Time-temperature relationships for step-down heating in normal and thermotolerant cells

Oncological hyperthermia: The correct dosing in clinical applications

The problem with the application of conventional hyperthermia in oncology is firmly connected to the dose definition, which conventionally uses the concept of the homogeneous (isothermal) temperature of the target. Its imprecise control and complex evaluation is the primary barrier to the extensive clinical applications. The aim of this study was to show the basis of the problems of the misleading dose concept. A clear clarification of the proper dose concept must begin with the description of the limitations of the present doses in conventional hyperthermia applications. The surmounting of the limits the dose of oncologic hyperthermia has to be based on the applicability of the Eyring transition state theory on thermal effects. In order to avoid the countereffects of thermal homeostasis, the use of precise heating on the nanoscale with highly efficient energy delivery is recommended. The nano‑scale heating allows for an energy‑based dose to control the process. The main aspects of the method are the following: i) It is not isothermal (no homogeneous heating); ii) malignant cells are heated selectively; and iii) it employs high heating efficacy, with less energy loss. The applied rigorous thermodynamical considerations show the proper terminology and dose concept of hyperthermia, which is based on the energy‑absorption (such as in the case of ionizing radiation) instead of the temperature‑based ideas. On the whole, according to the present study, the appropriate dose in oncological hyperthermia must use an energy‑based concept, as it is well‑known in all the ionizing radiation therapies. We propose the use of Gy (J/kg) in cases of non‑ionizing radiation (hyperthermia) as well.

Effect of adenosine 3':5'-cyclic monophosphate and inhibition of protein kinase A on heat sensitivity in H35 hepatoma cells

PURPOSE

To investigate the role of the cyclic adenosine 3':5'-monophosphate (AMP) signal transductions pathway in heat-induced cell death and the development of thermotolerance.

METHODS AND MATERIALS

Reuber H35 rat hepatoma cells were heated after preincubation with various compounds known to modulate the cyclic AMP signal transduction pathway. Cell survival was determined by colony-forming ability.

RESULTS

Preincubation of H35 cells with forskolin, a stimulator of adenylate cyclase, in combination with IBMX (3-isobutyl-1-methylxanthine), an inhibitor of cyclic AMP phosphodiesterase, results in thermosensitization. Similar results are obtained with various cyclic AMP analogs. Maximum thermosensitization occurs with 0.5 mM dibutyryl cyclic AMP (DBcAMP) after a preincubation period of 5 h and heating in the presence of the drug. The same relative degree of thermosensitization is found with 8-Cl-cAMP, but at a 10-fold lower concentration. Thermosensitization by DBcAMP is prevented by H89, a specific inhibitor of cyclic AMP-dependent protein kinase (PKA). Without additional cyclic AMP-inducing factors, H89 induces thermoprotection. None of the drug treatments are cytotoxic at 37 degrees C. DBcAMP does not affect the development of heat-induced thermotolerance but it reduces its expression to an extent similarly found in the observed thermosensitization in nonthermotolerant cells.

CONCLUSION

The results strongly indicate that the cyclic AMP signal transduction pathway is involved in the process of heat-induced cell death. DBcAMP reduces the expression of thermotolerance, but does not affect its induction.

Modulation of HSP68 gene expression after heat shock in thermosensitized and thermotolerant cells is not solely regulated by binding of HSF to HSE

Induction of heat shock proteins (HSP) is generally regarded as a consequence of binding of the heat shock transcription factor (HSF) to heat shock elements (HSE), i.e. to be a single hit induction. The activation of HSF and the induction of HSP68 mRNA were studied in non pretreated Reuber H35 rat hepatoma cells in a thermosensitized and in a thermotolerant state. It was found that HSF in Reuber H35 hepatoma cells already acquires maximum DNA binding activity at temperatures that are too low to induce HSP68 mRNA. Directly following heat shock cells are in a transient thermosensitized state. In this state a second stress of lower impact leads to even higher production of HSP68, which corresponds with a decreased decay rate HSF-HSE binding. Directly following the thermosensitized state cells become refractory. In this period a second stress of the same impact does lead to HSF-HSE binding but the production of HSP68 mRNA is lowered, while only higher-impact stresses lead to high inductions of the said mRNA. The results indicate that regulation of HSP68 gene transcription involves at least one additional event outside the acquisition of DNA-binding activity by HSF and that this process can thus be described as a multiple-hit occurrence.

Hepatoma cells adapted to proliferate under normally lethal hyperthermic stress conditions show rapid decay of thermoresistance and heat shock protein synthesis when returned to 37 degrees C

H35 hepatoma cultures were adapted to sustained growth at 41.3 degrees C. In these variant cells the 'basic' levels of various heat shock proteins (hsps), especially those of hsp60, 70 and 100, are significantly raised. These cells exhibit a thermoresistance comparable with the induced thermotolerance in normal hepatoma cells heat shocked at 42.5 degrees C for 30 min. However, this resistance of variant cells shows a rapid, exponential decay with a half-time of 2.2 h when the temperature is lowered to 37 degrees C, with a concomitant decrease of the synthesis of hsp60 and 70. Heat shock experiments with variant cells grown at 41.3 degrees C lead to increased thermoresistance and synthesis of hsps when further incubation was performed at the original temperature but not at 37 degrees C. In the latter case, only a 3-h delay in the onset of decay of thermoresistance is observed. However, when the variant cells were incubated at 37 degrees C prior to heat stress normal induction of thermoresistance and hsp synthesis return inversely proportional to the progression of thermoresistance decay. Thermoresistant cells thus seem to be valuable tools in the study of the down-regulation of thermoresistance as well as of hsp synthesis.

Sensitization to x-rays by sodium arsenite or heat in normal cells and in cells with an induced tolerance for heat and arsenite

In this study we compared sensitization to x-rays by heat or sodium arsenite and the effect of an induced heat or arsenite resistance on radiosensitization. Treatment of Reuber H35 hepatoma cells with either heat or arsenite causes a dose-dependent radiosensitization. Based on a comparison of isosurvival doses for arsenite and heat, arsenite causes a stronger enhancement of the radiosensitivity. Radiosensitization increases exponentially with increasing sensitizer dose. It is gradually lost when the time interval between irradiation and treatment with heat or arsenite increases, depending on the treatment sequence. For x-rays prior to heat, radiosensitization disappears approximately twice as fast as in the reverse case. Arsenite radiosensitization shows approximately the same kinetics for an isoeffective combination, but slightly longer times are needed for the complete clearance of the interaction. As with heat, an exposure to arsenite induces a stress response in cultured cells which results in the development of an increased tolerance towards a second exposure. Heat and arsenite induce self- as well as cross-tolerance. The reduction in arsenite or heat toxicity in tolerant cells is correlated with a reduction in radiosensitization. The mechanisms for heat and arsenite cytotoxicity appear to be different. A combination of non-toxic doses of heat and arsenite has a synergistic effect on the cytotoxicity. One hour incubation with 0.02 mM arsenite at 41 degrees C has the same cytotoxicity as 0.2 mM after 3 h incubation at 37 degrees C, and the amount of radiosensitization induced by these treatments is approximately the same.