Regression activity that is naturally present in vitreous becomes ineffective as patients develop proliferative diabetic retinopathy.

AIMS/HYPOTHESIS: The realisation that targeting agents in the vitreous is an effective approach to treating patients with diabetic retinopathy (DR) has increased awareness that changes in the composition/bioactivity of the vitreous is a contributor to the pathogenesis of DR. The overall goal of this study was to test the hypothesis that the vitreous has regression activity, and that lysophosphatidic acid (LPA) contributes to such activity. LPA is a bioactive phospholipid present in many biological fluids, and has been recently appreciated for its ability to promote regression of blood vessels. METHODS: Vitreous-mediated regression was monitored on tubes organised from primary retinal endothelial cells or neovessels that sprouted from retinal explants. LPA was quantified radioenzymatically. RESULTS: Bovine and human vitreous promoted regression of retinal explant vessels and of tubes organised from primary retinal endothelial cells. LPA was a substantial component of this regression activity. Comparing the regression activities of vitreous from patients with different stages of DR revealed that, as patients developed proliferative diabetic retinopathy (PDR), vitreous lost its ability to promote regression, even though the amount of LPA did not change. The underlying mechanism was a PDR-vitreous-mediated insensitivity to LPA, which could be overcome pharmacologically. CONCLUSIONS/INTERPRETATION: Our findings suggest that a decline in the responsiveness to regression factors such as LPA, which are naturally present in the vitreous, contributes to the pathogenesis of PDR.

Anti-apoptotic gene therapy prolongs survival of corneal endothelial cells during storage.

Corneal transplantation is the most common form of grafting performed worldwide. Corneal endothelial cells (EC) form a monolayer in the posterior portion of the cornea and are essential for corneal transparency. EC loss during storage before transplantation is a principal reason for rendering donor tissue unsuitable for transplantation, and apoptosis has been shown to be the major contributor to EC loss during storage and after transplantation. Therefore, the potential use of anti-apoptotic gene therapy to promote both graft storage and graft survival is of major interest. The goal of this study was to transduce human donor corneas in vitro to enhance EC survival during storage conditions used in eye banking. We utilized a lentiviral vector to perform gene transfer of baculoviral p35 or mammalian Bcl-xL to corneal endothelium in different storage conditions utilizing a lentiviral vector. Our results show significantly enhanced survival and prolonged retention of physiological EC morphology in cells expressing either p35 or Bcl-xL. The clinical application of this technology could lead to a higher availability of donor tissue for transplantation, extend storage periods and reduce graft failure after transplantation.

Growth-factor-dependent mitogenesis requires two distinct phases of signalling.

Prolonged and continuous exposure to growth factors is required to commit cells to the cell cycle. Here we show that the prolonged requirement for growth factor can be replaced with two short pulses of mitogen. The first pulse of growth factor moves the cell through the initial segment of the G0 to S interval. This initial pulse also makes cells responsive to a second pulse of growth factor, which engages components of the cell-cycle machinery necessary for progression into S phase. We also show that activation of MAP kinase kinase (MEK) and induction of the transcription factor c-Myc are sufficient to drive the first, but not the second, phase of signalling. Furthermore, synthetic phosphatidylinositol-3-OH kinase (PI(3)K) lipid products are sufficient to drive the second phase of signalling, but not the first. These findings suggest that there is a common signalling cascade by which mitogens drive arrested cells into the cell cycle, and that this cascade involves the temporally coordinated input of MEK, c-Myc and PI(3)K.

[Viral vectors for gene delivery to corneal endothelial cells]. / Virale Vektoren für den Gentransfer in korneale Endothelzellen.

BACKGROUND: Corneal endothelium is an interesting target for in vitro gene transfer strategies as it is readily accessible thanks to its anatomic structure as a monolayer and its direct contact to culture medium. Whereas the use of adenoviruses as viral vectors (carriers) to endothelial cells (EC) has been described as problematic as to its immunogenicity, lentiviruses and adeno-associated viruses (AAV) are potent vectors for the transfer of genetic DNA into EC. Lentiviral vectors, developed on the basis of HI-viruses, can integrate the transferred gene into the host DNA and thus lead to a permanent protein expression. Evaluating apathogen alternatives to lentiviral vectors for humans, we herein compared non-integrating AAV to lentiviral gene transfer. MATERIALS AND METHODS: A comparison was made of the kinetics of expression of a green fluorescent protein after transduction using a lentiviral vector and AAV 2 / 2 in a murine EC line, human EC line and human primary cells (flow cytometry). A proof of principle experiment was conducted to demonstrate the function after lentiviral gene transfer of the anti-apoptotic gene Bcl-xL. RESULTS: The kinetics of protein expression after transduction of EC using a lentiviral or an AAV vector show fundamental differences. Contrary to gene transfer using AAV, a high expression of the reporter protein was readily detectable only hours after transduction using the lentiral vector. In addition, we could demonstrate distinct differences in protein expression characteristics between human and murine EC as well as human EC line and primary human EC. Function could be demonstrated by showing a significant reduction in apoptosis in both murine and human EC. CONCLUSION: AAV vectors are an alternative to lentiviral vectors for gene transfer to corneal EC. Given a cultivation time of donor corneas of up to 4 weeks before transplantation, translation to eye banking, e. g., to decrease apoptosis in corneal allografts, is conceivable.

Protein kinase C mediates platelet-derived growth factor-induced tyrosine phosphorylation of p42.

One of the early events after stimulation of Swiss 3T3 cells with either platelet-derived growth factor (PDGF), 12-O-tetradecanoyl-phorbol-13-acetate (TPA), diacylglycerol, or several other mitogens is the near stoichiometric phosphorylation at tyrosine and serine of a scarce cytoplasmic protein (p42). TPA and diacylglycerol are known to directly stimulate the activity of a protein-serine/threonine kinase, protein kinase C (PKC). PDGF and several other mitogens stimulate tyrosine kinases directly and PKC indirectly. We have therefore examined the involvement of PKC in p42 tyrosine phosphorylation in Swiss 3T3 cells. Firstly, six agents which stimulated phosphorylation of p42 also stimulated phosphorylation of a known PKC substrate, an 80,000-Mr protein (p80). Secondly, in PKC-deficient cells (cells in which PKC activity was reduced to undetectable levels by prolonged exposure to TPA), PDGF-induced p42 phosphorylation was reduced three- to fourfold. Phosphoamino acid analysis of phosphorylated p42 from PDGF-stimulated PKC-deficient cells revealed primarily phosphoserine and only a trace of phosphotyrosine, suggesting that the reduction in PDGF-stimulated tyrosine phosphorylation of p42 resulting from PKC deficiency is greater than three- to fourfold. Finally, comparison of antiphosphotyrosine immunoprecipitates of PKC-deficient versus naive cells revealed that most other PDGF-induced tyrosine phosphorylation events were quite similar. These data suggest that mitogens such as PDGF, which directly stimulate phosphorylation of some proteins at tyrosine, induce p42 tyrosine phosphorylation via a cascade of events involving PKC.

Disruption of PDGF receptor trafficking by mutation of its PI-3 kinase binding sites.

Human platelet-derived growth factor receptors (PDGFRs) expressed in human Hep G2 cells internalized and concentrated in a juxtanuclear region near the Golgi network within 10 minutes after the cells were treated with PDGF. A PDGFR mutant (F5) that lacks high-affinity binding sites for the Src homology 2 domain-containing proteins phosphatidylinositol-3 kinase (PI-3 kinase), Ras guanosine triphosphatase activating protein, phospholipase C-gamma, and a phosphotyrosine phosphatase (Syp) remained at the cell periphery. Restoration of the PI-3 kinase binding sites on F5 completely restored the ability of the receptor to concentrate intracellularly. A PDGFR mutant lacking only PI-3 kinase binding sites failed to concentrate intracellularly. Thus, PI-3 kinase binding sites appear both necessary and sufficient for the normal endocytic trafficking of the activated PDGFR.

Binding of GAP to activated PDGF receptors.

The ras proto-oncogene products appear to relay intracellular signals via the Ras guanosine triphosphatase (GTPase) activator protein, GAP. In dog epithelial cells expressing human platelet-derived growth factor (PDGF) receptors, binding of PDGF caused approximately one-tenth of the total GAP molecules to complex with the receptor. Studies with mutant PDGF receptors showed that maximum association required both receptor kinase activity and phosphorylatable tyrosine residues at both the identified sites of receptor autophosphorylation.

The B cell antigen receptor complex: association of Ig-alpha and Ig-beta with distinct cytoplasmic effectors.

The B cell antigen receptor complex is a hetero-oligomeric structure composed of antigen binding, membrane immunoglobulin, and transducer-transporter substructures. The transducer-transporter substructure is composed of disulfide-linked dimers of immunoglobulin (Ig)-alpha and Ig-beta/gamma subunits that are products of the mb-1(alpha) and B29 (beta/gamma) genes. Although the receptor complex associates with Src family kinases that are activated after receptor ligation, the site of interaction of these and other cytoplasmic effector molecules with receptor subunits is unknown. The cytoplasmic tails of Ig-alpha and Ig-beta chains were found to associate with distinct sets of effector molecules. The Ig-alpha chain cytoplasmic domain bound to the Src family kinases Lyn and Fyn, phosphatidylinositol-3 kinase (PI-3 kinase), and an unidentified 38-kilodalton phosphoprotein; the cytoplasmic tail of Ig-beta bound PI-3 kinase and unidentified 40- and 42-kilodalton phosphoproteins. Binding activity was found to occur within a 26-amino acid sequence of Ig-alpha and Ig-beta that contains a motif [(Asp or Glu)-(any amino acid)7-(Asp or Glu)-Tyr-(any amino acid)3-Leu-(any amino acid)7-Tyr-(any amino acid)2-(Leu or Ile)] previously implicated in signal transduction via other receptors including the Fc epsilon receptor I and the T cell antigen receptor. These findings indicate that the subunits act independently to activate distinct second messenger pathways.

Receptor tyrosine kinases and their targets.

One of the ways in which higher eukaryotes receive messages from the environment is via cell surface receptor tyrosine kinases. These are transmembrane proteins with an extracellular binding domain that specifies the growth factor with which it will interact, and an intracellular domain that encodes the tyrosine kinase. The mechanism by which receptor tyrosine kinases direct intracellular signal relay appears to involve receptor autophosphorylation that permits the stable binding of SH2 domain containing signal transduction enzymes. Some of the more recent advances are summarized in this review.

Importance of the putative furin recognition site 742RNRR745 for antiangiogenic Sema3C activity in vitro.

Angiogenesis is one of the key processes in the growth and development of tumors. Class-3 semaphorins (Sema3) are characterized as axon guidance factors involved in tumor angiogenesis by interacting with the vascular endothelial growth factor signaling pathway. Sema3 proteins convey their regulatory signals by binding to neuropilins and plexins receptors, which are located on the effector cell. These processes are regulated by furin endoproteinases that cleave RXRR motifs within the Sema, plexin-semaphorins-integrin, and C-terminal basic domains of Sema3 protein. Several studies have shown that the furin-mediated processing of the basic domain of Sema3F and Sema3A is critical for association with receptors. It is unclear, however, if this mechanism can also be applied to other Sema3 proteins, including the main subject of this study, Sema3C. To address this question, we generated a variant of the full-length human Sema3C carrying point mutation R745A at the basic domain at the hypothetical furin recognition site 742RNRR745, which would disable the processing of Sema3C at this specific location. The effects produced by this mutation were tested in an in vitro angiogenesis assay together with the wild-type Sema3C, Sema3A, and Sema3F proteins. Our results showed that the inhibitory effect of Sema3C on microcapillary formation by human umbilical vein endothelial cells could be abrogated upon mutation at the Sema3C basic domain within putative furin cleavage site 742RNRR745, indicating that this site was essential for the Sema3 biological activity.

PDGF initiates two distinct phases of protein kinase C activity that make unequal contributions to the G0 to S transition.

BACKGROUND: Platelet-derived growth factor (PDGF) promotes cell-cycle progression by engaging signaling enzymes such as phospholipase Cgamma (PLCgamma). When activated, PLCgamma cleaves phosphatidylinositol-4,5-bisphosphate to produce inositol-1,4, 5-trisphosphate (IP(3)) and diacylglycerol (DAG). IP(3) stimulates the release of calcium from intracellular stores, which together with DAG activate some protein kinase C (PKC) family members. In this study we focused on putative downstream effectors of PLCgamma - PKC family members. We investigated whether, and when, DAG-responsive PKCs contribute to PDGF-dependent DNA synthesis. RESULTS: In HepG2 cells expressing wild-type PDGF beta receptors (betaPDGFRs), PDGF activated at least one PKC family member (PKCepsilon) at two distinct times - within 10 minutes after PDGF stimulation, and then for a longer duration between 5 and 9 hours. Blocking the early burst of PKC activity had no effect on PDGF-dependent DNA synthesis. In contrast, the DNA-synthesis response was reduced by 60-80% when the second phase of PKC activity was blocked. Similarly, DAG rescued PDGF-dependent DNA synthesis in the cells expressing a mitogenically incompetent mutant betaPDGFR, but only when DAG was added at times corresponding to the late phase of PKC activity. Our studies also indicate that the late phase of PKCepsilon activity can be induced by either phosphoinositide 3-kinase-dependent or DAG-dependent pathways in PDGF-stimulated HepG2 cells. CONCLUSIONS: We conclude that PDGF activates PKCs at two distinct times and that these two intervals of PKC activity make unequal contributions to the mitogenic response. The late phase of PKC activity is required for PDGF-dependent DNA synthesis, whereas the early phase of activity is dispensable.

PDGF induces an early and a late wave of PI 3-kinase activity, and only the late wave is required for progression through G1.

BACKGROUND: Platelet-derived growth factor (PDGF) triggers cytoskeletal rearrangements and chemotaxis within minutes. These events are at least in part due to the activation of phosphoinositide (PI) 3-kinase; there is good temporal correlation between these events and the accumulation of 3-phosphorylated products of the kinase. Prolonged and continuous PDGF exposure results in S-phase entry many hours after the initial burst of activity. Although early signals appear responsible for the early responses, they may not fully account for later responses, such as cell-cycle progression. RESULTS: We assessed when PI 3-kinase products accumulate in PDGF-stimulated cells. In addition to the previously identified early accumulation of products, we detected a second, prolonged wave of accumulation 3-7 hours after stimulation. To determine the relative contribution of each phase to PDGF-dependent DNA synthesis, we first developed an assay in which synthetic 3-phosphorylated lipids were used to rescue DNA synthesis in cells expressing a PDGF-receptor mutant. The lipids rescued DNA synthesis only when added 2-6 hours after PDGF. In addition, PI 3-kinase inhibitors failed to block PDGF-dependent DNA synthesis if added during the first wave of PI 3-kinase activity, but adding them later, in G1 phase, prevented PDGF-dependent cell-cycle progression. CONCLUSIONS: PDGF induces distinct waves of PI 3-kinase activity. The second wave is required for PDGF-dependent DNA synthesis, whereas the initial wave is not. One of the ways in which cells use PI 3-kinase to mediate distinct cellular responses seems to be by regulating when its products accumulate.

Src-like adaptor protein (Slap) is a negative regulator of mitogenesis.

The Src-like adaptor protein (Slap) is a recently identified adaptor protein containing Src homology 3 (SH3) and SH2 domains. Slap is found in a wide range of cell types and was shown to interact with the Eck receptor tyrosine kinase in a yeast two-hybrid interaction screen [1]. Here, we found that Slap is expressed in NIH3T3 cells and could associate with the activated platelet-derived growth factor (PDGF) receptor. Using mutated versions of the PDGF receptor and phosphopeptide competition experiments, we determined that Slap has the highest affinity for the Src-binding site of the PDGF receptor. Our inability to produce cell lines that stably expressed Slap suggested that Slap inhibited cell growth. We further investigated this issue by transiently expressing Slap by microinjection. Overexpression of Slap by this method inhibited DNA synthesis induced by PDGF and serum, whereas overexpression of the adaptor proteins Grb2 and Shc did not. Finally, microinjection of a Slap antibody into NIH3T3 cells that had been stimulated with suboptimal doses of growth factors potentiated the effects of the growth factors. These data suggest that, unlike other adaptor proteins, Slap is a negative regulator of signalling initiated by growth factors.

The GTPase-activating protein of Ras suppresses platelet-derived growth factor beta receptor signaling by silencing phospholipase C-gamma 1.

The beta receptor for platelet-derived growth factor (beta PDGFR) is activated by binding of PDGF and undergoes phosphorylation at multiple tyrosine residues. The tyrosine-phosphorylated receptor associates with numerous SH2-domain-containing proteins which include phospholipase C-gamma 1 (PLC gamma), the GTPase-activating protein of Ras (GAP), the p85 subunit of phosphatidylinositol 3 kinase (PI3K), the phosphotyrosine phosphatase Syp, and several other proteins. Our previous studies indicated that PI3K and PLC gamma were required for relay of the mitogenic signal of beta PDGFR, whereas GAP and Syp did not appear to be required for this response. In this study, we further investigated the role of GAP and Syp in mitogenic signaling by beta PDGFR. Focusing on the PLC gamma-dependent branch of beta PDGFR signaling, we constructed a series of mutant beta PDGFRs that contained the binding sites for pairs of the receptor-associated proteins: PLC gamma and PI3K, PLC gamma and GAP, or PLC gamma and Syp. Characterization of these mutants showed that while all receptors were catalytically active and bound similar amounts of PLC gamma, they differed dramatically in their ability to initiate DNA synthesis. This signaling deficiency related to an inability to efficiently tyrosine phosphorylate and activate PLC gamma. Surprisingly, the crippled receptor was the one that recruited PLC gamma and GAP. Thus, GAP functions to suppress signal relay by the beta PDGFR, and it does so by silencing PLC gamma. These findings demonstrate that the biological response to PDGF depends not only on the ability of the beta PDGFR to recruit signal relay enzymes but also on the blend of these receptor-associated proteins.

Activation of Src family members is not required for the platelet-derived growth factor beta receptor to initiate mitogenesis.

The basal activity of Src family kinases is readily detectable throughout the cell cycle and increases by two- to fivefold upon acute stimulation of cells with growth factors such as platelet-derived growth factor. Previous reports have demonstrated a requirement for Src activity for the G1/S and G2/M transitions. With a chimeric alpha-beta PDGF receptor (PDGFR) expressed in fibroblasts, we have investigated the importance of the PDGF-mediated increase in Src activity at the G0/G1 transition for subsequent cell cycle events. A mutant PDGFR chimera that was not able to detectably associate with or activate Src was compromised in its ability to mediate tyrosine phosphorylation of receptor-associated signaling molecules and initiated a submaximal activation of Erk. In contrast to these early cell cycle events, later responses such as entry of cells into S phase and cell proliferation proceeded normally when Src activity did not increase following acute stimulation with PDGF. We conclude that the initial burst of Src activity is required for efficient tyrosine phosphorylation of receptor-associated proteins such as PLCgamma, RasGAP, Shc, and SHP-2 and for maximal activation of Erk. Surprisingly, these events are not required for PDGF-dependent cell proliferation. Finally, later cell cycle events do not require that Src be activated at the G0/G1 transition and leave open the possibility that events such as the G1/S transition require the basal Src activity and/or activation of Src at later times in G1.

Tyrosines 1021 and 1009 are phosphorylation sites in the carboxy terminus of the platelet-derived growth factor receptor beta subunit and are required for binding of phospholipase C gamma and a 64-kilodalton protein, respectively.

Binding of platelet-derived growth factor (PDGF) to the PDGF receptor (PDGFR) beta subunit triggers receptor tyrosine phosphorylation and the stable association of a number of signal transduction molecules, including phospholipase C gamma (PLC gamma), the GTPase activating protein of ras (GAP), and phosphatidylinositol-3 kinase (PI3K). Previous reports have identified three PDGFR tyrosine phosphorylation sites in the kinase insert domain that are important for stable association of GAP and PI3K. Two of them, tyrosine (Y) 740, and Y-751 are required for the stable association of PI3K, while Y-771 is required for binding of GAP. Here we present data for two additional tyrosine phosphorylation sites, Y-1009 and Y-1021, that are both in the carboxy-terminal region of the PDGFR. Characterization of PDGFR mutants in which these phosphorylation sites are substituted with phenylalanine (F) indicated that Y-1021 and Y-1009 were required for the stable association of PLC gamma and a 64-kDa protein, respectively. An F-1009/F-1021 double mutant selectively failed to bind both PLC gamma and the 64-kDa protein, whereas all of the carboxy-terminal mutants bound wild-type levels of GAP and PI3K. The carboxy terminus encodes the complete binding site for PLC gamma, since a phosphorylated carboxy-terminal fusion protein selectively bound PLC gamma. To determine the biological consequences of failure to associate with PLC gamma, we measured PDGF-dependent inositol phosphate production and initiation of DNA synthesis. The PDGFR mutants that failed to associate with PLC gamma were not able to mediate the PDGF-dependent production of inositol phosphates. Since tyrosine phosphorylation of PLC gamma enhances its enzymatic activity, we speculated that PDGFR mutants that failed to activate PLC gamma were unable to mediate its tyrosine phosphorylation. Surprisingly, the F-1021 receptor mediated readily detectable levels of PDGF-dependent PLC gamma tyrosine phosphorylation. Thus, the production of inositol phosphates requires not only PLC gamma tyrosine phosphorylation but also its association with the PDGFR. Comparison of the mutant PDGFRs' abilities to initiate PDGF-dependent DNA synthesis indicated that failure to associate with PLC gamma and produce inositol phosphates diminished the mitogenic response by 30%. In contrast, preventing the PDGFR from binding the 64-kDa protein did not compromise PDGF-triggered DNA synthesis at saturating concentrations of PDGF. Thus, it appears that phosphorylation of the PDGFR at Y-1021 is required for the stable association of PLC gamma to the receptor's carboxy terminus, the production of inositol phosphates, and initiation of the maximal mitogenic response.

Phosphorylation of tyrosine 720 in the platelet-derived growth factor alpha receptor is required for binding of Grb2 and SHP-2 but not for activation of Ras or cell proliferation.

Following binding of platelet-derived growth factor (PDGF), the PDGF alpha receptor (alphaPDGFR) becomes tyrosine phosphorylated and associates with a number of signal transduction molecules, including phospholipase Cgamma-1 (PLCgamma-1), phosphatidylinositol 3-kinase (PI3K), the phosphotyrosine phosphatase SHP-2, Grb2, and Src. Here, we present data identifying a novel phosphorylation site in the kinase insert domain of the alphaPDGFR at tyrosine (Y) 720. We replaced this residue with phenylalanine and expressed the mutated receptor (F720) in Patch fibroblasts that do not express the alphaPDGFR. Characterization of the F720 mutant indicated that binding of two proteins, SHP-2 and Grb2, was severely impaired, whereas PLCgamma-1 and PI3K associated to wild-type levels. In addition, mutating Y720 to phenylalanine dramatically reduced PDGF-dependent tyrosine phosphorylation of SHP-2. Since Y720 was required for recruitment of two proteins, we investigated the mechanism by which these two proteins associated with the alphaPDGFR. SHP-2 bound the alphaPDGFR directly, whereas Grb2 associated indirectly, most probably via SHP-2, as Grb2 and SHP-2 coimmunoprecipitated when SHP-2 was tyrosine phosphorylated. We also compared the ability of the wild-type and F720 alphaPDGFRs to mediate a number of downstream events. Preventing the alphaPDGFR from recruiting SHP-2 and Grb2 did not compromise PDGF-AA-induced activation of Ras, initiation of DNA synthesis, or growth of cells in soft agar. We conclude that phosphorylation of the alphaPDGFR at Y720 is required for association of SHP-2 and Grb2 and tyrosine phosphorylation of SHP-2; however, these events are not required for the alphaPDGFR to activate Ras or initiate a proliferative response. In addition, these findings reveal that while SHP-2 binds to both of the receptors, it binds in different locations: to the carboxy terminus of the betaPDGFR but to the kinase insert of the alphaPDGFR.

GTPase-activating protein and phosphatidylinositol 3-kinase bind to distinct regions of the platelet-derived growth factor receptor beta subunit.

In response to binding of platelet-derived growth factor (PDGF), the PDGF receptor (PDGFR) beta subunit is phosphorylated on tyrosine residues and associates with numerous signal transduction enzymes, including the GTPase-activating protein of ras (GAP) and phosphatidylinositol 3-kinase (PI3K). Previous studies have shown that association of PI3K requires phosphorylation of tyrosine 751 (Y751) in the kinase insert and that this region of receptor forms at least a portion of the binding site for PI3K. In this study, the in vitro binding of GAP to the PDGFR was investigated. Like PI3K, GAP associates only with receptors that have been permitted to autophosphorylate, and GAP itself does not require tyrosine phosphate in order to stably associate with the phosphorylated PDGFR. To define which tyrosine residues are required for GAP binding, a panel of PDGFR phosphorylation site mutants was tested. Mutation of Y771 reduced the amount of GAP that associates to an undetectable level. In contrast, the F771 (phenylalanine at 771) mutant bound wild-type levels of PI3K, whereas the F740 and F751 mutants bound 3 and 23%, respectively, of the wild-type levels of PI3K but wild-type levels of GAP. The F740/F751 double mutant associated with wild-type levels of GAP, but no detectable PI3K activity, while the F740/F751/F771 triple mutant could not bind either GAP or PI3K. The in vitro and in vivo associations of GAP and PI3K activity to these PDGFR mutants were indistinguishable. The distinct tyrosine residue requirements suggest that GAP and PI3K bind different regions of the PDGFR. This possibility was also supported by the observation that the antibody to the PDGFR kinase insert Y751 region that blocks association of PI3K had only a minor effect on the in vitro binding of GAP. In addition, highly purified PI3K and GAP associated in the absence of other cellular proteins and neither cooperated nor competed with each other's binding to the PDGFR. Taken together, these studies indicate that GAP and PI3K bind directly to the PDGFR and have discrete binding sites that include portions of the kinase insert domain.

The hsp90 chaperone complex regulates intracellular localization of the dioxin receptor.

The molecular chaperone complex hsp90-p23 interacts with the dioxin receptor, a ligand-dependent basic helix-loop-helix (bHLH)/Per-Arnt-Sim domain transcription factor. Whereas biochemical and genetic evidence indicates that hsp90 is important for maintenance of a high-affinity ligand binding conformation of the dioxin receptor, the role of hsp90-associated proteins in regulation of the dioxin receptor function remains unclear. Here we demonstrate that the integrity of the hsp90 complex characterized by the presence of the hsp90-associated cochaperone p23 and additional cochaperone proteins is important for regulation of the intracellular localization of the dioxin receptor by two mechanisms. First, in the absence of ligand, the dioxin receptor-hsp90 complex was associated with the immunophilin-like protein XAP2 to mediate cytoplasmic retention of the dioxin receptor. Second, upon exposure to ligand, the p23-associated hsp90 complex mediated interaction of the dioxin receptor with the nuclear import receptor protein pendulin and subsequent nuclear translocation of the receptor. Interestingly, these two modes of regulation target two distinct functional domains of the dioxin receptor. Whereas the nuclear localization signal-containing and hsp90-interacting bHLH domain of the receptor regulates ligand-dependent nuclear import, the interaction of the p23-hsp90-XAP2 complex with the ligand binding domain of the dioxin receptor was essential to mediate cytoplasmic retention of the ligand-free receptor form. In conclusion, these data suggest a novel role of the hsp90 molecular chaperone complex in regulation of the intracellular localization of the dioxin receptor.

Ligand-independent activation of the platelet-derived growth factor beta receptor: requirements for bovine papillomavirus E5-induced mitogenic signaling.

The E5 protein of bovine papillomavirus type 1 binds to and activates the endogenous platelet-derived growth factor (PDGF) beta receptor in fibroblasts, resulting in cell transformation. We have developed a functional assay to test the ability of PDGF beta receptor mutants to mediate a mitogenic signal initiated by the E5 protein. Lymphoid Ba/F3 cells are strictly dependent on interleukin-3 for growth, but coexpression of the wild-type PDGF beta receptor and the E5 or v-sis-encoded protein generated a mitogenic signal which allowed Ba/F3-derived cells to proliferate in the absence of interleukin-3. In these cells, the E5 protein bound to and caused increased tyrosine phosphorylation of both the mature and the precursor forms of the wild-type PDGF beta receptor. The tyrosine kinase activity of the receptor was necessary for E5-induced receptor tyrosine phosphorylation and mitogenic activity but not for complex formation with the E5 protein. In contrast, the PDGF-binding domain of the receptor was not required for complex formation with the E5 protein, E5-induced tyrosine phosphorylation or mitogenic activity, demonstrating that E5-mediated receptor activation is ligand independent. Analysis of receptor mutants lacking various combinations of tyrosine phosphorylation sites revealed that the E5 and v-sis-encoded proteins display similar requirements for signaling and suggested that the wild-type PDGF beta receptor can generate multiple independent mitogenic signals. Importantly, these mutants dissociated two activities of the PDGF beta receptor tyrosine kinase, both of which are required for sustained mitogenic signaling: (i) receptor autophosphorylation and creation of binding sites for SH2 domain-containing proteins and (ii) phosphorylation of substrates other than the receptor itself.