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Biomedical subjects

A T Vaughan

Publications and source records attributed to A T Vaughan.

At least 19 recordsLinked to original sources
PubMed2001

Apoptotic triggers initiate translocations within the MLL gene involving the nonhomologous end joining repair system.

Translocations involving the MLL gene at 11q23 are a frequent finding in therapy-related leukemia and are concentrated within a short, 8.3-kb tract of DNA, the breakpoint cluster region. In addition, a specific site adjacent to exon 12 within this region of MLL is cleaved in cells undergoing apoptosis. We show here, using human TK6 lymphoblastoid cells, that irradiation and the apoptotic trigger anti-CD95 antibody are each able to initiate translocations at the MLL exon 12 cleavage site. The translocation junctions produced contain regions of microhomology consistent with operation of the nonhomologous end joining (NHEJ) repair process. Participation of the NHEJ process is supported by the identification of the NHEJ component DNA-PKcs at the site of apoptotic cleavage. Suppression of DNA-PKcs function by the phosphatidylinositol 3-kinase inhibitor wortmannin compromises DNA end joining, increases site-specific cleavage within MLL, and eliminates MLL-restricted translocations. We propose that activation of apoptotic effector nucleases alone is sufficient to generate proleukemogenic translocations and raises the possibility that some of these may persist in cells that evade apoptotic execution and survive.

Apoptosis↗
PubMed2001

Steroid-mediated inhibition of radiation-induced apoptosis in C4-1 cervical carcinoma cells is p53-dependent.

In human papillomavirus (HPV) infected cervical epithelial cells the synthetic steroid dexamethasone inhibits radiation-induced apoptosis and increases the transcription of HPV E6/E7, enhancing p53 degradation. The aim of this study was to determine if suppression of apoptosis was mechanistically linked to changes in p53. HPV 16 E6 or E6/E7 expression vectors were transiently transfected into C4-1 HPV 18-positive cervical carcinoma cells to mimic the enhanced transcription following steroid treatment. After irradiation, apoptosis was suppressed in these cells comparable to the effect observed after steroid treatment alone. To confirm whether loss of p53 was responsible for the inhibition of apoptosis, residual p53 in C4-1 cells was targeted by stable transfection with a dominant-negative p53 mutant. While radiation-induced apoptosis increased after mutant transfection, inhibition of programmed cell death by steroid treatment was either eliminated or substantially reduced. Steroid-dependent inhibition of radiation-induced apoptosis in carcinoma of the cervix involves E6 modulation of p53 expression and may adversely affect treatment.

Anti-Inflammatory Agents↗
PubMed2000

LINE L1 retrotransposable element is targeted during the initial stages of apoptotic DNA fragmentation.

Using a directional cloning strategy, DNA sequence information was obtained corresponding to the site of early radiation-induced apoptotic DNA fragmentation within the human lymphoblastoid cell line TK6. Data were obtained from 88 distinct clones comprising approximately 65 kbp of sequenced material. Analysis of all cloned material showed that sequences in the 10 bp immediately adjacent to the cleavage sites were enriched in short oligoT tracts. The proportion of repetitive DNA within the entire cloned material was found to be within the normal range. However the distribution of Alu and LINE repetitive DNA were biased to positions at or adjacent to the apoptotic cleavage site. In particular, a non-random distribution of five cleavage sites was found clustered within the second ORF of the LINE L1 that partially overlapped with two binding sites for the nuclear matrix-associated protein SATB1. Three other clones, containing alpha satellite elements, were also linked to a DNA matrix binding function. These data indicate that the site of chromatin loop formation at the nuclear matrix may be a specific target for early DNA fragmentation events during apoptosis.

Apoptosis↗
PubMed2000

P53 overexpression is associated with bulky tumor and poor local control in T1 glottic cancer.

PURPOSE: To study the role of two possible prognostic factors, p53 and tumor bulk, and their interaction with other tumor and treatment variables in early-stage laryngeal cancer patients treated with curative radiotherapy. METHODS: One hundred two patients with T1N0M0 squamous cell carcinoma of the glottic larynx treated with definitive radiotherapy were analyzed. p53 status in pretreatment biopsy specimens was assessed by immunohistochemistry (IHC) using mouse monoclonal antibody DO-7. Tumors were classified as small surface lesions or bulky tumors. All tumor-related and treatment-related variables which might influence the outcome were analyzed. Local control after definitive radiotherapy was the end point of the study. RESULTS: The local control at 5 years for the entire group of patients was 78% (80/102) and 91% (93/102) after surgical salvage. p53 overexpression by IHC was seen in 37% (38/102) of patients. Tumors were classified as small volume in 69 (68%) and bulky in 33 (32%) patients. Five-year local control was 48% for p53-positive patients as compared to 94% for p53-negative patients (p = 0.0001). Tumor bulk was the other important prognostic factor, with 5-year local control of 91% for small tumors and 48% for bulky tumors (p = 0.0001). Patients who had both p53 positivity and bulky tumors did worse, with a 5-year local control of 23% as compared to 92% for all other groups combined (p = 0.0001). Among other variables, only the length of radiation time was of borderline significance. CONCLUSION: Both p53 overexpression and tumor bulk are independent prognostic factors for local control in early-stage glottic cancer treated with curative radiotherapy. The precise relationship between a genetic event, the p53 mutation, and an observable phenotype expression such as tumor bulk needs to be further defined.

Aged↗
PubMed2000

RU486 increases radiosensitivity and restores apoptosis through modulation of HPV E6/E7 in dexamethasone-treated cervical carcinoma cells.

OBJECTIVE: Cervical carcinoma tumors containing radioresistant cells are associated with decreased local control and survival. Therefore, strategies to increase cell kill during radiotherapy have a clear rationale. It was previously determined that treatment with the corticosteroid dexamethasone increased radioresistance and decreased apoptosis in C4-1 cervical carcinoma cells. The goal of this study was to determine whether hormone antagonists, specifically Mifepristone (RU486), could reverse the effects of dexamethasone on clonogenic survival and apoptosis following gamma-irradiation. METHODS: Cervical carcinoma cell line C4-1 cells were exposed to 1 microM dexamethasone in the presence or absence of 1 microM Mifepristone (RU486), a hormone antagonist, and irradiation. Cells were analyzed for steroid-dependent HPV E6/E7 mRNA expression (by Northern blot analysis), clonogenic survival, and apoptosis (by Annexin V staining and the DNA fragmentation assay). In addition, p53 protein levels were determined by Western blot analysis. RESULTS: The hormone antagonist RU486 reversed dexamethasone-dependent upregulation of E6/E7 mRNA and restored radiation-induced p53 expression, apoptosis, and clonogenic survival to levels similar to those observed following irradiation alone. CONCLUSION: RU486 reverses glucocorticoid-dependent upregulation of HPV E6/E7, which corresponds to restoration of p53 expression, and restores radiosensitivity and apoptosis following gamma-irradiation. Therefore, it appears that along with radiation, RU486 may be a beneficial agent in the treatment of hormone-reactive cervical tumors.

Apoptosis↗
PubMed2000

Inhibition of radiation-induced apoptosis by dexamethasone in cervical carcinoma cell lines depends upon increased HPV E6/E7.

Through a glucocorticoid-responsive promoter, glucocorticoids can regulate the transcription of the human papillomavirus (HPV) E6 and E7 viral genes which target the tumour suppressor proteins p53 and Rb respectively. In C4-1 cells, the glucocorticoid dexamethasone up-regulated HPV E6/E7 mRNA and decreased radiation-induced apoptosis. In contrast, dexamethasone had no effect on apoptosis of cells that either lack the HPV genome (C33-a) or in which HPV E6/E7 transcription is repressed by dexamethasone (SW756). Irradiated C4-1 cells showed increased p53 expression, while dexamethasone treatment prior to irradiation decreased p53 protein expression. In addition, p21 mRNA was regulated by irradiation and dexamethasone in accordance with the observed changes in p53. Overall, glucocorticoids decreased radiation-induced apoptosis in cervical carcinoma cells which exhibit increased HPV E6/E7 transcription and decreased p53 expression. Therefore, in HPV-infected cervical epithelial cells, p53-dependent apoptosis appears to depend upon the levels of HPV E6/E7 mRNA.

Antineoplastic Agents, Hormonal↗
PubMed1999

Abortive apoptosis as an initiator of chromosomal translocations.

Apoptosis is a well-recognized regulator of a cell populations size and structure. Irreversible stages of apoptosis lead to activation of different enzymatic cascades, changes in cell morphology and DNA fragmentation. However, little is known about nuclear events which accompany the initial stages of apoptosis. These events are connected with introduction of limited amounts of double strand breaks into genomic DNA, some of which may be subsequently rejoined. We hypothesize here that the initial stages of apoptotic DNA fragmentation may be reversible and connected with the initiation of recombinational events and certain chromosomal translocations. The factors influencing apoptosis reversibility and cell survival after delivery of apoptotic stimuli may provide new insights into mechanisms of lymphocyte development and tumorigenesis.

Apoptosis↗
PubMed1998

Second primary tumors in laryngeal cancer: results of long-term follow-up.

PURPOSE: The development of second primary tumors (SPTs) is the most important factor determining the survival in early-stage head and neck cancer patients, whose first tumor has been successfully treated. New methods of examining genetic changes have raised doubts about the validity of the widely held field cancerization hypothesis as the cause of SPTs, and an alternative hypothesis of monoclonal origin has been proposed. The objectives of this study were to look at the pattern of development of SPTs and the possible factors influencing the incidence of SPTs and the survival in early-stage laryngeal cancer with long-term follow-up. METHODS AND MATERIALS: One hundred forty-four consecutive patients of T1N0M0 squamous cell carcinoma of the true vocal cord treated with definitive radiotherapy between 1976 and 1992 were analyzed. The incidence, time to development, and survival of aerodigestive and other SPTs were noted. p53 overexpression indicating a mutated p53 gene was analyzed by immunohistochemistry. RESULTS: With a median follow-up of 6 years (range 2-20 years), 42 patients developed a SPT, 24 in upper aerodigestive tract and lung and 18 at other sites. The actuarial incidence of developing a SPT at 5, 10, and 15 years was 23%, 44%, and 48.7% respectively. The median time interval for development of SPT in an upper aerodigestive tract was 21 months as opposed to 50 months for other sites (p = 0.02). The most common sites of SPTs included lung for upper aerodigestive tract; and prostate, followed by colon, for other sites. The actuarial risk of developing a nonaerodigestive SPT at 5 and 10 years was 35% and 55% respectively. p53 status affected neither the incidence of SPT nor the survival. SPTs were the leading cause of death in these early-stage laryngeal cancer patients. CONCLUSION: The origin of SPTs seems to be multifactorial, involving both the field cancerization effect and an increased baseline genetic predisposition. Until more reliable genetic markers are developed, chemoprevention remains the best treatment option at preventing SPTs in these early-stage patients.

Adult↗
PubMed1998

Association between DNA cleavage during apoptosis and regions of chromatin replication.

We have addressed the association between the site of DNA cleavage during apoptosis and DNA replication. DNA double strand breaks were introduced into chromatin containing pulse labeled nascent DNA by the induction of apoptosis or autocleavage of isolated nuclei. The location of these breaks in relation to nascent DNA were revealed by Bal31 exonuclease digestion at the cut sites. Our data show that Bal31 accessible cut sites are directly linked to regions enriched in nascent DNA. We suggest that these regions coincide with the termini of replication domains, possibly linked by strong DNA-matrix interactions with biophysically defined topological structures of 0.5-1.3 Mbp in size. The 50 kbp fragments that are commonly observed as products of apoptosis are also enriched in nascent DNA within internal regions but not at their termini. It is proposed that these fragments contain a subset of replicon DNA that is excised during apoptosis through recognition of their weak attachment to the nuclear matrix within the replication domain.

Apoptosis↗
PubMed1998

Is p53 an independent prognostic factor in patients with laryngeal carcinoma?

BACKGROUND: Carcinoma of the true vocal cord represents the earliest clinically recognizable invasive malignancy in the head and neck region and provides a unique model for studying possible prognostic genetic markers. The aim of this study was to determine whether p53 overexpression correlated with tumor recurrence in a homogenous population of patients with early stage glottic carcinoma treated with radiotherapy alone. METHODS: One hundred and fourteen patients with T1N0M0 squamous cell carcinoma of the glottis were treated with curative radiotherapy between 1976 and 1990. With a median follow-up of 6 years, actuarial local control was 80% with 23 local recurrences. Laryngeal biopsy specimens obtained prior to radiation therapy were analyzed retrospectively in 22 patients. Forty-five patients with local control were used as a control group. p53 overexpression indicating a mutated p53 gene was analyzed by immunohistochemistry using the mouse monoclonal antibody D0-7. RESULTS: Approximately 82% of carcinomas that recurred locally expressed p53 compared with only 29% of those with local control (P < 0.001). No significant relation was noted between p53 expression and histologic grade. Intensity of staining did not predict tumor recurrence. CONCLUSIONS: The authors believe that this case-controlled study demonstrated the role of p53 as an independent prognostic factor in patients with early stage glottic carcinoma.

Actuarial Analysis↗
PubMed1998

Mycoplasma infection can sensitize host cells to apoptosis through contribution of apoptotic-like endonuclease(s).

Mycoplasma infection may lead to various pathologies in a broad range of hosts. It has been shown that Mycoplasma may trigger cell death in cell cultures; however, the mechanism remains unknown. In the present paper we show that Mycoplasma infection of different lymphocyte and epithelial tumour cell lines leads to the inhibition of proliferation, and increased cell death, accompanied by DNA fragmentation and the morphological features of apoptosis. We also showed that this infection leads to an increased sensitivity of cells to various inducers of apoptosis targeting different signalling pathways. Finally, we show that increased apoptosis is associated with overexpression of an endonuclease produced by Mycoplasma. This endonuclease is recovered in the nuclear fraction of host cells, introduces mostly DSB and is active at neutral pH in the presence of divalent cations. Activation of this endonuclease is connected with limited proteolysis, which may be reproduced in vitro by snake venom serine proteinase.

Animals↗
PubMed1998

Mechanisms of induction of apoptotic DNA fragmentation.

PURPOSE: Despite its common use as an indicator of apoptosis, little is known about the mechanisms controlling apoptotic DNA fragmentation in irradiated cells. This review discusses the pathways of chromatin fragmentation, and the role of both nucleases and chromatin structure in this process. DEFINITIONS: DNA fragmentation linked to apoptosis is a combination of cleavage events excising both large DNA fragments within the range 0.4-1.0 Mbp and 50 kbp followed by random cuts within internucleosomal regions (i.e. DNA laddering). The first two cleavage steps can be detected in virtually all apoptotic cells, but DNA laddering is not ubiquitously observed. Endonucleases that mediate this cleavage of chromatin may be classified by substrate specificity, mode of DNA cleavage and their cofactor requirements. CONCLUSIONS: Three major pathways of DNA fragmentation are proposed and discussed: (1) upregulation of endonucleases, (2) their intranuclear/intracellular redistribution and (3) primary changes of chromatin structure.

Animals↗
PubMed1998

Synchronous block in DNA synthesis initiation with change in chromatin topology mediated by VP16.

The impact of chromatin topology on the DNA synthetic process was studied in the human squamous-cell carcinoma cell line SQ-20B. A 1-h exposure < or = 10 microM VP16 produced an increase in DNA supercoil tension, measured by recording laser light scatter from salt-extracted nuclei. This change was precisely paralleled by a decrease in DNA synthesis. The effects on both DNA supercoiling and DNA synthesis were suppressed at VP16 concentrations between 10 and 20 microM. The changes in DNA supercoiling and synthesis at VP16 concentrations -10 microM were eliminated by coincubation with mimosine, a DNA synthesis initiator poison. We conclude that brief exposure to low concentrations of VP16 disturbs the balance of torsional energy within discrete replicon domains by affecting normal topoisomerase II activity at sites of replication initiation. The resultant increase in negative supercoil tension mediates a topologic checkpoint, limiting the initiation of DNA synthesis. Such a checkpoint may be a common pathway for control, both during the normal replicative cycle and subsequent to DNA damage.

Cell Nucleus↗
PubMed1998

Transfection of a vector expressing wild-type p53 into cells of two human glioma cell lines enhances radiation toxicity.

Replication-deficient adenovirus (Adv5)-based vectors containing either wild-type p53 or the beta-gal marker gene were introduced into cells of the T98G (p53 mutant) and U87MG (p53 wild-type) human glioma cell lines. The wild-type p53 gene was successfully expressed in each cell line as shown by flow cytometry and Western blotting. The presence of the p53-expressing vector was toxic in both cell lines compared to control cells or to those containing the beta-gal vector. At levels of Adv5p53 vector that produced detectable toxicity, the effect of irradiation was enhanced, producing a twofold increase in cell killing. In the T98G cells, the presence of the p53 vector resulted in an increase in the number of cells undergoing apoptosis after irradiation, whereas a smaller and only additive response was observed in the U87MG cells. Conversely, an increase in micronucleus formation, indicating corrupt mitotic activity, was observed in irradiated Adv5p53-positive U87MG cells but not in T98G cells. These data suggest that p53-expressing vectors effectively enhance radiation lethality in these human glioma cell lines, but that the mechanism of action cannot be simply related to activation of the p53-dependent pathway to apoptosis.

Apoptosis↗
PubMed1997

Differential effects of BCL-2 on survival and proliferation of human B-lymphoma cells following gamma-irradiation.

Bcl-2 can inhibit apoptosis induced by a variety of stimuli, including radiation and its presence in tumour cells would be expected to indicate poor prognosis. Bcl-2-expressing tumours, however, are often low-grade and highly responsive to therapy. To investigate this apparent paradox, we analysed in vitro the responses of Burkitt lymphoma (BL) cells to gamma-irradiation in the presence and absence of Bcl-2. High-level expression of Bcl-2 was shown to promote BL cell survival following irradiation. However, a significant proportion of Bcl-2-rescued cells subsequently underwent apoptosis after an extended period in culture. In addition, in different BL lines, Bcl-2 was found either to promote or to inhibit long-term proliferative activity following gamma-irradiation. This differential regulation of proliferation correlated both with differential effects of Bcl-2 on the cell cycle and with differences in p53 status. Thus, by one week after irradiation, BL cells expressing only wild-type p53 (wt/wt) had arrested in G1, whereas those with a mutant allele (wt/mu) were arrested in all phases of the cell cycle. The proportion of Bcl-2-rescued cells that subsequently underwent apoptosis was reduced by ligation of CD40 at the time of irradiation in wt/wt BL cells, but not in wt/mu cells. CD40-ligation reduced both G1-arrest and apoptosis in parallel. These results indicate that, whilst Bcl-2 can delay apoptosis in BL cells following gamma-irradiation, the protein can also cause growth-arrest and thereby promote apoptosis. Long-term survival following Bcl-2-mediated rescue of gamma-irradiated cells may depend on p53 status and require additional death-repressing or growth-promoting signals.

CD40 Antigens↗
PubMed1997

Topologically constrained domains of supercoiled DNA in eukaryotic cells.

The size of supercoiled, topologically constrained DNA domains within the squamous carcinoma cell line SQ-20B were determined by direct comparison with a panel of irradiated supercoiled plasmid DNAs. Loss of supercoiling in plasmids was determined by gel electrophoresis and in cells by nucleoid flow cytometry. Comparison of dose-response data for plasmid relaxation with that obtained from SQ-20B cells enabled a direct estimation of supercoil target size in these cells. Plasmids pUCD9P (3.9 kbp), pXT-1 (10.1 kbp), pdBPV-MMT-neo (14.6 kbp), pRK290 (20.0 kbp), and R6K (38 kbp) were used and analyzed under the same exposure conditions as nucleoid DNA. Two sizes of topologically closed domains were found in nucleoids of 0.51+/-0.17Mbp and 1.34+/-0.3 Mbp. In an attempt to relate these large-scale organizations of DNA with function, cells were exposed to the DNA topoisomerase II inhibitor, VP16 and the G1/S cell cycle blocking agent mimosine. A 1 h exposure to VP16 was effective in reducing DNA synthesis which was associated with a parallel increase in nucleoid supercoiling. Addition of the G1 > S inhibitor mimosine enhanced both responses. It is concluded that chromosomes and interphase nuclei are organized into at least two sizes of topologically constrained domains of DNA which may have functional relevance to the control and execution of DNA synthesis.

Carcinoma, Squamous Cell↗
PubMed1996

Commitment to erythroid differentiation in mouse erythroleukemia cells is controlled by alterations in topoisomerase II alpha phosphorylation.

To explore the program of cell differentiation in Friend murine erythroleukemia (MEL) cells, we used three clonal variants: phorbol 12-myristate 13-acetate (PMA)-hypersensitive TS-19-101, PMA-resistant TR19-9, and hexamethylene bis-acetamide (HMBA)- and PMA-resistant DS19/R1. After treating TS19-101 cells with HMBA, topoisomerase II (topo II) enzymatic activity was dramatically reduced, and cells became terminally differentiated. The initial reduction in activity was soon followed by reduced topo II alpha phosphorylation, but only later did the protein level drop significantly. PMA, which completely blocked HMBA-induced differentiation in TS19-101 cells, increased the phosphorylation of topo II alpha and restored the enzymatic activity to its original levels. Reduced topo II activity and phosphorylation were also evident in HMBA-treated TR19-9 cells. PMA failed to restore topo II activity and phosphorylation to their original levels in TR19-9 cells. Predictably, the topo II activity and phosphorylation of DS19/R1 cells showed little change in response to HMBA or PMA treatment. Structural changes in chromatin became evident in sensitive cells 24 h after HMBA treatment, suggesting that alterations in topo II alpha phosphorylation may control cell differentiation by altering nuclear architecture.

Acetamides↗