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Key Points Many cell cycle proteins are overexpressed or overactive in human cancers, in particular, D-type and E-type cyclins, cyclin-dependent kinases (CDK4, CDK6 and CDK2), Polo-like kinase 1 (PLK1) and Aurora kinases (Aurora A and Aurora B). In transgenic mice, overexpression of several of these cell cycle proteins induces or contributes to tumorigenesis, revealing their prominent oncogenic roles. Some of these cell cycle proteins are also required for tumorigenesis, and their ablation in mice impairs tumour formation induced by specific genetic lesions or by carcinogen treatment, as demonstrated for several cyclins (D1, D2 and D3) and CDKs (CDK4, CDK6, CDK2 and CDK1), as well as for checkpoint kinase 1 (CHK1). Importantly, in some cases the continued presence of a cell cycle protein has also been shown to be required for tumour maintenance and progression, for example, for cyclin D1, cyclin D3 and CDK4, thereby providing a clear rationale for targeting these proteins in cancer treatment. Kinases involved in cell cycle checkpoint function such as CHK1 and WEE1 also constitute potential therapeutic targets. Their inhibition compromises checkpoint function, causes excessive DNA damage and eventually leads to apoptosis, particularly in cells with compromised p53 function. CDK4/6-selective inhibitors, such as palbociclib, ribociclib and abemaciclib, have shown significant benefits in clinical studies, particularly in breast cancer, but also in non-small-cell lung cancer, melanoma and head and neck squamous cell carcinoma. Importantly, following demonstration of a substantial improvement in progression-free survival, combination of palbociclib and letrozole received accelerated approval for first-line treatment of patients with advanced ER + HER2 − breast cancer. Inhibitors of PLK1, such as rigosertib and volasertib, have also shown encouraging results in clinical phase II/III studies for patients with myelodysplastic syndromes and acute myelogenous leukaemia, respectively, and several phase III trials are currently ongoing. Compounds targeting Aurora A, particularly alisertib, have been extensively studied in preclinical models and demonstrated synergy with many other targeted therapies, leading to tumour regression in various cancer models. Moreover, clinical studies revealed encouraging activity of alisertib in peripheral T cell lymphoma, non-Hodgkin lymphoma, non-small-cell lung cancer and breast cancer. Proteins regulating cell cycle progression are involved in the formation of most cancer types. This Review discusses the role of cell cycle proteins in cancer, the rationale for targeting them in cancer treatment, results of clinical trials, as well as future therapeutic potential of various cell cycle inhibitors. Cancer is characterized by uncontrolled tumour cell proliferation resulting from aberrant activity of various cell cycle proteins. Therefore, cell cycle regulators are considered attractive targets in cancer therapy. Intriguingly, animal models demonstrate that some of these proteins are not essential for proliferation of non-transformed cells and development of most tissues. By contrast, many cancers are uniquely dependent on these proteins and hence are selectively sensitive to their inhibition. After decades of research on the physiological functions of cell cycle proteins and their relevance for cancer, this knowledge recently translated into the first approved cancer therapeutic targeting of a direct regulator of the cell cycle. In this Review, we focus on proteins that directly regulate cell cycle progression (such as cyclin-dependent kinases (CDKs)), as well as checkpoint kinases, Aurora kinases and Polo-like kinases (PLKs). We discuss the role of cell cycle proteins in cancer, the rationale for targeting them in cancer treatment and results of clinical trials, as well as the future therapeutic potential of various cell cycle inhibitors.

Verteporfin (VP), a light-activated drug used in photodynamic therapy for the treatment of choroidal neovascular membranes, has also been shown to be an effective inhibitor of malignant cells. Recently, studies have demonstrated that, even without photo-activation, VP may still inhibit certain tumor cell lines, including ovarian cancer, hepatocarcinoma and retinoblastoma, through the inhibition of the YAP-TEAD complex. In this study, we examined the effects of VP without light activation on human glioma cell lines (LN229 and SNB19). Through western blot analysis, we identified that human glioma cells that were exposed to VP without light activation demonstrated a downregulation of YAP-TEAD-associated downstream signaling molecules, including c-myc, axl, CTGF, cyr61 and survivin and upregulation of the tumor growth inhibitor molecule p38 MAPK. In addition, we observed that expression of VEGFA and the pluripotent marker Oct-4 were also decreased. Verteporfin did not alter the Akt survival pathway or the mTor pathway but there was a modest increase in LC3-IIB, a marker of autophagosome biogenesis. This study suggests that verteporfin should be further explored as an adjuvant therapy for the treatment of glioblastoma.

In the cell cycle, there are two checkpoint arrests that allow cells to repair damaged DNA in order to maintain genomic integrity. Many cancer cells have defective G1 checkpoint mechanisms, thus depending on the G2 checkpoint far more than normal cells. G2 checkpoint abrogation is therefore a promising concept to preferably damage cancerous cells over normal cells. The main factor influencing the decision to enter mitosis is a complex composed of Cdk1 and cyclin B. Cdk1/CycB is regulated by various feedback mechanisms, in particular inhibitory phosphorylations at Thr14 and Tyr15 of Cdk1. In fact, Cdk1/CycB activity is restricted by the balance between WEE family kinases and Cdc25 phosphatases. The WEE kinase family consists of three proteins: WEE1, PKMYT1, and the less important WEE1B. WEE1 exclusively mediates phosphorylation at Tyr15, whereas PKMYT1 is dual-specific for Tyr15 as well as Thr14. Inhibition by a small molecule inhibitor is therefore proposed to be a promising option since WEE kinases bind Cdk1, altering equilibria and thus affecting G2/M transition.

Mutations in SPOP , the gene encoding a component of the E3 ubiquitin ligase complex, impair ubiquitination-dependent degradation of BRD2, BRD3 and BRD4 proteins and result in activation of ATK–mTORC1 signaling and resistance to BET inhibitors. Pharmacological blockade of AKT represents a viable strategy to restore the sensitivity of SPOP-mutant prostate tumors to BET inhibitors. These results, together with findings by Dai et al . and Janouskova et al ., uncover a new nongenetic mechanism of resistance to BET inhibition involving cancer-type-specific mutations in SPOP , and support the evaluation of SPOP mutation status to inform the administration of BET inhibitors in the clinic. Bromodomain and extraterminal domain (BET) protein inhibitors are emerging as promising anticancer therapies. The gene encoding the E3 ubiquitin ligase substrate-binding adaptor speckle-type POZ protein (SPOP) is the most frequently mutated in primary prostate cancer. Here we demonstrate that wild-type SPOP binds to and induces ubiquitination and proteasomal degradation of BET proteins (BRD2, BRD3 and BRD4) by recognizing a degron motif common among them. In contrast, prostate cancer–associated SPOP mutants show impaired binding to BET proteins, resulting in decreased proteasomal degradation and accumulation of these proteins in prostate cancer cell lines and patient specimens and causing resistance to BET inhibitors. Transcriptome and BRD4 cistrome analyses reveal enhanced expression of the GTPase RAC1 and cholesterol-biosynthesis-associated genes together with activation of AKT–mTORC1 signaling as a consequence of BRD4 stabilization. Our data show that resistance to BET inhibitors in SPOP -mutant prostate cancer can be overcome by combination with AKT inhibitors and further support the evaluation of SPOP mutations as biomarkers to guide BET-inhibitor-oriented therapy in patients with prostate cancer.

A multi-genomic approach identifies the addiction of KRAS -mutant lung cancer cells to XPO1-dependent nuclear export, offering a new therapeutic opportunity. Druggable targets in KRAS-driven tumours These authors use RNA interference screening of more than a hundred human non-small-cell lung cancer cell lines to identify phenotypic variations selectively required for the survival of cells carrying mutations in the KRAS gene. They find that KRAS-driven cancers are dependent on the nuclear export machinery. This vulnerability can be exploited by clinically available drugs targeting nuclear export receptor XPO-1, which inhibit tumour growth at least in part by promoting nuclear accumulation of NF-κB inhibitors. Conversely, some KRAS-driven tumours bypass this dependence through co-occurring mutations that result in YAP1 activation. This resistance mechanism can be countered by coadministration of the YAP1/TEAD inhibitor verteporfin. The common participation of oncogenic KRAS proteins in many of the most lethal human cancers, together with the ease of detecting somatic KRAS mutant alleles in patient samples, has spurred persistent and intensive efforts to develop drugs that inhibit KRAS activity 1 . However, advances have been hindered by the pervasive inter- and intra-lineage diversity in the targetable mechanisms that underlie KRAS-driven cancers, limited pharmacological accessibility of many candidate synthetic-lethal interactions and the swift emergence of unanticipated resistance mechanisms to otherwise effective targeted therapies. Here we demonstrate the acute and specific cell-autonomous addiction of KRAS -mutant non-small-cell lung cancer cells to receptor-dependent nuclear export. A multi-genomic, data-driven approach, utilizing 106 human non-small-cell lung cancer cell lines, was used to interrogate 4,725 biological processes with 39,760 short interfering RNA pools for those selectively required for the survival of KRAS -mutant cells that harbour a broad spectrum of phenotypic variation. Nuclear transport machinery was the sole process-level discriminator of statistical significance. Chemical perturbation of the nuclear export receptor XPO1 (also known as CRM1), with a clinically available drug, revealed a robust synthetic-lethal interaction with native or engineered oncogenic KRAS both in vitro and in vivo . The primary mechanism underpinning XPO1 inhibitor sensitivity was intolerance to the accumulation of nuclear IκBα (also known as NFKBIA), with consequent inhibition of NFκB transcription factor activity. Intrinsic resistance associated with concurrent FSTL5 mutations was detected and determined to be a consequence of YAP1 activation via a previously unappreciated FSTL5–Hippo pathway regulatory axis. This occurs in approximately 17% of KRAS -mutant lung cancers, and can be overcome with the co-administration of a YAP1–TEAD inhibitor. These findings indicate that clinically available XPO1 inhibitors are a promising therapeutic strategy for a considerable cohort of patients with lung cancer when coupled to genomics-guided patient selection and observation.

Inactivation of ARID1A and other components of the nuclear SWI/SNF protein complex occurs at very high frequencies in a variety of human malignancies, suggesting a widespread role for the SWI/SNF complex in tumour suppression 1 . However, the underlying mechanisms remain poorly understood. Here we show that ARID1A-containing SWI/SNF complex (ARID1A–SWI/SNF) operates as an inhibitor of the pro-oncogenic transcriptional coactivators YAP and TAZ 2 . Using a combination of gain- and loss-of-function approaches in several cellular contexts, we show that YAP/TAZ are necessary to induce the effects of the inactivation of the SWI/SNF complex, such as cell proliferation, acquisition of stem cell-like traits and liver tumorigenesis. We found that YAP/TAZ form a complex with SWI/SNF; this interaction is mediated by ARID1A and is alternative to the association of YAP/TAZ with the DNA-binding platform TEAD. Cellular mechanotransduction regulates the association between ARID1A–SWI/SNF and YAP/TAZ. The inhibitory interaction of ARID1A–SWI/SNF and YAP/TAZ is predominant in cells that experience low mechanical signalling, in which loss of ARID1A rescues the association between YAP/TAZ and TEAD. At high mechanical stress, nuclear F-actin binds to ARID1A–SWI/SNF, thereby preventing the formation of the ARID1A–SWI/SNF–YAP/TAZ complex, in favour of an association between TEAD and YAP/TAZ. We propose that a dual requirement must be met to fully enable the YAP/TAZ responses: promotion of nuclear accumulation of YAP/TAZ, for example, by loss of Hippo signalling, and inhibition of ARID1A–SWI/SNF, which can occur either through genetic inactivation or because of increased cell mechanics. This study offers a molecular framework in which mechanical signals that emerge at the tissue level together with genetic lesions activate YAP/TAZ to induce cell plasticity and tumorigenesis. The ARID1A-containing SWI/SNF complex operates as an inhibitor of the pro-oncogenic transcriptional coactivators YAP and TAZ; this interaction is regulated by cellular mechanotransduction.

Key Points RNA polymerase II (Pol II) elongation is a highly regulated process. Regulation of transcription is often mediated at the level of promoter-proximal pausing of Pol II, in which Pol II is paused approximately 30–60 nucleotides downstream of the transcription start site (TSS) and awaits recruitment of kinase positive transcription elongation factor-b (P-TEFb). P-TEFb is the main factor required to release paused Pol II from the promoter-proximal region, and can directly or indirectly be recruited by many factors, including bromodomain-containing protein 4 (BRD4) and the super elongation complex (SEC). Elongation rates throughout the gene body are not uniform but vary between, and within genes, and can range from ∼1 to 6 kb per minute. Transient slowdown of Pol II is observed up to 15 kb downstream of the TSS, at exons and near the poly(A) cleavage site. Elongation rates can affect co-transcriptional RNA processes such as splicing and termination, as well as genome stability. Pausing of RNA polymerase II (Pol II) in promoter-proximal regions and its release to initiate productive elongation are key steps in the regulation of transcription, and involve many factors. Evidence is now emerging that transcriptional elongation is highly dynamic. Elongation rates vary between genes and across the length of a gene, affecting splicing, termination and genome stability. Recent advances in sequencing techniques that measure nascent transcripts and that reveal the positioning of RNA polymerase II (Pol II) have shown that the pausing of Pol II in promoter-proximal regions and its release to initiate a phase of productive elongation are key steps in transcription regulation. Moreover, after the release of Pol II from the promoter-proximal region, elongation rates are highly dynamic throughout the transcription of a gene, and vary on a gene-by-gene basis. Interestingly, Pol II elongation rates affect co-transcriptional processes such as splicing, termination and genome stability. Increasing numbers of factors and regulatory mechanisms have been associated with the steps of transcription elongation by Pol II, revealing that elongation is a highly complex process. Elongation is thus now recognized as a key phase in the regulation of transcription by Pol II.

The mTOR complex 1 (mTORC1) pathway promotes cell growth in response to many cues, including amino acids, which act through the Rag guanosine triphosphatases (GTPases) to promote mTORC1 translocation to the lysosomal surface, its site of activation. Although progress has been made in identifying positive regulators of the Rags, it is unknown if negative factors also exist. Here, we identify GATOR as a complex that interacts with the Rags and is composed of two subcomplexes we call GATOR1 and -2. Inhibition of GATOR1 subunits (DEPDC5, Nprl2, and Nprl3) makes mTORC1 signaling resistant to amino acid deprivation. In contrast inhibition of GATOR2 subunits (Mios, WDR24, WDR59, Seh1l, and Sec13) suppresses mTORC1 signaling, and epistasis analysis shows that GATOR2 negatively regulates DEPDC5. GATOR1 has GTPase-activating protein (GAP) activity for RagA and RagB, and its components are mutated in human cancer. In cancer cells with inactivating mutations in GATOR1, mTORC1 is hyperactive and insensitive to amino acid starvation, and such cells are hypersensitive to rapamycin, an mTORC1 inhibitor. Thus, we identify a key negative regulator of the Rag GTPases and reveal that, like other mTORC1 regulators, Rag function can be deregulated in cancer.

Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive human malignancies. The Yes‐associated protein‐1 (YAP) plays a critical role in cell proliferation, apoptosis and angiogenesis. Verteporfin is a photosensitizer used in photodynamic therapy and also a small molecular inhibitor of the Hippo‐YAP pathway. However, little is known about whether verteporfin could inhibit YAP activity in PDAC cells. Our present results showed that verteporfin suppressed the proliferation of human PDAC PANC‐1 and SW1990 cells by arresting cells at the G1 phase, and inducing apoptosis in dose‐ and time‐dependent manners. Verteporfin also inhibited the tumor growth on the PDAC xenograft model. Treatment with verteporfin led to downregulation of cyclinD1 and cyclinE1, modulation of Bcl‐2 family proteins and activation of PARP. In addition, verteporfin exhibited an inhibitory effect on angiogenesis and vasculogenic mimicry via suppressing Ang2, MMP2, VE‐cadherin, and α‐SMA expression in vitro and in vivo. Mechanism studies demonstrated that verteporfin impaired YAP and TEAD interaction to suppress the expression of targeted genes. Our results provide a foundation for repurposing verteporfin as a promising anti‐tumor drug in the treatment of pancreatic cancer by targeting the Hippo pathway. Verteporfin suppresses cell survival, migration, angiogenesis and vasculogenic mimicry of pancreatic cancer via disrupting the YAP‐TEAD interaction. Verteporfin inhibits cell cycle progression by suppressing cyclinD1 and cyclinE1 expression. Verteporfin induces cell apoptosis by activating the intrinsic apoptosis pathway. Verteporfin also inhibits Ang2, MMP2, VE‐cadherin and α‐SMA expression to suppress angiogenesis and vasculogenic mimicry.

SUZ12, a component of the PRC2 complex, can also function as a tumour suppressor in certain tumours of the nervous system and melanomas. Interaction of PRC2 with Ras pathway The PRC2 complex, which regulates gene expression through chromatin modification, has been shown to play a pro-tumorigenic role in many tumours. Karen Cichowski and colleagues now show that SUZ12, a component of PRC2, can also function as a tumour suppressor in certain tumours of the nervous system and melanomas. Through deregulation of chromatin and thereby gene expression, SUZ12 loss cooperates with the loss of NF1, another tumour suppressor frequently lost in these tumours types. At the same time, SUZ12 loss renders tumours sensitive to drugs that target bromodomain proteins, which are currently being explored for a number of cancer types. This work reveals an unexpected connection between PRC2 and several components of the Ras pathway, as well as providing possible targets for epigenetic-based therapies. The polycomb repressive complex 2 (PRC2) exerts oncogenic effects in many tumour types 1 . However, loss-of-function mutations in PRC2 components occur in a subset of haematopoietic malignancies, suggesting that this complex plays a dichotomous and poorly understood role in cancer 2 , 3 . Here we provide genomic, cellular, and mouse modelling data demonstrating that the polycomb group gene SUZ12 functions as tumour suppressor in PNS tumours, high-grade gliomas and melanomas by cooperating with mutations in NF1 . NF1 encodes a Ras GTPase-activating protein (RasGAP) and its loss drives cancer by activating Ras 4 . We show that SUZ12 loss potentiates the effects of NF1 mutations by amplifying Ras-driven transcription through effects on chromatin. Importantly, however, SUZ12 inactivation also triggers an epigenetic switch that sensitizes these cancers to bromodomain inhibitors. Collectively, these studies not only reveal an unexpected connection between the PRC2 complex, NF1 and Ras, but also identify a promising epigenetic-based therapeutic strategy that may be exploited for a variety of cancers.