Disruption of Heat Shock Factor 1 Reveals an Essential Role in the Ubiquitin Proteolytic Pathway

Abstract

Inhibition of proteasome-mediated protein degradation machinery is a potent stress stimulus that causes accumulation of ubiquitinated proteins and increased expression of heat shock proteins (Hsps). Hsps play pivotal roles in homeostasis and protection in a cell, through their well-recognized properties as molecular chaperones. The inducible Hsp expression is regulated by the heat shock transcription factors (HSFs). Among mammalian HSFs, HSF1 has been shown to be important for regulation of the heat-induced stress gene expression, whereas the function of HSF2 in stress response is unclear. ). To date, however, no unambiguous evidence has been presented as to whether a single specific HSF or multiple members of the HSF family are required for transcriptional induction of heat shock genes when proteasome activity is down-regulated. Therefore, by using loss-of-function and gain-of-function strategies, we investigated the specific roles of mammalian HSFs in regulation of the ubiquitin-proteasome-mediated stress response. Here we demonstrate that HSF1, but not HSF2, is essential and sufficient for up-regulation of Hsp70 expression during down-regulation of the ubiquitin proteolytic pathway. We propose that specificity of HSF1 could be an important therapeutic target during disease pathogenesis associated with abnormal ubiquitin-dependent proteasome function.Regulation of protein degradation by the ubiquitin-proteasome pathway enables cells rapidly to reduce levels of defined proteins that control diverse processes, such as gene expression, cell signaling, immune responses, and stress adaptation. Therefore, proteasome-mediated degradation has to display a high degree of specificity, carried out by complex cascades of enzymes, toward its numerous substrates (6). Recently, a variety of inhibitors of the 26S proteasome have been identified. For example, the peptide aldehyde MG132 and the natural products lactacystin and its derivative clasto-lactacystin ␤-lactone all selectively inhibit degradation of proteins by the ubiquitin-proteasome pathway (15).When the ubiquitin-proteasome network is down-regulated, certain heat shock proteins, such as Hsp70, and other molecular chaperones are induced (5,13,14,21,22,45). Expression of heat shock genes is regulated at the transcriptional level by activation of the heat shock transcription factors (HSFs) (25,42). Unlike in yeast and Drosophila, the existence of multiple HSFs in eukaryotes suggests that the various HSFs might have specialized functions in response to distinct physiological and environmental stimuli. Among the three known mammalian HSFs (HSF1, HSF2, and HSF4), HSF1 mediates the stress response induced by heat, heavy metals, and oxidants, while HSF2 has been suggested to have a role as a developmental regulator (1,2,23,31,32,34,35,37,38), and the role of HSF4 is still unknown (27). Recently, the phenotype of homozygous HSF1-deficient (hsf1 Ϫ/Ϫ ) mice was reported to include defects of the chorioallantoic placenta and increased prenatal lethality, growth retardation, fe...

Heat Shock Protein 25 or Inducible Heat Shock Protein 70 Activates Heat Shock Factor 1

“…Because HSF1 induces Hsp70i and Hsp25 gene expression in response to heat stress, it has been suggested that HSPs may function in a negative feedback mechanism to return HSF1 to its inactive monomeric state (12,13). However, in the present study, ectopic transfection of HSP25 or HSP70i in RIF cells increased the expression of endogenous HSPs, such as HSP25 and HSP70i, through activation of HSF1.…”

“…In recent years, a conceptual framework for stress-induced activation and feedback repression of HSF1 has emerged (3,12,13). However, our recent study indicated that, in RIF cells that have relatively low HSP25 and inducible HSP70 (HSP70i) expressions, HSP25 or HSP70i mutually coregulated each other and increased their own protein expression through HSF1 activation (14).…”

Roles of heat shock factor 1 and 2 in response to proteasome inhibition: consequence on p53 stability

“…HSF1 deficiency dramatically prevented the induced expression of Hsp25 and Hsp70 after proteotoxic stress. This was in agreement with previous works showing that HSF1 is the major factor involved in proteotoxic response and that it could not be compensated by HSF2 alone (Pirkkala et al, 2000). As HSP25 and HSP70 promote cellular survival by preventing protein aggregation, facilitating refolding and directly inhibiting apoptotic signaling (Beere et al, 2000;Bruey et al, 2000), Hsf1 À/À cells lacked an important protective mechanism, which could explain the observed reduction in cell survival.…”

“…However, inhibition of the proteasome activates both HSF1 and HSF2 (Kawazoe et al, 1998). Proteasome inhibition leads to the accumulation of misfolded proteins and consequently induces expression of all the major HSPs (Bush et al, 1997), but Pirkkala et al (2000) clearly showed that HSF1, but not HSF2, has a key role in this induction. Nevertheless, to add to the complexity of this proteotoxic response, our group showed that an HSF1/HSF2 heterocomplex was present on the promoter of the chaperone gene clusterin, following proteasome inhibition (Loison et al, 2006).…”

Hsp70 Reduces α-Synuclein Aggregation and Toxicity

“…HSPs belong to the family of chaperone proteins and are important in both refolding misfolded proteins and directing proteins toward proteasomal degradation (23)(24)(25). HSPs can be protective in several neurodegeneration models (26 -29), and recent data in the fly model suggest that overexpression of the molecular chaperone Hsp70 protects against ␣-synuclein-induced degeneration (26,30).…”