Clusterin enhances proliferation of primary astrocytes through extracellular signal-regulated kinase activation

The Ins and Outs of Clusterin: Its Role in Cancer, Eye Diseases and Wound Healing

Clusterin (CLU) is a glycoprotein originally discovered in 1983 in ram testis fluid. Rapidly observed in other tissues, it was initially given various names based on its function in different tissues. In 1992, it was finally named CLU by consensus. Nearly omnipresent in human tissues, CLU is strongly expressed at fluid-tissue interfaces, including in the eye and in particular the cornea. Recent research has identified different forms of CLU, with the most prominent being a 75-80 kDa heterodimeric protein that is secreted. Another truncated version of CLU (55 kDa) is localized to the nucleus and exerts pro-apoptotic activities. CLU has been reported to be involved in various physiological processes such as sperm maturation, lipid transportation, complement inhibition and chaperone activity. CLU was also reported to exert important functions in tissue remodeling, cell-cell adhesion, cell-substratum interaction, cytoprotection, apoptotic cell death, cell proliferation and migration. Hence, this protein is sparking interest in tissue wound healing. Moreover, CLU gene expression is finely regulated by cytokines, growth factors and stress-inducing agents, leading to abnormally elevated levels of CLU in many states of cellular disturbance, including cancer and neurodegenerative conditions. In the eye, CLU expression has been reported as being severely increased in several pathologies, such as age-related macular degeneration and Fuch's corneal dystrophy, while it is depleted in others, such as pathologic keratinization. Nevertheless, the precise role of CLU in the development of ocular pathologies has yet to be deciphered. The question of whether CLU expression is influenced by these disorders or contributes to them remains open. In this article, we review the actual knowledge about CLU at both the protein and gene expression level in wound healing, and explore the possibility that CLU is a key factor in cancer and eye diseases. Understanding the expression and regulation of CLU could lead to the development of novel therapeutics for promoting wound healing.

The clusterin connectome: Emerging players in chondrocyte biology and putative exploratory biomarkers of osteoarthritis

Introduction

Clusterin is amoonlighting protein that hasmany functions. It is amultifunctional Q6 holdase chaperone glycoprotein that is present intracellularly and extracellularly in almost all bodily fluids. Clusterin is involved in lipid transport, cell differentiation, regulation of apoptosis, and clearance of cellular debris, and plays a protective role in ensuring cellular survival. However, the possible involvement of clusterin in arthritic disease remains unclear. Given the significant potential of clusterin as a biomarker of osteoarthritis (OA), a more detailed analysis of its complex network in an inflammatory environment, specifically in the context of OA, is required. Based on the molecular network of clusterin, this study aimed to identify interacting partners that could be developed into biomarker panels for OA.

Methods

The STRING database and Cytoscape were used to map and visualize the clusterin connectome. The Qiagen Ingenuity Pathway Analysis (IPA) software was used to analyze and study clusterinassociated signaling networks in OA. We also analyzed transcription factors known to modulate clusterin expression, which may be altered in OA.

Results

The top hits in the clusterin network were intracellular chaperones, aggregate-forming proteins, apoptosis regulators and complement proteins. Using a text-mining approach in Cytoscape, we identified additional interacting partners, including serum proteins, apolipoproteins, and heat shock proteins.

Discussion

Based on known interactions with proteins, we predicted potential novel components of the clusterin connectome in OA, including selenoprotein R, semaphorins, and meprins, which may be important for designing new prognostic or diagnostic biomarker panels.

Clusterin as a serum biomarker candidate contributes to the lung fibroblasts activation in chronic obstructive pulmonary disease

BACKGROUND

Fibrosis in the peripheral airways contributes to airflow limitation in patients with chronic obstructive pulmonary disease (COPD). However, the key proteins involved in its development are still poorly understood. Thus, we aimed to identify the differentially expressed proteins (DEPs) between smoker patients with and without COPD and elucidate the molecular mechanisms involved by investigating the effects of the identified biomarker candidate on lung fibroblasts.

METHODS

The potential DEPs were identified by isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomic analysis. The messenger RNA and protein levels of clusterin (CLU) in COPD patients and 12% cigarette smoke extract (CSE)-treated human bronchial epithelial cells were determined at the indicated time points. Furthermore, an in vitro COPD model was established via the administration of 8% CSE to normal human lung fibroblasts (NHLFs) at indicated time points. The effects of CSE treatment and CLU silencing on proliferation and activation of lung fibroblasts were analyzed.

RESULTS

A total of 144 DEPs were identified between COPD patients and normal smokers. The iTRAQ-based proteomics and bioinformatics analyses identified CLU as a serum biomarker candidate. We also discovered that CLU levels were significantly increased ( P   <  0.0001) in Global Initiative for Obstructive Lung Disease II, III, and IV patients and correlated ( P   <  0.0001) with forced expiratory volume in 1 s ( R  = -0.7705), residual volume (RV) ( R  = 0.6281), RV/total lung capacity ( R  = 0.5454), and computerized tomography emphysema ( R  = 0.7878). Similarly, CLU levels were significantly increased in CSE-treated cells at indicated time points ( P  < 0.0001). The CSE treatment significantly inhibited the proliferation, promoted the inflammatory response, differentiation of NHLFs, and collagen matrix deposition, and induced the apoptosis of NHLFs; however, these effects were partially reversed by CLU silencing.

CONCLUSION

Our findings suggest that CLU may play significant roles during airway fibrosis in COPD by regulating lung fibroblast activation.

Contribution of the Transcription Factors Sp1/Sp3 and AP-1 to Clusterin Gene Expression during Corneal Wound Healing of Tissue-Engineered Human Corneas

In order to reduce the need for donor corneas, understanding of corneal wound healing and development of an entirely tissue-engineered human cornea (hTECs) is of prime importance. In this study, we exploited the hTEC to determine how deep wound healing affects the transcriptional pattern of corneal epithelial cells through microarray analyses. We demonstrated that the gene encoding clusterin (CLU) has its expression dramatically repressed during closure of hTEC wounds. Western blot analyses confirmed a strong reduction in the expression of the clusterin isoforms after corneal damage and suggest that repression of CLU gene expression might be a prerequisite to hTEC wound closure. Transfection with segments from the human CLU gene promoter revealed the presence of three regulatory regions: a basal promoter and two more distal negative regulatory regions. The basal promoter bears DNA binding sites for very potent transcription factors (TFs): Activator Protein-1 (AP-1) and Specificity protein-1 and 3 (Sp1/Sp3). By exploiting electrophoretic mobility shift assays (EMSA), we demonstrated that AP-1 and Sp1/Sp3 have their DNA binding site overlapping with one another in the basal promoter of the CLU gene in hCECs. Interestingly, expression of both these TFs is reduced (at the protein level) during hTEC wound healing, thereby contributing to the extinction of CLU gene expression during that process. The results of this study contribute to a better understanding of the molecular mechanisms accounting for the repression of CLU gene expression during corneal wound healing.

Astrocytes in Alzheimer's Disease: Pathological Significance and Molecular Pathways

Astrocytes perform a wide variety of essential functions defining normal operation of the nervous system and are active contributors to the pathogenesis of neurodegenerative disorders such as Alzheimer's among others. Recent data provide compelling evidence that distinct astrocyte states are associated with specific stages of Alzheimer´s disease. The advent of transcriptomics technologies enables rapid progress in the characterisation of such pathological astrocyte states. In this review, we provide an overview of the origin, main functions, molecular and morphological features of astrocytes in physiological as well as pathological conditions related to Alzheimer´s disease. We will also explore the main roles of astrocytes in the pathogenesis of Alzheimer´s disease and summarize main transcriptional changes and altered molecular pathways observed in astrocytes during the course of the disease.

Clusterin overexpression protects against western diet-induced obesity and NAFLD

Obesity is a significant risk factor for various metabolic diseases and is closely related to non-alcoholic fatty liver disease (NAFLD) characterized by inflammation and oxidative stress. Clusterin is a multi-functional protein that is up-regulated in the pathogenesis of various metabolic diseases, including obesity and NAFLD. Our previous studies indicated that hepatocyte-specific overexpression of clusterin alleviates methionine choline-deficient (MCD) diet-induced non-alcoholic steatohepatitis (NASH) by activating nuclear factor erythroid 2-related factor 2 (Nrf2). Here we generated transgenic mice with whole-body clusterin overexpression (wCLU-tg) and investigated the role of clusterin in Western diet-induced obesity and NAFLD. We confirmed that obesity parameters and the spectrum of NAFLD of wCLU-tg mice were improved compared to wild type mice. Contrarily, clusterin deficiency deteriorated metabolic disruptions. We also found that clusterin activates target molecules for obesity and NAFLD, namely Nrf2 and AMPK, suggesting that clusterin protects against Western diet-induced obesity and NAFLD by activating Nrf2 and AMPK.

DNAJA4 deficiency augments hyperthermia-induced Clusterin and ERK activation: two critical protective factors of human keratinocytes from hyperthermia-induced injury

BACKGROUND

Hyperthermia upregulates DNAJA4, a member of heat shock proteins (HSPs) 40 family, in human keratinocytes and HPV-infected tissue. DNAJA4 deficiency enhances growth arrest induced by hyperthermia. Clusterin (CLU) and phosphorylated ERK (p-ERK) play a role in regulating cell proliferation and apoptosis, under environmental stress.

OBJECTIVES

To examine the downstream molecules and signalling pathways of DNAJA4 and assess their roles in cell cycle and apoptosis of keratinocytes in response to hyperthermia.

METHODS

Wild-type and DNAJA4-knockout (KO) HaCaT cells were exposed to either 44 °C (hyperthermia) or 37 °C (control) for 30 min. The expression levels of CLU and p-ERK were determined by RT-PCR and Western blotting. RNAi and PD98059 were used to inhibit the expression of CLU and p-ERK, respectively. Cell viability, cell cycle and apoptosis were analysed by MTS assay and flow cytometry. Fresh biopsy samples of human normal foreskin or condyloma acuminatum (CA) were utilized to examine the expression of CLU and p-ERK after ex vivo culture at 44 °C.

RESULTS

The expression of CLU and p-ERK was significantly increased by hyperthermia treatment at 44 °C in HaCaT cells, foreskin and HPV-infected tissues. In HaCaT cells subjected to hyperthermia, DNAJA4 deficiency further augmented the expression of CLU and p-ERK. CLU deficiency enhanced the p-ERK expression. Hyperthermia-induced CLU and p-ERK exerted protective roles mainly through inhibiting apoptosis and maintaining cell cycle, respectively.

CONCLUSIONS

In keratinocytes, CLU and p-ERK are induced by hyperthermia, an effect which can be further enhanced by DNAJA4 deficiency. CLU deficiency also increases p-ERK expression. Both CLU and p-ERK are critical protective factors of human keratinocytes from hyperthermia-induced injury.

Single-cell transcriptomic analysis of Alzheimer's disease

Alzheimer's disease is a pervasive neurodegenerative disorder, the molecular complexity of which remains poorly understood. Here, we analysed 80,660 single-nucleus transcriptomes from the prefrontal cortex of 48 individuals with varying degrees of Alzheimer's disease pathology. Across six major brain cell types, we identified transcriptionally distinct subpopulations, including those associated with pathology and characterized by regulators of myelination, inflammation, and neuron survival. The strongest disease-associated changes appeared early in pathological progression and were highly cell-type specific, whereas genes upregulated at late stages were common across cell types and primarily involved in the global stress response. Notably, we found that female cells were overrepresented in disease-associated subpopulations, and that transcriptional responses were substantially different between sexes in several cell types, including oligodendrocytes. Overall, myelination-related processes were recurrently perturbed in multiple cell types, suggesting that myelination has a key role in Alzheimer's disease pathophysiology. Our single-cell transcriptomic resource provides a blueprint for interrogating the molecular and cellular basis of Alzheimer's disease.

Diverse functions of clusterin promote and protect against the development of pulmonary fibrosis

Pulmonary fibrosis is a progressive scarring disorder of the lung with dismal prognosis and no curative therapy. Clusterin, an extracellular chaperone and regulator of cell functions, is reduced in bronchoalveolar lavage fluid of patients with pulmonary fibrosis. However, its distribution and role in normal and fibrotic human lung are incompletely characterized. Immunohistochemical localization of clusterin revealed strong staining associated with fibroblasts in control lung and morphologically normal areas of fibrotic lung but weak or undetectable staining in fibrotic regions and particularly fibroblastic foci. Clusterin also co-localized with elastin in vessel walls and additionally with amorphous elastin deposits in fibrotic lung. Analysis of primary lung fibroblast isolates in vitro confirmed the down-regulation of clusterin expression in fibrotic compared with control lung fibroblasts and further demonstrated that TGF-β1 is capable of down-regulating fibroblast clusterin expression. shRNA-mediated down-regulation of clusterin did not affect TGF-β1-induced fibroblast-myofibroblast differentiation but inhibited fibroblast proliferative responses and sensitized to apoptosis. Down-regulation of clusterin in fibrotic lung fibroblasts at least partly due to increased TGF-β1 may therefore represent an appropriate but insufficient response to limit fibroproliferation. Reduced expression of clusterin in the lung may also limit its extracellular chaperoning activity contributing to dysregulated deposition of extracellular matrix proteins.

Secretory clusterin is upregulated in rats with pulmonary arterial hypertension induced by systemic-to-pulmonary shunts and exerts important roles in pulmonary artery smooth muscle cells

AIM

Phenotype modification of pulmonary artery smooth muscle cells (PASMCs) (excessive proliferation, migration and impaired apoptosis) plays central roles in pulmonary vascular remodelling of pulmonary arterial hypertension (PAH); however, the potential mechanism and contributing factors involved in the phenotype alteration in PASMCs are still not completely elucidated. This study attempted to investigate the expression pattern of secretory clusterin (sCLU), a prosurvival protein, in systemic-to-pulmonary shunt-induced PAH rats and the potential roles of sCLU in pulmonary vascular remodelling.

METHODS

An original rat model of systemic-to-pulmonary shunt-induced PAH was established by combined surgery as we previously reported. Lung tissues were harvested at specific time points for real-time polymerase chain reaction, Western blot and immunohistochemisty analysis; meanwhile, plasma was collected for enzyme-linked immunosorbent assay. Cell culture experiments were performed using cultured human PASMCs (HPASMCs).

RESULTS

Expression of sCLU was significantly increased in lungs exposed to systemic-to-pulmonary shunt. Moreover, plasma sCLU levels were markedly elevated with the progression of PAH in rats and also presented a positive correlation with pulmonary hemodynamic indices. In vitro cell culture assay indicated that sCLU expression and secretion increased with the phenotype modification of HPASMCs; furthermore, sCLU promoted HPASMCs proliferation, migration and apoptosis resistance, at least in part, via Erk1/2 and Akt signalling pathways.

CONCLUSION

These results demonstrate that sCLU is functionally an important phenotype modulator of PASMCs, and its upregulation in lung tissues may exert a deteriorative role in pulmonary vascular remodelling.

Exposure to acoustic stimuli promotes the development and differentiation of neural stem cells from the cochlear nuclei through the clusterin pathway

Stem cell therapy has attracted widespread attention for a number of diseases. Recently, neural stem cells (NSCs) from the cochlear nuclei have been identified, indicating a potential direction for the treatment of sensorineural hearing loss. Acoustic stimuli play an important role in the development of the auditory system. In this study, we aimed to determine whether acoustic stimuli induce NSC development and differentiation through the upregulation of clusterin (CLU) in NSCs isolated from the cochlear nuclei. To further clarify the underlying mechanisms involved in the development and differentiation of NSCs exposed to acoustic stimuli, we successfully constructed animal models in which was CLU silenced by an intraperitoneal injection of shRNA targeting CLI. As expected, the NSCs from rats treated with LV-CLU shRNA exhibited a lower proliferation ratio when exposed to an augmented acoustic environment (AAE). Furthermore, the inhibition of cell apoptosis induced by exposure to AAE was abrogated after silencing the expression of the CLU gene. During the differentiation of acoustic stimuli-exposed stem cells into neurons, the number of astrocytes was significantly reduced, as evidenced by the expression of the cell markers, microtubule associated protein-2 (MAP-2) and glial fibrillary acidic protein (GFAP), which was markedly inhibited when the CLU gene was silenced. Our results indicate that acoustic stimuli may induce the development and differentiation of NSCs from the cochlear nucleus mainly through the CLU pathway. Our study suggests that CLU may be a novel target for the treatment of sensorineural hearing loss.

Promotion of cell proliferation by clusterin in the renal tissue repair phase after ischemia-reperfusion injury

Renal repair begins soon after the kidney suffers ischemia-reperfusion injury (IRI); however, its molecular pathways are not fully understood. Clusterin (Clu) is a chaperone protein with cytoprotective functions in renal IRI. The aim of this study was to investigate the role of Clu in renal repair after IRI. IRI was induced in the left kidneys of wild-type (WT) C57BL/6J (B6) vs. Clu knockout (KO) B6 mice by clamping the renal pedicles for 28–45 min at the body temperature of 32°C. The renal repair was assessed by histology and confirmed by renal function. Gene expression was examined using PCR array. Here, we show that following IRI, renal tubular damage and Clu expression in WT kidneys were induced at day 1, reached the maximum at day 3, and significantly diminished at day 7 along with normal function, whereas the tubular damage in Clu KO kidneys steadily increased from initiation of insult to the end of the experiment, when renal failure occurred. Renal repair in WT kidneys was positively correlated with an increase in Ki67+ proliferative tubular cells and survival from IRI. The functions of Clu in renal repair and renal tubular cell proliferation in cultures were associated with upregulation of a panel of genes that could positively regulate cell cycle progression and DNA damage repair, which might promote cell proliferation but not involve cell migration. In conclusion, these data suggest that Clu is required for renal tissue regeneration in the kidney repair phase after IRI, which is associated with promotion of tubular cell proliferation.

Clusterin plays an important role in clear renal cell cancer metastasis

OBJECTIVE

Clusterin (CLU) is implicated in regulating clear renal cell carcinoma (CRCC) progression and metastasis, yet the mechanisms are not elucidated. In the present study, we explored the potential role of CLU in CRCC metastasis.

METHODS

Levels of CLU mRNA and CLU protein were measured by RT-PCR and immunohistochemistry analysis in 22 CRCC with metastasis and 22 without metastasis and 22 samples of normal kidney tissue. After CLU silencing and re-expression, the migration and invasion in vitro and in vivo of Caki-2 cells were determined by wound healing assay, transwell migration assay and pulmonary nodule assay, respectively. The expression of pERK1/2 and MMP-9 were detected by RT-PCR and Western blot assay.

RESULTS

We found a significant increase of CLU and CLU mRNA expression in CRCC, and the expression of CLU is strongly correlated in patients with metastatic disease. We discovered that CLU-rich Caki-2 cells displayed higher invasive ability which prompted us to investigate if CLU silencing could reduce the migration and invasion in Caki-2 cells. Compared with the vector-transfected cells, CLU knocked-down (CLUi) cells showed reduced migration and invasion in vitro, as well as decreased metastatic potential in experimental metastasis. Re-expression of CLU in CLUi cells restored the invasive phenotypes. We found that MMP-9 was downregulated in CLUi cells. We also discovered that levels of activated ERK1/2 correlated with the rich expression of CLU and MMP-9.

CONCLUSION

Our data suggest that CLU may regulate aggressive behavior of human CRCC cells through modulating ERK1/2 signaling and MMP-9 expression.

Increased Production of Clusterin in Biopsies of Repair Tissue following Autologous Chondrocyte Implantation

OBJECTIVE

To characterize the immunolocalization of clusterin in the repair cartilage of patients having undergone autologous chondrocyte implantation (ACI) and evaluate correlation to clinical outcome.

DESIGN

Full-depth core biopsies of repair tissue were obtained from 38 patients who had undergone ACI at an average of 18 ± 13 months previously (range 8-67 months). The biopsies were snap frozen, cryosectioned, and clusterin production immunolocalized using a specific monoclonal clusterin antibody and compared with normal and osteoarthritic cartilage. Clinical outcome was assessed from patients preoperatively, at the time of biopsy, and annually postoperatively.

RESULTS

Intensity of immunostaining for clusterin decreased with age in healthy cartilage tissue. Clusterin was detected to a variable degree in 37 of the 38 ACI cartilage biopsies, in single and clustered chondrocytes, in the pericellular capsule and the cartilage extracellular matrix, as well as the osteocytes and osteoid within the bone. Chondrocytes in hyaline repair tissue were significantly more immunopositive than those in fibrocartilage repair tissue. Clinical outcome improved significantly post-ACI, but did not correlate with the presence of clusterin in the repair tissue.

CONCLUSIONS

These results demonstrate the presence of clusterin in actively repairing human cartilage and indicate a different distribution of clusterin in this tissue compared to normal cartilage. Variability in clusterin staining in the repair tissue could indicate different states of chondrogenic differentiation. The clinical significance of clusterin within repair tissue is difficult to assess, although the ideal functioning repair tissue morphology should resemble that of healthy adult cartilage.

Unglycosylated clusterin variant accumulates in the endoplasmic reticulum and induces cytotoxicity

Clusterin is a stress-responsive and highly glycosylated secretory protein that plays cytoprotective role in most body fluids. In addition to extracellular clusterin, several intracellular clusterin variants that are rather cytotoxic have been recently uncovered under diverse pathological conditions. Although these variants revealed heterogeneity in their glycan modification, its significance in many diseases remains to be validated. Here, we found that clusterin is differentially metabolized by two well-characterized ER stress inducers. Thapsigargin induced retrotranslocation and rapid degradation of clusterin from the endoplasmic reticulum, whereas tunicamycin failed to degrade but rather retained clusterin in the endoplasmic reticulum. Important sorting determinant for these processes proved to be N-glycan moieties that are required for the prevention of terminal misfolding and aggregation of clusterin in the endoplasmic reticulum. This study provides a mechanistic insight into the generation of noble cytotoxic variant of intracellular clusterin and an idea about molecular pathogenesis of diseases associated with chronic endoplasmic reticulum stress, such as neurodegeneration.

Stable knockdown of clusterin by vectorbased RNA interference in a human breast cancer cell line inhibits tumour cell invasion and metastasis

OBJECTIVE

Overexpression of the clusterin (CLU) gene occurs in breast cancer and is associated with lymph node metastasis. The present study explored the effect of CLU silencing on invasion and metastasis, and the relationship between CLU expression and the extracellular signal-regulated kinase (ERK) / matrix metalloproteinase-9 (MMP) signalling pathway in human breast cancer cells.

METHODS

A pcDNA3.1-based RNA interference approach was used to knockdown the CLU gene in MDA-231 cells (MDA-231-CLUi); control MDA-231 cells were transfected with an empty vector (MDA-231-Vec). Reverse transcription-polymerase chain reaction was used to assess CLU and MMP-9 mRNA levels, and Western blotting was used to analyse CLU, MMP-9 and ERK protein levels. Metastatic potential was evaluated using in vitro and in vivo models of invasion and metastasis.

RESULTS

Compared with MDA-231-Vec cells, the MDA-231-CLUi cells demonstrated reduced migration and invasion in vitro and decreased metastatic potential in vivo. Reintroduction and reexpression of the CLU gene into the MDA-231-CLUi cells restored the invasive phenotype. MMP-9 mRNA and protein levels were reduced in MDA-231-CLUi cells, and there was a correlation between activated ERK and CLU and MMP-9 protein levels.

CONCLUSION

CLU may regulate the aggressive behaviour of human breast cancer cells through modulation of ERK signalling and MMP9 expression.

Knockdown of clusterin sensitizes pancreatic cancer cells to gemcitabine chemotherapy by ERK1/2 inactivation

OBJECTIVES

To study the hypothesis that gemcitabine treatment augments the chemoresistance to gemcitabine by clusterin (sCLU) upregulation. Clusterin inhibition could augment the chemosensitivity of human pancreatic cancer cells by inhibition of clusterin-dependent pERK1/2 activation.

METHODS

Clusterin was silenced by serial concentration of OGX-011 transfection in pancreatic cancer MIAPaCa-2 and BxPC-3 cell lines, then treated with serial concentration of gemcitabine. After the cells were treated with OGX-011 for 8 h, the cells were then treated with 5 μM ERK inhibitor PD98059 for 18 h or transfected with a wt-pERK-expressing plasmid into these cells for 24 h, after which the cells were treated with 1.0 uM gemcitabine for 24-72 h. Cell proliferation was determined by MTT. Apoptosis was quantified by flow cytometry,.sCLU and pERK1/2 production was analyzed by western blot, and sCLU mRNA was analyzed by RT-PCR. Xenograft of established tumors was used to evaluate primary tumor growth and apoptosis after treatment with gemcitabine alone or in combination with OGX-011. Phosphorylated ERK1/2 and sCLU levels in tumor tissues were measured by TUNEL analysis.

RESULTS

As detected by MTT and FACS assay, a combination of gemcitabine + OGX-011 reflected the chemotherapeutic sensitivity and increased the gemcitabine -induced apoptosis in MIAPaCa-2 and BxPC-3 cells. Western blotting and RT-PCR analysis revealed that the expression of clusterin was higher in gemcitabine -resistant MIAPaCa-2 cells, however, decreased significantly after pretreatment with OGX-011. Furthermore, the OGX-011 or combination of gemcitabine + OGX-011 decreased the gemcitabine -induced activation of pERK1/2. wt-pERK-re-expression decreased OGX-011+ gemcitabine -induced apoptosis. Finally, OGX-011 in combination with gemcitabine substantially decreased the in vivo tumor growth and promoted apoptosis. Taken together, clusterin confers gmcitabine resistance in pancreatic cancer cells.

CONCLUSIONS

Knockdown of clusterin by OGX-011 transfection sensitizes pancreatic cancer cells to gemcitabine by inhibition of gemcitabine -induced clusterin-pERK1/2 activation.

Lipid metabolism and glial lipoproteins in the central nervous system

Lipoproteins in the central nervous system (CNS) are not incorporated from the blood but are formed mainly by glial cells within the CNS. In addition, cholesterol in the CNS is synthesized endogenously because the blood-brain barrier segregates the CNS from the peripheral circulation. Apolipoprotein (apo) E is a major apo in the CNS. In normal condition, apo E is secreted from glia, mainly from astrocytes, and forms cholesterol-rich lipoproteins by ATP-binding cassette transporters. Subsequently, apo E-containing glial lipoproteins supply cholesterol and other components to neurons via a receptor-mediated process. Recent findings demonstrated that receptors of the low density lipoprotein (LDL) receptor family not only internalize lipoproteins into the cells but also, like signaling receptors, transduce signals upon binding the ligands. In this review, the regulation of lipid homeostasis will be discussed as well as roles of lipoproteins and functions of receptors of LDL receptor family in the CNS. Furthermore, the relation between lipid metabolism and Alzheimer's disease (AD) is discussed.

Nitric oxide counteracts the hyperoxia-induced proliferation and proinflammatory responses of mouse astrocytes

Preclinical studies in the premature baboon evaluating the efficacy and potential toxicity of inhaled nitric oxide indicated a significant effect on astrocyte area density, suggesting phenotypic and functional changes in astrocytes upon exposure to nitric oxide. However, the effects of nitric oxide and oxygen, the two major therapeutic gases utilized in neonatal intensive care, on astrocyte morphology and function remain vastly unknown. Herein, we report that exposure of mouse neonatal cortical astrocytes to hyperoxia results in a proinflammatory phenotype and increase in proliferation without significant changes in cellular morphology or levels of intermediate filament proteins. The proinflammatory phenotype was evident by a significant increase in cellular levels of cyclooxygenase-2 and a concomitant increase in prostaglandin E(2) secretion, a decline in the intracellular and secreted levels of apolipoprotein E, and a significant increase in the intracellular levels of clusterin. This proinflammatory phenotype was not evident upon simultaneous exposure to hyperoxia and nitric oxide. These results suggest that exposure to nitric oxide in the setting of hyperoxia confers unrecognized beneficial effects by suppressing astrocytic inflammation.

Clusterin protects H9c2 cardiomyocytes from oxidative stress-induced apoptosis via Akt/GSK-3β signaling pathway

Clusterin is a secretory glycoprotein, which is highly up-regulated in a variety of normal and injury tissues undergoing apoptosis including infarct region of the myocardium. Here, we report that clusterin protects H9c2 cardiomyocytes from H2O2-induced apoptosis by triggering the activation of Akt and GSK-3β. Treatment with H2O2 induces apoptosis of H9c2 cells by promoting caspase cleavage and cytochrome c release from mitochondria. However, co-treatment with clusterin reverses the induction of apoptotic signaling by H2O2, thereby recovers cell viability. The protective effect of clusterin on H2O2-induced apoptosis is impaired by PI3K inhibitor LY294002, which effectively suppresses clusterin-induced activation of Akt and GSK-3β. In addition, the protective effect of clusterin is independent on its receptor megalin, because inhibition of megalin has no effect on clusterin-mediated Akt/GSK-3β phosphoylation and H9c2 cell viability. Collectively, these results suggest that clusterin has a role protecting cardiomyocytes from oxidative stress and the Akt/GSK-3β signaling mediates anti-apoptotic effect of clusterin.

Essential role of clusterin in pancreas regeneration

Based on our previous observations that clusterin induction accompanies pancreas regeneration in the rat, we sought to determine if regeneration might be impaired in mice that lacked clusterin. We studied the impact of absent clusterin on morphogenic and functional features of regenerating pancreas. Clusterin induction was accompanied in the regenerating pancreas by a robust development of new lobules with ductules, acini, and endocrine islets in wild type after partial pancreatectomy. In clusterin knock-out mice, however, pancreatectomy resulted in a poor formation of regenerating lobule. In particular, regeneration of beta-cells was also significantly reduced and was associated with persistent hyperglycemia. Duct cells obtained from pancreatectomized clusterin knock-out mice exhibited impaired beta-cell formation in vitro; this was restored by administration of exogenous clusterin. We suggest that clusterin plays a critical role to promote both exocrine and endocrine regeneration following pancreas injury, as well as for in vitro beta-cell regeneration.

Clusterin synergizes with IL-2 for the expansion and IFN-γ production of natural killer cells

CLU is a secreted, multifunctional protein implicated in several immunologic and pathologic conditions. As the level of serum CLU was shown to be elevated during inflammatory responses, we questioned if CLU might interact with circulating lymphocytes leading to functional consequences. To assess this possibility directly, mouse splenocytes and purified NK cells were cultured with varying dose of CLU, and its effect on cell proliferation was examined. Our data showed that CLU up-regulated DNA synthesis and expansion of NK cells significantly in response to a suboptimal, but not maximal, dose of IL-2, and CLU alone did not exhibit such effects. This CLU-mediated synergy required the co-presence of CLU at the onset of IL-2 stimulation and needed a continuous presence during the rest of the culture. Importantly, NK cells stimulated with CLU showed increased formation of cell clusters and a CD69 activation receptor, representing a higher cellular activation status compared with those from the control group. Furthermore, these NK cells displayed elevated IFN-γ production upon RMA/S tumor target exposures, implying that CLU regulates not only NK cell expansion but also effector function of NK cells. Collectively, our data present a previously unrecognized function of CLU as a novel regulator of NK cells via providing costimulation required for cell proliferation and IFN-γ secretion. Therefore, the role of CLU on NK cells should be taken into consideration for the previously observed, diverse functions of CLU in chronic inflammatory and autoimmune conditions.

Clusterin as a predictor for chemoradiotherapy sensitivity and patient survival in esophageal squamous cell carcinoma

Clusterin (CLU) is frequently overexpressed and correlates closely with chemotherapy and radiotherapy resistance and poor prognosis in many human cancers. However, the significance of CLU expression in chemoradiotherapy (CRT) sensitivity and its effect on the prognosis of esophageal squamous cell carcinoma (ESCC) are still unknown. In the present study, we used the methods of immunohistochemistry and terminal deoxyuridine triphosphate nick-end labeling assay to examine the expression status of CLU and apoptotic index in 110 pretreated biopsy specimens of ESCC patients treated with definitive CRT. High expression of CLU was observed in 42.7% of epithelium and 50.0% of stroma in ESCC. A significant association of high CLU stromal expression with large tumor size (P = 0.012) and locoregional progression (P = 0.001) was observed, and high epithelial expression of CLU showed a significant correlation with the lack of complete response (P = 0.028) and low apoptotic index (P = 0.001). Univariate analysis revealed that high CLU stromal expression was associated with poor locoregional progression-free survival, distant progression-free survival, and overall survival. Furthermore, ESCC patients with high CLU expression in both epithelium and stroma have the shortest survival time among the subgroups of different CLU expression status. In multivariate analysis, CLU stromal expression was evaluated as an independent prognostic factor for locoregional progression-free survival, distant progression-free survival, and overall survival. These findings suggest an important role for CLU, especially in stroma, in ESCC progression, and that high CLU epithelial expression might be a promising predictor of ESCC resistance to CRT.

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.

The Role of Clusterin inIn VitroIschemia of Human Retinal Endothelial Cells

Clusterin has been reported to be up-regulated in diverse pathophysiological stresses, but its role is controversial. In this study, we investigated the role of clusterin under in vitro ischemia of human retinal endothelial cells (HRECs). When HRECs were exposed to oxygen-glucose deprivation (OGD), clusterin expression increased, whereas von Willebrand factor (vWF), occludin, and zonula occludens (ZO-1) markedly decreased. Interestingly, loss of tight junction proteins and death of HRECs in OGD conditions were restored by clusterin treatment. Our results suggest that the enhanced clusterin in OGD conditions may play a protective role against ischemia-induced tight junction protein loss and HRECs death.

Epidermal growth factor receptor is involved in clusterin-induced astrocyte proliferation

We previously reported that clusterin enhances astrocyte proliferation and extracellular signal-regulated kinase (ERK) activity. It, however, remains largely unknown how clusterin promotes cell growth. Here, we investigate the signaling pathway and related molecules underlying astrocyte proliferation by clusterin. Exogenous clusterin stimulates Ras-dependent Raf-1/mitogen-activated protein kinase kinase (MEK)/ERK activation. Clusterin-induced astrocyte proliferation and ERK1/2 phosphorylation were abrogated by either AG1478 (an inhibitor of epidermal growth factor receptor, EGFR) or EGFR small interfering RNA. Furthermore, clusterin treatment provoked tyrosine phosphorylation of EGFR (pY(1173)), which was also blocked by AG1478. These results suggest that clusterin requires EGFR activation to deliver its mitogenic signal through the Ras/Raf-1/MEK/ERK signaling cascade in astrocytes.

Clusterin increases post-ischemic damages in organotypic hippocampal slice cultures

Clusterin or apolipoprotein J is a heterodimeric glycoprotein which is known to be increased during tissue involution in response to hormonal changes or injury and under circumstances leading to apoptosis. Previous studies in wild-type (WT) and clusterin-null (Clu-/-) mice indicated a protective role of clusterin over-expression in astrocytes lasting up to 90 days post-ischemia. However, in in vitro and in vivo models of neonatal hypoxia-ischemia, clusterin exacerbates necrotic cell death. We developed recombinant forms of clusterin and examined their effect on propidium iodide uptake, neuronal and synaptic markers as well as electrophysiological recordings in hippocampal slice cultures from Clu-/- and WT mice subjected to oxygen-glucose deprivation (OGD). WT mice displayed a marked up-regulation of clusterin associated with electrophysiological deficits and dramatic increase of propidium iodide uptake 5 days post-OGD. Immunocytochemical and western blot analyses revealed a substantial decrease of neuronal nuclei and synaptophysin immunoreactivity that predominated in WT mice. These findings contrasted with the relative post-OGD resistance of Clu-/- mice. The addition of biologically active recombinant forms of human clusterin for 24 h post-OGD led to the abolishment of the ischemic tolerance in Clu-/- slices. This deleterious effect of clusterin was reverted by the concomitant administration of the NMDA receptor antagonist, d-2-amino-5-phosphonopentanoate. The present data indicate that in an in vitro model of ischemia characterized by the predominance of NMDA-mediated cell death, clusterin exerts a negative effect on the structural integrity and functionality of hippocampal neurons.

The role of clusterin in retinal development and free radical damage

AIM

To assess the role of clusterin in retinal vascular development and in free radical damage in vivo and in vitro.

METHODS

The expression of clusterin, von Willebrand factor (vWF), flk-1, heat shock protein 27 (Hsp27) and heat shock protein 70 (Hsp70) was examined in the retinas of developing mice and oxygen-induced retinopathy (OIR) mice by immunofluorescence staining and western blot analysis. Hydrogen peroxide (H(2)O(2))-pretreated human retinal endothelial cells (HREC) and astrocytes were cultured in the presence or absence of exogenous clusterin, and then the cell viability was measured using the MTT assay and DAPI staining.

RESULTS

Clusterin was expressed mainly in the inner retina and co-localised with vWF, an endothelial cell marker. During the mouse developmental process, clusterin expression was decreased, which was similar to the expression of flk-1, vWF and Hsp27. Furthermore, in the OIR model, clusterin expression changed in a similar way to both vWF and Hsp27. Under hypoxic conditions, clusterin expression increased in HREC and astrocytes. In H(2)O(2)-pretreated HREC and astrocytes, clusterin protected against apoptotic cell death.

CONCLUSIONS

These results suggest that clusterin is associated with protection from apoptotic retinal cell death in retinal development and in free radical damage.