Anti-tumor effects of mAb against L-type amino acid transporter 1 (LAT1) bound to human and monkey LAT1 with dual avidity modes.

L-Type amino acid transporter 1 (LAT1) disulfide linked to CD98 heavy chain (hc) is highly expressed in most cancer cells, but weakly expressed in normal cells. In the present study, we developed novel anti-LAT1 mAbs and showed internalization activity, inhibitory effects of amino acid uptake and cell growth and antibody-dependent cellular cytotoxicity, as well as in vivo antitumor effects in athymic mice. Furthermore, we examined the reactivity of mAbs with LAT1 of Macaca fascicularis to evaluate possible side-effects of antihuman LAT1 mAbs in clinical trials. Antihuman LAT1 mAbs reacted with ACHN human and MK.P3 macaca kidney-derived cells, and this reactivity was significantly decreased by siRNAs against LAT1. Macaca LAT1 cDNA was cloned from MK.P3, and only two amino acid differences between human and macaca LAT1 were seen. RH7777 rat hepatoma and HEK293 human embryonic kidney cells expressing macaca LAT1 were established as stable transfectants, and antihuman LAT1 mAbs were equivalently reactive against transfectants expressing human or macaca LAT1. Dual (high and low) avidity modes were detected in transfectants expressing macaca LAT1, MK.P3, ACHN and HCT116 human colon cancer cells, and KA values were increased by anti-CD98hc mAb, suggesting anti-LAT1 mAbs detect an epitope on LAT1-CD98hc complexes on the cell surface. Based on these results, LAT1 may be a promising anticancer target and Macaca fascicularis can be used in preclinical studies with antihuman LAT1 mAbs.

Dasatinib Suppresses TGFß-Mediated Epithelial-Mesenchymal Transition in Alveolar Epithelial Cells and Inhibits Pulmonary Fibrosis.

PURPOSE: Transforming growth factor ß (TGFß)-mediated epithelial-mesenchymal transition (EMT) of alveolar epithelial cells contributes to pulmonary fibrosis. Dasatinib (DAS), a potent and broad-spectrum tyrosine kinase inhibitor, has been widely studied as an anti-cancer agent. However, the therapeutic application of DAS for pulmonary fibrosis has not been clarified. Our purpose here is to investigate the effect of DAS on TGFß1-induced EMT in human alveolar and bronchial epithelial cells in vitro and to evaluate the efficacy of DAS on lung fibrosis in vivo. METHODS: TGFß1-stimulated human alveolar epithelial (A549) and bronchial epithelial (BEAS-2B) cells were treated with or without DAS in vitro. Murine pulmonary fibrosis model was generated by injection of bleomycin (BLM). RESULTS: A549 and BEAS-2B cells exposed to TGFß1 underwent EMT, as indicated by downregulation of epithelial protein E-cadherin and induction of the mesenchymal proteins, fibronectin and type I and type IV collagen. These effects were dramatically suppressed by DAS treatment, which also prevented Smad2 and Smad3 phosphorylation. DAS inhibited TGFß1-induced cell motility and migration. Furthermore, DAS administration significantly attenuated lung fibrosis in mice by histological analysis. Treatment with DAS also significantly reduced the levels of collagen and fibronectin and phosphorylation of Smad2 in the lung tissues of the murine model. CONCLUSIONS: These findings suggest that DAS inhibited TGFß-mediated EMT of alveolar and bronchial epithelial cells and attenuated BLM-induced lung fibrosis in mice by suppressing the TGFß/Smad pathway. DAS may be a promising and novel anti-fibrotic agent for preventing lung fibrosis.

Genomic and expression analysis of microdissected inflammatory breast cancer.

Inflammatory breast cancer (IBC) is a unique clinical entity characterized by rapid onset of erythema and swelling of the breast often without an obvious breast mass. Many studies have examined and compared gene expression between IBC and non-IBC (nIBC), repeatedly finding clusters associated with receptor subtype, but no consistent gene signature associated with IBC has been validated. Here we compared microdissected IBC tumor cells to microdissected nIBC tumor cells matched based on estrogen and HER-2/neu receptor status. Gene expression analysis and comparative genomic hybridization were performed. An IBC gene set and genomic set were identified using a training set and validated on the remaining data. The IBC gene set was further tested using data from IBC consortium samples and publicly available data. Receptor driven clusters were identified in IBC; however, no IBC-specific gene signature was identified. Fifteen genes were correlated between increased genomic copy number and gene overexpression data. An expression-guided gene set upregulated in the IBC training set clustered the validation set into two clusters independent of receptor subtype but segregated only 75 % of samples in each group into IBC or nIBC. In a larger consortium cohort and in published data, the gene set failed to optimally enrich for IBC samples. However, this gene set had a high negative predictive value for excluding the diagnosis of IBC in publicly available data (100 %). An IBC enriched genomic data set accurately identified 10/16 cases in the validation data set. Even with microdissection, no IBC-specific gene signature distinguishes IBC from nIBC. Using microdissected data, a validated gene set was identified that is associated with IBC tumor cells. Inflammatory breast cancer comparative genomic hybridization data are presented, but a validated genomic data set that identifies IBC is not demonstrated.

The compound LG283 inhibits bleomycin-induced skin fibrosis via antagonizing TGF-ß signaling.

BACKGROUND: Systemic sclerosis (SSc) is a collagen disease that exhibits intractable fibrosis and vascular injury of the skin and internal organs. Transforming growth factor-ß (TGF-ß)/Smad signaling plays a central role in extracellular matrix (ECM) production by α-SMA-positive myofibroblasts. Myofibroblasts may be partially derived from various precursor cells in addition to resident fibroblasts. Recently, our high-throughput in vitro screening discovered a small compound, LG283, that may disrupt the differentiation of epithelial cells into myofibroblasts. This compound was originally generated as a curcumin derivative. METHODS: In this study, we investigated the effect of LG283 on inhibiting fibrosis and its mechanism. The action of LG283 on TGF-ß-dependent fibrogenic activity and epithelial-mesenchymal transition (EMT) was analyzed in vitro. The effects of LG283 were also examined in a bleomycin-induced skin fibrosis mouse model. RESULTS: LG283 suppressed TGF-ß-induced expression of ECM, α-SMA, and transcription factors Snail 1 and 2, and Smad3 phosphorylation in cultured human dermal fibroblasts. LG283 was also found to block EMT induction in cultured human epithelial cells. During these processes, Smad3 phosphorylation and/or expression of Snail 1 and 2 were inhibited by LG283 treatment. In the bleomycin-induced skin fibrosis model, oral administration of LG283 efficiently protected against the development of fibrosis and decrease of capillary vessels without significantly affecting cell infiltration or cytokine concentrations in the skin. No apparent adverse effects of LG283 were found. LG283 treatment remarkably inhibited the enhanced expression of α-SMA and phosphorylated Smad3, as well as those of Snail 1 and 2, in the bleomycin-injected skin. CONCLUSIONS: The LG283 compound exhibits antagonistic activity on fibrosis and vascular injury through inhibition of TGF-ß/Smad/Snail mesenchymal transition pathways and thus, may be a candidate therapeutic for the treatment of SSc. Although the involvement of EMT in the pathogenesis of SSc remains unclear, the screening of EMT regulatory compounds may be an attractive approach for SSc therapy.

Molecular mechanisms regulating dissociation of cell-cell junction of epithelial cells by oxidative stress.

Oxidative stress is regarded as a causative factor in aging and various degenerative diseases. Here, we show the mechanism by which oxidative stress induces disruption of cell-cell junctions using retinal pigment epithelial cells. We demonstrated that reactive oxygen species (ROS)-mediated activation of Src kinase increases the tyrosine phosphorylation state of p120-catenin and rapidly triggers translocation of p120-catenin and internalization of N-cadherin from the cell-cell adhesion sites to an early endosomal compartment. Endosomal accumulation of p120-catenin resulted in stress fiber formation and cell-cell dissociation through the activation of Rho/Rho kinase pathway. However, these cytoskeletal remodeling and cell-cell dissociation induced by oxidative stress were transient, due to the activation of nuclear factor-kappaB (NF-kappaB) and the expression of manganese superoxide dismutase (Mn-SOD). Using the NF-kappaB specific inhibitor DHMEQ, we found that NF-kappaB is part of a negative feedback loop to control intracellular ROS levels. Finally, we demonstrated that H(2)O(2) treatment alone does not induce the epithelial mesenchymal transition (EMT) in retinal pigment epithelial cells, which can be induced by TNF-alpha treatment. These findings suggest that oxidative stress is a crucial factor to induce the cell-cell dissociation, an initial step of EMT, but does not provide sufficient signals to establish and to maintain the EMT.

Tranilast Inhibits Pulmonary Fibrosis by Suppressing TGFß/SMAD2 Pathway.

PURPOSE: Idiopathic pulmonary fibrosis (IPF) is characterized by the accumulation of extracellular matrix (ECM) protein in the lungs. Transforming growth factor (TGF) ß-induced ECM protein synthesis contributes to the development of IPF. Tranilast, an anti-allergy drug, suppresses TGFß expression and inhibits interstitial renal fibrosis in animal models. However, the beneficial effects of tranilast or its mechanism as a therapy for pulmonary fibrosis have not been clarified. METHODS: We investigated the in vitro effect of tranilast on ECM production and TGFß/SMAD2 pathway in TGFß2-stimulated A549 human alveolar epithelial cells, using quantitative polymerase chain reaction, Western blotting, and immunofluorescence. In vitro observations were validated in the lungs of a murine pulmonary fibrosis model, which we developed by intravenous injection of bleomycin. RESULTS: Treatment with tranilast suppressed the expression of ECM proteins, such as fibronectin and type IV collagen, and attenuated SMAD2 phosphorylation in TGFß2-stimulated A549 cells. In addition, based on a wound healing assay in these cells, tranilast significantly inhibited cell motility, with foci formation that comprised of ECM proteins. Histological analyses revealed that the administration of tranilast significantly attenuated lung fibrosis in mice. Furthermore, tranilast treatment significantly reduced levels of TGFß, collagen, fibronectin, and phosphorylated SMAD2 in pulmonary fibrotic tissues in mice. CONCLUSION: These findings suggest that tranilast inhibits pulmonary fibrosis by suppressing TGFß/SMAD2-mediated ECM protein production, presenting tranilast as a promising and novel anti-fibrotic agent for the treatment of IPF.

Nintedanib inhibits epithelial-mesenchymal transition in A549 alveolar epithelial cells through regulation of the TGF-ß/Smad pathway.

BACKGROUND: Idiopathic pulmonary fibrosis (IPF) is a progressive fibrotic lung disorder. Recent studies have suggested that epithelial-mesenchymal transition (EMT) of alveolar epithelial cells influences development of pulmonary fibrosis, which is mediated by transforming growth factor ß (TGF-ß). Tumor necrosis factor α (TNF-α), an important proinflammatory cytokine in IPF, has been shown to enhance TGF-ß-induced EMT. Nintedanib, a multiple tyrosine kinase inhibitor that is currently used to treat IPF, has been shown to suppress EMT in various cancer cell lines. However, the mechanism of EMT inhibition by nintedanib and its effect on TGF-ß and TNF-α signaling pathways in alveolar epithelial cells have not been fully elucidated. METHODS: A549 alveolar epithelial cells were stimulated with TGF-ß2 and TNF-α, and the effects of nintedanib on global gene expression were evaluated using microarray analysis. Furthermore, Smad2/3 phosphorylation was assessed using western blotting. RESULTS: We found that in A549 cells, TGF-ß2 and TNF-α treatment induces EMT, which was inhibited by nintedanib. Gene ontology analysis showed that nintedanib significantly attenuates the gene expression of EMT-related cellular pathways and the TGF-ß signaling pathway, but not in the TNF-α-mediated signaling pathway. Furthermore, hierarchical cluster analysis revealed that EMT-related genes were attenuated in nintedanib-treated cells. Additionally, nintedanib was found to markedly suppress phosphorylation of Smad2/3. CONCLUSION: Nintedanib inhibits EMT by mediating EMT-related gene expression and the TGF-ß/Smad pathway in A549 alveolar epithelial cells.

Molecular analysis of multifocal prostate cancer by comparative genomic hybridization.

BACKGROUND: Prostate cancer is often multifocal and shows histological heterogeneity among different tumor foci within the same prostate. We analyzed the origin and molecular basis of multifocal prostate cancer and genomic alterations associated with tumor progression. METHODS: We examined 45 multifocal prostate cancer foci from 22 radical prostatectomy specimens by comparative genomic hybridization (CGH). Laser capture microdissection (LCM) was used to gather cancer cells from frozen prostate specimens. RESULTS: Frequent chromosomal alternations were losses of 2q21-24 (22.2%), 6q14-22 (60.0%), 8p12-22 (35.6%), 13q14-31 (44.4%) and 16q13-24 (24.4%) and gains of 8q21.3-24.3 (37.8%) and 7q21-33 (20.0%). Frequency of losses of 8p12-22 and 16q13-24 and gains of 8q21.3-24.3 were significantly higher in tumors with high Gleason score (GS) than in those with low GS (P < 0.01, P < 0.05, and P < 0.01, respectively). Tumors with losses of 8p12-22 or 13q14-31 displayed larger volume than those without such losses (P < 0.05 and P < 0.01, respectively). In comparison between different tumor foci within the same prostate, chromosomal alterations did not show completely the same pattern between any tumor foci, except for one case in which two of the three foci displayed no chromosomal abnormalities. More malignant tumors (high GS or extracapsular invasion) displayed significantly higher frequency of losses of 8p12-22 (P < 0.05). CONCLUSIONS: These results suggest that tumor foci within the same prostate represent independent tumors with differing clonal origin and that loss of 8p12-22 represents an important determinant of prostate cancer progression.

Blockade of TGF-ß/Smad signaling by the small compound HPH-15 ameliorates experimental skin fibrosis.

BACKGROUND: Transforming growth factor-ß (TGF-ß)/Smad signaling is well known to play a critical role in the pathogenesis of systemic sclerosis (SSc). We previously developed an artificial molecule, the histidine-pyridine-histidine ligand derivative HPH-15, which may have an antifibrotic effect. The purpose of the present study was to clarify the effects of this drug in human skin fibroblasts and in a preclinical model of SSc. METHODS: The effects of HPH-15 on expression of extracellular matrix components and TGF-ß signaling in human dermal fibroblasts were analyzed. The antifibrotic properties of HPH-15 and its mechanisms were also examined in a bleomycin-induced skin fibrosis mouse model. RESULTS: HPH-15 suppressed the TGF-ß-induced phosphorylation of Smad3 and inhibited the expression of collagen I, fibronectin 1, connective tissue growth factor, and α-smooth muscle actin induced by TGF-ß in cultured human skin fibroblasts. In the bleomycin-induced skin fibrosis model, oral administration of HPH-15 protected against the development of skin fibrosis and ameliorated established skin fibrosis. Additionally, HPH-15 suppressed the phosphorylation of Smad3 in various cells, including macrophages in the bleomycin-injected skin. Further, in the treated mice, dermal infiltration of proinflammatory macrophages (CD11b+Ly6Chi) and M2 profibrotic macrophages (CD11b+CD204+ or CD11b+CD206+) was significantly decreased during the early and late stages, respectively. HPH-15 treatment resulted in decreased messenger RNA (mRNA) expression of the M2 macrophage markers arginase 1 and Ym-1 in the skin, whereas it inversely augmented expression of Friend leukemia integration 1 and Krüppel-like factor 5 mRNAs, the transcription factors that repress collagen synthesis. No apparent adverse effects of HPH-15 were found during the treatment. CONCLUSIONS: HPH-15 may inhibit skin fibrosis by inhibiting the phosphorylation of Smad3 in dermal fibroblasts and possibly in macrophages. Our results demonstrate several positive qualities of HPH-15, including oral bioavailability, a good safety profile, and therapeutic effectiveness. Thus, this TGF-ß/Smad inhibitor is a potential candidate therapeutic for SSc clinical trials.

Impact of chromosome 17q deletion in the primary lesion of colorectal cancer on liver metastasis.

Colorectal cancer is a prevalent malignancy worldwide, and investigations are required to elucidate the underlying carcinogenic mechanisms. Amongst these mechanisms, de novo carcinogenesis and the adenoma to carcinoma sequence, are the most understood. Metastasis of colorectal cancer to the liver often results in fatality, therefore, it is important for any associated risk factors to be identified. Regarding the treatment of the disease, it is important to manage not only the primary colorectal tumor, but also the liver metastases. Previously, through gene variation analysis, chromosomal loss has been indicated to serve an important role in liver metastasis. Such analysis may aid in the prediction of liver metastasis risk, alongside individual responses to treatment, thus improving the management of colorectal cancer. In the present study, we aimed to clarify a cause of the liver metastasis of colorectal cancer using comparative genomic hybridization analysis. A total of 116 frozen samples were analyzed from patients with advanced colorectal cancer that underwent surgery from 2004 to 2011. The present study analyzed mutations within tumor suppressor genes non-metastatic gene 23 (NM23), deleted in colorectal carcinoma (DCC) and deleted in pancreatic carcinoma, locus 4 (DPC4), which are located on chromosomes 17 and 18 and have all been reported to affect liver metastasis of colorectal cancer. The association between chromosomal abnormalities (duplication and deletion) and liver metastasis of colorectal cancer was evaluated using comparative genomic hybridization. Cluster analysis indicated that the group of patients lacking the long arm of chromosome 17 demonstrated the highest rate of liver metastasis. No significant association was observed between the frequency of liver metastases for synchronous and heterochronous colorectal cancer cases and gene variation (P=0.206). However, when these liver metastasis cases were divided into the synchronous and heterochronous types, the ratio of each was significantly different between gene variation groups, classified by the existence of the 17q deletion (P=0.023). These results indicate that the deletion of 17q may act as a predictive marker of liver metastasis in postoperative states.

Spindle-cell ameloblastic carcinoma: A case report with immunohistochemical, ultrastructural, and comparative genomic hybridization analyses.

Spindle-cell ameloblastic carcinoma is a classification proposed for a group of rare odontogenic carcinomas with sarcomatoid components and is distinguished from odontogenic carcinosarcoma. We report a case of spindle-cell ameloblastic carcinoma of the right mandible that occurred in a 67-year-old Japanese man. Growth of the tumor was destructive, there was extensive lung metastasis, and the outcome was unfavorable. Ultrastructural and immunohistochemical examination showed the spindle-cell component of the tumor to be epithelial in character. A gain of 5q with amplification of 5q13 was detected in the tumor by comparative genomic hybridization.

Katanin p60 contributes to microtubule instability around the midbody and facilitates cytokinesis in rat cells.

The completion of cytokinesis is crucial for mitotic cell division. Cleavage furrow ingression is followed by the breaking and resealing of the intercellular bridge, but the detailed mechanism underlying this phenomenon remains unknown. Katanin is a microtubule-severing protein comprised of an AAA ATPase subunit and an accessory subunit designated as p60 and p80, respectively. Localization of katanin p60 was observed at the midzone to midbody from anaphase to cytokinesis in rat cells, and showed a ring-shaped distribution in the gap between the inside of the contractile ring and the central spindle bundle in telophase. Katanin p60 did not bind with p80 at the midzone or midbody, and localization was shown to be dependent on microtubules. At the central spindle and the midbody, no microtubule growth plus termini were seen with katanin p60, and microtubule density was inversely correlated with katanin p60 density in the region of katanin p60 localization that seemed to lead to microtubule destabilization at the midbody. Inhibition of katanin p60 resulted in incomplete cytokinesis by regression and thus caused the appearance of binucleate cells. These results suggest that katanin p60 contributes to microtubule instability at the midzone and midbody and facilitates cytokinesis in rat cells.

Status of the anaplastic lymphoma kinase (ALK) gene in inflammatory breast carcinoma.

BACKGROUND: Although preliminary reports suggest that ALK gene amplification may occur in inflammatory breast cancer (IBC), data are limited. We performed a comprehensive investigation of the status of ALK gene in IBC. METHODS: We used core biopsy (CB) samples from 30 IBC patients for immunohistochemistry (IHC), 25 of these samples for fluorescence in situ hybridization (FISH) of ALK gene rearrangement, 8 for chromosome 2 aneusomy, and 20 microdissected frozen CBs for array comparative genomic hybridization (CGH) and mRNA analysis. RESULTS: All 30 samples were negative for ALK protein expression by IHC. FISH analysis showed no EML4-ALK gene rearrangement in any samples, although 16 of the 25 samples (64%) contained 3 to 4 extra copies of the ALK gene, and chromosome 2 aneusomy was found in 7 of 8 samples that had extra copies of the ALK gene. Only 3 of the 20 samples showed evidence of mild ALK gene amplification by array CGH. mRNA analysis revealed that mRNA expression of ALK was not significantly higher in these samples compared with samples that showed no evidence of ALK gene amplification in CGH analysis, nor was mRNA expression of ALK significantly different in tumor compared with 5 normal breast samples (P > 0.05, t test). CONCLUSION: Our comprehensive evaluation suggests that ALK gene rearrangement did not occur in the IBC patients studied. The significance of our finding of mildly increased copy numbers of the ALK gene resulting from chromosome 2 aneusomy rather than mild amplification of the ALK gene requires further investigation.

Anti-tumor effect against human cancer xenografts by a fully human monoclonal antibody to a variant 8-epitope of CD44R1 expressed on cancer stem cells.

BACKGROUND: CD44 is a major cellular receptor for hyaluronic acids. The stem structure of CD44 encoded by ten normal exons can be enlarged by ten variant exons (v1-v10) by alternative splicing. We have succeeded in preparing MV5 fully human IgM and its class-switched GV5 IgG monoclonal antibody (mAb) recognizing the extracellular domain of a CD44R1 isoform that contains the inserted region coded by variant (v8, v9 and v10) exons and is expressed on the surface of various human epithelial cancer cells. METHODS AND PRINCIPAL FINDINGS: We demonstrated the growth inhibition of human cancer xenografts by a GV5 IgG mAb reshaped from an MV5 IgM. The epitope recognized by MV5 and GV5 was identified to a v8-coding region by the analysis of mAb binding to various recombinant CD44 proteins by enzyme-linked immunosorbent assay. GV5 showed preferential reactivity against various malignant human cells versus normal human cells assessed by flow cytometry and immunohistological analysis. When ME180 human uterine cervix carcinoma cells were subcutaneously inoculated to athymic mice with GV5, significant inhibition of tumor formation was observed. Furthermore, intraperitoneal injections of GV5markedly inhibited the growth of visible established tumors from HSC-3 human larynx carcinoma cells that had been subcutaneously transplanted one week before the first treatment with GV5. From in vitro experiments, antibody-dependent cellular cytotoxicity and internalization of CD44R1 seemed to be possible mechanisms for in vivo anti-tumor activity by GV5. CONCLUSIONS: CD44R1 is an excellent molecular target for mAb therapy of cancer, possibly superior to molecules targeted by existing therapeutic mAb, such as Trastuzumab and Cetuximab recognizing human epidermal growth factor receptor family.

Crystal structure of a chaperone complex that contributes to the assembly of yeast 20S proteasomes.

Eukaryotic 20S proteasomes are composed of two alpha-rings and two beta-rings, which form an alphabetabetaalpha stacked structure. Here we describe a proteasome-specific chaperone complex, designated Dmp1-Dmp2, in budding yeast. Dmp1-Dmp2 directly bound to the alpha5 subunit to facilitate alpha-ring formation. In Deltadmp1 cells, alpha-rings lacking alpha4 and decreased formation of 20S proteasomes were observed. Dmp1-Dmp2 interacted with proteasome precursors early during proteasome assembly and dissociated from the precursors before the formation of half-proteasomes. Notably, the crystallographic structures of Dmp1 and Dmp2 closely resemble that of PAC3-a mammalian proteasome-assembling chaperone; nonetheless, neither Dmp1 nor Dmp2 showed obvious sequence similarity to PAC3. The structure of the Dmp1-Dmp2-alpha5 complex reveals how this chaperone functions in proteasome assembly and why it dissociates from proteasome precursors before the beta-rings are assembled.

Regulation of CD8+ T cell development by thymus-specific proteasomes.

Proteasomes are responsible for generating peptides presented by the class I major histocompatibility complex (MHC) molecules of the immune system. Here, we report the identification of a previously unrecognized catalytic subunit called beta5t. beta5t is expressed exclusively in cortical thymic epithelial cells, which are responsible for the positive selection of developing thymocytes. Although the chymotrypsin-like activity of proteasomes is considered to be important for the production of peptides with high affinities for MHC class I clefts, incorporation of beta5t into proteasomes in place of beta5 or beta5i selectively reduces this activity. We also found that beta5t-deficient mice displayed defective development of CD8(+) T cells in the thymus. Our results suggest a key role for beta5t in generating the MHC class I-restricted CD8(+) T cell repertoire during thymic selection.

Cooperation of multiple chaperones required for the assembly of mammalian 20S proteasomes.

The 20S proteasome is a catalytic core of the 26S proteasome, a central enzyme in the degradation of ubiquitin-conjugated proteins. It is composed of 14 distinct gene products that form four stacked rings of seven subunits each, alpha(1-7)beta(1-7)beta(1-7)alpha(1-7). It is reported that the biogenesis of mammalian 20S proteasomes is assisted by proteasome-specific chaperones, named PAC1, PAC2, and hUmp1, but the details are still unknown. Here, we report the identification of a chaperone, designated PAC3, as a component of alpha rings. Although it can intrinsically bind directly to both alpha and beta subunits, PAC3 dissociates before the formation of half-proteasomes, a process coupled with the recruitment of beta subunits and hUmp1. Knockdown of PAC3 impaired alpha ring formation. Further, PAC1/2/3 triple knockdown resulted in the accumulation of disorganized half-proteasomes that are incompetent for dimerization. Our results describe a cooperative system of multiple chaperones involved in the correct assembly of mammalian 20S proteasomes.

Down-regulation of nuclear protein ICBP90 by p53/p21Cip1/WAF1-dependent DNA-damage checkpoint signals contributes to cell cycle arrest at G1/S transition.

Checkpoints, which monitor DNA damage and regulate cell cycle progression, ensure genomic integrity and prevent the propagation of transformed cells. DNA damage activates the p53-dependent checkpoint pathway that induces expression of p21Cip1/WAF1, resulting in cell cycle arrest at G1/S transition by inhibition of cdk activity and DNA replication. ICBP90 was identified as a nuclear protein that binds to the TopoII alpha gene promoter and is speculated to be involved in DNA replication. ICBP90 expression is cell cycle regulated in normal cells but stably high throughout cell cycle in various cancer cell lines. We here demonstrate that ICBP90 expression is down-regulated by the p53/p21Cip1/WAF1-dependent DNA damage checkpoint signals. The reduction of ICBP90 appeared to be caused by both transcriptional suppression and protein degradation. Adenoviral expression of p21Cip1/WAF1 directly led to ICBP90 reduction in p53-/- HCT116 cells without DNA damage. Furthermore, ICPB90 depletion by RNA interference significantly blocked G1/S transition after DNA damage in HeLa cells. The down-regulation of ICBP90 is an important mechanism for cell cycle arrest at G1/S transition, which is induced by the activation of a p53/p21Cip1/WAF1-dependent DNA-damage checkpoint. Deregulation of ICBP90 may impair the control of G1/S transition during checkpoint activation and lead to genomic instability.