Paul E. Kroeger

The Transcriptional regulation of heat shock genes: A plethora of heat shock factors and regulatory conditions

The inducible regulation of heat shock gene transcription is mediated by a family of heat shock factors (HSF) that respond to diverse forms of physiological and environmental stress including elevated temperature, amino acid analogs, heavy metals, oxidative stress, anti-inflammatory drugs, arachidonic acid, and a number of pathophysiological disease states. The vertebrate genome encodes a family of HSFs which are expressed ubiquitously, yet the DNA binding properties of each factor are negatively regulated and activated in response to specific conditions. This chapter will discuss the regulation of the HSF multi-gene family and the role of these transcriptional activators in the inducible expression of genes encoding heat shock proteins and molecular chaperones.

The Carboxyl-Terminal Transactivation Domain of Heat Shock Factor 1 Is Negatively Regulated and Stress Responsive

We have characterized a stress-responsive transcriptional activation domain of mouse heat shock factor 1 (HSF1) by using chimeric GAL4-HSF1 fusion proteins. Fusion of the GAL4 DNA-binding domain to residues 124 to 503 of HSF1 results in a chimeric factor that binds DNA yet lacks any transcriptional activity. Transactivation is acquired upon exposure to heat shock or by deletion of a negative regulatory domain including part of the DNA-binding-domain-proximal leucine zippers. Analysis of a collection of GAL4-HSF1 deletion mutants revealed the minimal region for the constitutive transcriptional activator to map within the extreme carboxyl-terminal 108 amino acids, corresponding to a region rich in acidic and hydrophobic residues. Loss of residues 395 to 425 or 451 to 503, which are located at either end of this activation domain, severely diminished activity, indicating that the entire domain is required for transactivation. The minimal activation domain of HSF1 also confers enhanced transcriptional response to heat shock or cadmium treatment. These results demonstrate that the transcriptional activation domain of HSF1 is negatively regulated and that the signal for stress induction is mediated by interactions between the amino-terminal negative regulator and the carboxyl-terminal transcriptional activation domain.

The Heat Shock Transcriptional Response

The heat shock response represents one of the most dramatic changes in gene expression and has served as a paradigm for inducible transcriptional responses. The response to temperature elevation, exposure to toxic agents, or other physiological stresses is universal and mediated through the induction of a highly conserved set of genes referred to as heat shock (HS) genes (Lindquist, 1986; Lindquist and Craig, 1988; Morimoto and Milarski, 1990; Morimoto, 1993; Morimoto et al, 1994). Studies in the early 1970’s had revealed that the elevation of temperature induces the synthesis of new polypeptides. It had been recognized early that these newly synthesized HS proteins (HSPs) are important to the survival response mounted by the cell. Perhaps the most significant effect of HS is on transcription. As the severity of the HS increases, the transcription of most genes is repressed and the genes coding for HSPs are transcriptionally induced 50 to 100-fold within minutes. HS has additional effects on mRNA stability and translational control which contribute to the preferential expression of HSPs (Lindquist, 1980; Storti et al, 1980; Banerji et al, 1984; Lindquist and Craig, 1988).

17 Regulation of Heat Shock Gene Transcription by a Family of Heat Shock Factors

I. INTRODUCTION All bacterial, plant, and animal cells must cope with rapid changes in their environment, including exposure to elevated temperatures, heavy metals, toxins, oxidants, and bacterial and viral infection, by a rapid and often dramatic change in the patterns of gene expression, resulting in the elevated synthesis of a family of heat shock proteins and molecular chaperones (Lindquist and Craig 1988; Morimoto et al. 1990). Heat shock proteins ensure survival under stressful conditions, which if left unchecked, lead to irreversible cell damage and untimely cell death. They have essential roles in protein biosynthesis, specifically in the synthesis, transport, and translocation of proteins and in the regulation of protein conformation, and are also referred to as molecular chaperones (Craig et al.; Langer and Neupert; Brodsky and Schekman; Gething et al.; Dice et al.; Hightower et al.; Georgopoulos et al.; Frydman and Hartl; Randall et al.; Willison and Kuboda; Bohen and Yamamoto; all this volume). Heat shock proteins constitute a surprisingly large fraction of the protein within a cell, amounting to 5–10% of the total protein mass in cells growing under ambient conditions. Yet, despite their abundance, the genes encoding heat shock proteins are rapidly induced in response to stressful conditions. The consequence of this genetic switch, which is activated in response to a perturbation of the physiological state of the cell, is the elevated expression of constitutively expressed heat shock proteins and the de novo induction of heat shock genes that are expressed primarily in response to stress. A...