Associate editor: H. BönischHeat shock proteins in the regulation of apoptosis: new strategies in tumor therapy: A comprehensive review
Introduction
Programmed cell death (PCD) is a common phenomenon in developmental processes or in normal physiological conditions, where the old or damaged cells have to be eliminated. Two distinct forms of cell death, apoptosis and necrosis, have been characterized. Apoptosis is induced by an array of extra- or intracellular stimuli. Organisms are equipped with their own physiological defense to cope with environmental stress and are under the control of genetic machinery in order to prevent or induce cell death depending upon the severity of the stress. Heat shock proteins (Hsp) are highly conserved and play a major role in cytoprotection. Apoptosis resistance is associated with the high expression of Hsp, hence, the present discussion is focused on the functions of Hsp in apoptosis regulation. We also review the pharmacological applications using Hsp inhibitors to induce apoptosis in various tumor models.
The eukaryotic stress response is highly conserved and involves the induction of Hsp. Hsp are detected in all living cells. Both in vivo and in vitro studies have shown that various stressors transiently increase the production of Hsp as protection against harmful insults. Hsp induction was first identified in the salivary glands of Drosophila melanogaster upon application of heat shock (Ritossa, 1962). The unique nature of Hsp synthesis was correlated with the acquisition of thermotolerance and cytoprotection. Later, the interest on Hsp tremendously expanded as more and more functions of normal resting cells involving Hsp were uncovered. The multifunctional roles of Hsp in cells show that Hsp is one of the major regulatory proteins in the cell, and vital functions of cell, such as the maintenance of the cell cycle, are associated with Hsp. The role of Hsp in cell proliferation was reviewed Pechan, 1991, Helmbrecht et al., 2000.
In mammalian cells, the stress response involves the induction of 5 major classes of Hsp families, namely, the small Hsp exemplified by Hsp27, Hsp60, Hsp70, Hsp90, and Hsp104 (Table 1; Lindquist & Craig, 1988, Craig et al., 1994, Morimoto et al., 1994, Scharf et al., 1998). Hsp synthesis is tightly regulated at the transcriptional level by heat shock factors (HSF). Although various HSF were reported, HSF-1 was shown to be the main regulator of the short-term induction of Hsp. In resting cells, HSF-1 is a monomer; however, active HSF-1 exists as a trimer and binds to the heat shock elements (the consensus DNA sequence for heat shock factor binding; Morimoto et al., 1992). In addition, some members of the HSF family help the long-term induction of Hsp or exhibit important roles in the regulation of gene expression and developmental processes (Morimoto, 1998).
Hsp function as molecular chaperones in regulating cellular homeostasis and promoting cell survival Hartl, 1996, Bukau & Horwich, 1998. Various studies demonstrate that Hsp-induced cytoprotection can be attributed partly to the suppression of apoptosis (Samali & Orrenius, 1998), but the precise mechanism of these effects remains to be elucidated. As an additional evidence showing the tight connection between cytoprotection and resistance to apoptosis, cells failing to respond to stress are sensitive to induced cell death via apoptosis (Sreedhar et al., 1999).
Cells typically die either by apoptosis or necrosis. These two forms of cell death are probably much closer to each other than previously thought (Proskuryakov et al., 2003). Both necrosis (where the cell membrane looses its integrity and the cell content is released causing an inflammatory response) and apoptosis (where the cell content remains “well-packed” in the apoptotic bodies and inflammation does not occur) (1) can be caused by the same pathophysiological exposures; (2) can be prevented by antiapoptotic mechanisms; and (3) can be transformed from one form to the other.
Several cell types exhibit only a limited number of replications in cell culture. Morphological and functional properties change until the cell reaches a nondividing—senescent—state Hayflick & Moorhead, 1961, Smith & Pereira-Smith, 1996. Senescent fibroblasts are resistant to PCD (Wang, 1995), are unable to undergo p53-dependent apoptosis, and are shifted to necrosis upon DNA damage (Seluanov et al., 2001). However, apoptosis resistance is not a general feature of senescent cells, which may also be apoptosis-prone depending on the cell type and apoptotic stimuli (Zhang et al., 2002). Senescent fibroblasts promote carcinogenesis of neighboring cells by secreting tumorigenic factors (Krtolica et al., 2001). Therefore, the accumulation of senescent cells may contribute to the age-dependent dramatic increase of cancer incidences.
As we discussed in Section 1.1, Hsp-induced cytoprotection can rescue cells from apoptosis. However, the protein folding capacity of Hsp may be exhausted due to a massive stress resulting in protein misfolding and aggregation, as well as during aging, where oxidized proteins accumulate, or in any chronic disease. This chaperone overload leads to gross disturbances in cellular life (Csermely, 2001a), and increasing severity of the malfunction in protein folding assistance may lead to cell senescence, apoptosis, or necrosis, respectively. Thus, Hsp may not only constitute the most ancient defense mechanism of our cells, but also behave as direct sensors of their functional competence. Various levels of chaperone overload may have an important contribution to the signals directing the cell to senescence, apoptosis, or necrosis (Soti et al., 2003a).
Apoptosis is a universal phenomenon. In principle, this “altruistic” form of cell death ensuring the survival of the whole organism by sacrificing some of its cells was considered to help multicellular organisms. However, bacteria, yeast, and unicellular protozoa also develop various forms of PCD, which is required in certain developmental processes (such as lysis of the mother cell in sporulation) or helps the survival of the whole colony upon environmental stress, like nutrient starving (Lewis, 2000). The protective role of Hsp is acting against apoptosis here as well: their induction effectively suppressed the autolysis of Escherichia coli (Powell & Young, 1991).
Phylogenetic analysis reveals a specific affinity of several eukaryotic proteins involved in apoptosis (such as metacaspases or the OMI/Htra2 protease), with homologues from α-proteobacteria, suggesting a mitochondrial origin of the respective genes. Exploration of the bacterial homologues of many apoptotic proteins, such as caspases, shows a greater diversity than seen in eukaryotes inferring a horizontal gene transfer of the respective genes from bacteria to eukaryotes. Considering these results, a double acquisition of most proteins involved in apoptotic machinery can be hypothesized as having the first event with the domestication of the promitochondrial endosymbiont and the second at the stage of the primitive multicellular eukaryote (Koonin & Aravind, 2002).
Apoptosis (Greek word meaning falling off) is an energy-dependent, ubiquitous physiological process genetically controlled by the expression of evolutionarily conserved genes, which either mediate or suppress the process of cell death. Apoptosis is well characterized by distinct morphological and physiological changes. The morphological changes include nuclear condensation, cell shrinkage, and membrane blebbing, whereas the physiological changes are fragmentation of nuclear DNA due to activation of specific endonucleases cleaving nuclear DNA into 80–200 oligonucleosomal fragments and the activation of caspases, resulting in partially digested proteolytic protein products (Table 2; Kerr et al., 1972, Evan & Littlewood, 1998, Lawen, 2003, Shivapurkar et al., 2003). Apoptosis is a highly regulated process, and it mainly responds to the initial stimulus followed by a cascade of events, hence, it can be divided into 3 phases: (1) the initiation phase (or signaling phase), which involves the activation of surface death receptors (mainly the tumor necrosis factor [TNF] family members), the mitochondrial pathway or the initiation of apoptosis by other stimuli (e.g., those affecting the endoplasmic reticulum [ER]); (2) the signal transduction phase (or preparation phase), where activation of initiator caspases and certain kinases/phosphatases takes place; followed by (3) the execution phase (or death phase), which involves the activation of effector caspases (Thornberry & Lazebnik, 1998). Therapeutic approaches to modulate apoptosis target the cross-roads of major signaling pathways (Dickson, 1998). Hsp are ideal targets, since they both contribute to the pathways themselves and act as chaperones for key molecules in apoptosis.
Section snippets
Heat shock proteins and caspase-dependent apoptosis
Hsp have an extremely complex role in the regulation of apoptosis (Fig. 1). At first glance, due to their cytoprotective role, they inhibit the apoptotic response. We will give many exciting examples of the molecular mechanisms of how Hsp inhibit key steps in the apoptotic cascade and how Hsp fight to maintain the physiological homeostasis of the cell, which is an important requirement for cell survival Wei et al., 1994, Punyiczki & Fesus, 1998, Samali & Orrenius, 1998, Jolly & Morimoto, 2000.
Heat shock proteins and caspase-independent apoptosis
In this section, we will review the emerging alternative pathways of apoptosis, which are not centered around caspase activation. We would like to apologize for this dissection, which will turn to be rather artificial in some cases. Obviously, the signaling pathways are interrelated and, therefore, “caspase-independent” pathways may sometimes converge with caspase-dependent ones. In Section 3.6, we will briefly summarize anoikis, where currently no caspase-independent pathways are known.
Heat shock proteins and antiapoptotic mediators
Hsp are involved not only in the regulation of the various proapoptotic pathways, but also in the maintenance and activation of antiapoptotic mediators (Fig. 3). To better understand the pleiotropic role of Hsp, we chose to give a separate summary of their role in the regulation of antiapoptotic pathways in the following section.
Modulation of modulators: effect of apoptosis on heat shock protein induction
The preceding sections gave numerous examples of how Hsp regulate apoptosis. However, key pro- and antiapoptotic processes also regulate Hsp synthesis. In this section, we will briefly summarize the (unfortunately rather limited) knowledge on the regulation of Hsp synthesis by the apoptotic process. In principle, apoptosis inhibits Hsp synthesis by down-regulating the respective transcription factor, HSF-1. Thus, activation of Fas inhibited heat-induced activation of HSF-1 and the up-regulation
Molecular mechanism of heat shock protein action
Hsp act as molecular chaperones preventing protein aggregation and promoting protein folding. Hsp almost never act alone: they tend to oligomerize, as well as to form chaperone complexes with each other. All major chaperone classes have their co-chaperones, which are smaller chaperone proteins usually regulating the folding cycle of the chaperone complex. Hsp have other general roles as well. Due to their pleiotropic action on the cell, they profoundly influence cellular homeostasis including
Heat shock proteins as pharmacological targets in apoptosis modulation
The preceding sections gave numerous examples on how Hsp can regulate the apoptotic process. This makes any pharmacological intervention, which regulates the synthesis or activity of Hsp, an exciting tool to modulate apoptosis in pathological conditions. Here, we summarize our knowledge of the role of Hsp in tumor cell survival and apoptosis. We also highlight, why Hsp inhibition does not lead to the death of the whole organism and vice versa, why Hsp activation is not causing tumor cell
Conclusions and perspectives
The involvement of Hsp in a multitude of intracellular actions places them as central coordinators in deciding the fate of cell. The level of various Hsp (the Hsp pattern), as well as the amount of Hsp, which are not occupied by damaged, misfolded proteins, can be critical in cytoprotection and cell survival. We need much more comparative investigations on the induction of various Hsp, as well as on their occupancy, to get a full picture of the optimal levels of these proteins. However, from
Acknowledgements
Work in the authors' laboratory is supported by research grants from the EU 6th Framework Program (FP6506850), the Hungarian Science Foundation (OTKA-T37357), from the Hungarian Ministry of Social Welfare (ETT-32/03), and from the International Centre for Genetic Engineering and Biotechnology (ICGEB, CRP/HUN 99-02). A.S.S. is a recipient of National Overseas Scholarship from Ministry of Social Justice and Empowerment, Government of India.
References (423)
- et al.
Three-dimensional structure of the apoptosome: implications for assembly, procaspase-9 binding, and activation
Mol Cell
(2002) - et al.
Halohydrin and oxime derivatives of radicicol: synthesis and antitumor activities
Bioorg Med Chem
(2002) - et al.
Suppression of ceramide-mediated apoptosis by HSP70
Mol Cell
(1999) - et al.
Effects of tissue transglutaminase on retinoic acid-induced cellular differentiation and protection against apoptosis
J Biol Chem
(2001) - et al.
ISO: a critical evaluation of the role of peptides in heat shock/chaperone protein-mediated tumor rejection
Curr Opin Immunol
(2003) - et al.
Akt forms an intracellular complex with heat shock protein 90 (Hsp90) and Cdc37 and is destabilized by inhibitors of Hsp90 function
J Biol Chem
(2002) - et al.
CD91 is a common receptor for heat shock proteins gp96, hsp90, hsp70, and calreticulin
Immunity
(2001) - et al.
Role of reactive oxygen species intermediates in activation-induced CD95(APO-1/Fas)ligand expression
J Biol Chem
(1998) - et al.
Stress management-heat shock protein-70 and the regulation of apoptosis
Trends Cell Biol
(2001) - et al.
Neuronal nitric-oxide synthase is regulated by the Hsp90-based chaperone system in vivo
J Biol Chem
(1999)
The endoplasmic reticulum: a multifunctional signaling organelle
Cell Calcium
Excision of DNA loop domains as a common step in caspase-dependent and -independent types of neuronal cell death
Brain Res Mol Brain Res
hsp90 is required for heme binding and activation of apo-neuronal nitric-oxide synthase: geldanamycin-mediated oxidant generation is unrelated to any action of hsp90
J Biol Chem
Heat shock protein-chaperoned peptides but not free peptides introduced into the cytosol are presented efficiently by major histocompatibility complex I molecules
J Biol Chem
Tissue transglutaminase protects against apoptosis by modifying the tumor suppressor protein p110 Rb
J Biol Chem
Hsp90 and Co—a holding for folding
Trends Biochem Sci
The Hsp70 and Hsp60 chaperone machines
Cell
Differential acquisition of antigenic peptides by Hsp70 and Hsc70 under oxidative conditions
J Biol Chem
Apoptosis-inducing factor (AIF): a novel caspase-independent death effector released from mitochondria
Biochimie
Hsp90's secrets unfold: new insights from structural and functional studies
Trends Cell Biol
C-terminal Hsp-interacting protein slows androgen receptor synthesis and reduces its rate of degradation
Arch Biochem Biophys
Hsp27 inhibits release of mitochondrial protein Smac in multiple myeloma cells and confers dexamethasone resistance
Blood
Heat shock protein 70 moderately enhances peptide binding and transport by the transporter associated with antigen processing
Immunol Lett
TNF-induced recruitment and activation of the IKK complex require Cdc37 and Hsp90
Mol Cell
LY294002-geldanamycin heterodimers as selective inhibitors of the PI3K and PI3K-related family
Bioorg Med Chem Lett
Development of a purine-scaffold novel class of Hsp90 binders that inhibit the proliferation of cancer cells and induce the degradation of Her2 tyrosine kinase
Bioorg Med Chem
Stable overexpression of the constitutive form of heat shock protein 70 confers oxidative protection
J Mol Cell Cardiol
Endothelial nitric oxide synthase: the Cinderella of inflammation?
Trends Pharmacol Sci
Disruption of Raf-1/heat shock protein 90 complex and Raf signaling by dexamethasone in mast cells
J Biol Chem
A retinal heat shock protein is associated with elements of the cytoskeleton and binds to calmodulin
Biochem Biophys Res Commun
Heat shock proteins and molecular chaperones: mediators of protein conformation and turnover in the cell
Cell
Chaperone overload as a possible contributor to civilization diseases
Trends Genet
The 90 kDa heat shock protein (hsp90) induces the condensation of the chromatin structure
Biochem Biophys Res Commun
Signalling and transport through the nuclear membrane
Biochim Biophys Acta
The 90-kDa molecular chaperone family: structure, function, and clinical applications. A comprehensive review
Pharmacol Ther
A population of rat liver lysosomes responsible for the selective uptake and degradation of cytosolic proteins
J Biol Chem
VEGF(165) promotes survival of leukemic cells by Hsp90-mediated induction of Bcl-2 expression and apoptosis inhibition
Blood
Apoptosis regulation and its applications to biotechnology
Trends Biotechnol
The human heat shock protein hsp70 interacts with HSF, the transcription factor that regulates heat shock gene expression
Genes Dev
Molecular chaperone complex at the lysosomal membrane is required for protein translocation
J Cell Sci
Identification and characterization of murine caspase-14, a new member of the caspase family
Cancer Res
A novel mechanism for chaperone-mediated telomerase during prostate cancer progression
Cancer Res
Overexpression of Hsp27 affects the metastatic phenotype of human melanoma cells in vitro
Cell Stress Chaperones
The growth arrest and down-regulation of c-myc transcription induced by ceramide are related events dependent on p21 induction, Rb underphosphorylation and E2F sequestering
Cell Death Differ
HSP90 interacts with and regulates the activity of heat shock factor 1 in Xenopus oocytes
Mol Cell Biol
The heat shock protein 90 antagonist geldanamycin alters chaperone association with p210bcr-abl and v-src proteins before their degradation by the proteasome
Cell Growth Differ
4-1BB and Ox40 are members of a tumor necrosis factor (TNF)-nerve growth factor receptor subfamily that bind TNF receptor-associated actors and activate nuclear factor kappaB
Mol Cell Biol
Hsp27: novel regulator of intracellular redox state
IUBMB Life
Death receptors: signaling and modulation
Science
Proteasome inhibitors differentially affect heat shock protein response in cancer cells
Int J Mol Med
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Currently on leave from the Centre for Cellular and Molecular Biology, Hyderabad 500 007, India.