Ceramide reverses brefeldin A (BFA) resistance in BFA-resistant cell lines

COPI Vesicle Disruption Inhibits Mineralization via mTORC1-Mediated Autophagy

Bone mineralization is a sophisticated regulated process composed of crystalline calcium phosphate and collagen fibril. Autophagy, an evolutionarily conserved degradation system, whereby double-membrane vesicles deliver intracellular macromolecules and organelles to lysosomes for degradation, has recently been shown to play an essential role in mineralization. However, the formation of autophagosomes in mineralization remains to be determined. Here, we show that Coat Protein Complex I (COPI), responsible for Golgi-to-ER transport, plays a pivotal role in autophagosome formation in mineralization. COPI vesicles were increased after osteoinduction, and COPI vesicle disruption impaired osteogenesis. Mechanistically, COPI regulates autophagy activity via the mTOR complex 1 (mTORC1) pathway, a key regulator of autophagy. Inhibition of mTOR1 rescues the impaired osteogenesis by activating autophagy. Collectively, our study highlights the functional importance of COPI in mineralization and identifies COPI as a potential therapeutic target for treating bone-related diseases.

Golgi bypass: skirting around the heart of classical secretion

Classical secretion consists of the delivery of transmembrane and soluble proteins to the plasma membrane and the extracellular medium, respectively, and is mediated by the organelles of the secretory pathway, the Endoplasmic Reticulum (ER), the ER exit sites, and the Golgi, as described by the Nobel Prize winner George Palade (Palade 1975). At the center of this transport route, the Golgi stack has a major role in modifying, processing, sorting, and dispatching newly synthesized proteins to their final destinations. More recently, however, it has become clear that an increasing number of transmembrane proteins reach the plasma membrane unconventionally, either by exiting the ER in non-COPII vesicles or by bypassing the Golgi. Here, we discuss the evidence for Golgi bypass and the possible physiological benefits of it. Intriguingly, at least during Drosophila development, Golgi bypass seems to be mediated by a Golgi protein, dGRASP, which is found ectopically localized to the plasma membrane.

Purification and characterization of a novel high molecular weight exotoxin produced by red tide phytoplankton, Alexandrium tamarense

Our recent studies have demonstrated that the aqueous extract prepared from Alexandrium tamarense, a harmful red tide phytoplankton, showed cytotoxicity on Vero cells. In this study, the toxic substance was purified from the culture supernatant of A. tamarense. Based on the gel-filtration profile, the molecular mass of a purified toxin was estimated to be about 1,000 kDa. On sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, a main band with molecular mass of 1,000 kDa was detected with periodic acid-Schiff (PAS) staining, but no protein bands were detected by Coomassie brilliant blue (CBB) protein staining. Sugar composition analysis of the toxin suggested that the toxin contains galactose, fucose, mannose, N-acetylglucosamine, xylose, and other minor saccharides, whereas no significant levels of amino acids were detected by amino acid analysis. These results suggest that the toxin is a polysaccharide-based compound. The toxin showed cytotoxic effects on various cell lines in a concentration-dependent manner. Among the cell lines tested, U937 cells were the most susceptible to the toxin. In U937 cells treated with the toxin, a typical apoptotic nuclear morphological change and DNA fragmentation were observed. This is the first report demonstrating that a polysaccharide-based toxin isolated from red tide phytoplankton can induce apoptotic cell death.

A critical role of a cellular membrane traffic protein in poliovirus RNA replication

Replication of many RNA viruses is accompanied by extensive remodeling of intracellular membranes. In poliovirus-infected cells, ER and Golgi stacks disappear, while new clusters of vesicle-like structures form sites for viral RNA synthesis. Virus replication is inhibited by brefeldin A (BFA), implicating some components(s) of the cellular secretory pathway in virus growth. Formation of characteristic vesicles induced by expression of viral proteins was not inhibited by BFA, but they were functionally deficient. GBF1, a guanine nucleotide exchange factor for the small cellular GTPases, Arf, is responsible for the sensitivity of virus infection to BFA, and is required for virus replication. Knockdown of GBF1 expression inhibited virus replication, which was rescued by catalytically active protein with an intact N-terminal sequence. We identified a mutation in GBF1 that allows growth of poliovirus in the presence of BFA. Interaction between GBF1 and viral protein 3A determined the outcome of infection in the presence of BFA.

Purification and characterization of photosensitizing hemolytic toxin from harmful red tide phytoplankton, Heterocapsa circularisquama

We have previously found that the methanol extract prepared from Heterocapsa circularisquama, a harmful red rid dinoflagellate, showed light-dependent hemolytic activity toward rabbit erythrocytes. Interestingly, the cytotoxicity of the extract against HeLa cells was also strictly light-dependent, and no significant toxic effect was observed even at very high concentration in the dark. In this study, the hemolytic agents present in the extract were purified by several chromatographic procedures. Final HPLC analysis revealed that there are at least two hemolytic toxins with different retention times (H2-a and H3-a), which have similar absorption spectra. Although H3-a fraction was contaminated by a trace amount of H2-a, H2-a fraction gave a single peak on a three-dimensional chromatogram. The results of fast atom bombardment (FAB)-mass spectrometry (MS) and electrospray ionization quadrupole time-of-flight (ESI Q-TOF) mass spectrometry analyses suggested that the molecular weight of H2-a was 566. Since nuclear magnetic resonance analysis indicated the presence of a pyrrole ring in H2-a molecule, H2-a may be a porphyrin derivative with similar structure of pyropheophorbide a methyl ester, a well-known photosensitizing hemolytic compound. However, some structural and physicochemical differences were observed between H2-a and pyropheophorbide a methyl ester. Since H2-a showed potent photosensitizing cytotoxicity toward HeLa cells in a concentration-dependent manner as well as hemolytic activity, H2-a may be a novel porphyrin derivative with photosensitizing hemolytic cytotoxicity.

Isolation and characterization of light-dependent hemolytic cytotoxin from harmful red tide phytoplankton Chattonella marina

Chattonella marina (C. marina), a raphidophycean flagellate, is a causative organism of red tide, and highly toxic to fish. In this study, we found that the cell-free methanol extract prepared from this flagellate exhibited potent hemolytic activity against rabbit erythrocytes. Interestingly, the hemolytic activity of the extract was absolutely light-dependent, and no hemolytic activity was detected in the dark even at very high concentration. Gel filtration chromatography of the methanol extract on a column of Sephadex LH-20 revealed that the extract contained hemagglutinin as well as hemolytic agents, and the substances responsible for these activities were separately eluted. These results suggest that the hemagglutinating and hemolytic activities were derived from distinct compounds. The hemolytic fraction obtained after gel filtration (F4) caused marked inhibition of the growth of C. marina itself and other species of phytoplanktons. Furthermore, F4 showed a potent cytotoxicity toward various mammalian cultured cell lines including human tumor cells (HeLa cells) in a dose-dependent manner. The cytotoxicity was also light-dependent, and no cytotoxic effect was exhibited in any cell lines tested in the dark. After further purification procedures via preparative thin-layer chromatography and subsequent HPLC, a major hemolytic agent was obtained as highly purified form. Since the methanol extracts prepared from other raphidophycean flagellates such as Heterosigma akashiwo, Olisthodiscus luteus, and Fibrocapsa japonica showed light-dependent hemolytic activity toward rabbit erythrocytes, it was suggested that the light-dependent hemolytic agents commonly exist at least in these raphidophycean flagellates.

Brefeldin A induces apoptosis and cell cycle blockade in glioblastoma cell lines

Brefeldin A (BFA), a fungal metabolite known to affect the structure and function of the Golgi apparatus, has recently been shown to induce apoptosis and cell growth inhibition in various human cell lines. Glioblastomas (GB) are cerebral tumors with poor prognosis, which display resistance to current therapies including radio- and chemotherapy. The objective of this study was to investigate BFA effects in three human GB cell lines (SA4, SA146 and U87MG cells). Compared with control cells, about 60% of cell growth inhibition was observed in BFA (100 ng/ml for 24 h)-exposed cells in the three cell lines. Furthermore, in SA4 and SA146 cells, BFA was able to induce a time- and dose-dependent apoptosis detected by DAPI staining, TUNEL assay and flow-cytometric analysis. Since p53 expression was not modified after BFA exposure, BFA-induced apoptosis may follow a p53-independent pathway, as already reported. In the same way, BFA did not alter Bcl-2, Bax and Mcl-1 expression. Cell cycle analysis revealed a cell cycle arrest in early G0/G1 phase with an increase in G0/G1 cell population (70% in control cells vs. 83% in exposed cells) associated with a decrease in the S cell population (14% in control cells vs. 5.5% in exposed cells). The Ki67 labeling index also confirmed the cell cycle blockade. Our results suggest that BFA may be a potent cell cycle modulator and inducer of apoptosis in GB cell lines, and therefore may become a promising candidate for the chemotherapeutic treatment of gliomas.

Regulation of connexin43 expression by c-fos and c-jun in myometrial cells

Labor is associated with a dramatic increase in the myometrial expression of connexin43 (Cx43) which is thought to mediate myocyte contractile coupling. The transcription factor c-fos is also dramatically increased prior to the onset of labor and is therefore a potential regulator of Cx43. The promoter region of Cx43 contains a conserved activator protein-1 (AP-1) site, which binds dimers of Fos and Jun proteins. We constructed expression vectors for c-Fos and c-Jun to investigate the role of these transcription factors in the regulation of Cx43 expression. These expression vectors were then co-transfected into SHM (syrian hamster myocyte) cells with a Cx43 promoter-Luciferase vector. The combinations of c-Fos and c-Jun proteins activated the Cx43 promoter while c-Jun alone had no effect on Cx43 promoter activity. Mutation of the AP-1 site was found to reduce this responsiveness. These data indicate that the transcription factors c-Fos and c-Jun are important in the regulation of Cx43 expression.

Possible factors responsible for the toxicity of Cochlodinium polykrikoides, a red tide phytoplankton

Cochlodinium polykrikoides, a harmful red tide dinoflagellate, is highly toxic to fish, but the toxic mechanism is still unknown. Recent study has suggested that C. polykrikoides generates reactive oxygen species (ROS) such as superoxide anion (O(2)(-)) and hydrogen peroxide (H(2)O(2)), and the ROS-mediated ichthyotoxicity has been proposed. In this study, we found that the levels of O(2)(-) and H(2)O(2) detected in C. polykrikoides were trace levels as compared with those of Chattonella marina which is well-known to produce ROS. Furthermore, no significant increase in O(2)(-) generation by C. polykrikoides was observed in the presence of lectins such as concanavalin A (Con A) and wheat germ agglutinin (WGA) or fish mucus prepared from skin and gill of yellowtail, whereas C. marina generated increased level of O(2)(-) responding to these stimuli. Interestingly, the cell-free aqueous extract prepared from C. polykrikoides showed toxic effect on the HeLa cells, but the extract of C. marina had no significant effect. Furthermore, gradual accumulation of polysaccharides in the medium was observed during the growth of C. polykrikoides, and the medium gradually became viscous, but no such changes were observed in the medium of C. marina. These results suggest that multiple factors may be responsible for the toxic mechanism of C. polykrikoides.

Photosensitizing hemolytic toxin in Heterocapsa circularisquama, a newly identified harmful red tide dinoflagellate

Red tides of Heterocapsa circularisquama (H. circularisquama), recently identified as a novel species of dinoflagellate, have frequently caused mass mortality of several species of bivalves in Japan, while no harmful effects of this flagellate on fish have been reported so far. We found that the cell-free ethanol extract prepared from H. circularisquama caused hemolysis of rabbit erythrocytes and demonstrated cytotoxic effects in HeLa cells and on the microzooplankton rotifer (B. plicatilis) in a dose- and time-dependent manner. Interestingly, the hemolytic activity and cytotoxic effects of the extract were completely dependent on the presence of light. When the experiments were conducted in the dark, no hemolysis was observed even at very high concentration of the extract. These results suggest that H. circularisquama has photosensitizing hemolytic toxin which can be easily extracted into ethanol. This may be the first report documenting the occurrence of photosensitizing hemolytic toxin in marine phytoplankton species.

Cholesterol regulates oxysterol binding protein (OSBP) phosphorylation and Golgi localization in Chinese hamster ovary cells: correlation with stimulation of sphingomyelin synthesis by 25-hydroxycholesterol

Sphingomyelin (SM) and cholesterol content is positively correlated in cellular membranes, and in several pathological and experimental conditions there is evidence for coregulation. The potential role of oxysterols and oxysterol binding protein (OSBP) in mediating the coregulation of cholesterol and SM was examined using Chinese hamster ovary (CHO) and cholesterol auxotrophic, sterol regulatory defective (SRD) 6 cells. SRD 6 cells grown in the presence or absence of cholesterol for 24 h displayed a 30-50% reduction in SM synthesis compared with control CHO 7 cells. SM synthesis in CHO 7 and cholesterol-supplemented SRD 6 cells was stimulated 2-fold by 25-hydroxycholesterol, but cholesterol-starved SRD 6 cells were unresponsive. Basal and 25-hydroxycholesterol-stimulated SM synthesis was also inhibited in lovastatin-treated wild-type CHO-K1 cells. Lack of 25-hydroxycholesterol activation of SM synthesis in cholesterol-starved SRD 6 and lovastatin-treated CHO-K1 cells was correlated with dephosphorylation of OSBP. In SRD 6 cells, this was evident after 12 h of cholesterol depletion, it occurred equally at all phosphorylation sites and was exacerbated by 25-hydroxycholesterol. Unlike CHO 7 cells, where OSBP was observed in small vesicles and the cytoplasm, OSBP in cholesterol-starved SRD 6 cells was constitutively localized in the Golgi apparatus. Supplementation with non-lipoprotein cholesterol promoted redistribution to vesicles and the cytoplasm. Similarly, OSBP in CHO-K1 cells grown in delipidated serum was predominantly in the Golgi apparatus. Low-density lipoprotein (LDL) supplementation of CHO-K1 cells caused the redistribution of OSBP to the cytoplasm and small vesicles, and this effect was blocked by pharmacological agents ¿3-beta-[2-(diethylamino)ethoxy]androst-5-en-17-one and progesterone¿, which inhibited LDL cholesterol efflux from lysosomes. The results showed that localization of OSBP between the Golgi apparatus and a cytoplasmic/vesicular compartment was responsive to changes in cholesterol content and trafficking. In cholesterol depleted SRD 6 cells, this was accompanied by dephosphorylation of OSBP and attenuation of 25-hydroxycholesterol activation of SM synthesis.

Comparative study of the metabolic pools of sphingomyelin and phosphatidylcholine sensitive to tumor necrosis factor

The metabolism and localization of the pools of sphingomyelin and phosphatidylcholine (PtdCho) which are hydrolyzed upon activation of the sphingomyelin signal transduction pathway were studied in human skin fibroblasts treated with tumor necrosis factor alpha (TNF-alpha). In a first series of experiments, cellular phospholipids were labeled with [3H]choline under conditions that inhibit the vesicular traffic to the plasma membrane. Thus, in human fibroblasts metabolically labeled in the presence of brefeldin A, monensin or at 20 degree C, the arrival of newly synthesized sphingomyelin to the cell surface was prevented, supporting previous conclusions for a vesicular mechanism of sphingomyelin transport to the plasma membrane. Under these conditions, TNF-alpha induced the hydrolysis of PtdCho but did not promote the hydrolysis of 3H-labeled sphingomyelin, suggesting that the sphingomyelin signaling pool resides in a compartment distal to the Golgi apparatus, and possibly in the plasma membrane. TNF was also unable to trigger the breakdown of a radioactive sphingomyelin, [ceramide-3H]sphingomyelin, exogenously added to the cells to label the exoplasmic side of the cell surface. However, TNF caused PtdCho and sphingomyelin degradation in fibroblasts that had been treated with bacterial sphingomyelinase to degrade the sphingomyelin pool of the external leaflet of the plasma membrane. A similar result was obtained at 4 degree C, i.e. under conditions which inhibit endocytosis, thereby excluding the endosomes as a potential site for TNF-induced sphingomyelin hydrolysis. Altogether, these results strongly argue for a localization of the sphingomyelin signaling pool at the inner leaflet of the plasma membrane, but neither in the endolyso-somal nor the Golgi compartments. In addition, when [3H]choline-labeled fibroblasts were treated under non-lytic conditions with bacterial phospholipase C to degrade the external pool of PtdCho, TNF was still able to stimulate the hydrolysis of PtdCho. This demonstrates that the pool of PtdCho involved in TNF-alpha signaling (and which is hydrolyzed concurrently with sphingomyelin to generate diacylglycerol), is not located in the outer leaflet of the plasma membrane.