Clusterin expression in differentiating smooth muscle cells

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.

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.

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.

Induction of clusterin expression by neuronal cell death in Zebrafish

Clusterin, a protein associated with multiple functions, is expressed in a wide variety of mammalian tissues. Although clusterin is known to be involved in neurodegenerative diseases, ageing, and tumorigenesis, a detailed analysis of the consequences of gain- or loss-of-function approaches has yet to be performed to understand the underlying mechanisms of clusterin functions. Since clusterin levels change in neurological diseases, it is likely that clusterin contributes to cell death and degeneration in general. Zebrafish was investigated as a model system to study human diseases. During development, zebrafish clusterin was expressed in the notochord and nervous system. Embryonic overexpression of clusterin by mRNA microinjection did not affect axis formation, whereas its knock-down by anti-sense morpholino treatment resulted in neuronal cell death. To analyze the function of clusterin in neurodegeneration, a transgenic zebrafish was investigated, in which nitroreductase expression is regulated under the control of a neuron-specific huC promoter which is active between the stages of early neuronal precursors and mature neurons. Nitroreductase turns metronidazole into a cytotoxic agent that induces cell death within 12 h. After metronidazole treatment, transgenic zebrafish showed neuron-specific cell death. Interestingly, we also observed a dramatic induction of clusterin expression in the brain and spinal cord in these fish, suggesting a direct or indirect role of clusterin in neuronal cell death and thus, more generally, in neurodegeneration.

The physiological roles of apolipoprotein J/clusterin in metabolic and cardiovascular diseases

Several isoforms of apolipoprotein J/clusterin (CLU) are encoded from a single gene located on chromosome 8 in humans. These isoforms are ubiquitously expressed in the tissues, and have been implicated in aging, neurodegenerative disorders, cancer progression, and metabolic/cardiovascular diseases including dyslipidemia, diabetes, atherosclerosis and myocardial infarction. The conventional secreted form of CLU (sCLU) is thought to be a component of high density lipoprotein-cholesterol. sCLU functions as a chaperone for misfolded proteins and it is thought to promote survival by reducing oxidative stress. Nuclear CLU, a truncated CLU formed by alternative splicing, is responsible for promoting apoptosis via a Bax-dependent pathway. There are putative regulatory sites in the promoter regions of CLU, which are occupied by transcription factors such as transforming growth factor (TGF)-β inhibitory element, activator protein-1, CLU-specific elements, and carbohydrate response element. However, the molecular mechanisms underlying the distinct roles of CLU in a variety of conditions remain unclear. Although the function of CLU in cancer or neurological disease has been studied intensively for three decades, physiological roles of CLU seem unexplored in the cardiovascular system and metabolic diseases. In this review, we will discuss general characteristics and regulations of CLU based on previous literature and assess the recent findings associated with its physiological roles in different tissues including the vasculature, heart, liver, kidney, adipose tissue, and brain.

Toll-like receptor 3 mediates expression of clusterin/apolipoprotein J in vascular smooth muscle cells stimulated with RNA released from necrotic cells

Clusterin/Apolipoprotein J is a protein that is upregulated in a broad spectrum of diverse pathological processes. The predominant form is a secreted glycoprotein (sCLU) with cytoprotective and anti-inflammatory properties which shows enhanced expression in vascular smooth muscle cells (VSMC) following aortic injury and in atherosclerotic disease. Recent evidence indicates that during atherosclerosis, Toll-like receptors (TLRs) are activated in vascular cells by endogenous ligands. Here, we analyzed whether CLU expression in VSMC is controlled by TLRs, and stimulated by factors associated with or released by necrotic cells. Activation of TLR3 by the synthetic RNA analogue polyinosinic-polycytidylic acid (poly(I:C)) in CRL2018 VSMC and in mice led to induction of CLU mRNA and protein synthesis, respectively. In TLR3-deficient 10A yolk sac cells, induction of CLU by poly(I:C) challenge depended on the ectopic expression of human TLR3. In mice lacking the TLR3-signaling adaptor protein TRIF (TIR-domain-containing adaptor protein inducing IFN-β) CLU induction by poly(I:C) was abrogated. In addition to poly(I:C) CLU gene expression in CRL2018 cells was induced by purified cellular RNA and RNA present in necrotic cell lysate. Our data indicate that cellular RNA following its release from necrotic cells in atherosclerotic lesions can act as an endogenous TLR3 ligand to induce CLU expression in VSMC and in vivo. Thus, they expand the view on TLR2 and TLR4 as known pro-atherosclerotic effectors toward TLR3. Conclusively, TLR3 activation induces expression of cytoprotective and anti-inflammatory CLU by VSMC and mice, to potentially counteract atherosclerotic pathology.

Chapter 5: Nuclear CLU (nCLU) and the fate of the cell

The possible biological role played by Clusterin (CLU) has been puzzling researchers for a long time since its first discovery and characterization. CLU has been often described as an "enigmatic" gene, a clear indication that too many aspects of this issue have been obscure or difficult to interpret for long. The good news is that this is certainly no longer true. Since the beginning, CLU was believed to play important roles in nearly all most important biological phenomena. The diversity, sometime the contradictions, of its biological action is now likely explained by the existence of different protein products all generated by the same single copy CLU gene. The relatively recent discovery that CLU can be retained inside the cell and targeted to many intracellular sites and organelles, including the nucleus, provided us a very different view from that solely deriving from its possible role in the outer cellular environment. In particular, nuclear localization of CLU (nCLU) was found to trigger cell death in many systems. In this chapter, a critical review of previous work will enable us to reinterpret old data and observations in the attempt to progressively unravelling the CLU "enigma" by considering its localization inside and outside the cell. The final picture would supposedly reconciliate different or alternative hypothesis. Starting with an "historical" approach demonstrating that nCLU was right under our eyes since the beginning, up to the more recent contributions we will describe which stimuli would inhibit secretion and maturation of CLU leading at least one protein product to target the nucleus and kill the cell. A better understanding of this complex issue is not an easy work, considering the thoughtfulness in reviewing the existing literature and the known controversial reports. We hope that the information contained in this article will be useful for the reader to enlighten this field.

Expression pattern of sulfated glycoprotein-2 (SGP-2) mRNA in rat testes exposed to endocrine disruptors

Sulfated glycoprotein-2 (SGP-2) is secreted in Sertoli cells and epididymal epithelial cells and plays important roles in the regulation of spermatogenesis and sperm maturation. To investigate whether endocrine disruptors affect spermatogenesis through an SGP-2-dependent mechanism, daily oral doses of testosterone (50, 200 and 1,000 microg/kg), flutamide (1, 5 and 25 mg/kg), ketoconazole (0.2, 1, 5 and 25 mg/kg), diethylhexylphthalate (10, 50 and 250 mg/kg), nonylphenol (10, 50, 100 and 250 mg/kg), octylphenol (10, 50 and 250 mg/kg), diethylstilbesterol (10, 20 and 40 microg/kg) or corn oil (control) were administered to 5 week-old, male Sprague-Dawley rats for 3 weeks. Following treatment with these endocrine disruptors, testicular expression of SGP-2 mRNA was analyzed using reverse transcription-polymerase chain reaction. Compared with the control, the lowest dose of testosterone (50 microg/kg/day) significantly increased expression of SGP-2 mRNA, whereas 200 and 1,000 microg/kg/day testosterone significantly decreased the expression (P<0.05). Flutamide, ketoconazole, diethylhexylphthalate, nonylphenol, octylphenol and diethylstilbesterol significantly decreased SGP-2 mRNA expression in testes at all doses studied, with the exception of 1 mg/kg/day flutamide (P<0.05). These results suggest that endocrine disruptors might decrease spermatogenesis in testes by decreasing expression of SGP-2 mRNA.

Challenge and promise: roles for clusterin in pathogenesis, progression and therapy of cancer

Clusterin (CLU) has been implicated in various cell functions involved in carcinogenesis and tumour progression. There are two known CLU protein isoforms generated in human cells. A nuclear form of CLU protein (nCLU) is proapoptotic, and a secretory form (sCLU) is prosurvival. CLU expression has been associated with tumorigenesis of various malignancies, including tumours of prostate, colon, and breast. Furthermore, CLU expression is modulated by many factors that are believed to regulate tumour growth and/or apoptosis, including 1,25-dihydroxyvitamin D3, transforming growth factor beta-1, ultraviolet radiation, and IR. sCLU upregulation appears to be a general molecular stress response. Presently, preliminary results indicate that therapeutic modalities targeting CLU may be effective in cancer treatment. However, such strategies should make sure that nCLU is not eliminated or reduced. This review summarizes our present understanding of the importance of CLU in various physiological functions including tumour growth, and discusses its relevance to future cancer therapy.

Activation of the Ca(2+)/calcineurin/NFAT2 pathway controls smooth muscle cell differentiation

Cellular mechanisms controlling smooth muscle cells (SMCs) phenotypic modulation are largely unknown. Intracellular Ca2+ movements are essential to ensure SMC functions; one of the roles of Ca2+ is to regulate calcineurin, which in turn induces nuclear localization of the nuclear factor of activated T-cell (NFAT). In order to investigate, during phenotypic differentiation of SMCs, the effect of calcineurin inhibition on NFAT2 nuclear translocation, we used a culture model of SMC differentiation in serum-free conditions. We show that the treatment of cultured SMC with the calcineurin inhibitor cyclosporine A induced their dedifferentiation while preventing their differentiation. These findings suggest that nuclear translocation of NFAT2 is dependent of calcineurin activity during the in vitro SMC differentiation kinetic and that the nuclear presence of NFAT2 is critical in the acquisition and maintenance of SMC differentiation.

Differential effects of clusterin/apolipoprotein J on cellular growth and survival

The secreted clusterin/apolipoprotein J (CLU) protein form is a ubiquitously expressed heterodimeric glycoprotein which is differentially regulated in many severe physiological disturbance states including cell death, ageing, cancer progression, and various neurological diseases. Despite extensive efforts CLU function remains an enigma, the main cause being the intriguingly distinct and usually opposed functions in various cell types and tissues. In the current report we investigated the effects of CLU on cellular growth and survival in three human osteosarcoma (OS) cell lines, namely KH OS, Sa OS, and U-2 OS that express very low, moderate, and high endogenous steady-state CLU amounts, respectively. We found that exposure of these established OS cell lines or primary OS cells to genotoxic stress results in CLU gene induction at distinct levels that correlate negatively to CLU endogenous amounts. Following CLU-forced overexpression by means of an artificial transgene, we found that although extracellular CLU inhibits cell death in all three OS cell lines, intracellular CLU has different effects on cellular proliferation and survival in these cell lines. Transgenic KH OS cell lines adapted to moderate intracellular CLU levels were growth-retarded and became resistant to genotoxic and oxidative stress. In contrast, transgenic Sa OS and U2 OS cell lines adapted to high intracellular CLU amounts were sensitive to genotoxic and oxidative stress. In these two cell lines, the proapoptotic CLU function could be rescued by caspase inhibition. To monitor the immediate effects of heterologous CLU overexpression prior to cell adaptation, we performed transient transfections in all three OS cell lines. We found that induction of high intracellular CLU amounts increases spontaneous apoptosis in KH OS cells and reduces DNA synthesis in all three cell lines assayed. On the basis of these novel findings we propose that although extracellular CLU as well as intracellular CLU at low/moderate levels is cytoprotective, CLU may become highly cytostatic and/or cytotoxic if it accumulates intracellularly in high amounts either by direct synthesis or by uptake from the extracellular milieu.

Involvement of clusterin in 15-deoxy-Δ12,14-prostaglandin J2-induced vascular smooth muscle cell differentiation

To establish an in vitro model of vascular smooth muscle cell (VSMC) differentiation, we examined the effect of 15-deoxy-delta12,14-prostaglandin J(2) (15d-PGJ(2)) on the expression of VSMC differentiation markers. After the addition of 15d-PGJ(2) to confluent human umbilical artery smooth muscle cells synchronized in the G(0) phase, cells showed a "hill and valley" appearance and thereafter aggregated and formed macroscopic nodules. Cells forming nodules expressed high levels of SM2, the most specific VSMC differentiation marker, comparable to medial VSMCs in vivo. 15d-PGJ(2) significantly increased the mRNA and protein expression levels of clusterin, a secreted glycoprotein reported to induce nodule formation and differentiation of VSMCs. Moreover, addition of an anti-clusterin antibody completely inhibited the nodule formation induced by 15d-PGJ(2) and induced apoptosis. Our results suggested that clusterin is involved in 15d-PGJ(2)-induced nodule formation and cell differentiation in VSMCs.

Association of Vimentin overexpression and hepatocellular carcinoma metastasis

The poor prognosis of hepatocellular carcinoma (HCC) has been associated with recurrence and metastasis. Recently, we established a pair of HCC cell lines from a primary (H2-P) and its matched metastatic (H2-M) HCC tumors. A high density of cDNA microarray with 9184 human cDNA was used to identify the differentially expressed genes between H2-P and H2-M. Comparing with H2-P, eight upregulated and six downregulated genes were detected in H2-M. One interesting finding is the overexpression of Vimentin (VIM), a well-defined intermediate filament, which has been linked to a more aggressive status in various tumors. The correlation of overexpression of VIM and HCC metastasis was studied by immunohistochemistry using a tissue microarray with 200 primary HCCs and 60 pairs of primary and matched metastatic HCC samples. Tissue microarray demonstrated that the overexpression of VIM was significantly associated with HCC metastasis (P<0.01). This finding strongly suggests that the overexpression of VIM may play an important role in the metastasis of HCC.

Clusterin/apolipoprotein J in human aging and cancer

Clusterin/Apolipoprotein J (ApoJ) is a heterodimeric highly conserved secreted glycoprotein being expressed in a wide variety of tissues and found in all human fluids. Despite being cloned since 1989, no genuine function has been attributed to ApoJ so far. The protein has been reportedly implicated in several diverse physiological processes such as sperm maturation, lipid transportation, complement inhibition, tissue remodeling, membrane recycling, cell-cell and cell-substratum interactions, stabilization of stressed proteins in a folding-competent state and promotion or inhibition of apoptosis. ApoJ gene is differentially regulated by cytokines, growth factors and stress-inducing agents, while another defining prominent and intriguing ApoJ feature is its upregulation in many severe physiological disturbances states and in several neurodegenerative conditions mostly related to advanced aging. Moreover, ApoJ accumulates during the viable growth arrested cellular state of senescence, that is thought to contribute to aging and to tumorigenesis suppression; paradoxically ApoJ is also upregulated in several cases of in vivo cancer progression and tumor formation. This review focuses on the reported data related to ApoJ cell-type and signal specific regulation, function and site of action in normal and cancer cells. We discuss the role of ApoJ during cellular senescence and tumorigenesis, especially under the light of the recently demonstrated various ApoJ intracellular protein forms and their interaction with molecules involved in signal transduction and DNA repair, raising the possibility that its overexpression during cellular senescence might cause a predisposition to cancer.

Multiple receptors mediate apoJ-dependent clearance of cellular debris into nonprofessional phagocytes

Phagocytosis of apoptotic, senescent, and dying cells by macrophages is a well characterized process. More recently it has been shown that in addition to macrophages vital neighboring cells in the affected tissue participate in the cellular clearance. While scavenger receptors have been shown to mediate uptake into macrophages, it is poorly understood how cellular debris is internalized by nonprofessional phagocytes. We here analyze the endocytic activity of vital fibroblasts and epithelial cells exposed to cellular debris and membrane remnants. We show a mutual stimulation in the endocytosis of debris and apolipoproteinJ (clusterin) in these cells. Experiments using RAP (receptor-associated protein) to block ligand binding to LRP and megalin as well as studies in LRP- and megalin-deficient cells suggest that the uptake of apoJ and cellular debris is mediated by megalin, LRP, and yet unidentified internalization mechanisms.

Apolipoprotein J inhibits the migration, adhesion, and proliferation of vascular smooth muscle cells

PURPOSE

Apolipoprotein J (ApoJ) is expressed during tissue injury and remodeling and has been implicated in vascular smooth muscle cell (VSMC) differentiation. Recently, the gene for ApoJ was identified as upregulated in distal anastomotic intimal hyperplasia after prosthetic arterial grafting. In this study we investigate the effect of ApoJ on VSMC migration, adhesion, proliferation, and gene expression.

METHODS

To study the effect of ApoJ on cell migration, we used a microchemotaxis chamber with an intervening 8-microm pore semipermeable polycarbonate membrane. Assays were performed with ApoJ alone (1-50 microg/mL) or in combination with platelet-derived growth factor homodimer bb (PDGF-bb; 10 ng/mL) or 2% fetal bovine serum (FBS; n = 8) in the lower wells. The influence of extracellular matrix interactions on ApoJ and chemotaxis was studied in a similar way, except membranes were collagen coated. Furthermore, cells were exposed to ApoJ 15 minutes before or after seeding on the coated membrane (n = 10). The influence of ApoJ on cell adhesion to collagen was assessed with cell exposure to ApoJ 15 minutes before or after seeding on a collagen-coated membrane (n = 10). Migration and adhesion were quantified by counting the number of cells per three independent high-power fields with light microscopy. The effect of ApoJ in 0.4% FBS with or without PDGF-bb on VSMC proliferation (n = 12) was assessed by means of [Methyl-(3)H] thymidine incorporation. The transcriptional profile of VSMCs in 2% FBS exposed to ApoJ and a control for 24 hours was analyzed with an oligonucleotide microarray containing 12,560 genes.

RESULTS

ApoJ alone was not chemotactic for VSMCs. Without collagen, ApoJ decreased the migration of VSMCs toward 2% FBS by 96% or more starting at 10 microg/mL (P < .05) and toward PDGF-bb by 60.9% or more starting at 25 microg/mL (P < .05) compared with the control. When collagen was introduced, ApoJ (25 microg/mL) decreased migration toward 2% FBS by 64% (P < .01) and toward PDGF-bb by 67.5% (P < .01) and decreased adhesion by 26.8% (P < .01) only when VSMCs in solution were exposed to ApoJ before placement on collagen. ApoJ did not induce VSMC proliferation. ApoJ alone decreased VSMC thymidine incorporation by 41.1% at 25 microg/mL (P < .05). ApoJ decreased thymidine incorporation of PDGF-bb stimulated VSMCs by 42.8% at 50 microg/mL (P < .05). Interleukin-8 and endothelin-1 were demonstrated by means of the microarray to be differentially expressed more than twofold in VSMCs that were exposed to ApoJ.

CONCLUSION

ApoJ is a potent inhibitor of VSMC migration, adhesion, and proliferation. Its genetic targets are linked to cell senescence and differentiation. Therefore, ApoJ may play a role, in part, in modulating the VSMC response to injury.

Clusterin gene transcription is activated by caudal-related homeobox genes in intestinal epithelium

Caudal-related homeobox (Cdx) proteins play an important role in development and differentiation of the intestinal epithelium. Using cDNA differential display, we identified clusterin as a prominently induced gene in a Cdx2-regulated cellular model of intestinal differentiation. Transfection experiments and DNA-protein interaction assays showed that clusterin is an immediate downstream target gene for Cdx proteins. The distribution of clusterin protein in the intestine was assessed during development and in the adult epithelium using immunohistochemistry. In the adult mouse epithelium, clusterin protein was localized in both crypt and villus compartments but not in interstitial cells of the intestinal mucosa. Together, these data suggest that clusterin is a direct target gene for Cdx homeobox proteins, and the pattern of clusterin protein expression suggests that it is associated with the differentiated state in the intestinal epithelium.

Identification of clusterin sequences mediating renal tubular cell interactions

Expression of the glycoprotein clusterin is markedly increased following tissue injury. One function of clusterin is to promote cell interactions which are perturbed in these pathologic settings. Clusterin causes cell aggregation and adhesion in vitro yet the molecular mechanism for this effect is not known. In order to identify the active site(s) of clusterin, 34 peptides, each 15 amino acid residues in length, were synthesized from hydrophilic regions of human clusterin. When studied individually, none of the peptides caused aggregation of LLC-PK1 cells, a porcine renal epithelial cell line. However, two out of the 34 peptides inhibited clusterin-induced cell aggregation in a dose-dependent manner. Scrambled versions of these two 'active' peptides did not inhibit cell aggregation. Seven peptides promoted cell adhesion. In conclusion, these findings provide evidence for novel amino acid sequences mediating clusterin-induced renal cell interactions.

Clusterin (Apo J) regulates vascular smooth muscle cell differentiation in vitro

Previously we reported a significant and substantial increase in the synthesis and secretion of clusterin in cultured porcine vascular smooth muscle cells (VSMC) during the time when the VSMC culture modulates from a proliferating monolayer morphology to a nodular cell culture morphology. That in vitro process appears to recapitulate some aspects of in vivo vascular remodeling in response to injury and is facilitated by the presence of a well-developed extracellular matrix. To directly test the hypothesis that clusterin regulates VSMC phenotypic modulation, cultured VSMC were stably transfected with an expression plasmid containing the full-length murine clusterin sequence in antisense orientation. Twenty-four clones were selected on the basis of neomycin resistance and characterized for clusterin expression and culture morphology. In contrast to clone SM-CLU18AS, which expresses a high level of clusterin and forms multicellular nodules, clone SM-CLU13AS expresses a low level of clusterin and does not form nodules even in the presence of a preformed collagen gel. Importantly, clusterin-negative SM-CLU13AS retains the ability to form nodules in an environment containing exogenous clusterin. SM-CLU13AS forms nodules when cultured in Matrigel (which contains clusterin) and in the presence of clusterin-containing conditioned media prepared from nodular SMC cultures or SM-CLU18AS cultures. These results demonstrate that clusterin is required for VSMC nodule formation and suggest that it may play a role in smooth muscle cell reorganization in the vascular wall.

Gp38k, a protein synthesized by vascular smooth muscle cells, stimulates directional migration of human umbilical vein endothelial cells

Gp38k is a 383-amino-acid secreted glycoprotein expressed by cultured vascular smooth muscle cells during the time of transition from a proliferating monolayer culture to a nonproliferating multilayered (differentiated) culture. Expression continues as the cell culture forms multicellular nodules. Because this transition period involves active cell migration, we evaluated the effects of exogenously added gp38k on vascular endothelial cell (HUVEC) migration and chemotaxis. Here we demonstrate that gp38k acts as a chemoattractant for HUVECs and stimulates cell migration in Boyden chambers at a level comparable to that achieved with the known endothelial cell chemoattractant bFGF. The migration effect is neutralized by the presence of a polyclonal anti-gp38k antibody. Because gp38k expression is also correlated with changes in culture morphology, we also assessed its ability to act as an agonist of HUVEC morphology using cultures growing on Matrigel. We report that gp38k stimulates endothelial cell tubulogenesis in this assay system. These results provide the first evidence that gp38k may function in angiogenesis by stimulating the migration and reorganization of vascular endothelial cells.

Transforming growth factor beta-1 up-regulates clusterin synthesis in thyroid epithelial cells

Porcine epithelial cells in primary culture seeded on plastic substratum form a monolayer containing pseudo-vesicles. When cultured in the presence of thyreotropin (TSH) thyrocytes adopted a follicular-like structure and formed clusters. Transforming growth factor beta-1 (TGFbeta1) induced a rapid spreading of the TSH-treated cells only. At the same time, TGFbeta1 enhanced clusterin protein and mRNA expression. The increase of clusterin synthesis was proportional to the TGFbeta1 concentration in the culture medium. Tunicamycin abolished the up-regulation of whole clusterin synthesis and morphological changes. The activator protein-1 binding site partly directed the TGFbeta1-stimulated clusterin expression. Phorbol ester caused rapid spreading of the cells with disappearance of vesicular and follicular structures. It decreased clusterin mRNA and protein expression, but increased Mr 45, 000 protein secretion in both TSH-treated and nontreated cells. Up-regulation of clusterin expression may be a marker of TGFbeta-mediated thyrocyte dedifferentiation.

Clusterin protects against oxidative stress in vitro through aggregative and nonaggregative properties

Perturbations of cell interactions, an early event in acute renal injury, have important pathophysiologic consequences. We hypothesized that promotion of cell interactions protects cells from injury. To test this hypothesis, a single cell suspension of LLC-PK1 cells (porcine proximal tubular cell line) treated with albumin (control) was compared to cells aggregated with fibrinogen or purified human clusterin (aggregation graded 0 to 4). Following aggregation, the cells were injured with 1.5 mM hydrogen peroxide (H2O2) for three hours. Cell aggregation induced by clusterin but not fibrinogen protected against oxidant injury by H2O2. Complete abrogation of cytotoxicity occurred at a clusterin concentration of 2.5 micrograms/ml, which resulted in an aggregation score of 1. In the absence of aggregation, clusterin at concentrations of 20 and 50 micrograms/ml, but not lower doses, partially protected against injury induced by H2O2. Cell aggregation induced by both clusterin and fibrinogen partially protected against endogenously generated oxidant stress induced by incubating LLC-PK1 cells with aminotriazole and 1-chloro-2,4-dinitrobenzene (CDNB). In conclusion, clusterin protects against models of oxidant stress in vitro, whether generated by exogenously administered hydrogen peroxide, or from endogenously produced peroxide, and such protective effects can accrue from aggregative and nonaggregative properties of clusterin.

Paraoxonase as a risk marker for cardiovascular disease: facts and hypotheses

Paraoxonase (PON1) is a Ca2+-dependent enzyme whose mechanism of action is incompletely elucidated. PON1 was originally found to be responsible for the hydrolysis of paraoxon, a catabolite of the insecticide parathion, but this enzyme is equally able to hydrolyze other substrates such as phenyl acetate. PON1 exhibits two sequence polymorphisms, Arg-->Gln 192 and Met-->Leu 55, respectively, of which the former is responsible for the distinct catalytic activity of the two corresponding allozymes against paraoxon. The PON1 gene is a member of a family of at least three related genes. Although the physiologic substrate of PON1 is unknown, a protective role against the oxidative degradation of serum lipoproteins has been attributed to this enzyme. Indeed, PON1 is a component of a spectrum of circulating high density lipoprotein particles and can hydrolyze oxidized phospholipids and cholesteryl ester hydroperoxides. Studies have been conducted to evaluate the possible "protective" role of PON, and especially the influence of the Arg-->Gln 192 polymorphism, in coronary artery disease. Results from these investigations are conflicting, and recent data suggest a complex pattern with influences from other polymorphisms in either the PON1 and/or the PON2 and PON3 genes, or even another region of the gene cluster. A number of related factors, which include the heterogeneity of the high density lipoprotein particles incorporating PON(s), the metabolism of associated apolipoproteins such as apoJ/clusterin, the respective roles of PON(s) and other high density lipoprotein-associated enzymes such as platelet-activating-factor acetyl-hydrolase and lecithin-cholesterol acyltransferase, modifications of high density lipoprotein composition and activity under acute-phase conditions, the dietary and environmental regulation of PON(s), and the actual in situ availability of PON in the atherosclerotic artery wall, must equally be taken into account.

Regulation of clusterin gene expression by transforming growth factor beta

Transforming growth factor beta (TGFbeta) induces the expression of a wide variety of genes in many cell types. Our previous studies have shown that TGFbeta stimulates both clusterin mRNA and protein levels, and induces its accumulation in the nucleus of CCL64 cells. To further investigate the molecular mechanism of clusterin mRNA induction by TGFbeta, we created a 1.3-kilobase rat clusterin promoter/luciferase reporter construct. We demonstrate that TGFbeta enhances luciferase activity 2.5-6-fold in transient transfection assays of epithelial, endothelial, and fibroblast cell lines. Deletional analysis reveals that an AP-1-binding site (5'-TGAGTCA) in the minimal promoter region is necessary for initiating transactivation by TGFbeta. A single T to G base mutation in the AP-1 site (5'-TGAGGCA) abolishes TGFbeta-induced clusterin promoter transactivation. In transcription factor decoy experiments, 23-mer oligonucleotides of wild type AP-1 reduce TGFbeta induction of clusterin mRNA levels and promoter transactivation, while an oligonucleotide containing the mutated AP-1 site has no effect. Two specific protein kinase C inhibitors, GF109203X and calphostin C, block TGFbeta-induced clusterin mRNA levels and promoter transactivation. Together these results indicate that TGFbeta regulates clusterin gene expression through an AP-1 site and its cognate transcription factor AP-1, and requires the involvement of protein kinase C.

Transforming growth factor beta (TGF beta)-induced nuclear localization of apolipoprotein J/clusterin in epithelial cells

Apolipoprotein J (apoJ)/clusterin was first identified as an 80 kDa secretory glycoprotein present in most body fluids. It has been implicated in a variety of physiological processes including cellular differentiation and apoptosis. We demonstrate here that in addition to the well characterized secreted form of the protein, there exists an intracellular, nuclear form of apoJ. This intracellular form of the protein is induced to accumulate in the nucleus of two epithelial cell lines (HepG2 and CCL64) in response to treatment with transforming growth factor beta (TGF beta). We demonstrate in vitro that apoJ protein can be translated from two in-frame ATG sites. Initiation from the first ATG encodes for the secretory form of apoJ and initiation from the second ATG, located 33 amino acids downstream of the first and lacking the hydrophobic signal sequence, encodes for a truncated apoJ protein. This shorter form of apoJ is not recognized by microsomes and therefore not glycosylated, and we postulate that it is retained intracellularly and targeted to the nucleus due to the presence of an SV40-like nuclear localization sequence (NLS). This mechanism of nuclear targeting of apoJ occurs in cells since the protein isolated from nuclei of TGF beta-treated cells and the in vitro-translated truncated form are identical by V8 protease analysis. These results suggest that the diverse physiological responses attributed to apoJ may be elicited through a common molecular mechanism involving a previously uncharacterized intracellular form of the protein.

Clusterin (SGP-2) induction in rat astroglial cells exposed to prion protein fragment 106-126

Prion-related encephalopathies are characterized by the accumulation of an abnormal prion protein isoform (PrPSc) associated with neuronal degeneration and astrogliosis. The synthetic peptide homologous to PrP fragment 106-126 (PrP 106-126) induced in vitro neuronal apoptosis and glial proliferation. We used Northern blot analysis and the RNA polymerase chain reaction to assess the expression of several genes associated with programmed cell death and proliferation. Blots of total RNA extracted from neuronal and astroglial cells exposed to PrP 106-126 for between 1 h and 7 days were hybridized with probes recognizing c-fos, c-jun, c-myc, p53, hsp-70 and bcl-2 mRNA. Except for a slight decrease in bcl-2 mRNA in neuronal cells, no change in other transcripts was evident. Since clusterin (apolipoprotein J) mRNA levels are increased in prion-related encephalopathies and clusterin immunoreactivity has been located in association with PrPSc in Gerstmann-Sträussler-Scheinker brain, the expression of clusterin was determined in neuronal and astroglial cells chronically exposed to PrP 106-126. Although the induction of clusterin has been involved in the apoptotic mechanism in other experimental conditions, its expression was unchanged in PrP 106-126-treated neurons, while a three-fold induction of clusterin mRNA was observed in astrocytes exposed to PrP 106-126. To investigate whether the clusterin up-regulation was simply associated with the astroglial proliferative stimulus of PrP 106-126 or was specifically induced by the peptide, we measured clusterin expression in astrocytes cultured in fetal calf serum-free medium and exposed to PrP 106-126 or fetal calf serum restoration. In this condition the PrP peptide, like fetal calf serum, increased the glial proliferation rate, but only PrP 106-126 doubled clusterin mRNA. The selectivity of this effect indicates that PrPSc is directly involved in the clusterin up-regulation seen in prion-related encephalopathies and is associated with astroglial cells.

Interaction of transforming growth factor beta receptors with apolipoprotein J/clusterin

Proteins mediating the transmission of the signal from an activated transforming growth factor beta (TGF beta) receptor complex have not been identified. Using a yeast interaction screen to search for proteins that associate with the type II TGF beta receptor (RII), we isolated a protein which was identical to apolipoprotein J (apoJ)/clusterin. ApoJ interacts with both the type I (RI) and type II (RII) TGF beta receptors but does not interact with the epidermal growth factor (EGF) receptor. The interaction between RII and apoJ occurs through the C-terminal 127 amino acids of RII. Deletion of this region, which contains the kinase insert 2 domain, abrogates binding to apoJ. The binding of apoJ to either the RI and the RII receptors is direct, not requiring other proteins, and is not specific for the alpha or beta subunit of apoJ since both subunits are effective in competing for binding. RI and RII fusion proteins are capable of precipitating the 60 kDa intracellular form of apoJ from [35S]methionine-labeled cellular lysates, suggesting that this form of the protein may play some role in TGF beta signaling or TGF beta receptor processing.

Identification of a 38-kDa heparin-binding glycoprotein (gp38k) in differentiating vascular smooth muscle cells as a member of a group of proteins associated with tissue remodeling

Cultured aortic smooth muscle cells (SMC) exhibit morphological and phenotypic modulation characterized by a change from a substrate attached monolayer culture to a multilayered nodular cell culture in which SMC are imbedded into the extracellular matrix. Associated with nodule formation is a change in the pattern of SMC gene expression including increased expression of a well characterized marker of smooth muscle cell differentiation, SM alpha-actin, and a 38-kDa glycoprotein (gp38k). gp38k has sequence homology with proteins reported to be correlated with tissue remodeling. To characterize the gp38k mRNA we designed degenerate oligonucleotides based on partial polypeptide sequencing to select a cDNA encoding the full-length gp38k. Southern analysis indicates that porcine gp38k is present as a single copy gene. Northern analysis indicates that the increase in gp38k is correlated with an increase in the steady state level of gp38k mRNA; and is present in cultures that have initiated the formation of multilayered foci and nodules. The correlation between SMC differentiation and gp38k expression is further established by using culture conditions that facilitate SMC differentiation. Cultures seeded onto reconstituted extracellular matrix show rapid formation of nodules and increased expression of gp38k mRNA. Comparison of the gp38k and cDNA sequences with nucleotide and protein sequences available through GenBank and SwissProt data banks revealed that molecules homologous to gp38k were present in human, mouse, bovine, and Drosophila tissues, suggesting that the gp38k may be a member of a gene family. Although a function for gp38k has not been identified, this report represents the first report of its correlation with a specific process important in phenotypic and morphological modulation of vascular SMC.