The heat shock response is a defense reaction activated by proteotoxic damage induced by physiological or environmental stress. Cells respond to the proteotoxic damage by elevated expression of heat shock proteins (Hsps) that function as molecular chaperones and maintain the vital homeostasis of protein folds. Heat shock factors (HSFs) are the main transcriptional regulators of the stress-induced expression of hsp genes. Mammalian HSF1 was originally identified as the transcriptional regulator of the heat shock response, whereas HSF2 has not been implicated a role in the stress response. Previously, we and others have demonstrated that HSF1 and HSF2 interact through their trimerization domains, but the functional consequence of this interaction remained unclear. We have now demonstrated on chromatin that both HSF1 and HSF2 were able to bind the hsp70 promoter not only in response to heat shock but also during hemin-induced differentiation of K562 erythroleukemia cells. In both cases an intact HSF1 was required in order to reach maximal levels of promoter occupancy, suggesting that HSF1 influences the DNA binding activity of HSF2. The functional consequence of the HSF1-HSF2 interplay was demonstrated by real-time reverse transcription-PCR analyses, which showed that HSF2 was able to modulate the HSF1-mediated expression of major hsp genes. Our results reveal, contrary to the predominant model, that HSF2 indeed participates in the transcriptional regulation of the heat shock response.The heat shock response enables the cell to cope with the deleterious effects of protein-damaging stresses, e.g. heat, heavy metals, and viral and bacterial infections. Characteristic of the heat shock response is a down-regulation of gene transcription and protein synthesis in general, whereas the transcription of a specific subset of genes called the heat shock genes is induced (1, 2). The classical heat shock genes include the hsp genes, but genome-wide analysis has revealed numerous other genes to be activated by heat and other stresses (3). During heat shock, the Hsps 3 function as molecular chaperones that bind to and aid the folding of damaged proteins, thereby preventing protein aggregation under stressful conditions (1, 4, 5). The induction of Hsps is mainly regulated by a family of heat shock transcription factors (HSFs), which bind to the heat shock elements (HSEs) on heat shock genes and other target genes (6 -9).Four HSFs (HSF1-4) exist in vertebrates. HSF1 and HSF2 are the most studied factors because of their co-expression in most tissues and cell lines (9). HSF3 has been found only in avian species, and HSF4 is the most recently identified member in mammals and expressed predominantly in lens and brain (10 -15). Mammalian HSF1 corresponds to the single HSF in yeast, nematode, and fruit fly (16 -19) and is considered the bona fide stress-activated transcription factor. Mice lacking HSF1 and fibroblasts derived from these animals are sensitive to stress and do not develop thermotolerance or display induction of Hsps upo...

Organisms respond to circumstances threatening the cellular protein homeostasis by activation of heat-shock transcription factors (HSFs), which play important roles in stress resistance, development, and longevity. Of the four HSFs in vertebrates (HSF1-4), HSF1 is activated by stress, whereas HSF2 lacks intrinsic stress responsiveness. The mechanism by which HSF2 is recruited to stress-inducible promoters and how HSF2 is activated is not known. However, changes in the HSF2 expression occur, coinciding with the functions of HSF2 in development. Here, we demonstrate that HSF1 and HSF2 form heterotrimers when bound to satellite III DNA in nuclear stress bodies, subnuclear structures in which HSF1 induces transcription. By depleting HSF2, we show that HSF1-HSF2 heterotrimerization is a mechanism regulating transcription. Upon stress, HSF2 DNA binding is HSF1 dependent. Intriguingly, when the elevated expression of HSF2 during development is mimicked, HSF2 binds to DNA and becomes transcriptionally competent. HSF2 activation leads to activation of also HSF1, revealing a functional interdependency that is mediated through the conserved trimerization domains of these factors. We propose that heterotrimerization of HSF1 and HSF2 integrates transcriptional activation in response to distinct stress and developmental stimuli.