Table of Contents

Volume 11, Issue 17

  September 1, 2012

  Pages 3143 - 3338

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 Issue Catalogue (MARC XML)

EDITORIALS: CELL CYCLE FEATURES

Open Access Article

Do transcription factories and TOP2B provide a recipe for chromosome translocations in therapy-related leukemia?

Ian G. Cowell and Caroline A. Austin
Pages 3143 - 3144
http://dx.doi.org/10.4161/cc.21477
Abstract | Full Text | PDF

Comment on: Cowell IG, et al. Proc Natl Acad Sci USA 2012; 109:8989-94.

Open Access Article

Turned on by genotoxic stress

Anna Travesa and Curt Wittenberg
Pages 3145 - 3146
http://dx.doi.org/10.4161/cc.21587
Abstract | Full Text | PDF

Comment on: Travesa A, et al. EMBO J 2012; 31:1811-22.

Open Access Article

Too much or too little: Harnessing senescence to control oncogene-driven cancer

Katherine M. Hannan and Richard B. Pearson
Pages 3147 - 3148
http://dx.doi.org/10.4161/cc.21588
Abstract | Full Text | PDF

Comment on: Astle MV, et al. Oncogene 2012; 31:1949-62.

Open Access Article

Lack of communication rusts and ages stem cells

Eri Taniguchi Ishikawa and Jose A. Cancelas
Pages 3149 - 3150
http://dx.doi.org/10.4161/cc.21589
Abstract | Full Text | PDF

Comment on: Ishikawa ET, et al. Proc Natl Acad Sci USA 2012; 109:9071-6.

Open Access Article

Glucose homeostasis and p75^NTR: The sweet side of neurotrophin receptor signaling

Bernat Baeza-Raja and Katerina Akassoglou
Pages 3151 - 3152
http://dx.doi.org/10.4161/cc.21590
Abstract | Full Text | PDF

Comment on: Baeza-Raja B, et al. Proc Natl Acad Sci USA 2012; 109:5838-43.




CELL CYCLE NEWS & VIEWS

Open Access Article

Ubiquitin and the DNA damage response: A new handle on histones

Simon Bekker-Jensen and Niels Mailand
Page 3153
http://dx.doi.org/10.4161/cc.21718
Abstract | Full Text | PDF

Comment on: Gatti M, et al. Cell Cycle 2012; 11:2538-44.

Open Access Article

Fenofibrate triggers apoptosis of glioblastoma cells in vitro: New insights for therapy

Antonio Giordano and Marcella Macaluso
Page 3154
http://dx.doi.org/10.4161/cc.21719
Abstract | Full Text | PDF

Comment on: Wilk A, et al. Cell Cycle 2012; 11:2660-71.

Open Access Article

Blood cell fate changes without cell cycle transition

Matthias Stadtfeld
Page 3155
http://dx.doi.org/10.4161/cc.21720
Abstract | Full Text | PDF

Comment on: Di Tullio A, et al. Cell Cycle 2012; 11:2739-46.

Open Access Article

The yin and yang of proliferation and differentiation: Cyclin D1 inhibits differentiation factors ChREBP and HNF4α

Frances M. Sladek
Page 3156
http://dx.doi.org/10.4161/cc.21721
Abstract | Full Text | PDF

Comment on: Hanse EA, et al. Cell Cycle 2012; 11:2681-90.

Open Access Article

Tetraploid cancer cell precursors in ovarian carcinoma

Maria Castedo, Laura Senovilla, Ilio Vitale and Guido Kroemer
Pages 3157 - 3158
http://dx.doi.org/10.4161/cc.21722
Abstract | Full Text | PDF

Comment on: Lv L, et al. Cell Cycle 2012; 11:2864-75.




EXTRA VIEWS

The role of S6K1 in ER-positive breast cancer

Marina K. Holz
Pages 3159 - 3165
http://dx.doi.org/10.4161/cc.21194
Abstract | Full Text | PDF

The 40S ribosomal S6 kinase 1 (S6K1) is a conserved serine/threonine protein kinase that belongs to the AGC family of protein kinases, which also includes Akt and many others. S6K1 is the principal kinase effector downstream of the mammalian target of rapamycin complex 1 (mTORC1). S6K1 is sensitive to a wide range of signaling inputs, including growth factors, amino acids, energy levels and hypoxia. S6K1 relays these signals to regulate a growing list of substrates and interacting proteins in control of oncogenic processes, such as cell growth and proliferation, cell survival and apoptosis and cell migration and invasion. Several lines of evidence suggest an important role for S6K1 in estrogen receptor (ER)-positive breast cancer. S6K1 directly phosphorylates and activates ERα. Furthermore, S6K1 expression is estrogenically regulated. Therefore, hyperactivation of mTORC1/S6K1 signaling may be closely related to ER-positive status in breast cancer and may be utilized as a marker for prognosis and a therapeutic target.

The checkpoint transcriptional response: Make sure to turn it off once you are satisfied

Marcus B. Smolka, Francisco M. Bastos de Oliveira, Michael R. Harris and Robertus A.M. de Bruin
Pages 3166 - 3174
http://dx.doi.org/10.4161/cc.21197
Abstract | Full Text | PDF

The replication checkpoint signaling network monitors the presence of replication-induced lesions to DNA and coordinates an elaborate cellular response that includes ample transcriptional reprogramming. Recent work has established two major groups of replication stress-induced genes in Saccharomyces cerevisiae, the DNA damage response (DDR) genes and G₁/S cell cycle (CC) genes. In both cases, transcriptional activation is mediated via checkpoint-dependent inhibition of a transcriptional repressor (Crt1 for DDR and Nrm1 for CC) that participates in negative feedback regulation. This repressor-mediated regulation enables transcription to be rapidly repressed once cells have dealt with the replication stress. The recent finding of a new class of CC genes, named “switch genes,” further uncovers a mode of transcription regulation that prevents overexpression of replication stress induced genes during G₁. Collectively, these findings highlight the need for mechanisms that tightly control replication stress-induced transcription, allowing rapid transcriptional activation during replication stress but also avoiding long-term hyperaccumulation of the induced protein product that may be detrimental to cell proliferation.

The role of mTOR signaling pathway in spinal cord injury

Haruo Kanno, Hiroshi Ozawa, Akira Sekiguchi, Seiji Yamaya, Satoshi Tateda, Kenichiro Yahata and Eiji Itoi
Pages 3175 - 3179
http://dx.doi.org/10.4161/cc.21262
Abstract | Full Text | PDF

The mammalian target of rapamycin (mTOR) signaling pathway plays an important role in multiple cellular functions, such as cell metabolism, proliferation and survival. Many previous studies have shown that mTOR regulates both neuroprotective and neuroregenerative functions in trauma and various diseases in the central nervous system (CNS). Recently, we reported that inhibition of mTOR using rapamycin reduces neural tissue damage and locomotor impairment after spinal cord injury (SCI) in mice. Our results demonstrated that the administration of rapamycin at four hours after injury significantly increases the activity of autophagy and reduces neuronal loss and cell death in the injured spinal cord. Furthermore, rapamycin-treated mice show significantly better locomotor function in the hindlimbs following SCI than vehicle-treated mice. These findings indicate that the inhibition of mTOR signaling using rapamycin during the acute phase of SCI produces neuroprotective effects and reduces secondary damage at lesion sites. However, the role of mTOR signaling in injured spinal cords has not yet been fully elucidated. Various functions are regulated by mTOR signaling in the CNS, and multiple pathophysiological processes occur following SCI. Here, we discuss several unresolved issues and review the evidence from related articles regarding the role and mechanisms of the mTOR signaling pathway in neuroprotection and neuroregeneration after SCI.

Mapping differentiation pathways from hematopoietic stem cells using Flk2/Flt3 lineage tracing

Scott W. Boyer, Anna E. Beaudin and E. Camilla Forsberg
Pages 3180 - 3188
http://dx.doi.org/10.4161/cc.21279
Abstract | Full Text | PDF

Genetic fate-mapping approaches provide a unique opportunity to assess differentiation pathways under physiological conditions. We have recently employed a lineage tracing approach to define hematopoietic differentiation pathways in relation to expression of the tyrosine kinase receptor Flk2.¹ Based on our examination of reporter activity across all stem, progenitor and mature populations in our Flk2-Cre lineage model, we concluded that all mature blood lineages are derived through a Flk2⁺ intermediate, both at steady-state and under stress conditions. Here, we re-examine in depth our initial conclusions and perform additional experiments to test alternative options of lineage specification. Our data unequivocally support the conclusion that onset of Flk2 expression results in loss of self-renewal but preservation of multilineage differentiation potential. We discuss the implications of these data for defining stem cell identity and lineage potential among hematopoietic populations.




REVIEWS

Regulation of transcription and chromatin structure by pRB: Here, there and everywhere

Srikanth Talluri and Frederick A. Dick
Pages 3189 - 3198
http://dx.doi.org/10.4161/cc.21263
Abstract | Full Text | PDF

Commitment to divide is one of the most crucial steps in the mammalian cell division cycle. It is critical for tissue and organismal homeostasis, and consequently is highly regulated. The vast majority of cancers evade proliferative control, further emphasizing the importance of the commitment step in cell cycle regulation. The Retinoblastoma (RB) tumor suppressor pathway regulates this decision-making step. Since being the subject of Knudson’s ‘two hit hypothesis’, there has been considerable interest in understanding pRB’s role in cancer. It is best known for repressing E2F dependent transcription of cell cycle genes. However, pRB’s role in controlling chromatin structure is expanding and bringing it into new regulatory paradigms. In this review we discuss pRB function through protein-protein interactions, at the level of transcriptional regulation of individual promoters and in organizing higher order chromatin domains.

Cistrome plasticity and mechanisms of cistrome reprogramming

Ivan Garcia-Bassets and Dong Wang
Pages 3199 - 3210
http://dx.doi.org/10.4161/cc.21281
Abstract | Full Text | PDF

Mammalian genomes contain thousands of cis-regulatory elements for each transcription factor (TF), but TFs only occupy a relatively small subset referred to as cistrome. Recent studies demonstrate that a TF cistrome might differ among different organisms, tissue types and individuals. In a cell, a TF cistrome might differ among different physiological states, pathological stages and between physiological and pathological conditions. It is, therefore, remarkable how highly plastic these binding profiles are, and how massively they can be reprogrammed in rapid response to intra/extracellular variations and during cell identity transitions and evolution. Biologically, cistrome reprogramming events tend to be followed by changes in transcriptional outputs, thus serving as transformative mechanisms to synchronically alter the biology of the cell. In this review, we discuss the molecular basis of cistrome plasticity and attempt to integrate the different mechanisms and biological conditions associated with cistrome reprogramming. Emerging data suggest that, when altered, these reprogramming events might be linked to tumor development and/or progression, which is a radical conceptual change in our mechanistic understanding of cancer and, potentially, other diseases.




REPORTS

Septin 7 is required for orderly meiosis in mouse oocytes

Sen Li, Xiang-Hong Ou, Liang Wei, Zhen-Bo Wang, Qing-Hua Zhang, Ying-Chun Ouyang, Yi Hou, Heide Schatten and Qing-Yuan Sun
Pages 3211 - 3218
http://dx.doi.org/10.4161/cc.21553
Abstract | Full Text | PDF

Septin 7 is a conserved GTP-binding protein. In this study, we examined the localization and functions of Septin 7 during mouse oocyte meiotic maturation. Immunofluorescent analysis showed that intrinsic Septin 7 localized to the spindles from the pro-MI stage to the MII stage. Knockdown of Septin 7 by siRNA microinjection caused abnormal spindles and affected extrusion of the first polar body. Septin 7 mRNA tagged with myc was injected into GV stage oocytes to overexpress Septin 7. Overexpressed Myc-Septin 7 localized to the spindle and beneath the plasma membrane displaying long filaments. Fluorescence intensity of spindle α-tubulin in myc-Septin 7-injected oocytes was weaker than that of the control group, demonstrating that Septin 7 may influence recruitment of α-tubulin to spindles. MII oocytes injected with myc-Septin 7 exhibited abnormal chromosome alignment, and parthenogenetic activation failed to allow extrusion of the second polar body, suggesting that overexpression of Septin 7 may affect extrusion of the polar body by disturbing the alignment of chromosomes and regulating α-tubulin recruitment to spindles. In summary, Septin 7 may regulate meiotic cell cycle progression by affecting microtubule cytoskeletal dynamics in mouse oocytes.

Deacetylase inhibitors modulate proliferation and self-renewal properties of leukemic stem and progenitor cells

Annette Romanski, Kerstin Schwarz, Maren Keller, Sarah Wietbrauk, Anja Vogel, Jessica Roos, Claudia Oancea, Boris Brill, Oliver H. Krämer, Hubert Serve, Martin Ruthardt and Gesine Bug
Pages 3219 - 3226
http://dx.doi.org/10.4161/cc.21565
Abstract | Full Text | PDF

Acute myeloid leukemia (AML) is a highly malignant disease that is not curable in the majority of patients. Numerous non-random genetic abnormalities are known, among which several translocations such as PLZF/RARα or AML1/ETO are known to aberrantly recruit histone deacetylases. Deacetylase inhibitors (DACi) are promising drugs leading to growth inhibition, cell cycle arrest, premature senescence and apoptosis in malignant cells. It is believed that DACi may have clinical efficacy by eradicating the most primitive population of leukemic stem and progenitor cells, possibly by interfering with self-renewal.

The aim of the study was to investigate the effects of DACi on leukemic stem and progenitor cells using murine transduction-transplantation models of hematopoietic cells harboring the leukemia-associated fusion proteins (LAFP) PLZF/RARα or a truncated AML1/ETO protein (AML1/ETO exon 9). We show that the self-renewal and short-term repopulation capacity of AML1/ETO- or PLZF/RARα-expressing Sca1⁺/lin^- stem and progenitor cells are profoundly inhibited by clinically applicable concentrations of the DACi dacinostat and vorinostat. To further investigate the mechanisms underlying these effects, we examined the impact of DACi on the transcription factor c-MYC and the Polycomb group protein BMI1, which are induced by LAFP and involved in leukemic transformation. In AML1/ETO or PLZF/RARα-positive 32D cells, DACi-mediated antiproliferative effects were associated with downregulation of BMI1 and c-MYC protein levels. Similar effects were demonstrated in primary samples of cytogenetically defined high-risk AML patients. In conclusion, DACi may be effective as maintenance therapy by negatively interfering with signaling pathways that control survival and proliferation of leukemic stem and progenitor cells.

LIN54 harboring a mutation in CHC domain is localized to the cytoplasm and inhibits cell cycle progression

Taira Matsuo, Hiroyo Kuramoto, Tsutomu Kumazaki, Youji Mitsui and Tomoko Takahashi
Pages 3227 - 3236
http://dx.doi.org/10.4161/cc.21569
Abstract | Full Text | PDF

The mammalian LIN complex (LINC) plays important roles in regulation of cell cycle genes. LIN54 is an essential core subunit of the LINC and has a DNA binding region (CHC domain), which consists of two cysteine-rich (CXC) domains separated by a short spacer. We generated various LIN54 mutants, such as CHC deletion mutant, and investigated their subcellular localizations and effects on cell cycle. Wild-type LIN54 was predominantly localized in the nucleus. We identified two nuclear localization signals (NLSs), both of which were required for nuclear localization of LIN54. Interestingly, deletion of one CXC domain resulted in an increased cytoplasmic localization. The cytoplasmic LIN54 mutant accumulated in the nucleus after leptomycin B treatment, suggesting CRM1-mediated nuclear export of LIN54. Point mutations (C525Y and C611Y) in conserved cysteine residues of CXC domain that abolish DNA binding activity also increased cytoplasmic localization. These data suggest that DNA binding activity of LIN54 is required for its nuclear retention. We also found that LIN54^C525Y and LIN54^C611Y inhibited cell cycle progression and led to abnormal nuclear morphology. Other CXC mutants also induced similar abnormalities in cell cycle progression. LIN54^C525Y led to a decreased expression of some G₂/M genes, whose expressions are regulated by LINC. This cell cycle inhibition was partially restored by overexpression of wild-type LIN54. These results suggest that abnormal cellular localization of LIN54 may have effects on LINC activity.

MEK2 regulates ribonucleotide reductase activity through functional interaction with ribonucleotide reductase small subunit p53R2

Chunmei Piao, Cha-Kyung Youn, Min Jin, Sang Pil Yoon, In-Youb Chang, Jung Hee Lee and Ho Jin You
Pages 3237 - 3249
http://dx.doi.org/10.4161/cc.21591
Abstract | Full Text | PDF

The p53R2 protein, a newly identified member of the ribonucleotide reductase family that provides nucleotides for DNA damage repair, is directly regulated by p53. We show that p53R2 is also regulated by a MEK2 (ERK kinase 2/MAP kinase kinase 2)-dependent pathway. Increased MEK1/2 phosphorylation by serum stimulation coincided with an increase in the RNR activity in U2OS and H1299 cells. The inhibition of MEK2 activity, either by treatment with a MEK inhibitor or by transfection with MEK2 siRNA, dramatically decreased the serum-stimulated RNR activity. Moreover, p53R2 siRNA, but not R2 siRNA, significantly inhibits serum-stimulated RNR activity, indicating that p53R2 is specifically regulated by a MEK2-dependent pathway. Co-immunoprecipitation analyses revealed that the MEK2 segment comprising amino acids 65–171 is critical for p53R2–MEK2 interaction, and the binding domain of MEK2 is required for MEK2-mediated increased RNR activity. Phosphorylation of MEK1/2 was greatly augmented by ionizing radiation, and RNR activity was concurrently increased. Ionizing radiation-induced RNR activity was markedly attenuated by transfection of MEK2 or p53R2 siRNA, but not R2 siRNA. These data show that MEK2 is an endogenous regulator of p53R2 and suggest that MEK2 may associate with p53R2 and upregulate its activity.

Disruption of the protein interaction between FAK and IGF-1R inhibits melanoma tumor growth

Deniz A. Ucar, Elena Kurenova, Timothy J. Garrett, William G. Cance, Carl Nyberg, Audrey Cox, Nicole Massoll, David A. Ostrov, Nicholas Lawrence, Saïd M. Sebti, Maria Zajac-Kaye and Steven N. Hochwald
Pages 3250 - 3259
http://dx.doi.org/10.4161/cc.21611
Abstract | Full Text | PDF

FAK (focal adhesion kinase) and IGF-1R (insulin-like growth factor receptor-1) directly interact with each other and thereby activate crucial signaling pathways that benefit cancer cells. Inhibition of FAK and IGF-1R function has been shown to significantly decrease cancer cell proliferation and increase sensitivity to chemotherapy and radiation treatment. As a novel approach in human melanoma, we evaluated the effect of a small-molecule compound that disrupts the protein interaction of FAK and IGF-1R.

Previously, using virtual screening and functional testing, we identified a lead compound (INT2–31) that targets the known FAK-IGF-1R protein interaction site. We studied the ability of this compound to disrupt FAK-IGF-1R protein interactions, inhibit downstream signaling, decrease human melanoma cell proliferation, alter cell cycle progression, induce apoptosis and decrease tumor growth in vivo.

INT2–31 blocked the interaction of FAK and IGF-1R in vitro and in vivo in melanoma cells and tumor xenografts through precluding the activation of IRS-1, leading to reduced phosphorylation of AKT upon IGF-1 stimulation. As a result, INT2–31 significantly inhibited cell proliferation and viability (range 0.05–10 μM). More importantly, 15 mg/kg of INT2–31 given for 21 d via intraperitoneal injection disrupted the interaction of FAK and IGF-1R and effectively decreased phosphorylation of tumor AKT, resulting in significant melanoma tumor regression in vivo.

Our data suggest that the FAK-IGF-1R protein interaction is an important target, and disruption of this interaction with a novel small molecule (INT2–31) has potential anti-neoplastic therapeutic effects in human melanoma.

Heat shock induces apoptosis in human embryonic stem cells but a premature senescence phenotype in their differentiated progeny

Larisa L. Alekseenko, Victoria I. Zemelko, Valery V. Zenin, Nataly A. Pugovkina, Irina V. Kozhukharova, Zoya V. Kovaleva, Tatiana M. Grinchuk, Irina I. Fridlyanskaya and Nikolay N. Nikolsky
Pages 3260 - 3269
http://dx.doi.org/10.4161/cc.21595
Abstract | Full Text | PDF

Embryonic stem cells (ESC) are able to self-renew and to differentiate into any cell type. To escape error transmission to future cell progeny, ESC require robust mechanisms to ensure genomic stability. It was stated that stress defense of mouse and human ESC against oxidative stress and irradiation is superior compared with differentiated cells. Here, we investigated heat shock response of human ESC (hESC) and their differentiated progeny. Fibroblast-like cells were generated by spontaneous hESC differentiation via embryoid bodies. Like normal human diploid fibroblasts, these cells have a finite lifespan in culture, undergo replicative senescence and die. We found that sublethal heat shock affected survival of both cell types, but in hESC it induced apoptosis, whereas in differentiated cells it produced cell cycle arrest and premature senescence phenotype. Heat shock survived hESC and differentiated cells restored the properties of initial cells. Heated hESC progeny exhibited pluripotent markers and the capacity to differentiate into the cells of three germ layers. Fibroblast-like cells resisted heat shock, proliferated for a limited number of passages and entered replicative senescence as unheated parental cells. Taken together, these results show for the first time that both hESC and their differentiated derivatives are sensitive to heat shock, but the mechanisms of their stress response are different: hESC undergo apoptosis, whereas differentiated cells under the same conditions exhibit stress-induced premature senescence (SIPS) phenotype. Both cell types that survived sublethal heat shock sustain parental cell properties.

Overexpression of a novel osteopetrosis-related gene CCDC154 suppresses cell proliferation by inducing G₂/M arrest

Wanqin Liao, Rongsen Zhao, Liting Lu, Rongrong Zhang, Jiawei Zou, Tao Xu, Changjie Wu, Jiajia Tang, Yuezhen Deng and Xincheng Lu
Pages 3270 - 3279
http://dx.doi.org/10.4161/cc.21642
Abstract | Full Text | PDF

Osteopetrosis, a disorder of skeletal bone, can cause death during childhood. We previously described a new spontaneous autosomal recessive osteopetrosis mouse mutant, “new toothless” (ntl). In this study, we reported for the first time the identification, cloning and characterization of the coiled-coil domain-containing 154 (CCDC154), a novel gene whose deletion of ~5 kb sequence including exons 1–6 was completely linked to the ntl mutant. The CCDC154 was conserved between mouse and human and is wildly expressed in mouse tissues. The cellular localization of CCDC154 was in the early endosomes. Overexpression of CCDC154 inhibited cell proliferation of HEK293 cells by inducing G₂/M arrest. CCDC154 also inhibited tumor cell growth, and the soft agar assay revealed a significant decrease of the colony size of Hela cells upon transfection of CCDC154. Our results indicate that CCDC154 is a novel osteopetrosis-related gene involved in cell cycle regulation and tumor suppression growth.

Metabolic reprogramming and two-compartment tumor metabolism: Opposing role(s) of HIF1α and HIF2α in tumor-associated fibroblasts and human breast cancer cells

Barbara Chiavarina, Ubaldo E. Martinez-Outschoorn, Diana Whitaker-Menezes, Anthony Howell, Herbert B. Tanowitz, Richard G. Pestell, Federica Sotgia and Michael P. Lisanti
Pages 3280 - 3289
http://dx.doi.org/10.4161/cc.21643
Abstract | Full Text | PDF

Hypoxia-inducible factor (HIF) 1α and 2α are transcription factors responsible for the cellular response to hypoxia. The functional roles of HIF1α and HIF2α in cancer are distinct and vary among different tumor types. The aim of this study was to evaluate the compartment-specific role(s) of HIF1α and HIF2α in breast cancer. To this end, immortalized human fibroblasts and MDA-MB-231 breast cancer cells carrying constitutively active HIF1α or HIF2α mutants were analyzed with respect to their metabolic function(s) and ability to promote tumor growth in an in vivo setting. We observed that activation of HIF1α, but not HIF2α, in stromal cells promotes a shift toward aerobic glycolysis, with increased L-lactate production and a loss of mitochondrial activity. In a xenograft model, HIF1α-activated fibroblasts promoted the tumor growth of co-injected MDA-MB-231 cells without an increase in angiogenesis. Conversely, HIF2α-activated stromal cells did not favor tumor growth and behaved as the empty vector controls. Similarly, activation of HIF1α, but not HIF2α, in MDA-MB-231 cells promoted a shift toward aerobic glycolysis, with increased glucose uptake and L-lactate production. In contrast, HIF2α activation in cancer cells increased the expression of EGFR, Ras and cyclin D1, which are known markers of tumor growth and cell cycle progression. In a xenograft model, HIF1α activation in MDA-MB-231 cells acted as a tumor suppressor, resulting in an almost 2-fold reduction in tumor mass and volume. Interestingly, HIF2α activation in MDA-MB-231 cells induced a significant ~2-fold-increase in tumor mass and volume. Analysis of mitochondrial activity in these tumor xenografts using COX (cytochrome C oxidase) staining demonstrated elevated mitochondrial oxidative metabolism (OXPHOS) in HIF2α-tumors. We conclude that the role(s) of HIF1α and HIF2α in tumorigenesis are compartment-specific. HIF1α acts as a tumor promoter in stromal cells but as a tumor suppressor in cancer cells. Conversely, HIF2α is a tumor promoter in cancer cells. Mechanistically, HIF1α-driven aerobic glycolysis in stromal cells supports cancer cell growth via the paracrine production of nutrients (such as L-lactate) that can “feed” cancer cells. However, HIF1α-driven aerobic glycolysis in cancer cells inhibits tumor growth. Finally, HIF2α activation in cancer cells induces the expression of known pro-oncogenic molecules and promotes the mitochondrial activity of cancer cells.

Mutant p53 is a transcriptional co-factor that binds to G-rich regulatory regions of active genes and generates transcriptional plasticity

Timo Quante, Benjamin Otto, Marie Brázdová, Iva Kejnovská, Wolfgang Deppert and Genrich V. Tolstonog
Pages 3290 - 3303
http://dx.doi.org/10.4161/cc.21646
Abstract | Full Text | PDF

The molecular mechanisms underlying mutant p53 (mutp53) “gain-of-function” (GOF) are still insufficiently understood, but there is evidence that mutp53 is a transcriptional regulator that is recruited by specialized transcription factors. Here we analyzed the binding sites of mutp53 and the epigenetic status of mutp53-regulated genes that had been identified by global expression profiling upon depletion of endogenous mutp53 (R273H) expression in U251 glioblastoma cells. We found that mutp53 preferentially and autonomously binds to G/C-rich DNA around transcription start sites (TSS) of many genes characterized by active chromatin marks (H3K4me3) and frequently associated with transcription-competent RNA polymerase II. Mutp53-bound regions overlap predominantly with CpG islands and are enriched in G4-motifs that are prone to form G-quadruplex structures. In line, mutp53 binds and stabilizes a well-characterized G-quadruplex structure in vitro. Hence, we assume that binding of mutp53 to G/C-rich DNA regions associated with a large set of cancer-relevant genes is an initial step in their regulation by mutp53. Using GAS1 and HTR2A as model genes, we show that mutp53 affects several parameters of active transcription. Finally, we discuss a dual mode model of mutp53 GOF, which includes both stochastic and deterministic components.

Featured Article - FREE

The RelB subunit of NFκB acts as a negative regulator of circadian gene expression

Marina M. Bellet, Loredana Zocchi and Paolo Sassone-Corsi
Pages 3304 - 3311
http://dx.doi.org/10.4161/cc.21669
Abstract | Full Text | PDF

The circadian system controls a large array of physiological and metabolic functions. The molecular organization of the circadian clock is complex, involving various elements organized in feedback regulatory loops. Here we demonstrate that the RelB subunit of NFκB acts as a repressor of circadian transcription. RelB physically interacts with the circadian activator BMAL1 in the presence of CLOCK to repress circadian gene expression at the promoter of the clock-controlled gene Dbp. The repression is independent of the circadian negative regulator CRY. Notably, RelB −/− fibroblasts have profound alterations of circadian genes expression. These findings reveal a previously unforeseen function for RelB as an important regulator of the mammalian circadian system in fibroblasts.

Mitomycin C potentiates TRAIL-induced apoptosis through p53-independent upregulation of death receptors: Evidence for the role of c-Jun N-terminal kinase activation

Hairong Cheng, Bo Hong, Lanlan Zhou, Joshua E. Allen, Guihua Tai, Robin Humphreys, David T. Dicker, Yingqiu Y. Liu and Wafik S. El-Deiry
Pages 3312 - 3323
http://dx.doi.org/10.4161/cc.21670
Abstract | Full Text | PDF

The discovery of the molecular targets of chemotherapeutic medicines and their chemical footprints can validate and improve the use of such medicines. In the present report, we investigated the effect of mitomycin C (MMC), a classical chemotherapeutic agent on cancer cell apoptosis induced by TRAIL. We found that MMC not only potentiated TRAIL-induced apoptosis in HCT116 (p53−/−) colon cancer cells but also sensitized TRAIL-resistant colon cancer cells HT-29 to the cytokine both in vitro and in vivo. MMC also augmented the pro-apoptotic effects of two TRAIL receptor agonist antibodies, mapatumumab and lexatumumab. At a mechanistic level, MMC downregulated cell survival proteins, including Bcl2, Mcl-1 and Bcl-XL, and upregulated pro-apoptotic proteins including Bax, Bim and the cell surface expression of TRAIL death receptors DR4 and DR5. Gene silencing of DR5 by short hairpin RNA reduced the apoptosis induced by combination treatment of MMC and TRAIL. Induction of DR4 and DR5 was independent of p53, Bax and Bim but was dependent on c-Jun N terminal kinase (JNK) as JNK pharmacological inhibition and siRNA abolished the induction of the TRAIL receptors by MMC.

Dephosphorylation of threonine-821 of the retinoblastoma tumor suppressor protein (Rb) is required for apoptosis induced by UV and Cdk inhibition

Brandon Lentine, Lisa Antonucci, Ray Hunce, Justina Edwards, Valerie Marallano and Nancy A. Krucher
Pages 3324 - 3330
http://dx.doi.org/10.4161/cc.21693
Abstract | Full Text | PDF

The Retinoblastoma protein (Rb) is important in the control of cell proliferation and apoptosis. Its activity is controlled by reversible phosphorylation on several serine and threonine residues. When Rb is hypophosphorylated, it inhibits proliferation by preventing passage through the G₁- S phase transition. Hyperphosphorylated Rb promotes cell cycle progression. The role of Rb phosphorylation in the control of apoptosis is largely unknown, although several apoptotic stimuli result in dephosphorylation of Rb. It may be that dephosphorylation of specific amino acids signals apoptosis vs. cell cycle arrest. Using glutamic acid mutagenesis, we have generated 15 single phosphorylation site mutants of Rb to alter serine/threonine to glutamic acid to mimic the phosphorylated state. By calcium phosphate transfection, mutant plasmids were introduced into C33A Rb-null cells, and apoptosis was induced using UV. Apoptosis was measured by ELISA detection of degraded DNA and by immunoblotting to assess proteolytic cleavage of PARP. Our results show that only mutation of threonine-821 to glutamic acid (T821E) blocked apoptosis by 50%, whereas other sites tested had little effect. In Rb-null Saos-2 and SKUT-1 cells, the T821E mutation also blocked apoptosis induced by the cdk inhibitor, Roscovitine, by 50%. In addition, we show that endogenous Rb is dephosphorylated on threonine-821 when cells are undergoing apoptosis. Thus, our data indicates that dephosphorylation of threonine-821 of Rb is required for cells to undergo apoptosis.

Human telomerase reverse transcriptase (hTERT) is a target gene of β-catenin in human colorectal tumors

Stefanie Jaitner, Jana A. Reiche, Achim J. Schäffauer, Elke Hiendlmeyer, Hermann Herbst, Thomas Brabletz, Thomas Kirchner and Andreas Jung
Pages 3331 - 3338
http://dx.doi.org/10.4161/cc.21790
Abstract | Full Text | PDF

The majority of colorectal cancers (CRCs) are characterized by a dysregulated canonical Wnt-signaling pathway leading to the stabilization and subsequent cellular increase and accumulation of β-catenin. After translocation into the nucleus, it acts as a transcription factor resulting in the expression of β-catenin target genes. These resemble most of the hallmarks of cancer except eternal life. The central mediator of this hallmark is hTERT (human telomerase reverse transcriptase). The hTERT gene is regulated, besides others, by the transcription factor c-Myc and, thus, indirectly via β-catenin as c-Myc is a β-catenin target gene. Interestingly, the expression patterns of hTERT and β-catenin, but not c-Myc are overlapping, probably because c-Myc is not only regulated by β-catenin, but also by many other transcription factors and pathways. Therefore, we argued that hTERT might be a direct target gene of β-catenin. In this study, we show evidence that β-catenin directly regulates the expression of the hTERT gene.