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J C Barrett

Publications and source records attributed to J C Barrett.

At least 235 records · Page 13Linked to original sources
PubMed1993

Investigation of the role of G1/S cell cycle mediators in cellular senescence.

Cellular senescence is a state of irreversible cell cycle arrest in which normal cells at the end of their lifespan fail to enter into DNA synthesis upon serum or growth factor stimulation. We examined whether proteins required for G1/S cell cycle progression were irreversibly down-regulated in senescent human fibroblasts. Both the 44- and 42-kDa forms of the MAP-kinase protein were expressed at similar levels in young and senescent cells. In contrast to young cells where both forms were phosphorylated on tyrosine in response to serum, the p42MAP-kinase was not tyrosine phosphorylated upon serum stimulation, whereas p44MAP-kinase was phosphorylated on tyrosine in serum-starved or serum-stimulated senescent cells. Examination of p53 protein in growing, quiescent, and senescent cells revealed no significant differences in levels between the different growth states. In contrast, cdk2 and cyclin A mRNAs were completely down-regulated in stimulated senescent fibroblasts, while the G1 cyclins, C, D1, and E mRNAs, were still expressed in stimulated senescent cells although at reduced levels compared to young cells. The expression of early G1 markers, but not late G1 markers, indicates that senescent cells may be blocked at a point in late G1. We investigated whether transfection of cyclin A, alone or in combination with cdc2, was sufficient for extension of lifespan or escape from senescence. Clones expressing the transfected human cyclin A or cdc2 genes senesced at a population doubling similar to controls, thereby showing that cyclin A or cdc2 expression alone was insufficient for escape from senescence.

Base Sequence↗
PubMed1993

Genetic and molecular basis for cellular senescence.

Normal human and rodent cells in culture exhibit a finite life span at the end of which they exhibit morphological changes and cease proliferating, a process termed cellular senescence or cellular aging. Many cancer cells differ from normal cells in that they do not senesce and have an indefinite life span in culture, suggesting that alterations relating to cellular senescence are involved in the neoplastic evolution of tumor cells. Recent experimental results strongly support a genetic basis for cellular senescence. Defects in the senescence program in transformed cells can be corrected by introduction of a specific chromosome from normal cells into the abnormal cells. Using this approach, possible senescence genes have been mapped to specific chromosomes. Cell cycle control genes that regulate entry into the DNA synthetic phase of the cell cycle must be altered in senescent cells. Recent findings suggest that phosphorylation of the retinoblastoma gene is altered in senescent cells. It is possible, but not yet proven, that aging at the cellular level contributes to the aging of the individual. Therefore, an understanding of cellular senescence at the genetic and molecular levels may provide new insights into both the cancer and aging processes.

Animals↗
PubMed1993

Inactivation of a tumor suppressor function in immortal Syrian hamster cells by N-methyl-N'-nitro-N-nitrosoguanidine and by 5-aza-2'-deoxycytidine.

Clonal lines of immortal Syrian hamster cells were previously isolated that either suppressed (supB+) tumorigenicity in hybrids with a malignant hamster cell line (BP6T) or had lost this suppression ability (supB-). Neither line was tumorigenic or showed anchorage-independent growth in normal growth medium. SupB- cells, but not supB+ cells, grew in agar supplemented with the growth factors EGF, PDGF and insulin (EPI), providing a selective assay for the supB- phenotype. After treatment of supB+ cells with either N-methyl-N'-nitro-N-nitrosoguanidine (10-300 ng/ml) or 5-aza-2'-deoxycytidine (25-250 ng/ml), and an expression period of 4-8 weeks, a dose-dependent increase in altered cells that grew in agar supplemented with EPI was observed. Cell lines derived from colonies in agar showed persistent EPI-stimulated growth in agar, and decreased suppression of growth in agar for hybrids with BP6T cells. Thus, carcinogen-induced loss of the tumor suppressor phenotype has been demonstrated.

Animals↗
PubMed1993

Differential growth control of normal, preneoplastic and neoplastic Syrian hamster embryo cells in serum-free media.

The objective of this work was to develop a serum-free medium that supported the growth of secondary Syrian hamster embryo (SHE) cells suitable for use in the morphological transformation assay (MTA). The current assay employs high levels of fetal bovine serum, each lot of which must be selected for its efficacy in the assay. To circumvent the problems associated with the use of serum, two serum-free media (CGM-4 and CGM-9) were developed. The approach used was to replace serum with more defined hormones and growth factors. DME/F12 with low NaHCO3 (pH 6.7) was chosen as a basal nutrient medium since this pH has been reported to give enhanced transformation. Both serum-free media supported the clonal high density growth of normal secondary SHE cells. These media were also found to support the growth of several established SHE cell lines exhibiting successive stages of neoplastic progression. However, significant differential responses of the individual cell lines to CGM-4 and CGM-9 were observed, whereas, in serum-supplemented medium all three lines responded similarly. These results raise the hope that CGM-4 and CGM-9 will be useful for studies of altered autocrine function during transformation and progression as well as for use in the transformation assay itself.

Animals↗
PubMed1993

Ability of peroxisome proliferators to induce cell transformation, chromosome aberrations and peroxisome proliferation in cultured Syrian hamster embryo cells.

Di(2-ethylhexyl)phthalate (DEHP), a commonly used plasticizer, induces proliferation of peroxisomes in liver cells and causes hepatocellular carcinomas when chronically administered in the diet to rodents. To examine possible mechanisms for DEHP-associated cancer, we have measured induction of morphological transformation, chromosome aberrations and peroxisome proliferations of cultured Syrian hamster embryo (SHE) cells by DEHP and other peroxisome proliferators. Morphological transformation of SHE cells was weakly induced by treatment for 48 h with DEHP and its metabolite mono(2-ethylhexyl)phthalate (MEHP). The transformation frequency by DEHP was enhanced by exogenous metabolic activation using rat liver postmitochondrial supernatants. Treatment for 24 h with DEHP resulted in chromosome aberrations of the cells only in the presence of exogenous metabolic activation. 2-(p-chlorophenoxy)-2-methylpropionic acid ethyl ester (clofibrate), a widely used hypolipidemic drug, failed to induce morphological transformation or chromosome aberrations of SHE cells. Treatment with [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio] acetic acid (WY-14,643), which is a more potent carcinogen than DEHP or clofibrate, elicited a lower frequency of morphological transformation than DEHP in the presence of exogenous metabolic activation but was more active than DEHP at inducing chromosome aberrations. Similar levels of peroxisome proliferation, as determined by an intensity of diaminobenzidine staining, were observed in cultures treated for 2 h with DEHP, MEHP, clofibrate or WY-14,643. These results suggest a possible involvement of genetic damage by DEHP metabolites in the induction of cell transformation of SHE cells by DEHP; however, no clear relationship among induction of peroxisome proliferation, carcinogenicity in vivo and cell transformation was observed. Although the ability to induce cell transformation and chromosomal mutations is not adequate to explain the carcinogenicity of this class of compounds, these biological effects may be contributory to the carcinogenic activities of peroxisome proliferators.

Animals↗
PubMed1993

Mechanisms of multistep carcinogenesis and carcinogen risk assessment.

Many different types of chemical exposures can increase the incidence of tumors in animals and humans, but usually a long period of time is required before the carcinogenic risk of an exposure is manifested. Both of these observations can be explained by a multistep/multigene model of carcinogenesis. In this model, a normal cell evolves into a cancer cell as the result of heritable changes in multiple, independent genes. The two-stage model of initiation and promotion for chemical carcinogenesis has provided a paradigm by which chemicals can act by qualitatively different mechanisms, but the process of carcinogenesis is now recognized as more complex than simply initiation and promotion. Even a three-stage model of initiation, promotion, and progression, which can be operationally defined, is not adequate to describe the carcinogenic process. The number of genes altered in a cancer cell compared to a normal cell is not known; recent evidence suggests that 3-10 genetic events are involved in common adult malignancies in humans. Two distinct classes of genes, protooncogenes and tumor-suppressor genes, are involved in the cancer process. Multiple oncogenes may be activated in a tumor, while multiple tumor-suppressor genes may be inactivated. Identification of the genes involved in carcinogenesis and elucidation of the mechanisms of their activation or inactivation allows a better understanding of how chemical carcinogens influence the process of neoplastic evolution. The findings of multiple genetic changes (including point mutations, chromosomal translocations, deletions, gene amplification, and numerical chromosome changes) in activated protooncogenes and inactivated tumor-suppressor genes provide experimental support for Boveri's somatic mutation theory of carcinogenesis. In addition to mutagenic mechanisms, chemicals may heritably alter cells by epigenetic mechanisms and enhance the clonal expansion of altered cells. Most chemical carcinogens operate via a combination of mechanisms, and even their primary mechanism of action may vary depending on the target tissues. The classification of chemicals by mechanism of action or by nongenotoxic or genotoxic activity has certain inherent difficulties because no classification of chemicals is exhaustive or definitive.

Animals↗
PubMed1993

Cell proliferation and chemical carcinogenesis: symposium overview.

Cancer, by definition, is a proliferative disease. The fundamental scientific issue explored at the international symposium "Cell Proliferation and Chemical Carcinogenesis" was the impact of chemically enhanced cell proliferation on the dynamic carcinogenic processes. This conference, held at the National Institute of Environmental Health Sciences January 14-16, 1992, provided an open forum for the exchange of new results, information, and ideas in four areas: a) general principles of cell division and carcinogenesis, b) critical evaluation of cell proliferation methodologies, c) cell proliferation and modeling of organ-specific carcinogenesis, and d) cell proliferation and human carcinogenesis. This overview summarizes key findings from that symposium. The general view expressed was that although cell proliferation is involved inextricably in the development of cancers, chemically enhanced cell division does not reliably predict carcinogenicity. Our knowledge of the multistep nature of carcinogenesis has advanced substantially during recent years; however, much still needs to be learned. A greater understanding of the cellular and molecular events in chemical carcinogenesis should improve all aspects of the overall risk assessment process, including extrapolations based on dose, species, and interindividual differences.

Animals↗
PubMed1993

Cell cycle controls: potential targets for chemical carcinogens?

The progression of the cell cycle is controlled by the action of both positive and negative growth regulators. The key players in this activity include a family of cyclins and cyclin-dependent kinases, which are themselves regulated by other kinases and phosphatases. Maintenance of balanced cell cycle controls may be directly linked to genomic stability. Loss of the check-points involved in cell cycle control may result in unrepaired DNA damage during DNA synthesis or mitosis leading to genetic mutations and contributing to carcinogenesis.

Animals↗
PubMed1993

Coordinate regulation of collagen II(alpha 1) and H19 expression in immortalized hamster cells.

Loss of tumor suppressor gene function is essential in the multistep progression of cells to neoplasia. Immortalized cells were established by carcinogen treatment of Syrian hamster embryo cells. At early passages, these nontumorigenic cells retained the ability to suppress tumorigenicity in cell hybrids with malignant cells. Upon passage and subcloning of these suppressor-positive (supB+) cells, variant clones that had lost tumor suppressor activity were isolated. These suppressor-negative (supB-) clones remained nontumorigenic. The mRNAs encoding collagen II(alpha 1a), a chondrocyte differentiation marker, and H19, a developmentally controlled gene, were more abundant in supB+ cells than in supB- cells. Nuclear run-on analysis indicated that the transcription of these genes is differentially regulated. Transient transfection experiments revealed that a cis-acting element in the rat collagen II 5' flanking sequences directs differentially regulated transcription. Gel retention analysis demonstrated the presence of a nuclear DNA-binding factor(s) that specifically recognizes a DNA sequence common to both the rat collagen II sequences and the mouse H19 enhancer. In one set of clones, transcriptional regulation could account for differential collagen II and H19 expression in supB+ and supB- cells. In another set of clones, posttranscriptional controls are responsible for the decreased expression of these genes in supB- cells. The emergence of two independent mechanisms that cause differential expression of collagen II and H19 related to tumor suppressor loss suggests that coordinate regulation of these genes, or others regulated by common mechanisms, may be important in tumor suppression.

Animals↗
PubMed1992

Localization of metastasis suppressor gene(s) for prostatic cancer to the short arm of human chromosome 11.

Previous studies using somatic cell hybridization of highly metastatic and nonmetastatic rat prostatic cancer cells demonstrated that the resultant hybrids were nonmetastatic if all of the parental chromosomes were retained. Somatic hybrid segregants which underwent nonrandom chromosomal losses reexpressed high metastatic ability. These results demonstrated that there are gene(s) the expression of which can suppress metastatic ability of prostatic cancer cells. To identify the location of homologous gene(s) in the human, specific human chromosomes were introduced into highly metastatic rat prostatic cancer cells using the microcell-mediated chromosome transfer. Introduction of human chromosome 11 into highly metastatic rat prostate cancer cells results in suppression of metastatic ability without suppression of the in vivo growth rate or tumorigenicity of the hybrid cells. Spontaneous deletion of portions of human chromosome 11 in some of the clones delineated the minimal portion of human chromosome 11 capable of suppressing prostatic cancer metastases as the region between 11p11.2-13 but not including the Wilms' tumor-1 locus.

Animals↗
PubMed1992

Modulation of the epidermal growth factor mitogenic response by metabolites of linoleic and arachidonic acid in Syrian hamster embryo fibroblasts. Differential effects in tumor suppressor gene (+) and (-) phenotypes.

Specific metabolites of arachidonic and linoleic acid have been proposed as serving a regulatory function in growth factor signal transduction in fibroblasts. In studies with Syrian hamster embryo (SHE) fibroblasts, we found lipoxygenase inhibitors to be potent blockers of epidermal growth factor (EGF)-dependent mitogenesis. Analytical chemical characterization of arachidonic and linoleic acid metabolism in SHE cells demonstrated that the major lipoxygenase product was 13-hydroxyoctadecadienoic acid (HODE). EGF stimulation of quiescent SHE cells resulted in an enhancement of HODE biosynthesis. The primary arachidonate products were prostaglandin E2 and F2 alpha formed via the cyclooxygenase pathway. Inhibition of cyclooxygenase activity did not alter the EGF-mitogenic response in SHE cells. Addition of lipoxygenase-derived linoleate metabolites (10(-10)-10(-6) M) produced a 2-4-fold potentiation of EGF-stimulated [3H]thymidine incorporation in SHE cells. Interestingly, the linoleate products did not enhance the EGF mitogenic effect in variant SHE cells that had lost tumor suppressor gene function. These results were confirmed by autoradiographic studies of DNA synthesis and suggest that loss of tumor suppressor phenotype correlates with a lack of responsiveness to linoleate products in signal transduction. In studies on the mechanism of EGF regulation of linoleic acid metabolism, inhibitors of EGF receptor tyrosine kinase activity were observed to block EGF-stimulated HODE biosynthesis. In addition, both cyclohexamide and actinomycin D attenuated the ability of EGF to increase linoleic acid metabolism in SHE cells. EGF induction of the linoleate pathway appears to be linked to activation of the EGF receptor and may be modulated at transcriptional or translational levels.

Animals↗
PubMed1992

Detection of frequent allelic loss on proximal chromosome 17q in sporadic breast carcinoma using microsatellite length polymorphisms.

Analyses of losses of heterozygosity and linkage studies have implicated a gene(s) on chromosome 17q in the genesis of sporadic and early-onset familial breast carcinomas, respectively. To define the critical region of 17q, we examined DNAs from a series of 20 sporadic breast carcinomas and corresponding blood samples for allelic losses of chromosome 17q using microsatellite length polymorphisms. With these highly informative markers (average heterozygosity, 0.73), we observed frequent deletions of 17q at several loci. We found that D17S250 was deleted in 50% (7 of 14), THRA1 in 79% (11 of 14), D17S579 in 59% (11 of 19), NME1 in 29% (5 of 17), MPO in 36% (4 of 11), and GH in 25% (4 of 16) in the tumor set examined. A common region of deletion was found that was flanked by D17S250 to D15S579. These markers have recently been localized to a 6-cM interval of proximal chromosome 17q in bands 17q11.2-q21 and map within the region of the early-onset familial breast cancer locus, implying that the same gene or genes may be involved in both sporadic and familial breast tumors. Thyroid hormone receptor alpha and retinoic acid receptor alpha are two potential candidate genes in this region.

Base Sequence↗
PubMed1992

Possible cellular and molecular mechanisms for asbestos carcinogenicity.

Asbestos fibers may exert their carcinogenic effects on mesothelial cells and bronchial epithelial cells by direct and indirect mechanisms. Direct effects can occur following the physical interaction of fibers with target cells or by the generation of free radicals from the fiber surface; indirect effects, following the interaction of fibers with inflammatory cells can result in the production of cellular mediators such as cytokines and various reactive oxygen species. As a result, target cells may be induced to proliferate and/or sustain genetic alterations, which lead to tumor development.

Animals↗
PubMed1992

Characterization of the Syrian hamster c-Ha-ras gene and intron-D-exon transcript.

The coding sequences as well as 5'- and 3'-flanking sequences of the Syrian hamster c-Ha-ras gene were deduced from cDNA clones derived from embryo fibroblast cell lines. Sequences of introns B, C, and D were obtained from genomic DNA after amplification by the polymerase chain reaction. Sequence comparisons with rat, mouse, and human c-Ha-ras genes revealed a high degree of homology. One of 12 cDNA clones contained intron-D-exon (IDX) sequences due to alternative splicing that would encode a p19 Ha-ras gene product. Conservation between species suggests a functional role for the IDX, possibly as a negative control of p21 Ha-ras expression.

Amino Acid Sequence↗
PubMed1992

Growth and transformation suppressor genes for BHK Syrian hamster cells on human chromosomes 1 and 11.

To map putative tumor suppressor genes for the near-diploid baby hamster kidney fibrosarcoma cell line BHK, we transferred five different normal human chromosomes (1, 3, 7, 11, and 12) into these tumor cells by microcell-mediated chromosome transfer. Transfer of human chromosome 1 into BHK cells resulted in suppression of cell growth both on plastic and in soft agar, indicating that chromosome 1 has a generalized effect on cell growth and thereby suppresses anchorage-independent growth. Selection against cells with an intact chromosome 1 was observed. In contrast, the introduction of chromosome 11 into BHK cells resulted in suppression of anchorage independence but not growth on plastic. Most chromosome-11 growth-suppressed BHK hybrids retained intact copies of human chromosome 11. Tumorigenic derivatives of chromosome 11 hybrids had lost this chromosome. Transfer of human chromosome 3, 7, or 12 into BHK cells did not correlate with growth suppression of BHK cells on plastic or in soft agar. Thus, we conclude that genes that suppress BHK-cell growth in general or in agar reside on human chromosomes 1 and 11, respectively.

Animals↗
PubMed1992

Complementation of a DNA repair deficiency in six human tumor cell lines by chromosome 11.

Human tumor cells, after x-irradiation during the G2 phase of the cell cycle, show an abnormally high frequency of persistent chromatid breaks and gaps resulting from deficient DNA repair. Addition of a single human chromosome 11 from normal fibroblasts by micro-cell fusion to cell lines from six different tumors resulted in efficient repair of the radiation-induced damage to the level in normal cells. For one of the cell lines, addition of the long arm of chromosome 11 was sufficient to restore repair efficiency. In four of the six tumor lines, restoration of efficient DNA repair by chromosome 11 was associated with tumor suppression in nude mice. These results suggest that chromosome 11 carries a DNA repair gene or genes that complement the repair deficiency of tumor cells and that this gene for at least one tumor is localized to the long arm.

Carcinoma, Renal Cell↗