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Mol. Cells, Vol. 23, No. 2, pp. 123-131
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Molecules
and
Cells
©KSMCB 2007
Heat Shock Responses for Understanding Diseases of Protein
Denaturation
Hee-Jung Kim, Na Rae Hwang, and Kong-Joo Lee*
The Center for Cell Signaling and Drug Discovery Research, College of Pharmacy and Division of Life and Pharmaceutical Sciences,
Ewha Womans University, Seoul 120-750, Korea.
(Received April 10, 2007; Accepted April 12, 2007)
Extracellular stresses induce heat shock response and
render cells resistant to lethal stresses. Heat shock response involves induction of heat shock proteins (Hsps).
Recently the roles of Hsps in neurodegenerative diseases
and cancer are attracting increasing attention and have
accelerated the study of heat shock response mechanism.
This review focuses on the stress sensing steps, molecules
involved in Hsps production, diseases related to Hsp malfunctions, and the potential of proteomics as a tool for
understanding the complex signaling pathways relevant
to these events.
Keywords: Heat Shock Factor; Heat Shock Protein; Heat
Shock Response; MAPK; Misfolding of Protein; ROS;
Ubiquitin-Proteasome System.
fide oxidants and amino acid analogues. Mild administration
of each stress can protect cells against subsequent administration of other lethal stresses. This phenomenon, called
cross-resistance, suggests that some kinds of stresses have
common cellular processes. Recent results suggest that aberrant heat shock responses are associated with diseases including cancer, neurodegenerative disorders, ischemia or
hypoxia, virus infection, inflammation and wound healing.
Therefore, the compounds that can down- or up-regulate heat
shock response and Hsp levels can be used in the treatment
of various diseases (Westerheide and Morimoto, 2005). It is
therefore important to understand the molecular events of
heat shock responses, including the signaling pathways for
suppression of protein synthesis, induction of Hsp synthesis
and molecular and cellular functions of Hsps. In this review,
we will focus on the molecular components in heat shock
responses and their cellular functions.
Introduction
Heat shock response is nature’s device to protect cells
against environmental and physiological stresses. Cells
under stress either mount heat shock response and survive,
or succumb to the stress and die (Lindquist, 1986). This
phenomenon is conserved through evolution. The sequential molecular events in heat shock response are extensively characterized. Its typical features include drastic
repression of normal transcription and translation pathways, and activation of heat shock gene family by heat
shock factor (HSF) (Lindquist and Craig, 1988). An initial,
nonlethal heat dose induces temporary resistance against
subsequent lethal heat shock. This phenomenon is called
as thermotolerance. These responses are also induced by
various protein-damaging stressors including heat shock,
hypoxia, heavy metals including sodium arsenite, disul* To whom correspondence should be addressed.
Tel: 82-2-3277-3038; Fax: 82-2-3277-3760
E-mail: [email protected]
Agents that initiate heat shock response
signaling
Ceramide Ceramide is the simple sphingolipid produced
mainly from sphingomyelin by sphingomyelinases or by a
de novo pathway. Membrane lipid ceramide has been proposed as a signaling molecule that converts extracellular
stresses into intracellular signals. In response to heat
shock, ceramide levels increased in normal HL-60 cells
but not in thermotolerant HL-60 cells (Kondo et al., 2000).
As this implies the possibility that ceramide is the second
messenger of heat shock, we examined the effect of ceraAbbreviations: EGF, epidermal growth factor; HPK, hematopoietic progenitor kinase; IL-1, interleukin 1; MEKK, mitogenactivated protein kinase kinase kinase; MKK, MAP kinase
kinase; MLK, mixed lineage kinase; TAK, TGF-activated
kinase; TNFalpha, tumor necrosis factor alpha; PDGF, plateletderived growth factor.
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Heat Shock Response and Related Diseases
mide in thermotolerant fibroblast cells. We found that
ceramide could induce cell death both in normal and
thermotolerant cells without heat shock response includeing
blockage of protein synthesis and Hsp synthesis (Kim and
Lee, 2002). This indicates that membrane disturbances that
induce cell death do not induce heat shock response. In yeast,
a transmembrane protein, Wsc1-Hcs77, acts as a heat shock
sensor. It is thought to sense cell wall stress and membrane
fluidity (Philip and Levin, 2001). Further studies are needed
to identify the membrane component involved in heat shock
response.
Reactive oxygen species (ROS) H2O2 has recently been
suggested as a second messenger generated by growth
factors and cytokines, including PDGF, EGF, angiopoietin-1, TNFα, and IL-1 in nonphagocytic cells (Han et al.,
2003; Kim et al., 2006; Rhee, 2006). Since many heat
shock response inducing stresses including heat shock,
hypoxia, sodium arsenite and mechanical stress, also induce the generation of ROS, ROS can be considered to be
heat shock response inducing molecules.
ROS, a defense system against infections by engulfing
and killing foreign microorganisms in phagocytic cells,
have been extensively studied. The ROS defense system
requires the NADPH oxidase complex (Nox), which generates H2O2 within the phagosome. Invasion by microorganisms leads to assembly of an active Nox complex, which
comprises a catalytic subunit, the integral membrane protein gp91 Phox, and regulatory proteins including the
small guanosine triphosphatase Rac at the plasma membrane. In nonphagocytic cells, gp91 Phox and its homologs mainly play a role in H2O2 generation by various
growth factors and cytokines, resulting in cell proliferation, differentiation, and migration (Rhee et al., 2000).
ROS is produced and acts as a signaling molecule for the
oxidation of proteins including phosphatase, kinase and
oxidoreductase, and for the regulation of the on-off switch
proteins in the signaling pathway (Bae et al., 1997; Chang
et al., 2002; Giannoni et al., 2005; Rhee et al., 2005).
The functions of ROS in heat shock response have been
extensively investigated, but the results are controversial.
One group claims that the activation of heat shock factor
(HSF) was induced by ROS in vitro and that the HSF1 is
multimerized by either heat shock or by oxidation by
H2O2 and that the oligomerized HSF1 can bind to DNA
(Ahn and Thiele, 2003). However, others claim that oxidation of HSF inhibits its DNA binding activity and all of
the cysteine mutants of HSF1 could be activated and bind
to DNA (Jacquier-Sarlin and Polla, 1996; Manalo et al.,
2002). Also, heat shock-induced kinase activation is not
antagonized by pretreatments with antioxidants (Huot et
al., 1995). We observed heat shock induced ROS production in mouse fibroblast cells, but Hsp expression induced
by H2O2 treatment was negligible (unpublished data). To
confirm these results, we examined heat shock responses
in Rat2 control cells and stably expressing Rac1 dominant
negative cells (Rat2-RacN17) which cannot generate ROS
in response to heat shock. Rat2-RacN17 cells were significantly more tolerant to heat shock than control Rat2 cells in
terms of cell survival and caspase-3 activation, but no typical heat shock responses, including Hsp expression and
repression and recovery of total protein synthesis were discerned. This indicates that ROS produced by Rac1 mediated pathway has indirect effect on heat shock induced
cell death and affects on the localization of intermediate
filament vimentin (Lee et al., 2001). One possible hypothesis is that deubiquitinating enzymes, which have
redox sensitive cysteines in their active sites, are deactivated by ROS, resulting in the accumulation of ubiquitinated misfolded proteins, which in turn induce Hsp expression and cell death in a dose-dependent manner. Another possible explanation is that ROS generated by heat
shock inactivates specific phosphatases and prolong the
phosphorylation status of signaling proteins. This possibility was validated using proteomics which showed increases in phosphorylated proteins following heat shock
(Kim et al., 2002) and H2O2 treatment (Kim et al., 2007).
Misfolded proteins Protein denaturation occurs in cells
treated with physical stressors such as heat shock, hypoxia, and chemicals such as sodium arsenite and amino
acid analogues. Denatured proteins disrupt cellular redox
homeostasis and increase ROS levels and ROS induces
protein misfolding. When misfolded proteins are produced, proteolytic machinery is turned on to remove them.
Cells have two main clearance systems, proteasomal degradation and autophagic degradation system (Rubinsztein,
2006). If the cell’s proteolytic machinery is unable to
eliminate all of the misfolded or denatured proteins, they
tend to aggregate through their abnormally exposed hydrophobic residues which are usually inside of the protein
structure to keep the proteins in solution in the cytoplasm
and nucleoplasm. Misfolded proteins are covalently modified with ubiquitin chains by E1, E2, and E3 ubiquitinconjugating enzymes. Chains of four or more ubiquitin
molecules in the protein are recognized by proteasome
shuttle chaperone CDC48/p97 (Richly et al., 2005; Song
et al., 2005) and targeted to the proteasome to be degraded. More severe protein denaturation is cleared by the
less restricted autophagy system which engulfs the cytosol containing the denatured proteins and causes lysis by
fusion with lysosome (Rubinsztein, 2006).
The misfolded proteins accumulate following stresses,
such as heat shock and proteasome inhibitors, induce
JNK2 activation and activated JNK2 hyperphosphorylates
heat shock factor 1 (HSF1). Together with HSF1, transcriptional activity of HSF2 is activated which results in
the expression of Hsps (Bush et al., 1997; Kim et al.,
1999; Mathew et al., 1998; Park and Liu, 2001). We confirmed this at the proteome level by examining protein
Hee-Jung Kim et al.
expression profiles after treating cells with proteasome
inhibitor, MG132, which results in accumulation of misfolded proteins. The Hsp synthesis profiles were same
after heat shock and proteasome inhibitor treatment.
Pagliari et al. (2005) showed that heat shock induced denaturation and oligomerization of proapoptotic protein Bax,
resulting in permeabilization of the mitochondrial membrane and cytochrome c release. This process was retarded
in presence of Hsps. A recent review hypothesized that heat
shock-induced denatured proteins recruit Hsps which are
bound to stress response molecules in normal conditions,
and causes the release of Hsps from the signaling molecules, activating or inactivating them (Sherman and Gabai,
2006). In this context, the amounts and kinds of misfolded
proteins can determine the fate of cells, namely, recovery or
death. It is necessary to examine the details of signaling
changes that determine the turning point leading to survival
or death of the cells.
Signaling pathways in heat shock
Mitogen activated kinase (MAPK) signaling cascades
play a central role in the regulation and determination of
cellular growth, differentiation, or apoptosis in numerous
physiological conditions. The three major members of
MAPK family are c-Jun N-terminal protein kinase (JNK),
extracellular signal-regulated kinase (ERK), and p38.
They are reported to play roles in stress induced signaling
pathways, especially in heat shock signaling pathway.
JNK In mammals, there are three isoforms of JNK, JNK1,
2 and 3 and their splicing variants. JNK1 and JNK2 are
ubiquitously expressed and JNK3 expression is restricted
to neuronal and heart tissues. Under general stressful conditions, they are activated by Ask1 (Dorion et al., 2002)
which in turn is activated by small guanosine triphosphate
(GTP)-binding proteins, including Cdc42, Rac, Rho and
Ras. They activate MEKKs, HPK, MLK, and TAK as well
as Ask1. These kinases phosphorylate and activate the
dual-specificity kinases MKK4 and MKK7, which in turn
phosphorylate JNK (Davis, 2000; Kyriakis and Avruch,
2001). Intracellular stimuli including endoplasmic reticulum (ER) stress, induced by accumulation of unfolded
proteins in the ER lumen also activate Ask1 and the
downstream signaling molecules (Urano et al., 2000). In
hyperthermal condition, JNK is activated by inhibition of
phosphatases including M3/6, VH1-related (VHR), and
mitogen-activated protein kinase phosphatase 7 (MKP7)
rather than by a upstream kinases (Chen et al., 2001b;
Meriin et al., 1999; Muda et al., 1996; Palacios et al., 2001;
Todd et al., 2002). JNK activation by inhibition of phosphatases is not induced by general stressors including UV
irradiation, osmotic shock, IL-1, and anisomycin but only by
protein-damaging stressors such as heat shock (Meriin et al.,
125
1999). JNK phosphorylates nuclear proteins such as c-Jun,
JunD, ATF2, PPARγ1, and nuclear hormone receptors and
non-nuclear proteins such as DCX, tau, I1ch, IRS-1, Bad,
Bim, and 14-3-3 proteins (Bogoyevitch, 2006).
JNK is known more as a member of proapoptotic
MAPK family member in contrast to ERK which is antiapoptotic protein and a member of another MAPK protein
family. However, evidence is accumulating that JNK also
has anti-apoptotic functions. As mentioned above, JNK2
activates HSFs and induces Hsp expressions. In mice deficient in both jnk1 and jnk2, hindbrain and forebrain regions showed enhanced apoptosis at E10.5 (Kuan et al.,
1999; Sabapathy et al., 1999). The antiapoptotic functions
of JNK are also detected in tumor cells (Bost et al., 1999;
Chen et al., 2001a; Potapova et al., 2002). This discrepancy can be explained by examination of its kinetics. We
showed that even very mild heat shock which does not
induce cell death, induces JNK activation, and the activation and deactivation of JNK occurs during the recovery
from heat shock (Kim and Lee, 2002). However, severe
heat shock, which induces prolonged JNK activation and
not inactivation, induces cell death. We therefore suggested that the activation of JNK followed by the deactivation is not a marker of apoptosis; rather it is possibly
the recovery signal in heat shock.
ERK Heat shock also activates antiapoptotic pathways
involving ERK. ERK is activated by Raf1-MEK-ERK
cascade and Raf1 is activated by agonist-independent
phosphorylation and activation of EGFR (Lin et al., 1997).
As in the case of JNK, ERK is activated by inhibition of
the ERK phosphatases, mitogen activated protein kinase
phosphatase 3 (MKP3) and MKP1 (Yaglom et al., 2003).
ERK phosphorylates Ser 307 of HSF1 and negatively
regulates its transcriptional activity (Chu et al., 1996;
1998).
p38 Another MAPK family member, p38, is activated in
the heat shock signaling pathway. Its activation is through
Ask1-MKK3/6-p38 cascade and Ask1 is released from glutathione S-transferase Mu1-1 by heat shock. Activated p38
phosphorylates MAPKAP kinase-2 which phosphorylates
Hsp27 (Landry et al., 1991). The phosphorylated Hsp27
stabilizes actin filament and mediates actin filament dynamics during stress (Lavoie et al., 1995). Another way in
which p38 functions as a thermotolerant related molecule
in cells is by desensitization of p38 activity.
Expression of Hsps and their functions
Activation of HSFs The induction of heat shock proteins
is regulated by a family of HSFs which bind to the heat
shock elements (HSEs) present on heat shock protein
genes (Pirkkala et al., 2001). Four HSFs (HSF1,-2, 4, and
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Heat Shock Response and Related Diseases
HSFY) exist in mammals. HSF1 is the best studied and
has been characterized as a bona fide transcription factor
in response to heat shock and other stresses. HSF1 performs cytoprotective and antiapoptic functions (Hu and
Mivechi, 2006; Kline and Morimoto, 1997). On the other
hand, because HSF2 is found in developing neuroectoderm and developing testis, it has been regarded to function separately from stress-related HSF1 and implicated in
differentiation and development (Kallio et al., 2002; Pirkkala et al., 2001). HSF4 has been recently identified and
shown to be predominantly expressed in lens and brain
(Hu and Mivechi, 2006; Nakai et al., 1997). HSFY is expressed in the testis and is analogous to HSF (Shinka et
al., 2004). Its function and regulation mechanism remain
to be elucidated.
In unstressed conditions, HSF1 exists as a latent monomer in a negatively regulated state in association with the
major Hsps (Hsp70 and Hsp90). Within minutes of heat
shock and other stresses, HSF1 is activated through a
multistep processes. First, HSF1 is derepressed through
the dissociation from Hsps, trimerized, and translocated
into the nucleus. Then, phosphorylated HSF1 on multiple
sites binds with high affinity to the heat shock elements
which are located in the promoter region of target genes,
leading to the transcription of target genes (Morimoto, 1998;
Pirkkala et al., 2001). HSF1 is known to be constitutively
phosphorylated on Ser 303 by GSK-3, Ser 307 by ERK, and
Ser 308 residue (Chu et al., 1996; 1998) and these modifications appear to be important for negative regulation of HSF1,
while another site, Ser 230 is phosphorylated by calcium/
calmodulin-dependent protein kinase II. Ser 326 and Ser 419
which are phosphorylated by polo-like kinase 1, are inducibly phosphorylated leading to the promotion of HSF1 activity (Guettouche et al., 2005; Holmberg et al., 2001; 2002;
Kim et al., 2005). HSF1 is also reported to be sumoylated
on Lys 298 depending on the phosphorylation of Ser 303
and 307 but the role of this modification remains to be
elucidated (Hietakangas et al., 2003; Hong et al., 2001).
When cells highly express chaperones such as Hsp90 and
Hsp70, HSF1 interacts with these chaperones and is negatively regulated by feedback control (Morimoto, 1998).
Thus, cells have intricate mechanisms to regulate chaperone expression and function which protect against various
stresses.
Cellular functions of Hsps Accumulation of misfolded
proteins in stressed cells, triggers expressions of Hsps,
which prevent protein aggregation and facilitate refolding
or elimination of misfolded proteins in their capacities as
chaperones. These Hsps are classified into six families
according to their approximate molecular masses which
include high-molecular-mass Hsps (≥ 100 kDa), Hsp90
(81 to 99 kDa), Hsp70 (65 to 80 kDa), Hsp60 (55 to 64
kDa), Hsp40 (35 to 54 kDa), and small Hsps (≤ 34 kDa)
(Minami et al., 1996). Hsps play various biological roles
in regulating protein assembly, folding and translocation
(Minami et al., 1996). Hsp100 chaperones share a common ATPase domain and belong to the AAA+ (adenosine
triphosphatases associated with diverse activities) family.
In yeast, Hsp104 controls protein aggregation and disaggregation, but no mammalian homologue has been identified until now (Shorter and Lindquist, 2004). Hsp90 chaperones function as pivotal elements in eukaryotic cells by
stabilizing misfolded proteins and regulating different
signaling proteins such as steroid hormone receptors, tyrosine kinases and calcineurin (Young and Hartl, 2002).
Hsp60 chaperones are heptameric complexes which possess a large central cavity in which protein folding is suggested to occur. Eukaryotic Hsp60 family members (group
I chaperonins) exist in the mitochondria in association
with a cofactor of the Hsp10 family. Other chaperones
(group II chaperonins) such as TRic which have no Hsp10
cofactor are found in the eukaryotic cytosol (Muchowski
and Wacker, 2005). Hsp70 chaperones have a conserved
N-terminal ATPase domain that binds and hydrolyses ATP,
and a C-terminal substrate-binding domain which contributes to stabilization and folding of substrates in association with their co-chaperone Hsp40s. In humans, 11
genes are reported to encode Hsp70 family members
which include constitutive cytosolic member heat shock
cognate (Hsc70), the stress-induced cytosolic Hsp70, the
endoplasmic reticulum-located glucose-regulated protein
78 (Grp78) and the mitochondrial Grp75 (Mayer and Bukau, 2005; Muchowski and Wacker, 2005). Hsp40 binds
Hsp70 through a conserved J-domain and promotes ATP
hydrolysis, leading to a conformational switch which allows the capture of non-native protein substrates (Minami
et al., 1996). Small heat shock proteins assemble into
large oligomeric structures and possess a conserved Cterminal α-crystallin domain that mediates assembly into
an oligomeric form (Clark and Muchowski, 2000; Horwitz, 1992).
Roles of Hsps in neurodegenerative
diseases and Cancer
Neurodegenerative diseases One of the characteristics of
neurodegenerative diseases such as Alzheimer’s disease
(AD), Parkinson’s disease (PD), familial amylotrophic
lateral sclerosis (FALS) and poly Q disease is the formation of plaques/inclusion bodies which co-localize with
various chaperones and components of the ubiquitinproteasome degradation system (Muchowski and Wacker,
2005). In AD, Hsp70 is found in the extracellular senile
plaques and is believed to play a role in phagocytic digestion of amyloid plaques by microglia (Kakimura et al.,
2002). The unusual extracellular accumulation of chaperones is suggested to be facilitated by calcium-induced
interaction with lipid rafts (Broquet et al., 2003). In AD,
Hee-Jung Kim et al.
127
Fig. 1. Stress induced signaling pathways. External stresses including heat shock, mechanical stress, hypoxia, sodium arsenite, and
amino acid analogues, induce the generation of ROS and denaturation of cellular proteins. Activations of signaling pathways in response to a stress vary depending on the strength of stress resulting in the generation of various amounts of ROS and denatured proteins. Weak stress activates cell proliferation or differentiation. Moderate stress induces Hsp expression through heat shock factor
mediated pathway and make cells tolerant to other stresses by blocking the ROS generation and recovering the misfolded proteins.
Strong stress which is overflowing the rescuing capacity of cells, induce cell death. However, the boundaries of the stress level are
varied depending on the tissue and cell types. These phenomena are related to various diseases. High expressions of Hsps and thermotolerance are common features of cancer cells and protein aggregation mediated cell death is characteristics of neurodegenerative diseases.
intracellular amyloid-β (Aβ) starts cellular dysfunction
before it accumulates in extracellular plaques. Grp78 interacts with amyloid precursor protein (APP) and inhibits
the secretion of Aβ40 and Aβ42, suggesting that Grp78
might retain APP in the endoplasmic reticulum and protect APP from β/γ-secretase cleavage into Aβ (Yang et al.,
1998). Tau, a neuronal microtubule binding protein that
contributes to microtubule stability in normal condition, is
hyperphosphorylated in pathologic conditions and accumulates in the neurofibrillary tangles, which are regarded
as a hallmark of AD. At the same time, Hsp27 is reported
to bind to a hyperphosphorylated tau variant in human
brain samples (Shimura et al., 2004). Down regulation of
Hsp70 and Hsp90 using RNA-mediated interference and
Hsp90 inhibitor, geldanamycin, induced tau aggregation,
and overexpression of these chaperones showed the opposite effect. This report suggests that Hsp70 and Hsp90
maintain tau in a soluble, functional conformation and
prevent tau aggregation (Dou et al., 2003).
The hallmark of PD is the accumulation of the αsynuclein, a protein in Lewy bodies. Chaperones certainly
have a role on the aggregation and toxicity of this protein
(Muchowski and Wacker, 2005). Overexpression of HDJ1
(an Hsp40) or Hsp70 in an α-synuclein/synphilin 1 cell
model predominantly decreases the number of cells with
inclusion bodies (McLean et al., 2002). Also, overexpression of Hsp70 decreases detergent-insoluble, high molecular mass α-synuclein species with the overall decrease
in total α-synuclein protein, implying that Hsp70 plays a
role in the promotion of refolding and/or degradation of
α-synuclein (Klucken et al., 2004). Auluck et al. (2002)
were the first to show that the protective effect of Hsp70 in
vivo D. melanogaster model, and demonstrate that endogenous chaperones modestly suppress α-synuclein mediated
neurodegeneration.
There are numerous studies on the effects of chaperones on the aggregation and toxicity of proteins with poly
Q expansions. Overexpression of Hsp40, Hsp70, and
Hsp27 has been shown to suppress the formation or the
toxicity of polyQ inclusion bodies (Jana et al., 2000; Wyttenbach et al., 2002). Studies with the Saccharomyces
cerevisiae system also showed that overexpression of
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Heat Shock Response and Related Diseases
Ssa1 (Hsp70) or Ydj1 (Hsp40) inhibits the formation of
inclusion bodies and promotes the accumulation of
smaller aggregates (Muchowski et al., 2000). An in vivo
study in D. melanogaster showed that endogenous Hsp70
provides protection against mutant polyglutamine toxicity
in a limited way, and overexpression significantly suppresses degeneration (Warrick et al., 1999). Also, it was
shown in a mouse model of poly Q disease that Hsp70 overexpression resulted in a significant amelioration of disease
pathology and phenotypic improvement (Cummings et al.,
2001). Clearly, molecular chaperones are involved in most
neurodegenerative diseases, providing protection at several
levels. This shows that modulators of chaperone expression
can have therapeutic benefits for neurodegenerative diseases.
Cancer Hsp expression levels are high in various tumors,
compared to normal cells. Overexpression of Hsps inhibits the apoptotic machinery and promotes the scavenge of
the misfolded proteins in proteasome-mediated degradation (Aghdassi et al., 2007), possibly inducing the resistance to chemotherapy. The depletion of Hsp27, Hsp70
and Hsp90 using RNA interference or inhibitors, induces
the cell growth arrest or cell death (Calderwood et al.,
2006; Garrido et al., 2006; Xiao et al., 2006). Thermotolerant cells induced by chronic heat shock stress showed
the inhibition of JNK activation and the rapid deactivation
in response to heat shock (Kim and Lee, 2002). Studies on
the up- and down-regulation of various Hsps, obviously
suggest that Hsps expression is well associated with the
resistance of cell death in various tumors. Therefore, the
strategy to inhibit the expression and biological function
of Hsps has great potential in cancer treatment.
Proteomics as a systemic approach to study
heat shock responses
As described above, the modulation of heat shock response is a valuable strategy for the treatment of diseases
involving protein misfolding. However, as the heat shock
responses include a series of signaling pathways for the
initiation, expression of Hsp and their feedback protection,
it is not a simple process (Fig. 1). The complicated procedures can be examined by systematic analysis using proteomics. We compared the proteins differentially expressed by heat shock, proteasome inhibitor MG132, and
hydrogen peroxide (Kim et al., in preparation) and proteins differentially phosphorylated by heat shock and by
hydrogen peroxide using proteomics (Kim et al., 2002;
2007). Heat shock and proteasome inhibition show very
similar protein expression profiles with distinct elevations
of Hsps levels. On the other hand, heat shock and hydrogen peroxide have share both common and different signaling molecules. A systemic analysis of these proteomic
results suggest that heat shock induced cellular response
is very similar to MG132 induced effect. However, in
case of hydrogen peroxide, as mentioned above, there are
some common and, at the same time, some different signaling molecules with heat shock. We think the common
signaling pathway contributes to cell death rather than
heat shock response. As the importance of Hsps in neurodegenerative diseases is recognized, we expect that further emphasis will be placed on systematic proteomic
studies and they will provide more candidate target proteins for drug development.
Concluding remarks
Recently, increasing attention is being paid to the importance of heat shock proteins in neurodegenerative disease
and cancer. Further studies of the mechanism of heat shock
response and the role of heat shock proteins can provide
new therapeutic approaches to the therapy of these diseases.
Accumulating data show that heat shock response is tightly
controlled by specific kinases and phosphatases, MAPK
activation. The heat shock response is distinct signaling
pathway discriminated from other extracellular stresses
such as ultraviolet (UV) and hyperosmolarity. This suggests that the possibility of modulating these diseases with
specific molecules participating in these pathways. Further
systemic studies on the cross-talk between the signaling
pathways should elucidate the overall regulation mechanisms underlying heat shock response and stress-induced
cell death, which, in turn, point to appropriate protein targets to be modulated in the therapy of neurodegerative disorders and cancer.
Acknowledgments We thank to Dr. Sri Ram for the correction
of the manuscript. This work was supported by KOSEF through
the Center for Cell Signaling & Drug Discovery Research (CCS
& DDR, R15-2006-002) at Ewha Womans University, by
KOSEF FPR05A2-480. Kim HJ and Hwang NR were supported
by the Brain Korea 21 Project.
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