The E-box-like sterol regulatory element mediates the insulin-stimulated expression of hepatic clusterin

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.

Clusterin and Related Scoring Index as Potential Early Predictors of Response to Sorafenib in Hepatocellular Carcinoma

Advanced hepatocellular carcinoma (HCC) remains a highly lethal malignancy, although several systemic therapeutic options are available, including sorafenib (SFN), which has been one of the standard treatment agents for almost a decade. As early prediction of response to SFN remains challenging, biomarkers that enable early prediction using a clinically feasible method are needed. Here, we report that the serum secretory form of clusterin (sCLU) protein and its related predictive index are potential beneficial biomarkers for early prediction of SFN response. Using high-throughput screening and subsequent multivariate analysis in the derivation cohort, we found that changes in the concentrations of CLU, vascular cell adhesion molecule-1 (VCAM1), and α-fetoprotein were significantly associated with response to SFN. Furthermore, we confirmed that an increase in CLU serum level 1 month after treatment initiation was significantly associated with shorter progression-free survival. In addition, "NR-index," which comprises these proteins, was evaluated as a tool for accurately predicting the efficacy of SFN and confirmed in the validation cohort. We also established SFN-resistant HepG2 cells (HepG2-SR) and found that sCLU significantly increased in HepG2-SR cells compared with normal HepG2 cells, and confirmed that HepG2-SR cells treated with SFN were resistant to apoptosis. The mechanism underlying activation of sCLU expression in acquired SFN resistance involves aberrant signaling and expression of Akt, mammalian target of rapamycin (mTOR), and a nutrient-related transcription factor, sterol regulatory element binding protein 1c (SREBP-1c). Furthermore, the PI3K and mTOR inhibitor BEZ235 markedly decreased sCLU expression in HepG2-SR cells. Conclusion: These results suggest that measurement of sCLU serum levels and the sCLU-related NR-index are promising clinical tools for the early prediction of SFN response in HCC. Additionally, sCLU-overexpressing HCC might be susceptible to mTOR inhibition.

Regulation of adipocyte differentiation by clusterin-mediated Krüppel-like factor 5 stabilization

Clusterin (CLU) is a heterodimeric glycoprotein involved in a range of biological processes. We investigated the function of CLU as a novel regulator of adipogenesis. CLU expression increased during 3T3-L1 preadipocyte differentiation. CLU overexpression promoted adipogenic differentiation of preadipocytes and increased the mRNA levels of adipogenic markers including peroxisome proliferator-activated receptor γ (Pparg) and CCAAT enhancer-binding protein α (Cebpa). Conversely, knockdown of CLU attenuated adipogenesis and reduced transcript levels of Pparg and Cebpa. However, the promoter activities of both the Pparg and the Cebpa gene were not affected by alteration of CLU expression on its own. Additionally, the protein level of Krüppel-like factor 5 (KLF5), an upstream transcription factor of Pparg and Cebpa involved in adipogenic differentiation, was upregulated by CLU overexpression, although the mRNA level of Klf5 was not altered by changes in the expression level of CLU. Cycloheximide chase assay showed that the increased level of KLF5 by CLU overexpression was due to decreased degradation of KLF5 protein. Interestingly, CLU increased the stability of KLF5 by decreasing KLF5 ubiquitination. CLU inhibited the interaction between KLF5 and F-box/WD repeat-containing protein 7, which is an E3 ubiquitin ligase that targets KLF5. The adipogenic role of CLU was also addressed in mesenchymal stem cells (MSCs) and Clu^-/- mouse embryonic fibroblasts (MEFs). Furthermore, CLU enhanced KLF5-mediated transcriptional activation of both the Cebpa and the Pparg promoter. Taken together, these results suggest that CLU is a novel regulator of adipocyte differentiation by modulating the protein stability of the adipogenic transcription factor KLF5.

Establishment of a HEK293 cell line by CRISPR/Cas9-mediated luciferase knock-in to study transcriptional regulation of the human SREBP1 gene

OBJECTIVES

To establish a HEK293 cell line with a luciferase knock-in reporter controlled by the endogenous SREBP1 promoter for investigating transcriptional regulation of the SREBP1 gene.

RESULTS

PCR confirmed the site-specific integration of a single copy of the exogenous luciferase gene into one allele of the genome and a 14 bp deletion of the targeted sequence in the other. Luciferase activity was directly correlated with the promoter activity of the endogenous SREBP1 gene in the HEK293-SREBP1-T2A-luciferase-KI cell line cell line.

CONCLUSIONS

We successfully generated a novel luciferase knock-in reporter system, which will be very useful for studying transcriptional regulation of the SREBP1 gene and for screening drugs or chemical molecules that regulate SREBP1 gene expression.

Regulation of Nampt expression by transcriptional coactivator NCOA6 in pancreatic β-cells

Nuclear receptor coactivator 6 (NCOA6) is a transcriptional coactivator and crucial for insulin secretion and glucose metabolism in pancreatic β-cells. However, the regulatory mechanism of β-cell function by NCOA6 is largely unknown. In this study, we found that the transcript levels of nicotinamide phosphoribosyltransferase (Nampt) were decreased in islets of NCOA6^+/- mice compared with NCOA6^+/+ mice. Moreover, NCOA6 overexpression increased the levels of Nampt transcripts in the mouse pancreatic β-cell line NIT-1. Promoter analyses showed that transcriptional activity of the Nampt promoter was stimulated by cooperation of sterol regulatory element binding protein-1c (SREBP-1c) and NCOA6. Additional studies using mutant promoters demonstrated that SREBP-1c activates Nampt promoter through the sterol regulatory element (SRE), but not through the E-box. Using chromatin immunoprecipitation assay, NCOA6 was also shown to be directly recruited to the SRE region of the Nampt promoter. Furthermore, treatment with nicotinamide mononucleotide (NMN), a product of the Nampt reaction and a key NAD⁺ intermediate, ameliorates glucose-stimulated insulin secretion from NCOA6^+/- islets. These results suggest that NCOA6 stimulates insulin secretion, at least partially, by modulating Nampt expression in pancreatic β-cells.

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.