The influence of environmental pH on the interaction and repair of heat and radiation damage

Differential heat sensitivity of normal and transformed Syrian golden hamster embryo cells in confluence

The heat sensitivities of normal and transformed cells derived from Syrian golden hamster embryo (SHE) cells were investigated. There were no differences in heat sensitivities between normal and transformed cells when they were exponentially growing. When confluent cultures were heated in situ, only normal cells showed a reduced heat sensitivity, 4-5 times less than that of exponentially growing cells. However, when these confluent cultures were trypsinized, diluted in medium and heated in suspension, the difference disappeared. When mixed cultures of normal and transformed S11 cells in confluence were heated at 43 degrees C, transformed S11 cells were selectively killed. The induction of HSP70 synthesis by heating in normal SHE cells and transformed counterparts were investigated in confluence using monoclonal antibody. The rate of enhanced HSP70 synthesis after heat treatment did not correlate with the difference in heat sensitivities between normal and transformed SHE cells in confluence as measured by cell survival. The results that normal SHE cells become more resistant to heat than their transformed counterparts only in confluence suggest that cell-to-cell contact plays an important role in the heat resistance of normal SHE cells in vivo.

A differential low pH effect on tumour cells grown in vivo and in vitro when treated with hyperthermia

The effectiveness of low extracellular pH in sensitizing cells to heat was studied using SCK mammary carcinoma cells of A/J mice. Solid tumours of 400-600 mg or cells grown in vitro for less than 13 weeks were dispersed to single cells and the in vivo- or in vitro-derived cells were suspended in a medium of pH 7.2 or 5.5-6.6 and then heated at 41-44 degrees C in vitro using a water bath. The dispersion procedure did not alter the heat sensitivity of these cells, but acidic medium of pH below 6.6 caused an increase in heat sensitivity. This effect of low pH decreased with increasing temperature of heating for both cell types. However, the effect was much smaller on in vivo-derived cells than that on in vitro-derived cells for any heating temperature tested. This reduction of the pH effect was also observed for cells derived from larger tumours as well as tumours at an early stage of growth in which the internal milieu was not acidic. This indicates that cellular adaptation to low intratumour pH was not the cause of the reduced pH effect; instead, factors other than low pH must cause the reduced pH effect seen in tumour-derived cells.

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.

Acid modification of thermal damage and its relationship to nutrient availability

The purpose of this present investigation was to determine if the presence of exogenous nutrients constitutes a prerequisite for acid modification of thermal damage and to determine if thermal cytotoxicity is affected by nutrient availability under acid conditions. To this end, Chinese hamster ovary cells (CHO) were heated (42 degrees or 43 degrees C) and/or irradiated in either McCoy's medium containing 10% fetal bovine serum or glucose free Hanks Balanced Salt Solution (HBSS). Both thermal sensitivity and thermal radiosensitization were increased when CHO cells were treated under acid conditions (e.g., pH 6.8) compared to alkaline conditions (e.g., pH 7.2), independent of the media used. Furthermore, decreasing nutrient availability increased thermal cytotoxicity, but the increase was greatest at pH 7.2 compared to pH 6.8. This study indicates that thermal sensitivity is more dependent upon pH than upon nutrient availability.

The relevance of tumour pH to the treatment of malignant disease

The wide range of tumour pH values that have been determined in human tumours is shown in Fig. 4. It can be seen that tumour pH values may be very low, or may fall in the same range as the values found in normal tissues. This means that pH-mediated modification of therapeutic effectiveness will be patient specific, rather than a general phenomenon. That the pH of the cellular environment might influence the effectiveness of various therapeutic agents is not a new idea. The data published in this field to date concerning such effects have been discussed extensively and are summarized in Table IV. Here we can see that low pH leads to decreased cell survival following treatment with hyperthermia, radiotherapy combined with hyperthermia, radiosensitizers and various chemotherapeutic agents. Conversely, low pH affords some protection against radiation and some drugs. Most of these data were, of necessity, derived from in vitro studies. In vivo studies are in most cases not feasible due to the difficulty of isolating the effect of one selected factor. Low tumour pH is, in vivo, generally assumed to be closely interlinked with tissue hypoxia and low blood-flow levels, each of which may individually influence the experimental outcome. Moreover, most of the aforementioned in vitro studies were conducted under well-oxygenated conditions. As previously mentioned, euoxic cells can, under certain conditions, maintain a pH gradient over the cell membrane. This collapses with the onset of hypoxia, leading to intracellular acidification. Low oxygen levels have been shown to be characteristic of many tumours. Within these limitations it is thus evident that tumour pH values could have far-reaching consequences for therapy. If the in vitro findings should prove to be relevant to the clinical situation various applications are possible. Pre-selection of patients less likely to respond to certain (toxic) chemotherapeutic agents, or conversely selection of agents that are more likely to be effective in the pH range of the tumour to be treated are two examples. Alternatively, the exploitation of low tumour pH values is a possibility. Agents that form or release toxic derivatives in areas of low pH, e.g., pH-sensitive liposomes, will work selectively in such areas. Tumour selective therapy may also be possible in patients with higher tumour pH values if the tumour pH can be lowered. This has been achieved experimentally by the administration of hyperthermia at temperatures above 42 degrees C, or by the administration of glucose.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of heat, radiation and pH on mouse mammary tumor cells

Experiments were done in vitro with cells derived from a C3H mouse mammary adenocarcinoma (MADCAP-37) to determine the influence of pH and sequencing on hyperthermia induced radiosensitization. The heat treatment was one hour at 42.5 degrees C in a precision controlled water bath. The heat treatment did not significantly reduce cell survival. Five Gy of x radiation was given at variable intervals from two hours before to two hours following heating. Survival curves under acid pH (6.4-6.7) and alkaline pH (7.2-7.4) were obtained for selected intervals, corresponding to points of interest in the sequencing study. Survival was greater for irradiation alone at acid pH than at alkaline pH. The greatest cell killing occurred when irradiation and hyperthermia were given simultaneously at either alkaline or acid pH. At alkaline pH, survival increased as the interval increased between heating and irradiation. At acid pH, radiation survival remained low for as much as two hours following heating. If tumors are at acid pH, the greatest differential effect should be obtained for irradiation following heating because cells at alkaline pH (normal) had approximately 10 times greater survival. Vascular changes caused by heating might alter that response.

Influence of pH and thermotolerance on the enhancement of X-ray induced inactivation of cultured mammalian cells by hyperthermia

The influence of pH and thermotolerance on thermal enhancement of radiation effects have been studied with M8013 cells, derived from a transplantable murine mammary carcinoma. The effect of combined heat-irradiation treatment of cells at different pH of the medium clearly shows an influence of the pH. Thermal enhancement of the effectiveness of radiation is relatively strong below pH 7.0 and above pH 7.75. In the physiological range pH 6.7-7.4 the influence of pH is not very large. This implies that combination of hyperthermia and radiotherapy in cancer treatment would not give much therapeutic advantage from low pH which is often observed in solid tumours as this low pH is rarely below pH 6.7. Sensitization of cells to the effects of heat treatment alone at a low pH of the cellular medium is greatly decreased when thermotolerance is induced in the cells by a prior heat treatment 4-6 hours before the final treatment. When the cells are in the thermotolerant state, induced by a prior heat treatment, not only resistance to heat treatment, but also resistance to combined heat-irradiation treatment is observed. The degree of this latter resistance is independent of the pH of the cellular medium during the combined heat-irradiation treatment. In spite of the resistance, strong thermal enhancement of radiation effects can be observed when cells are treated in the thermotolerant state by a strong heat treatment. Without thermotolerance such a strong heat treatment, whether combined with irradiation or not, caused so much effect that it was impossible to determine relative cellular survival. The enhancement of radiation effects by a strong heat treatment with cells in the thermotolerant state is largely independent of the pH of the cellular medium.