Relocalization of alpha B-crystallin by heat shock in ovarian carcinoma cells

Small Heat Shock Proteins Collaborate with FAIM to Prevent Accumulation of Misfolded Protein Aggregates

Cells and tissues are continuously subject to environmental insults, such as heat shock and oxidative stress, which cause the accumulation of cytotoxic, aggregated proteins. We previously found that Fas Apoptosis Inhibitory Molecule (FAIM) protects cells from stress-induced cell death by preventing abnormal generation of protein aggregates similar to the effect of small heat shock proteins (HSPs). Protein aggregates are often associated with neurodegenerative diseases, including Alzheimer's disease (AD). In this study, we sought to determine how FAIM protein dynamics change during cellular stress and how FAIM prevents the formation of amyloid-β aggregates/fibrils, one of the pathological hallmarks of AD. Here, we found that the majority of FAIM protein shifts to the detergent-insoluble fraction in response to cellular stress. A similar shift to the insoluble fraction was also observed in small heat shock protein (sHSP) family molecules, such as HSP27, after stress. We further demonstrate that FAIM is recruited to sHSP-containing complexes after cellular stress induction. These data suggest that FAIM might prevent protein aggregation in concert with sHSPs. In fact, we observed the additional effect of FAIM and HSP27 on the prevention of protein aggregates using an in vitro amyloid-β aggregation model system. Our work provides new insights into the interrelationships among FAIM, sHSPs, and amyloid-β aggregation.

Small heat-shock proteins: important players in regulating cellular proteostasis

Small heat-shock proteins (sHsps) are a diverse family of intra-cellular molecular chaperone proteins that play a critical role in mitigating and preventing protein aggregation under stress conditions such as elevated temperature, oxidation and infection. In doing so, they assist in the maintenance of protein homeostasis (proteostasis) thereby avoiding the deleterious effects that result from loss of protein function and/or protein aggregation. The chaperone properties of sHsps are therefore employed extensively in many tissues to prevent the development of diseases associated with protein aggregation. Significant progress has been made of late in understanding the structure and chaperone mechanism of sHsps. In this review, we discuss some of these advances, with a focus on mammalian sHsp hetero-oligomerisation, the mechanism by which sHsps act as molecular chaperones to prevent both amorphous and fibrillar protein aggregation, and the role of post-translational modifications in sHsp chaperone function, particularly in the context of disease.

Recruitment of phosphorylated small heat shock protein Hsp27 to nuclear speckles without stress

During stress, the mammalian small heat shock protein Hsp27 enters cell nuclei. The present study examines the requirements for entry of Hsp27 into nuclei of normal rat kidney (NRK) renal epithelial cells, and for its interactions with specific nuclear structures. We find that phosphorylation of Hsp27 is necessary for the efficient entry into nuclei during heat shock but not sufficient for efficient nuclear entry under control conditions. We further report that Hsp27 is recruited to an RNAse sensitive fraction of SC35 positive nuclear speckles, but not other intranuclear structures, in response to heat shock. Intriguingly, Hsp27 phosphorylation, in the absence of stress, is sufficient for recruitment to speckles found in post-anaphase stage mitotic cells. Additionally, pseudophosphorylated Hsp27 fused to a nuclear localization peptide (NLS) is recruited to nuclear speckles in unstressed interphase cells, but wildtype and nonphosphorylatable Hsp27 NLS fusion proteins are not. The expression of NLS-Hsp27 mutants does not enhance colony forming abilities of cells subjected to severe heat shock, but does regulate nuclear speckle morphology. These data demonstrate that phosphorylation, but not stress, mediates Hsp27 recruitment to an RNAse soluble fraction of nuclear speckles and support a site-specific role for Hsp27 within the nucleus.

Expression of the small heat-shock protein alphaB-crystallin in tauopathies with glial pathology

Intracellular accumulations of filamentous material composed of tau proteins are defining features of sporadic and familial neurodegenerative disorders termed "tauopathies." In Alzheimer's disease, the most common tauopathy, tau pathology is predominantly localized within neurons; however, robust glial pathology occurs in other tauopathies. Although the pathogenesis of tauopathies remains primarily unknown, molecular chaperones such as heat-shock proteins (HSPs) are implicated in these tau disorders as well as other neurodegenerative diseases characterized by the accumulation of insoluble protein aggregates such as alpha-synuclein in Parkinson's disease and polyglutamine in Huntington's disease. We analyzed a variety of tauopathies with antibodies to a panel of HSPs to determine their role in the pathogenesis of these disorders. Although HSPs are not found in neuronal tau inclusions, we demonstrate increased expression of the small HSP alphaB-crystallin in glial inclusions of both sporadic and familial tauopathies. alphaB-crystallin was observed in a subset of astrocytic and oligodendrocytic tau inclusions as well as the neuropil thread pathology in cellular processes, but the co-expression of alphaB-crystallin with tau inclusions was relatively specific to tauopathies with extensive glial pathology. Thus, increased alphaB-crystallin expression in glial tau inclusions may represent a response by glia to the accumulation of misfolded or aggregated tau protein that is linked to the pathogenesis of the glial pathology and distinct from mechanisms underlying neuronal tau pathology in neurodegenerative disease.

Nuclear staining for the small heat shock protein alphaB-crystallin colocalizes with splicing factor SC35

AlphaB-Crystallin has for a long time been considered a specific eye lens protein. Later on it appeared that this protein belongs to the family of the small heat shock proteins and that it occurs also extra-lenticularly in many different cell types. AlphaB-Crystallin is mainly present in the cytoplasm, but there are some indications that it might have a function in the nucleus too. However, till now its presence in the nucleus is uncertain. We therefore compared the localization of alphaB-crystallin in nine cell lines cultured under normal conditions using four different antisera. All four antisera gave a diffuse staining for alphaB-crystallin in the cytoplasm, but one of the antibodies consistently showed nuclear staining in eight of the cell types, in the form of distinct speckles. These speckles are equally pronounced in the different cell types, whether or not cytoplasmic alphaB-crystallin is present. Preabsorption of the antiserum with alphaB-crystallin abolished the staining. Furthermore we demonstrate that if only minor amounts of alphaB-crystallin are present, the protein seems to be located exclusively in the nucleus. However, in case of higher amounts of protein, alphaB-crystallin is distributed between cytoplasm and nucleus. The nuclear alphaB-crystallin exists, like the cytoplasmic alphaB-crystallin, in non-phosphorylated and phosphorylated forms, is Triton-insoluble but can be extracted by 2 M NaCl. These data suggest that alphaB-crystallin might be bound to the nuclear matrix per se or to nuclear matrix proteins via other proteins. In agreement with other nuclear matrix proteins, nuclear alphaB-crystallin staining turns diffuse upon mitosis and leaves the chromosomes unstained. Double staining experiments revealed colocalization of alphaB-crystallin with the splicing factor SC35 in nuclear speckles, suggesting a role for alphaB-crystallin in splicing or protection of the splicing machinery.

Inhibition of Rho-kinase induces alphaB-crystallin expression in lens epithelial cells

The small heat shock protein, alphaB-crystallin, has been shown to interact with actin and intermediate filament proteins. However, little is known regarding the cellular mechanisms regulating such interactions. In this study, we explored the role of the Rho/Rho-kinase pathway in alphaB-crystallin distribution and expression in porcine lens epithelial cells. alphaB-crystallin was distributed uniformly throughout the cytoplasm and did not exhibit any unique redistribution in response to actin depolymerization induced by Rho/Rho-kinase inhibitors (C3-exoenzyme or Y-27632) or by overexpression of the dominant negative mutant of Rho-kinase (DNRK) in porcine lens epithelial cells. Interestingly, alphaB-crystallin levels markedly increased in lens epithelial cells treated with the inhibitors of Rho/Rho-kinase proteins (lovastatin, Y-27632 or DNRK) while a protein kinase C inhibitor (GF109203x) was found to have no effect. Further, Y-27632 showed a dose (2-50 microM) response effect on alphaB-crystallin induction. Nocodazole, a microtubule-depolymerizing agent, elicited an increase in alphaB-crystallin levels but latrunculin, an actin depolymerizing agent, did not show any significant effect. Pretreatment with cycloheximide or genistein blocked the Rho-kinase inhibitor-induced increase in alphaB-crystallin protein levels. Rho-kinase inhibitor-induced increases in alphaB-crystallin levels were found to be associated with activation of P38 mitogen-activated protein kinase (MAPK). These results suggest that Rho/Rho-kinase negatively regulates alphaB-crystallin expression, and this response appears to be dependent on tyrosine-protein kinase and P38 MAPK function. Finally, alphaB-crystallin induction appears to be better correlated with the direct inhibition of Rho/Rho-kinase than with actin depolymerization per se.

Translocation of small heat shock proteins to the actin cytoskeleton upon proteasomal inhibition

The role of small heat shock proteins (sHsps) as molecular chaperones is still poorly understood. We therefore investigated the effect of proteasomal inhibition on sHsps in the rat cardiac myoblast cell line H9c2. Proteasomes are responsible for controlled degradation of intracellular proteins. Inhibition of their activities leads to accumulation of unfolded proteins, which can form insoluble "aggresomes" together with proteasomes and heat shock proteins Hsp70 and Hsp90. We here report that upon proteasome inhibition, alpha B-crystallin and Hsp25 translocate from the detergent-soluble cytosolic fraction to the detergent-insoluble nuclear/cytoskeletal fraction. Although phosphorylation of both alpha B-crystallin and Hsp25 is induced, this does not seem to be essential for the translocation. Immunocytochemistry revealed that alpha B-crystallin and Hsp25, which show a diffuse cytoplasmic staining in unstressed H9c2 cells, colocalize with F-actin upon proteasomal inhibition. After transfection in H9c2 cells, other sHsps (alpha A-crystallin, Hsp20, HspB2 and HspB3) showed similar translocation to the actin cytoskeleton. The redistribution of sHsps upon proteasomal inhibition may reflect a mechanism by which cells are protected from damaged intracellular proteins by sequestering them on the cytoskeleton.

Small heat shock proteins, the cytoskeleton, and inclusion body formation

Since first being implicated in central nervous system disease 10 years ago, much has been learned concerning the regulation and function of the small heat shock protein alpha B-crystallin. Neuropathological, cellular and molecular studies all now point to a functional relationship between alpha B-crystallin and intermediate filaments. alpha B-crystallin accumulation marks reactive astrocytes in general in a wide variety of disorders and specifically intermediate filament-based glial inclusion bodies such as Rosenthal fibres found in astrocytes in Alexander's disease. In vitro, alpha B-crystallin expression suppresses intermediate filament aggregation and can prevent or reverse experimentally induced glial inclusion body formation. Conversely, dysregulation of glial fibrillary acidic protein expression in vivo results in Rosenthal fibre formation and upregulation of endogenous alpha B-crystallin expression. These data and those from studies recently carried out on other tissues strongly suggest that one function of this small heat shock protein is to modulate intermediate filament organization under conditions of physiological stress and neurodegenerative disease.

The small heat shock proteins Hsp20 and alphaB-crystallin in cultured cardiac myocytes: differences in cellular localization and solubilization after heat stress

Hsp20, a recently described new member of the small heat shock protein superfamily, is abundant in heart, skeletal muscle types and smooth muscle. We investigated the intracellular localization of Hsp20 in cultured rat neonatal cardiac myocytes, under normal conditions and after stress. These cellular characteristics of Hsp20 were compared with those of its closest relative, alphaB-crystallin, which is also highly expressed in heart. Like alphaB-crystallin, Hsp20 is normally located in the cytoplasm of the cardiac myocytes. After a heat stress, a subpopulation of Hsp20 migrates into the nucleus, while another part remains in the cytoplasm. In very few cells a faint sarcomeric association of Hsp20 is observed. In contrast, as previously reported, alphaB-crystallin displays a very distinct cross-striated sarcomeric staining after the heat shock, but no nuclear migration. Also at the level of Triton solubility, differences exist between the two related proteins; while alphaB-crystallin, like other small heat shock proteins, becomes insoluble upon heat stress, Hsp20 remains largely soluble. Our results indicate that Hsp20 and alphaB-crystallin, despite their structural similarities, display conspicuous functional differences.

Loss of epithelial polarity is accompanied by differential association of proteins with intracellular membranes

Cellular membranes play an important role in the formation and maintenance of epithelial polarity, which is lost early during carcinogenesis. We set out to identify membrane proteins which are altered during loss of cell polarity in mammary epithelium. As a model system we used murine mammary epithelial cells expressing the conditional oncoprotein c-JunER, which induces a reversible loss of polarity upon beta-estradiol-driven activation [1]. When grown either in the absence or presence of hormone, these cells exhibit a polarized or unpolarized phenotype, respectively. Different membrane fractions of polarized or unpolarized cells were analyzed by two-dimensional electrophoresis (2-DE) and differentially expressed membrane proteins were identified. To distinguish between transmembrane orientation and peripheral attachment of these proteins, were performed extractions with carbonate at high pH or with Triton X-114. In addition, cytosolic proteins of both states were analyzed to investigate their differential association with distinct membrane fractions. We found ten protein spots preferentially or exclusively in polarized cells and 17 other proteins as being upregulated during loss of polarity. Some of the peripheral membrane proteins were identified by microsequencing. The resident Golgi protein nucleobindin and fructose-bisphosphate aldolase were preferentially associated with membranes of polarized cells, whereas alphaB crystallin was detected exclusively and in high amounts in unpolarized cells.

Ectopic expression of alpha B-crystallin in Chinese hamster ovary cells suggests a nuclear role for this protein

alpha B-crystallin (alpha B) is known to be a cytosolic, small heat shock-like multimeric protein that has anti-aggregation, chaperone-like properties. The expression of the alpha B-crystallin gene is developmentally regulated and is induced by a variety of stress stimuli. Importantly, alpha B-crystallin expression is enhanced during oncogenic transformation of cells, in a number of tumors, and most notably, in many neurodegenerative disorders, including Alzheimer's disease and multiple sclerosis. Other than its perceived role as a structural protein in the ocular lens, the actual function of alpha B-crystallin in cellular physiology remains unknown. We have stably transfected CHO cells with an inducible alpha B-cDNA-MMTV-promoter construct that allows the synthesis of recombinant alpha B-crystallin only upon exposure of these cells to dexamethasone. Using immunostaining and conventional and confocal microscopy, we have examined the subcellular distribution of the ectopically expressed alpha B-crystallin. We find that in addition to being in the cytoplasm, the protein resides in the nuclear interior in the interphase nucleus. Double labeling with anti alpha B-crystallin and anti-tubulin, concanavallin, and wheat germ agglutinin, respectively, revealed that during cell division alpha B-crystallin is excluded from condensed chromatin and the nascent nuclei. However, the protein again appears in the newly formed nuclei after the completion of cytokinesis suggesting a conditional, regulatory role for alpha B-crystallin in the nucleus.

The molecular chaperone alphaA-crystallin enhances lens epithelial cell growth and resistance to UVA stress

alphaA-Crystallin (alphaA) is a member of the small heat shock protein (sHSP) family and has the ability to prevent denatured proteins from aggregating in vitro. Lens epithelial cells express relatively low levels of alphaA, but in differentiated fiber cells, alphaA is the most abundant soluble protein. The lenses of alphaA-knock-out mice develop opacities at an early age, implying a critical role for alphaA in the maintenance of fiber cell transparency. However, the function of alpha-crystallin in the lens epithelium is unknown. To investigate the physiological function of alphaA in lens epithelial cells, we used the following two systems: alphaA knock-out (alphaA(-/-)) mouse lens epithelial cells and human lens epithelial cells that overexpress alphaA. The growth rate of alphaA(-/-) mouse lens epithelial cells was reduced by 50% compared with wild type cells. Cell cycle kinetics, measured by fluorescence-activated cell sorter analysis of propidium iodide-stained cells, indicated a relative deficiency of alphaA(-/-) cells in the G2/M phases. Exposure of mouse lens epithelial cells to physiological levels of UVA resulted in an increase in the number of apoptotic cells in the cultures. Four hours after irradiation the fraction of apoptotic cells in the alphaA(-/-) cultures was increased 40-fold over wild type. In cells lacking alphaA, UVA exposure modified F-actin, but actin was protected in cells expressing alphaA. Stably transfected cell lines overexpressing human alphaA were generated by transfecting extended life span human lens epithelial cells with the mammalian expression vector construct pCI-neoalphaA. Cells overexpressing alphaA were resistant to UVA stress, as determined by clonogenic survival. alphaA remained cytoplasmic after exposure to either UVA or thermal stress indicating that, unlike other sHSPs, the protective effect of alphaA was not associated with its relocalization to the nucleus. These results indicate that alphaA has important cellular functions in the lens over and above its well characterized role in refraction.

alpha B-crystallin and hsp25 in neonatal cardiac cells--differences in cellular localization under stress conditions

Two members of the small heat shock protein family, alpha B-crystallin and hsp25, occur at high levels in the mammalian heart. To try and understand any differences in functioning, we compared their properties in cultured rat neonatal cardiac myocytes. Both proteins are stress-inducible, but the level of hsp25 is only slightly increased in cultured cardiac myocytes subjected to hyperthermic stress, while alpha B-crystallin levels even remain unchanged. Phosphorylation of alpha B-crystallin and to a lesser extent also of hsp25 is induced after the heat shock. Directly after heat stress, alpha B-crystallin and hsp25 are partly found in detergent-insoluble fractions, representing cytoskeletal/nuclear structures. Additionally, we show by confocal laser scanning microscopy that alpha B-crystallin and hsp25 become associated with sarcomeric structures directly after the heat shock, indicating a cytoskeletal protective function. Four to six hours after the heat shock, both proteins reoccupy their original positions in the cytoplasm again. In contrast to alpha B-crystallin, hsp25 not only translocates to the cytoskeleton but also migrates to positions inside the nucleus. Despite the fact that both proteins are normally part of the same complex, their behavior in neonatal cardiac myocytes appears to be very different. The sarcomeric association of alpha B-crystallin occurs under milder conditions and persists for a longer period of time in comparison with hsp25. Our findings suggest that alpha B-crystallin and hsp25 are both involved in protection of the cytoskeleton during stress situations in the heart, although in different manners. In addition, hsp25 also plays a role inside the nucleus.

Bovine and human alpha-crystallins as molecular chaperones: prevention of the inactivation of glutathione reductase by fructation

With no measurable protein synthesis occurring in the centre of the lens, structural proteins and enzymes there will need to be stable for many years, if not decades, in order to maintain lens integrity and function. Recent work has indicated that alpha-crystallin, which is sequentially related to heat shock proteins, has chaperone-like properties in that it is capable of preventing heat-induced aggregation of various proteins, including other crystallins. Thus this universal vertebrate lens protein may contribute to maintenance of lens integrity by protecting other lens proteins from non-enzymic insults or the consequences thereof. We previously showed that the enzyme glutathione reductase was inactivated in a time-dependent manner when incubated with various sugars, suggesting glycation was responsible for this effect. In this paper we confirmed that this was the case. Using this enzyme model system, the inclusion of either bovine or human alpha-crystallin protected against the inactivation of glutathione reductase by fructation. This action was specific, with control proteins displaying no such protection. Use of high performance liquid chromatography supported the fact that alpha-crystallin did not act simply by mopping up free sugar but rather maintained the activity of the modified enzyme. Dose-dependent experiments indicated that human alpha-crystallin was more effective than its bovine counterpart, which might be expected considering the much longer lifespan of humans. The stoichiometry of the protection by both alpha-crystallins indicated that alpha-crystallin with glutathione reductase was not acting like GroEL as a large complex with a hydrophobic pore, but rather that individual subunits may be capable of acting as chaperones.

Immobilization of the C-terminal extension of bovine alphaA-crystallin reduces chaperone-like activity

alpha-Crystallins occur as multimeric complexes, which are able to suppress precipitation of unfolding proteins. Although the mechanism of this chaperone-like activity is unknown, the affinity of alpha-crystallin for aggregation-prone proteins is probably based on hydrophobic interactions. alpha-Crystallins expose a considerable hydrophobic surface to solution, but nevertheless they are very stable and highly soluble. An explanation for this paradox may be that alpha-crystallin subunits have a polar and unstructured C-terminal extension that functions as a sort of solubilizer. In this paper we have described five alphaA-crystallins in which charged and hydrophobic residues were inserted in the C-terminal extension. Introduction of lysine, arginine, and aspartate does not substantially influence chaperone-like activity. In contrast, introduction of a hydrophobic tryptophan greatly diminishes functional activity. CD experiments indicate that this mutant has a normal secondary structure and fluorescence measurements show that the inserted tryptophan is located in a polar environment. However, NMR spectroscopy clearly demonstrates that the presence of the tryptophan residue dramatically reduces the flexibility of the C-terminal extension. Furthermore, the introduction of this tryptophan results in a considerably decreased thermostability of the protein. We conclude that changing the polarity of the C-terminal extension of alphaA-crystallin by insertion of a highly hydrophobic residue can seriously disturb structural and functional integrity.

Spatial and temporal activity of the alpha B-crystallin/small heat shock protein gene promoter in transgenic mice

In order to study the spatial and temporal activity of the mouse alpha B-crystallin/small heat shock gene promoter during embryogenesis, we generated mice harboring a transgene consisting of approximately 4 kbp of alpha B-crystallin promoter sequence fused to the Escherichia coli lacZ reporter gene. beta-galactosidase activity was first observed in the heart rudiment of 8.5 days post coitum (d.p.c.) embryos. An identical expression pattern was obtained for the endogenous alpha B-crystallin gene by whole mount in situ hybridization. At 9.5 d.p.c., beta-galactosidase activity was detected in the lens placode, in the myotome of the somites, in Rathke's pouch (future anterior pituitary), and in some regions of oral ectoderm. We also examined the stress inducibility of the alpha B-crystallin promoter in vivo. Injection of sodium arsenite into mice resulted in increased endogenous alpha B-crystallin expression in the adrenal gland and possibly the liver. Our results indicate that visualization of beta-galactosidase activity provides an accurate reflection of endogenous alpha B-crystallin expression and demonstrate that the complex developmental pattern of mouse alpha B-crystallin gene expression is regulated at the transcriptional level. This expression pattern, coupled with the present literature which addresses functions of the protein, suggests a role for the alpha B-crystallin/small heat shock protein in intermediate filament turnover and cellular remodeling which occur during normal development and differentiation.

Aggregation of a subpopulation of vimentin filaments in cultured human skin fibroblasts derived from patients with giant axonal neuropathy

Giant axonal neuropathy (GAN) is a generalized disorder of intermediate filament networks which results in the formation of an ovoid aggregate in a large variety of cell types. We investigated the cytoskeletal organization of cultured skin fibroblasts derived from three GAN patients by indirect immunofluorescence, confocal, and electron microscopy. Whereas the organization of microfilaments seemed normal, the microtubule network appeared disorganized and tangled. The organization of the intermediate filament network, composed of vimentin, was probed with three antibodies directed against different epitopes: two vimentin-specific antibodies, a monoclonal antibody (mAb V9) and a polyclonal antibody, and a serum specific for all type III IFPs (PI serum). These experiments showed that 20% of cultured skin fibroblasts from GAN patients have a vimentin aggregate composed of densely packed filaments which coexists with a well-organized vimentin network. After depolymerization of microtubules with nocodazole, all fibroblasts from GAN patients contained a vimentin aggregate which seemed to arise from a subpopulation of vimentin filaments normally integrated in the vimentin network. Such aggregates were never observed in any condition in control fibroblasts. Moreover, the ultrastructural analysis of GAN cells revealed the presence of swollen mitochondria. We suggest that GAN may be due to a defect in a factor which stabilizes cytoplasmic intermediate filament networks, and we speculate on its identification and properties.

Conversion from oligomers to tetramers enhances autophosphorylation by lens alpha A-crystallin. Specificity between alpha A- and alpha B-crystallin subunits

Previously we showed that alpha-crystallins are autophosphorylated (Kantorow, M., and Piatigorsky, J. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 3112-3116). Here we report that addition of 1% deoxycholate converted alpha A-crystallin aggregates into 80-kDa tetramers which were 10-fold more active for autophosphorylation. Circular dichroism (CD) spectra of alpha-crystallin revealed little or no change in secondary and tertiary structures in 1% deoxycholate, alpha A2D, a truncated form of bovine alpha A that exists as a tetramer, was as active for autophosphorylation in the absence of deoxycholate as intact alpha A was in the presence of deoxycholate. At least one serine between amino acids 131 and 145 of bovine alpha A was autophosphorylated in peptide mapping experiments. Chicken alpha A-crystallin, which lacks the Ser-122 cAMP-dependent kinase site of bovine alpha A, was also autophosphorylated in the presence of deoxycholate. In contrast to alpha A-crystallin, autophosphorylation by alpha B-crystallin was not activated by deoxycholate despite its conversion to a tetrameric form, and alpha B was also more efficiently phosphorylated by cAMP-dependent kinase than alpha A. These data suggest metabolic differences between the alpha-crystallin subunits that may be related to specific expression of alpha A in the lens and ubiquitous expression of alpha B in numerous normal and diseased tissues.

Phosphorylation of alpha-crystallin in rat lenses is stimulated by H2O2 but phosphorylation has no effect on chaperone activity

Alpha crystallin (alpha), a phosphorylated structural protein of the lens, has been shown to be a chaperone preventing other lens proteins from aggregating. It is now demonstrated that with oxidative stress imposed on cultured rat lenses, the incorporation of labeled phosphate into the alpha polypeptide chains increased by two to four times over a 90-min period in comparison to control experiments. The phosphorylation rate of the B chain, alpha B, was twice that of the A chain, alpha A. However, phosphorylation of the alpha chains has an insignificant effect on the chaperone activity of alpha or the individual alpha A and alpha B chains as measured by suppressing the thermally induced aggregation of beta low or gamma crystallins. It was also found that the alpha A aggregates are more effective chaperones than the alpha B aggregates. The size of the macromolecules resulting from reaggregation of the isolated non-phosphorylated or phosphorylated alpha B chains are not markedly effected by phosphorylation. However, phosphorylation of the alpha A chain leads to a heterogeneous population with two major species, one similar in size to alpha A and another approximately twice as large. It is concluded that the phosphorylation of alpha is associated with some other function of the protein than that of chaperone activity and that this function may be linked to a protective response to oxidative stress.

The expression of alpha B-crystallin in epithelial tumours: a useful tumour marker?

Alpha B-crystallin is a lens protein showing homology with small heat-shock proteins. We have previously demonstrated its expression in non-lenticular normal and diseased human tissues by immunostaining with a polyclonal antibody. In view of the expression seen in normal renal tubular epithelium, we have assessed the immunoreactivity of a variety of epithelial tumours, to determine the usefulness of alpha B-crystallin as a specific renal tumour marker. Carcinomas arising from tissues which normally express alpha B-crystallin, such as colo-rectal and thyroid carcinomas, showed a varying pattern and degree of immunoreactivity. The most consistently positive tumours, however, with typically strong cytoplasmic and cell membrane staining, were renal cell carcinomas, 90 per cent of which showed positive immunoreactivity. This pattern of staining, while not absolutely specific, is a useful aid to the diagnosis of renal carcinoma. When a metastic deposit or a small biopsy is being assessed, anti-alpha B-crystallin may be included in a panel of antibodies, the pattern of staining of which may direct the search for the primary site of the tumour.

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.

Divergent effects of hyperosmolality on stress-response (heat shock) protein expression in cultured human tumor cells: an immunocytochemical study

Exposing cells to adverse conditions usually elicits expression of stress-response (heat shock) proteins (srp). Here we show that hyperosmolar growth conditions do not uniformly affect srp expression in MCF-7 and HeLa S3 cells, derived from carcinoma of the breast and cervix, respectively. Thus, whereas srp 27 expression was increased in MCF-7, but not in HeLa S3, the opposite was the case with srp 72. On the other hand, hyperosmolality did not induce alpha B-crystallin or ubiquitin in either cell line. These findings show that srp expression by the human tumor cells studied is non-coordinate, suggesting that each srp is independently modulated.

Coordinate and independent regulation of alpha B-crystallin and hsp27 expression in response to physiological stress

alpha-Crystallins share structural and functional properties with the stress protein hsp27. These polypeptides are expressed at low constitutive levels in many tissues including brain, and alpha B-crystallin and hsp27 can accumulate in central nervous system glia in a variety of neurological conditions. We report here that heat shock and exposure to transition metals result in an increase in the steady state mRNA level of alpha B-crystallin and hsp27 in primary cultures of rat forebrain astrocytes. Both exposure to tumour necrosis factor-alpha and hypertonic conditions result in alpha B-crystallin mRNA accumulation but no change in the hsp27 mRNA level. Under some of these conditions increased synthesis and accumulation of alpha B-crystallin and hsp27 protein are also evident. We are unable to detect alpha A-crystallin mRNA in resting or stressed astrocytes. A novel phenomenon involving a transitory change in stress protein mRNA mobility in Northern blots during induction is reported, which is stress type and cell type independent. The results demonstrate multiple stress regulation of alpha B-crystallin and hsp27 in cultured astrocytes, suggesting that they can legitimately be regarded as stress proteins in the central nervous system.

Elastase inhibition by the C-terminal domains of alpha-crystallin and small heat-shock protein

alpha-Crystallin, an abundant eye-lens protein and a stress protein in other tissues, shows structural and functional similarities with the small heat-shock proteins. One of the properties in common is the inhibition of elastase. We now report that the separated subunits of alpha-crystallin, alpha A and alpha B, also exhibit elastase inhibition, whereas phosphorylation of these subunits apparently has no influence on the inhibitory capacity. Furthermore, for both alpha A-crystallin and mouse HSP25 the putative C-terminal structural domain, comprising the major region of homology between these proteins, is sufficient to give elastase inhibition. With database search no homology could be found between the three proteins under investigation and any of the known consensus sequences of proteinase inhibitor families.

Distribution of alpha B-crystallin in the anterior segment of primate and bovine eyes

The presence and distribution of alpha B-crystallin in the anterior segment of human, monkey and bovine eyes was investigated immunocytochemically. In all three species the most intense staining was seen in the lens and in the nonpigmented and pigmented epithelial cells covering the tips of the pars plicata of the ciliary body. The staining intensity of the ciliary epithelial cells was comparable to that seen in the lens fibers. Strong labeling was also found in the corneal endothelium. In bovine eyes the presence of alpha B-crystallin in lens, ciliary epithelium of the pars plicata and corneal endothelium was also shown by biochemical analysis using SDS-PAGE and immunoblotting.