Tài liệu Báo cáo khoa học: Novel aspects of heat shock factors: DNA recognition, chromatin modulation and gene expression ppt

Novel aspects of heat shock factors: DNA recognition,chromatin modulation and gene expression Hiroshi Sakurai and Yasuaki Enoki Department of Clinical Laboratory Science, Kanazawa Univer

Novel aspects of heat shock factors: DNA recognition,

chromatin modulation and gene expression

Hiroshi Sakurai and Yasuaki Enoki

Department of Clinical Laboratory Science, Kanazawa University Graduate School of Medical Science, Ishikawa, Japan

Background

The heat shock factor (HSF) in eukaryotes is involved

not only in heat shock protein (HSP) gene expression

and stress resistance, but also in the expression of

genes with roles in cell maintenance and

differentia-tion, as well as in developmental processes HSF forms

a homotrimer that binds to gene promoters containing

a heat shock element (HSE), which is composed of

multiple inverted repeats of the pentanucleotide motif

nGAAn Functional conservation of HSFs among

eukaryotes has been revealed by the finding that HSFs

from various organisms, including insects, mammals

and plants, can substitute for yeast HSF in

Saccharo-myces[1–4]

HSF proteins contain two evolutionarily conserved

functional modules: the DNA-binding domain (DBD)

at the amino-terminus and the oligomerization domain

in the central region of the protein [1,4] The HSF DBD belongs to the ‘winged’ helix-turn-helix family of DNA-binding proteins and contains a three-helix bun-dle capped by a four-stranded antiparallel b-sheet, and

a flexible loop or ‘wing’ with a less ordered structure (Fig 1) [5,6] The second and third a-helices comprise the helix-turn-helix motif The oligomerization domain consists of arrays of hydrophobic heptad repeats (HRs), characteristic of helical coiled-coil structures [1,4,7] The HRs are divided into two subdomains: HR-A and HR-B The amino-terminal HR-A has the potential to form trimers independently of HR-B, and the carboxy-terminal HR-B can form large oligomers [7]

Keywords

chromatin; heat shock element; heat shock

transcription factor; histone; protein–DNA

interactions

Correspondence

H Sakurai, Department of Clinical

Laboratory Science, Kanazawa University

Graduate School of Medical Science,

5-11-80 Kodatsuno, Kanazawa, Ishikawa

920-0942, Japan

Fax: +81 76 234 4369

Tel: +81 76 265 2588

E-mail: sakurai@kenroku.kanazawa-u.ac.jp

(Received 10 May 2010, revised 9 July

2010, accepted 23 July 2010)

doi:10.1111/j.1742-4658.2010.07829.x

Heat shock factor (HSF) is an evolutionarily conserved stress-response reg-ulator that activates the transcription of heat shock protein genes, whose products maintain protein homeostasis under normal physiological condi-tions, 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 func-tion not only in stress resistance, but also in a broad range of biological processes Mammalian cells have four HSF family members involved in dif-ferent, but in some cases similar, biological functions, including stress resis-tance, cell differentiation and development Mammalian HSF family members exhibit differential specificity for different types of heat shock ele-ments, which, together with cell type-specific expression of HSFs is impor-tant 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

Abbreviations

3P, three perfect repeats; DBD, DNA-binding domain; HDAC1, ; HDAC2, ; HR, hydrophobic heptad repeat; HSE, heat shock element; HSF, heat shock factor; HSP, heat shock protein; ncRNA, noncoding RNA; PARP, poly(ADP)-ribose polymerase; Pol II, RNA polymerase II; SAGA, Spt-Ada-Gcn5 acetyltransferase; TFIIA, general transcription factor IIA.

Trimerization of HSF polypeptides is a prerequisite

for binding to the HSE Under physiological

condi-tions, stress-inducible HSFs, such as Drosophila HSF

and mammalian HSF1, are found in an inactive

mono-meric form In vitro analysis has shown that inactive

monomeric HSF directly senses heat and oxidative

stress, and becomes able to trimerize and bind to the

HSE [8,9] Drosophila HSF and mammalian HSF1

have a third HR, HR-C, adjacent to the

carboxy-ter-minus of the protein [10] The HR-C maintains HSFs

in a monomeric form by suppressing trimerization

through intramolecular coiled-coil interactions with the

HR-A⁄ B Under stress conditions, the HR-A ⁄ B–HR-C

interaction is disrupted, thereby leading to

intermolec-ular coiled-coil interactions of the HR-A⁄ B, although

the detailed molecular mechanisms are unknown

[10–12] In addition, protein–protein interactions with

HSPs and other proteins can regulate the monomeric–

trimeric transition of HSF in cells [2] Transcription

activation domains have been located in the HSFs of

several organisms; however, the transactivating ability

of the domain itself, even from stress-inducible HSFs,

is not stress sensitive [1,2,4] Rather, another region of

HSF possesses a stress-regulated inhibitory role that

represses the transactivating ability under physiological

conditions [13] Therefore, the transcriptional activity

of HSF is regulated at two steps: DNA-binding; and

acquisition of the transactivating ability

HSF–HSE interactions

The HSE is composed of multiple inverted repeats of

the nGAAn pentanucleotide that are positioned

con-tinuously without spacing Because each individual

DBD of a trimeric HSF binds to the sequence, the typ-ical HSE contains at least three nGAAn units [4] The helical repeat of the DNA (10.5 bp), the length of the unit (5 bp) and the inverted nature of the repeats, posi-tions all of the GAA units on the same side of the double helix, with the central GAA oriented in the opposite direction In vitro, Drosophila HSF is capable

of binding to two repeat sequences, but the affinity is significantly lower than binding to three repeat sequences [14] The crystal structure of the yeast Kluy-veromyces HSF DBD complexed with the sequence ggTTCtaGAAcc (containing one set of the nGAAn inverted repeat) has been determined [15] In the com-plex, the third helix of the DBD is docked into the major groove, approximately perpendicular to the heli-cal axis of the DNA The evolutionarily conserved arginine residue in this recognition helix makes two hydrogen bonds with G2 of the pentanucleotide

n1G2A3A4n5 (Fig 1) Additional direct contacts include van der Waals bonds between the arginine and

a conserved serine residue and the thymine comple-mentary to the third A3 The remaining contacts with the DNA are to the phosphate backbone Consistent with the structural data, in vitro-binding assays have shown that the order of importance of the bases in the nGAAn repeat is G2>A3>A4 [16–18] In addition, HSF binds with highest affinity when the n1 residue is adenine

HSF trimers bind to long arrays of the nGAAn sequence in a cooperative manner [19] In an electro-phoretic mobility shift assay, an HSE containing four continuous nGAAn units (four perfect repeats) was found to bind two trimers of Saccharomyces HSF, Drosophila HSF, or human HSF1, with two subunits

P F LT KLW TLVSDPDTDAL I W SPS G S FHVFDQGQ F AKEV LP KYF KH NNMAS V QLNMYG FR K VVHIEQGGLVKPERDDTE F QHP C LR

S3

P F VN KLW SMLNDDSNTKL I W AED G S FIVTNREE F VHQI LP KYF KH SNFAS V QLNMYG WH K VQDVKSGSIQSSSDDKWQ F ENEN F IR Sc

Hs1

P F VN KLW SMVNDKSNEKF I W STS G S IVVPNRER F VQEV LP KYF KH SNFAS V QLNMYG WH K VQDVKSGSMLSNNDSRWE F ENEN F KR Kl

P F LA KLW RLVDDADTNRL I W TKD G S FVIQNQAQ F AKEL LP LNY KH NNMAS I QLNMYG FH K ITSIDNGGL-RFDRDEIE F SHPF F KR Dm

P F LS KLW TLVEETHTNEF I W SQN G S FLVLDEQR F AKEI LP KYF KH NNMAS V QLNMYG FR K VVHIDSGIVKQERDGPVE F QHPY F KQ Hs2

P F LG KLW ALVGDPGTDHL I W SPS G S FLVSDQSR F AKEV LP QYF KH SNMAS V QLNMYG FR K VVSIEQGGLLRPERDHVE F QHPS F VR Hs4

DNA contact Aromatic-aromatic interaction

Disulfide bond formation in mammalian HSF1

S1

P F LT KLW ILVDDAVLDHV I W GKD G S FQIVNEET F AREV LP KYF KH NKITS I QLNMYG SR K VFALQTEKTSQENKISIE F QHPL F KR Mm3

Fig 1 Amino acid sequences of HSF DBDs The DBD contains three a-helices (H1, H2 and H3), four b-sheets (S1, S2, S3 and S4), a ‘turn’

of the helix-turn-helix motif and a disordered loop referred to as a ‘wing’ The linker region connects between the DBD and the HR-A ⁄ B motifs The amino acid sequences of the DBDs from Kluyveromyces HSF (Kl), Saccharomyces HSF (Sc), Drosophila HSF (Dm), human HSF1 (Hs1), HSF2 (Hs2) and HSF4 (Hs4), and mouse HSF3 (Mm3) are shown Residues making contacts with DNA are shown in red, residues that have aromatic–aromatic interactions are shown in blue, cysteine residues that form a disulfide bond are shown in green and conserved residues are shown in black.

(possibly one from each trimer) not making contact

with the DNA (Fig 2A, B) [14,20,21] The dissociation

constant of Drosophila HSF for an HSE containing six

or more units (2· 10)15m at 25C) is significantly

lower than that for an HSE containing three perfect

repeats (3P) (4· 10)12m at 25C) [19] Multiple

inverted repeats of nGAAn found at the promoter

regions of many HSP genes provide high

affinity-bind-ing sites for HSF

The DBD–DBD interaction is important in the

HSF–HSF and HSF–HSE interactions In HSF DBD–

HSE co-crystals, the protein–protein interface consists

of the helix 2 amino-terminus, the turn and the wing

[15] Unlike other winged helix-turn-helix proteins, the

wing of the HSF does not appear to contact the DNA

The wing of the Saccharomyces HSF is necessary for

efficient binding of a single trimer to the HSE [22], and

that of mammalian HSF1 is involved in trimer–trimer

interactions [23] The tryptophan and phenylalanine

residues in the mammalian HSF1 DBD form intermo-lecular aromatic–aromatic interactions, which stimulate trimerization (Fig 1) [24] Furthermore, two cysteine residues near the aromatic amino acids in HSF1 form intermolecular disulfide bonds in a stress-inducible manner [9,24] The aromatic amino acids are conserved

in HSFs of various organisms and would generally enhance HSF trimerization; however, the disulfide bond formation is specific for mammalian HSF1 because the cysteines are not conserved in the HSFs of other organisms (Fig 1) Mutations in the linker region that connects the DBD and HR-A⁄ B have been shown

to alter the monomer–trimer equilibrium of mamma-lian HSF1, indicating that this region is crucial for HSF1 trimerization [25]

HSE-specific transcriptional regulation

by Saccharomyces HSF

Saccharomyces HSF constitutively forms a trimer, localizes in the nucleus and binds to and regulates basal expression via HSEs of target genes [26] Deletion of the HSF gene is lethal to cells, even at physiological temperatures The genes targeted by HSF encode proteins that function in protein folding and degradation, detoxification, energy generation, carbohydrate metabolism and cell wall organization [27,28] Heat shock and oxidative stress lead to enhanced binding of HSF to HSEs in yeast [27,29] However, whether the trimerization status and⁄ or the affinity of each DBD for nGAAn changes under stress conditions is not known The activation domains at the amino- and carboxy-termini are required for the transient and sustained heat shock responses, respectively [30] Phosphorylation of HSF, regulated by heat and oxidative stress, is involved in the activation and inactivation of the transactivating ability [26] Interestingly, Saccharomyces HSF is unu-sual among transcriptional activators because it can bypass a need for critical general transcription factors and co-activators, including general transcription fac-tor IIA (TFIIA) [31], Kin28 (a C-terminal-domain kinase of TFIIH) [32], the Taf9 subunit of TFIID and Spt-Ada-Gcn5 acetyltransferase (SAGA) [33,34], and the Med17 and Med22 subunits of Mediator [32] Although the archetypical HSE is a sequence of con-tinuous perfect inverted repeats of nGAAn (perfect-type), the Saccharomyces HSF tolerates the presence of gaps between the units [28,35] The gap-type HSE con-sists of two inverted nGAAn units followed by another unit after a gap of 5 bp, and the step-type HSE con-sists of three direct units, each interrupted by 5 bp (Fig 2A) In the discontinuous gap- and step-type

Two trimers on 4P-type HSE

One trimer on step-type HSE

One trimer on gap-type HSE

One trimer on 3P-type HSE

4P type

A

B a

b

c

d

nTTCnnGAAnnTTCnnGAAn

3P type nTTCnnGAAnnTTCn

Gap type nTTCnnGAAnnnnnnnGAAn

Step type nTTCnnnnnnnTTCnnnnnnnTTCn

Fig 2 HSE types and HSF–HSE interactions (A) Nucleotide

sequences of the different types of HSEs The GAA and inverted

TTC sequences are indicated by bold uppercase letters with

arrows (B) HSF binding to the different types of HSEs Thick lines

represent DNA with GAA and inverted TTC sequences (white

squares) Gray ovals represent HSF monomers making contact with

the sequences, and light gray ovals represent HSF monomers that

do not make contact with the DNA On the gap- and step-type

HSEs, a single HSF trimer is in equilibrium between the two

bind-ing states that are shown.

HSEs, all three GAA units are positioned on the same

side of the double helix, as in the continuous HSE

HSF binds to discontinuous HSEs with similar affinity

for the continuous 3P-type HSE [20,28] On

discontin-uous HSEs, the HSF trimer dissociates from two units

and quickly rebinds to another two units, thereby

sta-bilizing the protein–DNA complex (Fig 2B) Gap-type

HSEs are involved in moderate stress-induced

tran-scription, whereas step-type HSEs are involved in basal

constitutive transcription and in low-level activation

[28,36] These discontinuous HSEs are widely used as

HSF-binding sequences, at least in the Saccharomyces

genome, because approximately half of the HSF target

genes contain these types of HSE [37]

The number of HSF trimers bound to an HSE

affects the subsequent acquisition of the

transactivat-ing capacity In HSF, a C-terminal modulator

domain is necessary for stress-induced

hyperphosph-orylation of the protein and for transcriptional

activa-tion of a gene by a single trimer bound to HSEs with

three nGAAn units (3P-, gap-, or step-type)

How-ever, the C-terminal modulator, and thereby

hyper-phosphorylation, are not necessary for transcriptional

activation by trimers cooperatively bound to HSEs

containing four or more repeat units [20,29] The

HR-A⁄ B is indispensable for binding of a single

tri-mer to three-unit HSEs and for HSF

hyperphosph-orylation, but HR-A⁄ B is not required for binding

HSEs with four or more units or for transactivation

at these HSEs These results indicate that

trimer–tri-mer interactions are implicated not only in the

coop-erative binding of HSF to the DNA, but also in

transcriptional activation [36] Although the role of

hyperphosphorylation in transcriptional activation is

not understood, this modification may induce a

con-formational change in a single trimer that converts it

to an active form As hyperphosphorylation is not

required for activation at HSEs containing four or

more repeats, a similar conformational change may

be mediated by trimer–trimer interactions The

essen-tial role of the HR-A⁄ B is to maintain the structural

integrity of the HSF trimer, because an HSF

deriva-tive containing the dimerization domain of

transcription factor Gcn4 instead of the HR-A⁄ B is

hyperphosphorylated and is capable of activating

transcription via three-unit HSEs [36]

HSE binding by the four mammalian

HSFs

Mammalian HSF1 is a bona fide stress-inducible HSF:

in response to stress, monomeric HSF1 located in the

cytoplasm trimerizes, translocates into the nucleus and

binds to the HSEs of HSP genes [10,11] Under physi-ological conditions, the activity of the carboxy-termi-nal activation domain is repressed by the regulatory region of the protein, and repression is relieved in response to heat shock [13] The transcriptional activ-ity of HSF1 is further regulated by covalent modifica-tions, including phosphorylation, SUMOylation and acetylation [2] It should be noted, however, that HSF1-null mice exhibit multiple phenotypes, including placentation defects, growth retardation and exagger-ated production of the pro-inflammatory cytokine tumour necrosis factor-a, without affecting basal HSP expression [38] Although HSF1 gains DNA-binding and transactivating abilities upon stress, a little HSF1 trimer is present constitutively in cells and binds to and regulates the expression of genes without any apparent stress intervention [1,3]

HSF2 and HSF4 may not be directly involved in the stress response, but rather in cell differentiation and development [3] In addition to covalent modifications, including phosphorylation, SUMOylation and ubiquiti-nation, the regulatory roles of HSF2 and HSF4 are dependent on their cellular concentrations [2] The non-DNA-binding form of HSF2 exists primarily in the cytoplasm as a dimer, which has been suggested to require an interaction between HR-A⁄ B and HR-C Although the signals responsible for activation of HSF2 remain enigmatic, activated HSF2 trimerizes and binds to HSEs [39] HSF2 possesses an activation domain at the carboxy-terminus, but the activity is sig-nificantly lower than the activation domain of HSF1 [2] During development, HSF2 is important for neural specification and spermatogenesis [3] HSF4 lacks the HR-C, and trimeric HSF4 is able to constitutively bind

to the HSEs of target genes [40] Alternative splicing

of the HSF4 transcript results in the production of two isoforms, HSF4a and HSF4b HSF4b has the potential to activate transcription, whereas HSF4a does not [2] HSF4 is required for ocular lens develop-ment and fibre cell differentiation [3]

The HSF3 gene was recently identified in mouse as

an orthologue of the chicken HSF3 gene; however, the human HSF3 gene is a pseudogene [41] Mouse HSF3 translocates into the nucleus upon heat shock and may activate nonclassical heat shock genes, but does not activate classical heat shock genes (i.e HSP genes)

HSF1 prefers long arrays of the nGAAn unit, while HSF2 prefers short arrays [18] In addition, a four per-fect repeat-type HSE binds two trimers of HSF1 or a single trimer of HSF2 [21] These differences are related to differences in the cooperativity of the tri-mers; the wing regions of the HSF1 and HSF2 DBDs

affect the cooperativity [23] HSF4 trimers exhibit

weak cooperativity [21,40]

The genomic sequences to which HSF1 binds are

continuous perfect inverted repeats of nGAAn [42] In

contrast, the HSF4-binding consensus sequence is

more ambiguous than that of HSF1 and HSF2 [43]

The human cA-crystallin and cC-crystallin promoters

contain HSEs of different configurations, and the

for-mer is recognized by both HSF1 and HSF4, while the

latter is preferentially recognized by HSF4 [21] The

mouse p35 gene, a specific target of HSF2, contains a

putative HSE that diverges from the canonical HSE

[44] The HSE specificity of three HSF members has

been systematically characterized using model 3P-,

gap- and step-type HSEs [21] HSF1 preferentially

binds to continuous 3P-type HSE, HSF2 exhibits a

slightly higher binding affinity to discontinuous HSEs

than does HSF1, and HSF4 efficiently recognizes

dis-continuous HSEs When HSF1, HSF2 and HSF4 are

expressed in yeast cells, transcription of various genes

is differentially regulated by the three HSFs and

corre-lated with the type of HSE, namely perfect, gap, or

step [21,45] These observations indicate that the

con-figuration of the nGAAn unit is an important

determi-nant of HSF–HSE interactions This is consistent with

the notion that although DNA-binding transcription

factors of the same family bind the same

highest-affin-ity sites, they prefer different lower-affinhighest-affin-ity sites, and

that low-affinity sites do contribute to factor binding

and gene expression [46]

Differences in the HSE-binding specificity are in part

explained by the oligomerization of HSFs When an

amino acid substitution is made in the HR-A⁄ B of

HSF4, the protein exhibits a reduced ability to trimerize

and is unable to bind to discontinuous gap- and step-type

HSEs, but binding to the continuous 3P-type HSE is not

affected [21] In contrast, an amino acid substitution in

the HR-C of HSF1 enables HSF1 to constitutively form

trimers that bind to discontinuous HSEs [45] Similar

results have been obtained by introducing

oligomeriza-tion-prone mutations into the HSF1 DBD [45] On

discontinuous HSEs, in which two pairs of two nGAAn

units provide a platform for binding two DBDs of a

single HSF trimer, stable oligomerization inhibits

disso-ciation and⁄ or stimulates rebinding of HSF (Fig 2B)

Interestingly, HSF1 and HSF2 form heterotrimers

via the HR-A⁄ B regions [47–49] When HSF1 and

HSF2 bind as a complex to satellite III DNA in

nuclear stress bodies, elevated expression of HSF2

causes transcriptional activation of noncoding satellite

III RNA by the heterotrimer [49] In addition, HSF2

modulates the activity of stress-induced HSF1 in a

gene-specific manner: the presence of HSF2 leads to

stimulation of HSP70 expression but to inhibition of HSP40 and HSP110 expression [48] Differences between HSF1 and HSF2 in HSE specificity may enable an HSF1–HSF2 heterotrimer to have a distinct HSE specificity

Binding of HSF to chromatin and changes in chromatin structure

Packaging of DNA into nucleosome arrays, which can

be folded into higher-order structures, provides way with which to tightly control access to the DNA sequence Many sequence-specific DNA-binding fac-tors, including HSF, as well as general transcription factors, cannot bind to nucleosomal DNA [50] In cells, there are various proteins and protein complexes that change chromatin structure: histone variants, histone-modifying enzymes, histone chaperones and chromatin remodelling complexes [51]

In yeast

In Saccharomyces cerevisiae cells, heat shock induces the binding of general transcription factors and RNA polymerase II (Pol II) to the HSP promoter, resulting

in rapid RNA synthesis [52] Chromatin regulators, including the SAGA histone–acetylase complex, the Rpd3 histone–deacetylase complex, ATP-dependent chromatin remodelling complexes (SWI⁄ SNF, ISW1 and RSC) and histone chaperones (Asf1, Spt6 and Spt16), are involved in histone acetylation and eviction [52–55] Stress-inducible or constitutive binding of HSF to a promoter is dependent on the structure of the HSE The HSP12 promoter possesses a low-affinity HSE and a binding site for the general stress response transcription factors Msn2 and Msn4 (Msn2⁄ 4) However, HSF and Msn2⁄ 4 are not preloaded on this promoter Heat-induced binding of HSF to the pro-moter in nucleosomes is assisted by Msn2⁄ 4 and the chromatin remodelling complexes SWI⁄ SNF, ISW1 and RSC [56,57] These remodellers play partially overlapping, but not redundant, function [57] In con-trast, the HSP82 promoter possesses a high-affinity HSE, which constitutively binds HSF The promoter contains remodelled nucleosomes exhibiting DNase I hypersensitivity, and the gene is transcribed at low levels under physiological conditions [58,59] Binding

of HSF to the HSP82 promoter has been suggested to occur soon after DNA replication and before nucleo-somes have assembled, in the S⁄ G2 phase of the cell cycle [59], depending on ISW1 and RSC activities [57]

In response to heat shock, HSF occupancy at HSE-containing promoters increases, the SAGA and

SWI⁄ SNF complexes are recruited to both the

pro-moter and coding regions, and histones are acetylated

and displaced [56–58] However, the presence of SAGA

and SWI⁄ SNF, and a high density of Pol II within the

coding region, are not sufficient to elicit histone

dis-placement, suggesting that histone eviction is

modu-lated by factors that are not linked to elongating

polymerase [55] Recently, a candidate factor was

iden-tified in Drosophila [60, see below] The

Rpd3-contain-ing histone–deacetylase complex, which deacetylates

histones to repress transcription, is also recruited to

induced genes It is possible that the opposing activities

of histone acetylase and deacetylase modulate

chroma-tin structure and fine-tune transcription [58] The

his-tone chaperones Asf1, Spt6 and Spt16 are involved in

histone eviction and redeposition, and inactivation of

Spt16 leads to sustained transcription [61]

In Drosophila

Studies in Drosophila provide many insights into how

HSF and Pol II overcome the nucleosome barrier In

brief, on polytene chromosomes of Drosophila, heat

shock leads to visible changes in the heat shock loci,

termed ‘‘puffing’’ Puffing reflects the changes in

chro-matin structure that lead to the disruption of

nucleo-somes along the coding region of HSP genes Unlike

yeast HSP genes, the DNase I hypersensitive region of

the hsp70 promoter of nonheat-shocked cells contains

paused Pol II, which is transcriptionally engaged and

paused 20–40 bp downstream of its initiation site [62–

64] The promoter-bound GAGA factor is required for

efficient pause-site entry of Pol II Upon heat shock,

escape of the paused polymerase from the site is

trig-gered by HSF-mediated recruitment of positive

tran-scription elongation factor-b (P-TEFb), which

phosphorylates the polymerase [62–64] Concomitant

nucleosome loss at hsp70 requires poly(ADP)-ribose

polymerase (PARP), which is capable of binding

nucleo-somes and is important for puff formation [60] The

possible roles of PARP in nucleosome loss are as

fol-lows, (a) PARP prebound to nucleosomes is released

from chromatin to reverse any repressive effects of the

chromatin structure, (b) PARP ADP-ribosylates

histones and thus destabilizes nucleosomes or (c)

ADP-ribose polymers generated by PARP, which look

much like a nucleic acid, bind to histones, facilitating

nucleosome dissociation [60]

In mammals

Similarly to Drosophila, the promoter region of

mam-malian HSP70 is hypersensitive to DNase I and

contains paused polymerase [62–64] Although GAGA factor-like DNA-binding activity has been reported, the involvement of a mammalian GAGA factor-like protein in polymerase pausing has not been docu-mented [65] Instead, the HSP70 promoter is bound by HSF2 in mitotic cells, which prevents compaction at the site by condensin and maintains the gene in a tran-scription-competent state, thereby enabling robust acti-vation by HSF1 if cells in the early G1 phase are exposed to a stressor [66] The bookmarking function

of HSF2 is achieved by recruitment of protein phos-phatase 2A to inhibit nearby condesin complexes by dephosphorylation The bookmarking of the HSP70 promoter and other HSE-containing promoters by HSF2 is different from that of many active promoters, which are marked by binding of TATA-binding pro-tein and recruitment of propro-tein phosphatase 2A [67,68] In mouse lens, HSF4 binds to nonclassical heat shock genes and induces demethylation of histone H3 lysine 9; methylation of this residue is correlated with transcriptionally repressed chromatin [43] This HSF4 function is required for heat-induced transcription of these genes, in part by facilitating HSF1 binding via chromatin modification The roles of HSF1 in chroma-tin modulation have been shown: under unstressed conditions, HSF1 binds to the promoter of the non-heat-shock interleukin-6 gene and functions in the opening of its chromatin structure through recruitment

of the CBP histone acetyltransferase and the BRG1-containing SWI⁄ SNF chromatin remodeller [69] It is unknown whether binding of HSF4 and HSF1 to DNA is maintained in mitotic chromatin

Upon transcriptional activation, HSF1 bound to the HSP70 promoter recruits the BRG1-containing SWI⁄ SNF chromatin remodeller and the CBP ⁄ p300 histone acetyltransferases [70,71] In the HSF1 activa-tion domain, phenylalanine residues are involved in promoting elongation mediated by Pol II and recruit-ment of the SWI⁄ SNF complex, while acidic amino acids are involved in transcriptional initiation [70] Interestingly, mouse HSF3, which fails to activate clas-sical heat shock genes (including HSP70), does not bind SWI⁄ SNF, suggesting the importance of the chro-matin remodeller in HSF member-specific transcription [41] Histone acetylation at the HSP70 promoter occurs under treatment with heat and arsenite; how-ever, phosphorylation of histone H3 occurs as a result

of treatment with arsenite [72] The arsenite-induced phosphorylation of H3 and HSP70 transcription is dependent on p38 mitogen-activated protein kinase activation However, it remains to be explored how phosphorylation of H3 is involved in arsenite-induced, but not in heat-induced, transcription by HSF1

During attenuation of RNA synthesis, the chaperone

HSP70 behaves as a corepressor of HSF1 in a

nega-tive-feedback loop and interacts with CoREST, a

com-ponent of a histone deacetylase complex containing

HDAC1 and HDAC2 [73] CoREST binds to the

HSP70 promoter at low levels under physiological

conditions and suppresses basal expression

Global transcriptional changes and

histone modifications in heat-shocked

mammalian cells

When cells are subjected to heat shock, transcription

of many mRNA genes is rapidly repressed In contrast,

the levels of the noncoding RNAs (ncRNAs) – B2

RNA and Alu RNA – which are transcribed from

short interspersed elements (SINEs) by RNA

polymer-ase III, increpolymer-ase in mouse and human cells,

respec-tively Both ncRNAs are transacting transcriptional

repressors during the heat shock response: they block

transcription by binding Pol II and entering complexes

at some mRNA gene promoters [74,75] On the

acti-vated mouse and human HSP70 promoters, however,

Pol II is present, but B2 RNA and Alu RNA are

absent, implying that mechanisms must exist to

over-come ncRNA repression on the promoter, for example,

degradation of the ncRNA repressors by an RNase,

removal of the ncRNA repressors by an RNA helicase,

or binding of another ncRNA to Pol II to displace the

ncRNA repressors [75] In this context, it is

notewor-thy that the ncRNA heat shock RNA-1 (HSR1) is an

important regulator for heat-induced HSF1

trimeriza-tion and HSP expression [76]

A global deacetylation of core histones is associated

with heat shock This is achieved by HSF1 and the

his-tone deacetylases HDAC1 and HDAC2 [77] In

heat-shocked cells, HSF1 binds to HDAC1 and HDAC2,

and their histone-deacetylase activities increase The

heat-induced deacetylation of histone H4 is not

observed in HSF1 knockout mouse cells In addition,

loss of HSF1 causes hyperacetylation of histone H4 in

nonheat-shocked cells [77] The relationship between

HSF1 and histone deacetylases has also been shown in

human breast cancer cells: treatment of cells with the

transforming factor heregulin leads to increases in the

levels of HSF1 and MTA1 (a component of the NuRD

complex containing HDAC1 and HDAC2) proteins

and to binding of HSF1 to MTA1 [78] The complex

is recruited to the promoters of estrogen-responsive

genes and participates in the repression of

transcrip-tion, an effect linked to metastasis [78] Therefore,

HSF1 is a potent regulator of global histone

acetyla-tion⁄ deacetylation in stressed and unstressed cells

Conclusion

In the HSF trimer, binding of each DBD to a 5-bp unit (nGAAn) is necessary to achieve a stable protein– DNA interaction, because monomeric HSF binds to the repeat unit with significantly less affinity than does the trimer In addition to the continuous repeat units, three discontinuous units in the gap- and step-type HSEs can be bound by HSFs Efficient trimerization

of HSFs is important to establish interactions with the gap- and step-type HSEs The different types of HSEs result in the induction, by yeast HSFs, of different lev-els of gene expression and probably in distinguishing target genes by mammalian HSFs In response to stress, yeast HSF bound to an HSE recruits the gen-eral transcription factors and Pol II to the promoter, while the target promoters of mammalian HSF1 con-tain paused polymerase that switches to productive elongation in response to activated HSF1 Chromatin regulators that disrupt the nucleosome structure facili-tate the binding of HSF, general transcription factors and Pol II to the promoters, and subsequent RNA synthesis by polymerase In mammalian cells, HSF1 target promoters are also bound by HSF2 and HSF4 under physiological conditions, suggesting that these factors play a positive role in the opening of the chro-matin structure and in stress-induced transcription by HSF1 In addition, HSF1 itself associates with histone deacetylases and regulates global histone acetylation Therefore, mammalian HSFs function not only as transcription activators but also as chromatin-modu-lating factors

Acknowledgements

This work was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan to

H Sakurai

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