Analysis of the resistance to heat and hydrogen peroxide stresses in COS cells transiently expressing wild type or deletion mutants of the Drosophila 27-kDa heat-shock protein

Hsp27 negatively regulates cell death by interacting with cytochrome c

Mammalian cells respond to stress by accumulating or activating a set of highly conserved proteins known as heat-shock proteins (HSPs). Several of these proteins interfere negatively with apoptosis. We show that the small HSP known as Hsp27 inhibits cytochrome-c-mediated activation of caspases in the cytosol. Hsp27 does not interfere with granzyme-B-induced activation of caspases, nor with apoptosis-inducing factor-mediated, caspase-independent, nuclear changes. Hsp27 binds to cytochrome c released from the mitochondria to the cytosol and prevents cytochrome-c-mediated interaction of Apaf-1 with procaspase-9. Thus, Hsp27 interferes specifically with the mitochondrial pathway of caspase-dependent cell death.

Identification and characterization of a novel protein from Sertoli cells, PASS1, that associates with mammalian small stress protein hsp27

hsp27 is involved in development of tolerance to stress, possibly by its involvement in molecular chaperoning, maintenance of glutathione status, and/or modulation of microfilament structure and function. We hypothesize that hsp27 function depends on specific association with other proteins. To discover proteins that associate with hsp27, we made a differentiated rat Sertoli cell cDNA expression library and screened it using the yeast two-hybrid system. We obtained a cDNA coding for a novel protein of 428 amino acids that we have named PASS1 (protein associated with small stress proteins 1). BLAST searches did not reveal major similarity of PASS1 to any known protein, but the cDNA sequence matched several mouse EST clones and shares 34% homology with a Caenorhabditis elegans genomic sequence. In vitro, bacterially expressed glutathione S-transferase-PASS1 fusion protein bound to hsp27, and hsp27 was co-immunoprecipitated with c-Myc-tagged PASS1 overexpressed in several cell lines. The region of PASS1 responsible for association with hsp27 was identified as existing predominantly between amino acids 108 and 208 of PASS1. Northern hybridization and Western blot analysis demonstrated that PASS1 is expressed in several tissues, with the highest expression occurring in testis, primarily in Sertoli cells. The presence of a 1.4-kilobase PASS1 mRNA in kidney as well as the 1. 8-kilobase mRNA seen in other tissues suggests that alternate splicing may occur in this organ. Ectopic expression of PASS1 in two cultured cell lines was observed to inhibit the ability of hsp27 to protect cells against heat shock, indicating that PASS1 does interact with hsp27 in the live cell.

Review: Age-related cataract: immunity and lens epithelium-derived growth factor (LEDGF)

This short review summarizes our recent work and relevant publications on autoimmunity and cataract. A complete review of this subject is beyond the scope of this paper. Age-related cataract (ARC) is the leading cause of world blindness. In spite of more than fifty years of basic and clinical research, there is no nonsurgical intervention to prevent or treat ARC, but there is a better understanding of the manifold complexities of this age-related condition. ARC is a multifactorial condition in which incidence and progress are modified by factors such as age, sex, radiation [visible, ultraviolet (UV), and X-ray], oxidation, physical trauma, diet, and medications. The lens contains at least three different cell types: central epithelial cells, dividing germinative epithelial cells, and fiber cells. The central epithelial cells covering the anterior axial part of the lens do not divide but survive throughout life. The bulk of the lens comprises anucleate fiber cells, differentiated germinative epithelial cells, which have undergone an apoptosis-like change "diffoptosis" to become elongated, crystallin-rich, organelle-deficient, cells. The epithelial cells and their active transport mechanisms maintain lens homeostasis and clarity. The survival mechanisms of the central lens epithelial cells (LECs) are unknown. In other cells, growth or survival factors, when present, enhance survival and, when absent or deficient, induce programmed cell death "apoptosis". Many developing mammalian cells produce signal proteins, or require signal proteins from other cells, to avoid apoptosis. Although much is known about the role of growth factors in the lens, less is known about how such signals are involved in the survival and death of LECs. We have hypothesized that LECs, like other mammalian cells, use signal proteins to regulate growth, survival, and apoptosis, and we have begun a search for such molecules. Furthermore, we have hypothesized that such factors, if found, may also be involved in the death of LECs, the consequent alteration of lens homeostasis and, eventually, certain types of ARC.

Implication of Bcl-2 family genes in basal and D-amphetamine-induced apoptosis in preneoplastic and neoplastic rat liver lesions

Molecular mechanisms of basal and D-amphetamine (AMPH)-induced apoptosis were studied in rat liver nodules, 12 (N12) and 30 (N30) weeks after initiation, and in hepatocellular carcinoma (HCC) induced by diethylnitrosamine in rats subjected to resistant hepatocyte model. Basal apoptosis in hematoxylin/eosin- and propidium iodide-stained sections was higher in nodules and HCC than in normal livers. It sharply increased in all tissues 4 hours after AMPH treatment (10 mg/kg), and declined to basal levels at 8 to 12 hours in liver and N12, but remained high up to 18 hours in N30 and HCC. c-myc, Tgf-alpha, p53, and Bcl-X(S) messenger RNA (mRNA) levels were higher, and Bcl-2 mRNA was lower in N12 and/or N30 and HCC than in normal liver. Four hours after AMPH injection, increase in c-myc and decreases in Bcl-2 and Bcl-X(L) mRNAs occurred in all tissues, whereas p53, Bax, and Bcl-X(S) mRNAs increased in N30 and HCC. These changes disappeared in liver and N12 at 18 hours, but persisted in N30 and HCC. c-Myc, P53, Bcl-2, and Bax proteins in normal liver and HCC +/- AMPH showed similar patterns. Tgf-beta1, Tgf-beta-RIII, CD95, and CD95L mRNA levels underwent slight or no changes in any tissue +/- AMPH. Basal Hsp27 expression was high in nodules and HCC, and was stimulated by AMPH in liver and N12, but not in N30 and HCC. These data suggest a role of dysregulation of Bcl-2 family genes and, at least in atypical lesions, of p53 overexpression, in basal and AMPH-induced apoptosis in nodules and HCCs. Hsp27 does not appear to sufficiently protect atypical lesions against apoptosis.

Heat shock proteins in human cancer

The heat shock proteins (hsp) are ubiquitous molecules induced in cells exposed to sublethal heat shock, present in all living cells, and highly conserved during evolution. Their function is to protect cells from environmental stress damage by binding to partially denatured proteins, dissociating protein aggregates, to regulate the correct folding, and to cooperate in transporting newly synthesized polypeptides to the target organelles. The molecular chaperones are involved in numerous diseases, including cancer, revealing changes of expression. In this review, we mainly describe the relationship of hsp expression with human cancer, and discuss what is known about their post-translational modifications according to malignancies.

Functional characterization of Xenopus small heat shock protein, Hsp30C: the carboxyl end is required for stability and chaperone activity

Small heat shock proteins protect cells from stress presumably by acting as molecular chaperones. Here we report on the functional characterization of a developmentally regulated, heat-inducible member of the Xenopus small heat shock protein family, Hsp30C. An expression vector containing the open reading frame of the Hsp30C gene was expressed in Escherichia coli. These bacterial cells displayed greater thermoresistance than wild type or plasmid-containing cells. Purified recombinant protein, 30C, was recovered as multimeric complexes which inhibited heat-induced aggregation of either citrate synthase or luciferase as determined by light scattering assays. Additionally, 30C attenuated but did not reverse heat-induced inactivation of enzyme activity. In contrast to an N-terminal deletion mutant, removal of the last 25 amino acids from the C-terminal end of 30C severely impaired its chaperone activity. Furthermore, heat-treated concentrated solutions of the C-terminal mutant formed nonfunctional complexes and precipitated from solution. Immunoblot and gel filtration analysis indicated that 30C binds with and maintains the solubility of luciferase preventing it from forming heat-induced aggregates. Coimmunoprecipitation experiments suggested that the carboxyl region is necessary for 30C to interact with target proteins. These results clearly indicate a molecular chaperone role for Xenopus Hsp30C and provide evidence that its activity requires the carboxyl terminal region.

Role of small heat shock protein HSP25 in radioresistance and glutathione-redox cycle

Expression of heat shock proteins (HSPs) has been shown to protect mammalian cells exposed to a variety of stress stimuli. Among various HSPs, small HSPs from diverse species were shown to protect cells against oxidative stress. Here, we show that the overexpression of the mouse small hsp gene, hsp25, provides protection against ionizing radiation. Our results demonstrate that the radiation survival of the L929 cells stably transfected with hsp25 was enhanced compared with that of the parental or vector transfected control, L25#1 cells. Our results also demonstrate that the radiation-induced apoptosis was reduced in HSP25 overexpressors. A detailed analysis of glutathione composition of those clones that overexpressed HSP25 revealed the increases of the glutathione pool, which primarily resulted from the increase of reduced glutathione. Our data suggest that higher content of GSH in HSP25 overexpressors was because of a faster reduction of oxidized glutathione (GSSG) to GSH rather than an increased de novo synthesis of GSH. The activities of glutathione reductase (GRd) and glutathione peroxidase (GPx) were greater in HSP25 overexpressors but the activity of gamma-glutamylcysteine synthetase was similar between the transfectants and the control cells. Consistent with our view, a steady state ratio of the GSH/GSSG was greater in the transfectants in comparison with the control L25#1 cells. A difference in the relative ratio became more significant after exposure to the ionizing radiation. To our knowledge, this study provides the first experimental evidence in support of the hypothesis that small HSP plays a key role in radioresistance by modulating the metabolism of glutathione. Based on the results obtained from the current investigation, we propose that HSP25 helps facilitate the glutathione-redox cycle and therefore, enhances glutathione utilization and maintains the cellular glutathione pool in favor of the reduced states.

Small heat shock proteins, IbpA and IbpB, are involved in resistances to heat and superoxide stresses in Escherichia coli

To investigate the function of Escherichia coli small heat shock proteins, IbpA and IbpB, we constructed ibpA-, ibpB- and ibpAB-overexpressing strains and also an ibpAB-disrupted strain. The ibpA-, ibpB- and ibpAB-overexpressing strains were found to be resistant not only to heat but also to superoxide stress. However, the ibpAB-disrupted strain was not more sensitive to these stresses than the wild-type strain. The heat sensitivity of a rpoH amber mutant was partially suppressed by the overexpression of plac::ibpAB. These results suggest that IbpA and IbpB may be involved in the resistances to heat and oxidative stress.

Ischemic acute renal failure induces differential expression of small heat shock proteins

AlphaB-crystallin and heat shock protein (hsp) 25 are structurally and functionally related small stress proteins induced by a variety of insults, including heat and ischemia. Cytoprotection by these two hsp is thought to result from molecular chaperoning and/or cytoskeletal stabilization. Because renal ischemia is characterized by disruption of the renal tubular cell actin cytoskeleton, this study was conducted to determine the localization and quantify the expression and phosphorylation of both hsp in renal cortex, isolated glomeruli, outer medulla, and inner medulla of rats after bilateral renal ischemia. Sham-operated kidneys had similarly small amounts of hsp25 and alphaB-crystallin in cortex and glomeruli, with substantially greater amounts of alphaB-crystallin versus hsp25 in outer and inner medulla. Ischemia resulted in significantly increased hsp25 (and hsp70i) but variable alphaB-crystallin levels in cortex and outer medulla, and progressively decreased glomerular hsp25 phosphorylation. In sham-operated kidneys, hsp25 localized to glomeruli, vessels, and collecting ducts, with alphaB-crystallin primarily in medullary thin limbs and collecting ducts. After ischemia, hsp25 accumulated in proximal tubules in cortex and outer medulla, while alphaB-crystallin labeling became nonhomogeneous in outer medulla, and increased in Bowman's capsule. It is concluded that: (1) There is striking differential expression of hsp25 and alphaB-crystallin in various renal compartments; and (2) Renal ischemia results in differential accumulation of hsp25 and alphaB-crystallin, with hsp25 part of a generalized stress response in renal proximal tubular cells, which may play a role in recovery from ischemia-induced actin filament disruption.

Subunit exchange of small heat shock proteins. Analysis of oligomer formation of alphaA-crystallin and Hsp27 by fluorescence resonance energy transfer and site-directed truncations

alphaA-Crystallin, a member of the small heat shock protein (sHsp) family, is a large multimeric protein composed of 30-40 identical subunits. Its quaternary structure is highly dynamic, with subunits capable of freely and rapidly exchanging between oligomers. We report here the development of a fluorescence resonance energy transfer method for measuring structural compatibility between alphaA-crystallin and other proteins. We found that Hsp27 and alphaB-crystallin readily exchanged with fluorescence-labeled alphaA-crystallin, but not with other proteins structurally unrelated to sHsps. Truncation of 19 residues from the N terminus or 10 residues from the C terminus of alphaA-crystallin did not significantly change its subunit organization or exchange rate constant. In contrast, removal of the first 56 or more residues converts alphaA-crystallin into a predominantly small multimeric form consisting of three or four subunits, with a concomitant loss of exchange activity. These findings suggest residues 20-56 are essential for the formation of large oligomers and the exchange of subunits. Similar results were obtained with truncated Hsp27 lacking the first 87 residues. We further showed that the exchange rate is independent of alphaA-crystallin concentration, suggesting subunit dissociation may be the rate-limiting step in the exchange reaction. Our findings reveal a quarternary structure of alphaA-crystallin, consisting of small multimers of alphaA-crystallin subunits in a dynamic equilibrium with the oligomeric complex.

HSP27 inhibits cytochrome c-dependent activation of procaspase-9

We have previously shown that the small heat shock protein HSP27 inhibited apoptotic pathways triggered by a variety of stimuli in mammalian cells. The present study demonstrates that HSP27 overexpression decreases U937 human leukemic cell sensitivity to etoposide-induced cytotoxicity by preventing apoptosis. As observed for Bcl-2, HSP27 overexpression delays poly(ADP-ribose)polymerase cleavage and procaspase-3 activation. In contrast with Bcl-2, HSP27 overexpression does not prevent etoposide-induced cytochrome c release from the mitochondria. In a cell-free system, addition of cytochrome c and dATP to cytosolic extracts from untreated cells induces the proteolytic activation of procaspase-3 in both control and bcl-2-transfected U937 cells but fails to activate procaspase-3 in HSP27-overexpressing cells. Immunodepletion of HSP27 from cytosolic extracts increases cytochrome c/dATP-mediated activation of procaspase-3. Overexpression of HSP27 also prevents procaspase-9 activation. In the cell-free system, immunodepletion of HSP27 increases LEDH-AFC peptide cleavage activity triggered by cytochrome c/dATP treatment. We conclude that HSP27 inhibits etoposide-induced apoptosis by preventing cytochrome c and dATP-triggered activity of caspase-9, downstream of cytochrome c release.

Ras-mediated signaling pathway regulates the expression of a low-molecular-weight heat-shock protein in fission yeast

In fission yeast, Schizosaccharomyces pombe, deficiency of ras1 gene causes an abnormal cell shape and abolishes mating ability. However, target genes of this signaling pathway are largely unknown because of the lack of an appropriate analysis system. To overcome this problem, we have started a novel project to categorize entire genes based on their expression levels under different growth conditions. Using this strategy, we screened genes whose expression levels were affected in the presence or absence of the ras1 gene product. For this purpose, we utilized high-density arrays of clones covering the entire genome of the fission yeast, and probed with labelled cDNA derived from various strains and growth conditions. Here, we demonstrate the detection of a low-molecular-weight heat-shock protein gene, hsp16, whose expression is very likely to be regulated by a ras-mediated signaling pathway, but not by the heat-shock response.

On the path to the heat shock response: destabilization and formation of partially folded protein intermediates, a consequence of protein thiol modification

This review discusses the initial events that occur during oxidative stress that induce the synthesis of heat shock proteins. The focus is on non-native oxidation or modification of protein thiols and the destablization that can result. Proteins that contain non-native modified thiols can become destablized such that they unfold into molten globule-like intermediates at or below 37 degrees C, relieving Hsf-1 negative regulation, and inducing Hsp transcription.

Mammalian small stress proteins protect against oxidative stress through their ability to increase glucose-6-phosphate dehydrogenase activity and by maintaining optimal cellular detoxifying machinery

The protective activity of small stress proteins (sHsp) against H2O2-mediated cell death in the highly sensitive murine L929 fibroblast has been analyzed. We report here that the human Hsp27- and murine Hsp25-mediated rise in glutathione (GSH) levels as well as the maintenance of this redox modulator in its reduced form was directly responsible for the protection observed at the level of cell morphology and mitochondrial membrane potential. sHsp expression also buffered the increase in protein oxidation following H2O2 treatment and protected several key enzymes against inactivation. In this case, however, the protection necessitated both an increase in GSH and the presence of sHsp per se since the pattern of protection against protein oxidation mediated by a simple GSH increase was different from that induced by sHsp expression. Among the enzymes analyzed, we noticed that sHsp significantly increased glucose-6-phosphate dehydrogenase (G6PD) activity and to a lesser extent glutathione reductase and glutathione transferase activities. Moreover, an increased GSH level was observed in G6PD-overexpressing L929 cell clones. Taken together our results suggest that sHsp protect against oxidative stress through a G6PD-dependent ability to increase and uphold GSH in its reduced form and by using this redox modulator as an essential parameter of their in vivo chaperone activity against oxidized proteins.

Sulfomucins in the human body

Mucins are widely distributed in mucous secretion fluids or are associated with plasma membranes. Up to now 9 genes of epithelial mucins have been identified, distributed over five chromosomes. Superposed on the genetic diversity, each type of mucin displays heterogeneity in oligosaccharide composition, including the terminal sugar residues. On top of that there is variation between individuals brought about by blood group antigens. Heterogeneity is further incited by the degree of sulfation. This tremendous structural heterogeneity endows mucin molecules with properties suggestive for a multifunctional role. The major biological function assigned to mucins is still the protection of tissues covered by the mucous gel. Current knowledge on the specific biological functions of the sulfate residues is fragmentary and periphrastic. Glycosylation including sulfation appears to be subject to modification under pathological conditions. There is evidence that sulfation rate-limits bacterial degradation of mucins. Moreover, accumulating data focus towards their involvement in recognition phenomena. Sulfate residues on blood group related structures provoke specific epitopes for selective interaction with microorganisms e.g. Helicobacter pylori. A distinct class of mucins acts as ligands for selectins, crucial in cellular recognition processes like cellular homing of lymphocytes. Whereas in earlier days mucins were only seen as water-binding molecules, protecting the underlying mucosa against harmful agents, the current picture of these molecules is characterized by the selective interaction with their environment, including epithelial-, and endothelial cells and microorganisms, thereby regulating a great number of biological processes. However, the specific role of sulfate remains to be further elucidated.

hsp27 as a switch between differentiation and apoptosis in murine embryonic stem cells

Small stress proteins are developmentally regulated and linked to cell growth and differentiation. The early phase of murine embryonic stem (ES) cell differentiation, characterized by a gradual growth arrest, is accompanied with hsp27 transient accumulation. This differentiation process also correlated with changes in hsp27 phosphorylation and oligomerization. The role of hsp27 was investigated in ES clones stably transfected with murine or human hsp27 genes, placed in sense or antisense orientation. Several clones were obtained that either underexpressed endogenous murine hsp27 or overexpressed murine or human hsp27. Maintained undifferentiated, these clones showed similar growth rates. We report here that hsp27 constitutive overexpression enhanced the differentiation-mediated decreased rate of ES cell proliferation but did not alter morphological changes. In contrast, hsp27 underexpression, which attenuated cell growth arrest, induced differentiation abortion because of an overall cell death by apoptosis. Recently, we showed that hsp27 interfered with cell death probably because of its ability to modulate intracellular glutathione. hsp27 accumulation during ES cell differentiation was also correlated with an increase in glutathione, which was attenuated by hsp27 down-expression. Hence, hsp27 transient expression seems essential for preventing differentiating ES cells from undergoing apoptosis, a switch that may be redox regulated.

Large unphosphorylated aggregates as the active form of hsp27 which controls intracellular reactive oxygen species and glutathione levels and generates a protection against TNFalpha in NIH-3T3-ras cells

The mammalian small stress protein hsp27 is an oligomeric phosphoprotein which interferes with the cell death induced by several stimuli. In that sense, we and others have recently shown that human hsp27 expression induced cellular protection against tumor necrosis factor (TNFalpha), a protection which depends on the ability of hsp27 to decrease the level of reactive oxygen species and increase that of glutathione. Here, we have analyzed unphosphorylatable mutants of human hsp27 in which serines 15, 78, and 82 were replaced by alanines, glycines, or aspartic acids. Depending on the amino acid which was used to substitute the serine sites, a different pattern of hsp27 structural organization was observed. Alanine substitution generated large hsp27 aggregates while glycine and aspartic acid did the reverse. Hence, these phosphorylatable serine residues can be considered as key elements affecting hsp27 structural organization. Only the large aggregates of hsp27 were able to modulate reactive oxygen species and glutathione and generated cellular protection against TNFalpha. Moreover, using drugs that modulate the intracellular level of glutathione, we show that an increase in glutathione by itself was sufficient to generate large hsp27 structures while the reverse was observed in the case of glutathione deprivation.

Subunit exchange of alphaA-crystallin

alpha-Crystallin, the major protein in the mammalian lens, is a molecular chaperone that can bind denaturing proteins and prevent their aggregation. Like other structurally related small heat shock proteins, each alpha-crystallin molecule is composed of an average of 40 subunits that can undergo extensive reorganization. In this study we used fluorescence resonance energy transfer to monitor the rapid exchange of recombinant alpha-crystallin subunits. We labeled alphaA-crystallin with stilbene iodoacetamide (4-acetamido-4'-((iodoacetyl)amino)stilbene-2,2'-disulfonic acid), which serves as an energy donor and with lucifer yellow iodoacetamide, which serves as an energy acceptor. Upon mixing the two populations of labeled alphaA-crystallin, we observed a reversible, time-dependent decrease in stilbene iodoacetamide emission intensity and a concomitant increase in lucifer yellow iodoacetamide fluorescence. This result is indicative of an exchange reaction that brings the fluorescent alphaA-crystallin subunits close to each other. We further showed that the exchange reaction is strongly dependent on temperature, with a rate constant of 0.075 min-1 at 37 degrees C and an activation energy of 60 kcal/mol. The subunit exchange is independent of pH and calcium concentration but decreases at low and high ionic strength, suggesting the involvement of both ionic and hydrophobic interactions. It is also markedly reduced by the binding of large denatured proteins. The degree of inhibition is directly proportional to the molecular mass and the amount of bound polypeptide, suggesting an interaction of several alphaA-crystallin subunits with multiple binding sites of the denaturing protein. Our findings reveal a dynamic organization of alphaA-crystallin subunits, which may be a key factor in preventing protein aggregation during denaturation.

UVB irradiation induces changes in cellular localization and phosphorylation of mouse HSP27

We investigated the induction, cellular localization and phosphorylation of a low-molecular weight stress protein (heat shock protein 27, HSP27) by UVB (290-320 nm, max. 312 nm) irradiation stress using immunoblot and indirect immunofluorescence analysis in in vivo and in vitro experiments. The HSP27 was constitutively expressed and distributed in the cytoplasmic fraction of Pam 212 cells (mouse keratinocyte line) or dorsal skin. The increase in the cytoplasm HSP27 level induced by UVB irradiation was less than two-fold that in nonirradiated controls. On the other hand, the translocation of HSP27 from cytoplasm to the nucleus or perinuclear area was time- and dose-dependently induced by UVB irradiation. After UVB irradiation, three isoforms having different isoelectric points were detected in nucleic HSP27 by two-dimensional immunoblotting. The most basic isoform was the unphosphorylated type and the two acidic isoforms were phosphorylated, suggesting that HSP27 is phosphorylated in response to UVB irradiation and accumulates in or around the nucleus as a phosphorylated isoform. These results suggest that the translocation and phosphorylation of HSP27 are induced in response to UVB-irradiation stress.

Overexpression of the small heat shock protein, hsp27, confers resistance to hyperthermia, but not to oxidative stress and UV-induced cell death, in a stably transfected squamous cell carcinoma cell line

The 27 kD heat shock protein (hsp27) is expressed in human keratinocytes in association with differentiation in vitro and in situ. This study was conducted to investigate whether the expression of hsp27 in keratinocytes is associated with increased resistance to the deleterious effects of heat and UV radiation. A transfection vector carrying the human gene for hsp27, under the control of hsp27 as well as the SV40 promoter (pSG2711, M. Jäättelä et al., EMBO J. 11 (1992) 3507-3512), was introduced together with a neomycin-resistance gene into the squamous cell carcinoma cell line A431. Cells were exposed to either UVA, UVB, head (45 degrees C, 4 h) or hydrogen peroxide (0.025-0.5 mM) and the percentage of surviving cells was determined. Overexpression of hsp27 induced increased resistance to hyperthermia, but not to hydrogen peroxide-mediated oxidative injury. When cells were exposed to increasing amounts of UVA (5-80 J cm-2) and UVB (4-64 mJ cm-2), the percentage of surviving cells was identical for clones overexpressing hsp27 and control clones. From these data, we conclude that hsp27 is a mediator of thermotolerance, but does not protect keratinocytes from UV-induced cell death.

Inhibition of colony formation of NIH 3T3 cells by the expression of the small molecular weight heat shock protein HSP27: involvement of its phosphorylation and aggregation at the C-terminal region

The ectopic expression of the small molecular weight heat shock protein HSP27 reportedly confers resistance to heat and other types of stress, but our recent findings indicated that it rendered human immortalized fibroblast cells (KMST-6) more sensitive to oxidative stress and caused irreversible growth arrest (Arata et al., 1995, J. Cell. Physiol., 163:458-465). To clarify the relationship between HSP27 and growth regulation, we investigated the effect of overexpression of HSP27 and its mutants on the growth potential of several cell lines. Mammalian expression vectors of the wild-type, hypophosphorylatable, or C-terminal deletion mutants of human HSP27 were constructed from the pRc/CMV plasmid that contained the neomycin-resistant gene. The plasmid was introduced into mouse fibroblasts (NIH 3T3), normal human fibroblasts (TIG-3), Chinese hamster ovary (CHO-K1), or mammary tumor cells (MCF-7), which were then selected in medium containing G418. The number of drug-resistant colonies was significantly decreased by transfection with the expression vector for wild-type HSP27 compared with vector alone, whereas the overexpression of HSP27 in CHO-K1 cells had essentially no effect. The expression vectors of an hypophosphorylatable mutant (pKSm, human HSP27 gene in which codons for Ser-15, -78, and -82 were converted to code for Gly by site-directed mutagenesis) as well as C-terminal deletion mutants in which 12-36 amino acid residues from the C-terminus were deleted had no significant effect on the colony-forming efficiency of NIH 3T3 cells. Cells isolated from G418-resistant colonies formed by transfection of NIH 3T3 cells with the HSP27 expression vector expressed no detectable levels of wild-type HSP27 and did not form stable clonal transformants expressing high levels of HSP27 from NIH 3T3 cells. In contrast, several clones expressing high levels of HSP27 were obtained from CHO-K1 cells transfected with the HSP27 expression vector. In KMST-6 clones expressing high levels of HSP27, the wild-type HSP27 formed aggregates with a mean molecular mass of about 200 kDa as determined by gel filtration, and the size of the oligomers changed with oxidative stress. On the other hand, the size of aggregates of HSP27 encoded by pKSm or C-terminal deletion mutants did not change. These observations indicated that the forced expression of wild-type HSP27 participates in inhibiting the growth of some cell types and that the inhibition may be associated with its phosphorylation and aggregation.

Stage-specific localization of the small heat shock protein Hsp27 during oogenesis in Drosophila melanogaster

The developmental and heat shock-induced expression of the small heat shock protein Hsp27 was investigated by confocal microscopy of whole-mount immunostained preparations of ovarioles during oogenesis in Drosophila melanogaster. In unstressed flies, Hsp27 was mainly associated with germline nurse cells throughout egg development. A small group of somatic follicle cells also expressed Hsp27 specifically at stages 8 to 10 of oogenesis. Interestingly, this Hsp showed a different intracellular localization depending on the stages of egg chamber development. Thus Hsp27 was localized in the nucleus of nurse cells during the first stages of oogenesis (from germarium to stage 6) whereas it showed a perinuclear and cytoplasmic localization from stage 8. After a heat shock, Hsp27 accumulated in somatic follicle cells surrounding the egg chamber whereas the expression of this small Hsp did not seem to be enhanced in nurse cells. The stage-dependent pattern of intracellular localization of Hsp27 observed in nurse cells of unstressed flies was also observed following heat shock. At late stages of oogenesis, Hsp27 also showed a perinuclear distribution in follicle and nurse cells after heat stress. These observations suggest that different factors may modulate the expression and intracellular distribution of Hsp27. This modulation may be associated with the specific activities occurring in each particular cell type throughout oogenesis during both normal development and under heat shock conditions.

Stationary phase-associated protein expression in Mycobacterium tuberculosis: function of the mycobacterial alpha-crystallin homolog

The majority of active tuberculosis cases arise as a result of reactivation of latent organisms which are quiescent within the host. The ability of mycobacteria to survive extended periods without active replication is a complex process whose details await elucidation. We used two-dimensional gel electrophoresis to examine both steady-state protein composition and time-dependent protein synthetic profiles in aging cultures of virulent Mycobacterium tuberculosis. At least seven proteins were maximally synthesized 1 to 2 weeks following the end of log-phase growth. One of these proteins accumulated to become a predominant stationary-phase protein. N-terminal amino acid sequencing and immunoreactivity identified this protein as the 16-kDa alpha-crystallin-like small heat shock protein. The gene for this protein was shown to be limited to the slowly growing M. tuberculosis complex of organisms as assessed by Southern blotting. Overexpression of this protein in wild-type M. tuberculosis resulted in a slower decline in viability following the end of log-phase growth. Accumulation of this protein was observed in log-phase cultures following a shift to oxygen-limiting conditions but not by other external stimuli. The protein was purified to homogeneity from overexpressing M. smegmatis in two steps and shown to have a significant ability to suppress the thermal denaturation of alcohol dehydrogenase. Collectively, these results suggest that the mycobacterial alpha-crystallin protein may play a role in enhancing long-term protein stability and therefore long-term survival of M. tuberculosis.

Small stress proteins as novel regulators of apoptosis. Heat shock protein 27 blocks Fas/APO-1- and staurosporine-induced cell death

Small stress protein expression enhances the survival of mammalian cells exposed to numerous injuries that induce necrotic cell death. The cell surface receptor Fas/APO-1 and its ligand have been recently identified as important mediators of apoptosis. Here, we show that constitutive expression of human heat shock protein (hsp)27 in murine L929 cells blocks Fas/APO-1-mediated cell death. Expression of human hsp27 prevented anti-APO-1-induced DNA fragmentation and morphological changes. These results strongly suggest that human hsp27 acts as a cellular inhibitor of Fas/APO-1-induced apoptosis. We also report that the expression of small stress proteins from different species, such as human hsp27, Drosophila Dhsp27, or human alphaB-crystallin, confers resistance to apoptotic cell death induced by staurosporine, a protein kinase C inhibitor. Hence, small stress proteins are novel regulators that are able to block apoptosis induced by different pathways.

Altered expression of glomerular heat shock protein 27 in experimental nephrotic syndrome

Although nephrotic syndrome is a very common kidney disease, little is known about the molecular changes occurring within glomerular capillary loops during development of disease. The characteristic histologic change is retraction (effacement) of the distal "foot" processes of glomerular epithelial cells (GEC) which surround the capillary loops. The GEC foot processes are an essential part of the kidney's filtration barrier, and their structure is regulated primarily by actin microfilaments, cytoskeletal proteins present in high concentrations in foot processes. Actin polymerization has been reported to be regulated via phosphorylation of the low molecular weight heat shock protein, hsp27. We localized hsp27 within normal rat GECs using immunofluorescence and immunoelectron microscopy. Induction of nephrotic syndrome and GEC foot process effacement using the puromycin aminonucleoside rat model resulted in significant increases in: (a) renal cortical hsp27 mRNA expression (826 +/- 233%, x +/- SEM, P < 0.01 vs. control); (b) glomerular hsp27 protein expression (87 +/- 2%, P < 0.001 vs. control); and (c) glomerular hsp27 phosphorylation (101 +/- 32%, P < 0.05 vs. control). These findings support the hypothesis that hsp27, by regulating GEC foot process actin polymerization, may be important in maintaining normal foot process structure, and regulating pathophysiologic GEC cytoskeletal changes during development of nephrotic syndrome.

The anti-cancer drug cisplatin induces H25 in Ehrlich ascites tumor cells by a mechanism different from transcriptional stimulation influencing predominantly H25 translation

Treatment of Ehrlich ascites tumor (EAT) cells with the anti-cancer drug cisplatin induces an increase of the intracellular level of the small heat shock protein Hsp25 without stimulating the general stress response. The mechanism of this induction process was investigated at the levels of gene transcription, protein synthesis and stability. We show that an increased synthesis of Hsp25 is predominantly responsible for the increased intracellular level of this protein. In addition, there is a slightly increased metabolic stability of Hsp25 in cisplatin-treated EAT cells. In contrast to the mechanism of Hsp25 induction by heat shock and other chemical stresses, stimulated synthesis of Hsp25 after treatment with cisplatin is not the result of increased transcription of the hsp25 gene. Cisplatin treatment does not significantly influence the oligomerization of heat shock transcription factors 1 and 2, hsp25 promoter activity or hsp25 mRNA stability, as judged by cross-linking experiments, reporter gene assay and Northern blot analysis. Hence, cisplatin specifically induces Hsp25 synthesis at the level of mRNA translation without any changes in hsp25 gene transcription.

Heat-induced chaperone activity of HSP90

The 90-kDa stress protein, HSP90, is a major cytosolic protein ubiquitously distributed in all species. Using two substrate proteins, dihydrofolate reductase (DHFR) and firefly luciferase, we demonstrate here that HSP90 newly acquires a chaperone activity when incubated at temperatures higher than 46 degrees C, which is coupled with self-oligomerization of HSP90. While chemically denatured DHFR refolds spontaneously upon dilution from denaturant, oligomerized HSP90 bound DHFR during the process of refolding and prevented it from renaturation. DHFR was released from the complex with HSP90 by incubating with GroEL/ES complexes in an ATP-dependent manner and refolded into the native form. alpha-Casein inhibited the binding of DHFR to HSP90 and also chased DHFR from the complex with HSP90. These results suggest that HSP90 binds substrates to maintain them in a folding-competent structure. Furthermore, we found that HSP90 prevents luciferase from irreversible thermal denaturation and enables it to refold when postincubated with reticulocyte lysates. This heat-induced chaperone activity of HSP90 associated with its oligomerization may have a pivotal role in protection of cells from thermal damages.

Intracellular reactive oxygen species as apparent modulators of heat-shock protein 27 (hsp27) structural organization and phosphorylation in basal and tumour necrosis factor alpha-treated T47D human carcinoma cells

The small stress protein heat-shock protein 27 (hsp27) is an oligomeric phosphoprotein, constitutively expressed in most human cells, which enhances cellular resistance to tumour necrosis factor alpha (TNF alpha). This phenomenon correlates with dramatic changes in hsp27 cellular location, structural organization and phosphorylation. To gain a better understanding of the molecular mechanisms regulating these properties of hsp27, we investigated whether they were a consequence of the intracellular production of reactive oxygen species (ROS) generated by TNF alpha. Here, we report that, in T47D carcinoma cell lines, the rapid burst of intracellular ROS production and changes in hsp27 locale, structural organization and phosphoisoform composition induced by TNF alpha were abolished by the overexpression of the antioxidant enzyme seleno-glutathione peroxidase (GSHPx). These effects were greatly diminished when GSHPx-expressing cells were grown in the absence of selenium, a cofactor that is essential for seleno-GSHPx activity, indicating that they are directly linked to the increased GSHPx activity. Moreover, in growing T47D cells, GSHPx expression induced intracellular redistribution of hsp27 and decreased the phosphorylation of this protein without altering its pattern of oligomerization. In contrast, the heat-mediated phosphorylation of hsp27 was not altered by decreased intracellular ROS levels. Hence, in growing and TNF-treated cells, several hsp27 properties appear to be modulated by fluctuations in intracellular ROS levels.

Resistance of Ehrlich tumor cells to apoptosis can be due to accumulation of heat shock proteins

Previously we have found that stationary Ehrlich ascites carcinoma (EAC) cells in vivo accumulated heat shock proteins (HSPs) and became resistant to necrotic death induced by prolonged energy deprivation of hyperthermia. Here we report that apoptotic death induced by nutrient starvation, transient ATP depletion, heat shock and a microtubule-disrupting drug, vinblastine, was also suppressed in stationary EAC cells comparing with exponential cells. When exponential (sensitive) cells were subjected to short-term heating with recovery to accumulate inducible form of HSP70, they also became resistant to all of the employed apoptosis-inducing exposures, and an inhibitor of cytosolic protein synthesis, cycloheximide, prevented acquisition of the resistance. It is suggested that in vivo accumulation of HSPs in stationary tumor cells can be endogenous protective device against apoptotic death induced by starvation or some anticancer treatments.

Thermal sensitivity in NIH 3T3 fibroblasts transformed by the v-fos oncogene. Correlation with reduced accumulation of 68-kDa and 25-kDa stress proteins after heat shock

The effect of v-fos transformation on the cellular response to heat shock has been investigated. NIH 3T3 fibroblasts were transfected with the FBR p75gag-fos gene fusion under the control of the long terminal repeat (LTR) promoter of Finkel-Biskin-Reilly (FBR) murine sarcoma virus and with the gene encoding hygromycin resistance. Several hygromycin-resistant clone isolates, that expressed various levels of p75gag-fos oncoprotein, were analyzed as they displayed properties of transformed cells, such as altered morphology, shorter doubling time, serum-independent growth and foci formation in soft agar. The thermal response of these clones was compared to that of the control cells expressing the hygromycin-resistance gene only. Here, we report that the v-fos-transformed clones displayed an enhanced thermosensitivity which resulted in a reduced tolerance to thermal stress. Heat-treated v-fos-transformed cells displayed a decreased expression and accumulation of the major stress proteins Hsp68 (68-kDa heat-shock protein) and Hsp25 which probably resulted of a reduced accumulation of the corresponding mRNAs. This effect was particularly intense at the level of Hsp25. These alterations in cell survival and stress-protein expression appeared correlated to the level of p75gag-fos. At least for Hsp68, the transcription of this gene was not found altered by v-fos expression suggesting that this oncogene increases the turn-over of Hsp68 mRNA. After the heat-shock treatment, v-fos transformation also reduced the time period during which the constitutively expressed stress protein Hsc70 redistributes inside the nucleus. Since Hsp68 and Hsp25 are molecular chaperones that in vivo protect cells against the deleterious effects of heat shock, it is conceivable that their reduced accumulation and altered cellular distribution following heat shock may contribute, at least in part, to the thermosensitivity of v-fos-transformed NIH 3T3 fibroblasts.

1H NMR spectroscopy reveals that mouse Hsp25 has a flexible C-terminal extension of 18 amino acids

The small heat-shock proteins (Hsps) exist as large aggregates and function by interacting and stabilising non-native proteins in a chaperone-like manner. Two-dimensional 1H NMR spectroscopy of mouse Hsp25 reveals that the last 18 amino acids have great flexibility with motion that is essentially independent of the domain core of the protein. The lens protein, alpha-crystallin, is homologous to Hsp25 and its two subunits also have flexible C-terminal extensions. The flexible region in Hsp25 encompasses exactly that expected from sequence comparison with alpha-crystallin implying that both proteins have similar structures and that the C-terminal extensions could be of functional importance for both proteins.

Transient acquired thermotolerance in Drosophila, correlated with rapid degradation of Hsp70 during recovery

Acquired thermotolerance, measured either as increased cell viability following a lethal heat shock or by translational thermotolerance, appears rapidly following a 'priming' heat treatment, but also decays rapidly. 4 hours after priming heating thermotolerance is reduced by > 50% and by 9 hours it is virtually undetectable. Heat-shock protein 70 (Hsp70) turns over with a half-life of approximately 2 hours, and the decline in its intracellular abundance parallels the loss of acquired thermotolerance. Continuous heat shock extends the half-life of Hsp70 to approximately 7 hours. When Hsp70 is expressed at normal temperature using a metallothionein promoter, only partial acquired translational thermotolerance results. The results suggest that acquired thermotolerance is tightly regulated in Drosophila and partly, but not wholly, determined by post-translational regulation of Hsp70 levels.

Effects of the overexpression of the small heat shock protein, HSP27, on the sensitivity of human fibroblast cells exposed to oxidative stress

The role of the human small heat shock protein (HSP27) in oxidative stress was examined using stable transformants of an immortalized human fibroblast cell line (KMST-6) isolated by transfection of HSP27 expression vectors. Several stable transformants that expressed high or low levels of HSP27 protein were obtained. Clones expressing high levels of HSP27 were more sensitive to growth inhibition by a low dose of hydrogen peroxide (0.1 mM) than those expressing low levels. Clones expressing high levels of HSP27 did not acquire obvious resistance to hyperthermy and cytotoxic agents, except for one (#13), in which resistance to cytotoxic agents was increased. The level of phosphorylated HSP27 in clones expressing high levels of this protein increased at 30 min and was sustained even 4 hours after exposing the cells to 0.1 mM of hydrogen peroxide. On the other hand, the levels in clones expressing low levels of HSP27 were reduced within 4 hours after exposure to hydrogen peroxide. Furthermore, overexpression of nonphosphorylatable mutant HSP27 did not affect sensitivity to oxidative stress. These results suggested that constitutively high expression of HSP27 in KMST-6 cells make them susceptible to oxidative stress resulting in growth arrest, and this mechanism could involve the phosphorylation of HSP27.

Tumor necrosis factor-alpha induces changes in the phosphorylation, cellular localization, and oligomerization of human hsp27, a stress protein that confers cellular resistance to this cytokine

The stress protein hsp27 is constitutively expressed in several human cells and shows a rapid phosphorylation following treatment with tumor necrosis factor-alpha (TNF-alpha). hsp27 usually displays native molecular mass ranging from 100 to 700 kDa. Here, we have analyzed the TNF-alpha-mediated changes in the phosphorylation, cellular localization, and structural organization of hsp27 in HeLa cells. We report that the TNF-alpha-mediated hsp27 phosphorylation is a long-lasting phenomenon that correlates with the cytostatic effect of this cytokine. Following TNF-alpha treatment, the rapid phosphorylation of hsp27 occurred concomitantly with complex changes in the intracellular distribution and structural organization of this protein. This resulted in the quantitative redistribution of hsp27 toward the soluble phase of the cytoplasm. In addition, during the first 2 h of TNF-alpha treatment, a transient increase in the native molecular mass of most hsp27 molecules (< or = 700 kDa) occurred. Then, by 4 h of TNF-alpha treatment, the native size of this stress protein drastically regressed (< 200 kDa). During this phenomenon, the phosphorylated isoforms of hsp27 remained concentrated in the small or medium-sized oligomers (< 300 kDa) of this protein. We also analyzed the properties of human hsp27 in transfected murine L929 cell lines that constitutively express this protein. In these cells, TNF-alpha induced modifications in the phosphorylation, intracellular distribution, and oligomerization of human hsp27 similar to those observed in HeLa cells. Moreover, the expression of hsp27 in L929 cells was found to correlate with a reduced cytotoxicity of this cytokine. Hence, the complex changes in the phosphorylation, intracellular locale and structural organization of human hsp27 may be related to the protective activity of this protein against the deleterious effects induced by TNF-alpha.

Disruption of the gene for hsp30, an alpha-crystallin-related heat shock protein of Neurospora crassa, causes defects in thermotolerance

The alpha-crystallin-related heat shock proteins are produced by all eukaryotes, but the role of these proteins in thermoprotection remains unclear. To investigate the function of one of these proteins, we disrupted expression of the single-copy hsp30 gene of Neurospora crassa, using repeat-induced point mutagenesis, and we generated and characterized mutant strains that were deficient in hsp30 synthesis. These strains could grow at high temperature and they acquired thermotolerance from a heat shock. However, the hsp30-defective strains proved to be extremely sensitive to the combined stresses of high temperature and carbohydrate limitation, enforced by the addition of a nonmetabolizable glucose analogue. Under these conditions, their survival was reduced by 90% compared with wild-type cells. This sensitive phenotype was reversed by reintroduction of a functional hsp30 gene into the mutant strains. The mutant cells contained mitochondria from which a 22-kDa protein was readily extracted with detergents, in contrast to its retention by the mitochondria of wild-type cells. Antibodies against hsp30 coimmunoprecipitated a protein also of approximately 22 kDa from wild-type cells. Results of this study suggest that hsp30 may be important for efficient carbohydrate utilization during high temperature stress and that it may interact with other mitochondrial membrane proteins and function as a protein chaperone.

The expanding small heat-shock protein family, and structure predictions of the conserved "alpha-crystallin domain"

The ever-increasing number of proteins identified as belonging to the family of small heat-shock proteins (shsps) and alpha-crystallins enables us to reassess the phylogeny of this ubiquitous protein family. While the prokaryotic and fungal representatives are not properly resolved, most of the plant and animal shsps and related proteins are clearly grouped in distinct clades, reflecting a history of repeated gene duplications. The members of the shsp family are characterized by the presence of a conserved homologous "alpha-crystallin domain," which sometimes is present in duplicate. Predictions are made of secondary structure and solvent accessibility of this domain, which together with hydropathy profiles and intron positions support the presence of two similar hydrophobic beta-sheet-rich motifs, connected by a hydrophilic alpha-helical region. Together with an overview of the newly characterized members of the shsp family, these data help to define this family as being involved as stable structural proteins and as molecular chaperones during normal development and induced under pathological and stressful conditions.

Characterization of 45-kDa/54-kDa HSP27 kinase, a stress-sensitive kinase which may activate the phosphorylation-dependent protective function of mammalian 27-kDa heat-shock protein HSP27

Heat-shock protein 27 (HSP27) is a major target of phosphorylation upon cell stimulation with a variety of agents and has been suggested to have a phosphorylation-regulated function at the level of actin filaments. Here we investigated comparatively the mechanisms of HSP27 phosphorylation by oxidative stresses, exposures to tumor necrosis factor (TNF), heat shock and growth factors. Extracts of Chinese hamster or human cells exposed to H2O2, xanthine/xanthine oxidase, menadione or TNF contained up to 15-fold more HSP27 kinase activity than comparable extracts obtained from control cells. Induction of HSP27 kinase activity by TNF or H2O2 was completely inhibited by first treating the cells with the antioxidant N-acetyl-L-cysteine, suggesting that generation of reactive oxygen metabolites was the key triggering element of this induction. In contrast, prior treatment with acetylcysteine had no or little effect on the induction by thrombin, serum and heat shock. The kinase activity in extracts of cells stimulated by heat shock, H2O2, sodium arsenite, TNF or growth factors was identified by in-gel renaturation and purified approximately 8000-fold by sequential chromatography. In all cases, the induced kinase activity was entirely associated with two polypeptides of 45 kDa and 54 kDa, identified as mitogen-activated-protein kinase-activated protein (MAPKAP) kinase-2 based on its reactivation in vitro by 42/44-kDa MAP kinases, its antigenic properties and its substrate specificity. The 45/54-kDa HSP27 kinase may play an important role in the cell response to oxidative stress. Overexpression of the wild-type HSP27 but not of a nonphosphorylatable form of human HSP27 in Chinese hamster cells conferred resistance to actin fragmentation by oxidative stress generated by H2O2. It is concluded that activation of the 45/54-kDa HSP27 kinase is a common mechanism of HSP27 phosphorylation to which converge both oxyradical-dependent and oxyradical-independent pathways and which may participate in a homeostatic response to stress at the level of actin microfilament.

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.

Assisting spontaneity: the role of Hsp90 and small Hsps as molecular chaperones

Hsp90 and small Hsps are two abundant types of eukaryotic stress protein whose function has remained largely enigmatic. In the cell, Hsp90 exists in a complex (with other Hsps and prolyl isomerases) possibly implicated in interactions with non-native proteins. Recent biochemical analysis of both Hsp90 and small Hsps has revealed that they may act as ATP-independent molecular chaperones involved in protein folding and unfolding events.

The serum-induced phosphorylation of mammalian hsp27 correlates with changes in its intracellular localization and levels of oligomerization

The oligomeric small heat-shock protein hsp27, also denoted hsp28, is constitutively expressed in several mammalian cells and displays a phosphorylation status that is related to cellular growth and differentiation. This protein is related to alpha-crystallin and has strong sequence similarity with an in vitro inhibitor of actin polymerization. Here, we have analyzed hsp27 phosphorylation, cellular localization and structural organization following serum stimulation of serum-starved HeLa cells. hsp27 is dephosphorylated in starved cells and quantitatively recovered in the form of small structures (< 200 kDa) present in the soluble phase of the cytoplasm. Immediately after the addition of serum to starved cells, a rapid phosphorylation and complex changes in the intracellular distribution and structural organization of hsp27 are observed. Phosphorylation essentially occurs at the level of small hsp27 structures (< 200 kDa) and is concomitant with the increased molecular mass (up to 700 kDa) of a fraction of this protein. Serum treatment also induced the detergent-sensitive association of another fraction of hsp27, still in the form of small and dephosphorylated structures, with cellular particulate fractions. Contrasting with these observations, hsp70 had the tendency to concentrate into nucleoli during serum starvation.