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

Ligand-independent activation of platelet-derived growth factor receptor β promotes vitreous-induced contraction of retinal pigment epithelial cells

BACKGROUND

Epiretinal membranes in patients with proliferative vitreoretinopathy (PVR) consist of extracellular matrix and a number of cell types including retinal pigment epithelial (RPE) cells and fibroblasts, whose contraction causes retinal detachment. In RPE cells depletion of platelet-derived growth factor (PDGF) receptor (PDGFR)β suppresses vitreous-induced Akt activation, whereas in fibroblasts Akt activation through indirect activation of PDGFRα by growth factors outside the PDGF family (non-PDGFs) plays an essential role in experimental PVR. Whether non-PDGFs in the vitreous, however, were also able to activate PDGFRβ in RPE cells remained elusive.

METHODS

The CRISPR/Cas9 technology was utilized to edit a genomic PDGFRB locus in RPE cells derived from an epiretinal membrane (RPEM) from a patient with PVR, and a retroviral vector was used to express a truncated PDGFRβ short of a PDGF-binding domain in the RPEM cells lacking PDGFRβ. Western blot was employed to analyze expression of PDGFRβ and α-smooth muscle actin, and signaling events (p-PDGFRβ and p-Akt). Cellular assays (proliferation, migration and contraction) were also applied in this study.

RESULTS

Expression of a truncated PDGFRβ lacking a PDGF-binding domain in the RPEM cells whose PDGFRB gene has been silent using the CRISPR/Cas9 technology restores vitreous-induced Akt activation as well as cell proliferation, epithelial-mesenchymal transition, migration and contraction. In addition, we show that scavenging reactive oxygen species (ROS) with N-acetyl-cysteine and inhibiting Src family kinases (SFKs) with their specific inhibitor SU6656 blunt the vitreous-induced activation of the truncated PDGFRβ and Akt as well as the cellular events related to the PVR pathogenesis. These discoveries suggest that in RPE cells PDGFRβ can be activated indirectly by non-PDGFs in the vitreous via an intracellular pathway of ROS/SFKs to facilitate the development of PVR, thereby providing novel opportunities for PVR therapeutics.

CONCLUSION

The data shown here will improve our understanding of the mechanism by which PDGFRβ can be activated by non-PDGFs in the vitreous via an intracellular route of ROS/SFKs and provide a conceptual foundation for preventing PVR by inhibiting PDGFRβ transactivation (ligand-independent activation).

Activation of the PDGF β Receptor by a Persistent Artificial Signal Peptide

Most eukaryotic transmembrane and secreted proteins contain N-terminal signal peptides that mediate insertion of the nascent translation products into the membrane of the endoplasmic reticulum. After membrane insertion, signal peptides typically are cleaved from the mature protein and degraded. Here, we tested whether a small hydrophobic protein selected for growth promoting activity in mammalian cells retained transforming activity while also acting as a signal peptide. We replaced the signal peptide of the PDGF β receptor (PDGFβR) with a previously described 29-residue artificial transmembrane protein named 9C3 that can activate the PDGFβR in trans. We showed that a modified version of 9C3 at the N-terminus of the PDGFβR can function as a signal peptide, as assessed by its ability to support high level expression, glycosylation, and cell surface localization of the PDGFβR. The 9C3 signal peptide retains its ability to interact with the transmembrane domain of the PDGFβR and cause receptor activation and cell proliferation. Cleavage of the 9C3 signal peptide from the mature receptor is not required for these activities. However, signal peptide cleavage does occur in some molecules, and the cleaved signal peptide can persist in cells and activate a co-expressed PDGFβR in trans. Our finding that a hydrophobic sequence can display signal peptide and transforming activity suggest that some naturally occurring signal peptides may also display additional biological activities by interacting with the transmembrane domains of target proteins.

Biologically Active Ultra-Simple Proteins Reveal Principles of Transmembrane Domain Interactions

Specific interactions between the helical membrane-spanning domains of transmembrane proteins play central roles in the proper folding and oligomerization of these proteins. However, the relationship between the hydrophobic amino acid sequences of transmembrane domains and their functional interactions is in most cases unknown. Here, we use ultra-simple artificial proteins to systematically study the sequence basis for transmembrane domain interactions. We show that most short homopolymeric polyleucine transmembrane proteins containing single amino acid substitutions can activate the platelet-derived growth factor β receptor or the erythropoietin receptor in cultured mouse cells, resulting in cell transformation or proliferation. These proteins displayed complex patterns of activity that were markedly affected by seemingly minor sequence differences in the ultra-simple protein itself or in the transmembrane domain of the target receptor, and the effects of these sequence differences are not additive. In addition, specific leucine residues along the length of these proteins are required for activity, and the positions of these required leucines differ based on the identity and position of the central substituted amino acid. Our results suggest that these ultra-simple proteins use a variety of molecular mechanisms to activate the same target and that diversification of transmembrane domain sequences over the course of evolution minimized off-target interactions.

Beyond Channel Activity: Protein-Protein Interactions Involving Viroporins

Viroporins are short polypeptides encoded by viruses. These small membrane proteins assemble into oligomers that can permeabilize cellular lipid bilayers, disrupting the physiology of the host to the advantage of the virus. Consequently, efforts during the last few decades have been focused towards the discovery of viroporin channel inhibitors, but in general these have not been successful to produce licensed drugs. Viroporins are also involved in viral pathogenesis by engaging in critical interactions with viral proteins, or disrupting normal host cellular pathways through coordinated interactions with host proteins. These protein-protein interactions (PPIs) may become alternative attractive drug targets for the development of antivirals. In this sense, while thus far most antiviral molecules have targeted viral proteins, focus is moving towards targeting host proteins that are essential for virus replication. In principle, this largely would overcome the problem of resistance, with the possibility of using repositioned existing drugs. The precise role of these PPIs, their strain- and host- specificities, and the structural determination of the complexes involved, are areas that will keep the fields of virology and structural biology occupied for years to come. In the present review, we provide an update of the efforts in the characterization of the main PPIs for most viroporins, as well as the role of viroporins in these PPIs interactions.

Two transmembrane dimers of the bovine papillomavirus E5 oncoprotein clamp the PDGF β receptor in an active dimeric conformation

The dimeric 44-residue E5 protein of bovine papillomavirus is the smallest known naturally occurring oncoprotein. This transmembrane protein binds to the transmembrane domain (TMD) of the platelet-derived growth factor β receptor (PDGFβR), causing dimerization and activation of the receptor. Here, we use Rosetta membrane modeling and all-atom molecular dynamics simulations in a membrane environment to develop a chemically detailed model of the E5 protein/PDGFβR complex. In this model, an active dimer of the PDGFβR TMD is sandwiched between two dimers of the E5 protein. Biochemical experiments showed that the major PDGFβR TMD complex in mouse cells contains two E5 dimers and that binding the PDGFβR TMD to the E5 protein is necessary and sufficient to recruit both E5 dimers into the complex. These results demonstrate how E5 binding induces receptor dimerization and define a molecular mechanism of receptor activation based on specific interactions between TMDs.

Pericyte-fibroblast transition promotes tumor growth and metastasis

Vascular pericytes, an important cellular component in the tumor microenvironment, are often associated with tumor vasculatures, and their functions in cancer invasion and metastasis are poorly understood. Here we show that PDGF-BB induces pericyte-fibroblast transition (PFT), which significantly contributes to tumor invasion and metastasis. Gain- and loss-of-function experiments demonstrate that PDGF-BB-PDGFRβ signaling promotes PFT both in vitro and in in vivo tumors. Genome-wide expression analysis indicates that PDGF-BB-activated pericytes acquire mesenchymal progenitor features. Pharmacological inhibition and genetic deletion of PDGFRβ ablate the PDGF-BB-induced PFT. Genetic tracing of pericytes with two independent mouse strains, TN-AP-CreERT2:R26R-tdTomato and NG2-CreERT2:R26R-tdTomato, shows that PFT cells gain stromal fibroblast and myofibroblast markers in tumors. Importantly, coimplantation of PFT cells with less-invasive tumor cells in mice markedly promotes tumor dissemination and invasion, leading to an increased number of circulating tumor cells and metastasis. Our findings reveal a mechanism of vascular pericytes in PDGF-BB-promoted cancer invasion and metastasis by inducing PFT, and thus targeting PFT may offer a new treatment option of cancer metastasis.

The PDGF-BB-SOX7 axis-modulated IL-33 in pericytes and stromal cells promotes metastasis through tumour-associated macrophages

Signalling molecules and pathways that mediate crosstalk between various tumour cellular compartments in cancer metastasis remain largely unknown. We report a mechanism of the interaction between perivascular cells and tumour-associated macrophages (TAMs) in promoting metastasis through the IL-33–ST2-dependent pathway in xenograft mouse models of cancer. IL-33 is the highest upregulated gene through activation of SOX7 transcription factor in PDGF-BB-stimulated pericytes. Gain- and loss-of-function experiments validate that IL-33 promotes metastasis through recruitment of TAMs. Pharmacological inhibition of the IL-33–ST2 signalling by a soluble ST2 significantly inhibits TAMs and metastasis. Genetic deletion of host IL-33 in mice also blocks PDGF-BB-induced TAM recruitment and metastasis. These findings shed light on the role of tumour stroma in promoting metastasis and have therapeutic implications for cancer therapy.

Elevated IL-33 levels have been correlated with metastasis and poor prognosis. Here the authors show in mouse tumour xenograft models that PDGF-BB produced by tumour cells induces IL-33 via Sox7 in tumour pericytes, and IL-33 promotes metastasis through its effects on tumour-associated macrophages.

Biologically active LIL proteins built with minimal chemical diversity

We have constructed 26-amino acid transmembrane proteins that specifically transform cells but consist of only two different amino acids. Most proteins are long polymers of amino acids with 20 or more chemically distinct side-chains. The artificial transmembrane proteins reported here are the simplest known proteins with specific biological activity, consisting solely of an initiating methionine followed by specific sequences of leucines and isoleucines, two hydrophobic amino acids that differ only by the position of a methyl group. We designate these proteins containing leucine (L) and isoleucine (I) as LIL proteins. These proteins functionally interact with the transmembrane domain of the platelet-derived growth factor β-receptor and specifically activate the receptor to transform cells. Complete mutagenesis of these proteins identified individual amino acids required for activity, and a protein consisting solely of leucines, except for a single isoleucine at a particular position, transformed cells. These surprisingly simple proteins define the minimal chemical diversity sufficient to construct proteins with specific biological activity and change our view of what can constitute an active protein in a cellular context.

Viral miniproteins

Many viruses encode short transmembrane proteins that play vital roles in virus replication or virulence. Because many of these proteins are less than 50 amino acids long and not homologous to cellular proteins, their open reading frames were often overlooked during the initial annotation of viral genomes. Some of these proteins oligomerize in membranes and form ion channels. Other miniproteins bind to cellular transmembrane proteins and modulate their activity, whereas still others have an unknown mechanism of action. Based on the underlying principles of transmembrane miniprotein structure, it is possible to build artificial small transmembrane proteins that modulate a variety of biological processes. These findings suggest that short transmembrane proteins provide a versatile mechanism to regulate a wide range of cellular activities, and we speculate that cells also express many similar proteins that have not yet been discovered.

De novo selection of oncogenes

All cellular proteins are derived from preexisting ones by natural selection. Because of the random nature of this process, many potentially useful protein structures never arose or were discarded during evolution. Here, we used a single round of genetic selection in mouse cells to isolate chemically simple, biologically active transmembrane proteins that do not contain any amino acid sequences from preexisting proteins. We screened a retroviral library expressing hundreds of thousands of proteins consisting of hydrophobic amino acids in random order to isolate four 29-aa proteins that induced focus formation in mouse and human fibroblasts and tumors in mice. These proteins share no amino acid sequences with known cellular or viral proteins, and the simplest of them contains only seven different amino acids. They transformed cells by forming a stable complex with the platelet-derived growth factor β receptor transmembrane domain and causing ligand-independent receptor activation. We term this approach de novo selection and suggest that it can be used to generate structures and activities not observed in nature, create prototypes for novel research reagents and therapeutics, and provide insight into cell biology, transmembrane protein-protein interactions, and possibly virus evolution and the origin of life.

Compensatory mutants of the bovine papillomavirus E5 protein and the platelet-derived growth factor β receptor reveal a complex direct transmembrane interaction

The 44-amino-acid E5 protein of bovine papillomavirus is a dimeric transmembrane protein that exists in a stable complex with the platelet-derived growth factor (PDGF) β receptor, causing receptor activation and cell transformation. The transmembrane domain of the PDGF β receptor is required for complex formation, but it is not known if the two proteins contact one another directly. Here, we studied a PDGF β receptor mutant containing a leucine-to-isoleucine substitution in its transmembrane domain, which prevents complex formation with the wild-type E5 protein in mouse BaF3 cells and inhibits receptor activation by the E5 protein. We selected E5 mutants containing either a small deletion or multiple substitution mutations that restored binding to the mutant PDGF β receptor, resulting in receptor activation and growth factor independence. These E5 mutants displayed lower activity with PDGF β receptor mutants containing other transmembrane substitutions in the vicinity of the original mutation, and one of them cooperated with a receptor mutant containing a distal mutation in the juxtamembrane domain. These results provide strong genetic evidence that the transmembrane domains of the E5 protein and the PDGF β receptor contact one another directly. They also demonstrate that different mutations in the E5 protein allow it to tolerate the same mutation in the PDGF β receptor transmembrane domain and that a mutation in the E5 protein can allow it to tolerate different mutations in the PDGF β receptor. Thus, the rules governing direct interactions between transmembrane helices are complex and not restricted to local interactions.

A single amino acid substitution converts a transmembrane protein activator of the platelet-derived growth factor β receptor into an inhibitor

Receptors for PDGF play an important role in cell proliferation and migration and have been implicated in certain cancers. The 44-amino acid E5 protein of bovine papillomavirus binds to and activates the PDGFβ receptor (PDGFβR), resulting in oncogenic transformation of cultured fibroblasts. Previously, we isolated an artificial 36-amino acid transmembrane protein, pTM36-4, which transforms cells because of its ability to activate the PDGFβR despite limited sequence similarity to E5. Here, we demonstrated complex formation between the PDGFβR and three pTM36-4 mutants: T21E, T21Q, and T21N. T21Q retained wild type transforming activity and activated the PDGFβR in a ligand-independent manner as a consequence of binding to the transmembrane domain of the PDGFβR, but T21E and T21N were severely defective. In fact, T21N substantially inhibited E5-induced PDGFβR activation and transformation in both mouse and human fibroblasts. T21N did not prevent E5 from binding to the receptor, and genetic evidence suggested that T21N and E5 bind to nonidentical sites in the transmembrane domain of the receptor. T21N also inhibited transformation and PDGFβR activation induced by v-Sis, a viral homologue of PDGF-BB, as well as PDGF-induced mitogenesis and signaling by preventing phosphorylation of the PDGFβR at particular tyrosine residues. These results demonstrated that T21N acts as a novel inhibitor of the PDGFβR and validated a new strategy for designing highly specific short transmembrane protein inhibitors of growth factor receptors and possibly other transmembrane proteins.

The E5 proteins

The E5 proteins are short transmembrane proteins encoded by many animal and human papillomaviruses. These proteins display transforming activity in cultured cells and animals, and they presumably also play a role in the productive virus life cycle. The E5 proteins are thought to act by modulating the activity of cellular proteins. Here, we describe the biological activities of the best-studied E5 proteins and discuss the evidence implicating specific protein targets and pathways in mediating these activities. The primary target of the 44-amino acid BPV1 E5 protein is the PDGF β receptor, whereas the EGF receptor appears to be an important target of the 83-amino acid HPV16 E5 protein. Both E5 proteins also bind to the vacuolar ATPase and affect MHC class I expression and cell-cell communication. Continued studies of the E5 proteins will elucidate important aspects of transmembrane protein-protein interactions, cellular signal transduction, cell biology, virus replication, and tumorigenesis.

Papillomavirus E5: the smallest oncoprotein with many functions

Papillomaviruses (PVs) are established agents of human and animal cancers. They infect cutaneous and mucous epithelia. High Risk (HR) Human PVs (HPVs) are consistently associated with cancer of the uterine cervix, but are also involved in the etiopathogenesis of other cancer types. The early oncoproteins of PVs: E5, E6 and E7 are known to contribute to tumour progression. While the oncogenic activities of E6 and E7 are well characterised, the role of E5 is still rather nebulous. The widespread causal association of PVs with cancer makes their study worthwhile not only in humans but also in animal model systems. The Bovine PV (BPV) system has been the most useful animal model in understanding the oncogenic potential of PVs due to the pivotal role of its E5 oncoprotein in cell transformation. This review will highlight the differences between HPV-16 E5 (16E5) and E5 from other PVs, primarily from BPV. It will discuss the targeting of E5 as a possible therapeutic agent.

Lack in Efficacy for Imatinib Mesylate as Second-Line Treatment of Recurrent or Metastatic Cervical Cancer Expressing Platelet-Derived Growth Factor Receptor α

Imatinib mesylate inhibits platelet-derived growth factor receptor (PDGFR), and there are evidences that the PDGFR participates in development and progression of cervical cancer. This pilot study was set to evaluate the efficacy in response rate and progression-free survival of imatinib. A secondary end point was to evaluate its safety as second-line treatment of recurrent or metastatic cervical cancer expressing PDGFRα. Imatinib mesylate was administered in daily dosages of 600 mg. Response was evaluated by positron emission tomography/computed tomography every two 28-day courses, and toxicity was evaluated weekly and thereafter. Twelve patients were included in the study. The median age was 49.8 years; all but 1 tumor were squamous cell carcinomas. First-line palliative chemotherapy with carboplatin-paclitaxel was the most frequently used scheme (75.0%). Ten (83.3%) had pelvic and systemic disease, whereas only 2 had systemic disease alone. All patients expressed the PDGFRα in more than 10% of malignant cells, whereas only 4 coexpressed the PDGFRβ. No patient showed response. A single patient having metastatic disease in the lung showed stabilization for 6 months to then progressing in bone. No severe toxicities were seen except for the patient with worsening of bleeding from proctitis. Grades 1 and 2 gastrointestinal toxicities were common. Despite lack of activity of single-agent imatinib, further studies in cervical cancer are deserved to better define the status of imatinib targets in this tumor and to investigate its activity in combination with cytotoxic drugs.

The canine papillomavirus e5 protein signals from the endoplasmic reticulum

The recently discovered Canis familiaris papillomavirus (PV) type 2 (CfPV2) provides a unique opportunity to study PV gene functions in vitro and in vivo. Unlike the previously characterized canine oral PV, CfPV2 contains an E5 open reading frame and is associated with progression to squamous cell carcinoma. In the current study, we have expressed and characterized the CfPV2-encoded E5 protein, a small, hydrophobic, 41-amino-acid polypeptide. We demonstrate that, similar to the E5 protein from high-risk human PV type 16, the CfPV2 E5 protein is localized in the endoplasmic reticulum (ER) and that its expression decreases keratinocyte proliferation and cell life span. E5 expression also increases the percentage of cells in the G(1) phase of the cell cycle, with a concomitant decrease in the percentage of cells in S phase. To identify a potential mechanism for E5-mediated growth inhibition from the ER, we developed a real-time PCR method to quantify the splicing of XBP1 mRNA as a measure of ER stress. We found that the CfPV2 E5 protein induced ER stress and that this, as well as the observed growth inhibition, is tempered significantly by coexpression of the CfPV2 E6 and E7 genes. It is possible that the spatial/temporal regulation of E6/E7 gene expression during keratinocyte differentiation might therefore modulate E5 activity and ER stress.

Artificial transmembrane oncoproteins smaller than the bovine papillomavirus E5 protein redefine sequence requirements for activation of the platelet-derived growth factor beta receptor

The bovine papillomavirus E5 protein (BPV E5) is a 44-amino-acid homodimeric transmembrane protein that binds directly to the transmembrane domain of the platelet-derived growth factor (PDGF) beta receptor and induces ligand-independent receptor activation. Three specific features of BPV E5 are considered important for its ability to activate the PDGF beta receptor and transform mouse fibroblasts: a pair of C-terminal cysteines, a transmembrane glutamine, and a juxtamembrane aspartic acid. By using a new genetic technique to screen libraries expressing artificial transmembrane proteins for activators of the PDGF beta receptor, we isolated much smaller proteins, from 32 to 36 residues, that lack all three of these features yet still dimerize noncovalently, specifically activate the PDGF beta receptor via its transmembrane domain, and transform cells efficiently. The primary amino acid sequence of BPV E5 is virtually unrecognizable in some of these proteins, which share as few as seven consecutive amino acids with the viral protein. Thus, small artificial proteins that bear little resemblance to a viral oncoprotein can nevertheless productively interact with the same cellular target. We speculate that similar cellular proteins may exist but have been overlooked due to their small size and hydrophobicity.

The bovine papillomavirus E5 protein and the PDGF beta receptor: it takes two to tango

The extremely hydrophobic, 44-amino acid bovine papillomavirus (BPV) E5 protein is the smallest known oncoprotein, which orchestrates cell transformation by causing ligand-independent activation of a cellular receptor tyrosine kinase, the platelet-derived growth factor beta receptor (PDGFbetaR). The E5 protein forms a dimer in transformed cells and is essentially an isolated membrane-spanning segment that binds directly to the transmembrane domain of the PDGFbetaR, inducing receptor dimerization, autophosphorylation, and sustained mitogenic signaling. There are few sequence constraints for activity as long as the overall hydrophobicity of the E5 protein and its ability to dimerize are preserved. Nevertheless, the E5 protein is highly specific for the PDGFbetaR and does not activate other cellular proteins. Genetic screens of thousands of small, artificial hydrophobic proteins with randomized transmembrane domains inserted into an E5 scaffold identified proteins with diverse transmembrane sequences that activate the PDGFbetaR, including some activators as small as 32-amino acids. Analysis of these novel proteins has provided new insight into the requirements for PDGFbetaR activation and specific transmembrane recognition in general. These results suggest that small, transmembrane proteins can be constructed and selected that specifically bind to other cellular or viral transmembrane target proteins. By using this approach, we have isolated a 44-amino acid artificial transmembrane protein that appears to activate the human erythropoietin receptor. Studies of the tiny, hydrophobic BPV E5 protein have not only revealed a novel mechanism of viral oncogenesis, but have also suggested that it may be possible to develop artificial small proteins that specifically modulate much larger target proteins by acting within cellular or viral membranes.

Bovine papillomaviruses, papillomas and cancer in cattle

Bovine papillomaviruses (BPV) are DNA oncogenic viruses inducing hyperplastic benign lesions of both cutaneous and mucosal epithelia in cattle. Ten (BPV 1-10) different viral genotypes have been characterised so far. BPV 1-10 are all strictly species-specific but BPV 1/2 may also infect equids inducing fibroblastic tumours. These benign lesions generally regress but may also occasionally persist, leading to a high risk of evolving into cancer, particularly in the presence of environmental carcinogenic co-factors. Among these, bracken fern is the most extensively studied. The synergism between immunosuppressants and carcinogenic principles from bracken fern and the virus has been experimentally demonstrated for both urinary bladder and alimentary canal cancer in cows whose diets were based on this plant. BPV associated tumours have veterinary and agricultural relevance in their own right, although they have also been studied as a relevant model of Human papillomavirus (HPV). Recent insights into BPV biology have paved the way to new fields of speculation on the role of these viruses in neoplastic transformation of cells other than epithelial ones. This review will briefly summarise BPV genome organization, will describe in greater detail the functions of viral oncoproteins, the interaction between the virus and co-carcinogens in tumour development; relevant aspects of immunity and vaccines will also be discussed.

CuZn-SOD suppresses the bovine papillomavirus-induced proliferation of fibroblasts

Eukaryotic cells continuously produce reactive oxygen species (ROS) and have mechanisms to control ROS levels. ROS have been shown to mediate cell proliferation and transformation. We studied the effect of CuZn-superoxide dismutase (CuZnSOD) on the focus-forming ability of bovine papillomavirus (BPV-1) wtDNA and hypertransforming mutant of its major oncoprotein E5, E5-17S. We found that CuZnSOD suppresses the focus-forming ability of BPV-1 wtDNA and E5 oncoprotein. Significantly fewer foci were detected in pCGCuZnSOD- and BPV-1 DNA-cotransfected cell culture compare to BPV-1 DNA-transfected cell culture (p<0.001). CuZnSOD decreases the rate of cell proliferation in both non-transformed C127 and BPV-1- and E5-transformed cell lines. CuZnSOD decelerates cell entry into the S phase of the cell cycle and has a suppressing effect on the actively dividing cells. As the transformed cells proliferate faster than normal cells when confluent, CuZnSOD inhibits the growth of foci. These results indicate that superoxide radicals may be involved in signaling for cell proliferation and that SOD suppresses cell proliferation.

Expression of platelet derived growth factor family members and the potential role of imatinib mesylate for cervical cancer

BACKGROUND

Despite significant achievements in the treatment of cervical cancer, it is still a deadly disease; hence newer therapeutical modalities are needed. Preliminary investigations suggest that platelet-derived growth factor (PDGF) might have a role in the development of cervical cancer, therefore it is important to determine whether this growth factor pathway is functional and its targeting with imatinib mesylate leads to growth inhibition of cervical cancer cells.

RESULTS

PDGF receptors (PDGFR) and their ligands are frequently expressed in cervical cancer and the majority exhibited a combination of family members co-expression. A number of intronic and exonic variations but no known mutations in the coding sequence of the PDGFRalpha gene were found in cancer cell lines and primary tumors. Growth assays demonstrated that PDGFBB induces growth stimulation that can be blocked by imatinib and that this tyrosine kinase inhibitor on its own inhibits cell growth. These effects were associated with the phosphorylation status of the receptor.

CONCLUSION

The PDGFR system may have a role in the pathogenesis of cervical cancer as their members are frequently expressed in this tumor and cervical cancer lines are growth inhibited by the PDGFR antagonist imatinib.

Roles of tyrosine 589 and 591 in STAT5 activation and transformation mediated by FLT3-ITD

Acquired mutations in the FLT3 receptor tyrosine kinase are common in acute myeloid leukemia and result in constitutive activation. The most frequent mechanism of activation is disruption of the juxtamembrane autoregulatory domain by internal tandem duplications (ITDs). FLT3-ITDs confer factor-independent growth to hematopoietic cells and induce a myeloproliferative syndrome in murine bone marrow transplant models. We and others have observed that FLT3-ITD activates STAT5 and its downstream effectors, whereas ligand-stimulated wild-type FLT3 (FLT3WT) does not. In vitro mapping of tyrosine phosphorylation sites in FLT3-ITD identified 2 candidate STAT5 docking sites within the juxtamembrane domain that are disrupted by the ITD. Tyrosine to phenylalanine substitution of residues 589 and 591 in the context of the FLT3-ITD did not affect tyrosine kinase activity, but abrogated STAT5 activation. Furthermore, FLT3-ITD-Y589/591F was incapable of inducing a myeloproliferative phenotype when transduced into primary murine bone marrow cells, whereas FLT3-ITD induced myeloproliferative disease with a median latency of 50 days. Thus, the conformational change in the FLT3 juxtamembrane domain induced by the ITD activates the kinase through dysregulation of autoinhibition and results in qualitative differences in signal transduction through STAT5 that are essential for the transforming potential of FLT3-ITD in vivo.

Bovine papillomavirus E5 oncoprotein binds to the activated form of the platelet-derived growth factor beta receptor in naturally occurring bovine urinary bladder tumours

Studies regarding the functions of the bovine papillomavirus (BPV) E5 oncoprotein in vivo are lacking and no E5-mediated mechanism underlying epithelial carcinogenesis is known. We have shown that BPV-2 DNA is present in the majority of naturally occurring urinary bladder tumours of cattle and that E5 is expressed in the cancer cells. Here we show that the interaction between the platelet-derived growth factor (PDGF) beta receptor and BPV E5, described in vitro in cultured cells, takes place in vivo in bovine urinary bladder cancers. In these cancers, E5 and PDGF beta receptor colocalize, as shown by confocal microscopy, and physically interact, as shown by coimmunoprecipitation. Furthermore, the PDGF beta receptor associated with E5 is highly phosphorylated, suggesting the functional activation of the receptor upon E5 interaction. Our results demonstrate, for the first time, that E5-PDGF beta receptor interaction occurs during the natural history of bovine urinary bladder tumours, suggesting an important role for E5 in carcinogenesis. Finally, the system provides a suitable animal model of papillomavirus-associated cancer to test therapeutic vaccination against E5. Successful bladder tumour regression would provide a valuable model for therapeutic vaccination against papillomavirus-associated tumours.

Bovine papillomavirus type 1 oncoprotein E5 stimulates the utilization of superoxide radicals in the mouse fibroblast cell line C127

The major transforming protein of bovine papillomavirus type 1 (BPV-1) is a small hydrophobic polypeptide, the E5 gene product, localized in the cellular membranes and modulating various pathways in the cell. Many studies have shown that reactive oxygen species (ROS) are essential in several biological processes, including cell transformation by oncogenes, but unregulated ROS are highly toxic to cells. We studied the effect of the bovine papillomavirus protein E5 and its mutants on the level of the superoxide radicals in the mouse fibroblast cell line C127. The superoxide level in C127 cells transfected with the E5-expressing plasmids were measured by nitroblue tetrazolium reduction. Relative concentrations of intracellular peroxide were determined by using 2,7-dichlorofluorescin diacetate. Our results showed that all transforming mutants of E5 reduced the level of superoxide in C127 cells, besides the activity of superoxide dismutase (SOD) and level of peroxides was not altered. In the presence of neopterin, an inhibitor of the superoxide-producing enzymes, the reduction of superoxide level correlated with the transforming ability of the E5-mutants. The inhibitor of the protein tyrosine kinase, tyrphostin 25 and inhibitors of oxygenases of the arachidonic acid metabolism, aspirin and nordihydroguaiaretic acid, blocked the effect of BPV-1 E5. We conclude that BPV-1 E5 and its transforming mutants are able to modulate the level of superoxide and stimulate the utilization of superoxide through protein tyrosine kinases and oxygenases of the arachidonic acid metabolism.

Modulation of cell function by small transmembrane proteins modeled on the bovine papillomavirus E5 protein

Viruses have been subjected to intense study because of their medical importance and because they can provide fundamental insights into normal and pathological cellular processes. Indeed, much of our knowledge about basic cellular biology and biochemistry was acquired through the study of viruses, and some of medicine's greatest triumphs and challenges involve viruses. Since viruses have evolved to exploit important cell processes, they can provide tools and approaches to manipulate cell function. The small transmembrane E5 protein of bovine papillomavirus type 1 transforms cells by a unique mechanism involving ligand-independent activation of the platelet-derived growth factor beta receptor. Experiments summarized in this review suggest that it may be possible to use the E5 protein as a model to design an entirely new class of small, modular transmembrane proteins with novel biological activities.

Imatinib mesylate (Gleevec) downregulates telomerase activity and inhibits proliferation in telomerase-expressing cell lines

Imatinib mesylate (IM) is a tyrosine kinase inhibitor, which inhibits phosphorylation of downstream proteins involved in BCR-ABL signal transduction. It has proved beneficial in treating patients with chronic myeloid leukaemia (CML). In addition, IM demonstrates activity against malignant cells expressing c-kit and platelet-derived growth factor receptor (PDGF-R). The activity of IM in the blastic crisis of CML and against various myeloma cell lines suggests that this drug may also target other cellular components. In the light of the important role of telomerase in malignant transformation, we evaluated the effect of IM on telomerase activity (TA) and regulation in various malignant cell lines. Imatinib mesylate caused a dose-dependent inhibition of TA (up to 90% at a concentration of 15 μM IM) in c-kit-expressing SK-N-MC (Ewing sarcoma), SK-MEL-28 (melanoma), RPMI 8226 (myeloma), MCF-7 (breast cancer) and HSC 536/N (Fanconi anaemia) cells as well as in ba/F3 (murine pro-B cells), which do not express c-kit, BCR-ABL or PDGF-R. Imatinib mesylate did not affect the activity of other DNA polymerases. Inhibition of TA was associated with 50% inhibition of proliferation. The inhibition of proliferation was associated with a decrease in the S-phase of the cell cycle and an accumulation of cells in the G2/M phase. No apoptosis was observed. Inhibition of TA was caused mainly by post-translational modifications: dephosphorylation of AKT and, to a smaller extent, by early downregulation of hTERT (the catalytic subunit of the enzyme) transcription. Other steps of telomerase regulation were not affected by IM. This study demonstrates an additional cellular target of IM, not necessarily mediated via known tyrosine kinases, that causes inhibition of TA and cell proliferation.

Are transforming properties of the bovine papillomavirus E5 protein shared by E5 from high-risk human papillomavirus type 16?

The E5 proteins of bovine papillomavirus type 1 (BPV-1) and human papillomavirus type 16 (HPV-16) are small (44-83 amino acids), hydrophobic polypeptides that localize to membranes of the Golgi apparatus and endoplasmic reticulum, respectively. While the oncogenic properties of BPV-1 E5 have been characterized in detail, less is known about HPV-16 E5 due to its low expression in mammalian cells. Using codon-optimized HPV-16 E5 DNA, we have generated stable fibroblast cell lines that express equivalent levels of epitope-tagged BPV-1 and HPV-16 E5 proteins. In contrast to BPV-1 E5, HPV-16 E5 does not activate growth factor receptors, phosphoinositide 3-kinase or c-Src, and fails to induce focus formation, although it does promote anchorage-independent growth in soft agar. These variant activities are apparently unrelated to differences in intracellular localization of the E5 proteins since retargeting HPV-16 E5 to the Golgi apparatus does not induce focus formation.

Productive interaction between transmembrane mutants of the bovine papillomavirus E5 protein and the platelet-derived growth factor beta receptor

The bovine papillomavirus E5 protein is a 44-amino-acid transmembrane protein that transforms cells by binding to the transmembrane region of the cellular platelet-derived growth factor (PDGF) beta receptor, resulting in sustained receptor signaling. However, there are published reports that certain mutants with amino acid substitutions in the membrane-spanning segment of the E5 protein transform cells without activating the PDGF beta receptor. We re-examined several of these transmembrane mutants, and here we present five lines of evidence that these mutants do in fact activate the PDGF beta receptor, resulting in cellular signaling and transformation.

Specific locations of hydrophilic amino acids in constructed transmembrane ligands of the platelet-derived growth factor beta receptor

The 44 amino acid E5 transmembrane protein is the primary oncogene product of bovine papillomavirus. Homodimers of the E5 protein activate the cellular PDGF beta receptor tyrosine kinase by binding to its transmembrane domain and inducing receptor dimerization, resulting in cellular transformation. To investigate the role of transmembrane hydrophilic amino acids in receptor activation, we constructed a library of dimeric small transmembrane proteins in which 16 transmembrane amino acids of the E5 protein were replaced with random, predominantly hydrophobic amino acids. A low level of hydrophilic amino acids was encoded at each of the randomized positions, including position 17, which is an essential glutamine in the wild-type E5 protein. Library proteins that induced transformation in mouse C127 cells stably bound and activated the PDGF beta receptor. Strikingly, 35% of the transforming clones had a hydrophilic amino acid at position 17, highlighting the importance of this position in activation of the PDGF beta receptor. Hydrophilic amino acids in other transforming proteins were found adjacent to position 17 or at position 14 or 21, which are in the E5 homodimer interface. Approximately 22% of the transforming proteins lacked hydrophilic amino acids. The hydrophilic amino acids in the transforming clones appear to be important for driving homodimerization, binding to the PDGF beta receptor, or both. Interestingly, several of the library proteins bound and activated PDGF beta receptor transmembrane mutants that were not activated by the wild-type E5 protein. These experiments identified transmembrane proteins that activate the PDGF beta receptor and revealed the importance of hydrophilic amino acids at specific positions in the transmembrane sequence. Our identification of transformation-competent transmembrane proteins with altered specificity suggests that this approach may allow the creation and identification of transmembrane proteins that modulate the activity of a variety of receptor tyrosine kinases.

Selection and characterization of small random transmembrane proteins that bind and activate the platelet-derived growth factor beta receptor

Growth factor receptors are typically activated by the binding of soluble ligands to the extracellular domain of the receptor, but certain viral transmembrane proteins can induce growth factor receptor activation by binding to the receptor transmembrane domain. For example, homodimers of the transmembrane 44-amino acid bovine papillomavirus E5 protein bind the transmembrane region of the PDGF beta receptor tyrosine kinase, causing receptor dimerization, phosphorylation, and cell transformation. To determine whether it is possible to select novel biologically active transmembrane proteins that can activate growth factor receptors, we constructed and identified small proteins with random hydrophobic transmembrane domains that can bind and activate the PDGF beta receptor. Remarkably, cell transformation was induced by approximately 10% of the clones in a library in which 15 transmembrane amino acid residues of the E5 protein were replaced with random hydrophobic sequences. The transformation-competent transmembrane proteins formed dimers and stably bound and activated the PDGF beta receptor. Genetic studies demonstrated that the biological activity of the transformation-competent proteins depended on specific interactions with the transmembrane domain of the PDGF beta receptor. A consensus sequence distinct from the wild-type E5 sequence was identified that restored transforming activity to a non-transforming poly-leucine transmembrane sequence, indicating that divergent transmembrane sequence motifs can activate the PDGF beta receptor. Molecular modeling suggested that diverse transforming sequences shared similar protein structure, including the same homodimer interface as the wild-type E5 protein. These experiments have identified novel proteins with transmembrane sequences distinct from the E5 protein that can activate the PDGF beta receptor and transform cells. More generally, this approach may allow the creation and identification of small proteins that modulate the activity of a variety of cellular transmembrane proteins.

Molecular examination of the transmembrane requirements of the platelet-derived growth factor beta receptor for a productive interaction with the bovine papillomavirus E5 oncoprotein

The small transmembrane E5 protein of bovine papillomavirus (BPV) transforms cells by forming a stable complex with and activating the platelet-derived growth factor beta receptor (PDGFbetaR). The E5/PDGFbetaR interaction is thought to involve specific physical contacts between the transmembrane domains of the two proteins. Lys(499) at the extracellular juxtamembrane position and Thr(513) within the transmembrane domain of the PDGFbetaR are required for the interaction and are predicted to contact analogously positioned residues in the E5 protein. Here, mutagenic analysis of the transmembrane region of the PDGFbetaR was performed to further characterize the nature of the E5/PDGFbetaR interaction. We show that the receptor transmembrane domain, with minimal extracellular and intracellular sequence, is sufficient for the interaction. In addition, we provide evidence that the polar nature of Thr(513) as well as its positioning along the transmembrane alpha-helix is important for the interaction. We also identify the receptor transmembrane amino acids Ile(506) and Leu(520) as additional requirements for the interaction. Because Lys(499), Thr(513), Ile(506), and Leu(520) all align along the same face of the predicted PDGFbetaR transmembrane alpha-helix, our data support the model that the PDGFbetaR contacts the E5 protein via multiple amino acids along a single alpha-helical interface.

Definition of an inhibitory juxtamembrane WW-like domain in the platelet-derived growth factor beta receptor

A variety of tumors contain activating mutations in the cytoplasmic juxtamembrane domain of the type III family of receptor-tyrosine kinases, and some constructed mutations in this domain induce ligand-independent receptor activation. To explore the role of this domain in regulation of receptor activity, we subjected the juxtamembrane domain of the murine platelet-derived growth factor (PDGF) beta receptor to alanine-scanning mutagenesis. The mutant receptors were expressed in Ba/F3 cells and tested for constitutive tyrosine phosphorylation, association with phosphatidylinositol 3'-kinase, and their ability to induce cell survival and proliferation in the absence of interleukin-3. The mutant receptors accumulated to similar levels and appeared to undergo a normal PDGF-induced increase in tyrosine phosphorylation. Alanine substitutions at numerous positions located throughout the juxtamembrane domain caused constitutive receptor activation, as did an alanine insertion in the membrane-proximal segment of the juxtamembrane domain and a six-amino acid deletion in the center of the domain. It is possible to model the PDGF receptor juxtamembrane domain as a short alpha-helix followed by a three-stranded beta-sheet very similar to the known structures of WW domains. Strikingly, the activating mutations clustered in the central portions of the first and second beta strands and along one face of the beta-sheet, whereas the loops connecting the strands were largely devoid of mutationally sensitive positions. These findings provide strong support for the model that the activating mutations in the juxtamembrane region stimulate receptor activity by disrupting an inhibitory WW-like domain.

Multiple transmembrane amino acid requirements suggest a highly specific interaction between the bovine papillomavirus E5 oncoprotein and the platelet-derived growth factor beta receptor

The bovine papillomavirus E5 protein activates the cellular platelet-derived growth factor beta receptor (PDGFbetaR) tyrosine kinase in a ligand-independent manner. Evidence suggests that the small transmembrane E5 protein homodimerizes and physically interacts with the transmembrane domain of the PDGFbetaR, thereby inducing constitutive dimerization and activation of this receptor. Amino acids in the receptor previously found to be required for the PDGFbetaR-E5 interaction are a transmembrane Thr513 and a juxtamembrane Lys499. Here, we sought to determine if these are the only two receptor amino acids required for an interaction with the E5 protein. Substitution of large portions of the PDGFbetaR transmembrane domain indicated that additional amino acids in both the amino and carboxyl halves of the receptor transmembrane domain are required for a productive interaction with the E5 protein. Indeed, individual amino acid substitutions in the receptor transmembrane domain identified roles for the extracellular proximal transmembrane residues in the interaction. These data suggest that multiple amino acids within the transmembrane domain of the PDGFbetaR are required for a stable interaction with the E5 protein. These may be involved in direct protein-protein contacts or may support the proper transmembrane alpha-helical conformation for optimal positioning of the primary amino acid requirements.

The presence of a single tyrosine residue at the carboxyl domain of vascular endothelial growth factor receptor-2/FLK-1 regulates its autophosphorylation and activation of signaling molecules

Vascular endothelial growth factor receptor (VEGFR)-2 plays a critical role in vasculogenesis during embryonic development and pathological angiogenesis, but little is known about the molecular mechanisms governing its functions. Here we investigated the role of tyrosine 1212 on mouse VEGFR-2 autophosphorylation and its signal transduction relay in endothelial cells. Mutation of tyrosine 1212 on VEGFR-2 to phenylalanine severely impaired the ligand-dependent autophosphorylation of VEGFR-2 and its ability to associate with and activate Src. This mutation also reduced the VEGFR-2 ability to phosphorylate phospholipase Cgamma1 and mitogen-activated protein kinase (MAPK). Unlike mutation of tyrosine 1212 to phenylalanine, replacement of tyrosine 1212 with glutamic acid preserved the ligand-dependent activation of VEGFR-2 and activation of VEGFR-2-associated signaling proteins including Src, phospholipase Cgamma1, and MAPK. Further analysis showed that Src activation is not required for activation of VEGFR-2, since cells co-expressing wild type receptor with kinase dead Src or wild type Src displayed no apparent effect in the ligand-dependent autophosphorylation of VEGFR-2. Similarly, expression of wild type VEGFR-2 in fibroblast (SYF) cells obtained from the triple knockout Src family kinases showed normal ligand-dependent autophosphorylation. Collectively, these results suggest that phosphorylation of tyrosine 1212 of VEGFR-2 plays a crucial role in the activation of VEGFR-2 and subsequently VEGFR-2-mediated angiogenesis.

c-Src activation by the E5 oncoprotein enables transformation independently of PDGF receptor activation

The E5 oncoprotein of bovine papillomavirus type 1 is a Golgi-resident, hydrophobic polypeptide that can transform immortalized fibroblasts by activating endogenous platelet-derived growth factor receptor beta (PDGF-R). However, the existence of E5 mutants that dissociate transformation from PDGF-R activation implies that there are additional mechanism(s) by which E5 can transform cells. We now show that both wt E5, and transforming E5 mutants that are defective for PDGF-R activation, constitutively activate endogenous c-Src in NIH3T3 cell lines to levels normally associated with acute growth factor stimulation. The ubiquitous Src family protein tyrosine kinase (PTK) Fyn is not activated by these E5 constructs, nor are focal adhesion kinase and endogenous receptor PTKs for insulin, epidermal growth factor, basic fibroblast growth factor and insulin-like growth factor. We further demonstrate that transforming activity of the L26A E5 mutant, which is highly defective for PDGF-R activation, depends on its ability to activate Src. L26A E5 does not transform SYF cells that are deficient for Src, Fyn and Yes, unless Src expression is reconstituted, and does not transform NIH3T3 cells in which Src PTK activity is maintained at a basal level by means of kinase-defective K295R Src overexpression.

The TEL/PDGFbetaR fusion in chronic myelomonocytic leukemia signals through STAT5-dependent and STAT5-independent pathways

The TEL/PDGFbetaR gene, which encodes a fusion protein containing the ETS-family member TEL fused to the protein-tyrosine kinase domain of the platelet-derived growth factor receptor-beta (PDGFbetaR), confers interleukin 3 (IL-3)-independent growth on Ba/F3 hematopoietic cells. TEL/PDGFbetaR mutants have been generated that contain tyrosine-to-phenylalanine (Tyr-->Phe) substitutions at phosphorylation sites present in the native PDGFbetaR to assess the role of these sites in cell transformation by TEL/PDGFbetaR. Similar to previous findings in a murine bone marrow transplantation model, full transformation of Ba/F3 cells to IL-3-independent survival and proliferation required the TEL/PDGFbetaR juxtamembrane and carboxy terminal phosphorylation sites. In contrast to previous reports concerning comparable mutants in the native PDGFbetaR, each of the TEL/PDGFbetaR mutants is fully active as a protein-tyrosine kinase. Expression of the TEL/PDGFbetaR fusion protein causes hyperphosphorylation and activation of signal transducer and activator of transcription (STAT5), and this activation of STAT5 requires the juxtamembrane Tyr579 and Tyr581 in the TEL/PDGFbetaR fusion. Hyperphosphosphorylation of phospholipase Cgamma (PLCgamma) and the p85 subunit of phosphatidylinositol 3-kinase (PI3K) requires the carboxy terminal tyrosine residues of TEL/PDGFbetaR. Thus, full transformation of Ba/F3 cells by TEL/PDGFbetaR requires engagement of PI3K and PLCgamma and activation of STAT5. Taken together with the growth properties of cells transformed by the TEL/PDGFbetaR variants, these findings indicate that a minimal combination of these signaling intermediates contributes to hematopoietic transformation by the wild-type TEL/PDGFbetaR fusion. (Blood. 2001;98:3390-3397)

Mechanisms of cell transformation by papillomavirus E5 proteins

The papillomavirus E5 proteins are short, hydrophobic transforming proteins. The transmembrane E5 protein encoded by bovine papillomavirus transforms cells by activating the platelet-derived growth factor beta receptor tyrosine kinase in a ligand-independent fashion. The bovine papillomavirus E5 protein forms a stable complex with the receptor, thereby inducing receptor dimerization and activation, trans-phosphorylation, and recruitment of cellular signaling proteins to the receptor. The E5 proteins of the human papillomaviruses also appear to affect the activity of growth factor receptors and their signaling pathways. The interaction of papillomavirus E5 proteins with a subunit of the vacuolar ATPase may also contribute to transformation. Further analysis of these unique mechanisms of viral transformation will yield new insight into the regulation of growth factor receptor activity and cellular signal transduction pathways.

Cell transformation by the E5/E8 protein of bovine papillomavirus type 4. p27(Kip1), Elevated through increased protein synthesis is sequestered by cyclin D1-CDK4 complexes

The E5/E8 hydrophobic protein of BPV-4 is, at only 42 residues, the smallest transforming protein identified to date. Transformation of NIH-3T3 cells by E5/E8 correlates with up-regulation of both cyclin A-associated kinase activity and, unusually, p27(Kip1) (p27) but does not rely on changes in cyclin E or cyclin E-CDK2 activity. Here we have examined how p27 is prevented from functioning efficiently as a CDK2 inhibitor, and we investigated the mechanisms used to achieve elevated p27 expression in E5/E8 cells. Our results show that normal subcellular targeting of p27 is not subverted in E5/E8 cells, and p27 retains its ability to inhibit both cyclin E-CDK2 and cyclin A-CDK activities upon release from heat-labile complexes. E5/E8 cells also have elevated levels of cyclins D1 and D3, and high levels of nuclear p27 are tolerated because the inhibitor is sequestered within an elevated pool of cyclin D1-CDK4 complexes, a significant portion of which retain kinase activity. In agreement with this, pRB is constitutively hyperphosphorylated in E5/E8 cells in vivo. The increased steady-state level of p27 is achieved largely through an increased rate of protein synthesis and does not rely on changes in p27 mRNA levels or protein half-life. This is the first report of enhanced p27 synthesis as the main mechanism for increasing protein levels in continuously cycling cells. Our results are consistent with a model in which E5/E8 promotes a coordinated elevation of cyclin D1-CDK4 and p27, as well as cyclin A-associated kinase activity, which act in concert to allow continued proliferation in the absence of mitogens.

Identification of the transmembrane dimer interface of the bovine papillomavirus E5 protein

We have developed a genetic method to determine the active orientation of dimeric transmembrane protein helices. The bovine papillomavirus E5 protein, a 44-amino acid homodimeric protein that appears to traverse membranes as a left-handed coiled-coil, transforms fibroblasts by binding and activating the platelet-derived growth factor (PDGF) beta receptor. A heterologous dimerization domain was used to force E5 monomers to adopt all seven possible symmetric coiled-coil registries relative to one another within the dimer. Focus formation assays demonstrated that dimerization of the E5 protein is required for transformation and identified a single preferred orientation of the monomers. The essential glutamine residue at position 17 resided in the dimer interface in this active orientation. The active chimera formed complexes with the PDGF beta receptor and induced receptor tyrosine phosphorylation. We also identified E5-like structures that underwent non-productive interactions with the receptor.

H4(D10S170), a gene frequently rearranged in papillary thyroid carcinoma, is fused to the platelet-derived growth factor receptor beta gene in atypical chronic myeloid leukemia with t(5;10)(q33;q22)

The molecular cloning of the t(5;10)(q33;q22) associated with atypical chronic myeloid leukemia (CML) is reported. Fluorescence in situ hybridization (FISH), Southern blot, and reverse transcriptase- polymerase chain reaction analysis demonstrated that the translocation resulted in an H4/platelet-derived growth factor receptor betaR (PDGFbetaR) fusion transcript that incorporated 5' sequences from H4 fused in frame to 3' PDGFbetaR sequences encoding the transmembrane, WW-like, and tyrosine kinase domains. FISH combined with immunophenotype analysis showed that t(5;10)(q33;q22) was present in CD13(+) and CD14(+) cells but was not observed in CD3(+) or CD19(+) cells. H4 has previously been implicated in pathogenesis of papillary thyroid carcinoma as a fusion partner of RET. The H4/RET fusion incorporates 101 amino acids of H4, predicted to encode a leucine zipper dimerization domain, whereas the H4/PDGFbetaR fusion incorporated an additional 267 amino acids of H4. Retroviral transduction of H4/PDGFbetaR, but not a kinase-inactive mutant, conferred factor-independent growth to Ba/F3 cells and caused a T-cell lymphoblastic lymphoma in a murine bone marrow transplantation assay of transformation. Mutational analysis showed that the amino-terminal H4 leucine zipper domain (amino acids 55-93), as well as H4 amino acids 101 to 386, was required for efficient induction of factor-independent growth of Ba/F3 cells. Tryptophan-to-alanine substitutions in the PDGFbetaR WW-like domain at positions 566/593, or tyrosine-to-phenylalanine substitutions at PDGFbetaR positions 579/581 impaired factor-independent growth of Ba/F3 cells. H4/PDGFbetaR is an oncoprotein expressed in t(5;10)(q33;q22) atypical CML and requires dimerization motifs in the H4 moiety, as well as residues implicated in signal transduction by PDGFbetaR, for efficient induction of factor-independent growth of Ba/F3 cells. (Blood. 2001;97:3910-3918)

The requirement of tyrosines 579 and 581 for maximal ligand-dependent activation of the betaPDGFR is influenced by noncytoplasmic regions of the receptor

Mutating tyrosines 579 and 581 of the beta platelet-derived growth factor receptor (betaPDGFR) tyrosine kinase to phenylalanines (the F2 mutation) impair activation of the receptor in response to ligand, but mutation of the analogous tyrosines in the alphaPDGFR has no effect on ligand-dependent receptor activation. We have found that the F2 mutation has only a modest effect on ligand-dependent activation of a chimeric PDGFR composed of the extracellular and transmembrane domains of the alphaPDGFR and the cytoplasmic domain of the betaPDGFR by three measures: (1) the ability to phosphorylate endogenous and exogenous protein substrates in vitro, (2) phosphorylation of tyrosine 857, and (3) binding of the effector proteins PLCgamma, RasGAP, and SHP-2. Conversely, the F2 mutation substantially impairs ligand-dependent activation of chimeric PDGFRs that consist of either the extracellular domain alone or the extracellular and transmembrane domains of the betaPDGFR and all remaining sequence from the alphaPDGFR by two measures: (1) phosphorylation of endogenous protein substrates in vitro and (2) binding of PLCgamma and SHP-2. Our results indicate that the requirement of tyrosines 579 and 581 for maximal activation of the betaPDGFR in response to ligand is primarily determined by noncytoplasmic regions of the receptor.

Bovine papillomavirus oncoprotein E5 affects the arachidonic acid metabolism in cells

The bovine papillomavirus type 1 (BPV-1) oncoprotein encoded by the E5 ORF is a small highly hydrophobic protein, which is capable of inducing oncogenic transformation of cells. We studied the effect of the BPV-1 E5 protein expression on the arachidonic acid metabolism in monkey (COS1) and human (C33A) cells. At relatively low protein concentrations the phospholipase A(2) (PLA(2)) activity and the arachidonic acid (AA) metabolism are activated. E5 mutant proteins, lacking cysteines responsible for the dimerisation of the protein (C37S, C37SC39S), and truncated E5, lacking the C-terminal region, are non-transforming and unable to stimulate the PLA(2) activity and AA metabolism. The transformation-defective mutant D33V, which does not activate the platelet-derived growth factor receptor (PDGFR), activates AA metabolism like wt E5. Our data suggest that the BPV-1 E5 protein could stimulate the AA metabolism independently of PDGF receptor.

The platelet-derived growth factor beta receptor as a target of the bovine papillomavirus E5 protein

The 44-amino acid E5 protein of bovine papillomavirus is a homo-dimeric, transmembrane protein that transforms cells by activating the platelet-derived growth factor ss receptor in a ligand-independent fashion. The E5 protein induces receptor activation by forming a stable complex with the receptor, thereby inducing receptor dimerization, trans-phosphorylation of tyrosine residues in the cytoplasmic domain of the receptor, and recruitment of cellular SH2 domain-containing proteins into a signal transduction complex. Direct interactions between specific transmembrane and juxtamembrane amino acids in the E5 protein and the PDGF ss receptor appear to drive complex formation and dimerization of the receptor. Further analysis of this unique mechanism of viral transformation promises to yield new insight into the regulation of growth factor receptor activity and cellular signal transduction pathways.

Expression of epidermal growth factor and platelet-derived growth factor receptors during cervical carcinogenesis

Altered expression of epidermal growth factor receptor (EGFR) is common in a variety of epithelial malignancies, including cervical cancer. However, the prognostic significance of EGFR expression is controversial for cervical cancer. Platelet-derived growth factor receptor (PDGFR) expression status is unknown in cervical cancer. Our results demonstrated that expression of EGFR and PDGFR was greatly enhanced in vivo and in organotypic cultures of low-grade cervical dysplastic tissues, but levels were decreased in high-grade lesions. To our knowledge, this is the first report identifying the expression of PDGFR in human epithelium. When low-grade dysplastic organotypic culture tissues were induced to differentiate more completely, EGFR expression, but not PDGFR expression, was relocalized to the basal layer as seen in normal tissues. Differentiation also induced phosphorylation of EGFR but not PDGFR. Our results suggest a role for EGFR and PDGFR during the early stages of cervical carcinogenesis, and demonstrate the facility of organotypic cultures to study the role of these growth factors in the development of cervical cancer.

TEL/PDGFbetaR fusion protein activates STAT1 and STAT5: a common mechanism for transformation by tyrosine kinase fusion proteins

OBJECTIVE

TEL/PDGFbetaR is a tyrosine kinase fusion protein associated with the pathogenesis of chronic myelomonocytic leukemia. The following experiments were undertaken to understand the mechanisms whereby TEL/PDGFbetaR transforms cells.

MATERIALS AND METHODS

Activation of JAK and STAT proteins was studied in an interleukin 3 (IL-3)-dependent cell line, Ba/F3, transformed to IL-3 independence by TEL/PDGFbetaR.

RESULTS

TEL/PDGFbetaR activates STAT1 and STAT5 in transformed Ba/F3 cells through a JAK-independent pathway. Activation of STAT proteins requires the kinase activity of TEL/PDGFbetaR. JAK1, JAK2, JAK3, and TYK2 are not phosphorylated by TEL/PDGFbetaR. However, TEL/PDGFbetaR can phosphorylate STAT5 in transiently transfected COS cells, suggesting that TEL/PDGFbetaR may itself be the kinase involved in tyrosine phosphorylation of STAT proteins. In contrast, native PDGFbetaR stimulated by PDGF ligand does not activate STAT proteins to a significant degree in this hematopoietic context. STAT1 and STAT5 also are activated by TEL/ABL and TEL/JAK2 fusion proteins associated with human leukemia.

CONCLUSIONS

STAT activation may be a common mechanism of transformation by leukemogenic tyrosine kinase fusion proteins.

Bovine papillomavirus E5 protein induces the formation of signal transduction complexes containing dimeric activated platelet-derived growth factor beta receptor and associated signaling proteins

The bovine papillomavirus E5 protein binds to the cellular platelet-derived growth factor (PDGF) beta receptor, resulting in constitutive activation of the receptor and cell growth transformation. By subjecting extracts from E5-transformed or PDGF-treated cells to velocity sedimentation in sucrose gradients, activated PDGF beta receptor complexes were separated from monomeric, inactive receptor. Rapidly sedimenting activated complexes contained oligomeric (apparently dimeric), tyrosine-phosphorylated PDGF beta receptor, the E5 protein, and associated cellular signaling proteins including the p85 subunit of phosphoinositol 3'-kinase, phospholipase Cgamma, and Ras-GTPase activating protein. These signaling proteins made the major contribution to the increased sedimentation rate of the activated receptor complexes. Pairwise analysis of components of these complexes indicated that multiple signaling proteins and the E5 protein were simultaneously present in the activated complexes. Our results also showed that the E5 protein and PDGF activated only a small fraction of the total PDGF beta receptor, that not all receptor molecules associated with the E5 protein were tyrosine-phosphorylated, and that signaling proteins could bind to hemiphosphorylated receptor dimers. On the basis of these results, we propose a model for the assembly of multiprotein, activated PDGF beta receptor complexes in response to the E5 protein.

Virocrine transformation: the intersection between viral transforming proteins and cellular signal transduction pathways

This review describes a mechanism of viral transformation involving activation of cellular signaling pathways. We focus on four viral oncoproteins: the E5 protein of bovine papillomavirus, which activates the platelet-derived growth factor beta receptor; gp55 of spleen focus forming virus, which activates the erythropoietin receptor; polyoma virus middle T antigen, which resembles an activated receptor tyrosine kinase; and LMP-1 of Epstein-Barr virus, which mimics an activated tumor necrosis factor receptor. These examples indicate that diverse viruses induce cell transformation by activating cellular signal transduction pathways. Study of this mechanism of viral transformation will provide new insights into viral tumorigenesis and cellular signal transduction.

E5 oncoprotein mutants activate phosphoinositide 3-kinase independently of platelet-derived growth factor receptor activation

The E5 oncoprotein of bovine papillomavirus type 1 is a Golgi-resident, 44-amino acid polypeptide that can transform fibroblast cell lines by activating endogenous platelet-derived growth factor receptor beta (PDGF-R). However, the recent discovery of E5 mutants that exhibit strong transforming activity but minimal PDGF-R tyrosine phosphorylation indicates that E5 can potentially use additional signal transduction pathway(s) to transform cells. We now show that two classes of E5 mutants, despite poorly activating the PDGF-R, induce tyrosine phosphorylation and activation of phosphoinositide 3-kinase (PI 3-K) and that this activation is resistant to a selective inhibitor of PDGF-R kinase activity, tyrphostin AG1296. Consistent with this independence from PDGF-R signaling, the E5 mutants fail to induce significant cell proliferation in the absence of PDGF, unlike wild-type E5 or the sis oncoprotein. Despite differences in growth factor requirements, however, both wild-type E5 and mutant E5 cell lines form colonies in agarose. Interestingly, activation of PI 3-K occurs without concomitant activation of the ras-dependent mitogen-activated protein kinase pathway. The known ability of constitutively activated PI 3-K to induce anchorage-independent cell proliferation suggests a mechanism by which the mutant E5 proteins transform cells.