Intracellular clusterin causes juxtanuclear aggregate formation and mitochondrial alteration

The Influence of Clusterin Glycosylation Variability on Selected Pathophysiological Processes in the Human Body

The present review gathers together the most important information about variability in clusterin molecular structure, its profile, and the degree of glycosylation occurring in human tissues and body fluids in the context of the utility of these characteristics as potential diagnostic biomarkers of selected pathophysiological conditions. The carbohydrate part of clusterin plays a crucial role in many biological processes such as endocytosis and apoptosis. Many pathologies associated with neurodegeneration, carcinogenesis, metabolic diseases, and civilizational diseases (e.g., cardiovascular incidents and male infertility) have been described as causes of homeostasis disturbance, in which the glycan part of clusterin plays a very important role. The results of the discussed studies suggest that glycoproteomic analysis of clusterin may help differentiate the severity of hippocampal atrophy, detect the causes of infertility with an immune background, and monitor the development of cancer. Understanding the mechanism of clusterin (CLU) action and its binding epitopes may enable to indicate new therapeutic goals. The carbohydrate part of clusterin is considered necessary to maintain its proper molecular conformation, structural stability, and proper systemic and/or local biological activity. Taking into account the wide spectrum of CLU action and its participation in many processes in the human body, further studies on clusterin glycosylation variability are needed to better understand the molecular mechanisms of many pathophysiological conditions. They can also provide the opportunity to find new biomarkers and enrich the panel of diagnostic parameters for diseases that still pose a challenge for modern medicine.

CAG Repeat Instability in the Peripheral and Central Nervous System of Transgenic Huntington’s Disease Monkeys

Huntington’s Disease (HD) is an autosomal dominant disease that results in severe neurodegeneration with no cure. HD is caused by the expanded CAG trinucleotide repeat (TNR) on the Huntingtin gene (HTT). Although the somatic and germline expansion of the CAG repeats has been well-documented, the underlying mechanisms had not been fully delineated. Increased CAG repeat length is associated with a more severe phenotype, greater TNR instability, and earlier age of onset. The direct relationship between CAG repeat length and molecular pathogenesis makes TNR instability a useful measure of symptom severity and tissue susceptibility. Thus, we examined the tissue-specific TNR instability of transgenic nonhuman primate models of Huntington’s disease. Our data show a similar profile of CAG repeat expansion in both rHD1 and rHD7, where high instability was observed in testis, liver, caudate, and putamen. CAG repeat expansion was observed in all tissue samples, and tissue- and CAG repeat size-dependent expansion was observed. Correlation analysis of CAG repeat expansion and the gene expression profile of four genes in different tissues, clusterin (CLU), transferrin (TF), ribosomal protein lateral stalk subunit P1 (RPLP1), and ribosomal protein L13a (RPL13A), showed a strong correlation with CAG repeat instability. Overall, our data, along with previously published studies, can be used for studying the biology of CAG repeat instability and identifying new therapeutic targets.

Clusterin levels in undernourished SH-SY5Y cells

Food-related disorders are increasingly common in developed societies, and the psychological component of these disorders has been gaining increasing attention. Both overnourishment with high-fat diets and perinatal undernourishment in mice have been linked to a higher motivation toward food, resulting in an alteration in food intake. Clusterin (CLU), a multifaced protein, is overexpressed in the nucleus accumbens (NAc) of over-fed rats, as well as in those that suffered chronic undernutrition. Moreover, an increase of this protein was observed in the plasma of obese patients with food addiction, suggesting the implication of CLU in this eating disorder. To characterize CLU’s cellular mechanisms, in vitro experiments of undernutrition were performed using dopaminergic SH-SY5Y cells. To mimic in vivo dietary conditions, cells were treated with different fetal bovine serum (FBS) concentrations, resulting in control (C group) diet (10% FBS), undernourishment (U group) diet (0.5% FBS), and undernourishment diet followed by restoration of control diet (UC group) (0.5 + 10% FBS). Undernourishment compromised cell viability and proliferation, and concomitantly increased CLU secretion as well as the cytosolic pool of the protein, while decreasing the mitochondrial level. The restoration of normal conditions tended to recover cell physiology, and the normal levels and distribution of CLU. This research study is a step forward toward the characterization of clusterin as a potential marker for food addiction and nutritional status.

Clusterin overexpression in mice exacerbates diabetic phenotypes but suppresses tumor progression in a mouse melanoma model

Clusterin (CLU) is an ATP-independent small heat shock protein-like chaperone, which functions both intra- and extra-cellularly. Consequently, it has been functionally involved in several physiological (including aging), as well as in pathological conditions and most age-related diseases, e.g., cancer, neurodegeneration, and metabolic syndrome. To address CLU function at an in vivo model we established CLU transgenic (Tg) mice bearing ubiquitous or pancreas-targeted CLU overexpression (OE). Our downstream analyses in established Tg lines showed that ubiquitous or pancreas-targeted CLU OE in mice affected antioxidant, proteostatic and metabolic pathways. Targeted OE of CLU in the pancreas, which also resulted in CLU upregulation in the liver likely via systemic effects, increased basal glucose levels in the circulation and exacerbated diabetic phenotypes. Furthermore, by establishing a syngeneic melanoma mouse tumor model we found that ubiquitous CLU OE suppressed melanoma cells growth, indicating a likely tumor suppressor function in early phases of tumorigenesis. Our observations provide in vivo evidence corroborating the notion that CLU is a potential modulator of metabolic and/or proteostatic pathways playing an important role in diabetes and tumorigenesis.

Novel nucleolar localization of clusterin and its associated functions in human oral cancers: An in vitro and in silico analysis

Clusterin (CLU), a multifunctional chaperonic glycoprotein associated with diverse cellular functions has been shown to act as an oncogene or tumour suppressor gene in different cancers, implying a dual role in tumorigenesis. Here, we investigated the expression of CLU isoforms, their subcellular localization and functional significance in oral cancer cells. Significant downregulation of secretory CLU (sCLU) transcripts was observed in oral cancer cell lines and tumours versus normal cells while the nuclear CLU (nCLU) transcripts were undetectable. We demonstrated for the first time the nucleolar localization of sCLU, its response to different nucleolar stresses and association with cajal bodies post nucleolar stress. Functionally, knockdown of CLU revealed its negative association with ribosome biogenesis implying a possible tumour suppressor like role in oral cancers. Further, loss of sCLU in these cells also resulted in altered nuclear morphology and shrunken tubulin filaments. In addition, the levels of nucleolar Nucleophosmin 1(NPM1) and Fibrillarin, known to regulate nuclear morphology were downregulated indicating a possible role of sCLU in their stabilization. Further, an in silico docking approach to gain insights into the interaction of sCLU with nucleolar proteins NPM1, Fibrillarin, UBF and Nucleolin, revealed the involvement of a conserved region comprising of amino acid residues 140-155 of sCLU β-chain, specifically via the Phe152 residue in hydrophobic interactions with these client nucleolar proteins indicating a possible stabilizing or regulatory role of sCLU. SIGNIFICANCE OF THE STUDY: This is the first study to demonstrate the nucleolar localization of sCLU and its associated functions in oral cancer cells. Downregulation of sCLU in oral cancer tissues and cell lines, and its negative association with ribogenesis suggest its tumour suppressor like role in oral cancers. The possible role of sCLU in stabilization or regulation of different nucleolar proteins thereby impacting their functions is also implicated.

Transcriptomic Changes Related to Cellular Processes with Particular Emphasis on Cell Activation in Lysosomal Storage Diseases from the Group of Mucopolysaccharidoses

Although mucopolysaccharidoses (MPS), inherited metabolic diseases from the group of lysosomal storage diseases (LSD), are monogenic disorders, recent studies indicated that their molecular mechanisms are complicated. Storage of glycosaminoglycans (GAGs), arising from a deficiency in one of the enzymes involved in the degradation of these compounds, is the primary cause of each MPS type. However, dysfunctions of various cellular organelles and disturbance of cellular processes have been reported which contribute considerably to pathomechanisms of the disease. Here, we present a complex transcriptomic analysis in which all types and subtypes of MPS were investigated, with special emphasis on genes related to cell activation processes. Complex changes in expression of these genes were found in fibroblasts of all MPS types, with number of transcripts revealing higher or lower levels (relative to control fibroblasts) between 19 and over 50, depending on MPS type. Genes in which expression was significantly affected in most MPS types code for proteins involved in following processes, classified according to Gene Ontology knowledge database: cell activation, cell growth, cell recognition, and cell division. Levels of some transcripts (including CD9, CLU, MME and others) were especially significantly changed (over five times relative to controls). Our results are discussed in the light of molecular pathomechanisms of MPS, indicating that secondary and/or tertiary changes, relative to GAG storage, might significantly modulate cellular dysfunctions and contribute to molecular mechanisms of the disease. This may influence the efficacy of various therapies and suggests why various treatments are not fully effective in improving the complex symptoms of MPS.

Dynamics of clusterin protein expression in the brain and plasma following experimental traumatic brain injury

Progress in the preclinical and clinical development of neuroprotective and antiepileptogenic treatments for traumatic brain injury (TBI) necessitates the discovery of prognostic biomarkers for post-injury outcome. Our previous mRNA-seq data revealed a 1.8–2.5 fold increase in clusterin mRNA expression in lesioned brain areas in rats with lateral fluid-percussion injury (FPI)-induced TBI. On this basis, we hypothesized that TBI leads to increases in the brain levels of clusterin protein, and consequently, increased plasma clusterin levels. For evaluation, we induced TBI in adult male Sprague-Dawley rats (n = 80) by lateral FPI. We validated our mRNA-seq findings with RT-qPCR, confirming increased clusterin mRNA levels in the perilesional cortex (FC 3.3, p < 0.01) and ipsilateral thalamus (FC 2.4, p < 0.05) at 3 months post-TBI. Immunohistochemistry revealed a marked increase in extracellular clusterin protein expression in the perilesional cortex and ipsilateral hippocampus (7d to 1 month post-TBI), and ipsilateral thalamus (14d to 12 months post-TBI). In the thalamus, punctate immunoreactivity was most intense around activated microglia and mitochondria. Enzyme-linked immunoassays indicated that an acute 15% reduction, rather than an increase in plasma clusterin levels differentiated animals with TBI from sham-operated controls (AUC 0.851, p < 0.05). Our findings suggest that plasma clusterin is a candidate biomarker for acute TBI diagnosis.

Brain clusterin protein isoforms and mitochondrial localization

Clusterin (CLU), or apolipoprotein J (ApoJ), is the third most predominant genetic risk factor associated with late-onset Alzheimer’s disease (LOAD). In this study, we use multiple rodent and human brain tissue and neural cell models to demonstrate that CLU is expressed as multiple isoforms that have distinct cellular or subcellular localizations in the brain. Of particular significance, we identify a non-glycosylated 45 kDa CLU isoform (mitoCLU) that is localized to the mitochondrial matrix and expressed in both rodent and human neurons and astrocytes. In addition, we show that rodent mitoCLU is translated from a non-canonical CUG (Leu) start site in Exon 3, a site that coincides with an AUG (Met) in human CLU. Last, we reveal that mitoCLU is present at the gene and protein level in the currently available CLU^–/– mouse model. Collectively, these data provide foundational knowledge that is integral in elucidating the relationship between CLU and the development of LOAD.

Knockout of apolipoprotein A-I decreases parenchymal and vascular β-amyloid pathology in the Tg2576 mouse model of Alzheimer's disease

AIMS

Apolipoprotein A-I (apoA-I), the principal apolipoprotein associated with high-density lipoproteins in the periphery, is also found at high concentrations in the cerebrospinal fluid. Previous studies have reported either no impact or vascular-specific effects of apoA-I knockout (KO) on β-amyloid (Aβ) pathology. However, the putative mechanism(s) by which apoA-I may influence Aβ deposition is unknown.

METHODS

We evaluated the effect of apoA-I deletion on Aβ pathology, Aβ production and clearance from the brain in the Tg2576 mouse model of Alzheimer's disease (AD).

RESULTS

Contrary to previous reports, deletion of the APOA1 gene significantly reduced concentrations of insoluble Aβ40 and Aβ42 and reduced plaque load in both the parenchyma and blood vessels of apoA-I KO × Tg2576 mice compared to Tg2576 animals. This was not due to decreased Aβ production or alterations in Aβ species. Levels of soluble clusterin/apoJ were significantly higher in neurons of apoA-I KO mice compared to both wildtype (WT) and apoA-I KO × Tg2576 mice. In addition, clearance of Aβ along intramural periarterial drainage pathways was significantly higher in apoA-I KO mice compared to WT animals.

CONCLUSION

These data suggest that deletion of apoA-I is associated with increased clearance of Aβ and reduced parenchymal and vascular Aβ pathology in the Tg2576 model. These results suggest that peripheral dyslipidaemia can modulate the expression of apolipoproteins in the brain and may influence Aβ clearance and aggregation in AD.

Clusterin in Alzheimer's Disease: Mechanisms, Genetics, and Lessons From Other Pathologies

Clusterin (CLU) or APOJ is a multifunctional glycoprotein that has been implicated in several physiological and pathological states, including Alzheimer's disease (AD). With a prominent extracellular chaperone function, additional roles have been discussed for clusterin, including lipid transport and immune modulation, and it is involved in pathways common to several diseases such as cell death and survival, oxidative stress, and proteotoxic stress. Although clusterin is normally a secreted protein, it has also been found intracellularly under certain stress conditions. Multiple hypotheses have been proposed regarding the origin of intracellular clusterin, including specific biogenic processes leading to alternative transcripts and protein isoforms, but these lines of research are incomplete and contradictory. Current consensus is that intracellular clusterin is most likely to have exited the secretory pathway at some point or to have re-entered the cell after secretion. Clusterin's relationship with amyloid beta (Aβ) has been of great interest to the AD field, including clusterin's apparent role in altering Aβ aggregation and/or clearance. Additionally, clusterin has been more recently identified as a mediator of Aβ toxicity, as evidenced by the neuroprotective effect of CLU knockdown and knockout in rodent and human iPSC-derived neurons. CLU is also the third most significant genetic risk factor for late onset AD and several variants have been identified in CLU. Although the exact contribution of these variants to altered AD risk is unclear, some have been linked to altered CLU expression at both mRNA and protein levels, altered cognitive and memory function, and altered brain structure. The apparent complexity of clusterin's biogenesis, the lack of clarity over the origin of the intracellular clusterin species, and the number of pathophysiological functions attributed to clusterin have all contributed to the challenge of understanding the role of clusterin in AD pathophysiology. Here, we highlight clusterin's relevance to AD by discussing the evidence linking clusterin to AD, as well as drawing parallels on how the role of clusterin in other diseases and pathways may help us understand its biological function(s) in association with AD.

BACE2, a conditional β-secretase, contributes to Alzheimer's disease pathogenesis

Deposition of amyloid-β protein (Aβ) to form neuritic plaques is the characteristic neuropathology of Alzheimer's disease (AD). Aβ is generated from amyloid precursor protein (APP) by β- and γ-secretase cleavages. BACE1 is the β-secretase and its inhibition induces severe side effects, whereas its homolog BACE2 normally suppresses Aβ by cleaving APP/Aβ at the θ-site (Phe20) within the Aβ domain. Here, we report that BACE2 also processes APP at the β site, and the juxtamembrane helix (JH) of APP inhibits its β-secretase activity, enabling BACE2 to cleave nascent APP and aggravate AD symptoms. JH-disrupting mutations and clusterin binding to JH triggered BACE2-mediated β-cleavage. Both BACE2 and clusterin were elevated in aged mouse brains, and enhanced β-cleavage during aging. Therefore, BACE2 contributes to AD pathogenesis as a conditional β-secretase and could be a preventive and therapeutic target for AD without the side effects of BACE1 inhibition.

Exfoliation Syndrome: A Disease of Autophagy and LOXL1 Proteopathy

Exfoliation syndrome (XFS) is an age-related disease involving the deposition of aggregated fibrillar material (exfoliation material) at extracellular matrices in tissues that synthesize elastic fibers. Its main morbidity is in the eye, where exfoliation material accumulations form on the surface of the ciliary body, iris, and lens. Exfoliation glaucoma (XFG) occurs in a high proportion of persons with XFS and can be a rapidly progressing disease. Worldwide, XFG accounts for about 25% of open-angle glaucoma cases. XFS and XFG show a sharp age-dependence, similarly to the many age-related diseases classified as aggregopathies. Progress in understanding the cellular bases for XFS/XFG has been slowed by a lack of experimental models. Working with primary human tenon fibroblasts (TF) derived from trabeculectomies of XFG patients and age-matched primary open-glaucoma controls, we found that TF from XFG cells display many of the functional features observed in cells from other protein aggregate diseases, such as Parkinson, Alzheimer, Huntington, and age-related macular degeneration. We have documented defects in lysosomal positioning, microtubule organization, autophagy processing rate, and mitochondrial health. In regard to failure of lysosomal and autophagosome positioning in XFG cells, we have found that XFG TF are unable to establish the transnuclear microtubule organizing center that is required for efficient centripetal vesicular locomotion along microtubules. In regard to potential sources of the autophagy malfunction, we have directed our attention to a potential role of the lysyl oxidase-like 1 protein (LOXL1), the elastic fiber catalyst that displays variant-dependent association with risk for XFG. Our experiments show that (a) in XFG cells, a substantial fraction of LOXL1 is processed for degradation by the autophagic system; (b) most of the LOXL1 N-terminus domain exists in a highly disordered state, a condition known to greatly increase the frequency of polypeptide misfolding; (c) that maximum misfolding occurs at amino acid position 153, the location of the high risk variant G153D; and (d) that replacement of glycine (G) by aspartate (D) there results in a substantial decrease in disorder within the 20 amino acid surrounding domain. Finally, we show that clusterin, a protein that can be induced by the presence of intracellular, or extracellular aggregates, is uniformly overexpressed in XFG TF. The implications of our results for a theory relating XFG to cellular aggregopathy are discussed.

C-terminally mutated tubby protein accumulates in aggresomes

The tubby protein (Tub), a putative transcription factor, plays important roles in the maintenance and function of neuronal cells. A splicing defect-causing mutation in the 3′-end of the tubby gene, which is predicted to disrupt the carboxy-terminal region of the Tub protein, causes maturity-onset obesity, blindness, and deafness in mice. Although this pathological Tub mutation leads to a loss of function, the precise mechanism has not yet been investigated. Here, we found that the mutant Tub proteins were mostly localized to puncta found in the perinuclear region and that the C-terminus was important for its solubility. Immunocytochemical analysis revealed that puncta of mutant Tub co-localized with the aggresome. Moreover, whereas wild-type Tub was translocated to the nucleus by extracellular signaling, the mutant forms failed to undergo such translocation. Taken together, our results suggest that the malfunctions of the Tub mutant are caused by its misfolding and subsequent localization to aggresomes.

Clusterin: full-length protein and one of its chains show opposing effects on cellular lipid accumulation

Proteins, made up of either single or multiple chains, are designed to carry out specific biological functions. We found an interesting example of a two-chain protein where administration of one of its chains leads to a diametrically opposite outcome than that reported for the full-length protein. Clusterin is a highly glycosylated protein consisting of two chains, α- and β-clusterin. We have investigated the conformational features, cellular localization, lipid accumulation, in vivo effects and histological changes upon administration of recombinant individual chains of clusterin. We demonstrate that recombinant α- and β-chains exhibit structural and functional differences and differ in their sub-cellular localization. Full-length clusterin is known to lower lipid levels. In contrast, we find that β-chain-treated cells accumulate 2-fold more lipid than controls. Interestingly, α-chain-treated cells do not show such increase. Rabbits injected with β-chain, but not α-chain, show ~40% increase in weight, with adipocyte hypertrophy, liver and kidney steatosis. Many, sometimes contrasting, roles are ascribed to clusterin in obesity, metabolic syndrome and related conditions. Our findings of differential localization and activities of individual chains of clusterin should help in understanding better the roles of clusterin in metabolism.

Mitochondrial Membrane Potential and Nuclear and Gene Expression Changes During Human Disc Cell Apoptosis: In Vitro and In Vivo Annulus Findings

STUDY DESIGN

A study using cultured human annulus cells and human annular tissue.

OBJECTIVE

To further explore and define mitochondrial mechanisms related to disc cell apoptosis in vitro and in vivo.

SUMMARY OF BACKGROUND DATA

Mitochondrial-dependent intrinsic signaling pathways are a well-recognized component of apoptosis (programmed cell death). Disc cell apoptosis is important because it is a major mechanism by which cell numbers decrease during disc degeneration. Our objective was to further explore and define mitochondrial mechanisms related to disc cell apoptosis.

METHODS

High-content screening techniques were used to study nuclear morphology and mitochondrial membrane potentials in cultured annulus cells. Gene expression in annulus tissue was studied with microarray analysis.

RESULTS

Cultured cells showed significantly increased nuclear size (an indicator of apoptosis) with increasing Thompson grade (P < 0.00001 by analysis of variance). A significant negative correlation for mitochondrial potential (which results from the difference in electrical potential generated by the electrochemical gradient across the inner membrane of the mitochondrion) versus Thompson grade was identified in cultured human annulus cells in control conditions (r = 0.356, P < 0.0001). When exposed to the K ionophore valinomycin at sublethal levels to induce apoptosis, a significant reduction in mitochondrial potential was identified versus nontreated cells. Gene expression patterns in more degenerated Thompson grade III, IV, and V discs versus healthier grade I and II discs showed significant upregulation of a number of genes with well-recognized apoptosis roles in mitochondrial potential decline (ITM2B, beta-2-microglobulin, and cathepsin B, DAP, GAS1, and PDCD5) and TNF-α associations (cathepsin B, RAC1, and PPT1).

CONCLUSION

Data presented here show the in vivo expression of apoptosis-related genes associated with the loss of mitochondrial membrane integrity and decreased mitochondrial membrane potential with increasing Thompson scores. These data, which mimic our novel, direct cell-based in vitro findings, stress the importance of mitochondrial changes related to apoptosis and TNF-α during human disc degeneration.

LEVEL OF EVIDENCE

N/A.

INS-gene mutations: from genetics and beta cell biology to clinical disease

A growing list of insulin gene mutations causing a new form of monogenic diabetes has drawn increasing attention over the past seven years. The mutations have been identified in the untranslated regions of the insulin gene as well as the coding sequence of preproinsulin including within the signal peptide, insulin B-chain, C-peptide, insulin A-chain, and the proteolytic cleavage sites both for signal peptidase and the prohormone convertases. These mutations affect a variety of different steps of insulin biosynthesis in pancreatic beta cells. Importantly, although many of these mutations cause proinsulin misfolding with early onset autosomal dominant diabetes, some of the mutant alleles appear to engage different cellular and molecular mechanisms that underlie beta cell failure and diabetes. In this article, we review the most recent advances in the field and discuss challenges as well as potential strategies to prevent/delay the development and progression of autosomal dominant diabetes caused by INS-gene mutations. It is worth noting that although diabetes caused by INS gene mutations is rare, increasing evidence suggests that defects in the pathway of insulin biosynthesis may also be involved in the progression of more common types of diabetes. Collectively, the (pre)proinsulin mutants provide insightful molecular models to better understand the pathogenesis of all forms of diabetes in which preproinsulin processing defects, proinsulin misfolding, and ER stress are involved.

Serum clusterin as a tumor marker and prognostic factor for patients with esophageal cancer

Background. Recent studies have revealed that clusterin is implicated in many physiological and pathological processes, including tumorigenesis. However, the relationship between serum clusterin expression and esophageal squamous cell carcinoma (ESCC) is unclear. Methods. The serum clusterin concentrations of 87 ESCC patients and 136 healthy individuals were examined. An independent-samples Mann-Whitney U test was used to compare serum clusterin concentrations of ESCC patients to those of healthy controls. Univariate analysis was conducted using the log-rank test and multivariate analyses were performed using the Cox proportional hazards model. Results. In healthy controls, the mean clusterin concentration was 288.8 ± 75.1 μg/mL, while in the ESCC patients, the mean clusterin concentration was higher at 412.3 ± 159.4 μg/mL (P < 0.0001). The 1-, 2-, and 4-year survival rates for the 87 ESCC patients were 89.70%, 80.00%, and 54.50%. Serum clusterin had an optimal diagnostic cut-off point (serum clusterin concentration = 335.5 μg/mL) for esophageal squamous cell carcinoma with sensitivity of 71.26% and specificity of 77.94%. And higher serum clusterin concentration (>500 μg/mL) indicated better prognosis (P = 0.030). Conclusions. Clusterin may play a key role during tumorigenesis and tumor progression of ESCC and it could be applied in clinical work as a tumor marker and prognostic factor.

Secreted clusterin (sCLU) regulates cell proliferation and chemosensitivity to cisplatin by modulating ERK1/2 signals in human osteosarcoma cells

Background

Several studies have shown that secreted clusterin (sCLU) up-regulation in multi-drug resistant osteosarcoma (OS) cells relates to enhanced drug resistance. Furthermore, sCLU silencing directed against sCLU induces significant reduction of cellular growth and sensitizes OS cells to chemotherapy. However, the molecular mechanisms underlying the effect of sCLU on OS cells are not known.

Methods

To evaluate the roles and possible mechanisms of sCLU in chemoresistance of OS cells to cisplatin (DPP), we utilized RNA interference to knockdown sCLU expression in the sCLU-rich U-2 OS cells and to overexpress sCLU in the sCLU-poorer KH OS cells, and further assessed the cell viability and chemosensitivity to DDP as well as possible signaling transduction pathways.

Results

The data showed that sCLU depletion inhibited growth and sensitized sCLU-rich U-2 OS cells to cisplatin in vitro and in vivo by inducing inactivation of ERK1/2, and sCLU overexpression promoted growth and increased resistance of sCLU-less KH OS cells to cisplatin in vitro and in vivo by activation of ERK1/2.

Conclusions

The data also suggests critical roles of sCLU in OS cell chemoresistance to DPP and raises the possibility of sCLU depletion as a promising approach to OS therapy.

Inhibition of intracellular clusterin attenuates cell death in nephropathic cystinosis

Nephropathic cystinosis, characterized by accumulation of cystine in the lysosomes, is caused by mutations in CTNS. The molecular and cellular mechanisms underlying proximal tubular dysfunction and progressive renal failure in nephropathic cystinosis are largely unclear, and increasing evidence supports the notion that cystine accumulation alone is not responsible for the end organ injury in cystinosis. We previously identified clusterin as potentially involved in nephropathic cystinosis. Here, we studied the expression of clusterin in renal proximal tubular epithelial cells obtained from patients with nephropathic cystinosis. The cytoprotective secretory form of clusterin, as evaluated by Western blot analysis, was low or absent in cystinosis cells compared with normal primary cells. Confocal microscopy revealed elevated levels of intracellular clusterin in cystinosis cells. Clusterin in cystinosis cells localized to the nucleus and cytoplasm and showed a filamentous and punctate aggresome-like pattern compared with diffuse cytoplasmic staining in normal cells. In kidney biopsy samples from patients with nephropathic cystinosis, clusterin protein expression was mainly limited to the proximal tubular cells. Furthermore, expression of clusterin overlapped with the expression of apoptotic proteins (apoptosis-inducing factor and cleaved caspase-3) and autophagy proteins (LC3 II and p62). Silencing of the clusterin gene resulted in a significant increase in cell viability and attenuation of apoptosis in cystinosis cells. Results of this study identify clusterin as a pivotal factor in the cell injury mechanism of nephropathic cystinosis and provide evidence linking cellular stress and injury to Fanconi syndrome and progressive renal injury in nephropathic cystinosis.

Effects of everolimus on macrophage-derived foam cell behavior

PURPOSE

The purpose of this study was to investigate the effects of everolimus on foam cell (FC) viability, mRNA levels, and inflammatory cytokine production to better understand its potential inhibitory effects on atheroma progression.

METHODS AND MATERIALS

Human THP1 macrophage-derived FC were formed using acetylated LDL (acLDL, 100 μg/mL) for 72 hours, followed by everolimus treatment (10(-5)-10(-11) M) for 24 hours. FC viability was quantified using fluorescent calcein AM/DAPI staining. FC lysates and media supernatants were analyzed for apoptosis and necrosis using a Cell Death ELISA(PLUS) assay. FC lysates and media supernatants were also analyzed for inflammatory cytokine (IL1β, IL8, MCP1, TNFα) mRNA levels and protein expression using quantitative reverse transcription real-time polymerase chain reaction (QPCR) and a Procarta® immunoassay, respectively. mRNA levels of autophagy (MAP1LC3), apoptosis (survivin, clusterin), and matrix degradation (MMP1, MMP9) markers were evaluated by Quantigene® Plex assay and verified with QPCR. Additionally, hypercholesterolemic rabbits received everolimus-eluting stents (EES) for 28 or 60 days. RAM-11 immunohistochemical staining was performed to compare %RAM-11 positive area between stented sections and unstented proximal sections. Statistical significance was calculated using one-way ANOVA (p≤0.05).

RESULTS

Calcein AM/DAPI staining showed that FC exposed to everolimus (10(-5) M) had significantly decreased viability compared to control. FC apoptosis was significantly increased at a high dose of everolimus (10(-5)M), with no necrotic effects at any dose tested. Everolimus did not affect endothelial (HUVEC) and smooth muscle (HCASMC) cell apoptosis or necrosis. Everolimus (10(-5)M) significantly increased MAP1LC3, caused an increased trend in clusterin (p=0.10), and significantly decreased survivin and MMP1 mRNA levels in FC. MCP1 cytokine mRNA levels and secreted protein expression was significantly decreased by everolimus (10(-5) M) in FC. Percentage of RAM-11 positive area exhibited a reduction trend within sections stented with EES compared to unstented proximal sections at 60 days (p=0.09).

CONCLUSION

Everolimus, a potent anti-proliferative agent used in drug-eluting stents and bioresorbable vascular scaffolds, may inhibit atheroma progression and/or promote atheroma stabilization through diminished viability of FC, decreased matrix degradation, and reduced pro-inflammatory cytokine secretion.

EXECUTIVE SUMMARY

We explored the effects of everolimus on the behavior of human THP1 macrophage-derived foam cells in culture, including cell viability, mRNA levels, and pro-inflammatory cytokine production. We conclude that everolimus, a potent anti-proliferative agent used in drug-eluting stents/bioresorbable vascular scaffolds, may potentially inhibit atheroma progression and/or promote atheroma stabilization through diminished viability of foam cells, decreased matrix degradation, and reduced pro-inflammatory cytokine secretion.

Inefficient translocation of preproinsulin contributes to pancreatic β cell failure and late-onset diabetes

Among the defects in the early events of insulin biosynthesis, proinsulin misfolding and endoplasmic reticulum (ER) stress have drawn increasing attention as causes of β cell failure. However, no studies have yet addressed potential defects at the cytosolic entry point of preproinsulin into the secretory pathway. Here, we provide the first evidence that inefficient translocation of preproinsulin (caused by loss of a positive charge in the n region of its signal sequence) contributes to β cell failure and diabetes. Specifically, we find that, after targeting to the ER membrane, preproinsulin signal peptide (SP) mutants associated with autosomal dominant late-onset diabetes fail to be fully translocated across the ER membrane. The newly synthesized, untranslocated preproinsulin remains strongly associated with the ER membrane, exposing its proinsulin moiety to the cytosol. Rather than accumulating in the ER and inducing ER stress, untranslocated preproinsulin accumulates in a juxtanuclear compartment distinct from the Golgi complex, induces the expression of heat shock protein 70 (HSP70), and promotes β cell death. Restoring an N-terminal positive charge to the mutant preproinsulin SP significantly improves the translocation defect. These findings not only reveal a novel molecular pathogenesis of β cell failure and diabetes but also provide the first evidence of the physiological and pathological significance of the SP n region positive charge of secretory proteins.

Expression of the protein chaperone, clusterin, in spinal cord cells constitutively and following cellular stress, and upregulation by treatment with Hsp90 inhibitor

Clusterin, a protein chaperone found at high levels in physiological fluids, is expressed in nervous tissue and upregulated in several neurological diseases. To assess relevance to amyotrophic lateral sclerosis (ALS) and other motor neuron disorders, clusterin expression was evaluated using long-term dissociated cultures of murine spinal cord and SOD1(G93A) transgenic mice, a model of familial ALS. Motor neurons and astrocytes constitutively expressed nuclear and cytoplasmic forms of clusterin, and secreted clusterin accumulated in culture media. Although clusterin can be stress inducible, heat shock failed to increase levels in these neural cell compartments despite robust upregulation of stress-inducible Hsp70 (HspA1) in non-neuronal cells. In common with HSPs, clusterin was upregulated by treatment with the Hsp90 inhibitor, geldanamycin, and thus could contribute to the neuroprotection previously identified for such compounds in disease models. Clusterin expression was not altered in cultured motor neurons expressing SOD1(G93A) by gene transfer or in presymptomatic SOD1(G93A) transgenic mice; however, clusterin immunolabeling was weakly increased in lumbar spinal cord of overtly symptomatic mice. More striking, mutant SOD1 inclusions, a pathological hallmark, were strongly labeled by anti-clusterin. Since secreted, as well as intracellular, mutant SOD1 contributes to toxicity, the extracellular chaperoning property of clusterin could be important for folding and clearance of SOD1 and other misfolded proteins in the extracellular space. Evaluation of chaperone-based therapies should include evaluation of clusterin as well as HSPs, using experimental models that replicate the control mechanisms operant in the cells and tissue of interest.

Cotransin induces accumulation of a cytotoxic clusterin variant that cotranslationally rerouted to the cytosol

Although clusterin (CLU) was originally identified as a secreted glycoprotein that plays cytoprotective role, several intracellular CLU variants have been recently identified in the diverse pathological conditions. The mechanistic basis of these variants is now believed to be alternative splicing and retrotranslocation. Here, we uncovered, an unglycosylated and signal sequence-unprocessed, CLU variant in the cytosol. This variant proved to be a product that cotranslationally rerouted to the cytosol during translocation. Cytosolic CLU was prone to aggregation at peri-nuclear region of cells and induced cell death. Signal sequence is shown to be an important determinant for cytosolic CLU generation and aggregation. These results provide not only a new mechanistic insight into the cytosolic CLU generation but also an idea for therapeutic mislocalization of CLU as a strategy for cancer treatment.

Small interfering RNA targeted to secretory clusterin blocks tumor growth, motility, and invasion in breast cancer

Clusterin/apolipoprotein J (Clu) is a ubiquitously expressed secreted heterodimeric glycoprotein that is implicated in several physiological processes. It has been reported that the elevated level of secreted clusterin (sClu) protein is associated with poor survival in breast cancer patients and can induce metastasis in rodent models. In this study, we investigated the effects of sClu inhibition with small interfering RNAs (siRNAs) on cell motility, invasion, and growth in vitro and in vivo. MDA-MB-231 cells were transfected with pSuper-siRNA/sClu. Cell survival and proliferation were examined by 3-(4,5-dimethyl-thiazol-2yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium and clonogenic survival assay. The results showed that sClu silencing significantly inhibited the proliferation of MDA-MB-231 cells. The invasion and migration ability were also dramatically decreased, which was detected by matrigel assays. TUNEL staining and caspase-3 activity assay demonstrated that sClu silencing also could increase the apoptosis rate of cells, resulting in the inhibition of cell growth. We also determined the effects of sClu silencing on tumor growth and metastatic progression in an orthotopic breast cancer model. The results showed that orthotopic primary tumors derived from MDA-MB-231/pSuper sClu siRNA cells grew significantly slower than tumors derived from parental MDA-MB-231 or MDA-MB-231/pSuper scramble siRNA cells, and metastasize less to the lungs. These data suggest that secretory clusterin plays a significant role in tumor growth and metastatic progression. Knocking-down sClu gene expression may provide a valuable method for breast cancer therapy.

Implication of clusterin in TNF-α response of rheumatoid synovitis: lesson from in vitro knock-down of clusterin in human synovial fibroblast cells

Recently clusterin (CLU) was reported to be an inhibitor of NF-κB pathway and involved in rheumatoid arthritis (RA) synovitis. This study was designed to decipher the molecular network linked to CLU expression in FLS (fibroblast-like synoviocytes) and evaluate the consequences of its low expression in conditions of TNF-α stimulation. FLS were transfected with siRNA for CLU or not and cultured for 24 and 48 h with TNF-α or not. Pan-genomic gene expression was assayed by DNA microarray. The gene network around CLU and gene interactions were analyzed with the Ingenuity Pathway Analysis software. Downregulation of CLU resulted in modification of the expression of genes known to be directly linked to CLU and for almost 5% of the tested genes (857 out of 17,225); the upregulation of a small group of gene (e.g., TIAM1) emphasizes the hypothetical role of CLU in the pseudotumoral characteristic of FLS. The comparison of gene expression with or without TNF stimulation allowed the classification of sampled with good concordance. Moreover, differential comparison showed that CLU downregulation in RA led to a profound modification of the TNF-α response as three sets of genes emerged: 497 genes modulated by siCLU transfection with TNF stimulation, 356 genes modified because of TNF stimulation only, and 484 genes modulated during TNF stimulation with CLU expression (e.g., IL-8 and Wnt signaling genes). Using a global two-way ANOVA we could identify a set of genes defining a molecular signature of TNF response directly influenced by CLU. These results (based on differential gene expression patterns) argue that CLU downregulation in FLS alters their aggressiveness in RA synovitis.

CRM1 protein-mediated regulation of nuclear clusterin (nCLU), an ionizing radiation-stimulated, Bax-dependent pro-death factor

Expression of the clusterin (CLU) gene results in the synthesis of a conventional secretory isoform set (pre- and mature secretory clusterin proteins, psCLU/sCLU), as well as another set of intracellular isoforms, appearing in the cytoplasm (pre-nuclear CLU, pnCLU) and in the nucleus as an ∼55-kDa mature nuclear clusterin (nCLU) form. These two isoform sets have opposing cell functions: pro-survival and pro-death, respectively. Although much is known about the regulation and function of sCLU as a pro-survival factor, the regulation and function of endogenous nCLU in cell death are relatively unexplored. Here, we show that depletion of endogenous nCLU protein using siRNA specific to its truncated mRNA increased clonogenic survival of ionizing radiation (IR)-exposed cells. nCLU-mediated apoptosis was Bax-dependent, and lethality correlated with accumulation of mature nCLU protein. nCLU accumulation was regulated by CRM1 because binding between CRM1 and nCLU proteins was significantly diminished by leptomycin B (LMB), and nuclear levels of nCLU protein were significantly enhanced by LMB and IR co-treatment. Moreover, LMB treatment significantly enhanced IR-induced nCLU-mediated cell death responses. Importantly, bax(-/-) and bax(-/-)/bak(-/-) double knock-out cells were resistant to nCLU-mediated cell death, whereas bak(-/-) or wild-type bax(+/+)/bak(+/+) cells were hypersensitive. The regulation of nCLU by CRM1 nuclear export/import may explain recent clinical results showing that highly malignant tumors have lost the ability to accumulate nCLU levels, thereby avoiding growth inhibition and cell death.

Proteasome inhibition drives HDAC6-dependent recruitment of tau to aggresomes

Lesions containing aggregated and hyperphosphorylated tau protein are characteristic of neurodegenerative tauopathies. We have developed a cellular model of pathological tau deposition and clearance by overexpressing wild type human tau in HEK293 cells. When proteasome activity is inhibited, HEK293/tau cells accumulate tau protein in structures that bear many of the hallmarks of aggresomes. These include recruitment of tau into large spherical inclusions, accumulation of the retrograde motor protein dynein at the centrosome, formation of an intermediate filament cage around inclusions, and clustering of mitochondria at the aggresome. Tau aggresomes form rapidly and can be cleared upon relief of proteasome inhibition. We observe recruitment of pathological misfolded phospho-tau species to aggresomes. Immunoblotting reveals accumulation of detergent insoluble aggregated tau species. Knockdown of histone deacetylase 6, a protein known to interact with tau, reveals a requirement for HDAC6 activity in tau aggresome formation. Direct observation of the accumulation and clearance of abnormal tau species will allow us to dissect the cellular and molecular mechanisms at work in clearing aggresomal tau and its similarity to disease relevant pathological tau clearance mechanisms.

Recruitment of the oncoprotein v-ErbA to aggresomes

Aggresome formation, a cellular response to misfolded protein aggregates, is linked to cancer and neurodegenerative disorders. Previously we showed that Gag-v-ErbA (v-ErbA), a retroviral variant of the thyroid hormone receptor (TRα1), accumulates in and sequesters TRα1 into cytoplasmic foci. Here, we show that foci represent v-ErbA targeting to aggresomes. v-ErbA colocalizes with aggresomal markers, proteasomes, hsp70, HDAC6, and mitochondria. Foci have hallmark characteristics of aggresomes: formation is microtubule-dependent, accelerated by proteasome inhibitors, and they disrupt intermediate filaments. Proteasome-mediated degradation is critical for clearance of v-ErbA and T(3)-dependent TRα1 clearance. Our studies highlight v-ErbA's complex mode of action: the oncoprotein is highly mobile and trafficks between the nucleus, cytoplasm, and aggresome, carrying out distinct activities within each compartment. Dynamic trafficking to aggresomes contributes to the dominant negative activity of v-ErbA and may be enhanced by the viral Gag sequence. These studies provide insight into novel modes of oncogenesis across multiple cellular compartments.

Extracellular chaperones

The maintenance of the levels and correct folding state of proteins (proteostasis) is a fundamental prerequisite for life. Life has evolved complex mechanisms to maintain proteostasis and many of these that operate inside cells are now well understood. The same cannot yet be said of corresponding processes in extracellular fluids of the human body, where inappropriate protein aggregation is known to underpin many serious diseases such as Alzheimer's disease, type II diabetes and prion diseases. Recent research has uncovered a growing family of abundant extracellular chaperones in body fluids which appear to selectively bind to exposed regions of hydrophobicity on misfolded proteins to inhibit their toxicity and prevent them from aggregating to form insoluble deposits. These extracellular chaperones are also implicated in clearing the soluble, stabilized misfolded proteins from body fluids via receptor-mediated endocytosis for subsequent lysosomal degradation. Recent work also raises the possibility that extracellular chaperones may play roles in modulating the immune response. Future work will better define the in vivo functions of extracellular chaperones in proteostasis and immunology and pave the way for the development of new treatments for serious diseases.

Extracellular Chaperones

The maintenance of the levels and correct folding state of proteins (proteostasis) is a fundamental prerequisite for life. Life has evolved complex mechanisms to maintain proteostasis and many of these that operate inside cells are now well understood. The same cannot yet be said of corresponding processes in extracellular fluids of the human body, where inappropriate protein aggregation is known to underpin many serious diseases such as Alzheimer's disease, type II diabetes and prion diseases. Recent research has uncovered a growing family of abundant extracellular chaperones in body fluids which appear to selectively bind to exposed regions of hydrophobicity on misfolded proteins to inhibit their toxicity and prevent them from aggregating to form insoluble deposits. These extracellular chaperones are also implicated in clearing the soluble, stabilized misfolded proteins from body fluids via receptor-mediated endocytosis for subsequent lysosomal degradation. Recent work also raises the possibility that extracellular chaperones may play roles in modulating the immune response. Future work will better define the in vivo functions of extracellular chaperones in proteostasis and immunology and pave the way for the development of new treatments for serious diseases.

Clusterin immunoexpression and its clinical significance in patients with non-small cell lung cancer

Clusterin is an enigmatic glycoprotein with a nearly ubiquitous tissue distribution. It plays important roles in various pathophysiological processes, including tissue remodeling, reproduction, lipid transport, complement regulation, and apoptosis. Clusterin appears to have two main isoforms that result from alternative splicing. The secreted and nuclear forms of clusterin have been reported to play different roles in human malignancies. The purpose of this study was to examine clusterin immunoexpression and its clinical significance in a group of Chinese patients with non-small cell lung cancer (NSCLC). Tissue samples from the primary tumors of 121 patients with completely resected NSCLC were obtained. Clusterin protein expression was evaluated by immunohistochemical staining with an antibody against all clusterin isoforms. Staining patterns were observed and graded based on intensity and density and were correlated with clinical and pathological data. Both cytoplasmic and nuclear clusterin immunostaining patterns were observed. Clusterin staining was observed only in the cytoplasm in 70 patients (57.9%), only in the nucleus in 27 patients (22.3%), and in both the cytoplasm and nucleus in 16 patients (13.2%). A significant association was observed between positive cytoplasmic clusterin expression and histologic type as indicated by adenocarcinomas that were more likely to have clusterin staining only in the cytoplasm. Clusterin immunostaining was neither associated with recurrence-free survival (RFS) nor overall survival of patients by univariate or multivariate analysis. For patients undergoing chemotherapy, those with only cytoplasmic clusterin staining had worse survival than other patients. In conclusion, both cytoplasmic and nuclear immunostaining patterns of clusterin were detected in the tumors of patients with NSCLC. Adenocarcinomas were more likely to have only cytoplasmic staining. The immunoexpression of clusterin was not associated with prognosis, and cytoplasm-only immunostaining of clusterin was inversely correlated with chemosensitivity in this group of patients.

Clusterin facilitates COMMD1 and I-kappaB degradation to enhance NF-kappaB activity in prostate cancer cells

Secretory clusterin (sCLU) is a stress-activated, cytoprotective chaperone that confers broad-spectrum cancer treatment resistance, and its targeted inhibitor (OGX-011) is currently in phase II trials for prostate, lung, and breast cancer. However, the molecular mechanisms by which sCLU inhibits treatment-induced apoptosis in prostate cancer remain incompletely defined. We report that sCLU increases NF-kappaB nuclear translocation and transcriptional activity by serving as a ubiquitin-binding protein that enhances COMMD1 and I-kappaB proteasomal degradation by interacting with members of the SCF-betaTrCP E3 ligase family. Knockdown of sCLU in prostate cancer cells stabilizes COMMD1 and I-kappaB, thereby sequestrating NF-kappaB in the cytoplasm and decreasing NF-kappaB transcriptional activity. Comparative microarray profiling of sCLU-overexpressing and sCLU-knockdown prostate cancer cells confirmed that the expression of many NF-kappaB-regulated genes positively correlates with sCLU levels. We propose that elevated levels of sCLU promote prostate cancer cell survival by facilitating degradation of COMMD1 and I-kappaB, thereby activating the canonical NF-kappaB pathway.

Chapter 9: Oxidative stress in malignant progression: The role of Clusterin, a sensitive cellular biosensor of free radicals

Clusterin/Apolipoprotein J (CLU) gene is expressed in most human tissues and encodes for two protein isoforms; a conventional heterodimeric secreted glycoprotein and a truncated nuclear form. CLU has been functionally implicated in several physiological processes as well as in many pathological conditions including ageing, diabetes, atherosclerosis, degenerative diseases, and tumorigenesis. A major link of all these, otherwise unrelated, diseases is that they are characterized by increased oxidative injury due to impaired balance between production and disposal of reactive oxygen or nitrogen species. Besides the aforementioned diseases, CLU gene is differentially regulated by a wide variety of stimuli which may also promote the production of reactive species including cytokines, interleukins, growth factors, heat shock, radiation, oxidants, and chemotherapeutic drugs. Although at low concentration reactive species may contribute to normal cell signaling and homeostasis, at increased amounts they promote genomic instability, chronic inflammation, lipid oxidation, and amorphous aggregation of target proteins predisposing thus cells for carcinogenesis or other age-related disorders. CLU seems to intervene to these processes due to its small heat-shock protein-like chaperone activity being demonstrated by its property to inhibit protein aggregation and precipitation, a main feature of oxidant injury. The combined presence of many potential regulatory elements in the CLU gene promoter, including a Heat-Shock Transcription Factor-1 and an Activator Protein-1 element, indicates that CLU gene is an extremely sensitive cellular biosensor of even minute alterations in the cellular oxidative load. This review focuses on CLU regulation by oxidative injury that is the common molecular link of most, if not all, pathological conditions where CLU has been functionally implicated.

Chapter 6: The chaperone action of Clusterin and its putative role in quality control of extracellular protein folding

The function(s) of clusterin may depend upon its topological location. A variety of intracellular "isoforms" of clusterin have been reported but further work is required to better define their identity. The secreted form of clusterin has a potent ability to inhibit both amorphous and amyloid protein aggregation. In the case of amorphous protein aggregation, clusterin forms stable, soluble high-molecular-weight complexes with misfolded client proteins. Clusterin expression is increased during many types of physiological and pathological stresses and is thought to function as an extracellular chaperone (EC). The pathology of a variety of serious human diseases is thought to arise as a consequence of the inappropriate aggregation of specific extracellular proteins (e.g., Abeta peptide in Alzheimer's disease and beta(2)-microglobulin in dialysis-related amyloidosis). We have proposed that together with other abundant ECs (e.g., haptoglobin and alpha(2)-macroglobulin), clusterin forms part of a previously unknown quality-control (QC) system for protein folding that mediates the recognition and disposal of extracellular misfolded proteins via receptor-mediated endocytosis and lysosomal degradation. Characterizing the mechanisms of this extracellular QC system will thus have major implications for our understanding of diseases of this type and may eventually lead to the development of new therapies.

Clusterin: a forgotten player in Alzheimer's disease

Clusterin, also known as apolipoprotein J, is a versatile chaperone molecule which contains several amphipathic and coiled-coil alpha-helices, typical characteristics of small heat shock proteins. In addition, clusterin has three large intrinsic disordered regions, so-called molten globule domains, which can stabilize stressed protein structures. Twenty years ago, it was demonstrated that the expression of clusterin was clearly increased in Alzheimer's disease (AD). Later it was observed that clusterin can bind amyloid-beta peptides and prevent their fibrillization. Clusterin is also involved in the clearance of amyloid-beta peptides and fibrils by binding to megalin receptors and enhancing their endocytosis within glial cells. Clusterin is a complement inhibitor and can suppress complement activation observed in AD. Clusterin is also present in lipoprotein particles and regulates cholesterol and lipid metabolism of brain which is disturbed in AD. Clusterin is a stress-induced chaperone which is normally secreted but in conditions of cellular stress, it can be transported to cytoplasm where it can bind to Bax protein and inhibit neuronal apoptosis. Clusterin can also bind to Smad2/3 proteins and potentiate the neuroprotective TGFbeta signaling. An alternative splicing can produce a variant isoform of clusterin which can be translocated to nuclei where it induces apoptosis. The role of nuclear clusterin in AD needs to be elucidated. We will review here the extensive literature linking clusterin to AD and examine the recent progress in clusterin research with the respect to AD pathology. Though clusterin can be viewed as a multipotent guardian of brain, it is unable to prevent the progressive neuropathology in chronic AD.

Transcriptional and posttranslational regulation of clusterin by the two main cellular proteolytic pathways

Clusterin/apolipoprotein J (CLU) is a secreted glycoprotein associated with many severe physiological disturbances that represent states of increased oxidative stress, such as aging, cancer, atherosclerosis, diabetes, and renal and neurodegenerative diseases. The aim of our work was to examine the effect of proteasome and lysosome inhibition on CLU expression and to determine whether those proteolytic pathways are implicated in CLU gene regulation and protein degradation. To this end we used two different model systems, namely the U-2 OS osteosarcoma cell line and the WI38 primary human embryonic lung fibroblasts. We report that proteasome inhibition promotes both heat-shock factor 1 (HSF-1)-dependent CLU gene expression induction and protein accumulation due to reduced degradation. In contrast, lysosome inhibition results in elevated levels of CLU protein but does not affect the CLU mRNA levels. We also provide direct evidence that both the intracellular precursor, psCLU, and the mature secreted, sCLU, isoforms constitute proteolytic substrates of the proteasome and the lysosome. Overall our findings indicate that CLU overexpression after proteasome inhibition relates to both positive gene transcriptional regulation by HSF-1 and posttranslational protein accumulation due to reduced proteasomal and lysosomal degradation.

Limbic structures are prone to age-related impairments in proteasome activity and neuronal ubiquitinated inclusions in SAMP10 mouse: a model of cerebral degeneration

AIMS

Neurodegenerative diseases are characterized by ubiquitinated inclusions in selective brain regions. Here we investigated whether the dysfunction of the ubiquitin proteasome system might be involved in the pathogenesis and regional selectivity of neuronal ubiquitinated inclusions using the SAMP10 strain of mouse, an inbred model of age-related cerebral degeneration.

METHODS

By comparing SAMP10 mice at various ages with SAMR1 and C57BL mice as normal brain ageing controls, we studied morphological features and distribution of inclusions. We measured tissue proteasome activity in different brain regions of mice at various ages by fluorogenic substrate assays. We induced inclusions in cultured neurones by inhibiting the proteasome and analysed changes in the dendritic morphology.

RESULTS

Inclusions were formed in association with lipofuscin in neuronal perikarya and occurred most frequently in the limbic-related forebrain structures. There were sparse inclusion-bearing neurones in the non-limbic forebrain. In aged SAMR1 and C57BL, there were far fewer inclusions in the limbic-related forebrain than in aged SAMP10. The proteasome activity in the limbic-related forebrain decreased much more rapidly and remarkably upon ageing (26% activity was detected in 17-month-old compared with 3-month-old mice) in SAMP10 than in SAMR1. The proteasome activity in the non-limbic forebrain did not change significantly with advancing age in either SAMP10 or SAMR1. Proteasomal inhibition enhanced the formation of ubiquitinated inclusions in cultured neurones. Neurones bearing inclusions had shortened neurites.

CONCLUSIONS

We propose that the regional selectivity of proteasomal impairment is causally related to the selectivity of inclusion formation and associated dendritic degeneration in neurones of ageing SAMP10 mice.

Multiple pathways regulating the anti-apoptotic protein clusterin in breast cancer

Cancer chemotherapy inhibits tumor growth, in part, by triggering apoptosis, and anti-apoptotic proteins reduce the effectiveness of chemotherapy. Clusterin, a chaperone-like protein that binds to apoptotic and DNA repair proteins, is induced by chemotherapy and promotes tumor cell survival. Histone deacetylase inhibitors (HDIs) such as sodium butyrate and suberoylanilide hydroxamic acid (SAHA) are pharmacological agents that induce differentiation and apoptosis in cancer cells by altering chromatin structure, and we have found that combinations of chemotherapeutic drugs such as doxorubicin and HDIs efficiently induce apoptosis, even though they paradoxically induce high levels of clusterin. The hyper-expressed form of clusterin localizes to mitochondria, inhibits cytochrome c release, and is inhibited by the proteasome. When HDIs are used as single agents, clusterin suppresses cytochrome c release and apoptosis. However, doxorubicin/HDI-induced apoptosis is not inhibited by clusterin, and clusterin-resistant apoptosis corresponds with markers of the extrinsic/receptor-mediated apoptotic pathway. Thus, chemotherapy-HDI combinations are capable of overcoming an innate anti-apoptotic pathway of tumor cells, suggesting that chemotherapy-HDI combinations have potential for treating advanced stage breast cancer.

Stress-Induced Retrotranslocation of Clusterin/ApoJ into the Cytosol

Clusterin is a usually secreted glycoprotein with chaperone properties. Recently, it has been suggested that clusterin isoforms reside in the nuclear and cytosolic compartments of human cell types, where they can influence various cellular programs including DNA repair, transcription and apoptosis. Several mechanisms have been proposed to explain this atypical location, including alternative transcription initiation and alternative splicing. However, none of these have been unequivocally established as occurring in live cells. Here we provide direct experimental evidence that in live intact cells, under certain stress conditions, clusterin can evade the secretion pathway and reach the cytosol. This was demonstrated using several complementary approaches. Flow cytometry and selective permeabilization of U251 cell membranes with digitonin allowed detection of cytosolic clusterin in stressed U251 cells. In addition, a stringent enzymatic assay reliant upon the exclusively cytosolic deubiquitinase enzymes confirmed that clusterin synthesized with its hydrophobic secretion signal sequence can reach the cytosol of U251 cells. The retrotranslocation of clusterin is likely to occur through a mechanism similar to the endoplasmic reticulum (ER)-associated protein degradation pathway and involves passage through the Golgi apparatus. We also report that the ER-associated ubiquitin ligase Hrd1/synoviolin can interact with, and ubiquitinate clusterin. The possible biological functions of these novel behaviours of clusterin are discussed.

Regulation of clusterin/apolipoprotein J, a functional homologue to the small heat shock proteins, by oxidative stress in ageing and age-related diseases

Clusterin/apolipoprotein J (CLU) gene has a nearly ubiquitous expression pattern in human tissues. The two main CLU protein isoforms in human cells include the conventional glycosylated secreted heterodimer (sCLU) and a truncated nuclear form (nCLU). CLU has been implicated in various physiological processes and in many severe physiological disturbance states including ageing, cancer progression, vascular damage, diabetes, kidney and neuron degeneration. Although unrelated in their etiology and clinical manifestation, these diseases represent states of increased oxidative stress, which in turn, promotes amorphous aggregation of target proteins, increased genomic instability and high rates of cellular death. Among the various properties attributed to CLU so far, those mostly investigated and invariably appreciated are its small heat shock proteins-like chaperone activity and its involvement in cell death regulation, which are both directly correlated to the main features of oxidant injury. Moreover, the presence of both a heat shock transcription factor-1 and an activator protein-1 element in the CLU gene promoter indicate that CLU gene can be an extremely sensitive biosensor to reactive oxygen species. This review emphasizes on CLU gene regulation by oxidative stress that is the common link between all pathological conditions where CLU has been implicated.

Characterization and functional consequences of underexpression of clusterin in rheumatoid arthritis

We previously compared by microarray analysis gene expression in rheumatoid arthritis (RA) and osteoarthritis (OA) tissues. Among the set of genes identified as a molecular signature of RA, clusterin (clu) was one of the most differentially expressed. In the present study we sought to assess the expression and the role of CLU (mRNA and protein) in the affected joints and in cultured fibroblast-like synoviocytes (FLS) and to determine its functional role. Quantitative RT-PCR, Northern blot, in situ hybridization, immunohistochemistry, and Western blot were used to specify and quantify the expression of CLU in ex vivo synovial tissue. In synovial tissue, the protein was predominantly expressed by synoviocytes and it was detected in synovial fluids. Both full-length and spliced isoform CLU mRNA levels of expression were lower in RA tissues compared with OA and healthy synovium. In synovium and in cultured FLS, the overexpression of CLU concerned all protein isoforms in OA whereas in RA, the intracellular forms of the protein were barely detectable. Transgenic overexpression of CLU in RA FLS promoted apoptosis within 24 h. We observed that CLU knockdown with small interfering RNA promoted IL-6 and IL-8 production. CLU interacted with phosphorylated IkappaBalpha. Differential expression of CLU by OA and RA FLS appeared to be an intrinsic property of the cells. Expression of intracellular isoforms of CLU is differentially regulated between OA and RA. We propose that in RA joints, high levels of extracellular CLU and low expression of intracellular CLU may enhance NF-kappaB activation and survival of the synoviocytes.

The leader sequence triggers and enhances several functions of clusterin and is instrumental in the progression of human prostate cancer in vivo and in vitro

OBJECTIVE

To investigate the role of the leader sequence (which during clusterin biosynthesis facilitates its proper post-translational processing and secretion) in the functional activities of clusterin, a ubiquitous secretory glycoprotein with many biological functions, reported to be pro-apoptotic and anti-apoptotic in target cells, but for which the dual mechanism remains unclear.

MATERIALS AND METHODS

We designed an expression vector starting from the second in-frame ATG on the full-length human clusterin cDNA that was capable of driving the expression of both the full-length and the truncated isoforms of clusterin. We established stable expression clones of the androgen-dependent prostate cancer line LNCaP expressing clusterin with and without the leader sequence. This induced expression provided an opportunity to evaluate both the in vivo and in vitro actions of clusterin expression.

RESULTS

The LNCaP cells expressing clusterin with the leader sequence resisted apoptosis induced by tumour necrosis factor (TNF)-alpha, but clones with no leader sequence were highly susceptible to TNF-alpha-induced apoptosis. Furthermore, in the absence of the leader sequence, the expressed clusterin had a molecular weight consistent with that of the predicted holoprotein (40 kDa), suggesting a compromised post-translational processing with diffuse distribution throughout the cytoplasm. However, cells transfected with the full-length vector expressed clusterin of 60 and 35 kDa variants, and located exclusively in the Golgi apparatus. In vivo, only the overexpression of the full-length clusterin is anti-apoptotic and stimulates the proliferation of tumour.

CONCLUSION

The leader sequence is important in determining the functions of clusterin, which include anti-apoptotic and anti-necrotic properties. The lack of the leader sequence allowed the incompletely processed clusterin to induce apoptosis in target cells; without the leader sequence, clusterin functions differently. Thus, the leader sequence is a trigger for many functions of clusterin in the progression of human prostate cancer cells.

Molecular morphology and toxicity of cytoplasmic prion protein aggregates in neuronal and non-neuronal cells

Recent studies have revealed that accumulation of prion protein (PrP) in the cytoplasm results in the production of aggregates that are insoluble in non-ionic detergents and partially resistant to proteinase K. Transgenic mice expressing PrP in the cytoplasm develop severe ataxia with cerebellar degeneration and gliosis, suggesting that cytoplasmic PrP may play a role in the pathogenesis of prion diseases. The mechanism of cytoplasmic PrP neurotoxicity is not known. In this report, we determined the molecular morphology of cytoplasmic PrP aggregates by immunofluorescence and electron microscopy, in neuronal and non-neuronal cells. Transient expression of cytoplasmic PrP produced juxtanuclear aggregates reminiscent of aggresomes in human embryonic kidney 293 cells, human neuroblastoma BE2-M17 cells and mouse neuroblastoma N2a cells. Time course studies revealed that discrete aggregates form first throughout the cytoplasm, and then coalesce to form an aggresome. Aggresomes containing cytoplasmic PrP were 1-5-microm inclusion bodies and were filled with electron-dense particles. Cytoplasmic PrP aggregates induced mitochondrial clustering, reorganization of intermediate filaments, prevented the secretion of wild-type PrP molecules and diverted these molecules to the cytoplasm. Cytoplasmic PrP decreased the viability of neuronal and non-neuronal cells. We conclude that any event leading to accumulation of PrP in the cytoplasm is likely to result in cell death.

Kinase activity is required for the toxic effects of mutant LRRK2/dardarin

Mutations in the LRRK2 gene, coding for dardarin, cause dominantly inherited Parkinson's disease (PD). Dardarin is a large protein, and mutations are found throughout the gene including the kinase domain. However, it is not clear if kinase activity is important for the damaging effects of pathogenic mutations. In this study, we noted two cellular phenotypes associated with mutant dardarin. First, pathogenic mutations increase the tendency of dardarin to form inclusion bodies. Secondly, neurons and neuronal cell lines undergo cell death after expression of mutant protein. Manipulating activity by replacing the kinase domain with a 'kinase-dead' version blocks inclusion body formation and strongly delays cell death. This predicts that kinase inhibitors will be useful therapeutic agents in patients with LRRK2 mutations and, perhaps, in sporadic PD. We also show that dardarin protein is expressed within human midbrain neurons and that C-terminal epitopes are also found in some Lewy bodies.

Up-regulation of the clusterin gene after proteotoxic stress: implication of HSF1-HSF2 heterocomplexes

Clusterin is a secreted protein chaperone up-regulated in several pathologies, including cancer and neurodegenerative diseases. The present study shows that accumulation of aberrant proteins, caused by the proteasome inhibitor MG132 or the incorporation of the amino acid analogue AZC (L-azetidine-2-carboxylic acid), increased both clusterin protein and mRNA levels in the human glial cell line U-251 MG. Consistently, MG132 treatment was capable of stimulating a 1.3 kb clusterin gene promoter. Promoter deletion and mutation studies revealed a critical MG132-responsive region between -218 and -106 bp, which contains a particular heat-shock element, named CLE for 'clusterin element'. Gel mobility-shift assays demonstrated that MG132 and AZC treatments induced the formation of a protein complex that bound to CLE. As shown by supershift and chromatin-immunoprecipitation experiments, CLE is bound by HSF1 (heat-shock factor 1) and HSF2 upon proteasome inhibition. Furthermore, co-immunoprecipitation assays indicated that these two transcription factors interact. Gel-filtration analyses revealed that the HSF1-HSF2 heterocomplexes bound to CLE after proteasome inhibition have the same apparent mass as HSF1 homotrimers after heat shock, suggesting that HSF1 and HSF2 could heterotrimerize. Therefore these studies indicate that the clusterin is a good candidate to be part of a cellular defence mechanism against neurodegenerative diseases associated with misfolded protein accumulation or decrease in proteasome activity.

Antisense oligodeoxynucleotide therapy targeting clusterin gene for prostate cancer: Vancouver experience from discovery to clinic

BACKGROUND

The objective of this study was to review our experience in the development of antisense (AS) oligodeoxynucleotide (ODN) therapy for prostate cancer targeting antiapoptotic gene, clusterin.

METHODS

We initially summarized our data demonstrating that clusterin could be an optimal therapeutic target for prostate cancer, then presented the process of developing AS ODN therapy using several preclinical animal models. Finally, the preliminary data of the recently completed phase I clinical trial using AS clusterin ODN as well as the future prospects of this therapy are discussed.

RESULTS

Expression of clusterin was highly up-regulated after androgen withdrawal and during progression to androgen-independence, but low or absent in untreated tissues in both prostate cancer animal model systems and human clinical specimens. Introduction of the clusterin gene into human prostate cancer cells confers resistance to several therapeutic stimuli, including androgen ablation, chemotherapy and radiation. AS ODN targeting the translation initiation site of the clusterin gene markedly inhibited clusterin expression in prostate cancer cells in a dose-dependent and sequence-specific manner. Systemic treatment with AS clusterin ODN enhanced the effects of several conventional therapies through the effective induction of apoptosis in prostate cancer xenograft models. Based on these findings, a phase I clinical trial was completed using AS clusterin ODN incorporating 2'-O-(2-methoxy)ethyl-gapmer backbone (OGX-011), showing up to 90% suppression of clusterin in prostate cancer.

CONCLUSIONS

The data described above identified clusterin as an antiapoptotic gene up-regulated in an adaptive cell survival manner following various cell death triggers that helps confer a phenotype resistant to therapeutic stimuli. Inhibition of clusterin expression using AS ODN technology enhances apoptosis induced by several conventional treatments, resulting in the delay of AI progression and improved survival. Clinical trials using AS ODN confirm potent suppression of clusterin expression and phase II studies will begin in early 2005.