Laura A. Lopez-Garcia

3,5-Diphenylpent-2-enoic acids as allosteric activators of the protein kinase PDK1: structure-activity relationships and thermodynamic characterization of binding as paradigms for PIF-binding pocket-targeting compounds.

The modulation of protein kinase activities by low molecular weight compounds is a major goal of current pharmaceutical developments. In this line, important efforts are directed to the development of drugs targeting the conserved ATP binding site. However, there is very little experience on targeting allosteric, regulatory sites, different from the ATP binding site, in protein kinases. Here we describe the synthesis, cell-free activation potency, and calorimetric binding analysis of 3,5-diphenylpent-2-enoic acids and derivatives as allosteric modulators of the phosphoinositide-dependent kinase-1 (PDK1) catalytic activity. Our SAR results combined with thermodynamic binding analyses revealed both favorable binding enthalpy and entropy and confirmed the PIF-binding pocket of PDK1 as a druggable site. In conclusion, we defined the minimal structural requirements for compounds to bind to the PIF-binding pocket and to act as allosteric modulators and identified two new lead structures (12Z and 13Z) with predominating binding enthalpy.

Substrate-Selective Inhibition of Protein Kinase PDK1 by Small Compounds that Bind to the PIF-Pocket Allosteric Docking Site

The PIF-pocket of AGC protein kinases participates in the physiologic mechanism of regulation by acting as a docking site for substrates and as a switch for the transduction of the conformational changes needed for activation or inhibition. We describe the effects of compounds that bind to the PIF-pocket of PDK1. In vitro, PS210 is a potent activator of PDK1, and the crystal structure of the PDK1-ATP-PS210 complex shows that PS210 stimulates the closure of the kinase domain. However, in cells, the prodrug of PS210 (PS423) acts as a substrate-selective inhibitor of PDK1, inhibiting the phosphorylation and activation of S6K, which requires docking to the PIF-pocket, but not affecting PKB/Akt. This work describes a tool to study the dynamics of PDK1 activity and a potential approach for drug discovery.

4-benzimidazolyl-3-phenylbutanoic acids as novel PIF-pocket-targeting allosteric inhibitors of protein kinase PKCζ.

Protein kinase inhibitors with an allosteric mode of action are expected to reach, in many cases, higher selectivity for the target enzyme than ATP-competitive compounds. Therefore, basic research is aiming at identifying and establishing novel sites on the catalytic domain of protein kinases which might be targeted by allosteric inhibitors. We previously published the first structure-activity relationships (SARs) for allosteric activators of protein kinase PDK1. Here, we present the design, synthesis, and SAR data on a series of novel compounds, 4-benzimidazolyl-3-phenylbutanoic acids, that inhibit the atypical protein kinace C (PKC) ζ via binding to the PIF-pocket. Key positions were identified in the compounds that can be modified to increase potency and selectivity. Some congeners showed a high selectivity toward PKCζ, lacking inhibition of the most closely related isoform, PKCι, and of further AGC kinases. Furthermore, evidence is provided that these compounds are also active toward cellular PKCζ without loss of potency compared to the cell-free assay.

Regulation of the interaction between protein kinase C-related protein kinase 2(PRK2) and its upstream kinase, 3-phosphoinositide-dependent protein kinase 1 (PDK1)

The members of the AGC kinase family frequently exhibit three conserved phosphorylation sites: the activation loop, the hydrophobic motif (HM), and the zipper (Z)/turn-motif (TM) phosphorylation site. 3-Phosphoinositide-dependent protein kinase 1 (PDK1) phosphorylates the activation loop of numerous AGC kinases, including the protein kinase C-related protein kinases (PRKs). Here we studied the docking interaction between PDK1 and PRK2 and analyzed the mechanisms that regulate this interaction. In vivo labeling of recombinant PRK2 by 32Pi revealed phosphorylation at two sites, the activation loop and the Z/TM in the C-terminal extension. We provide evidence that phosphorylation of the Z/TM site of PRK2 inhibits its interaction with PDK1. Our studies further provide a mechanistic model to explain different steps in the docking interaction and regulation. Interestingly, we found that the mechanism that negatively regulates the docking interaction of PRK2 to the upstream kinase PDK1 is directly linked to the activation mechanism of PRK2 itself. Finally, our results indicate that the mechanisms underlying the regulation of the interaction between PRK2 and PDK1 are specific for PRK2 and do not apply for other AGC kinases.

2-(3-Oxo-1,3-diphenylpropyl)malonic acids as potent allosteric ligands of the PIF pocket of phosphoinositide-dependent kinase-1: development and prodrug concept.

The protein kinase C-related kinase 2 (PRK2)-interacting fragment (PIF) pocket of phosphoinositide-dependent kinase-1 (PDK1) was proposed as a novel target site for allosteric modulators. In the present work, we describe the design, synthesis, and structure-activity relationship of a series of 2-(3-oxo-1,3-diphenylpropyl)malonic acids as potent allosteric activators binding to the PIF pocket. Some congeners displayed AC(50) values for PDK1 activation in the submicromolar range. The potency of the best compounds to stabilize PDK1 in a thermal stability shift assay was in the same order of magnitude as that of the PIF pocket binding peptide PIFtide, suggesting comparable binding affinities to the PIF pocket. The crystal structure of PDK1 in complex with compound 4h revealed that additional ionic interactions are mainly responsible for the increased potency compared to the monocarboxylate analogues. Notably, several compounds displayed high selectivity for PDK1. Employing a prodrug strategy, we were able to corroborate the novel mechanism of action in cells.

PLK1 phosphorylates PAX3-FOXO1, the inhibition of which triggers regression of alveolar Rhabdomyosarcoma.

Pediatric tumors harbor very low numbers of somatic mutations and therefore offer few targets to improve therapeutic management with targeted drugs. In particular, outcomes remain dismal for patients with metastatic alveolar rhabdomyosarcoma (aRMS), where the chimeric transcription factor PAX3/7-FOXO1 has been implicated but problematic to target. In this report, we addressed this challenge by developing a two-armed screen for druggable upstream regulatory kinases in the PAX3/7-FOXO1 pathway. Screening libraries of kinome siRNA and small molecules, we defined PLK1 as an upstream-acting regulator. Mechanistically, PLK1 interacted with and phosphorylated PAX3-FOXO1 at the novel site S503, leading to protein stabilization. Notably, PLK1 inhibition led to elevated ubiquitination and rapid proteasomal degradation of the PAX3-FOXO1 chimeric oncoprotein. On this basis, we embarked on a preclinical validation of PLK1 as a target in a xenograft mouse model of aRMS, where the PLK1 inhibitor BI 2536 reduced PAX3-FOXO1-mediated gene expression and elicited tumor regression. Clinically, analysis of human aRMS tumor biopsies documented high PLK1 expression to offer prognostic significance for both event-free survival and overall survival. Taken together, these preclinical studies validate the PLK1-PAX3-FOXO1 axis as a rational target to treat aRMS.

PI3K/AKT signaling modulates transcriptional expression of EWS/FLI1 through specificity protein 1

Ewing sarcoma (ES) is the second most frequent bone cancer in childhood and is characterized by the presence of the balanced translocation t(11;22)(q24;q12) in more than 85% of cases, generating a dysregulated transcription factor EWS/FLI1. This fusion protein is an essential oncogenic component of ES development which is necessary for tumor cell maintenance and represents an attractive therapeutic target. To search for modulators of EWS/FLI1 activity we screened a library of 153 targeted compounds and identified inhibitors of the PI3K pathway to directly modulate EWS/FLI1 transcription. Surprisingly, treatment of four different ES cell lines with BEZ235 resulted in down regulation of EWS/FLI1 mRNA and protein by ∼50% with subsequent modulation of target gene expression. Analysis of the EWS/FLI1 promoter region (−2239/+67) using various deletion constructs identified two 14bp minimal elements as being important for EWS/FLI1 transcription. We identified SP1 as modulator of EWS/FLI1 gene expression and demonstrated direct binding to one of these regions in the EWS/FLI1 promoter by EMSA and ChIP experiments. These results provide the first insights on the transcriptional regulation of EWS/FLI1, an area that has not been investigated so far, and offer an additional molecular explanation for the known sensitivity of ES cell lines to PI3K inhibition.

Proteasomal Degradation of the EWS-FLI1 Fusion Protein Is Regulated by a Single Lysine Residue

E-26 transformation-specific (ETS) proteins are transcription factors directing gene expression through their conserved DNA binding domain. They are implicated as truncated forms or interchromosomal rearrangements in a variety of tumors including Ewing sarcoma, a pediatric tumor of the bone. Tumor cells express the chimeric oncoprotein EWS-FLI1 from a specific t(22;11)(q24;12) translocation. EWS-FLI1 harbors a strong transactivation domain from EWSR1 and the DNA-binding ETS domain of FLI1 in the C-terminal part of the protein. Although Ewing cells are crucially dependent on continuous expression of EWS-FLI1, its regulation of turnover has not been characterized in detail. Here, we identify the EWS-FLI1 protein as a substrate of the ubiquitin-proteasome system with a characteristic polyubiquitination pattern. Using a global protein stability approach, we determined the half-life of EWS-FLI1 to lie between 2 and 4 h, whereas full-length EWSR1 and FLI1 were more stable. By mass spectrometry, we identified two ubiquitin acceptor lysine residues of which only mutation of Lys-380 in the ETS domain of the FLI1 part abolished EWS-FLI1 ubiquitination and stabilized the protein posttranslationally. Expression of this highly stable mutant protein in Ewing cells while simultaneously depleting the endogenous wild type protein differentially modulates two subgroups of target genes to be either EWS-FLI1 protein-dependent or turnover-dependent. The majority of target genes are in an unaltered state and cannot be further activated. Our study provides novel insights into EWS-FLI1 turnover, a critical pathway in Ewing sarcoma pathogenesis, and lays new ground to develop novel therapeutic strategies in Ewing sarcoma.

Abstract 478: EWS/FLI1 transcription is modulated by the PI3K pathway via SP1 in Ewing sarcoma

Ewing sarcoma (ES) is the second most frequent bone cancer in childhood and it is characterized by the presence of the balanced t(11;22)(q24;q12) translocation in more than 85% of cases, generating a dysregulated transcription factor EWS/FLI1. ES belongs to small-round-blue-cell tumors and it is a very aggressive osteolytic cancer with early tendency for development of metastasis. Mostly it affects bones such as pelvis, femour and ribs but can also arise in soft tissues, mainly in adults. EWS/FLI1 is an essential oncogenic component of ES development which is necessary for tumor cell maintenance, through inappropriate regulation of target genes that are crucial for the fully malignant phenotype. Therefore, EWS/FLI1 represents an attractive therapeutic target. Screening of a small library of 153 targeted compounds has identified inhibitors of the PI3K pathway as main modulators of EWS/FLI1 activity and surprisingly this was due to an effect of the compound on EWS/FLI1 transcription. Indeed, treatment of four different ES cell lines with BEZ235 (PI3K-mTOR inhibitor) resulted in downregulation of EWS/FLI1 by 50% expression with subsequent modulation of target gene expression. Analysis of the EWS/FLI1 promoter region (-2239/+67) using various deletion constructs identified two 14bp minimal elements, named Del2 and Del23 region, as being important for EWS/FLI1 transcription. Based on in silico prediction and on in vivo data, we were able to predict 4 transcription factor candidates to bind this region and being responsive to PI3K regulation. Among these, SP1 was identified as modulator of EWS/FLI1 gene expression by siRNA. Various approches such as qRT-PCR, Western Blot and Immunofluorescence confirmed a reduction of EWS/FLI1 in Ewing cells when PI3Kαγδ and SP1 were depleted; Electrophoretic Mobility Shift Assay (EMSA) and Chromatine Immuno Precipitation (ChIP) both confirmed that the transcription factor SP1 is indeed binding to Del23 region of the EWS/FLI1 promoter. In summary, our results provide the very first insights on the transcriptional regulation of EWS/FLI1, an area that has not been investigated so far, and offer a molecular explanation for the known sensitivity of ES cell lines to PI3K inhibition. Citation Format: Chiara Giorgi, Alexander Boro, Laura A. Lopez-Garcia, Beat W. Schaefer, Felix K. Niggli. EWS/FLI1 transcription is modulated by the PI3K pathway via SP1 in Ewing sarcoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 478. doi:10.1158/1538-7445.AM2015-478

Abstract 3966: PLK1 regulates PAX3-FOXO1 stability and its inhibition mediates regression of alveolar rhabdomyosarcoma xenograft tumors

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Oncogenic addiction provides an opportunity to develop new treatment options, especially for childhood cancers. Pediatric tumors contain a lower number of oncogenic mutations compared to most adult cancers, suggesting stronger dependency on individual oncogenes, such as chimeric transcription factors that have the ability to control multiple oncogenic pathways. Taking advantage of this addiction, targeting of oncogenic transcription factors becomes a new powerful strategy for therapy of translocation positive pediatric tumors like alveolar rhabdomyosarcoma (aRMS), which is characterized by a very dismal prognosis. As transcription factors are considered to be difficult to target, we developed an approach to screen for druggable upstream regulators like kinases. In two parallel activity reporter assays, using a kinome siRNA library and a small molecule library, we identified PLK1 to contribute to the activity of PAX3-FOXO1, the oncogenic transcription factor specific to aRMS. Mechanistically, we were able to demonstrate a direct interaction of the two proteins by co-immunoprecipitation. PLK1 silencing or inhibition increased degradation of PAX3-FOXO1, which suggests a stabilization of PAX3-FOXO1 due to phosphorylation by PLK1. Candidate phosphorylation sites were identified by in vitro kinase assays and mass spectrometric analysis and site-specific mutagenesis is currently utilized to validate the impact of these sites on fusion protein turnover. To highlight PLK1 as therapeutic target in aRMS, in vivo treatment studies using the PLK1 inhibitor BI 2536 resulted in complete tumor regression in two aRMS xenograft mouse models. Finally, tissue microarray analysis of human aRMS tumor biopsies showed PLK1 to be overexpressed and predictive of overall survival. Hence, our preclinical studies validate PLK1 as highly relevant drug target in alveolar rhabdomyosarcoma, similar to recent findings in other pediatric tumors. Our data suggests that PLK1 inhibitors warrant further investigations to evaluate the potential to be translated into a clinical setting. Citation Format: Verena Thalhammer, David Herrero-Martin, Regina Hecker, Dominik Laubscher, Laura Lopez-Garcia, Marco Wachtel, Peter Bode, Beat Schafer. PLK1 regulates PAX3-FOXO1 stability and its inhibition mediates regression of alveolar rhabdomyosarcoma xenograft tumors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3966. doi:10.1158/1538-7445.AM2014-3966