Elevated Expression of Heat Shock Factor (HSF) 2A Stimulates HSF1-induced Transcription during Stress

Abstract

Heat shock factor 2 (HSF2) belongs to a family of structurally related transcription factors, which share the property of binding to heat shock elements in the promoters of hsp molecular chaperone genes. However, unlike HSF1, which is essential for hsp gene transcription, the cellular functions of HSF2 are not well known. Here we show that human HSF2, although an ineffective activator of the hsp70 promoter in vitro and in vivo in the absence of stress, participates in the activation of the hsp70 promoter by heat shock. HSF2 was not, however, activated by heat shock in cells deficient in functional HSF1, suggesting a requirement for HSF1 in HSF2-mediated transcriptional enhancement. In addition, HSF2 regulation involves differential activity of two isoforms, HSF2A and HSF2B, which arise from alternative splicing of a common hsf2 gene. Under basal conditions, both HSF2 isoforms are ineffective in activating the hsp70 transcription. However, heat shock differentially activates HSF2A in vivo. This phenomenon appears to be physiologically significant, as human myeloprogenitor cells differentiating along the erythroid lineage express HSF2A de novo and undergo a large increase in capacity to activate the hsp70 promoter. Our experiments further show that HSF1 is physically associated with HSF2 in the cell and that such binding is enhanced by heat shock. Our data suggest a mechanism involving the formation of heterocomplexes between HSF1 and HSF2 with enhanced activity to activate the hsp70 promoter when compared with HSF1 or HSF2 homotrimers.The heat shock transcription factor (HSF) 1 family was first discovered in experiments investigating proteins binding to the promoters of hsp genes (1-3). HSF isolated from yeast, Drosophila, murine, and human cells was shown to be involved in binding to the heat shock response elements (HSE) in heat shock promoters and activating transcription (3-9). Since the isolation of hsf from Saccharomyces cerevisiae, related genes have been found in a wide range of species and a hsf multigene family has now been described (10). Members of the hsf family share structural domains and functional properties including a highly conserved N-terminal DNA binding domain and a heptad repeat "leucine zipper" structure adjacent to the DNA binding domain that regulates trimerization and the ability to bind DNA (10 -12). In mammalian and avian cells, there are multiple hsf family members and four distinct hsf genes have been described to date (6, 7, 13-15). These include heat shock factor 1 (hsf1) with a crucial role in the stress response and other, structurally related genes (hsf2, hsf3, and hsf4) (10, 14 -19).The cellular functions of the hsf2 product still remain largely unknown. Some studies show that HSF2 increases in concentration and becomes activated to a DNA binding form in differentiating K-562 myeloprogenitor cells and that this increase correlates with an increase in transcription of HSP70 (18,19). Although these studies suggest that increases in HSF2 expression may be involved in the activat...

Heat Shock Factor 2 (HSF2) Contributes to Inducible Expression of hsp Genes through Interplay with HSF1

“…By forming heterocomplexes with different compositions, a gradient of transactivation could be achieved. The transcriptional capacity of HSF1 in reporter assays is stronger than that of HSF2, and functional interaction on the level of transactivation had been studied previously only in reporter assays (37,56). Our results regarding the role of HSF2 in the heat-induced activation of hsp70 promoter are in line with the reporter assays performed by He et al (37), who reported that overexpression of the HSF2-␣ isoform in the presence of HSF1 enhanced heat-induced hsp70BЈ promoter activity.…”

“…Similar to HSF1, HSF2 has been shown to translocate to punctuated structures in the nucleus of heat-shocked HeLa cells (35). Subsequently, we observed that during heat shock, HSF1 and HSF2 co-localize in the nuclear stress bodies, and a direct interaction between HSF1 and HSF2 during both control and heat shock treatment has been demonstrated (36,37). Reporter assays have suggested that HSF2 is able to potentiate HSF1-mediated transactivation (37).…”

“…Unlike McMillan et al (52), who used primary MEFs, our study was conducted with immortalized MEFs. The additive effects of HSF2 on hsp70 might be a characteristic of transformed cells, because potentiating effects of HSF2 on HSF1-mediated transcription have been observed in immortalized MEFs, K562 (this study), and mouse erythroleukemia cells (37). Hsp70 and other Hsps are known to act as potent survival proteins protecting cells against numerous death-inducing stimuli (57), and perhaps HSF2, in addition to HSF1, should be considered as an important regulator in diseases such as cancer and neurodegeneration, where dysregulation of molecular chaperones is evident (58,59).…”

“…Subsequently, we observed that during heat shock, HSF1 and HSF2 co-localize in the nuclear stress bodies, and a direct interaction between HSF1 and HSF2 during both control and heat shock treatment has been demonstrated (36,37). Reporter assays have suggested that HSF2 is able to potentiate HSF1-mediated transactivation (37). However, evidence for a functional relationship between these two factors on endogenous promoters is still missing.…”

Inducible heat shock protein 70 kD and inducible nitric oxide synthase in hemorrhage/resuscitation-induced injury

“…HSF2 has two isoforms due to splicing: HSF2A and HSF2B [26]. HSFs have a binding domain, a helical trimerization surface, and a short conserved element.…”