Bone marrow stromal cell-mediated gene therapy for hemophilia A: in vitro expression of human factor VIII with high biological activity requires the inclusion of the proteolytic site at amino acid 1648.

To evaluate the potential of the ex vivo bone marrow stromal cell (BMSC) system as a gene therapy for hemophilia A, we studied the in vitro expression of human factor VIII (hFVIII) in canine BMSCs following transfection with plasmid vectors and transduction with retroviral vectors. Vectors were composed of B domain-deleted forms of hFVIII that either retain or delete the proteolytic site at amino acid 1648. On transfection of BMSCs, vectors supported expression and secretion of similar levels of up to 386 mU/10(6) cells/24 hr, even though only 3-9% of the cells expressed hFVIII while 42-48% of transfected cells harbored plasmid vector. Much higher percentages (approximately 70%) of cells expressing hFVIII were achieved when BMSCs were transduced by retroviral vectors, resulting in expression and secretion as high as 1000-4000 mU/10(6) cells/24 hr. Western analysis demonstrated that the B domain-deleted forms possessing the proteolytic site were secreted predominantly as heavy and light chain heterodimers that resemble native forms found in plasma. In contrast, the hFVIII lacking the proteolytic site was expressed mostly as unprocessed, single heavy-light chains. Both hFVIII forms were correctly cleaved and activated by thrombin. The proteolyzed hFVIII form possessed > or = 93% normal biological activity while the unproteolyzed form possessed consistently less than 55% normal biological activity and was therefore considered less suitable for therapeutic application. These results demonstrate that the BMSC system has potential utility in gene therapy for hemophilia A and stress the importance of selecting the appropriate hFVIII structure for prospective clinical use.

Retroviral vector-modified bone marrow stromal cells secrete biologically active factor IX in vitro and transiently deliver therapeutic levels of human factor IX to the plasma of dogs after reinfusion.

Canine bone marrow stromal cells (BMSCs), transduced ex vivo with retroviral vectors, expressed and secreted biologically active human and canine coagulation factor IX (hFIX and cFIX) in vitro, and on autologous reinfusion expressed hFIX into the circulation of normal (nonhemophiliac) dogs. Human FIX, when expressed in vitro by BMSCs of two dogs at 1.22 and 1.39 microg/10(6) cells/24 hr in medium supplemented with vitamin K, respectively, exhibited 28.1 and 27.3% normal biological activity as determined on the basis of a one-stage clotting assay. BMSCs of two additional dogs expressed 1.54 and 4.81 microg of cFIX/10(6) cells/24 hr in vitamin K-supplemented medium and the expressed cFIX possessed 58.4 and 32.9% normal activity, respectively. Between 2.33 and 3.35 x 10(8) transduced BMSCs, expressing 1.22 and 2.61 microg of hFIX/10(6) cells/24 hr or 3.24 and 7.82 microg of cFIX/10(6) cells/24 hr were reintroduced into the four donor dogs by intravenous infusion. Human FIX was detected in plasma for 7 or 12 days after BMSC reinfusion, with peak levels of 85.8 and 233.0 ng/ml observed at 2 days. Canine anti-hFIX antibodies, which were detected as early as 2-4 days after reinfusion of BMSCs expressing hFIX, may have masked potentially longer duration expression in vivo. Peak plasma levels of hFIX represented 2.1 and 5.8% normal human hFIX levels. When adjusted for percent normal one-stage clotting activity determined in vitro, these levels represented 0.6 and 1.6% normal human hFIX activity levels. Thus, we have demonstrated that retroviral vector-modified BMSCs can deliver human therapeutic levels of hFIX to the circulation of dogs.

Enhanced growth of canine bone marrow stromal cell cultures in the presence of acidic fibroblast growth factor and heparin.

The ex vivo establishment, expansion, transduction, and reintroduction of autologous bone marrow stromal cells offers a potential efficacious system for somatic cell gene therapy. It is likely that any ex vivo system will require the use of large numbers of cells which express high levels of transgene products. We present a method for routine expansion of canine bone marrow stromal cells, established from initial 10-20 ml marrow aspirates, to greater than 10(9) cells. This high level expansion of cell cultures uses the stimulatory effect of acidic fibroblast growth factor (aFGF) and heparin. In the absence of these factors, stromal cell cultures grow actively for only 1 to 2 passages, become flattened in morphology, and expand to only 10(8) cells. In the presence of heparin (5 U/ml), aFGF exerts its effect over a wide range of concentrations (0.1-10 ng/ml) in a dose-dependent manner. The stimulatory effect is dependent on the presence of both aFGF and heparin. Immunocytochemical and cytochemical analyses phenotypically characterize these stromal cells as bone marrow stromal myofibroblasts. Stromal cells grown in the presence of aFGF and heparin grow actively and maintain a fibroblast-like morphology for a number of passages, transduce efficiently with a human growth hormone (hGH) expression vector, and express and secrete high levels of hGH. Human marrow stromal cells were also established and expanded by the same culture method. This culture method should be of great value in somatic cell gene therapy for the delivery of secreted gene products to the plasma of large mammals.

Systemic delivery of human growth hormone or human factor IX in dogs by reintroduced genetically modified autologous bone marrow stromal cells.

Canine bone marrow stromal cells were expanded to numbers in excess of 10(9) cells from the initial 10-20 ml of marrow aspirates and transfected to express high levels of human growth hormone (hGH) in vitro. Ex vivo-modified marrow stromal cells were used in a gene therapy model system for the systemic delivery of transgene products in dogs. Adherent bone marrow stromal cell cultures, established and expanded from iliac crest marrow aspirates from each of 8 dogs, were transfected with a hGH gene plasmid expression vector and shown to express from 0.54-3.84 micrograms/10(6) cells per 24 hr hGH in vitro. The transfected plasmid vector does not possess a eukaryotic origin of replication nor does it possess sequences required for efficient integration into the host cell genome. As such, expression was expected to be transient. Transfected cells were autologously reintroduced into each dog by either infusion into a foreleg vein or directly into iliac crest marrow. In two cases, the stromal cells were cryopreserved following transfection, and subsequently thawed and infused. In one case, the expanded stromal cells were first cryopreserved, and then thawed, recultured, transfected, and infused. Reintroduced cell numbers ranged from 2.2 x 10(7) to 2.6 x 10(9), with total hGH expression capacities ranging from 62 to 1,400 micrograms/24 hr. Plasma of each of the dogs contained detectable hGH for a mean of 3.1 days (SD +/- 0.8 day) ranging from 2 to 5 days following reinfusion of cells. Peak plasma levels ranged from 0.10 to 1.76 ng/ml. Similar hGH expression values, based upon total expression capacity of the cells infused and dog body weight, were obtained for all dogs. Vector-modified stromal cells were detectable, by polymerase chain reaction (PCR) analysis, in the peripheral circulation following reinfusion in all 4 dogs analyzed. In 3 of the dogs, modified stromal cells were detected for 8.5-15 weeks. In addition, modified stromal cells were detected in iliac crest marrow of 2 dogs for 9 and 13 weeks, respectively, following reinfusion. In another experiment, cultured bone marrow stromal cells were transfected with a human factor IX (hFIX) plasmid vector. Modified cells (5.57 x 10(8)), with a total hFIX expression capacity of 281 micrograms/24 hr, were reinfused, resulting in detectable hFIX in plasma continuously for 9 days with a peak level of 8 ng/ml on day 1. These results demonstrate that the ex vivo bone marrow stromal cell system is a potentially powerful method by which to deliver secreted transgene product to the systemic circulation of large animals.

Co-integration of beta-lactoglobulin/human serum albumin hybrid genes with the entire beta-lactoglobulin gene or the matrix attachment region element: repression of human serum albumin and beta-lactoglobulin expression in the mammary gland and dual regulation of the transgenes.

The effect of co-integration of the entire beta-lactoglobulin (BLG) gene or matrix attachment region (MAR) sequences on the expression of various BLG/ human serum albumin (HSA) gene constructs was tested in transgenic mice. These former sequences were chosen because of their reported ability to insulate transgenes from the neighboring host genomic DNA sequences and/or to provide a more permissive transcriptional environment. When introduced alone, a cDNA-based BLG/HSA construct was expressed in 60% of transgenic strains and HSA was secreted at levels up to 0.3 mg/ml into the milk. Upon co-integration with either the entire BLG gene or MAR element, HSA RNA and protein expression were completely abrogated. While the co-integrated BLG gene suppressed the proportion of expresser strains carrying cDNA as well as genomic BLG/HSA constructs, the MAR element only exerted its negative effect on the cDNA-based BLG/HSA construct. In transgenics expressing both HSA and BLG, the tissue specificity and developmental patterns of BLG expression were altered and resembled the less stringent pattern of the BLG/HSA expression. These results demonstrate that rescue of transgene expression through co-integration with BLG or MAR sequences do not apply universally.

Elements within the beta-lactoglobulin gene inhibit expression of human serum albumin cDNA and minigenes in transfected cells but rescue their expression in the mammary gland of transgenic mice.

Two new beta-lactoglobulin (BLG)/human serum albumin (HSA) hybrid gene vectors were constructed and tested for expression in COS-7 cells and in transgenic mice. The HSA sequences were inserted between the second and sixth BLG exons. Transient transfection experiments with these vectors as well as a series of additional vectors with either the BLG 5'- or 3'- intragenic sequences revealed that sequences within BLG exon 1/intron 1/exon 2 abrogated BLG- directed HSA expression in vitro, regardless of the presence of HSA introns or the origin of the 3' polyadenylation signal. In contrast, the same BLG expression cassette enabled the efficient expression of HSA cDNA or minigene in the mammary gland of transgenic mice with subsequent secretion of the corresponding protein into the milk of 56 and 82%, respectively of the mouse strains at levels up to 0.3 mg/ml. Previous attempts to express HSA cDNA inserted into exon 1 of the BLG gene had failed [Shani,M., Barash,I., Nathan,M., Ricca,G., Searfoss,G.H., Dekel,I., Faerman,A., Givol,D. and Hurwitz,D.R. (1992) Transgenic Res. 1, 195- 208]. The new BLG expression cassette conferred more stringent tissue specific expression than previously described BLG/HSA constructs [Barash,I, Faerman,A., Ratovitsky,T, Puzis,R., Nathan,M., Hurwitz,D.R. and Shani, M. (1994) Transgenic Res. 3, 141-151]. However, it was not able to insulate the transgenes from the surrounding host DNA sequences and did not result in copy number dependent expression in transgenics. Together, the in vitro and in vivo results suggest both positive and negative regulatory elements within the BLG intragenic sequences evaluated. The new BLG construct represents an extremely valuable vector for the efficient expression of cDNAs in the mammary gland of transgenic animals.

Dramatic heterogeneity of transgene expression in the mammary gland of lactating mice: a model system to study the synthetic activity of mammary epithelial cells.

We studied the expression of human serum albumin (HSA) driven by the ovine beta-lactoglobulin promoter in the mammary glands of lactating mice from five independent transgenic strains, by employing combined in situ hybridization and immunostaining techniques. Four strains displayed a heterogeneous pattern of expression: mice of strains 91 and 92 expressed the transgene in only a fraction of the lobules, whereas in strains 69 and 83 all lobules contained cells expressing HSA. In all four strains the patterns of expression within expressing lobules corresponded to the morphology of alveolar cells and to the extent of local milk secretion, suggesting that filling of alveolus with secreted material was accompanied by asynchronous downregulation of transgene expression. In situ hybridization to the endogenous milk protein genes alpha-lactalbumin, beta-casein, and whey acidic protein revealed a uniform pattern of expression in lactating mammary glands of transgeneic and in four out of five non-transgeneic mice. In the fifth control mouse, we detected downregulation of gene expression in lobules containing alveoli distended by secreted milk. The pattern of expression of the three endogenous genes was greatly disturbed after a short (3-hr) unilateral closure of mammary glands, and very much resembled the pattern of expression of the HSA transgenes. These results demonstrate that transgeneic mice provide a useful model to study the factors that regulate the synthetic activity of mammary epithelial cells.

Developmental regulation of the ovine beta-lactoglobulin/human serum albumin transgene is distinct from that of the beta-lactoglobulin and the endogenous beta-casein genes in the mammary gland of transgenic mice.

We compared the developmental pattern of expression of the sheep beta-lactoglobulin (BLG), the chimeric BLG/human serum albumin (HSA), and the endogenous murine beta-casein genes in the mammary gland of virgin, pregnant and lactating transgenic mice, both at the RNA (expression) and protein (synthesis and secretion) levels. The BLG and casein genes were expressed at very low levels in virgin animals and during early stages of pregnancy. The increase in the expression of these genes started at the second half of pregnancy and reached a peak between the end of pregnancy and day 10 of lactation. The accumulation of their RNA coincided with that of the corresponding proteins, indicating a transcriptional control of expression of these genes. The expression and secretion patterns of the endogenous casein gene in transgenic and nontransgenic mice were indistinguishable. The hybrid BLG/HSA gene constructs displayed distinct patterns of expression in virgin animals and at early stage of pregnancy, from that of the BLG transgene or the endogenous mouse milk protein gene. High levels of expression (17-60% of that on day 18 of pregnancy) were detected in the mammary gland of virgin animals. At day 5 of pregnancy there was a dramatic decrease in HSA synthesis and secretion in all transgenic strains tested. The down-regulation, revealed by immunoprecipitation and immunohistochemical studies, demonstrated that at that stage of pregnancy only 10-18% of ductal structures contained HSA expressing cells in contrast to the majority of ducts expressing HSA in virgin animals. These morphological studies also demonstrated that the down-regulation in HSA synthesis and secretion was correlated with the transition from ducts comprised of a single layer of epithelial cells (characteristic of the virgin state) to ducts composed of multilayers of such cells. In two of the three transgenic strains tested, the down-regulation at the protein level was associated with a similar decrease in HSA transcripts. In the exceptional strain no. 23, HSA transcripts continued accumulating even at this stage. The differences in the control of expression at the RNA level between these transgenic strains were also confirmed by in situ hybridization. Our results suggest the involvement of at least two regulatory mechanisms effective at early stages of gestation in the control of expression/secretion of the HSA transgene targeted for expression in the mammary gland by the BLG milk protein promoter. These putative mechanisms may play key roles in the interplay between normal mammogenesis and lactogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Specific combinations of human serum albumin introns direct high level expression of albumin in transfected COS cells and in the milk of transgenic mice.

A new series of expression vectors, each comprised of the beta-lactoglobulin (BLG) promoter driving one of a variety of human serum albumin (HSA) minigenes or the entire gene, were evaluated for their ability to direct expression of HSA in vitro in COS tissue culture cells and into the milk of transgenic mice. Vectors directed a hierarchy of expression levels in vitro, dependent upon the specific complement of HSA introns included. HSA introns acted in a synergistic manner. In addition, minigenes comprised of specific subsets of introns were more efficacious than the entire HSA gene with all of its introns. Transgenic mice expressed as much as 10 mg ml-1 of HSA in their milk. Vectors comprised of specific intron subsets directed levels at 1 mg ml-1 or greater in the milk of 20% of generated transgenics. A statistical correlation between the expression level trend in vitro with the trend of expression in vivo (% which express) at detectable levels (p = 0.0015) and at the level of greater than 0.1 mg ml-1 (p = 0.0156) was demonstrated. A weak correlation existed (p = 0.0526) at in vivo levels of 1 mg ml-1 or greater. These new vectors are expected to direct the production of high levels of HSA in the milk of a large percentage of generated transgenic dairy animals.

Ectopic expression of beta-lactoglobulin/human serum albumin fusion genes in transgenic mice: hormonal regulation and in situ localization.

We produced transgenic mice carrying the native sheep beta-lactoglobulin (BLG) or fusion genes composed of the BLG promoter and human serum albumin (HSA) minigenes. BLG was expressed exclusively in the mammary glands of the virgin and lactating transgenic mice evaluated. In contrast, transgenic females carrying the BLG/HSA fusion constructs also expressed the HSA RNA ectopically in skeletal muscle, kidney, brain, spleen, salivary gland and skin. Ectopic expression of HSA RNA was detected only in strains that express the transgene in the mammary gland. There was no obvious correlation between the level of the HSA RNA expressed in the mammary gland and that found ectopically. In three transgenic strains analysed, the expression of HSA RNA in kidney and skeletal muscle increased during pregnancy and lactation, whereas in the brain HSA expression decreased during lactation in one of the strains. HSA protein was synthesized in skeletal muscle and skin of strain #23 and its level was higher in lactating mice compared with virgin mice. Expression of HSA was also analysed in males and was found to be more stringently controlled than in females of the same strains. In situ hybridization analyses localized the expressed transgene in the skin, kidney, brain and salivary glands of various transgenic strains. Distinct strain-specific and cell-type specific HSA expression patterns were observed in the skin. This is in contrast to the exclusive expression of the HSA transgene in epithelial cells surrounding the alveoli of the mammary gland. Taken together, these results suggest that the absence of sufficient mammary-specific regulatory elements in the BLG promoter sequences and/or the juxtaposition of the BLG promoter with the HSA coding sequences leads to novel tissue- and cell-specific expression in ectopic tissues of transgenic mice.

Synthesis and secretion of human serum albumin by mammary gland explants of virgin and lactating transgenic mice.

Transgenic mice were produced, carrying hybrid genes comprised of the ovine beta-lactoglobulin (BLG) milk protein gene promoter and human serum albumin (HSA) coding sequences. In situ hybridization revealed high levels of BLG/HSA hybrid mRNA, confined to the epithelial cells of the lactating mammary gland with a several hundred fold lower concentration in virgin mammary glands. During the first 24 h in culture, exceptionally high levels of HSA were secreted from explants of virgin mice, independent of hormonal control. HSA secretion was reduced considerably during subsequent days in culture and became dependent on the presence of insulin, hydrocortisone and prolactin. This temporal and hormonal pattern of regulation of HSA was different than that found for the secretion of caseins. In contrast to the vast difference in the mRNA content, the amount of HSA secreted from explants derived from lactating mice during the first 24 h in culture was only 2- to 5-fold higher than that found with explants from virgin transgenic mice, suggesting post-transcriptional control of HSA synthesis. The high-level synthesis and secretion of HSA in mammary explants of lactating mice was also dependent on the presence of insulin, hydrocortisone and prolactin. This study confirms previous suggestion that mammary explants from virgin transgenics may serve as a powerful tool for screening the potential of transgenic animals to secrete foreign proteins in their milk.

Real-time measurements of kinetics of EGF binding to soluble EGF receptor monomers and dimers support the dimerization model for receptor activation.

We have tested one aspect of the allosteric dimerization model for the activation of EGF receptor (EGFR) by EGF: whether EGF binding favors dimerization of the receptor. For this to be true, EGF molecules must bind with higher affinity to dimeric receptors than to monomeric receptors. We have tested this directly in a defined system using the soluble, extracellular ligand binding domain of EGFR monomers (sEGFR) and sEGFR dimers stabilized by treatment with a covalent cross-linking agent. We describe real-time kinetic measurements of EGF binding to receptor monomers and dimers employing the method of total internal reflection (surface plasmon resonance). Our data show that sEGFR dimers bound EGF with 30-40-fold higher affinity [KD = (2-3) x 10(-8) M] than did sEGFR monomers. The enhanced binding affinity of sEGFR dimers resulted mainly from a reduced off-rate with k(off) = 0.001 s-1 for sEGFR dimers as compared to k(off) = 0.06 s-1 for sEGFR monomers. These measurements indicate that dimerization of sEGFR increases its affinity for EGF by prolonging the amount of time that EGF remains bound to the receptor. This provides evidence that EGF binding stabilizes receptor dimerization and provides further support for the allosteric dimerization model as a mechanism for ligand induced receptor activation.

Aggregation-induced activation of the epidermal growth factor receptor protein tyrosine kinase.

Various agents are able to stimulate the EGF receptor protein tyrosine kinase in the absence of ligand binding. To characterize their mechanism of action, we investigated their effects on the kinase activity of the intracellular domain of the EGF receptor (EGFR-IC). EGFR-IC (67 kDa) lacking the extracellular domain and transmembrane segment of the EGF receptor, but retaining kinase and autophosphorylation domains, was produced and purified as a soluble, cytoplasmic protein from Sf9 insect cells infected with a recombinant baculovirus. EGFR-IC was able to undergo autophosphorylation in a manner similar to full-length EGFR. Synthetic substrate peptides showed similar affinity to EGFR-IC as to the full-length receptor. The activity of the EGFR-IC was found to be dependent on divalent cations, Mn2+ being a more potent activator than Mg2+. Agents capable of aggregating the kinase by direct interaction (cross-linking antibodies, polycations) or through altering the surrounding solvent structure and thereby decreasing protein solubility [ammonium sulfate, poly(ethylene glycol), 2-methyl-2,4-pentanediol] activated the kinase in a manner which correlated with their ability to precipitate the EGFR intracellular domain. The widely different chemical nature of these agents suggests that they do not act by direct interaction with specific allosteric regulatory sites, but rather by facilitating the interactions between kinase molecules. These results support the hypothesis that full-length receptor aggregation itself, induced by ligand binding to the extracellular domain, results in intracellular domain interactions and the activation of kinase activity.

Expression of human serum albumin in the milk of transgenic mice.

We have tested the feasibility of producing large quantities of human serum albumin (HSA) in the milk of transgenic livestock by generating transgenic mice as a model system. The sheep beta-lactoglobulin (BLG) 5'-regulatory promoter sequences were used to support expression of BLG or HSA in transgenic mice. Transgenic animals generated from the entire BLG gene including 3, 5.5 or 10.8 kb of 5'-sequences demonstrated that 3 kb of 5'-sequences were sufficient to support high levels of expression of BLG, and that the longer 5'-sequences did not improve upon the levels of expression. As such, the 3 kb 5'-sequences were used to drive expression of HSA in BLG-HSA constructs. HSA was not detectably expressed in eight transgenic lines generated from a BLG-HSA construct containing the HSA cDNA. Two transgenic lines of 26 generated, using five different constructs, with an HSA minigene possessing the first intron expressed HSA in their milk. One of these expressed HSA at high levels (2.5 mg ml-1) and has stably transmitted this ability to its progeny. A high percentage of transgenic mouse lines (four of six) generated from a vector containing an HSA minigene possessing introns 1 and 2 expressed HSA in their milk at levels which ranged from 1 to 35 micrograms ml-1. In a similar trend, levels of expression of HSA by transfected tissue culture cells from BLG-HSA vectors containing an introduced SV40 enhancer were low with the HSA cDNA, increased with the HSA minigene with intron 1 and increased further with the minigene containing introns 1 and 2. This study demonstrates that high levels of HSA can be expressed in the milk of transgenic animals, that introns of the HSA gene play a role in its expression and that transfected cell lines may be used to quickly evaluate the relative expression efficiencies of various vector constructs intended for future transgenic evaluation.

EGF induces increased ligand binding affinity and dimerization of soluble epidermal growth factor (EGF) receptor extracellular domain.

The binding of epidermal growth factor (EGF) to its cell surface receptor (EGF-R) results in a number of intracellular responses including the activation of the receptor intracellular tyrosine kinase. Receptor oligomerization induced by ligand binding has been suggested to play an important role in signal transduction. However, the mechanisms involved in oligomerization and signal transduction are poorly understood. We have produced and purified several milligrams of recombinant extracellular domain of the EGF receptor (EGF-Rx) using the baculovirus/insect cell expression system. The baculovirus-generated EGF-Rx is glycosylated, has had its signal peptide correctly cleaved, and exhibits a dissociation constant for EGF similar to that for solubilized full-length receptor, of about 100 nM. The binding of EGF to EGF-Rx leads to the formation of receptor dimers and higher oligomerization states which are irreversibly captured using the covalent cross-linking agent disuccinimidyl suberate. Interestingly, purified receptor monomers and dimers, stabilized by the cross-linker in the presence of EGF, exhibit increased binding affinity toward EGF as compared with receptor monomers which have not been exposed to EGF. It appears that the high affinity state of receptor can be maintained by the covalent cross-linking agent. These results indicate that in addition to ligand binding, the extracellular domain of EGF receptor possesses the inherent ability to undergo ligand-induced dimerization and that the low affinity state is converted to a high affinity state by EGF.

Epidermal growth factor (EGF) induces oligomerization of soluble, extracellular, ligand-binding domain of EGF receptor. A low resolution projection structure of the ligand-binding domain.

Ligand-induced oligomerization is a universal phenomenon among growth factor receptors. Although the mechanism involved is yet to be defined, much evidence indicates that receptor oligomerization plays a crucial role in receptor activation and signal transduction. Here we show that epidermal growth factor (EGF) is able to stimulate the oligomerization of a recombinant, soluble, extracellular ligand-binding domain of EGF receptor. Covalent cross-linking experiments, analysis by sodium dodecyl sulfate-gel electrophoresis, size exclusion chromatography, and electron microscopy demonstrate that receptor dimers, trimers and larger multimers are formed in response to EGF. This establishes that receptor oligomerization is an intrinsic property of the extracellular ligand-binding domain of EGF receptor. Ligand-induced conformational change in the extracellular domain will stimulate receptor-receptor interactions. This may bring about the allosteric change involved in signal transduction from the extracellular domain across the plasma membrane, resulting in the activation of the cytoplasmic kinase domain. Electron microscopic images of individual extracellular ligand-binding domains appear as clusters of four similarly-sized stain-excluding areas arranged around a central, relatively less stain-excluded area. This suggests that the extracellular ligand-binding domain is structurally composed of four separate domains.

Autophosphorylation of the intracellular domain of the epidermal growth factor receptor results in different effects on its tyrosine kinase activity with various peptide substrates. Phosphorylation of peptides representing Tyr(P) sites of phospholipase C-gamma.

The effect of autophosphorylation on the tyrosine kinase activity of the epidermal growth factor receptor (EGFR) is not well understood. We previously demonstrated that phospholipase C-gamma physically associates with the EGF-activated EGFR, but not with a kinase-negative mutant of the EGFR, and, moreover, that only the tyrosine-phosphorylated EGFR is able to associate with phospholipase C-gamma. We have now investigated the effect of autophosphorylation on the tyrosine kinase activity of the EGFR by employing the purified kinase-active intracellular domain of the EGFR (EGFR-IC) produced by a baculovirus expression system. Synthetic peptides, including ones which contain the individual major tyrosine phosphorylation sites of phospholipase C-gamma, were used as substrates. We found that the extensively prephosphorylated EGFR-IC exhibited similar reaction kinetics to the unphosphorylated EGFR-IC when angiotensin II was used as a nonspecific substrate. In contrast there was a clear stimulation of kinase activity due to autophosphorylation of the EGFR-IC when peptides representing either the major autophosphorylation site of the EGFR or the EGFR phosphorylation sites of phospholipase C-gamma were used as substrates. However, the modes of stimulation for these peptides differed. The binding affinity (Km) for the unphosphorylated EGFR-IC for the peptide containing Tyr-771 of phospholipase C-gamma was relatively poor compared with other peptides, but improved 5-6-fold when the EGFR-IC was prephosphorylated. On the other hand, autophosphorylation improved the reaction velocity (Vm) of the phosphorylation of other peptides by 2-3-fold, with little or no increase in affinity. These results suggest that autophosphorylation of the EGFR may induce a conformational change of its kinase domain which enhances its kinase activity with exogenous substrates and may induce association with phospholipase C-gamma by increasing its affinity to a domain containing Tyr-771.

The tyrosine phosphorylated carboxyterminus of the EGF receptor is a binding site for GAP and PLC-gamma.

Phospholipase C-gamma (PLC-gamma) and GTPase activating protein (GAP) are substrates of EGF, PDGF and other growth factor receptors. Since either PLC-gamma or GAP also bind to the activated receptors it was suggested that their SH2 domains are mediating this association. We attempted to delineate the specific region of the EGF receptor that is responsible for the binding, utilizing EGF receptor mutants, PLC-gamma, and a bacterially expressed TRP E fusion protein containing the SH2 domains of GAP. As previously shown, tyrosine autophosphorylation of the wild-type receptor wsa crucial in mediating the association and in agreement, a kinase negative EGF receptor could bind PLC-gamma or TRP E GAP SH2, but only when cross tyrosine phosphorylated by an active EGF receptor kinase. The importance of autophosphorylation for association was confirmed by demonstrating that a carboxy-terminal deletion of the EGFR missing four autophosphorylation sites bound these proteins poorly. To study the role of EGF receptor autophosphorylation further, a 203 amino acid EGF receptor fragment was generated with cyanogen bromide that contained all known tyrosine autophosphorylation sites. This fragment bound both TRP E GAP SH2 and PLC-gamma but only when tyrosine phosphorylated. This data localizes a major binding site for SH2 domain containing proteins to the carboxy-terminus of the EGF receptor and points to the importance of tyrosine phosphorylation in mediating this association.

Generation of recombinant cytoplasmic domain of epidermal growth factor receptor with intrinsic protein tyrosine kinase activity.

We have generated a recombinant baculovirus using the high expression vector pVL941 containing the complementary DNA encoding the intracellular domain of the human epidermal growth factor receptor (EGFR-IC). Upon infection of Spodoptera frugiperda insect cells, protein tyrosine kinase-active EGFR-IC was produced. The expressed protein has a molecular weight of 61,000 and is specifically recognized by antibodies directed against peptides representing different regions of human EGFR-IC. Upon sonication of infected cells, EGFR-IC was detected in both the soluble and insoluble fractions of the cell lysate. About 20-50% of the expressed EGFR-IC was soluble. Metabolic labeling and protein analyses showed that EGFR-IC comprised 7% of newly synthesized proteins in the cytoplasmic lysate and 0.1-0.2% of the total soluble protein. We have used a three-step purification procedure (fast-Q-Sepharose and phenyl-Sepharose column chromatographies and 30% ammonium sulfate precipitation) to purify EGFR-IC to 85% purity with 15-20% recovery from the initial soluble lysate. A yield of 3-4 mg of purified EGFR-IC has been consistently produced from 20 roller bottles with 2-4 x 10(8) infected cells/bottle. The tyrosine kinase activity was retained through purification. The enzyme demonstrated much higher autophosphorylation activity in the presence of Mn2+ than Mg2+. Phosphopeptide mapping revealed the same autophosphorylation sites utilized by EGFR-IC as those identified in wild-type EGFR. EGFR-IC-catalyzed phosphorylation of either a synthetic peptide representing the major autophosphorylation site or angiotensin II showed that the baculovirus-expressed EGFR-IC exhibits similar enzymatic kinetic characteristics to the intact activated EGFR kinase.

Expression of mitogenically active human recombinant platelet-derived growth factor A-chain.

Expression vectors encoding cDNAs for the human platelet-derived growth factor (PDGF) A-chain (pJKl) and murine dihydrofolate reductase (pTKDHFR) were cotransfected into dihydrofolate reductase-deficient Chinese hamster ovary cells. Methotrexate-induced coamplification of clones, expressing PDGF A-chain resulted in enhanced levels of A-chain-specific DNA, RNA and protein. A 30,500 Mr protein was immunoprecipitated with PDGF antisera from the conditioned media of metabolically labeled cells. Reducing conditions resolved the A-chain-specific protein into two polypeptides of 16,500 and 17,000 Mr, confirming the homodimeric nature of the recombinant A-chain protein. The recombinant PDGF A-chain produced constitutively by these amplified clones proved to be mitogenically active. The secretion of a recombinant PDGF A-chain into conditioned media may provide a continuous and abundant source of PDGF A.A dimers, normally produced by specific tissues in only minute quantities. Future purification of the recombinant homodimeric A-chain will allow the assessment of its ability to function in clinical applications such as wound healing.