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Biomedical subjects

G L Chan

Publications and source records attributed to G L Chan.

At least 55 records · Page 3Linked to original sources
PubMed1985

T4 DNA polymerase (3'-5') exonuclease, an enzyme for the detection and quantitation of stable DNA lesions: the ultraviolet light example.

Ultraviolet light irradiation of DNA results in the formation of two major types of photoproducts, cyclobutane dimers and 6-4' [pyrimidin-2'-one] -pyrimidine photoproducts. The enzyme T4 DNA polymerase possesses a 3' to 5' exonuclease activity and hydrolyzes both single and double stranded DNA in the absence of deoxynucleotide triphosphate substrates. Here we describe the use of T4 DNA polymerase associated exonuclease for the detection and quantitation of UV light-induced damage on both single and double stranded DNA. Hydrolysis of UV-irradiated single or double stranded DNA by the DNA polymerase associated exonuclease is quantitatively blocked by both cyclobutane dimers and (6-4) photoproducts. The enzyme terminates digestion of UV-irradiated DNA at the 3' pyrimidine of both cyclobutane dimers and (6-4) photoproducts. For a given photoproduct site, the induction of cyclobutane dimers was the same for both single and double stranded DNA. A similar relationship was also found for the induction of (6-4) photoproducts. These results suggest that the T4 DNA polymerase proofreading activity alone cannot remove these UV photoproducts present on DNA templates, but instead must function together with enzymes such as the T4 pyrimidine dimer-specific endonuclease in the repair of DNA photoproducts. The T4 DNA polymerase associated exonuclease should be useful for the analysis of a wide variety of bulky, stable DNA adducts.

Base Sequence↗
PubMed1984

Antigenic similarity between cells transformed by ultraviolet radiation in vitro and in vivo.

Cells of the 10T 1/2 mouse fibroblast line transformed in vitro by ultraviolet radiation are antigenically similar to those from skin cancers produced in mice by repeated exposure to ultraviolet radiation. Both types of tumor cells grew preferentially in ultraviolet-irradiated syngeneic mice relative to untreated animals, and both were recognized by ultraviolet radiation-induced tumor-specific suppressor lymphocytes. These properties were not shared by 10T 1/2 cells transformed in vitro by x-rays or 3-methylcholanthrene.

Animals↗
PubMed1984

Anaemia and menstrual blood loss studies in women using multiload Cu250 and progestasert IUDs.

Two to 9 months following insertion of the ML Cu250 (n = 54) and Alza T (n = 30) IUDs, mean menstrual blood losses (MBL) were 46.3 ml and 32.7 ml, respectively (p less than 0.005). Significantly more of the Alza T users had losses below 40 ml and they also perceived their menses to be lighter. Based on a haemoglobin level of 12gm/dL, the upper normal limit of MBL was about 40 ml. These findings are consistent with earlier observations that during the first year, ferritin levels fall in ML Cu250 users whereas they are unaltered in Alza T users.

Adult↗
PubMed1983

Cultured diploid fibroblasts from patients with the nevoid basal cell carcinoma syndrome are hypersensitive to killing by ionizing radiation.

Nevoid basal cell carcinoma syndrome (NBCCS) is an autosomal dominant disease. About 20% of the gene carriers studied developed medulloblastoma before the age of 5 years. Clinical follow-up of these patients, treated with radiotherapy, revealed a predisposition to radiogenic basal cell carcinomas with an unusually short latent period of 6 months to 3 years. The authors have therefore cultured skin fibroblasts from 5 NBCCS patients and measured their radiosensitivity in terms of clonogenic survival. Our results showed that, compared with 6 normal controls, the NBCCS cells were hypersensitive to X-rays. The average D0 (the inverse of the slope of the survival curve) for the NBCCS cells was 98 rads, compared with 142 rads for the normal controls and 44 rads for an ataxia telangiectasia (AT) strain. The average D10 values (the dose required to reduce survival to 10%) were 258, 351, and 123 rads for the NBCCS, normal, and AT strains, respectively. Unscheduled DNA synthesis measurements showed that NBCCS cells were not defective in excision repair of X-ray-damaged DNA. Pulse labeling index measurements showed that NBCCS cells were abnormally inhibited in the initiation of DNA synthesis following X-irradiation. The mechanisms underlying the radiosensitivity of NBCCS differ in several respects from those of AT. NBCCS appears to be potentially a useful model for studying the cellular processes that are important in radiation carcinogenesis.

Adolescent↗
PubMed1982

Dissociated occurrence of single-gene mutation and oncogenic transformation in C3H 10T1/2 cells exposed to ultraviolet light and caffeine.

We examined the relationship of cytotoxicity, mutagenesis, and malignant transformation by measuring in parallel clonogenic survival, mutation to ouabain resistance, and malignant transformation in cultured C3H mouse 10T1/2 cells. Exposure to caffeine alone for 48 hours was cytotoxic and induced transformation in a dose-dependent manner. However, this same treatment did not induce any detectable ouabain-resistant mutants. When caffeine was present for 48 hours immediately following UV irradiation, alkaline sucrose gradient sedimentation of DNA showed that postreplication repair was inhibited. This inhibition of repair was correlated with reduced survival and inhibition of mutation induction, but the transformation frequencies were either unaltered or potentiated, depending on the UV dose and caffeine concentration. Thus, these experiments demonstrate that gene mutation and malignant transformation in 10T1/2 cells can be dissociated. We suggest that the mechanism of transformation of 10T1/2 cells is nonmutagenic in nature.

Animals↗
PubMed1982

Further studies on the survival of non-proliferating human diploid fibroblasts irradiated with ultraviolet light.

Labelling index data showed that in AG1518 cells, a diploid human fibroblast strain, there was a lag period of at least 14 hours between subculture from the density-inhibited plateau phase of growth and entry into DNA synthesis. Cells irradiated with 254 nm wavelength U.V. light 8 hours after subculture did not exhibit the same degree of resistance as cells irradiated in plateau phase and subcultured immediately. When cells were arrested from proliferation by maintenance in an arginine and glutamine deficient medium for 72 hours, they were nearly as resistant to U.V. light as plateau phase cells although maintenance in this medium for 24 hours after irradiation supported further recovery only after low U.V. doses. One strain of Cockayne syndrome fibroblasts was found to be resistant to U.V light in plateau phase while another strain was found to have the same survival response whether it was irradiated in the plateau or log phase of growth.

Cell Line↗
PubMed1981

The influence of cellular proliferative history on the susceptibility to oncogenic transformation.

C3H mouse 10T1/2 clone 8 cells were serially transferred from passage 11 to 15 with 5 x 10(4) cells seeded per 60-mm dish at each passage. One group of cells was passaged as soon as confluence was reached. Two other groups were kept for 3 or 6 days in confluence at each passage before subculture. Cloning efficiency was found to increase progressively with passage of all three groups. At the 15th passage, cells from all three groups were harvested just prior to confluence, irradiated with ultraviolet light, and assayed for clonogenic survival and malignant transformation. Survival response was the same for all three groups, but cells which were kept constantly proliferating in previous passages were found to be much more susceptible to transformation. These results suggest that the susceptibility of these cells to transformation is influenced by their proliferative history; in particular, intermittent growth quiescence in previous passages decreased this susceptibility.

Animals↗
PubMed1981

Complex interactions of caffeine and its structural analogs with ultraviolet light in cell killing.

We measured the clonogenic survival response of cultured mouse 10T1/2 cells exposed to UV light and caffeine post-treatment. When 0.5 and 1 mM caffeine were present for 24 h immediately following UV, the D0 values of the biphasic survival curves suggest that one subpopulation was sensitized and one subpopulation was protected from killing by UV light. A cloned survivor from the radioprotected subpopulation responded to UV plus caffeine in identical manner as the parent cells. When the caffeine exposure was prolonged to 48 h, only the radiosensitizing effect was observed. Two demethylated analogs of caffeine were also tested. The response of 10T1/2 cells to 1 mM theophylline present for 24 h after UV irradiation was approximately the same as that for the same treatment with 1 mM caffeine. However, prolonging the theophylline exposure to 48 h failed to produce the same kind of potentiation of cell killing as that observed for caffeine. Xanthine by itself was as toxic to 10T1/2 cells as caffeine, but had no synergistic effect as caffeine when given to UV-irradiated cells for 24 or 48 h. It is therefore unlikely that all the effects of caffeine on UV-irradiated cells are mediated by its demethylated metabolites.

Animals↗
PubMed1979

Resistance of plateau-phase human normal and xeroderma pigmentosum fibroblasts to the cytotoxic effect of ultraviolet light.

Clonogenic survival response to 254-nm ultraviolet light was measured in 2 strains of repair-proficient normal human fibroblasts and 4 strains of xeroderma pigmentosum (XP) fibroblasts belonging to complementation groups A, C, D and variant. In all strains except XPA, cells irradiated in plateau phase and subcultured immediately were much more resistant to the lethal effect of UV than cells irradiated in the exponential phase of growth. Typically, 10-20% of plateau-phase cells were extremely resistant. When the cultures were held in plateau phase for 24 h after irradiation and before subculture, there was a further enhance of survival. By use of a UV-specific endonuclease assay, no difference was found in the number of DNA lesions induced in exponentially growing and plateau cultures by the same dose of UV light. Thus plateau-phase cells appear to be more efficient in their DNA-repair capability than cells in exponential growth. XP group A cells were uniquely found to be deficient in the processes which lead to plateau-phase resistance. Since plateau-phase repair was not lacking in XP groups C, D and variant, it may be related to a DNA-repair process different from that which is responsible for the overall UV sensitivity of these cells.

Cell Cycle↗
PubMed1979

Response of plateau-phase mouse embryo fibroblasts to ultra-violet light.

Clonogenic survival was measured in plateau-phase cultures of the 10T1/2 mouse cell line exposed to 254 nm ultra-violet light. The survival curve was found to be biphasic, Do for the two components being 37 and 1191 erg/mm2 respectively. This extreme resistance at higher doses can only be partly accounted for by the increased cytoplasmic absorption of U.V.L. due to an increased thickness of plateau-phase cells. When the cultures were held for 24 hours in plateau phase in conditioned medium after irradiation, recovery yielding a 1.4-fold enhancement of survival was found at higher doses. This recovery process was inhibited by neither caffeine nor cycloheximide. When caffeine was given for 48 hours after sub-culture, the effect on survival was also negligible. We propose that this plateau-phase recovery process is associated with excision repair of DNA adducts induced by U.V.L. Delayed sub-culturing favours the excision mode of repair and renders the post-replication mode less necessary.

Animals↗