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

Stressor-specific enhancement of hsp induction by low doses of stressors in conditions of self- and cross-sensitization

In this paper, the pattern of induction of heat shock proteins (hsps) was studied in cultured Reuber H35 rat hepatoma cells by sequential application of different stressors. We analyzed whether a specific stress condition is able to induce an enhanced sensitivity to a subsequent application of a low dose of either the same or another stressor (self-sensitization and cross-sensitization, respectively). As a measure of sensitization, the stimulation of hsp induction was employed. Three different stressor conditions (heat shock, sodium arsenite and cadmium chloride) were used in doses which exerted a similar impact on overall protein synthesis. A synergistic effect in induction of the synthesis of various hsps was observed when a high stressor dose was followed by an 8-h incubation in a lower stressor dose in both self- and cross-sensitization experiments. The low-dose conditions used as second treatments did not induce any responses in non-pretreated cells. Studies in cultured cells have demonstrated stressor-specific hsp induction patterns. In this study we analyzed whether the pattern of hsps induced by the low-dose condition is characteristic for the first sensitizing stressor or for the secondary stressor applied in a low dose. The pattern of hsps which was induced above the level of the high-dose effect, due to the incubation with the secondary applied low-dose condition, was found to be characteristic for the secondary stressor and not for the sensitizing primary treatment. These results are of importance for an improved understanding of the regulation of heat shock protein synthesis in conditions of self- and cross-sensitization, as well as for a proper use of hsps as biomarkers of exposure to environmental stress.

A mathematical model of the hsp70 regulation in the cell

A mathematical model of the regulation process of the heat shock protein hsp70 in the cell is presented. The model describes the damaging effect of elevated temperature on proteins; the interaction of free hsp70 with injured proteins and its chaperone role in nascent protein translation; the relation between the amount of free hsp70 and the formation of the activated trimer form of the heat shock factor protein (HSF); the binding of activated HSF with the heat shock elements on the DNA; the transcription of mRNA of hsp70 and the synthesis of hsp70. The reaction of the model to a temporal rise in temperature shows an initial decline and a subsequent sharp rise to an ultimately increased level of free hsp70 in the cell. The response of the model to both a single and two consecutive heat shocks appears to closely resemble experimental data on hsp70 synthesis. This general agreement demonstrates the structure of the model to be sound and suitable as a basis for further modelling the complex tolerance mechanism of the cell.

Enhancement of the stress response by minute amounts of cadmium in sensitized Reuber H35 hepatoma cells

The aim of this study was to determine whether the cadmium-induced cellular stress response can be modulated by the subsequent application of low concentrations of the same ion. It is shown that exposure of Reuber H35 rat hepatoma cells to cadmium concentrations of 10 or 30 microM for 1 h leads to a biphasic change in their sensitivity towards a second exposure to cadmium, an initial sensitization is followed by development of tolerance towards the secondary treatment with cadmium. Furthermore, incubations for 1 h in the presence of 10 microM of cadmium induce the synthesis of the major heat shock proteins except for hsp60. A step-down cadmium regime, i.e. a pretreatment of 1 h with 10 or 30 microM immediately followed by incubations with lower concentrations of cadmium (ranging from 0.03 to 1 microM), leads to additional increases in hsp synthesis. Since no effect of these low concentrations was observed on hsp synthesis in non-pretreated cells, the effect of a step-down treatment thus results in a higher effect on hsp synthesis than could be expected based on their summation. The sensitized cells also develop a higher level of tolerance in the presence of the above mentioned low concentrations of cadmium. It can be concluded that during the transient period of enhanced sensitivity, low concentrations of the original stressor enhance the synthesis of hsps and thus induce higher levels of tolerance in comparison with cells which only received the primary cadmium treatment.

Is heat shock protein re-induction during tolerance related to the stressor-specific induction of heat shock proteins?

The existence of stressor-specific induction programs of heat shock proteins (hsps) leads us to analyze the possible occurrence of a stressor-specific tolerance induced by either heat shock, arsenite, or cadmium. As a measure of this tolerance re-induction of hsps was studied. In this paper, we tested whether the refractory state is either valid for each specific hsp (implying independent regulation of every member of the heat shock protein family) or extends from small subsets of the hsp-family to even larger groups of proteins (indicating a more common denominator in their regulation). (re-)induction of hsps does not seem to be regulated at the level of each individual hsp since differences in induced synthesis of hsps between two stressor conditions are not supplemented systematically upon the sequential application of the two stressors. The most notable example in this respect is hsp60. A pretreatment with cadmium, which hardly induces synthesis of this hsp, does induce a tolerance to (re)-induction by heat shock, which normally induces hsp60. This suggests the existence of a more common denominator regulating the coordinate expression of at least some hsps. From our data we conclude that the degree, but not the pattern, of hsp re-induction is influenced by the type of stressor used in the pretreatment. The pattern of hsps induced by a secondary applied stressor still shows most of its stressor-specificity and seems to be independent of any pretreatment. The possible implications of stressor-specificity are discussed.