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Scott W Lowe

Publications and source records attributed to Scott W Lowe.

At least 19 recordsLinked to original sources
PubMed2005

Slugging it out: fine tuning the p53-PUMA death connection.

In response to DNA damage, the tumor suppressor p53 elicits a complex cellular response. In this issue of Cell, Wu et al. (2005) show that the transcription factor SLUG is induced by p53 and protects hematopoietic progenitor cells from apoptosis triggered by DNA damage. SLUG exerts this protective role by repressing Puma, a proapoptotic target of p53. PUMA is also a key coordinator of apoptosis mediated by both nuclear and cytoplasmic functions of p53 (Chi-puk et al., 2005).

Animals↗
PubMed2005

Defined genetic events associated with the spontaneous in vitro transformation of ElA/Ras-expressing human IMR90 fibroblasts.

In contrast to rodent cells, normal human fibroblasts are generally resistant to neoplastic transformation in vitro. Here, we report the derivation and characterization of a spontaneously transformed cell line from normal human IMR90 fibroblasts transduced with E1A and Ras oncogenes. Unlike the parental, non-tumorigenic E1A/Ras-expressing IMR90 cells, these spontaneously transformed cells displayed aberrant growth potential in vitro and were capable of tumorigenesis in vivo. In contrast to the parental E1A/Ras-expressing cells, both the spontaneously transformed cells and cells derived from resultant tumors displayed specific t(7q;8q) and t(5q;17) structural chromosomal changes. Chromosome 8q contains c-Myc, which is capable of activating the telomerase catalytic subunit hTERT. Notably, upregulation of c-Myc, hTERT and telomerase activity were detected only in the tumorigenic cells. Transduction of Myc siRNA into the tumorigenic cells led to a concomitant downregulation of hTERT. Furthermore, transduction of Myc or hTERT into the non-tumorigenic E1A/Ras-expressing IMR90 cells was able to confer tumorigenesis on these cells. These studies suggest that the t(7;8) translocation may result in Myc overexpression and its subsequent activation of hTERT, which may contribute to the tumorigenicity of the IMR90 cells. Furthermore, this report describes additional successful neoplastic transformation of human IMR90 fibroblasts by defined genetic elements. The spontaneously transformed cells we have derived provide a valuable model system for the study of neoplastic transformation.

Adenovirus E1A Proteins↗
PubMed2005

Probing tumor phenotypes using stable and regulated synthetic microRNA precursors.

RNA interference is a powerful method for suppressing gene expression in mammalian cells. Stable knock-down can be achieved by continuous expression of synthetic short hairpin RNAs, typically from RNA polymerase III promoters. But primary microRNA transcripts, which are endogenous triggers of RNA interference, are normally synthesized by RNA polymerase II. Here we show that RNA polymerase II promoters expressing rationally designed primary microRNA-based short hairpin RNAs produce potent, stable and regulatable gene knock-down in cultured cells and in animals, even when present at a single copy in the genome. Most notably, by tightly regulating Trp53 knock-down using tetracycline-based systems, we show that cultured mouse fibroblasts can be switched between proliferative and senescent states and that tumors induced by Trp53 suppression and cooperating oncogenes regress upon re-expression of Trp53. In practice, this primary microRNA-based short hairpin RNA vector system is markedly similar to cDNA overexpression systems and is a powerful tool for studying gene function in cells and animals.

Animals↗
PubMed2005

p63 deficiency activates a program of cellular senescence and leads to accelerated aging.

The p53 tumor suppressor plays a key role in organismal aging. A cellular mechanism postulated to drive the aging process is cellular senescence, mediated in part by p53. Although senescent cells accumulate in elderly individuals, most studies have relied on correlating in vitro senescence assays with in vivo phenotypes of aging. Here, using two different mouse models in which the p53-related protein p63 is compromised, we demonstrate that cellular senescence and organismal aging are intimately linked and that these processes are mediated by p63 loss. We found that p63(+/-) mice have a shortened life span and display features of accelerated aging. Both germline and somatically induced p63 deficiency activates widespread cellular senescence with enhanced expression of senescent markers SA-beta-gal, PML, and p16(INK4a). Using an inducible tissue-specific p63 conditional model, we further show that p63 deficiency induces cellular senescence and causes accelerated aging phenotypes in the adult. Our results thus suggest a causative link between cellular senescence and aging in vivo, and demonstrate that p63 deficiency accelerates this process.

Aging↗
PubMed2005

Evasion of the p53 tumour surveillance network by tumour-derived MYC mutants.

The c-Myc oncoprotein promotes proliferation and apoptosis, such that mutations that disable apoptotic programmes often cooperate with MYC during tumorigenesis. Here we report that two common mutant MYC alleles derived from human Burkitt's lymphoma uncouple proliferation from apoptosis and, as a result, are more effective than wild-type MYC at promoting B cell lymphomagenesis in mice. Mutant MYC proteins retain their ability to stimulate proliferation and activate p53, but are defective at promoting apoptosis due to a failure to induce the BH3-only protein Bim (a member of the B cell lymphoma 2 (Bcl2) family) and effectively inhibit Bcl2. Disruption of apoptosis through enforced expression of Bcl2, or loss of either Bim or p53 function, enables wild-type MYC to produce lymphomas as efficiently as mutant MYC. These data show how parallel apoptotic pathways act together to suppress MYC-induced transformation, and how mutant MYC proteins, by selectively disabling a p53-independent pathway, enable tumour cells to evade p53 action during lymphomagenesis.

Adoptive Transfer↗
PubMed2005

Differential regulation of noxa in normal melanocytes and melanoma cells by proteasome inhibition: therapeutic implications.

Melanoma is the most aggressive form of skin cancer and advanced stages are invariably resistant to conventional therapeutic agents. Using bortezomib as a prototypic proteasome inhibitor, we have identified a novel and critical role of the proteasome in the maintenance of the malignant phenotype of melanoma cells that could have direct translational implications. Thus, melanoma cells from early, intermediate, and late stages of the disease could not sustain proteasome inhibition and underwent an effective activation of caspase-dependent and -independent death programs. This effect was tumor cell selective, because under similar conditions, normal melanocytes remained viable. Intriguingly, and despite of interfering with a cellular machinery in charge of controlling the half-life of the vast majority of cellular proteins, bortezomib did not promote a generalized disruption of melanoma-associated survival factors (including NF-kappaB, Bcl-2, Bcl-x(L), XIAP, TRAF-2, or FLIP). Instead, we identified a dramatic induction in vitro and in vivo of the BH3-only protein Noxa in melanoma cells (but not in normal melanocytes) in response to proteasome inhibition. RNA interference validated a critical role of Noxa for the cytotoxic effect of bortezomib. Notably, the proteasome-dependent regulation of Noxa was found to extend to other tumor types, and it could not be recapitulated by standard chemotherapeutic drugs. In summary, our results revealed Noxa as a new biomarker to gauge the efficacy of bortezomib specifically in tumor cells, and provide a new strategy to overcome tumor chemoresistance.

Animals↗
PubMed2005

Recurrent chromosomal aberrations in INK4a/ARF defective primary lymphomas predict drug responses in vivo.

Predicting responsiveness to anticancer therapy based on molecular findings at diagnosis is important to optimize treatment decisions. Although clinical outcome correlates with distinct mutations in some cancer entities, treatment responses within these lesion-stratified subgroups still remain heterogeneous, underscoring the need for additional prognosticators. We previously demonstrated that defined genetic defects at the INK4a/ARF locus, which encodes the tumor suppressors p16INK4a and ARF, not only accelerated lymphomagenesis in the Emu-myc transgenic mouse but also interfered with treatment sensitivity. In this study, we take a nonbiased genome-wide approach to examine whether the responsiveness of these lymphomas can be further stratified based on cytogenetic information at diagnosis. Indeed, using spectral karyotyping, comparative genomic hybridization, and fluorescence in situ hybridization in 38 primary lymphomas, we find recurrent cytogenetic alterations that refine the predictive value of INK4a/ARF lesions on drug responses in vivo: gain of chromosome 14, which was never detected in INK4a/ARFnull lymphomas, defined an ARFnull subgroup with superior treatment outcome. Gain of chromosome 6 was identified as a recurrent chromosomal aberration that predisposed ARFnull tumors to their subsequent INK4a loss during therapy. These data illustrate how cytogenetic information from cancer specimens might complement established prognostic markers and may improve anticancer treatment strategies.

Animals↗
PubMed2005

A microRNA polycistron as a potential human oncogene.

To date, more than 200 microRNAs have been described in humans; however, the precise functions of these regulatory, non-coding RNAs remains largely obscure. One cluster of microRNAs, the mir-17-92 polycistron, is located in a region of DNA that is amplified in human B-cell lymphomas. Here we compared B-cell lymphoma samples and cell lines to normal tissues, and found that the levels of the primary or mature microRNAs derived from the mir-17-92 locus are often substantially increased in these cancers. Enforced expression of the mir-17-92 cluster acted with c-myc expression to accelerate tumour development in a mouse B-cell lymphoma model. Tumours derived from haematopoietic stem cells expressing a subset of the mir-17-92 cluster and c-myc could be distinguished by an absence of apoptosis that was otherwise prevalent in c-myc-induced lymphomas. Together, these studies indicate that non-coding RNAs, specifically microRNAs, can modulate tumour formation, and implicate the mir-17-92 cluster as a potential human oncogene.

Animals↗
PubMed2005

p400 is required for E1A to promote apoptosis.

The adenovirus E1A oncoprotein promotes proliferation and transformation by binding cellular proteins, including members of the retinoblastoma protein family, the p300/CREB-binding protein transcriptional coactivators, and the p400-TRRAP chromatin-remodeling complex. E1A also promotes apoptosis, in part, by engaging the ARF-p53 tumor suppressor pathway. We show that E1A induces ARF and p53 and promotes apoptosis in normal fibroblasts by physically associating with the retinoblastoma protein and a p400-TRRAP complex and that its interaction with p300 is largely dispensable for these effects. We further show that E1A increases p400 expression and, conversely, that suppression of p400 using stable RNA interference reduces the levels of ARF, p53, and apoptosis in E1A-expressing cells. Therefore, whereas E1A inactivates the retinoblastoma protein, it requires p400 to efficiently promote cell death. These results identify p400 as a regulator of the ARF-p53 pathway and a component of the cellular machinery that couples proliferation to cell death.

ADP-Ribosylation Factors↗
PubMed2005

The p400 E1A-associated protein is a novel component of the p53 --> p21 senescence pathway.

Adenovirus E1A-associated p400 belongs to the SWI2/SNF2 family of chromatin remodeling proteins. Here, we report that p400 is a component of the p53-p21(WAF1/CIP1/sid1) pathway, regulating the p21 transcription and senescence induction program. Acute depletion of p400 expression by shRNA (short hairpin RNA) synthesis led to premature senescence of untransformed human fibroblasts, whose features include G1 arrest, p21 induction, senescence-associated heterochromatic foci (SAHF), and beta-gal staining. Importantly, p400shRNA-induced premature senescence phenotypes were rescued by coexpression of p53-shRNA or p21-shRNA. Furthermore, p400 complex colocalized with p53 on the p21 promoter. These data suggest that the p400 complex inhibits p53 --> p21 transcription and the development of premature senescence.

Cell Cycle Proteins↗
PubMed2005

A conserved element in Myc that negatively regulates its proapoptotic activity.

Myc is an oncoprotein transcription factor that promotes cell proliferation and apoptosis. Analysis of highly conserved elements within vertebrate Myc proteins has been instrumental in defining the functions of the Myc protein. Here, we probe the role of a highly conserved, but little studied, element within the central region of c-Myc, termed 'Myc box III' (MbIII). We show that MbIII is important for transcriptional repression by Myc, and for transformation both in vitro and in a mouse model of lymphomagenesis. Curiously, disruption of MbIII decreases transformation activity by increasing the efficiency with which Myc can induce apoptosis, suggesting that MbIII is a negative regulator of programmed cell death. These findings reveal a role for MbIII in Myc biology, and establish that the oncogenic capacity of Myc is linked directly to its ability to temper the apoptotic response.

Animals↗
PubMed2004

Intrinsic tumour suppression.

Mutations that drive uncontrolled cell-cycle progression are requisite events in tumorigenesis. But evolution has installed in the proliferative programmes of mammalian cells a variety of innate tumour-suppressive mechanisms that trigger apoptosis or senescence, should proliferation become aberrant. These contingent processes rely on a series of sensors and transducers that act in a coordinated network to target the machinery responsible for apoptosis and cell-cycle arrest at different points. Although oncogenic mutations that disable such networks can have profound and varied effects on tumour evolution, they may leave intact latent tumour-suppressive potential that can be harnessed therapeutically.

Animals↗
PubMed2004

Rb inactivation promotes genomic instability by uncoupling cell cycle progression from mitotic control.

Advanced human cancers are invariably aneuploid, in that they harbour cells with abnormal chromosome numbers. However, the molecular defects underlying this trait, and whether they are a cause or a consequence of the malignant phenotype, are not clear. Mutations that disable the retinoblastoma (Rb) pathway are also common in human cancers. These mutations promote tumour development by deregulating the E2F family of transcription factors leading to uncontrolled cell cycle progression. We show that the mitotic checkpoint protein Mad2 is a direct E2F target and, as a consequence, is aberrantly expressed in cells with Rb pathway defects. Concordantly, Mad2 is overexpressed in several tumour types, where it correlates with high E2F activity and poor patient prognosis. Generation of Rb pathway lesions in normal and transformed cells produces aberrant Mad2 expression and mitotic defects leading to aneuploidy, such that elevated Mad2 contributes directly to these defects. These results demonstrate how chromosome instability can arise as a by-product of defects in cell cycle control that compromise the accuracy of mitosis, and suggest a new model to explain the frequent appearance of aneuploidy in human cancer.

Aneuploidy↗
PubMed2004

Reversing drug resistance in vivo.

Apoptotic defects occur in oncogenesis and contribute to drug resistance. We have shown that Bcl-2, Akt, and the translational regulator eIF4E cooperate with Myc during lymphomagenesis and are potent inducers of drug resistance. Interestingly, lymphomas expressing Akt, but not those expressing Bcl-2 are sensitized to chemotherapy-induced apoptosis by the mTOR inhibitor rapamycin, an effect that is countered by eIF4E. These results provide in vivo validation for a strategy to reverse drug resistance in human cancers and highlight the potential role of translational deregulation in oncogenesis and resistance. They also illustrate the importance of tailoring cancer therapy based on tumor genotype.

Animals↗
PubMed2004

Suppression of tumorigenesis by the p53 target PUMA.

The p53 tumor suppressor regulates diverse antiproliferative processes such that cells acquiring p53 mutations have impaired cell-cycle checkpoints, senescence, apoptosis, and genomic stability. Here, we use stable RNA interference to examine the role of PUMA, a p53 target gene and proapoptotic member of the Bcl2 family, in p53-mediated tumor suppression. PUMA short hairpin RNAs (shRNAs) efficiently suppressed PUMA expression and p53-dependent apoptosis but did not impair nonapoptotic functions of p53. Like p53 shRNAs, PUMA shRNAs promoted oncogenic transformation of primary murine fibroblasts by the E1A/ras oncogene combination and dramatically accelerated myc-induced lymphomagenesis without disrupting p53-dependent cell-cycle arrest. However, the ability of PUMA to execute p53 tumor suppressor functions was variable because, in contrast to p53 shRNAs, PUMA shRNAs were unable to cooperate with oncogenic ras in transformation. These results demonstrate that the p53 effector functions involved in tumor suppression are context dependent and, in some settings, depend heavily on the expression of a single proapoptotic effector. Additionally, they demonstrate the utility of RNA interference for evaluating putative tumor suppressor genes in vivo.

Animals↗
PubMed2004

Survival signalling by Akt and eIF4E in oncogenesis and cancer therapy.

Evading apoptosis is considered to be a hallmark of cancer, because mutations in apoptotic regulators invariably accompany tumorigenesis. Many chemotherapeutic agents induce apoptosis, and so disruption of apoptosis during tumour evolution can promote drug resistance. For example, Akt is an apoptotic regulator that is activated in many cancers and may promote drug resistance in vitro. Nevertheless, how Akt disables apoptosis and its contribution to clinical drug resistance are unclear. Using a murine lymphoma model, we show that Akt promotes tumorigenesis and drug resistance by disrupting apoptosis, and that disruption of Akt signalling using the mTOR inhibitor rapamycin reverses chemoresistance in lymphomas expressing Akt, but not in those with other apoptotic defects. eIF4E, a translational regulator that acts downstream of Akt and mTOR, recapitulates Akt's action in tumorigenesis and drug resistance, but is unable to confer sensitivity to rapamycin and chemotherapy. These results establish Akt signalling through mTOR and eIF4E as an important mechanism of oncogenesis and drug resistance in vivo, and reveal how targeting apoptotic programmes can restore drug sensitivity in a genotype-dependent manner.

Animals↗
PubMed2004

Executing cell senescence.

Senescence is a permanent form of cell cycle arrest that limits the proliferation of damaged cells and may contribute to tumor suppression and aging. We recently demonstrated that some senescent cell types undergo dramatic changes in chromatin organization that are dependent on the retinoblastoma protein and are associated with the stable repression of some E2F target genes. Here we show how these changes might contribute to the stability of the senescent state.

Animals↗