Increased HSF1 expression predicts shorter disease-specific survival of prostate cancer patients following radical prostatectomy.

Prostate cancer is a highly heterogeneous disease and the clinical outcome is varying. While current prognostic tools are regarded insufficient, there is a critical need for markers that would aid prognostication and patient risk-stratification. Heat shock transcription factor 1 (HSF1) is crucial for cellular homeostasis, but also a driver of oncogenesis. The clinical relevance of HSF1 in prostate cancer is, however, unknown. Here, we identified HSF1 as a potential biomarker in mRNA expression datasets on prostate cancer. Clinical validation was performed on tissue microarrays from independent cohorts: one constructed from radical prostatectomies from 478 patients with long term follow-up, and another comprising of regionally advanced to distant metastatic samples. Associations with clinical variables and disease outcomes were investigated. Increased nuclear HSF1 expression correlated with disease advancement and aggressiveness and was, independently from established clinicopathological variables, predictive of both early initiation of secondary therapy and poor disease-specific survival. In a joint model with the clinical Cancer of the Prostate Risk Assessment post-Surgical (CAPRA-S) score, nuclear HSF1 remained a predictive factor of shortened disease-specific survival. The results suggest that nuclear HSF1 expression could serve as a novel prognostic marker for patient risk-stratification on disease progression and survival after radical prostatectomy.

Expression of HSF2 decreases in mitosis to enable stress-inducible transcription and cell survival.

Unless mitigated, external and physiological stresses are detrimental for cells, especially in mitosis, resulting in chromosomal missegregation, aneuploidy, or apoptosis. Heat shock proteins (Hsps) maintain protein homeostasis and promote cell survival. Hsps are transcriptionally regulated by heat shock factors (HSFs). Of these, HSF1 is the master regulator and HSF2 modulates Hsp expression by interacting with HSF1. Due to global inhibition of transcription in mitosis, including HSF1-mediated expression of Hsps, mitotic cells are highly vulnerable to stress. Here, we show that cells can counteract transcriptional silencing and protect themselves against proteotoxicity in mitosis. We found that the condensed chromatin of HSF2-deficient cells is accessible for HSF1 and RNA polymerase II, allowing stress-inducible Hsp expression. Consequently, HSF2-deficient cells exposed to acute stress display diminished mitotic errors and have a survival advantage. We also show that HSF2 expression declines during mitosis in several but not all human cell lines, which corresponds to the Hsp70 induction and protection against stress-induced mitotic abnormalities and apoptosis.

Anaphase-promoting complex/cyclosome participates in the acute response to protein-damaging stress.

The ubiquitin E3 ligase anaphase-promoting complex/cyclosome (APC/C) drives degradation of cell cycle regulators in cycling cells by associating with the coactivators Cdc20 and Cdh1. Although a plethora of APC/C substrates have been identified, only a few transcriptional regulators are described as direct targets of APC/C-dependent ubiquitination. Here we show that APC/C, through substrate recognition by both Cdc20 and Cdh1, mediates ubiquitination and degradation of heat shock factor 2 (HSF2), a transcription factor that binds to the Hsp70 promoter. The interaction between HSF2 and the APC/C subunit Cdc27 and coactivator Cdc20 is enhanced by moderate heat stress, and the degradation of HSF2 is induced during the acute phase of the heat shock response, leading to clearance of HSF2 from the Hsp70 promoter. Remarkably, Cdc20 and the proteasome 20S core α2 subunit are recruited to the Hsp70 promoter in a heat shock-inducible manner. Moreover, the heat shock-induced expression of Hsp70 is increased when Cdc20 is silenced by a specific small interfering RNA (siRNA). Our results provide the first evidence for participation of APC/C in the acute response to protein-damaging stress.

Regulation of the members of the mammalian heat shock factor family.

Regulation of gene expression is fundamental in all living organisms and is facilitated by transcription factors, the single largest group of proteins in humans. For cell- and stimulus-specific gene regulation, strict control of the transcription factors themselves is crucial. Heat shock factors are a family of transcription factors best known as master regulators of induced gene expression during the heat shock response. This evolutionary conserved cellular stress response is characterized by massive production of heat shock proteins, which function as cytoprotective molecular chaperones against various proteotoxic stresses. In addition to promoting cell survival under stressful conditions, heat shock factors are involved in the regulation of life span and progression of cancer and they are also important for developmental processes such as gametogenesis, neurogenesis and maintenance of sensory organs. Here, we review the regulatory mechanisms steering the activities of the mammalian heat shock factors 1­4.

miR-18, a member of Oncomir-1, targets heat shock transcription factor 2 in spermatogenesis.

miR-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.

Heterotrimerization of heat-shock factors 1 and 2 provides a transcriptional switch in response to distinct stimuli.

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.

Heat shock factor 2 (HSF2) contributes to inducible expression of hsp genes through interplay with HSF1.

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.

Clustering of heat-shock factors.

Clusterin is a ubiquitous glycoprotein found in most physiological fluids and tissues. Although not fully understood, the function of clusterin seems to be related to its ability to bind a wide variety of molecules. Since clusterin has been found associated with extracellular protein aggregates, a role as a molecular chaperone has been proposed. In this issue of the Biochemical Journal, Le Dréan and colleagues demonstrate an up-regulation of clusterin in neuronal cells exposed to proteotoxic stress that results in unfolded protein accumulation and proteasome impairment, both commonly associated with neurodegenerative diseases. Interestingly, expression of clusterin was found to be regulated by two members of the HSF (heat-shock factor) family, HSF1 and HSF2, which possibly form a trimeric complex on the clusterin promoter. The study proposes clusterin as a player in a cellular defence mechanism against harmful protein accumulation, and highlights the importance of elucidating further the exact role of clusterin and the intriguing interaction between HSF1 and HSF2.