Epithelial sphingolipid sorting is insensitive to reorganization of the Golgi by nocodazole, but is abolished by monensin in MDCK cells and by brefeldin A in Caco-2 cells

Golgi-Targeting Anticancer Natural Products

The Golgi apparatus plays an important role in maintaining cell homeostasis by serving as a biosynthetic center for glycans, lipids and post-translationally modified proteins and as a sorting center for vesicular transport of proteins to specific destinations. Moreover, it provides a signaling hub that facilitates not only membrane trafficking processes but also cellular response pathways to various types of stresses. Altered signaling at the Golgi apparatus has emerged as a key regulator of tumor growth and survival. Among the small molecules that can specifically perturb or modulate Golgi proteins and organization, natural products with anticancer property have been identified as powerful chemical probes in deciphering Golgi-related pathways and, in particular, recently described Golgi stress response pathways. In this review, we highlight a set of Golgi-targeting natural products that enabled the characterization of the Golgi-mediated signaling events leading to cancer cell death and discuss the potential for selectively exploiting these pathways for the development of novel chemotherapeutic agents.

Decoupling polarization of the Golgi apparatus and GM1 in the plasma membrane

Cell polarization is a process of coordinated cellular rearrangements that prepare the cell for migration. GM1 is synthesized in the Golgi apparatus and localized in membrane microdomains that appear at the leading edge of polarized cells, but the mechanism by which GM1 accumulates asymmetrically is unknown. The Golgi apparatus itself becomes oriented toward the leading edge during cell polarization, which is thought to contribute to plasma membrane asymmetry. Using quantitative image analysis techniques, we measure the extent of polarization of the Golgi apparatus and GM1 in the plasma membrane simultaneously in individual cells subject to a wound assay. We find that GM1 polarization starts just 10 min after stimulation with growth factors, while Golgi apparatus polarization takes 30 min. Drugs that block Golgi polarization or function have no effect on GM1 polarization, and, conversely, inhibiting GM1 polarization does not affect Golgi apparatus polarization. Evaluation of Golgi apparatus and GM1 polarization in single cells reveals no correlation between the two events. Our results indicate that Golgi apparatus and GM1 polarization are controlled by distinct intracellular cascades involving the Ras/Raf/MEK/ERK and the PI3K/Akt/mTOR pathways, respectively. Analysis of cell migration and invasion suggest that MEK/ERK activation is crucial for two dimensional migration, while PI3K activation drives three dimensional invasion, and no cumulative effect is observed from blocking both simultaneously. The independent biochemical control of GM1 polarity by PI3K and Golgi apparatus polarity by MEK/ERK may act synergistically to regulate and reinforce directional selection in cell migration.

IcsB, secreted via the type III secretion system, is chaperoned by IpgA and required at the post-invasion stage of Shigella pathogenicity

Shigella deliver a subset of effector proteins such as IpaA, IpaB and IpaC via the type III secretion system (TTSS) into host cells during the infection of colonic epithelial cells. Many bacterial effectors including some from Shigella require specific chaperones for protection from degradation and targeting to the TTSS. In this study, we have investigated the role of the icsB gene located upstream of the ipaBCDA operon in Shigella infection because the role of IcsB as a virulence factor remains unknown. Here, we found that the IcsB protein is secreted via the TTSS of Shigella in vitro and in vivo. We show that IpgA protein encoded by ipgA, the gene immediately downstream of icsB, serves as the chaperone required for the stabilization and secretion of IcsB. We have shown that IcsB binds to IpgA in bacterial cytosol and the binding site is in the middle of the IcsB protein. Intriguingly, although its significance in Shigella pathogenicity is as yet unclear, the icsB gene can be read-through into the ipgA gene to create a translational fusion protein. Furthermore, the contribution of IcsB to the pathogenicity of Shigella was demonstrated by plaque-forming assay and the Sereny test. The ability of the icsB mutant to form plaques was greatly reduced compared with that of the wild type in MDCK cell monolayers. Furthermore, when guinea pig eyes were infected with a non-polar icsB mutant, the bacteria failed to provoke keratoconjunctivitis. These results suggest that IcsB is secreted via the TTSS, chaperoned by IpgA, and required at the post-invasion stage of Shigella pathogenicity

Sphingolipid transport in eukaryotic cells

Sphingolipids constitute a sizeable fraction of the membrane lipids in all eukaryotes and are indispensable for eukaryotic life. First of all, the involvement of sphingolipids in organizing the lateral domain structure of membranes appears essential for processes like protein sorting and membrane signaling. In addition, recognition events between complex glycosphingolipids and glycoproteins are thought to be required for tissue differentiation in higher eukaryotes and for other specific cell interactions. Finally, upon certain stimuli like stress or receptor activation, sphingolipids give rise to a variety of second messengers with effects on cellular homeostasis. All sphingolipid actions are governed by their local concentration. The intricate control of their intracellular topology by the proteins responsible for their synthesis, hydrolysis and intracellular transport is the topic of this review.

Phospholipid composition of purified Chlamydia trachomatis mimics that of the eucaryotic host cell

Chlamydia trachomatis is an obligate intracellular eubacterial parasite capable of infecting a wide range of eucaryotic host cells. Purified chlamydiae contain several lipids typically found in eucaryotes, and it has been established that eucaryotic lipids are transported from the host cell to the parasite. In this report, we examine the phospholipid composition of C. trachomatis purified from host cells grown under a variety of conditions in which the cellular phospholipid composition was altered. A mutant CHO cell line, with a thermolabile CDP-choline synthetase, was used to show that decreased host cell phosphatidylcholine levels had no significant effect on C. trachomatis growth. However, less phosphatidylcholine was transported to the parasite and purified elementary bodies contained decreased levels of phosphatidylcholine. Brefeldin A, fumonisin B1, and exogenous sphingomyelinase were used to alter levels of host cell sphingomyelin. None of the agents had a significant effect on C. trachomatis replication. Treatment with fumonisin B1 and exogenous sphingomyelinase resulted in decreased levels of host cell sphingomyelin. This had no effect on glycerophospholipid trafficking to chlamydiae; however, sphingomyelin trafficking was reduced and elementary bodies purified from treated cells had reduced sphingomyelin content. Exposure to brefeldin A, which had no adverse effect on chlamydia growth, resulted in an increase in cellular levels of sphingomyelin and a concomitant increase in the amount of sphingomyelin in purified chlamydiae. Under the experimental conditions used, brefeldin A treatment had only a small effect on sphingomyelin trafficking to the host cell surface or to C. trachomatis. Thus, the final phospholipid composition of purified C. trachomatis mimics that of the host cell in which it is grown.

Actin filaments and microtubules are involved in different membrane traffic pathways that transport sphingolipids to the apical surface of polarized HepG2 cells

In polarized HepG2 hepatoma cells, sphingolipids are transported to the apical, bile canalicular membrane by two different transport routes, as revealed with fluorescently tagged sphingolipid analogs. One route involves direct, transcytosis-independent transport of Golgi-derived glucosylceramide and sphingomyelin, whereas the other involves basolateral to apical transcytosis of both sphingolipids. We show that these distinct routes display a different sensitivity toward nocodazole and cytochalasin D, implying a specific transport dependence on either microtubules or actin filaments, respectively. Thus, nocodazole strongly inhibited the direct route, whereas sphingolipid transport by transcytosis was hardly affected. Moreover, nocodazole blocked "hyperpolarization," i.e., the enlargement of the apical membrane surface, which is induced by treating cells with dibutyryl-cAMP. By contrast, the transcytotic route but not the direct route was inhibited by cytochalasin D. The actin-dependent step during transcytotic lipid transport probably occurs at an early endocytic event at the basolateral plasma membrane, because total lipid uptake and fluid phase endocytosis of horseradish peroxidase from this membrane were inhibited by cytochalasin D as well. In summary, the results show that the two sphingolipid transport pathways to the apical membrane must have a different requirement for cytoskeletal elements.

Brefeldin A (BFA) inhibits basolateral membrane (BLM) delivery and dimerization of transcobalamin II receptor in human intestinal epithelial Caco-2 cells. BFA effects on BLM cholesterol content

Brefeldin A (BFA) treatment of Caco-2 cells (5 microg/ml for 12 h) reduced by 90% the cholesterol, but not the phospholipid (PL), levels of the basolateral membrane (BLM), thus altering its PL/cholesterol molar ratio from 2.6 to 22.0, and decreasing its steady state fluorescent anisotropy (rs) from 0.27 to 0.15. BFA treatment for 12 h also resulted in complete loss of transcobalamin II receptor (TC II-R) activity/protein levels in the BLM and the disappearance of trans-Golgi network (TGN) morphology as revealed by confocal immunofluorescence microscopy using antibody to TGN 38. However, BFA treatment had no effect on either total cellular cholesterol, TC II-R activity, or PL levels. When cells treated with BFA for 12 h were exposed to BFA-free medium for 0-24 h, all of the effects were reversed, including reappearance of normal TGN morphology. TC II-R delivered to the BLM during this period was progressively sialylated and changed its physical state from a monomer (8 h) to a dimer (12 h), coinciding with increased delivery (11-53 pmol) of cholesterol to the BLM and an increase in the BLM rs from 0.15 to 0.21. These results indicate that cholesterol, but not PL, delivery to the BLM of Caco-2 cells is BFA-sensitive, and cholesterol, by influencing the higher order of the BLM, is essential for TC II-R dimerization.

The subcellular sites of sphingomyelin synthesis in BHK cells

The subcellular distributions of the enzymes which synthesise sphingomyelin (SM) and glucosylceramide (GluCer) from ceramide have been assessed in BHK cells. On a sucrose density gradient GluCer synthase (a marker of the cis/medial Golgi apparatus) and the trans-Golgi marker galactosyltransferase showed an similar monotonic distribution. In contrast, SM synthase showed two peaks of activity, a minor one which migrated with the Golgi markers and a major one which had a density close to that of plasma membrane markers (sphingomyelin, cholesterol, PtdSer, ganglioside GM3 and alkaline phosphodiesterase). When cell homogenates were treated with digitonin, the sedimentation characteristics of the Golgi markers was largely unaffected whereas the plasma membrane markers and the main peak of SM synthase activity were shifted to higher density. In contrast, when cells were treated with brefeldin A (BFA) the Golgi markers were shifted to higher density but not the plasma membrane markers or the main peak of SM synthase. These results suggest that the bulk of SM synthase activity in BHK cells is not associated with the Golgi cisternae but with a cell compartment which is relatively rich in cholesterol (e.g., plasma membrane, endosomes or trans-Golgi network.) Further experiments in which cells were treated with sphingomyelinase provided evidence that SM synthase activity was in an internal compartment and not at the plasma membrane.

Disruption of microtubules reveals two independent apical targeting mechanisms for G-protein-coupled receptors in polarized renal epithelial cells

G-protein-coupled receptors demonstrate differing trafficking itineraries in polarized Madin-Darby canine kidney (MDCK II) cells. The alpha2A adrenergic receptor (alpha2AAR) is directly delivered to the basolateral subdomain; the A1 adenosine receptor (A1AdoR) is apically enriched in its targeting; and the alpha2BAR subtype is randomly delivered to both domains but selectively retained basolaterally (Keefer, J. R., and Limbird, L. E. (1993) J. Biol. Chem. 268, 11340-11347; Saunders, C., Keefer, J. R., Kennedy, A. P., Wells, J. N., and Limbird, L. E. (1996) J. Biol. Chem. 271, 995-1002; Wozniak, M., and Limbird, L. E. (1996) J. Biol. Chem. 271, 5017-5024). The present studies explore the role of the polarized cytoskeleton in localization of G-protein-coupled receptors in MDCK II cells. Nocodazole or colchicine, which disrupt microtubules, did not perturb lateral localization of alpha2AR subtypes but led to a relocalization the A1AdoR to the basolateral surface, revealed by immunocytochemical and metabolic labeling strategies. Conversely, the apical component of the random delivery of alpha2BAR was not affected by these agents, suggesting microtubule-dependent and -independent apical targeting mechanisms for G-protein-coupled receptors in polarized cells. Apparent rerouting of the apically targeted A1AdoR was selective for microtubule-disrupting agents, since cytochalasin D, which disrupts actin polymerization, did not alter A1AdoR or alpha2BAR localization or targeting. These data suggest that multiple apical targeting mechanisms exist for G-protein-coupled receptors and that microtubule-disrupting agents serve as tools to probe their different trafficking mechanisms.

Transport of sphingomyelin to the cell surface is inhibited by brefeldin A and in mitosis, where C6-NBD-sphingomyelin is translocated across the plasma membrane by a multidrug transporter activity

Sphingomyelin is a major lipid of the mammalian cell surface. The view that sphingomyelin, after synthesis in the Golgi lumen, reaches the outer leaflet of the plasma membrane on the inside of carrier vesicles has been challenged by inconsistencies in the results of transport studies. To investigate whether an alternative pathway to the cell surface exists for sphingomyelin, brefeldin A and mitotic cells were used to block vesicular traffic between the Golgi complex and the plasma membrane. Exogenous sphingomyelinase was applied in the cold to assay for the presence of sphingomyelin on the surface of CHO cells. Newly synthesized radiolabeled sphingomyelin was found to equilibrate with cell surface sphingomyelin within 1.5 hours at 37 degrees C. Brefeldin A and mitosis inhibited this transport but, surprisingly, not the surface appearance of the short-chain sphingomyelin analog N-6[7-nitro-2,1,3-benzoxadiazol-4-yl]aminohexanoyl(C6-NBD)-sphingo myelin as assayed by depletion of this lipid in the medium by the scavenger albumin. Transport of C6-NBD-sphingomyelin in the presence of brefeldin A was blocked by cyclosporin A and PSC 833, inhibitors of the multidrug resistance P-glycoprotein. The same was observed in HepG2 and HeLa cells, and for short-chain glucosylceramide, which demonstrates the general nature of the transporter-dependent sphingolipid translocation across the plasma membrane.

Intracellular lipid heterogeneity caused by topology of synthesis and specificity in transport. Example: sphingolipids

The differences in lipid composition between intracellular membranes cannot be adequately explained by local synthesis and degradation. Especially in the case of sphingolipids, which are synthesized in the Golgi complex but enriched on the cell surface and in endocytotic organelles, there is evidence for a cellular machinery that preferentially shuttles these lipids in vesicles to the cell surface. The machinery appears to involve the formation of domains of sphingolipid and cholesterol in the lumenal leaflet of Golgi membranes. Several pieces of evidence suggest that the selective anterograde transport of plasma membrane proteins may be mechanistically related to the sphingolipid domains.

Monensin and brefeldin A inhibit high density lipoprotein-mediated cholesterol efflux from cholesterol-enriched cells. Implications for intracellular cholesterol transport

Mechanisms and pathways of excess cholesterol removal from intracellular sites of accumulation to extracellular cholesterol acceptors remain poorly defined. To gain further insights, compounds known to affect cellular protein transport pathways were tested for their effects on high density lipoprotein (HDL)-mediated cholesterol efflux from cultured cells enriched with cholesterol. Monensin, nigericin, and brefeldin A inhibited the ability of HDL to decrease cellular cholesterol esterification, stimulate sterol biosynthesis, and promote the efflux of labeled cholesterol and cholesterol mass from fibroblasts and smooth muscle cells. HDL-mediated decrease in cell cholesterol esterification was inhibited up to 80% by these compounds compared with control incubations over an HDL concentration of 5-100 micrograms/ml and up to 18 h of incubation. Up-regulation of sterol biosynthesis after depletion of cell cholesterol by HDL increased over 10-fold; however, inclusion of monensin or brefeldin A during the incubation completely prevented the increase of sterol biosynthesis by HDL. Efflux of [3H]cholesterol to HDL from prelabeled cells was inhibited up to 40% by these compounds, and this effect persisted when cholesterol esterification was blocked. Similarly, monensin and brefeldin A inhibited up to 50% of HDL-mediated cholesterol mass efflux relative to controls. Treatment of cells with cholesterol oxidase demonstrated an increase of intracellular cholesterol after exposure to monensin or nigericin and to a lesser extent with brefeldin A. These data show that monensin, nigericin, and brefeldin A sequester cholesterol from sites normally available for efflux by HDL. Since these compounds act by disruption of Golgi complex structure and function, a role for this intracellular organelle in transport of cholesterol between intracellular sites and the plasma membrane for eventual removal by extracellular acceptors such as HDL is suggested.

Isoprenoid lipid metabolism in the retina: dynamics of squalene and cholesterol incorporation and turnover in frog rod outer segment membranes

Frogs were injected intravitreally with [3H]acetate, and the formation of [3H]-labeled squalene and cholesterol in the retina and their incorporation into rod outer segment (ROS) membranes were evaluated biochemically over a 60-day time course. ROS [3H]squalene specific activity was maximal by 1-3 days, then declined with a half-time of approximately 20-30 days. In contrast, the specific activity of ROS [3H]cholesterol initially increased to a level substantially less than that of [3H]squalene, and then remained constant. Thus, ROS squalene appears to turn over without obligatory conversion to, or coturnover with, ROS cholesterol. When [3H]acetate was injected into one eye, radiolabel in non-saponifiable lipids of the contralateral retina represented < 1% of those recovered from the ipsilateral retina; hence, systemic contributions to de novo synthesis were obviated. Long-term (> or = 8 hr) in vitro incubations of isolated retinas with [3H]acetate resulted in incorporation of [3H]-labeled sterols and squalene into ROS, at levels comparable to those observed in ROS from companion incubated eyecup preparations and from retinas 8 hr after intravitreal injection of [3H]acetate. These results demonstrate that the in vitro system faithfully reflects the in vivo biosynthetic capacity with respect to isoprenoid lipid metabolism, and suggest that de novo synthesis within the neural retina is responsible for generating most, if not all, of the [3H]squalene and [3H]cholesterol formed under the given conditions. Treatment of retinas in vitro with brefeldin A or energy poisons blocked transport of newly synthesized opsin, but not squalene, to the ROS. Furthermore, frogs maintained at 8 degrees C exhibited marked suppression of incorporation of newly synthesized protein into the ROS, while [3H]squalene incorporation was only minimally reduced, compared with frogs maintained at 22 degrees C. These results are consistent with prior findings that suggest that lipids are transported to the ROS by a mechanism distinct and independent from that employed for intracellular trafficking of opsin and other ROS-destined membrane proteins.

Is plasma membrane lipid composition defined in the exocytic or the endocytic pathway?

Compared with intracellular membranes, the plasma membrane is rich in cholesterol and sphingomyelin. How does this distinct composition arise? Here David Allan and Karl-Josef Kallen take a critical view of the belief that these lipids arrive at the plasma membrane via vesicular traffic from the Golgi complex and propose instead that they may be accreted in the endocytic recycling pathway.

Transport and sorting of membrane lipids

The lipid composition of cellular membranes may seem unnecessarily complex. However, the lipid composition of each membrane is carefully regulated by local metabolism and specificity in transport, marking the functional significance for the cell. Recent research has revealed unexpected discoveries concerning the topology of lipid synthesis, specificity in lipid transport, and the function of lipid and protein microdomains in sorting.