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T Jacks

Publications and source records attributed to T Jacks.

At least 19 recordsLinked to original sources
PubMed1997

p53-independent apoptosis induced by paclitaxel through an indirect mechanism.

The efficacy of chemotherapeutic agents may be determined by a number of different factors, including the genotype of the tumor cell. The p53 tumor suppressor gene frequently is mutated in human tumors, and this may contribute to chemotherapeutic resistance. We tested the requirement for wild-type p53 in the response of tumor cells to treatment with paclitaxel (trade name Taxol), an antineoplastic agent that stabilizes cellular microtubules. Although paclitaxel is broadly effective against human tumor xenografts in mice, including some known to carry p53 mutations, we found that p53-containing mouse tumor cells were significantly more sensitive to direct treatment with this drug than were p53-deficient tumor cells. In an attempt to reconcile this apparent discrepancy, we examined the requirement for p53 in the cytotoxic effects of tumor necrosis factor alpha (TNF-alpha), a cytokine released from murine macrophages upon paclitaxel treatment. Conditioned medium from paclitaxel-treated macrophages was capable of inducing p53-independent apoptosis when applied to transformed mouse embryonic fibroblasts and was inhibitable by antibodies against TNF-alpha. Furthermore, in response to direct treatment with TNF-alpha, both wild-type and p53-deficient tumor cells underwent apoptosis to similar extents and with similar kinetics. Our results suggest that the efficacy of paclitaxel in vivo may be due not only to its microtubule-stabilizing activity, but its ability to activate local release of an apoptosis-inducing cytokine.

Animals↗
PubMed1997

The Nf2 tumor suppressor gene product is essential for extraembryonic development immediately prior to gastrulation.

The neurofibromatosis type II (NF2) tumor suppressor encodes a putative cytoskeletal associated protein, the loss of which leads to the development of Schwann cell tumors associated with NF2 in humans. The NF2 protein merlin belongs to the band 4.1 family of proteins that link membrane proteins to the cytoskeleton and are thought to be involved in dynamic cytoskeletal reorganization. Beyond its membership in this family, however, the function of merlin remains poorly understood. In order to analyze the function of merlin during embryogenesis and to develop a system to study merlin function in detail, we have disrupted the mouse Nf2 gene by homologous recombination in embryonic stem cells. Most embryos homozygous for a mutation at the Nf2 locus fail between embryonic days 6.5 and 7.0, exhibiting a collapsed extraembryonic region and the absence of organized extraembryonic ectoderm. The embryo proper continues to develop, but fails to initiate gastrulation. These observations are supported by the expression patterns of markers of the extraembryonic lineage and the lack of expression of mesodermal markers in the mutant embryos. Mosaic studies demonstrate that merlin function is not required cell autonomously in mesoderm, and support the proposition that merlin function is essential for the development of extraembryonic structures during early mouse development.

Animals↗
PubMed1997

A mouse model for the learning and memory deficits associated with neurofibromatosis type I.

Neurofibromatosis type I (NF1) is one of the most commonly inherited neurological disorders in humans, affecting approximately one in 4,000 individuals. NF1 results in a complex cluster of developmental and tumour syndromes that include benign neurofibromas, hyperpigmentation of melanocytes and hamartomas of the iris. Some NF1 patients may also show neurologic lesions, such as optic pathway gliomas, dural ectasia and aqueduct stenosis. Importantly, learning disabilities occur in 30% to 45% of patients with NF1, even in the absence of any apparent neural pathology. The learning disabilities may include a depression in mean IQ scores, visuoperceptual problems and impairments in spatial cognitive abilities. Spatial learning has been assessed with a variety of cognitive tasks and the most consistent spatial learning deficits have been observed with the Judgement of Line Orientation test. It is important to note that some of these deficits could be secondary to developmental abnormalities and other neurological problems, such as poor motor coordination and attentional deficits. Previous studies have suggested a role for neurofibromin in brain function. First, the expression of the Nf1 gene is largely restricted to neuronal tissues in the adult. Second, this GTPase activating protein may act as a negative regulator of neurotrophin-mediated signalling. Third, immunohistochemical studies suggest that activation of astrocytes may be common in the brain of NF1 patients. Here, we show that the Nf1+/- mutation also affects learning and memory in mice. As in humans, the learning and memory deficits of the Nf1+/- mice are restricted to specific types of learning, they are not fully penetrant, they can be compensated for with extended training, and they do not involve deficits in simple associative learning.

Acoustic Stimulation↗
PubMed1997

Aberrant Ras regulation and reduced p190 tyrosine phosphorylation in cells lacking p120-Gap.

The Ras guanine nucleotide-binding protein functions as a molecular switch in signalling downstream of protein-tyrosine kinases. Ras is activated by exchange of GDP for GTP and is turned off by hydrolysis of bound GTP to GDP. Ras itself has a low intrinsic GTPase activity that can be stimulated by GTPase-activating proteins (GAPs), including p120-Gap and neurofibromin. These GAPs possess a common catalytic domain but contain distinct regulatory elements that may couple different external signals to control of the Ras pathway. p120-Gap, for example, has two N-terminal SH2 domains that directly recognize phosphotyrosine motifs on activated growth factor receptors and cytoplasmic phosphoproteins. To analyze the role of p120-Gap in Ras regulation in vivo, we have used fibroblasts derived from mouse embryos with a null mutation in the gene for p120-Gap (Gap). Platelet-derived growth factor stimulation of Gap-/- cells led to an abnormally large increase in the level of Ras-GTP and in the duration of mitogen-activated protein (MAP) kinase activation compared with wild-type cells, suggesting that p120-Gap is specifically activated following growth factor stimulation. Induction of DNA synthesis in response to platelet-derived growth factor and morphological transformation by the v-src and EJ-ras oncogenes were not significantly affected by the absence of p120-Gap. However, we found that normal tyrosine phosphorylation of p190-rhoGap, a cytoplasmic protein that associates with the p120-Gap SH2 domains, was dependent on the presence of p120-Gap. Our results suggest that p120-Gap has specific functions in downregulating the Ras/MAP kinase pathway following growth factor stimulation, and in modulating the phosphorylation of p190-rhoGap, but is not required for mitogenic signalling.

Animals↗
PubMed1997

Absence of p53 in a mouse mammary tumor model promotes tumor cell proliferation without affecting apoptosis.

Loss or mutation of p53 may have multiple biological and genetic effects that result in accelerated tumor progression. Loss of p53 in some tumors has been correlated with a marked decrease in tumor cell apoptosis. p53 loss may also accelerate tumor growth through an increase in cell proliferation rates. To examine the effects of p53 loss on tumor progression in a controlled experimental context, we previously crossed p53-deficient mice to mammary tumor-susceptible Wnt-1 transgenic (TG) mice. The resulting female Wnt-1 TG offspring of this cross all developed mammary tumors, regardless of p53 status (p53+/+, p53+/-, or p53-/-). However, female p53-/- Wnt-1 TG mice developed tumors much sooner than their p53+/+ counterparts. In this report, we demonstrate that the average growth rates of tumors missing (p53-/-) or losing p53 (p53+/- with loss of heterozygosity) are accelerated compared to tumors with both wild-type p53 alleles (p53+/+). This accelerated growth rate appears to be due primarily to increases in rates of tumor cell proliferation. Tumor cell apoptotic levels were modest and were not measurably different in the presence or absence of wild-type p53. These results differ substantially from other mouse tumor models in which p53 loss was closely correlated with accelerated growth rates through attenuated apoptosis. Thus, the mechanisms by which p53 loss influences tumor progression may differ, depending on the tissue type and/or the oncogenic pathways involved.

Animals↗
PubMed1996

Loss of Rb activates both p53-dependent and independent cell death pathways in the developing mouse nervous system.

Extensive apoptosis occurs in the nervous system of mouse embryos homozygous mutant for a targeted disruption of the retinoblastoma (Rb) gene. This cell death is present in both the central (CNS) and peripheral nervous systems (PNS) and is associated with abnormal S phase entry of normally post-mitotic neurons. Aberrant proliferation in the CNS correlates with increased free E2F DNA binding activity and increased expression of cyclin E, an E2F target gene and critical cell cycle regulator. Cell death in the CNS is accompanied by increased levels of the p53 tumor suppressor gene product and increased expression of the p53 target gene, p21Waf-1/Cip-1. However, induction of p53 is not observed in the PNS of Rb-mutant embryos, nor does loss of p53 function inhibit cell death in the PNS. Surprisingly, p21Waf-1/Cip-1 is induced in the sensory ganglia of Rb-mutant embryos in a p53-independent manner. Although loss of p53 gene function prevents cell death in the CNS of Rb-mutant embryos, it does not restore normal proliferative control.

Animals↗
PubMed1996

Mutations in the p53 and SCID genes cooperate in tumorigenesis.

DNA damage can cause mutations that contribute to cellular transformation and tumorigenesis. The p53 tumor suppressor acts to protect the organism from DNA damage by inducing either G1 arrest to facilitate DNA repair or by activating physiological cell death (apoptosis). Consistent with this critical function of p53, mice lacking p53 are predisposed to developing tumors, particularly lymphoma. The severe combined immune deficiency (scid) focus encodes the catalytic subunit of DNA protein kinase (DNA-PKcs), a protein complex that has a role in the cellular response to DNA damage. Cells from scid mice are hypersensitive to radiation and scid lymphocytes fail to develop from precursors because they are unable to properly join DNA-coding ends during antigen receptor gene rearrangement. We examined the combined effect of loss of p53 and loss of DNA-PKcs on lymphocyte development and tumorigenesis by generating p53-/- scid mice. Our data demonstrate that loss of p53 promotes T-cell development in scid mice but does not noticeably affect B lymphopoiesis. Moreover, scid cells are able to induce p53 protein expression and activate G1 arrest or apoptosis in response to ionizing radiation, indicating that DNA-PKcs is not essential for these responses to DNA damage. Furthermore, p53-/- scid double mutant mice develop lymphoma earlier than p53-/- littermates, demonstrating that loss of these two genes can cooperate in tumorigenesis. Collectively, these results provide evidence for an unsuspected role of p53 as a checkpoint regulator in early T-cell development and demonstrate that loss of an additional component of the cellular response to DNA damage can cooperate with loss of p53 in lymphomagenesis.

Animals↗
PubMed1996

Transcriptional activation by p53, but not induction of the p21 gene, is essential for oncogene-mediated apoptosis.

The p53 tumor suppressor limits cellular proliferation by inducing either G1 arrest or apoptosis, depending on the cellular context. To determine if these pathways are mechanistically distinct, we have examined the effects of different p53 mutants in p53 null primary mouse embryo fibroblasts. We chose this system as it is highly physiological and ensures that the interpretation of the results will not be confounded by the presence of endogenous p53 or oncoproteins which target p53. Using single cell microinjection assays for both G1 arrest and apoptosis, with loss-of-function and chimeric gain-of-function mutants, we have demonstrated that transcriptional activation is critical for both processes. Replacement of the p53 activation domain with that of VP16, or replacement of the p53 oligomerization domain with that of GCN4, reconstituted both G1 arrest and apoptosis activities. However, despite the importance of transcriptional activation in both processes, the target gene requirements are different. The p21 cyclin-dependent kinase inhibitor, which has been shown to be a direct target of p53 and a component of the radiation-induced G1 arrest response, is dispensable for oncogene-induced apoptosis, suggesting that these two p53-dependent transcriptional pathways are distinct.

Adenovirus E1A Proteins↗
PubMed1996

Targeted disruption of p107: functional overlap between p107 and Rb.

To explore the physiological role of p107, a member of retinoblastoma gene (Rb) family, we disrupted the mouse gene by homologous recombination in embryonic stem cells. p107 homozygous mutant mice were viable, fertile, and displayed no obvious abnormalities. To investigate possible functional overlap between p107 and Rb, mice with mutations at both loci were generated. Rb+/-;p107-/- mice have a pronounced growth retardation and increased mortality rate during the first 3 weeks after birth. The Rb+/-;p107-/- pups that survive to adulthood did not show any altered tumor predisposition when compared with Rb+/- mice but developed multiple dysplastic lesions of the retina. Embryos homozygous for both Rb and p107 died at approximately 11.5 days of gestation, 2 days earlier than embryos homozygous for Rb alone. Histological examination revealed accelerated apoptosis in the liver and the central nervous system of Rb-/-;p107-/- embryos relative to Rb-/- embryos. These results provide the first in vivo evidence that p107 and Rb have overlapping functions in some tissues of the developing and adult mouse.

Actins↗
PubMed1996

Shared role of the pRB-related p130 and p107 proteins in limb development.

The p130 protein shares extensive sequence similarity with pRB, the product of the retinoblastoma gene, and is a major E2F-associated protein in quiescent cells. To investigate its biological function, we have mutated p130 via gene targeting in the mouse. Homozygous mutation of p130 had little discernible effect on development or on the growth of mouse embryo fibroblasts in culture. Much of the E2F activity that normally associates with p130 in serum-starved mouse embryo fibroblasts associated instead with the highly related p107 protein. To determine whether p130 and p107 have overlapping biological roles, we produced mice having simultaneous inactivation of the p130 and p107 genes. Such mice exhibited deregulated chondrocyte growth, defective endochondral bone development, shortened limbs, and neonatal lethality. These findings indicate that p130 and p107 play an important role in limb development through their abilities to control chondrocyte proliferation. Thus, in certain settings p107 and p130 perform growth-regulatory functions that are not fulfilled by pRB.

Animals↗
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↗