Heat shock factors (HSFs) are the master regulators of transcription under protein-damaging conditions, acting in an environment where the overall transcription is silenced. We determined the genomewide transcriptional program that is rapidly provoked by HSF1 and HSF2 under acute stress in human cells. Our results revealed the molecular mechanisms that maintain cellular homeostasis, including HSF1-driven induction of polyubiquitin genes, as well as HSF1-and HSF2-mediated expression patterns of cochaperones, transcriptional regulators, and signaling molecules. We characterized the genomewide transcriptional response to stress also in mitotic cells where the chromatin is tightly compacted. We found a radically limited binding and transactivating capacity of HSF1, leaving mitotic cells highly susceptible to proteotoxicity. In contrast, HSF2 occupied hundreds of loci in the mitotic cells and localized to the condensed chromatin also in meiosis. These results highlight the importance of the cell cycle phase in transcriptional responses and identify the specific mechanisms for HSF1 and HSF2 in transcriptional orchestration. Moreover, we propose that HSF2 is an epigenetic regulator directing transcription throughout cell cycle progression.ChIP-seq | ENCODE | human genome | proteostasis C ells exposed to proteotoxic conditions provoke a rapid and transient response to maintain homeostasis. The stress response induces profound cellular adaptation as cytoskeleton and membranes are reorganized, cell cycle progression is stalled, and the global transcription and translation are silenced (1, 2). Despite the silenced chromatin environment, the stressed cell mounts a transcriptional program that involves induction of genes coding for heat shock proteins (HSPs). HSPs are molecular chaperones and proteases that assist in protein folding and maintain cellular structures and molecular functions (3).Heat shock factor 1 (HSF1) is an evolutionarily well-conserved transcription factor that is rapidly activated by stress and absolutely required for the stress-induced HSP expression (4). Aberrant HSF1 levels are associated with stress sensitivity, aging, neurodegenerative diseases, and cancer (5-9). Instead of a single HSF in yeasts and invertebrates, vertebrates contain a family of four members, HSF1-4. HSF2 and HSF4 are involved in corticogenesis, spermatogenesis, and formation of sensory epithelium, and they have primarily been considered as developmental factors (10-14). HSF1 and HSF2 share high sequence homology of the DNA-binding and oligomerization domains and are able to form heterotrimers at the chromatin (15, 16). Moreover, HSF2 participates in the regulation of stress-responsive genes and is required for proper protein clearance also at febrile temperatures (17,18). Although HSF1 and HSF2 have been shown to interplay on the heat shock elements (HSEs) of the target loci, their impacts on transcription of chaperone genes are remarkably different; HSF1 is a potent inducer of transcription, whereas HSF2 is a poor transactivator o...

SUMMARYmiR-18 belongs to the Oncomir-1 or miR-17~92 cluster that is intimately associated with the occurrence and progression of different types of cancer. However, the physiological roles of the Oncomir-1 cluster and its individual miRNAs are largely unknown. Here, we describe a novel function for miR-18 in mouse. We show that miR-18 directly targets heat shock factor 2 (HSF2), a transcription factor that influences a wide range of developmental processes including embryogenesis and gametogenesis. Furthermore, we show that miR-18 is highly abundant in testis, displaying distinct cell-type-specific expression during the epithelial cycle that constitutes spermatogenesis. Expression of HSF2 and of miR-18 exhibit an inverse correlation during spermatogenesis, indicating that, in germ cells, HSF2 is downregulated by miR-18. To investigate the in vivo function of miR-18 we developed a novel method, T-GIST, and demonstrate that inhibition of miR-18 in intact seminiferous tubules leads to increased HSF2 protein levels and altered expression of HSF2 target genes. Our results reveal that miR-18 regulates HSF2 activity in spermatogenesis and link miR-18 to HSF2-mediated physiological processes such as male germ cell maturation.