Heat shock factors: integrators of cell stress, development and lifespan

Abstract

Heat shock factors (HSFs) are essential for all organisms to survive exposures to acute stress. They are best known as inducible transcriptional regulators of genes encoding molecular chaperones and other stress proteins. Four members of the HSF family are also important for normal development and lifespan-enhancing pathways, and the repertoire of HSF targets has thus expanded well beyond the heat shock genes. These unexpected observations have uncovered complex layers of post-translational regulation of HSFs that integrate the metabolic state of the cell with stress biology, and in doing so control fundamental aspects of the health of the proteome and ageing.

Figure 1. The mammalian HSF machinery

An overview of the mammalian heat shock factor (HSF) family members and their biological functions. HSFs contribute to multiple normal physiological processes and pathologies through direct regulation of their target genes. The HSF target genes that have been identified in vivo are shown. HSF1 was originally recognized as the principal stress-responsive regulator of the heat shock response, but now HSF2 is known to modulate HSF1-mediated expression of heat shock protein (HSP) genes through heterocomplex formation. On heat shock, HSF1 and HSF2 accumulate into nuclear stress bodies (NSBs), where they bind to satellite III repeats. HSF1 is also a regulator of immune responses and cancer. So far, the regulation of HSP genes in ageing has most intensively been examined in Caenorhabditis elegans. Both HSF1 and HSF2 have been ascribed regulatory functions in several developmental processes, such as oogenesis, spermatogenesis and corticogenesis. HSF4 is involved in the development of different sensory organs in cooperation with HSF1, but has no role in the heat shock response. Murine HSF3 is the most recently identified mammalian HSF, which participates in the heat shock response by binding to the PDZ domain-containing 3 (Pdzk3) promoter. Currently, HSF3 is not known to crosstalk with any member of the HSF family, and is therefore placed separately from the other HSFs. Crygf, crystallin γF; Fgf7, fibroblast growth factor 7; Il-6, interleukin-6; MSYq, male-specific long arm of the mouse Y chromosome.

Figure 2. Members of the mammalian HSF family

a | A phylogenetic tree showing the species-specific relationship of heat shock factors (HSFs) among higher eukaryotes. Two recently found, but still poorly characterized, family members are: HSFY, which is located on the human Y chromosome and on the murine chromosome 2 (HSFY2), and HSFX, which has only been found on the human X chromosome^–. HSFY and HSFX exist in two identical copies on their respective chromosome. The phylogenetic tree was generated in CLUSTAL W and gaps were excluded from all phylogenetic analyses. The numbers represent bootstrap values (1000 bootstrap replicates were carried out). b | A schematic of the functional domains of the human and murine HSF family members. The conserved domains of distinct HSFs are indicated: the DNA-binding domain (DBD), the oligomerization domain (heptad repeat A (HR-A) and HR-B) and the carboxy-terminal HR-C. All HSFs contain the characteristic helix-loop-helix DBD. HSF1–HSF4 contain Leu zipper-like HR domains, which are required for homotrimerization or heterotrimerization. Yeast Hsf is included as a comparison. Image in part a is modified, with permission, from REF. © (2010) The American Society for Cell Biology.

Figure 3. HSF1 undergoes multiple PTMs on activation

a | An overview of heat shock factor 1 (HSF1)-related post-translational modifications (PTMs). Some of the identified sites for acetylation (A), phosphorylation (P) and sumoylation (S) are indicated, as well as the phosphorylation-dependent sumoylation motif (PDSM). The DNA-binding domain (DBD) and the heptad repeats (HR-A and HR-B, and HR-C) are indicated as in FIG. 2, as well as the regulatory domain (RD) and activation domains (AD1 and AD2). b | The HSF1 activation and attenuation cycle, involving trimerization, multiple PTMs and feedback from heat shock proteins (HSPs). In the resting state, HSF1 is a monomer in both the cytoplasm and nucleus. Monomeric HSF1 is already a phosphoprotein under non-stress conditions and it interacts with HSP90. On stress, HSF1 dissociates from the HSP90 complex, allowing HSF1 to trimerize and bind to the heat shock elements (HSEs) in HSP genes. Several PTMs, such as phosphorylation and sumoylation, are involved in regulating the transactivation capacity of HSF1. HSF1 acquires transcriptional activity, which is abrogated during the attenuation phase. Attenuation involves two regulatory steps: negative feedback from HSPs, which represses the transactivation of DNA-bound HSF1, and inhibition of DNA binding by the acetylation of Lys80 in the DBD of HSF1. The sirtuin SIRT1 regulates the attenuation phase of the heat shock response by preventing HSF1 acetylation.

Figure 4. Interactions between different HSFs provide distinct functional modes in transcriptional regulation

On stress, heat shock factor 1 (HSF1) is activated and HSF1–HSF2 heterotrimers are formed. Heat shock stress diminishes the levels of HSF2 and restricts heterotrimerization by limiting the availability of HSF2. Biochemical characterization of HSF2 has revealed that, unlike HSF1, which undergoes a monomer-to-trimer transition, HSF2 is mainly converted from a dimer to a trimer on activation. In certain developmental processes, such as corticogenesis and spermatogenesis, HSF2 levels are elevated in specific cell types and tissues, leading to activation of HSF2. Increased HSF2 expression then induces the formation of heterotrimers with HSF1. It has therefore been suggested that HSF1–HSF2 heterotrimerization provides a switch that integrates the transcriptional activation in response to specific stimuli.