N-glycosylation of the human granulocyte-macrophage colony-stimulating factor receptor alpha subunit is essential for ligand binding and signal transduction

Genome wide CRISPR screening reveals a role for sialylation in the tumorigenesis and chemoresistance of acute myeloid leukemia cells

Aberrant activation of cytokine and growth factor signal transduction pathways confers enhanced survival and proliferation properties to acute myeloid leukemia (AML) cells. However, the mechanisms underlying the deregulation of signaling pathways in leukemia cells are unclear. To identify genes capable of independently supporting cytokine-independent growth, we employed a genome-wide CRISPR/Cas9-mediated loss-of-function screen in GM-CSF-dependent human AML TF-1 cells. More than 182 genes (p < 0.01) were found to suppress the cytokine-independent growth of TF-1 cells. Among the top hits, genes encoding key factors involved in sialylation biosynthesis were identified; these included CMAS, SLC35A1, NANS, and GNE. Knockout of either CMAS or SLC35A1 enabled cytokine-independent proliferation and survival of AML cells. Furthermore, NSG (NOD/SCID/IL2Rγ^-/-) mice injected with CMAS or SLC35A1-knockout TF-1 cells exhibited a shorter survival than mice injected with wild-type cells. Mechanistically, abrogation of sialylation biosynthesis in TF-1 cells induced a strong activation of ERK signaling, which sensitized cells to MEK inhibitors but conferred resistance to JAK inhibitors. Further, the surface level of α2,3-linked sialic acids was negatively correlated with the sensitivity of AML cell lines to MEK/ERK inhibitors. We also found that sialylation modulated the expression and stability of the CSF2 receptor. Together, these results demonstrate a novel role of sialylation in regulating oncogenic transformation and drug resistance development in leukemia. We propose that altered sialylation could serve as a biomarker for targeted anti-leukemic therapy.

The Pleiotropic Effects of the GM-CSF Rheostat on Myeloid Cell Differentiation and Function: More Than a Numbers Game

Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) is a myelopoietic growth factor that has pleiotropic effects not only in promoting the differentiation of immature precursors into polymorphonuclear neutrophils (PMNs), monocytes/macrophages (MØs) and dendritic cells (DCs), but also in controlling the function of fully mature myeloid cells. This broad spectrum of GM-CSF action may elicit paradoxical outcomes-both immunostimulation and immunosuppression-in infection, inflammation, and cancer. The complexity of GM-CSF action remains to be fully unraveled. Several aspects of GM-CSF action could contribute to its diverse biological consequences. Firstly, GM-CSF as a single cytokine affects development of most myeloid cells from progenitors to mature immune cells. Secondly, GM-CSF activates JAK2/STAT5 and also activate multiple signaling modules and transcriptional factors that direct different biological processes. Thirdly, GM-CSF can be produced by different cell types including tumor cells in response to different environmental cues; thus, GM-CSF quantity can vary greatly under different pathophysiological settings. Finally, GM-CSF signaling is also fine-tuned by other less defined feedback mechanisms. In this review, we will discuss the role of GM-CSF in orchestrating the differentiation, survival, and proliferation during the generation of multiple lineages of myeloid cells (PMNs, MØs, and DCs). We will also discuss the role of GM-CSF in regulating the function of DCs and the functional polarization of MØs. We highlight how the dose of GM-CSF and corresponding signal strength acts as a rheostat to fine-tune cell fate, and thus the way GM-CSF may best be targeted for immuno-intervention in infection, inflammation and cancer.

Proteolytic Origin of the Soluble Human IL-6R In Vivo and a Decisive Role of N-Glycosylation

Signaling of the cytokine interleukin-6 (IL-6) via its soluble IL-6 receptor (sIL-6R) is responsible for the proinflammatory properties of IL-6 and constitutes an attractive therapeutic target, but how the sIL-6R is generated in vivo remains largely unclear. Here, we use liquid chromatography–mass spectrometry to identify an sIL-6R form in human serum that originates from proteolytic cleavage, map its cleavage site between Pro-355 and Val-356, and determine the occupancy of all O- and N-glycosylation sites of the human sIL-6R. The metalloprotease a disintegrin and metalloproteinase 17 (ADAM17) uses this cleavage site in vitro, and mutation of Val-356 is sufficient to completely abrogate IL-6R proteolysis. N- and O-glycosylation were dispensable for signaling of the IL-6R, but proteolysis was orchestrated by an N- and O-glycosylated sequon near the cleavage site and an N-glycan exosite in domain D1. Proteolysis of an IL-6R completely devoid of glycans is significantly impaired. Thus, glycosylation is an important regulator for sIL-6R generation.

Interleukin-6 (IL-6) is a cytokine secreted by our body upon infection or trauma to stimulate the immune system response. IL-6 is partially responsible for fever and triggers inflammation in many diseases. It activates its target cells via the membrane-bound IL-6 receptor (IL-6R), and soluble forms of this receptor (sIL-6R) are present in high amounts in the serum of healthy individuals and mediate the inflammatory response in all cells of the human body. However, it remains unclear how the soluble form of this cytokine is generated in humans. In this study, we isolate sIL-6R from human serum and show that the majority is produced via cleavage of the membrane-bound IL-6R by a protease. We identify the exact cleavage site and find that it is identical to a cleavage site used by the metalloprotease ADAM17. We further show that glycosylation, a post-transcriptional modification, is dispensable for the transport and biological function of IL-6R and map the occupancy of all O- and N-glycosylation sites. However, we find that only a single N-glycan is critically involved in the regulation of proteolysis by ADAM17 and conclude that glycosylation is an important regulator for sIL-6R generation.

The Thrombopoietin Receptor: Structural Basis of Traffic and Activation by Ligand, Mutations, Agonists, and Mutated Calreticulin

A well-functioning hematopoietic system requires a certain robustness and flexibility to maintain appropriate quantities of functional mature blood cells, such as red blood cells and platelets. This review focuses on the cytokine receptor that plays a significant role in thrombopoiesis: the receptor for thrombopoietin (TPO-R; also known as MPL). Here, we survey the work to date to understand how this receptor functions at a molecular level throughout its lifecycle, from traffic to the cell surface, dimerization and binding cognate cytokine via its extracellular domain, through to its subsequent activation of associated Janus kinases and initiation of downstream signaling pathways, as well as the regulation of these processes. Atomic level resolution structures of TPO-R have remained elusive. The identification of disease-causing mutations in the receptor has, however, offered some insight into structure and function relationships, as has artificial means of receptor activation, through TPO mimetics, transmembrane-targeting receptor agonists, and engineering in dimerization domains. More recently, a novel activation mechanism was identified whereby mutated forms of calreticulin form complexes with TPO-R via its extracellular N-glycosylated domain. Such complexes traffic pathologically in the cell and persistently activate JAK2, downstream signal transducers and activators of transcription (STATs), and other pathways. This pathologic TPO-R activation is associated with a large fraction of human myeloproliferative neoplasms.

Phyllodes tumor of the breast: role of Axl and ST6GalNAcII in the development of mammary phyllodes tumors

Phyllodes tumor exhibits an aggressive growth. The expression of many biological markers has been explored to discriminate between different grades of phyllodes tumor and to predict their behavior. The purpose of this study was to evaluate the implications of Axl and ST6GalNAcII in phyllodes tumors. Real-time PCR, Western blot, and immunohistochemical were used to analyze differential expression of ST6GalNAcII and Axl in phyllodes tumor (PT) cell lines and tissue specimens. RNAi assay, ECM invasion assay, and tumorigenicity assay were used to analyze the altered expression of ST6GalNAcII gene effects on the expression of Axl and invasive ability of phyllodes tumor cells in vitro and in vivo. Compared to benign tumors, borderline and malignant ones showed a remarkable increase in mRNA levels of Axl and ST6GalNAcII gene, and it was higher in malignant tumor cells than in borderline tumor cells. When ST6GalNAcII was silenced, compared to the control, the expression level of Axl was significantly reduced in malignant tumor cell transfectants and knockdown of ST6GalNAcII gene significantly inhibited invasive activity in malignant tumor cells. The high expression of ST6GalNAcII and Axl was significantly correlated with tumor grade and distance metastasis by immunohistochemical analysis. Axl and ST6GalNAcII expression increases with increasing tumor grade in mammary phyllodes tumors. ST6GalNAc II might be participated in the glycosylation of Axl, and this Axl glycosylation may mediate the tumorigenicity, invasion, and distant metastasis of PT cells.

Axl glycosylation mediates tumor cell proliferation, invasion and lymphatic metastasis in murine hepatocellular carcinoma

AIM: To investigate the effects of Axl deglycosylation on tumor lymphatic metastases in mouse hepatocellular carcinoma cell lines.

METHODS: Western blotting was used to analyze the expression profile of Axl glycoprotein in mouse hepatocellular carcinoma cell line Hca-F treated with tunicamycin and PNGase F 3-(4,5)-dimethylthiazol(-zyl)-3,5-diphenyltetrazolium bromide (MTT) assay, extracellular matrix (ECM) invasion assay (in vitro) and tumor metastasis assay (in vivo) were utilized to evaluate the effect of Axl deglycosylation on the Hca-F cell proliferation, invasion and lymphatic metastasis.

RESULTS: Tunicamycin and PNGase F treatment markedly inhibited Axl glycoprotein synthesis and expression, proliferation, invasion, and lymphatic metastasis both in vitro and in vivo. In the MTT assay, proliferation was apparent in untreated Hca-F cells compared with treated Hca-F cells. In the ECM invasion assay (in vitro), treated cells passed through the ECMatrix gel in significantly smaller numbers than untreated cells (tunicamycin 5 μg/mL: 68 ± 8 vs 80 ± 9, P = 0.0222; 10 μg/mL: 50 ± 6 vs 80 ± 9, P = 0.0003; 20 μg/mL: 41 ± 4 vs 80 ± 9, P = 0.0001); (PNGase F 8 h: 66 ± 7 vs 82 ± 8, P = 0.0098; 16 h: 49 ± 4 vs 82 ± 8, P = 0.0001; 24 h: 34 ± 3 vs 82 ± 8, P = 0.0001). In the tumor metastasis assay (in vivo), average lymph node weights of the untreated Hca-F group compared with treated Hca-F groups (tunicamycin 5 μg/mL: 0.84 ± 0.21 g vs 0.72 ± 0.19 g, P = 0.3237; 10 μg/mL: 0.84 ± 0.21 g vs 0.54 ± 0.11 g, P = 0.0113; 20 μg/mL: 0.84 ± 0.21 g vs 0.42 ± 0.06 g, P = 0.0008); (PNGase F 8 h: 0.79 ± 0.15 g vs 0.63 ± 0.13 g, P = 0.0766; 16 h: 0.79 ± 0.15 g vs 0.49 ± 0.10 g, P = 0.0022; 24 h: 0.79 ± 0.15 g vs 0.39 ± 0.05 g, P = 0.0001). Also, average lymph node volumes of the untreated Hca-F group compared with treated Hca-F groups (tunicamycin 5 μg/mL: 815 ± 61 mm³vs 680 ± 59 mm³, P = 0.0613; 10 μg/mL: 815 ± 61 mm³vs 580 ± 29 mm³, P = 0.0001; 20 μg/mL: 815 ± 61 mm³vs 395 ± 12 mm³, P = 0.0001); (PNGase F 8 h: 670 ± 56 mm³vs 581 ± 48 mm³, P = 0.0532; 16 h: 670 ± 56 mm³vs 412 ± 22 mm³, P = 0.0001; 24 h: 670 ± 56 mm³vs 323 ± 11 mm³, P = 0.0001).

CONCLUSION: Alteration of Axl glycosylation can attenuate neoplastic lymphatic metastasis. Axl N-glycans may be a universal target for chemotherapy.

Extracellular domain N-glycosylation controls human thrombopoietin receptor cell surface levels

The thrombopoietin receptor (TpoR) is a type I transmembrane protein that mediates the signaling functions of thrombopoietin (Tpo) in regulating megakaryocyte differentiation, platelet formation, and hematopoietic stem cell renewal. We probed the role of each of the four extracellular domain putative N-glycosylation sites for cell surface localization and function of the receptor. Single N-glycosylation mutants at any of the four sites were able to acquire the mature N-glycosylated pattern, but exhibited a decreased Tpo-dependent JAK2-STAT response in stably transduced Ba/F3 or Ba/F3-JAK2 cell lines. The ability of JAK2 to promote cell surface localization and stability of TpoR required the first N-glycosylation site (Asn117). In contrast, the third N-glycosylation site (Asn298) decreased receptor maturation and stability. TpoR mutants lacking three N-glycosylation sites were defective in maturation, but N-glycosylation on the single remaining site could be detected by sensitivity to PNGaseF. The TpoR mutant defective in all four N-glycosylation sites was severely impaired in plasma membrane localization and was degraded by the proteasome. N-glycosylation receptor mutants are not misfolded as, once localized on the cell surface in overexpression conditions, they can bind and respond to Tpo. Our data indicate that extracellular domain N-glycosylation sites regulate in a combinatorial manner cell surface localization of TpoR. We discuss how mutations around TpoR N-glycosylation sites might contribute to inefficient receptor traffic and disease.

Naturally-occurring mutation in the calcium-sensing receptor reveals the significance of extracellular domain loop III region for class C G-protein-coupled receptor function

CONTEXT

Inactivating mutations of the calcium-sensing receptor (CaSR) cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Most mutations are clustered in the N-terminal and Cys-rich regions of the extracellular domain (ECD) and seven-transmembrane domain. Disease-causing mutations are uncommon in the C terminus of ECD.

OBJECTIVE

The aim of the study was to characterize the CaSR mutations causing neonatal severe hyperparathyroidism in a consanguineous family.

METHODS

Parathyroid glands from the index patient were stained for CaSR protein. The CaSR gene was sequenced, mutations were recreated in CaSR cDNA, and HEK293 cells were transfected with the CaSR mutant expression vector. Cellular CaSR targeting was detected by immunoblotting and immunocytochemistry; CaSR activity was assayed by inositol phosphate accumulation, MAPK activation, and single-cell microfluorimetry.

RESULTS

Immunocytochemistry showed reduced intracellular CaSR in patient parathyroids. An in-frame homozygous deletion/insertion mutation, c.1031 > 1034 (delACAAinsT), replaced His344-Asn345 with a single Leu in CaSR loop III. The mutant reduced cell surface expression of CaSR in transfected HEK293 cells. Inositol phosphate accumulation, MAPK activation, and single-cell microfluorimetry revealed blunted signaling responses of the mutant receptor to changes in extracellular Ca(2+) concentration.

CONCLUSION

Deletion of His344-Asn345 in the ECD loop III region affects cell surface targeting of CaSR in transfected cells and in affected parathyroid glands. Absence of conserved Asn345 may interfere with CaSR folding or glycosylation, leading to poor protein targeting to the cell membrane. This loss-of-function mutant indicates that the ECD loop III is required for CaSR activity.

Expression of pheromone biosynthesis activating neuropeptide and its receptor (PBANR) mRNA in adult female Spodoptera exigua (Lepidoptera: Noctuidae)

The full-length cDNA of pheromone biosynthesis activating neuropeptide receptor (PBANR) was cloned from the beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae); it included an open reading frame of 1,053 bp encoding 350 amino acids. The PBANR of S. exigua (SePBANR) was structurally characteristic of G protein-coupled receptor and its amino acid sequence shared 98% identity with the PBANR of Spodoptera littoralis. Both pheromone biosynthesis activating neuropeptide (PBAN) and PBANR mRNA abundance were measured in the brain-subesophageal ganglion complex, pheromone gland, ventral nerve cord, and ovary of S. exigua female moths by real-time RT-PCR. The abundance of PBAN mRNA in brain-subesophageal ganglion complex and PBANR mRNA in pheromone gland was significantly greater compared to other tissues, suggesting that the ligand-receptor relationship of PBAN and PBANR exists quantitatively in S. exigua. Both PBAN and PBANR expression displayed a remarkable diurnal rhythm, for they were low and stable during the photophase (07:00-21:00) and increased markedly during the scotophase (with a maximum abundance at 23:30) in 3-day-old female moths. The abundance of PBAN and PBANR increased steadily from the 1st day to the 5th day of the adult female life. The pattern of both diurnal and daily expression of PBAN and PBANR mRNA were coincident with enhanced capacity of sex pheromone release and mating of S. exigua moths during the same period. We infer from these results that pheromone biosynthesis and release in S. exigua is regulated by PBAN via up-regulating synthesis.

Hereditary pulmonary alveolar proteinosis: pathogenesis, presentation, diagnosis, and therapy

RATIONALE

We identified a 6-year-old girl with pulmonary alveolar proteinosis (PAP), impaired granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor function, and increased GM-CSF.

OBJECTIVES

Increased serum GM-CSF may be useful to identify individuals with PAP caused by GM-CSF receptor dysfunction.

METHODS

We screened 187 patients referred to us for measurement of GM-CSF autoantibodies to diagnose autoimmune PAP. Five were children with PAP and increased serum GM-CSF but without GM-CSF autoantibodies or any disease causing secondary PAP; all were studied with family members, subsequently identified patients, and controls.

MEASUREMENT AND MAIN RESULTS

Eight children (seven female, one male) were identified with PAP caused by recessive CSF2RA mutations. Six presented with progressive dyspnea of insidious onset at 4.8 ± 1.6 years and two were asymptomatic at ages 5 and 8 years. Radiologic and histopathologic manifestations were similar to those of autoimmune PAP. Molecular analysis demonstrated that GM-CSF signaling was absent in six and severely reduced in two patients. The GM-CSF receptor β chain was detected in all patients, whereas the α chain was absent in six and abnormal in two, paralleling the GM-CSF signaling defects. Genetic analysis revealed multiple distinct CSF2RA abnormalities, including missense, duplication, frameshift, and nonsense mutations; exon and gene deletion; and cryptic alternative splicing. All symptomatic patients responded well to whole-lung lavage therapy.

CONCLUSIONS

CSF2RA mutations cause a genetic form of PAP presenting as insidious, progressive dyspnea in children that can be diagnosed by a combination of characteristic radiologic findings and blood tests and treated successfully by whole-lung lavage.

N-linked glycosylation is essential for the stability but not the signaling function of the interleukin-6 signal transducer glycoprotein 130

N-Linked glycosylation is an important determinant of protein structure and function. The interleukin-6 signal transducer glycoprotein 130 (gp130) is a common co-receptor for cytokines of the interleukin (IL)-6 family and is N-glycosylated at 9 of 11 potential sites. Whereas N-glycosylation of the extracellular domains D1-D3 of gp130 has been shown to be dispensable for binding of the gp130 ligand IL-6 and its cognate receptor in vitro, the role of the N-linked glycans on domains D4 and D6 is still unclear. We have mutated the asparagines of all nine functional N-glycosylation sites of gp130 to glutamine and systematically analyzed the consequences of deleted N-glycosylation (dNG) in both cellular gp130 and in a soluble gp130-IgG1-Fc fusion protein (sgp130Fc). Our results show that sgp130Fc-dNG is inherently unstable and degrades rapidly under conditions that do not harm wild-type sgp130Fc. Consistently, the bulk of cellular gp130-dNG is not transported to the plasma membrane but is degraded in the proteasome. However, the small quantities of gp130-dNG, which do reach the cell surface, are still able to activate the key gp130 signaling target signal transducer and activator of transcription-3 (STAT3) upon binding of the agonistic complex of IL-6 and soluble IL-6 receptor. In conclusion, N-linked glycosylation is required for the stability but not the signal-transducing function of gp130.

N-glycans modulate the activation of gp130 in mouse embryonic neural precursor cells

gp130 is a ubiquitously expressed glycoprotein and signal transducer of interleukin 6 family of cytokines. It has been reported that gp130 has 11 potential N-glycosylation sites in the extracellular domain, and nine of them are actually N-glycosylated. However, the structure and functional role of the carbohydrate chains carried by gp130 are totally unknown. In this study, we examined the functional role of N-glycans of gp130 in mouse neuroepithelial cells. In neuroepithelial cells treated with tunicamycin, an N-glycosylation inhibitor, unglycosylated form of gp130 was detected. The unglycosylated gp130 was not phosphorylated in response to leukemia inhibitory factor stimulation. Although the unglycosylated gp130 was found to be expressed on the cell surface, it could not form a heterodimer with leukemia inhibitory factor receptor. These results suggest that N-glycans are required for the activation, but not for the localization, of gp130 in neuroepithelial cells.

Familial pulmonary alveolar proteinosis caused by mutations in CSF2RA

Primary pulmonary alveolar proteinosis (PAP) is a rare syndrome characterized by accumulation of surfactant in the lungs that is presumed to be mediated by disruption of granulocyte/macrophage colony-stimulating factor (GM-CSF) signaling based on studies in genetically modified mice. The effects of GM-CSF are mediated by heterologous receptors composed of GM-CSF binding (GM-CSF-Rα) and nonbinding affinity-enhancing (GM-CSF-Rβ) subunits. We describe PAP, failure to thrive, and increased GM-CSF levels in two sisters aged 6 and 8 yr with abnormalities of both GM-CSF-Rα–encoding alleles (CSF2RA). One was a 1.6-Mb deletion in the pseudoautosomal region of one maternal X chromosome encompassing CSF2RA. The other, a point mutation in the paternal X chromosome allele encoding a G174R substitution, altered an N-linked glycosylation site within the cytokine binding domain and glycosylation of GM-CSF-Rα, severely reducing GM-CSF binding, receptor signaling, and GM-CSF–dependent functions in primary myeloid cells. Transfection of cloned cDNAs faithfully reproduced the signaling defect at physiological GM-CSF concentrations. Interestingly, at high GM-CSF concentrations similar to those observed in the index patient, signaling was partially rescued, thereby providing a molecular explanation for the slow progression of disease in these children. These results establish that GM-CSF signaling is critical for surfactant homeostasis in humans and demonstrate that mutations in CSF2RA cause familial PAP.

Clarification of the role of N-glycans on the common beta-subunit of the human IL-3, IL-5 and GM-CSF receptors and the murine IL-3 beta-receptor in ligand-binding and receptor activation

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3 and IL-5 are related cytokines that play key roles in regulating the differentiation, proliferation, survival and activation of myeloid blood cells. The cell surface receptors for these cytokines are composed of cytokine-specific alpha-subunits and a common beta-receptor (betac), a shared subunit that is essential for receptor signaling in response to GM-CSF, IL-3 and IL-5. Previous studies have reached conflicting conclusions as to whether N-glycosylation of the betac-subunit is necessary for functional GM-CSF, IL-3 and IL-5 receptors. We sought to clarify whether betac N-glycosylation plays a role in receptor function, since all structural studies of human betac to date have utilized recombinant protein lacking N-glycosylation at Asn(328). Here, by eliminating individual N-glycans in human betac and the related murine homolog, beta(IL-3), we demonstrate unequivocally that ligand-binding and receptor activation are not critically dependent on individual N-glycosylation sites within the beta-subunit although the data do not preclude the possibility that N-glycans may exert some sort of fine control. These studies support the biological relevance of the X-ray crystal structures of the human betac domain 4 and the complete ectodomain, both of which lack N-glycosylation at Asn(328).

Contribution of leptin receptor N-linked glycans to leptin binding

The extracellular domain of the human leptin receptor (Ob-R) contains 20 potential N-glycosylation sites whose role in leptin binding remains to be elucidated. We found that a mammalian cell-expressed sOb-R (soluble Ob-R) fragment (residues 22-839 of the extracellular domain) bound leptin with a dissociation constant of 1.8 nM. This binding was inhibited by Con A (concanavalin A) or wheatgerm agglutinin. Treatment of sOb-R with peptide N-glycosidase F reduced leptin binding by approximately 80% concurrently with N-linked glycan removal. The human megakaryoblastic cell line, MEG-01, expresses two forms of the Ob-R, of approx. 170 and 130 kDa molecular mass. Endo H (endoglycosidase H) treatment and cell culture with alpha-glucosidase inhibitors demonstrated that N-linked glycans are of the complex mature type in the 170 kDa form and of the high-mannose type in the 130 kDa form. Both isoforms bound leptin, but not after peptide N-glycosidase F treatment. An insect-cell-expressed sOb-R fragment, consisting of the Ig (immunoglobulin), CRH2 (second cytokine receptor homology) and FNIII (fibronectin type III) domains, bound leptin with affinity similar to that of the entire extracellular domain, but this function was abolished after N-linked glycan removal. The same treatment had no effect on the leptin-binding activity of the isolated CRH2 domain. Our findings show that N-linked glycans within Ig and/or FNIII domains regulate Ob-R function, but are not involved in essential interactions with the ligand.

Characterization of a novel gene, BcMF7, that is expressed preferentially in pollen of Brassica campestris L. ssp. chinensis Makino

Pollen formation is important for plant sexual reproduction. To identify the genes that are involved in pollen formation, we performed the genome-wide transcriptional profiling in the flower buds of both male meiotic cytokinesis (mmc) mutant and its wild-type plants of Brassica campestris L. ssp. chinensis, syn. B. rapa L. ssp. chinensis. cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis showed that the mmc mutation resulted in changes in expression of a variety of genes. BcMF7, a transcript-derived fragment (TDF) accumulated in the wild-type flower buds was further characterized. The BcMF7 gene has 1161 bp in length with two introns. The full-length BcMF7 cDNA has 609 bp in length and encodes a protein of 129 amino acids. The deduced amino acid sequence of BcMF7 protein shares no similarity to any function-known protein in Swiss-Prot database, but has 8 protein kinase C phosphorylation sites, 2 caselin kinase II phosphorylation sites, 2 tyrosine kinase phosphorylation sites, 2 N-glycosylation sites and 2 N-myristolyation sites. Spatial and temporal expression patterns analysis showed that BcMF7 was expressed exclusively in pollen. The expression signal of BcMF7 was first detected at the tetrad stage of microspore development, reached a peak level at the uninucleate stage, and decreased to a slightly low level at the mature pollen stage. All these results show that BcMF7 may play a certain role in the signal transduction during pollen development.

N-linked glycosylation of IL-13R alpha2 is essential for optimal IL-13 inhibitory activity

A high-affinity receptor for interleukin (IL)-13 (interleukin-13R alpha 2) is over-expressed in disease-related fibroblasts and neoplastic cells and is involved in cancer, allergic, and inflammatory diseases. The extracellular domain of IL-13R alpha2 (ECD alpha2) could be cleaved, which serves as a decoy receptor. We have expressed and purified ECD alpha2 in both Escherichia coli (E. coli) and mammalian systems as a soluble fragment and studied its biological activities. Although both products of ECD alpha2 showed IL-13 inhibitory activities, mammalian cell-derived ECD alpha2 appeared to be superior compared with purified protein from E. coli. When expressed in E. coli, ECD alpha2 appeared to be a monomer of 42 but a 60 kDa protein when purified from mammalian cells due to heavy glycosylation. The purified glycosylated ECD alpha2 efficiently inhibited IL-13-induced STAT6 phosphorylation in immune and Hodgkin's lymphoma cell lines, IL-13 binding, and cytotoxicity of IL-13 cytotoxin in various cancer cell lines. The improved potency of mammalian cell-derived ECD alpha2 was shown over ECD alpha2/Fc fusion protein. The N-linked glycosylation of ECD alpha2 was found to be essential for optimal IL-13 inhibitory activity as deglycosylation by PNGase F showed lower activity. ECD alpha2 did not inhibit IL-4-induced STAT6 phosphorylation, indicating that inhibitory effects of ECD alpha2 are receptor specific. These results indicate that glycosylated ECD alpha2 can serve as a potent inhibitor of IL-13 in a variety of conditions in which IL-13 is a key mediator, e.g., pulmonary, allergic, fibrotic, and neoplastic diseases.

Glycosylation, disulfide bond formation, and the presence of a WSXWS-like motif in the orf virus GIF protein are critical for maintaining the integrity of Binding to ovine granulocyte-macrophage colony-stimulating factor and interleukin-2

Orf virus (ORFV), the type species of the family Parapoxviridae, encodes a protein (GIF) that binds and inhibits the ovine cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2). There is no obvious sequence homology between the ORFV protein and any known mammalian GM-CSF- or IL-2-binding proteins. We demonstrate here that many of the biochemical properties of mammalian GM-CSF receptors that are required for efficient binding of GM-CSF are also critical to the GIF protein for binding to ovine GM-CSF (ovGM-CSF). Site-directed mutagenesis of the GIF protein demonstrated, first, the importance of disulfide bonds, and second, that a sequence motif (WDPWV), related to the WSXWS motif of the type 1 cytokine receptor superfamily, was necessary for biological activity. Finally, glycosylation of the GIF protein was also critical for binding to GM-CSF.

The influence of N-linked glycosylation on the function of platelet glycoprotein VI

Using recombinant human glycoprotein VI (GPVI), we evaluated the effect of N-linked glycosylation at the consensus site Asparagine92-Glycine-Serine94 (N92GS94) on binding of this platelet-specific receptor to its ligands, human type I collagen, collagen-related peptide (CRP), and the snake venom C-type lectin convulxin (CVX). In COS-7 cells transiently transfected with GPVI, deglycosylation with peptide-N-glycosidase F (PNGase F; specific for complex N-linked glycans) or tunicamycin decreases the molecular weight of GPVI and reduces transfected COS-7 cell binding to both CRP and CVX. In stably transfected Dami cells, the substitutions N92A or S94A, but not L95H, resulted in a 30% to 40% decrease in adhesion to CVX, but a 90% or greater decrease in adhesion to CRP and a 65% to 70% decrease in adhesion to type I collagen. Treatment with PNGase F, but not Endoglycosidase H (Endo H) (specific for high-mannose N-linked glycans), produced an equivalent decrease in molecular weight. Neither N92A nor S94A affected the expression of GPVI, based on the direct binding of murine anti-human GPVI monoclonal antibody 204-11 to transfected Dami cells. These findings indicate that N-linked glycosylation at N92 in human GPVI is not required for surface expression, but contributes to maximal adhesion to type I collagen, CRP and, to a lesser extent, CVX.

A model of the ternary complex of interleukin-10 with its soluble receptors

BACKGROUND

Interleukin-10 (IL-10) is a cytokine whose main biological function is to suppress the immune response by induction of a signal(s) leading to inhibition of synthesis of a number of cytokines and their cellular receptors. Signal transduction is initiated upon formation of a ternary complex of IL-10 with two of its receptor chains, IL-10R1 and IL-10R2, expressed on the cell membrane. The affinity of IL-10R1 toward IL-10 is very high, which allowed determination of the crystal structure of IL-10 complexed with the extracellular/soluble domain of IL-10R1, while the affinity of IL-10R2 toward either IL-10 or IL-10/sIL-10R1 complex is quite low. This so far has prevented any attempts to obtain structural information about the ternary complex of IL-10 with its receptor chains.

RESULTS

Structures of the second soluble receptor chain of interleukin-10 (sIL-10R2) and the ternary complex of IL-10/sIL-10R1/sIL-10R2 have been generated by homology modeling, which allowed us to identify residues involved in ligand-receptor and receptor-receptor interactions.

CONCLUSION

The previously experimentally determined structure of the intermediate/binary complex IL-10/sIL-10R1 is the same in the ternary complex. There are two binding sites for the second receptor chain on the surface of the IL-10/sIL-10R1 complex, involving both IL-10 and sIL-10R1. Most of the interactions are hydrophilic in nature, although each interface includes two internal hydrophobic clusters. The distance between C-termini of the receptor chains is 25 A, which is common for known structures of ternary complexes of other cytokines. The structure is likely to represent the biologically active signaling complex of IL-10 with its receptor on the surface of the cell membrane.

The role of N-linked glycosylation in determining the surface expression, G protein interaction and effector coupling of the alpha (alpha) isoform of the human thromboxane A(2) receptor

In humans, thromboxane (TX) A(2) signals through two TXA(2) receptor (TP) isoforms, termed TPalpha and TPbeta, that diverge exclusively within the carboxyl terminal cytoplasmic domains. The amino terminal extracellular region of the TPs contains two highly conserved Asn (N)-linked glycosylation sites at Asn(4) and Asn(16). While it has been established that impairment of N-glycosylation of TPalpha significantly affects ligand binding/intracellular signalling, previous studies did not ascertain whether N-linked glycosylation was critical for ligand binding per se or whether it was required for the intracellular trafficking and the functional expression of TPalpha on the plasma membrane (PM). In the current study, we investigated the role of N-linked glycosylation in determining the functional expression of TPalpha, by assessment of its ligand binding, G protein coupling and intracellular signalling properties, correlating it with the level of antigenic TPalpha protein expressed on the PM and/or retained intracellularly. From our data, we conclude that N-glycosylation of either Asn(4) or Asn(16) is required and sufficient for expression of functionally active TPalpha on the PM while the fully non-glycosylated TPalpha(N4,N16-Q4,Q16) is almost completely retained within the endoplasmic reticulum (ER) and remains functionally inactive, failing to associate with its coupling G protein Galpha(q) and, in turn, failing to mediate phospholipase (PL) Cbeta activation.

N-linked glycosylations at Asn(26) and Asn(114) of human MD-2 are required for toll-like receptor 4-mediated activation of NF-kappaB by lipopolysaccharide

MD-2 is physically associated with Toll-like receptor 4 (TLR4) and is required for TLR4-mediated LPS signaling. Western blotting analysis revealed the presence of three forms of human (h)MD-2 with different electrophoretic mobilities. After N-glycosidase treatment of the cellular extract prepared from cells expressing hMD-2, only a single form with the fastest mobility was detected. Mutation of either one of two potential glycosylation sites (Asn(26) and Asn(114)) of MD-2 resulted in the disappearance of the slowest mobility form, and only the fastest form was detected in hMD-2 carrying mutations at both Asn(26) and Asn(114). Although these mutants were expressed on the cell surface and maintained its ability to associate with human TLR4, these mutations or tunicamycin treatment substantially impaired the ability of MD-2 to complement TLR4-mediated activation of NF-kappaB by LPS. LPS binding to cells expressing CD14, TLR4, and MD-2 was unaffected by these mutations. These observations demonstrate that hMD-2 undergoes N-linked glycosylation at Asn(26) and Asn(114), and that these glycosylations are crucial for TLR4-mediated signal transduction of LPS.

A systematic approach to the complete study of a signaling molecule: ribosomal p90rsk as an example

Ribosomal p90rsk is a kinase of central importance in transducing mitogenic signals from an activated receptor to the cell nucleus and for protein synthesis. Here, we analyze the optimal steps to fully describe this kinase in both normal neutrophils and leukemic cell lines. These are: (i) immunological analyses (immunoblotting and immunoprecipitation); (ii) enzyme activity assays (in vitro and "in-gel"); and (iii) immunobiochemical combination methods (immunoprecipitation/kinase assay, immunoprecipitation/"in-gel" assay and ion exchange chromatography/immunoblotting). For the enzyme assays, we describe a novel method to measure ribosomal p90rsk kinase activity "in-gel", based on a renatured-protein method that allows for the direct quantitation of enzyme activity. Finally, we present an algorithm that can be readily implemented to the quantification of the extent of stimulation of a kinase in response to a particular extracellular stimuli. In our case, it was found that activation of p90rsk was higher in proliferating leukemic cells than in mature neutrophils, indicating that a suppression of key signal transduction links could contribute to the maturational arrest typical of acute leukemia. All the techniques and strategies described here for p90rsk could be easily extrapolated to the study of any signal transduction molecule, provided it has a phosphotransferase activity.

O-Glycosylation of G-protein-coupled receptor, octopus rhodopsin. Direct analysis by FAB mass spectrometry

In addition to the N-glycan that is evidently conserved in G-protein-coupled receptors (GPCRs), O-glycans in the N-terminus of GPCRs have been suggested. Using a combination of enzymatic and manual Edman degradation in conjunction with G-protein coupled receptor mass spectrometry, the structure and sites of O-glycans in octopus rhodopsin are determined. Two N-acetylgalactosamine residues are O-linked to Thr4 and Thr5 in the N-terminus of octopus rhodopsin. Further, we found chicken iodopsin, but not bovine rhodopsin, contains N-acetylgalactosamine. This is the first direct evidence to determine the structure and sites of O-glycans in GPCRs.

Tunicamycin treatment reduces intracellular glutathione levels: effect on the metastatic potential of the rhabdomyosarcoma cell line S4MH

Highly metastatic cells are known to overexpress certain Asn-linked oligosaccharides in the plasmatic membrane. Another phenotypic characteristic of malignant cells consists in the expression of high levels of intracellular glutathione (GSH). The aim of the present work was to demonstrate that the inhibition of N-glycosylation induces changes in intracellular GSH levels, and in turn participates in the inhibition of the metastatic potential of tumor cells by tunicamycin treatment. Firstly, we demonstrated that in comparison to the poorly metastatic cell line F21, the highly metastatic cells S4MH express a higher number of Asn-linked beta1-6 branched oligosaccharides and sialic acid (SA) and/or chitobiose oligosaccharides in glycoproteins involved in the regulation of the adhesion efficiency of tumor cells on endothelial cells and extracellular matrix. Our results showed that the decrease in S4MH cell adhesion efficiency on endothelial cells and extracellular matrix after the inhibition of N-glycan processing by tunicamycin treatment was caused by: (1) inhibition of the expression of N-glycan structures recognized by endothelial endogenous lectins, including beta1-6 branched oligosaccharides and SA and/or chitobiose oligosaccharides, and (2) redistribution of cell surface glycoproteins with beta1-6 branched oligosaccharides and/or SA and/or chitobiose oligosaccharides in their structures, caused by the depletion of intracellular GSH levels. The latter condition prevents the organization of these glycoproteins in the plasmatic membrane of S4MH cells necessary for anchoring to the substratum.

Molecular characterization of a granulocyte macrophage–colony-stimulating factor receptor α subunit-associated protein, GRAP

The granulocyte macrophage–colony-stimulating factor receptor (GM-CSF-R) is a heterodimer composed of 2 subunits,  and β, and ligand binding to the high-affinity receptor leads to signalling for the multiple actions of GM-CSF on target cells. In order to explore the role of the  subunit in signalling, we used a yeast-2-hybrid system to identify proteins interacting with the intracellular domain of the GMR-. A cDNA encoding a predicted protein of 198 amino acids, designated GRAP (GM-CSFreceptor  subunit-associatedprotein), was isolated in experiments using the intracellular portion of GMR- as bait. The interaction between GRAP and GMR- was confirmed by coimmunoprecipitation in mammalian cells. GRAP mRNA is widely expressed in normal human and mouse tissues and in neoplastic human cell lines, but it is not restricted to cells or tissues that express GM-CSF receptors. Three discrete GRAP mRNA species were detected in human tissues and cells, with estimated sizes of 3.3, 3.1, and 1.3 kb. GRAP is highly conserved throughout evolution, and homologues are found in yeast. The GRAP locus in Saccharomyces cerevisiae was disrupted, and mutant yeast cells showed an inappropriate stress response under normal culture conditions, manifested by early accumulation of glycogen during the logarithmic growth phase. GRAP is, therefore, a highly conserved and widely expressed protein that binds to the intracellular domain of GMR-, and it appears to play an important role in cellular metabolism.

Molecular characterization of a granulocyte macrophage–colony-stimulating factor receptor α subunit-associated protein, GRAP

The granulocyte macrophage–colony-stimulating factor receptor (GM-CSF-R) is a heterodimer composed of 2 subunits,  and β, and ligand binding to the high-affinity receptor leads to signalling for the multiple actions of GM-CSF on target cells. In order to explore the role of the  subunit in signalling, we used a yeast-2-hybrid system to identify proteins interacting with the intracellular domain of the GMR-. A cDNA encoding a predicted protein of 198 amino acids, designated GRAP (GM-CSFreceptor  subunit-associatedprotein), was isolated in experiments using the intracellular portion of GMR- as bait. The interaction between GRAP and GMR- was confirmed by coimmunoprecipitation in mammalian cells. GRAP mRNA is widely expressed in normal human and mouse tissues and in neoplastic human cell lines, but it is not restricted to cells or tissues that express GM-CSF receptors. Three discrete GRAP mRNA species were detected in human tissues and cells, with estimated sizes of 3.3, 3.1, and 1.3 kb. GRAP is highly conserved throughout evolution, and homologues are found in yeast. The GRAP locus in Saccharomyces cerevisiae was disrupted, and mutant yeast cells showed an inappropriate stress response under normal culture conditions, manifested by early accumulation of glycogen during the logarithmic growth phase. GRAP is, therefore, a highly conserved and widely expressed protein that binds to the intracellular domain of GMR-, and it appears to play an important role in cellular metabolism.

CD164 monoclonal antibodies that block hemopoietic progenitor cell adhesion and proliferation interact with the first mucin domain of the CD164 receptor

The novel sialomucin, CD164, functions as both an adhesion receptor on human CD34+ cell subsets in bone marrow and as a potent negative regulator of CD34+ hemopoietic progenitor cell proliferation. These diverse effects are mediated by at least two functional epitopes defined by the mAbs, 103B2/9E10 and 105A5. We report here the precise epitope mapping of these mAbs together with that of two other CD164 mAbs, N6B6 and 67D2. Using newly defined CD164 splice variants and a set of soluble recombinant chimeric proteins encoded by exons 1-6 of the CD164 gene, we demonstrate that the 105A5 and 103B2/9E10 functional epitopes map to distinct glycosylated regions within the first mucin domain of CD164. The N6B6 and 67D2 mAbs, in contrast, recognize closely associated and complex epitopes that rely on the conformational integrity of the CD164 molecule and encompass the cysteine-rich regions encoded by exons 2 and 3. On the basis of their sensitivities to reducing agents and to sialidase, O-sialoglycoprotease, and N-glycanase treatments, we have characterized CD164 epitopes and grouped them into three classes by analogy with CD34 epitope classification. The class I 105A5 epitope is sialidase, O-glycosidase, and O-sialoglycoprotease sensitive; the class II 103B2/9E10 epitope is N-glycanase, O-glycosidase, and O-sialoglycoprotease sensitive; and the class III N6B6 and 67D2 epitopes are not removed by such enzyme treatments. Collectively, this study indicates that the previously observed differential expression of CD164 epitopes in adult tissues is linked with cell type specific post-translational modifications and suggests a role for epitope-associated carbohydrate structures in CD164 function.

High-affinity binding to the GM-CSF receptor requires intact N-glycosylation sites in the extracellular domain of the β subunit

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor consists of 2 glycoprotein subunits, GMR and GMRβ. GMR in isolation binds to GM-CSF with low affinity. GMRβ does not bind GM-CSF by itself, but forms a high-affinity receptor in association with GMR. Previously, it was found that N-glycosylation of GMR is essential for ligand binding. The present study investigated the role of N-glycosylation of the β subunit on GM-CSF receptor function. GMRβ has 3 potential N-glycosylation sites in the extracellular domain at Asn58, Asn191, and Asn346. Single mutants and triple mutants were constructed, converting asparagine in the target sites to aspartic acid or alanine. A single mutation at any of the 3 consensus N-glycosylation sites abolished high-affinity GM-CSF binding in transfected COS cells. Immunofluorescence and subcellular fractionation studies demonstrated that all of the GMRβ mutants were faithfully expressed on the cell surface. Reduction of apparent molecular weight of the triple mutant proteins was consistent with loss of N-glycosylation. Intact N-glycosylation sites of GMRβ in the extracellular domain are not required for cell surface targeting but are essential for high-affinity GM-CSF binding.

High-affinity binding to the GM-CSF receptor requires intact N-glycosylation sites in the extracellular domain of the β subunit

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor consists of 2 glycoprotein subunits, GMR and GMRβ. GMR in isolation binds to GM-CSF with low affinity. GMRβ does not bind GM-CSF by itself, but forms a high-affinity receptor in association with GMR. Previously, it was found that N-glycosylation of GMR is essential for ligand binding. The present study investigated the role of N-glycosylation of the β subunit on GM-CSF receptor function. GMRβ has 3 potential N-glycosylation sites in the extracellular domain at Asn58, Asn191, and Asn346. Single mutants and triple mutants were constructed, converting asparagine in the target sites to aspartic acid or alanine. A single mutation at any of the 3 consensus N-glycosylation sites abolished high-affinity GM-CSF binding in transfected COS cells. Immunofluorescence and subcellular fractionation studies demonstrated that all of the GMRβ mutants were faithfully expressed on the cell surface. Reduction of apparent molecular weight of the triple mutant proteins was consistent with loss of N-glycosylation. Intact N-glycosylation sites of GMRβ in the extracellular domain are not required for cell surface targeting but are essential for high-affinity GM-CSF binding.

Biosynthesis and post-translational processing of lectin-like oxidized low density lipoprotein receptor-1 (LOX-1). N-linked glycosylation affects cell-surface expression and ligand binding

LOX-1 (lectin-like oxidized low density lipoprotein receptor-1) is a type II membrane protein belonging to the C-type lectin family that can act as a cell-surface receptor for atherogenic oxidized low density lipoprotein (Ox-LDL) and may play crucial roles in atherogenesis. In this study, we show, by pulse-chase labeling and glycosidase digestion, that LOX-1 is synthesized as a 40-kDa precursor protein with N-linked high mannose carbohydrate chains (pre-LOX-1), which is subsequently further glycosylated and processed into the 48-kDa mature form within 40 min. Furthermore, when treated with an N-glycosylation inhibitor, tunicamycin, both tumor necrosis factor-alpha-activated bovine aortic endothelial cells and CHO-K1 cells stably expressing bovine LOX-1 (BLOX-1-CHO) exclusively produced a 32-kDa deglycosylated form of LOX-1. Cell enzyme-linked immunosorbent assay, flow cytometry, and immunofluorescence confocal microscopy demonstrated that the deglycosylated form of LOX-1 is not efficiently transported to the cell surface, but is retained in the endoplasmic reticulum or Golgi apparatus in tumor necrosis factor-alpha-activated bovine aortic endothelial cells, but not in BLOX-1-CHO cells. Radiolabeled Ox-LDL binding studies revealed that the deglycosylated form of LOX-1 expressed on the cell surface of BLOX-1-CHO cells has a reduced affinity for Ox-LDL binding. Taken together, N-linked glycosylation appears to play key roles in the cell-surface expression and ligand binding of LOX-1.

Kinetic Resolution of Two Mechanisms for High-Affinity Granulocyte-Macrophage Colony-Stimulating Factor Binding to Its Receptor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an important hematopoietic cytokine that exerts its effects by interaction with the GM-CSF receptor (GMR) on the surface of responsive cells. The GM-CSF receptor consists of two subunits: GMR, which binds GM-CSF with low affinity, and GMRβ, which lacks intrinsic ligand-binding capability but complexes with GMR to form a high-affinity receptor (GMR/β). We conducted dynamic kinetic analyses of GM-CSF receptors to define the role of GMRβ in the interaction of ligand and receptor. Our data show that GMR/β exhibits a higher kon than GMR, indicating that GMRβ facilitates ligand acquisition to the binding pocket. Heterogeneity with regard to GM-CSF dissociation from GMR/β points to the presence of loose and tight ligand-receptor complexes in high-affinity binding. Although the loose complex has a koff similar to GMR, the lower koffindicates that GMRβ inhibits GM-CSF release from the tight receptor complex. The two rates of ligand dissociation may provide for discrete mechanisms of interaction between GM-CSF and its high-affinity receptor. These results show that the β subunit functions to stabilize ligand binding as well as to facilitate ligand acquisition.

Kinetic Resolution of Two Mechanisms for High-Affinity Granulocyte-Macrophage Colony-Stimulating Factor Binding to Its Receptor

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an important hematopoietic cytokine that exerts its effects by interaction with the GM-CSF receptor (GMR) on the surface of responsive cells. The GM-CSF receptor consists of two subunits: GMR, which binds GM-CSF with low affinity, and GMRβ, which lacks intrinsic ligand-binding capability but complexes with GMR to form a high-affinity receptor (GMR/β). We conducted dynamic kinetic analyses of GM-CSF receptors to define the role of GMRβ in the interaction of ligand and receptor. Our data show that GMR/β exhibits a higher kon than GMR, indicating that GMRβ facilitates ligand acquisition to the binding pocket. Heterogeneity with regard to GM-CSF dissociation from GMR/β points to the presence of loose and tight ligand-receptor complexes in high-affinity binding. Although the loose complex has a koff similar to GMR, the lower koffindicates that GMRβ inhibits GM-CSF release from the tight receptor complex. The two rates of ligand dissociation may provide for discrete mechanisms of interaction between GM-CSF and its high-affinity receptor. These results show that the β subunit functions to stabilize ligand binding as well as to facilitate ligand acquisition.

Preferential uptake and accumulation of oxidized vitamin C by THP-1 monocytic cells

THP-1 cells preferentially accumulate vitamin C in its oxidized form. The uptake displays first-order kinetics and leads to a build-up of an outward concentration gradient which is stable in the absence of extracellular vitamin. The transport is faster than reduction by extracellular glutathione or by added cytosolic extract, and glutathione-depleted cells show the same uptake rates as control cells. In addition, energy depletion or oxidation of intracellular sulfhydryls does not inhibit accumulation of ascorbate. The accumulation, however, always occurs in the reduced form. The affinity for dehydroascorbate is lower (Km 450 microM vs 60 microM) than for reduced ascorbate, but the maximal rate is more than 30 times higher (581 compared to 19 pmol.min-1 per 106 cells), and it is independent of sodium, whereas the uptake of ascorbate is not. The sodium gradient also allows accumulation of reduced ascorbate. Inhibitors of glucose transport by the GLUT-1 transporter also inhibit uptake of dehydroascorbate (DHA), but there are some inconsistencies, because the Ki-values are higher than reported for the isolated transporter and one inhibitor (deoxyglucose) is noncompetitive. The preferential uptake of the dehydro-form of the vitamin may be useful for situations where this short-lived metabolite is formed by oxidation in the environment.

Protein glycosylation: implications for in vivo functions and therapeutic applications

The glycosylation machinery in eukaryotic cells is available to all proteins that enter the secretory pathway. There is a growing interest in diseases caused by defective glycosylation, and in therapeutic glycoproteins produced through recombinant DNA technology route. The choice of a bioprocess for commercial production of recombinant glycoprotein is determined by a variety of factors, such as intrinsic biological properties of the protein being expressed and the purpose for which it is intended, and also the economic target. This review summarizes recent development and understanding related to synthesis of glycans, their functions, diseases, and various expression systems and characterization of glycans. The second section covers processing of N- and O-glycans and the factors that regulate protein glycosylation. The third section deals with in vivo functions of protein glycosylation, which includes protein folding and stability, receptor functioning, cell adhesion and signal transduction. Malfunctioning of glycosylation machinery and the resultant diseases are the subject of the fourth section. The next section covers the various expression systems exploited for the glycoproteins: it includes yeasts, mammalian cells, insect cells, plants and an amoeboid organism. Biopharmaceutical properties of therapeutic proteins are discussed in the sixth section. In vitro protein glycosylation and the characterization of glycan structures are the subject matters for the last two sections, respectively.

Identification of the sites of N-linked glycosylation on the human calcium receptor and assessment of their role in cell surface expression and signal transduction

The human calcium receptor (hCaR) is a G-protein-coupled receptor containing 11 potential N-linked glycosylation sites in the large extracellular domain. The number of potential N-linked glycosylation sites actually modified, and the effect on cell surface expression and signal transduction of blocking glycosylation at these sites, was examined by site-directed mutagenesis. Asparagine residues of the consensus sequences (Asn-Xaa-Ser/Thr) for N-linked glycosylation were mutated to glutamine individually and in various combinations to disrupt the potential N-linked glycosylation sites in the context of the full-length receptor. The cDNA constructs were transiently transfected into HEK-293 cells lacking endogeneous hCaR, and expressed receptors were analyzed by mobility differences on immunoblots, glycosidase digestion, intact cell enzyme-linked immunoassay, and extracellular calcium-stimulated phosphoinositide hydrolysis assay. Immunoblot analyses and glycosidase digestion studies of the wild type versus mutant receptors demonstrate that, of the 11 potential sites for N-linked glycosylation, eight sites (Asn-90, -130, -261, -287, -446, -468, -488, and -541) are glycosylated; the three remaining sites (Asn-386, -400, and -594) may not be efficiently glycosylated in the native receptor. Sequential mutagenesis of multiple N-linked glycosylation sites and analyses by immunoblotting, immunofluorescence, biotinylation of cell surface proteins, and intact cell enzyme-linked immunoassay indicated that disruption of as few as three glycosylation sites impairs proper processing and expression of the receptor at the cell surface. Disruption of five glycosylation sites reduced cell surface expression by 50-90% depending on which five sites were disrupted. Phosphoinositide hydrolysis assay results for various glycosylation-defective mutant receptors in general correlated well with the level of cell surface expression. Our results demonstrate that among 11 potential N-linked glycosylation sites on the hCaR, eight sites are actually utilized; glycosylation of at least three sites is critical for cell surface expression of the receptor, but glycosylation does not appear to be critical for signal transduction.

Human Trop-2 is a tumor-associated calcium signal transducer

Trop-2/EGP-1/GA733-1 is a recently identified cell surface glycoprotein highly expressed by human carcinomas. The cytoplasmic tail of Trop-2 possesses potential serine and tyrosine phosphorylation sites and a phosphatidyl-inositol binding consensus sequence. Thus, we investigated whether Trop-2 might be a functional signaling molecule. Using the fluorescent probe Fura-2, we assayed the cytoplasmic calcium levels in human cancer cells stimulated with anti-Trop-2 or control antibodies. Three anti-Trop-2 MAbs, Rs7-7G11, MOv16 and 162-46.2 specifically induced a transient intracellular calcium level increment in up to 40% of the experiments performed. Polyclonal antisera recognizing recombinant Trop-2 molecules possessed a much lower stimulation efficiency. The average latency of antibody-induced Ca2+ rise for OvCa-432 cells was 64+/-26 sec. Internal Ca2+ concentrations reached peaks of 190+/-24 nM vs. basal levels of 61+/-4 nM and returned to baseline within 193+/-37 sec. Similar values were obtained in MCF-7 cells. For comparison, stimulation of P2-purinergic receptors on MCF-7 and OvCa-432 cells induced a Ca2+ rise in most cases, leading to average internal Ca2+ concentrations of 297+/-41 and 391+/-39 nM, respectively. Our findings show that Trop-2 transduces an intracellular calcium signal, are consistent with the hypothesis that it acts as a cell surface receptor and support a search for a physiological ligand.

Human Trop-2 is a tumor-associated calcium signal transducer

Trop-2/EGP-1/GA733-1 is a recently identified cell surface glycoprotein highly expressed by human carcinomas. The cytoplasmic tail of Trop-2 possesses potential serine and tyrosine phosphorylation sites and a phosphatidyl-inositol binding consensus sequence. Thus, we investigated whether Trop-2 might be a functional signaling molecule. Using the fluorescent probe Fura-2, we assayed the cytoplasmic calcium levels in human cancer cells stimulated with anti-Trop-2 or control antibodies. Three anti-Trop-2 MAbs, Rs7-7G11, MOv16 and 162-46.2 specifically induced a transient intracellular calcium level increment in up to 40% of the experiments performed. Polyclonal antisera recognizing recombinant Trop-2 molecules possessed a much lower stimulation efficiency. The average latency of antibody-induced Ca2+ rise for OvCa-432 cells was 64+/-26 sec. Internal Ca2+ concentrations reached peaks of 190+/-24 nM vs. basal levels of 61+/-4 nM and returned to baseline within 193+/-37 sec. Similar values were obtained in MCF-7 cells. For comparison, stimulation of P2-purinergic receptors on MCF-7 and OvCa-432 cells induced a Ca2+ rise in most cases, leading to average internal Ca2+ concentrations of 297+/-41 and 391+/-39 nM, respectively. Our findings show that Trop-2 transduces an intracellular calcium signal, are consistent with the hypothesis that it acts as a cell surface receptor and support a search for a physiological ligand.

N-glycosylation of the prolactin receptor is not required for activation of gene transcription but is crucial for its cell surface targeting

The functional importance of the three oligosaccharide chains linked to Asn35, Asn80 and Asn108, of the long form of the PRL receptor (PRLR) was investigated by individual or multiple substitutions of asparagyl residues using site-directed mutagenesis and transient transfection of these mutated forms of PRLR in monkey kidney cells, Chinese hamster ovary, and human 293 fibroblast cells that exhibit different levels of protein expression. Scatchard analysis performed on monkey kidney cells revealed that the mutants possess the same affinity for PRL as compared with wild-type PRLR. A strong reduction (90%) of the aglycosylated PRLR expression at the cell surface of monkey kidney or human 293 cells was observed. Immunohistochemistry experiments using an anti-PRLR monoclonal antibody showed an accumulation of the deglycosylated receptor in the Golgi area of transfected monkey kidney cells. Upon PRL stimulation, the aglycosylated PRLR associated with Janus kinase 2 was phosphorylated and was able to activate a beta-casein gene promoter in transfected 293 fibroblast cells. The active form of the PRLR was thus acquired independently of glycosylation. By contrast, no functional activity was detectable in transfected Chinese hamster ovary cells that expressed low levels of PRLR. These studies demonstrate that the glycosylation on the asparagyl residues of the extracellular domain of the PRLR is crucial for its cell surface localization and may affect signal transduction, depending on the cell line.

Emerging paradigms in granulocyte-macrophage colony-stimulating factor signaling

The myelomonocytic lineage of hematopoiesis is regulated by the growth factor granulocyte-macrophage colony-stimulating factor (GM-CSF). This cytokine has proven to be safe for use in coordination with the treatments for bone marrow transplants and acute myelogenous leukemia. GM-CSF and related cytokines operate through specific receptors in the membranes of target cells of the myelopoietic lineages (both immature and mature cells). The exact signal transduction mechanisms in the cell are only beginning to be clarified and involve a plethora of signaling molecules. With a wealth of new information from studies in GM-CSF-induced cell activation, three major experimental approaches are emerging as gold standards in the exploration of those signaling pathways initiated by hematopoietic growth factors. We consider here: (1) a protein-protein interaction, as exemplified by the association between the cytokine membrane receptor and JAK kinase; (2) a covalent modification of an enzyme, as studied in the phosphorylation of MAP kinase; and (3) a protein-DNA interaction, as demonstrated by the translocation of STAT from the cytosol to the nucleus where it can bind to the promoters of specific genes.