SOLUBLE RECOMBINANT ALPHA BETA ADHESION RECEPTOR

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a novel purified recombinant αvβ3 adhesion receptor which shows an unimpaired ligand binding activity, and a process for preparing said soluble non-membrane bound receptor in excellent yields by recombinant techniques using a baculovirus-insect cell expression system.

[0002] The so-synthesized soluble receptor may be used very easily as screening tool for new therapeutic compounds which may inhibit the natural αvβ3 adhesion receptor.

[0003] The invention relates, furthermore, to a corresponding process for preparing recombinant full-length αvβ3 adhesion receptor in excellent yields, additionally using detergents to solve the membrane bound receptor from the surface of the host cell.

BACKGROUND OF THE INVENTION

[0004] Integrins are a super-family of cell surface adhesion receptors which control the attachment of cells with the solid extracellular environment both to the extracellular matrix (ECM), and to other cells. Adhesion is of fundamental importance to a cell; it provides anchorage and cues for migration and signals for growth and differentiation. Integrins are directly involved in numerous normal and pathological events, such as cell attachment, migration to blood clotting, inflammation, embryogenesis or cancer growth and metastasis, and as such, they are primary targets for therapeutic intervention.

[0005] Integrins are heterodimeric integral transmembrane glycoproteins, consisting of non-covalently linked α and β subunits. The integrins are classified in four overlapping subfamilies, containing the β1, β2, β3 or αv chains, and a particular cell may express several different integrins from each subfamily.

[0006] The last decade has shown that integrins are major receptors involved in cell adhesion and so may be a suitable target for therapeutic intervention. Reports concerning integrins are given, e.g., by E. Ruoslahti (J. Clin. Invest., 1991, 87) and R. O. Hynes (Cell, 1992, 69).

[0007] The αv-series integrins are now seen to be a major subfamily, with both classical, and novel functions. In melanoma cell lines, for example, enhanced expression of the αvβ3 integrin correlated with tumorigenicity and metastatic properties (e.g. Felding-Habermann et al. 1992, J. Clin. Invest. 89, 2018; Marshall et al. 1991, Int. J. Cancer 49, 924), suggesting the involvement of αv-containing integrins in some steps of the tumor cell metastasis process. A similar role is discussed for carcinoma adhesion and spreading and colorectal cancer, respectively, which is one of the most common epithelial malignancies.

[0008] The αv-series integrins seem to exclusively recognize ligands bearing the RGD- (NH2—arginine-glycine-aspartic acid—COOH) tripeptide sequences, including those in vitronectin (αvβ1, αvβ3, αvβ5), fibronection (αvβ1, αvβ3, αvβ5, αvβ6) and von Willebrand factor, fibrinogen and osteopontin (αvβ3) (e.g., Busk et al. 1992, J. Biol. Chem. 267, 5790; Smith and Cheresh 1990, J. Biol. Chem. 265, 2168). Function blocking anti-integrin antibodies, e.g., antibodies directed against the αvβ3 integrin are also known (e.g., Cheresh and Spiro 1987, J. Biol. Chem. 262, 17703; Chuntharapai et al. 1993, Exp. Cell Res. 205, 345; EP 95 119 233).

[0009] The disruption of ligand-integrin interaction by peptides and antibodies has highlighted the important roles of αvβ3 integrin, “the vitronectin receptor,” processes as diverse as tumor growth and metastasis, viral infection, osteoporosis and angiogenesis (Felding-Habermann and Cheresh 1993, Curr. Opin. Cell Biol. 5, 864; Brooks et al. 1994, Cell 79, 1157; Brooks et al. 1994, Science 264, 569). The emergence of αvβ3 as a potential target for therapeutic intervention has thus led to a search for αvβ3 antagonists—a process that requires purified αvβ3. The source of biochemical amounts of αvβ3 has usually been human placental tissue. In common with most integral membrane proteins, integrins can only be obtained as active solubilized molecules in the presence of non-ionic detergents and therefore purification of αvβ3 from placenta for drug screening is both cumbersome, costly, and a considerable health risk (Mitjans et al. 1995, J. Cell Sci. 108, 2825).

[0010] Although recombinant integrins have been expressed already in eukaryotic systems, such as in CHO cells cells (O'Toole et al. 1990, Cell Regul. 1, 883) and in embryonic kidney 293 cells (King et al. 1994, J. Bone Miner Res. 9, 381) the preparation of biochemically satisfactory amounts has not been achieved by recombinant technology.

[0011] There have been no reports on the biological activity of truncated soluble αvβ3 constructs, and full-length αvβ3 receptor apparently has not been produced by recombinant technology in biochemically important amounts.

[0012] Thus, it would be desirable to provide a soluble recombinant αvβ3 receptor which can be produced in biochemically useful amounts and in a purified quality by a comparably easy and riskless process. Moreover, such a process would be also advantageous for producing full-length αvβ3 receptor.

SUMMARY OF THE INVENTION

[0013] This invention describes for the first time high-level expression and purification of biologically competent soluble αvβ3 using the baculovirus-insect cell expression system. To achieve high yields of recombinant proteins processed in a manner similar to mammalian cells, over-expression of αvβ3 was achieved by utilizing the baculovirus polyhedrin gene as a strong viral promotor. The soluble molecule retains the ligand binding activity and specificity of the native placental and recombinant full length receptors, and can be purified in the absence of detergents, thus removing many possible ligation artefacts and producing a molecule suitable for high throughput screening and high resolution structural analysis.

[0014] Thus it is object of the present invention to provide an essentially purified soluble recombinant αvβ3 adhesion receptor with unimpaired ligand binding activity. A corresponding receptor which derives from human origin is a preferred embodiment of the invention.

[0015] The receptor according to the invention comprises the αv chain and the β3 chain of said receptor, each shortened at its C-terminus by a portion containing the transmenmbrane domain or a portion thereof and the complete cytoplasmatic domain of each individual chain. The amino acid and DNA sequences of the mature human αv and β3 protein chain as well their transmembrane and cytoplasmatic domains are known.

[0016] Especially, it is a preferred object of the invention to make available a soluble human recombinant receptor comprising a truncated αv chain containing approximately 957 amino and a truncated β3 chain containing approximately 692 amino acids, each calculated from the N-terminal of the corresponding mature protein chain. From this preferred soluble receptor the essentially complete transmembrane domain as well as the complete cytoplasmatic domain of the mature protein chains has been removed. However, the invention includes also soluble receptors comprising portions of the transmembrane domain (see below). The invention, however, is not limited to human αvβ3 receptor. According to the process described and claimed here it is no problem to generate (αvβ3 receptors deriving from non-human origin.

[0017] Furthermore, a corresponding human receptor can be produced by the present invention which is obtainable by a process as defined below and in the claims. This process according to the invention provides highly purified soluble recombinant human αvβ3 adhesion receptor with unimpaired ligand binding activity in good yields, and is characterized by the following steps:

[0018] (i) a first cDNA coding for the αv chain of said receptor, shortened by a portion comprising at least 52 amino acids calculated from the C-terminus, and a second cDNA coding for the β3 chain of said receptor, shortened by a portion comprising at least 61 amino acids calculated from the C-terminus, is sub-cloned into a baculovirus transfer vector of a baculovirus expression system,

[0019] (ii) said vector comprising said first and/or second DNA is transfered into the genomic DNA of a baculovirus of said expression system,

[0020] (iii) insect cells are infected with said complete recombinant baculovirus,

[0021] (iv) said infected insect cells are cultivated in a culture medium and said heteromeric truncated αvβ3 receptor is expressed into the medium, and

[0022] (v) said receptor expressed is isolated from the medium and purified by antibody affinity chromatography, the antibody used hereby is specific to the human αvβ3 adhesion receptor or its individual component chains.

[0023] As another aspect of the invention a similar process for the preparation of recombinant full-length αvβ3 receptor, especially of human origin, is disclosed. This process is characterized by the following steps:

[0024] (i) a first cDNA coding for the complete αv chain of said receptor and a second cDNA coding for the complete β3 chain of said receptor, is sub-cloned into a baculovirus transfer vector of a baculovirus expression system,

[0025] (ii) said vector comprising said first and/or second DNA is transfered into the genomic DNA of a baculovirus of said expression system,

[0026] (iii) insect cells are infected with said complete recombinant baculovirus,

[0027] (iv) said infected insect cells are cultivated in a culture medium, said cell membrane-bound receptor expressed is solubilized by a detergent, and

[0028] (v) said solubilized receptor is purified from the cell-free medium by antibody affinity chromatography, the antibody used hereby is specific to the human αvβ3 adhesion receptor or its individual component chains.

[0029] The αvβ3 receptors according to the invention are optimally suitable for the screening of compounds, e.g., therapeutic compounds, which may inhibit the natural receptor. Therefore, the invention relates to the use of said soluble receptors for the discovery of said compounds. As mentioned above such compounds according to the invention are, above all, linear or cyclic peptides comprising one or more RGD units or they are non-peptidic analogues which show the same biological activities against the αvβ3 adhesion receptor as the RGD peptides (see below).

[0030] Finally the invention relates to a process of screening therapeutic compounds for their capability to inhibit the natural αvβ3 receptor by using said soluble αv623 receptor as defined above and below and a therapeutic compound, especially RGD-peptides or non-peptidic analogues thereof, which may inhibit said αvβ3 receptor.

1Abbreviations:αVβ3integrin17E6mAbAP3mAbLM609mAbP4C19mAbP1F6mAbαV(FL)full-length αV chainαV(SL)short-length (truncated) αV chainβ3(FL)full-length β3 chainβ3(SL)short-length (truncated) β3 chainBacPAKBaculovirus expression systemSF9, HighFiveInsect cellsM.O.I.Multiplicity of Infection

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, wherein:

[0032]
FIG. 1 shows a diagram of the αvβ3 heterodimer showing the sites for truncation of the αv and β3 chains. The αv chain is normally post translationally cleaved into heavy (αvHC) and light (αvLC) chains that are linked with a disulfide (S—S) bond.

[0033] EC=Extracellular; PM=Plasma membrane; IC=Intracellular; IAC=Integrin Associated Cytoskeleton; Lig=Ligand

[0034]
FIG. 2 shows

[0035] (A) Mock infected Sf9 cells (lane 1) or Sf9 cells infected with null recombinant (lane 2), αv(FL) (lane 3) or βv(FL) (lane 4) were metabolically labelled with 35S-labelled amino acids for 2 hours at 40 hours post infection. The cell extracts were resolved by SDS-PAGE under reducing conditions. The molecular mass in kDa of standard protein markers are indicated on the right.

[0036] (B) Sf9 cells infected with αv(FL) (lanes 1 and 2), αv(SL) (lane 3 and 4), β3(FL) (lane 5 and 6) and β3(SL) (lane 7 and 8) were either treated with 10 μM Tunicamycin in DMSO (lanes 1, 3, 5 and 7) or with DMSO alone (lanes 2, 4, 6 and 8). The infected cells were metabolically pulse labelled as above. Standard protein markers are indicated on the right.

[0037]
FIG. 3 shows cell shape change after co-infection. Sf9 cells infected with either null recombinant virus (A), αv(FL)+β3(FL) (B) or αv(SL)+β3(SL) (C) at 48 hours post infection were photographed under phase contrast.

[0038]
FIG. 4 shows immunoprecipitation of cell surface and soluble αvβ3. Cell extracts (lanes 1-5) and cell conditioned medium (lanes 6-10) prepared from High Five cells co-infected with αv(FL)+β3(FL) (lanes 1 and 6), αv(FL)+β3(SL) (lanes 2 and 7), αv(SL)+β3(FL) (lanes 3 and 8), αv(SL)+β3(SL) (lanes 4 and 9) or null recombinant (lanes 5 and 10). αvβ3 complex was immunoprecipitated using monoclonal antibody LM609 and resolved by SDS-PAGE under non-reducing conditions, transfered onto PVDF membrane and detected with Streptavidin-HRP conjugate.

[0039]
FIG. 5 shows analysis of purified receptors. Epitope and purity analysis of purified full length and truncated αvβ3 by monoclonal antibodies. Increasing receptor concentrations were immobilized and analyzed with antibodies LM609 (anti-αvβ3), 17E6 (anti-αv), AP3 (anti-β3), P4C10 (anti-β1) and P1F6 (anti-αvβ5). Vertical axis: optical density at 450 nm; horizontal axis: concentration of receptor coating (μg/ml).

[0040] (◯) αvβ3 placenta, (▪) αv(FL)×β3(FL), (♦) αv(SL)×β3(SL)

[0041]
FIG. 6 shows analysis of purified receptors. Analysis of purified receptors by SDS-PAGE: Lane 1 and 5 HMW, lane 2 and 6 αvβ3 placenta, lane 3 and 7 αv(FL)×β3(FL), lane 4 and 8 αv(SL)×β3(SL). Lane 1, 3, 5, 7 are under nonreducing, lane 2, 4, 6, 8 are under reducing conditions.

[0042]
FIG. 7 shows activity test of purified recombinant αvβ3. Ligand binding to immobilized receptors: Biotinylated vitronectin, fibrinogen and fibronectin were allowed to bind to immobilized receptors. Bound ligand was detected using an anti-biotin antibody. Vertical axis: ligand binding in optical density units (405 nm), horizontal axis: vitronectin, fibrinogen, fibronectin concentration (μg/ml).

[0043] (◯) αvβ3 placenta, (▪) αv(FL)×β3(FL), (♦) αvΔ(SL)×β3Δ(SL).

[0044]
FIG. 8 shows effect of different RGD-peptides on vitronectin binding to different receptor preparations. Biotinylated vitronectin was incubated in parallel with incerasing concentrations of several peptides as indicated. Vertical axis: VN binding % of control, horizontal axis: peptide concentration;

[0045] —&Circlesolid;—αvβ3 placenta

[0046] —▪—αvβ3 recombinant truncated (1st preparing)

[0047] —▴—αvβ3 recombinant truncated (2nd preparing)

DETAILED DESCRIPTION

[0048] Microorganisms, cell lines, plasmids, phagemids, promoters, resistance markers, replication origins or other fragments of vectors which are mentioned in this application are normally commer-cially or otherwise generally available. In some cases the above-mentioned materials are not directly purchasable. However, they are used here only as examples in order to demonstrate properties or effects of the objects according to the invention, and are not essential to fulfill the requirements of disclosure. They can be replaced, as a rule, by other suitable generally obtainable tools and biological materials.

[0049] The invention includes also DNA and amino acid sequences having slightly varied or altered sequences or mutants and variants which can be obtained intentionally or randomly by chemical or physical processes. Generally, all such mutants and variants are included which show the described properties and functions.

[0050] The techniques and methods which are essential according to the invention are described in detail in the specification. Other techniques which are not described in detail correspond to known standard methods which are well known to a person skilled in the art, or are described more in detail in the cited references and patent applications and in the standard literature (e.g., Sambrook et al. 1989: Molecular Cloning, a laboratory manual, Cold Spring Harbor: Cold Spring Harbor Laboratory Press; Harlow and Lane 1988: Antibodies—A Laboratory Manual, Cold Spring Harbor: Cold Spring Harbor Laboratory Press).

[0051] Integrin αv and β3 cDNAs are known and available (e.g., Fitzgerald et al 1987, J. Biol. Chem. 262, 3936 (β3) or Suzuki et al. 1987, J. Biol. Chem. 262, 14080 (αv)), or can be produced by known methods, for example, by nucleotide synthesizers.

[0052] The human αv mature full-length protein chain according to the invention has 1018 amino acids, the transmembrane domain has approximately 29 amino acids and the cytoplasmic domain has approximately 32 amino acids. Cytoplasmic and transmembrane regions have about 61 amino acids. Cytoplasmic and transmembrane regions are located at the N-terminus of the chain. Thus, the preferred truncated βv chain according to the invention has approximately 957 amino acids.

[0053] The human β3 mature full-length protein chain according to the invention has 762 amino acids, the transmembrane domain has, like the αv chain, approximately 29 amino acids and the cytoplasmic domain has approximately 41 amino acids. Cytoplasmic and transmembrane regions have, therefore, 70 amino acids. Cytoplasmic and transmembrane regions are located at the N-terminus of the chain. Thus, the preferred truncated β3 chain according to the invention has approximately 692 amino acids.

[0054] According to the invention chains are preferred from which the complete cytoplasmic and transmembrane domain is removed. However, it is also possible to remove the complete cytoplasmic domain and only a portion of the amino acids of the transmembrane domain. Preferably, the truncated αv or β3 chain comprises additionally up 1 to 10 amino acids, especially 1 to 5 amino acids deriving from said transmembrane domain of each chain.

[0055] According to their nature the truncated αvβ3 receptor chains are glycosylated. The degree of glycosylation depends on their origin and the host cell system used.

[0056] The baculovirus expression system used according to the invention is also well known and commonly available. As a preferred baculovirus expression system, the BacPAK system from Clontech Laboratories, Inc. (supplied by Cambridge Bioscience, UK) is used. However, in principle, other baculovirus systems can be used.

[0057] According to the invention insect cells are infected with recombinant baculovirus DNA. In principal, all insect cell systems are suitable, however, those systems are preferred which guarantee a high infection by the virus system and a good and stable expression. Sf9 cells, and preferably High Five cells, which both are commercially available, are suitable insect cells. According to the invention, Sf9 cells are used according to the invention preferably for sub-cloning steps, whereas High Five cells are used, preferably for final expression steps (see Examples).

[0058] Sf9 cells (purchasable e.g. from Invitrogen Corporation) are, for example, maintained in TC100 supplemented with 10% insect cell-qualified Foetal Bovine Serum (FBS, Life Technologies, Inc.) and High Five cells (e.g. BTI-TN-5B1-4 from Invitrogen Corporation) are maintained, for example, in Express Five medium (Life Technologies, Inc.) supplemented with 16.5 mM L-Glutamine (Life Technologies, Inc.), Penicillin (50 IU/ml) and Streptomycin (50 μg/ml).

[0059] The generation of recombinant human αvβ3 receptor by means of a baculovirus-insect cell expression system is carried out, for example as follows:

[0060] The baculovirus system is used analogously to the methods given in Kidd and Emery (1993, Appl. Biochem. Biotechnol. 42, 137; O'Reilly et al., 1992: Baculovirus expression vectors: a laboratory manual. Oxford University Press, Inc., New York).

[0061] βv and β3 cDNAs truncated at positions coding for the transmembrane and extracellular interface (FIG. 1) are prepared by PCR. The αv and β3 truncated (αv(SL) and β3(SL)) and the full length (αv(FL) and β3(FL) cDNAs are sub-cloned into baculovirus transfer vector pBacPAK9. It is also possible according to the invention to clone the cDNA of each chain into seperate vectors in order to avoid stability problems. However, cloning into one single vector is preferred, since this guarantees better that equimolar amounts of each chain are produced. Surprisingly, the present results show that the vector comprising the DNA sequences of both chains are, although rather large, stable enough.

[0062] The truncated αv and β3 cDNAs usually comprise a signal sequence (thus, in case where the truncation comprises the complete transmembrane and cytoplasmic domain, coding for 987 αv chain amino acids [957 for mature protein+30 for signal peptide] and 718 β3 chain amino acids [692 for mature protein+26 for signal peptide]). Recombinant baculovirus expressing either full length or truncated αv or β3 chains are prepared by co-transfection with linearized baculovirus BacPAK6 genomic DNA followed by enrichment by plaque purification and virus amplification in Sf9 cells or High Five cells (O'Reilly et al., 1992: Baculovirus expression vectors: a laboratory manual. Oxford University Press, Inc., New York). Accurate integration of the transfer plasmids is confirmed by PCR and Southern blotting of the recombinant viral genomic DNA.

[0063] Metabolic labelling of Sf9/High Five cells expressing full length αv chain shows two major recombinant proteins of approximately 110 kDa and 150 kDa (FIG. 2A, lane 3). Similarly, Sf9 cells expressing full length β3 show two bands of approximately 90 kDa and 125 kDa (FIG. 2A, lane 4). Comparison of mock infected (lane 1) and null recombinant virus infected (lane 2) also shows the degree of suppression of the host cell genome during the very late phase of the viral life cycle. Treatment of cells expressing either individual full length or truncated forms of αv or β3 with tunicamycin (FIG. 2B, lanes 1, 3, 5 and 7) inhibits the production of the slower migrating band, indicating that the 150/110 kDa bands (FIG. 2B, lanes 2 and 4) and the 125/90 kDa (FIG. 2B, lanes 6 and 8) correspond to glycosylated and unglycosylated forms of the recombinant proteins. This result confirms several reports that have suggested that the proportion of recombinant protein in baculovirus expression systems that undergo post-translational modifications, such as N-glycosylation, decreases during later stages of the virus life cycle (Jarvis and Finn 1995, Virology 212, 500). The relatively large difference in size between the glycosylated and unglycosylated forms of αv and β3 indicated that the proteins undergo substantial glycosylation, which is also in keeping with the presence in αv of thirteen and in β3 of ten potential N-glycosylation sites.

[0064] Integrins determine and modulate the cell shape by acting as structural links between the extracellular matrix and the cytoskeleton (Schwartz et al. 1995, Ann. Rev. Cell Dev. Biol. 11, 549; Montgomery et al. 1994, Proc. Natl. Acad. Sci. USA 91, 8856). In later phases of baculovirus infection, Sf9/High Five cells are usually rounded and loosely attached (FIG. 3A), as were the cells infected with either of the αv or β3 expressing recombinant baculovirus alone or with the null recombinant virus (FIG. 3A). However, insect cells co-infected with αv(FL)+β3(FL) or αv(SL)+β3(FL) are firmly attached and exhibit a more spread cell shape (FIG. 3B and 3C). No shape change is observed in cells co-infected with αv(FL)+β3(SL) or αv(SL)+β3(SL). These results support the notion that the cytoplasmic domain of the α-chain is responsible for negative regulation of the affinity for the cytoskeleton of the β-chain cytoplasmic domain (Filardo and Cheresh 1994, J. Biol. Chem. 269, 4641; Akiyama et al. 1994, J. Biol. Chem. 269, 15961; LaFlamme et al. 1994, J. Cell Biol. 126, 1287). αvβ3 binds many ligands that have the peptide sequence Arg-Gly-Asp (RGD). Correspondingly, cell shape change of αv and β3 virus infected Sf9 cells on vitronectin coated plates are inhibited when challenged with RGD peptides (Table 1).

2TABLE 1−+RGD+nRGDαV(FL) + β3(FL)++−++αV(FL) + β3(SL)+/−−+/−αV(SL) + β3(FL)+++−+++αV(SL) + β3(SL)−−−Null-Recombinant−−−

[0065] Control peptides, cyc(RGEfV) and cyc(RβADfV) where glutamate was substituted for aspartate, or β-alanine for glycine have no effect on cell shape.

[0066] High Five cells, known to support higher level of production of baculovirus of expressed secreted proteins than Sf9 cells (Davis et al. 1993, In Vitro Cell Dev. Bio. Anim. 29A, 388), are co-infected with various combinations of recombinant baculovirus expressing the full length and truncated αv and β3. Immunoprecipitation of the αvβ3 complex with the MAb LM609 from cell extracts or from cell conditioned medium (FIG. 4) shows that the recombinant αvβ3 heterodimer is on the cell surface when either one or both the individual chains are full length (FIG. 4, lanes 1-3). More importantly, co-expression of truncated αv with truncated β3 results in secretion of a soluble heterodimer (FIG. 4, lane 9). Production of soluble αvβ3 in this serum free environment greatly simplified the downstream processing and also introduced a degree of flexibility in establishing large scale screening procedures for anti-αvβ3 compounds.

[0067] The novel truncated αvβ3 receptors according to the invention as well as their full-length variants are purified after recombinant expression by antibody affinity chromatography. Since the expression system according to the invention is very effective in producing large amounts of said receptor and only very small amounts of foreign proteins, and especially free from other human proteins, a single purification step is usually sufficient to obtain a highly purified product.

[0068] Antibody affinity chromatography is a well known technique: an antibody with a desired specificity to a certain antigen epitope is coupled enzymatically or chemically to a suitable activated support matrix, eg. modified agarose, sepharose, etc. Suitable antibodies according to the invention must have a good specificity to the αvβ3 receptor or to the single αv chain or the single β3 chain.

[0069] Many such antibodies are known, some of them are public domain antibodies and, therefore, without problems available. AP3 (anti-β3), LM609 (anti-αvβ3), P4C10 (anti-β1), P1F6 (anti-αvβ5) and 17E6 (anti-αv) have been characterized in detail (e.g., Mitjans et al. 1995, J. Cell Sci. 108, 2825). Monoclonal antibody 17E6 is an antibody which is produced by a hybridoma cell line having the designation 272-17E6. The cell line was deposited under accession number DSM ACC2160 at the Deutsche Sammlung für Mikroorganismen, Braunschweig, FRG. MAb 17E6 is, in addition, object of the European patent application 0 719 859. Anti-β3 antibody Aβ3 is deposited at the ATCC under accession number HB-242. Antibodies 17E6 and AP3 are especially suitable according to the invention, because they are easily available. LM609 (Cheresh and Spiro 1987, J. Biol. Chem. 262, 17703) is also of special interest since it is directed to the alpha as well as to the beta chain.

[0070] After having coupled said antibody to the support, the clear cell-free medium of the infected and lysed insect cells, containing the truncated or the solubilized full-length recombinant receptor according to the invention, is applied to a column filled with said support, and is recirculated several times over the column to bind the expressed receptor to the immobilized antibody. The purified αvβ3 receptor is washed thereafter by means of an ionic buffer from the column and can be used directly for ligand binding and other experiments.

[0071] Human vitronectin, fibronectin and fibrinogen can be purified from human plasma (e.g., Yatohgo et al. 1988, Cell Struct. Funct. 13, 281; Ruoslahti et al. 1982, Methods Enzymol 82 Pt A, 803; Ruoslahti 1988, Ann. Rev. Biochem. 57, 375; Kazal et al. 1963, Proc. Soc. Exp. Biol. Med. 113, 989).

[0072] In detail the purification and characterization of recombinant human αvβ3 is carried out for example, as follows:

[0073] Antibody affinity chromatography is carried out using 17E6, Aβ3 or LM609 antibodies. αvβ3 receptor from human placenta is, for example, purified by affinity chromatography according to the method of Smith and Cheresh (1988, J. Biol. Chem. 263, 18726).

[0074] Full length recombinant αvβ3 can be purified from non-ionic detergent solublilized extracts of High Five cells co-infected with αv(FL)+β3(FL).

[0075] Soluble truncated αvβ3 can be purified from High Five cells co-infected with αv(SL)+β3(SL) without using any detergent.

[0076] In ELISA, truncated soluble and full length solubilized recombinant αvβ3 are both recognized by the same antibodies (LM609, 17E6 and AP3) and with similar affinity, as judged by ELISA titres, as solubilized placental αvβ3. Each fails to react with anti-β1 (P4C10) or with anti-αvβ5-specific (P1F6) antibodies indicating conservation of epitopes and low contamination with other integrins (FIG. 5).

[0077] Purified placenta αvβ3 dissociates into two bands of the a-chain at 160 kDa and of the β-chain at 95 kDa when resolved by SDS-PAGE under non-reducing conditions (FIG. 6, lane 2). Purified full length recombinant αvβ3 (lane 3) dissociates into αv chain of approximately 140 kDa and a β3 chain at 90 kDa. The lower molecular weights may be due to incomplete or variant glycosylation of recombinant proteins expressed in baculovirus infected insect cells. Purified soluble recombinant αvβ3 dissociates into a truncated αv chain of approximately 120 kDa and a truncated β3 chain of 80 kDa.

[0078] The αv chain normally undergoes post translational cleavage into heavy and light chains, linked by a disulfide bridge (FIG. 1) (Hynes 1992, Cell 69, 11). Therefore, under reducing conditions the α chain of placental αvβ3 further dissociates into the heavy chain of 140 kDa and the light chain of 25 kDa (FIG. 6, lane 6) while the β3 chain migrates at 110 kDa. As the transmembrane truncation in the et chain lies within the light chain, both full length and truncated recombinant α chains should migrate under reducing conditions at ˜140 kDa. However, the αv chains of full length and soluble receptors exhibit the same molecular weights under both reducing and non-reducing conditions (lane 7 and 8), suggesting that there had only been partial post-translational cleavage of the αv chain. The faint band of approximately 120 kDa in lanes 7 and 8 may represent the αv heavy chain. This discrepancy between the placental and recombinant αvβ3 may be due to inefficient proteolytic processing during the very late phase of the baculovirus infection (O'Reilly et al. 1992, Baculuvirus Expression Vectors: A Laboratory Manual. Oxford University Press, Inc., New York).

3TABLE 2depicts an overview of the SDS-PAGE results:kD (±10%)kD (±10%)human αVβ3 chainsnon-reducedreducedPlacentaαV(FL)160140 (hc), 25 (lc)β3(FL)95110recombinantαV(FL)140140αV(SL)120140β3(FL)90105β3(SL)8095

[0079] According to the process of the invention recombinant shortened αvβ3 receptor and full-length receptor can be prepared. In order to demonstrate that so-prepared receptors, above all the truncated form, show the ligand specifities of the natural receptor, which was isolated, e.g., from human placenta and used as comparison embodiment, a binding assay has to be established. Ligands known to be competitive inhibitors of the αvβ3 receptor are used therefor. As mentioned above, linear and cyclic peptides containing the RGD amino acid sequence (arginine-glycine-asparagine) are excellent inhibitors of said receptor.

[0080] Suitable peptides are, for example: GRGDSPK, Echistatin, cyclic-RGDfV, cyclic-RGDfNMeV, cyclic-RβADfV, KTADC(Trt)PRNPHKGPAT, GRGESPK, cyclic-KGDfV, and cyclic-RGEfV. Additionally, also non-peptidic compounds which may mimick RGD-peptides and bind to the αvβ3 receptor can be used. Such RGD-peptides and non-peptidic analogues are known (e.g. from European Appln. No. 96 113 972, European Publication Nos. 0578 083, 0632 053, 0478 363, 0710 657, 0741 133 and PCT Publication No. WO 94/12181) and/or can be synthesized by known standard methods. The ligands are modified by marker molecules or atoms according to methods known in the prior art. Preferably, biotinylated ligands are used.

[0081] The recombinant truncated αvβ3 receptor or its full-length version are immobilized, for example by adsorption at microtitre plates, and reacted with said marked ligands. The αvβ3 receptors according to the invention bind said ligands strongly, very similar to the natural receptor isolated from human placenta, whereas non-RGD peptides are not bound. This result demonstrates that the receptors, obtainable by the claimed process, are functionally intact with respect to their specific ligand binding capacity.

[0082] In detail, measurement of the biological activity of recombinant truncated αvβ3 receptor is carried out, for example, as follows:

[0083] The truncated and full length recombinant αvβ3 and placenta αvβ3 are immobilized and binding of biotinylated ligands is examined. Three purified RGD-containing ligands—vitronectin, fibrinogen and fibronectin—that are known to bind to αv3 are used. The ligands bind both to the full length and soluble receptor preparations in a concentration dependent manner, and with overlapping concentration dependencies and saturation concentrations similar to those of placental αvβ3 (FIG. 7). Vitronectin is used for testing specific interaction with the recombinant αvβ3. Biotinylated vitronectin and increasing concentrations of above-specified peptides or non-peptidic analogues are incubated in parallel with immobilized receptor. The compounds are able to inhibit vitronectin binding to placenta αvβ3 to full length and to soluble recombinant αvβ3 (FIG. 8) and are about 4-5 orders of magnitude more active than the β-alanine-to-glycine control peptide. Each receptor shows an IC50-value for cyc(RGDfV) in the low nanomolar (1-5 nM) range.

[0084] In the same way as described here for known αvβ3 receptor specific ligands it is possible to screen for new drugs and therapeutic compounds which are regarded to be potent αvβ3 inhibitors.

[0085] This invention shows that the baculovirus insect cell expression system can be used to express high amounts of fully functional human recombinant αvβ3 integrins as soluble receptors secreted into the medium or, as alternative, on the cell surface. The double (SL)-truncated forms of the receptor are secreted as a complex into the medium as shown by precipitation and purification on an αvβ3-complex specific antibody (LM609, 17E6, AP3).

[0086] Under optimized conditions yields (soluble as well as full-length receptor) of 1-10 mg, preferably 2-5 mg/L culture medium (supernatant) can be obtained. This routine production in biochemical amounts of soluble αvβ3 opens the way for high throughput screening using αvβ3 and for structural analysis of this important receptor. The yield of natural receptor isolated from plancenta tissue is about 1-2 mg/kg tissue. Truncated and full-length recombinant αvβ3 proteins obtained by the process according to this invention have a purity of 95-99%, preferably 97-99%, as assessed by SDS-PAGE and ELISA. The soluble recombinant receptor is approximately 4-fold less sensitive to inhibitor than the full length recombinant molecules. A further advantage of the truncated soluble receptor is that it can be prepared without using detergents. The necessary non-ionic detergents are usually very expensive (e.g., ca. $500/10 g octyl-β-D-glyco-pyranoside). Approximately 25 g of detergent are necessary for solubilizing 3-4 mg receptor from placenta tissue.

[0087] Cells co-infected with recombinant virus with one full length and one truncated α- and β-chain (αv(FL)×β3(SL) or αv(SL)×β3(FL)) express hetero-dimeric receptor on their surface. But only cells containing either both full length chains, or full length β-chain and truncated α-chain show shape change when adhering on vitronectin. That this adhesion and spreading was αvβ3-specific was shown by adding RGD-containing peptides which specifically block the interaction between ligand and the native receptor while RGE- or RAD-peptides do not. Cells with fill length α-chain and truncated β-chain do not spread.

[0088] ELISA shows that the β3-specific antibody AP3 (Newman et al. 1985, Blood 65, 227) and αv-specific antibody 17E6 (Mitjans et al. 1995, J. Cell Sci. 108, 2825) and the complex-specific antibody LM609 (Cheresh and Spiro 1987, J. Biol. Chem. 262, 1770323) recognized both full length and truncated receptors, indicating that the recombinant integrins had the same epitopes as placenta αvβ3. SDS-PAGE shows under non-reducing conditions the bands typical for αv- and β3-chains. The recombinant full length a chain has a lower molecular weight compared to the placenta full length a chain (approx. 140 kDa compared to 160 kDa) which could be explained by a decreased or different glycosylation in insect cells. Both full length and soluble recombinant αvβ3 receptors are able to specifically bind the ligands vitronectin, fibrinogen and fibronectin, the vitronectin binding was specifically inhibited by RGD-containing peptides with the same concentration dependency as the placental receptor.

[0089] With this demonstration of the specificity and similar saturation concentrations for ligand binding for full length and truncated human (αvβ3 receptor it is now possible to embark on large scale preparation of functional recombinant soluble αvβ3 in the baculovirus insect cell system without the labor and the health risks associated using human placental tissue. Furthermore, the system described by this invention also allows expression of modified or chimeric receptors that could prove very useful for investigating at molecular level the ligand binding and the subsequent signal transduction processes.

[0090] In the foregoing and in the following examples, all temperatures are set forth uncorrected in degrees Celsius; and, unless otherwise indicated, all parts and percentages are by weight.

[0091] The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding European application No. 96 119 700. 1, filed Dec. 9, 1996 is hereby incorporated by reference.

EXAMPLES

Example 1

[0092] Construction of Recombinant Transfer Vectors

[0093] αv cDNA was excised as an EcoRI fragment from αvpcDNA-1Neo(Felding-Habermann 1992, J. Clin. Invest. 89, 2018) and cloned into the transfer plasmid pBacPAK8 (Clontech). The resulting recombinant was termed αv(FL)(pBac8). The integrin β3 cDNA was excised as a XbaI fragment from β3pcDNA-1Neo and cloned into the transfer plasmid pBacPAK9 (Clontech). The transmembrane truncated αv and β3 cDNAs were constructed using Polymerase Chain Reaction (PCR) using Pfu Thermostable DNA polymerase (Stratagene). Truncated αv cDNA coding for amino acids 1-987 of mature αv (signal esquence included) was generated using oligonucleotide primers:

45′-GAC CAG CAT TTA CAG TGA-3′ [forward] and5′ CA CAG GTC TAG ACT ATG GCT GAA TGC CCC AGG-3′

[0094] [reverse primer, containing translation stop codon (bold) after that coding for aa 987 followed by XbaI restriction site (underlined)].

[0095] The PCR product was digested with SalI and XbaI restriction enzymes and the fragment was cloned into SalI/XbaI digested movpcDNA-1Neo. The truncated αv cDNA was sub-cloned into EcoRI/XbaI sites of pBacPAC9 and the resulting clone was termed αv(SL) (pBac9). Truncated β3 cDNA coding for aa 1-718 of mature β3 (signal sequence includes) was generated using oligonucleotide primers:

55′-GCG CGC AAG CTT GCC GCC ACC ATG CGA GCG CGGCCG-3′

[0096] [forward primer containing the translation start codon (bold) and HindIII restriction side (underlined)] and

65′-GAT CGA TCT AGA CTA GGT CAG GGC CCT TGG GACACT-3′

[0097] [reverse primer also containing translation stop codon (bold) after that coding for aa 718 and XbaI restriction site (underlined)]. The PCR product was digested with HindIII and XbaI and cloned into HindIII/XbaI sites of pSK+(Stratagene). The resulting clone, termed β3(SL)(pSK+), was digested with HindIII and the ends were subsequently polished by filling in with Klenow fragment of DNA polymerase I (Boehringer Mannheim). The blunt-end linearized plasmid was then digested with XbaI restriction enzyme and the fragment was sub-cloned into SmaI/XbaI sites of pBacPAK9. The resulting clone was termed β3(SL)(pBac9). The PCR amplified regions of both αv(SL)(pBac9) and β3(SL)(pBac9) were verified by DNA sequencing and all the constructs were tested for expression of correct sized protein by in vitro transcription and translation from bacteriophage T7 promoter using TNT Coupled Reticulocyte Lysate System (Promega).

Example 2

[0098] Generation of Recombinant Baculovirus

[0099] BacPAK6 (Clontech) baculovirus genomic DNA was prepared from high titre virus stock (Page and Murphy 1990: Methods in Molecular Biology. Humana Press, Clifton, N.J., USA) and linearized with Bsu36I (Kitts and Possee 1993, Biotechniques 14, 810).

[0100] Recombinant baculovirus clones expressing full length and truncated αv and β3 integrins were prepared according to the Clontech BacPAK product protocol. A null recombinant baculovirus clone was also prepared using pBacPAK9 as transfer plasmid for use as negative control. The correct recombinant viral clones were identified by PCR and the virus was propagated and titered in Sf9 cells (O'Reilly et al. 1992, Baculuvirus expression vectors: a laboratory manual. Oxford University Press, Inc., New York).

Example 3

[0101] Metabolic Pulse Labelling of Proteins Expressed in Sf9 Cells

[0102] Metabolic labelling of Sf9 cells was performed as described (Summer and Smith 1987: A manual of methods for baculovirus vectors and insect cell procedures. Texas Agricultural Experiment Station Bulletin B-1555). Where indicated, tunicamycin (Sigma) was present at 10 μg/ml for 16 hours before and during pulse labelling. Cells were lysed with 100 μl of buffer [1% Nonidet® P-40, 1 mM CaCl2, 150 mM NaCl, 0.4 mM Pefabloc® (Boehringer Mann-heim), 10 μg/ml Leupeptin and E64 (Sigma), 10 mM Tris/HCl; pH 7.4; Nonidet P-40 is a detergent and Pefabloc is a protease inhibitor].The lysate was centrifuged at 14000×g in an Eppendorf microfuge at 4° C. for 10 minutes and resolved by electrophoresis in 8% SDS-poly-acrylamide gel under reducing conditions, fixed, and dried before autoradiography (Sambrook et al. 1989: Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory Press, Cold Sring Harbor, New York).

Example 4

[0103] Protein Expression in Insect Cells

[0104] Recombinant proteins were expressed in High Five cells, either as monolayer or in suspension. Cells were co-infected with αv- and β3-expressing recombinant baculovirus, each at M.O.I. of 5-20 (that means: one cell is infected with 5-20 virus DNA), in a minimum amount of medium with gentle agitation. After two hours the viral inoculum was removed and replaced with fresh Express Five medium, the infected cells were incubated at 27° C. for 48-64 hours. For isolation of membrane bound recombinant integrin, the cells were harvested by centrifugation (1000×g, 10 minutes) and the cell pellet was used. For isolation of the soluble recombinant integrin the cell-free supernatant was used.

Example 5

[0105] Immunoprecipitation

[0106] Immunoprecipitation of surface-biotinylated cell extract and cell conditioned medium was carried out with monoclonal antibody LM609 which recognises the extracellular domains of intact αvβ3-complex, linked to Affigel® beads (BIO-RAD) (Mitjans et al. 1995, J. Cell Sci. 108, 2825). After SDS-PAGE and Western blotting onto Hybond-PVDF membrane (Towbin et al. 1979, Proc. Natl. Acad. Sci. USA 76, 4350), the biotinylated proteins were detected by Enhanced-Chemiluminescence using ECL Western Blotting Detection Reagents (Amersham) as per manufacturers instructions.

Example 6

[0107] Cell Shape Change Studies

[0108] Sf9 cells (1×106 cells/well) were allowed to adhere to six-well plates either virgin or coated with purified human plasma vitronectin (5 μg/ml in PBS; 2 hours) followed by blocking of non-specific sites (3% BSA (w/v) in PBS), then infected with various virus combinations. The cells were observed 48 hours post-infection for cell shape change. Where indicated, the medium was supplemented with 10 μM cyclic peptides as competitive inhibitors of αv-integrin-ligand interaction [cyc(RGDfV), cyc(RβADfV) or cyc(RGEfv), where small letters are D-amino acids].

Example 7

[0109] Purification of Recombinant Integrins

[0110] High Five cells expressing full length αvβ3 were harvested, washed with PBS, and then lysed in ice cold lysis buffer [100 mM n-Octyl-β-D-glucoside, 1 mM CaCl2, 2 mM Pefabloc in PBS, pH 7.4] for 1 h at 4° C. The lysate was centrifuged (10,000×g; 45 min at 4° C.) and the supernatant recirculated overnight at 4° C. over a 17E6-antibody affinity column pre-equilibrated with lysis buffer. After washing [20 ml bed volumes, 2×10 cm column, 50 mM n-Octyl-β-D-glucoside, 2 mM Pefabloc, 2 mM CaCl2 in PBS, pH 7.4]bound protein was eluted [50 mM OG, 2 mM Pefabloc®, 2 mM CaCl2, 50 mM Na-Acetate, pH 3.1], the eluant monitored at 280 nm, and the fractions were immediately neutralized (1:50 volume 3M Tris-HCl pH 8.8). Peak fractions were pooled, dialyzed [10 mM OG, 1 mM CaCl2 in PBS, pH 7.4] and concentrated, then analyzed by SDS-PAGE. Aliquots at approx. 1 mg/ml protein were stored at −80° C. Protein concentration was determined against BSA standards with the BCA protein assay (Pierce).

[0111] Soluble truncated recombinant αvβ3 was purified by recirculating the cell free medium of infected High Five cells over an 17E6 affinity column at 4° C. The washing, elution and analysis were as described for the full length αvβ3 except that no detergent was present in the buffers. ELISA analysis of the purified receptors used goat-anti-mouse IgG (H+L) HRP conjugate (BIO-RAD) and 3,3′,5,5′-Tetramethylbenzidine-dihydrochloride (Sigma) as substrate.

Example 8

[0112] The purification of soluble and transmembrane full length αvβ3 receptor was achieved by using a LM609 antibody column.

Example 9

[0113] Preparation of an Antibody Affinity Chromatography Column

[0114] The support matrix (Affigel® BIORAD) was washed with 500 ml cold PBS. Purified antibody was incubated together with the support matrix (5 mg/ml gel) under circular rotation during ca. 12 hours. The supernatant was removed and still active groups on the matrix were blocked with 0.1 M ethanolamine. The support matrix was washed alternately several times with 0.01 M Tris-HCl pH 8.0 and 0.01 M sodium acetate pH 4.5 and was then directly used.

Example 10

[0115] Integrin Ligand Binding and Competition Assays: Biotinylation of Ligands or Antibodies

[0116] Proteins in PBS were diluted with 5-fold concentrated ligation buffer [500 mM NaCl, 500 mM NaHCO3] to 1 mg/ml protein. Freshly prepared N-hydroxysuccinimidobiotin (Sigma) [1 mg/ml in DMSO] was added to 0.1 mg/ml and incubated for 2 h at 20° C. After dialysis [PBS, 0.025% NaN3], protein concentration was determined. The biotinylated proteins were stored at 4° C.

[0117] Ligand Binding assay Integrins were diluted to 1 μg/ml in coating buffer [150 mM NaCl, 1 mM CaCl2, 1 mM MgCl2, 10 μM MnCl2, 20 mM Tris-HCl; pH 7.4) and 100 μl were adsorbed overnight at 4° C. to 96-well microtitre plates. The plates were washed once with binding buffer [0.1 % (w/v) BSA in coating buffer] and blocked with blocking buffer (coating buffer containing 3% (w/v) BSA) 2 h at 37° C. After rinsing with binding buffer, serially diluted biotinylated ligands were added. After incubation (3 h, 37° C.), unbound ligand was washed from the plate with binding buffer, and bound biotin was detected by incubation with anti-biotin-alkaline-phosphatase conjugated antibody and detection of bound antibody with p-Nitro-phenyl-phosphate (BIO-RAD) substrate.

Example 11

[0118] Integrin Ligand Binding and Competition Assays: Competition Assays

[0119] Integrins were immobilized as described above. Serially diluted cyclic peptides were added in parallel with biotinylated vitronectin (1 μg/ml). After 3 h incubation at 37° C., bound ligand was detected as described above. Assays were performed in triplicate and repeated several times.

[0120] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

SOLUBLE RECOMBINANT ALPHA BETA ADHESION RECEPTOR

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