Retinal degeneration is slowed in transgenic rats by AAV-mediated delivery of FGF-2

PURPOSE

We evaluated adeno-associated virus (AAV)-mediated gene transfer of basic fibroblast growth factor (FGF-2) as a therapy for photoreceptor degeneration in a transgenic rat model of retinitis pigmentosa.

METHODS

Recombinant adeno-associated virus vector (rAAV) incorporating a constitutive cytomegalovirus (CMV) promoter was used to transfer the bovine FGF-2 gene to photoreceptors. AAV was administered by subretinal injection to transgenic rats (TgN S334ter-4) at postnatal day 15 (P15). Control eyes were uninjected, injected with PBS, or AAV-LacZ. Eyes were examined by histopathology, morphometric analysis, and electroretinography at P60.

RESULTS

Expression of recombinant FGF-2 slowed the rate of photoreceptor degeneration. Morphologic studies demonstrated significantly more photoreceptors surviving in eyes injected with AAV-FGF-2 than in controls. Insignificant rescue effects were seen in retinas injected with buffer only. No significant inflammatory response or neovascularization was detected. Electroretinographic (ERG) responses of eyes injected with AAV-FGF-2 were increased compared with uninjected eyes; however, these amplitudes were not significantly larger than eyes receiving an AAV-LacZ control vector.

CONCLUSIONS

Transduction of retinal cells with AAV-FGF-2 reduces the rate of photoreceptor degeneration in an S334ter-4 animal model. Despite the lack of significantly increased ERG amplitudes from eyes expressing FGF-2, a greater number of surviving photoreceptors was demonstrated. Delivery of FGF-2 using recombinant AAV has potential as a therapy for retinal degeneration.

Dose response to a single intramuscular injection of recombinant adeno-associated virus-erythropoietin in monkeys

BACKGROUND

Anemia is a significant problem in many disease states. Erythropoietin (Epo) has been used in the treatment of anemia associated with numerous chronic diseases. This study investigates the dose-response profiles of a single intramuscular (im) injection of a recombinant adeno-associated virus vector (rAAV) containing the Epo gene with the goal of achieving a sustained elevation of hematocrit (Hct).

METHODS

Cynomolgus (cm) monkeys were given single injections of different doses of rAAV-cm-Epo. The biological effect of Epo gene expression was monitored by determining the Hct levels and circulating hormone levels by ELISA. Antibody to the rAAV capsid protein was also measured over the 41-week period of the experiment.

RESULTS

Epo expression was noted only when 2 x 10(11) or more particles were injected. Epo was noted to be increased as soon as 1 week postinjection and was maximum in 6 to 8 weeks. This level of expression remained constant for nearly 20 weeks. Animals given the highest dose of rAAV developed a higher Hct over the first 8 weeks postinjection than those given an intermediate dose. However, the maximum levels of hemoglobin were the same. There was a weak correlation between amount of rAAV injected and capsid antibody response.

CONCLUSIONS

AAV vectors are able to transduce skeletal muscle and are capable of achieving sustained expression and systemic delivery of a therapeutic protein following a single im administration. Dose responses to rAAV-Epo are achievable, although a threshold inoculum of virus is necessary to produce an effect and the therapeutic window is narrow.

Lipitoids--novel cationic lipids for cellular delivery of plasmid DNA in vitro

BACKGROUND

Although synthetic nonviral vectors hold promise for the delivery of plasmid DNA, their gene-transfer efficiencies are far from matching those of viruses. To systematically investigate the structure-activity relationship of cationic lipids, a small library of cationic lipid-peptoid conjugates (lipitoids) was synthesized. The compounds were evaluated for their ability to form complexes with plasmid DNA and to mediate DNA transfer in vitro.

RESULTS

Lipid-peptoid conjugates were conveniently prepared in high yield using solid-phase synthesis. Several lipitoids condensed plasmid DNA into 100 nm spherical particles and protected the DNA and DNase digestion. A subset of lipitoids with a repeated (aminoethyl, neutral, neutral) sidechain trimer motif conjugated with dimyristoyl phosphatidyl-ethanolamine (DMPE) mediated DNA transfer with high efficiency.

CONCLUSIONS

Automated solid-phase synthesis of cationic lipids allowed the rapid synthesis of a diverse set of transfection reagents. The most active compound DMPE-(Nae-Nmpe-Nmpe)3 (Nae, N-aminoethyl glycine; Nmpe, N-p-methoxyphenethyl-glycine) is more efficient than lipofectin or DMRIE-C (two commercial cationic lipid transfection reagents) and is active in the presence and absence of serum. The activity in the presence of serum suggests potential for applications in vivo.

Adeno-associated virus-mediated delivery of erythropoietin leads to sustained elevation of hematocrit in nonhuman primates

Recombinant adeno-associated virus encoding the monkey erythropoietin gene (rAAV-cm-Epo) was generated and tested for its potential to confer long-term expression of the gene product following intramuscular injection. A single intramuscular injection of 2 x 10(12) rAAV-cm-Epo particles into two baboons led to sustained high circulating Epo levels and a concomitant increase in hematocrit. The hematocrits reached 62 and 75% by week 10 (from pre-injection values of 38 and 40%, respectively) and remained elevated throughout the study period (28 weeks). Circulating Epo levels were also elevated throughout the study period. Our data demonstrate the potential for long-term gene expression in large animals by a single intramuscular injection of a recombinant adeno-associated virus (rAAV) vector.

Transient immunosuppression allows transgene expression following readministration of adeno-associated viral vectors

Adeno-associated viral (AAV) vectors have much promise in gene therapy. Among the many properties that make AAV an ideal vector for gene therapy are its ability to infect both dividing and nondividing cells and the longevity of expression in tissues such as brain, skeletal muscle, and liver. However, like other viral vectors, readministration of vector is limited because of the host's immune response to viral components of the vector. Using class I, class II, and CD40 ligand (CD40L)-deficient mice, we demonstrate that neutralizing antibodies to the viral capsid proteins prevent transgene expression following readministration of rAAV vectors. Transient immunosuppression of mice by treatment with antibody to CD4 at the time of primary infection allowed transgene expression after readministration of rAAV vectors to animals. Transient immunosuppression with antibody to CD40L had only a modest effect on the efficacy of readministration. The ability to readminister virus was inversely correlated with both AAV capsid enzyme-linked immunosorbent assay titers and AAV neutralizing antibody titers. These studies demonstrate that readministration of rAAV can be accomplished by down regulating the anti-AAV immune response and suggest the use of repeated administration of rAAV as a viable form of therapy for the treatment of chronic diseases.

Long-term correction of obesity and diabetes in genetically obese mice by a single intramuscular injection of recombinant adeno-associated virus encoding mouse leptin

The ob/ob mouse is genetically deficient in leptin and exhibits a phenotype that includes obesity and non-insulin-dependent diabetes mellitus. This phenotype closely resembles the morbid obesity seen in humans. In this study, we demonstrate that a single intramuscular injection of a recombinant adeno-associated virus (AAV) vector encoding mouse leptin (rAAV-leptin) in ob/ob mice leads to prevention of obesity and diabetes. The treated animals show normalization of metabolic abnormalities including hyperglycemia, insulin resistance, impaired glucose tolerance, and lethargy. The effects of a single injection have lasted through the 6-month course of the study. At all time points measured the circulating levels of leptin in the serum were similar to age-matched control C57 mice. These results demonstrate that maintenance of normal levels of leptin (2-5 ng/ml) in the circulation can prevent both the onset of obesity and associated non-insulin-dependent diabetes. Thus a single injection of a rAAV vector expressing a therapeutic gene can lead to complete and long-term correction of a genetic disorder. Our study demonstrates the long-term correction of a disease caused by a genetic defect and proves the feasibility of using rAAV-based vectors for the treatment of chronic disorders like obesity.

Genetic immunization with adeno-associated virus vectors expressing herpes simplex virus type 2 glycoproteins B and D

Intramuscular injection of mice with an adeno-associated virus (AAV) vector expressing herpes simplex virus type 2 glycoprotein B led to the generation of both gB-specific major histocompatibility complex class I-restricted cytotoxic T lymphocytes and anti-gB antibody. AAV-mediated immunization was more potent than plasmid DNA or protein in generating antibody responses.

Angiotensin II type 1 receptor signals through Raf-1 by a protein kinase C-dependent, Ras-independent mechanism

To understand the molecular mechanism by which the angiotensin II (AII) type 1 receptor (AT1 receptor) transduces its biological signal, we examined the role of various signaling molecules involved in AT1 receptor signaling in Chinese hamster ovary cells stably transfected with the AT1 receptor. AT1 receptor-transfected cells responded to AII treatment by inhibiting adenylyl cyclase, increasing the intracellular Ca2+ concentration, and activating protein kinase C (PKC) alpha and PKC epsilon. AII also activated the c-fos gene and mitogen-activated protein (MAP) kinases. The activation of PKC, the c-fos gene, and MAP kinases was blocked by inhibition of PKC induced by pretreatment with 12-O-tetradecanoylphorbol-13-acetate but not by pretreatment with pertussis toxin, suggesting that PKC couples to the activation of the the c-fos gene and MAP kinases. In addition, AII activated Raf-1 and MAP kinase kinase in a PKC-dependent manner. A dominant negative mutant of Ras had no effect on AII-induced MAP kinase or c-fos gene activation. Thus, the AT1 receptor signals through Raf-1 and its downstream signaling molecules by a PKC-dependent mechanism that does not involve Ras activation.

Viability of transgenic mice expressing a platelet derived growth factor (PDGF) antagonist in plasma

BACKGROUND

Many studies have implicated PDGF in the development of diseases such as atherosclerosis. Previously, we showed in tissue culture that the soluble extracellular domain of the PDGF beta-receptor is capable of binding BB-PDGF with high affinity; therefore antagonizing the ability of BB-PDGF to stimulate cell growth.

METHODS

This work describes the efforts of expressing the soluble extracellular domain of the PDGF beta-receptor in transgenic mice. Driven by the albumin promoter, which is activated relatively late during embryonic development, the secreted form of the PDGF receptor protein was detected in plasma of the homozygous mice at a high concentration (approximately 60 micrograms/microL or approximately 545 nm).

RESULTS

Plasma from these transgenic mice was capable of blocking PDGF-induced receptor autophosphorylation in tissue culture. The mice appeared to be healthy, demonstrating that full PDGF beta-receptor function is not required for viability.

CONCLUSION

By expressing a high level of a soluble form of the extracellular domain of the PDGF receptor in transgenic mice, we have established a novel animal model that will allow us to gain insight into the role of the PDGF receptor in vascular diseases and other diseases involving PDGF stimulated cell proliferation.

Receptor tyrosine kinase stimulates cell-matrix adhesion by phosphatidylinositol 3 kinase and phospholipase C-gamma 1 pathways

Receptor tyrosine kinases are known to be important in growth and differentiation. We have recently found that c-kit, the tyrosine kinase receptor for steel factor, also regulates cell-matrix adhesion. Because Steel factor helps regulate cell migration and localization, this may be an important biologic function. Integrin adhesiveness is regulated within minutes by c-kit. The signaling pathways for tyrosine kinase stimulation of integrin adhesiveness and their relation to pathways that regulate growth and differentiation over much longer time periods remain uncharacterized. We have studied the effector pathways by which receptor tyrosine kinases regulate cell-matrix adhesion using wild-type and mutant forms of the platelet-derived growth factor (PDGF) receptor, which is closely related to c-kit. The PDGF receptor expressed in mast cells is as potent as c-kit in stimulating adhesion to fibronectin. We show that induction of adhesion is regulated through two independent pathways of phosphatidylinositol 3 kinase (PI3K) and phospholipase C-gamma 1 (PLC gamma)-protein kinase C by elimination of autophosphorylation sites required for activation of PI3K and PLC gamma or in combination with downregulation of protein kinase C or wortmannin. By contrast, a receptor mutated in both the PI3K and PLC gamma association sites can still stimulate mast cell growth, indicating a crucial role of these effector molecules in regulating adhesion rather than cell growth.

Identification of a domain in the angiotensin II type 1 receptor determining Gq coupling by the use of receptor chimeras

The angiotensin II type 1 (AT1R) and type 2 (AT2R) receptors belong to the seven transmembrane receptor superfamily. Previous studies have suggested that the AT1R couples to a Gq signaling pathway, whereas the AT2R does not associate with Gq. To identify the role that individual intracellular domains play in AT1R function, AT1R/AT2R chimeric receptors were prepared by substitution of intracellular loops. CHO cells expressing these chimeras were used to test angiotensin II-induced c-fos expression and Ca2+ mobilization which are involved in the AT1R signaling pathway through Gq coupling. Substitution of the second intracellular loop (IC2) and the cytoplasmic tail between the two receptors did not affect AT1R function. However, exchange of the third intracellular loop (IC3) resulted in the loss of function in the AT1R and conferred to the AT2R the ability to constitutively activate the fos promoter. These findings suggest that the third intracellular loop of the AT1R is critical for Gq coupling. Substitution of discrete amino acid sequences of the third intracellular loop indicate that its N-terminal and C-terminal portions, especially the seven amino acids 219-225 in the N-terminal portion, are important for AT1R function, and that the intermediate portion of this loop is not required for Gq coupling.

Identification of Tyr-397 as the primary site of tyrosine phosphorylation and pp60src association in the focal adhesion kinase, pp125FAK

A number of cellular processes, such as proliferation, differentiation, and transformation, are regulated by cell-extracellular matrix interactions. Previous studies have identified a novel tyrosine kinase, the focal adhesion kinase p125FAK, as a component of cell adhesion plaques. p125FAK was identified as a 125-kDa tyrosine-phosphorylated protein in cells transformed by the v-src oncogene. p125FAK is an intracellular protein composed of three domains: a central domain with homology to protein tyrosine kinases, flanked by two noncatalytic domains of 400 amino acids which bear no significant homology to previously cloned proteins. p125FAK is believed to play an important regulatory role in cell adhesion because it localizes to cell adhesion plaques and because its phosphorylation on tyrosine residues is regulated by binding of cell surface integrins to the extracellular matrix. Recent studies have shown that Src, through its SH2 domain, stably associates with pp125FAK and that this association prevents dephosphorylation of pp125FAK in vitro by protein tyrosine phosphatases. In this report, we identify Tyr-397 as the primary in vivo and in vitro site of p125FAK tyrosine phosphorylation and association with Src. Substituting phenylalanine for tyrosine at position 397 significantly reduces p125FAK tyrosine phosphorylation and association with Src but does not abolish p125FAK kinase activity. In addition, p125FAK kinase is able to trans-phosphorylate Tyr-397 in vitro in a kinase-deficient p125FAK variant. Phosphorylation of Tyr-397 provides a site [Y(P)AEI] that fits the consensus sequence for the binding of Src.

Platelet-derived growth factor triggers translocation of the insulin-regulatable glucose transporter (type 4) predominantly through phosphatidylinositol 3-kinase binding sites on the receptor

Insulin is the only known hormone which rapidly stimulates glucose uptake in target tissues, mainly by translocation to the cell surface of the intracellular insulin-regulatable glucose transporter (glucose transporter type 4, GLUT4). We have developed a cell line for direct, sensitive detection of GLUT4 on the cell surface. We have suggested that insulin-activated phosphatidylinositol (PI) 3-kinase may be involved in the signaling pathway of insulin-stimulated GLUT4 translocation. We report that platelet-derived growth factor (PDGF), which stimulates PI 3-kinase activity, triggers GLUT4 translocation in Chinese hamster ovary (CHO) cells stably overexpressing the PDGF receptor and in 3T3-L1 mouse adipocytes. Using mutant PDGF receptors that cannot bind to Ras-GTPase-activating protein, phospholipase C-gamma, and PI 3-kinase, respectively, we obtained evidence that PI 3-kinase binding sites play a key role in the signaling pathway of PDGF-stimulated GLUT4 translocation in the CHO cell system.

Activation of Na+/H+ exchange by platelet-derived growth factor involves phosphatidylinositol 3'-kinase and phospholipase C gamma

The effect of site-specific mutations in the mouse platelet-derived growth factor (PDGF) beta-receptor on activation of the Na+/H+ exchanger was examined in normal murine mammary gland epithelial (NMuMG) and Chinese hamster ovary (CHO) cells. These cells, which do not normally express PDGF receptors, were stably transfected with PDGF beta-receptor cDNA. Intracellular pH and Ca2+ were monitored using fluorescent probes. In both NMuMG and CHO cells expressing wild-type PDGF beta-receptors, PDGF B/B activated the amiloride-sensitive Na+/H+ exchanger. In both cell types, cell alkalinization was reduced by approximately 50% with a receptor mutant Y708F,Y719F which cannot bind phosphatidylinositol (PI) 3'-kinase. An inhibitor of PI 3'-kinase, LY294002, also inhibited alkalinization by 43% in cells with wild-type, but not Y708F,Y719F receptors. PDGF-induced intracellular Ca2+ release was not affected by this mutation. Both alkalinization and Ca2+ release were reduced by nearly 100% with the mutant Y977F,Y989F, which cannot bind phospholipase C gamma (PLC gamma). Y739F, a mutant that fails to bind the GTPase-activating protein did not affect PDGF-induced alkalinization. In protein kinase C (PKC) down-regulated NMuMG cells (wild-type receptor), PDGF no longer activated the Na+/H+ exchanger. In contrast, in PKC down-regulated CHO cells (wild-type receptor), PDGF-induced alkalinization was attenuated by only 37%. This residual activity was unaffected by the Y708F,Y719F mutation, but was completely eliminated by removal of medium Ca2+. These findings indicate that phospholipase C gamma is essential for activation of Na+/H+ exchange. PI 3'-kinase participates in PKC-dependent activation of Na+/H+ exchange by PDGF. In CHO cells, there is a second, Ca(2+)-dependent mechanism for activation of the exchanger.

Evidence for both tyrosine kinase and G-protein-coupled pathways leading to starfish egg activation

To investigate possible pathways leading to egg activation at fertilization, the ability of exogenously introduced tyrosine kinase and G-protein-coupled receptors to mimic events of fertilization was examined. Oocytes of the starfish Asterina miniata were injected with RNA for a chimeric receptor consisting of the extracellular domain of the beta form of the mouse platelet-derived growth factor (PDGF) receptor and the transmembrane/intracellular domain of the human fibroblast growth factor (FGF) receptor, or with RNA for the rat serotonin 1c receptor. These oocytes were cultured for 1 to 3 days and then matured with 1-methyladenine. In response to PDGF or serotonin, the injected eggs underwent responses like those at fertilization: cortical granule exocytosis, a rise in intracellular free calcium, and DNA synthesis. Some of these artificially activated eggs cleaved, and some of the PDGF-activated eggs were observed to form larvae. A PDGF/FGF receptor with a point mutation which eliminated its ability to interact with phospholipase C-gamma did not cause fertilization-like responses. Thus components of a signaling pathway involving phospholipase C-gamma, characteristic of tyrosine kinase receptors, as well as components of a pathway involving a G-protein and phospholipase C-beta, characteristic of G-protein-coupled receptors, appear to be present in starfish eggs. Either or both could function in egg activation at fertilization.

The interaction of small domains between the subunits of phosphatidylinositol 3-kinase determines enzyme activity

Previous studies have suggested that the two subunits of phosphatidylinositol (PI) 3-kinase, p85 and p110, function as localizing and catalytic subunits, respectively. Using recombinant p85 and p110 molecules, we have reconstituted the specific interaction between the two subunits of mouse PI 3-kinase in cells and in vitro. We have previously shown that the region between the two Src homology 2 (SH2) domains of p85 is able to form a functional complex with the 110-kDa subunit in vivo. In this report, we identify the corresponding domain in p110 which directs the binding to p85. We demonstrate that the interactive domains in p85 and p110 are less than 103 and 124 amino acids, respectively, in size. We also show that the association of p85 and p110 mediated by these domains is critical for PI 3-kinase activity. Surprisingly, a complex between a 102-amino-acid segment of p85 and the full-length p110 molecule is catalytically active, whereas p110 alone has no activity. In addition to the catalytic domain in the carboxy-terminal region, 123 amino acids at the amino terminus of p110 were required for catalytic activity and were sufficient for the interaction with p85. These results indicate that the 85-kDa subunit, previously thought to have only a linking role in localizing the p110 catalytic subunit, is an important component of the catalytic complex.

Regulation of chemotaxis by the platelet-derived growth factor receptor-beta

Chemotaxis is an important component of wound healing, development, immunity and metastasis, yet the signalling pathways that mediate chemotaxis are poorly understood. Platelet-derived growth factor (PDGF) acts both as a mitogen and a chemoattractant. Upon stimulation, the tyrosine kinase PDGF receptor-beta (PDGFR-beta) autophosphorylates and forms a complex that includes SII2(Src homology 2)-domain-containing proteins such as the phosphatidylinositol-specific phospholipase C-gamma, Ras-GTPase-activating protein (GAP), and phosphatidylinositol-3-OH kinase. Specific tyrosine-to-phenylalanine substitutions in the PDGFR-beta can prevent binding of one SH2-domain-containing protein without affecting binding of other receptor-associated proteins. Here we use phospholipase C-gamma and PDGFR-beta mutants to map specific tyrosines involved in both positive and negative regulation of chemotaxis towards the PDGF-BB homodimer. Our results indicate that a delicate balance of migration-promoting (phospholipase C-gamma and phosphatidylinositol-3-OH kinase) and migration-suppressing (GAP) activities are recruited by the PDGFR-beta to drive chemotaxis towards PDGF-BB.

Dominant-negative mutations of platelet-derived growth factor (PDGF) receptors. Inhibition of receptor function by ligand-dependent formation of heterodimers between PDGF alpha- and beta-receptors

We showed previously that a truncated form of the platelet-derived growth factor (PDGF) beta-receptor lacking its kinase region can form a nonfunctional heterodimer with the wild-type beta-receptor and thereby inhibit its signal transduction. In this paper we investigated whether the truncated form of either alpha- or beta-receptor could block the function of the other type of wild-type PDGF receptor. When the truncated alpha-receptor was expressed in Xenopus oocytes in excess over either the wild-type alpha- or beta-receptor, the Ca2+ mobilization signal elicited by either the wild-type alpha- or beta-receptor was completely blocked. The truncated beta-receptor abolished signaling by the wild-type alpha-receptor in response to PDGF-AB or -BB. However signal transduction by the alpha-receptor in response to PDGF-AA was not affected by the truncated beta-receptor. In the presence of PDGF-AB or -BB, both the wild-type and truncated beta-receptors formed a heterologous complex with the alpha-receptor in intact cells. A kinase-inactive beta-receptor (an ATP-binding site mutation) became cross-phosphorylated on tyrosine residues by the co-expressed wild-type alpha-receptor in response to PDGF-AB or -BB but not in response to PDGF-AA. These findings indicate that the alpha-and beta-receptors interact in response to PDGF-AB or -BB and are consistent with the hypothesis that the truncated alpha-receptor inhibits function of the wild-type beta-receptor through formation of a ligand-dependent nonfunctional alpha beta-heterodimer. A similar heterodimer can form between the truncated beta-receptor and the wild-type alpha-receptor. These observations provide useful information for future studies using dominant-negative mutations of PDGF receptors to selectively inhibit the actions of specific PDGFs in animals.

Signal transduction through a biomolecular receptor tyrosine protein kinase composed of a platelet-derived growth factor receptor-CD4 chimera and the nonreceptor tyrosine protein kinase Lck

We have generated a novel "receptor tyrosine kinase" by fusing the extracellular and transmembrane domain of the mouse platelet-derived growth factor receptor (PDGFR) to the cytoplasmic domain of CD4 and coexpressing the construct with the murine cytoplasmic tyrosine protein kinase p56lck. NMuMG cells, which are mouse mammary gland epithelial cells that lack endogenous platelet-derived growth factor (PDGF) receptor expression, were stably transfected with both PDGFR-CD4 and p56lck. The PDGFR-CD4 chimeric protein was expressed at the cell surface and formed a complex with p56lck. Addition of PDGF to these cells led to increased tyrosine phosphorylation of a 56-kDa protein likely to be p56lck and several unidentified cellular proteins. The enzymatic activity of p56lck was increased after treatment with PDGF, indicating that dimerization (or oligomerization) mediated by ligand binding at the cell surface is capable of inducing the activation not only of receptor tyrosine kinases but nonreceptor tyrosine kinases as well. However, the PDGFR-CD4.p56lck complex was, in contrast to the wild type PDGF receptor, not able to induce a PDGF-dependent mitogenic response or DNA synthesis in NMuMG cells. Analysis of several known substrates of the PDGFR-signaling pathway indicates an early block in the transduction of the signal generated by p56lck.

A region of the 85-kilodalton (kDa) subunit of phosphatidylinositol 3-kinase binds the 110-kDa catalytic subunit in vivo

Phosphatidylinositol (PI) 3-kinase is a heterodimer consisting of an 85-kDa subunit (p85) and 110-kDa subunit (p110). The 85-kDa noncatalytic subunit, which contains two Src homology 2 (SH2) domains, one SH3 domain, and a domain homologous to the carboxy terminus of the breakpoint cluster region gene product, is known to mediate the association of the PI 3-kinase complex with activated growth factor receptors. We previously demonstrated that the C-terminal SH2 domain of p85 is responsible for the interaction of PI 3-kinase with phosphorylated platelet-derived growth factor receptor. To define the region in p85 that directs the complex formation with the PI 3-kinase catalytic subunit, a series of truncated p85 mutants was analyzed for association with p110 in vivo. We found that a fragment of p85 containing the region between the two SH2 domains was sufficient to promote the interaction with p110 in vivo. The complex between the fragment of p85 and p110 had PI 3-kinase activity that was comparable in magnitude to the activity of p110 associated with full-length p85. The binding with p110 was abolished when this domain in p85 was disrupted. These results identify a novel structural and functional element that is responsible for localizing the catalytic subunit of PI 3-kinase.

Platelet-derived growth factor receptor mediates activation of ras through different signaling pathways in different cell types

A series of pieces of evidence have shown that Ras protein acts as a transducer of the platelet-derived growth factor (PDGF) receptor-mediated signaling pathway: (i) formation of Ras.GTP is detected immediately on PDGF stimulation, and (ii) a dominant inhibitory mutant Ras, as well as a neutralizing anti-Ras antibody, can interfere with PDGF-induced responses. On the other hand, several signal transducing molecules including phosphatidylinositol 3-kinase (PI3-K), GTPase-activating protein (GAP), and phospholipase C gamma (PLC gamma) bind directly to the PDGF receptor and become tyrosine phosphorylated. Recently, it was shown that specific phosphorylated tyrosines of the PDGF receptor are responsible for interaction between the receptor and each signaling molecule. However, the roles of these signaling molecules have not been elucidated, and it remains unclear which molecules are implicated in the Ras pathway. In this study, we measured Ras activation in cell lines expressing mutant PDGF receptors that are deficient in coupling with specific molecules. In fibroblast CHO cells, a mutant receptor (Y708F/Y719F [PI3-K-binding sites]) was unable to stimulate Ras, whereas another mutant (Y739F [the GAP-binding site]) could do so, suggesting an indispensable role of PI3-K or a protein that binds to the same sites as PI3-K for PDGF-stimulated Ras activation. By contrast, both of the above mutants were capable of stimulating Ras protein in a pro-B-cell line, BaF3. Furthermore, a mutant receptor (Y977F/Y989F [PLC gamma-binding sites]) could fully activate Ras, and the direct activation of protein kinase C and calcium mobilization had almost no effect on the GDP/GTP state of Ras in this cell line. These results suggest that, in the pro-B-cell transfectants, each of the above pathways (PI3-K, GAP, and PLC gamma) can be eliminated without a loss of Ras activation. It remains unclear whether another unknown essential pathway which regulates Ras protein exists within BaF3 cells. Therefore, it is likely that several different PDGF receptor-mediated signaling pathways function upstream of Ras, and the extent of the contribution of each pathway for the regulation of Ras may differ among different cell types.

Modification of the 85-kilodalton subunit of phosphatidylinositol-3 kinase in platelet-derived growth factor-stimulated cells

The activated platelet-derived growth factor (PDGF) receptor physically associates with p85, a subunit of phosphatidylinositol-3 kinase. Although this interaction may activate phosphatidylinositol-kinase and is crucial for PDGF-induced mitogenesis, it has not been shown whether p85 is modified in the process. p85 contains two SH2 (Src homology) domains, designated SH2-N and SH2-C. Recent experiments have shown that the SH2-C domain alone determines high-affinity binding of p85 to the PDGF receptor. The function of SH2-N, which binds receptors with lower affinity, is unknown. In this study, using a receptor-blotting technique, we find that p85 is modified by PDGF stimulation of intact cells. This modification involves inhibition of binding of the SH2-N region of p85 to the PDGF receptor. Studies with vanadate suggest that tyrosine phosphorylation of p85 is responsible for the modification of p85 detected by receptor blotting. Furthermore, recombinant p85 is modified in a similar manner when it is tyrosine phosphorylated in vitro by PDGF receptors. Tyrosine phosphorylation of p85 does not block binding of the SH2-C domain and therefore does not release p85 from high-affinity binding sites on the receptor in vitro. Instead, phosphorylation may regulate the ability of the SH2-N of p85 to bind to a different portion of the PDGF receptor or to another molecule in the signaling complex. This study provides the first evidence that p85 is tyrosine phosphorylated upon PDGF stimulation of cells and suggests that tyrosine phosphorylation of p85 regulates its activity or its interaction with other proteins.

Distinct phosphotyrosines on a growth factor receptor bind to specific molecules that mediate different signaling pathways

The receptor for platelet-derived growth factor (PDGF) binds two proteins containing SH2 domains, GTPase activating protein (GAP) and phosphatidylinositol 3-kinase (PI3-kinase). The sites on the receptor that mediate this interaction were identified by using phosphotyrosine-containing peptides representing receptor sequences to block specifically binding of either PI3-kinase or GAP. These results suggested that PI3-kinase binds two phosphotyrosine residues, each located in a 5 aa motif with an essential methionine at the fourth position C-terminal to the tyrosine. Point mutations at these sites caused a selective elimination of PI3-kinase binding and loss of PDGF-stimulated DNA synthesis. Mutation of the binding site for GAP prevented the receptor from associating with or phosphorylating GAP, but had no effect on PI3-kinase binding and little effect on DNA synthesis. Therefore, GAP and PI3-kinase interact with the receptor by binding to different phosphotyrosine-containing sequence motifs.

The C-terminal SH2 domain of p85 accounts for the high affinity and specificity of the binding of phosphatidylinositol 3-kinase to phosphorylated platelet-derived growth factor beta receptor

Upon stimulation by its ligand, the platelet-derived growth factor (PDGF) receptor associates with the 85-kDa subunit of phosphatidylinositol (PI) 3-kinase. The 85-kDa protein (p85) contains two Src homology 2 (SH2) domains and one SH3 domain. To define the part of p85 that interacts with the PDGF receptor, a series of truncated p85 mutants was analyzed for association with immobilized PDGF receptor in vitro. We found that a fragment of p85 that contains a single Src homology domain, the C-terminal SH2 domain (SH2-C), was sufficient for directing the high-affinity interaction with the receptor. Half-maximal binding of SH2-C to the receptor was observed at an SH2-C concentration of 0.06 nM. SH2-C, like full-length p85, was able to distinguish between wild-type PDGF receptor and a mutant receptor lacking the PI 3-kinase binding site. An excess of SH2-C blocked binding of full-length p85 and PI 3-kinase to the receptor but did not interfere with the binding of two other SH2-containing proteins, phospholipase C-gamma and GTPase-activating protein. These results demonstrate that a region of p85 containing a single SH2 domain accounts both for the high affinity and specificity of binding of PI 3-kinase to the PDGF receptor.

The fms-like tyrosine kinase, a receptor for vascular endothelial growth factor

The fms-like tyrosine kinase (Flt) is a transmembrane receptor in the tyrosine kinase family. Expression of flt complementary DNA in COS cells conferred specific, high-affinity binding of vascular endothelial growth factor, also known as vascular permeability factor (VEGF-VPF), a factor that induces vascular permeability when injected in the guinea pig skin and stimulates endothelial cell proliferation. Expression of Flt in Xenopus laevis oocytes caused the oocytes to release calcium in response to VEGF-VPF. These findings show that flt encodes a receptor for VEGF-VPF.

Inhibition of PDGF beta receptor signal transduction by coexpression of a truncated receptor

A mutated form of the platelet-derived growth factor (PDGF) beta receptor lacking most of its cytoplasmic domain was tested for its ability to block wild-type PDGF receptor function. PDGF induced the formation of complexes consisting of wild-type and truncated receptors. Such complexes were defective in autophosphorylation. When truncated receptors were expressed in excess compared to wild-type receptors, stimulation by PDGF of receptor autophosphorylation, association of phosphatidylinositol-3 kinase with the receptor, and calcium mobilization were blocked. Thus, a truncated receptor can inactivate wild-type receptor function by forming ligand-dependent receptor complexes (probably heterodimers) that are incapable of mediating the early steps of signal transduction.

[Symptomatic right aortic arch in adults]

A 54-year-old female presented with dyspnea on exertion and wheezing, with a negative response to inhaled beta-2 stimulating drugs and intravenous glucocorticoids. The major values of basal spirometric study were normal; however, the morphology of the flow-volume curve showed intrathoracic obstruction of large airways, with negative response to beta stimulating drugs. The bronchial provocative test with histamine was normal. Chest radiographs only showed a mild widening of the median mediastinum with left anterior tracheal displacement. Nuclear magnetic resonance showed a right aortic arch with tracheal strangulation.

cDNA cloning of a novel 85 kd protein that has SH2 domains and regulates binding of PI3-kinase to the PDGF beta-receptor

Using immobilized PDGF receptor as an affinity reagent, we purified an 85 kd protein (p85) from cell lysates and we cloned its cDNA. The protein contains an SH3 domain and two SH2 domains that are homologous to domains found in several receptor-associated enzymes. Recombinant p85 overexpressed in mammalian cells inhibited the binding of endogenous p85 and a 110 kd protein to the receptor and also blocked the association of PI3-kinase activity with the receptor. Experiments with receptor mutants and with short peptides derived from the kinase insert region of the PDGF receptor showed that the recombinant p85 binds to a well-defined phosphotyrosine-containing sequence of the receptor. p85 appears to be the subunit of PI3-kinase that links the enzyme to the ligand-activated receptor.

A phosphatidylinositol-3 kinase binds to platelet-derived growth factor receptors through a specific receptor sequence containing phosphotyrosine

Platelet-derived growth factor (PDGF) stimulates autophosphorylation of the PDGF receptor and association of the receptor with several cytoplasmic molecules, including phosphatidylinositol-3 kinase (PI3 kinase). In this study we examined the association of PI3 kinase with immunoprecipitated autophosphorylated PDGF receptor in vitro. The PI3 kinase from cell lysates bound to the wild-type receptor but not to a mutant receptor that had a deletion of the kinase insert region. A protein of an apparent size of 85 kDa bound to the receptor, consistent with previous observations that a protein of this size is associated with PI3 kinase activity. In addition, 110- and 74-kDa proteins bound to the phosphorylated receptor. Dephosphorylated receptors lost the ability to bind PI3 kinase activity as well as the 85-kDa protein. A 20-amino-acid peptide composed of a sequence in the kinase insert region that included one of the autophosphorylation sites of the receptor (tyrosine 719) as well as a nearby tyrosine (Y708) blocked the binding of PI3 kinase to the receptor, but only when the peptide was phosphorylated on tyrosine residues. A scrambled version of the peptide did not block PI3 kinase binding to the receptor even when it was phosphorylated on tyrosine. These tyrosine-phosphorylated peptides did not block binding of phospholipase C-gamma or GTPase-activating protein to the receptor. In separate experiments (receptor blots), soluble radiolabeled receptor bound specifically to an 85-kDa protein present in sodium dodecyl sulfate-polyacrylamide gel electrophoresis-fractionated 3T3 cell lysates that were transferred to nitrocellulose paper. The binding was blocked by the same tyrosine-phosphorylated peptides that prevented binding of PI3 kinase activity to immobilized receptors. These findings show that the PDGF receptor binds directly to an 85-kDa protein and to a PI3 kinase activity through specific sequences in the kinase insert region. The association of a 110-kDa protein with the receptor also involve these sequences, suggesting that this protein may be a subunit of the PI3 kinase. Phosphotyrosine is an essential structure required for the interactions of these proteins with the PDGF receptor.

Chromosomal localization of the gene for AA-type platelet-derived growth factor receptor (PDGFRA) in humans and mice

The full-length cDNA of the receptor for human AA-type platelet-derived growth factor (PDGF) was used to assign the PDGFRA gene to region q11----q21 of human chromosome 4 and to mouse Chromosome 5 by somatic cell hybrid analysis. Since the same region also contains the c-kit oncogene homolog KIT, we carried out pulsed-field gel electrophoresis to determine the physical distance between the two genes in human DNA. The two probes, when successively applied to the same filters, hybridized to a 450-kb EagI-fragment but not to other common restriction fragments. The genes are separated by at least one NotI, one XhoI, and one SalI site.

Interactions of growth factor receptors with cytoplasmic signaling molecules

The first step in the action of many growth factors is to bind to the receptors and to stimulate autophosphorylation of the receptors on tyrosine residues. The receptors then form high-affinity physical complexes with cytoplasmic signaling molecules (Fig. 8). It is not clear whether the function of the complexes is to localize signaling molecules at the plasma membrane or to position the molecules to be favored substrates of the receptor. It is also not necessarily true that each receptor molecule binds more than one signaling molecule at a time. We have shown that each of the signaling molecules that binds to the PDGF receptor recognizes a specific site in the receptor cytoplasmic domain. A phosphotyrosine on the receptor is an important determinant of the interaction with the signaling molecule. However, the specificity of the interaction is determined by the receptor sequence surrounding each phosphotyrosine, especially the sequences on the carboxy-terminal side of the tyrosine. SH2 regions of the signaling molecules appear to bind directly to the specific recognition sequences on the receptor. Thus, the intracellular protein-protein interactions that depend on SH2 domains binding to phosphotyrosine are not as random as we once believed but are part of a highly specific system of interactions between tyrosine-phosphorylated proteins and SH2-containing signaling proteins. A major role of tyrosine kinase appears to be in creating specific recognition sites that bind SH2 domains. By elucidating the specificity of these interactions, we have been able to selectively block some interactions while allowing others to occur.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of phosphotyrosine residues during protein sequence analysis

Synthetic tyrosine-phosphorylated peptides were subjected to protein sequence analysis using a gas-phase sequencer and on-line phenylthiohydantoin (PTH) amino acid analysis. Our data show that phosphotyrosine is stable to the gas-phase sequencing chemistry and can be detected as its PTH-derivative during routine sequence analysis without the need of prior tyrosine radiolabeling.

Platelet derived growth factor (PDGF) autocrine components in human tumor cell lines

Tumor cells may stimulate their own proliferation through an autocrine mechanism by simultaneously producing growth factors and growth factor receptors. We now report that numerous human tumor-derived cell lines simultaneously express the genes for platelet-derived growth factor (PDGF) A and B chains and the PDGF receptor (PDGF-R). Measurement of mRNA transcribed from these genes showed that among 16 malignant glioma cell lines tested, 15 expressed the PDGF A gene, 12 expressed the PDGF B gene, and 13 expressed the PDGF-R gene. Of three osteosarcoma lines, three expressed PDGF A, two expressed PDGF B, and three expressed PDGF-R. For eight malignant melanoma lines, seven expressed PDGF A, five expressed PDGF B, and three expressed PDGF-R genes. Thus, 13 of 16 malignant glioma, 3 of 3 osteosarcomas, and 3 of 8 malignant melanoma cell lines expressed the PDGF receptor gene and either or both PDGF genes. Five cell lines were tested for production of biologically active PDGF and PDGF receptor protein. Media conditioned by each of the five cell lines induced tyrosine phosphorylation of a protein identical in size to the PDGF receptor. These five cell lines also produced PDGF receptor protein as measured by Western blot analysis or metabolic labeling and immunoprecipitation using PDGF-R antibodies. The PDGF receptors of these cell lines were activated by human platelet PDGF or by recombinant AA or BB homodimers. Intracellular interaction of these receptors with the growth factor simultaneously produced may provide continuous stimulation to the proliferation of these cells.

Mutations of the platelet-derived growth factor receptor that cause a loss of ligand-induced conformational change, subtle changes in kinase activity, and impaired ability to stimulate DNA synthesis

The wild-type and two mitogenic-defective mutants of the type beta receptor for platelet-derived growth factor (PDGF) were expressed in Chinese hamster ovary cells. In the first mutant, delta Ki, 82 of 104 amino acids in the kinase insert region were deleted. This mutant was recently reported to be defective in mediating DNA synthesis. In the second mutant, Y825F, tyrosine 825 was converted to phenylalanine by a point mutation. We report here that this mutant is also defective in mediating PDGF-stimulated DNA synthesis. Both mutants were capable of eliciting many of the early responses to PDGF, including receptor autophosphorylation. However, neither mutant was capable of undergoing PDGF-stimulated change in receptor conformation or of phosphorylating exogenous substrate in an in vitro assay. These data suggest that changes in receptor conformation and efficient utilization of specific tyrosine kinase substrates are important for the stimulation of cell proliferation of PDGF and that phosphorylation of tyrosine 825 may be involved in signal transduction.

Direct activation of the serine/threonine kinase activity of Raf-1 through tyrosine phosphorylation by the PDGF beta-receptor

We have examined the interaction between the serine/threonine kinase proto-oncogene product Raf-1 and the tyrosine kinase PDGF beta-receptor. Raf-1 tyrosine phosphorylation and kinase activity were increased by PDGF treatment of 3T3 cells or CHO cells expressing wild-type PDGF receptors but not mutant receptors defective in transmitting mitogenic signals, suggesting that the increase in Raf-1 kinase activity is a significant event in PDGF-induced mitogenesis. Concurrent with these increases, Raf-1 associated with the ligand-activated PDGF receptor. Furthermore, both mammalian Raf-1 and Raf-1 expressed using a recombinant baculoviral vector, associated in vitro with baculoviral-expressed PDGF receptor. This association was markedly decreased by prior phosphatase treatment of the receptor. Following incubation of partially purified baculoviral-expressed PDGF receptor with partially purified Raf-1, Raf-1 became phosphorylated on tyrosine and its serine/threonine kinase activity increased 4- to 6-fold. This is the first demonstration of the direct modulation of a protein activity by a growth factor receptor tyrosine kinase.

Role of phosphatidylinositol kinase in PDGF receptor signal transduction

The molecules with which the platelet-derived growth factor (PDGF) receptor interacts to elicit the biochemical reactions responsible for cell proliferation have not been identified. Antisera directed against specific PDGF receptor peptides coprecipitated a phosphatidylinositol (PI) kinase and the PDGF receptor. Immunoprecipitates from PDGF-stimulated cells contained 10 to 50 times as much PI kinase as those from unstimulated cells. Mutation of the PDGF receptor by deletion of its kinase insert region resulted in a receptor markedly less effective than the wild type in eliciting cell proliferation and defective in PDGF-stimulated PI kinase, but still capable of PDGF-induced receptor autophosphorylation and phosphoinositide hydrolysis. These data show that the PDGF receptor is physically associated with a PDGF-sensitive PI kinase that is distinct from tyrosine kinase and is not required for PDGF-induced PI hydrolysis. The finding that the mutant PDGF receptor missing the kinase insert domain elicited known early biochemical responses to PDGF, but did not associate with or regulate PI kinase, suggests a novel role for the receptor-associated PI kinase in the transmission of mitogenic signals.

Role of tyrosine kinase and membrane-spanning domains in signal transduction by the platelet-derived growth factor receptor

Three types of mutations were introduced into the platelet-derived growth factor (PDGF) receptor to cause a loss of PDGF-stimulated tyrosine kinase activity: (i) a point mutation of the ATP-binding site, (ii) a deletion of the carboxyl-terminal region, and (iii) replacement of the membrane-spanning sequences by analogous transmembrane sequences of other receptors. Transfectants expressing mutated receptors bind, 125I-labeled PDGF with a high affinity but had no PDGF-sensitive tyrosine kinase activity, phosphatidylinositol turnover, increase in the intracellular calcium concentration, change in cellular pH, or stimulation of DNA synthesis. However, PDGF-induced receptor down regulation was normal in the mutant cells. These results indicate that the transmembrane sequence has a specific signal-transducing function other than merely serving as a membrane anchor and that the receptor kinase activity is necessary for most responses to PDGF but is not required for receptor down regulation.

Phosphatidylinositol linkage of a truncated form of the platelet-derived growth factor receptor

The platelet-derived growth factor (PDGF) receptor is usually anchored to the plasma membrane through a membrane-spanning hydrophobic amino acid sequence that splits the molecule into two approximately equal pieces, an amino-terminal external domain that contains the binding site for PDGF and a carboxyl-terminal cytoplasmic domain that includes the tyrosine kinase coding sequences. Here we report the expression of a truncated PDGF receptor that consists of the extracellular domain without the transmembrane and cytoplasmic domains. Unexpectedly, this form of the receptor that lacks a hydrophobic membrane-anchoring sequence was bound to the membrane and was not secreted into the culture media. Conventional methods to dissociate noncovalent protein-protein interactions failed to release the protein from the membrane. When the transmembrane and cytoplasmic sequences were artificially deleted from the PDGF receptor, the truncated extracellular domain was anchored to the membrane through phospholipids and could be released by phospholipase C treatment. This truncated form of the receptor bound PDGF with an affinity 5-20-fold lower than the full-length receptor.

Ligand activation causes a phosphorylation-dependent change in platelet-derived growth factor receptor conformation

The effect of ligand binding on platelet-derived growth factor (PDGF) receptor conformation was examined using peptide antibodies directed against specific receptor domains. Antiserum 83, which was directed to the receptor's carboxyl terminus (residues 934-951), preferentially immunoprecipitated the ligand-activated form of the PDGF receptor from 35S-labeled BALB/c 3T3 cells. By contrast, two antisera directed against other receptor sequences precipitated unactivated and activated receptors equally well. Denatured receptors were recognized equally by all antisera, even 83. Thus, ligand activation caused a change in PDGF receptor conformation that enhanced accessibility of the antibody to the carboxyl terminus. The activated receptor conformation was induced by three different forms of PDGF (AA and BB homodimers and AB heterodimers) and was reversed by suramin, a polyanionic compound that dissociates PDGF from the receptor. The inhibitory effect of suramin on receptor conformation was abolished by the phosphatase inhibitor, sodium orthovanadate, suggesting that receptor phosphorylation mediated the conformational change. In a cell-free assay, the change in receptor conformation was induced by PDGF only in the presence of ATP and was inhibited by adenyl-5'-yl imidodiphosphate, a nonhydrolyzable analog of ATP. The functional significance of receptor conformation was examined in Chinese hamster ovary fibroblasts transfected with wild-type or mutated forms of the PDGF receptor. When receptor tyrosine kinase activity was abolished by a mutation of the ATP binding site the receptor no longer underwent PDGF-induced conformational change and did not mediate PDGF-induced mitogenesis even though 125I-PDGF binding was normal. These findings show that ligand binding elicits a phosphorylation-dependent change in PDGF receptor conformation that may be important for receptor function.

A PDGF receptor domain essential for mitogenesis but not for many other responses to PDGF

The receptors for mesenchymal growth factors contain tyrosine kinase coding sequences and exhibit ligand-activated tyrosine kinase activity. A variety of mutations of the epidermal growth factor, insulin and platelet-derived growth factor (PDGF) (manuscript in preparation) receptors that have resulted in a loss of tyrosine kinase activity have produced a concomitant loss of growth factor-stimulated DNA synthesis. Comparison of amino acid sequences of tyrosine kinases shows that these regions in the PDGF receptors in mouse and human contain an insert of unknown function. We have deleted this region, and expressed the altered form of the receptor in fibroblasts which lack PDGF receptors. This had no effect on a number of responses to PDGF, but cells bearing the mutant receptor did not proliferate or synthesize DNA in response to PDGF. This demonstrates that the insert is essential in PDGF-induced mitogenesis, and that PDGF stimulation of the mutant receptor tyrosine kinase and phosphatidylinositol turnover are not sufficient to elicit a mitogenic response to PDGF.

A common PDGF receptor is activated by homodimeric A and B forms of PDGF

The human platelet-derived growth factor (PDGF) receptor complementary DNA was cloned and expressed by transfection of Chinese hamster ovary (CHO) fibroblasts. The ability of CHO cells expressing the human receptor complementary DNA (CHO-HR5) to interact with different recombinant forms of PDGF (AA and BB homodimers) was tested. Both forms of PDGF bind to the transfected receptor, stimulate the receptor tyrosine kinase activity, and elicit a mitogenic response in a manner that was indistinguishable from the responses of Balb/c 3T3 cells to AA and BB forms of PDGF can be attributed to a single type of receptor and show that the AA form, like the BB form, is a true mitogen.

Platelet-derived growth factor receptors expressed by cDNA transfection couple to a diverse group of cellular responses associated with cell proliferation

We have expressed the mouse platelet-derived growth factor (PDGF) receptor cDNA under transcriptional control of simian virus 40 promoter in Chinese hamster ovary cells, a cell type that lacks PDGF receptors. Stable transfectants expressed receptor protein that was recognized by antireceptor antibodies. PDGF stimulated a diverse group of responses in the transfected cells including tyrosine phosphorylation of the receptor and a 32-kDa cellular substrate, receptor internalization, phosphoinositide turnover, cytoplasmic alkalinization, changes in the intracellular calcium concentration, thymidine uptake into DNA, and cell division. There were no responses to PDGF in the control Chinese hamster ovary cells. After prolonged exposure to PDGF, the transfectants became refractory to subsequent PDGF stimulation and the receptor protein was no longer detectable. These data validate the authenticity of the PDGF receptor cDNA, show that transfection of a single type of receptor cDNA sequence confers PDGF sensitivity to a diverse group of cellular responses, and demonstrate that effector molecules that mediate the PDGF responses are present in cells that lack PDGF receptors.

The stimulation of paracrine and autocrine mitogenic pathways by the platelet-derived growth factor receptor

Platelet-derived growth factor (PDGF) and PDGF-like polypeptides stimulate cell proliferation through paracrine and autocrine pathways. Each of these pathways is mediated by the PDGF receptor. Recently, cDNA clones for the receptor have been isolated and sequenced. The receptor gene on chromosome 5 is transcribed into a single 5.2 kb mRNA. The translated product, which is processed through at least one identifiable precursor, is expressed at the cell surface and is rapidly degraded. When activated by PDGF, the receptor mediates a group of diverse intracellular reactions. The receptor domains that mediate tyrosine kinase activity can be identified in the amino acid sequence of the receptor. However, the domains that mediate other PDGF-stimulated responses, such as turnover of phosphatidylinositol and enhanced expression of the c-myc and c-fos genes, have not been determined. Recently, a full-length receptor cDNA clone has been expressed in cells that normally lack PDGF receptors. This expression system should provide an approach to studies of the function of specific receptor domains and should help determine the relationship among the intracellular reactions stimulated by PDGF.