Distinct stimulus-specific histone modifications at hsp70 chromatin targeted by the transcription factor heat shock factor-1

MosSCI-mediated exogenous gene expression is modulated by genomic positioning

Although the Mos1-mediated single-copy insertion (MosSCI) technique has been widely used to generate stable transgenic Caenorhabditis elegans strains, the link between stability of expression and integration site still needs to be explored. Here, experimental evidence is provided that transgenes are not able to match the level of transcription of their native counterpart, and that insertions at certain locations can result in an external stress-mediated increase in expression. Insertion site ttTi5605 on chromosome II was shown to be a superior location, at least when introducing reproduction related genes. Thus, this study provides a reference for the selection of an optimal site for MosSCI which provides acceptable expression performance whilst minimizing undesirable secondary effects.

Molecular mechanisms driving transcriptional stress responses

Proteotoxic stress, that is, stress caused by protein misfolding and aggregation, triggers the rapid and global reprogramming of transcription at genes and enhancers. Genome-wide assays that track transcriptionally engaged RNA polymerase II (Pol II) at nucleotide resolution have provided key insights into the underlying molecular mechanisms that regulate transcriptional responses to stress. In addition, recent kinetic analyses of transcriptional control under heat stress have shown how cells 'prewire' and rapidly execute genome-wide changes in transcription while concurrently becoming poised for recovery. The regulation of Pol II at genes and enhancers in response to heat stress is coupled to chromatin modification and compartmentalization, as well as to co-transcriptional RNA processing. These mechanistic features seem to apply broadly to other coordinated genome-regulatory responses.

NRF2 transcriptionally activates the heat shock factor 1 promoter under oxidative stress and affects survival and migration potential of MCF7 cells

Functional up-regulation of heat shock factor 1 (HSF1) activity through different posttranslational modifications has been implicated in the survival and proliferation of various cancers. It is increasingly recognized that the HSF1 gene is also up-regulated at the transcriptional level, a phenomenon correlated with poor prognosis for patients with different cancers, including breast cancer. Here, we analyzed the transcriptional up-regulation of HSF1 in human cells upon arsenite- or peroxide-induced oxidative stress. Sequential promoter truncation coupled with bioinformatics analysis revealed that this activation is mediated by two antioxidant response elements (AREs) located between 1707 and 1530 bp upstream of the transcription start site of the HSF1 gene. Using shRNA-mediated down-regulation, ChIP of NRF2, site-directed mutagenesis of the AREs, and DNase I footprinting of the HSF1 promoter, we confirmed that nuclear factor erythroid-derived 2-like 2 (NRF2, also known as NFE2L2) interacts with these AREs and up-regulates HSF1 expression. We also found that BRM/SWI2-related gene 1 (BRG1), a catalytic subunit of SWI2/SNF2-like chromatin remodeler, is involved in this process. We further show that NRF2-dependent HSF1 gene regulation plays a crucial role in cancer cell biology, as interference with NRF2-mediated HSF1 activation compromised survival, migration potential, and the epithelial-to-mesenchymal transition and autophagy in MCF7 breast cancer cells exposed to oxidative stress. Taken together, our findings unravel the mechanistic basis of HSF1 gene regulation in cancer cells and provide molecular evidence supporting a direct interaction between HSF1 and NRF2, critical regulators of two cytoprotective mechanisms exploited by cancer cells.

Roles of heat shock factor 1 beyond the heat shock response

Various stress factors leading to protein damage induce the activation of an evolutionarily conserved cell protective mechanism, the heat shock response (HSR), to maintain protein homeostasis in virtually all eukaryotic cells. Heat shock factor 1 (HSF1) plays a central role in the HSR. HSF1 was initially known as a transcription factor that upregulates genes encoding heat shock proteins (HSPs), also called molecular chaperones, which assist in refolding or degrading injured intracellular proteins. However, recent accumulating evidence indicates multiple additional functions for HSF1 beyond the activation of HSPs. Here, we present a nearly comprehensive list of non-HSP-related target genes of HSF1 identified so far. Through controlling these targets, HSF1 acts in diverse stress-induced cellular processes and molecular mechanisms, including the endoplasmic reticulum unfolded protein response and ubiquitin-proteasome system, multidrug resistance, autophagy, apoptosis, immune response, cell growth arrest, differentiation underlying developmental diapause, chromatin remodelling, cancer development, and ageing. Hence, HSF1 emerges as a major orchestrator of cellular stress response pathways.

Epigenetics and cytoprotection with heat acclimation

Studying “phenotypic plasticity” involves comparison of traits expressed in response to environmental fluctuations and aims to understand tolerance and survival in new settings. Reversible phenotypic changes that enable individuals to match their phenotype to environmental demands throughout life can be artificially induced, i.e., acclimation or occur naturally, i.e., acclimatization. The onset and achievement of acclimatory homeostasis are determined by molecular programs that induce the acclimated transcriptome. In heat acclimation, much evidence suggests that epigenetic mechanisms are powerful players in these processes. Epigenetic mechanisms affect the accessibility of the DNA to transcription factors, thereby regulating gene expression and controlling the phenotype. The heat-acclimated phenotype confers cytoprotection against novel stressors via cross-tolerance mechanisms, by attenuation of the initial damage and/or by accelerating spontaneous recovery through the release of help signals. This indispensable acclimatory feature has a memory and can be rapidly reestablished after the loss of acclimation and the return to the physiological preacclimated phenotype. The transcriptional landscape of the deacclimated phenotype includes constitutive transcriptional activation of epigenetic bookmarks. Heat shock protein (HSP) 70/HSP90/heat shock factor 1 memory protocol demonstrated constitutive histone H4 acetylation on hsp70 and hsp90 promotors. Novel players in the heat acclimation setup are poly(ADP-ribose)ribose polymerase 1 affecting chromatin condensation, DNA linker histones from the histone H1 cluster, and transcription factors associated with the P38 pathway. We suggest that these orchestrated responses maintain euchromatin and proteostasis during deacclimation and predispose to rapid reacclimation and cytoprotection. These mechanisms represent within-life epigenetic adaptations and cytoprotective memory.

Dynamic expression patterns of Pax6 during spermatogenesis in the mouse

Spermatogenesis is a series of complex processes to generate mature sperm, and various molecules play crucial roles in regulating these processes. Previous studies imply a possibility that a transcriptional factor Pax6, a key player of brain and sensory organ development, could be involved in spermatogenesis, but neither expression nor function of Pax6 in the adult testis has been examined yet. In the present study, we described for the first time Pax6 expression dynamics in the adult mouse testis. Using cell-type-specific markers, the expression of Pax6 was detected in 67.0% of promyelocytic leukemia zinc finger (Plzf)-positive type A spermatogonia. The expression of Pax6 was also observed in p63-positive spermatocytes and round spermatids. We did not detect any expression of Pax6 in Sox9-positive Sertoli cells or in elongated spermatids and mature sperm. High-resolution analyses revealed that Pax6 formed a single dot-like structure during mid-phase of the pachytene spermatocyte. This dot-like structure co-localized with γH2A.X demarcating XY body, a domain in which X and Y chromosomes are silenced and compartmentalized. These results may suggest a novel role of Pax6 in spermatogenesis.

The wonderous chaperones: A highlight on therapeutics of cancer and potentially malignant disorders

Diverse environmental and physiological factors are known to induce the transcription of a set of genes encoding special protective molecules known as "molecular chaperones" within our cells. Literature abounds in evidence regarding the varied roles; these "guides" can effectively perform in our system. Highly conserved through evolution, from the prokaryotes to the eukaryotes, these make perfect study tools for verifying their role in both the pathogenesis as well as the therapeutics of varied neurodegenerative, autoimmune and potentially malignant disorders and varied cancer states. We present a concise review of this ever dynamic molecule, highlighting the probable role in a potentially malignant disorder, oral lichen planus.

Heat shock factor 2 is a stress-responsive mediator of neuronal migration defects in models of fetal alcohol syndrome

Fetal alcohol spectrum disorder (FASD) is a frequent cause of mental retardation. However, the molecular mechanisms underlying brain development defects induced by maternal alcohol consumption during pregnancy are unclear. We used normal and Hsf2-deficient mice and cell systems to uncover a pivotal role for heat shock factor 2 (HSF2) in radial neuronal migration defects in the cortex, a hallmark of fetal alcohol exposure. Upon fetal alcohol exposure, HSF2 is essential for the triggering of HSF1 activation, which is accompanied by distinctive post-translational modifications, and HSF2 steers the formation of atypical alcohol-specific HSF1-HSF2 heterocomplexes. This perturbs the in vivo binding of HSF2 to heat shock elements (HSEs) in genes that control neuronal migration in normal conditions, such as p35 or the MAPs (microtubule-associated proteins, such as Dclk1 and Dcx), and alters their expression. In the absence of HSF2, migration defects as well as alterations in gene expression are reduced. Thus, HSF2, as a sensor for alcohol stress in the fetal brain, acts as a mediator of the neuronal migration defects associated with FASD.

Epigenetic effects of cadmium in cancer: focus on melanoma

Cadmium is a highly toxic heavy metal, which has a destroying impact on organs. Exposure to cadmium causes severe health problems to human beings due to its ubiquitous environmental presence and features of the pathologies associated with pro-longed exposure. Cadmium is a well-established carcinogen, although the underlying mechanisms have not been fully under-stood yet. Recently, there has been considerable interest in the impact of this environmental pollutant on the epigenome. Be-cause of the role of epigenetic alterations in regulating gene expression, there is a potential for the integration of cadmium-induced epigenetic alterations as critical elements in the cancer risk assessment process. Here, after a brief review of the ma-jor diseases related to cadmium exposure, we focus our interest on the carcinogenic potential of this heavy metal. Among the several proposed pathogenetic mechanisms, particular attention is given to epigenetic alterations, including changes in DNA methylation, histone modifications and non-coding RNA expression. We review evidence for a link between cadmium-induced epigenetic changes and cell transformation, with special emphasis on melanoma. DNA methylation, with reduced expression of key genes that regulate cell proliferation and apoptosis, has emerged as a possible cadmium-induced epigenetic mechanism in melanoma. A wider comprehension of mechanisms related to this common environmental contaminant would allow a better cancer risk evaluation.

Toll-like receptor agonists and febrile range hyperthermia synergize to induce heat shock protein 70 expression and extracellular release

Heat shock protein (Hsp) 70 expression can be stimulated by febrile range temperature (FRT). Hsp70 has been shown to be elevated in serum of patients with sepsis, and when released from cells, extracellular Hsp70 exerts endotoxin-like effects through Toll-like receptor 4 (TLR4) receptors. Circulating TLR agonists and fever both persist for the first several days of sepsis, and each can activate Hsp70 expression; however, the effect of combined exposure to FRT and TLR agonists on Hsp70 expression is unknown. We found that concurrent exposure to FRT (39.5 °C) and agonists for TLR4 (LPS), TLR2 (Pam3Cys), or TLR3 (poly(IC)) synergized to increase Hsp70 expression and extracellular release in RAW264.7 macrophages. The increase in Hsp70 expression was associated with activation of p38 and ERK MAP kinases, phosphorylation of histone H3, and increased recruitment of HSF1 to the Hsp70 promoter. Pretreatment with the p38 MAPK inhibitor SB283580 but not the ERK pathway inhibitor UO126 significantly reduced Hsp70 gene modification and Hsp70 expression in RAW cells co-exposed to LPS and FRT. In mice challenged with intratracheal LPS and then exposed to febrile range hyperthermia (core temperature, ∼39.5 °C), Hsp70 levels in lung tissue and in cell-free lung lavage were increased compared with mice exposed to either hyperthermia or LPS alone. We propose a model of how enhanced Hsp70 expression and extracellular release in patients concurrently exposed to fever and TLR agonists may contribute to the pathogenesis of sepsis.

Transcriptional regulation of small HSP-HSF1 and beyond

The members of the small heat shock protein (sHSP) family are molecular chaperones that play major roles in development, stress responses, and diseases, and have been envisioned as targets for therapy, particularly in cancer. The molecular mechanisms that regulate their transcription, in normal, stress, or pathological conditions, are characterized by extreme complexity and subtlety. Although historically linked to the heat shock transcription factors (HSFs), the stress-induced or developmental expression of the diverse members, including HSPB1/Hsp27/Hsp25, αA-crystallin/HSPB4, and αB-crystallin/HSPB5, relies on the combinatory effects of many transcription factors. Coupled with remarkably different cis-element architectures in the sHsp regulatory regions, they confer to each member its developmental expression or stress-inducibility. For example, multiple regulatory pathways coordinate the spatio-temporal expression of mouse αA-, αB-crystallin, and Hsp25 genes during lens development, through the action of master genes, like the large Maf family proteins and Pax6, but also HSF4. The inducibility of Hsp27 and αB-crystallin transcription by various stresses is exerted by HSF-dependent mechanisms, by which concomitant induction of Hsp27 and αB-crystallin expression is observed. In contrast, HSF-independent pathways can lead to αB-crystallin expression, but not to Hsp27 induction. Not surprisingly, deregulation of the expression of sHSP is associated with various pathologies, including cancer, neurodegenerative, or cardiac diseases. However, many questions remain to be addressed, and further elucidation of the developmental mechanisms of sHsp gene transcription might help to unravel the tissue- and stage-specific functions of this fascinating class of proteins, which might prove to be crucial for future therapeutic strategies. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.

Dynamic acetylation of lysine-4-trimethylated histone H3 and H3 variant biology in a simple multicellular eukaryote

Dynamic acetylation of all lysine-4-trimethylated histone H3 is a complex phenomenon involved in Immediate-early gene induction in metazoan eukaryotes. Higher eukaryotes express repeated copies of three closely related H3 variants, inaccessible to genetic analysis. We demonstrate conservation of these phenomena in Dictyostelium which has three single-copy H3 variant genes. Although dynamic acetylation is targeted to two H3 variants which are K4-trimethylated, K9-acetylation is preferentially targeted to one. In cells lacking Set1 methyltransferase and any detectable K4-trimethylation, dynamic acetylation is lost demonstrating a direct link between the two. Gene replacement to express mutated H3 variants reveals a novel interaction between K4-trimethylation on different variants. Cells expressing only one variant show defects in growth, and in induction of a UV-inducible gene, demonstrating the functional importance of variant expression. These studies confirm that dynamic acetylation targeted to H3K4me3 arose early in evolution and reveal a very high level of specificity of histone variant utilization in a simple multicellular eukaryote.

Regulation of survival gene hsp70

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.

Inducible hsp70 in the regulation of cancer cell survival: analysis of chaperone induction, expression and activity

Understanding the mechanisms that control stress is central to realize how cells respond to environmental and physiological insults. All the more important is to reveal how tumour cells withstand their harsher growth conditions and cope with drug-induced apoptosis, since resistance to chemotherapy is the foremost complication when curing cancer. Intensive research on tumour biology over the past number of years has provided significant insights into the molecular events that occur during oncogenesis, and resistance to anti-cancer drugs has been shown to often rely on stress response and expression of inducible heat shock proteins (HSPs). However, with respect to the mechanisms guarding cancer cells against proteotoxic stresses and the modulatory effects that allow their survival, much remains to be defined. Heat shock proteins are molecules responsible for folding newly synthesized polypeptides under physiological conditions and misfolded proteins under stress, but their role in maintaining the transformed phenotype often goes beyond their conventional chaperone activity. Expression of inducible HSPs is known to correlate with limited sensitivity to apoptosis induced by diverse cytotoxic agents and dismal prognosis of several tumour types, however whether cancer cells survive because of the constitutive expression of heat shock proteins or the ability to induce them when adapting to the hostile microenvironment remains to be elucidated. Clear is that tumours appear nowadays more "addicted" to heat shock proteins than previously envisaged, and targeting HSPs represents a powerful approach and a future challenge for sensitizing tumours to therapy. This review will focus on the anti-apoptotic role of heat shock 70kDa protein (Hsp70), and how regulatory factors that control inducible Hsp70 synthesis, expression and activity may be relevant for response to stress and survival of cancer cells.

Altered chromatin architecture underlies progressive impairment of the heat shock response in mouse models of Huntington disease

Huntington disease (HD) is a devastating neurodegenerative disorder for which there are no disease-modifying treatments. Previous studies have proposed that activation of the heat shock response (HSR) via the transcription factor heat shock factor 1 (HSF1) may be of therapeutic benefit. However, the effect of disease progression on the HSR and the therapeutic potential of this pathway are currently unknown. Here, we used a brain-penetrating HSP90 inhibitor and physiological, molecular, and behavioral readouts to demonstrate that pharmacological activation of HSF1 improves huntingtin aggregate load, motor performance, and other HD-related phenotypes in the R6/2 mouse model of HD. However, the beneficial effects of this treatment were transient and diminished with disease progression. Molecular analyses to understand the transient nature of these effects revealed altered chromatin architecture, reduced HSF1 binding, and impaired HSR accompanied disease progression in both the R6/2 transgenic and HdhQ150 knockin mouse models of HD. Taken together, our findings reveal that the HSR, a major inducible regulator of protein homeostasis and longevity, is disrupted in HD. Consequently, pharmacological induction of HSF1 as a therapeutic approach to HD is more complex than was previously anticipated.

Environmental epigenetics in metal exposure

Although it is widely accepted that chronic exposure to arsenite, nickel, chromium and cadmium increases cancer incidence in individuals, the molecular mechanisms underlying their ability to transform cells remain largely unknown. Carcinogenic metals are typically weak mutagens, suggesting that genetic-based mechanisms may not be primarily responsible for metal-induced carcinogenesis. Growing evidence shows that environmental metal exposure involves changes in epigenetic marks, which may lead to a possible link between heritable changes in gene expression and disease susceptibility and development. Here, we review recent advances in the understanding of metal exposure affecting epigenetic marks and discuss establishment of heritable gene expression in metal-induced carcinogenesis.

Heat perception and signalling in plants: a tortuous path to thermotolerance

An accurate assessment of the rising ambient temperature by plant cells is crucial for the timely activation of various molecular defences before the appearance of heat damage. Recent findings have allowed a better understanding of the early cellular events that take place at the beginning of mild temperature rise, to timely express heat-shock proteins (HSPs), which will, in turn, confer thermotolerance to the plant. Here, we discuss the key components of the heat signalling pathway and suggest a model in which a primary sensory role is carried out by the plasma membrane and various secondary messengers, such as Ca(2+) ions, nitric oxide (NO) and hydrogen peroxide (H(2)O(2)). We also describe the role of downstream components, such as calmodulins, mitogen-activated protein kinases and Hsp90, in the activation of heat-shock transcription factors (HSFs). The data gathered for land plants suggest that, following temperature elevation, the heat signal is probably transduced by several pathways that will, however, coalesce into the final activation of HSFs, the expression of HSPs and the onset of cellular thermotolerance.

Implication of heat shock factors in tumorigenesis: therapeutical potential

Heat Shock Factors (HSF) form a family of transcription factors (four in mammals) which were named according to the discovery of their activation by a heat shock. HSFs trigger the expression of genes encoding Heat Shock Proteins (HSPs) that function as molecular chaperones, contributing to establish a cytoprotective state to various proteotoxic stresses and in pathological conditions. Increasing evidence indicates that this ancient transcriptional protective program acts genome-widely and performs unexpected functions in the absence of experimentally defined stress. Indeed, HSFs are able to re-shape cellular pathways controlling longevity, growth, metabolism and development. The most well studied HSF, HSF1, has been found at elevated levels in tumors with high metastatic potential and is associated with poor prognosis. This is partly explained by the above-mentioned cytoprotective (HSP-dependent) function that may enable cancer cells to adapt to the initial oncogenic stress and to support malignant transformation. Nevertheless, HSF1 operates as major multifaceted enhancers of tumorigenesis through, not only the induction of classical heat shock genes, but also of “non-classical” targets. Indeed, in cancer cells, HSF1 regulates genes involved in core cellular functions including proliferation, survival, migration, protein synthesis, signal transduction, and glucose metabolism, making HSF1 a very attractive target in cancer therapy. In this review, we describe the different physiological roles of HSFs as well as the recent discoveries in term of non-cogenic potential of these HSFs, more specifically associated to the activation of “non-classical” HSF target genes. We also present an update on the compounds with potent HSF1-modulating activity of potential interest as anti-cancer therapeutic agents.

Histone code pathway involving H3 S28 phosphorylation and K27 acetylation activates transcription and antagonizes polycomb silencing

Histone H3 phosphorylation is a critical step that couples signal transduction pathways to gene regulation. To specifically assess the transcriptional regulatory functions of H3 phosphorylation, we developed an in vivo targeting approach and found that the H3 kinase MSK1 is a direct and potent transcriptional activator. Targeting of this H3 kinase to the endogenous c-fos promoter is sufficient to activate its expression without the need of upstream signaling. Moreover, targeting MSK1 to the α-globin promoter induces H3 S28 phosphorylation and reactivates expression of this polycomb-silenced gene. Importantly, we discovered a mechanism whereby H3 S28 phosphorylation not only displaces binding of the polycomb-repressive complexes, but it also induces a methyl-acetylation switch of the adjacent K27 residue. Our findings show that signal transduction activation can directly regulate polycomb silencing through a specific histone code-mediated mechanism.

Hyperthermia: malformations to chaperones

Hyperthermia has been known to induce malformations in numerous animal models as well being associated with human abnormalities. This was apparent particularly when the hyperthermia exposure was during the early stages of neural development. Although it was recognized relatively early that these exposures induced cell death, the specific molecular mechanism of how a brief heat exposure was translated in to specific cellular functions remains largely unknown. While our understanding of the events that govern how cells react to heat, or stresses in general, has increased, there is much that remains undiscovered. In this brief review, animal and clinical observations are outlined as are some of the scientific explorations that were undertaken to characterize, define, and better understand the morphological, biochemical, and molecular effects of hyperthermia on the developing embryo.

Roles of heat shock factors in gametogenesis and development

Heat shock factors form a family of transcription factors (four in mammals), which were named according to the first discovery of their activation by heat shock. As a result of the universality and robustness of their response to heat shock, the stress-dependent activation of heat shock factor became a ‘paradigm’: by binding to conserved DNA sequences (heat shock elements), heat shock factors trigger the expression of genes encoding heat shock proteins that function as molecular chaperones, contributing to establish a cytoprotective state to various proteotoxic stress and in several pathological conditions. Besides their roles in the stress response, heat shock factors perform crucial roles during gametogenesis and development in physiological conditions. First, during these process, in stress conditions, they are either proactive for survival or, conversely, for apoptotic process, allowing elimination or, inversely, protection of certain cell populations in a way that prevents the formation of damaged gametes and secure future reproductive success. Second, heat shock factors display subtle interplay in a tissue- and stage-specific manner, in regulating very specific sets of heat shock genes, but also many other genes encoding growth factors or involved in cytoskeletal dynamics. Third, they act not only by their classical transcription factor activities, but are necessary for the establishment of chromatin structure and, likely, genome stability. Finally, in contrast to the heat shock gene paradigm, heat shock elements bound by heat shock factors in developmental process turn out to be extremely dispersed in the genome, which is susceptible to lead to the future definition of ‘developmental heat shock element’.

Novel aspects of heat shock factors: DNA recognition, chromatin modulation and gene expression

Heat shock factor (HSF) is an evolutionarily conserved stress-response regulator that activates the transcription of heat shock protein genes, whose products maintain protein homeostasis under normal physiological conditions, as well as under conditions of stress. The promoter regions of the target genes contain a heat shock element consisting of multiple inverted repeats of the pentanucleotide sequence nGAAn. A single HSF of yeast can bind to heat shock elements that differ in the configuration of the nGAAn units and can regulate the transcription of various genes that function not only in stress resistance, but also in a broad range of biological processes. Mammalian cells have four HSF family members involved in different, but in some cases similar, biological functions, including stress resistance, cell differentiation and development. Mammalian HSF family members exhibit differential specificity for different types of heat shock elements, which, together with cell type-specific expression of HSFs is important in determining the target genes of each HSF. This minireview focuses on the molecular mechanisms of DNA recognition, chromatin modulation and gene expression by yeast and mammalian HSFs.

Regulation of the members of the mammalian heat shock factor family

Regulation of gene expression is fundamental in all living organisms and is facilitated by transcription factors, the single largest group of proteins in humans. For cell- and stimulus-specific gene regulation, strict control of the transcription factors themselves is crucial. Heat shock factors are a family of transcription factors best known as master regulators of induced gene expression during the heat shock response. This evolutionary conserved cellular stress response is characterized by massive production of heat shock proteins, which function as cytoprotective molecular chaperones against various proteotoxic stresses. In addition to promoting cell survival under stressful conditions, heat shock factors are involved in the regulation of life span and progression of cancer and they are also important for developmental processes such as gametogenesis, neurogenesis and maintenance of sensory organs. Here, we review the regulatory mechanisms steering the activities of the mammalian heat shock factors 1–4.

Protein homeostasis in models of aging and age-related conformational disease

The stability of the proteome is crucial to the health of the cell, and contributes significantly to the lifespan of the organism. Aging and many age-related diseases have in common the expression of misfolded and damaged proteins. The chronic expression of damaged proteins during disease can have devastating consequences on protein homeostasis (proteostasis), resulting in disruption ofnumerous biological processes. This chapter discusses our current understanding of the various contributors to protein misfolding, and the mechanisms by which misfolding, and accompanied aggregation/toxicity, is accelerated by stress and aging. Invertebrate models have been instrumental in studying the processes related to aggregation and toxicity of disease-associated proteins and how dysregulation ofproteostasis leads to neurodegenerative diseases of aging.

Posttranslational modifications in histones underlie heat acclimation-mediated cytoprotective memory

We have demonstrated that heat acclimation (AC) causes selective, long-lasting, transcriptional changes in cytoprotective and chromatin remodeling-associated genes, which maintain their AC transcriptome profile, despite the loss of the AC phenotype (Tetievsky et al. Physiol Genomics 34: 78-87, 2008). We postulated that AC memory involves upstream epigenetic information, which predisposes to rapid reacclimation (ReAC) and cytoprotective memory. Here we tested the hypothesis that posttranslational histone modifications are linked to this process. Rats subjected to AC (34°C for 2 or 30 days), deacclimation (DeAC; 24°C, 30 days), and ReAC (34°C, 2 days), and untreated controls were used. Histone H4 lysine acetylation and histone H3 acetylation and phosphorylation in the heat shock element (HSE) of the promoters of heat shock protein-70 (hsp70) and -90 (hsp90) genes were examined. Histone acetyltransferase recruitment of TIP60 (60-kDa histone acetyltransferase-interactive protein), the catalytic subunit of NuH4, was used to validate acetylation. Heat shock factor-1 (HSF-1)-HSE binding to the hsp70 and hsp90 genes was measured to confirm HSF-1 binding to euchromatin. Our results indicate that, while histone H3Ser10 phosphorylation occurred during the AC 2-day phase, AC constitutively elevated histone H4 acetylation in the HSE of hsp70 and hsp90 promoters. HSF-1-HSE binding was detected in the hsp70 gene throughout AC-DeAC-ReAC. The hsp90 gene lacked HSF-1 binding during DeAC, but resumed a high binding level upon ReAC. HSP-90 is a critical cytoprotective protein, and the HSF-1-hsp90 binding profile matched levels of this protein. We conclude that, while early histone H3 phosphorylation is probably required for subsequent histone H4 acetylation, the constitutively acetylated histone H4 and the preserved euchromatin state throughout AC-DeAC-ReAC predispose to rapid cytoprotective acclimatory memory.

Signal Transduction Pathways Leading to Heat Shock Transcription

Heat shock proteins (HSP) are essential for intracellular protein folding during stress and protect cells from denaturation and aggregation cascades that can lead to cell death. HSP genes are regulated at the transcriptional level by heat shock transcription factor 1 (HSF1) that is activated by stress and binds to heat shock elements in HSP genes. The activation of HSF1 during heat shock involves conversion from an inert monomer to a DNA binding trimer through a series of intramolecular folding rearrangements. However, the trigger for HSF1 at the molecular level is unclear and hypotheses for this process include reversal of feedback inhibition of HSF1 by molecular chaperones and heat-induced binding to large non-coding RNAs. Heat shock also causes a profound modulation in cell signaling pathways that lead to protein kinase activation and phosphorylation of HSF1 at a number of regulatory serine residues. HSP genes themselves exist in an accessible chromatin conformation already bound to RNA polymerase II. The RNA polymerase II is paused on HSP promoters after transcribing a short RNA sequence proximal to the promoter. Activation by heat shock involves HSF1 binding to the promoter and release of the paused RNA polymerase II followed by further rounds of transcriptional initiation and elongation. HSF1 is thus involved in both initiation and elongation of HSP RNA transcripts. Recent studies indicate important roles for histone modifications on HSP genes during heat shock. Histone modification occurs rapidly after stress and may be involved in promoting nucleosome remodeling on HSP promoters and in the open reading frames of HSP genes. Understanding these processes may be key to evaluating mechanisms of deregulated HSP expression that plays a key role in neurodegeneration and cancer.

cAMP-response element-binding protein (CREB) controls MSK1-mediated phosphorylation of histone H3 at the c-fos promoter in vitro

The rapid induction of the c-fos gene correlates with phosphorylations of histone H3 and HMGN1 by mitogen- and stress-activated protein kinases. We have used a cell-free system to dissect the mechanism by which MSK1 phosphorylates histone H3 within the c-fos chromatin. Here, we show that the reconstituted c-fos chromatin presents a strong barrier to histone H3 phosphorylation by MSK1; however, the activators (serum response factor, Elk-1, cAMP-response element-binding protein (CREB), and ATF1) bound on their cognate sites recruit MSK1 to phosphorylate histone H3 at Ser-10 within the chromatin. This activator-dependent phosphorylation of histone H3 is enhanced by HMGN1 and occurs preferentially near the promoter region. Among the four activators, CREB plays a predominant role in MSK1-mediated phosphorylation of histone H3, and the phosphorylation of Ser-133 in CREB is essential for this process. Mutational analyses of MSK1 show that its N-terminal inhibition domain is critical for the kinase to phosphorylate chromatin-embedded histone H3 in a CREB-dependent manner, indicating the presence of an intricate regulatory network for MSK1-mediated phosphorylation of histone H3.

Febrile-range temperature modifies cytokine gene expression in LPS-stimulated macrophages by differentially modifying NF-{kappa}B recruitment to cytokine gene promoters

We previously showed that exposure to febrile-range temperatures (FRT, 39.5-40 degrees C) reduces LPS-induced TNF-alpha expression, in part through the direct interaction of heat shock factor-1 (HSF1) with the TNF-alpha gene promoter. However, it is not known whether exposure to FRT also modifies more proximal LPS-induced signaling events. Using HSF1-null mice, we confirmed that HSF1 is required for FRT-induced repression of TNF-alpha in vitro by LPS-stimulated bone marrow-derived macrophages and in vivo in mice challenged intratracheally with LPS. Exposing LPS-stimulated RAW 264.7 mouse macrophages to FRT reduced TNF-alpha expression while increasing IL-1beta expression despite the two genes sharing a common myeloid differentiation protein-88 (MyD88)-dependent pathway. Global activation of the three LPS-induced signaling intermediates that lead to cytokine gene expression, ERK and p38 MAPKs and NF-kappaB, was not affected by exposing RAW 264.7 cells to FRT as assessed by ERK and p38 phosphorylation and NF-kappaB in vitro DNA-binding activity and activation of a NF-kappaB-dependent synthetic promoter. However, chromatin immunoprecipitation (ChIP) analysis demonstrated that exposure to FRT reduced LPS-induced recruitment of NF-kappaB p65 to the TNF-alpha promoter while simultaneously increasing its recruitment to the IL-1beta promoter. These data suggest that FRT exerts its effects on cytokine gene expression in a gene-specific manner through distal effects on promoter activation rather than proximal receptor activation and signal transduction.

Regulation of heat shock gene transcription in neuronal cells

The heat shock protein (HSP) molecular chaperones are the primary cellular defense against damage to the proteome, initiating refolding of denatured proteins and regulating degradation after severe protein damage. Many neurodegenerative disorders involve aberrant protein folding and protein damage, which accumulates in an age-dependent manner. Ageing is associated with the decrease in activity of the heat shock transcription factors (HSF) that regulate HSP gene transcription. Neuronal cells seem particularly vulnerable in this sense as HSF activity and HSP expression are relatively weak in such cells and motor neurons appear to require input of HSP secreted from adjacent glial cells to maintain adequate molecular chaperone levels. It may be significant that motor neurons have been shown to be the sensitive cells in the ageing of Drosophila and C. elegans and that these organisms may acquire extended lifespans with over-expression of small heat shock proteins and HSF1. HSF1 transcriptional activity has been discussed in neuronal cells, concentrating on the regulation and activity of HSF1 and HSF2 and their role in HSP expression, during neurodegenerative diseases and as mediators of cell survival.

Analysis of HSF4 binding regions reveals its necessity for gene regulation during development and heat shock response in mouse lenses

Heat shock transcription factors (HSFs) regulate gene expression in response to heat shock and in physiological conditions. In mammals, HSF1 is required for heat-mediated induction of classic heat shock genes; however, we do not know the molecular mechanisms by which HSF4 regulates gene expression or the biological consequences of its binding to chromatin. Here, we identified that HSF4 binds to various genomic regions, including the introns and distal parts of protein-coding genes in vivo in mouse lenses, and a substantial numbers of the regions were also occupied by HSF1 and HSF2. HSF4 regulated expression of some genes at a developmental stage when HSF1 and HSF2 expression decreased. Although HSF4 binding did not affect expression of many genes, it induces demethylated status of histone H3K9 on the binding regions. Unexpectedly, a lot of HSF4 targets were induced by heat shock treatment, and HSF4 is required for induction of a set of non-classic heat shock genes in response to heat shock, in part by facilitating HSF1 binding through chromatin modification. These results suggest novel mechanisms of gene regulation controlled by HSF4 in non-classic heat shock response and in lens development.

A transcription cofactor required for the heat-shock response

The Stress-responsive activator of p300 (Strap) is a transcription cofactor that has an important role in the control of DNA damage response through its ability to regulate p53 activity. Here, we report that Strap is inducible by heat shock and stimulates the transcription of heat-shock genes. A chromatin-associated complex involving heat-shock factor 1 (HSF1), Strap and the p300 coactivator assembles on the heat-shock protein 70 (hsp70) promoter, and Strap augments HSF1 binding and chromatin acetylation in Hsp genes, most probably through the p300 histone acetyltransferase. Cells depleted of Strap do not survive under heat-shock conditions. These results indicate that Strap is an essential cofactor that acts at the level of chromatin control to regulate heat-shock-responsive transcription.

Chromosomal protein HMGN1 enhances the heat shock-induced remodeling of Hsp70 chromatin

The nucleosome-binding protein HMGN1 affects the structure and function of chromatin; however, its role in regulating specific gene expression in living cells is not fully understood. Here we use embryonic fibroblasts from Hmgn1(+/+) and Hmgn1(-/-) mice to examine the effect of HMGN1 on the heat shock-induced transcriptional activation of Hsp70, a well characterized gene known to undergo a rapid chromatin re-structuring during transcriptional activation. We find that loss of HMGN1 decreases the levels of Hsp70 transcripts at the early stages of heat shock. HMGN1 enhances the rate of heat shockinduced changes in the Hsp70 chromatin but does not affect the chromatin structure before induction, an indication that it does not predispose the gene to rapid activation. Heat shock elevates the levels of H3K14 acetylation in the Hsp70 chromatin of wild type cells more efficiently than in the chromatin of Hmgn1(-/-) cells, whereas treatment with histone deacetylase inhibitors abrogates the effects of HMGN1 on the heat shock response. We suggest that HMGN1 enhances the rate of heat shock-induced H3K14 acetylation in the Hsp70 promoter, thereby enhancing the rate of chromatin remodeling and the subsequent transcription during the early rounds of Hsp70 activation when the gene is still associated with histones in a nucleosomal conformation.

Stress-dependent dynamics of global chromatin remodeling in yeast: dual role for SWI/SNF in the heat shock stress response

Although chromatin structure is known to affect transcriptional activity, it is not clear how broadly patterns of changes in histone modifications and nucleosome occupancy affect the dynamic regulation of transcription in response to perturbations. The identity and role of chromatin remodelers that mediate some of these changes are also unclear. Here, we performed temporal genome-wide analyses of gene expression, nucleosome occupancy, and histone H4 acetylation during the response of yeast (Saccharomyces cerevisiae) to different stresses and report several findings. First, a large class of predominantly ribosomal protein genes, whose transcription was repressed during both heat shock and stationary phase, showed strikingly contrasting histone acetylation patterns. Second, the SWI/SNF complex was required for normal activation as well as repression of genes during heat shock, and loss of SWI/SNF delayed chromatin remodeling at the promoters of activated genes. Third, Snf2 was recruited to ribosomal protein genes and Hsf1 target genes, and its occupancy of this large set of genes was altered during heat shock. Our results suggest a broad and direct dual role for SWI/SNF in chromatin remodeling, during heat shock activation as well as repression, at promoters and coding regions.

Dynamic histone H3 methylation during gene induction: HYPB/Setd2 mediates all H3K36 trimethylation

Understanding the function of histone modifications across inducible genes in mammalian cells requires quantitative, comparative analysis of their fate during gene activation and identification of enzymes responsible. We produced high-resolution comparative maps of the distribution and dynamics of H3K4me3, H3K36me3, H3K79me2 and H3K9ac across c-fos and c-jun upon gene induction in murine fibroblasts. In unstimulated cells, continuous turnover of H3K9 acetylation occurs on all K4-trimethylated histone H3 tails; distribution of both modifications coincides across promoter and 5′ part of the coding region. In contrast, K36- and K79-methylated H3 tails, which are not dynamically acetylated, are restricted to the coding regions of these genes. Upon stimulation, transcription-dependent increases in H3K4 and H3K36 trimethylation are seen across coding regions, peaking at 5′ and 3′ ends, respectively. Addressing molecular mechanisms involved, we find that Huntingtin-interacting protein HYPB/Setd2 is responsible for virtually all global and transcription-dependent H3K36 trimethylation, but not H3K36-mono- or dimethylation, in these cells. These studies reveal four distinct layers of histone modification across inducible mammalian genes and show that HYPB/Setd2 is responsible for H3K36 trimethylation throughout the mouse nucleus.

Phosphorylation of histone H3 at Ser10 facilitates RNA polymerase II release from promoter-proximal pausing in Drosophila

The Drosophila JIL-1 kinase is known to phosphorylate histone H3 at Ser10 (H3S10) during interphase. This modification is associated with transcriptional activation, but its function is not well understood. Here we present evidence suggesting that JIl-1-mediated H3S10 phosphorylation is dependent on chromatin remodeling by the brahma complex and is required during early transcription elongation to release RNA polymerase II (Pol II) from promoter-proximal pausing. JIL-1 localizes to transcriptionally active regions and is required for activation of the E75A ecdysone-responsive and hsp70 heat-shock genes. The heat-shock transcription factor, the promoter-paused form of Pol II (Pol IIo(ser5)), and the pausing factor DSIF (DRB sensitivity-inducing factor) are still present at the hsp70 loci in JIL-1-null mutants, whereas levels of the elongating form of Pol II (Pol IIo(ser2)) and the P-TEFb kinase are dramatically reduced. These observations suggest that phosphorylation of H3S10 takes place after transcription initiation but prior to recruitment of P-TEFb and productive elongation. Western analyses of global levels of both forms of Pol II further suggest that JIL-1 plays a general role in early elongation of a broad range of genes. Taken together, the results introduce H3S10 phosphorylation by JIL-1 as a hallmark of early transcription elongation in Drosophila.

Heat shock transcription factor 1 opens chromatin structure of interleukin-6 promoter to facilitate binding of an activator or a repressor

Heat shock transcription factor 1 (HSF1) not only regulates expression of heat shock genes in response to elevated temperature, but is also involved in developmental processes by regulating genes such as cytokine genes. However, we did not know how HSF1 regulates non-heat shock genes. Here, we show that constitutive HSF1 binding to the interleukin (IL)-6 promoter is necessary for its maximal induction by lipopolysaccharide (LPS) stimulation in mouse embryo fibroblasts and peritoneal macrophages. Lack of HSF1 inhibited LPS-induced in vivo binding of an activator NF-kappaB and a repressor ATF3 to IL-6 promoter. Neither NF-kappaB nor ATF3 binds to the IL-6 promoter in unstimulated HSF1-null cells even if they were overexpressed. Treatment with histone deacetylase inhibitor or a DNA methylation inhibitor restored LPS-induced IL-6 expression in HSF1-null cells, and histone modification enzymes were recruited on the IL-6 promoter in the presence of HSF1. Consistently, chromatin structure of the IL-6 promoter in the presence of HSF1 was more open than that in its absence. These results indicate that HSF1 partially opens the chromatin structure of the IL-6 promoter for an activator or a repressor to bind to it, and provides a novel mechanism of gene regulation by HSF1.

Heat Shock Factor 1 as a Coordinator of Stress and Developmental Pathways

The transition from normal growth conditions to stressful conditions is accompanied by a robust upregulation of heat shock proteins, which dampen the cytotoxicity caused by misfolded and denatured proteins. The most prominent part of this transition occurs on the transcriptional level. In mammals, protein-damaging stress leads to the activation of heat shock factor 1 (HSF1), which binds to upstream regulatory sequences in the promoters of heat shock genes. The activation of HSF1 proceeds through a multi-step pathway, involving a monomer-to-trimer transition, nuclear accumulation and extensive posttranslational modifications. In addition to its established role as the main regulator of heat shock genes, new data link HSF 1 to developmental pathways. In this chapter, we examine the established stress-related functions and prospect the intriguing role of HSF 1 as a developmental coordinator.

ATF2 is required for amino acid-regulated transcription by orchestrating specific histone acetylation

The transcriptional activation of CHOP (a CCAAT/enhancer-binding protein-related gene) by amino acid deprivation involves the activating transcription factor 2 (ATF2) and the activating transcription factor 4 (ATF4) binding the amino acid response element (AARE) within the promoter. Using a chromatin immunoprecipitation approach, we report that in vivo binding of phospho-ATF2 and ATF4 to CHOP AARE are associated with acetylation of histones H4 and H2B in response to amino acid starvation. A time course analysis reveals that ATF2 phosphorylation precedes histone acetylation, ATF4 binding and the increase in CHOP mRNA. We also show that ATF4 binding and histone acetylation are two independent events that are required for the CHOP induction upon amino acid starvation. Using ATF2-deficient mouse embryonic fibroblasts, we demonstrate that ATF2 is essential in the acetylation of histone H4 and H2B in vivo. The role of ATF2 on histone H4 acetylation is dependent on its binding to the AARE and can be extended to other amino acid regulated genes. Thus, ATF2 is involved in promoting the modification of the chromatin structure to enhance the transcription of a number of amino acid-regulated genes.

Effects of histone deacetylase inhibitors on transcriptional regulation of the hsp70 gene in Drosophila

Histone acetyltransferases/deacetylases contribute to the activation or inactivation of transcription by modifying the structure of chromatin. Here we examined the effects of histone deacetylase inhibitors (HDIs), trichostatin A, and sodium butyrate on hsp70 gene transcriptional regulation in Drosophila. The chromatin immunoprecipitation assays revealed that HDI treatments induced the hyperacetylation of histone H3 at the promoter and the transcribing regions of hsp70 gene, increased the accessibility of heat-shock factor to target heat-shock element, and promoted the RNA polymerase II-mediated transcription. Moreover, the quantitative real-time PCR confirmed that the HDI-induced hyperacetylation of histone H3 enhanced both the basal and the inducible expression of hsp70 mRNA level. In addition, the acetylation level of histone H3 at the promoter exhibited a fluctuated change upon the time of heat shock. These experimental data implicated a causal link between histone acetylation and enhanced transcription initiation of hsp70 gene in Drosophila.

Chromatin modification of the trefoil factor 1 gene in human breast cancer cells by the Ras/mitogen-activated protein kinase pathway

Histone H3 phosphorylation is a downstream response to activation of the Ras/mitogen-activated protein kinase (MAPK) pathway. This modification is thought to have a role in chromatin remodeling and in the initiation of gene transcription. In MCF-7 breast cancer cells, we observed that phosphorylated histone H3 (phospho-H3) at Ser(10) but not Ser(28) increased with phorbol ester (12-O-tetradecanoylphorbol-13-acetate, TPA) treatment. Although phosphorylated extracellular signal-regulated kinase 1/2 levels in these cells cultured under estradiol deplete and replete conditions displayed no change, a significant induction was observed after TPA treatment. Furthermore, whereas both estradiol and TPA increased trefoil factor 1 (TFF1) mRNA levels in these cells, only TPA-induced and not estradiol-induced TFF1 expression was inhibited by the H3 kinase mitogen and stress activated protein kinase (MSK) inhibitor H89 and MAPK kinase inhibitor UO126, showing the involvement of the Ras/MAPK following TPA induction. Mutation of the activator protein 1 (AP-1) binding site abrogated the TPA-induced transcriptional response of the luciferase reporter gene under the control of the TFF1 promoter, showing the requirement for the AP-1 site. In chromatin immunoprecipitation assays, estradiol treatment resulted in the association of the estrogen receptor-alpha (ERalpha) and acetylated H3 with the TFF1 promoter. The levels of phospho-H3 and MSK1 associated with the TFF1 promoter were moderately increased. In the presence of TPA, whereas ERalpha was not bound to the promoter, a strong association of acetylated and/or phospho-H3, MSK1, and c-Jun was observed. These results show that although both stimuli lead to TFF1 gene activation, estradiol and TPA exert their effects on TFF1 gene expression by different mechanisms.

Expression of heat-shock protein 70 during limb development and regeneration in the axolotl

Urodele amphibians (e.g., axolotls) have the unique ability, among vertebrates, to regenerate perfectly many parts of their body after amputation. The limb has been the most widely studied regenerating structure in these organisms and provides an ideal model in which to understand how vertebrates can regenerate complex tissues. The present study focuses on Hsp-70, a member of the stress-related heat-shock protein family. This protein is normally induced after a stress or trauma such as heat-shock, ultraviolet irradiation, or wounding. Thus, studying its expression during axolotl limb regeneration, a response to an important traumatic event (amputation), is of great interest to further understand how the regenerative process is mediated. Using molecular biology and biochemical techniques, we have characterized both the spatiotemporal and quantitative expression patterns of Hsp-70 in axolotl development and regeneration. Our results show that Hsp-70 is expressed and regulated during axolotl development as in other vertebrates. Our data also demonstrate an up-regulation of the RNA transcript for Hsp-70 during limb regeneration as early as 24 hr after amputation that is maintained up to early differentiation. We also demonstrate a similar pattern of expression for the protein during regeneration. Finally, we show that axolotl Hsp-70 is induced threefold after heat-shock as observed in other vertebrates.

Abrogation of Heat Shock Protein 70 Induction as a Strategy to Increase Antileukemia Activity of Heat Shock Protein 90 Inhibitor 17-Allylamino-Demethoxy Geldanamycin

17-Allylamino-demethoxy geldanamycin (17-AAG) inhibits the chaperone association of heat shock protein 90 (hsp90) with the heat shock factor-1 (HSF-1), which induces the mRNA and protein levels of hsp70. Increased hsp70 levels inhibit death receptor and mitochondria-initiated signaling for apoptosis. Here, we show that ectopic overexpression of hsp70 in human acute myelogenous leukemia HL-60 cells (HL-60/hsp70) and high endogenous hsp70 levels in Bcr-Abl-expressing cultured CML-BC K562 cells confers resistance to 17-AAG-induced apoptosis. In HL-60/hsp70 cells, hsp70 was bound to Bax, inhibited 17-AAG-mediated Bax conformation change and mitochondrial localization, thereby inhibiting the mitochondria-initiated events of apoptosis. Treatment with 17-AAG attenuated the levels of phospho-AKT, AKT, and c-Raf but increased hsp70 levels to a similar extent in the control HL-60/Neo and HL-60/hsp70 cells. Pretreatment with 17-AAG, which induced hsp70, inhibited 1-beta-D-arabinofuranosylcytosine or etoposide-induced apoptosis in HL-60 cells. Stable transfection of a small interfering RNA (siRNA) to hsp70 completely abrogated the endogenous levels of hsp70 and blocked 17-AAG-mediated hsp70 induction, resulting in sensitizing K562/siRNA-hsp70 cells to 17-AAG-induced apoptosis. This was associated with decreased binding of Bax to hsp70 and increased 17-AAG-induced Bax conformation change. 17-AAG-mediated decline in the levels of AKT, c-Raf, and Bcr-Abl was similar in K562 and K562/siRNA-hsp70 cells. Cotreatment with KNK437, a benzylidine lactam inhibitor of hsp70 induction and thermotolerance, attenuated 17-AAG-mediated hsp70 induction and increased 17-AAG-induced apoptosis and loss of clonogenic survival of HL-60 cells. Collectively, these data indicate that induction of hsp70 attenuates the apoptotic effects of 17-AAG, and abrogation of hsp70 induction significantly enhances the antileukemia activity of 17-AAG.

Histone modifications as a platform for cancer therapy

Tumorigenesis and metastasis are a progression of events resulting from alterations in the processing of the genetic information. These alterations result from stable genetic changes (mutations) involving tumor suppressor genes and oncogenes (e.g., ras, BRAF) and potentially reversible epigenetic changes, which are modifications in gene function without a change in the DNA sequence. Mutations of genes coding for proteins that directly or indirectly influence epigenetic processes will alter the cell's gene expression program. Epigenetic mechanisms often altered in cancer cells are DNA methylation and histone modifications (acetylation, methylation, phosphorylation). This article will review the potential of these reversible epigenetic processes as targets for cancer therapies.

Role of histone modifications in marking and activating genes through mitosis

The global inhibition of transcription at the mitotic phase of the cell cycle occurs together with the general displacement of transcription factors from the mitotic chromatin. Nevertheless, the DNase- and potassium permanganate-hypersensitive sites are maintained on potentially active promoters during mitosis, helping to mark active genes at this stage of the cell cycle. Our study focuses on the role of histone acetylation and H3 (Lys-4) methylation in the maintenance of the competency of these active genes during mitosis. To this end we have analyzed histone modifications across the promoters and coding regions of constitutively active, inducible, and inactive genes in mitotic arrested cells. Our results show that basal histone modifications are maintained during mitosis at promoters and coding regions of the active and inducible RNA polymerase II-transcribed genes. In addition we have demonstrated that, together with H3 acetylation and H3 (Lys-4) methylation, H4 (Lys-12) acetylation at the coding regions contributes to the formation of a stable mark on active genes at this stage of the cell cycle. Finally, analysis of cyclin B1 gene activation during mitosis revealed that the former occurs with a strong increase of H3 (Lys-4) trimethylation but not H3 or H4 acetylation, suggesting that histone methyltransferases are active during this stage. These data demonstrate a critical role of histone acetylation and H3 (Lys-4) methylation during mitosis in marking and activating genes during the mitotic stage of the cell cycle.