Dynamin 1 is important for microtubule organization and stabilization in glomerular podocytes.

Dynamin 1 is a neuronal endocytic protein that participates in vesicle formation by scission of invaginated membranes. Dynamin 1 is also expressed in the kidney; however, its physiological significance to this organ remains unknown. Here, we show that dynamin 1 is crucial for microtubule organization and stabilization in glomerular podocytes. By immunofluorescence and immunoelectron microscopy, dynamin 1 was concentrated at microtubules at primary processes in rat podocytes. By immunofluorescence of differentiated mouse podocytes (MPCs), dynamin 1 was often colocalized with microtubule bundles, which radially arranged toward periphery of expanded podocyte. In dynamin 1-depleted MPCs by RNAi, α-tubulin showed a dispersed linear filament-like localization, and microtubule bundles were rarely observed. Furthermore, dynamin 1 depletion resulted in the formation of discontinuous, short acetylated α-tubulin fragments, and the decrease of microtubule-rich protrusions. Dynamins 1 and 2 double-knockout podocytes showed dispersed acetylated α-tubulin and rare protrusions. In vitro, dynamin 1 polymerized around microtubules and cross-linked them into bundles, and increased their resistance to the disassembly-inducing reagents Ca2+ and podophyllotoxin. In addition, overexpression and depletion of dynamin 1 in MPCs increased and decreased the nocodazole resistance of microtubules, respectively. These results suggest that dynamin 1 supports the microtubule bundle formation and participates in the stabilization of microtubules.

Internalization of AMPA-type Glutamate Receptor in the MIN6 Pancreatic ß-cell Line.

The activity of AMPA-type glutamate receptor is involved in insulin release from pancreatic ß-cells. However, the mechanism and dynamics that underlie AMPA receptor-mediated insulin release in ß-cells is largely unknown. Here, we show that AMPA induces internalization of glutamate receptor 2/3 (GluR2/3), AMPA receptor subtype, in the mouse ß-cell line MIN6. Immunofluorescence experiments showed that GluR2/3 appeared as fine dots that were distributed throughout MIN6 cells. Intracellular GluR2/3 co-localized with AP2 and clathrin, markers for clathrin-coated pits and vesicles. Immunoelectron microscopy revealed that GluR2/3 was also localized at plasma membrane. Surface biotinylation and immunofluorescence measurements showed that addition of AMPA caused an approximate 1.8-fold increase in GluR2/3 internalization under low-glucose conditions. Furthermore, internalized GluR2 largely co-localized with EEA1, an early endosome marker. In addition, GluR2/3 co-immunoprecipitated with cortactin, a F-actin binding protein. Depletion of cortactin by RNAi in MIN6 cells altered the intracellular distribution of GluR2/3, suggesting that cortactin is involved in internalization of GluR2/3 in MIN6 cells. Taken together, our results suggest that pancreatic ß-cells adjust the amount of AMPA-type GluR2/3 on the cell surface to regulate the receptive capability of the cell for glutamate.Key words: endocytosis, GluR2, AMPA, cortactin, MIN6.

Exogenous DKK-3/REIC inhibits Wnt/ß-catenin signaling and cell proliferation in human kidney cancer KPK1.

The third member of the Dickkopf family (DKK-3), also known as reduced expression in immortalized cells (REIC), is a tumor suppressor present in a variety of tumor cells. Regarding the regulation of the Wnt/ß-catenin signaling pathway, exogenous DKK-1 and DKK-2 are reported to inhibit Wnt signaling by binding the associated effectors. However, whether exogenous DKK-3 inhibits Wnt signaling remains unclear. A recombinant protein of human full-length DKK-3 was used to investigate the exogenous effects of the protein in vitro in KPK1 human renal cell carcinoma cells. It was demonstrated that the expression of phosphorylated (p-)ß-catenin (inactive form as the transcriptional factor) was increased in KPK1 cells treated with the exogenous DKK-3 protein. The levels of non-p-ß-catenin (activated form of ß-catenin) were consistently decreased. It was revealed that the expression of transcription factor (TCF) 1 and c-Myc, the downstream transcription factors of the Wnt/ß-catenin signaling pathway, was inhibited following treatment with DKK-3. A cancer cell viability assay confirmed the anti-proliferative effects of exogenous DKK-3 protein, which was consistent with a suppressed Wnt/ß-catenin signaling cascade. In addition, as low-density lipoprotein receptor-related protein 6 (LRP6) is a receptor of DKK-1 and DKK-2 and their interaction on the cell surface inhibits Wnt/ß-catenin signaling, it was examined whether the exogenous DKK-3 protein affects LRP6-mediated Wnt/ß-catenin signaling. The LRP6 gene was silenced and the effects of DKK-3 on the time course of the upregulation of p-ß-catenin expression were subsequently analyzed. Notably, LRP6 depletion elevated the base level of p-ß-catenin; however, there was no significant effect on its upregulation course or expression pattern. These findings indicate that exogenous DKK-3 upregulates p-ß-catenin and inhibits Wnt/ß-catenin signaling in an LRP6-independent manner. Therefore, exogenous DKK-3 protein may inhibit the proliferation of KPK1 cells via inactivating Wnt/ß-catenin signaling.

Overexpression of REIC/Dkk-3 suppresses the expression of CD147 and inhibits the proliferation of human bladder cancer cells.

Our group previously developed an adenoviral vector encoding the REIC/Dkk-3 gene (Ad-REIC), a tumor suppressor, for cancer gene therapy. The Ad-REIC agent induces apoptosis and inhibits invasion in a number of cancer cell lines; however, the molecular mechanisms underlying its effects remain unclear. Cluster of differentiation 147 (CD147), also known as extracellular matrix metalloproteinase inducer (EMMPRIN), is a key molecule that promotes cancer proliferation and invasion. In order to elucidate the therapeutic mechanism of Ad-REIC, its effect on the expression of CD147 in human bladder cancer KK47 cells was investigated. Treatment with Ad-REIC markedly downregulated the expression of CD147 and significantly inhibited cellular proliferation. Since the expression of CD147 is reported to be under the positive control of mitogen-activated protein kinase (MAPK) signaling and the c-Myc protein, the correlations between the expression of CD147 and the activation of MAPKs or the expression of c-Myc were examined. Unexpectedly, no positive correlation was observed between the level of CD147 and the potential regulators that were assessed, indicating that another signaling pathway is responsible for the downregulation of CD147. The results from the present study demonstrate that Ad-REIC treatment can significantly downregulate the expression of CD147 in bladder cancer cells. Downregulation of the cancer-progression factor CD147 may be a novel mechanism that underlies the therapeutic effects of Ad-REIC treatment.

The Downregulation of the Expression of CD147 by Tumor Suppressor REIC/Dkk-3, and Its Implication in Human Prostate Cancer Cell Growth Inhibition.

The cluster of differentiation 147 (CD147), also known as EMMPRIN, is a key molecule that promotes cancer progression. We previously developed an adenoviral vector encoding a tumor suppressor REIC/Dkk-3 gene (Ad-REIC) for cancer gene therapy. The therapeutic effects are based on suppressing the growth of cancer cells, but, the underlying molecular mechanism has not been fully clarified. To elucidate this mechanism, we investigated the effects of Ad-REIC on the expression of CD147 in LNCaP prostate cancer cells. Western blotting revealed that the expression of CD147 was significantly suppressed by Ad-REIC. Ad-REIC also suppressed the cell growth of LNCaP cells. Since other researchers have demonstrated that phosphorylated mitogen-activated protein kinases (MAPKs) and c-Myc protein positively regulate the expression of CD147, we investigated the correlation between the CD147 level and the activation of MAPK and c-Myc expression. Unexpectedly, no positive correlation was observed between CD147 and its possible regulators, suggesting that another signaling pathway was involved in the downregulation of CD147. This is the first study to show the downregulation of CD147 by Ad-REIC in prostate cancer cells. At least some of the therapeutic effects of Ad-REIC may be due to the downregulation of the cancer-progression factor, CD147.

The cysteine-rich core domain of REIC/Dkk-3 is critical for its effect on monocyte differentiation and tumor regression.

Reduced expression in immortalized cells (REIC)/Dickkopf (Dkk)-3 is a tumor-suppressor gene and has been studied as a promising therapeutic gene for cancer gene therapy. Intratumoral injection of an adenovirus vector carrying the human REIC/Dkk-3 gene (Ad-REIC) elicits cancer cell-specific apoptosis and anticancer immune responses. The cytokine-like effect of secretory REIC/Dkk-3 on the induction of dendritic cell (DC)-like cell differentiation from monocytes plays a role in systemic anticancer immunity. In the present study, we generated recombinant full-length and N-terminally truncated REIC/Dkk-3 to characterize the biological activity of the protein. During the purification procedure, we identified a 17 kDa cysteine-rich stable product (C17-REIC) showing limited degradation. Further analysis showed that the C17-REIC domain was sufficient for the induction of DC-like cell differentiation from monocytes. Concomitant with the differentiation of DCs, the REIC/Dkk-3 protein induced the phosphorylation of glycogen synthase kinase 3ß (GSK-3ß) and signal transducers and activators of transcription (STAT) at a level comparable to that of granulocyte/macrophage colony-stimulating factor. In a mouse model of subcutaneous renal adenocarcinoma, intraperitoneal injection of full-length and C17-REIC proteins exerted anticancer effects in parallel with the activation of immunocompetent cells such as DCs and cytotoxic T lymphocytes in peripheral blood. Taken together, our results indicate that the stable cysteine-rich core region of REIC/Dkk-3 is responsible for the induction of anticancer immune responses. Because REIC/Dkk-3 is a naturally circulating serum protein, the upregulation REIC/Dkk-3 protein expression could be a promising option for cancer therapy.

A vaccine strategy with multiple prostatic acid phosphatase-fused cytokines for prostate cancer treatment.

Immunotherapy is one of the attractive treatment strategies for advanced prostate cancer. The US Food and Drug Administration (FDA) previously approved the therapeutic vaccine, sipuleucel-T, which is composed of autologous antigen-presenting cells cultured with a fusion protein [prostatic acid phosphatase (PAP) and granulocyte-macrophage colony-stimulating factor (GMCSF)]. Although sipuleucel-T has been shown to prolong the median survival of patients for 4.1 months, more robust therapeutic effects may be expected by modifying the vaccination protocol. In the present study, we aimed to develop and validate a novel vaccination strategy using multiple PAP-fused cytokines for prostate cancer treatment. Using a super gene expression (SGE) system that we previously established to amplify the production of a recombinant protein, significant amounts of PAP-fused cytokines [human GMCSF, interleukin-2 (IL2), IL4, IL7 and mouse GMCSF and IL4] were obtained. We examined the activity of the fusion proteins in vitro to validate their cytokine functions. A significant upregulation of dendritic cell differentiation from monocytes was achieved by PAP-GMCSF when used with the other PAP-fused cytokines. The PAP-fused human IL2 significantly increased the proliferation of lymphocytes, as determined by flow cytometry. We also investigated the in vivo therapeutic effects of multiple PAP-fused cytokines in a mouse prostate cancer model bearing prostate-specific antigen (PSA)- and PAP-expressing tumors. The simultaneous intraperitoneal administration of PAP-GMCSF, -IL2, -IL4 and -IL7 significantly prevented tumor induction and inhibited the tumor growth in the PAP-expressing tumors, yet not in the PSA-expressing tumors. The in vivo therapeutic effects with the multiple PAP-fused cytokines were superior to the effects of PAP-GMCSF alone. We thus demonstrated the advantages of the combined use of multiple PAP-fused cytokines including PAP-GMCSF, and propose a promising prostatic antigen-vaccination strategy to enhance the therapeutic effects.

Dramatic increase in expression of a transgene by insertion of promoters downstream of the cargo gene.

For expression of genes in mammalian cells, various vectors have been developed using promoters including CMV, EF-1α, and CAG promoters and have been widely used. However, such expression vectors sometimes fail to attain sufficient expression levels depending on the nature of cargo genes and/or on host cell types. In the present study, we aimed to develop a potent promoter system that enables high expression levels of cargo genes ubiquitously in many different cell types. We found that insertion of an additional promoter downstream of a cargo gene greatly enhanced the expression levels. Among the constructs we tested, C-TSC cassette (C: CMV-RU5' located upstream; TSC: another promoter unit composed of triple tandem promoters, hTERT, SV40, and CMV, located downstream of the cDNA plus a polyadenylation signal) had the most potent capability, showing far higher efficiency than that of potent conventional vector systems. The results indicate that the new expression system is useful for production of recombinant proteins in mammalian cells and for application as a gene therapeutic measure.

A novel gene expression system strongly enhances the anticancer effects of a REIC/Dkk-3-encoding adenoviral vector.

Gene expression systems with various promoters, including the cytomegalovirus (CMV) promoter, have been developed to increase the gene expression in a variety of normal and cancer cells. In particular, in the clinical trials of cancer gene therapy, a more efficient and robust gene expression system is required to achieve sufficient therapeutic outcomes. By inserting the triple translational enhancer sequences of human telomerase reverse transcriptase (hTERT), Simian virus 40 (SV40) and CMV downstream of the sequence of the BGH polyA, we were able to develop a novel gene expression system that significantly enhances the expression of the genes of interest. We termed this novel gene expression cassette the super gene expression (SGE) system, and herein verify the utility of the SGE cassette for a replication-deficient adenoviral vector. We newly developed an adenoviral vector expressing the tumor suppressor, reduced expression in immortalized cells (REIC)/Dickkopf-3 (Dkk-3), based on the CMV promoter-driven SGE system (Ad-SGE-REIC) and compared the therapeutic utility of Ad-SGE-REIC with that of the conventional adenoviral vectors (Ad-CMV-REIC or Ad-CAG-REIC). The results demonstrated that the CMV promoter-SGE system allows for more potent gene expression, and that the Ad-SGE-REIC is superior to conventional adenoviral systems in terms of the REIC protein expression and therapeutic effects. Since the SGE cassette can be applied for the expression of various therapeutic genes using various vector systems, we believe that this novel system will become an innovative tool in the field of gene expression and gene therapy.

N'-[4-(dipropylamino)benzylidene]-2-hydroxybenzohydrazide is a dynamin GTPase inhibitor that suppresses cancer cell migration and invasion by inhibiting actin polymerization.

Dynasore, a specific dynamin GTPase inhibitor, suppresses lamellipodia formation and cancer cell invasion by destabilizing actin filaments. In search for novel dynamin inhibitors that suppress actin dynamics more efficiently, dynasore analogues were screened. N'-[4-(dipropylamino)benzylidene]-2-hydroxybenzohydrazide (DBHA) markedly reduced in vitro actin polymerization, and dose-dependently inhibited phosphatidylserine-stimulated dynamin GTPase activity. DBHA significantly suppressed both the recruitment of dynamin 2 to the leading edge in U2OS cells and ruffle formation in H1299 cells. Furthermore, DBHA suppressed both the migration and invasion of H1299 cells by approximately 70%. Furthermore, intratumoral DBHA delivery significantly repressed tumor growth. DBHA was much less cytotoxic than dynasore. These results strongly suggest that DBHA inhibits dynamin-dependent actin polymerization by altering the interactions between dynamin and lipid membranes. DBHA and its derivative may be potential candidates for potent anti-cancer drugs.

Preclinical biodistribution and safety study of reduced expression in immortalized cells/Dickkopf-3-encoding adenoviral vector for prostate cancer gene therapy.

The biodistribution and safety of adenoviral vectors encoding the human REIC/Dkk-3 tumor suppressor gene (Ad-REIC) were examined in this preclinical study for in situ prostate cancer gene therapy. First, the in vitro apoptotic effects of Ad-REIC in normal and cancer cells derived from the prostate and liver were examined. Significant apoptotic effects were observed at 100 MOI (multiplicity of infection) in prostate cancer cells (LNCaP, PC3) and hepatoma cells (HEP3B and HEPG2); however, no effects were seen in normal cells. To analyze the safety of intraprostatic Ad-REIC administration, the biodistribution and histology after Ad-REIC injection were evaluated in various organs of normal male C57BL6 mice. In a supporting study, vector dissemination following intravenous injection of Ad-REIC into tail veins was determined. To evaluate whether Ad-REIC was present in the collected tissue specimens, human REIC gene detection was performed using DNA-PCR. Intraprostatic treatment administered at lower doses showed vector biodistribution into the colon, urinary bladder and prostate. At higher doses, vector dissemination was observed in tissues more distant from the prostate, including the lung, thymus, heart, liver and adrenal gland. After intravenous injection of Ad-REIC, dissemination was observed in the liver and spleen. These results indicate that the biodistribution of Ad-REIC is determined by the dose and route of administration. Although acute inflammatory effects were observed in the prostate after intraprostatic administration at higher doses, no abnormal histological findings were noted in the other tissues, including those of intravenously treated mice. Regarding the safety of Ad-REIC administration, no deaths and no signs of toxicity or unusual behavior were observed in the mice in any treatment group. Based on these preclinical experiments, adenovirus-mediated in situ REIC/Dkk-3 gene therapy is considered to be safe for use as a treatment for human prostate cancer.

A novel gene expression system for detecting viable bladder cancer cells.

A novel transcriptional system was developed that can robustly enhance cancer-specific gene expression. In the system, hTERT promoter-driven gene expression was enhanced by an advanced two-step transcriptional amplification (TSTA). This construct was used to develop a novel system for detection of bladder cancer cells. The current study evaluated the advanced TSTA system by examining the cancer-specific gene transcription in various bladder cancer cell lines. The system significantly enhanced cancer-specific luciferase gene expression in the bladder cancer cell lines in comparison to the previous expression system of one-step or conventional TSTA. The fold gain of the enhancement was significantly correlated to the telomerase activity of the cell lines. A green fluorescent protein (GFP) gene encoding plasmid vector was constructed where hTERT promoter-driving transcription is enhanced by the advanced TSTA to utilize the system for the imaging and detection of viable bladder cancer cells. The advanced TSTA-hTERT-GFP plasmid successfully induced cancer-specific gene expression, showing robust GFP expression in human bladder cancer cell lines, but no visible GFP expression in normal bladder urothelial cells. The control GFP plasmid with a CMV promoter yielded GFP expression in both normal bladder cells and cancer cells. The advanced TSTA-hTERT-GFP plasmid allowed selective visualization of viable human bladder cancer cells in mixed cell culture containing 10- and 100-fold more normal bladder urothelial cells. These findings indicate that the advanced TSTA-hTERT expressional system is a valuable tool for detecting viable bladder cancer cells. The current system can be applied for in vitro detection of bladder cancer cells in urine and other types of cancer cells disseminated in vivo.

Dynamin 2 associates with microtubules at mitosis and regulates cell cycle progression.

Dynamin, a ~100 kDa large GTPase, is known as a key player for membrane traffic. Recent evidence shows that dynamin also regulates the dynamic instability of microtubules by a mechanism independent of membrane traffic. As microtubules are highly dynamic during mitosis, we investigated whether the regulation of microtubules by dynamin is essential for cell cycle progression. Dynamin 2 intensely localized at the mitotic spindle, and the localization depended on its proline-rich domain (PRD), which is required for microtubule association. The deletion of PRD resulted in the impairment of cytokinesis, whereby the mutant had less effect on endocytosis. Interestingly, dominant-negative dynamin (K44A), which blocks membrane traffic but has no effect on microtubules, also blocked cytokinesis. On the other hand, the deletion of the middle domain, which binds to γ-tubulin, impaired the entry into mitosis. As both deletion mutants had no significant effect on endocytosis, dynamin 2 may participate in cell cycle progression by regulating the microtubules. These data suggest that dynamin may play a key role for cell cycle progression by two distinct pathways, membrane traffic and cytoskeleton.

Expression pattern of REIC/Dkk-3 in various cell types and the implications of the soluble form in prostatic acinar development.

The tumor suppressor REIC/Dkk-3 is a secretory protein which was originally identified to be downregulated in human immortalized cells. In the present study, we investigated the expression pattern of REIC/Dkk-3 in various cell types to characterize its physiological functions. We first examined the expression level of REIC/Dkk-3 in a broad range of cancer cell types and confirmed that it was significantly downregulated in all of the cell types. We also examined the tissue distribution pattern in a variety of normal mouse organs. Ubiquitous REIC/Dkk-3 protein expression was observed in the organs. The expression was abundant in the liver, heart and brain tissue, but was absent in the spleen and peripheral blood mononuclear cells. The immunohistochemical analyses revealed that the subcellular localization of REIC/Dkk-3 had a punctate pattern around the nucleus, indicating its association with secretory vesicles. In cancer cells stably transfected with REIC/Dkk-3, the protein was predominantly localized to the endoplasmic reticulum (ER) under observation with confocal microscopy. Because REIC/Dkk-3 was found to be abundantly expressed in the acinar epithelial cells of the mouse prostate, we analyzed the effects of recombinant REIC/Dkk-3 protein on the acinar morphogenesis of RWPE-1 cells, which are derived from human normal prostate epithelium. Statistically significant acinar growth was observed in the culture condition with 10 µg/ml REIC/Dkk-3 protein, implicating the soluble form in prostatic acinar development. Current results suggest that REIC/Dkk-3 may play a role in regulating the morphological process of normal tissue architecture through an autocrine and/or paracrine manner.

Dynasore, a dynamin inhibitor, suppresses lamellipodia formation and cancer cell invasion by destabilizing actin filaments.

Dynamic remodeling of actin filaments are bases for a variety of cellular events including cell motility and cancer invasion, and the regulation of actin dynamics implies dynamin, well characterized endocytotic protein. Here we report that dynasore, a inhibitor of dynamin GTPase, potently destabilizes F-actin in vitro, and it severely inhibits the formation of pseudopodia and cancer cell invasion, both of which are supported by active F-actin formation. Dynasore rapidly disrupted F-actin formed in brain cytosol in vitro, and the dynasore's effect on F-actin was indirect. Dynasore significantly suppressed serum-induced lamellipodia formation in U2OS cell. Dynasore also destabilized F-actin in resting cells, which caused the retraction of the plasma membrane. A certain amount of dynamin 2 in U2OS cells localized along F-actin, and co-localized with cortactin, a physiological binding partner of dynamin and F-actin. However, these associations of dynamin were partially disrupted by dynasore treatment. Furthermore, invasion activity of H1080 cell, a lung cancer cell line, was suppressed by approximately 40% with dynasore treatment. These results strongly suggest that dynasore potently destabilizes F-actin, and the effect implies dynamin. Dynasore or its derivative would be suitable candidates as potent anti-cancer drugs.

Immunological aspects of REIC/Dkk-3 in monocyte differentiation and tumor regression.

The REIC/Dkk-3 gene has been reported to be a tumor suppressor and the expression is significantly down-regulated in a broad range of cancer cell types. The protein is secretory, but the physiological function remains unclear. This study demonstrated that recombinant REIC/Dkk-3 protein induced the differentiation of human CD14+ monocytes into a novel cell type (REIC/Dkk-3Mo). REIC/Dkk-3Mo resembles immature dendritic cells generated with IL-4 and GM-CSF. Both these cell populations exhibit similar proportions of CD11c+, CD40+, CD86+ and HLA-DR+ cells and endocytic capacity, but REIC/Dkk-3Mo is negative for CD1a antigen. An analysis of the signal transducers and activators of transcription (STAT) pathways revealed that REIC/Dkk-3 induces phosphorylation of STAT 1 and STAT 3. Furthermore, intratumoral administration of REIC/Dkk-3 protein significantly suppressed tumor growth with CD11c+ and CD8+ (dendritic and killer T cell marker, respectively) cell accumulation and enhanced anti-cancer cytolytic activity of splenocytes. These data indicated a cytokine-like role of REIC/Dkk-3 protein in monocyte differentiation that might be exploited therapeutically.

CaM kinase I alpha-induced phosphorylation of Drp1 regulates mitochondrial morphology.

Mitochondria are dynamic organelles that frequently move, divide, and fuse with one another to maintain their architecture and functions. However, the signaling mechanisms involved in these processes are still not well characterized. In this study, we analyze mitochondrial dynamics and morphology in neurons. Using time-lapse imaging, we find that Ca2+ influx through voltage-dependent Ca2+ channels (VDCCs) causes a rapid halt in mitochondrial movement and induces mitochondrial fission. VDCC-associated Ca2+ signaling stimulates phosphorylation of dynamin-related protein 1 (Drp1) at serine 600 via activation of Ca2+/calmodulin-dependent protein kinase Ialpha (CaMKIalpha). In neurons and HeLa cells, phosphorylation of Drp1 at serine 600 is associated with an increase in Drp1 translocation to mitochondria, whereas in vitro, phosphorylation of Drp1 results in an increase in its affinity for Fis1. CaMKIalpha is a widely expressed protein kinase, suggesting that Ca2+ is likely to be functionally important in the control of mitochondrial dynamics through regulation of Drp1 phosphorylation in neurons and other cell types.

Regulation of clathrin-mediated endocytosis by p53.

The p53 gene encodes a multi-functional protein to prevent tumorigenesis. Although there have been many reports of the nuclear functions of p53, little is known about the cytosolic functions of p53. Here, we found that p53 is present in cytosol as well as nuclei under unstressed conditions and binds to clathrin heavy chain (CHC). CHC is known to play a role in receptor-mediated endocytosis. Based on our findings, we examined the effect of p53 on clathrin-mediated endocytosis of epidermal growth factor receptor (EGFR). Surprisingly, p53 co-localized with CHC at the plasma membrane in response to EGF stimulation. In cells with ablated p53 expression by RNAi, EGFR internalization was delayed and intracellular signaling from EGFR was altered. Thus, our findings provide evidence that cytosolic p53 may participate in the regulation of clathrin-mediated endocytosis to control the correct signaling from EGFR.

Involvement of calcineurin in glutamate-induced mitochondrial dynamics in neurons.

Alterations in the morphology and movement of mitochondria influence neuronal viability. However, the precise mechanisms of such alterations are unclear. In this study, we showed calcineurin was involved in the regulation of mitochondrial dynamics. Glutamate stimulation inhibited mitochondrial movement and decreased mitochondrial length in neurons. FK506 and cyclosporine A, calcineurin inhibitors, attenuated the effects of glutamate on mitochondrial dynamics. It was also found that glutamate treatment dephosphorylated, a proapoptotic protein, Bad and promoted its translocation to mitochondria in neurons via calcineurin. These results provide important new insights into intracellular signaling pathways that regulate mitochondrial dynamics and neuronal cell death.

Amphiphysin 1 is important for actin polymerization during phagocytosis.

Amphiphysin 1 is involved in clathrin-mediated endocytosis. In this study, we demonstrate that amphiphysin 1 is essential for cellular phagocytosis and that it is critical for actin polymerization. Phagocytosis in Sertoli cells was induced by stimulating phosphatidylserine receptors. This stimulation led to the formation of actin-rich structures, including ruffles, phagocytic cups, and phagosomes, all of which showed an accumulation of amphiphysin 1. Knocking out amphiphysin 1 by RNA interference in the cells resulted in the reduction of ruffle formation, actin polymerization, and phagocytosis. Phagocytosis was also drastically decreased in amph 1 (-/-) Sertoli cells. In addition, phosphatidylinositol-4,5-bisphosphate-induced actin polymerization was decreased in the knockout testis cytosol. The addition of recombinant amphiphysin 1 to the cytosol restored the polymerization process. Ruffle formation in small interfering RNA-treated cells was recovered by the expression of constitutively active Rac1, suggesting that amphiphysin 1 functions upstream of the protein. These findings support that amphiphysin 1 is important in the regulation of actin dynamics and that it is required for phagocytosis.