Biomedical subjects

W C Dewey

Publications and source records attributed to W C Dewey.

At least 37 records · Page 2Linked to original sources

PubMed1995

Thermotolerant cells possess an enhanced capacity to repair heat-induced alterations to centrosome structure and function.

To study the mechanisms of thermotolerance, the adaptive response by which cells become transiently resistant to killing by heat shock, we have focused on the centrosome, an organelle whose disorganization is closely correlated with thermal killing in Chinese hamster ovary (CHO) cells. Centrosome structure was studied by use of antisera directed against pericentrin, a 220 Kd protein of the pericentriolar material (PCM). Centrosome function was measured in intact cells by performing microtubule regrowth following exposure to the drug nocodazole. Immediately following heating at 45 degrees C for 4-18 min, centrosomal staining by antipericentrin decreased. Thereafter, staining gradually recovered, although abnormal configurations of staining appeared in heated cultures 10-20 h later. In contrast, abnormal patterns of staining rarely developed in thermotolerant cultures. Centriole number was not perturbed by heat, indicating that the heat effect was specific for the PCM. Heat also caused an immediate reduction in the number of microtubules nucleated by the PCM. As for staining by antipericentrin, microtubule nucleation recovered during 3-20 h at 37 degrees C after heating. The immediate, heat-induced decrease in antipericentrin staining or microtubule nucleation was similar in thermotolerant and nontolerant cells. In contrast, the inhibition for both endpoints recovered to control levels much more quickly in thermotolerant cells than in nontolerant cells. Furthermore, new protein synthesis was not required for the recovery of microtubule nucleation. These data show that thermotolerant cells have an enhanced capacity to repair thermal damage to centrosome structure and function, and suggest that a faster rate of recovery prevents disorganization of the PCM that is observed in nontolerant cells several hours after heating.

Adaptation, Physiological↗

PubMed1995

Analysis by pulsed-field gel electrophoresis of DNA double-strand breaks induced by heat and/or X-irradiation in bulk and replicating DNA of CHO cells.

For a given amount of cell killing, heat alone (10-80 min, 45.5 degrees C) induced very few double-strand breaks (dsbs) compared with X-rays. Furthermore, 10 min at 45.5 degrees C immediately prior to X-rays caused only a 1.3-fold increase in the slope of the X-ray-induced dsb dose-response curve, i.e. 0.67 +/- 0.006 (95% confidence) dsbs/100Mbp/Gy for heated cells compared with 0.53 +/- 0.005 for unheated control cells. However, this same heat treatment caused > 5-fold inhibition in the rate of repair of dsbs induced by 60-Gy X-rays, with the degree of inhibition being much less in thermotolerant (TT) cells than in non-tolerant (NT) cells. This reduced inhibition of repair in TT cells correlated with the more rapid removal of excess nuclear protein from nuclei isolated from TT cells than from NT cells. These results plus a TT ratio of 2-3 for both heat-induced radiosensitization and heat-inhibition of repairing dsbs are consistent with the hypothesis that heat radiosensitization results primarily from heat aggregation of nuclear protein interfering with access of repair enzymes to DNA dsbs. The selective heat-radiosensitization of S-phase cells, however, may result from an increase in radiation-induced dsbs in or near replicating regions. For example, a preferential increase in dsbs in replicating DNA compared with bulk DNA was found following either hyperthermia alone (10-30 min, 45.5 degrees C) or a combined treatment (10 min, 45.5 degrees C before 60 Gy). A 30-min treatment at 45.5 degrees C induced dsbs equivalent to approximately 10 Gy in replicating DNA compared with 3-5 Gy in bulk DNA. When cells were heated immediately before irradiation, the increase in dsbs induced in the replicating DNA by 60 Gy was equivalent to 200 Gy. We hypothesize that the observed 2-fold increase in single-stranded regions in replicating DNA after heat resulted in radiation selectively inducing dsbs at or near the replication fork where the heat-induced increase in single-stranded DNA should occur. Thus, this preferential increase in dsbs in the replicating DNA by heat alone and especially when heat was combined with radiation may explain at least in part, the high sensitivity of S-phase cells to heat killing and heat radiosensitization.

Animals↗

PubMed1995

Methods for the quantification of DNA double-strand breaks determined from the distribution of DNA fragment sizes measured by pulsed-field gel electrophoresis.

Different methods were used for evaluating data for DNA double-strand breaks (DSBs), as obtained by pulsed-field gel electrophoresis (PFGE) after X irradiation of Chinese hamster ovary cells. A total of 60 data points in the dose range of 0 to 116 Gy, along with repair data for 30 and 60 Gy, were analyzed by four methods: (1) percentage of DNA released from the plug, (2) specific size markers (percentage of DNA less than specific sizes, (3) fragment size distributions and (4) shape of the molecular weight (M) distributions. With the last method, both the slope and the intercept of the logarithm of the amount of radioactive DNA/delta M/M plotted as a function of M were used for calculating DSBs/100 Mbp. The slope and the intercept analyses differ in that the former is relatively independent of DNA trapped in the agarose plugs, i.e. cannot be released by doses of 100-150 Gy, whereas the intercept is dependent on the percentage of DNA trapped. Also, calculations of DSBs/100 Mbp for methods 1, 2 and 3 depend on the amount of DNA trapped in the plug. However, the slope method is unreliable for doses below about 20 Gy, and the scatter of data points is much greater than that obtained by the intercept method and by methods 1, 2 and 3. Therefore, the fragment size distribution and the specific size marker methods give the most consistent results, with 0.49 +/- 0.03 (95% CI) (DSBs/100 Mbp)/Gy. With the specific size marker method, however, care must be taken in selection of size markers in relation to the levels of DSBs of interest. Assuming randomly distributed DSBs, all four methods gave essentially the same results; i.e., the dose response was linear with a calculated level of 0.5-0.6 (DSBs/100 Mbp)/Gy, which is the same as 0.47-0.62 determined previously by calibrating with 125IdU.

Animals↗

PubMed1995

Repair of DNA double-strand breaks: errors encountered in the determination of half-life times in pulsed-field gel electrophoresis and neutral filter elution.

For theoretical reasons, it is incorrect to define experimentally the half-life times of DNA double-strand breaks (DSBs) as the half-life time of an amount of DNA. This is illustrated by one example of human DNA, where the half-life for first-order kinetics of the disappearance of DSBs has been assumed to be 10 min. Experimental sources of errors and their influence on experimental results are analyzed. Some experimental situations may lead to serious misinterpretation data. The differential decreases in fractions released (amounts of DNA) as often followed in pulsed-field gel electrophoresis depend on run conditions, background and level of DSB induction and are a function of time itself--a time function that is unrelated to the half-life of DSBs. It is shown that, using the decrease of a measured amount of DNA, one may obtain practically any value for the half-life time.

Animals↗

PubMed1994

Hyperthermia can reduce cytotoxicity from etoposide without a corresponding reduction in the number of topoisomerase II-DNA cleavable complexes.

The chemotherapeutic drug etoposide (VP-16) causes the equilibrium reaction between noncleavable and cleavable topoisomerase II-DNA complexes to shift in favor of the cleavabel complex [H. Zang, P. D'Arpa, and L.F. Liu, Cancer Cells (Cold Spring Harbor), 2:23-27, 1990]. Pulsed-field gel electrophoresis was used to study induction and removal of cleavable complexes in cells heated before, during, or after VP-16 treatment. Pulsed-field gel electrophoresis results were evaluated both as the fraction of activity (DNA) released from the plug and as the number of double-strand breaks (DSBs) calculated from molecular weight distributions; both end points led to the same conclusions. When cells were heated at 42 degrees C during treatment with VP-16 (12 micrograms/ml up to 60 min), a slight decrease in cleavable complexes (from 30 to 20 DSBs/100 megabase pairs) was detected immediately after treatment when compared with cells treated with the drug at 37 degree C. Furthermore, heating at 42 degrees C caused a slight decrease in drug cytotoxicity as measured by less than a 2-fold increase in clonogenic survival. When cells were heated for 10 min at 45.5 degrees C prior to or after treatment with the drug, there was a reduction (approximately 50%) immediately after treatment in the number of DSBs/100 megabase pairs compared with unheated cells. The rate of removal of cleavable complexes was decreased slightly by heat. After 120 min at 37 degrees C, the number of DSB/100 megabase pairs decreased to approximately 6 for both unheated cells and those heated prior to drug treatment and to approximately 8 for cells heated after drug treatment. In agreement with a low effect of heat on the number of cleavable complexes after drug treatment, there was no significant effect of this heating protocol on drug cytotoxicity. However, heating at 45.5 degrees C prior to drug treatment at 37 degrees C protected cells from drug cytotoxicity (e.g., increased survival after 12 micrograms/ml for 60 min by approximately 100-fold) despite the similarity in the induction and rate of removal of cleavable complexes when compared with nonheated cells. Thus, when cells are heated prior to administration of VP-16, drug cytotoxicity does not correlate with the number of cleavable complexes measured either immediately after treatment or 180 min later when approximately 75% of the initial number have been removed. Finally, since hyperthermia can actually decrease drug cytotoxicity, the use of hyperthermia as an adjuvant to chemotherapy involving topoisomerase II poisons, such as VP-16, should be approached with caution.

Animals↗

PubMed1994

A charge-coupled-device camera image analysis system for quantifying DNA distributions in agarose gels after pulsed-field gel electrophoresis.

A charge-coupled-device camera system was coupled to a personal computer and, with uniformity in illumination and detection (within 4-8%) along each lane, was used for quantifying the distribution of DNA molecules that migrate from the PFGE well (plug) into the lane at distances varying from 1 to 50 mm (with 0.5 mm/pixel). By using a specially designed transmission filter for transmitting 470-725 nm fluorescence from ethidium bromide-stained DNA while eliminating most of the fluorescence (< 400 nm) from the agarose gel, and by using neutral density filters to prevent saturation of the camera, the fluorescence intensity is linearly related to the amount of DNA varying from approximately 0.03 micrograms in a 3-mm-diameter cylindrical plug 5 mm long (equal to background) to approximately 4 micrograms (where ethidium bromide staining saturates). The percentage DNA released from the plug and distribution in the lane (with 1-2 mm resolution) obtained by quantifying DNA fluorescence were not significantly different from the same data obtained by analysis of radioactivity of the same DNA labeled with [3H]dThd. However, scattering of fluorescence from one lane into an adjacent lane 3 mm away and as far as 10 mm from the plug into the lane presented a problem. This problem was overcome by using a form with slots to cover every other lane when the images were obtained and either (1) cutting the lane from the plug and moving it 15 mm away or (2) imaging the intact gel and applying a correction for approximately 7% of the fluorescence from the plug tailing out approximately 10 mm beyond the first 1 mm in the lane. In addition, the following were required: (1) carefully controlled staining and destaining procedures, and (2) a low background that is obtained as an average uniform background in each lane 5 mm beyond where DNA migration stops.

DNA↗

PubMed1994

The effects of incorporation of bromodeoxyuridine into mammalian DNA on the migration patterns of DNA fragments subjected to pulsed-field gel electrophoresis after X irradiation or cutting with a restriction enzyme.

Incorporation of bromodeoxyuridine (BrdU) into DNA in both Chinese hamster ovary (CHO) cells and human melanoma (U1) cells reduced the rate of DNA migration in transverse alternating-field electrophoresis (TAFE) agarose gel optimized for separating molecules larger than 2 Mb. This "BrdU migration effect" was independent of the method of damaging the DNA; i.e., the effect was observed after irradiation of the cells or treatment of the plugs containing DNA with the restriction enzyme, Mlu-1, and similar results were found for CHO and U1 cells. However, when the amount of cutting of DNA was estimated from the amount of DNA migrating from the plugs, a difference between U1 cells and CHO cells was observed in that incorporation of BrdU enhanced the cutting of DNA by either irradiation or Mlu-1 digestion for U1 cells but not for CHO cells. Therefore, the "BrdU migration effect" could not be attributed to an increase in large molecules because of reduced cutting of the BrdU-labeled DNA. The decrease in migration rate during pulsed-field gel electrophoresis when BrdU replaces thymidine in the DNA is hypothesized to result from the increase in negative charge on BrdU-labeled DNA. An increase in electron charge density is expected to increase the reorientation time of BrdU-labeled DNA fragments during each voltage pulse due to an increase in the elongation length of the DNA caused by an increase in electronegativity.

Animals↗

PubMed1993

Cell killing and chromosomal aberrations induced in Chinese hamster ovary cells by treating with cisplatin at 41.5 degrees C during the G1 or late S phase.

Variation in sensitivity to cisplatin during the cell cycle was studied in synchronous Chinese hamster ovary cells treated during G1 or late S for 1 h at 41.5 degrees C with cisplatin (0.25-1.25 micrograms/ml, 0.8-4.2 x 10(-6) M). The cells were assayed for cell killing and chromosomal aberrations. Either they were plated for colony survival, or colcemid was added from 12 to 40 h after plating followed by fixation 4 h later for analysis of chromosomal aberrations after the cells completed 1 or 2 cycles (i.e., first or second mitosis). When the cells were treated either in G1 or late S, the cells entering metaphase exhibited primarily chromatid-type deletions and exchanges. However, aberrations were observed primarily in the first mitosis when cells were treated in G1 compared with aberrations being observed in both the first and second mitoses when cells were treated in late S. For a given amount of cytotoxicity or cytological damage, the cisplatin concentration at 41.5 degrees C could be reduced 4-6-fold compared with treatment at 37 degrees C. For low cisplatin concentrations of less than 0.5-0.7 microgram/ml (survival, approximately 0.3), heat killing predominated, and cells treated in S phase were more sensitive than those treated in G1. However, for cisplatin concentrations greater than 0.5-0.7 microgram/ml, cisplatin cytotoxicity predominated, and for both cell killing and chromosomal aberrations, the cells treated in G1 were approximately 1.5 times more sensitive than those treated in late S. Furthermore, the positive correlation between survival and aberration frequency was similar for cells treated at 37 degrees C or 41.5 degrees C in either G1 or late S. These results suggest that cisplatin administered at 37 degrees C or 41.5 degrees C causes cell lethality primarily by the induction of chromosomal aberrations.

Animals↗

PubMed1993

Changes in intracellular free calcium during hyperthermia: effects of local anesthetics and induction of thermotolerance.

We wished to determine if local anesthetics (LAs) induced changes in intracellular free calcium ([fCa2+i]) that could have an effect on cell killing by hyperthermia. Flow cytometry was used to measure [fCa2+i] of mouse NIH 3T3 cells during heating at 45.5 degrees C. In both non-tolerant and thermotolerant cells, heating caused a rapid increase (within 1 min) in [fCa2+i] of approximately 100 nM, which remained relatively constant during 25 min of continued heating; however, survival was higher in thermotolerant cells. Procaine, lidocaine, and tetracaine had no effect on survival or [fCa2+i] of cells kept at 37 degrees C up to 25 min. Cells heated with procaine and lidocaine showed no difference in [fCa2+i] compared to cells heated without LAs but were greatly sensitized to killing. Cells heated with tetracaine became permeable to trypan blue within 10-15 min of heating. We conclude that heat sensitization by LAs does not involve changes in [fCa2+i]. Furthermore, these studies reject the hypothesis that changes in [fCa2+i] are involved in heat-induced cell killing.

3T3 Cells↗

PubMed1993

Molecular mechanisms for the induction of chromosomal aberrations in CHO cells heated in S phase.

Chromosomal aberrations are induced by heat only when Chinese hamster ovary cells are heated in S phase of the cell cycle. Studies on the hyperthermic inhibition of cellular DNA replication have indicated that four molecular aspects of DNA replication are affected after heating. New replicon initiation and DNA chain elongation are inhibited; the fork displacement rate is very sensitive to heat-inactivation; and finally, there is almost a two-fold increase in single-stranded regions in the replicating DNA after heating. From a comparison of these altered processes between S phase cells and heated G1 cells, which do not die from chromosomal aberrations, our current hypothesis involves 3 steps for the chromosomal aberration induction process. The first critical step is the persistent increase of single-stranded regions in the replicating DNA. Then, we hypothesize that the second step is the creation of transient double strand breaks (DSBs) induced at sites opposite these regions by endogenous endonucleases. Finally, the third step requires that improper repair of these DSBs occurs from either nonrepair or misrepair which then leads to the final chromosomal aberrations seen in the first mitosis after treatment. We believe that this 3 step induction process is common for any cytotoxic agent that induces chromosomal aberrations after DNA replication has been inhibited.

Animals↗

PubMed1993

Content of nonhistone protein in nuclei after hyperthermic treatment.

When nuclei were isolated from Chinese hamster ovary cells after being heated, there was a large increase in the amount of 3H-tryptophan labeled nonhistone protein in the nucleus relative to the whole cell. After 15 min or 30 min of heating at 45.5 degrees C, the nuclear nonhistone protein content increased by 1.6 or 1.8, respectively. In contrast, when the nuclear nonhistone protein content was determined in the intact cell by using autoradiography to quantify 3H-tryptophan labeled protein in the nucleus and cytoplasm in sections of fixed cells, the nuclear nonhistone protein content increased by only 1.14 or 1.28 for 15 or 30 min at 45.5 degrees C, respectively. Therefore, heat does not induce a massive movement of cytoplasmic protein into the nucleus.

Animals↗

PubMed1993

Nuclear protein redistribution in heat-shocked cells.

An increase was observed in the total protein mass of nuclei isolated from Chinese hamster ovary cells heated at 45 degrees C or 45.5 degrees C. An increase in the fractional recovery of DNA polymerase alpha and beta, and of DNA topoisomerase activity coincided with this increase in the protein mass of nuclei from heated cells. Nuclear protein mass which was soluble in 2.0 M NaCl decreased 0.5 fold, while DNA-associated and nuclear matrix-associated protein mass increased 2.2 and 3.4 fold, respectively. The results indicate that the increase in nuclear protein mass observed in nuclei from heated cells is due in part to an increased binding, or precipitation, of nuclear proteins onto the cell's DNA and nuclear matrix.

Animals↗

PubMed1993

Heat shock alters centrosome organization leading to mitotic dysfunction and cell death.

To identify the cellular target(s) responsible for thermal killing in the G1 phase of the cell cycle, synchronous cultures of Chinese hamster ovary cells (CHO) were heat shocked and studied for one cell cycle by time-lapse videomicroscopy and immunocytochemistry. At the first mitosis post-heating, the fraction of cells giving rise to multinucleated progeny approximately equaled the nonclonogenic fraction. In addition, the cells yielding multinucleated progeny were delayed in prophase-metaphase relative to the cells yielding two uninucleated progeny (clonogenic cells). To study the basis for the delay in prophase-metaphase and subsequent formation of multinucleated cells, cells in mitosis were examined by immunofluorescence for spindle abnormalities. Multipolar mitotic spindles and chromosome misalignment were induced by heat. All multiple spindle poles induced by heat stained for pericentriolar material (PCM), the microtubule nucleating material of centrosomes. Heated cells in mitosis also contained additional foci of PCM which were not associated with the spindle. Cells made thermotolerant by a nonlethal heat shock were resistant to both thermal killing and the induction of multiple foci of PCM. Quantitative analysis revealed a good correlation between the fraction of cells with multipolar spindles, the fraction with more than two foci of PCM, and the nonclonogenic fraction. These data indicate that heat-induced alterations to the PCM of centrosomes resulted in multipolar mitotic spindles, delay in prophase-metaphase, and formation of multinucleated cells which were nonclonogenic. These results identify the centrosome as a G1 target for cell killing.

Animals↗

PubMed1993

Cisplatin induced cell killing and chromosomal aberrations in CHO cells: treated during G1 or S phase.

Variation in sensitivity to cisplatin during the cell cycle was studied in synchronous CHO cells treated during G1 or late S phase. The cells were assayed for cell killing, cell-cycle delay, and chromosomal aberrations after they were treated with cisplatin (1-12 micrograms/ml) for 1 h at 37 degrees C. They were either plated for colony survival, or colcemid was added from 12 to 40 h after plating followed by fixation 4 h later for analysis of chromosomal aberrations after the cells completed 1 or 2 cycles (i.e. first or second mitosis). Cells treated with 6 micrograms/ml exhibited about a 10-h delay during the first cycle after treatment during G1 compared with about 3 h during the first cycle and 6 h during the second cycle after treatment during late S. In both cases, cells entering metaphase exhibited predominantly chromatid-type breaks and exchanges. For both cell killing and chromosomal aberrations, the cells in G1 were 1.5-1.6 times more sensitive than those treated in late S, with 1 aberration per cell corresponding to about 37% survival. However, the exchanges and breaks were observed primarily in the first mitosis when cells were treated in G1 compared with the second mitosis when cells were treated in late S. These results suggest that DNA replication opposite cisplatin cross-links in the DNA results in lethal chromosomal aberrations.

Animals↗

PubMed1993

Plasma membrane activities retained after lethal heat shock.

Cultures of Chinese hamster ovary (CHO) cells were examined to determine if heat killing could be attributed to severe damage in the plasma membrane. Three independent transport activities of the plasma membrane were measured. Glucose transport into the cells (measured with the non-metabolizable analogue 3-O-methyl-D-glucose) was stimulated rather than inhibited by heat. Most of the stimulation was found after non-toxic heat doses. Although amino acid transport (measured with the non-metabolizable analogue 2-aminoisobutyric acid) was slightly inhibited by heat, heat-sterilized cells were able to accumulate high intracellular concentrations. Cellular uptake of the nucleoside uridine was unaffected for at least 4 h after heating. In contrast, its incorporation into RNA was immediately inhibited. To further study plasma membrane damage, cells were either heated or treated with drugs which localize to the plasma membrane, ionophore A23187 or amphotericin B. The mode of cell killing by heat was radically different from that of the two drugs: heat-sterilized cells retained a phase-bright morphology and excluded the viability dye trypan blue while drug-killed cells rapidly became phase-dark and absorbed the dye. These results add to a growing list of plasma membrane activities which are retained in heat-sterilized cells, and suggest that the initial thermal damage responsible for cell killing is at an alternate site(s).

Amino Acids↗

PubMed1993

A comparison of the enhancement of radiation sensitivity and DNA polymerase inactivation by hyperthermia in human glioma cells.

Two human glioma cell lines (U87MG and U373MG) were evaluated for their thermal enhancement of radiation sensitivity and its correlation to the degree of inactivation of DNA polymerase alpha and beta. The data showed that hyperthermia increased radiation sensitivity in a time- and temperature-dependent manner. The differential heat sensitivity of the two cell lines was reflected in the degree of polymerase inactivation. Polymerase inactivation was also dependent on time and temperature and was greater for polymerase beta than alpha. The degree of polymerase inactivation correlated well with the thermal enhancement ratio (TER) calculated at the 1.0% survival level. This correlation was poor for the TER at the 50% survival level. The correlations were better for polymerase beta than alpha. The small differences in thermal sensitivity between the two cell lines primarily at 41 and 42 degrees C could not be explained by correlation between polymerase inactivation and TER. Incubation between hyperthermia and irradiation resulted in recovery of polymerase activity and loss of radiosensitization. Levels of polymerase beta after hyperthermia may be used to predict thermal enhancement of radiosensitivity for low survival levels, but possibly not in the shoulder region of the radiation survival curve. Small cell line-dependent differences in thermal sensitivity may not be resolved in these comparisons.

Brain Neoplasms↗

PubMed1993

Critical steps for induction of chromosomal aberrations in CHO cells heated in S phase.

The following four effects on DNA replication are observed in cells heated in S phase of the cell cycle: (1) inhibition of replicon initiation, (2) delay in DNA chain elongation into multicluster-sized molecules > 160S, (3) reduction in fork displacement rate, and (4) increase in single-stranded regions in replicating DNA. Since cells heated in S phase manifest chromosomal aberrations when they enter metaphase, whereas cells heated in G1 do not, we attempted to determine if the effects on DNA replication are critical for the induction of chromosomal aberrations by studying these same effects during DNA replication when synchronous CHO cells had been heated (10 min at 45.5 degrees C) in G1 phase. Following a heat-induced G1 block (12 h), we found previously that when the cells entered S phase, replicon initiation was functional and chain elongation into multicluster-sized molecules > 160S was delayed but completed during S phase. In the present study, we find that the fork displacement rate was near normal and that there was no increase in single-stranded DNA. Additionally, an increase in excess nuclear protein induced in the heated G1-phase cells returns to a normal level by about 12 h, just prior to when the cells enter S phase. Since the excess nuclear protein remains for many hours in heated S-phase cells, we hypothesize that the excess nuclear protein is responsible for the drastic reduction in the fork displacement rate and the associated increase in single-stranded DNA. Furthermore, we hypothesize that this persistent increase in single-stranded DNA during replication is a critical step for the induction of chromosomal aberrations in heated S-phase cells. Consistent with this hypothesis, we observed that aphidicolin (1-2 micrograms/ml) treatment of S-phase cells for 13-16 h, which results in a twofold increase in single-stranded DNA during the inhibition of DNA synthesis, also induces chromosomal aberrations. Possibly, endogenous endonucleolytic attack occurs opposite these sites of single-stranded DNA, thus creating double-strand breaks which either can remain unrepaired or are misrepaired to account for the chromatid breaks and exchanges, respectively, observed as cells complete their cell cycle and enter metaphase.

Animals↗

PubMed1993

Similar heat-induced cell cycle delays in the clonogenic and nonclonogenic fractions of a thermotolerant population.

We have tested the theory that a faster rate of recovery from thermal damage in thermotolerant cells relative to nontolerant cells is the critical factor which confers heat resistance. We measured the rate of recovery from the heat-induced cell cycle delay in Chinese hamster ovary cells, since this delay is shortened in thermotolerant cells (i.e., exhibits thermotolerance) and can be measured in individual cells. Individual thermotolerant cells were followed by discontinuous microscopic observation from shortly after heating, through the first mitosis after heating, until a colony formed or failed to form by 8 days. The heat-induced delay in the clonogenic and nonclonogenic fractions was the same. This shows that the rate of recovery from the cell cycle delay was not the determining factor as to whether or not a cell survived to form a colony. Either additional factors are involved or the rate of recovery from the cell cycle delay plays no role in cell survival. These data show the importance of determining whether a faster rate of repair of thermal damage is specific for the clonogenic fraction, since most if not all types of thermal damage are likely to be repaired more quickly when thermotolerant and nontolerant cells are compared at isodose.

Acclimatization↗