Diverse effects of Stat1 on the regulation of hsp90alpha gene under heat shock.

Stat1 has been known as a regulator of gene expression and a mediator of IFNgamma signaling in mammalian cells, while its effect in a heat shock response remains unclear. We used RNAi knockdown, point mutations, ChIP and promoter activity assays to study the effect of Stat1 on the heat-shock induction of the hsp90alpha gene under heat shock conditions. We found that Stat1 regulates the heat shock induction of its target genes, the hsp90alpha gene in a heat shock response while the constitutive activity of the gene remains unaffected. The result of Stat1 in complex with Stat3 and HSF1 that bound at the GAS to lead a moderate heat shock induction was designated as an "intrinsic" induction of the hsp90alpha gene. Additionally a reduced or an elevated level of heat shock induction was also controlled by the Stat1 on hsp90alpha. These diverse effects on the hsp90alpha gene were a "reduced" induction with over-expressed Stat1 elicited by transfection of wild-type Stat1 or IFNgamma treatment, bound at the GAS as homodimer; and an "enhanced" heat shock induction with a mutation-mediated prohibition of Stat1/GAS binding. In conclusion, the status and efficacy of Stat1 bound at the GAS of its target gene are pivotal in determining the impact of Stat1 under heat shock. The results provided the first evidence on the tumor suppressor Stat1 that it could play diverse roles on its target genes under heat shock that also shed lights on patients with fever or under thermotherapy.

Blocking HSF1 by dominant-negative mutant to sensitize tumor cells to hyperthermia.

Heat shock protein 70 (HSP70), an antiapoptotic chaperon protein, is highly expressed in human breast tumors and renders them resistant to such therapy as hyperthermia. In the present study, we inhibited the expression of HSP70 by blocking the heat shock transcription factor 1 (HSF1) function with its dominant-negative mutant (mHSF1) in Bcap37 cells, a thermotolerant breast cancer cell line. Here we report that retrovirus-mediated transfer of mHSF1 led to massive cell death of Bcap37 after hyperthermia. mHSF1 sensitized Bcap37 cells to hyperthermia by promoting apoptosis induced by heat shock. We also examined the efficacy of mHSF1 gene therapy in the nude mouse. mHSF1 transfection led to diminution of tumor growth with hyperthermia therapy. Thus, disrupting HSF1 in combination with hyperthermia may open new possibilities for treatment of cancers that have acquired resistance to heat treatment.