Abstract 5014: Profile of ARX001822, a highly potent, selective and orally bioavailable A2a antagonist effective in preventing adenosinergic suppression of T cell activation

Adenosine is elevated in the tumour microenvironment and plays a critical role in suppressing T cell function through high affinity interaction with the A2a receptor. Genetic deficiency of A2a in mice is associated with enhanced cytotoxic responses and reduced tumour burden in syngeneic models. These effects are mimicked by small molecule A2a antagonists and some of these compounds are currently being evaluated in clinical trials for the treatment of solid tumours, particularly in combination with checkpoint inhibitors. However, the high levels of adenosine in the tumour microenvironment can dramatically reduce the effectiveness of competitive A2a antagonists. The challenge therefore is to identify highly potent and selective A2a antagonists which retain potency in the presence of high concentrations of adenosine and therefore have the potential to nullify the adenosinergic pathway within the tumour microenvironment ARX001822 binds A2a with high affinity (Ki=0.9 nM) and with greater than 660-fold selectivity over A1, A2b and A3. In a functional assay utilising CHO cells expressing recombinant A2a ARX001822 inhibited cAMP production in response to the selective A2a agonist CGS21680 in a competitive manner (KB= 0.3nM). Activation of A2a leads to the suppression of T cell-derived cytokine production and ARX001822 prevented this suppression even in the presence of high concentration of the adenosine receptor ligand NECA (IC50=38 nM). ARX001822 was also active in human whole blood, preventing NECA-mediated elevation of pCREB in CD8+ T cells and restoring production of interferon-γ with a potency 5-20 times higher than that of competitor molecules undergoing clinical evaluation in cancer. ARX001822 was orally bioavailable in rats and mice and was effective in inhibiting elevation of pCREB in mouse CD8+ T cells in an ex vivo pharmacodynamic assay. ARX001822 is a highly potent and selective A2a antagonist which is effective in preventing adenosinergic mediated suppression of cytokine production in the presence of high concentrations of adenosine receptor ligands and a full complement of plasma proteins. Knowledge of whole blood potency on both pCREB and interferon-γ modulation combined with the exposure required for activity in the pharmacodynamic model is helpful in estimating the clinical exposure required for A2a receptor blockade and downstream events related to modulating T cell function.

Citation Format: Peter M. Finan, Roy Pettipher, Jonathan White, Viral Patel, Ben Moulton, Soraya Porres, Karolina Gherbi, Elizabeth M. Rosethorne, Steven J. Charlton, Clive McCarthy. Profile of ARX001822, a highly potent, selective and orally bioavailable A2a antagonist effective in preventing adenosinergic suppression of T cell activation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5014.

Potent, Selective, and Orally Bioavailable Inhibitors of VPS34 Provide Chemical Tools to Modulate Autophagy in Vivo

Autophagy is a dynamic process that regulates lysosomal-dependent degradation of cellular components. Until recently the study of autophagy has been hampered by the lack of reliable pharmacological tools, but selective inhibitors are now available to modulate the PI 3-kinase VPS34, which is required for autophagy. Here we describe the discovery of potent and selective VPS34 inhibitors, their pharmacokinetic (PK) properties, and ability to inhibit autophagy in cellular and mouse models.

PIKfyve, a class III PI kinase, is the target of the small molecular IL-12/IL-23 inhibitor apilimod and a player in Toll-like receptor signaling

Toll-like receptor (TLR) signaling is a key component of innate immunity. Aberrant TLR activation leads to immune disorders via dysregulation of cytokine production, such as IL-12/IL-23. Herein, we identify and characterize PIKfyve, a lipid kinase, as a critical player in TLR signaling using apilimod as an affinity tool. Apilimod is a potent small molecular inhibitor of IL-12/IL-23 with an unknown target and has been evaluated in clinical trials for patients with Crohn's disease or rheumatoid arthritis. Using a chemical genetic approach, we show that it binds to PIKfyve and blocks its phosphotransferase activity, leading to selective inhibition of IL-12/IL-23p40. Pharmacological or genetic inactivation of PIKfyve is necessary and sufficient for suppression of IL-12/IL-23p40 expression. Thus, we have uncovered a phosphoinositide-mediated regulatory mechanism that controls TLR signaling.

RAD001 enhances the potency of BEZ235 to inhibit mTOR signaling and tumor growth

The mammalian target of rapamycin (mTOR) is regulated by oncogenic growth factor signals and plays a pivotal role in controlling cellular metabolism, growth and survival. Everolimus (RAD001) is an allosteric mTOR inhibitor that has shown marked efficacy in certain cancers but is unable to completely inhibit mTOR activity. ATP-competitive mTOR inhibitors such as NVP-BEZ235 can block rapamycin-insensitive mTOR readouts and have entered clinical development as anti-cancer agents. Here, we show the degree to which RAD001 and BEZ235 can be synergistically combined to inhibit mTOR pathway activation, cell proliferation and tumor growth, both in vitro and in vivo. RAD001 and BEZ235 synergized in cancer lines representing different lineages and genetic backgrounds. Strong synergy is seen in neuronal, renal, breast, lung, and haematopoietic cancer cells harboring abnormalities in PTEN, VHL, LKB1, Her2, or KRAS. Critically, in the presence of RAD001, the mTOR-4EBP1 pathway and tumorigenesis can be fully inhibited using lower doses of BEZ235. This is relevant since RAD001 is relatively well tolerated in patients while the toxicity profiles of ATP-competitive mTOR inhibitors are currently unknown.

Genetic ablation of PI3Kγ results in defective IL-17RA signalling in T lymphocytes and increased IL-17 levels

The signalling molecule PI3Kγ has been reported to play a key role in the immune system and the inflammatory response. In particular, it facilitates the migration of haemato-poietic cells to the site of inflammation. In this study, we reveal a novel role for PI3Kγ in the regulation of the pro-inflammatory cytokine IL-17. Loss of PI3Kγ or expression of a catalytically inactive mutant of PI3Kγ in mice led to increased IL-17 production both in vitro and in vivo in response to various stimuli. The kinetic profile was unaltered from WT cells, with no effect on proliferation or other cytokines. Elevated levels of IL-17 were not due to an aberrant expansion of IL-17-producing cells. Furthermore, we also identified an increase in IL-17RA expression on PI3Kγ(-/-) CD4(+) T cells, yet these cells exhibited impaired PI3K-dependent signalling in response to IL-17A, and subsequent NF-κB phosphorylation. In vivo, instillation of recombinant IL-17 into the airways of mice lacking PI3Kγ signalling also resulted in reduced phosphorylation of Akt. Cell influx in response to IL-17 was also reduced in PI3Kγ(-/-) lungs. These data demonstrate PI3Kγ-dependent signalling downstream of IL-17RA, which plays a pivotal role in regulating IL-17 production in T cells.

TBC1D7 is a third subunit of the TSC1-TSC2 complex upstream of mTORC1

The tuberous sclerosis complex (TSC) tumor suppressors form the TSC1-TSC2 complex, which limits cell growth in response to poor growth conditions. Through its GTPase-activating protein (GAP) activity toward Rheb, this complex inhibits the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1), a key promoter of cell growth. Here, we identify and biochemically characterize TBC1D7 as a stably associated and ubiquitous third core subunit of the TSC1-TSC2 complex. We demonstrate that the TSC1-TSC2-TBC1D7 (TSC-TBC) complex is the functional complex that senses specific cellular growth conditions and possesses Rheb-GAP activity. Sequencing analyses of samples from TSC patients suggest that TBC1D7 is unlikely to represent TSC3. TBC1D7 knockdown decreases the association of TSC1 and TSC2 leading to decreased Rheb-GAP activity, without effects on the localization of TSC2 to the lysosome. Like the other TSC-TBC components, TBC1D7 knockdown results in increased mTORC1 signaling, delayed induction of autophagy, and enhanced cell growth under poor growth conditions.

RNF146 is a poly(ADP-ribose)-directed E3 ligase that regulates axin degradation and Wnt signalling

The Wnt/β-catenin signalling pathway plays essential roles in embryonic development and adult tissue homeostasis, and deregulation of this pathway has been linked to cancer. Axin is a concentration-limiting component of the β-catenin destruction complex, and its stability is regulated by tankyrase. However, the molecular mechanism by which tankyrase-dependent poly(ADP-ribosyl)ation (PARsylation) is coupled to ubiquitylation and degradation of axin remains undefined. Here, we identify RNF146, a RING-domain E3 ubiquitin ligase, as a positive regulator of Wnt signalling. RNF146 promotes Wnt signalling by mediating tankyrase-dependent degradation of axin. Mechanistically, RNF146 directly interacts with poly(ADP-ribose) through its WWE domain, and promotes degradation of PARsylated proteins. Using proteomics approaches, we have identified BLZF1 and CASC3 as further substrates targeted by tankyrase and RNF146 for degradation. Thus, identification of RNF146 as a PARsylation-directed E3 ligase establishes a molecular paradigm that links tankyrase-dependent PARsylation to ubiquitylation. RNF146-dependent protein degradation may emerge as a major mechanism by which tankyrase exerts its function.

COT drives resistance to RAF inhibition through MAP kinase pathway reactivation

Oncogenic mutations in the serine/threonine kinase B-RAF (also known as BRAF) are found in 50-70% of malignant melanomas. Pre-clinical studies have demonstrated that the B-RAF(V600E) mutation predicts a dependency on the mitogen-activated protein kinase (MAPK) signalling cascade in melanoma-an observation that has been validated by the success of RAF and MEK inhibitors in clinical trials. However, clinical responses to targeted anticancer therapeutics are frequently confounded by de novo or acquired resistance. Identification of resistance mechanisms in a manner that elucidates alternative 'druggable' targets may inform effective long-term treatment strategies. Here we expressed ∼600 kinase and kinase-related open reading frames (ORFs) in parallel to interrogate resistance to a selective RAF kinase inhibitor. We identified MAP3K8 (the gene encoding COT/Tpl2) as a MAPK pathway agonist that drives resistance to RAF inhibition in B-RAF(V600E) cell lines. COT activates ERK primarily through MEK-dependent mechanisms that do not require RAF signalling. Moreover, COT expression is associated with de novo resistance in B-RAF(V600E) cultured cell lines and acquired resistance in melanoma cells and tissue obtained from relapsing patients following treatment with MEK or RAF inhibitors. We further identify combinatorial MAPK pathway inhibition or targeting of COT kinase activity as possible therapeutic strategies for reducing MAPK pathway activation in this setting. Together, these results provide new insights into resistance mechanisms involving the MAPK pathway and articulate an integrative approach through which high-throughput functional screens may inform the development of novel therapeutic strategies.

Activation of a metabolic gene regulatory network downstream of mTOR complex 1

Aberrant activation of the mammalian target of rapamycin complex 1 (mTORC1) is a common molecular event in a variety of pathological settings, including genetic tumor syndromes, cancer, and obesity. However, the cell-intrinsic consequences of mTORC1 activation remain poorly defined. Through a combination of unbiased genomic, metabolomic, and bioinformatic approaches, we demonstrate that mTORC1 activation is sufficient to stimulate specific metabolic pathways, including glycolysis, the oxidative arm of the pentose phosphate pathway, and de novo lipid biosynthesis. This is achieved through the activation of a transcriptional program affecting metabolic gene targets of hypoxia-inducible factor (HIF1alpha) and sterol regulatory element-binding protein (SREBP1 and SREBP2). We find that SREBP1 and 2 promote proliferation downstream of mTORC1, and the activation of these transcription factors is mediated by S6K1. Therefore, in addition to promoting protein synthesis, mTORC1 activates specific bioenergetic and anabolic cellular processes that are likely to contribute to human physiology and disease.

Bidirectional transport of amino acids regulates mTOR and autophagy

Amino acids are required for activation of the mammalian target of rapamycin (mTOR) kinase which regulates protein translation, cell growth, and autophagy. Cell surface transporters that allow amino acids to enter the cell and signal to mTOR are unknown. We show that cellular uptake of L-glutamine and its subsequent rapid efflux in the presence of essential amino acids (EAA) is the rate-limiting step that activates mTOR. L-glutamine uptake is regulated by SLC1A5 and loss of SLC1A5 function inhibits cell growth and activates autophagy. The molecular basis for L-glutamine sensitivity is due to SLC7A5/SLC3A2, a bidirectional transporter that regulates the simultaneous efflux of L-glutamine out of cells and transport of L-leucine/EAA into cells. Certain tumor cell lines with high basal cellular levels of L-glutamine bypass the need for L-glutamine uptake and are primed for mTOR activation. Thus, L-glutamine flux regulates mTOR, translation and autophagy to coordinate cell growth and proliferation.

Down-regulation of class II phosphoinositide 3-kinase alpha expression below a critical threshold induces apoptotic cell death

Members of the phosphoinositide 3-kinase (PI3K) family collectively control multiple cellular responses, including proliferation, growth, chemotaxis, and survival. These diverse effects can partly be attributed to the broad range of downstream effectors being regulated by the products of these lipid kinases, the 3'-phosphoinositides. However, an additional layer of complexity is introduced by the existence of multiple PI3K enzyme isoforms. Much has been learned over the last years on the roles of the classes I and III PI3K members in cellular signaling, but little is known about the isoform-specific tasks done by the class II PI3Ks (C2alpha, beta, and gamma). In this study, we used quantitative reverse transcription-PCR and RNA interference in mammalian cells to gain further insight into the function of these lesser studied PI3K enzymes. We find that PI3K-C2alpha, but not PI3K-C2beta, has an important role in controlling cell survival and by using a panel of RNA interference reagents, we were able to determine a critical threshold of PI3K-C2alpha mRNA levels, below which the apoptotic program is switched on, via the intrinsic cell death pathway. In addition, knockdown of PI3K-C2alpha to levels that by themselves do not induce apoptosis sensitize cells to the anticancer agent Taxol (paclitaxel). Lastly, we report that lowering the levels of PI3K-C2alpha in a number of cancer cell lines reduces their proliferation and cell viability, arguing that PI3K inhibitors targeting not only the class Ialpha isoform but also class IIalpha may contribute to an effective anticancer strategy.

PI 3-kinase inhibition: a therapeutic target for respiratory disease

Asthma and COPD (chronic obstructive pulmonary disease) are a growing major health burden, which, despite improvements in disease management, still require new effective treatments. As our understanding of the cellular and molecular processes which govern respiratory diseases improves, the range of potential therapeutic targets increase. PI 3-kinases (phosphoinositide 3-kinases) are a family of closely related enzymes, which play pivotal roles in a diverse array of cellular mechanisms. In the present paper, we review the evidence for PI 3-kinase involvement in various cellular processes underlying asthma and COPD generated through inhibitor studies and gene-targeting approaches, and discuss the prospects for PI 3-kinase inhibition as a future therapeutic strategy for the treatment of respiratory disease.

A Nuclear SH3 Domain-binding Protein That Colocalizes with mRNA Splicing Factors and Intermediate Filament-containing Perinuclear Networks

A protein (SNP70) has been isolated that binds to the Src homology domain 3 of p47(phox), p85alpha, and c-src. Cloning and sequencing of the polypeptide revealed it to be a 70-kDa protein that has a number of potential domains, including Src homology 3 binding motifs and several nuclear localization signals. Immunofluorescence using anti-peptide antibodies revealed SNP70 to be primarily concentrated in the nucleus but excluded from nucleoli, in interphase cells. However, it was distributed throughout the cytoplasm in dividing cells. Extraction and subfractionation experiments indicated that SNP70 did not bind directly to DNA but did bind to poly(G)-rich oligonucleotides and was resistant to extraction with non-ionic detergents but was solubilized by treatment with RNase, high salt, or ammonium sulfate. Double-immunofluorescence experiments showed that SNP70 co-localized with two pre-mRNA splicing factors SC35 and U2B" within the nucleus. A population of SNP70 was found outside the nucleus, and double-immunofluorescence and immunoelectron microscopy demonstrated that it associated with vimentin-containing intermediate filaments, particularly those surrounding the nucleus. The data suggest that SNP70 associates with nuclear or perinuclear filaments and may play a role in the regulation of pre-mRNA processing.

Identification and characterisation of a novel splice variant of synaptojanin1

Synaptojanin1, the major constitutively active PtdInsP3 5-phosphatase activity in rat brain, is one of two closely related proteins both extensively spliced in their C-terminal proline rich domain. We describe here the discovery of a novel splice variant of synaptojanin1 which misses the major N-terminal part of the SAC1 domain. This deltaSAC-synaptojanin1 is expressed in rat brain tissue as shown by Northern and Western analysis. However, the deletion of the SAC1 domain does not alter PtdInsP3 5-phosphatase activity demonstrating that the SAC1 domain is not necessary for catalytic function.

Synaptojanin is the major constitutively active phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase in rodent brain

The major constitutive phosphatidylinositol-3,4,5-P3 (PtdIns) 5-phosphatase activity was purified and subjected to peptide sequence analysis providing extensive amino acid sequence which was subsequently used for cloning the cDNA. Peptide and cDNA sequences revealed that the purified PtdIns(3,4,5)P3 5-phosphatase was identical to a splice variant of a recently cloned inositol polyphosphate 5-phosphatase termed synaptojanin. Since synaptojanin is not known to possess PtdIns(3,4,5)P3 5-phosphatase activity, we verified that the purified PtdIns(3,4,5)P3 5-phosphatase activity and synaptojanin are identical by Western blot using specific antibodies raised against synaptojanin sequences. Immunoprecipitation from crude lysates of rat brain tissue showed that synaptojanin accounts for the major part of the active PtdIns(3, 4,5)P3 5-phosphatase activity. It is also shown that the protein is localized to the soluble fraction. Expression of a truncated recombinant protein demonstrates that the conserved 5-phosphatase region of the synaptojanin gene expresses PtdIns(3,4,5)P3 5-phosphatase activity. However, immunological analysis demonstrates that the PtdIns(3,4,5)P3 5-phosphatase activity expressed from the synaptojanin gene in brain is due to a particular splice variant which contains a 16-amino acid insert as shown by immunoprecipitation using a specific antibody raised against this particular splice variant.

Identification of regions of the Wiskott-Aldrich syndrome protein responsible for association with selected Src homology 3 domains

Src homology 3 (SH3) domains have been shown to mediate selected interactions between signaling molecules and are essential for the activation of a number of receptor-driven pathways. The Wiskott-Aldrich syndrome protein was identified as a protein that associated selectively with the SH3 domains derived from c-Src, p85alpha, phospholipase Cgamma1, and c-Fgr. Significantly reduced association was detected to the N-terminal SH3 domain and the tandem SH3 domains of p47(phox), and no binding was detected to the SH3 domain of n-Src, the C-terminal SH3 domain of p47(phox), or either of the SH3 domains of p67(phox). Three peptides corresponding to potential Wiskott-Aldrich syndrome protein SH3 domain binding motifs were found to inhibit its association with c-Src, Fgr, and phospholipase Cgamma1 SH3 domains, but not the p85alpha SH3 domain. These peptides have the sequences MRRQEPLPPPPPPSRG, TGRSGPLPPPPPGA, and KGRSGPLPPVPLGI and show homology with other SH3 domain binding motifs. It is possible that the intracellular association of Wiskott-Aldrich syndrome protein with other signaling proteins is mediated by its SH3 domain-binding regions, and this may play a role in its putative function as a regulatory molecule in immune cells.

Characterization of Grb2-binding proteins in human platelets activated by Fc gamma RIIA cross-linking

Glutathione-S-transferase (GST)-Grb2 fusion proteins have been used to identify the potential role of Grb2-binding proteins in platelet activation by the platelet low-affinity IgG receptor, Fc gamma RIIA. Two tyrosine phosphoproteins of 38 and 63 kD bind to the SH2 domain of Grb2 following Fc gamma RIIA stimulation of platelets. Both are located in the particulate fraction following platelet activation and are also able to bind to a GST-construct containing the SH2 and SH3 domains of phospholipase C gamma 1. p38 also forms a complex with the tyrosine kinase csk in stimulated cells and is a substrate for the kinase. The SH3 domains of Grb2 form a stable complex with SOS1 and two proteins of 75 kD and 120 kD, which undergo tyrosine phosphorylation in Fc gamma RIIA stimulated cells. The 75-kD protein is recognized by antibodies to SLP-76, which has recently been isolated from T cells and sequenced. Tyrosine phosphorylation of p38 and p63 is also observed in platelets stimulated by the tyrosine kinase-linked receptor agonist collagen and by the G protein-coupled receptor agonist thrombin, although phosphorylation of SLP-76 is only observed in collagen-stimulated platelets. p38 and p63 may provide a docking site for Grb2, thereby linking Grb2 SH3-binding proteins SOS1, SLP-76, and p120 to downstream signalling events.

Sam68 from an immortalised B-cell line associates with a subset of SH3 domains

The binding of proteins from an immortalised B-cell line to a panel of SH3 domains was investigated in vitro. One of the most prominent SH3 domain binding proteins was a 68 kD polypeptide which strongly associated with the SH3 domains of c-src, p85a and p47phox and weakly with the SH3 domain of PLCgamma and n-src with undetectable binding to the other SH3 domains tested. Immunoblotting identified this protein as human Sam68. The ability of proline-rich peptides homologous to the Sam68 primary sequence to inhibit the binding of Sam68 to SH3 domains was investigated. Only one peptide inhibited binding of Sam68 to the p85alpha SH3 domain, whereas several peptides inhibited binding of Sam68 to c-src SH3 domain, suggesting that Sam68 uses different proline-rich motifs to bind to different SH3 domains. A peptide derived from residues 32-44 of Sam68 which fits the class II SH3 domain binding consensus sequence inhibited binding of Sam68 to both p85alpha SH3 domain and c-src SH3 domain, but with differential potency, suggesting a differential affinity of these SH3 domains for this proline-rich motif.

Tyrosine phosphatase antagonist-induced activation of the neutrophil NADPH oxidase: a possible role for protein kinase C

To investigate the role of tyrosine phosphorylation in polymorphonuclear leucocyte (PMN) activation we have examined the effect of the potent tyrosine phosphatase (PTPase) inhibitor, vanadyl hydroperoxide, on PMN function. Western blotting of vanadyl hydroperoxide-treated PMN showed that there was a rapid dose-dependent increase in tyrosine-phosphorylated proteins. Vanadyl hydroperoxide also induced superoxide production in PMN over the range 10-100 microM, similar to the concentrations that also induced tyrosine phosphorylation. The tyrosine kinase inhibitor erbstatin totally inhibited the respiratory burst induced by vandyl hydroperoxide, showing that tyrosine kinase activity was necessary for superoxide production. The protein kinase C (PKC) inhibitors chelerythrine and bisidolylmaleimide inhibited the vanadyl hydroperoxide-induced respiratory burst with an inhibitory concentration of 50% (IC50) close to that for PKC inhibition without affecting tyrosine phosphorylation. These results indicate a possible role for PKC in vanadyl hydroperoxide-mediated superoxide production, and that any PKC involvement is downstream of tyrosine phosphorylation. These results further demonstrate that inhibition of phosphotyrosine phosphatases results in the activation of a functional response, indicating a critical role for phosphotyrosine phosphatases in PMN stimulation.

Molecular cloning, sequence determination and heterologous expression of nucleoside diphosphate kinase from Pisum sativum

Protein sequence data derived from the N-terminal region of a 17 kDa polypeptide associated with the microsomal membrane fraction from Pisum sativum was used to design degenerate oligonucleotides which were used to amplify P. sativum cDNA via the polymerase chain reaction (PCR). Amplified cDNA was used as a probe to screen a P. sativum cDNA library and a cDNA clone, NDK-P1 was isolated and sequenced. The protein encoded by NDK-P1 had a calculated molecular mass of 16,485 Da and possessed substantial homology with nucleoside diphosphate kinases (NDKs) isolated and cloned from other sources. High levels of expression of NDK-P1 protein were achieved in Escherichia coli using a T7-driven expression system. Recombinant NDK-P1 protein was shown to possess NDK activity and had similar biochemical characteristics to NDKs isolated from other sources. The Michaelis constants for a variety of nucleoside diphosphate (NDP) substrates were found to be broadly similar to those reported for other NDKs, with thymidine nucleotides being the substrates of greatest affinity.