which augment sensitivity to subsequent ionizing radiation (IR) but are not required to elicit cytotoxicity. We have proposed that RR suppression would be as effective as dFdCyd for radiosensitization. RR is a heterodimeric tetramer composed of the regulatory and active site subunit R1 paired with either R2 or its p53-inducible homolog, p53R2, as the catalytic and rate-limiting subunit. We used a RNAi approach to suppress either R2 or p53R2 (≥ 90% suppression of either R2 or p53R2, with little effect (≤20%) on expression of its homolog) in two p53 wild type cell lines, MCF7 breast carcinoma and A549 lung carcinoma. This approach produced equivalent radiosensitization, dATP depletion, cytotoxicity and increase in DNA mismatches compared to inactivation of RR by dFdCyd (IC50). These results reinforce our prior finding that radiosensitization with dFdCyd is the result of inhibition of RR and not incorporation into DNA or cytotoxicity. We then proposed that R2 but not p53R2 suppression would radiosensitize mutant p53 tumor cells. Interestingly, R2 shRNA suppression did not radiosensitize mutant p53 MCF7/ADR cells (radiation enhancement ratio (RER) = 1.07 ± 0.06). Despite the mutant p53 status of MCF7/ADR cells, p53R2 was elevated (≥ 50%) following suppression of R2 compared to untreated cells (no shRNA). We hypothesized that this increase in p53R2 permits RR to continue producing dNTPs for DNA replication and repair in the absence of R2, thus preventing radiosensitization. Indeed, simultaneous suppression of R2 and p53R2 produced excellent radiosensitization (RER = 1.68 ± 0.04 (shRNAs) vs. 1.6 ± 0.01 (dFdCyd (IC50)), and similar cytotoxicity (surviving fraction (SF) = 76.5 ± 8.3% (shRNA) vs. 71.3 ± 6.7% (dFdCyd)). Impressively, suppression of p53R2 alone produced excellent radiosensitization (RER= 1.76 ± 0.30) with similar cytotoxicity (SF = 84 ± 20%) compared to the double knockdown or dFdCyd. Considering that most solid tumors express mutant p53, suppression of p53R2 instead of R2 may be a more effective method for radiosensitization. These studies suggest that deoxynucleotide biosynthesis is regulated differently in p53 wild type compared to mutant p53 tumor cells, and may be used to maintain DNA replication and repair and prevent anticancer efficacy after damage induced by IR or other DNA damaging agents. The mechanism by which mutant p53 tumor cells upregulate p53R2 warrants further investigation.

Note: This abstract was not presented at the meeting.

Citation Format: Sheryl A. Flanagan, Jeffrey J. Ackroyd, Sudha Mannava, Mikhail A. Nikiforov, Donna S. Shewach. Suppression of p53R2 but not R2 radiosensitizes mutant p53 tumor cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 847. doi:10.1158/1538-7445.AM2014-847

Nrf2 amplifies oxidative stress via induction of Klf9

Reactive oxygen species (ROS) activate NF-E2-related transcription factor 2 (Nrf2), a key transcriptional regulator driving antioxidant gene expression and protection from oxidant injury. Here, we report that in response to elevation of intracellular ROS above a critical threshold, Nrf2 stimulates expression of transcription Kruppel-like factor 9 (Klf9), resulting in further Klf9-dependent increases in ROS and subsequent cell death. We demonstrated that Klf9 independently causes increased ROS levels in various types of cultured cells and in mouse tissues and is required for pathogenesis of bleomycin-induced pulmonary fibrosis in mice. Mechanistically, Klf9 binds to the promoters and alters the expression of several genes involved in the metabolism of ROS, including suppression of thioredoxin reductase 2, an enzyme participating in ROS clearance. Our data reveal an Nrf2-dependent feedforward regulation of ROS and identify Klf9 as a ubiquitous regulator of oxidative stress and lung injury.

A purine nucleotide biosynthesis enzyme guanosine monophosphate reductase is a suppressor of melanoma invasion

Melanoma is one of the most aggressive types of human cancers, and the mechanisms underlying melanoma invasive phenotype are not completely understood. Here, we report that expression of guanosine monophosphate reductase (GMPR), an enzyme involved in de novo biosynthesis of purine nucleotides, was downregulated in the invasive stages of human melanoma. Loss- and gain-of-function experiments revealed that GMPR downregulates the amounts of several GTP-bound (active) Rho-GTPases and suppresses the ability of melanoma cells to form invadopodia, degrade extracellular matrix, invade in vitro, and grow as tumor xenografts in vivo. Mechanistically, we demonstrated that GMPR partially depletes intracellular GTP pools. Pharmacological inhibition of de novo GTP biosynthesis suppressed whereas addition of exogenous guanosine increased invasion of melanoma cells as well as cells from other cancer types. Our data identify GMPR as a melanoma invasion suppressor and establish a link between guanosine metabolism and Rho-GTPase-dependent melanoma cell invasion.

Recent progress in genetics of aging, senescence and longevity: focusing on cancer-related genes

It is widely believed that aging results from the accumulation of molecular damage, including damage of DNA and mitochondria and accumulation of molecular garbage both inside and outside of the cell. Recently, this paradigm is being replaced by the “hyperfunction theory”, which postulates that aging is caused by activation of signal transduction pathways such as TOR (Target of Rapamycin). These pathways consist of different enzymes, mostly kinases, but also phosphatases, deacetylases, GTPases, and some other molecules that cause overactivation of normal cellular functions. Overactivation of these sensory signal transduction pathways can cause cellular senescence, age-related diseases, including cancer, and shorten life span. Here we review some of the numerous very recent publications on the role of signal transduction molecules in aging and age-related diseases. As was emphasized by the author of the “hyperfunction model”, many (or actually all) of them also play roles in cancer. So these “participants” in pro-aging signaling pathways are actually very well acquainted to cancer researchers. A cancer-related journal such as Oncotarget is the perfect place for publication of such experimental studies, reviews and perspectives, as it can bridge the gap between cancer and aging researchers.

Ribonucleotide reductase and thymidylate synthase or exogenous deoxyribonucleosides reduce DNA damage and senescence caused by C-MYC depletion

The down-regulation of dominant oncogenes, including C-MYC, in tumor cells often leads to the induction of senescence via mechanisms that are not completely identified. In the current study, we demonstrate that MYC-depleted melanoma cells undergo extensive DNA damage that is caused by the underexpression of thymidylate synthase (TS) and ribonucleotide reductase (RR) and subsequent depletion of deoxyribonucleoside triphosphate pools. Simultaneous genetic inhibition of TS and RR in melanoma cells induced DNA damage and senescence phenotypes very similar to the ones caused by MYC-depletion. Reciprocally, overexpression of TS and RR in melanoma cells or addition of deoxyribo-nucleosides to culture media substantially inhibited DNA damage and senescence-associated phenotypes caused by C-MYC depletion. Our data demonstrate the essential role of TS and RR in C-MYC-dependent suppression of senescence in melanoma cells.

Cell-based methods for the identification of MYC-inhibitory small molecules

Oncoproteins encoded by dominant oncogenes have long been considered as targets for chemotherapeutic intervention. However, oncogenic transcription factors have often been dismissed as "undruggable." Members of Myc family of transcription factors have been identified as promising targets for cancer chemotherapy in multiple publications reporting the requirement of Myc proteins for maintenance of almost every type of tumor. Here, we describe cell-based approaches to identify c-Myc small molecule inhibitors by screening complex libraries of diverse small molecules based on Myc functionality and specificity.

Depletion of deoxyribonucleotide pools is an endogenous source of DNA damage in cells undergoing oncogene-induced senescence

In normal human cells, oncogene-induced senescence (OIS) depends on induction of DNA damage response. Oxidative stress and hyperreplication of genomic DNA have been proposed as major causes of DNA damage in OIS cells. Here, we report that down-regulation of deoxyribonucleoside pools is another endogenous source of DNA damage in normal human fibroblasts (NHFs) undergoing HRAS(G12V)-induced senescence. NHF-HRAS(G12V) cells underexpressed thymidylate synthase (TS) and ribonucleotide reductase (RR), two enzymes required for the entire de novo deoxyribonucleotide biosynthesis, and possessed low dNTP levels. Chromatin at the promoters of the genes encoding TS and RR was enriched with retinoblastoma tumor suppressor protein and histone H3 tri-methylated at lysine 9. Importantly, ectopic coexpression of TS and RR or addition of deoxyribonucleosides substantially suppressed DNA damage, senescence-associated phenotypes, and proliferation arrest in two types of NHF-expressing HRAS(G12V). Reciprocally, short hairpin RNA-mediated suppression of TS and RR caused DNA damage and senescence in NHFs, although less efficiently than HRAS(G12V). However, overexpression of TS and RR in quiescent NHFs did not overcome proliferation arrest, suggesting that unlike quiescence, OIS requires depletion of dNTP pools and activated DNA replication. Our data identify a previously unknown role of deoxyribonucleotides in regulation of OIS.

Controversial aspects of oncogene-induced senescence

Oncogene-induced senescence (OIS) is a fail-safe mechanism that is developed to suppress cell proliferation caused by aberrant activation of oncoproteins in normal cells. Most of the available literature considers senescence to be caused by activated RAS or RAF proteins. In the current review, we will discuss some of the controversial aspects of RAS- or RAF-induced senescence in different types of normal cells: are tumor suppressors important for OIS? What is the role of DNA damage in OIS? Are there different types of OIS?

Short hairpin RNA suppression of thymidylate synthase produces DNA mismatches and results in excellent radiosensitization

PURPOSE

To determine the effect of short hairpin ribonucleic acid (shRNA)-mediated suppression of thymidylate synthase (TS) on cytotoxicity and radiosensitization and the mechanism by which these events occur.

METHODS AND MATERIALS

shRNA suppression of TS was compared with 5-fluoro-2'-deoxyuridine (FdUrd) inactivation of TS with or without ionizing radiation in HCT116 and HT29 colon cancer cells. Cytotoxicity and radiosensitization were measured by clonogenic assay. Cell cycle effects were measured by flow cytometry. The effects of FdUrd or shRNA suppression of TS on dNTP deoxynucleotide triphosphate imbalances and consequent nucleotide misincorporations into deoxyribonucleic acid (DNA) were analyzed by high-pressure liquid chromatography and as pSP189 plasmid mutations, respectively.

RESULTS

TS shRNA produced profound (≥ 90%) and prolonged (≥ 8 days) suppression of TS in HCT116 and HT29 cells, whereas FdUrd increased TS expression. TS shRNA also produced more specific and prolonged effects on dNTPs deoxynucleotide triphosphates compared with FdUrd. TS shRNA suppression allowed accumulation of cells in S-phase, although its effects were not as long-lasting as those of FdUrd. Both treatments resulted in phosphorylation of Chk1. TS shRNA alone was less cytotoxic than FdUrd but was equally effective as FdUrd in eliciting radiosensitization (radiation enhancement ratio: TS shRNA, 1.5-1.7; FdUrd, 1.4-1.6). TS shRNA and FdUrd produced a similar increase in the number and type of pSP189 mutations.

CONCLUSIONS

TS shRNA produced less cytotoxicity than FdUrd but was equally effective at radiosensitizing tumor cells. Thus, the inhibitory effect of FdUrd on TS alone is sufficient to elicit radiosensitization with FdUrd, but it only partially explains FdUrd-mediated cytotoxicity and cell cycle inhibition. The increase in DNA mismatches after TS shRNA or FdUrd supports a causal and sufficient role for the depletion of dTTP thymidine triphosphate and consequent DNA mismatches underlying radiosensitization. Importantly, shRNA suppression of TS avoids FP-mediated TS elevation and its negative prognostic role. These studies support the further exploration of TS suppression as a novel radiosensitizing strategy.

Krüppel-like factor 9 and progesterone receptor coregulation of decidualizing endometrial stromal cells: implications for the pathogenesis of endometriosis

CONTEXT

Endometriosis is characterized by progesterone resistance and associated with infertility. Krüppel-like factor 9 (KLF9) is a progesterone receptor (PGR)-interacting protein, and mice null for Klf9 are subfertile. Whether loss of KLF9 expression contributes to progesterone resistance of eutopic endometrium of women with endometriosis is unknown.

OBJECTIVE

The aims were to investigate 1) KLF9 expression in eutopic endometrium of women with and without endometriosis, 2) effects of attenuated KLF9 expression on WNT-signaling component expression and on WNT inhibitor Dickkopf-1 promoter activity in human endometrial stromal cells (HESC), and 3) PGR and KLF9 coregulation of the stromal transcriptome network.

METHODS

Transcript levels of KLF9, PGR, and WNT signaling components were measured in eutopic endometrium of women with and without endometriosis. Transcript and protein levels of WNT signaling components in HESC transfected with KLF9 and/or PGR small interfering RNA were analyzed by quantitative RT-PCR and Western blot. KLF9 and PGR coregulation of Dickkopf-1 promoter activity was evaluated using human Dickkopf-1-luciferase promoter/reporter constructs and by chromatin immunoprecipitation. KLF9 and PGR signaling networks were analyzed by gene expression array profiling.

RESULTS

Eutopic endometrium from women with endometriosis had reduced expression of KLF9 mRNA together with those of PGR-B, WNT4, WNT2, and DKK1. KLF9 and PGR were recruited to the DKK1 promoter and modified each other's transactivity. In HESC, KLF9 and PGR coregulated components of the WNT, cytokine, and IGF gene networks that are implicated in endometriosis and infertility.

CONCLUSION

Loss of KLF9 coregulation of endometrial stromal PGR-responsive gene networks may underlie progesterone resistance in endometriosis.

Transcriptional Regulation of CRD-BP by c-myc: Implications for c-myc Functions

The coding region determinant binding protein, CRD-BP, is a multifunctional RNA binding protein involved in different processes such as mRNA turnover, translation control, and localization. It is mostly expressed in fetal and neonatal tissues, where it regulates many transcripts essential for normal embryonic development. CRD-BP is scarce or absent in normal adult tissues but reactivated and/or overexpressed in various neoplastic and preneoplastic tumors and in most cell lines. Its expression has been associated with the most aggressive form of some cancers. CRD-BP is an important regulator of different genes including a variety of oncogenes or proto-oncogenes (c-myc, β-TrCP1, GLI1, etc.). Regulation of CRD-BP expression is critical for proper control of its targets as its overexpression may play an important role in abnormal cell proliferation, suppression of apoptosis, invasion, and metastasis. Molecular bases of the regulatory mechanisms governing CRD-BP expression are still not completely elucidated. In this article, we have identified c-myc as a novel transcriptional regulator of CRD-BP. We show that c-myc binds to CRD-BP promoter and induces its transcription. This induction of CRD-BP expression contributes to the role of c-myc in the regulation of translation, increase in cell size, and acceleration of cell cycle progression via a mechanism involving upregulation of β-TrCP1 levels and activities and accelerated degradation of PDCD4.

Abstract 1390: shRNA mediated suppression of thymidylate synthase results in an increased sensitivity to ionizing radiation similar to 5-fluoro-2′-deoxyuridine

The fluoropyrimidines (FPs), 5-fluorouracil (5-FU) and 5-fluoro-2’-deoxyuridine (FdUrd), are widely used as radiosensitizers in the treatment of gastrointestinal cancer. The FPs exert their cytotoxic effects through activation to 5-fluoro-2’-deoxyuridine 5’-monophosphate (FdUMP), an inhibitor of thymidylate synthase (TS), resulting in depletion of dTTP and inhibition of DNA synthesis. Incorporation of fluorodeoxyuridine 5’-triphosphate (FdUTP) and dUTP into DNA can also elicit cytotoxicity. Radiosensitization by FdUrd correlates with dTTP depletion and S-phase arrest, but not with cytotoxicity. We hypothesized that depletion of dTTP leads to incorrect nucleotide incorporation into DNA which, if not repaired, augments cell death following irradiation. This hypothesis was supported by our findings that mismatch repair (MMR)-deficient HCT116 were better radiosensitized by FdUrd compared to MMR-proficient HCT 116 cells. Furthermore, FdUrd produced nucleotide misincorporations in DNA, measured directly as pSP189 plasmid mutations in HCT116 and SW620 cells, but only at radiosensitizing concentrations. We wished to determine whether suppression of TS protein would be as effective as FPs as a radiosensitizing strategy.

To selectively inhibit TS we used each of two shRNAs, TS1 and TS2, both of which produced a ≥ 90% decrease in TS expression. Suppression of TS expression in HCT116 and HT29 cells elicited an increase in sensitivity to ionizing radiation (IR) that was similar to the increase observed with FdUrd (IC50) (Radiation Enhancement Ratio (RER) = HCT116: 1.4 ± 0.08 (TS1), 1.5 ± 0.04 (TS2) vs. 1.5 ± 0.08 (FdUrd); HT29: 1.4 ± 0.06 (TS1), 1.4 ± 0.04 (TS2) vs. 1.6 ± 0.3 (FdUrd)). Additionally, a similar increase in pSP189 plasmid mutations was observed following suppression of TS, and FdUrd (IC50) in both cell lines. S-phase accumulation following TS suppression was slightly attenuated compared to drug (HCT116: 70% (TS1), 60% (TS2) vs. 75% (FdUrd); HT29: 55% (TS1), 75% (TS2) vs. 85% (FdUrd)). FdUrd and TS suppression produced a similar initial depletion of dTTP (HCT116: 65% (TS1), 70% (TS2) vs. 40% (FdUrd); HT29: 75% (TS1), 50% (TS2) vs. 50% (FdUrd)), however dTTP levels rebounded with FdUrd, but remained low with TS suppression. dATP levels increased profoundly with FdUrd, but decreased slightly with TS suppression. Despite these differences in dNTPs, TS suppression produced less cytotoxicity than FdUrd (IC50), but both produced a similar increase in sensitivity to IR.

These results support our hypothesis that the mechanism of radiosensitzation by FdUrd is the decrease in dTTP, and not the incorporation of the drug into DNA or its cytotoxicity. Therefore, clinical treatment with FdUrd and IR could be titrated to maximize DNA mismatches in tumors rather than cytotoxicity, or TS could be targeted directly by shRNA methods to decrease normal tissue toxicity.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1390.

C-MYC overexpression is required for continuous suppression of oncogene-induced senescence in melanoma cells

Malignant melanomas often harbor activating mutations in BRAF (V600E) or, less frequently, in NRAS (Q61R). Intriguingly, the same mutations have been detected at higher incidences in benign nevi, which are largely composed of senescent melanocytes. Overexpression of BRAF(V600E) or NRAS(Q61R) in human melanocytes in vitro has been shown to induce senescence, although via different mechanisms. How oncogene-induced senescence is overcome during melanoma progression remains unclear. Here, we report that in the majority of analysed BRAF(V600E)- or NRAS(Q61R)-expressing melanoma cells, C-MYC depletion induced different yet overlapping sets of senescence phenotypes that are characteristic of normal melanocytes undergoing senescence due to overexpression of BRAF(V600E) or NRAS(Q61R), respectively. These senescence phenotypes were p16(INK4A)- or p53-independent, however, several of them were suppressed by genetic or pharmacological inhibition of BRAF(V600E) or phosphoinositide 3-kinase pathways, including rapamycin-mediated inhibition of mTOR-raptor in NRAS(Q61R)-expressing melanoma cells. Reciprocally, overexpression of C-MYC in normal melanocytes suppressed BRAF(V600E)-induced senescence more efficiently than NRAS(Q61R)-induced senescence, which agrees with the generally higher rates of activating mutations in BRAF than NRAS gene in human cutaneous melanomas. Our data suggest that one of the major functions of C-MYC overexpression in melanoma progression is to continuous suppress BRAF(V600E)- or NRAS(Q61R)-dependent senescence programs.

MLH1 deficiency enhances radiosensitization with 5-fluorodeoxyuridine by increasing DNA mismatches

The antitumor drug 5-fluoro-2'-deoxyuridine (FdUrd) also sensitizes tumor cells to ionizing radiation in vitro and in vivo. Although radiosensitization with FdUrd requires dTTP depletion and S-phase arrest, the exact mechanism by which these events produce radiosensitization remains unknown. We hypothesized that the depletion of dTTP produces DNA mismatches that, if not repaired before irradiation, would result in radiosensitization. We evaluated this hypothesis in mismatch repair (MMR)-deficient HCT116 0-1 cells that lack the expression of the required MMR protein MLH1 (inactive MLH1), and in MMR-proficient (wild-type MLH1) HCT116 1-2 cells. Although HCT116 0-1 cells were less sensitive to FdUrd (IC(50) = 3.5 microM) versus HCT116 1-2 cells (IC(50) = 0.75 microM), when irradiation followed FdUrd (IC(50)) the MLH1-inactivated cells exhibited greater radiosensitization compared with MMR-wild-type cells [radiation enhancement ratio (RER) = 1.8 +/- 0.28 versus 1.1 +/- 0.1, respectively] and an increase (> or =8-fold) in nucleotide misincorporations. In SW620 cells and HCT116 1-2 MLH1-wild-type cells, FdUrd (IC(50)) did not produce radiosensitization nor did it increase the mutation frequency, but after short hairpin RNA-directed suppression of MLH1 this concentration produced excellent radiosensitization (RER = 1.6 +/- 0.10 and 1.5 +/- 0.06, respectively) and an increase in nucleotide misincorporations (8-fold and 6-fold, respectively). Incubation with higher concentrations of FdUrd (IC(90)) after suppression of MLH1 produced a further increase in ionizing radiation sensitivity in both SW620 and HCT116 1-2 cells (RER = 1.8 +/- 0.03 and 1.7 +/- 0.13, respectively) and nucleotide misincorporations (>10-fold in both cell lines). These results demonstrate an important role for MLH1 and implicate mismatches in radiosensitization by FdUrd.

Direct role of nucleotide metabolism in C-MYC-dependent proliferation of melanoma cells

To identify C-MYC targets rate-limiting for proliferation of malignant melanoma, we stably inhibited C-MYC in several human metastatic melanoma lines via lentivirus-based shRNAs approximately to the levels detected in normal melanocytes. C-MYC depletion did not significantly affect levels of E2F1 protein reported to regulate expression of many S-phase specific genes, but resulted in the repression of several genes encoding enzymes rate-limiting for dNTP metabolism. These included thymidylate synthase (TS), inosine monophosphate dehydrogenase 2 (IMPDH2) and phosphoribosyl pyrophosphate synthetase 2 (PRPS2). C-MYC depletion also resulted in reduction in the amounts of deoxyribonucleoside triphosphates (dNTPs) and inhibition of proliferation. shRNA-mediated suppression of TS, IMPDH2 or PRPS2 resulted in the decrease of dNTP pools and retardation of the cell cycle progression of melanoma cells in a manner similar to that of C-MYC-depletion in those cells. Reciprocally, concurrent overexpression of cDNAs for TS, IMPDH2 and PRPS2 delayed proliferative arrest caused by inhibition of C-MYC in melanoma cells. Overexpression of C-MYC in normal melanocytes enhanced expression of the above enzymes and increased individual dNTP pools. Analysis of in vivo C-MYC interactions with TS, IMPDH2 and PRPS2 genes confirmed that they are direct C-MYC targets. Moreover, all three proteins express at higher levels in cells from several metastatic melanoma lines compared to normal melanocytes. Our data establish a novel functional link between C-MYC and dNTP metabolism and identify its role in proliferation of tumor cells.

c-Myc depletion inhibits proliferation of human tumor cells at various stages of the cell cycle

A major role for c-Myc in the proliferation of normal cells is attributed to its ability to promote progression through G(1) and into S phase of the cell cycle. The absolute requirement of c-Myc for cell cycle progression in human tumor cells has not been comprehensively addressed. In the present work, we used a lentiviral-based short hairpin RNA (shRNA) expression vector to stably reduce c-Myc expression in a large number of human tumor cell lines and in three different types of normal human cells. In all cases, cell proliferation was severely inhibited, with normal cells ultimately undergoing G(0)/G(1) growth arrest. In contrast, tumor cells demonstrated a much more variable cell cycle response with cells from several lines accumulating in S or G(2)/M phases. Moreover, in some tumor lines, the phase of cell cycle arrest caused by inhibition of c-Myc could be altered by depleting tumor suppressor protein p53 or its transcriptional target p21(CIP/WAF). Our data suggest that, as in the case of normal cells, c-Myc is essential for sustaining proliferation of human tumor cells. However its rate-limiting role in cell cycle control is variable and is reliant upon the status of other cell cycle regulators.

Anti-oncogenic role of the endoplasmic reticulum differentially activated by mutations in the MAPK pathway

Dysfunction of the endoplasmic reticulum (ER) has been reported in a variety of human pathologies, including cancer. However, the contribution of the ER to the early stages of normal cell transformation is largely unknown. Using primary human melanocytes and biopsies of human naevi (moles), we show that the extent of ER stress induced by cellular oncogenes may define the mechanism of activation of premature senescence. Specifically, we found that oncogenic forms of HRAS (HRAS(G12V)) but not its downstream target BRAF (BRAF(V600E)), engaged a rapid cell-cycle arrest that was associated with massive vacuolization and expansion of the ER. However, neither p53, p16(INK4a) nor classical senescence markers--such as foci of heterochromatin or DNA damage--were able to account for the specific response of melanocytes to HRAS(G12V). Instead, HRAS(G12V)-driven senescence was mediated by the ER-associated unfolded protein response (UPR). The impact of HRAS on the UPR was selective, as it was poorly induced by activated NRAS (more frequently mutated in melanoma than HRAS). These results argue against premature senescence as a converging mechanism of response to activating oncogenes and support a direct role of the ER as a gatekeeper of tumour control.

The Mad and Myc basic domains are functionally equivalent

The Myc/Max/Mad family of transcription factors plays a fundamental role in the regulation of cell proliferation, oncogenic transformation, and cell differentiation. However, it remains unclear whether different heterodimers, such as Myc/Max and Mad/Max, recognize the same or different target genes in vivo. We show by chromatin immunoprecipitation that Myc target genes are also recognized by Mad1 in differentiated HL60 cells. We also substituted the complete basic region of Myc for the corresponding region of Mad. Wild-type c-Myc was then compared with c-Myc(Mad-BR) in oncogenic transformation, regulation of cell proliferation, induction of apoptosis, activation of chromosomal gene expression, and direct binding to chromosomal sites by chromatin immunoprecipitation. We find that the wild-type c-Myc and c-Myc/MadBR proteins have indistinguishable biological activity and target gene recognition in vivo. These data are consistent with a model in which Myc and Mad regulate a common set of target genes.

A functional screen for Myc-responsive genes reveals serine hydroxymethyltransferase, a major source of the one-carbon unit for cell metabolism

A cDNA library enriched with Myc-responsive cDNAs but depleted of myc cDNAs was used in a functional screen for growth enhancement in c-myc-null cells. A cDNA clone for mitochondrial serine hydroxymethyltransferase (mSHMT) that was capable of partial complementation of the growth defects of c-myc-null cells was identified. Expression analysis and chromatin immunoprecipitation demonstrated that mSHMT is a direct Myc target gene. Furthermore, a separate gene encoding the cytoplasmic isoform of the same enzyme is also a direct target of Myc regulation. SHMT enzymes are the major source of the one-carbon unit required for folate metabolism and for the biosynthesis of nucleotides and amino acids. Our data establish a novel functional link between Myc and the regulation of cellular metabolism.

TRRAP-dependent and TRRAP-independent transcriptional activation by Myc family oncoproteins

We demonstrate that transformation-transactivation domain-associated protein (TRRAP) binding and the recruitment of histone H3 and H4 acetyltransferase activities are required for the transactivation of a silent telomerase reverse transcriptase (TERT) gene in exponentially growing human fibroblasts by c-Myc or N-Myc protein. However, recruitment of TRRAP by c- or N-Myc is dispensable for the partial induction of several basally expressed genes in exponentially growing primary and immortalized fibroblasts. Furthermore, recruitment of TRRAP is required for c-Myc- or N-Myc-mediated oncogenic transformation but not for the partial restoration of the growth defect in myc-null fibroblasts. A segment of the adenovirus E1A protein fused to a transformation-defective N-Myc protein carrying a small deletion in the transactivation domain specifically restores interaction with TRRAP, activates the silent TERT gene, induces acetylation of histones H3 and H4 at the TERT promoter, and transforms primary cells. Accordingly, wild-type L-Myc is much less efficient in TRRAP binding, activation of the silent TERT gene, and transformation of primary fibroblasts. Nevertheless, L-Myc is a potent activator of several basally expressed genes and can fully restore the growth defect of myc-null cells. These results suggest a differential requirement for TRRAP for several Myc-mediated activities.

Apoptosis inhibitor as a suppressor of tumor progression: expression of Bcl-2 eliminates selective advantages for p53-deficient cells in the tumor

Inactivation of p53 and expression of Bcl-2, frequently occurring during tumor progression, have different prognostic value: while inactivation of p53 is generally associated with unfavorable prognosis, expression of Bcl-2 often correlates with better clinical outcome and delays selection of metastatic variants of experimental tumors. To analyze the mechanisms underlying the "anti-progression" function of Bcl-2, we engineered tumor cell variants differing in their p53 status and Bcl-2 expression and compared their expansion in experimental tumors. Although neither p53 suppression nor Bcl-2-expression altered cell growth properties in vitro, both variants showed rapid accumulation in growing tumors in vivo, presumably due to their resistance to hypoxia. However, no expansion of p53-deficient variants occurred in the tumors formed by Bcl-2-overexpressing cells, indicating that p53 deficiency has no selective advantages in the Bcl-2-expressing environment. Importantly, expression of Bcl-2, unlike p53 suppression, did not lead to genomic instability as judged by the frequencies of gene amplification. Thus, acquisition of Bcl-2 expression is as advantageous for tumor cell growth in vivo as is p53 inactivation but does not affect genomic stability and creates the environment restrictive for the expansion of genetically unstable and potentially malignant p53-deficient cells, causing a delay in tumor progression and explaining the different prognostic value of Bcl-2 and p53.

Complementation of Myc-dependent cell proliferation by cDNA expression library screening

The targeted knockout of the c-myc gene from rat fibroblasts leads to a stable defect in cell proliferation. We used complex cDNA libraries expressed from retroviral vectors and an efficient sorting procedure to rapidly select for cDNAs that can restore the growth rate of c-myc deficient cells. All of the biologically active cDNAs contained either c-myc or N-myc, suggesting that no other cellular genes can effectively bypass the requirement for c-myc in fibroblast proliferation. This approach provides a powerful screening method for cell cycle changes in genetically defined systems.

A novel chromodomain protein, pdd3p, associates with internal eliminated sequences during macronuclear development in Tetrahymena thermophila

Conversion of the germ line micronuclear genome into the genome of a somatic macronucleus in Tetrahymena thermophila requires several DNA rearrangement processes. These include (i) excision and subsequent elimination of several thousand internal eliminated sequences (IESs) scattered throughout the micronuclear genome and (ii) breakage of the micronuclear chromosomes into hundreds of DNA fragments, followed by de novo telomere addition to their ends. Chromosome breakage sequences (Cbs) that determine the sites of breakage and short regions of DNA adjacent to them are also eliminated. Both processes occur concomitantly in the developing macronucleus. Two stage-specific protein factors involved in germ line DNA elimination have been described previously. Pdd1p and Pdd2p (for programmed DNA degradation) physically associate with internal eliminated sequences in transient electron-dense structures in the developing macronucleus. Here, we report the purification, sequence analysis, and characterization of Pdd3p, a novel developmentally regulated, chromodomain-containing polypeptide. Pdd3p colocalizes with Pdd1p in the peripheral regions of DNA elimination structures, but is also found more internally. DNA cross-linked and immunoprecipitated with Pdd1p- or Pdd3p-specific antibodies is enriched in IESs, but not Cbs, suggesting that different protein factors are involved in elimination of these two groups of sequences.

Excision of micronuclear-specific DNA requires parental expression of pdd2p and occurs independently from DNA replication in Tetrahymena thermophila

Elimination of germ-line DNA segments is an essential step in the somatic development of many organisms and in the terminal differentiation of several specialized cell types. In binuclear ciliates, including Tetrahymena thermophila, DNA elimination occurs during the conversion of the germ-line micronuclear genome into the somatic genome of the new macronucleus. Little is known about molecular determinants and regulatory mechanisms involved in this process. Pdd2p is one of a small set of Tetrahymena polypeptides whose time of synthesis, nuclear localization, and physical association with sequences destined for elimination suggest an involvement in the DNA elimination process. In this study, we report that loss of parental expression of Pdd2p leads to the perturbation of several DNA rearrangement processes in developing zygotic macronuclei, including excision of internal eliminated sequences, excision of chromosome breakage sequences, and endoreplication of the new macronuclear genome and eventually results in lethality of the progeny. We demonstrate that excision and elimination of micronuclear-specific DNA occurs independently of endoreplication of the new macronuclear genome that takes place during the same period of time. Thus, our data indicate that parental expression of Pdd2p is required for successful DNA elimination and development of somatic nuclei.

Parental expression of the chromodomain protein Pdd1p is required for completion of programmed DNA elimination and nuclear differentiation

Thousands of DNA elimination events occur during somatic differentiation of many ciliated protozoa. In Tetrahymena, the eliminated DNA aggregates into submacronuclear structures containing the protein Pdd1p, a member of the chromodomain family. We disrupted somatic copies of PDD1, eliminating parental expression of the gene early in the sexual phase of the life cycle. Even though zygotic expression, from the undisrupted germline PDD1 copy, is activated before DNA elimination normally occurs, the somatic knockout cells suffer defects in DNA elimination, genome endoduplication, and nuclear resorption, and eventually die, demonstrating that PDD1 is essential and suggesting Pdd1p is directly involved in establishing a chromatin structure required for DNA elimination.

A nuclear protein involved in apoptotic-like DNA degradation in Stylonychia: implications for similar mechanisms in differentiating and starved cells

Ciliates are unicellular eukaryotic organisms containing two types of nuclei: macronuclei and micronuclei. After the sexual pathway takes place, a new macronucleus is formed from a zygote nucleus, whereas the old macronucleus is degraded and resorbed. In the course of macronuclear differentiation, polytene chromosomes are synthesized that become degraded again after some hours. Most of the DNA is eliminated, and the remaining DNA is fragmented into small DNA molecules that are amplified to a high copy number in the new macronucleus. The protein Pdd1p (programmed DNA degradation protein 1) from Tetrahymena has been shown to be present in macronuclear anlagen in the DNA degradation stage and also in the old macronuclei, which are resorbed during the formation of the new macronucleus. In this study the identification and localization of a Pdd1p homologous protein in Stylonychia (Spdd1p) is described. Spdd1p is localized in the precursor nuclei in the DNA elimination stage and in the old macronuclei during their degradation, but also in macronuclei and micronuclei of starved cells. In all of these nuclei, apoptotic-like DNA breakdown was detected. These data suggest that Spdd1p is a general factor involved in programmed DNA degradation in Stylonychia.

Cell transforming genes and tumor progression: in vivo unified secondary phenotypic cell changes

We have earlier shown that Syrian hamster cells spontaneously transformed in vitro during in vivo progression, acquire in 1 step, along with highly increased tumorigenicity, 2 new properties characterizing the [H2O2CA + tPGE(S)] phenotype, i.e., a high H2O2 catabolizing (antioxidant) activity and the ability to release PGE2 upon contact with NK cells. In contrast, RSV-SR-(v-src)-transformed cells acquire the [H2O2CA + PGE(S)] phenotype and high tumorigenicity during in vitro transformation, i.e., without preliminary in vivo selection. In the present study, the possible influence of different transforming genes on the rates of subsequent in vivo tumor progression was studied using cells in vitro transformed by SV40, BAV-3, or transduced by activated genes Ha-ras, p53, myc and bcl-2. The expression of the [H2O2CA + PGE(S)] phenotype, the extent of tumorigenic and spontaneous metastasizing activities were examined before and during in vivo cells selection in s.c. growing tumors. Our results demonstrate that: (1) after in vitro transformation all cell lines (except v-src) were negative for the expression of [H2O2CA + PGE(S)] phenotype and remained equally low-tumorigenic; (2) independently of the types of genes initially transforming the cells, in vivo tumor progression was consistently leading to the replacement of parental cells by cells expressing the [H2O2CA + PGE(S)] phenotype, to 30-200 times increased tumorigenicity and less frequently to metastasizing; (3) the time necessary for selection of cells expressing this phenotype was the same (about 180 days in vivo) for all transformants, except bcl-2; the latter reaching similar values after a significant delay. Thus, common secondary src-like phenotypic cell changes, regardless of initially cell transforming genes are necessarily selected during tumor progression in vivo.

Suppression of apoptosis by bcl-2 does not prevent p53-mediated control of experimental metastasis and anchorage dependence

Mutations in the p53 tumor suppressor gene are frequently associated with the metastatic stage of tumor progression. Inactivation of p53 was shown to promote metastasis under experimental conditions. To determine the p53 functions that are involved in the control of tumor metastasis, we compared properties of three types of transformed mouse fibroblasts: with intact p53, with p53-mediated apoptosis suppressed by bcl-2 and with p53 inactivated by dominant negative mutants. Although expression of bcl-2 blocked apoptosis in detached cells and increased tumor cell survival in the blood circulation, it was insufficient to affect the ability of p53 to cause cell cycle arrest in detached cells and suppress experimental metastasis. For the suppression of metastasis complete inactivation of p53 was required. We conclude that the apoptotic function of p53 is dispensable for the p53-dependent suppression of experimental metastasis that is presumably achieved by controlling anchorage dependence. These data provide a possible explanation to dramatic differences in values of bcl-2 and mutant p53 as prognostic markers in human cancer.

p53 modulation of anchorage independent growth and experimental metastasis

Death in circulation is one of the natural barriers preventing dissemination of tumor cells and formation of metastases. One of the negative factors acting in circulation is the loss of cell contact with natural substrate which can be imitated in vitro by the incubation of cells in suspension or in semi-solid media. Normal mouse fibroblasts (MEFs) stay viable in suspension and undergo p53-independent G1 growth arrest. Transformation with Ela and ras oncogenes leads to the abrogation of this arrest and to the p53-dependent apoptosis occurring in G1 phase of the cell cycle. Suppression of apoptosis by p53 gene knock-out, transduction of dominant negative p53 mutant or bcl-2 prevents death in suspension and greatly induces frequency of colony formation in semi-solid media. The ability of cells to undergo apoptosis does not correlate with their tumorigenicity in nude mice but does correlate with their ability to survive in lungs of intravenously injected mice and to form experimental metastases. We suggest that abrogation of a p53-mediated apoptosis facilitates experimental metastasis by promoting survival of tumor cells in circulation.

Mechanisms of unusually high antioxidant activity of RSV-SR-transformed cells and of its suppression by activated p21ras

We have previously demonstrated that hamster embryo fibroblasts (HEFs) transformed by Rous Sarcoma virus, Schmidt-Ruppin strain (RSV-SR) are highly resistant to damage by H202 (H2O2R), (in contrast to HEFs transformed spontaneously, or by bovine adenovirus and SV40), while N-ras transfection of RSV-SR transformants leads to suppression of pp6Ov-scr and of H2O2R. In this study we have examined (1) mechanisms of antioxidant activity (AOA) of HEFs transformed by these agents and (2) the possible role of the v-src gene in unusually high AOA of RSV-SR transformants and of activated ras oncogenes in its suppression. All transformants exhibit increased catalase and glutathione peroxidase (GP) activities, while SOD, glutathione and glutathione reductase (GR) were reduced. As compared with other transformants, the significantly higher catalase and the low SOD activities were characteristic of RSV-SR-transformants, while an increase in GP was observed in all types of transformants. Correspondingly, RSV-SR-transformants showed an extremely high H202-catabolizing activity (H2O2CA) and no lipid peroxidation chain reaction (LPCR). N-ras-induced suppression of pp60v-scr of RSV-SR-transformed HEFs coincided with the suppression of catalase, GP, H202 and H202CA. However, suppression of catalase and GP was also observed in N-ras- and Ha-ras-transfected, spontaneously transformed HEFs. Thus, extremely high catalase activity and suppression of LPCR are apparently the main mechanisms of the unusually high H202R of RSV-SR transformants, while its suppression by activated ras oncogenes may also take place in some transformants, free of v-src activity.

ART-CH: a VL30 in chickens?

The complete sequence of ART-CH, a recently found chicken retrotransposon (A. V. Gudkov, E. A. Komarova, M. A. Nikiforov, and T. E. Zaitsevskaya, J. Virol. 66:1726-1736, 1992), was characterized. ART-CH has the structure of a 3,300-bp-long provirus, including two 388-bp long terminal repeats (LTRs) (U3, 245 bp; R region, 17 bp; and U5, 126 bp), a tRNA(Trp)-binding site, and a polypurine tract, similar to avian leukosis viruses. At least some of the approximately 50 genomic copies of ART-CH are transcribed into polyadenylated RNA, which is initiated and terminated at the expected sites within the LTRs. In contrast to the regulatory sequences involved in proviral expression and replication, the internal regions of ART-CH seem to be completely defective. Several short regions of homology with avian leukosis virus genes, most of which encode gag-related sequences, were found among different reading frames of ART-CH, which are not organized like regular retroviral genes. Both sequence analysis and restriction fragment length polymorphism analysis revealed a high degree of sequence (97% homology) and structural similarity among members of the ART-CH family, indicating their common origin and recent penetration into chicken DNA. ART-CH sequences were detected in mouse cells infected with Rous sarcoma virus produced by an ART-CH-expressing Rous sarcoma. These data are consistent with the hypothesis that ART-CH belongs to a class of defective retrotransposons whose replication strategy requires the use of helper viruses. They might originate from an avian leukosis virus-related retrovirus which completely lost its coding capacities as a result of multiple mutations and deletions. These features apparently group ART-CH with the VL30 retrotransposons of rodents.