Biomedical subjects

T Jacks

Publications and source records attributed to T Jacks.

At least 91 records · Page 5Linked to original sources

PubMed1996

Tumor induction and tissue atrophy in mice lacking E2F-1.

The retinoblastoma tumor suppressor protein (pRB) is a transcriptional repressor that regulates gene expression by physically associating with transcription factors such as E2F family members. Although pRB and its upstream regulators are commonly mutated in human cancer, the physiological role of the pRB-E2F pathway is unknown. To address the function of E2F-1 and pRB/E2F-1 complexes in vivo, we have produced mice homozygous for a nonfunctional E2F-1 allele. Mice lacking E2F-1 are viable and fertile, yet experience testicular atrophy and exocrine gland dysplasia. Surprisingly, mice lacking E2F-1 develop a broad and unusual spectrum of tumors. Although overexpression of E2F-1 in tissue culture cells can stimulate cell proliferation and be oncogenic, loss of E2F-1 in mice results in tumorigenesis, demonstrating that E2F-1 also functions as a tumor suppressor.

Animals↗

PubMed1996

Identification and cloning of EI24, a gene induced by p53 in etoposide-treated cells.

To search for candidate genes involved in p53-mediated apoptosis, the differential display technique was used to identify RNA species whose expression was altered in murine NIH3T3 cells treated with the cytotoxic drug etoposide. We report here the isolation and characterization of EI24, a novel gene whose 2.4 kb mRNA is induced following etoposide treatment. Induction of EI24 mRNA by etoposide required expression of wild-type p53 in murine embryonic fibroblasts which had been transformed with the oncogenes E1A and T24 H-ras; and overexpression of functional p53 in these cells was sufficient to induce expression of the EI24 mRNA. The EI24 mRNA was also induced in a p53-dependent manner by ionizing irradiation of primary murine thymocytes. Isolation of a full-length EI24 cDNA revealed that its protein product bears homology to CELF37C12.2, a Caenorhabditis elegans protein of unknown function.

3T3 Cells↗

PubMed1996

Hypoxia-mediated selection of cells with diminished apoptotic potential in solid tumours.

Apoptosis is a genetically encoded programme of cell death that can be activated under physiological conditions and may be an important safeguard against tumour development. Regions of low oxygen (hypoxia) and necrosis are common features of solid tumours. Here we report that hypoxia induces apoptosis in oncogenically transformed cells and that further genetic alterations, such as loss of the p53 tumour-suppressor gene or overexpression of the apoptosis-inhibitor protein Bcl-2, substantially reduce hypoxia-induced cell death. Hypoxia also selects for cells with defects in apoptosis, because small numbers of transformed cells lacking p53 overtake similar cells expressing wild-type p53 when treated with hypoxia. Furthermore, highly apoptotic regions strongly correlate with hypoxic regions in transplanted tumours expressing wild-type p53, whereas little apoptosis occurs in hypoxic regions of p53-deficient tumours. We propose that hypoxia provides a physiological selective pressure in tumours for the expansion of variants that have lost their apoptotic potential, and in particular for cells acquiring p53 mutations.

Animals↗

PubMed1996

Nf1 gene targeting: toward models and mechanisms.

Patients with neurofibromatosis type I develop multiple benign nerve sheath tumors and are predisposed to a number of malignancies. Since loss-of-function mutations in the NF1 gene appear to be responsible for the disease, NF1 has been classified as a tumor suppressor. Several strategies involving the targeted disruption of the murine homologue have been used in an attempt to establish an animal model for the disease and four types of animals have been generated: (1) Nf1 +/- animals, (2) NF1 -/- embryos, (3) Nf1 -/- chimeras, and (4) mice transplanted with Nf1 -/- hematopoietic stem cells. In addition to yielding mice which mimic various aspects of the human disease, each of these approaches has contributed to a better understanding of the normal function of Nf1.

Animals↗

PubMed1996

Lessons from the p53 mutant mouse.

The use of the mouse as a model organism in cancer research has a long and productive history, from the earliest studies of chemical carcinogenesis to the recent advances in gene targeting. Many of the basic principles of tumorigenesis have been formed in whole or in part through the study of tumor development in the mouse. Over the past decade, the major experimental approach has been to generate cancer-prone strains, either through transgenic technologies or, more recently, gene targeting. Here, I will review the state of the field of gene targeting of tumor-suppressor genes and concentrate on the p53 mutant strains and the lessons learned from the p53 mutant mouse.

Animals↗

PubMed1996

Mechanisms of carcinogenesis and the mutant mouse.

Gene targeting in embryonic stem cells has allowed the creation of several new mouse strains carrying mutations in the murine homologs of human tumor suppressor genes. The phenotypes of these mutant strains are reviewed. In addition, recent experiments utilizing the ApcMin and p53 mutant animals to address specific mechanisms of carcinogenesis are discussed.

Adenomatous Polyposis Coli↗

PubMed1996

Loss of NF1 results in activation of the Ras signaling pathway and leads to aberrant growth in haematopoietic cells.

Individuals with neurofibromatosis type 1 (NF1) are predisposed to certain cancers including juvenile chronic myelogenous leukaemia (JCML). The NF1 tumour-suppressor gene encodes a protein (neurofibromin) that accelerates GTP hydrolysis on Ras proteins. Here we show that primary leukaemic cells from children with NF1 show a selective decrease in NF1-like GTPase activating protein (GAP) activity for Ras but retain normal cellular GAP activity. Leukaemic cells also show an elevated percentage of Ras in the GTP-bound conformation. JCML cells are hypersensitive to granulocyte-macrophage colony stimulating factor (GM-CSF), and we observed a similar pattern of aberrant growth in haematopoietic cells from Nf1-/- mouse embryos. These data define a specific role for neurofibromin in negatively regulating GM-CSF signaling through Ras in haematopoietic cells and they suggest that hypersensitivity to GM-CSF may be a primary event in the development of JCML.

Animals↗

PubMed1996

Skeletal muscle cells lacking the retinoblastoma protein display defects in muscle gene expression and accumulate in S and G2 phases of the cell cycle.

Viral oncoproteins that inactivate the retinoblastoma tumor suppressor protein (pRb) family both block skeletal muscle differentiation and promote cell cycle progression. To clarify the dependence of terminal differentiation on the presence of the different pRb-related proteins, we have studied myogenesis using isogenic primary fibroblasts derived from mouse embryos individually deficient for pRb, p107, or p130. When ectopically expressed in fibroblasts lacking pRb, MyoD induces an aberrant skeletal muscle differentiation program characterized by normal expression of early differentiation markers such as myogenin and p21, but attenuated expression of late differentiation markers such as myosin heavy chain (MHC). Similar defects in MHC expression were not observed in cells lacking either p107 or p130, indicating that the defect is specific to the loss of pRb. In contrast to wild-type, p107-deficient, or p130-deficient differentiated myocytes that are permanently withdrawn from the cell cycle, differentiated myocytes lacking pRb accumulate in S and G2 phases and express extremely high levels of cyclins A and B, cyclin-dependent kinase (Cdk2), and Cdc2, but fail to readily proceed to mitosis. Administration of caffeine, an agent that removes inhibitory phosphorylations on inactive Cdc2/cyclin B complexes, specifically induced mitotic catastrophe in pRb-deficient myocytes, consistent with the observation that the majority of pRb-deficient myocytes arrest in S and G2. Together, these findings indicate that pRb is required for the expression of late skeletal muscle differentiation markers and for the inhibition of DNA synthesis, but that a pRb-independent mechanism restricts entry of differentiated myocytes into mitosis.

Animals↗

PubMed1996

Altered cell cycle kinetics, gene expression, and G1 restriction point regulation in Rb-deficient fibroblasts.

Fibroblasts prepared from retinoblastoma (Rb) gene-negative mouse embryos exhibit a shorter G1 phase of the growth cycle and smaller size than wild-type cells. In addition, the mutant cells are no longer inhibited by low levels of cycloheximide at any point in G1 but do remain sensitive to serum withdrawal until late in G1. Certain cell cycle-regulated genes showed no temporal or quantitative differences in expression. In contrast, cyclin E expression in Rb-deficient cells is deregulated in two ways. Cyclin E mRNA is generally derepressed in mutant cells and reaches peak levels about 6 h earlier in G1 than in wild-type cells. Moreover, cyclin E protein levels are higher in the Rb-/- cells than would be predicted from the levels of its mRNA. Thus, the selective growth advantage conferred by Rb gene deletion during tumorigenesis may be explained in part by changes in the regulation of cyclin E. In addition, the mechanisms defining the restriction point of late G1 may consist of at least two molecular events, one cycloheximide sensitive and pRb dependent and the other serum sensitive and pRb independent.

Animals↗

PubMed1996

Tumor suppressor gene mutations in mice.

Over the past several years, a number of human tumor suppressor genes have been cloned and characterized. Germline mutations in tumor suppressor genes strongly predispose to cancer, and they are also mutated somatically in sporadic forms of the disease. In order to create animal models for the familial cancer syndromes caused by inherited mutations in these genes as well as to determine their role in embryogenesis, the homologues of several members of this class have been mutated in the mouse. The initial characterization of the heterozygous and homozygous phenotypes caused by these mutations has led to important insights into the mechanisms by which tumor suppressor genes participate in normal development and how their loss contributes to tumorigenesis.

Animals↗

PubMed1996

MK-0677, a potent, novel, orally active growth hormone (GH) secretagogue: GH, insulin-like growth factor I, and other hormonal responses in beagles.

MK-0677, a spiroindoline sulfonamide, is a novel, orally active GH secretagogue. The effects of MK-0677 on serum GH and other hormones after oral and iv single dose administrations in beagles were evaluated. After oral administration in a balanced eight-dog crossover study, treatment with MK-0677 significantly increased peak GH concentrations, with a 5.3-fold increase (mean +/- SEM, 10.5 +/- 1.9 ng/ml) at the 0.25 mg/kg dose, a 9.0-fold increase (18.0 +/- 3.3 ng/ml) at the 0.50 mg/kg dose, and a 15.8-fold increase (31.6 +/- 5.8 ng/ml) at the 1.0 mg/kg dose. Total GH release, expressed as the area under the curve, showed similar significant increases over the effect of the water placebo. A single oral 1 mg/kg dose in three dogs induced a mean GH peak of 27.6 +/- 1.5 ng/ml at 120 min, and GH levels remained elevated up to 360 min after treatment. Insulin-like growth factor I (IGF-I) levels were significantly increased by 30% at 480 min after treatment. Cortisol levels were increased 2.4-fold over pretreatment levels. After i.v. administration, compared to the saline control group which had a mean (+/- SEM) serum GH peak of 3.8 +/- 0.7 ng/ml, MK-0677 at 0.25 mg/kg significantly increased (P < 0.05) peak GH concentrations 20.4-fold (77.4 +/- 13.7 ng/ml). Total GH release, expressed as the area under the curve, showed a similar increase. The mean peak GH level was recorded 10 min after treatment, with GH levels elevated up to 180 min after treatment. IGF-I levels were significantly elevated by 25% 360 min after the administration of MK-0677. Cortisol levels were increased 2.3-fold over pretreatment levels. Insulin and glucose levels were higher, LH and PRL levels were unaltered, and T4 levels were marginally lower; the levels of each of these hormones remained within the normal ranges for dogs throughout the experiment. In summary, MK-0677 is a potent GH secretagogue that induces an immediate, large, long lasting increase in GH levels when administered orally or i.v. In contrast to GH-releasing peptide-6 and benzolactam secretagogues, GH levels were elevated up to 360 min after treatment, and this was associated with a significant increase in IGF-I levels. Cortisol levels were increased; however, the increases were modest compared to those in GH.

Administration, Oral↗

PubMed1996

Modulation of pulsatile GH release through a novel receptor in hypothalamus and pituitary gland.

Hormone replacement should provide a serum hormone profile similar to that found in normal physiology. This is generally impractical because hormones are usually released episodically and therefore require frequent administration. However, rather than replacing the hormone directly, in theory, one could administer a mimic or amplifier of the pulse generator that controls pulsatile release of the particular hormone. Using growth hormone (GH) as a paradigm we sought such a mimetic that would provide episodic GH release when administered by the oral route. A GH secretagogue MK0677, is described that has these ideal properties; following oral administration MK0677 amplifies episodic GH release. Mechanistically, it synergizes with growth hormone releasing hormone (GHRH) through a receptor and signal transduction pathway distinct from that of GHRH and is a functional antagonist of somatostatin (SRIF). MK0677 also acts on the arcuate nucleus and appears to stimulate GHRH release. By using 35S-MK0677, a new G-protein coupled receptor for MK0677 was characterized in the plasma membrane fraction of pituitary and hypothalamic tissue. The receptor is present in very low abundance and couples to phospholipase C. Other ligands selective for this receptor also cause synchronization of well-defined pathways leading to GH release. Repeated oral treatment of dogs once daily with MK0677 initiates amplified pulsatile GH release accompanied by increases in IGF-1 that are sustained. The unique biological properties of MK0677 and other synthetic ligands that bind to the same receptor force us to predict that these ligands mimic a naturally occurring hormone that regulates pulsatile GH release. Understanding the regulatory mechanisms involved in this paradigm has broad implications for the control of pulsatile rhythms in the endocrine system.

Amino Acid Sequence↗

PubMed1995

Vascular system defects and neuronal apoptosis in mice lacking ras GTPase-activating protein.

The gene encoding p120-rasGAP, a negative regulator of Ras, has been disrupted in mice. This Gap mutation affects the ability of endothelial cells to organize into a highly vascularized network and results in extensive neuronal cell death. Mutati ons in the Gap and Nf1 genes have a synergistic effect, such that embryos homozygous for mutations in both genes show an exacerbated Gap phenotype. Thus rasGAP and neurofibromin act together to regulate Ras activity during embryonic development.

Animals↗

PubMed1995

Radiation-induced cell cycle arrest compromised by p21 deficiency.

The protein p21 is a dual inhibitor of cyclin-dependent kinases and proliferating-cell nuclear antigen (PCNA), both of which are required for passage through the cell cycle. The p21 gene is under the transcriptional control of p53 (ref. 5), suggesting that p21 might promote p53-dependent cell cycle arrest or apoptosis. p21 has also been implicated in cell senescence and in cell-cycle withdrawal upon terminal differentiation. Here we investigate the role of p21 in these processes using chimaeric mice composed partly of p21-/- and partly of p21+/+ cells. Immunohistochemical studies of the p21+/+ and p21-/- components of adult small intestine indicated that deletion of p21 has no detectable effect on the migration-associated differentiation of the four principal intestinal epithelial cell lineages or on p53-dependent apoptosis following irradiation. However, p21-/- mouse embryo fibroblasts are impaired in their ability to undergo G1 arrest following DNA damage.

Animals↗

PubMed1995

Characterization of immature thymocyte lines derived from T-cell receptor or recombination activating gene 1 and p53 double mutant mice.

The T-cell receptor (TCR) beta chain is instrumental in the progression of thymocyte differentiation from the CD4-CD8- to the CD4+CD8+ stage. This differentiation step may involve cell surface expression of novel CD3-TCR complexes. To facilitate biochemical characterization of these complexes, we established cell lines from thymic lymphomas originating from mice carrying a mutation in the p53 gene on the one hand and a mutation in TCR-alpha, TCR-beta, or the recombination activating gene 1 (RAG-1) on the other hand. The cell lines were CD4+CD8+ and appeared to be monoclonal. A cell line derived from a RAG-1 x p53 double mutant thymic lymphoma expressed low levels of CD3-epsilon, -gamma, and -delta on the surface. TCR-alpha x p53 double mutant cell lines were found to express complexes consisting of TCR-beta chains associated with CD3-epsilon, -gamma, and -delta chains and CD3-zeta zeta dimers. These lines will be useful tools to study the molecular structure and signal transducing properties of partial CD3-TCR complexes expressed on the surface of immature thymocytes.

Animals↗

PubMed1995

p53-dependent and independent expression of p21 during cell growth, differentiation, and DNA damage.

Expression of p21 has been shown to be up-regulated by the p53 tumor suppressor gene in vitro in response to DNA-damaging agents. However, p21 expression can be regulated independently of p53, and here we show that expression of p21 in various tissues during development and in the adult mouse occurs in the absence of p53 function. However, most tissues tested did require p53 for p21 induction following exposure of the whole animal to gamma irradiation. These results show that normal tissue expression of p21 to high levels is not dependent on p53 and confirm that induction of p21 by DNA-damaging agents does require p53. p21 is expressed upon differentiation of p53-deficient murine erythroleukemia (MEL) cells, and the kinetics of induction of p21 in this system suggest that it may be involved in the growth arrest that precedes terminal differentiation. The gene is up-regulated in mouse fibroblasts in response to serum restimulation but the kinetics and levels of induction differ between wild-type and mutant cells. Expression of p21 message following serum restimulation is superinducible by cycloheximide in wild-type but not in p53-deficient cells. The increases in p21 mRNA are reflected in changes in p21 protein levels. p21 expression also appears to be regulated at the post-transcriptional level because moderate increases in mRNA expression, during differentiation of MEL cells and upon serum restimulation of fibroblasts, are followed by large increases in protein levels. Regulation of the mouse p21 promoter by p53 depends on two critical p53-binding sites located 1.95 and 2.85 kb upstream from the transcriptional initiation site. The sequences mediating serum responsiveness of the promoter map to a region containing the proximal p53 site. p53 appears to play a critical role in p21 induction following DNA damage. Moreover, p21 can be regulated independently of p53 in several situations including during normal tissue development, following serum stimulation, and during cellular differentiation.

Animals↗

PubMed1995

A subset of p53-deficient embryos exhibit exencephaly.

Defects in neural tube formation are among the most common malformations leading to infant mortality. Although numerous genetic loci appear to contribute to the defects observed in humans and in animal model systems, few of the genes involved have been characterized at the molecular level. Mice lacking the p53 tumour suppressor gene are predisposed to tumours, but the viability of these animals indicates that p53 function is not essential for embryonic development. Here, we demonstrate that a fraction of p53-deficient embryos in fact do not develop normally. These animals display defects in neural tube closure resulting in an overgrowth of neural tissue in the region of the mid-brain, a condition known as exencephaly.

Animals↗

PubMed1994

p53 status and the efficacy of cancer therapy in vivo.

The therapeutic responsiveness of genetically defined tumors expressing or devoid of the p53 tumor suppressor gene was compared in immunocompromised mice. Tumors expressing the p53 gene contained a high proportion of apoptotic cells and typically regressed after treatment with gamma radiation or adriamycin. In contrast, p53-deficient tumors treated with the same regimens continued to enlarge and contained few apoptotic cells. Acquired mutations in p53 were associated with both treatment resistance and relapse in p53-expressing tumors. These results establish that defects in apoptosis, here caused by the inactivation of p53, can produce treatment-resistant tumors and suggest that p53 status may be an important determinant of tumor response to therapy.

Animals↗