MODULATION OF HSP68 GENE-EXPRESSION AFTER HEAT-SHOCK IN THERMOSENSITIZED AND THERMOTOLERANT CELLS IS NOT SOLELY REGULATED BY BINDING OF HSFTO HSE

Induction of heat shock proteins (HSP) is generally regarded as a cons equence of binding of the heat shock transcription factor (HSF) to hea t shock elements (HSE), i.e. to be a single hit induction. The activat ion of HSF and the induction of HSP68 mRNA were studied in non pretrea ted Reuber H35 rat hepatoma cells in a thermosensitized and in a therm otolerant state. It was found that HSF in Reuber H35 hepatoma cells al ready acquires maximum DNA binding activity at temperatures that are t oo low to induce HSP68 mRNA. Directly following heat shock cells are i n a transient thermosensitized state. In this state a second stress of lower impact leads to even higher production of HSP68, which correspo nds with a decreased decay rate HSF-HSE binding. Directly following th e thermosensitized state cells become refractory. In this period a sec ond stress of the same impact does lead to HSF-HSE binding but the pro duction of HSP68 mRNA is lowered, while only higher-impact stresses le ad to high inductions of the said mRNA. The results indicate that regu lation of HSP68 gene transcription involves at least one additional ev ent outside the acquisition of DNA-binding activity by HSF and that th is process can thus be described as a multiple-hit occurrence.