The HSP40 family chaperone isoform DNAJB6b prevents neuronal cells from tau aggregation

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disorder with clinical presentations of progressive cognitive and memory deterioration. The pathologic hallmarks of AD include tau neurofibrillary tangles and amyloid plaque depositions in the hippocampus and associated neocortex. The neuronal aggregated tau observed in AD cells suggests that the protein folding problem is a major cause of AD. J-domain-containing proteins (JDPs) are the largest family of cochaperones, which play a vital role in specifying and directing HSP70 chaperone functions. JDPs bind substrates and deliver them to HSP70. The association of JDP and HSP70 opens the substrate-binding domain of HSP70 to help the loading of the clients. However, in the initial HSP70 cycle, which JDP delivers tau to the HSP70 system in neuronal cells remains unclear.

RESULTS

We screened the requirement of a diverse panel of JDPs for preventing tau aggregation in the human neuroblastoma cell line SH-SY5Y by a filter retardation method. Interestingly, knockdown of DNAJB6, one of the JDPs, displayed tau aggregation and overexpression of DNAJB6b, one of the isoforms generated from the DNAJB6 gene by alternative splicing, reduced tau aggregation. Further, the tau bimolecular fluorescence complementation assay confirmed the DNAJB6b-dependent tau clearance. The co-immunoprecipitation and the proximity ligation assay demonstrated the protein-protein interaction between tau and the chaperone-cochaperone complex. The J-domain of DNAJB6b was critical for preventing tau aggregation. Moreover, reduced DNAJB6 expression and increased tau aggregation were detected in an age-dependent manner in immunohistochemical analysis of the hippocampus tissues of a mouse model of tau pathology.

CONCLUSIONS

In summary, downregulation of DNAJB6b increases the insoluble form of tau, while overexpression of DNAJB6b reduces tau aggregation. Moreover, DNAJB6b associates with tau. Therefore, this study reveals that DNAJB6b is a direct sensor for its client tau in the HSP70 folding system in neuronal cells, thus helping to prevent AD.

Bone Marrow Histology in Hemophagocytic Lymphohistiocytosis

CONTEXT.—

Bone marrow (BM) samples are obtained through aspiration and trephine biopsy. Hemophagocytic lymphohistiocytosis (HLH) has been largely studied in BM aspirate smears.

OBJECTIVE.—

To investigate the histologic features of HLH in trephine biopsy.

DESIGN.—

Patients with hemophagocytosis in BM aspirate smears were assigned to HLH (n = 127) and non-HLH (n = 203) groups. We quantified hematoxylin-eosin and CD68 immunohistochemical staining of their trephine biopsies.

RESULTS.—

No significant correlation was noted in the hemophagocytosis count between aspirate smears and trephine biopsies. Compared with the non-HLH group, the HLH group had a higher hemophagocytosis count (13 versus 9 per tissue section, P = .046), lower percentage of the adipocytic area (36.7% versus 50.3%, P < .001), and higher percentage of the foamy area (19.1% versus 14.5%, P < .001). The HLH group had more histiocyte infiltrates (total histiocyte density, 9.2% versus 7.3%; P < .001) and more fat-infiltrating histiocytes (histiocyte density of the fat-associated part [HD-FA], 7.6% versus 6.2%; P < .001). We identified the following poor prognostic factors in the HLH group: age 50 years or older (median overall survival [mOS], 95 versus 499 days; P = .04), Epstein-Barr virus-positive T-cell lymphoproliferative diseases (EBV+TLPDs) (mOS, 51 versus 425 days; P < .001), hemophagocytosis count of 6 or higher per tissue section (mOS, 66 versus 435 days; P = .02), and HD-FA of 9% or greater (mOS, 61 versus 359 days; P = .02). Multivariate analysis revealed that age 50 years or older (hazard ratio [HR], 2.38; P < .001), EBV+TLPDs (HR, 2.07; P < .001), and hemophagocytosis count of 6 or higher per tissue section (HR, 2.07; P = .002) were independent prognostic factors for HLH.

CONCLUSIONS.—

The HLH group had higher hemophagocytic activity, higher cellularity, a more foamy appearance, more histiocyte infiltrates, and more fat-infiltrating histiocytes. High hemophagocytic activity and marked histiocyte infiltrates in the BM fat were associated with poorer prognosis.

An HSP90 cochaperone Ids2 maintains the stability of mitochondrial DNA and ATP synthase

BACKGROUND

Proteostasis unbalance and mitochondrial dysfunction are two hallmarks of aging. While the chaperone folds and activates its clients, it is the cochaperone that determines the specificity of the clients. Ids2 is an HSP90's cochaperone controlling mitochondrial functions, but no in vivo clients of Ids2 have been reported yet.

RESULTS

We performed a screen of the databases of HSP90 physical interactors, mitochondrial components, and mutants with respiratory defect, and identified Atp3, a subunit of the complex V ATP synthase, as a client of Ids2. Deletion of IDS2 destabilizes Atp3, and an α-helix at the middle region of Ids2 recruits Atp3 to the folding system. Shortage of Ids2 or Atp3 leads to the loss of mitochondrial DNA. The intermembrane space protease Yme1 is critical to maintaining the Atp3 protein level. Moreover, Ids2 is highly induced when cells carry out oxidative respiration.

CONCLUSIONS

These findings discover a cochaperone essentially for maintaining the stability of mitochondrial DNA and the proteostasis of the electron transport chain-crosstalk between two hallmarks of aging.

A Clinicopathological Study of Cytomegalovirus Lymphadenitis and Tonsillitis and Their Association with Epstein-Barr Virus

Introduction

Histopathological characteristics of cytomegalovirus (CMV) lymphadenitis have been well described. Rare studies have reported the immune status and clinical features. Clinically, experts believed that CMV lymphadenitis develops in immunocompromised and immunocompetent patients. Infectious mononucleosis (IM)-like syndrome is the most well-known clinical presentation.

Methods

We reviewed archived CMV immunohistochemical stains on lymphoid tissues. The clinicopathological features of CMV-positive cases were studied.

Results

For lymph nodes, we detected CMV in 29% (5/17) allogeneic peripheral blood hematopoietic stem cell transplantation (PBSCT) recipients, 29% (4/14) post-autologous PBSCT patients, 13% (6/47) patients treated with intravenous chemotherapy, and 9% (9/96) immunocompetent patients. We detected CMV in 7% (2/24) of tonsils but not in the nasopharynx, tongue base, or spleen specimens. The patients with iatrogenic immunodeficiency ranged from 37 to 76 years old. CMV infections developed a few years after lymphoma treatment (median duration after allogeneic PBSCT, 932 days; after autologous PBSCT, 370 days; and after chemotherapy, 626 days). The most common clinical presentation was neck mass (13/25, 42%), followed by asymptomatic image finding (10/25, 40%). Positron emission tomography/computed tomography (PET/CT) scan showed increased uptake compared to the liver in all patients (11/11, 100%). Of 10 lymphoma patients, 8 (80%) had a Deauville score of 4–5; they accounted for 30% (8/27) of lymphoma patients with false-positive PET/CT scan results. All cases were self-limiting. 96% (23/25) cases had Epstein–Barr virus coinfection, and EBER-positive cells were predominantly in a few germinal centers.

Conclusions

Cytomegalovirus (CMV) lymphadenitis and tonsillitis were subclinical infections, not primary CMV infection with IM-like syndrome. The lymphadenopathy typically developed a few years after lymphoma treatments in the middle-aged and the elderly. The lesions mimicked lymphoma relapse in PET scans. Therefore, recognizing CMV infection in lymphoid tissues is of clinical importance.

Graphic abstract

Clinical and functional characterization of a novel STUB1 frameshift mutation in autosomal dominant spinocerebellar ataxia type 48 (SCA48)

BACKGROUND

Heterozygous pathogenic variants in STUB1 are implicated in autosomal dominant spinocerebellar ataxia type 48 (SCA48), which is a rare familial ataxia disorder. We investigated the clinical, genetic and functional characteristics of STUB1 mutations identified from a Taiwanese ataxia cohort.

METHODS

We performed whole genome sequencing in a genetically undiagnosed family with an autosomal dominant ataxia syndrome. Further Sanger sequencing of all exons and intron-exon boundary junctions of STUB1 in 249 unrelated patients with cerebellar ataxia was performed. The pathogenicity of the identified novel STUB1 variant was investigated.

RESULTS

We identified a novel heterozygous frameshift variant, c.832del (p.Glu278fs), in STUB1 in two patients from the same family. This rare mutation is located in the U-box of the carboxyl terminus of the Hsc70-interacting protein (CHIP) protein, which is encoded by STUB1. Further in vitro experiments demonstrated that this novel heterozygous STUB1 frameshift variant impairs the CHIP protein's activity and its interaction with the E2 ubiquitin ligase, UbE2D1, leading to neuronal accumulation of tau and α-synuclein, caspase-3 activation, and promoting cellular apoptosis through a dominant-negative pathogenic effect. The in vivo study revealed the influence of the CHIP expression level on the differentiation and migration of cerebellar granule neuron progenitors during cerebellar development.

CONCLUSIONS

Our findings provide clinical, genetic, and a mechanistic insight linking the novel heterozygous STUB1 frameshift mutation at the highly conserved U-box domain of CHIP as the cause of autosomal dominant SCA48. Our results further stress the importance of CHIP activity in neuronal protein homeostasis and cerebellar functions.

Nuclear envelope tethering inhibits the formation of ALT-associated PML bodies in ALT cells

Telomere length homeostasis is essential for maintaining genomic stability and cancer proliferation. Telomerase-negative cancer cells undergo recombination-mediated alternative lengthening of telomeres. Telomeres associate with the nuclear envelope through the shelterin RAP1 and nuclear envelope SUN1 proteins. However, how the associations between telomeres and the nuclear envelope affect the progression of telomere recombination is not understood. Here, we show that telomere anchorage might inhibit telomere-telomere recombination. SUN1 depletion stimulates the formation of alternative lengthening of telomeres-associated promyelocytic leukemia bodies in ALT cells. In contrast, overexpression of a telomere-nuclear envelope-tethering chimera protein, RAP1-SUN1, suppresses APB formation. Moreover, inhibition of this nuclear envelope attachment alleviates the requirement of TOP3α for resolving the supercoiling pressure during telomere recombination. A coimmunoprecipitation assay revealed that the SUN1 N-terminal nucleoplasmic domain interacts with the RAP1 middle coil domain, and phosphorylation-mimetic mutations in RAP1 inhibit this interaction. However, abolishing the RAP1-SUN1 interaction does not hinder APB formation, which hints at the existence of another SUN1-dependent telomere anchorage pathway. In summary, our results reveal an inhibitory role of telomere-nuclear envelope association in telomere-telomere recombination and imply the presence of redundant pathways for the telomere-nuclear envelope association in ALT cells.

Glucose intake hampers PKA-regulated HSP90 chaperone activity

Aging is an intricate phenomenon associated with the gradual loss of physiological functions, and both nutrient sensing and proteostasis control lifespan. Although multiple approaches have facilitated the identification of candidate genes that govern longevity, the molecular mechanisms that link aging pathways are still elusive. Here, we conducted a quantitative mass spectrometry screen and identified all phosphorylation/dephosphorylation sites on yeast proteins that significantly responded to calorie restriction, a well-established approach to extend lifespan. Functional screening of 135 potential regulators uncovered that Ids2 is activated by PP2C under CR and inactivated by PKA under glucose intake. ids2Δ or ids2 phosphomimetic cells displayed heat sensitivity and lifespan shortening. Ids2 serves as a co-chaperone to form a complex with Hsc82 or the redundant Hsp82, and phosphorylation impedes its association with chaperone HSP90. Thus, PP2C and PKA may orchestrate glucose sensing and protein folding to enable cells to maintain protein quality for sustained longevity.

PARP1 controls KLF4-mediated telomerase expression in stem cells and cancer cells

Telomerase is highly expressed in cancer and embryonic stem cells (ESCs) and implicated in controlling genome integrity, cancer formation and stemness. Previous studies identified that Krüppel-like transcription factor 4 (KLF4) activates telomerase reverse transcriptase (TERT) expression and contributes to the maintenance of self-renewal in ESCs. However, little is known about how KLF4 regulates TERT expression. Here, we discover poly(ADP-ribose) polymerase 1 (PARP1) as a novel KLF4-interacting partner. Knockdown of PARP1 reduces TERT expression and telomerase activity not only in cancer cells, but also in human and mouse ESCs. Recruitment of KLF4 to TERT promoter is reduced in PARP1-suppressed cells. The poly(ADP-ribose) polymerase activity is dispensable, while the oligo(ADP-ribose) polymerase activity is required for the PARP1- and KLF4-mediated TERT activation. Repression of Parp1 in mouse ESCs decreases expression of pluripotent markers and induces differentiation. These results suggest that PARP1 recruits KLF4 to activate telomerase expression and stem cell pluripotency, indicating a positive regulatory role of the PARP1–KLF4 complex in telomerase expression in cancer and stem cells.

Telomere shortening triggers a feedback loop to enhance end protection

Telomere homeostasis is controlled by both telomerase machinery and end protection. Telomere shortening induces DNA damage sensing kinases ATM/ATR for telomerase recruitment. Yet, whether telomere shortening also governs end protection is poorly understood. Here we discover that yeast ATM/ATR controls end protection. Rap1 is phosphorylated by Tel1 and Mec1 kinases at serine 731, and this regulation is stimulated by DNA damage and telomere shortening. Compromised Rap1 phosphorylation hampers the interaction between Rap1 and its interacting partner Rif1, which thereby disturbs the end protection. As expected, reduction of Rap1–Rif1 association impairs telomere length regulation and increases telomere–telomere recombination. These results indicate that ATM/ATR DNA damage checkpoint signal contributes to telomere protection by strengthening the Rap1–Rif1 interaction at short telomeres, and the checkpoint signal oversees both telomerase recruitment and end capping pathways to maintain telomere homeostasis.

Yeast Cip1 is activated by environmental stress to inhibit Cdk1-G1 cyclins via Mcm1 and Msn2/4

Upon environmental changes, proliferating cells delay cell cycle to prevent further damage accumulation. Yeast Cip1 is a Cdk1 and Cln2-associated protein. However, the function and regulation of Cip1 are still poorly understood. Here we report that Cip1 expression is co-regulated by the cell-cycle-mediated factor Mcm1 and the stress-mediated factors Msn2/4. Overexpression of Cip1 arrests cell cycle through inhibition of Cdk1–G1 cyclin complexes at G1 stage and the stress-activated protein kinase-dependent Cip1 T65, T69, and T73 phosphorylation may strengthen the Cip1and Cdk1–G1 cyclin interaction. Cip1 accumulation mainly targets Cdk1–Cln3 complex to prevent Whi5 phosphorylation and inhibit early G1 progression. Under osmotic stress, Cip1 expression triggers transient G1 delay which plays a functionally redundant role with another hyperosmolar activated CKI, Sic1. These findings indicate that Cip1 functions similarly to mammalian p21 as a stress-induced CDK inhibitor to decelerate cell cycle through G1 cyclins to cope with environmental stresses.

A G1 cell cycle regulatory kinase Cip1 has been identified in budding yeast but how this is regulated is unclear. Here the authors identify cell cycle (Mcm1) and stress-mediated (Msn 2/4) transcription factors as regulating Cip1, causing stress induced CDK inhibition and delay in cell cycle progression.

SMYD3 Promotes Homologous Recombination via Regulation of H3K4-mediated Gene Expression

SMYD3 is a methyltransferase highly expressed in many types of cancer. It usually functions as an oncogenic protein to promote cell cycle, cell proliferation, and metastasis. Here, we show that SMYD3 modulates another hallmark of cancer, DNA repair, by stimulating transcription of genes involved in multiple steps of homologous recombination. Deficiency of SMYD3 induces DNA-damage hypersensitivity, decreases levels of repair foci, and leads to impairment of homologous recombination. Moreover, the regulation of homologous recombination-related genes is via the methylation of H3K4 at the target gene promoters. These data imply that, besides its reported oncogenic abilities, SMYD3 may maintain genome integrity by ensuring expression levels of HR proteins to cope with the high demand of restart of stalled replication forks in cancers.

K63-polyubiquitinated HAUSP deubiquitinates HIF-1α and dictates H3K56 acetylation promoting hypoxia-induced tumour progression

Intratumoural hypoxia induces HIF-1α and promotes tumour progression, metastasis and treatment resistance. HIF-1α stability is regulated by VHL-E3 ligase-mediated ubiquitin-dependent degradation; however, the hypoxia-regulated deubiquitinase that stabilizes HIF-1α has not been identified. Here we report that HAUSP (USP7) deubiquitinase deubiquitinates HIF-1α to increase its stability, induce epithelial-mesenchymal transition and promote metastasis. Hypoxia induces K63-linked polyubiquitinated HAUSP at lysine 443 to enhance its functions. Knockdown of HAUSP decreases acetylation of histone 3 lysine 56 (H3K56Ac). K63-polyubiquitinated HAUSP interacts with a ubiquitin receptor CBP to specifically mediate H3K56 acetylation. ChIP-seq analysis of HAUSP and HIF-1α binding reveals two motifs responsive to hypoxia. HectH9 is the E3 ligase for HAUSP and a prognostic marker together with HIF-1α. This report demonstrates that hypoxia-induced K63-polyubiquitinated HAUSP deubiquitinates HIF-1α and causes CBP-mediated H3K56 acetylation on HIF-1α target gene promoters to promote EMT/metastasis, further defining HAUSP as a therapeutic target in hypoxia-induced tumour progression.

SMYD3-Mediated H2A.Z.1 Methylation Promotes Cell Cycle and Cancer Proliferation

SMYD3 methyltransferase is nearly undetectable in normal human tissues but highly expressed in several cancers, including breast cancer, although its contributions to pathogenesis in this setting are unclear. Here we report that histone H2A.Z.1 is a substrate of SMYD3 that supports malignancy. SMYD3-mediated dimethylation of H2A.Z.1 at lysine 101 (H2A.Z.1K101me2) increased stability by preventing binding to the removal chaperone ANP32E and facilitating its interaction with histone H3. Moreover, a microarray analysis identified cyclin A1 as a target coregulated by SMYD3 and H2A.Z.1K101me2. The colocalization of SMYD3 and H2A.Z.1K101me2 at the promoter of cyclin A1 activated its expression and G₁-S progression. Enforced expression of cyclin A1 in cells containing mutant H2A.Z.1 rescued tumor formation in a mouse model. Our findings suggest that SMYD3-mediated H2A.Z.1K101 dimethylation activates cyclin A1 expression and contributes to driving the proliferation of breast cancer cells. Cancer Res; 76(20); 6043-53. ©2016 AACR.

Genistein suppresses the proliferation of telomerase-negative cells

In both tumor and yeast cells that lack telomerase, telomeres are maintained via an alternative recombination mechanism. In this study, we tested genistein, a potential TOP2 inhibitor required for telomere–telomere recombination, on the repression of telomere–telomere recombination. Genistein on the repression of type II recombination on a tlc1 yeast strain was examined by the telomeric DNA structures using Southern blot analysis. Telomere patterns of freshly dissected tlc1 spores containing an empty plasmid (pYES2) or a yeast TOP2 (yTOP2) plasmid were analyzed. The results indicated that the reintroduction of TOP2 recovered the type II pattern, implying genistein in the blockage of type II survivors in the tlc1 strain. The effects of genistein on both tlc1 and tlc1 rad 51 strains in liquid and solid mediums were also examined. Finally, treatment of 10 μmol/L of genistein showed inhibitory effect on the growth of telomerase‐negative U2OS alternative lengthening of telomere (ALT) cells, but not in telomerase‐positive HCT116 cells. These results provide evidences that the inhibitory effects of genistein on telomerase‐negative cells depend on type II recombination pathway in yeast and the ALT pathway in human tumors.

miR-1236 regulates hypoxia-induced epithelial-mesenchymal transition and cell migration/invasion through repressing SENP1 and HDAC3

Intratumoral hypoxia induces epithelial-mesenchymal transition and promotes cancer metastasis. MicroRNAs (miRNAs) are endogenous, single-strand RNA molecules that regulate gene expression. MiRNAs control cell growth, proliferation, differentiation and cell death and may function as oncogenes or tumor suppressors. HDAC3 and SENP1 are two molecules involved in hypoxia-induced EMT and HIF-1α stability, respectively. In this report, we show that miR-1236 plays a critical role in hypoxia-induced EMT and metastasis. MiRNA prediction programs TargetScan and miRanda show that miR-1236 may target HDAC3 and SENP1. MiR-1236 represses the luciferase activity of reporter constructs containing 3'UTR of HDAC3 and SENP1 as well as the expression levels of HDAC3 and SENP1. MiR-1236 abolishes hypoxia-induced EMT and inhibits migration and invasion activity of tumor cells. Hypoxia represses miR-1236 expression. The promoter region of miR-1236 is identified as the NELFE promoter. Twist1, an EMT regulator activated by hypoxia/HIF-1α, is shown to repress the reporter construct driven by the NELFE promoter. The binding site of Twist1 in the NELFE promoter is identified and chromatin immunoprecipitation assays show the direct binding of Twist1 to this site. Overexpression or knockdown of Twist1 in stable cell lines shows the inverse correlation between Twist1 and miR-1236 expression. These results identify a miRNA that regulates hypoxia-induced EMT and metastasis through repressing HDAC3 and SENP1 expression and present a regulatory network that involves many key players in hypoxia-induced EMT.

Acquisition of tumorigenic potential and enhancement of angiogenesis in pulmonary stem/progenitor cells through Oct-4 hyperexpression

Cancer stem cells, also known as cancer initiating cells (CICs), are considered to be responsible for tumor growth and chemoresistance. Different hypotheses have been proposed to explain the origin of CICs, including mutations in adult stem/progenitor cells or the acquisition of stem-like characteristics in differentiated cells; however, studies have yielded conflicting identification for CICs and have little information for the origin to generate CICs. Part of the difficulty in identifying CICs may stem from the fact that the CICs studied have been largely derived from cancer cell lines or well-developed tumors. In previous studies, we have reported the enrichment of mouse pulmonary stem/progenitor cells (mPSCs) by using serum-free primary selection culture followed by FACS isolation using the coxsackievirus/adenovirus receptor (CAR) as the positive selection marker. Here, we demonstrated that overexpression of the pluripotent transcription factor Oct-4 is sufficient to induce CAR⁺/mPSCs transformation, which we name CAR⁺/mPSCs^Oct-4_hi. These transformed cells possess cancer initiating and chemoresistance potential, as well as exhibiting remarkable expression of certain proangiogenic factors, including angiopoietins (ANGs) and VEGF, and enhanced angiogenic potential. Moreover, CAR⁺/mPSCs^Oct-4_hi actively participated in tumor blood vessel formation and triggered a novel angiogenic mechanism, the angiopoietins/Tie2 signaling pathway. These study provide critical evidence supporting the possible origin to generate CICs, and help elucidate the pathways responsible for CICs-mediated blood vessel formation.

Nijmegen breakage syndrome protein 1 (NBS1) modulates hypoxia inducible factor-1α (HIF-1α) stability and promotes in vitro migration and invasion under ionizing radiation

Hypoxia-inducible factor (HIF) is a heterodimer transcription factor complex that monitors the cellular response to the oxygen levels in cells. Hypoxia-inducible factor-1α (HIF-1α) has been shown to be stabilized by ionizing radiation (IR) and its stabilization promotes tumor progression and metastasis. Nijmegen breakage syndrome protein 1 (NBS1), a component of the MRE11-RAD50-NBS1 complex, plays an important role in the cellular response to DNA damage but its overexpression contributes to transformation and has been found to correlate with metastasis. However, whether NBS1 participates in IR-induced metastasis needs to be further determined. The aim of this study is to investigate whether radiation-induced HIF-1α stabilization is regulated by NBS1 and thereby promotes tumor cell migration/invasion. Here, we show that both NBS1 and HIF-1α expression are up-regulated after exposure to IR, and NBS1 increases HIF-1α expression at the protein level. In addition, IR treatment promotes the epithelial-mesenchymal transition (EMT) and in vitro cell migration and invasion activity, which could be abolished by suppression of NBS1. Furthermore, NBS1 directly interacts with HIF-1α and reduces the ubiquitination of HIF-1α⋅ Co-expression of HIF-1α and NBS1 in primary tumors of patients with lung adenocarcinoma correlates with a worse prognosis. These results provide a new function of NBS1 in stabilizing HIF-1α under IR, which leads to enhanced cancer cell migration and invasion.

Instability of succinate dehydrogenase in SDHD polymorphism connects reactive oxygen species production to nuclear and mitochondrial genomic mutations in yeast

AIMS

Mitochondrial succinate dehydrogenase (SDH) is an essential complex of the electron transport chain and tricarboxylic acid cycle. Mutations in the human SDH subunit D frequently lead to paraganglioma (PGL), but the mechanistic consequences of the majority of SDHD polymorphisms have yet to be unraveled. In addition to the originally discovered yeast SDHD subunit Sdh4, a conserved homolog, Shh4, has recently been identified in budding yeast. To assess the pathogenic significance of SDHD mutations in PGL patients, we performed functional studies in yeast.

RESULTS

SDHD protein expression was reduced in SDHD-related carotid body tumor tissues. A BLAST search of SDHD to the yeast protein database revealed a novel protein, Shh4, that may have a function similar to human SDHD and yeast Sdh4. The missense SDHD mutations identified in PGL patients were created in Sdh4 and Shh4, and, surprisingly, a severe respiratory incompetence and reduced expression of the mutant protein was observed in the sdh4Δ strain expressing shh4. Although shh4Δ cells showed no respiratory-deficient phenotypes, deletion of SHH4 in sdh4Δ cells further abolished mitochondrial function. Remarkably, sdh4Δ shh4Δ strains exhibited increased reactive oxygen species (ROS) production, nuclear DNA instability, mtDNA mutability, and decreased chronological lifespan.

INNOVATION AND CONCLUSION

SDHD mutations are associated with protein and nuclear and mitochondrial genomic instability and increase ROS production in our yeast model. These findings reinforce our understanding of the mechanisms underlying PGL tumorigenesis and point to the yeast Shh4 as a good model to investigate the possible pathogenic relevance of SDHD in PGL polymorphisms.

TET1 regulates hypoxia-induced epithelial-mesenchymal transition by acting as a co-activator

BACKGROUND

Hypoxia induces the epithelial-mesenchymal transition, EMT, to promote cancer metastasis. In addition to transcriptional regulation mediated by hypoxia-inducible factors, HIFs, other epigenetic mechanisms of gene regulation, such as histone modifications and DNA methylation, are utilized under hypoxia. However, whether DNA demethylation mediated by TET1, a DNA dioxygenase converting 5-methylcytosine, 5mC, into 5-hydroxymethylcytosine, 5hmC, plays a role in hypoxia-induced EMT is largely unknown.

RESULTS

We show that TET1 regulates hypoxia-responsive gene expression. Hypoxia/HIF-2α regulates the expression of TET1. Knockdown of TET1 mitigates hypoxia-induced EMT. RNA sequencing and 5hmC sequencing identified the set of TET1-regulated genes. Cholesterol metabolic process genes are among the genes that showed high prevalence and statistical significance. We characterize one of the genes, INSIG1 (insulin induced gene 1), to confirm its expression and the 5hmC levels in its promoter. Knockdown of INSIG1 also mitigates hypoxia-induced EMT. Finally, TET1 is shown to be a transcriptional co-activator that interacts with HIF-1α and HIF-2α to enhance their transactivation activity independent of its enzymatic activity. TET1 acts as a co-activator to further enhance the expression of INSIG1 together with HIF-2α. We define the domain in HIF-1α that interacts with TET1 and map the domain in TET1 that confers transactivation to a 200 amino acid region that contains a CXXC domain. The TET1 catalytically inactive mutant is capable of rescuing hypoxia-induced EMT in TET1 knockdown cells.

CONCLUSIONS

These findings demonstrate that TET1 serves as a transcription co-activator to regulate hypoxia-responsive gene expression and EMT, in addition to its role in demethylating 5mC.

DNA topoisomerase III alpha regulates p53-mediated tumor suppression

PURPOSE

Human DNA topoisomerase III alpha (hTOP3α) is involved in DNA repair surveillance and cell-cycle checkpoints possibly through formatting complex with tumor suppressors. However, its role in cancer development remained unsolved.

EXPERIMENTAL DESIGN

Coimmunoprecipitation, sucrose gradient, chromatin immunoprecipitation (ChIP), real time PCR, and immunoblotting analyses were performed to determine interactions of hTOP3α with p53. Paired cell lines with different hTOP3α levels were generated via ectopic expression and short hairpin RNA (shRNA)-mediated knockdown approaches. Cellular tumorigenic properties were analyzed using cell counting, colony formation, senescence, soft agar assays, and mouse xenograft models.

RESULTS

The hTOP3α isozyme binds to p53 and cofractionizes with p53 in gradients differing from fractions containing hTOP3α and BLM. Knockdown of hTOP3α expression (sh-hTOP3α) caused a higher anchorage-independent growth of nontumorigenic RHEK-1 cells. Similarly, sh-hTOP3α and ectopic expression of hTOP3α in cancer cell lines caused increased and reduced tumorigenic abilities, respectively. Genetic and mutation experiments revealed that functional hTOP3α, p53, and p21 are required for this tumor-suppressive activity. Mechanism-wise, ChIP data revealed that hTOP3α binds to the p53 and p21 promoters and positively regulates their expression. Two proteins affect promoter recruitments of each other and collaborate in p21 expression. Moreover, sh-hTOP3α and sh-p53 in AGS cells caused a similar reduction in senescence and hTOP3α mRNA levels were lower in gastric and renal tumor samples.

CONCLUSION

We concluded that hTOP3α interacts with p53, regulates p53 and p21 expression, and contributes to the p53-mediated tumor suppression.

Global analysis of cdc14 dephosphorylation sites reveals essential regulatory role in mitosis and cytokinesis

Degradation of the M phase cyclins triggers the exit from M phase. Cdc14 is the major phosphatase required for the exit from the M phase. One of the functions of Cdc14 is to dephosphorylate and activate the Cdh1/APC/C complex, resulting in the degradation of the M phase cyclins. However, other crucial targets of Cdc14 for mitosis and cytokinesis remain to be elucidated. Here we systematically analyzed the positions of dephosphorylation sites for Cdc14 in the budding yeast Saccharomyces cerevisiae. Quantitative mass spectrometry identified a total of 835 dephosphorylation sites on 455 potential Cdc14 substrates in vivo. We validated two events, and through functional studies we discovered that Cdc14-mediated dephosphorylation of Smc4 and Bud3 is essential for proper mitosis and cytokinesis, respectively. These results provide insight into the Cdc14-mediated pathways for exiting the M phase.

Interaction between NBS1 and the mTOR/Rictor/SIN1 complex through specific domains

Nijmegen breakage syndrome (NBS) is a chromosomal-instability syndrome. The NBS gene product, NBS1 (p95 or nibrin), is a part of the Mre11-Rad50-NBS1 complex. SIN1 is a component of the mTOR/Rictor/SIN1 complex mediating the activation of Akt. Here we show that NBS1 interacted with mTOR, Rictor, and SIN1. The specific domains of mTOR, Rictor, or SIN1 interacted with the internal domain (a.a. 221-402) of NBS1. Sucrose density gradient showed that NBS1 was located in the same fractions as the mTOR/Rictor/SIN1 complex. Knockdown of NBS1 decreased the levels of phosphorylated Akt and its downstream targets. Ionizing radiation (IR) increased the NBS1 levels and activated Akt activity. These results demonstrate that NBS1 interacts with the mTOR/Rictor/SIN1 complex through the a.a. 221–402 domain and contributes to the activation of Akt activity.

Epigenetic reprogramming and post-transcriptional regulation during the epithelial-mesenchymal transition

The epithelial-mesenchymal transition (EMT) is a developmental process that is important for organ development, metastasis, cancer stemness, and organ fibrosis. The EMT process is regulated by different signaling pathways as well as by various epigenetic and post-transcriptional mechanisms. Here, we review recent progress describing the role of different chromatin modifiers in various signaling events leading to EMT, including hypoxia, transforming growth factor (TGF)-β, Notch, and Wnt. We also discuss post-transcriptional mechanisms, such as RNA alternative splicing and the effects of miRNAs in EMT regulation. Furthermore, we highlight on-going and future work aimed at a detailed understanding of the epigenetic and post-transcriptional mechanisms that regulate EMT. This work will shed new light on the cellular and tumorigenic processes affected by EMT misregulation.

Interplay between HDAC3 and WDR5 is essential for hypoxia-induced epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is important for organ development, metastasis, cancer stemness, and organ fibrosis. Molecular mechanisms to coordinately regulate hypoxia-induced EMT remain elusive. Here, we show that HIF-1α-induced histone deacetylase 3 (hdac3) is essential for hypoxia-induced EMT and metastatic phenotypes. Change of specific chromatin states is associated with hypoxia-induced EMT. Under hypoxia, HDAC3 interacts with hypoxia-induced WDR5, recruits the histone methyltransferase (HMT) complex to increase histone H3 lysine 4 (H3K4)-specific HMT activity, and activates mesenchymal gene expression. HDAC3 also serves as an essential corepressor to repress epithelial gene expression. Knockdown of WDR5 abolishes mesenchymal gene activation but not epithelial gene repression during hypoxia. These results indicate that hypoxia induces different chromatin modifiers to coordinately regulate EMT through distinct mechanisms.

Krüppel-like transcription factor 4 contributes to maintenance of telomerase activity in stem cells

The zinc finger Krüppel-like transcription factor 4 (KLF4) has been implicated in cancer formation and stem cell regulation. However, the function of KLF4 in tumorigenesis and stem cell regulation are poorly understood due to limited knowledge of its targets in these cells. In this study, we have revealed a surprising link between KLF4 and regulation of telomerase that offers important insight into how KLF4 contributes to cancer formation and stem cell regulation. KLF4 sufficiently activated expression of the human telomerase catalytic subunit, human telomerase reverse transcriptase (hTERT), in telomerase-low alternative lengthening of telomeres (ALT), and fibroblast cells, while downregulation of KLF4 reduced its expression in cancerous and stem cells, which normally exhibits high expression. Furthermore, KLF4-dependent induction of hTERT was mediated by a KLF4 binding site in the proximal promoter region of hTERT. In human embryonic stem cells, expression of hTERT replaced KLF4 function to maintain their self-renewal. Therefore, our findings demonstrate that hTERT is one of the major targets of KLF4 in cancer and stem cells to maintain long-term proliferation potential.

Intragenic transcription of a noncoding RNA modulates expression of ASP3 in budding yeast

Inter- and intragenic noncoding transcription is widespread in eukaryotic genomes; however, the purpose of these types of transcription is still poorly understood. Here, we show that intragenic sense-oriented transcription within the budding yeast ASP3 coding region regulates a constitutively and immediately accessible promoter for the transcription of full-length ASP3. Expression of this short intragenic transcript is independent of GATA transcription factors, which are essential for the activation of full-length ASP3, and independent of RNA polymerase II (RNAPII). Furthermore, we found that an intragenic control element is required for the expression of this noncoding RNA (ncRNA). Continuous expression of the short ncRNA maintains a high level of trimethylation of histone H3 at lysine 4 (H3K4me3) at the ASP3 promoter and makes this region more accessible for RNAPII to transcribe the full-length ASP3. Our results show for the first time that intragenic noncoding transcription promotes gene expression.

Recruitment of Rad51 and Rad52 to short telomeres triggers a Mec1-mediated hypersensitivity to double-stranded DNA breaks in senescent budding yeast

Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the action of telomerase. In both mammalian tumor and yeast cells that lack telomerase, telomeres are maintained by an alternative recombination mechanism. Here we demonstrated that the budding yeast Saccharomyces cerevisiae type I survivors derived from telomerase-deficient cells were hypersensitive to DNA damaging agents. Assays to track telomere lengths and drug sensitivity of telomerase-deficient cells from spore colonies to survivors suggested a correlation between telomere shortening and bleomycin sensitivity. Our genetic studies demonstrated that this sensitivity depends on Mec1, which signals checkpoint activation, leading to prolonged cell-cycle arrest in senescent budding yeasts. Moreover, we also observed that when cells equipped with short telomeres, recruitments of homologous recombination proteins, Rad51 and Rad52, were reduced at an HO-endonuclease-catalyzed double-strand break (DSB), while their associations were increased at chromosome ends. These results suggested that the sensitive phenotype may be attributed to the sequestration of repair proteins to compromised telomeres, thus limiting the repair capacity at bona fide DSB sites.

Sumoylation of the BLM ortholog, Sgs1, promotes telomere-telomere recombination in budding yeast

BLM and WRN are members of the RecQ family of DNA helicases, and in humans their loss is associated with syndromes characterized by genome instability and cancer predisposition. As the only RecQ DNA helicase in the yeast Saccharomyces cerevisiae, Sgs1 is known to safeguard genome integrity through its role in DNA recombination. Interestingly, WRN, BLM and Sgs1 are all known to be modified by the small ubiquitin-related modifier (SUMO), although the significance of this posttranslational modification remains elusive. Here, we demonstrate that Sgs1 is specifically sumoylated under the stress of DNA double strand breaks. The major SUMO attachment site in Sgs1 is lysine 621, which lies between the Top3 binding domain and the DNA helicase domain. Surprisingly, sumoylation of K621 was found to be uniquely required for Sgs1's role in telomere-telomere recombination. In contrast, sumoylation was dispensable for Sgs1's roles in DNA damage tolerance, supppression of direct repeat and rDNA recombination, and promotion of top3Delta slow growth. Our results demonstrate that although modification by SUMO is a conserved feature of RecQ family DNA helicases, the major sites of modification are located on different domains of the protein in different organisms. We suggest that sumoylation of different domains of RecQ DNA helicases from different organisms contributes to conserved roles in regulating telomeric recombination.

Rapid Cdc13 turnover and telomere length homeostasis are controlled by Cdk1-mediated phosphorylation of Cdc13

Budding yeast telomerase is mainly activated by Tel1/Mec1 (yeast ATM/ATR) on Cdc13 from late S to G2 phase of the cell cycle. Here, we demonstrated that the telomerase-recruitment domain of Cdc13 is also phosphorylated by Cdk1 at the same cell cycle stage as the Tel1/Mec1-dependent regulation. Phosphor-specific gel analysis demonstrated that Cdk1 phosphorylates residues 308 and 336 of Cdc13. The residue T308 of Cdc13 is critical for efficient Mec1-mediated S306 phosphorylation in vitro. Phenotypic analysis in vivo revealed that the mutations in the Cdc13 S/TP motifs phosphorylated by Cdk1 caused cell cycle delay and telomere shortening and these phenotypes could be partially restored by the replacement with a negative charge residue. In the absence of Ku or Tel1, Cdk1-mediated phosphorylation of Cdc13 showed no effect on telomere length maintenance. Moreover, this Cdk1-mediated phosphorylation was required to promote the regular turnover of Cdc13. Together these results demonstrate that Cdk1 phosphorylates the telomerase recruitment domain of Cdc13, thereby preserves optimal function and expression level of Cdc13 for precise telomere replication and cell cycle progression.

Proofreading activity of DNA polymerase Pol2 mediates 3'-end processing during nonhomologous end joining in yeast

Genotoxic agents that cause double-strand breaks (DSBs) often generate damage at the break termini. Processing enzymes, including nucleases and polymerases, must remove damaged bases and/or add new bases before completion of repair. Artemis is a nuclease involved in mammalian nonhomologous end joining (NHEJ), but in Saccharomyces cerevisiae the nucleases and polymerases involved in NHEJ pathways are poorly understood. Only Pol4 has been shown to fill the gap that may form by imprecise pairing of overhanging 3' DNA ends. We previously developed a chromosomal DSB assay in yeast to study factors involved in NHEJ. Here, we use this system to examine DNA polymerases required for NHEJ in yeast. We demonstrate that Pol2 is another major DNA polymerase involved in imprecise end joining. Pol1 modulates both imprecise end joining and more complex chromosomal rearrangements, and Pol3 is primarily involved in NHEJ-mediated chromosomal rearrangements. While Pol4 is the major polymerase to fill the gap that may form by imprecise pairing of overhanging 3' DNA ends, Pol2 is important for the recession of 3' flaps that can form during imprecise pairing. Indeed, a mutation in the 3'-5' exonuclease domain of Pol2 dramatically reduces the frequency of end joins formed with initial 3' flaps. Thus, Pol2 performs a key 3' end-processing step in NHEJ.

Activation of phosphoinositide 3-kinase by the NBS1 DNA repair protein through a novel activation motif

Class IA phosphoinositide 3-kinases (PI 3-kinases) are key signaling components downstream of tyrosine kinases and Ras, regulating many different cellular functions and contributing to tumorigenesis. Class IA PI 3-kinases are heterodimers comprised of a p85 regulatory and a p110 catalytic subunit. Nijmegen breakage syndrome (NBS) is a chromosomal instability syndrome associated with cancer predisposition, radiosensitivity, microcephaly, and growth retardation. The NBS gene product p95 (also known as NBS1) is part of the Mre11-Rad50-Nbs1 complex, a central player associated with double-strand break repair. We previously demonstrated that NBS1 overexpression induces transformation through activation of PI 3-kinase/Akt. In this study, we show that NBS1 directly interacts, through a highly conserved C-terminal motif (aa 653-669) of NBS1, with the N-terminal domain (aa 1-108) of the p110alpha catalytic subunit of PI 3-kinase, and stimulates PI 3-kinase activity. Mutations of different regions of the conserved motif abolish the ability of NBS1 to activate PI 3-kinase in vitro and in vivo. Co-expression of NBS1/p110alpha/p-Akt is observed in certain percentage of head and neck cancer patient samples. These results demonstrate that NBS1 can function as an adaptor/activator of p110alpha PI 3-kinase through a novel activation motif, consistent with its possible role in cell transformation and tumorigenesis.

Determination of the differential estrogenicity of isoflavonoids by E2-ER-ERE-dependent gene expression in recombinant yeast and MCF-7 human breast cancer cells

The use of phytoestrogens-containing natural sources as alternative hormone replacement therapy (HRT) has been a subject of debate for decades. Development of assays to characterize these phytoestrogens is required. In this study, differential estrogenicities of five isoflavonoids found in red clover and soy, including biochanin A, daidzein, formononetin, genistein and glycitein were examined in a yeast-based screen system with a classical palindromic estrogen response element (ERE)-ADE2 reporter and in a MCF-7 cell culture system with mRNA levels of ER-dependent genes compared. In a yeast-based assay, five isoflavonoids showed various extents of estrogenic potencies. A collection of primary estrogen receptor (ER)-regulated genes by estradiol (E2), including hTERT, c-MYC, BCL2 and Ha-ras (oncogenic) and quinone reductase (QR), human complement 3 (C3) and COX7RP (non-oncogenic) were selected as marker genes for a MCF-7 cell-based endogenous gene expression assay. The results indicated that the mRNA levels of these E2-ER-ERE-dependent marker genes were regulated differentially by five isoflavonoids, leading to distinct expression patterns, which are also significantly different from that of E2. Moreover, the anti-estrogenic effects of biochanin A and formononetin on E2-induced transcriptions of marker genes in MCF-7 cells were also displayed. Taken together, these results are significant for these naturally occurring isoflavonoids regarding the issues of safety and efficacy.

The telomerase-recruitment domain of the telomere binding protein Cdc13 is regulated by Mec1p/Tel1p-dependent phosphorylation

The DNA damage-responsive protein kinases ATM and ATR phosphorylate SQ/TQ motifs that lie in clusters in most of their in vivo targets. Budding yeast Cdc13p contains two clusters of SQ/TQ motifs, suggesting that it might be a target of Mec1p/Tel1p (yeast ATR/ATM). Here we demonstrated that the telomerase recruitment domain of Cdc13p is phosphorylated by Mec1p and Tel1p. Gel analysis showed that Cdc13p contains a Mec1/Tel1-dependent post-translational modification. Using an immunoprecipitate (IP)-kinase assay, we showed that Mec1p phosphorylates Cdc13p on serine 225, 249, 255 and 306, and Tel1p phosphorylates Cdc13p on serine 225, 249 and 255 in vitro. Phenotypic analysis in vivo revealed that the mutations in the Cdc13p SQ motifs phosphorylated by Mec1p and Tel1p caused multiple telomere and growth defects. In addition, normal telomere length and growth could be restored by expressing a Cdc13-Est1p hybrid protein. These results demonstrate the telomerase recruitment domain of Cdc13p as an important new telomere-specific target of Mec1p/Tel1p.

Importin KPNA2, NBS1, DNA Repair and Tumorigenesis

During the past 20 years, the MRE11-RAD50-NBS1 complex has become an increasingly important focus in basic and clinical cancer research. One main conceptual step forward was made with the discovery of NBS1 and the understanding of its critical pathophysiological role in Nijmegen breakage syndrome. Major efforts were carried out to define the role in DNA repair of this complex. Recently, basic research has continuously extended our understanding of the complexity of the NBS1 complex. MRE11-RAD50-NBS1 complex can no longer be viewed as having a single role in DNA damage repair since it also serves as a sensor and a mediator in cell cycle checkpoint signaling. Meanwhile, studies have challenged the concept that NBS1 only functions as a tumor suppressor in preserving genome integrity in the nucleus. It may also provide an oncogenic role in the cytoplasm which is associated with the PI3-kinase/AKT-activation pathway. Consistent with this aspect, a growing body of clinical evidence suggests that NBS1 contains a deleterious character that depends on its subcellular localization. This review focuses on recent experimental evidences demonstrating how NBS1 is translocated into the nucleus by an importin KPNA2 which mediates NBS1 subcellular localization and the functions of the NBS1 complex in tumorigenesis.

ShyA, a membrane protein for proper septation of hyphae in Streptomyces

The life cycle of Streptomyces involves the formation of filamentous substrate and aerial hyphae. Following cessation of growth of an aerial hypha, multiple septation occurs at the tip to produce a chain of unigenomic spores. A gene, shyA, which influences several aspects of this growth, was isolated and partially characterized in Streptomyces coelicolor. The gene product is a representative of a well-conserved family of small actinomycete proteins. The shyA mutant sporulates normally but displays hyper septum formation and altered spore-chain morphology. Biochemical separation experiments and immunofluorescence staining demonstrated that the shyA gene product locates at cell membranes. Moreover, yeast two-hybrid screen and GST-pull-down assay showed that ShyA can interact with itself. Altogether, ShyA belongs to a new family of membrane-associated proteins which plays a role in morphological differentiation in actinomycetes.

Involvement of Topoisomerase III in Telomere-Telomere Recombination

Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the action of telomerase. In both mammalian tumor and yeast cells that lack telomerase, telomeres are maintained by an alternative (ALT) recombination mechanism. In yeast, Sgs1p and its associated type IA topoisomerase, Top3p, may work coordinately in removing Holliday junction intermediates from a crossover-producing recombination pathway. Previous studies have also indicated that Sgs1 helicase acts in a telomere recombination pathway. Here we show that topoisomerase III is involved in telomere-telomere recombination. The recovery of telomere recombination-dependent survivors in a telomerase-minus yeast strain was dependent on Top3p catalytic activity. Moreover, the RIF1 and RIF2 genes are required for the establishment of TOP3/SGS1-dependent telomere-telomere recombination. In human Saos-2 ALT cells, human topoisomerase IIIalpha (hTOP3alpha) also contributes to telomere recombination. Strikingly, the telomerase activity is clearly enhanced in surviving si-hTOP3alpha Saos-2 ALT cells. Altogether, the present results suggest a potential role for hTOP3alpha in dissociating telomeric structures in telomerase-deficient cells, providing therapeutic implications in human tumors.

Telomere configuration influences the choice of telomere maintenance pathways

Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the action of telomerase. In yeast cells that lack telomerase, telomeres are maintained by alternative type I and type II recombination mechanisms. Previous studies identified several proteins to control the choice between two types of recombinations. Here, we demonstrate that configuration of telomeres also plays a role to determine the fate of telomere replication in progeny. When diploid yeasts from mating equip with a specific type of telomeric structure in their genomes, they prefer to maintain this type of telomere replication in their descendants. While inherited telomere structure is easier to be utilized in progeny at the beginning stage, the telomeres in type I diploids can gradually switch to the type II cells in liquid culture. Importantly, the TLC1/tlc1 yeast cells develop type II survivors suggesting that haploid insufficiency of telomerase RNA component, which is similar to a type of dyskeratosis congenital in human. Altogether, our results suggest that both protein factors and substrate availability contribute to the choice among telomere replication pathways in yeast.

Increased NBS1 expression is a marker of aggressive head and neck cancer and overexpression of NBS1 contributes to transformation

PURPOSE

Head and neck squamous cell carcinoma (HNSCC) represents the sixth most frequent type of cancer worldwide. However, the molecular genetic alterations underlying its malignant behavior and progression are little known. We showed previously that c-MYC directly activates the expression of the DNA double-strand break repair gene NBS1, and NBS1 overexpression contributes to transformation. Here, we investigate the role of NBS1 overexpression in HNSCC.

EXPERIMENTAL DESIGN

Immunohistochemistry analysis of NBS1 expression was done in 81 locally advanced HNSCC patients. Real-time PCR and Western blot analysis were used to confirm immunohistochemistry results. Human hypopharyngeal cancer cell lines (FADU) with overexpressing NBS1 (FADUNBS) or inducible short interference RNA to repress endogenous NBS1 (FADUNBSi) were generated by stable transfection. Soft agar clonogenicity assay was used to determine the transformation activity. Western blot analysis and phosphatidylinositol 3-kinase (PI3K) assay were done to evaluate the signaling pathways that were involved.

RESULTS

NBS1 overexpression was identified in 45% of advanced HNSCC patients. It was an independent marker of poor prognosis. NBS1 expression levels correlated with the transformation activity of FADU clones and also correlated with the phosphorylation levels of Akt and its downstream target mammalian target of rapamycin (mTOR). PI3K activity was increased in NBS1-overexpressing FADU clones. NBS1 overexpression also correlated with increased Akt phosphorylation levels in tumor samples.

CONCLUSIONS

Increased NBS1 expression is a significant prognostic marker of advanced HNSCC, and the underlying mechanism may involve the activation of the PI3K/Akt pathway.

Importin KPNA2 Is Required for Proper Nuclear Localization and Multiple Functions of NBS1

Nijmegen breakage syndrome (NBS) is a chromosomal-instability syndrome associated with cancer predisposition, radiosensitivity, microcephaly, and growth retardation. The NBS gene product, NBS1, is a component of the MRE11-RAD50-NBS1 (MRN) complex, a central player associated with double strand break (DSB) repair. In response to radiation, NBS1 is phosphorylated by ATM, and the MRN complex relocalizes to form punctate nuclear foci for DNA repair. NBS1 controls both the nuclear localization of the MRN complexes and radiation-induced focus formation. We report here that the KPNA2 (importin alpha1) is important for the normal nuclear localization of the MRN complex and its proper formation of the nuclear foci. KPNA2 is the only member of the importin alpha family that physically interacts with NBS1, and the KPNA2-mediated nucleus localization sequence (NLS) is mapped to amino acid residues 461-467 of NBS1 that is sufficient for both the interaction with KPNA2 and the proper nuclear localization. Inhibition of KPNA2 or blockage of the KPNA2 interaction with NBS1 results in a reduction of radiation-induced nuclear focus accumulation, DSB repair, and cell cycle checkpoint signaling of NBS1. Collectively, our results strongly suggest that an interaction with KPNA2 contributes to nuclear localization and multiple tumor suppression functions of the NBS1 complex.

Extrachromosomal telomeric circles contribute to Rad52-, Rad50-, and polymerase delta-mediated telomere-telomere recombination in Saccharomyces cerevisiae

Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the telomerase reverse transcriptase. In both tumor and yeast cells that lack telomerase, telomeres are maintained by an alternative recombination mechanism. By using an in vivo inducible Cre-loxP system to generate and trace the fate of marked telomeric DNA-containing rings, the efficiency of telomere-telomere recombination can be determined quantitatively. We show that the telomeric loci are the primary sites at which a marked telomeric ring-containing DNA is observed among wild-type and surviving cells lacking telomerase. Marked telomeric DNAs can be transferred to telomeres and form tandem arrays through Rad52-, Rad50-, and polymerase delta-mediated recombination. Moreover, increases of extrachromosomal telomeric and Y' rings were observed in telomerase-deficient cells. These results imply that telomeres can use looped-out telomeric rings to promote telomere-telomere recombination in telomerase-deficient Saccharomyces cerevisiae.

Overexpression of NBS1 Contributes to Transformation through the Activation of Phosphatidylinositol 3-Kinase/Akt

Nijmegen breakage syndrome (NBS) is a chromosomal instability syndrome associated with cancer predisposition, radiosensitivity, microcephaly, and growth retardation. The NBS gene product, NBS1 (p95) or nibrin, is a part of the hMre11 complex, a central player associated with double strand break repair. We previously demonstrated that c-Myc directly activates NBS1 expression. Here we have shown that constitutive expression of NBS1 in Rat1a and HeLa cells induces/enhances their transformation. Repression of endogenous NBS1 levels using short interference RNA reduces the transformation activity of two tumor cell lines. Increased NBS1 expression is observed in 40-52% of non-small cell lung carcinoma, hepatoma, and esophageal cancer samples. NBS1 overexpression stimulates phosphatidylinositol (PI) 3-kinase activity, leading to increased phosphorylation levels of Akt and its downstream targets such as glycogen synthase kinase 3beta and mammalian target of rapamycin in different cell lines and tumor samples. Transformation induced by NBS1 overexpression can be inhibited by a PI3-kinase inhibitor (LY294002). Repression of endogenous Akt expression by short interference RNA decreases the transformation activity of Rat1a cells overexpressing NBS1. These results indicate that overexpression of NBS1 is an oncogenic event that contributes to transformation through the activation of PI3-kinase/Akt.

Inhibition of estradiol-induced mammary proliferation by dibenzoylmethane through the E 2 –ER–ERE-dependent pathway

The phytochemical dibenzoylmethane (DBM) has been shown to inhibit 7,12-dimethylbenz[a]anthracene induced mammary tumorigenesis in Sencar mice. However, the molecular basis of this activity is still elusive. In the present study, we demonstrated that DBM inhibits estradiol (E2)-induced incorporation of bromodeoxyuridine into mammary DNA in immature female Sencar mice by 52%, when 10 micromol of DBM was intraperitoneally injected into mice prior to the injection of E2. Examination of the influence of DBM on the expressions of E2-ERE-dependent oncogenes in MCF-7 cells indicated that DBM inhibits the E2-induced cell growth as well as the expressions of four oncogenes, telomerase, c-myc, Ha-ras and bcl-2. Further mechanistic study using chromatin immunoprecipitation assay demonstrated that DBM acts as a pure antagonist by attenuating the binding of estrogen receptor to the estrogen response elements in the regulatory regions of c-myc, hTERT and bcl-2 genes in vivo. Taken together, our results strongly suggest that DBM plays an inhibitory role in E2-induced proliferations, which establishes DBM as a model molecule for studying the antiestrogenic drugs.

Direct Activation of HSP90A Transcription by c-Myc Contributes to c-Myc-induced Transformation

The c-myc proto-oncogene encodes a ubiquitous transcription factor involved in the control of cell growth and differentiation and implicated in inducing tumorigenesis. Understanding the function of c-Myc and its role in cancer depends upon the identification of c-Myc target genes. Heat shock protein 90 (HSP90) is involved in the folding of proteins such as signal transduction molecules (Src, Raf1, cdk4) and steroid receptors and in enhancing the activity of telomerase and nitric-oxide synthase. Here we show that c-Myc directly activates HSP90A transcription. c-Myc-mediated induction of HSP90A transcription occurs in different tissues, is independent of cell proliferation, and is mediated by a c-Myc binding site in the proximal promoter region of HSP90A gene. Overexpression of HSP90A in Rat1a cells induces transformation. Short interference RNA of HSP90A/Hsp86alpha reduces transformation activity in HeLa and RatMyc cells. These results indicate that by induction of HSP90A c-Myc may control the activity of multiple signal pathways involved in cellular transformation.

An argument against routine sentinel node mapping for DCIS

Indications for sentinel lymph node mapping (SLNM) for patients with ductal carcinoma in situ (DCIS) of the breast are controversial. We reviewed our institutional experience with SLNM for DCIS to determine the node positive rate and clarify indications for nodal staging in patients with DCIS. Since 1998 we have used SLNM to stage breast cancer patients using both blue dye and radiocolloid. In DCIS patients, SLNM has been reserved for patients considered at high risk for harboring coexistent invasive carcinoma or treated by mastectomy. All sentinel nodes were evaluated with serial sectioning, hematoxylin and eosin staining, and immunohistochemical evaluation for cytokeratins. We identified 44 patients with 46 cases of DCIS (two patients with bilateral disease). SLNM identified at least one sentinel node in all cases. In all cases, the sentinel node(s) were negative for axillary metastasis. We calculated the binomial probability of observing 0 of 46 cases as negative when the expected incidence according to published reports in the surgical literature was 13 per cent and found a P value of <0.01. Based on this case-series observation, we conclude SLNM should not be routinely performed for patients with DCIS. We now use SLNM only for DCIS patients treated by mastectomy.

c-Myc directly regulates the transcription of the NBS1 gene involved in DNA double-strand break repair

The c-myc proto-oncogene encodes a ubiquitous transcription factor involved in the control of cell growth and implicated in inducing tumorigenesis. Understanding the function of c-Myc and its role in cancer depends upon the identification of c-Myc target genes. Nijmegen breakage syndrome (NBS) is a chromosomal-instability syndrome associated with cancer predisposition, radiosensitivity, and chromosomal instability. The NBS gene product, NBS1 (p95 or nibrin), is a part of the hMre11 complex, a central player associated with double-strand break (DSB) repair. NBS1 contains domains characteristic for proteins involved in DNA repair, recombination, and replication. Here we show that c-Myc directly activates NBS1. c-Myc-mediated induction of NBS1 gene transcription occurs in different tissues, is independent of cell proliferation, and is mediated by a c-Myc binding site in the intron 1 region of NBS1 gene. Overexpression of NBS1 in Rat1a cells increased cell proliferation. These results indicate that NBS1 is a direct transcriptional target of c-Myc and links the function of c-Myc to the regulation of DNA DSB repair pathway operating during DNA replication.

Est1p as a cell cycle-regulated activator of telomere-bound telomerase

In Saccharomyces cerevisiae, the telomerase components Est2p, TLC1 RNA, Est1p, and Est3p are thought to form a complex that acts late during chromosome replication (S phase) upon recruitment by Cdc13p, a telomeric DNA binding protein. Consistent with this model, we show that Est1p, Est2p, and Cdc13p are telomere-associated at this time. However, Est2p, but not Est1p, also binds telomeres before late S phase. The cdc13-2 allele has been proposed to be defective in recruitment, yet Est1p and Est2p telomere association persists in cdc13-2 cells. These findings suggest a model in which Est1p binds telomeres late in S phase and interacts with Cdc13p to convert inactive, telomere-bound Est2p to an active form.

Involvement of replicative polymerases, Tel1p, Mec1p, Cdc13p, and the Ku complex in telomere-telomere recombination

Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the action of the reverse transcriptase telomerase. In both tumor and yeast cells that lack telomerase, telomeres are maintained by an alternative recombination mechanism. Genetic studies have led to the identification of DNA polymerases, cell cycle checkpoint proteins, and telomere binding proteins involved in the telomerase pathway. However, how these proteins affect telomere-telomere recombination has not been identified to date. Using an assay to trace the in vivo recombinational products throughout the course of survivor development, we show here that three major replicative polymerases, alpha, delta, and epsilon, play roles in telomere-telomere recombination and that each causes different effects and phenotypes when they as well as the telomerase are defective. Polymerase delta appears to be the main activity for telomere extension, since neither type I nor type II survivors arising via telomere-telomere recombination were seen in its absence. The frequency of type I versus type II is altered in the polymerase alpha and epsilon mutants relative to the wild type. Each prefers to develop a particular type of survivor. Moreover, type II recombination is mediated by the cell cycle checkpoint proteins Tel1 and Mec1, and telomere-telomere recombination is regulated by telomere binding protein Cdc13 and the Ku complex. Together, our results suggest that coordination between DNA replication machinery, DNA damage signaling, DNA recombination machinery, and the telomere protein-DNA complex allows telomere recombination to repair telomeric ends in the absence of telomerase.