Protein neddylation as a therapeutic target in pulmonary and extrapulmonary small cell carcinomas.

Small cell lung carcinoma (SCLC) is among the most lethal of all solid tumor malignancies. In an effort to identify novel therapeutic approaches for this recalcitrant cancer type, we applied genome-scale CRISPR/Cas9 inactivation screens to cell lines that we derived from a murine model of SCLC. SCLC cells were particularly sensitive to the deletion of NEDD8 and other neddylation pathway genes. Genetic suppression or pharmacological inhibition of this pathway using MLN4924 caused cell death not only in mouse SCLC cell lines but also in patient-derived xenograft (PDX) models of pulmonary and extrapulmonary small cell carcinoma treated ex vivo or in vivo. A subset of PDX models were exceptionally sensitive to neddylation inhibition. Neddylation inhibition suppressed expression of major regulators of neuroendocrine cell state such as INSM1 and ASCL1, which a subset of SCLC rely upon for cell proliferation and survival. To identify potential mechanisms of resistance to neddylation inhibition, we performed a genome-scale CRISPR/Cas9 suppressor screen. Deletion of components of the COP9 signalosome strongly mitigated the effects of neddylation inhibition in small cell carcinoma, including the ability of MLN4924 to suppress neuroendocrine transcriptional program expression. This work identifies neddylation as a regulator of neuroendocrine cell state and potential therapeutic target for small cell carcinomas.

MAX Functions as a Tumor Suppressor and Rewires Metabolism in Small Cell Lung Cancer.

Small cell lung cancer (SCLC) is a highly aggressive and lethal neoplasm. To identify candidate tumor suppressors we applied CRISPR/Cas9 gene inactivation screens to a cellular model of early-stage SCLC. Among the top hits was MAX, the obligate heterodimerization partner for MYC family proteins that is mutated in human SCLC. Max deletion increases growth and transformation in cells and dramatically accelerates SCLC progression in an Rb1/Trp53-deleted mouse model. In contrast, deletion of Max abrogates tumorigenesis in MYCL-overexpressing SCLC. Max deletion in SCLC resulted in derepression of metabolic genes involved in serine and one-carbon metabolism. By increasing serine biosynthesis, Max-deleted cells exhibit resistance to serine depletion. Thus, Max loss results in metabolic rewiring and context-specific tumor suppression.

Targeting NOTCH activation in small cell lung cancer through LSD1 inhibition.

Small cell lung cancer (SCLC) is a recalcitrant, aggressive neuroendocrine-type cancer for which little change to first-line standard-of-care treatment has occurred within the last few decades. Unlike nonsmall cell lung cancer (NSCLC), SCLC harbors few actionable mutations for therapeutic intervention. Lysine-specific histone demethylase 1A (LSD1 also known as KDM1A) inhibitors were previously shown to have selective activity in SCLC models, but the underlying mechanism was elusive. Here, we found that exposure to the selective LSD1 inhibitor ORY-1001 activated the NOTCH pathway, resulting in the suppression of the transcription factor ASCL1 and the repression of SCLC tumorigenesis. Our analyses revealed that LSD1 bound to the NOTCH1 locus, thereby suppressing NOTCH1 expression and downstream signaling. Reactivation of NOTCH signaling with the LSD1 inhibitor reduced the expression of ASCL1 and neuroendocrine cell lineage genes. Knockdown studies confirmed the pharmacological inhibitor-based results. In vivo, sensitivity to LSD1 inhibition in SCLC patient-derived xenograft (PDX) models correlated with the extent of consequential NOTCH pathway activation and repression of a neuroendocrine phenotype. Complete and durable tumor regression occurred with ORY-1001-induced NOTCH activation in a chemoresistant PDX model. Our findings reveal how LSD1 inhibitors function in this tumor and support their potential as a new and targeted therapy for SCLC.

Crebbp Loss Drives Small Cell Lung Cancer and Increases Sensitivity to HDAC Inhibition.

CREBBP, encoding an acetyltransferase, is among the most frequently mutated genes in small cell lung cancer (SCLC), a deadly neuroendocrine tumor type. We report acceleration of SCLC upon Crebbp inactivation in an autochthonous mouse model. Extending these observations beyond the lung, broad Crebbp deletion in mouse neuroendocrine cells cooperated with Rb1/Trp53 loss to promote neuroendocrine thyroid and pituitary carcinomas. Gene expression analyses showed that Crebbp loss results in reduced expression of tight junction and cell adhesion genes, including Cdh1, across neuroendocrine tumor types, whereas suppression of Cdh1 promoted transformation in SCLC. CDH1 and other adhesion genes exhibited reduced histone acetylation with Crebbp inactivation. Treatment with the histone deacetylase (HDAC) inhibitor Pracinostat increased histone acetylation and restored CDH1 expression. In addition, a subset of Rb1/Trp53/Crebbp-deficient SCLC exhibited exceptional responses to Pracinostat in vivo Thus, CREBBP acts as a potent tumor suppressor in SCLC, and inactivation of CREBBP enhances responses to a targeted therapy.Significance: Our findings demonstrate that CREBBP loss in SCLC reduces histone acetylation and transcription of cellular adhesion genes, while driving tumorigenesis. These effects can be partially restored by HDAC inhibition, which exhibited enhanced effectiveness in Crebbp-deleted tumors. These data provide a rationale for selectively treating CREBBP-mutant SCLC with HDAC inhibitors. Cancer Discov; 8(11); 1422-37. ©2018 AACR. This article is highlighted in the In This Issue feature, p. 1333.

Small Cell Lung Cancer Exhibits Frequent Inactivating Mutations in the Histone Methyltransferase KMT2D/MLL2: CALGB 151111 (Alliance).

INTRODUCTION: SCLC is a lethal neuroendocrine tumor type that is highly prone to metastasis. There is an urgency to understand the mutated genes that promote SCLC, as there are no approved targeted therapies yet available. SCLC is rarely resected, limiting the number of samples available for genomic analyses of somatic mutations. METHODS: To identify potential driver mutations in human SCLC we sequenced the whole exomes of 18 primary SCLCs and seven cell lines along with matched normal controls. We extended these data by resequencing a panel of genes across 40 primary SCLCs and 48 cell lines. RESULTS: We report frequent mutations in the lysine methyltransferase 2D gene (KMT2D) (also known as MLL2), a key regulator of transcriptional enhancer function. KMT2D exhibited truncating nonsense/frameshift/splice site mutations in 8% of SCLC tumors and 17% of SCLC cell lines. We found that KMT2D mutation in human SCLC cell lines was associated with reduced lysine methyltransferase 2D protein levels and reduced monomethylation of histone H3 lysine 4, a mark associated with transcriptional enhancers. We also found mutations in other genes associated with transcriptional enhancer control, including CREB binding protein gene (CREBBP), E1A binding protein p300 gene (EP300), and chromodomain helicase DNA binding protein 7 gene (CHD7), and we report mutations in additional chromatin remodeling genes such as polybromo 1 gene (PBRM1). CONCLUSIONS: These data indicate that KMT2D is one of the major mutated genes in SCLC, and they point to perturbation of transcriptional enhancer control as potentially contributing to SCLC.

Treating transcriptional addiction in small cell lung cancer.

Small cell lung cancer (SCLC) is a devastating tumor type with great therapeutic need. In this issue of Cancer Cell, Christensen and colleagues identify THZ1, a CDK7 inhibitor, as a potential therapy for SCLC. Using cells and mouse models, the authors show exquisite sensitivity of SCLC to transcriptional inhibition.

[New physiopathological roles for the PLA2R1 receptor in cancer and membranous nephropathy]. / Nouveaux rôles physiopathologiques pour le récepteur PLA2R1 dans le cancer et la glomérulonéphrite extramembraneuse.

PLA2R1 is a large transmembrane receptor of 180-kDa that belongs to the superfamily of C-type lectins. It was discovered because of its high affinity for secreted phospholipases A2 (sPLA2), enzymes that play a key role in lipid mediator synthesis. Early PLA2R1 physiological roles include the clearance of sPLA2 from the extracellular medium and/or promotion of their actions. Over the last four years, two independent studies suggested that PLA2R1 plays a role in cancer as a tumor gene suppressor and is the major target antigen of auto-immune antibodies involved in idiopathic membranous nephropathy, a severe human kidney disease. These novel findings shed light on PLA2R1 and pave the way for its use as a reliable biomarker and an attractive therapeutic target in these diseases.

Endoplasmic reticulum calcium release through ITPR2 channels leads to mitochondrial calcium accumulation and senescence.

Senescence is involved in various pathophysiological conditions. Besides loss of retinoblastoma and p53 pathways, little is known about other pathways involved in senescence. Here we identify two calcium channels; inositol 1,4,5-trisphosphate receptor, type 2 (ITPR2) (also known as inositol 1,4,5-triphosphate receptor 2 (IP3R2)) and mitochondrial calcium uniporter (MCU) as new senescence regulators in a loss-of-function genetic screen. We show that loss of ITPR2, known to mediate endoplasmic reticulum (ER) calcium release, as well as loss of MCU, necessary for mitochondrial calcium uptake, enable escape from oncogene-induced senescence (OIS). During OIS, ITPR2 triggers calcium release from the ER, followed by mitochondrial calcium accumulation through MCU channels. Mitochondrial calcium accumulation leads to a subsequent decrease in mitochondrial membrane potential, reactive oxygen species accumulation and senescence. This ER-mitochondria calcium transport is not restricted to OIS, but is also involved in replicative senescence. Our results show a functional role of calcium release by the ITPR2 channel and its subsequent accumulation in the mitochondria.

Repression of PLA2R1 by c-MYC and HIF-2alpha promotes cancer growth.

Loss of secreted phospholipase A2 receptor (PLA2R1) has recently been found to render human primary cells more resistant to senescence whereas increased PLA2R1 expression is able to induce cell cycle arrest, cancer cell death or blockage of cancer cell transformation in vitro, suggesting that PLA2R1 displays tumor suppressive activities. Here we report that PLA2R1 expression strongly decreases in samples of human renal cell carcinoma (RCC). Knockdown of PLA2R1 increases renal cancer cell tumorigenicity supporting a role of PLA2R1 loss to promote in vivo RCC growth. Most RCC result from Von Hippel-Lindau (VHL) tumor suppressor loss-of-function and subsequent gain-of-function of the oncogenic HIF-2alpha/c-MYC pathway. Here, by genetically manipulating VHL, HIF-2alpha and c-MYC, we demonstrate that loss of VHL, stabilization of HIF-2alpha and subsequent increased c-MYC activity, binding and transcriptional repression, through induction of PLA2R1 DNA methylation closed to PLA2R1 transcriptional start site, results in decreased PLA2R1 transcription. Our results describe for the first time an oncogenic pathway leading to PLA2R1 transcriptional repression and the importance of this repression for tumor growth.

PTEN is a potent suppressor of small cell lung cancer.

UNLABELLED: Small cell lung carcinoma (SCLC) is a highly metastatic tumor type with neuroendocrine features and a dismal prognosis. PTEN mutations and PIK3CA activating mutations have been reported in SCLC but the functional relevance of this pathway is unknown. The PTEN/PIK3CA pathway was interrogated using an AdenoCre-driven mouse model of SCLC harboring inactivated Rb and p53. Inactivation of one allele of PTEN in Rb/p53-deleted mice led to accelerated SCLC with frequent metastasis to the liver. In contrast with the high mutation burden reported in human SCLC, exome analyses revealed a low number of protein-altering mutations in mouse SCLC. Inactivation of both alleles of PTEN in the Rb/p53-deleted system led to nonmetastatic adenocarcinoma with neuroendocrine differentiation. This study reveals a critical role for the PTEN/PI3K pathway in both SCLC and lung adenocarcinoma and provides an ideal system to test the phosphoinositide 3-kinase (PI3K) pathway inhibitors as targeted therapy for subsets of patients with SCLC. IMPLICATIONS: The ability of PTEN inactivation to accelerate SCLC in a genetic mouse model suggests that targeting the PTEN pathway is a therapeutic option for a subset of human patients with SCLC. VISUAL OVERVIEW: http://mcr.aacrjournals.org/content/early/2014/04/28/1541-7786.MCR-13-0554/F1.large.jpg.

PLA2R1 mediates tumor suppression by activating JAK2.

Little is known about the physiological role of the phospholipase A2 receptor (PLA2R1). PLA2R1 has been described as regulating the replicative senescence, a telomerase-dependent proliferation arrest. The downstream PLA2R1 signaling and its role in cancer are currently unknown. Senescence induction in response to activated oncogenes is a failsafe program of tumor suppression that must be bypassed for tumorigenesis. We now present evidence that PLA2R1 functions in vitro as a tumor suppressor, the depletion of which is sufficient to escape oncogene-induced senescence (OIS), thereby facilitating oncogenic cell transformation. Furthermore, mice that are genetically deficient in PLA2R1 display increased sensitivity to RAS-induced tumorigenesis by facilitating OIS escape, highlighting its physiological role as a tumor suppressor. Unexpectedly, PLA2R1 activated JAK2 and its effector signaling, with PLA2R1-mediated inhibition of cell transformation largely reverted in JAK2-depleted cells. This finding was unexpected as the JAK2 pathway has been associated mainly with protumoral functions and several inhibitors are currently in clinical trials. Taken together, our findings uncover an unanticipated tumor suppressive role for PLA2R1 that is mediated by targeting downstream JAK2 effector signaling.

PLA2R1 kills cancer cells by inducing mitochondrial stress.

Little is known about the biological functions of the phospholipase A2 receptor (PLA2R1) except that it has the ability to bind a few secreted phospholipases A2 (sPLA2's). We have previously shown that PLA2R1 regulates senescence in normal human cells. In this study, we investigated the ability of PLA2R1 to control cancer cell growth. Analysis of expression in cancer cells indicates a marked PLA2R1 decrease in breast cancer cell lines compared to normal or nontransformed human mammary epithelial cells. Accordingly, PLA2R1 ectopic expression in PLA2R1-negative breast cancer cell lines led to apoptosis, whereas a prosenescence response was predominantly triggered in normal cells. PLA2R1 structure-function studies and the use of chemical inhibitors of sPLA2-related signaling pathways suggest that the effect of PLA2R1 is sPLA2-independent. Functional experiments demonstrate that PLA2R1 regulation of cell death is driven by a reactive oxygen species (ROS)-dependent mechanism. While screening for ROS-producing complexes involved in PLA2R1 biological responses, we identified a critical role for the mitochondrial electron transport chain in PLA2R1-induced ROS production and cell death. Taken together, this set of data provides evidence for an important role of PLA2R1 in controlling cancer cell death by influencing mitochondrial biology.

Platelet-derived growth factor B induces senescence and transformation in normal human fibroblasts.

Normal cells enter a senescent state upon aberrant oncogenic signals and this response inhibits tumor initiation and progression. It is now well admitted that intracellular and membrane localized oncogenes can illicit oncogene induced senescence. However, the effect of mitogenic growth factor on cellular senescence is so far largely unknown. Here we show that normal human dermal fibroblasts display a complex response to Platelet derived growth factor B (PDGFB) expression. Indeed, PDGFB expression induces, in the same cell population, both senescence and cellular transformation. Remarkably both populations are sustained with passages suggesting that transformed cells eventually enter a senescent state. This senescence state is p53 dependent as inhibiting the p53 pathway blocks the ability of PDGFB to induce senescence and results in strong cellular transformation increase upon PDGFB expression. The relevance of these observations is supported by the fact that human dermatofibrosarcoma protuberans, skin tumors arising from constitutive PDGFB production with little aggressiveness, also display some senescence hallmarks. Together these data support the view that PDGFB, a mitogenic growth factor, has a limited ability to induce senescence. We propose that this low level of senescence might decrease the transforming ability of this factor without totally abolishing it.

Potassium channel KCNA1 modulates oncogene-induced senescence and transformation.

Oncogene-induced senescence (OIS) constitutes a failsafe program that restricts tumor development. However, the mechanisms that link oncogenesis to senescence are not completely understood. We carried out a loss-of-function genetic screen that identified the potassium channel KCNA1 as a determinant of OIS escape that can license tumor growth. Oncogenic stress triggers an increase in KCNA1 expression and its relocation from the cytoplasm to the membrane. Mechanistically, this relocation is due to a loss of protein kinase A (PKA)-induced phosphorylation at residue S446 of KCNA1. Accordingly, sustaining PKA activity or expressing a KCNA1 phosphomimetic mutant maintained KCNA1 in the cytoplasm and caused escape from OIS. KCNA1 relocation to the membrane induced a change in membrane potential that invariably resulted in cellular senescence. Restoring KCNA1 expression in transformation-competent cells triggered variation in membrane potential and blocked RAS-induced transformation, and PKA activation suppressed both effects. Furthermore, KCNA1 expression was reduced in human cancers, and this decrease correlated with an increase in breast cancer aggressiveness. Taken together, our results identify a novel pathway that restricts oncogenesis through a potassium channel-dependent senescence pathway.

Regulation of ploidy and senescence by the AMPK-related kinase NUAK1.

Senescence is an irreversible cell-cycle arrest that is elicited by a wide range of factors, including replicative exhaustion. Emerging evidences suggest that cellular senescence contributes to ageing and acts as a tumour suppressor mechanism. To identify novel genes regulating senescence, we performed a loss-of-function screen on normal human diploid fibroblasts. We show that downregulation of the AMPK-related protein kinase 5 (ARK5 or NUAK1) results in extension of the cellular replicative lifespan. Interestingly, the levels of NUAK1 are upregulated during senescence whereas its ectopic expression triggers a premature senescence. Cells that constitutively express NUAK1 suffer gross aneuploidies and show diminished expression of the genomic stability regulator LATS1, whereas depletion of NUAK1 with shRNA exerts opposite effects. Interestingly, a dominant-negative form of LATS1 phenocopies NUAK1 effects. Moreover, we show that NUAK1 phosphorylates LATS1 at S464 and this has a role in controlling its stability. In summary, our work highlights a novel role for NUAK1 in the control of cellular senescence and cellular ploidy.

A genetic screen identifies topoisomerase 1 as a regulator of senescence.

Normal cell growth can be permanently blocked when cells enter a state known as senescence. This phenomenon can be triggered by various stresses, such as replicative exhaustion, oncogenic stimulation, or oxidative stress. Senescence prevents transmission of aberrant signals to daughter cells and thus prevents irreversible damage that could favor cancer development. To identify new genetic events controlling senescence, we have performed a loss-of-function genetic screen on normal human cells. We report that knockdown of topoisomerase I (Top1) results in an increased replicative potential associated with a decrease in senescence markers and a diminished DNA damage response. In addition, Top1 depletion also favors a bypass of oncogene-induced senescence. Conversely, Top1 constitutive expression induces growth arrest, the appearance of a senescence marker, and an activation of the DNA damage response. Altogether, these results reveal an unanticipated function of Top1 in regulating senescence.

The M-type receptor PLA2R regulates senescence through the p53 pathway.

Senescence is a stable proliferative arrest induced by various stresses such as telomere erosion, oncogenic or oxidative stress. Compelling evidence suggests that it acts as a barrier against tumour development. Describing new mechanisms that favour an escape from senescence can thus reveal new insights into tumorigenesis. To identify new genes controlling the senescence programme, we performed a loss-of-function genetic screen in primary human fibroblasts. We report that knockdown of the M-type receptor PLA2R (phospholipase A2 receptor) prevents the onset of replicative senescence and diminishes stress-induced senescence. Interestingly, expression of PLA2R increases during replicative senescence, and its ectopic expression results in premature senescence. We show that PLA2R regulates senescence in a reactive oxygen species-DNA damage-p53-dependent manner. Taken together, our study identifies PLA2R as a potential new tumour suppressor gene crucial in the induction of cellular senescence through the activation of the p53 pathway.

Chemokine signaling via the CXCR2 receptor reinforces senescence.

Cells enter senescence, a state of stable proliferative arrest, in response to a variety of cellular stresses, including telomere erosion, DNA damage, and oncogenic signaling, which acts as a barrier against malignant transformation in vivo. To identify genes controlling senescence, we conducted an unbiased screen for small hairpin RNAs that extend the life span of primary human fibroblasts. Here, we report that knocking down the chemokine receptor CXCR2 (IL8RB) alleviates both replicative and oncogene-induced senescence (OIS) and diminishes the DNA-damage response. Conversely, ectopic expression of CXCR2 results in premature senescence via a p53-dependent mechanism. Cells undergoing OIS secrete multiple CXCR2-binding chemokines in a program that is regulated by the NF-kappaB and C/EBPbeta transcription factors and coordinately induce CXCR2 expression. CXCR2 upregulation is also observed in preneoplastic lesions in vivo. These results suggest that senescent cells activate a self-amplifying secretory network in which CXCR2-binding chemokines reinforce growth arrest.