Wild-type Mouse Embryonic Fibroblast Cells Research Articles

Tributyltin activates the Keap1-Nrf2 pathway via a macroautophagy-independent reduction in Keap1.

Tributyltin (TBT) is an environmental chemical, which was used as an antifouling agent for ships. Although its use has been banned, it is still persistently present in ocean sediments. Although TBT reportedly causes various toxicity in mammals, few studies on the mechanisms of biological response against TBT toxicity exist. The well-established Keap1-Nrf2 pathway is activated as a cytoprotective mechanism under stressful conditions. The relationship between TBT and the Keap1-Nrf2 pathway remains unclear. In the present study, we evaluated the effect of TBT on the Keap1-Nrf2 pathway. TBT reduced Keap1 protein expression in Neuro2a cells, a mouse neuroblastoma cell line, after 6 hr without altering mRNA expression levels. TBT also promoted the nuclear translocation of Nrf2, a transcription factor for antioxidant proteins, after 12 hr and augmented the expression of heme oxygenase 1, a downstream protein of Nrf2. Furthermore, TBT decreased Keap1 levels in mouse embryonic fibroblast (MEF) cells, with the knockout of Atg5, which is essential for macroautophagy, as well as in wild-type MEF cells. These results suggest that TBT activates the Keap1-Nrf2 pathway via the reduction in the Keap1 protein level in a macroautophagy-independent manner. The Keap1-Nrf2 pathway is activated by conformational changes in Keap1 induced by reactive oxygen species or electrophiles. Furthermore, any unutilized Keap1 protein is degraded by macroautophagy. Understanding the novel mechanism governing the macroautophagy-independent reduction in Keap1 by TBT may provide insights into the unresolved biological response mechanism against TBT toxicity and the activation mechanism of the Keap1-Nrf2 pathway.

Biodegradable Polymer Theranostic Fluorescent Nanoprobe for Direct Visualization and Quantitative Determination of Antimicrobial Activity.

We report a biodegradable fluorescent theranostic nanoprobe design strategy for simultaneous visualization and quantitative determination of antibacterial activity for the treatment of bacterial infections. Cationic-charged polycaprolactone (PCL) was tailor-made through ring-opening polymerization methodology, and it was self-assembled into well-defined tiny 5.0 ± 0.1 nm aqueous nanoparticles (NPs) having a zeta potential of +45 mV. Excellent bactericidal activity at 10.0 ng/mL concentration was accomplished in Gram-negative bacterium Escherichia coli (E. coli) while maintaining their nonhemolytic nature in mice red blood cells (RBC) and their nontoxic trend in wild-type mouse embryonic fibroblast cells with a selectivity index of >104. Electron microscopic studies are evident of the E. coli membrane disruption mechanism by the cationic NP with respect to their high selectivity for antibacterial activity. Anionic biomarker 8-hydroxy-pyrene-1,3,6-trisulfonic acid (HPTS) was loaded in the cationic PCL NP via electrostatic interaction to yield a new fluorescent theranostic nanoprobe to accomplish both therapeutics and diagnostics together in a single nanosystem. The theranostic NP was readily degradable by a bacteria-secreted lipase enzyme as well as by lysosomal esterase enzymes at the intracellular compartments in <12 h and support their suitability for biomedical application. In the absence of bactericidal activity, the theranostic nanoprobe functions exclusively as a biomarker to exhibit strong green-fluorescent signals in live E. coli. Once it became active, the theranostic probe induces membrane disruption on E. coli, which enabled the costaining of nuclei by red fluorescent propidium iodide. As a result, live and dead bacteria could be visualized via green and orange signals (merging of red+green), respectively, during the course of the antibacterial activity by the theranostic probe. This has enabled the development of a new image-based fluorescence assay to directly visualize and quantitatively estimate the real-time antibacterial activity. Time-dependent bactericidal activity was coupled with selective photoexcitation in a confocal microscope to demonstrate the proof-of-concept of the working principle of a theranostic probe in E. coli. This new theranostic nanoprobe creates a new platform for the simultaneous probing and treating of bacterial infections in a single nanodesign, which is very useful for a long-term impact in healthcare applications.

Nanosecond pulsed electric fields induce the integrated stress response via reactive oxygen species-mediated heme-regulated inhibitor (HRI) activation.

The integrated stress response (ISR) is one of the most important cytoprotective mechanisms and is integrated by phosphorylation of the α subunit of eukaryotic translation initiation factor 2 (eIF2α). Four eIF2α kinases, heme-regulated inhibitor (HRI), double-stranded RNA-dependent protein kinase (PKR), PKR-like endoplasmic reticulum kinase (PERK), and general control nonderepressible 2 (GCN2), are activated in response to several stress conditions. We previously reported that nanosecond pulsed electric fields (nsPEFs) are a potential therapeutic tool for ISR activation. In this study, we examined which eIF2α kinase is activated by nsPEF treatment. To assess the responsible eIF2α kinase, we used previously established eIF2α kinase quadruple knockout (4KO) and single eIF2α kinase-rescued 4KO mouse embryonic fibroblast (MEF) cells. nsPEFs 70 ns in duration with 30 kV/cm electric fields caused eIF2α phosphorylation in wild-type (WT) MEF cells. On the other hand, nsPEF-induced eIF2α phosphorylation was completely abolished in 4KO MEF cells and was recovered by HRI overexpression. CM-H2DCFDA staining showed that nsPEFs generated reactive oxygen species (ROS), which activated HRI. nsPEF-induced eIF2α phosphorylation was blocked by treatment with the ROS scavenger N-acetyl-L-cysteine (NAC). Our results indicate that the eIF2α kinase HRI is responsible for nsPEF-induced ISR activation and is activated by nsPEF-generated ROS.

Ochratoxin A Sequentially Activates Autophagy and the Ubiquitin-Proteasome System.

Ochratoxin A (OTA) is a carcinogenic mycotoxin, which is produced by Aspergillus and Penicillium genera of fungi and commonly contaminates food and feed. We and others have previously shown that OTA causes sustained activation of PI3K/AKT and MAPK/ERK1-2 signaling pathways in different cell types and animal models. Given the close relationship between cellular signaling activity and protein stability, we were curious whether increased PI3K/AKT and MAPK/ERK1-2 signaling may be the result of OTA-stimulated alterations in proteolytic activity. We show that both of the major proteolytic systems, autophagy, and the ubiquitin-proteasome system (UPS), are activated upon OTA exposure in human kidney proximal tubule HK-2 and mouse embryonic fibroblast (MEF) cells. OTA stimulates transient autophagic activity at early time points of treatment but autophagic activity subsides after 6 h even in the sustained presence of OTA. Interestingly, OTA exposure also results in increased cell death in wild-type MEF cells but not in autophagy-halted Atg5-deficient cells, suggesting that autophagy exerts a pro-death effect on OTA-induced cytotoxicity. In addition, prolonged OTA exposure decreased ubiquitinated protein levels by increasing proteasomal activity. Using purified and cellular proteasomes, we observed enhanced chymotrypsin-, caspase-, and trypsin-like activities of the 26S but not the 20S proteasome in the presence of OTA. However, in the cellular context, increased proteasomal activity depended on prior induction of autophagy. Our results suggest that autophagy and subsequent UPS activation are responsible for sustained activation of PI3K/AKT and MAPK/ERK1-2 pathways through regulating the levels of critical phosphatases VHR/DUSP3, DUSP4, and PHLPP, which are known to be involved in OTA toxicity and carcinogenicity.

Enniatin B1-induced lysosomal membrane permeabilization in mouse embryonic fibroblasts

The mycotoxin enniatin B1 (ENN B1) is widely present in grain-based feed and food products. In the present study, we have investigated how this lipophilic and ionophoric molecule can affect the lysosomal stability and chaperone-mediated autophagy (CMA) in wild-type (WT) and in lysosome-associated membrane proteins (LAMP)-1/2 double-deficient (DD) mouse embryonic fibroblasts (MEF). The cell viability and lysosomal pH were assessed using the Neutral Red (NR) cytotoxicity assay and the LysoSensor® Yellow/Blue DND-160, respectively. Changes in the expression of the CMA-related components LAMP-2 and the chaperones heat shock cognate (hsc) 70 and heat shock protein (hsp) 90 were determined in cytosolic extracts by immunoblotting. In the NR assay, LAMP-1/2 DD MEF cells were significantly less sensitive to ENN B1 than WT MEF cells after 24h exposure to ENN B1 at levels of 2.5-10μmol/L. Exposure to ENN B1 at concentrations below the half maximal effective concentration (EC50) (1.5-1.7μmol/L) increased the lysosomal pH in WT MEF, but not in LAMP-1/2 DD cells, suggesting that lysosomal LAMP-2 is an early target of ENN B1-induced lysosomal alkalization and cytotoxicity in MEF cells. Additionally, cytosolic hsp90 and LAMP-2 levels slightly increased after exposure for 4h, indicating lysosomal membrane permeabilization (LMP). In summary, it appeared that ENN B1 can destabilize the LAMP-2 complex in the lysosomal membrane at concentrations close to the EC50, resulting in the alkalinization of lysosomes, partial LMP, and thereby leakage of CMA-associated components into the cytosol.

A porous collagen-carboxymethyl cellulose/hydroxyapatite composite for bone tissue engineering by bi-molecular template method

Inspired by the mechanism of bone formation, a porous collagen-carboxymethyl cellulose/hydroxyapatite (Col-CMC/HA) composite was designed and fabricated using a biomimetic template of Col and CMC protein-polysaccharide bi-molecules. The morphology, composition and physical properties of Col-CMC/HA composites were characterized systematically. It was found that the nano-HA homogenously distributed on the surface of Col-CMC bi-templates while the composite presented 3D porous structure with pore size from 100 μm to 300 μm. The porosities of composites were located at the range of 71%–85%. Besides, the compressive strength of composites was highly depended on the ratio of Col to CMC in the organic template. The optimized composite in respect to physical properties showed a compressive strength as high as 7.06 MPa, quite close to that of natural bone. The high relative growth rate of wild-type mouse embryonic fibroblasts cells was found for the composite, indicating a good biocompatibility. The organic-inorganic composite also behaved good in collagenase resistance and could be biodegraded in 8 weeks, with about 50% of initial weight left at the ratio of Col to CMC of 1:9. The results demonstrated that the Col-CMC/HA composite by bi-molecular template method was a rational and safe method to prepare biomaterials with tunable properties.

Biotin-Tagged Polysaccharide Vesicular Nanocarriers for Receptor-Mediated Anticancer Drug Delivery in Cancer Cells.

Biotin-conjugated multistimuli-responsive polysaccharide vesicular nanocarriers are designed and developed, for the first time, to accomplish receptor-mediated endocytosis in cancer cells and to deliver anticancer drugs to intracellular compartments. For this purpose, a new renewable hydrophobic unit was custom designed with redox-degradable disulfide and enzyme-biodegradable aliphatic ester chemical linkages, and it was conjugated along with biotin on the dextran backbone. The dextran derivative self-assembled into nanovesicles of <200 nm in size, which were characterized by dynamic and static light scattering, electron, and atomic force microscopes. Avidin-HABA assay established the high affinity of biotin-tagged dextran vesicles toward membrane-receptors up to 25 nM concentration. Doxorubicin-hydrochloride (DOX.HCl)-loaded dextran vesicles exhibited stable formulation in phosphate-buffered saline (PBS) and fetal bovine serum (FBS). Redox-degradation by glutathione (GSH) showed 60% drug release, whereas lysosomal esterase enzyme enabled >98% drug release in 12 h. Confocal microscope and flow cytometry-assisted time-dependent cellular uptake studies revealed that the biotin-receptors overexpressed in cervical cancer cells (HeLa) exhibited larger drug accumulation through the receptor-assisted endocytosis process. This process enabled the delivery of higher amount of DOX and significantly enhanced the killing in cancer cells (HeLa) compared to wild-type mouse embryonic fibroblast cells (WT-MEF, normal cells). Control experiments such as biotin pretreatment in cancer cells and energy-suppressed cellular uptake at 4 °C further supported the occurrence of receptor-mediated endocytosis by the biotin-tagged polymer vesicles. This report provides first insights into the targeted polysaccharide vesicle platform, and the proof-of-concept is successfully demonstrated in biotin receptor-overexpressed cervical cancer cells.

Inactivation of DNA-PK by knockdown DNA-PKcs or NU7441 impairs non-homologous end-joining of radiation-induced double strand break repair.

The DNA-dependent protein kinase (DNA-PK) complex plays a pivotal role in non-homologous end-joining (NHEJ) repair. We investigated the mechanism of NU7441, a highly selective DNA-PK inhibitor, in NHEJ-competent mouse embryonic fibroblast (MEF) cells and NHEJ-deficient cells and explored the feasibility of its application in radiosensitizing nasopharyngeal carcinoma (NPC) cells. We generated wild-type and DNA-PKcs−/− MEF cells. Clonogenic survival assays, flow cytometry, and immunoblotting were performed to study the effect of NU7441 on survival, cell cycle, and DNA repair. NU7441 profoundly radiosensitized wild-type MEF cells and SUNE-1 cells, but not DNA-PKcs−/− MEF cells. NU7441 significantly suppressed radiation-induced DSB repair post-irradiation through unrepaired and lethal DNA damage, the cell cycle arrest. The effect was associated with the activation of cell cycle checkpoints. The present study revealed a mechanism by which inhibition of DNA-PK sensitizes cells to irradiation suggesting that radiotherapy in combination with DNA-PK inhibitor is a promising paradigm for the management of NPC which merits further investigation.

The deubiquitinating enzyme Usp14 allosterically inhibits multiple proteasomal activities and ubiquitin-independent proteolysis

The proteasome-associated deubiquitinating enzyme Usp14/Ubp6 inhibits protein degradation by catalyzing substrate deubiquitination and by poorly understood allosteric actions. However, upon binding a ubiquitin chain, Usp14 enhances proteasomal degradation by stimulating ATP and peptide degradation. These studies were undertaken to clarify these seemingly opposite regulatory roles of Usp14 and their importance. To learn how the presence of Usp14 on 26S proteasomes influences its different activities, we compared enzymatic and regulatory properties of 26S proteasomes purified from wild-type mouse embryonic fibroblast cells and those lacking Usp14. The proteasomes lacking Usp14 had higher basal peptidase activity than WT 26S, and this activity was stimulated to a greater extent by adenosine 5'-O-(thiotriphosphate) (ATPγS) than with WT particles. These differences were clear even though Usp14 is present on only a minor fraction (30-40%) of the 26S in WT mouse embryonic fibroblast cells. Addition of purified Usp14 to the WT and Usp14-defficient proteasomes reduced both their basal peptidase activity and the stimulation by ATPγS. Usp14 inhibits these processes allosterically because a catalytically inactive Usp14 mutant also inhibited them. Proteasomes lacking Usp14 also exhibited greater deubiquitinating activity by Rpn11 and greater basal ATPase activity than WT particles. ATP hydrolysis by WT proteasomes is activated if they bind a ubiquitinated protein, which is loosely folded. Surprisingly, proteasomes lacking Usp14 could be activated by such proteins even without a ubiquitin chain present. Furthermore, proteasomes lacking Usp14 are much more active in degrading non-ubiquitinated proteins (e.g. Sic1) than WT particles. Thus, without a ubiquitinated substrate present, Usp14 suppresses multiple proteasomal activities, especially basal ATP consumption and degradation of non-ubiquitinated proteins. These allosteric effects thus reduce ATP hydrolysis by inactive proteasomes and nonspecific proteolysis and enhance proteasomal specificity for ubiquitinated proteins.

Abstract B11: Oncogenic KRAS regulates 4E-BP1, a repressor of cap-dependent translation, independently of growth factor activity

Abstract Oncogenic mutations in K-Ras often occur at high frequencies in human cancer resulting in the sustained activation of the ERK1/2 mitogen-activated protein kinase (MAPK) signaling pathway. However, the unique dependencies on upstream or downstream oncogenic signaling amongst the mutant K-Ras proteins remains elusive and often masked by the remaining wild-type allele, other Ras isoforms, or additional secondary genetic alterations. Thus, we employed an isogenic mouse embryonic fibroblast (MEFs) system devoid of H-, N-, and K-Ras alleles and reconstituted with a single Ras isoform to examine the dependencies of growth factors on K-Ras mediated control of translational regulation. Although oncogenic K-Ras G12D and G12V mutant MEF lines proliferated at twice the rate of K-Ras4b wild-type MEF cells, growth factor deprivation led to a cytostatic effect on the proliferation of all MEF lines. Analysis of signaling downstream of K-Ras revealed that phosphorylation of 4E-BP1, eIF4E, and ERK1/2 occurred in a growth factor-independent manner, which stood in contrast to the substantive effects of growth factor inhibition on AKT, p70S6K, and ribosomal protein S6 phosphorylation. Moreover, growth factor deprivation resulted in a marginal decrease of global protein synthesis with negligible effects on cap-dependent translation initiation in all K-Ras MEF cell lines. Taken together, these data suggest that oncogenic K-Ras signaling regulates 4E-BP1 phosphorylation independently of growth factor and mTORC1/2 effects. Citation Format: Jillian M. Silva, Rachel K. Bagni, Davide Ruggero, Frank McCormick. Oncogenic KRAS regulates 4E-BP1, a repressor of cap-dependent translation, independently of growth factor activity. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr B11.

Toll-Like Receptor Gene Expression during Trichinella spiralis Infection.

In Trichinella spiralis infection, type 2 helper T (Th2) cell-related and regulatory T (Treg) cell-related immune responses are the most important immune events. In order to clarify which Toll-like receptors (TLRs) are closely associated with these responses, we analyzed the expression of mouse TLR genes in the small intestine and muscle tissue during T. spiralis infection. In addition, the expression of several chemokine- and cytokine-encoding genes, which are related to Th2 and Treg cell mediated immune responses, were analyzed in mouse embryonic fibroblasts (MEFs) isolated from myeloid differentiation factor 88 (MyD88)/TIR-associated proteins (TIRAP) and Toll receptor-associated activator of interferons (TRIF) adapter protein deficient and wild type (WT) mice. The results showed significantly increased TLR4 and TLR9 gene expression in the small intestine after 2 weeks of T. spiralis infection. In the muscle, TLR1, TLR2, TLR5, and TLR9 gene expression significantly increased after 4 weeks of infection. Only the expression of the TLR4 and TLR9 genes was significantly elevated in WT MEF cells after treatment with excretory-secretory (ES) proteins. Gene expression for Th2 chemokine genes were highly enhanced by ES proteins in WT MEF cells, while this elevation was slightly reduced in MyD88/TIRAP-/- MEF cells, and quite substantially decreased in TRIF-/- MEF cells. In contrast, IL-10 and TGF-β expression levels were not elevated in MyD88/TIRAP-/- MEF cells. In conclusion, we suggest that TLR4 and TLR9 might be closely linked to Th2 cell and Treg cell mediated immune responses, although additional data are needed to convincingly prove this observation.

Core-shell polymer nanoparticles for prevention of GSH drug detoxification and cisplatin delivery to breast cancer cells.

Platinum drug delivery against the detoxification of cytoplasmic thiols is urgently required for achieving efficacy in breast cancer treatment that is over expressed by glutathione (GSH, thiol-oligopeptide). GSH-resistant polymer-cisplatin core-shell nanoparticles were custom designed based on biodegradable carboxylic functional polycaprolactone (PCL)-block-poly(ethylene glycol) diblock copolymers. The core of the nanoparticle was fixed as 100 carboxylic units and the shell part was varied using various molecular weight poly(ethylene glycol) monomethyl ethers (MW of PEGs = 100-5000 g mol(-1)) as initiator in the ring-opening polymerization. The complexation of cisplatin aquo species with the diblocks produced core-shell nanoparticles of 75 nm core with precise size control the particles up to 190 nm. The core-shell nanoparticles were found to be stable in saline solution and PBS and they exhibited enhanced stability with increase in the PEG shell thickness at the periphery. The hydrophobic PCL layer on the periphery of the cisplatin core behaved as a protecting layer against the cytoplasmic thiol residues (GSH and cysteine) and exhibited <5% of drug detoxification. In vitro drug-release studies revealed that the core-shell nanoparticles were ruptured upon exposure to lysosomal enzymes like esterase at the intracellular compartments. Cytotoxicity studies were performed both in normal wild-type mouse embryonic fibroblast cells (Wt-MEFs), and breast cancer (MCF-7) and cervical cancer (HeLa) cell lines. Free cisplatin and polymer drug core-shell nanoparticles showed similar cytotoxicity effects in the HeLa cells. In MCF-7 cells, the free cisplatin drug exhibited 50% cell death whereas complete cell death (100%) was accomplished by the polymer-cisplatin core-shell nanoparticles. Confocal microscopic images confirmed that the core-shell nanoparticles were taken up by the MCF-7 and HeLa cells and they were accumulated both at the cytoplasm as well at peri-nuclear environments. The present investigation lays a new foundation for the polymer-based core-shell nanoparticles approach for overcoming detoxification in platinum drugs for the treatment of GSH over-expressed breast cancer cells.

Methionine sulfoxide reductase B3 deficiency inhibits cell growth through the activation of p53–p21 and p27 pathways

Methionine sulfoxide reductase B3 (MsrB3) is an oxidoreductase in the endoplasmic reticulum that catalyzes the stereospecific reduction of methionine-R-sulfoxide to methionine. Here, we report the critical role and mechanisms of MsrB3 in cell proliferation. The deletion of MsrB3 led to a significant decrease in cell proliferation in mouse embryonic fibroblast (MEF) cells. MsrB3-knockout MEF cells showed increased p53 protein levels, compared to wild-type MEF cells, which subsequently elevated the protein level of cyclin-dependent kinase inhibitor p21. In addition, MsrB3 deficiency enhanced the protein level of p27, another cell cycle regulator, and caused cell cycle arrest at the G1 stage. The inhibitory effect of MsrB3 deficiency on cell proliferation through the activation of p53–p21 and p27 pathways was also confirmed in primary human dermal fibroblasts. Collectively, the data suggest that MsrB3 is a regulator of cell growth through the p53–p21 and p27 pathways.

Cross-talk between Aryl Hydrocarbon Receptor and the Inflammatory Response: A ROLE FOR NUCLEAR FACTOR-κB

The aryl hydrocarbon receptor (AhR) is involved in the regulation of immune responses, T-cell differentiation, and immunity. Here, we show that inflammatory stimuli such as LPS induce the expression of AhR in human dendritic cells (DC) associated with an AhR-dependent increase of CYP1A1 (cytochrome P4501A1). In vivo data confirmed the elevated expression of AhR by LPS and the LPS-enhanced 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated induction of CYP1A1 in thymus of B6 mice. Inhibition of nuclear factor-κB (NF-κB) repressed both normal and LPS-enhanced, TCDD-inducible, AhR-dependent gene expression and canonical pathway control of RelA-regulated AhR-responsive gene expression. LPS-mediated induction of AhR was NF-κB-dependent, as shown in mouse embryonic fibroblasts (MEFs) derived from Rel null mice. AhR expression and TCDD-mediated induction of CYP1A1 was significantly reduced in RelA-deficient MEF compared with wild type MEF cells and ectopic expression of RelA restored the expression of AhR and induction of CYP1A1 in MEF RelA null cells. Promoter analysis of the human AhR gene identified three putative NF-κB-binding elements upstream of the transcription start site. Mutation analysis of the AhR promoter identified one NF-κB site as responsible for mediating the induction of AhR expression by LPS and electrophoretic shift assays demonstrated that this NF-κB motif is recognized by the RelA/p50 heterodimer. Our results show for the first time that NF-κB RelA is a critical component regulating the expression of AhR and the induction of AhR-dependent gene expression in immune cells illustrating the interaction of AhR and NF-κB signaling.

Tumor suppressor WWOX participates in cell/cell recognition and migration

Metastatic cancer cells are essentially devoid of tumor suppressor WWOX expression, and this appears to be associated with their enhanced migration. A portion of WWOX is localized at the cell membrane/cytoskeleton area. By time‐lapse microscopy, we showed that wild type Wwox+/+ mouse embryonic fibroblast (MEF) cells migrated collectively, whereas the knockout Wwox−/− cells migrated individually. When wild type MEF cells encountered knockout cells, the knockout cells failed to recognize the wild type, although they are from the same mouse strain. The knockout Wwox−/− MEF cells reached out with their pseudopodia to touch the wild type from a distance (5–20 μm), and then moved backward significantly faster and divided simultaneously. The event was also shown in many WWOX‐negative malignant cancer cells upon encountering with WWOX‐expressing cells. Restoration of WWOX with ectopic gene expression or treatment with an inhibitor of DNA methylation restored collective migration. Molecular analysis revealed that when cells overexpressing a short N‐terminal segment of WWOX, they attracted upcoming visiting cells. But, they repelled the visitors if a short segment in the C‐terminal SDR domain of WWOX was overexpressed. Thus, WWOX participates in cell/cell recognition, and conformational alteration of membrane‐associated WWOX affects cell behavior. (Supported in part by NHRI, NSC and DOH, Taiwan, and DoD, USA)

C-IAP1 Binds and Processes PCSK9 Protein: Linking the c-IAP1 in a TNF-α Pathway to PCSK9-Mediated LDLR Degradation Pathway

Recent genetic studies have shown that PCSK9, one of the key genes in cholesterol metabolism, plays a critical role by controlling the level of low-density lipoprotein receptor. However, how PCSK9 mediates LDLR degradation is still unknown. By combining a shotgun proteomic method and differential analysis of natural occurring mutations of the PCSK9 gene, we found that an E3 ubiquitin ligase c-IAP1 binds and processes PCSK9 protein. One of the ‘gain-of-function’ mutations, S127R, is defective with respect to binding to c-IAP1, and thus has defective autocatalytic activity. Knockdown of c-IAP1 impairs PCSK9 processing and autocatalytic cleavage. In c-IAP1 null mouse embryonic fibroblasts (MEFs), there is a dramatic decrease in secreted mature PCSK9 protein accompanied by a significant increase in LDLR protein levels compared with matched wild-type MEF cells. c-IAP1 also acts as an E3 ligase for ubiquitination of PCSK9. Ubiquitin containing only lysine-27 mediated PCSK9 ubiquitination by c-IAP1. Given K27-linked polyubiquitination promotes lysosomal localization, the finding indicates the c-IAP1 acts on both secretion of PCSK9 and its lysosomal localization. The novel pathway described here will open new avenues for exploring novel disease treatments.

Distinct roles of mitochondria- and ER-localized Bcl-xLin apoptosis resistance and Ca2+homeostasis

Bcl-2 proteins are major regulators of cellular responses to intrinsic and extrinsic apoptotic stimuli. Among them, overexpression of the antiapoptotic protein Bcl-x(L) modulates intracellular Ca(2+) homeostasis and organelle-specific apoptotic signaling pathways. However, the specific activities of Bcl-x(L) at mitochondria and the endoplasmic reticulum (ER) have not been fully defined. To further explore this, we generated mouse embryonic fibroblast (MEF) cell lines deficient in Bcl-x(L) expression (Bcl-x-KO). Deficiency in Bcl-x(L) expression did not induce compensatory changes in the expression of other Bcl-2 proteins, and Bcl-x-KO MEF cells showed increased sensitivity to various apoptotic stimuli compared with wild-type MEF cells. Targeting Bcl-x(L) at mitochondria but not at the ER restored apoptosis protection in Bcl-x-KO MEF cells to the degree observed in wild-type MEF cells. However, expression of ER-targeted Bcl-x(L) but not mitochondrially targeted Bcl-x(L) was required to restore Ca(2+) homeostasis in Bcl-x-KO MEF cells. Of importance, ER-targeted Bcl-x(L) was able to protect cells against death stimuli in the presence of endogenous Bcl-x(L). These data indicate that mitochondrial Bcl-x(L) can regulate apoptosis independently of ER Bcl-x(L) and that when localized exclusively at the ER, Bcl-x(L) impinges on Ca(2+) homeostasis but does not affect apoptosis unless Bcl-x(L) is present in additional cellular compartments.

Ubiquitin- and MDM2 E3 Ligase-independent Proteasomal Turnover of Nucleostemin in Response to GTP Depletion

Nucleostemin (NS) is a nucleolar GTP-binding protein essential for ribosomal biogenesis, proliferation, and animal embryogenesis. It remains largely unclear how this protein is regulated. While working on its role in suppression of MDM2 and activation of p53, we observed that NS protein (but not mRNA) levels decreased drastically in response to GTP depletion. When trying to further elucidate the molecular mechanism(s) underlying this unusual phenomenon, we found that NS was degraded independently of ubiquitin and MDM2 upon GTP depletion. First, depletion of GTP by treating cells with mycophenolic acid decreased the level of NS without apparently affecting the levels of other nucleolar proteins. Second, mutant NS defective in GTP binding and exported to the nucleoplasm was much less stable than wild-type NS. Although NS was ubiquitinated in cells, its polyubiquitination was independent of Lys-48 or Lys-63 in the ubiquitin molecule. Inactivation of E1 in E1 temperature-sensitive mouse embryonic fibroblast (MEF) cells failed to prevent the proteasomal degradation of NS. The proteasomal turnover of NS was also MDM2-independent, as its half-life in p53/MDM2 double knock-out MEF cells was the same as that in wild-type MEF cells. Moreover, NS ubiquitination was MDM2-independent. Mycophenolic acid or doxorubicin induced NS degradation in various human cancerous cells regardless of the status of MDM2. Hence, these results indicate that NS undergoes a ubiquitin- and MDM2-independent proteasomal degradation when intracellular GTP levels are markedly reduced and also suggest that ubiquitination of NS may be involved in regulation of its function rather than stability.

Smad7-deficient mice show growth retardation with reduced viability

Smad7 is an inhibitory molecule induced by members of the transforming growth factor-β (TGF-β) family, including TGF-β, activin, nodal and bone morphogenetic proteins (BMPs). To elucidate the in vivo functions of Smad7, we generated conditional Smad7-knockout mice in which the Mad homology 2 (MH2) domain and the poly (A) signal sequence were flanked with loxP sites (floxed). The Smad7-floxed mice exhibited no obvious phenotype. Smad7 total-null mice on a C57BL/6 background died within a few days of birth, whereas mice with an ICR background developed to adulthood but were significantly smaller than wild-type mice. Unexpectedly, phospho-Smad2 and phospho-Smad3 were decreased in Smad7-deficient mouse embryonic fibroblast (MEF) cells, whereas phospho-Smad1/5/8 was similarly expressed in wild-type and Smad7-deficient MEF cells. Moreover, expression levels of TGF-β type I receptor (ALK5) were higher in Smad7-deficient MEF cells than in wild-type MEF cells. Plasminogen activator inhibitor-1 (PAI-1) and inhibitor of differentiation-1 (Id-1) mRNA were similarly expressed in wild-type and Smad7-deficient MEF cells. Some differences were observed in mitogen-activated protein kinase (MAPK)-signalling between wild-type and Smad7-deficient MEF cells. We demonstrated that Smad7 plays an important role in normal mouse growth and provide a useful tool for analysing Smad7 functions in vivo.

The Lipid Raft-Associated Protein CD98 Is Required for Vaccinia Virus Endocytosis

Mature vaccinia virus (vaccinia MV) infects a broad range of animals in vivo and cell cultures in vitro; however, the cellular receptors that determine vaccinia MV tropism and entry pathways are poorly characterized. Here, we performed quantitative proteomic analyses of lipid raft-associated proteins upon vaccinia MV entry into HeLa cells. We found that a type II membrane glycoprotein, CD98, is enriched in lipid rafts upon vaccinia MV infection compared to mock-infected HeLa cells. The knockdown of CD98 expression in HeLa cells significantly reduced vaccinia MV entry. Furthermore, CD98 knockout (KO) mouse embryonic fibroblasts (MEFs) also exhibited reduced vaccinia MV infectivity without affecting MV attachment to cells, suggesting a role for CD98 in the postbinding step of virus entry. Further characterization with inhibitors and dominant negative proteins that block different endocytic pathways revealed that vaccinia MV entry into MEFs occurs through a clathrin-independent, caveolin-independent, dynamin-dependent, fluid-phase endocytic pathway, implying that CD98 plays a specific role in the vaccinia MV endocytic pathway. Infections of wild-type and CD98 KO MEF cells with different strains of vaccinia MV provided further evidence that CD98 plays a specific role in MV endocytosis but not in plasma membrane fusion. Finally, different CD98-C69 chimeric proteins were expressed in CD98 KO MEFs, but none were able to reconstitute MV infectivity, suggesting that the overall structure of the CD98 protein is required for vaccinia MV endocytosis.