The small heat shock proteins and their role in human disease

Authors


T. H. MacRae, Department of Biology, Dalhousie University Halifax, Nova Scotia B3H 4J1, Canada
Fax: +1 902 4943736
Tel: +1 902 4946525
E-mail: tmacrae@dal.ca

Abstract

Small heat shock proteins (sHSPs) function as molecular chaperones, preventing stress induced aggregation of partially denatured proteins and promoting their return to native conformations when favorable conditions pertain. Sequence similarity between sHSPs resides predominately in an internal stretch of residues termed the α-crystallin domain, a region usually flanked by two extensions. The poorly conserved N-terminal extension influences oligomer construction and chaperone activity, whereas the flexible C-terminal extension stabilizes quaternary structure and enhances protein/substrate complex solubility. sHSP polypeptides assemble into dynamic oligomers which undergo subunit exchange and they bind a wide range of cellular substrates. As molecular chaperones, the sHSPs protect protein structure and activity, thereby preventing disease, but they may contribute to cell malfunction when perturbed. For example, sHSPs prevent cataract in the mammalian lens and guard against ischemic and reperfusion injury due to heart attack and stroke. On the other hand, mutated sHSPs are implicated in diseases such as desmin-related myopathy and they have an uncertain relationship to neurological disorders including Parkinson's and Alzheimer's disease. This review explores the involvement of sHSPs in disease and their potential for therapeutic intervention.

Abbreviations
17-AAG

17-allylamino-17-demethoxygeldanamycin

amyloid-β

AGE

advanced glycation end-product

ALS

amyotrophic lateral sclerosis

CAT

cancer/testis antigen

GFAP

glial fibrillary acidic protein

HMM

high molecular weight

IFN-γ

interferon-γ

MS

multiple sclerosis

sHSP

small heat shock protein

SOD

superoxide dismutase

Within the molecular chaperone family, sHSPs constitute a structurally divergent group characterized by a conserved sequence of 80–100 amino acid residues termed the α-crystallin domain [1–8]. The α-crystallin domain, duplicated in the unusual example of parasitic flatworms (Platyhelminthes) [9], is located toward a highly flexible, variable, C-terminal extension, and is usually preceded by a poorly conserved N-terminal region. The molecular mass of sHSP subunits ranges from 12 to 43 kDa, and they assemble into large, dynamic complexes up to 1 MDa. sHSP secondary structure is dominated by β-strands with limited α-helical content, and β-sheets within the α-crystallin domain mediate dimer formation. Crystallization of two sHSPs has contributed significantly to the description of oligomerization, quaternary structure, subunit exchange, and chaperone activity. Characterization of a highly conserved arginine is also an important outcome of crystallization and related studies because mutation of this residue has profound effects on sHSP function and contributes to certain diseases [10–16].

The sHSPs are molecular chaperones, storing aggregation prone proteins as folding competent intermediates and conferring enhanced stress resistance on cells by suppressing aggregation of denaturing proteins, actions associated with oligomerization and subunit exchange [17–20]. Functional studies of the sHSPs are more limited than for other chaperones, but this is changing as the application of genomics and proteomics reveals sHSP characteristics and their medical importance emerges. In this context, 10 sHSPs, termed HspB1–10, many of which are constitutively present at high levels in muscle and implicated in disease, are found in humans [2,21–23]. Intracellular quantities and cellular localizations of sHSPs change in response to development, physiological stressors such as anoxia/hypoxia, heat and oxidation, and in relation to pathological status. sHSPs interact with many essential cell structures and it follows from such promiscuity that functional disruption and inappropriate association of these molecular chaperones with substrates will foster disease. Therefore, this review considers the role of sHSPs in several human medical conditions and it ends with a discussion of their therapeutic potential.

sHSPs and cataract

sHSP mutation and post-translational change contribute to cataract development in the mammalian lens, a transparent organ with refractive characteristics specialized to focus visible light [5,24–28]. Lens tissue derives from cells containing large amounts of densely packed proteins known as α-, β- and γ-crystallins, which function for the lifespan of an organism and are essential for vision. Lens transparency, viscosity and refractive index depend on crystallins, their interactions with one another, with membranes [13,29], and with cell components such as actin [30] and the intermediate filament proteins CP49 and filensin [31]. α-crystallins maintain lens transparency by serving interdependently as structural elements and molecular chaperones. As α-crystallin chaperoning capability declines, lens proteins are more likely to aggregate, a characteristic linking cataract to other protein folding diseases [24]. That is, amyloid fibrils arise in solutions of bovine lens α-, β- and γ-crystallins under mild denaturing conditions, as might happen upon sHSP post-translational modification, leading to aggregation in the presence of reduced chaperoning ability [32]. What is more, post-translational changes reduce crystallin solubility, contributing to less effective protein packing. The evidence strongly favors the belief that perturbation of αA- and αB-crystallin reduces lens transparency and generates cataract, the leading cause of blindness worldwide. As these aberrant processes become better understood through continued study of the α-crystallins, methods to counter cataract development are certain to emerge.

Cataract and α-crystallin post-translational changes

Posttranslational modifications of αA- and αB-crystallin, including truncation [33–37], deamidation [36,38–42], oxidation [40,43–46], glycation [46–53], phosphorylation [33] and racemization/isomerization [54,55], promote cataract formation in aging organisms through modification of chaperone activity and solubility [24,35,40,41,47,56]. α-crystallin post-translational changes, with a corresponding effect on lens transparency, occur during diabetes where chaperone activity decreases in reverse correlation to glucose levels [52]. Glycation, the nonenzymatic addition of sugars to proteins, is enhanced in rat and human lenses during diabetes, causing protein cross-linking and advanced glycation end-products (AGE), a change engendered by methylglyoxal interaction with lysine and arginine residues [51]. Glycation in vitro limits the chaperone activity of human, calf and rabbit lens α-crystallins [46,51], as does methylglyoxal treatment of calf lens in organ culture, with corresponding reduction in protein stability [48,49]. However, in other studies, glycation of C-terminal lysines does not disrupt α-crystallin chaperoning [53] and activity increases when the protein is modified in vitro[48,50], suggesting in contrast to prevailing theories that post-translational modifications are an aging related protective mechanism for long-lived lens proteins.

Demonstrating definitive causal relationships between sHSP post-translational modifications and function is difficult, a problem confounding the analyses of other proteins such as tubulin [57,58], but progress has been made. Truncated α-crystallin from lenses of ICR/f rats, a strain with hereditary cataract, exhibits reduced chaperone activity against heat-induced aggregation of βL-crystallin from the same source [35]. Truncated α-crystallin functional loss can be rationalized in light of sHSP N- and C-terminal region properties, and reduced chaperoning links truncation to cataract. α-crystallin deamidation involves the nonenzymatic conversion of asparagine to either aspartate or isoaspartate, and glutamine becomes glutamic acid, prevalent changes during cataract formation and aging [36]. The use of site-directed mutagenesis to generate variants N146D and N78D/N146D demonstrates deamidation significantly impacts bacterially produced human αB-crystallin, whereas the single modification N78D has little effect [38]. In comparison to wild type, oligomer size increases and chaperone activity decreases in N146D and N78D/N146D mutants, suggesting deamidation disrupts lens αB-crystallin packing and chaperoning, thereby compounding the role of this post-translational change as a causative agent of cataract. Mutations N101D, N123D, and N101D/N123D of human αA-crystallin also reduce chaperone action and enlarge oligomers, with N101D effects greater than N123D [39]. Negative charges introduced by deamidation disturb tertiary structure, contributing to functional changes and to cataract. Site-directed mutagenesis was employed to examine oxidation of αA-crystallin, a protein with two cysteine residues [44] and where intrapolypeptide disulfides [45] and mixed glutathione disulfides [59] curtail chaperone activity. Exposing wild-type α-crystallin and mutants C113I, C142I and C131I/C142I to hydrogen peroxide demonstrates disulfide-dependent dimerizations are less important in production of high molecular mass (HMM) protein aggregates accompanying cataract than are secondary structural changes generated upon tryptophan and tyrosine oxidation. Additionally, α-crystallin dimerization promoted by calcium-activated transglutaminase eliminates chaperone activity, suggesting a role in reduced lens transparency and cataract [56]. Oxidation and transglutaminase induced cross-linking may coordinately transform lens α-crystallin chaperone activity and packing, magnifying the consequences of these changes and promoting cataract formation more than anticipated.

Evidence linking cataract and α-crystallin post-translational changes is compelling, but there are examples of extensive α-crystallin modification before disease appears, and cataract associated protein changes may occur subsequent to lens α-crystallin denaturation rather than before [24,42]. In spite of these observations, the prevalence of post-translational changes in lens α-crystallins argues forcefully for a major role in cataract and their study remains important if the disease is to be fully understood. Potential exists for development of therapeutic applications such as the use of carnosine to disaggregate glycated α-crystallin [47] and employing agents that prevent post-translational changes [40].

Cataract and α-crystallin mutations

The mutation responsible for autosomal dominant congenital cataract, a common cause of infant blindness, localizes to the αA-crystallin gene (CRYAA) [60]. An R116C substitution renders αA-crystallin defective in chaperone function [11–13], but impaired chaperoning may not completely explain cataract development [10,61]. Another dominant mutation in human αA-crystallin associated with cataract, R49C, is the first shown to lie outside the α-crystallin domain [61]. This change causes lens central core nuclear opacities, as does the R116C mutation. However, in contrast to R116C αA-crystallin, the R49C variant localizes to the cell nucleus and the cytoplasm, superficially suggesting a relationship to neurodegenerative disorders characterized by intranuclear glutamine-repeats [61]. The αB-crystallin gene, CRYAB, described later in the context of desmin-related myopathy, is associated with cataract when possessing an R120G mutation [15,62,63]. αB-Crystallin R120 corresponds to αA-crystallin R116 and both are conserved α-crystallin domain arginines. R120G αB-crystallin permits intermediate filament self association in vitro, although binding of the modified protein to filaments increases in comparison to wild-type αB-crystallin [15,16,64], and this may encourage cataract.

As a prelude to examination of protein recognition by modified α-crystallins, results obtained by mammalian two-hybrid analyses demonstrate that interaction of αA- and αB-crystallin with one another is about three times stronger than the engagement of either chaperone with the prominent lens proteins, βB2-crystallin or γC-crystallin [65,66]. Moreover, αB-crystallin self-interaction occurs essentially independent of the polypeptide's N-terminus, but self-association of αA-crystallin requires this domain [66]. Attachment of R116C αA-crystallin to Hsp27 and αB-crystallin increases in comparison to wild type, while binding to γC-crystallin and βB2-crystallin decreases. Reaction of R120G αB-crystallin with βB2-crystallin is moderately enhanced, but there is no change in recognition of γC-crystallin and Hsp27, and association with αA- and αB-crystallin declines. The altered interplay with other crystallins illustrates that R116C αA-crystallin and R120G αB-crystallin, both observed in congenital cataract, maintain lens protein solubility less effectively and promote cataract development.

Lens size drops off in mice homozygous for αA-crystallin gene loss [αA (–/–)], a characteristic correlated with 50% reduction in lens epithelial cell growth and enhanced sensitivity to apoptotic death [67,68]. The lenses of αA (–/–) mice become opaque with age and contain many inclusion bodies reactive with antibody to αB-crystallin, but not to β- and γ-crystallin, suggesting an important role for αA-crystallin in maintaining lens transparency [69]. Over-expression of αA-crystallin protects stably transfected cells against UVA radiation, whereas αA (–/–) lens epithelial cells have greater sensitivity to photo-oxidative stress, exhibiting more apoptosis and actin filament modifications. Synthesis of exogenous human αA-crystallin in lens epithelial cells of the same species counters UVB-induced apoptosis by favoring action of the AKT kinase pathway, potentially explaining results obtained with knock-out mice [70]. αB-Crystallin (–/–) mice develop skeletal muscle dystrophy but not cataract [71] and they are hyperproliferative, with tetraploid or higher ploidy cells and enhanced susceptibility to apoptosis [72,73]. αB-Crystallin may protect cells from genomic instability. In contrast to the situation with αA-crystallin depletion, there is no apparent effect on the actin cytoskeleton in αB-crystallin (–/–) mice, but abnormal mitotic spindles occur, demarcating a relationship between αB-crystallin and tubulin. Interestingly, synthesis of exogenous αB-crystallin in human lens epithelial cells hinders UVA-induced activation of the RAF/MEK/ERK signal transduction pathway and reduces apoptosis substantially, implicating the chaperone in protection against programmed cell death [70].

sHSPs and desmin-related myopathy

An R120G mutation in αB-crystallin, an abundant protein in nonocular tissues such as skeletal and cardiac muscle [2,21–23], gives rise to inherited, adult onset, desmin-related myopathy, a neuromuscular disorder where desmin, an intermediate filament protein, aggregates with αB-crystallin [63]. The mutation disrupts αB-crystallin structure, chaperone activity and intermediate filament interaction, demonstrating the functional importance of residue R120 [14–16,62,74]. This was the first sHSP mutation shown to cause inherited human muscle disease, but two additional dominant negative αB-crystallin mutations have since been linked to myofibrillar myopathy, but not cardiomyopathy [75]. The αB-crystallin C-terminus is truncated by 13 residues in one case and 25 in another, a region important for sHSP solubilization, chaperone activity and oligomer formation.

R120G αB-crystallin synthesis in hearts of transgenic mice induces desmin-related cardiomyopathy [74,76], potentiating desmin and αB-crystallin aggregation, myofibril derangement, compromised muscle action, and heart failure. Study of transgenic mice containing mutations in both desmin and αB-crystallin signifies that the sHSP prevents aggregation of misfolded desmin [77]. A nuclear role for αB-crystallin during cardiomyopathy is also possible because the R120G mutant inhibits speckle formation by the wild-type chaperone in several transfected cell lines [78]. Speckles are thought to participate in RNA transcription and splicing. Cardiomyocyte transfection with adenovirus encoding R120G αB-crystallin promotes microtubule-dependent production of intracellular aggresomes [79]. These structures, appearing in cardiomyocytes of dilated and hypertrophic cardiomyopathies, are characteristic of amyloid-related neurodegenerative conditions, indicating relationships between these two major types of disease and implying common roles for aggregate-associated sHSPs. Furthermore, aggregates stain weakly for desmin, suggesting the concept of desmin-related cardiomyopathies as desmin-based should be reconsidered [79]. In line with this proposal, R120G αB-crystallin localizes to insoluble inclusions when expressed in transiently transfected HeLa cells [80]. These inclusions lack the type III intermediate filament proteins, desmin and vimentin, differing from previously described aggresomes because ubiquitin is absent and formation is microtubule-independent. These HeLa cell inclusions are solubilized by Hsp27 coexpression, indicating R120G αB-crystallin is chaperoned. R120G αB-crystallin is disorganized and aggresome-like inclusions develop in cultured nonmuscle cells deficient in desmin, again demonstrating inclusion body construction independent of intermediate fialments [62]. Interestingly, inclusion body formation is slowed by αB-crystallin, Hsp27 and HspB8, offering a molecular explanation for the delayed adult-onset of desmin-related myopathy through chaperone action.

sHSPs and ischemia/reperfusion injury

Ischemia/reperfusion injury to cells during heart attack and stroke is far reaching and includes protein/enzyme denaturation, perturbation of oxidoreductive status, mitochondrial deterioration, cytoskeleton disruption and membrane lipid peroxidation [81]. sHSP over-expression in transgenic animals and cultured cardiomyocytes, the latter by transfection with adenovirus vectors, shields heart cells against apoptosis and necrosis upon ischemia/reperfusion injury [74,81–84]. Over expressed wild-type and nonphosphorylatable Hsp27 were equally effective in safeguarding contractile activity and cell integrity, as determined by retention of creatine kinase activity in transgenic mice hearts during ischemia/reperfusion [81]. sHSP phosphorylation status may have little influence on the ability of Hsp27 to protect myocardial cells of these transgenic mice during ischemia/reperfusion, although nonphosphorylatable Hsp27 variants produce larger oligomers on average than wild type, a trend accentuated by the stress of ischemia/reperfusion, and there is a potential effect on how well cells cope with oxidative stress.

Gene deletion experiments indicate sHSPs defend cells against ischemia/reperfusion injury. That is, the hearts of double knock-out mice lacking the abundant sHSPs, αB-crystallin and HspB2, develop as expected under nonstress conditions and show normal contractility [85]. However, when exposed to ischemia and reperfusion, hearts from these animals display reduced contractility and less glutathione, accompanied by greater necrosis and apoptosis due to free radical production. The need for either or both αB-crystallin and HspB2 for optimal recovery from heart attack is apparent. Phosphorylated Hsp20, known to associate with and stabilize actin [86], and αB-crystallin [87], arrest β-agonist-induced apoptosis experienced by heart failure patients, probably by inhibiting caspase-3 activation. Five mammalian sHSPs, namely αB-crystallin (HspB5), MKBP (HspB2), Hsp25 (HspB1), Hsp20 (HspB6) and cv Hsp (HspB7) translocate from heart cell cytosol to myofibrils during ischemia, with varying localization to Z-lines, I-bands, and intercalated discs. Binding to microfibrils is tight and sHSPs may save stressed heart cells from harm by stabilizing sarcomeres [36,88,89]. Microtubule preservation by αB-crystallin, but not Hsp27, occurs during ischemia [90], but the role played by microtubule disruption in cell injury is uncertain, possibly representing a reversible situation with minor implications for patient survival [91].

sHSPs and neurological disease

Maintaining the appropriate intracellular complement of functional proteins depends upon proteolytic enzymes and molecular chaperones [92]. If either one or both malfunction, potential exists for tissue-specific build-up of protein aggregates termed amyloid. Such accumulations typify neurodegenerative or ‘conformational’ diseases, of which Parkinson's, Alzheimer's and other tauopathies, Huntington's, amyotrophic lateral sclerosis (ALS), and the prion disorders, are examples [93–102]. Deposits are fibrillar, enriched in β-pleated sheet, and some contain neurofilament proteins as in desmin-related myopathy inclusions and Parkinson's associated Lewy bodies. Protein deposits observed in neurological diseases may be harmful, beneficial or of no consequence.

Alzheimer's is characterized by amyloid-β peptide (Aβ) in extracellular senile plaques and tau in neurofibrillary tangles, aggregates that are major morphological indicators of the disease [103]. Alzheimer's disease is the most common tauopathy, a group of familial neurodegenerative conditions distinguished by intracellular filamentous bodies composed of tau, a low molecular weight microtubule-associated protein [104]. Neurons are the predominant location of tau pathology in Alzheimer's, but glial pathology manifests in corticobasal degeneration and progressive supranuclear palsy. Increased αB-crystallin, and to a lesser extent Hsp27, appear in the latter, conceivably in response to aberrant tau. αB-Crystallin and Hsp27, up-regulated in Alzheimer's brains and localizing to astrocytes and degenerating neurons [104–109], interact with Aβ and occur in amyloid plaque, thereby affecting amyloid production [107,110,111].

Mass spectrometry reveals that three Hsp16 family members, in addition to other molecular chaperones, coimmunoprecipitate with human Aβ in transgenic Caenorhabditis elegans[112]. sHSP expression is induced by the presence of Aβ, which is associated with progressive worm paralysis, and the proteins colocalize intracellularly, suggesting a role for molecular chaperones in Aβ toxicity and metabolism. Human recombinant αB-crystallin also interacts with Aβin vitro, and as shown by thioflavin T fluorescence and far-CD measurements, αB-crystallin promotes β-sheet formation by Aβ[110]. Samples were not examined by electron microscopy during this work, so αB-crystallin effects on Aβ fibril formation and aggregation, although indicated by Aβ secondary structural changes, are unknown. Thioflavine T fluorescence assays and electron microscopy demonstrated that human Hsp27 inhibits Aβ amyloidogenesis in vitro much more effectively than α-crystallin, which is almost without effect [113]. Nonetheless, study of Hsp27 suggests aging-related reduction in chaperone activity contributes to Alzheimer's pathogenesis. αB-Crystallin inhibits Aβ fibril formation in vitro, although β-sheet content and neuronal toxicity of Aβ preparations increase. Possibly, αB-crystallin/Aβ complexes maintain Aβ as a toxic nonfibrillar protein and Aβ toxicity is independent of fibril formation. In this scenario, sHSPs exacerbate rather than diminish, Alzheimer's symptoms [111].

sHSPs have been investigated in neurological diseases other than Alzheimer's, but to lesser extents. The childhood leukodystrophy, Alexander's disease, manifests amplified expression of Hsp27 and αB-crystallin in the brain, and astrocytes display Rosenthal fibers where αB-crystallin and Hsp27 interact with glial fibrillary acidic protein (GFAP) [108,109,114,115]. Augmented αB-crystallin discriminates neurons in Creutzfeldt–Jakob disease and spinal cord astrocytes in amyotrophic lateral sclerosis (ALS) [108]. αB-Crystallin binds mutated Cu/Zn-superoxide dismutase (SOD-1) characteristic of familial ALS [116]. Moreover, a mouse model of familial ALS displays down-regulation of sHSPs in motor neurons and up-regulation in astrocytes. Mouse Hsp25 colocalizes with mutant SOD-1 [117], similar to results obtained with a cultured neuronal cell line [118]. Interaction with mutant, but not wild-type SOD-1 may limit antiapoptotic potential and decrease cell protection by Hsp25. In another example, Hsp27 and αB-crystallin appear in Parkinson's disease with severe dementia [119]. sHSPs and neurological diseases are evidently linked, but consequences are uncertain. Chaperoning can prevent or promote aggregate creation, and either outcome may be favorable or unfavorable, depending on the disease. As a case in point, formation of huntingtin-containing inclusion bodies in Huntington's disease encourages cell survival, whereas monomers and small inclusion bodies of huntingtin, a protein possessing abnormal polyQ repeats, are toxic, an effect potentially mediated by transcription factor destabilization [96,99,120]. Prevention of abnormal protein aggregation obviously does not always benefit cells, an observation with important implications when choosing therapeutic approaches to neurological diseases.

Nerve demyelination presents in multiple sclerosis (MS), a chronic autoimmune neurological condition involving brain and spinal cord inflammation. T cells from MS patients express a dominant response to αB-crystallin, a major autoantigen affiliated with central nervous system myelin, the disease target [121,122]. In contrast to healthy individuals, αB-crystallin resides in oligodendrocytes and astrocytes [122] and αB-crystallin mRNA is the most prevalent transcript found uniquely in MS plaques [123]. Moreover, MS characteristics are influenced by the αB-crystallin genotype with promoter polymorphisms affecting the disease [124]. αB-Crystallin is not thought to cause demyelination directly, but may enhance the inflammatory response and its effects. Antibodies to αB-crystallin and other elevated proteins could serve as confirmation markers for MS diagnosis, and this will assist in disease treatment [125].

sHSP mutations are linked to distal motor neuropathies, genetically heterogeneous diseases of the peripheral nervous system bringing about nerve degeneration and distal limb muscle atrophy [126–128]. HspB8 (Hsp22) mutation K141N exists in two families with distal hereditary motor neuropathy and a second mutation, K141E, is found in two other pedigrees [127]. K141 dwells in the α-crystallin domain and is equivalent to αA-crystallin R116 and αB-crystallin R120, amino acid residues described previously as associated with human disease. The K141N mutant of HspB8 binds more strongly to HspB1 than does its wild-type counterpart, and when expressed in cultured COS cells the K141N variant dramatically increases cytoplasmic and perinuclear aggregate number. Neuronal N2a cell viability is compromised by K141E HspB8 and less so by the K141N mutant. It is not known if neuronal aggregates form in distal motor neuropathies, nor is HspB8 function understood, however, mutations to K141 are linked to motor neuropathies. Mutations S135F, R127W, T151I and P182L in HspB1 (Hsp27) were subsequently discovered in families with distal hereditary motor neuropathy [128]. Individuals with the genetically and clinically heterogeneous syndrome, Charot–Marie–Tooth disease, the most common inherited motor and sensory neuropathy, contain HspB8 K141N, as in distal hereditary motor neuropathy [126], as well as S135F and R136W in HspB1 [128]. All HspB1 mutations, with exception of P182L in the C-terminal extension, are quartered in the α-crystallin domain near residue R140. Neuronal N2a cells transfected with S135F HspB1 are less viable than cells expressing wild-type HspB1, symptomatic of distal motor neuropathies and Charot–Marie–Tooth disease being caused by mutation induced, premature axonal degeneration. Multinucleated cells almost double upon expression of the S135F HspB1 mutant and intermediate filament arrangement is affected adversely in an adrenal carcinoma cell line, implicating cytoskeleton disruption in these diseases.

sHSPs and cancer

Based on the consequences of molecular chaperone induction in diseased (stressed) cells, the relationship between cancer and sHSPs is worthy of examination. One area receiving attention is sHSP value in clinical prognosis of individual cancers and of cancers at different developmental stages. By example, a strong correlation exists between lymph node involvement and high αB-crystallin levels in primary breast carcinoma specimens, but measuring only the sHSP inadequately predicts patient outcome [129]. Elevated Hsp27 expression indicates good prognosis in other studies [130,131], contrasting results where increased sHSP indicates aggressive tumor behavior and poor prognosis [132–139], findings that undoubtedly reflect differences between cancers and experimental methods. Interestingly, HspB9, a testis cell-specific mammalian sHSP under normal circumstances, occurs in tumors of several tissues and may be a cancer/testis antigen (CAT) [140]. CATs include many proteins typically synthesized in primitive germ cells; malignant transformation reactivates CAT genes and the proteins reappear in tumors. CAT effects on disease progression and their worth in prognosis are unknown. Overall, sHSPs tend to lack reliability as prognostic indicators for cancers, but the approach has use especially as sHSPs and other proteins indicating poor prognosis are potential therapeutic targets.

sHSPs modulate metastatic potential and tumor progression. Enhanced Hsp27 expression in human melanoma cell lines decreases invasiveness, reduces matrix metalloproteinases in vitro and eliminates production of αvβ3 integrin, a protein missing in normal melanocytes but often manufactured during the invasive phase [141]. Hsp27 over expression in melanoma cells prevents E-cadherin loss, and synthesis of the adhesion molecule MUC18/MCAM, which correlates with metastatic potential, is disrupted [142]. The cumulative data indicate Hsp27 slows A375 melanoma cell growth in vitro, lowers tumor appearance rate in mice [143] and inhibits tumor progression. In another example, Hsp27 increases MDA-MB-231 breast cancer cell metastasis [135]. Concurrently, MMP-9, a zinc dependent endoprotease capable of degrading several extracellular matrix proteins and enhancing tumor cell invasion, is amplified, while Yes, a Src tyrosine kinase related to cell adhesion and invasion, declines. Reconstitution of Yes in Hsp27 over-expressing cells by transfection reduces MMP-9, signifying mediation of Hsp27 effects by the Yes signaling cascade. Intriguingly, enhancing chondrocyte Hsp25 lowers growth rate, modifies morphology, lessens adhesion and disrupts differentiation, but leaves actin distribution unaffected. These observations have implications for metastatic potential as reduced adhesion leads to cell release from tumors and spreading throughout the organism [144].

sHSP induced drug resistance is of concern for patients undergoing cancer chemotherapy [145,146]. Rat sarcoma cells exhibit less cell death than either rat lymphoma or mouse breast carcinoma cells upon treatment with the anticancer drugs doxorubicin and lovastatin [132]. Among the three cancers, sarcoma cells possess the most Hsp25, the rodent equivalent of human Hsp27, and the protein builds up upon drug treatment, suggestive of a role in cell survival. In another case, a murine melanoma line of low metastatic potential over-expressing Hsp25 displays enhanced susceptibility to interleukin stimulated dDX-5+ natural killer cells, thought to perform malignant disease immune surveillance and control. In contrast, a related murine melanoma cell line with high metastatic potential and enhanced Hsp25 expression is no more susceptible to interleukin stimulated natural killer cells than controls not over expressing the sHSP [147]. The difference is apparently unrelated to Hsp25 surface display because protein prevalence at the cytoplasmic membrane is independent of metastatic potential and over-expression. Such findings demonstrate difficulties in extrapolating the implications of sHSP effects from one cancer to another while hinting at treatments. sHSP associated diseases are summarized in Table 1.

Table 1.  sHSP modifications associated with disease. Many diseases are associated with changes to sHSPs occurring either as a result of mutation or by post-translational changes, and these are outlined below and described in the text of the review. In addition, changes in the amounts of sHSPs, unaccompanied by a structural change in the protein per se, are observed in cancers and neurological diseases such as Alzheimer's, Alexander's, Creutzfeldt-Jakob, amyotrophic lateral sclerosis, Parkinson's and multiple sclerosis. These diseases are described in the review but not listed in the table. ΔC13, ΔC25, mutations resulting in loss of 13 and 25 amino acid residues, respectively, from the C-terminus of αB-crystallin. Hsp22, HspB8; Hsp25/27, HspB1.
DiseasesHSP modification
Post-translation changeMutationReferences
CataractTruncation [33–37]
Deamidation [36,38–42]
Oxidation [40,43–46]
Glycation [46–53]
Phosphorylation [33]
Racemization/isomerization [54,55]
αA-crystallin R116C[60]
αA-crystallin R49C[61]
αB-crystallin R120G[16]
Desmin-related myopathy αB-crystallin R120G[14–16,62,63,74]
αB-crystallin ΔC13[75]
αB-crystallin ΔC25[75]
Desmin-related cardiomyopathy αB-crystallin R120G[74,76]
Distal hereditary motor neuropathies Hsp25 S135F[128]
Hsp25 R127W[128]
Hsp25 T151I[128]
Hsp25 P182L[128]
Hsp22 K141N[127]
Hsp22 K141E[127]
Charot–Marie–Tooth disease Hsp25 S135F[128]
Hsp25 R136W[128]
Hsp22 K141N[126]

Therapeutic implications of sHSPs and other molecular chaperones

Temperature induced synthesis of sHSPs protects against ischemia/reperfusion damage to the heart, brain, and kidney [148]. Hsp27 microinjection enhances neuron survival upon stress exposure and reduces apoptosis, demonstrating the protein's importance in cell maintenance [149]. sHSPs prevent aggregation of oxidized and damaged proteins as organism's age, extending life-span and delaying disease onset [150]. These observations suggest sHSP utility as early diagnostic markers and therapeutic targets. Novel approaches include the use of reagents that modify chaperones structurally and functionally, the modulation of signaling pathways regulating sHSP properties such as phosphorylation, and changing the level of sHSP synthesis [26].

Suppression of sHSPs indicating poor cancer prognosis could be important for treatment. For example, the down regulation of Hsp27 by interferon-γ (IFN-γ) in oral squamous cell carcinoma lines enhances drug effectiveness [134]. Hsp27 is thought to protect against drug induced apoptosis and once either removed or reduced by IFN-γ exposure, cells gain sensitivity to anticancer drugs such as cisplatin. The importance of combination therapy consisting of sHSP reduction and drug exposure is demonstrated, however, INF-γ induced lowering of Hsp27 may be specific to oral squamous cell carcinomas, consequently limiting this potential therapeutic approach. The metabolite, pantethine, increases α-crystallin chaperone activity and aids prevention of rat lens opacification [26,151]. Other therapeutic possibilities include alteration of cellular Ca2+ balance through membrane transport protein effectors and changing sHSP function by nucleotide and anti-inflammatory drug application [26]. SAPK2/p38 kinase stimulation leads to sHSP phosphorylation and oligomer size alteration [152], suggesting that drug-dependent regulation of kinases and phosphatases improves sHSP protection [26]. Hsp20 phosporylation at serine 16 guards against agonist induced cardiac apoptosis, implicating the sHSP as a therapeutic target in treatment of heart failure [86]. The development of pharmaceuticals which modify and/or stimulate sHSPs is feasible and this depends on more extensive characterization of chaperone sites interacting with metabolites, nucleotides and drugs.

The therapeutic application of sHSPs is further suggested by study of other molecular chaperones, with disruption of HSPs that protect deregulated intracellular signaling proteins and transcription factors involved in malignant phenotypes, as examples [153,154]. Perturbation of high-affinity Hsp90 in tumors, but not healthy cells, causes ubiquitination and proteasomal degradation of chaperone binding proteins, enhancing drug antitumor activity. The first Hsp90-reactive drug to reach phase I trials, 17-allylamino-17-demethoxygeldanamycin (17-AAG, NSC 330507), modifies this molecular chaperone while exhibiting limited human toxicity. The hydroxylamine derivative, arimoclomol, delays ALS progression in mice with Cu/Zn superoxide dismutase-1 mutations and induces synthesis of Hsp70 and Hsp90, but not Hsp27 [155]. The hydroxylamine derivatives potentiate HSP expression during stress by prolonging the time heat shock transcription factor-1 (HSF-1) binds gene promoters, presumably increasing HSPs and protecting cells from protein misfolding. The macrocyclic antifungal antibiotic, radicicol, induces HSP expression in neonatal rat cardiomyocytes and shelters cells from the effects of simulated ischemia [156]. Radicicol frees HSF-1 from Hsp90. In contrast to many Hsp90 clients, liberated HSF-1 evades degradation, undergoes activation and enhances HSP gene expression, thereby inducing heat shock response. The HSPs increased upon radicicol exposure of rat neonatal cardiomyocytes are unknown, but protection from simulated ischemia is independent of Hsp90 over-expression [156]. Stimulation of HSP synthesis by drug-induced disruption of Hsp90 may promote sHSP synthesis leading to beneficial therapeutic effects.

sHSP delivery by gene therapy is being tested in animal models and a catheter-based clinical approach for infusion of adenoviral vectors has promise for treatment of congestive heart failure [157]. In a procedural variation, recombinant adeno-associated virus vectors containing an extracellular superoxide dismutase (SOD) are administered by intramyocardial injection, yielding long lasting protection against ischemia/reperfusion injury in rats [158]. Pre-emptive gene therapy strategies, where SOD or other therapeutic proteins are produced in patients at high risk for ischemic/reperfusion injury associated with coronary artery disease and related chronic ailments, hold medical potential.

Extracellular HSPs indicate necrosis, inducing significant immune response upon cell surface receptor recognition and initiating internal signaling cascades. Many peptides generated by degradation of self and nonself bind HSPs noncovalently, indicating cells of origin and cause of destruction, while effectively stimulating the immune system [159–164]. Tumor cell HSPs and client proteins/peptides have been used to synthesize oncophage vaccines, and when injected into patients immune responses against cells containing HSP-associated proteins are promoted, an approach that may facilitate cancer treatment. The delivery of constitutively active HSF-1 enhances tumor cell HSP expression and augments tumor immunoantigenicity, perhaps by limiting phagocytosis of apoptotic cells [161]. If HSF-1 is employed therapeutically only one gene must be introduced to effect expression of several HSP genes, all with the capacity to enhance HSP synthesis and immunogenecity. sHSPs have also been considered for delivery of antigens and the design of vaccines directed against protein targets in HIV infection [163]. The therapeutic implications associated with HSPs, are provocative, and efforts to exploit molecular chaperones, including the sHSPs, in disease amelioration are underway.

Conclusions

sHSPs were described previously as the ‘forgotten chaperones’, but this is no longer true. Two sHSPs have been crystallized, opening the door to more informed interpretation of results obtained by site-directed mutagenesis and other molecular probing. The functions of sHSP domains and individual amino acid residues are becoming clearer, as is the molecular basis of oligomerization. The implications of oligomer assembly and disassembly as chaperoning prerequisites are under study, sHSP substrates have been identified, and the role of ATP-dependent chaperones in substrate release and refolding revealed. sHSPs operate in the front lines of cell defense, protecting proteins during stress and providing opportunities for salvage. As molecular chaperones, sHSPs have the potential to guard cells from disease, but when perturbed or as residents of aberrant cells, they may promote disease. For example, sHSPs defend against ischemia, oxidative damage and apoptosis, but post-translational modifications and gene mutations cause cataract and desmin-related myopathies. Disease involvement suggests therapeutic exploitation of sHSPs, but this remains poorly explored, as is generally true for other HSPs. However, as sHSPs are better understood, opportunities for disease prevention and treatment become more apparent, and this, along with their fundamental importance in stress physiology, means that sHSPs will not be forgotten for some time to come.

Acknowledgements

The work was supported by a Natural Sciences and Engineering Research Council of Canada Discovery Grant, a Nova Scotia Health Research Foundation/Canadian Institutes of Health Research Regional Partnership Plan Grant, and a Heart and Stroke Foundation of Nova Scotia Grant to THM and a NSHRF Student Fellowship to YS.

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