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T Jacks

Publications and source records attributed to T Jacks.

At least 55 records · Page 3Linked to original sources
PubMed1999

Increases in circulating insulin-like growth factor I levels by the oral growth hormone secretagogue MK-0677 in the beagle are dependent upon pituitary mediation.

It has been well established that the spiroindoline sulfonamide MK-0677 stimulates GH secretion from the pituitary both in vitro and in vivo. MK-0677 has also been shown to increase serum insulin-like growth factor I (IGF-I) and cortisol levels in vivo; these increases are assumed to be driven by the increased serum GH and ACTH levels, respectively. However, such increases could also be due to a direct stimulatory action of MK-0677 at the level of the liver and adrenal cortex. To address this possibility, we investigated whether MK-0677 increased IGF-I and cortisol levels in hypophysectomized dogs. Baseline GH, IGF-I, and cortisol responses to MK-0677 (1 mg/kg, orally) were initially determined. Hypophysectomy (hypox; n = 7) or sham surgery (sham; n = 5) was then carried out. Six days postsurgery, the GH and cortisol responses to MK-0677 were reevaluated in each dog. In addition, each dog was treated with porcine GH (PST; 0.1 IU/kg, s.c.) to confirm the responsiveness of the GH-IGF-I axis. The mean peak GH increases in response to MK-0677 in the presham dogs (83.7 +/- 19.2 ng/ml), post-sham dogs (108 +/- 26.2 ng/ml), and pre-hypox dogs (121.2 +/- 13.6 ng/ml) were not significantly different. Mean peak GH levels were unchanged after MK-0677 administration in the hypox dogs (2.3 +/- 0.7 ng/ml). Before surgery, serum IGF-I levels increased to 243 +/- 27 and 224 +/- 47 ng/ml in the sham and hypox groups, respectively, after MK-0677 administration. Surgery was associated with a marked (> or =50%) decrease in serum IGF-I levels. MK-0677 administration increased IGF-I levels in the sham dogs from 78 +/- 14 to 187 +/- 31 ng/ml, whereas IGF-I levels remained unchanged (17.7 +/- 2.4 ng/ml) in the-hypox dogs. PST treatment increased IGF-I levels in the sham dogs from 162 +/- 30 to 325 +/- 32 ng/ml. In the hypox dogs PST treatment restored IGF-I to physiological levels (from 17.7 +/- 2.4 to 199 +/- 41 ng/ml). Cortisol was increased after MK-0677 administration 3.7-fold in the pre-sham, 3.6-fold in the post-sham, and 3.6-fold in the pre-hypox dogs, but no increase was seen in the post-hypox dogs. ACTH GEL administration (2.2 U/kg, i.m.) to hypox dogs returned cortisol to normal physiological levels, demonstrating the functional integrity of the adrenal cortex. This study demonstrates that the GH secretagogue MK-0677 does not directly stimulate an increase in serum IGF-I or cortisol levels, but depends upon the presence of an intact pituitary.

Adrenocorticotropic Hormone↗
PubMed1998

Nf1 regulates hematopoietic progenitor cell growth and ras signaling in response to multiple cytokines.

Neurofibromin, the protein encoded by the NF1 tumor-suppressor gene, negatively regulates the output of p21(ras) (Ras) proteins by accelerating the hydrolysis of active Ras-guanosine triphosphate to inactive Ras-guanosine diphosphate. Children with neurofibromatosis type 1 (NF1) are predisposed to juvenile chronic myelogenous leukemia (JCML) and other malignant myeloid disorders, and heterozygous Nf1 knockout mice spontaneously develop a myeloid disorder that resembles JCML. Both human and murine leukemias show loss of the normal allele. JCML cells and Nf1-/- hematopoietic cells isolated from fetal livers selectively form abnormally high numbers of colonies derived from granulocyte-macrophage progenitors in cultures supplemented with low concentrations of granulocyte-macrophage colony stimulating factor (GM-CSF). Taken together, these data suggest that neurofibromin is required to downregulate Ras activation in myeloid cells exposed to GM-CSF. We have investigated the growth and proliferation of purified populations of hematopoietic progenitor cells isolated from Nf1 knockout mice in response to the cytokines interleukin (IL)-3 and stem cell factor (SCF), as well as to GM-CSF. We found abnormal proliferation of both immature and lineage-restricted progenitor populations, and we observed increased synergy between SCF and either IL-3 or GM-CSF in Nf1-/- progenitors. Nf1-/- fetal livers also showed an absolute increase in the numbers of immature progenitors. We further demonstrate constitutive activation of the Ras-Raf-MAP (mitogen-activated protein) kinase signaling pathway in primary c-kit+ Nf1-/- progenitors and hyperactivation of MAP kinase after growth factor stimulation. The results of these experiments in primary hematopoietic cells implicate Nf1 as playing a central role in regulating the proliferation and survival of primitive and lineage-restricted myeloid progenitors in response to multiple cytokines by modulating Ras output.

Animals↗
PubMed1998

p53-Dependent and -independent responses to cisplatin in mouse testicular teratocarcinoma cells.

Testicular cancers respond favorably to chemotherapy with the platinum-containing drug cis-diamminedichloroplatinum(II) (cisplatin). One factor that could explain the efficacy of cisplatin is the low frequency of p53 mutations observed in this tumor type. The present study examines the p53-mediated responses in murine testicular teratocarcinoma cells exposed to the drug. Cisplatin treatment of teratocarcinoma cells with a wild-type p53 gene resulted in accumulation of the p53 protein through posttranscriptional mechanisms; induction of p53-target genes was also observed. Drug treatment resulted in rapid apoptosis in p53-wild-type cells but not in p53(-/-) teratocarcinoma cells. In the latter cells, cisplatin exposure caused prolonged cell cycle arrest accompanied by induction of the p21 gene. Clonogenic assays demonstrated that the p53 mutation did not confer resistance to cisplatin. These experiments suggest that cisplatin inhibits cellular proliferation of testicular teratocarcinoma cells by two possible mechanisms, p53-dependent apoptosis and p53-independent cell cycle arrest.

Animals↗
PubMed1998

p21 is a critical CDK2 regulator essential for proliferation control in Rb-deficient cells.

Proliferation in mammalian cells is controlled primarily in the G1-phase of the cell cycle through the action of the G1 cyclin-dependent kinases, CDK4 and CDK2. To explore the mechanism of cellular response to extrinsic factors, specific loss of function mutations were generated in two negative regulators of G1 progression, p21 and pRB. Individually, these mutations were shown to have significant effects in G1 regulation, and when combined, Rb and p21 mutations caused more profound defects in G1. Moreover, cells deficient for pRB and p21 were uniquely capable of anchorage-independent growth. In contrast, combined absence of pRB and p21 function was not sufficient to overcome contact inhibition of growth nor for tumor formation in nude mice. Finally, animals with the genotype Rb+/-;p21(-/-) succumbed to tumors more rapidly than Rb+/- mice, suggesting that in certain contexts mutations in these two cell cycle regulators can cooperate in tumor development.

Animals↗
PubMed1998

Mice heterozygous for a mutation at the Nf2 tumor suppressor locus develop a range of highly metastatic tumors.

A role for the membrane/cytoskeleton interface in the development and progression of cancer is established, yet poorly understood. The neurofibromatosis type II (NF2) tumor suppressor gene encodes a member of the ezrin/radixin/moesin (ERM) family of membrane/cytoskeleton linker proteins thought to be important for cell adhesion and motility. We report that in contrast to the narrow spectrum of benign tumors in human NF2 patients, Nf2 heterozygous mice develop a variety of malignant tumors. Using the fact that Nf2 is linked to the p53 tumor suppressor locus in the mouse we have also investigated the effects of genetic linkage of cancer-predisposing mutations on tumorigenesis and examined the genetic pathway to tumor formation involving Nf2 loss. Importantly, we observed a very high rate of metastasis associated with Nf2 deficiency, with or without loss of p53 function, and we provide experimental evidence supporting a role for Nf2 loss in metastatic potential. Together, our results suggest an important role for the NF2 tumor suppressor, and perhaps the ERM family in tumor formation and metastasis.

Animals↗
PubMed1998

Regulation of the neurofibromatosis type 2 tumor suppressor protein, merlin, by adhesion and growth arrest stimuli.

The neurofibromatosis type 2 tumor suppressor gene is inactivated in the development of familial and sporadic schwannomas and meningiomas. The encoded protein, Merlin, is closely related to the Ezrin, Radixin, and Moesin family of membrane/cytoskeletal linker proteins. Examination of Merlin in several cell lines revealed that the protein migrates as two distinct species near 70 kDa. Phosphatase treatment and orthophosphate labeling demonstrated that the species with decreased mobility is phosphorylated. Given Merlin's localization to cortical actin structures, we examined the effect of cell-cell contact or other forms of growth arrest on Merlin expression and post-translational modification. Under conditions of confluency or serum deprivation, the levels of phosphorylated and unphosphorylated Merlin species increased significantly. Cells arrested in G1 by other methods or other phases of the cell cycle did not show changes in Merlin levels. Furthermore, loss of adhesion resulted in a nearly complete dephosphorylation of Merlin, which was reversed upon re-plating of cells, suggesting Merlin phosphorylation may be responsive to cell spreading or changes in cell shape. Thus, the tumor suppressor function of Merlin may involve the regulation of cellular responses to cues such as cell-cell contact, growth factor microenvironment, or changes in cell shape.

3T3 Cells↗
PubMed1998

Potent 3-spiropiperidine growth hormone secretagogues.

Systematic SAR studies of the different regioisomers and homologues of the spiro(indane-1,4-piperidine) moiety in the growth hormone secretagogue L-162,752 are presented. Among them, spiro(3H-1-benzopyran-2,3-piperidine) was found to afford secretagogues with low nanomolar in vitro activity.

Animals↗
PubMed1998

The retinoblastoma gene family: cousins with overlapping interests.

The retinoblastoma tumor suppressor gene (RB1) and its relatives, p107 and p130, encode a family of proteins that share several properties, including the capacity to regulate E2F-dependent transcription and inhibit cell-cycle progression. Although RB1 inactivation is widely implicated in human cancer, the growth regulatory functions of p107 and p130, and the functional relationships within the gene family, are emerging only recently. Here we review studies of RB1 gene family function, with emphasis on in vivo experiments that explore shared and distinct functions within this family.

Animals↗
PubMed1998

Tumor suppressor mutations in mice: the next generation.

Mouse strains carrying tumor suppressor mutations genetically mimic familial forms of human cancer. New tumor suppressors have and will be identified and mutated in the mouse; however, it is clear that future investigation will focus on a new generation of experiments aimed at improving existing models, and using them to delineate the molecular pathways to tumorigenesis and to test the value of rationally designed drug therapies.

Animals↗
PubMed1998

Mutation of E2f-1 suppresses apoptosis and inappropriate S phase entry and extends survival of Rb-deficient mouse embryos.

Mice mutant for the Rb tumor suppressor gene die in mid-gestation with defects in erythropoiesis, cell cycle control, and apoptosis. We show here that embryos mutant for both Rb and its downstream target E2f-1 demonstrate significant suppression of apoptosis and S phase entry in certain tissues compared to Rb mutants, implicating E2f-1 as a critical mediator of these effects. Up-regulation of the p53 pathway, required for cell death in these cells in Rb mutants, is also suppressed in the Rb/E2f-1 double mutants. However, double mutants have defects in cell cycle regulation and apoptosis in some tissues and die at approximately E17.0 with anemia and defective skeletal muscle and lung development, demonstrating that E2F-1 regulation is not the sole function of pRB in development.

Animals↗
PubMed1998

Loss of E2F-1 reduces tumorigenesis and extends the lifespan of Rb1(+/-)mice.

Mutation of the retinoblastoma tumour-suppressor gene (RB) leads to the deregulation of many proteins and transcription factors that interact with the retinoblastoma gene product (pRB), including members of the E2F transcription factor family. As pRB is known to repress E2F transcriptional activity and overexpression of E2F is sufficient for cell cycle progression, it is thought that pRB suppresses growth in part by repressing E2F-mediated transcription. Previously, we reported that loss of E2f1 in mice results in tissue-specific tumour induction and tissue atrophy, demonstrating that E2F-1 normally controls growth both positively and negatively in a tissue-specific fashion. To determine whether E2F-1 deregulation--as a result of loss of pRB--promotes proliferation in vivo, we have tested whether loss of E2f1 interferes with the pituitary and thyroid tumorigenesis that occurs in Rb1(+/-) mice. We have found that loss of E2f1 reduces the frequency of pituitary and thyroid tumours, and greatly lengthens the lifespan of Rb1(+/-); E2f1(-/-) animals, demonstrating that E2F-1 is an important downstream target of pRB during tumorigenesis. Furthermore, loss of E2f1 reduces a previously reported strain-dependent difference in Rb1(+/-) lifespan, suggesting that E2f1 or an E2F-1-regulated gene acts as a genetic modifier between the 129/Sv and C57BL/6 strains.

Animals↗
PubMed1998

RhoA-dependent phosphorylation and relocalization of ERM proteins into apical membrane/actin protrusions in fibroblasts.

The ERM proteins (ezrin, radixin, and moesin) are a group of band 4. 1-related proteins that are proposed to function as membrane/cytoskeletal linkers. Previous biochemical studies have implicated RhoA in regulating the association of ERM proteins with their membrane targets. However, the specific effect and mechanism of action of this regulation is unclear. We show that lysophosphatidic acid stimulation of serum-starved NIH3T3 cells resulted in relocalization of radixin into apical membrane/actin protrusions, which was blocked by inactivation of Rho by C3 transferase. An activated allele of RhoA, but not Rac or CDC42Hs, was sufficient to induce apical membrane/actin protrusions and localize radixin or moesin into these structures in both Rat1 and NIH3T3 cells. Lysophosphatidic acid treatment led to phosphorylation of radixin preceding its redistribution into apical protrusions. Significantly, cotransfection of RhoAV14 or C3 transferase with radixin and moesin revealed that RhoA activity is necessary and sufficient for their phosphorylation. These findings reveal a novel function of RhoA in reorganizing the apical actin cytoskeleton and suggest that this function may be mediated through phosphorylation of ERM proteins.

3T3 Cells↗
PubMed1998

p130 is dispensable in peripheral T lymphocytes: evidence for functional compensation by p107 and pRB.

The proteins encoded by the retinoblastoma gene family, pRB, p107, and p130, have been implicated in the regulation of cellular proliferation, differentiation, and transformation. Because interactions between p130 and E2F transcription factors have been proposed to play a role in the establishment and/or maintenance of quiescence in human peripheral T lymphocytes, we examined lymphoid differentiation and proliferation in p130-deficient mice. We show that p130-/- T cells proliferate normally in culture and exhibit normal cell-mediated immune function in vivo. However, p130-/- T lymphocytes expressed elevated levels of p107, and the characteristic p130-E2F DNA binding complex was replaced by a p107-E2F complex. Adoptive transfer of fetal liver lymphoid progenitors allowed us to circumvent the neonatal lethality associated with loss of p130 and p107 and to analyze the phenotype of p130-/-;p107-/- peripheral T lymphocytes. These cells achieved a quiescent state, exhibited derepression of a subset of E2F target genes, and were hypersensitive to concanavalin A stimulation. Interestingly, a significant portion of the E2F-4 in p130-/-;p107-/- T cells was detected in a complex with pRB and an as-yet-unidentified protein. These findings provide a biochemical basis for functional compensation between pRB family proteins.

Animals↗
PubMed1998

Characterization of the p53-dependent postmitotic checkpoint following spindle disruption.

The p53 tumor suppressor gene product is known to act as part of a cell cycle checkpoint in G1 following DNA damage. In order to investigate a proposed novel role for p53 as a checkpoint at mitosis following disruption of the mitotic spindle, we have used time-lapse videomicroscopy to show that both p53+/+ and p53-/- murine fibroblasts treated with the spindle drug nocodazole undergo transient arrest at mitosis for the same length of time. Thus, p53 does not participate in checkpoint function at mitosis. However, p53 does play a critical role in nocodazole-treated cells which have exited mitotic arrest without undergoing cytokinesis and have thereby adapted. We have determined that in nocodazole-treated, adapted cells, p53 is required during a specific time window to prevent cells from reentering the cell cycle and initiating another round of DNA synthesis. Despite having 4N DNA content, adapted cells are similar to G1 cells in that they have upregulated cyclin E expression and hypophosphorylated Rb protein. The mechanism of the p53-dependent arrest in nocodazole-treated adapted cells requires the cyclin-dependent kinase inhibitor p21, as p21-/- fibroblasts fail to arrest in response to nocodazole treatment and become polyploid. Moreover, p21 is required to a similar extent to maintain cell cycle arrest after either nocodazole treatment or irradiation. Thus, the p53-dependent checkpoint following spindle disruption functionally overlaps with the p53-dependent checkpoint following DNA damage.

Animals↗