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Biomedical subjects

G C Li

Publications and source records attributed to G C Li.

At least 145 records · Page 8Linked to original sources
PubMed1977

Cellular inactivation by ultrasound.

The lethal effect of ultrasound (US) on mammalian cells has received relatively little attention. Understandably, potential genetic aspects of US have been of prime concern to physicians who use US as a diagnostic tool; at the average power densities involved (<<1 W cm(-2)) little, if any cell killing is to be expected. There have been sporadic attempts to use higher intensities ( approximately 1 W cm(-2)) as a treatment modality in cancer therapy, but those experiments seem to have been based on inadequate cellular studies. The effects of US usually were evaluated in terms of morphological criteria rather than on quantitative determination of the loss of viability as measured by colony formation. There are few reports of the effects of US on survival of mammalian cells, and none specifically examine hyperthermic interaction. With the increased interest in hyperthermia for tumour therapy, attention has been directed towards the use of ultrasound to achieve tumour heating. In preliminary experiments in which US was used to heat the EMT6 sarcoma and KHJJ carcinoma in mice, we found a high percentage of tumour cures with short (approximately 30 min) treatments at temperatures (43-44 degrees C) where in vitro results of hyperthermia-induced cell killing would not have led to a prediction of any cures. We therefore initiated an investigation of the effects of US on survival of Chinese hamster cells to see if direct cell killing by US could explain our in vivo results, or, as in the case of radiofrequency (RF) electromagnetic heating, we would be forced to invoke host response(8). In particular, we examined the thermal and non-thermal components of cellular inactivation by US. We report here that there is a definite non-thermal cytotoxic effect of US. Its relative contribution to cell killing is a highly nonlinear function of the temperature of the cellular milieu. The survival curves show clearly that, beyond an initial threshold, small changes in temperature and/or US intensity can give rise to impressive changes in survival values. The threshold nature of the data strongly suggests that by means of overlapping beams, ultrasound energy could be delivered to tumour tissue to achieve massive cell killings while sparing normal tissue outside the tumour volume to a degree far exceeding that of conventional techniques.

Animals↗
PubMed1977

Cytotoxicity of commonly used solvents at elevated temperatures.

At 43 degrees C (but not at 41 degrees C), organic solvents used to dissolve water-insoluble chemotherapeutic agents become themselves lethal to cells. This finding is not unique to Chinese hamster cells (HA-1); mouse mammary sarcoma cells (EMT-6) behave similarly. The solvent concentrations involved are in the range of those needed to make drug solutions. Hence experiments measuring drug-cell interactions at elevated temperatures must include controls which independently measure solvent effects.

Cell Line↗
PubMed1977

Interaction of amphotericin B and 43 degrees hyperthermia.

At 43 degrees (but not at 41 degrees), the polyene antibiotic amphotericin B effectively inactivates mammalian cells in vitro even at doses which are used prophylactically, routinely, and continuously in some tissue culture laboratories. The greatly enhanced killing may reflect interactions between the drug and hyperthermia at the level of the cells' plasma membrane. A similar enhancement of cell killing at 43 degrees was seen when cells were exposed to nonisotonic salt solutions. Another polyene, nystatin, shows no temperature dependence, at least over the dose range examined, while another antifungal agent, polymyxin B, does so only at very high doses. The in vitro thermosensibility of cells to amphotericin B is reflected in vivo: EMT-6 murine tumor cells were killed much more efficiently in situ at 43 than at 37 degrees. Amphotericin B may be a useful agent in multiple drug thermochemotherapy.

Amphotericin B↗
PubMedDate not supplied

Heat does not induce synthesis of heat shock proteins or thermotolerance in the earliest stage of mouse embryo development.

To study a possible role of HSP in the thermal response of mouse embryos, we examined heat survival and HSP synthesis of mouse embryos at the one-cell stage or the blastocyst stage. One-cell embryos were extremely heat sensitive and synthesized HSP at very low levels or not at all. At that developmental stage neither thermotolerance nor HSP synthesis could be induced by heat shock. In contrast, unheated blastocysts synthesized HSP constitutively, were comparatively heat resistant, and both thermotolerance and enhanced rate of HSP synthesis were induced by a non-lethal heat exposure. Our data demonstrate a correlation between HSP synthesis, thermal sensitivity and thermotolerance in this system, and strengthen the suggestion that gene activation of HSP synthesis is closely related to the differentiation process.

Animals↗
PubMedDate not supplied

Effect of irradiation on thermal sensitivity of bone marrow progenitors.

The heat sensitivity of murine CFU-GM and CFU-E following 2.5 Gy of total body irradiation (TBI) was studied. C3H f/Sed female mice were treated with 2.5 Gy TBI and femoral bone marrow was heated in vitro at 43 degrees C. CFU-GM show heat radiosensitization when bone marrow was heated immediately following irradiation. There was a brief decline in heat and radiation interaction when cells were heated 3 hours following 2.5 Gy of TBI, but heat radiosensitization returned to its maximum from 1 to 2 days following irradiation and remained significantly different from the control on days 5 and 7 following irradiation. The heat and radiation interaction disappeared by 30 days. CFU-E shows significant heat radiosensitization only on day 2 following 2.5 Gy of TBI. Total nucleated cells per femur showed a decrease by 70 per cent in days 1 to 2 following TBI, recovered to control values by day 5, and did not correlate with the changes in heat radiosensitization. Cell cycle analysis of CFU-GM using hydroxyurea showed no significant changes in cell cycle parameters on days 1 and 2 following 2.5 Gy, when maximum heat sensitization was observed. It is concluded that bone marrow progenitors may respond in a different way from other normal tissues to heat and irradiation sequencing, and that these differences must be considered when designing clinical trials.

Animals↗
PubMedDate not supplied

Re-induction of hsp70 synthesis: an assay for thermotolerance.

Of the many heat shock proteins (HSPs), hsp70 appears to correlate best with heat resistance, either permanent or transient. We have investigated various approaches to quantify the concentration of hsp 70, and examined the relationship between hsp70 and cells' thermal sensitivity during the development and decay of thermotolerance in model systems. Here, experiments were performed to determine the possibility of using the rate of synthesis of hsp70 after a second test heat shock to predict the kinetics of thermotolerance. Specifically, we studied the relationship between the retained thermotolerance in a murine tumor cell line SQ-1 and a human tumor cell line, HCT-8, after fractionated heat doses and the cells' ability to re-initiate synthesis of hsp70 in response to an additional test heat dose in vitro. Monolayers of cells were exposed to a first heat treatment (e.g., 41 degrees C, 4 h) and then incubated at 37 degrees C for 0-72 h. At various times after the first heat treatment, cells were either challenged with a 45 degrees C, 45 min heat shock to assess the residual thermotolerance by colony formation, or labelled with [35S]methionine before or after an additional test heat dose (e.g. 43.5 degrees C, 15 min). We found that the cells' ability to re-initiate hsp70 synthesis in response to the test heat shock inversely correlated with retained thermotolerance. Our data suggest the level of hsp70 in thermotolerant cells regulates the rate of synthesis of additional hsp70 in response to the subsequent heat challenge. Furthermore, the results showed that the rate of re-induction of hsp70 synthesis after a test shock can be used as a rapid measure of retained thermotolerance. This study suggests an approach for quantifying the level of retained thermotolerance during a course of fractionated hyperthermia.

Animals↗
PubMedDate not supplied

Heat sensitivity, thermotolerance and protein synthesis of granulocyte and macrophage progenitors from mice and from long-term bone marrow cultures.

Ex-vivo purging of leukaemic cells by single or fractionated heating during bone marrow transplantation for acute myeloid or lymphoblastic leukaemias may be possible since many leukaemic cells may be more sensitive to killing by heat than normal bone marrow colony-forming unit-granulocyte and macrophage (CFU-GM). In these studies we have compared heat response, thermotolerance response and heat shock protein (HSP) synthesis of bone marrow progenitors from intact mice or bone marrow obtained from 37 degrees C or 33 degrees C long-term bone marrow cultures (LTBMCs). Such studies were done to examine whether CFU-GM responses to heat are influenced by in vitro growth conditions. In terms of heat response CFU-GM progenitors from LTBMCs showed increased heat sensitivity when compared to CFU-GM from mice, with CFU-GM from 33 degrees C cultures being more heat-sensitive than those from 37 degrees C cultures. The kinetics of thermotolerance development and decay in CFU-GM were similar from all three systems. Thermotolerance was maximum at 3-9 h, began to decay by 24 h and was absent by 48 h from cells obtained from mice and LTBMCs grown at 37 degrees C. However, CFU-GM from 33 degrees C LTBMCs showed a delay in decay of thermotolerance. The kinetics of the development and decay of thermotolerance remained as above and was independent of incubation conditions whether bone marrow was left in situ or incubated in vitro in medium containing various colony-stimulating factors (CSF) or medium containing no CSF. The synthesis of HSPs was measured by one- and two-dimensional gel electrophoresis. Synthesis of HSP-70 kd was detected following a triggering dose of heat at 0-3 h for bone marrow heated in vivo and 0-6 h for bone marrow from LTBMCs. These studies indicate that CFU-GM heat and thermotolerance response was influenced by the temperature at which the cultures were maintained but not by other differences present in their microenvironment.

Animals↗
PubMedDate not supplied

Thermal response of yeast cells overexpressing hsp70 genes.

We have performed experiments to examine whether the overexpression of different members of the hsp70 family (SSA1, SSA2 and SSA4) in yeast Saccharomyces cerevisiae protects cells from thermal stress. Yeast cells were transformed with plasmids containing the SSA1 or SSA4 gene which was placed under the control of a galactose-inducible GAL1 promoter. In galactose, transformed yeast cells successfully overexpressed these hsp70 proteins at 23 degrees C, their normal growth temperature. When cell survival after heat shock at 50 or 56 degrees C was examined, our results showed that overexpression of SSA1 or SSA4 protein did not protect yeast cells against thermal stress, nor affect the cells' ability to develop thermotolerance. In contrast, thermal sensitivity was modified significantly by growing cells in galactose. Cellular survival of these cells after a 50 degrees C, 30-min heat treatment was 10(4)-fold higher than that of the same strains grown in glucose. In order to study the effects of overexpression of hsp70 on the thermal response of yeast cells independent of the carbon source, we eliminated the glucose-galactose-associated differences in thermal sensitivity by cloning the aforementioned hsp70 genes under the control of a glyceraldehyde phosphatase GAP promoter. S. cerevisiae transformed with these plasmids overexpressed the appropriate hsp70 gene in glucose as well as in galactose at 23 degrees C. Again, the overexpression of hsp70 neither protected cells from thermal stress, nor had a significant effect on the development of thermotolerance.

Fungal Proteins↗
PubMedDate not supplied

Rat fibroblasts transfected with the human 70-kDa heat shock gene exhibit altered translation and eukaryotic initiation factor 2 alpha phosphorylation following heat shock.

Heat shock inhibits translation in a wide variety of cells. After heating, eukaryotic initiation factor 2-alpha (eIF-2 alpha) becomes phosphorylated which prevents the binding of Met-tRNA to the 40s ribosomal subunit inhibiting initiation of translation. Thermotolerant cells demonstrate resistance to inhibition of translation by additional heating suggesting that heat shock proteins may help to maintain translational integrity following thermal stress. Here we have examined the effects of increased intracellular levels of hsp70 protein on translation and eIF-2 alpha phosphorylation using rat fibroblasts stably transfected with a cloned human hsp70 gene. We observed a decrease in the rate of translational inhibition following heat shock in both hsp70-transfected and thermotolerant cells. Upon recovery at 37 degrees C, both hsp70-transfected and thermotolerant cells exhibit a faster rate of translational recovery. Utilizing slab gel isoelectric focusing coupled with immunoblotting we demonstrate that 45 degrees C heat shock leads to a rapid 4-5-fold increase in eIF-2 alpha phosphorylation, with little difference seen between control cells and hsp70-transfected cells. However, dephosphorylation of eIF-2 alpha occurs faster in the hsp70-transfected cells. These results suggest that hsp70 may play a role in facilitating the dephosphorylation of eIF-2 alpha as well as reversing the inhibition of translation following heat shock.

Animals↗