Differential arrest of secretory protein transport in cultured rat hepatocytes by azide treatment

N-WASP inhibitor wiskostatin nonselectively perturbs membrane transport by decreasing cellular ATP levels

Wiskott-Aldrich syndrome protein (WASP) and WAVE stimulate actin-related protein (Arp)2/3-mediated actin polymerization, leading to diverse downstream effects, including the formation and remodeling of cell surface protrusions, modulation of cell migration, and intracytoplasmic propulsion of organelles and pathogens. Selective inhibitors of individual Arp2/3 activators would enable more exact dissection of WASP- and WAVE-dependent cellular pathways and are potential therapeutic targets for viral pathogenesis. Wiskostatin is a recently described chemical inhibitor that selectively inhibits neuronal WASP (N-WASP)-mediated actin polymerization in vitro. A growing number of recent studies have utilized this drug in vivo to uncover novel cellular functions for N-WASP; however, the selectivity of wiskostatin in intact cells has not been carefully explored. In our studies with this drug, we observed rapid and dose-dependent inhibition of N-WASP-dependent membrane trafficking steps. Additionally, however, we found that addition of wiskostatin inhibited numerous other cellular functions that are not believed to be N-WASP dependent. Further studies revealed that wiskostatin treatment caused a rapid, profound, and irreversible decrease in cellular ATP levels, consistent with its global effects on cell function. Our data caution against the use of this drug as a selective perturbant of N-WASP-dependent actin dynamics in vivo.

Nordihydroguaiaretic acid blocks protein transport in the secretory pathway causing redistribution of Golgi proteins into the endoplasmic reticulum

We have investigated the effect of nordihydroguaiaretic acid (NDGA), an inhibitor of lipoxygenase, on the intracellular protein transport and the structure of the Golgi complex. Pulse-chase experiments and immunoelectron microscopy showed that NDGA strongly inhibits the transport of newly synthesized secretory proteins to the Golgi complex resulting in their accumulation in the endoplasmic reticulum (ER). Despite their retention in the ER, oligosaccharides of secretory and ER-resident proteins were processed to endoglycosidase H-resistant forms, raising the possibility that oligosaccharide-processing enzymes are redistributed from the Golgi to the ER. Morphological observations further revealed that alpha-mannosidase II (a cis/medial-Golgi marker), but not TGN38 (a trans-Golgi network marker), rapidly redistributes to the ER in the presence of NDGA, resulting in the disappearance of the characteristic Golgi structure. Upon removal of the drug, the Golgi complex was reassembled into the normal structure as judged by perinuclear staining of alpha-mannosidase II and by restoration of the secretion. These effects of NDGA are quite similar to those of brefeldin A. However, unlike brefeldin A, NDGA did not cause a dissociation of beta-coatomer protein, a subunit of coatomer, from the Golgi membrane. On the contrary, NDGA exerted the stabilizing effect on beta-coatomer protein/membrane interaction against the dissociation caused by brefeldin A and ATP depletion. Taken together, these results indicate that NDGA is a potent agent disrupting the structure and function of the Golgi complex with a mechanism different from those known for other drugs reported so far.

Relationship between cellular ATP content and cellular functions of primary cultured rat hepatocytes in hypoxia

The importance of oxygen in maintaining the functional integrity of hepatocytes has been well established in a variety of experimental models, such as in vivo, perfused liver and isolated hepatocytes. However, one of the shortcomings of these systems is their short life span. Therefore, we have examined the effects of long-term hypoxia on cellular adenine nucleotide content and cellular functions, such as albumin production, urea production and DNA synthesis, in adult rat hepatocytes in primary culture. Hepatocytes were cultured at a density of 11 x 10(4) and 5 x 10(4) cells/0.18 mL per cm2 for the study of albumin and urea production and DNA synthesis, respectively, at various oxygen tensions (20, 12, 8 and 5%) for 24 h. Cellular ATP content in cultured hepatocytes in hypoxia gradually declined, corresponding to the decrease in oxygen tension, and the cellular ATP level at 5% oxygen was approximately 20% of that at 20% oxygen. Albumin production also decreased in parallel with the decrease in cellular ATP content in cultured hepatocytes in hypoxia. However, even when cellular ATP content gradually declined corresponding with the decrease in oxygen tension in cultured hepatocytes in hypoxia, such as at 8 or 5% oxygen, urea production remained at a high level; in contrast, DNA synthesis was completely suppressed. These results suggest that the cellular ATP content decreases in cultured hepatocytes during long-term hypoxia in relation to oxygen tension and that the relationship between decreased ATP levels and liver function in cultured hepatocytes during hypoxia differs for albumin production, urea production and DNA synthesis.

ATP depletion causes translational immobilization of cell surface transferrin receptors in K562 cells

We have used quantitative fluorescence microscopy and fluorescence photobleaching recovery to examine the role of metabolic energy in the translational movement of transferrin receptors in the plasma membrane of K562 erythroleukemia cells. Cellular ATP depletion caused a significant decrease in the translational mobility of cell surface transferrin receptors and a significant increase in the number of receptors on the cell surface. ATP repletion restored receptor translational mobility and cell surface expression to control values. Inhibition of ATP hydrolases by orthovanadate also immobilized cell surface transferrin receptors and altered cell surface receptor expression, in a concentration-dependent manner. Vanadate-induced changes in receptor mobility and cell surface expression were reversible upon washing out the drug. Cellular ATP depletion did not affect the translational mobility of plasma membrane glycophorins or a fluorescent phospholipid analogue. We conclude that the translational movement of cell surface transferrin receptors specifically requires metabolic energy and ATP hydrolysis.

Proteolytic cleavage of haptoglobin occurs in a subcompartment of the endoplasmic reticulum: evidence from membrane fusion in vitro

The primary translation product of haptoglobin mRNA is a 45-kD polypeptide which is proteolytically cleaved shortly after its synthesis. Previous studies have indicated that the cleavage of this proform of haptoglobin occurs in the ER. In an attempt to characterize the cleaving enzyme, we found that upon incubation of microsomes from rat hepatocytes pulse labeled with [35S]methionine, little cleavage of labeled prohaptoglobin occurred. In contrast, when cells whose cytoplasmic proteins had been released by saponin treatment were incubated, 30-40% of the prohaptoglobin was cleaved. The addition of GTP caused a twofold stimulation, which was abolished by the nonhydrolyzable analog GTP gamma S. With a homogenate of the cells, the addition of GTP resulted in a fourfold stimulation of the degree of cleavage--from 15 to 60%. Differential centrifugation revealed that most of the cleaving activity resided in membranes sedimenting similarly to mitochondria and to a small fraction of the ER. These rapidly sedimenting membranes were therefore prepared from a rat liver homogenate. Upon treatment with high salt, light membranes were released which, when incubated with microsomes of pulse-labeled hepatocytes in the presence of detergent (and in the absence of GTP), induced specific cleavage of prohaptoglobin. The cleaving enzyme had an alkaline pH optimum indicating that it was not of lysosomal origin. These results suggest that cleavage of prohaptoglobin occurs in a subcompartment of the ER. Apparently, the connection between this compartment and the bulk of the ER is broken upon saponin treatment or homogenization but can be reestablished through a process requiring GTP hydrolysis.

Release of stress proteins from Mesocestoides corti is a brefeldin A-inhibitable process: evidence for active export of stress proteins

Substantial evidence indicates that molecules released by infectious organisms affect virulence and influence immunity to infection. The characterization of extracellular molecules and their mechanism of release is therefore critical to understanding host-parasite relationships. The cestode parasite Mesocestoides corti is known to release at the larval stage several molecules including the heat shock proteins hsp70 and hsp60. In this report, it is shown that several molecules released by M. corti, including 70- and 60-kDa proteins, are induced by a temperature shift from room temperature to 37 degrees C. Such a shift is comparable to the thermal stress of parasites transmitted from insect vector to vertebrate host. By drug inhibition studies and Western blot (immunoblot) analyses, it is shown that M. corti hsp70 and hsp60 as well as other released molecules are actively exported. The active release of stress proteins by parasites has not been described and may play a critical role in the disease process.

Effect of monensin on plant Golgi: re-examination of the monensin-induced changes in cisternal architecture and functional activities of the Golgi apparatus of sycamore suspension-cultured cells

We have re-examined the effects of the ionophore monensin on the Golgi apparatus of sycamore maple suspension-cultured cells using a combination of high pressure freezing, immunocytochemical and biochemical techniques. Exposure of the cells to 10 microM monensin, which reduces protein secretion by approximately 90%, resulted first in the swelling of the trans-Golgi network, then of the trans-most trans-cisterna, the remaining trans-cisternae, and finally of the cis and medial cisternae. We postulate that these different rates of swelling reflect an underlying hierarchy of compartmental acidification with the trans-Golgi network being the most acidic compartment. Recovery occurred in the reverse sequence. Previous studies have suggested that the large swollen vesicles that accumulate in the cytoplasm of monensin-treated cells arise from the swelling and detachment of entire trans-cisternae. However, based on the many membrane blebbing configurations seen in association with the trans-Golgi network and the trans-Golgi cisternae of monensin-treated cells, and the fact that the surface area of the trans-Golgi cisternae is about five times greater than the surface area of the swollen vesicles, it appears that the swollen vesicles are produced by a budding mechanism. After 35–40 min of monensin treatment, cells with smaller, non-swollen, compact Golgi stacks began to appear and rapidly increased in number, contributing > 60% of the cell population after 60 min and > 80% after 100 min. In contrast, large numbers of swollen vesicles persisted in the cytoplasm of all cells for over 100 min. Since azide treatment of monensin-treated cells can prematurely induce the unswelling response and cellular ATP levels drop substantially after 45 min of monensin treatment, we propose that un-swelling of the Golgi stacks is due to a monensin-induced decline in ATP levels in the cells. Immunocytochemical labeling of the high pressure frozen cells with anti-xyloglucan antibodies demonstrated that the concentration of xyloglucan, a hemicellulose, in the swollen vesicles increased with time. This increase in vesicle contents may explain why these swollen vesicles do not contract in parallel with the Golgi stacks. In vivo labeling experiments with [3H]fucose, [3H]UDP-glucose and [3H]leucine demonstrated that monensin-treatment not only inhibited protein secretion, but also cellulose synthesis. Protein synthesis, on the other hand, was reduced only slightly during the first 30 min of treatment, but quite strongly between 30 and 60 min, consistent with the observed drop in ATP levels after > 40 min of exposure to monensin.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the nuclear translocation of acidic fibroblast growth factor

The subcellular localization of human acidic FGF (aFGF; FGF-1) expressed to high levels by using a bacteriophage T7 RNA polymerase-driven vaccinia virus expression system was studied in BHK21 and HeLa cells. Acidic FGF was detected by immunoblotting or immunofluorescence using an affinity-purified rabbit polyclonal antibody. The nuclei of most transfected cells, but not nuclei of control cells, were strongly immunoreactive. The nuclear accumulation of aFGF was confirmed by subcellular fractionation and immunoblotting, indicating that about 50% of the expressed protein was located in the nuclei at 12 h after transfection. It has previously been reported that a putative N-terminal nuclear localization sequence (NLS) in aFGF is required for full mitogenic activity (Imamura et al., Science 249, 1567–1570, 1990). We found that deletion of the first 27 residues including the putative NLS did not prevent the nuclear translocation of aFGF in either cell type. This observation suggests that the putative NLS sequence is not essential for targeting aFGF to the cell nucleus. To analyze further the mechanism of nuclear import, purified aFGF was microinjected into the cytoplasm of growing BHK21 cells under various conditions. In chilled (4 degrees C) or ATP-depleted cells, the injected aFGF entered the nucleus with similar efficiency to that in control cells at 37 degrees C. This suggests that aFGF, which has a molecular mass of only 16,500, enters the cell nucleus by free diffusion, and possibly becomes trapped by binding to some nuclear structures. When added exogenously to growing BHK21 cells, aFGF was not localized to the nucleus. Instead, a punctate staining pattern in the cytosol was observed, reminiscent of that in the endosomal-lysosomal compartments. In addition, a diffuse extracellular surface-staining was evident. This result demonstrates that receptor-mediated endocytosis of aFGF does not result in its translocation to the nucleus, as has been reported for basic FGF.

Biosynthesis of bikunin (urinary trypsin inhibitor) in rat hepatocytes

One of the major sulfated proteins secreted by rat hepatocytes contains a low-sulfated chondroitin sulfate chain and its apparent molecular mass upon sodium dodecyl sulfate/polyacrylamide gel electrophoresis shifts from 40 to 28 kDa upon chondroitinase ABC treatment (E. M. Sjöberg and E. Fries, 1990, Biochem. J. 272, 113-118). These properties suggest that this protein is the rat homologue of the major trypsin inhibitor of human urine which was recently named bikunin. In serum, bikunin occurs mainly as a subunit of the pre-alpha-inhibitor and the inter-alpha-inhibitor; in these proteins it is covalently linked to the other polypeptides through its chondroitin sulfate chain. Bikunin has been shown to be synthesized by liver cells as a 42-kDa precursor, in which it is linked to alpha 1-microglobulin by two basic amino acids. We have isolated bikunin from rat urine and prepared antibodies against it. In rat hepatocytes pulse-labeled with [35S]methionine, these antibodies precipitated a labeled protein of 42 kDa. Upon chase, three different labeled proteins were recognized by the antibodies in the medium: one protein of 40 kDa (free bikunin), one of 125 kDa (presumably pre-alpha-inhibitor), and one greater than 240 kDa (possibly a protein related to the inter-alpha-inhibitor). Pulse-chase experiments with [35S]sulfate showed that these proteins occurred intracellularly as precursors containing alpha 1-microglobulin. These results demonstrate that the completion of the chondroitin sulfate chain and its coupling to other polypeptide chains occur before the cleavage of the alpha 1-microglobulin/bikunin precursor.

Absorptive-mediated endocytosis of an adrenocorticotropic hormone (ACTH) analogue, ebiratide, into the blood-brain barrier: studies with monolayers of primary cultured bovine brain capillary endothelial cells

The internalization of a neuromodulatory adrenocorticotropic hormone (ACTH) analogue, [125I]ebiratide (H-Met(O2)-Glu[125I]His-Phe-D-Lys-Phe-NH(CH2)2NH2), was examined in cultured monolayers of bovine brain capillary endothelial cells (BCEC). HPLC analysis of the incubation solution showed that [125I]ebiratide was not metabolized during the incubation with BCEC. The acid-resistant binding of [125I]ebiratide to BCEC increased with time for 120 min and showed a significant dependence on temperature and medium osmolarity. Pretreatment of BCEC with dansylcadaverine or phenylarsine oxide, endocytosis inhibitors, and 2,4-dinitrophenol, a metabolic inhibitor, decreased significantly the acid-resistant binding of [125I]ebiratide. The acid-resistant binding of [125I]ebiratide was saturable in the presence of unlabeled ebiratide (100 nM-1 mM). The maximal internalization capacity (Bmax) at 30 min was 7.96 +/- 3.27 pmol/mg of protein with a half-saturation constant (Kd) of 15.9 +/- 6.4 microM. The acid-resistant binding was inhibited by basic peptides such as poly-L-lysine, protamine, histone, and ACTH but was not inhibited by poly-L-glutamic acid, insulin, or transferrin. These results confirmed that ebiratide is transported through the blood-brain barrier via an absorptive-mediated endocytosis.

ATP is required for receptor-mediated endocytosis in intact cells

We have demonstrated a requirement for cellular ATP in the receptor-mediated endocytosis of transferrin. This has been accomplished using a novel assay for endocytosis based on acquisition of resistance to the membrane impermeable reducing agent, glutathione (GSH). Diferric-transferrin was conjugated to biotin via a cleavable disulfide bond and iodinated. Internalization of 125I-biotin-S-S-transferrin (125I-BSST) was quantitated by adsorption to avidin-Sepharose after treatment of cells with GSH. Receptor-mediated endocytosis of 125I-BSST was severely inhibited in ATP-depleted cells. Similar results were obtained when ATP was depleted by incubation of cells either under a N2-atmosphere or in the presence of NaN3 and NaF. The latter treatment, alone, also resulted in a loss of surface transferrin receptors which could not be correlated to reductions in cellular ATP. In contrast to the acquisition of GSH resistance, the apparent internalization of 125I-BSST as assessed by inaccessibility to antitransferrin antibodies reached control levels in ATP-depleted cells. Our biochemical and morphological data suggested that, although ATP is required for receptor-mediated endocytosis, in ATP-depleted cells ligands can become efficiently sequestered into deeply invaginated pits that are inaccessible to large probes such as antibodies, but remain accessible to small molecules such as GSH.

One of the major sulphated proteins secreted by rat hepatocytes contains low-sulphated chondroitin sulphate

When isolated hepatocytes are incubated with 35SO4(2-), a specific set of secretory proteins is labelled. One of these proteins is electrophoretically heterogeneous, with an apparent molecular mass of 35-45 kDa [Marcks von Würtemberg & Fries (1989) Biochemistry 28, 4088-4093]. Here we report that treatment with chondroitinase ABC converted the broad electrophoretic band of this protein, with a 50-60% loss of radioactivity, into a relatively homogeneous band with a molecular mass of 28 kDa. Size determination by gel chromatography of the protein's oligosaccharide chain (released by alkali treatment) indicated that it contained about 40 hexose units. Similar analysis of the enzyme-resistant oligosaccharide chain remaining linked to the protein after chondroitinase ABC treatment indicated a size of between six and eight hexose units. These observations suggest that the protein's oligosaccharide chain carries only three or four sulphate groups, of which one or two are located close to the polypeptide chain. Consistent with this hypothesis, the free oligosaccharide behaved like a low-sulphated glycosaminoglycan upon ion-exchange chromatography.

Capping of surface receptors and concomitant cortical tension are generated by conventional myosin

We have investigated the role of cytoskeletal contraction in the capping of surface proteins crosslinked by concanavalin A on mutant Dictyostelium cells lacking conventional myosin. Measurements of cellular deformability to indicate the development of cortical tension show that cells of the wild-type parental strain, AX4, stiffen early during capping and relax back towards the softer resting state as the process is completed. Mutant cells lacking myosin (mhcA-) have a lower resting-state stiffness, and fail to stiffen and to cap crosslinked proteins on binding concanavalin A. Hence conventional myosin is essential both for capping and for the concomitant increase in cell stiffness. Furthermore, depletion of cellular ATP by azide causes a 'rigor' contraction in AX4 cells which makes them stiffen and become spherical. By contrast, the mhcA- cells fail to respond in these ways. These measurements of cortical tension in non-muscle cells can thus be directly correlated with the presence of conventional myosin, demonstrating that contractile tension generated by myosin can drive both a change of cell shape and the capping of crosslinked surface receptors.

Brefeldin A redistributes resident and itinerant Golgi proteins to the endoplasmic reticulum

Brefeldin A (BFA) has been reported to block protein transport from the ER and cause disassembly of the Golgi complex. We have examined the effects of BFA on the transport and processing of the vesicular stomatitis virus G protein, a model integral membrane protein. Delivery of G protein to the cell surface was reversibly blocked by 6 micrograms/ml BFA. Pulse-label experiments revealed that in the presence of BFA, G protein became completely resistant to endoglycosidase H digestion. Addition of sialic acid, a trans-Golgi event, was not observed. Despite processing by cis- and medial Golgi enzymes, G protein was localized by indirect immunofluorescence to a reticular distribution characteristic of the ER. By preventing transport of G protein from the ER with the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone or by use of the temperature-sensitive mutant ts045, which is restricted to the ER at 40 degrees C, we showed that processing of G protein occurred in the ER and was not due to retention of newly synthesized Golgi enzymes. Rather, redistribution of preexisting cis and medial Golgi enzymes to the ER occurred as soon as 2.5 min after addition of BFA, and was complete by 10-15 min. Delivery of Golgi enzymes to the ER was energy dependent and occurred only at temperatures greater than or equal to 20 degrees C. BFA also induced retrograde transport of G protein from the medial Golgi to the ER. Golgi enzymes were completely recovered from the ER 10 min after removal of BFA. These findings demonstrate that BFA induces retrograde transport of both resident and itinerant Golgi proteins to the ER in a fully reversible manner.