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I R Radford

Publications and source records attributed to I R Radford.

At least 19 recordsLinked to original sources
PubMed2006

Intestinal crypt properties fit a model that incorporates replicative ageing and deep and proximate stem cells.

A model of intestinal crypt organization is suggested based on the assumption that stem cells have a finite replicative life span. The model assumes the existence in a crypt of a quiescent ('deep') stem cell and a few more actively cycling ('proximate') stem cells. Monte Carlo computer simulation of published intestinal crypt mutagenesis data is used to test the model. The results of the simulation indicate that stabilization of the crypt mutant phenotype following treatment with external mutagen is consistent with a stem cell replicative life span of about 40 divisions for mouse colon and 90-100 divisions for mouse small intestine, corresponding to a deep stem cell cycle time of about 3.9 and 8.5 weeks for colon and small intestine, respectively. Simulation of the data obtained for human colorectal crypts suggests that the proximate stem cell cycle time is about 80 h, assuming a replicative life span of 50-150 divisions, and that the deep stem cell divides approximately every 30 weeks.

Animals↗
PubMed2006

An enteroendocrine cell-based model for a quiescent intestinal stem cell niche.

We have shown that the kinetics of conversion of intestinal crypt cell populations to a partially or wholly mutant phenotype are consistent with a model in which each crypt contains an infrequently dividing 'deep' stem cell that is the progenitor of several more frequently dividing 'proximate' stem cells. An assumption of our model is that each deep stem cell exists in a growth inhibitory niche. We have used information from the literature to develop a model for a quiescent intestinal stem cell niche. This niche is postulated to be primarily defined by an enteroendocrine cell type that maintains stem cell quiescence by secretion of growth inhibitory peptides such as somatostatin and guanylin/uroguanylin. Consistent with this model, there is evidence that the proteins postulated as defining a growth-inhibitory stem cell niche can act as intestinal tumour suppressors. Confirmation that a growth-inhibitory niche does exist would have important implications for our understanding of intestinal homeostasis and tumorigenesis.

Animals↗
PubMed2004

Chromosomal rearrangement as the basis for human tumourigenesis.

PURPOSE: To develop a model for the initiation of human tumourigenesis that is consistent with various observations that are difficult to reconcile with current models. CONCLUSIONS: A novel model of tumourigenesis was developed that includes three basic postulates: (1) tumourigenesis is initiated by recombinogenic DNA lesions, (2) potentially recombinogenic DNA lesions in transcribed regions of the genome can be converted into chromosomal rearrangements and (3) chromosomal rearrangements alone are insufficient for tumourigenesis but can initiate a mutator/recombinator phenotype.

Aneuploidy↗
PubMed2002

DNA lesion complexity and induction of apoptosis by ionizing radiation.

PURPOSE: To determine whether murine lymphoid cell lines can discriminate between high- and low-LET (linear energy transfer) radiation-induced DNA lesions. MATERIALS AND METHODS: Sensitivity to killing by DNA-incorporated 3H and 125I decays, accumulated during storage in the gas phase of a liquid nitrogen tank, was determined by clonogenic survival assay. RESULTS: Induction of a lethal event in the STRij-4-2.2, WEHI-22.1, and L5178Y-R cell lines required approximately 30 times more 3H than 125I decays. Hence, the same ratio of 3H to 125I decays was found irrespective of whether the cell lines contained mutant or wild-type p53 and irrespective of whether they underwent rapid interphase or mitosis-related apoptosis after irradiation. The 18-81 cell line differed in showing a ratio of around 21 and it is argued that this may be a consequence of v-ABL over-expression. The assumption that DNA-incorporated 3H and 125I decays are low- and high-LET-like events respectively was confirmed by the similar sensitivity of L5178Y-R and -S cells to killing by 125I decays in contrast with their difference in sensitivity to 3H decays. CONCLUSIONS: The difference in lethal effectiveness between DNA-incorporated 3H and 125I decays can be explained by the hypothesis that simple DSB (double-strand breaks) are non-lethal and that cell killing is attributable to complex DSB. The low-LET radiation-specific sensitization of L5178Y-S cells may reflect defective repair of a DNA lesion class (presumably simple DSB) that is differentially induced by high- and low-LET radiation and is non-lethal to cells with normal repair capacity.

Animals↗
PubMed2002

Model for the initiation of ionizing radiation-induced apoptosis in lymphoid cells by complex DNA double-strand breaks.

PURPOSE: To present a model for the molecular events that lead to the induction of apoptosis in irradiated lymphoid cells based on the assumption that the process is triggered by complex DNA double-strand breaks (DSB). OUTLINE OF THE MODEL: * Cellular DNA repair mechanisms have difficulty rejoining complex DSB because of the nature of the end groups on such breaks. * Association between p53 and DNA topoisomerase I (topo I) can occur at complex DSB in open regions of the genome and the enzymic activity of such associations is not suppressed by polyADP-ribosylation. * Binding of p53 and topo I at a complex DSB results in the transient trapping of a DNA-topo I cleavage complex. * Transiently trapped DNA-topo I cleavage complexes at complex DSB are reversed following association with topo I bound elsewhere in the genome, thus initiating a misrejoining event. * Topo I-mediated DNA misrejoining creates a structure that activates p53. Initiation of rapid interphase apoptosis requires that the inducing signal from activated p53 exceeds a threshold level. * Initiation of rapid interphase apoptosis is regulated by poly(ADP-ribose) polymerase.

Animals↗
PubMed2002

Response of mammalian cell lines that undergo necrosis after ionizing irradiation to DNA-incorporated tritium and iodine-125 decays: implications for models of radiation action.

PURPOSE: To test the validity of enzymatic DNA repair-based models of ionizing radiation-induced mammalian cell killing. MATERIALS AND METHODS: Sensitivity to ionizing radiation-induced killing was determined by clonogenic survival assay. RESULTS: A panel of seven cell lines that have apparently normal enzymatic DNA repair capability and die predominantly by necrosis were shown to have marked differences in their sensitivity to killing by gamma- or X-irradiation. Despite such differences, the pseudo-diploid cell lines CHO, V79, and Vero (when corrected for differences in cell cycle distribution) and normal mouse embryo fibroblasts all had comparable D(o) values for killing by DNA-incorporated (125)I that were in the range of 57-66 decays; whilst the near-tetraploid or-triploid B16-F1, RUC-2 and SQ-20B cell lines had D(o) values of around 130 (125)I decays. The sensitivity of these cell lines to killing by DNA-incorporated (3)H decays was similarly related to ploidy. CONCLUSIONS: Differences in sensitivity to killing by DNA-incorporated (3)H or (125)I decays between the cell lines tested were primarily related to differences in ploidy and, second, to differences in cell cycle distribution or nuclear volume. The data do not support suggestions that generalized variability in an hypothetical aspect of enzymatic DNA repair can explain differences in sensitivity to killing by external-beam, low-linear energy transfer ionizing radiation.

Animals↗
PubMed2002

Transcription-based model for the induction of interchromosomal exchange events by ionizing irradiation in mammalian cell lines that undergo necrosis.

PURPOSE: To develop a model of the molecular events leading to ionizing radiation-induced interchromosomal exchange in mammalian cells that undergo necrosis. OUTLINE OF THE MODEL: DNA double-strand breaks (DSB), primarily those involving multiple individual damage sites (i.e. complex DSB), are postulated to be the critical initiating lesion. Only those DSB occurring in transcription units that are associated with transcription factories are postulated to induce chromosomal exchange events. It is suggested that such DSB are brought into contact with a DNA topoisomerase I (topo I) molecule through RNA polymerase II (pol II)-catalysed transcription and give rise to trapped DNA-topo I cleavage complexes. Trapped complexes are postulated to interact with another topo I molecule on a temporarily inactive transcription unit at the same transcription factory leading to DNA cleavage and subsequent strand exchange between the cleavage complexes. Interchromosomal exchange events will occur when the cleavage complexes are on transcription units located on different chromosomes. OUTCOMES: The model can explain a variety of phenomena that includes: the increased resistance to killing by ionizing radiation shown by quiescent cells (i.e. potentially lethal damage repair); the effect of ploidy on sensitivity to killing by DSB; the effect of chasing time on sensitivity to killing by DNA-incorporated (125)I decays after pulse labelling; and the size of the target for ionizing radiation-induced cell killing.

Animals↗
PubMed2000

Computerized video time-lapse microscopy studies of ionizing radiation-induced rapid-interphase and mitosis-related apoptosis in lymphoid cells.

Computerized video time-lapse (CVTL) microscopy of X-irradiated cultures of cells of the murine lymphoma cell lines ST4 and L5178Y-S and the human lymphoid cell line MOLT-4 demonstrated that these cells exhibit a wide disparity in the timing of induction and execution of radiation-induced cell death that included rapid-interphase apoptosis, delayed apoptosis, and postmitotic apoptosis. ST4 cells that received 2.5 or 4 Gy of X radiation underwent rapid-interphase apoptosis within 2 h. Apoptosis commenced with a 10-20-min burst of membrane blebbing followed by swelling for 2-4 h and cell collapse. No apoptotic bodies were formed. After a dose of 1 Gy, approximately 90% of ST4 cells died by rapid-interphase apoptosis, while the remainder completed several rounds of cell division prior to cell death. Postmitotic death of ST4 cells occurred with the same morphological sequence of events as during rapid-interphase apoptosis induced by doses of 1-4 Gy. In contrast, L5178Y-S and MOLT-4 cells that received 4 Gy underwent apoptosis more slowly, with a complex series of events occurring over 30-60 h. Only 3% of L5178Y-S cells and 24% of MOLT-4 cells underwent apoptosis without attempting cell division. The cells became abnormally large during a long G(2)-phase delay, and then most of the cells (76-97%) attempted to divide for the first or second time at approximately 18-30 h postirradiation. However, either mitosis failed or division was aberrant; i.e., the large cells divided into three or four fragments which eventually fused together. This process was followed by several rounds of complex and unpredictable membrane blebbing, gross distortions of shape, fragmentation-refusion events, and formation of apoptotic bodies, after which the cells collapsed at 36-60 h postirradiation.

Animals↗
PubMed2000

Gd-Tex Pharmacyclics Inc.

Pharmacyclics is developing Gd-Tex (gadolinium texaphyrin) as a radiosensitizer for the potential treatment of various cancers including brain metastases and primary brain tumors, pancreatic tumors, lung tumors and pediatric cancers [196711], [348919]. The compound entered phase III pivotal trials for brain metastases in September 1998 [323929]. Phase I clinical trials for the treatment of primary brain tumors and pancreatic cancer have been initiated while several trials in other cancer types are in the planning stages [367716]. In September 1998, Pharmacyclics announced the initiation of a pivotal phase III trial for the treatment of patients with brain metastases. This multicenter trial originally included 30 sites in the US, Canada and Europe, and was expected to enroll 425 patients. The FDA agreed that this trial qualified for Fast Track review if efficacy end-points are met [301265]. By October 2000, nearly all 450 patients in 50 sites had been completed [375959], [387023]. In September 2000, Pharmacyclics and the National Cancer Institute (NCI) initiated two phase I trials of Gd-Tex. The first was to determine the safety of two different dosing regimens of the drug during preoperative radiotherapy after induction chemotherapy in patients with stage IIA non-small cell lung cancer (NSCLC). The second would examine the use of Gd-Tex in combination with stereotactic Gamma Knife radiosurgery in patients with primary brain tumors known as glioblastoma multiforme [381561]. A phase Ib/II trial, for brain metastases, was conducted in America and France, and involved over 100 patients. At the ASCO 1998 meeting, interim tumor response data were presented for 37 patients. The overall tumor response rate (complete plus partial response rate) was 73%. Furthermore, MRI scanning confirmed that Gd-Tex accumulated selectively in tumors [287459]. Full results were announced in October 1998 at the American Society of Therapeutic Radiology and Oncology. Following ten daily injections followed by whole brain radiation, 77.7% of patients demonstrated a tumor response defined as greater than 50% reduction in tumor volume. Gd-Tex was well tolerated, and liver enzyme elevation was the dose-limiting effect, which was reversible. Death due to tumor progression was seen in 15% of the Gd-Tex group as opposed to 35% in the control group [302872]. In November 1999, Pharmacyclics commenced a phase I trial of Gd-Tex injection, sponsored by the NCI, for treating children with intrinsic pontine glioma. The goals of the phase I dose-ranging study were to determine the Gd-Tex dose and administration schedule that can be safely administered with radiation and to evaluate the localization of Gd-Tex in affected tumors using MRI [348035]. In March 1997 the Decision Network of the NCI voted to sponsor additional clinical indications including adult and pediatric brain tumors, as well as cancers involving the lung, head & neck, pancreas and prostrate. Two phase I trials of Gd-Tex for the treatment of primary brain tumors commenced in August 1998 under a CRADA with the NCI [237538], [295592], [348919]. Pharmacyclics is collaborating with the NCI under a CRADA in phase I trials in primary brain tumors and pancreatic tumors [323929], [323952], [346596]. Analysts expected a filing to occur by the end of 1999 or early 2000, with sales in 2001 [303186].

Animals↗
PubMed1999

Initiation of ionizing radiation-induced apoptosis: DNA damage-mediated or does ceramide have a role?

PURPOSE: To critically review the data supporting membrane damage-induced increases in ceramide levels as the primary initiator of ionizing radiation-induced apoptosis and to point out that there is compelling evidence supporting the involvement of DNA damage in this process. CONCLUSIONS: There is now a significant literature suggesting that irradiation of cells can quickly lead to a modest, transitory increase in the level of the putative second messenger ceramide. These results have been used to support the views that membrane damage is the primary trigger for radiation-induced apoptosis and that DNA damage is irrelevant to this process. It is argued, however, that the data are inadequate to support such conclusions because it is questionable whether the induced levels of ceramide are toxic and because the ceramide hypothesis cannot convincingly explain the delayed apoptosis, dependent on events such as mitosis, that is shown by many cell lines. In contrast, it is suggested that the sensitivity of some cell types to the induction of apoptosis by DNA-targeted radiation damage, the relationship between p53 status and radiation response, and the influence of enzymatic DNA repair capability on susceptibility to apoptosis, argue strongly that DNA damage is relevant to the triggering of apoptosis.

Animals↗
PubMed1999

Selection and sequencing of interchromosomal rearrangements from gamma-irradiated normal human fibroblasts.

PURPOSE: To determine the sequences that flank sites of interchromosomal DNA rearrangements and to determine the relative frequency of inter- and intrachromosomal rearrangements induced by 30 Gy gamma-irradiation in a region 5' from exon I of the c-myc gene in normal human fibroblasts (IMR-90). MATERIALS AND METHODS: A modification of an inverse polymerase chain reaction (PCR) procedure, developed previously to detect rearrangements, was used. Inverse PCR products were re-amplified using primers designed to determine whether the product was a result of an inter- or intrachromosomal rearrangement. Possible interchromosomal rearrangements were then sequenced. RESULTS AND CONCLUSIONS: Four of 12 different products analyzed were potentially derived from interchromosomal rearrangements, while the remainder derived from intrachromosomal rearrangements. For three of the potential interchromosomal rearrangements, the sequence recombining with c-myc was unidentified, while in the other case the sequence was homologous to an L1 element. The frequencies of inter- and intrachromosomal rearrangements induced by 30 Gy gamma-irradiation in a 2 kbp region flanking the c-myc gene of IMR-90 cells were calculated to be at least 1.6x10(-4) and 3.3x10(-4) respectively. No clear association between sequence context and sites of radiation-induced rearrangement was found; however, two of the four sequenced rearrangements involved breakpoints in the 5'-flanking region of c-myc that occurred immediately after the sequence AAAGG.

Base Sequence↗
PubMed1999

Importance of DNA damage in the induction of apoptosis by ionizing radiation: effect of the scid mutation and DNA ploidy on the radiosensitivity of murine lymphoid cell lines.

PURPOSE: To study the effects of the murine scid mutation and DNA ploidy on the susceptibility of lymphoid cell lines to induction of apoptosis by ionizing radiation and thereby to determine whether DNA lesions are critical initiators of apoptosis. MATERIALS AND METHODS: Sensitivity to killing and rapidity of induction of apoptosis following y-irradiation or DNA-associated 125I decays were compared in pre-B and pre-T cell lines derived from wild-type mice and from mice homozygous or heterozygous for the scid mutation. Effects of differences in DNA ploidy on the same endpoints were studied using pseudo-diploid and -tetraploid clones of a murine pre-T cell line. RESULTS: Pairs of pre-B- and pre-T cell-derived lines that expressed wild-type p53 and underwent rapid interphase apoptosis after irradiation were identified. In both cases, the scid homozygous cell lines were more sensitive to killing, suggesting that DNA repair capability influences susceptibility to induction of apoptosis. Increasing DNA ploidy in a cell line that undergoes rapid interphase apoptosis produced a corresponding increase in the number of DNA lesions required to produce a lethal event; again consistent with DNA being the target for radiation action. CONCLUSION: DNA damage is an important, if not the sole, initiator of external beam ionizing radiation-induced apoptosis.

Animals↗
PubMed1998

Explaining differences in sensitivity to killing by ionizing radiation between human lymphoid cell lines.

We surveyed five human hematopoietic cell lines (HSB-2, MOLT-4, Reh, CEM, and HL-60) to determine whether any simple correlates with sensitivity to killing by gamma-irradiation might be revealed. The clonogenic survival gamma-ray dose-response curves for these cell lines cover a wide range of sensitivities. Consistent with previous results for murine hematopoietic cell lines, there was a clear correlation between the rapidity with which irradiation induced apoptosis and clonogenic radiosensitivity of a cell line, although the relationship between timing of apoptosis and radiosensitivity differed between human and murine cell lines. Flow cytometric determination of cell cycle distribution after irradiation showed that differences between human hematopoietic cell lines, in the rate of induction of apoptosis, were generally related to the functioning of cell cycle checkpoints. Whereas the rapidly dying and radiosensitive HSB-2 cell line underwent apoptosis from different points in the cell cycle, the more slowly dying cell lines showed a variety of cell cycle arrest profiles and initiated apoptosis after accumulation of cells in the G2 phase. The lag-phase between arrest in G2 and induction of apoptosis was comparable for MOLT-4, Reh, and CEM; however, HL-60 cells showed a markedly longer G2 arrest that correlated with their greater radioresistance. The results suggest that the total length of time available for DNA damage repair (irrespective of whether this time accrues as blockage in G1, S, or G2), prior to potential activation of apoptosis, is a critical determinant of radiosensitivity in human hematopoietic cell lines. Comparison of the p53 status of these cell lines suggested that mutations in the TP53 gene are contributing to the delay of induction of apoptosis seen in the more radioresistant cell lines. The sensitivity of MOLT-4 and HL-60 cells to killing by DNA-associated 125I decays was determined and was found to correlate with the relative sensitivity of these lines to gamma-irradiation. The highly localized deposition of energy by 125I decays argues that DNA damage is a potent initiator of apoptosis in these cell lines. The results presented suggest that differences in the radiosensitivity of the cell lines examined reflect differences in the rapidity of induction of apoptosis and that radiation-induced cell death in hematopoietic cells can be explained as a response to DNA damage.

Apoptosis↗
PubMed1998

Detection and sequencing of ionizing radiation-induced DNA rearrangements using the inverse polymerase chain reaction.

PURPOSE: To develop a procedure, using the inverse polymerase chain reaction, to detect and sequence ionizing radiation-induced DNA rearrangements without prior phenotypic selection of mutant cells. METHOD: Normal human fibroblast cells (IMR-90) were given 30Gy of gamma-irradiation and then incubated at 37 degrees C for 23h to allow DNA repair. Rearrangements of the sequence 5' to the c-myc gene were examined by amplifying the region using inverse PCR followed by DNA sequencing. RESULTS: Approximately fivefold more PCR products were amplified from the DNA of cells given 30 Gy of gamma-irradiation and allowed 23 h for repair than were obtained from cells that were either unirradiated or were irradiated and then lysed immediately. PCR products from seven putative radiation-induced DNA rearrangements were sequenced. Of these products, one contained an unidentified sequence (a possible inter-chromosomal rearrangement) whilst the other products appeared to derive from episomes or duplication events (possible intra-chromosomal rearrangements). The sequencing data suggested that the sites of DNA rearrangement breakpoints were non-randomly distributed and possibly associated with topoisomerase I consensus cleavage sequences. There was a significant level of direct homology between the sequences flanking the breakpoints. CONCLUSIONS: The procedure developed was able to detect both inter- and intra-chromosomal rearrangements.

Base Sequence↗
PubMed1996

Radiation induced apoptosis.

The response to ionising radiation, in terms of level of cell killing, depends on a number of factors that may be grouped into those that are genetically controlled, radiation quality and dosage, and environmental factors. There is a range of genetically controlled cellular properties such as stage of differentiation, mutations in specific genes (such as p53 and bcl-2) and stage of transformation that will determine the ability of the target cell to enter apoptosis. The so-called normal cells, are usually more radiosensitive and the majority of the cell population will enter into an apoptotic death. However, in response to high doses of ionising radiation and complex DNA damage as produced by high-LET radiation, an increased fraction of these cells will die by necrosis. There are several examples of environmental factors with relevance for the combined action of radiation and xenobiotics on carcinogenesis and in tumour therapy. In the case of normal cells, agents such as growth factors and tumour promoters, may decrease radiosensitivity. For certain type of tumour cells, radiation sensitivity can be increased in the presence of agents such as hormones, and the cells may die an apoptotic death. Removal of heavily compromised cells is essential to prevent a potential spreading of mutated clones. However, if apoptosis is inhibited (e.g., by tumour promoter), an increased fraction of damaged cells carrying genotoxic lesions may survive. This would significantly increase the risk of proliferation of precancerous cells. As discussed above, it is probably incorrect to make predictions about relative radiosensitivity based solely on mode of death. Intrinsic characteristics deriving from the cell type of origin of a line may be more important in determining radiosensitivity. The rapidly increasing knowledge about the process of radiation induced apoptosis has opened new frontiers in radiation biology, genetic toxicology, and cancer therapy and strongly motivates further research in this field.

Animals↗
PubMed1995

Increasing the susceptibility of the rat 208F fibroblast cell line to radiation-induced apoptosis does not alter its clonogenic survival dose-response.

Recent studies have suggested a correlation between the rate and incidence of apoptosis and the radiation response of particular cell lines. However, we found that increasing the rate of induction of apoptosis in the fibroblast line 208F, by transfecting it with human c-myc, did not lead to a change in its clonogenic survival dose-response for either gamma-irradiation or 125I-induced DNA damage. It was also found that expression of mutant (T24) Ha-ras in the 208F line appeared to decrease the level of apoptosis per mitosis after irradiation and inhibited the formation of nucleosomal ladders, but did not affect either the onset of the morphological features of apoptosis or the clonogenic survival dose-response of the cells to either gamma-irradiation or 125I-induced DNA damage. Our findings suggest that it may be incorrect to make predictions about the radiosensitivity of cells based only on knowledge of their mode of death.

Animals↗
PubMed1994

Radiation response of mouse lymphoid and myeloid cell lines. Part I. Sensitivity to killing by ionizing radiation, rate of loss of viability, and cell type of origin.

The sensitivity of 10 mouse lymphoid or myeloid cell lines to gamma-ray- and DNA-associated 125I-decay-induced clonogenic cell killing have been compared with their rate of loss of viability (membrane integrity) and with their putative cell type of origin. The pseudodiploid haematopoietic cell lines showed D0 values for 125I-induced DNA double-strand breakage (dsb) that ranged from 7.7 +/- 0.7 to 40.8 +/- 2.8 decays. These lines generally appeared to be more sensitive to killing by radiation-induced DNA dsb than are fibroblast-like cell lines. The increased sensitivity of haematopoietic cell lines to killing by DNA dsb may be related to their mode of death (apoptosis versus necrosis). Mode of cell death may thus be an important factor in determining the 'inherent radiosensitivity' of normal cells/tissues. Haematopoietic cell lines that undergo rapid interphase apoptotic death showed extreme sensitivity to DNA dsb. The latter cell lines were found to have derived from immature lymphoid cells, and it is speculated that their high radiosensitivity might reflect the action of a mechanism that normally eliminates cells containing illegitimate V(D)J recombinase-induced DNA dsb.

Animals↗