Characterization of the human transferrin receptor produced in a baculovirus expression system

Generation of bioactive peptides by biological libraries

Biological libraries are powerful tools for discovery of new ligands as well as for identification of cellular interaction partners. Since the first development of the first biological libraries in form of phage displays, numerous biological libraries have been developed. For the development of new ligands, the usage of synthetic oligonucleotides is the method of choice. Generation of random oligonucleotides has been refined and various strategies for random oligonucleotide design were developed. We trace the progress and design of new strategies for the generation of random oligonucleotides, and include a look at arising diversity biases. On the other hand, genomic libraries are widely employed for investigation of cellular protein-protein interactions and targeted search of proteomic binding partners. Expression of random peptides and proteins in a linear form or integrated in a scaffold can be facilitated both in vitro and in vivo. A typical in vitro system, ribosome display, provides the largest available library size. In vivo methods comprise smaller libraries, the size of which depends on their transformation efficiency. Libraries in different hosts such as phage, bacteria, yeast, insect cells, mammalian cells exhibit higher biosynthetic capabilities. The latest library systems are compared and their strengths and limitations are reviewed.

Characterization of bioactive recombinant woodchuck interleukin-2 amplified by RLM-RACE and produced in eukaryotic expression system

Woodchucks (Marmota monax) infected with woodchuck hepatitis virus (WHV) represent a highly valuable laboratory model of hepatitis B virus (HBV) infection, in which molecular, immunological and pathological events occurring in infected humans are adequately reflected. To advance studies on T cell immune responses and propagation of hepadnavirus in T lymphocytes in this animal model, we determined the complete sequence of woodchuck interleukin-2 (wIL-2) cDNA by utilizing RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE) reaction. The wIL-2 sequence revealed a single open reading frame encoding for the predicted precursor protein comprised of a signal peptide and a 134 amino acid-long mature protein. The mature wIL-2 protein produced in the Escherichia coli expression system, designated as ec-rwIL-2, was found to be immunogenic but not biologically active. In contrast, precursor wIL-2 protein cloned into baculovirus transfer vector and expressed in Sf9 cells, designated as bac-rwIL-2, demonstrated functional competence. Further, bac-rwIL-2 was able to stimulate proliferation and to induce multiple daughter cell generations in woodchuck T cells, as well as facilitated the survival of standard IL-2-dependent mouse CTLL-2 cells in culture. Western blot analysis of bac-rwIL-2 using antibodies generated against ec-rwIL-2 revealed a single protein band of 15.5kDa. The availability of biologically active recombinant wIL-2 should facilitate ex vivo studies on functional competence of woodchuck T lymphocytes derived from different stages of hepadnaviral hepatitis and assist in recognizing their contribution to the pathogenesis of liver injury in the woodchuck model of hepatitis B.

Effects of transferrin receptor blockade on cancer cell proliferation and hypoxia-inducible factor function and their differential regulation by ascorbate

Cellular iron is needed for cell survival and hydroxylation of hypoxia-inducible factor-1alpha (HIF-alpha) by prolyl hydroxylases (PHD). One mechanism of iron uptake is mediated by the cell surface transferrin receptor (TfR). Because iron is required for cell growth and suppression of HIF-alpha levels, we tested the effects of the two anti-TfR monoclonal antibodies (mAb) E2.3 and A27.15 on growth of breast cancer cells and induction of HIF-alpha and hypoxia-regulated genes. Treatment with both mAbs together synergistically inhibited cell proliferation in a dose-responsive manner by up to 80% following 8 days of exposure, up-regulated HIF-1alpha and HIF transcription targets, down-regulated TfR expression, and down-regulated cellular labile iron pool by 60%. Because combined treatment with anti-TfR mAbs resulted in the up-regulation of the hypoxia pathway, which may increase tumor angiogenesis, we analyzed the effects of ascorbate on cell viability and HIF-1alpha levels in cells treated with both anti-TfR mAbs together, as ascorbate has been shown to be required by PHD enzymes for full catalytic activity. Ascorbate at physiologic concentrations (25 micromol/L) suppressed HIF-1alpha protein levels and HIF transcriptional targets in anti-TfR mAb-treated cells but did not suppress the antiproliferative effect of the mAbs. These results indicate that the addition of ascorbate increased the activity of the PHD enzymes in down-regulating HIF but not the proliferation of iron-starved anti-TfR mAb-treated cells. The use of anti-TfR mAbs and ascorbate in inhibiting both cell proliferation and HIF-1alpha and angiogenesis under normoxic conditions may be of therapeutic use.

Receptor-mediated endocytosis and intracellular trafficking of lipoproteins and transferrin in insect cells

While the intracellular pathways of ligands after receptor-mediated endocytosis have been studied extensively in mammalian cells, in insect cells these pathways are largely unknown. We transfected Drosophila Schneider line 2 (S2) cells with the human low-density lipoprotein (LDL) receptor (LDLR) and transferrin (Tf) receptor (TfR), and used endocytosis of LDL and Tf as markers. After endocytosis in mammalian cells, LDL is degraded in lysosomes, whereas Tf is recycled. Fluorescence microscopy analysis revealed that LDL and Tf are internalized by S2 cells transfected with LDLR or TfR, respectively. In transfectants simultaneously expressing LDLR and TfR, both ligands colocalize in endosomes immediately after endocytic uptake, and their location remained unchanged after a chase. Similar results were obtained with Spodoptera frugiperda Sf9 cells that were transfected with TfR, suggesting that Tf is retained intracellularly by both cell lines. The insect lipoprotein, lipophorin, is recycled upon lipophorin receptor (LpR)-mediated endocytosis by mammalian cells, however, not after endocytosis by LpR-expressing S2 transfectants, suggesting that this recycling mechanism is cell-type specific. LpR is endogenously expressed by fat body tissue of Locusta migratoria for a limited period after an ecdysis. A chase following endocytosis of labeled lipophorin by isolated fat body tissue at this developmental stage resulted in a significant decrease of lipophorin-containing vesicles, indicative of recycling of the ligand.

Transferrin receptor overexpression enhances transferrin responsiveness and the metastatic growth of a rat mammary adenocarcinoma cell line

We previously found that breast cancer cell transferrin receptor expression and proliferative response to transferrin often correlated with metastatic capability. To further explore this, we transfected mammary tumor cells with a cDNA coding for the transferrin receptor and examined the effects of its overexpression on various cellular properties. A human transferrin receptor expression plasmid was made by excising the cDNA for the receptor from pcDTR1 and ligating it into the multiple cloning site of pcDNAINeo. The resulting construct was transfected into the poorly metastatic rat MTLn2 line that expresses low endogenous levels of rat transferrin receptor, and transfection-induced receptor expression was ascertained using antibodies specific for the human protein. Approximately 50% of the initial geneticin-resistant transfected MTLn2 cells overexpressed human transferrin receptor protein. High expressors were further isolated by four sequential FACS sorts. The final cell population expressed approximately 3-7 times more cell surface transferrin receptor than did vector transfected controls. Both lines proliferated at the same rate in normal (medium plus 5% FBS) culture conditions. However, in serum-free conditions, the transferrin receptor overexpressor cells displayed a pronounced proliferative response to transferrin whereas the control line did not. When injected into the mammary fat pads of female nude mice, cells from both lines formed micrometastases to the lung that were specifically visualized by immunohistochemical staining of rat cytokeratin 17. This revealed that the transferrin receptor transfected line formed larger lesions of this nature than did cells from the vector transfected controls. When injected into the tail vein of female nude mice, the transferrin receptor overexpressors likewise formed gross lung metastases of remarkably greater size than did the vector only transfectants. Overexpression of cell surface human transferrin receptor on MTLn2 cells appeared to affect their in vitro growth response to transferrin and their ability to grow at a secondary site in vivo.

Expression, characterization, processing and immunogenicity of an insulin-dependent diabetes mellitus autoantigen, IA-2, in Sf-9 cells

Autoantibodies to a 64-kD protein and a 40-kD tryptic fragment from pancreatic islets have been detected at high frequency in the sera of patients with insulin-dependent diabetes mellitus (IDDM). IA-2, a newly isolated transmembrane protein tyrosine phosphatase, is a major islet cell autoantigen in IDDM and the precursor of a 40-kD tryptic fragment. To express large quantities of recombinant IA-2 protein and analyse post-translational modifications we expressed full-length human IA-2 in baculovirus-infected Sf-9 cells. IA-2 expression was analysed by Western blot and by immunoprecipitation of 35S-methionine-radiolabelled proteins with rabbit antisera or IDDM sera. A 120-kD IA-2 protein was detected during the early, but not the late, phase of the infection. Pulse-chase experiments showed that the 120-kD protein was processed into fragments of 64 kD and smaller fragments of approximately 50 kD, 38 kD and 32 kD. The 64-kD fragment appeared as a doublet. Tunicamycin and PNGase F treatment down-shifted the 120-kD protein and the 64-kD doublet into lower molecular weight bands, suggesting that both were glycosylated. Trypsin treatment converted the 120-kD protein and the 64-kD doublet into a 40-kD fragment. Baculovirus-expressed IA-2 was as sensitive or slightly more sensitive than in vitro translated IA-2 in detecting autoantibodies to IA-2: 66% of sera from newly diagnosed IDDM patients reacted with baculovirus-expressed IA-2 compared with 59% of the same sera which reacted with in vitro translated IA-2. It is concluded that baculovirus-expressed IA-2 is a good source of autoantigen and that a number of lower molecular weight fragments with which IDDM autoantibodies react are derived from the 120-kD full-length IA-2 molecule.

Baculovirus surface display: construction and screening of a eukaryotic epitope library

The baculovirus expression system was utilized to serve as a tool for ligand selection, demonstrating the applicability of the system to the generation and screening of eukaryotic expression libraries. The HIV-1-gp41 epitope 'ELDKWA', specific for the neutralizing human mAb 2F5, was inserted into the antigenic site B of influenza virus hemagglutinin and expressed on the surface of baculovirus infected insect cells. In order to improve the antigenicity of the epitope within the hemagglutinin, and therefore enhance the specific binding of 2F5, we inserted three additional, random amino acids adjacent to the epitope. This pool of hemagglutinin genes was directly cloned into the baculovirus Ac-omega. To identify distinct proteins displayed on the cellular surface, we developed a screening protocol to select for specific binding capacity of individual viral clones. Using fluorescence activated cell sorting (FACS) we isolated a baculovirus clone displaying the epitope with markedly increased binding capacity out of a pool of 8000 variants in only one sorting step. Binding properties of the identified ligand were examined by FACS performing a competition assay.

Human transferrin receptor is active and plasma membrane-targeted in yeast

The human transferrin receptor, a type II plasma membrane protein which mediates iron transport in human cells, was expressed in the yeast Saccharomyces cerevisiae. The transferrin receptor synthesized by yeast cells was posttranslationally modified comparable to the native receptor with respect to glycosylation and dimer formation. The location of the expressed receptor in the yeast plasma membrane indicates that the targeting of this type II membrane protein shares similar mechanisms in yeast and mammalian cells. The yeast-expressed transferrin receptor showed binding activity towards its natural ligand, transferrin in an ELISA binding assay.

Functional analysis of human/chicken transferrin receptor chimeras indicates that the carboxy-terminal region is important for ligand binding

Chimeric human/chicken transferrin receptors have been constructed using the polymerase chain reaction. Different regions of the 671-residue external domain of the human transferrin receptor were replaced by the corresponding sequences from the chicken transferrin receptor. As chicken transferrin receptors do not bind human transferrin, functional analysis of such chimeric receptors provides an approach to define the ligand-binding site of the human transferrin receptor. Four of 16 chimeric human/chicken transferrin receptors expressed in chick embryo fibroblasts were efficiently transported to the plasma membrane and displayed on the cell surface. Studies of the four chimeric receptors indicated that binding of human transferrin was abolished if the carboxy terminal 192 amino acids of the human transferrin receptor (residues 569-760) were replaced with the corresponding region from the chicken transferrin receptor. Further, a chimeric receptor in which the carboxy-terminal 72 residues were derived from the chicken transferrin receptor exhibited a 16-fold decrease in binding affinity for human transferrin. In contrast, analysis of the other two chimeric receptors showed that 340 amino acids of the human transferrin receptor external domain more proximal to the transmembrane region (residues 151-490) could be replaced with the corresponding region from the chicken transferrin receptor without loss of high-affinity ligand binding. In contrast, two mAbs against the human transferrin receptor external domain, B3/25 and D65.3, that do not compete with transferrin binding, do not bind the chimeric transferrin receptors in which the membrane proximal part is replaced by chicken sequences, while they do bind the two other chimeric transferrin receptors with high affinity. These data indicate that sequence differences in the carboxy-terminal region of human and chicken transferrin receptor external domains are important for the species specificity of transferrin binding and imply that this portion of the human transferrin receptor is critical for ligand binding.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

Characterization of the oligosaccharide structures on bee venom phospholipase A2

The N-linked oligosaccharide structures on bee venom phospholipase A2 were investigated. The oligosaccharides on purified phospholipase A2 were released by hydrazinolysis and labeled in vitro by reduction with NaB3H4. Following purification, the labeled oligosaccharides were characterized by size exclusion chromatography in combination with digestion with specific glycosidases. Linkage positions were determined by methylation analysis. Four types of structures were identified on the molecule, all of which were of truncated high-mannose type and none of which contained any alpha-(1-->2)-linked mannose residues. The majority of the structures were Man3 oligosaccharides with (43%) or without (38%) a fucose residue linked alpha-(1-->6) to the reducing N-acetylglucosamine. The remaining 19% of the oligosaccharides on the molecule were identified as a Man5 oligosaccharide without core fucose (9.6%) and a core-fucosylated Man4 structure (9.2%).

Analysis of soluble human and mouse interferon-gamma receptors expressed in eukaryotic cells

The extracellular domains of the human and mouse interferon-gamma receptors were produced in insect Spodoptera frugiperda cells infected with recombinant baculoviruses and in mammalian Chinese-hamster-ovary cells. The receptors expressed in both systems are secreted into the culture medium. Their signal peptides are cleaved off and the proteins show heterogeneity in glycosylation which, however, does not affect the capacity to bind interferon gamma or specific antibodies. The soluble mouse receptors exhibit binding capacities similar to those of cell-surface-anchored receptors, whereas the human receptors exhibit a lower binding capacity. All soluble receptors inhibit the binding of interferon gamma to cellular receptors and neutralize the antiviral activity exerted by interferon gamma. These receptors could therefore be useful for structure/function analyses and in vivo studies.

Modulation of cell-surface transferrin receptor by the imino sugar N-butyldeoxynojirimycin

The imino sugar, N-butyldeoxynojirimycin, is an inhibitor of the glycoprotein-processing enzyme glucosidase I and exhibits anti-(human immunodeficiency virus) activity in vitro. We have investigated the effect(s) of this compound on cell-surface glycoproteins by flow cytometry. We observed selective modulation of the transferrin receptor in response to treatment with 0.5 mM N-butyldeoxynojirimycin resulting in reduced cell-surface transferrin-receptor expression. The receptor modulation was dose dependent, resulted in reduced 59Fe uptake by treated cells and was fully reversible within 24 h of culture in the absence of the compound. Pulse/chase analysis in conjunction with endoglycosidase-H digestion demonstrated that transferrin-receptor glycosylation was altered following N-butyldeoxynojirimycin treatment, which is compatible with glucosidase inhibition. In addition, modulation of transferrin receptor in response to N-butyldeoxynojirimycin was not confined to a single cell line, but was also observed with certain human lymphoid and myeloid cell lines. Mechanism(s) of action of the imino sugar resulting in reduced cell-surface transferrin-receptor expression are discussed.

Monoclonal antibodies against defined epitopes of the human transferrin receptor cytoplasmic tail

Three murine monoclonal antibodies (mAbs) against the 61-residue amino-terminal cytoplasmic tail of the human transferrin receptor (TR) have been produced by immunization of mice with recombinant human TR produced in a baculovirus expression system. Mutant human TRs expressed in chick embryo fibroblasts (CEFs) with point mutations or deletions in their cytoplasmic tails have been used to map the epitopes defined by each of the mAbs. One mAb, H68.4, previously shown to block receptor internalization, binds proximal to the carboxy-terminal side of the YTRF internalization signal of TR. The second mAb, H73.2, binds near to the carboxy-terminal side of the H68.4 epitope, whereas the third mAb, 160.1, binds closer to the transmembrane region. H68.4 and H73.2 are auto-antibodies consistent with their epitopes mapping to a region of the human TR that has an identical amino acid sequence to the mouse TR. All three mAbs crossreact with the cytoplasmic tail of Chinese hamster TR. Double labelling of recombinant human TRs on chick embryo fibroblast (CEF) cell membrane preparations with B3/35 and H68.4 antibody-gold conjugates established that receptors in clathrin-coated pits were not labeled with H68.4, implying that associated coated pit proteins may block binding of this mAb.

Processing and antigenicity of entire and anchor-free spike glycoprotein S of coronavirus TGEV expressed by recombinant baculovirus

The S gene of transmissible gastroenteritis virus (TGEV) was inserted into the genome of Autographa californica nuclear polyhedrosis virus (AcNPV) using the transfer plasmid pVL941. Infection of Sf9 insect cells with the recombinant virus resulted in the synthesis of a 175K polypeptide which was able to trimerize and was transported to the cell surface as is the authentic TGEV S protein. Despite the lack of complete carbohydrate processing, the recombinant S protein exhibited antigenic properties similar to TGEV S and induced high levels of neutralizing antibodies in immunized rats. Engineering a deletion (70 amino acids) into the carboxy-terminus containing the membrane anchor of the polypeptide allowed its secretion. The oligomerization process and the antigenic profile of the anchor-free S protein were shown to be partially altered.

Purification and biochemical characterization of a soluble mouse interferon-gamma receptor produced in insect cells

The extracellular domain of the mouse interferon gamma receptor comprising amino acids 17-243 of the protein was produced in Spodoptera frugiperda cells infected with a recombinant baculovirus. The receptor was mainly secreted into the culture medium and was purified to homogeneity in several hundred milligram amounts. The purification procedure involved four chromatography steps and delivered a soluble and active receptor with an overall recovery of 30%. From each purification run, two pools of soluble receptor with the same interferon gamma binding capacity were isolated. Under reducing electrophoretic conditions the protein of pool I migrates as two bands of molecular masses 32 and 34 kDa and of pool II as two bands of 30 and 32 kDa. The soluble receptor of both pools carries a heterogeneous glycosylation. After deglycosylation it appears as one protein band of 27 kDa. N-linked carbohydrates contribute about 6 kDa and O-linked carbohydrates 1 kDa to its molecular mass. The nonreduced protein specifically binds interferon gamma on ligand blots and in a solid-phase binding system and competes for the binding of radiolabeled interferon gamma to the cell surface receptor. The soluble mouse interferon gamma receptor exists as a monomer in physiological buffer and binds interferon gamma in its dimeric form. It is stable at room temperature and against tryptic digestion, but is very sensitive to proteinase K digestion. The soluble mouse interferon gamma receptor produced in the insect/baculovirus expression system may prove useful to study the function of interferon gamma receptor as an antagonist of endogenous interferon gamma in the treatment of immunological and inflammatory disorders.

Expression of the cystic fibrosis gene in non-epithelial invertebrate cells produces a regulated anion conductance

The nature of involvement of the cystic fibrosis gene product (CFTR) in epithelial anion transport is not yet understood. We have expressed CFTR in Sf9 insect cells using the baculovirus expression vector system. Reactivity with antibodies against 12 different epitopes spanning the entire sequence suggested that the complete polypeptide chain was synthesized. Immunogold labeling showed localization to both cell-surface and intracellular membranes. Concomitant with CFTR expression, these cells exhibited a new cAMP-stimulated anion permeability. This conductance, monitored both by radioiodide efflux and patch clamping, strongly resembled that present in several CFTR-expressing human epithelial cells. These findings demonstrate that CFTR can function in heterologous nonepithelial cells and lend support to the possibility that CFTR may itself be a regulated anion channel.

Functional expression of Shaker K+ channels in a baculovirus-infected insect cell line

We constructed a recombinant baculovirus, A. californica nuclear polyhedrosis virus, containing the Drosophila Shaker H4 K+ channel cDNA under control of the polyhedrin promoter. When infected with this recombinant baculovirus, the cell line Sf9, derived from the army-worm caterpillar S. frugiperda, expresses fully functional Shaker transient K+ currents, as assayed by whole-cell recording. K+ currents begin to appear at about 15 hr after infection, and they continue to increase over the next 3 days. Over the same period of time, a 75 kd band appears on SDS gels stained with Coomassie blue. The identity of this band as a Shaker gene product is confirmed by Western blot analysis using an anti-Shaker antiserum. The 75 kd band accounts for a substantial fraction of the membrane protein in Shaker-infected Sf9 cells. These results give hope that the baculovirus system, which has been used successfully for high-level expression of soluble proteins from higher eukaryotes, may be appropriate for producing large amounts of cloned ion channel proteins as well.

Expression of the human multidrug transporter in insect cells by a recombinant baculovirus

The plasma membrane associated human multidrug resistance (MDR1) gene product, known as the 170-kDa P-glycoprotein or the multidrug transporter, acts as an ATP-dependent efflux pump for various cytotoxic agents. We expressed recombinant human multidrug transporter in a baculovirus expression system to obtain large quantities and further investigate its structure and mechanism of action. MDR1 cDNA was inserted into the genome of the Autographa californica nuclear polyhedrosis virus under the control of the polyhedrin promoter. Spodoptera frugiperda insect cells synthesized high levels of recombinant multidrug transporter 2-3 days after infection. The transporter was localized by immunocytochemical methods on the external surface of the plasma membranes, in the Golgi apparatus, and within the nuclear envelope. The human multidrug transporter expressed in insect cells is not susceptible to endoglycosidase F treatment and has a lower apparent molecular weight of 140,000, corresponding to the nonglycosylated precursor of its authentic counterpart expressed in multidrug-resistant cells. Labeling experiments showed that the recombinant multidrug transporter is phosphorylated and can be photoaffinity labeled by [3H]-azidopine, presumably at the same two sites as the native protein. Various drugs and reversing agents (e.g., daunomycin greater than verapamil greater than vinblastine approximately vincristine) compete with the [3H]azidopine binding reaction when added in excess, indicating that the recombinant human multidrug transporter expressed in insect cells is functionally similar to its authentic counterpart.

Expression and purification of biologically active v-sis/platelet-derived growth factor B protein by using a baculovirus vector system

Malignant transformation induced by simian sarcoma virus is mediated by its v-sis protein, the monkey homolog of the platelet-derived growth factor (PDGF) B chain. By use of an appropriately engineered baculovirus expression vector, the v-sis protein was expressed in the insect cell line Spodoptera frugiperda (Sf9) at a level 50- to 100-fold higher than that observed with overexpression in mammalian-cell transfectants. The sis protein produced by Sf9 cells underwent processing similar to that observed in mammalian cells, including efficient disulfide-linked dimer formation. Moreover, the recombinant sis protein was capable of binding PDGF receptors and inducing DNA synthesis as efficiently as PDGF-B synthesized by mammalian cells. A significant fraction of sis protein was released from Sf9 cells, which made possible a one-step immunoaffinity purification to near homogeneity with a 40% recovery of biological activity. These results demonstrate that a protein whose normal processing requires both intrachain and interchain disulfide-bridge formation can be efficiently expressed in a biologically active form in insect cells by using a baculovirus vector system.