Correction: A high affinity pan-PI3K binding module supports selective targeted protein degradation of PI3Kα.

[This corrects the article DOI: 10.1039/D3SC04629J.].

A high affinity pan-PI3K binding module supports selective targeted protein degradation of PI3Kα.

Class I phosphoinositide 3-kinases (PI3Ks) control cellular growth, but are also essential in insulin signaling and glucose homeostasis. Pan-PI3K inhibitors thus generate substantial adverse effects, a reality that has plagued drug development against this target class. We present here evidence that a high affinity binding module with the capacity to target all class I PI3K isoforms can facilitate selective degradation of the most frequently mutated class I isoform, PI3Kα, when incorporated into a cereblon-targeted (CRBN) degrader. A systematic proteomics study guided the fine tuning of molecular features to optimize degrader selectivity and potency. Our work resulted in the creation of WJ112-14, a PI3Kα-specific nanomolar degrader that should serve as an important research tool for studying PI3K biology. Given the toxicities observed in the clinic with unselective PI3Kα inhibitors, the results here offer a new approach toward selectively targeting this frequently mutated oncogenic driver.

Investigation of morpholine isosters for the development of a potent, selective and metabolically stable mTOR kinase inhibitor.

Upregulation of mechanistic target of rapamycin (mTOR) signaling drives various types of cancers and neurological diseases. Rapamycin and its analogues (rapalogs) are first generation mTOR inhibitors, and selectively block mTOR complex 1 (TORC1) by an allosteric mechanism. In contrast, second generation ATP-binding site inhibitors of mTOR kinase (TORKi) target both TORC1 and TORC2. Here, we explore 3,6-dihydro-2H-pyran (DHP) and tetrahydro-2H-pyran (THP) as isosteres of the morpholine moiety to unlock a novel chemical space for TORKi generation. A library of DHP- and THP-substituted triazines was prepared, and molecular modelling provided a rational for a structure activity relationship study. Finally, compound 11b [5-(4-(3-oxa-8-azabicyclo[3.2.1]octan-8-yl)-6-(tetrahydro-2H-pyran-4-yl)-1,3,5-triazin-2-yl)-4-(difluoromethyl)pyridin-2-amine] was selected due its potency and selectivity for mTOR kinase over the structurally related class I phosphoinositide 3-kinases (PI3Ks) isoforms. 11b displayed high metabolic stability towards CYP1A1 degradation, which is of advantage in drug development. After oral administration to male Sprague Dawley rats, 11b reached high concentrations both in plasma and brain, revealing an excellent oral bioavailability. In a metabolic stability assay using human hepatocytes, 11b was more stable than PQR620, the first-in-class brain penetrant TORKi. Compound 11b also displayed dose-dependent anti-proliferative activity in splenic marginal zone lymphoma (SMZL) cell lines as single agent and when combined with BCL2 inhibition (venetoclax). Our results identify the THP-substituted triazine core as a novel scaffold for the development of metabolically stable TORKi for the treatment of chronic diseases and cancers driven by mTOR deregulation and requiring drug distribution also to the central nervous system.

Covalent Proximity Scanning of a Distal Cysteine to Target PI3Kα.

Covalent protein kinase inhibitors exploit currently noncatalytic cysteines in the adenosine 5'-triphosphate (ATP)-binding site via electrophiles directly appended to a reversible-inhibitor scaffold. Here, we delineate a path to target solvent-exposed cysteines at a distance >10 Å from an ATP-site-directed core module and produce potent covalent phosphoinositide 3-kinase α (PI3Kα) inhibitors. First, reactive warheads are used to reach out to Cys862 on PI3Kα, and second, enones are replaced with druglike warheads while linkers are optimized. The systematic investigation of intrinsic warhead reactivity (kchem), rate of covalent bond formation and proximity (kinact and reaction space volume Vr), and integration of structure data, kinetic and structural modeling, led to the guided identification of high-quality, covalent chemical probes. A novel stochastic approach provided direct access to the calculation of overall reaction rates as a function of kchem, kinact, Ki, and Vr, which was validated with compounds with varied linker lengths. X-ray crystallography, protein mass spectrometry (MS), and NanoBRET assays confirmed covalent bond formation of the acrylamide warhead and Cys862. In rat liver microsomes, compounds 19 and 22 outperformed the rapidly metabolized CNX-1351, the only known PI3Kα irreversible inhibitor. Washout experiments in cancer cell lines with mutated, constitutively activated PI3Kα showed a long-lasting inhibition of PI3Kα. In SKOV3 cells, compounds 19 and 22 revealed PI3Kß-dependent signaling, which was sensitive to TGX221. Compounds 19 and 22 thus qualify as specific chemical probes to explore PI3Kα-selective signaling branches. The proposed approach is generally suited to develop covalent tools targeting distal, unexplored Cys residues in biologically active enzymes.

SRP54 mutations induce congenital neutropenia via dominant-negative effects on XBP1 splicing.

Heterozygous de novo missense variants of SRP54 were recently identified in patients with congenital neutropenia (CN) who display symptoms that overlap with Shwachman-Diamond syndrome (SDS). Here, we investigate srp54 knockout zebrafish as the first in vivo model of SRP54 deficiency. srp54-/- zebrafish experience embryonic lethality and display multisystemic developmental defects along with severe neutropenia. In contrast, srp54+/- zebrafish are viable, fertile, and show only mild neutropenia. Interestingly, injection of human SRP54 messenger RNAs (mRNAs) that carry mutations observed in patients (T115A, T117Δ, and G226E) aggravated neutropenia and induced pancreatic defects in srp54+/- fish, mimicking the corresponding human clinical phenotypes. These data suggest that the various phenotypes observed in patients may be a result of mutation-specific dominant-negative effects on the functionality of the residual wild-type SRP54 protein. Overexpression of mutated SRP54 also consistently induced neutropenia in wild-type fish and impaired the granulocytic maturation of human promyelocytic HL-60 cells and healthy cord blood-derived CD34+ hematopoietic stem and progenitor cells. Mechanistically, srp54-mutant fish and human cells show impaired unconventional splicing of the transcription factor X-box binding protein 1 (Xbp1). Moreover, xbp1 morphants recapitulate phenotypes observed in srp54 deficiency and, importantly, injection of spliced, but not unspliced, xbp1 mRNA rescues neutropenia in srp54+/- zebrafish. Together, these data indicate that SRP54 is critical for the development of various tissues, with neutrophils reacting most sensitively to the loss of SRP54. The heterogenic phenotypes observed in patients that range from mild CN to SDS-like disease may be the result of different dominant-negative effects of mutated SRP54 proteins on downstream XBP1 splicing, which represents a potential therapeutic target.

SOX2 and p53 Expression Control Converges in PI3K/AKT Signaling with Versatile Implications for Stemness and Cancer.

Stemness and reprogramming involve transcriptional master regulators that suppress cell differentiation while promoting self-renewal. A distinguished example thereof is SOX2, a high mobility group (HMG)-box transcription factor (TF), whose subcellular localization and turnover regulation in embryonic, induced-pluripotent, and cancer stem cells (ESCs, iPSCs, and CSCs, respectively) is mediated by the PI3K/AKT/SOX2 axis, a stem cell-specific branch of the PI3K/AKT signaling pathway. Further effector functions associated with PI3K/AKT induction include cell cycle progression, cellular (mass) growth, and the suppression of apoptosis. Apoptosis, however, is a central element of DNA damage response (DDR), where it provides a default mechanism for cell clearance when DNA integrity cannot be maintained. A key player in DDR is tumor suppressor p53, which accumulates upon DNA-damage and is counter-balanced by PI3K/AKT enforced turnover. Accordingly, stemness sustaining SOX2 expression and p53-dependent DDR mechanisms show molecular-functional overlap in PI3K/AKT signaling. This constellation proves challenging for stem cells whose genomic integrity is a functional imperative for normative ontogenesis. Unresolved mutations in stem and early progenitor cells may in fact provoke transformation and cancer development. Such mechanisms are also particularly relevant for iPSCs, where genetic changes imposed through somatic cell reprogramming may promote DNA damage. The current review aims to summarize the latest advances in the understanding of PI3K/AKT/SOX2-driven stemness and its intertwined relations to p53-signaling in DDR under conditions of pluripotency, reprogramming, and transformation.

SOX2 protein biochemistry in stemness, reprogramming, and cancer: the PI3K/AKT/SOX2 axis and beyond.

Research of the past view years expanded our understanding of the various physiological functions the cell-fate determining transcription factor SOX2 exerts in ontogenesis, reprogramming, and cancer. However, while scientific reports featuring novel and exciting aspects of SOX2-driven biology are published in near weekly routine, investigations in the underlying protein-biochemical processes that transiently tailor SOX2 activity to situational demand are underrepresented and have not yet been comprehensively summarized. Largely unrecognizable to modern array or sequencing-based technology, various protein secondary modifications and concomitant function modulations have been reported for SOX2. The chemical modifications imposed onto SOX2 are inherently heterogeneous, comprising singular or clustered events of phosphorylation, methylation, acetylation, ubiquitination, SUMOylation, PARPylation, and O-glycosylation that reciprocally affect each other and critically impact SOX2 functionality, often in a tissue and species-specific manner. One recurring regulatory principle though is the canonical PI3K/AKT signaling axis to which SOX2 relates in various entangled, albeit not exclusive ways. Here we provide a comprehensive review of the current knowledge on SOX2 protein modifications, their proposed relationship to the PI3K/AKT pathway, and regulatory influence on SOX2 with regards to stemness, reprogramming, and cancer.

Publisher Correction: Absence of NKG2D ligands defines leukaemia stem cells and mediates their immune evasion.

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

Absence of NKG2D ligands defines leukaemia stem cells and mediates their immune evasion.

Patients with acute myeloid leukaemia (AML) often achieve remission after therapy, but subsequently die of relapse1 that is driven by chemotherapy-resistant leukaemic stem cells (LSCs)2,3. LSCs are defined by their capacity to initiate leukaemia in immunocompromised mice4. However, this precludes analyses of their interaction with lymphocytes as components of anti-tumour immunity5, which LSCs must escape to induce cancer. Here we demonstrate that stemness and immune evasion are closely intertwined in AML. Using xenografts of human AML as well as syngeneic mouse models of leukaemia, we show that ligands of the danger detector NKG2D-a critical mediator of anti-tumour immunity by cytotoxic lymphocytes, such as NK cells6-9-are generally expressed on bulk AML cells but not on LSCs. AML cells with LSC properties can be isolated by their lack of expression of NKG2D ligands (NKG2DLs) in both CD34-expressing and non-CD34-expressing cases of AML. AML cells that express NKG2DLs are cleared by NK cells, whereas NKG2DL-negative leukaemic cells isolated from the same individual escape cell killing by NK cells. These NKG2DL-negative AML cells show an immature morphology, display molecular and functional stemness characteristics, and can initiate serially re-transplantable leukaemia and survive chemotherapy in patient-derived xenotransplant models. Mechanistically, poly-ADP-ribose polymerase 1 (PARP1) represses expression of NKG2DLs. Genetic or pharmacologic inhibition of PARP1 induces NKG2DLs on the LSC surface but not on healthy or pre-leukaemic cells. Treatment with PARP1 inhibitors, followed by transfer of polyclonal NK cells, suppresses leukaemogenesis in patient-derived xenotransplant models. In summary, our data link the LSC concept to immune escape and provide a strong rationale for targeting therapy-resistant LSCs by PARP1 inhibition, which renders them amenable to control by NK cells in vivo.

Regulation of glioma cell invasion by 3q26 gene products PIK3CA, SOX2 and OPA1.

Diffuse gliomas progress by invading neighboring brain tissue to promote postoperative relapse. Transcription factor SOX2 is highly expressed in invasive gliomas and maps to chromosome region 3q26 together with the genes for PI3K/AKT signaling activator PIK3CA and effector molecules of mitochondria fusion and cell invasion, MFN1 and OPA1. Gene copy number analysis at 3q26 from 129 glioma patient biopsies revealed mutually exclusive SOX2 amplifications (26%) and OPA1 losses (19%). Both forced SOX2 expression and OPA1 inactivation increased LN319 glioma cell invasion in vitro and promoted cell dispersion in vivo in xenotransplanted D. rerio embryos. While PI3 kinase activity sustained SOX2 expression, pharmacological PI3K/AKT pathway inhibition decreased invasion and resulted in SOX2 nucleus-to-cytoplasm translocation in an mTORC1-independent manner. Chromatin immunoprecipitation and luciferase reporter gene assays together demonstrated that SOX2 trans-activates PIK3CA and OPA1. Thus, SOX2 activates PI3K/AKT signaling in a positive feedback loop, while OPA1 deletion is interpreted to counteract OPA1 trans-activation. Remarkably, neuroimaging of human gliomas with high SOX2 or low OPA1 genomic imbalances revealed significantly larger necrotic tumor zone volumes, corresponding to higher invasive capacities of tumors, while autologous necrotic cells are capable of inducing higher invasion in SOX2 overexpressing or OPA1 knocked-down relative to parental LN319. We thus propose necrosis volume as a surrogate marker for the assessment of glioma invasive potential. Whereas glioma invasion is activated by a PI3K/AKT-SOX2 loop, it is reduced by a cryptic invasion suppressor SOX2-OPA1 pathway. Thus, PI3K/AKT-SOX2 and mitochondria fission represent connected signaling networks regulating glioma invasion.

Prominent Oncogenic Roles of EVI1 in Breast Carcinoma.

Overexpression of the EVI1 oncogene is associated typically with aggressive myeloid leukemia, but is also detectable in breast carcinoma where its contributions are unexplored. Analyzing a tissue microarray of 608 breast carcinoma patient specimens, we documented EVI1 overexpression in both estrogen receptor-positive (ER+) and estrogen receptor-negative (ER-) breast carcinomas. Here, we report prognostic relevance of EVI1 overexpression in triple-negative breast carcinoma but not in the HER2-positive breast carcinoma subset. In human breast cancer cells, EVI1 silencing reduced proliferation, apoptosis resistance, and tumorigenicity, effects rescued by estrogen supplementation in ER+ breast carcinoma cells. Estrogen addition restored ERK phosphorylation in EVI1-silenced cells, suggesting that EVI1 and estradiol signaling merge in MAPK activation. Conversely, EVI1 silencing had no effect on constitutive ERK activity in HER2+ breast carcinoma cells. Microarray analyses revealed G-protein-coupled receptor (GPR) signaling as a prominent EVI1 effector mechanism in breast carcinoma. Among others, the GPR54-ligand KISS1 was identified as a direct transcriptional target of EVI1, which together with other EVI1-dependent cell motility factors such as RHOJ regulated breast carcinoma cell migration. Overall, our results establish the oncogenic contributions of EVI1 in ER- and HER2-negative subsets of breast cancer. Cancer Res; 77(8); 2148-60. ©2017 AACR.

Molecular and functional interactions between AKT and SOX2 in breast carcinoma.

The transcription factor SOX2 is a key regulator of pluripotency in embryonic stem cells and plays important roles in early organogenesis. Recently, SOX2 expression was documented in various cancers and suggested as a cancer stem cell (CSC) marker. Here we identify the Ser/Thr-kinase AKT as an upstream regulator of SOX2 protein turnover in breast carcinoma (BC). SOX2 and pAKT are co-expressed and co-regulated in breast CSCs and depletion of either reduces clonogenicity. Ectopic SOX2 expression restores clonogenicity and in vivo tumorigenicity of AKT-inhibited cells, suggesting that SOX2 acts as a functional downstream AKT target. Mechanistically, we show that AKT physically interacts with the SOX2 protein to modulate its subcellular distribution. AKT kinase inhibition results in enhanced cytoplasmic retention of SOX2, presumably via impaired nuclear import, and in successive cytoplasmic proteasomal degradation of the protein. In line, blockade of either nuclear transport or proteasomal degradation rescues SOX2 expression in AKT-inhibited BC cells. Finally, AKT inhibitors efficiently suppress the growth of SOX2-expressing putative cancer stem cells, whereas conventional chemotherapeutics select for this population. Together, our results suggest the AKT/SOX2 molecular axis as a regulator of BC clonogenicity and AKT inhibitors as promising drugs for the treatment of SOX2-positive BC.

A kinetic study of Am(III)/humic colloid interactions.

The interaction kinetics of the Am(III) ion with aquatic humic colloids is investigated under near-natural conditions by column experiments with a sandy aquifer sample rich in humic substancesforthe appraisal of the migration behavior of Am. The association and dissociation kinetics of the Am ion onto and from humic colloids control the migration of colloid-borne Am. As the contact time between Am and humic colloids prior to introduction into a column is increased, the mobility of colloid-borne Am is enhanced and hence the recovery of Am in the effluent increases. On the other hand, an increase of the migration time and residence time in column, respectively, reduces the Am recovery. Considering these experimental results a refined version of the kinetic model KICAM (Kinetically Controlled Availability Model), which suggests different Am binding modes with humic colloids, was developed. Applying KICAM it is possible to predict static and dynamic experiments affected by the kinetically controlled Am/humic colloid interactions over the range of 1 h up to several months. However, to apply these experimental results to long-term conditions, the Am binding scheme as proposed in KICAM needs to be verified. This paper provides, therefore, a basis for a better understanding of the colloid-borne Am migration in porous aquifer systems.