Regulation of survival gene hsp70

Abstract

Rapid expression of the survival gene, inducible heat shock protein 70 (hsp70), is critical for mounting cytoprotection against severe cellular stress, like elevated temperature. Hsp70 protein chaperones the refolding of heat-denatured peptides to minimize proteolytic degradation as a part of an eukaryotically conserved phenomenon referred to as the heat shock response. The physiologic stress associated with exercise, which can include elevated temperature, mechanical damage, hypoxia, lowered pH, and reactive oxygen species generation, may promote protein unfolding, leading to hsp70 gene expression in skeletal myofibers. Although the pre-transcriptional activation of hsp70 gene expression has been thoroughly reviewed, discussion of downstream hsp70 gene regulation is less extensive. The purpose of this brief review was to examine all levels of hsp70 gene regulation in response to heat stress and exercise with a special focus on skeletal myofibers where data are available. In general, while heat stress represses bulk gene expression, hsp70 mRNA expression is enhanced. Post-transcriptionally, intronless hsp70 mRNA circumvents a host of decay pathways, as well as heat stress-repressed pre-mRNA splicing and nuclear export. Pre-translationally, hsp70 mRNA is excluded from stress granules and preferentially translated during heat stress-repressed global cap-dependent translation. Post-translationally, nascent Hsp70 protein is thermodynamically stable at elevated temperatures, allowing for the commencement of chaperoning activity early after synthesis to attenuate the heat shock response and protect against subsequent injury. This review demonstrates that hsp70 mRNA expression is closely coupled with functional protein translation.

Fig. 1

At rest, global intron splicing, (as coupled with Pol-II), and cap-dependent translation (80S) proceed at basal rates in the skeletal myofiber. Under these conditions, the hsp70 gene nuclear factor, HSF1, activity is negatively regulated in both the nucleus and cytoplasm via constitutive phosphorylation (black beads) by pERK1/2, GSK-3β, and other kinases? and via sequestration by Hsp70, Hsp90, and other binding proteins (BP?). In this state, HSF1 sustains transactivational inhibition, causing Pol-II to remain in a promoter-paused state (tethered to PIC), thus suppressing downstream hsp70 mRNA and protein expression. In response to heat stress, global intron splicing (as coupled with Pol-II) and cap-dependent translation (80S) become impaired. Under these conditions, HSF1 becomes activated via homotrimerization, gain in phosphorylative status (white beads) via CAMKII and other kinases?, and transactivation upon binding to the HSE promoter. In doing so, activated HSF1 promotes Pol-II escape from PIC and, thus, vigorous increase in hsp70 mRNA transcriptional rate. Heat stress promotes hyperacetylation of the hsp70 gene (CH₃COR), which enhances HSF1 access to the HSE. Accumulation of hsp70 mRNA is thought to result in protein production. Post-transcriptionally, hsp70 mRNA concentrates near the nucleus and circumvents heat stress-related nuclear retention of bulk mRNA, degradation as afforded by extension of poly(A)-tail (AAA), and destabilization via protection by PKR. Pre-translationally, hsp70 mRNA avoids inclusion into stress granules and is preferentially translated via an IRES. Post-translationally, nascent Hsp70 protein localizes to SERCA1a calcium pump and myofibrils in the cytoplasm and commences chaperoning activity early after synthesis as coupled with ATP hydrolysis to promote the refolding of denatured peptides. The role of exercise? in inducing hsp70 mRNA and protein expression by the aforementioned mechanisms remains unclear in the skeletal myofiber. For definition of terms, please see the main text