CHK1 Inhibitor Blocks Phosphorylation of FAM122A and Promotes Replication Stress

Manganese-coordinated nanoparticles loaded with CHK1 inhibitor dually activate cGAS-STING pathway and enhance efficacy of immune checkpoint therapy

Notable advancements have been made in utilizing immune checkpoint blockade (ICB) for the treatment of various cancers. However, the overall response rates and therapeutic effectiveness remain unsatisfactory. One cause is the inadequate immune environment characterized by poor T cell infiltration in tumors. To address these limitations, enhancing immune infiltration is crucial for optimizing the therapeutic efficacy of ICB. Activating the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway is essential for initiating immune response and has become a potential target for developing combination therapies with ICB. In this study, we designed and fabricated manganese-containing nanoparticles loaded with the CHK1 inhibitor PF477736, which were subsequently encapsulated with macrophage membrane (PF/MMSN@MPM). This innovative design achieved excellent tumor targeting and demonstrated potent antitumor effects. The combination therapy dually amplified the cGAS-STING pathway, causing a cascade of enhanced therapeutic effects against tumors. Furthermore, single-cell mass cytometry (CyTOF) analysis revealed that PF/MMSN@MPM enhanced the activation and infiltration of immune cells. Moreover, the combination of PF/MMSN@MPM with anti-PD-1 (αPD-1) exhibited a stronger therapeutic effect compared to αPD-1 alone. PF/MMSN@MPM precisely and synergistically activated the cGAS-STING pathway, significantly improving therapeutic efficacy of ICB, and offering promising potential for tumor therapy.

Combining cryo-electron microscopy (cryo-EM) with orthogonal solution state methods to define the molecular basis of the phosphoprotein phosphatase family regulation and substrate specificity

Protein phosphatases are dynamic enzymes that exhibit complex regulatory mechanisms, with disruptions in these regulatory processes associated with disease. It is now clear that many phosphatases assemble into large macromolecular complexes via the interaction of phosphatase-specific regulatory proteins and substrates containing short linear motifs (SLiMs) or short helical motifs (SHelMs). Here, we review how cryo-electron microscopy (cryo-EM) integrated with orthogonal methods to study dynamic protein-protein interactions (NMR spectroscopy, hydrogen-deuterium exchange mass spectrometry, among others) is leading to new discoveries about the mechanisms controlling phosphatase assembly, substrate recruitment and dephosphorylation and, in turn, are providing novel strategies for targeting phosphatase-related diseases. This review focuses on the recently determined structures and regulation of the phosphoprotein phosphatase (PPP) family of ser/thr phosphatases-PP1, PP2A, Calcineurin and PP5.

PPP2R1A mutations cause ATR inhibitor sensitivity in ovarian clear cell carcinoma

Identification of ARID1A/ATR synthetic lethality led to ATR inhibitor phase II trials in ovarian clear cell carcinoma (OCCC), a cancer of unmet need. Using multiple CRISPR-Cas9 mutagenesis and interference screens, we show that inactivation of protein phosphatase 2A (PP2A) subunits, including PPP2R1A, enhance ATRi sensitivity in ARID1A mutant OCCC. Analysis of a new OCCC cohort indicates that 52% possess oncogenic PPP2R1A p.R183 mutations and of these, one half possessed both ARID1A as well as PPP2R1A mutations. Using CRISPR-prime editing to generate new isogenic models of PPP2R1A mutant OCCC, we found that PPP2R1A p.R183W and p.R183P mutations cause ATRi-induced S phase stress, premature mitotic entry, genomic instability and ATRi sensitivity in OCCC tumour cells. p.R183 mutation also enhanced both in vitro and in vivo ATRi sensitivity in preclinical models of ARID1A mutant OCCC. These results argue for the assessment of PPP2R1A mutations as a biomarker of ATRi sensitivity.

CHAMP1 complex directs heterochromatin assembly and promotes homology-directed DNA repair

The CHAMP1 complex, a little-known but highly conserved protein complex consisting of CHAMP1, POGZ, and HP1α, is enriched in heterochromatin though its cellular function in these regions of the genome remain unknown. Here we show that the CHAMP complex promotes heterochromatin assembly at multiple chromosomal sites, including centromeres and telomeres, and promotes homology-directed repair (HDR) of DNA double strand breaks (DSBs) in these regions. The CHAMP1 complex is also required for heterochromatin assembly and DSB repair in highly-specialized chromosomal regions, such as the highly-compacted telomeres of ALT (Alternative Lengthening of Telomeres) positive tumor cells. Moreover, the CHAMP1 complex binds and recruits the writer methyltransferase SETDB1 to heterochromatin regions of the genome and is required for efficient DSB repair at these sites. Importantly, peripheral blood lymphocytes from individuals with CHAMP1 syndrome, an inherited neurologic disorder resulting from heterozygous mutations in CHAMP1, also exhibit defective heterochromatin clustering and defective repair of DSBs, suggesting that a defect in DNA repair underlies this syndrome. Taken together, the CHAMP1 complex has a specific role in heterochromatin assembly and the enhancement of HDR in heterochromatin.

A comprehensive review of current therapeutic strategies in cancers targeting DNA damage response mechanisms in head and neck squamous cell cancer

The DNA damage response (DDR) is an essential mechanism for maintaining genomic stability. Although DDR-targeted therapeutic strategies are being developed in several familial cancers, evaluation of their utility in head and neck squamous cell cancer (HNSCC) is lagging. This review briefly summarizes the mechanisms of DDR and the current knowledge on discovering DDR-related predictive biomarkers in HNSCC. This review also presents novel therapeutic strategies targeting DDR pathways for HNSCC based on the synthetic lethal concept. The combination of DDR inhibitors with cytotoxic treatments such as radiotherapy, chemotherapy, and immune checkpoint inhibitors is being evaluated, and several clinical trials are ongoing in patients with HNSCC. While DDR inhibitors are considered promising treatment options, resistance to these drugs is frequently observed, and their mechanisms are currently active research areas. A better understanding of the correlation between DDR pathways and cancer biology provides new therapeutic strategies for personalized medicine in HNSCC.

Succinate dehydrogenase deficiency-driven succinate accumulation induces drug resistance in acute myeloid leukemia via ubiquitin-cullin regulation

Drug resistance is vital for the poor prognosis of acute myeloid leukemia (AML) patients, but the underlying mechanism remains poorly understood. Given the unique microenvironment of bone marrow, we reasoned that drug resistance of AML might rely on distinct metabolic processes. Here, we identify succinate dehydrogenase (SDH) deficiency and over-cumulative succinate as typical features in AML, with a marked function in causing the resistance of AML cells to various anti-cancer therapies. Mechanistically, succinate promotes the accumulation of oncogenic proteins in a manner that precedes transcriptional activation. This function is mediated by succinate-triggered upregulation of ubiquitin-conjugating enzyme E2M (UBC12) phosphorylation, which impairs its E2 function in cullins neddylation. Notably, decreasing succinate by fludarabine can restore the sensitivity of anti-cancer drugs in SDH-deficient AML. Together, we uncover the function of succinate in driving drug resistance by regulating p-UBC12/cullin activity, and indicate reshaping succinate metabolism as a promising treatment for SDH-deficient AML.

Molecular Mechanism of PP2A/B55α Phosphatase Inhibition by IER5

PP2A serine/threonine phosphatases are heterotrimeric complexes that execute many essential physiologic functions. These activities are modulated by additional regulatory proteins, such as ARPP19, FAM122A, and IER5. Here, we report the cryoelectron microscopy structure of a complex of PP2A/B55α with the N-terminal structured region of IER5 (IER5-N50), which occludes a surface on B55α used for substrate recruitment, and show that IER5-N50 inhibits PP2A/B55α catalyzed dephosphorylation of pTau in biochemical assays. Mutations of full-length IER5 that disrupt its PP2A/B55α interface interfere with co-immunoprecipitation of PP2A/B55α. These mutations and deletions that remove the nuclear localization sequence of IER5 suppress cellular events such as KRT1 expression that depend on association of IER5 with PP2A/B55α. Querying the Alphafold2 predicted structure database identified SERTA domain proteins as high-confidence PP2A/B55α-binding structural homologs of IER5-N50. These studies define the molecular basis of PP2A/B55α inhibition by IER5-family proteins and suggest a roadmap for selective pharmacologic modulation of PP2A/B55α complexes.

FAM122A functions as a tumor suppressor in oral squamous cell carcinoma

Family with sequence similarity 122a (FAM122A), identified as an endogenous inhibitor of protein phosphatase 2A (PP2A) previously, is involved in multiple important physiological processes, and essential for the growth of acute myeloid leukemia and hepatocellular carcinoma cells. However, the function of FAM122A in oral squamous cell carcinoma (OSCC) is undetermined. In this study, by analyzing TCGA and GEO databases, we found that the expression of FAM122A was significantly down-regulated in head and neck squamous cell carcinoma and OSCC patients, meanwhile this low expression was tightly associated with the poor prognosis and advanced clinical stage during OSCC development. The similar low expression pattern of FAM122A could also been seen in OSCC cell lines compared with normal human oral keratinocytes. Further, we demonstrated that FAM122A knockdown significantly promoted the growth, clonogenic potential as well as migration capabilities of OSCC cells, while these alterations could be rescued by the re-expression of FAM122A. Over-expression of FAM122A suppressed OSCC cell proliferation and migration. FAM122A also inhibited the epithelial-mesenchymal transition (EMT) in OSCC cells by the up-regulation of epithelial marker E-cadherin and down-regulation of mesenchymal markers Fibronectin and Vimentin, which is presumably mediated by transforming growth factor β receptor 3 (TGFBR3), a novel tumor suppressor. In addition, FAM122A could induce T cell infiltration in OSCC, indicating that FAM122A might influence the immune cell activity of tumor environment and further interfere the tumor development. Collectively, our results suggest that FAM122A functions as a tumor suppressor in OSCC and possibly acts as a predictive biomarker for the diagnosis and/or treatment of OSCC.

The ribotoxic stress response drives UV-mediated cell death

While ultraviolet (UV) radiation damages DNA, eliciting the DNA damage response (DDR), it also damages RNA, triggering transcriptome-wide ribosomal collisions and eliciting a ribotoxic stress response (RSR). However, the relative contributions, timing, and regulation of these pathways in determining cell fate is unclear. Here we use time-resolved phosphoproteomic, chemical-genetic, single-cell imaging, and biochemical approaches to create a chronological atlas of signaling events activated in cells responding to UV damage. We discover that UV-induced apoptosis is mediated by the RSR kinase ZAK and not through the DDR. We identify two negative-feedback modules that regulate ZAK-mediated apoptosis: (1) GCN2 activation limits ribosomal collisions and attenuates ZAK-mediated RSR and (2) ZAK activity leads to phosphodegron autophosphorylation and its subsequent degradation. These events tune ZAK's activity to collision levels to establish regimes of homeostasis, tolerance, and death, revealing its key role as the cellular sentinel for nucleic acid damage.

FAM122A ensures cell cycle interphase progression and checkpoint control by inhibiting B55α/PP2A through helical motifs

The Ser/Thr protein phosphatase 2 A (PP2A) regulates the dephosphorylation of many phosphoproteins. Substrate recognition are mediated by B regulatory subunits. Here, we report the identification of a substrate conserved motif [RK]-V-x-x-[VI]-R in FAM122A, an inhibitor of B55α/PP2A. This motif is necessary for FAM122A binding to B55α, and computational structure prediction suggests the motif, which is helical, blocks substrate docking to the same site. In this model, FAM122A also spatially constrains substrate access by occluding the catalytic subunit. Consistently, FAM122A functions as a competitive inhibitor as it prevents substrate binding and dephosphorylation of CDK substrates by B55α/PP2A in cell lysates. FAM122A deficiency in human cell lines reduces the proliferation rate, cell cycle progression, and hinders G1/S and intra-S phase cell cycle checkpoints. FAM122A-KO in HEK293 cells attenuates CHK1 and CHK2 activation in response to replication stress. Overall, these data strongly suggest that FAM122A is a short helical motif (SHeM)-dependent, substrate-competitive inhibitor of B55α/PP2A that suppresses multiple functions of B55α in the DNA damage response and in timely progression through the cell cycle interphase.

Increases in cyclin A/Cdk activity and in PP2A-B55 inhibition by FAM122A are key mitosis-inducing events

Entry into mitosis has been classically attributed to the activation of a cyclin B/Cdk1 amplification loop via a partial pool of this kinase becoming active at the end of G2 phase. However, how this initial pool is activated is still unknown. Here we discovered a new role of the recently identified PP2A-B55 inhibitor FAM122A in triggering mitotic entry. Accordingly, depletion of the orthologue of FAM122A in C. elegans prevents entry into mitosis in germline stem cells. Moreover, data from Xenopus egg extracts strongly suggest that FAM122A-dependent inhibition of PP2A-B55 could be the initial event promoting mitotic entry. Inhibition of this phosphatase allows subsequent phosphorylation of early mitotic substrates by cyclin A/Cdk, resulting in full cyclin B/Cdk1 and Greatwall (Gwl) kinase activation. Subsequent to Greatwall activation, Arpp19/ENSA become phosphorylated and now compete with FAM122A, promoting its dissociation from PP2A-B55 and taking over its phosphatase inhibition role until the end of mitosis.

Small molecule agents for triple negative breast cancer: Current status and future prospects

Triple-negative breast cancer (TNBC) is a subtype of breast cancer with poor prognosis. The number of cases increased by 2.26 million in 2020, making it the most commonly diagnosed cancer type in the world. TNBCs lack hormone receptor (HR) and human epidermal growth factor 2 (HER2), which limits treatment options. Currently, paclitaxel-based drugs combined with other chemotherapeutics remain the main treatment for TNBC. There is currently no consensus on the best therapeutic regimen for TNBC. However, there have been successful clinical trials exploring large-molecule monoclonal antibodies, small-molecule targeted drugs, and novel antibody-drug conjugate (ADC). Although monoclonal antibodies have produced clinical success, their large molecular weight can limit therapeutic benefits. It is worth noting that in the past 30 years, the FDA has approved small molecule drugs for HER2-positive breast cancers. The lack of effective targets and the occurrence of drug resistance pose significant challenges in the treatment of TNBC. To improve the prognosis of TNBC, it is crucial to search for effective targets and to overcome drug resistance. This review examines the clinical efficacy, adverse effects, resistance mechanisms, and potential solutions of targeted small molecule drugs in both monotherapies and combination therapies. New therapeutic targets, including nuclear export protein 1 (XPO1) and hedgehog (Hh), are emerging as potential options for researchers and become integrated into clinical trials for TNBC. Additionally, there is growing interest in the potential of targeted protein degradation chimeras (PROTACs), degraders of rogue proteins, as a future therapy direction. This review provides potentially valuable insights with clinical implications.

Cryo-EM structures of PP2A:B55-FAM122A and PP2A:B55-ARPP19

Progression through the cell cycle is controlled by regulated and abrupt changes in phosphorylation1. Mitotic entry is initiated by increased phosphorylation of mitotic proteins, a process driven by kinases2, whereas mitotic exit is achieved by counteracting dephosphorylation, a process driven by phosphatases, especially PP2A:B553. Although the role of kinases in mitotic entry is well established, recent data have shown that mitosis is only successfully initiated when the counterbalancing phosphatases are also inhibited4. Inhibition of PP2A:B55 is achieved by the intrinsically disordered proteins ARPP195,6 and FAM122A7. Despite their critical roles in mitosis, the mechanisms by which they achieve PP2A:B55 inhibition is unknown. Here, we report the single-particle cryo-electron microscopy structures of PP2A:B55 bound to phosphorylated ARPP19 and FAM122A. Consistent with our complementary NMR spectroscopy studies, both intrinsically disordered proteins bind PP2A:B55, but do so in highly distinct manners, leveraging multiple distinct binding sites on B55. Our extensive structural, biophysical and biochemical data explain how substrates and inhibitors are recruited to PP2A:B55 and provide a molecular roadmap for the development of therapeutic interventions for PP2A:B55-related diseases.

Adavosertib and beyond: Biomarkers, drug combination and toxicity of WEE1 inhibitors

WEE1 kinase is renowned as an S-G2 checkpoint inhibitor activated by ATR-CHK1 in response to replication stress. WEE1 inhibition enhances replication stress and effectively circumvents checkpoints into mitosis, which triggers significant genetic impairs and culminates in cell death. This approach has been validated clinically for its promising anti-tumor efficacy across various cancer types, notably in cases of ovarian cancers. Nonetheless, the initial stage of clinical trials has shown that the first-in-human WEE1 inhibitor adavosertib is limited by dose-limiting adverse events. As a result, recent efforts have been made to explore predictive biomarkers and smart combination schedules to alleviate adverse effects. In this review, we focused on the exploration of therapeutic biomarkers, as well as schedules of combination utilizing WEE1 inhibitors and canonical anticancer drugs, according to the latest preclinical and clinical studies, indicating that the optimal application of WEE1 inhibitors will likely be as part of dose-reducing combination and be tailored to specific patient populations.

Cryo-EM structures of PP2A:B55-FAM122A and PP2A:B55-ARPP19

Progression through the cell cycle is controlled by regulated and abrupt changes in phosphorylation.1 Mitotic entry is initiated by increased phosphorylation of mitotic proteins, a process driven by kinases,2 while mitotic exit is achieved by counteracting dephosphorylation, a process driven by phosphatases, especially PP2A:B55.3 While the role of kinases in mitotic entry is well-established, recent data have shown that mitosis is only successfully initiated when the counterbalancing phosphatases are also inhibited.4 For PP2A:B55, inhibition is achieved by the two intrinsically disordered proteins (IDPs), ARPP19 (phosphorylation-dependent)6,7 and FAM122A5 (inhibition is phosphorylation-independent). Despite their critical roles in mitosis, the mechanisms by which they achieve PP2A:B55 inhibition is unknown. Here, we report the cryo-electron microscopy structures of PP2A:B55 bound to phosphorylated ARPP19 and FAM122A. Consistent with our complementary NMR spectroscopy studies both IDPs bind PP2A:B55, but do so in highly distinct manners, unexpectedly leveraging multiple distinct binding sites on B55. Our extensive structural, biophysical and biochemical data explain how substrates and inhibitors are recruited to PP2A:B55 and provides a molecular roadmap for the development of therapeutic interventions for PP2A:B55 related diseases.

Phosphatases maintain low catalytic activity of SGK1: DNA damage resets the balance in favor of phosphorylation

The serum- and glucocorticoid-induced kinase 1 (SGK1) promotes cell survival under stress conditions and facilitates the emergence of drug resistance in cancer. The underlying mechanisms of these observations are not fully understood. In this study, we found that SGK1 activity is suppressed by the action of the S/T phosphatases PP5 and PP2A, which constantly dephosphorylate SGK1. Using newly developed anti-phospho SGK1 antibodies and inhibitors of phosphatases, we determined that the high degree of dephosphorylation is caused by two factors: the tendency of SGK1 to unfold, which makes it dependent on Hsp90 chaperone complexes composed of four proteins, Hsp90/CDC37/PP5/SGK1, and where the phosphatase PP5 persistently dephosphorylates SGK1 within the complex. SGK1 binding to PP2A regulatory subunits B55γ and B55δ brings PP2A catalytic subunit close to exposed SGK1 phosphoresidues. A further association of phosphorylated pS37-FAM122A-an endogenous inhibitor of PP2A-to the holoenzyme diminishes dephosphorylation of SGK1 mediated by PP2A. Our study also reveals that genotoxic stress can reverse the dominant impact of phosphatases over kinases by activating the DNA-dependent protein kinase, which enhances mTORC2 activity directed to SGK1. Thus, our results provide insight into a molecular pathway that enables SGK1 to gain phosphorylation and catalytic activity and promote cell survival, potentially diminishing the efficacy of cancer treatments. As the DNA damage response operates in many cancer cells and is further induced by chemotherapies, the findings of this study could have significant implications for the development of novel cancer therapies targeting SGK1.

Chronic treatment with ATR and CHK1 inhibitors does not substantially increase the mutational burden of human cells

DNA replication stress (RS) entails the frequent slow down and arrest of replication forks by a variety of conditions that hinder accurate and processive genome duplication. Elevated RS leads to genome instability, replication catastrophe and eventually cell death. RS is particularly prevalent in cancer cells and its exacerbation to unsustainable levels by chemotherapeutic agents remains a cornerstone of cancer treatments. The adverse consequences of RS are normally prevented by the ATR and CHK1 checkpoint kinases that stabilize stressed forks, suppress origin firing and promote cell cycle arrest when replication is perturbed. Specific inhibitors of these kinases have been developed and shown to potentiate RS and cell death in multiple in vitro cancer settings. Ongoing clinical trials are now probing their efficacy against various cancer types, either as single agents or in combination with mainstay chemotherapeutics. Despite their promise as valuable additions to the anti-cancer pharmacopoeia, we still lack a genome-wide view of the potential mutagenicity of these new drugs. To investigate this question, we performed chronic long-term treatments of TP53-depleted human cancer cells with ATR and CHK1 inhibitors (ATRi, AZD6738/ceralasertib and CHK1i, MK8776/SCH-900776). ATR or CHK1 inhibition did not significantly increase the mutational burden of cells, nor generate specific mutational signatures. Indeed, no notable changes in the numbers of base substitutions, short insertions/deletions and larger scale rearrangements were observed despite induction of replication-associated DNA breaks during treatments. Interestingly, ATR inhibition did induce a slight increase in closely-spaced mutations, a feature previously attributed to translesion synthesis DNA polymerases. The results suggest that ATRi and CHK1i do not have substantial mutagenic effects in vitro when used as standalone agents.

CDC6 as a Key Inhibitory Regulator of CDK1 Activation Dynamics and the Timing of Mitotic Entry and Progression

Timely mitosis is critically important for early embryo development. It is regulated by the activity of the conserved protein kinase CDK1. The dynamics of CDK1 activation must be precisely controlled to assure physiologic and timely entry into mitosis. Recently, a known S-phase regulator CDC6 emerged as a key player in mitotic CDK1 activation cascade in early embryonic divisions, operating together with Xic1 as a CDK1 inhibitor upstream of the Aurora A and PLK1, both CDK1 activators. Herein, we review the molecular mechanisms that underlie the control of mitotic timing, with special emphasis on how CDC6/Xic1 function impacts CDK1 regulatory network in the Xenopus system. We focus on the presence of two independent mechanisms inhibiting the dynamics of CDK1 activation, namely Wee1/Myt1- and CDC6/Xic1-dependent, and how they cooperate with CDK1-activating mechanisms. As a result, we propose a comprehensive model integrating CDC6/Xic1-dependent inhibition into the CDK1-activation cascade. The physiological dynamics of CDK1 activation appear to be controlled by the system of multiple inhibitors and activators, and their integrated modulation ensures concomitantly both the robustness and certain flexibility of the control of this process. Identification of multiple activators and inhibitors of CDK1 upon M-phase entry allows for a better understanding of why cells divide at a specific time and how the pathways involved in the timely regulation of cell division are all integrated to precisely tune the control of mitotic events.

Identification of indocyanine green as a STT3B inhibitor against mushroom α-amanitin cytotoxicity

The "death cap", Amanita phalloides, is the world's most poisonous mushroom, responsible for 90% of mushroom-related fatalities. The most fatal component of the death cap is α-amanitin. Despite its lethal effect, the exact mechanisms of how α-amanitin poisons humans remain unclear, leading to no specific antidote available for treatment. Here we show that STT3B is required for α-amanitin toxicity and its inhibitor, indocyanine green (ICG), can be used as a specific antidote. By combining a genome-wide CRISPR screen with an in silico drug screening and in vivo functional validation, we discover that N-glycan biosynthesis pathway and its key component, STT3B, play a crucial role in α-amanitin toxicity and that ICG is a STT3B inhibitor. Furthermore, we demonstrate that ICG is effective in blocking the toxic effect of α-amanitin in cells, liver organoids, and male mice, resulting in an overall increase in animal survival. Together, by combining a genome-wide CRISPR screen for α-amanitin toxicity with an in silico drug screen and functional validation in vivo, our study highlights ICG as a STT3B inhibitor against the mushroom toxin.

FAM122A Is Required for Mesendodermal and Cardiac Differentiation of Embryonic Stem Cells

Mesendodermal specification and cardiac differentiation are key issues for developmental biology and heart regeneration medicine. Previously, we demonstrated that FAM122A, a highly conserved housekeeping gene, is an endogenous inhibitor of protein phosphatase 2A (PP2A) and participates in multifaceted physiological and pathological processes. However, the in vivo function of FAM122A is largely unknown. In this study, we observed that Fam122 deletion resulted in embryonic lethality with severe defects of cardiovascular developments and significantly attenuated cardiac functions in conditional cardiac-specific knockout mice. More importantly, Fam122a deficiency impaired mesendodermal specification and cardiac differentiation from mouse embryonic stem cells but showed no influence on pluripotent identity. Mechanical investigation revealed that the impaired differentiation potential was caused by the dysregulation of histone modification and Wnt and Hippo signaling pathways through modulation of PP2A activity. These findings suggest that FAM122A is a novel and critical regulator in mesendodermal specification and cardiac differentiation. This research not only significantly extends our understanding of the regulatory network of mesendodermal/cardiac differentiation but also proposes the potential significance of FAM122A in cardiac regeneration.

Comparative landscape of genetic dependencies in human and chimpanzee stem cells

Comparative studies of great apes provide a window into our evolutionary past, but the extent and identity of cellular differences that emerged during hominin evolution remain largely unexplored. We established a comparative loss-of-function approach to evaluate whether changes in human cells alter requirements for essential genes. By performing genome-wide CRISPR interference screens in human and chimpanzee pluripotent stem cells, we identified 75 genes with species-specific effects on cellular proliferation. These genes comprised coherent processes, including cell cycle progression and lysosomal signaling, which we determined to be human-derived by comparison with orangutan cells. Human-specific robustness to CDK2 and CCNE1 depletion persisted in neural progenitor cells, providing support for the G1-phase length hypothesis as a potential evolutionary mechanism in human brain expansion. Our findings demonstrate that evolutionary changes in human cells can reshape the landscape of essential genes and establish a platform for systematically uncovering latent cellular and molecular differences between species.

FAM122A ensures cell cycle interphase progression and checkpoint control as a SLiM-dependent substrate-competitive inhibitor to the B55⍺/PP2A phosphatase

The Ser/Thr protein phosphatase 2A (PP2A) is a highly conserved collection of heterotrimeric holoenzymes responsible for the dephosphorylation of many regulated phosphoproteins. Substrate recognition and the integration of regulatory cues are mediated by B regulatory subunits that are complexed to the catalytic subunit (C) by a scaffold protein (A). PP2A/B55 substrate recruitment was thought to be mediated by charge-charge interactions between the surface of B55α and its substrates. Challenging this view, we recently discovered a conserved SLiM [ RK ]- V -x-x-[ VI ]- R in a range of proteins, including substrates such as the retinoblastoma-related protein p107 and TAU (Fowle et al. eLife 2021;10:e63181). Here we report the identification of this SLiM in FAM122A, an inhibitor of B55α/PP2A. This conserved SLiM is necessary for FAM122A binding to B55α in vitro and in cells. Computational structure prediction with AlphaFold2 predicts an interaction consistent with the mutational and biochemical data and supports a mechanism whereby FAM122A uses the 'SLiM' in the form of a short α-helix to dock to the B55α top groove. In this model, FAM122A spatially constrains substrate access by occluding the catalytic subunit with a second α-helix immediately adjacent to helix 1. Consistently, FAM122A functions as a competitive inhibitor as it prevents binding of substrates in in vitro competition assays and the dephosphorylation of CDK substrates by B55α/PP2A in cell lysates. Ablation of FAM122A in human cell lines reduces the rate of proliferation, progression through cell cycle transitions and abrogates G1/S and intra-S phase cell cycle checkpoints. FAM122A-KO in HEK293 cells results in attenuation of CHK1 and CHK2 activation in response to replication stress. Overall, these data strongly suggest that FAM122A is a 'SLiM'-dependent, substrate-competitive inhibitor of B55α/PP2A that suppresses multiple functions of B55α in the DNA damage response and in timely progression through the cell cycle interphase.

Targeting replication stress in cancer therapy

Replication stress is a major cause of genomic instability and a crucial vulnerability of cancer cells. This vulnerability can be therapeutically targeted by inhibiting kinases that coordinate the DNA damage response with cell cycle control, including ATR, CHK1, WEE1 and MYT1 checkpoint kinases. In addition, inhibiting the DNA damage response releases DNA fragments into the cytoplasm, eliciting an innate immune response. Therefore, several ATR, CHK1, WEE1 and MYT1 inhibitors are undergoing clinical evaluation as monotherapies or in combination with chemotherapy, poly[ADP-ribose]polymerase (PARP) inhibitors, or immune checkpoint inhibitors to capitalize on high replication stress, overcome therapeutic resistance and promote effective antitumour immunity. Here, we review current and emerging approaches for targeting replication stress in cancer, from preclinical and biomarker development to clinical trial evaluation.

Combination DNA Damage Response (DDR) Inhibitors to Overcome Drug Resistance in Ovarian Cancer

The DNA damage response (DDR) results in activation of a series of key target kinases that respond to different DNA damage insults. DDR inhibitors such as PARP inhibitors lead to the accumulation of DNA damage in tumor cells and ultimately apoptosis. However, responses to DDRi monotherapy in the clinic are not durable and resistance ultimately develops. DDRi-DDRi combinations such as PARPi-ATRi, PAPRi-WEE1i and PARPi-AsiDNA can overcome multiple resistance mechanisms to PARP inhibition. In addition, DDRi-DDRi combinations can provide viable treatment options for patients with platinum-resistant disease. In the present chapter we discuss rationale of DDRi-DDRi strategies that capitalize on genomic alterations found in ovarian cancer and other solid tumors and may provide in the near future new treatment options for these patients.

Phosphoproteomic dysregulation in Huntington's disease mice is rescued by environmental enrichment

Huntington's disease is a fatal autosomal-dominant neurodegenerative disorder, characterized by neuronal cell dysfunction and loss, primarily in the striatum, cortex and hippocampus, causing motor, cognitive and psychiatric impairments. Unfortunately, no treatments are yet available to modify the progression of the disease. Recent evidence from Huntington's disease mouse models suggests that protein phosphorylation (catalysed by kinases and hydrolysed by phosphatases) might be dysregulated, making this major post-translational modification a potential area of interest to find novel therapeutic targets. Furthermore, environmental enrichment, used to model an active lifestyle in preclinical models, has been shown to alleviate Huntington's disease-related motor and cognitive symptoms. However, the molecular mechanisms leading to these therapeutic effects are still largely unknown. In this study, we applied a phosphoproteomics approach combined with proteomic analyses on brain samples from pre-motor symptomatic R6/1 Huntington's disease male mice and their wild-type littermates, after being housed either in environmental enrichment conditions, or in standard housing conditions from 4 to 8 weeks of age (n = 6 per group). We hypothesized that protein phosphorylation dysregulations occur prior to motor onset in this mouse model, in two highly affected brain regions, the striatum and hippocampus. Furthermore, we hypothesized that these phosphoproteome alterations are rescued by environmental enrichment. When comparing 8-week-old Huntington's disease mice and wild-type mice in standard housing conditions, our analysis revealed 229 differentially phosphorylated peptides in the striatum, compared with only 15 differentially phosphorylated peptides in the hippocampus (statistical thresholds fold discovery rate 0.05, fold change 1.5). At the same disease stage, minor differences were found in protein levels, with 24 and 22 proteins dysregulated in the striatum and hippocampus, respectively. Notably, we found no differences in striatal protein phosphorylation and protein expression when comparing Huntington's disease mice and their wild-type littermates in environmentally enriched conditions. In the hippocampus, only four peptides were differentially phosphorylated between the two genotypes under environmentally enriched conditions, and 22 proteins were differentially expressed. Together, our data indicates that protein phosphorylation dysregulations occur in the striatum of Huntington's disease mice, prior to motor symptoms, and that the kinases and phosphatases leading to these changes in protein phosphorylation might be viable drug targets to consider for this disorder. Furthermore, we show that an early environmental intervention was able to rescue the changes observed in protein expression and phosphorylation in the striatum of Huntington's disease mice and might underlie the beneficial effects of environmental enrichment, thus identifying novel therapeutic targets.

A Phase 2 study of prexasertib (LY2606368) in platinum resistant or refractory recurrent ovarian cancer

OBJECTIVE

High-grade serous ovarian cancer, the most frequent type of ovarian cancer, has a poor prognosis and novel treatments are needed for patients with platinum resistant/refractory disease. New therapeutic strategies targeting cell cycle checkpoints, including CHK1 inhibition with prexasertib, may help improve clinical response and overcome resistance.

METHODS

Patients with ovarian cancer (N = 169) were assigned to 4 cohorts as part of the Phase 2 multicenter trial (NCT03414047): Cohort 1: platinum resistant, BRCA-wildtype with ≥3 lines prior therapy; Cohort 2: platinum resistant BRCA-wildtype with <3 lines prior therapy; Cohort 3: platinum resistant, BRCA-mutated with prior PARP inhibitor therapy; Cohort 4: platinum refractory, BRCA-mutated, or BRCA-wildtype with any number of prior therapy lines. The primary endpoint was objective response rate (ORR) and secondary endpoints included disease control rate (DCR), and safety. DNA from tumor biopsies was sequenced to identify biomarkers.

RESULTS

The ORR in platinum resistant patients (Cohorts 1--3) was 12.1%, and 6.9% in platinum refractory patients. In platinum resistant patients, DCR was 37.1%, and consistent across cohorts. In platinum refractory patients, DCR was 31.0%. Consistent with the prexasertib mechanism of action, the most common treatment related adverse events of all grades included thrombocytopenia, neutropenia, fatigue, nausea, and anemia.

CONCLUSIONS

Prexasertib demonstrated durable single agent activity in a subset of patients with recurrent ovarian cancer regardless of clinical characteristics, BRCA status, or prior therapies, including PARPi. There was no obvious correlation with genomic alterations in responders vs non-responders, emphasizing the need for alternative biomarker approaches for responder identification.

CHAMP1 binds to REV7/FANCV and promotes homologous recombination repair

A critical determinant of DNA repair pathway choice is REV7, an adaptor that binds to various DNA repair proteins through its C-terminal seatbelt domain. The REV7 seatbelt binds to either REV3, activating translesion synthesis, or to SHLD3, activating non-homologous end joining (NHEJ) repair. Recent studies have identified another REV7 seatbelt-binding protein, CHAMP1 (chromosome alignment-maintaining phosphoprotein 1), though its possible role in DNA repair is unknown. Here, we show that binding of CHAMP1 to REV7 activates homologous recombination (HR) repair. Mechanistically, CHAMP1 binds directly to REV7 and reduces the level of the Shieldin complex, causing an increase in double-strand break end resection. CHAMP1 also interacts with POGZ in a heterochromatin complex further promoting HR repair. Importantly, in human tumors, CHAMP1 overexpression promotes HR, confers poly (ADP-ribose) polymerase inhibitor resistance, and correlates with poor prognosis. Thus, by binding to either SHLD3 or CHAMP1 through its seatbelt, the REV7 protein can promote either NHEJ or HR repair, respectively.

Feng et al. demonstrate that CHAMP1 promotes homologous recombination by binding to REV7 and reducing the level of the Shieldin complex, causing an increase in double-strand break end resection. CHAMP1 and POGZ form a complex to further promote HR. Upregulation of CHAMP1 expression is a mechanism of acquired PARP inhibitor resistance.

DNA Damage Repair and Current Therapeutic Approaches in Gastric Cancer: A Comprehensive Review

DNA in cells is frequently damaged by endogenous and exogenous agents. However, comprehensive mechanisms to combat and repair DNA damage have evolved to ensure genomic stability and integrity. Improper DNA damage repair may result in various diseases, including some types of tumors and autoimmune diseases. Therefore, DNA damage repair mechanisms have been proposed as novel antitumor drug targets. To date, numerous drugs targeting DNA damage mechanisms have been developed. For example, PARP inhibitors that elicit synthetic lethality are widely used in individualized cancer therapies. In this review, we describe the latent DNA damage repair mechanisms in gastric cancer, the types of DNA damage that can contribute to the development of gastric cancer, and new therapeutic approaches for gastric cancer that target DNA damage repair pathways.

The SKP2-p27 axis defines susceptibility to cell death upon CHK1 inhibition

Checkpoint kinase 1 (CHK1; encoded by CHEK1) is an essential gene that monitors DNA replication fidelity and prevents mitotic entry in the presence of under-replicated DNA or exogenous DNA damage. Cancer cells deficient in p53 tumor suppressor function reportedly develop a strong dependency on CHK1 for proper cell cycle progression and maintenance of genome integrity, sparking interest in developing kinase inhibitors. Pharmacological inhibition of CHK1 triggers B-Cell CLL/Lymphoma 2 (BCL2)-regulated cell death in malignant cells largely independently of p53, and has been suggested to kill p53-deficient cancer cells even more effectively. Next to p53 status, our knowledge about factors predicting cancer cell responsiveness to CHK1 inhibitors is limited. Here, we conducted a genome-wide CRISPR/Cas9-based loss-of-function screen to identify genes defining sensitivity to chemical CHK1 inhibitors. Next to the proapoptotic BCL2 family member, BCL2 Binding Component 3 (BBC3; also known as PUMA), the F-box protein S-phase Kinase-Associated Protein 2 (SKP2) was validated to tune the cellular response to CHK1 inhibition. SKP2 is best known for degradation of the Cyclin-dependent Kinase Inhibitor 1B (CDKN1B; also known as p27), thereby promoting G1-S transition and cell cycle progression in response to mitogens. Loss of SKP2 resulted in the predicted increase in p27 protein levels, coinciding with reduced DNA damage upon CHK1-inhibitor treatment and reduced cell death in S-phase. Conversely, overexpression of SKP2, which consequently results in reduced p27 protein levels, enhanced cell death susceptibility to CHK1 inhibition. We propose that assessing SKP2 and p27 expression levels in human malignancies will help to predict the responsiveness to CHK1-inhibitor treatment.

Replication Stress: A Review of Novel Targets to Enhance Radiosensitivity-From Bench to Clinic

DNA replication is a process fundamental in all living organisms in which deregulation, known as replication stress, often leads to genomic instability, a hallmark of cancer. Most malignant tumors sustain persistent proliferation and tolerate replication stress via increasing reliance to the replication stress response. So whilst replication stress induces genomic instability and tumorigenesis, the replication stress response exhibits a unique cancer-specific vulnerability that can be targeted to induce catastrophic cell proliferation. Radiation therapy, most used in cancer treatment, induces a plethora of DNA lesions that affect DNA integrity and, in-turn, DNA replication. Owing to radiation dose limitations for specific organs and tumor tissue resistance, the therapeutic window is narrow. Thus, a means to eliminate or reduce tumor radioresistance is urgently needed. Current research trends have highlighted the potential of combining replication stress regulators with radiation therapy to capitalize on the high replication stress of tumors. Here, we review the current body of evidence regarding the role of replication stress in tumor progression and discuss potential means of enhancing tumor radiosensitivity by targeting the replication stress response. We offer new insights into the possibility of combining radiation therapy with replication stress drugs for clinical use.

CIGB-300-Regulated Proteome Reveals Common and Tailored Response Patterns of AML Cells to CK2 Inhibition

Protein kinase CK2 is a highly pleiotropic and ubiquitously expressed Ser/Thr kinase with instrumental roles in normal and pathological states, including neoplastic phenotype in solid tumor and hematological malignancies. In line with previous reports, CK2 has been suggested as an attractive prognostic marker and molecular target in acute myeloid leukemia (AML), a blood malignant disorder that remains as an unmet medical need. Accordingly, this work investigates the complex landscape of molecular and cellular perturbations supporting the antileukemic effect exerted by CK2 inhibition in AML cells. To identify and functionally characterize the proteomic profile differentially modulated by the CK2 peptide-based inhibitor CIGB-300, we carried out LC-MS/MS and bioinformatic analysis in human cell lines representing two differentiation stages and major AML subtypes. Using this approach, 109 and 129 proteins were identified as significantly modulated in HL-60 and OCI-AML3 cells, respectively. In both proteomic profiles, proteins related to apoptotic cell death, cell cycle progression, and transcriptional/translational processes appeared represented, in agreement with previous results showing the impact of CIGB-300 in AML cell proliferation and viability. Of note, a group of proteins involved in intracellular redox homeostasis was specifically identified in HL-60 cell-regulated proteome, and flow cytometric analysis also confirmed a differential effect of CIGB-300 over reactive oxygen species (ROS) production in AML cells. Thus, oxidative stress might play a relevant role on CIGB-300-induced apoptosis in HL-60 but not in OCI-AML3 cells. Importantly, these findings provide first-hand insights concerning the CIGB-300 antileukemic effect and draw attention to the existence of both common and tailored response patterns triggered by CK2 inhibition in different AML backgrounds, a phenomenon of particular relevance with regard to the pharmacologic blockade of CK2 and personalized medicine.

Opportunities for Utilization of DNA Repair Inhibitors in Homologous Recombination Repair-Deficient and Proficient Pancreatic Adenocarcinoma

Pancreatic cancer is rapidly progressive and notoriously difficult to treat with cytotoxic chemotherapy and targeted agents. Recent demonstration of the efficacy of maintenance PARP inhibition in germline BRCA mutated pancreatic cancer has raised hopes that increased understanding of the DNA damage response pathway will lead to new therapies in both homologous recombination (HR) repair-deficient and proficient pancreatic cancer. Here, we review the potential mechanisms of exploiting HR deficiency, replicative stress, and DNA damage-mediated immune activation through targeted inhibition of DNA repair regulatory proteins.

Unprecedented Monoterpenoid Polyprenylated Acylphloroglucinols with a Rare 6/6/5/4 Tetracyclic Core, Enhanced MCF-7 Cells' Sensitivity to Camptothecin by Inhibiting the DNA Damage Response

(±)-Hypersines A-C (1-3), the three pairs of enantiomerically pure monoterpenoid polyprenylated acylphloroglucinols with an unprecedented 6/6/5/4 fused ring system, were isolated from Hypericum elodeoides. Their structures, including absolute configurations, were elucidated by comprehensive spectroscopic data, single-crystal X-ray diffraction, and quantum chemical calculations. The plausible, biosynthetic pathway of 1-3 was proposed. Moreover, the bioactivity evaluation indicated that 1a might be a novel DNA damage response inhibitor, and could enhance MCF-7 cell sensitivity to the anticancer agent, camptothecin.

Valproic Acid Regulates HR and Cell Cycle Through MUS81-pRPA2 Pathway in Response to Hydroxyurea

Breast cancer is the primary problem threatening women’s health. The combined application of valproic acid (VPA) and hydroxyurea (HU) has a synergistic effect on killing breast cancer cells, but the molecular mechanism remains elusive. Replication protein A2 phosphorylation (pRPA2), is essential for homologous recombination (HR) repair and cell cycle. Here we showed that in response to HU, the VPA significantly decreased the tumor cells survival, and promoted S-phase slippage, which was associated with the decrease of pCHK1 and WEE1/pCDK1-mediated checkpoint kinases phosphorylation pathway and inhibited pRPA2/Rad51-mediated HR repair pathway; the mutation of pRPA2 significantly diminished the above effect, indicating that VPA-caused HU sensitization was pRPA2 dependent. It was further found that VPA and HU combination treatment also resulted in the decrease of endonuclease MUS81. After MUS81 elimination, not only the level of pRPA2 was abolished in response to HU treatment, but also VPA-caused HU sensitization was significantly down-regulated through pRPA2-mediated checkpoint kinases phosphorylation and HR repair pathways. In addition, the VPA altered the tumor microenvironment and reduced tumor burden by recruiting macrophages to tumor sites; the Kaplan-Meier analysis showed that patients with high pRPA2 expression had significantly worse survival. Overall, our findings demonstrated that VPA influences HR repair and cell cycle through down-regulating MUS81-pRPA2 pathway in response to HU treatment.

Genetic vulnerabilities upon inhibition of DNA damage response

Because of essential roles of DNA damage response (DDR) in the maintenance of genomic integrity, cellular homeostasis, and tumor suppression, targeting DDR has become a promising therapeutic strategy for cancer treatment. However, the benefits of cancer therapy targeting DDR are limited mainly due to the lack of predictive biomarkers. To address this challenge, we performed CRISPR screens to search for genetic vulnerabilities that affect cells’ response to DDR inhibition. By undertaking CRISPR screens with inhibitors targeting key DDR mediators, i.e. ATR, ATM, DNAPK and CHK1, we obtained a global and unbiased view of genetic interactions with DDR inhibition. Specifically, we identified YWHAE loss as a key determinant of sensitivity to CHK1 inhibition. We showed that KLHL15 loss protects cells from DNA damage induced by ATM inhibition. Moreover, we validated that APEX1 loss sensitizes cells to DNAPK inhibition. Additionally, we compared the synergistic effects of combining different DDR inhibitors and found that an ATM inhibitor plus a PARP inhibitor induced dramatic levels of cell death, probably through promoting apoptosis. Our results enhance the understanding of DDR pathways and will facilitate the use of DDR-targeting agents in cancer therapy.

USP7 limits CDK1 activity throughout the cell cycle

Chemical inhibitors of the deubiquitinase USP7 are currently being developed as anticancer agents based on their capacity to stabilize P53. Regardless of this activity, USP7 inhibitors also generate DNA damage in a p53-independent manner. However, the mechanism of this genotoxicity and its contribution to the anticancer effects of USP7 inhibitors are still under debate. Here we show that, surprisingly, even if USP7 inhibitors stop DNA replication, they also induce a widespread activation of CDK1 throughout the cell cycle, which leads to DNA damage and is toxic for mammalian cells. In addition, USP7 interacts with the phosphatase PP2A and supports its active localization in the cytoplasm. Accordingly, inhibition of USP7 or PP2A triggers very similar changes of the phosphoproteome, including a widespread increase in the phosphorylation of CDK1 targets. Importantly, the toxicity of USP7 inhibitors is alleviated by lowering CDK1 activity or by chemical activation of PP2A. Our work reveals that USP7 limits CDK1 activity at all cell cycle stages, providing a novel mechanism that explains the toxicity of USP7 inhibitors through untimely activation of CDK1.

Expanding roles of cell cycle checkpoint inhibitors in radiation oncology

PURPOSE

Radiation-induced activation of cell cycle checkpoints have been of long-standing interest. The WEE1, CHK1 and ATR kinases are key factors in cell cycle checkpoint regulation and are essential for the S and G2 checkpoints. Here, we review the rationale for why inhibitors of WEE1, CHK1 and ATR could be beneficial in combination with radiation.

CONCLUSIONS

Combined treatment with radiation and inhibitors of these kinases results in checkpoint abrogation and subsequent mitotic catastrophe. This might selectively radiosensitize tumor cells, as they often lack the p53-dependent G1 checkpoint and therefore rely more on the G2 checkpoint to repair DNA damage. Further affecting the repair of radiation damage, inhibition of WEE1, CHK1 or ATR also specifically suppresses the homologous recombination repair pathway. Moreover, inhibition of these kinases can induce massive replication stress during S phase of the cell cycle, likely contributing to eliminate radioresistant S phase cells. Intriguingly, recent findings suggest that cell cycle checkpoint inhibitors in combination with radiation can also enhance anti-tumor immune effects. Altogether, the expanding knowledge about the functional roles of WEE1, CHK1 and ATR inhibitors support that they are promising candidates for use in combination with radiation treatment.

MMB-FOXM1-driven premature mitosis is required for CHK1 inhibitor sensitivity

To identify genes whose loss confers resistance to CHK1 inhibitors, we perform genome-wide CRISPR-Cas9 screens in non-small-cell lung cancer (NSCLC) cell lines treated with the CHK1 inhibitor prexasertib (CHK1i). Five of the top six hits of the screens, MYBL2 (B-MYB), LIN54, FOXM1, cyclin A2 (CCNA2), and CDC25B, are cell-cycle-regulated genes that contribute to entry into mitosis. Knockout of MMB-FOXM1 complex components LIN54 and FOXM1 reduce CHK1i-induced DNA replication stress markers and premature mitosis during Late S phase. Activation of a feedback loop between the MMB-FOXM1 complex and CDK1 is required for CHK1i-induced premature mitosis in Late S phase and subsequent replication catastrophe, indicating that dysregulation of the S to M transition is necessary for CHK1 inhibitor sensitivity. These findings provide mechanistic insights into small molecule inhibitors currently studied in clinical trials and provide rationale for combination therapies.

Branigan et al., by using genome-wide CRISPR screens, identify the MMB-FOXM1 complex as being required for CHK1 inhibitor (CHK1i) sensitivity. Their study shows that CHK1i-induced premature activation of the G2/M transcriptional program by this complex triggers a breakdown in the separation of DNA synthesis and mitosis, leading to replication catastrophe.

Jatrophane Diterpenoids from Euphorbia peplus as Multidrug Resistance Modulators with Inhibitory Effects on the ATR-Chk-1 Pathway

Twelve undescribed jatrophane diterpenoids, euphpepluones A-L (1-12), together with seven known analogues (13-19), were isolated from the whole plant of Euphorbia peplus, and their structures were elucidated by spectroscopic studies. The absolute configurations of 1 and 4 were assigned by X-ray crystallographic analysis. All isolates were investigated for their inhibitory effects against the ATR-Chk1 pathway using a Western blotting assay. As a result, 1, 2, 5, 8, 10, and 16 were found to suppress the camptothecin (CPT)-induced phosphorylation of Chk1, indicating that these compounds inhibit the activation of the ATR-Chk1 pathway. A preliminary structure-activity relationship (SAR) study of the isolates was conducted. When compound 10 and CPT were combined, apoptosis was induced in A549 cells with PARP cleavage, while there was no apoptotic effect by treatment with CPT or 10 alone. The data obtained indicate that 10 potentiates the chemotherapeutic sensitivity of A549 cells to CPT.

Comparative Activity and Off-Target Effects in Cells of the CHK1 Inhibitors MK-8776, SRA737, and LY2606368

DNA damage activates the checkpoint protein CHK1 to arrest cell cycle progression, providing time for repair and recovery. Consequently, inhibitors of CHK1 (CHK1i) enhance damage-induced cell death. Additionally, CHK1i elicits single agent cytotoxicity in some cell lines. We compared three CHK1i that have undergone clinical trials and exhibited different toxicities. Each CHK1i inhibits other targets at higher concentrations, and whether these contribute to the toxicity is unknown. We compared their sensitivity in a panel of cell lines, their efficacy at inhibiting CHK1 and CHK2, and their ability to induce DNA damage and abrogate damage-induced S phase arrest. Published in vitro kinase analyses were a poor predictor of selectivity and potency in cells. LY2606368 was far more potent at inhibiting CHK1 and inducing growth arrest, while all three CHK1i inhibited CHK2 at concentrations 10- (MK-8776 and SRA737) to 100- (LY2606368) fold higher. MK-8776 and SRA737 exhibited similar off-target effects: higher concentrations demonstrated transient protection from growth inhibition, circumvented DNA damage, and prevented checkpoint abrogation, possibly due to inhibition of CDK2. Acquired resistance to LY2606368 resulted in limited cross-resistance to other CHK1i. LY2606368-resistant cells still abrogated DNA damage-induced S phase arrest, which requires low CDK2 activity, whereas inappropriately high CDK2 activity is responsible for sensitivity to CHK1i alone. All three CHK1i inhibited protein synthesis in a sensitive cell line correlating with cell death, whereas resistant cells failed to inhibit protein synthesis and underwent transient cytostasis. LY2606368 appears to be the most selective CHK1i, suggesting that further clinical development of this drug is warranted.

Should WEE(1) CHK(1) in on the FAM(122A)ily?

Li et al. (2020) elucidate the resistance mechanisms to small-molecule inhibitors targeting the G2/M cell cycle checkpoint kinase, CHK1, in a variety of non-small cell lung cancer cell lines using CRISPR-mediated genetic approaches and identify biomarkers of response.