Sustained plasma hepcidin suppression and iron elevation by Anticalin-derived hepcidin antagonist in cynomolgus monkey

BACKGROUND AND PURPOSE

Anaemia of chronic disease (ACD) has been linked to iron-restricted erythropoiesis imposed by high circulating levels of hepcidin, a 25 amino acid hepatocyte-derived peptide that controls systemic iron homeostasis. Here, we report the engineering of the human lipocalin-derived, small protein-based anticalin PRS-080 hepcidin antagonist with high affinity and selectivity.

EXPERIMENTAL APPROACH

Anticalin- and hepcidin-specific pharmacokinetic (PK)/pharmacodynamic modelling (PD) was used to design and select the suitable drug candidate based on t_1/2 extension and duration of hepcidin suppression. The development of a novel free hepcidin assay enabled accurate analysis of bioactive hepcidin suppression and elucidation of the observed plasma iron levels after PRS-080-PEG30 administration in vivo.

KEY RESULTS

PRS-080 had a hepcidin-binding affinity of 0.07 nM and, after coupling to 30 kD PEG (PRS-080-PEG30), a t_1/2 of 43 h in cynomolgus monkeys. Dose-dependent iron mobilization and hepcidin suppression were observed after a single i.v. dose of PRS-080-PEG30 in cynomolgus monkeys. Importantly, in these animals, suppression of free hepcidin and subsequent plasma iron elevation were sustained during repeated s.c. dosing. After repeated dosing and followed by a treatment-free interval, all iron parameters returned to pre-dose values.

CONCLUSIONS AND IMPLICATIONS

In conclusion, we developed a dose-dependent and safe approach for the direct suppression of hepcidin, resulting in prolonged iron mobilization to alleviate iron-restricted erythropoiesis that can address the root cause of ACD. PRS-080-PEG30 is currently in early clinical development.

Multimodality multiparametric imaging of early tumor response to a novel antiangiogenic therapy based on anticalins

Anticalins are a novel class of targeted protein therapeutics. The PEGylated Anticalin Angiocal (PRS-050-PEG40) is directed against VEGF-A. The purpose of our study was to compare the performance of diffusion weighted imaging (DWI), dynamic contrast enhanced magnetic resonance imaging (DCE)-MRI and positron emission tomography with the tracer [18F]fluorodeoxyglucose (FDG-PET) for monitoring early response to antiangiogenic therapy with PRS-050-PEG40. 31 mice were implanted subcutaneously with A673 rhabdomyosarcoma xenografts and underwent DWI, DCE-MRI and FDG-PET before and 2 days after i.p. injection of PRS-050-PEG40 (n = 13), Avastin (n = 6) or PBS (n = 12). Tumor size was measured manually with a caliper. Imaging results were correlated with histopathology. In the results, the tumor size was not significantly different in the treatment groups when compared to the control group on day 2 after therapy onset (P = 0.09). In contrast the imaging modalities DWI, DCE-MRI and FDG-PET showed significant differences between the therapeutic compared to the control group as early as 2 days after therapy onset (P<0.001). There was a strong correlation of the early changes in DWI, DCE-MRI and FDG-PET at day 2 after therapy onset and the change in tumor size at the end of therapy (r = -0.58, 0.71 and 0.67 respectively). The imaging results were confirmed by histopathology, showing early necrosis and necroptosis in the tumors. Thus multimodality multiparametric imaging was able to predict therapeutic success of PRS-050-PEG40 and Avastin as early as 2 days after onset of therapy and thus promising for monitoring early response of antiangiogenic therapy.

First-in-human phase I study of PRS-050 (Angiocal), an Anticalin targeting and antagonizing VEGF-A, in patients with advanced solid tumors

Background

To report the nonrandomized first-in-human phase I trial of PRS-050, a novel, rationally engineered Anticalin based on human tear lipocalin that targets and antagonizes vascular endothelial growth factor A (VEGF-A).

Methods

Patients with advanced solid tumors received PRS-050 at 0.1 mg/kg to 10 mg/kg by IV in successive dosing cohorts according to the 3+3 escalation scheme. The primary end point was safety.

Results

Twenty-six patients were enrolled; 25 were evaluable. Two patients experienced dose-limiting toxicity, comprising grade (G) 3 hypertension and G3 pyrexia, respectively. The maximum tolerated dose was not reached. Most commonly reported treatment-emergent adverse events (AEs) included chills (52%; G3, 4%), fatigue (52%; G3, 4%), hypertension (44%; G3, 16%), and nausea (40%, all G1/2). No anti–PRS-050 antibodies following multiple administration of the drug were detected. PRS-050 showed dose-proportional pharmacokinetics (PK), with a terminal half-life of approximately 6 days. Free VEGF-A was detectable at baseline in 9/25 patients, becoming rapidly undetectable after PRS-050 infusion for up to 3 weeks. VEGF-A/PRS-050 complex was detectable for up to 3 weeks at all dose levels, including in patients without detectable baseline-free VEGF-A. We also detected a significant reduction in circulating matrix metalloproteinase 2, suggesting this end point could be a pharmacodynamic (PD) marker of the drug’s activity.

Conclusions

PRS-050, a novel Anticalin with high affinity for VEGF-A, was well-tolerated when administered at the highest dose tested, 10 mg/kg. Based on target engagement and PK/PD data, the recommended phase II dose is 5 mg/kg every 2 weeks administered as a 120-minute infusion.

Trial Registration

ClinicalTrials.gov NCT01141257 http://clinicaltrials.gov/ct2/show/NCT01141257

Improved mass spectrometry assay for plasma hepcidin: detection and characterization of a novel hepcidin isoform

Mass spectrometry (MS)-based assays for the quantification of the iron regulatory hormone hepcidin are pivotal to discriminate between the bioactive 25-amino acid form that can effectively block the sole iron transporter ferroportin and other naturally occurring smaller isoforms without a known role in iron metabolism. Here we describe the design, validation and use of a novel stable hepcidin-25⁺⁴⁰ isotope as internal standard for quantification. Importantly, the relative large mass shift of 40 Da makes this isotope also suitable for easy-to-use medium resolution linear time-of-flight (TOF) platforms. As expected, implementation of hepcidin-25⁺⁴⁰ as internal standard in our weak cation exchange (WCX) TOF MS method yielded very low inter/intra run coefficients of variation. Surprisingly, however, in samples from kidney disease patients, we detected a novel peak (m/z 2673.9) with low intensity that could be identified as hepcidin-24 and had previously remained unnoticed due to peak interference with the formerly used internal standard. Using a cell-based bioassay it was shown that synthetic hepcidin-24 was, like the -22 and -20 isoforms, a significantly less potent inducer of ferroportin degradation than hepcidin-25. During prolonged storage of plasma at room temperature, we observed that a decrease in plasma hepcidin-25 was paralleled by an increase in the levels of the hepcidin-24, -22 and -20 isoforms. This provides first evidence that all determinants for the conversion of hepcidin-25 to smaller inactive isoforms are present in the circulation, which may contribute to the functional suppression of hepcidin-25, that is significantly elevated in patients with renal impairment. The present update of our hepcidin TOF MS assay together with improved insights in the source and preparation of the internal standard, and sample stability will further improve our understanding of circulating hepcidin and pave the way towards further optimization and standardization of plasma hepcidin assays.

Abstract 3875: Exploiting the Anticalin therapeutic protein platform for the treatment of cMet ligand-independent and dependent tumors - discovery and characterization of a highly specific and potent c-Met antagonist with drug-like properties

Background: Activation of the c-Met oncogenic pathway has been implicated in the development of aggressive cancers which are difficult to treat with current chemotherapies. Dimerization of c-Met receptor upon binding of Hepatocyte Growth Factor (HGF) leads to the stimulation of proliferative, migratory and survival pathways implicated in tumor development. Moreover it has recently been discovered that patients who become resistant / nonresponsive to therapies such as EGFR or VEGF inhibitors often show an enhanced c-Met expression. This has led to an increased interest in developing novel therapies that target the c-Met pathway. However, most existing drug modalities are confounded by their inability to specifically target and/or antagonize this pathway. Anticalins, a novel class of small biologics, are hypothesized to be ‘fit for purpose’ for developing highly specific and potent antagonists of cancer pathways. A monovalent Anticalin c-Met antagonist displaying efficacy in both ligand-dependent and independent cancer models has been developed. Methods/Results: Here we describe the in vitro and in vivo characterisation of the Anticalin c-Met antagonist PRS-110. In protein-based binding assays PRS-110 specifically binds to c-Met with high affinity and blocks HGF interaction (IC50 3.4 ± 0.7 nM). HUVEC cell proliferation assays demonstrated that PRS-110 efficiently antagonizes HGF-mediated growth. As a monovalent antagonist PRS-110 does not induce the c-Met pathway in the absence of ligand by receptor dimerization - an unwanted activation that can occur with bivalent antibodies. In mice, rats and non-human primates, PEGylated PRS-110 displayed favourable plasma elimination half-life profiles of 41 hours, 61 hours and 72 hours (T½α) respectively, with no signs of macrotoxicity. In vivo administration of PRS-110 resulted in significant, dose-dependent tumor growth inhibition in multiple xenograft models representative of ligand-dependent and ligand-independent c-Met activation. Analysis of c-Met protein levels on xenograft biopsy samples demonstrated a significant reduction in total c-Met (p&lt;0.01) and pMet (p&lt;0.01) following therapy with PRS-110. Furthermore, safety profiling using in silico epitope analysis and ex vivo human donor PBMC screening demonstrated a low risk of potential immunogenicity when compared to other approved biologic agents. Conclusion: Inhibition of the c-Met pathway by PRS-110 leads to attenuation of cell proliferation, dose-dependent tumour growth inhibition and a reduction in c-Met phosphorylation. Taken together these data indicate that the c-Met inhibitor PRS-110 has potentially broad anticancer activity which warrants evaluation in patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3875. doi:1538-7445.AM2012-3875

Abstract 3621: A monovalent Anticalin antagonist: A novel approach for inhibition of c-Met-dependent tumor growth

The c-MET receptor has important functions in embryonic development and adult organ regeneration but its tyrosine kinase activity is often aberrantly activated in human malignancies. Activation is associated with enhanced tumor cell proliferation, migration and the promotion of an invasive cell phenotype resulting in increased metastasis and diminished survival.

Both animal studies and clinical studies suggest that inhibition of this oncogene product could be beneficial and small molecules that target the enzyme activity of the kinase receptor and biologics that block the HGF/cMet pathway are under development.

Each of these has advantages and disadvantages in reducing the activity of this pathway. While the current biologics in clinical development display target specificity rarely achievable with small molecule inhibitors they are costly and often times complex to develop/manufacture, often lack the penetration of small molecules, and under some circumstances can have partial agonist activity. While small molecules are generally inexpensive to produce, specificity and toxicity, especially with kinase inhibitors, remains a problem. To address the possible limitations of these current therapeutic strategies directed at attenuation of this important oncogenic pathway, we present an alternative approach, the development of a cMet Anticalin (PRS-110).

Anticalins are targeted muteins based on the lipocalin protein scaffold and represent a new class of biologics. All lipocalins are made up of a single beta-barrel domain with four loops protruding from the opening of the calyx although they share limited sequence identity. The lipocalin family has naturally evolved to perform various binding functions. The loop residues of human tear lipocalin were randomized to generate a combinatorial library and successive rounds of panning and screening were employed to generate an Anticalin with high affinity and specificity for c-MET.

We show that this cMet Anticalin binds to cells expressing this kinase receptor with both high affinity and specificity. Importantly, we show, using a xenograft strategy, that the cMet Anticalin inhibits the growth of established tumors expressing this receptor.

Because of the small size and stable three-dimensional structure, this molecule can be produced and purified from E. coli with limited steps, high titer, and a line of sight to late stage development manufacturability and product launch.

In summary, our data suggest that the cMet Anticalin displays the specificity of anti cMet/HGF antibodies, but because of its structure, and small size it exemplifies some of the advantages of small molecules directed at this important tumor target.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3621. doi:10.1158/1538-7445.AM2011-3621

Abstract 3632: Nonclinical pharmacokinetics and safety of Angiocal, the first VEGF-A-specific alternative scaffold protein entering clinical development for cancer

Anticalins are high affinity scaffold proteins based on human lipocalins and represent a new class of biologics. Although lipocalins share limited sequence identity among each other they are all made up of a single conserved beta-barrel. The four loops protruding from the opening of the calyx contribute to the observed various binding functions of different lipocalins. The loop residues of human tear lipocalin were randomized to generate a combinatorial library and successive rounds of panning and screening were employed to generate an Anticalin with high affinity and specificity for VEGF-A, Angiocal. Angiocal was found to antagonize VEGF-A function by preventing its interaction with cognate membrane receptors. Following the initial characterization in cell-based and in vivo models as communicated previously (AACR 2008 #4077, AACR 2009 #2318) we sought to estimate the human pharmacokinetics of Angiocal by interspecies scaling. Therefore, PK profiles in mice, rats and cynomolgus monkeys were obtained and will be presented.

Therapeutic antibodies such as bevacizumab are usually well tolerated while significant toxicities occur in a subset of patients, for example leading to an enhanced risk for thromboembolic events. It has been demonstrated that bevacizumab forms multimeric immune complexes in vivo which may be a cause for these effects. Furthermore, immune complex deposition in glomeruli of the kidney may cause glomerulosclerosis. With respect to thromboembolic complications of bevacizumab use, these were mirrored in human FcgammaIIa receptor transgenic mice where administration of complexes between heparin, Bevacizumab and heparin-binding isoforms of its VEGF-A target lead to platelet aggregation and thrombosis (Meyer, T. et al. J. Thromb. Haemost. 2009, 7: 171-181). Angiocal was tested in the same model and does not cause any of these adverse effects on platelets in vivo.

GLP toxicology studies with Angiocal were performed in two relevant species to determine the NOAEL. In addition, we did not observe effects on inflammatory cytokines or complement factors in these studies while exaggerated pharmacodynamic effects consistent with potent VEGF neutralization were seen. Angiocal is currently undergoing phase Ib clinical testing in cancer patients to establish Anticalins as novel and safe class of therapeutic molecules.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3632. doi:10.1158/1538-7445.AM2011-3632

Identification of proangiogenic genes and pathways by high-throughput functional genomics: TBK1 and the IRF3 pathway

A genome-wide phenotype screen was used to identify factors and pathways that induce proliferation of human umbilical vein endothelial cells (HUVEC). HUVEC proliferation is a recognized marker for factors that modulate vascularization. Screening "hits" included known proangiogenic factors, such as VEGF, FGF1, and FGF2 and additional factors for which a direct association with angiogenesis was not previously described. These include the kinase TBK1 as well as Toll-like receptor adaptor molecule and IFN regulatory factor 3. All three proteins belong to one signaling pathway that mediates induction of gene expression, including a mixture of secreted factors, which, in concert, mediate proliferative activity toward endothelial cells. TBK1 as the "trigger" of this pathway is induced under hypoxic conditions and expressed at significant levels in many solid tumors. This pattern of expression and the decreased expression of angiogenic factors in cultured cells upon RNA-interference-mediated ablation suggests that TBK1 is important for vascularization and subsequent tumor growth and a target for cancer therapy.

Development of secreted proteins as biotherapeutic agents

As one of the most important classes of proteins, secreted factors account for about one-tenth of the human genome, 3000 - 4000 in total, including factors of signalling pathways, blood coagulation and immune defence, as well as digestive enzymes and components of the extracellular matrix. Secreted proteins are a rich source of new therapeutics and drug targets, and are currently the focus of major drug discovery programmes throughout the industry. Many of the most important novel drugs developed in biotechnology have resulted from the application of secreted proteins as therapeutics. Secreted proteins often circulate throughout the body and, therefore, have access to most organs and tissues. Because of that, many of the factors are themselves therapeutic agents. This paper gives an overview on the features and functions of human secreted proteins and peptides, as well as strategies by which to discover additional therapeutic proteins from the human 'secretome'. Furthermore, a variety of examples are provided for the therapeutic use of recombinant secreted proteins as 'biologicals', including features and applications of recombinant antibodies, erythropoietin, insulin, interferon, plasminogen activators, growth hormone and colony-stimulating factors.

Analysis of biological effects and signaling properties of Flt-1 (VEGFR-1) and KDR (VEGFR-2). A reassessment using novel receptor-specific vascular endothelial growth factor mutants

Endothelial cells express two related vascular endothelial growth factor (VEGF) receptor tyrosine kinases, KDR (kinase-insert domain containing receptor, or VEGFR-2) and Flt-1 (fms-like tyrosine kinase, or VEGFR-1). Although considerable experimental evidence links KDR activation to endothelial cell mitogenesis, there is still significant uncertainty concerning the role of individual VEGF receptors for other biological effects such as vascular permeability. VEGF mutants that bind to either KDR or Flt-1 with high selectivity were used to determine which of the two receptors serves to mediate different VEGF functions. In addition to mediating mitogenic signaling, selective KDR activation was sufficient for the activation of intracellular signaling pathways implicated in cell migration. KDR stimulation caused tyrosine phosphorylation of both phosphatidylinositol 3-kinase and phospholipase Cgamma in primary endothelial cells and stimulated cell migration. KDR-selective VEGF was also able to induce angiogenesis in the rat cornea to an extent indistinguishable from wild type VEGF. We also demonstrate that KDR, but not Flt-1, stimulation is responsible for the induction of vascular permeability by VEGF.

A repressor sequence in the juxtamembrane domain of Flt-1 (VEGFR-1) constitutively inhibits vascular endothelial growth factor-dependent phosphatidylinositol 3'-kinase activation and endothelial cell migration

Vascular endothelial growth factor (VEGF) has two highly homologous tyrosine kinase receptors: Flt-1 (VEGFR-1) and KDR (VEGFR-2). KDR is strongly phosphorylated on tyrosines and can transmit mitogenic and motogenic signals following VEGF binding, while Flt-1 is markedly less effective in mediating such functions. To dissect the regions that account for the differences between the two receptors, we generated a series of chimeric Flt-1-KDR molecules. We found that the juxtamembrane region of Flt-1 prevents key signaling functions. When the juxtamembrane region of Flt-1 is replaced by that of KDR, Flt-1 becomes competent to mediate endothelial cell migration and phosphatidylinositol 3'-kinase activation in response to VEGF. Further mutational analysis shows that a short divergent sequence is responsible for such repressor function. However, mutant Flt-1 receptors lacking this sequence do not transmit effective proliferative signals, suggesting that this receptor function is regulated separately. These results define a novel functional domain that serves to repress Flt-1 activity in endothelial cells.

Multiple ras effector pathways contribute to G(1) cell cycle progression

The involvement of Ras in the activation of multiple early signaling pathways is well understood, but it is less clear how the various Ras effectors interact with the cell cycle machinery to cause G(1) progression. Ras-mediated activation of extracellular-regulated kinase/mitogen-activated protein kinase has been implicated in cyclin D(1) up-regulation, but there is little extracellular-regulated kinase activity during the later stages of G(1), when cyclin D(1) expression becomes maximal, implying that other effector pathways may also be important in cyclin D(1) induction. We have addressed the involvement of Ras effectors from the phosphatidylinositol (PI) 3-kinase and Ral-GDS families in G(1) progression and compared it to that of the Raf/mitogen-activated protein kinase pathway. PI 3-kinase activity is required for the expression of endogenous cyclin D(1) and for S phase entry following serum stimulation of quiescent NIH 3T3 fibroblasts. Activated PI 3-kinase induces cyclin D(1) transcription and E2F activity, at least in part mediated by the serine/threonine kinase Akt/PKB, and to a lesser extent the Rho family GTPase Rac. In addition, both activated Ral-GDS-like factor and Raf stimulate cyclin D(1) transcription and E2F activity and act in synergy with PI 3-kinase. Therefore, multiple cooperating pathways mediate the effects of Ras on progression through the cell cycle.

Selective response of ternary complex factor Sap1a to different mitogen-activated protein kinase subgroups

Mitogenic and stres signals results in the activation of extracellular signal-regulated kinases (ERKs) and stress-activated protein kinase/c-Jun N-terminal kinases (SAPK/JNKs), respectively, which are two subgroups of the mitogen-activated protein kinases. A nuclear target of mitogen-activated protein (MAP) kinases is the ternary complex factor Elk-1, which underlies its involvement in the regulation of c-fos gene expression by mitogenic and stress signals. A second ternary complex factor, Sap1a, is coexpressed with Elk-1 in several cell types and shares attributes of Elk-1, the significance of which is not clear. Here we show that Sap1a is phosphorylated efficiently by ERKs but not by SAPK/JNKs. Serum response factor-dependent ternary complex formation by Sap1a is stimulated by ERK phosphorylation but not by SAPK/JNKs. Moreover, Sap1a-mediated transcription is activated by mitogenic signals but not by cell stress. These results suggest that Sap1a and Elk-1 have distinct physiological functions.

Phosphorylation-dependent formation of a quaternary complex at the c-fos SRE

The rapid and transient induction of the human proto-oncogene c-fos in response to a variety of stimuli depends on the serum responses element (SRE). In vivo footprinting experiments show that this promoter element is bound by a multicomponent complex including the serum response factor (SRF) and a ternary complex factor such as Elk-1. SRF is thought to recruit a ternary complex factor monomer into an asymmetric complex. In this report, we describe a quaternary complex over the SRE which, in addition to an SRF dimer, contains two Elk-1 molecules. Its formation at the SRE is strictly dependent on phosphorylation of S-383 in the Elk-1 regulatory domain and appears to involve a weak intermolecular association between the two Elk-1 molecules. The influence of mutations in Elk-1 on quaternary complex formation in vitro correlates with their effect on the induction of c-fos reporter expression in response to mitogenic stimuli in vivo.

Activation of ternary complex factor Elk-1 by stress-activated protein kinases

BACKGROUND

The mammalian response to stress results in the activation of stress-activated protein kinases (also known as cJun N-terminal kinases; SAPKs or JNKs), which are a sub-group of the mitogen-activated protein (MAP) kinase family. The SAPKs are involved in the upregulation of activity of the transcription factor AP-1 by post-translational modification of two of its components, cJun and ATF2. AP-1 activity can also be elevated by increased expression of the Fos protein, a further AP-1 component. Elk-1 (also called p62TCF), a transcription factor involved in the induction of the expression from the c-fos promoter through the promoter's serum response element, is known to be activated as a result of phosphorylation by the MAP kinases ERK1 and ERK2. However, induction of c-fos expression in response to noxious agents takes place in the absence of ERK activation. We therefore investigated whether SAPKs similarly upregulate c-fos expression by phosphorylating Elk-1.

RESULTS

Elk-1 is activated in response to stimuli other than mitogenic signals. Both p46SAPK and p54SAPK interact physically with, and phosphorylate, Elk-1. The capacity of Elk-1 to form a ternary complex with serum response factor in vitro is thereby elevated. In vivo, selective activation of SAPKs stimulates formation of the ternary complex containing Elk-1, serum response factor and the serum response element, and enhances Elk-1-dependent transcription. Expression of the SAPK upstream-activator kinase, MEKK1, induces SAPK activation and c-fos transcription in the absence of ERK activity. Phosphopeptide mapping of Elk-1 phosphorylated with p46SAPK or p54SAPK reveals Ser383, a residue critical for ternary complex formation and transcriptional activation, to be the major phosphorylation site.

CONCLUSION

Elk-1 responds to stress-induced, as well as mitogenic, signals by stimulating c-fos transcription through the serum response element. Phosphorylation of Elk-1 by SAPKs and the ensuing expression of Fos protein thus constitutes an additional mechanism by which cells can upregulate AP-1 activity in response to stress.

ERK phosphorylation potentiates Elk-1-mediated ternary complex formation and transactivation

Induction of the human c-fos proto-oncogene by mitogens depends on the formation of a ternary complex by p62TCF with the serum response factor (SRF) and the serum response element (SRE). We demonstrate that Elk-1, a protein closely related to p62TCF in function, is a nuclear target of two members of the MAP kinase family, ERK1 and ERK2. Phosphorylation of Elk-1 increases the yield of ternary complex in vitro. At least five residues in the C-terminal domain of Elk-1 are phosphorylated upon growth factor stimulation of NIH3T3 cells. These residues are also phosphorylated by purified ERK1 in vitro, as determined by a combination of phosphopeptide sequencing and 2-D peptide mapping. Conversion of two of these phospho-acceptor sites to alanine impairs the formation of ternary complexes by the resulting Elk-1 proteins. Removal of these serine residues also drastically diminishes activation of the c-fos promoter in epidermal growth factor-treated cells. Analogous mutations at other sites impair activation to a lesser extent without affecting ternary complex formation in vitro. Our results indicate that phosphorylation regulates ternary complex formation by Elk-1, which is a prerequisite for the manifestation of its transactivation potential at the c-fos SRE.

Identification of amino acids essential for DNA binding and dimerization in p67SRF: implications for a novel DNA-binding motif

The serum response factor (p67SRF) binds to a palindromic sequence in the c-fos serum response element (SRE). A second protein, p62TCF binds in conjunction with p67SRF to form a ternary complex, and it is through this complex that growth factor-induced transcriptional activation of c-fos is thought to take place. A 90-amino-acid peptide, coreSRF, is capable for dimerizing, binding DNA, and recruiting p62TCF. By using extensive site-directed mutagenesis we have investigated the role of individual coreSRF amino acids in DNA binding. Mutant phenotypes were defined by gel retardation and cross-linking analyses. Our results have identified residues essential for either DNA binding or dimerization. Three essential basic amino acids whose conservative mutation severely reduced DNA binding were identified. Evidence which is consistent with these residues being on the face of a DNA binding alpha-helix is presented. A phenylalanine residue and a hexameric hydrophobic box are identified as essential for dimerization. The amino acid phasing is consistent with the dimerization interface being presented as a continuous region on a beta-strand. A putative second alpha-helix acts as a linker between these two regions. This study indicates that p67SRF is a member of a protein family which, in common with many DNA binding proteins, utilize an alpha-helix for DNA binding. However, this alpha-helix is contained within a novel domain structure.

Phosphorylation of transcription factor p62TCF by MAP kinase stimulates ternary complex formation at c-fos promoter

Transcription of the proto-oncogene c-fos is stimulated rapidly and transiently by serum growth factors and mitogens. Critical for this response is the serum-response element which is bound in vivo in a ternary complex containing the transcription factors p67SRF and p62TCF (ref. 2). Disruption of the ternary complex correlates with impaired induction by serum and phorbol ester. Mitogen-activated protein (MAP) kinase is a serine/threonine kinase which is activated 1-5 minutes after treatment of cells with mitogens and growth factors that induce re-entry into the cell cycle, making MAP kinase a candidate for the transmission of proliferative signals. Here we show that p62TCF is phosphorylated by MAP kinase in vitro and that phosphorylation results in enhanced ternary complex formation. Serum-starved Swiss 3T3 cells treated with epidermal growth factor, which induces MAP kinase in these cells, are induced to express c-fos and yield p62TCF active in ternary complex formation. In contrast, treatment of Swiss 3T3 cells with insulin, which does not activate MAP kinase under these conditions, does not lead to enhanced ternary complex formation nor does it induce c-fos transcription. Our results link the expression of the human c-fos proto-oncogene to signal transduction pathways known to be activated before its own induction.

Mutations in the DnaA binding sites of the replication origin of Escherichia coli

Mutations (base changes) were introduced into the four DnaA binding sites (DnaA boxes) of the Escherichia coli replication origin, oriC. Mutations in a single DnaA box did not impair the ability of these origins to replicate in vivo and in vitro. A combination of mutations in two DnaA boxes, R1 and R4, resulted in slower growth of the oriC plasmid-bearing host cells. DnaA protein interaction with mutant and wild-type DnaA boxes was analyzed by DNase I footprinting. Binding of DnaA protein to a mutated DnaA box R1 was not affected by a mutation in DnaA box R4 and vice versa. Mutations in DnaA boxes R1 and R4 did not modify the ability of the DnaA protein to bind to other DnaA boxes in oriC.

The FIS protein binds and bends the origin of chromosomal DNA replication, oriC, of Escherichia coli

The FIS protein (factor for inversion stimulation) is known to stimulate site-specific recombination processes, such as the inversion of the G segment of bacteriophage Mu, by binding to specific enhancer sequences. It has also been shown to activate transcription from rRNA promoters both in vitro and in vivo. We have identified a specific binding site for FIS in the center of the origin of chromosomal DNA replication, oriC. The DNA bends upon FIS binding. Occupation of the FIS site and binding of DnaA, the initiator protein, to its adjacent binding site (R3) are mutually exclusive. A fis mutant strain can not be efficiently transformed with plasmids which carry and replicate from oriC, suggesting that FIS is required for minichromosome replication.

Localized DNA melting and structural pertubations in the origin of replication, oriC, of Escherichia coli in vitro and in vivo

The leftmost region of the Escherichia coli origin of DNA replication (oriC) contains three tandemly repeated AT-rich 13mers which have been shown to become single-stranded during the early stages of initiation in vitro. Melting is induced by the ATP form of DnaA, the initiator protein of DNA replication. KMnO4 was used to probe for single-stranded regions and altered DNA conformation during the initiation of DNA replication at oriC in vitro and in vivo. Unpairing in the AT-rich 13mer region is thermodynamically stable even in the absence of DnaA protein, but only when divalent cations are omitted from the reaction. In the presence of Mg2+, oriC melting is strictly DnaA dependent. The sensitive region is distinct from that detected in the absence of DnaA as it is located further to the left within the minimal origin. In addition, the DNA is severely distorted between the three 13mers and the IHF binding site in oriC. A change of conformation can also be observed during the initiation of DNA replication in vivo. This is the first in vivo evidence for a structural change at the 13mers during initiation complex formation.

The complex of oriC DNA with the DnaA initiator protein

We describe several experimental approaches relating to the early steps in the initiation of DNA replication at oriC. 1) A matrix is given which enables calculatation of the relative affinity of DnaA boxes for DnaA protein; 2) base changes within single Dna A boxes in oriC have little effect on oriC function; 3) mutations which change the distance between DnaA boxes inactivate oriC, but changes by one helical turn (+ and -) result in near wild-type oriC activity; 4) a Fis binding site was located at oriC coordinates 206-220; 5) KMnO4 probing demonstrates Dna-A-dependent unwinding in the left part of oriC in vivo and in vitro.