Dithiothreitol treatment of Madin-Darby canine kidney cells reversibly blocks export from the endoplasmic reticulum but does not affect vectorial targeting of secretory proteins

Reduced glutathione disrupts the intracellular trafficking of tyrosinase and tyrosinase-related protein-1 but not dopachrome tautomerase and Pmel17 to melanosomes, which results in the attenuation of melanization

We previously reported that treatment of B16 melanotic melanoma cells with reduced glutathione (GSH) converts them to amelanotic cells without any significant down-regulation of tyrosinase activity. To characterize the cellular mechanism(s) involved, we determined the intracellular distribution of melanocyte-specific proteins, especially in melanin synthesis-specific organelles, termed melanosomes by subcellular fractionation followed by Western blotting and confocal laser microscopy (CFLM). In the melanosome-rich large granule fraction and in highly purified melanosome fractions, while GSH-induced amelanotic B16 cells have significantly diminished levels of protein/activity of tyrosinase and tyrosinase-related protein-1 compared with control melanized B16 cells, there was substantially no difference in the distribution and levels of dopachrome tautomerase and the processed isoform of Pmel17 (HMB45) between control melanized and GSH-induced amelanotic B16 cells. Analysis of merged images obtained by CFLM revealed that whereas tyrosinase, Pmel17 and dopachrome tautomerase colocalize with each other in the control melanized B16 cells, tyrosinase does not colocalize with Pmel17 or its processed isoform and with dopachrome tautomerase in GSH-induced amelanotic B16 cells. The sum of these findings suggests that reduced glutathione selectively disrupts the intracellular trafficking of tyrosinase and tyrosinase-related protein-1 but not dopachrome tautomerase and Pmel17 to melanosomes, which results in the attenuation of melanization, probably serving as a putative model for oculocutaneous albinism type 4.

Chapter 4: Regulation of Clusterin activity by calcium

In this chapter, the attention is put on Ca(2+) effect on Clusterin (CLU) activity. We showed that two CLU forms (secreted and nuclear) are differently regulated by Ca(2+) and that Ca(2+) fluxes affect CLU gene expression. A secretory form (sCLU) protects cell viability whereas nuclear form (nCLU) is proapoptotic. Based on available data we suggest, that different CLU forms play opposite roles, depending on intracellular Ca(2+) concentration, time-course of Ca(2+) current, intracellular Ca(2+) compartmentalization, and final Ca(2+) targets. Discussion will be motivated on how CLU acts on cell in response to Ca(2+) waves. The impact of Ca(2+) on CLU gene activity and transcription, posttranscriptional modifications, translation of CLU mRNA, and posttranslational changes as well as biological effects of CLU will be discussed. We will also examine how Ca(2+) signal and Ca(2+)-dependent proteins are attributable to changes in CLU characteristics. Some elucidation of CLU gene activity, CLU protein formation, maturation, secretion, and intracellular translocations in response to Ca(2+) is presented. In response to cell stress (i.e., DNA damage) CLU gene is activated. We assume that commonly upregulated mRNA for nCLU versus sCLU and vice versa are dependent on Ca(2+) accessibility and its intracellular distribution. It looks as if at low intracellular Ca(2+) the delay in cell cycle allows more time for DNA repair; otherwise, cells undergo nCLU-dependent apoptosis. If cells are about to survive, intrinsic apoptosis is abrogated by sCLU interacting with activated Bax. In conclusion, a narrow range of intracellular Ca(2+) concentrations is responsible for the decision whether nCLU is mobilized (apoptosis) or sCLU is appointed to improve survival. Since the discovery of CLU, a huge research progress has been done. Nonetheless we feel that much work is left ahead before remaining uncertainties related to Ca(2+) signal and the respective roles of CLU proteins are unraveled.

Effect of N-Acetylcystein on Butyrate-Treated Chinese Hamster Ovary Cells To Improve the Production of Recombinant Human Interferon-β-1a

Sodium butyrate (NaBu) is used as a productivity enhancer for the production of therapeutic recombinant proteins in Chinese hamster ovary (CHO) cells. However, NaBu is well-known for having a cytotoxic effect, thereby inducing apoptosis. As an endeavor to reduce this defect, we studied 11 antioxidants known for inhibiting apoptosis, according to a Plackett-Burman statistical design on CHO cells producing recombinant interferon-beta-1a (IFN-beta). None of the antioxidants that we tested were as effective as N-acetylcystein (NAC) from the point of view of maintaining long-term survival of CHO cells and increasing the production of IFN-beta. In 7.5-L perfusion bioreactor cultures, the addition of NaBu and NAC elongated the culture period to almost 200 h throughout production phase and increased the production yield by 2-fold compared to control cultures containing only NaBu. Glycosylation patterns of produced IFN-beta at each run were also compared in IEF analysis. IEF profiles of where NaBu and NAC were added showed to be more isoforms with a lower pI than those of the control run. The sialic acid content was also increased by 17.7% according to HPLC analysis. Taken together, the data obtained demonstrate that the addition of NAC has positive effects on the elongation of the culture period, improving the production and increasing the sialylation of IFN-beta in NaBu-treated CHO cells.

Oxygen free radicals and redox biology of organelles

The presence and supposed roles of reactive oxygen species (ROS) were reported in literature in a myriad of instances. However, the breadth and depth of their involvement in cellular physiology and pathology, as well as their relationship to the redox environment can only be guessed from specialized reports. Whatever their circumstances of formation or consequences, ROS seem to be conspicuous components of intracellular milieu. We sought to verify this assertion, by collecting the available evidence derived from the most recent publications in the biomedical field. Unlike other reviews with similar objectives, we centered our analysis on the subcellular compartments, namely on organelles, grouped according to their major functions. Thus, plasma membrane is a major source of ROS through NAD(P)H oxidases located on either side. Enzymes of the same class displaying low activity, as well as their components, are also present free in cytoplasm, regulating the actin cytoskeleton and cell motility. Mitochondria can be a major source of ROS, mainly in processes leading to apoptosis. The protein synthetic pathway (endoplasmic reticulum and Golgi apparatus), including the nucleus, as well as protein turnover, are all exquisitely sensitive to ROS-related redox conditions. The same applies to the degradation pathways represented by lysosomes and peroxisomes. Therefore, ROS cannot be perceived anymore as a mere harmful consequence of external factors, or byproducts of altered cellular metabolism. This may explain why the indiscriminate use of anti-oxidants did not produce the expected "beneficial" results in many medical applications attempted so far, underlying the need for a deeper apprehension of the biological roles of ROS, particularly in the context of the higher cellular order of organelles.

Evidence for disulfide involvement in the regulation of intramolecular autolytic processing by human adamalysin19/ADAM19

Human adamalysin 19 (a disintegrin and metalloproteinase 19, hADAM19) is activated by furin-mediated cleavage of the prodomain followed by an autolytic processing within the cysteine-rich domain at Glu586-Ser587, which occurs intramolecularly, producing an NH2 terminal fragment (N-fragment) associated with its COOH-terminal fragment (C-fragment), most likely through disulfide bonds. When stable Madin-Darby canine kidney (MDCK) transfectants overexpressing soluble hADAM19 were treated with dithiothreitol (DTT) or with media at pH 6.5, 7.5, or 8.5, the secretion and folding of the enzyme were not affected. Autolytic processing was blocked by DTT and pH 6.5 media, which favor disulfide reduction, but was increased by pH 8.5 media, which promotes disulfide formation. Cys605, Cys633, Cys639, and Cys643 of the C-fragment appear to be partially responsible for the covalent association between the C-fragment and the N-fragment. A new autolytic processing site at Lys543-Val544 was identified in soluble mutants when these cysteine residues were individually mutated to serine residues. Shed fragments were also detectable in the media from MDCK cells stably expressing the full-length Cys633Ser mutant. Ilomastat/GM6001 inhibited hADAM19 with an IC50 of 447 nM, but scarcely affected the shedding process. The cysteine-rich domain likely forms disulfide bonds to regulate the autolytic processing and shedding of hADAM19.

Homocysteine increases the expression of vascular endothelial growth factor by a mechanism involving endoplasmic reticulum stress and transcription factor ATF4

Vascular endothelial growth factor (VEGF) plays a key role in the development and progression of diabetic retinopathy. We previously demonstrated that amino acid deprivation and other inducers of endoplasmic reticulum-stress (ER stress) up-regulate the expression of VEGF in the retinal-pigmented epithelial cell line ARPE-19. Because homocysteine causes ER stress, we hypothesized that VEGF expression is increased by ambient homocysteine. dl-Homocysteine-induced VEGF expression was investigated in confluent ARPE-19 cultures. Northern analysis showed that homocysteine increased steady state VEGF mRNA levels 4.4-fold. Other thiol-containing compounds, including l-homocysteine thiolactone and DTT, induced VEGF expression 7.9- and 8.8-fold. Transcriptional run-on assays and mRNA decay studies demonstrated that the increase in VEGF mRNA levels was caused by increased transcription rather than mRNA stabilization. VEGF mRNA induction paralleled that of the ER-stress gene GRP78. Homocysteine treatment caused transient phosphorylation of eIF2alpha and an increase in ATF4 protein level. Overexpression of a dominant-negative ATF4 abolished the VEGF response to homocysteine treatment and to amino acid deprivation. VEGF mRNA expression by ATF4-/- MEF did not respond to homocysteine treatment and the response was restored with expression of wild-type ATF4. These studies indicate that expression of the pro-angiogenic factor VEGF is increased by homocysteine and other thiol-containing reductive compounds via ATF4-dependent activation of VEGF transcription.

Folding and maturation of tyrosinase-related protein-1 are regulated by the post-translational formation of disulfide bonds and by N-glycan processing

In this study we have explored the endoplasmic reticulum associated events accompanying the maturation of the tyrosinase-related protein-1 (TRP-1) nascent chain synthesized in mouse melanoma cells. We show that TRP-1 folding process occurs much more rapidly than for tyrosinase, a highly homologous protein, being completed post-translationally by the formation of critical disulfide bonds. In cells pretreated with dithiothreitol (DTT), unfolded TRP-1 is retained in the endoplasmic reticulum by a prolonged interaction with calnexin and BiP before being targeted for degradation. The TRP-1 chain was able to fold into DTT-resistant conformations both in the presence or absence of alpha-glucosidase inhibitors, but folding occurred through different pathways. During the normal folding pathway, TRP-1 interacts with calnexin. In the presence of alpha-glucosidase inhibitors, the interaction with calnexin is prevented, with TRP-1 folding being assisted by BiP. In this case, the process has similar kinetics to that of untreated TRP-1 and yields a compact form insensitive to DTT as well. However, this form has different thermal denaturation properties than the native conformation. We conclude that disulfide bridge burring is crucial for the TRP-1 export. This suggests that although various folding pathways may complete this process, the native form may be acquired only through the normal unperturbed pathway.

150-kDa oxygen-regulated protein (ORP150) functions as a novel molecular chaperone in MDCK cells

To assess the participation of the 150-kDa oxygen-regulated protein (ORP150) in protein transport, its function in Madin-Darby canine kidney (MDCK) cells was studied. Exposure of MDCK cells to hypoxia resulted in an increase of ORP150 antigen and increased binding of ORP150 to GP80/clusterin (80-kDa glycoprotein), a natural secretory protein in this cell line. In ORP150 antisense transformant MDCK cells, GP80 was retained within the endoplasmic reticulum after exposure to hypoxia. Metabolic labeling showed the delay of GP80 maturation in antisense transformants in hypoxia, whereas its matured form was detected in wild-type cells, indicating a role of ORP150 in protein transport, especially in hypoxia. The affinity chromatographic analysis of ORP150 suggested its ability to bind to ATP-agarose. Furthermore, the ATP hydrolysis analysis showed that ORP150 can release GP80 at a lower ATP concentration. These data indicate that ORP150 may function as a unique molecular chaperone in renal epithelial cells by facilitating protein transport/maturation in an environment where less ATP is accessible.

Basic mechanisms of secretion: sorting into the regulated secretory pathway

Targeting proteins to their correct cellular location is crucial for their biological function. In neuroendocrine cells, proteins can be secreted by either the constitutive or the regulated secretory pathways but the mechanism(s) whereby proteins are sorted into either pathway is unclear. In this review we discuss the possibility that sorting is either an active process occurring at the level of the trans-Golgi network, or that sorting occurs passively in the immature granules. The possible involvement of protein-lipid interactions in the sorting process is also raised.

Key words: lipid rafts, regulated secretory pathway, secretion, sorting receptors, sorting signals, trans-Golgi network.

Conformational changes in apolipoprotein B modulate intracellular assembly and degradation of ApoB-containing lipoprotein particles in HepG2 cells

The linkage between the conformation of apolipoprotein B100 (apoB) and the intracellular assembly and degradation of apoB-containing lipoproteins was investigated in the present study. Disruption of disulfide bond formation in newly synthesized apoB molecules through the use of the reducing agent DTT resulted in a decrease in the secretion of apoB-containing lipoproteins from HepG2 cells compared with control cells. The synthesis of total apoB (apoB100 plus nascent chains), as well as a number of control proteins, such as albumin and alpha 1-antitrypsin, was decreased significantly in DTT-treated cells. However, the intracellular accumulation of full-length apoB100 molecules was not inhibited in the presence of DTT. Subcellular fractionation indicated that apoB molecules isolated from the microsomes of DTT-treated cells had an increased association with the microsomal membrane compared with apoB isolated from untreated cells. Analysis of the distribution of apoB-containing lipoproteins from the lumen of isolated microsomes demonstrated that in the presence of DTT, there was a shift in the distribution, such that there was a decrease in the formation of HDL-sized (lipid-poor) apoB-containing lipoproteins and a decrease in the formation of LDL/VLDL apoB particles. Alterations in apoB conformation and their impact on degradation were also investigated by using DTT and by inhibiting N-linked glycosylation with tunicamycin. DTT appeared to change the rate and pattern of apoB degradation. Degradation was accelerated in both intact and permeabilized HepG2 cells. ApoB degradation occurred in DTT-treated permeabilized cells without the usual generation of the 70-kD and 335-kD fragments and was largely N-acetyl-leucyl-leucyl-norleucinal (ALLN) insensitive. In tunicamycin-treated cells, DTT further accelerated the degradation of unglycosylated apoB. Overall, the data suggest that the misfolding of apoB may prevent the proper association of apoB with lipids, resulting in impairment of the assembly of mature apoB-containing lipoproteins. Alteration in the conformation of apoB also appears to alter the degradation pathway of apoB, such that the protein is degraded through a pathway that is at least in part ALLN insensitive.

Effect of disulfide bonds of transcobalamin II receptor on its activity and basolateral targeting in human intestinal epithelial Caco-2 cells

Transcobalamin II-receptor (TC II-R) contains 10 half-cysteines, of which 8 are involved in intramolecular disulfide bonding. Reduction followed by alkylation with N-ethylmaleimide (NEM) of the 62-kDa TC II-R monomer in vitro or treatment of human intestinal epithelial Caco-2 cells with low concentrations (10(-6) M) of NEM resulted in TC II-R exhibiting a loss of ligand binding and an increase in its apparent molecular mass by 10 kDa to 72 kDa. Domain-specific biotinylation studies using NEM-treated filter-grown cells revealed loss of TC II-R but not cation-independent mannose 6-phosphate receptor protein at the basolateral cell surface. Pulse-chase labeling of NEM-treated cells with [35S]methionine revealed that the modified 72-kDa TC II-R, like the native 62-kDa TC II-R in untreated cells, turned over rapidly with a t1/2 of 7.5 h and was sensitive to treatment with peptide N-glycosidase F, sialidase alone, or sialidase and O-glycanase but not to treatment with endoglycosidase H. Labeled 72-kDa TC II-R, which was retained intracellularly following treatment of Caco-2 cells with methyl methanethiosulfonate, returned to the basolateral cell surface following withdrawal of cells from methyl methanethiosulfonate treatment and exposure to dithiothreitol. Based on these results, we suggest that formation and maintenance of intramolecular disulfide bonds of TC II-R is important for its acquisition of ligand binding and post-trans-Golgi trafficking to basolateral surface membranes but not for its turnover and exit from the endoplasmic reticulum or trafficking through the Golgi.

Folding of the amino-terminal domain of apolipoprotein B initiates microsomal triglyceride transfer protein-dependent lipid transfer to nascent very low density lipoprotein

The initial assembly of apolipoprotein B100 (apoB) into lipoprotein particles occurs cotranslationally. To examine steps required to initiate this process, the intracellular folding and assembly of the amino-terminal 28% of apoB (apoB28) was examined using several criteria including nonreducing gel electrophoresis, sensitivity to dithiothreitol (DTT)-mediated reduction, and buoyant density gradient centrifugation. In hepatoma cells, after a 1-min pulse with radiolabeled amino acids, labeled apoB28 migrated during gel electrophoresis in the folded position and was resistant to reduction in vivo with 2 mM DTT. A similar rate and extent of folding was observed in Chinese hamster ovary cells, a microsomal triglyceride transfer protein (MTP)-negative cell line that can neither lipidate nor efficiently secrete apoB28. Amino-terminal folding of apoB28 was essential for its subsequent intracellular lipidation as apoB28 synthesized in hepatoma cells under reducing conditions remained lipid poor (d > 1.25 g/ml) and was retained intracellularly. Upon DTT removal, reduced apoB28 underwent a process of rapid (t1/2 approximately 2 min) post-translational folding followed by a slower process of MTP-dependent lipidation. As with the cotranslational assembly pathway, post-translational lipidation of apoB28 displayed a strict dependence upon amino-terminal folding. We conclude that: 1) folding of the amino-terminal disulfide bonded domain of apoB is achieved prior to the completion of translation and is independent of MTP and events associated with buoyant lipoprotein formation and 2) domain-specific folding of apoBs amino-terminal region is required to initiate MTP-dependent lipid transfer to nascent apoB in the hepatic endoplasmic reticulum.

Quality control of glycosylphosphatidylinositol anchor attachment in mammalian cells: a biochemical study

hGHDAF28 is a chimaeric protein consisting of human growth hormone fused to a crippled signal sequence for glycosylphosphatidylinositol (GPI)-anchor addition from decay-accelerating factor, and serves as a model for quality control of GPI-anchor addition. hGHDAF28 is retained in a pre-Golgi compartment and degraded intracellularly by a mechanism with similarity to that for other endoplasmic reticulum (ER)-retained proteins (Field, Moran, Lee, Keller and Caras (1994) J. Biol. Chem. 269, 10830-10837). We have studied the specific pathway of degradation for hGHDAF28 using a number of compounds which affect protein folding and trafficking pathways in eukaryotic cells. We found that high concentrations of dithiothreitol (DTT) accelerated loss of hGHDAF28 by degradation from cell lysates, without promoting secretion or alteration of disulphide-bond distribution, in contrast to a number of other examples of ER-retained proteins where DTT alters disulphide-bond formation. Additionally, degradation of hGHDAF28 was sensitive to pH, being promoted at pH 6.0 and inhibited at pH 8.0; however, the latter effect was transient, indicating incomplete blockade. Degradation was also partially enhanced by depletion of ER calcium with thapsigargin, but this was again a partial and transient effect. Furthermore, degradation was temperature sensitive, with a gradual decrease in rate observed at lower temperatures. However, a sharp decrease in turnover between 15 degrees C and 20 degrees C, indicative of a requirement for transport to a post-ER compartment, was not observed. Degradation of hGHDAF28 was insensitive to treatment with nocodozole or compounds preventing cytoplasmic autophagy, suggesting that ER degradation is independent of classical autophagy and microtubule-dependent processes. In addition, disruption of N-glycosylation with tunicamycin, or inhibition of processing of immature N-glycan chains with castanospermine or deoxynojirimycin, had little effect on the stability of hGHDAF28, suggesting that disruption of the BiP/calnexin quality-control system by bulk cellular secretory proteins does not influence the ER-degradation pathway of hGHDAF28. Intermolecular hGHDAF28 cysteine bonds result in the formation of aggregates which are probably important in the retention of the molecule. The insensitivity of this structure to reduction in vivo, together with the enhanced degradation rate, indicates that DTT mediates its effect on stability via a molecule involved in degradation of hGHDAF28, possibly a thiol-sensitive protease.

Biochemical characterization and intracellular localization of the Menkes disease protein

Menkes disease is a fatal neurodegenerative disorder of childhood due to the absence or dysfunction of a putative copper-transporting P-type ATPase encoded on the X chromosome. To elucidate the biosynthesis and subcellular localization of this protein, polyclonal antisera were generated against a bacterial fusion protein encoding the 4th to 6th copper-binding domains in the amino terminus of the human Menkes protein. RNA blot analysis revealed abundant Menkes gene expression in several cell lines, and immunoblotting studies utilizing this antiserum readily detected a 178-kDa protein in lysates from these cells. Pulse-chase studies indicate that this protein is synthesized as a single-chain polypeptide which is modified by N-linked glycosylation to a mature endoglycosidase H-resistant form. Sucrose gradient fractionation of HeLa cell lysates followed by immunoblotting of individual fractions with antibodies to proteins of known intracellular location identified the Menkes ATPase in fractions similar to those containing the cation-independent mannose-6-phosphate receptor. Consistent with this observation, confocal immunofluorescence studies of these same cells localized this protein to the trans-Golgi network and a vesicular compartment with no expression in the nucleus or on the plasma membrane. Taken together, these data provide a unique model of copper transport into the secretory pathway of mammalian cells which is compatible with clinical observations in affected patients and with recent data on homologous proteins identified in prokaryotes and yeast.

Clusterin gene expression in apoptotic MDCK cells is dependent on the apoptosis-inducing stimulus

Clusterin (Apolipoprotein J, complement lysis inhibitor) is a widely expressed multifunctional glycoprotein. The expression of clusterin mRNA has been reported to be elevated in a broad spectrum of apoptotic or degenerative tissues. More recently, it was shown that within these tissues clusterin is expressed in the surviving rather than in the dying cells, and that clusterin gene expression is actually down-regulated in the apoptotic cells. We have studied the expression of the clusterin gene in apoptotic MDCK cells. Cell death was initiated by three different stimuli: application of the steroid hormone antagonist RU 486, activation of protein kinase C by the application of the phorbol ester TPA, and--since clusterin is involved in lipid and cholesterol transport--perturbation of cell membranes by cholesterol. We show that clusterin gene expression is repressed in cells undergoing apoptosis in response to the application of RU 486 and TPA, but is unchanged in cells in which apoptosis has been triggered by cholesterol treatment.