The relationship of N-linked glycosylation and association with heavy chain binding protein (BiP) to the secretion of Factor VIII (FVIII), von Willebrand Factor (vWF), and tissue plasminogen activator (tPA) was studied in Chinese hamster ovary (CHO) cells. FVIII has a heavily glycosylated region containing 20 clustered potential N-linked glycosylation sites. A significant proportion of FVIII was detected in a stable complex with BiP and not secreted. Deletion of the heavily glycosylated region resulted in reduced association with BiP and more efficient secretion. Tunicamycin treatment of cells producing this deleted form of FVIII resulted in stable association of unglycosylated FVIII with BiP and inhibition of efficient secretion. vWF contains 17 potential N-linked glycosylation sites scattered throughout the molecule. vWF was transiently associated with BiP and efficiently secreted demonstrating that CHO cells are competent to secrete a highly glycosylated protein. tPA, which has three utilized N-linked glycosylation sites, exhibited low level association with BiP and was efficiently secreted. Disruption of N-linked glycosylation of tPA by either site-directed mutagenesis or tunicamycin treatment resulted in reduced levels of secretion and increased association with BiP. This effect was enhanced by high levels of tPA expression. The glycosylation state and extent of association with BiP could be correlated with secretion efficiency.
Although many eukaryotic proteins have been secreted by transfected bacterial cells, little is known about how a bacterial protein is treated as it passes through the secretory pathway when expressed in a eukaryotic cell. The eukaryotic N-glycosylation system could interfere with folding and secretion of prokaryotic proteins whose sequence has not been adapted for glycosylation in structurally appropriate locations. Here we show that such interference does indeed occur for chondroitinase ABC from the bacterium Proteus vulgaris, and can be overcome by eliminating potential N-glycosylation sites. Chondroitinase ABC was heavily glycosylated when expressed in mammalian cells or in a mammalian translation system, and this process prevented secretion of functional enzyme. Directed mutagenesis of selected N-glycosylation sites allowed efficient secretion of active chondroitinase. As these proteoglycans are known to inhibit regeneration of axons in the mammalian central nervous system, the modified chondroitinase gene is a potential tool for gene therapy to promote neural regeneration, ultimately in human spinal cord injury.
Glycosylation; Protein secretion; Endoplasmic reticulum; Chondroitinase; Spinal cord injury
Prostate cancer is the second most common cause of cancer-associated deaths in men and signalling via a transcription factor called androgen receptor (AR) is an important driver of the disease. Androgen treatment is known to affect the expression and activity of other oncogenes including receptor tyrosine kinases (RTKs). In this study we report that AR-positive prostate cancer cell-lines express 50% higher levels of enzymes in the hexosamine biosynthesis pathway (HBP) than AR-negative prostate cell-lines. HBP produces hexosamines that are used by endoplasmic reticulum and golgi enzymes to glycosylate proteins targeted to plasma-membrane and secretion. Inhibition of O-linked glycosylation by ST045849 or N-linked glycosylation with tunicamycin decreased cell viability by 20%. In addition, tunicamycin inhibited the androgen-induced expression of AR target genes KLK3 and CaMKK2 by 50%. RTKs have been shown to enhance AR activity and we used an antibody array to identify changes in the phosphorylation status of RTKs in response to androgen stimulation. Hormone treatment increased the activity of Insulin like Growth Factor 1-Receptor (IGF-1R) ten-fold and this was associated with a concomitant increase in the N-linked glycosylation of the receptor, analyzed by lectin enrichment experiments. Glycosylation is known to be important for the processing and stability of RTKs. Inhibition of N-linked glycosylation resulted in accumulation of IGF-1R pro-receptor with altered mobility as shown by immunoprecipitation. Confocal imaging revealed that androgen induced plasma-membrane localization of IGF-1R was blocked by tunicamycin. In conclusion we have established that the glycosylation of IGF-1R is necessary for the full activation of the receptor in response to androgen treatment and that perturbing this process can break the feedback loop between AR and IGF-1R activation in prostate cells. Achieving similar results selectively in a clinical setting will be an important challenge in the future.
Asparagine-linked glycosylation is a form of covalent modification that distinguishes proteins that are either membrane bound or are in cellular compartments topologically outside of the cell from those proteins that remain soluble in the cytoplasm. This type of glycosylation occurs stepwise, with core oligosaccharide added in the endoplasmic reticulum and subsequent modifications occurring in the golgi. We used tunicamycin, an inhibitor of one of the earliest steps in the synthesis of N-linked oligosaccharide, to select for mutants that are resistant to this antibiotic. Genetic, biochemical, and physiological experiments led to the following conclusions. The synthesis of N-linked oligosaccharide is an essential function in cells. In contrast to mammalian cells, yeast cells do not transport tunicamycin by a glucosamine transport function. We identified a gene, ALG7, that is probably the structural gene for UDP-N-acetylglucosamine-1-P transferase, the enzyme inhibited by tunicamycin. Dominant mutations in this gene result in increased activity of the transferase and loss of the ability of the cell to sporulate. In addition, we identified another gene, TUN1, in which recessive mutations result in resistance to tunicamycin. The ALG7 and TUN1 genes both map on chromosome VII.
Yeast secretory mutants sec53 and sec59 define a posttranslational stage in the penetration of glycoprotein precursors into the endoplasmic reticulum (ER). In the previous report we showed that at the restrictive temperature (37 degrees C) these mutants accumulate enzymatically inactive and incompletely glycosylated forms of the secretory enzyme invertase and the vacuolar enzyme carboxypeptidase Y. Cell fractionation experiments reveal that these precursor forms remain firmly bound to the ER membrane. However, upon return to the permissive temperature (24 degrees C), the invertase precursors are glycosylated, become partially active, and are secreted. Thermoreversible conversion does not require protein synthesis, but does require energy. In contrast to the effect of these mutations, inhibition of oligosaccharide synthesis with tunicamycin at 37 degrees C causes irreversible accumulation of unglycosylated invertase. The effect of the drug is exaggerated by high temperature since unglycosylated invertase synthesized in the presence of tunicamycin at 25 degrees C is secreted. A portion of the invertase polypeptide accumulated at 37 degrees C is preserved when membranes from sec53 and sec59 are treated with trypsin. In the presence of Triton X-100 or saponin, the invertase is degraded completely. The protected fragment appears to represent a portion of the invertase polypeptide that is embedded in or firmly associated with the ER membrane. This association may develop early during the synthesis of invertase, so that in the absence of translocation, some of the completed polypeptide chain remains exposed on the cytoplasmic surface of the ER.
We transformed Saccharomyces cerevisiae with a high-copy-number plasmid carrying either the wild-type gene coding for a repressible cell surface acid phosphatase or two modified genes whose products lack a 13- or 14-amino-acid segment spanning or immediately adjacent to the signal peptidase cleavage site. The wild-type gene product underwent proteolytic cleavage of the signal peptide, core glycosylation, and outer chain glycosylation. The deletion spanning the signal peptidase cleavage site led to an unprocessed protein. This modified protein exhibited core glycosylation, whereas its outer chain glycosylation was severely inhibited. Secretion of the deleted protein was impaired, and active enzyme accumulated within the cell. The deletion immediately adjacent to the signal peptidase cleavage site exhibited only a small decrease in the efficiency of processing and had no effect on the efficiency of secretion.
We have previously shown that Chinese hamster ovary (CHO) cells are resistant to infection by gibbon ape leukemia virus and amphotropic pseudotype retroviral vectors because of the secretion of factors that inhibit retrovirus infection. Such factors were not secreted by any mouse or human cell lines tested. Secretion of the inhibitors and resistance to infection are abrogated by treatment of CHO cells with the glycosylation inhibitor tunicamycin. Here we show that the inhibitory activities against gibbon ape leukemia virus and amphotropic viruses are partially separable and that glycosylation mutations in CHO cells mimic the effects of tunicamycin treatment. We find that several hamster cell lines derived from both Chinese and Syrian hamsters secrete inhibitors of retrovirus infection, showing that these inhibitors are not unique to the CHO cell line. Inhibitory factors are also present in the sera of Chinese and Syrian hamsters but were not detected in bovine serum. These results suggest the presence of specific factors that function to inhibit retrovirus infection in hamsters.
Lysyl oxidase (LOX) is secreted as a proenzyme (proLOX) that is proteolytically processed in the extracellular milieu to release the propeptide and mature, active LOX. LOX oxidizes lysyl residues of a number of protein substrates in the extracellular matrix and on the cell surface, which impacts several physiological and disease states. Although the LOX propeptide (LOX-PP) is glycosylated, little is known about the role of this modification in LOX secretion and activity. To gain insight into this issue, cells were transfected with native, full-length LOX cDNA (pre-pro-LOX), the N-glycosylation null pre-[N/Q]pro-LOX cDNA and the deletion mutant pre-LOX cDNA, referred to as secretory LOX, in which mature LOX is targeted to the secretory pathway without its N-terminal propeptide sequence. The results show that glycosylation of the LOX-PP is not required for secretion and extracellular processing of pro-LOX but it is required for optimal enzyme activity of the resulting mature LOX. Complete deletion of the propeptide sequence prevents mature LOX from exiting the endoplasmic reticulum (ER). Taken together, our study points out the requirement of the LOX-PP for pro-LOX exit from the ER and is the first to highlight the influence of LOX-PP glycosylation on LOX enzyme activity.
Endoplasmic reticulum; ER-associated protein degradation; Lysyl Oxidase; Propeptide; Glycosylation; subcellular localization
Many biopharmaceuticals are now produced as secreted glycoproteins from mammalian cell culture. The glycosylation profile of these proteins is essential to ensure structural stability and biological and clinical activity. However, the ability to control the glycosylation is limited by our understanding of the parameters that affect the heterogeneity of added glycan structures. It is clear that the glycosylation process is affected by a number of factors including the 3-dimensional structure of the protein, the enzyme repertoire of the host cell, the transit time in the Golgi and the availability of intracellular sugar-nucleotide donors. From a process development perspective there are many culture parameters that can be controlled to enable a consistent glycosylation profile to emerge from each batch culture. A further, but more difficult goal is to control the culture conditions to enable the enrichment of specific glycoforms identified with desirable biological activities. The purpose of this paper is to discuss the cellular metabolism associated with protein glycosylation and review the attempts to manipulate, control or engineer this metabolism to allow the expression of human glycosylation profiles in producer lines such as genetically engineered Chinese hamster ovary (CHO) cells.
Antennarity; CHO cells; Fucosylation; Galactosylation; Glycoprotein; Glycosylation; Golgi; N-glycan; O-glycan; Sequon; Sialylation
Carboxypeptidase Y from Saccharomyces cerevisiae contains 14% mannose, the only neutral sugar present. An antiserum can be raised in rabbits which reacts with both the protein and the sugar moieties of the enzyme. This antiserum also precipitates yeast invertase and yeast cell wall mannan. Thus carboxypeptidase Y, which is known to be localized in yeast vacuoles, is very probably a mannoprotein. Tunicamycin inhibits the apparent formation of carboxypeptidase Y to a similar extent as that of the externally localized mannoprotein, invertase. No accumulation of an inactive nonglycosylated or partly glycosylated carboxypeptidase Y occurs as determined by the immunoprecipitation technique. Tunicamycin also inhibits the apparent formation of proteinase A, whereas it does not affect the increase in the activities of a number of other enzymes. It is suggested that in the synthesis of glycoproteins there exists a regulatory link between the synthesis of their polypeptide chains and the reactions involved in their glycosylation.
The yeast Pichia pastoris is a common host for the recombinant production of biopharmaceuticals, capable of performing posttranslational modifications like glycosylation of secreted proteins. However, the activity of the OCH1 encoded α-1,6-mannosyltransferase triggers hypermannosylation of secreted proteins at great heterogeneity, considerably hampering downstream processing and reproducibility. Horseradish peroxidases are versatile enzymes with applications in diagnostics, bioremediation and cancer treatment. Despite the importance of these enzymes, they are still isolated from plant at low yields with different biochemical properties. Here we show the production of homogeneous glycoprotein species of recombinant horseradish peroxidase by using a P. pastoris platform strain in which OCH1 was deleted. This och1 knockout strain showed a growth impaired phenotype and considerable rearrangements of cell wall components, but nevertheless secreted more homogeneously glycosylated protein carrying mainly Man8 instead of Man10 N-glycans as a dominant core glycan structure at a volumetric productivity of 70% of the wildtype strain.
Previous work showed that tunicamycin suppresses glycosylation of laminin. In the present work, the role of glycosylation in the secretion of laminin and in the disulfide bonding of laminin subunits was studied, using tunicamycin to inhibit glycosylation. Tunicamycin inhibited extensively the secretion of laminin into culture medium and extracellular matrix even though the treated cells contained higher concentrations of laminin than the control cells. The laminin subunits synthesized in the presence of tunicamycin were disulfide bonded. Thus, suppression of glycosylation did not adversely affect disulfide bonding of the subunits, but did decrease the secretion of laminin. Glycosidases were also used to remove the carbohydrate of laminin to study the role of carbohydrate in the stability of laminin and in its interaction with another extracellular matrix component, heparin. The glycosidases removed about 73% of [3H]glucosamine. Both glycosidase-treated and untreated laminin were stable when incubated with cell lysate or culture medium. The glycosidase-treated laminin bound as efficiently as the untreated laminin to heparin. These results suggest that the presence of a carbohydrate moiety, at least at the level found in untreated laminin, is not essential in binding to heparin or in protecting laminin from proteolytic degradation in the cell or culture medium.
Angiotensin-I-converting enzyme (ACE) plays a critical role in the regulation of blood pressure through its central role in the renin-angiotensin and kallikrein-kinin systems. ACE contains two domains, the N and C domains, both of which are heavily glycosylated. Structural studies of ACE have been fraught with severe difficulties because of surface glycosylation of the protein. In order to investigate the role of glycosylation in the N domain and to create suitable forms for crystallization, we have investigated the importance of the 10 potential N-linked glycan sites using enzymatic deglycosylation, limited proteolysis, and mass spectrometry. A number of glycosylation mutants were generated via site-directed mutagenesis, expressed in CHO cells, and analyzed for enzymatic activity and thermal stability. At least eight of 10 of the potential glycan sites are glycosylated; three C-terminal sites were sufficient for expression of active N domain, whereas two N-terminal sites are important for its thermal stability. The minimally glycosylated Ndom389 construct was highly suitable for crystallization studies. The structure in the presence of an N domain-selective phosphinic inhibitor RXP407 was determined to 2.0 Å resolution. The Ndom389 structure revealed a hinge region that may contribute to the breathing motion proposed for substrate binding.
Glycosylation; Metalloprotease; Mutagenesis Mechanisms; Protease Inhibitor; Protein Domains; Protein Structure; Angiotensin-converting Enzyme; Phosphinic Peptide; Thermal Stability
The role of the glomerular visceral epithelial cell in the physiologic turnover and pathologic breakdown of the glomerular extracellular matrix has remained largely unexplored. In this study a 98-kD neutral proteinase secreted by cultured rat visceral glomerular epithelial cells was shown to be a calcium, zinc-dependent enzyme secreted in latent form. In addition, the protein was heavily glycosylated and demonstrated proteolytic activity against Type I gelatin, Type IV collagen gelatin, and fibronectin. The similarity in molecular mass and substrate specificities to the 92-kD human matrix metalloproteinase-9 (MMP-9, or gelatinase B) suggested the identity of this activity, which was confirmed by immunoprecipitation and Northern blot analysis. The differences in molecular mass (98 vs. 92 kD) were not due to species-specific differences in glycosylation patterns, since cultured rat peritoneal macrophages secreted MMP-9 as a 92-kD enzyme. Furthermore, transfection of the human MMP-9 cDNA into rat glomerular epithelial cells yielded the 98-kD product. Using a specific monoclonal anti-MMP-9 antibody and in situ reverse transcription (ISRT) analysis of MMP-9 mRNA, the expression of this enzyme was evaluated in vivo. Normal rat glomeruli expressed little immunohistochemical or ISRT staining for MMP-9, while in rats with passive Heymann nephritis there was a major increase in MMP-9 protein and mRNA staining within the visceral epithelial cells. The temporal patterns of MMP-9 expression correlated with the period of proteinuria associated with this model, suggesting that a causal relationship may exist between GEC MMP-9 expression and changes in glomerular capillary permeability.
The importance of carbohydrate in the secretion of immunoglobulin A (IgA) has previously been suggested by results of studies with tunicamycin, which prevents N-linked glycosylation of all cell glycoproteins. To directly evaluate the role of individual oligosaccharides in the secretion of IgA, we have used site-directed mutagenesis to selectively eliminate the two N-linked attachment sites reported to be glycosylated in alpha heavy chains. Transfected wild-type and mutant alpha genes were expressed in kappa light-chain-producing MPC-11 variant myeloma cells, and secretion kinetics of the IgAs were compared. Removal of either or both glycosylation sites led to intracellular alpha heavy-chain degradation and a 90 to 95% inhibition of IgA secretion. These results reveal that both N-linked oligosaccharides of the alpha heavy chain are essential for intracellular stability and normal secretion of IgA. This suggests that the key function of carbohydrate here is to maintain proper conformation of the glycoprotein. We also found that when expressed in the MPC-11 variant cells, alpha heavy chains were glycosylated at a third, normally unused site.
Glycosylation is one of the most abundant protein modifications in nature, having roles in protein stability, secretion and function. Alterations in mucin-type O-glycosylation are responsible for a number of human diseases and developmental defects, as well as associated with certain types of cancer. However, the mechanistic role of this form of glycosylation in many of these instances is unclear. Here we describe how one glycosyltransferase responsible for initiating mucin-type O-glycosylation (pgant3), specifically modulates integrin-mediated cell adhesion by influencing the secretion and localization of an integrin ligand. The integrin ligand Tiggrin, is normally O-glycosylated and localized to the basal matrix, where adhesion of two opposing cell layers takes place. In pgant3 mutants, Tiggrin is no longer O-glycosylated and fails to be properly secreted to the basal cell layer interface, resulting in disruption of proper cell adhesion. pgant3-mediated effects are dependent upon the enzymatic activity of PGANT3 and cannot be rescued by another pgant family member, indicating a unique role for this glycosyltransferase. These results provide in vivo evidence for the role of O-glycosylation in the secretion of specific extracellular matrix proteins, that thereby influences the composition of the cellular “microenvironment” and modulates cell adhesion events. The studies described in this review provide insight into the long-standing association between aberrant O-glycosylation and tumorigenesis, as changes in tumor environment and cell adhesion are hallmarks of cancer progression.
Candida albicans is a common opportunistic pathogen of the human body and is the frequent causative agent of candidiasis. Typically, these infections are associated with the formation of biofilms on both host tissues and implanted biomaterials. As a result of the intrinsic resistance of C. albicans biofilms to most antifungal agents, new strategies are needed to combat these infections.
Here we have used a 96-well microtitre plate model of C. albicans biofilm formation to study the inhibitory effect of tunicamycin, a nucleoside antibiotic that inhibits N-linked glycosylation affecting cell wall and secreted proteins, on C. albicans biofilm formation. A proteomic approach was used to study the effect of tunicamycin on levels of glycosylation of key secreted mannoproteins in the biofilm matrix.
Our results revealed that physiological concentrations of tunicamycin displayed significant inhibitory effects on biofilm development and maintenance, while not affecting overall cell growth or morphology. However, tunicamycin exerted a minimal effect on fully mature, pre-formed C. albicans biofilms.
The effect of tunicamycin on the C. albicans biofilm mode of growth demonstrates the importance of N-linked glycosylation in the developmental stages of biofilm formation. In addition, our results indicate that N-linked glycosylation represents an attractive target for the development of alternative strategies for the prevention of biofilm formation by this important pathogenic fungus.
microbial communities; N-glycosylation; cell wall mannoproteins; candidiasis
Chloride channel–2 (ClC-2) is a pH- and voltage-activated chloride channel that is highly expressed in mammalian fetal airway epithelia during the period of maximal fluid secretion. A high level of luminal ClC-2 protein expression is maintained by the SP1 transcription factor until SP1 and ClC-2 decline rapidly at birth. Using fetal (preII-19) and adult (L2) rat lung Type 2 cell lines, we demonstrate that the active higher-molecular-weight 105-kD isoform of SP1 is phosphorylated and glycosylated. Exposure of either cell line to high-dose glutamine is sufficient to induce glycosylation of SP1 and to induce and maintain ClC-2. Exposure to tunicamycin to inhibit SP1 glycosylation reduces ClC-2 expression. We also demonstrate that in vivo ClC-2 expression is similarly regulated. SP1 from 6-wk-old murine lung (high ClC-2 expression) is hyperphosphorylated and hyperglycosylated compared with SP1 from 16–wk-old lung (low ClC-2 expression). Our results support the hypothesis that glycosylation of SP1 produces the 105-kD isoform of SP1 and is involved in regulating ClC-2 gene expression.
chloride channel; transcription factor; cystic fibrosis; mouse; lung development
Growth factor stimulation moves O-glycosylation initiation enzymes (GalNac-Ts) from the Golgi to the ER in a Src-dependent fashion, increasing protein O-glycosylation.
After growth factor stimulation, kinases are activated to regulate multiple aspects of cell physiology. Activated Src is present on Golgi membranes, but its function here remains unclear. We find that Src regulates mucin-type protein O-glycosylation through redistribution of the initiating enzymes, polypeptide N-acetylgalactosaminyl transferases (GalNac-Ts), from the Golgi to the ER. Redistribution occurs after stimulation with EGF or PDGF in a Src-dependent manner and in cells with constitutively elevated Src activity. All GalNac-T family enzymes tested are affected, whereas multiple other glycosylation enzymes are not displaced from the Golgi. Upon Src activation, the COP-I coat is also redistributed in punctate structures that colocalize with GalNac-Ts and a dominant-negative Arf1 isoform, Arf1(Q71L), efficiently blocks GalNac-T redistribution, indicating that Src activates a COP-I–dependent trafficking event. Finally, Src activation increases O-glycosylation initiation as seen by lectin staining and metabolic labeling. We propose that growth factor stimulation regulates O-glycosylation initiation in a Src-dependent fashion by GalNac-T redistribution to the ER.
Chinese hamster ovary (CHO) cells are resistant to infections by gibbon ape leukemia virus (GALV) and amphotropic murine leukemia virus (A-MLV) unless they are pretreated with tunicamycin, an inhibitor of N-linked glycosylation. These viruses use the related sodium-phosphate symporters Pit1 and Pit2, respectively, as receptors in nonhamster cells, and evidence has suggested that the corresponding transporters of CHO cells may be masked by tunicamycin-sensitive secreted inhibitors. Although the E36 line of Chinese hamster cells was reported to secrete the putative Pit2 inhibitor and to be sensitive to the inhibitory CHO factors, E36 cells are highly susceptible to both GALV and A-MLV in the absence of tunicamycin. Moreover, expression of E36 Pit2 in CHO cells conferred tunicamycin-independent susceptibilities to both viruses. Based on the latter results, it was suggested that E36 Pit2 must functionally differ from the endogenous Pit2 of CHO cells. To test these ideas, we analyzed the receptor properties of CHO Pit1 and Pit2 in CHO cells. Surprisingly, and counterintuitively, transfection of a CHO Pit2 expression vector into CHO cells conferred strong susceptibility to both GALV and A-MLV, and similar overexpression of CHO Pit1 conferred susceptibility to GALV. Thus, CHO Pit2 is a promiscuous functional receptor for both viruses, and CHO Pit1 is a functional receptor for GALV. Similarly, we found that the natural resistance of Mus dunni tail fibroblasts to subgroup C feline leukemia viruses (FeLV-C) was eliminated simply by overexpression of the endogenous FeLV-C receptor homologue. These results demonstrate a novel and simple method to unmask latent retroviral receptor activities that occur in some cells. Specifically, resistances to retroviruses that are caused by subthreshold levels of receptor expression or by stoichiometrically limited masking or interference mechanisms can be efficiently overcome simply by overexpressing the endogenous receptors in the same cells.
Chinese hamster ovary (CHO) cells are resistant to infection by all of the major classes of murine retroviruses and are partially resistant to infection by gibbon ape leukemia virus. Treatment of CHO cells with the glycosylation inhibitor tunicamycin rendered these cells susceptible to infection by retroviral vectors with ecotropic, xenotropic, and amphotropic host ranges and increased the titer of gibbon ape leukemia virus pseudotyped vectors 10-fold. Vectors having a polytropic host range did not infect CHO cells in the presence or absence of tunicamycin, showing that the effect of tunicamycin was specific and related to the pseudotype of the vector. We present evidence for three mechanisms of resistance to infection: lack of viral receptors on CHO cells, the presence of nonfunctional receptors which can be made functional by treatment with tunicamycin, and the secretion of a protein factor that blocks retroviral infection of CHO cells. Several criteria indicate that the secreted inhibitor is not an interferon, and secretion of this factor was not detected in several other cell lines that were examined.
The influence of N glycosylation on the production of yeast acid phosphatase was studied. A set of synthetic hypoglycosylation mutants was generated by oligonucleotide-directed mutagenesis of the 12 putative sequons (Asn-X-Ser/Thr). Derepression of the hypoglycosylation mutants and analysis of their molecular sizes showed that all 12 sequons of the wild-type acid phosphatase are glycosylated. Activity measurements in combination with pulse-chase experiments revealed that the specific activity was not impaired by the introduced amino acid exchanges. However, absence of N glycosylation severely affected protein folding. Protein folding was found to be the rate-limiting factor in acid phosphatase secretion, and improper folding resulted in irreversible retention of malfolded acid phosphatase in the endoplasmic reticulum. With a decreasing number of attached glycan chains, less active acid phosphatase was secreted. Efficiency of correct folding was shown to be temperature dependent; i.e., lower temperatures could compensate for the reduction in attached oligosaccharides. In addition, protein folding and stability were shown to depend on both the number and the position of the attached oligosaccharides. N glycosylation was found to occur in a process independent of secondary structures, and thus our data support the model of a cotranslocational mechanism of glycosylation.
The Anaplastic Lymphoma Kinase (ALK) is an orphan receptor tyrosine kinase, which undergoes post-translational N-linked glycosylation. The catalytic domain of ALK was originally identified in the t(2;5) translocation that produces the unglycosylated oncogenic protein NPM-ALK, which occurs in Anaplastic Large Cell Lymphoma (ALCL). Recently, both germline and somatic activating missense mutations of ALK have been identified in neuroblastoma (NB), a pediatric cancer arising from neural crest cells. Moreover, we previously reported that ALK expression is significantly upregulated in advanced/metastatic NB. We hypothesized that ALK function may depend on N-linked glycosylation and that disruption of this post-translational modification would impair ALK activation, regardless the presence of either gene mutations or overexpression.
We employed tunicamycin to inhibit N-linked glycosylation. The following ALK-positive NB cell lines were used: SH-SY5Y and KELLY (ALK mutation F1174L), UKF-NB3 (ALK mutation R1275Q) and NB1 (ALK amplification). As a control, we used the NB cell lines LA1-5S and NB5 (no ALK expression), and the ALCL cell line SU-DHL1 (NPM-ALK).
Tunicamycin treatment of ALK-positive NB cells resulted in a hypoglycosylated ALK band and in decreased amounts of mature full size receptor. Concomitantly, we observed a marked reduction of mature ALK phosphorylation. On the contrary, tunicamycin had no effects on NPM-ALK phosphorylation in SU-DHL1 cells. Moreover, phosphorylation levels of ALK downstream effectors (AKT, ERK1/2, STAT3) were clearly impaired only in ALK mutated/amplified NB cell lines, whereas no significant reduction was observed in both ALK-negative and NPM-ALK-positive cell lines. Furthermore, inhibition of N-linked glycosylation considerably impaired cell viability only of ALK mutated/amplified NB cells. Finally, the cleavage of the Poly-ADP-ribose-polymerase (PARP) suggested that apoptotic pathways may be involved in cell death.
In this study we showed that inhibition of N-linked glycosylation affects ALK phosphorylation and disrupts downstream pro-survival signaling, indicating that inhibition of this post-translational modification may be a promising therapeutic approach. However, as tunicamycin is not a likely candidate for clinical use other approaches to alter N-linked glycosylation need to be explored. Future studies will assess whether the efficacy in inhibiting ALK activity might be enhanced by the combination of ALK specific small molecule and N-linked glycosylation inhibitors.
The epithelial chloride channel CFTR is a glycoprotein that is modified by two N-linked oligosaccharides. The most common mutant CFTR protein in patients with cystic fibrosis, ΔF508, is misfolded and retained by ER quality control. As oligosaccharide moieties of glycoproteins are known to mediate interactions with ER lectin chaperones, we investigated the role of N-linked glycosylation in the processing of wild-type and ΔF508 CFTR. We found that N-glycosylation and ER lectin interactions are not major determinants of trafficking of wild-type and ΔF508 from the ER to the plasma membrane. Unglycosylated CFTR, generated by removal of glycosylation sites or treatment of cells with the N-glycosylation inhibitor tunicamycin, did not bind calnexin, but did traffic to the cell surface and exhibited chloride channel activity. Most importantly, unglycosylated Δ F508 CFTR still could not escape quality control in the early secretory pathway and remained associated with the ER. However, the absence of N-linked oligosaccharides did reduce the stability of wild-type CFTR, causing significantly more-rapid turnover in post-ER compartments. Surprisingly, the individual N-linked carbohydrates do not play equivalent roles and modulate the fate of the wild-type protein in different ways in its early biosynthetic pathway.
CFTR; Glycosylation; Glycoprotein; Processing; Calnexin; EDEM
P-glycoprotein (P-gp), also known as ABCB1, is a member of the ABC transporter family of proteins. P-gp is an ATP-dependent drug efflux pump that is localized to the plasma membrane of mammalian cells and confers multidrug resistance in neoplastic cells. P-gp is a 140-kDa polypeptide that is glycosylated to a final molecular weight of 170 kDa. Our experimental model used two variants of L1210 cells in which overexpression of P-gp was achieved: either by adaptation of parental cells (S) to vincristine (R) or by transfection with the human gene encoding P-gp (T). R and T cells were found to differ from S cells in transglycosylation reactions in our recent studies. The effects of tunicamycin on glycosylation, drug efflux activity and cellular localization of P-gp in R and T cells were examined in the present study. Treatment with tunicamycin caused less concentration-dependent cellular damage to R and T cells compared with S cells. Tunicamycin inhibited P-gp N-glycosylation in both of the P-gp-positive cells. However, tunicamycin treatment did not alter either the P-gp cellular localization to the plasma membrane or the P-gp transport activity. The present paper brings evidence that independently on the mode of P-gp expression (selection with drugs or transfection with a gene encoding P-gp) in L1210 cells, tunicamycin induces inhibition of N-glycosylation of this protein, without altering its function as plasma membrane drug efflux pump.
P-gp (MDR1); tunicamycin; N-glycosylation; L1210