Increased genomic instability and reshaping of tissue microenvironment underlie oncogenic properties of Arid1a mutations

Oncogenic mutations accumulating in many chromatin-associated proteins have been identified in different tumor types. With a mutation rate from 10 to 57%, ARID1A has been widely considered a tumor suppressor gene. However, whether this role is mainly due to its transcriptional-related activities or its ability to preserve genome integrity is still a matter of intense debate. Here, we show that ARID1A is largely dispensable for preserving enhancer-dependent transcriptional regulation, being ARID1B sufficient and required to compensate for ARID1A loss. We provide in vivo evidence that ARID1A is mainly required to preserve genomic integrity in adult tissues. ARID1A loss primarily results in DNA damage accumulation, interferon type I response activation, and chronic inflammation leading to tumor formation. Our data suggest that in healthy tissues, the increased genomic instability that follows ARID1A mutations and the selective pressure imposed by the microenvironment might result in the emergence of aggressive, possibly immune-resistant, tumors.

Rarγ -Foxa1 signaling promotes luminal identity in prostate progenitors and is disrupted in prostate cancer

Retinoic acid (RA) signaling is a master regulator of vertebrate development with crucial roles in directing body axis orientation and tissue differentiation, including in the reproductive system. However, a mechanistic understanding of how RA signaling promotes cell lineage identity in different tissues is often missing. Here, leveraging prostate organoid technology, we demonstrated that RA signaling orchestrates the commitment of adult mouse prostate progenitors to glandular identity, epithelial barrier integrity, and ultimately, proper specification of the prostatic lumen. Mechanistically, RA-dependent RARγ activation promotes the expression of the pioneer factor Foxa1, which synergizes with the androgen pathway for proper luminal expansion, cytoarchitecture and function. FOXA1 nucleotide variants are common in human prostate and breast cancers and considered driver mutations, though their pathogenic mechanism is incompletely understood. Combining functional genetics experiments with structural modeling of FOXA1 folding and chromatin binding analyses, we discovered that FOXA1 F254E255 is a loss-of-function mutation leading to compromised transcriptional function and lack of luminal fate commitment of prostate progenitors. Overall, we define RA as a crucial instructive signal for glandular identity in adult prostate progenitors. We propose deregulation of vitamin A metabolism as a risk factor for benign and malignant prostate disease, and identified cancer associated FOXA1 indels affecting residue F254 as loss-of-function mutations promoting dedifferentiation of adult prostate progenitors. Summary: Retinoic acid signaling orchestrates luminal differentiation of adult prostate progenitors.

LncRNA EPR regulates intestinal mucus production and protects against inflammation and tumorigenesis

The long non-coding RNA EPR is expressed in epithelial tissues, binds to chromatin and controls distinct biological activities in mouse mammary gland cells. Because of its high expression in the intestine, in this study we have generated a colon-specific conditional targeted deletion (EPR cKO) to evaluate EPR in vivo functions in mice. EPR cKO mice display epithelium hyperproliferation, impaired mucus production and secretion, as well as inflammatory infiltration in the proximal portion of the large intestine. RNA sequencing analysis reveals a rearrangement of the colon crypt transcriptome with strong reduction of goblet cell-specific factors including those involved in the synthesis, assembly, transport and control of mucus proteins. Further, colon mucosa integrity and permeability are impaired in EPR cKO mice, and this results in higher susceptibility to dextran sodium sulfate (DSS)-induced colitis and tumor formation. Human EPR is down-regulated in human cancer cell lines as well as in human cancers, and overexpression of EPR in a colon cancer cell line results in enhanced expression of pro-apoptotic genes. Mechanistically, we show that EPR directly interacts with select genes involved in mucus metabolism whose expression is reduced in EPR cKO mice and that EPR deletion causes tridimensional chromatin organization changes.

Prdm16-mediated H3K9 methylation controls fibro-adipogenic progenitors identity during skeletal muscle repair

H3K9 methylation maintains cell identity orchestrating stable silencing and anchoring of alternate fate genes within the heterochromatic compartment underneath the nuclear lamina (NL). However, how cell type-specific genomic regions are specifically targeted to the NL is still elusive. Using fibro-adipogenic progenitors (FAPs) as a model, we identified Prdm16 as a nuclear envelope protein that anchors H3K9-methylated chromatin in a cell-specific manner. We show that Prdm16 mediates FAP developmental capacities by orchestrating lamina-associated domain organization and heterochromatin sequestration at the nuclear periphery. We found that Prdm16 localizes at the NL where it cooperates with the H3K9 methyltransferases G9a/GLP to mediate tethering and silencing of myogenic genes, thus repressing an alternative myogenic fate in FAPs. Genetic and pharmacological disruption of this repressive pathway confers to FAP myogenic competence, preventing fibro-adipogenic degeneration of dystrophic muscles. In summary, we reveal a druggable mechanism of heterochromatin perinuclear sequestration exploitable to reprogram FAPs in vivo.

Cooperation Between MYC and β-Catenin in Liver Tumorigenesis Requires Yap/Taz

BACKGROUND AND AIMS

Activation of MYC and catenin beta-1 (CTNNB1, encoding β-catenin) can co-occur in liver cancer, but how these oncogenes cooperate in tumorigenesis remains unclear.

APPROACH AND RESULTS

We generated a mouse model allowing conditional activation of MYC and WNT/β-catenin signaling (through either β-catenin activation or loss of APC - adenomatous polyposis coli) upon expression of CRE recombinase in the liver and monitored their effects on hepatocyte proliferation, apoptosis, gene expression profiles, and tumorigenesis. Activation of WNT/β-catenin signaling strongly accelerated MYC-driven carcinogenesis in the liver. Both pathways also cooperated in promoting cellular transformation in vitro, demonstrating their cell-autonomous action. Short-term induction of MYC and β-catenin in hepatocytes, followed by RNA-sequencing profiling, allowed the identification of a "Myc/β-catenin signature," composed of a discrete set of Myc-activated genes whose expression increased in the presence of active β-catenin. Notably, this signature enriched for targets of Yes-associated protein (Yap) and transcriptional coactivator with PDZ-binding motif (Taz), two transcriptional coactivators known to be activated by WNT/β-catenin signaling and to cooperate with MYC in mitogenic activation and liver transformation. Consistent with these regulatory connections, Yap/Taz accumulated upon Myc/β-catenin activation and were required not only for the ensuing proliferative response, but also for tumor cell growth and survival. Finally, the Myc/β-catenin signature was enriched in a subset of human hepatocellular carcinomas characterized by comparatively poor prognosis.

CONCLUSIONS

Myc and β-catenin show a strong cooperative action in liver carcinogenesis, with Yap and Taz serving as mediators of this effect. These findings warrant efforts toward therapeutic targeting of Yap/Taz in aggressive liver tumors marked by elevated Myc/β-catenin activity.

Loss of PRC1 activity in different stem cell compartments activates a common transcriptional program with cell type-dependent outcomes

Polycomb repressive complexes are evolutionarily conserved complexes that maintain transcriptional repression during development and differentiation to establish and preserve cell identity. We recently described the fundamental role of PRC1 in preserving intestinal stem cell identity through the inhibition of non-lineage-specific transcription factors. To further elucidate the role of PRC1 in adult stem cell maintenance, we now investigated its role in LGR5⁺ hair follicle stem cells during regeneration. We show that PRC1 depletion severely affects hair regeneration and, different from intestinal stem cells, derepression of its targets induces the ectopic activation of an epidermal-specific program. Our data support a general role of PRC1 in preserving stem cell identity that is shared between different compartments. However, the final outcome of the ectopic activation of non-lineage-specific transcription factors observed upon loss of PRC1 is largely context-dependent and likely related to the transcription factors repertoire and specific epigenetic landscape of different cellular compartments.

Maintenance of leukemic cell identity by the activity of the Polycomb complex PRC1 in mice

Leukemia is a complex heterogeneous disease often driven by the expression of oncogenic fusion proteins with different molecular and biochemical properties. Whereas several fusion proteins induce leukemogenesis by activating Hox gene expression (Hox-activating fusions), others impinge on different pathways that do not involve the activation of Hox genes (non-Hox-activating fusions). It has been postulated that one of the main oncogenic properties of the HOXA9 transcription factor is its ability to control the expression of the p16/p19 tumor suppressor locus (Cdkn2a), thereby compensating Polycomb-mediated repression, which is dispensable for leukemias induced by Hox-activating fusions. We show, by genetically depleting the H2A ubiquitin ligase subunits of the Polycomb repressive complex 1 (PRC1), Ring1a and Ring1b, that Hoxa9 activation cannot repress Cdkn2a expression in the absence of PRC1 and its dependent deposition of H2AK119 monoubiquitination (H2AK119Ub). This demonstrates the essential role of PRC1 activity in supporting the oncogenic potential of Hox-activating fusion proteins. By combining genetic tools with genome-wide location and transcription analyses, we further show that PRC1 activity is required for the leukemogenic potential of both Hox-activating and non-Hox-activating fusions, thus preventing the differentiation of leukemic cells independently of the expression of the Cdkn2a locus. Overall, our results genetically demonstrate that PRC1 activity and the deposition of H2AK119Ub are critical factors that maintain the undifferentiated identity of cancer cells, positively sustaining the progression of different types of leukemia.

PRC2 preserves intestinal progenitors and restricts secretory lineage commitment

Chromatin modifications shape cell heterogeneity by activating and repressing defined sets of genes involved in cell proliferation, differentiation and development. Polycomb-repressive complexes (PRCs) act synergistically during development and differentiation by maintaining transcriptional repression of common genes. PRC2 exerts this activity by catalysing H3K27 trimethylation. Here, we show that in the intestinal epithelium PRC2 is required to sustain progenitor cell proliferation and the correct balance between secretory and absorptive lineage differentiation programs. Using genetic models, we show that PRC2 activity is largely dispensable for intestinal stem cell maintenance but is strictly required for radiation-induced regeneration by preventing Cdkn2a transcription. Combining these models with genomewide molecular analysis, we further demonstrate that preferential accumulation of secretory cells does not result from impaired proliferation of progenitor cells induced by Cdkn2a activation but rather from direct regulation of transcription factors responsible for secretory lineage commitment. Overall, our data uncover a dual role of PRC2 in intestinal homeostasis highlighting the importance of this repressive layer in controlling cell plasticity and lineage choices in adult tissues.

Polycomb Complex PRC1 Preserves Intestinal Stem Cell Identity by Sustaining Wnt/β-Catenin Transcriptional Activity

Polycomb repressive complexes (PRCs) are among the most important gatekeepers of establishing and maintaining cell identity in metazoans. PRC1, which plays a dominant role in this context, executes its functions via multiple subcomplexes, which all contribute to H2AK119 mono-ubiquitination (H2Aubq). Despite our comprehensive knowledge of PRC1-dependent H2Aubq in embryonic stem cells and during early development, its role in adult stem cells still remains poorly characterized. Here we show that PRC1 activity is required for the integrity of the intestinal epithelium, regulating stem cell self-renewal via a cell-autonomous mechanism that is independent from Cdkn2a expression. By dissecting the PRC1-dependent transcription program in intestinal stem cells, we demonstrate that PRC1 represses a large number of non-lineage-specific transcription factors that directly affect β-catenin/Tcf transcriptional activity. Our data reveal that PRC1 preserves Wnt/β-catenin activity in adult stem cells to maintain intestinal homeostasis and supports tumor formation induced by the constitutive activation of this pathway.

A novel AMPK-dependent FoxO3A-SIRT3 intramitochondrial complex sensing glucose levels

Reduction of nutrient intake without malnutrition positively influences lifespan and healthspan from yeast to mice and exerts some beneficial effects also in humans. The AMPK-FoxO axis is one of the evolutionarily conserved nutrient-sensing pathways, and the FOXO3A locus is associated with human longevity. Interestingly, FoxO3A has been reported to be also a mitochondrial protein in mammalian cells and tissues. Here we report that glucose restriction triggers FoxO3A accumulation into mitochondria of fibroblasts and skeletal myotubes in an AMPK-dependent manner. A low-glucose regimen induces the formation of a protein complex containing FoxO3A, SIRT3, and mitochondrial RNA polymerase (mtRNAPol) at mitochondrial DNA-regulatory regions causing activation of the mitochondrial genome and a subsequent increase in mitochondrial respiration. Consistently, mitochondrial transcription increases in skeletal muscle of fasted mice, with a mitochondrial DNA-bound FoxO3A/SIRT3/mtRNAPol complex detectable also in vivo. Our results unveil a mitochondrial arm of the AMPK-FoxO3A axis acting as a recovery mechanism to sustain energy metabolism upon nutrient restriction.

Epigenetic methylations and their connections with metabolism

Metabolic pathways play fundamental roles in several processes that regulate cell physiology and adaptation to environmental changes. Altered metabolic pathways predispose to several different pathologies ranging from diabetes to cancer. Specific transcriptional programs tightly regulate the enzymes involved in cell metabolism and dictate cell fate regulating the differentiation into specialized cell types that contribute to metabolic adaptation in higher organisms. For these reasons, it is of extreme importance to identify signaling pathways and transcription factors that positively and negatively regulate metabolism. Genomic organization allows a plethora of different strategies to regulate transcription. Importantly, large evidence suggests that the quality of diet and the caloric regimen can influence the epigenetic state of our genome and that certain metabolic pathways are also epigenetically controlled reveling a tight crosstalk between metabolism and epigenomes. Here we focus our attention on methylation-based epigenetic reactions, on how different metabolic pathways control these activities, and how these can influence metabolism. Altogether, the recent discoveries linking these apparent distant areas reveal that an exciting field of research is emerging.

Tet proteins connect the O-linked N-acetylglucosamine transferase Ogt to chromatin in embryonic stem cells

O-linked N-acetylglucosamine (O-GlcNAc) transferase (Ogt) activity is essential for embryonic stem cell (ESC) viability and mouse development. Ogt is present both in the cytoplasm and the nucleus of different cell types and catalyzes serine and threonine glycosylation. We have characterized the biochemical features of nuclear Ogt and identified the ten-eleven translocation (TET) proteins Tet1 and Tet2 as stable partners of Ogt in the nucleus of ESCs. We show at a genome-wide level that Ogt preferentially associates with Tet1 to genes promoters in close proximity of CpG-rich transcription start sites. These regions are characterized by low levels of DNA modification, suggesting a link between Tet1 and Ogt activities in regulating CpG island methylation. Finally, we show that Tet1 is required for binding of Ogt to chromatin affecting Tet1 activity. Taken together, our data characterize how O-GlcNAcylation is recruited to chromatin and interacts with the activity of 5-methylcytosine hydroxylases.

Blocking p38/ERK crosstalk affects colorectal cancer growth by inducing apoptosis in vitro and in preclinical mouse models

We recently demonstrated that p38α is required to maintain colorectal cancer (CRC) metabolism, as its inhibition leads to FoxO3A activation, autophagy, cell death, and tumor growth reduction both in vitro and in vivo. Here we show that inhibition of p38α is followed by TRAIL-mediated activation of caspase-8 and FoxO3A-dependent HER3 upregulation with consequent overactivation of the MEK-ERK1/2 survival pathway. p38α and MEK combined inhibition specifically induces apoptosis by enabling TRAIL signaling propagation through t-Bid and caspase-3, and fosters cell death in CRC cells and preclinical mouse models. Current MEK1-directed pharmacological strategies could thus be exploited, in combination with p38α inhibition, to develop new approaches for CRC treatment.

p38alpha is required for ovarian cancer cell metabolism and survival

INTRODUCTION

Ovarian cancer is highly sensitive to chemotherapy but also shows a high rate of recurrence and drug resistance. These negative outcomes mostly depend on altered apoptotic pathways, making the design of new therapeutic strategies based on the induction of other types of cell death highly desirable. Several lines of research are now addressing cancer-specific features to specifically target tumor cells, thus reducing adverse effects. In this light, a great deal of attention has been devoted to the metabolic reprogramming occurring in cancer cells, which display increased levels of glycolysis compared with their normal counterparts. We recently showed that inhibition of p38alpha impairs key metabolic functions of colorectal cancer cells, inducing growth arrest, autophagy, and cell death both in vivo and in vitro. These effects are mediated by a switch from hypoxia-inducible factor 1alpha (HIF1alpha) to forkhead transcription factor O (FoxO)-dependent transcription.

METHODS

We first characterized p38 expression in OVCAR-3, A2780, and SKOV-3 ovarian cancer cell lines. Then, we treated these cells with the p38alpha/p38beta-specific inhibitor SB202190 and performed a morphological, proliferation, and survival analyses. Finally, we studied HIF1alpha and FoxO3A expressions and signaling pathways to evaluate their role in SB202190-induced effects.

RESULTS

p38alpha blockade induces the formation of intracellular autophagic vacuoles and reduces growth and viability of ovarian cancer cells. As in colorectal cancer, the underlying molecular mechanism seems to rely on a shift from HIF1alpha- to FoxO3A-dependent transcription, which is promoted by the activation of the adenosine monophosphate-activated protein kinase pathway.

CONCLUSIONS

These data corroborate the hypothesis that pharmacological modulation of genes involved in cancer-specific homeostasis, such as p38alpha, might be exploited to design new therapeutic approaches to cancer treatment.

The AMPK-FoxO3A axis as a target for cancer treatment

FoxO proteins are an evolutionarily conserved subfamily of transcription factors involved in tumor suppression, regulation of energy metabolism and development in several tissues, and are mainly regulated by phosphorylation-dependent nuclear/cytoplasmic shuttling. The transcriptional activity of FoxO3A, one of the four members of the family, is further modulated by AMPK, one of the key regulators of cellular metabolism, which basically shifts cell machinery from energy-consuming to energy-producing pathways. We recently demonstrated that the AMPK/FoxO3A energy sensor pathway is still inducible in human cancer cells in response to metabolic stress, as it becomes activated in colorectal and ovarian cancer cells in response to the inhibition of p38α. Activation of the FoxO3A transcriptional program initially induces autophagy as an attempt to retain energy to survive, whereas under persistent stress conditions it triggers autophagic cell death. In this review, we focus on the connections between AMPK and FoxO3A, describing their central role as modulators of fundamental processes such as stress resistance, cell metabolism, autophagy and cell death, and highlighting the therapeutic potential of pharmacological modulation of the AMPK-FoxO3A axis.

Inhibition of p38alpha unveils an AMPK-FoxO3A axis linking autophagy to cancer-specific metabolism

Autophagy is an essential process for the maintenance of cellular and metabolic homeostasis. Indeed, it is required for the recovery of ATP-generating substrates in cells subjected to different types of stress insults. Thus, the activity of the autophagic machinery strongly depends on the metabolic status of the cell.(1) It has been proposed that this principle applies not only to normal, but also to cancer cells,(2) despite the profound differences in their metabolism. Cancer cells predominantly produce ATP through the constitutive activation of aerobic glycolysis, a process that generally relies on the stabilization and activation of the transcription factor HIF1alpha, which regulates the expression of glycolytic genes.(3) We recently showed that p38alpha is required to sustain the expression of HIF1alpha target genes, and that its inhibition causes a rapid drop in ATP levels in colorectal cancer cells (CRCs). This acute energy need triggers AMPK-dependent nuclear accumulation of FoxO3A and subsequent activation of its transcriptional program, leading to sequential induction of autophagy, cell cycle arrest and cell death. In vivo, pharmacological blockade of p38alpha has both a cytostatic and cytotoxic effect on colorectal neoplasms, associated with nuclear enrichment of FoxO3A and expression of its target genes p21 and PTEN.(4) Our data suggest that CRCs impaired in their glycolytic metabolism trigger autophagy as a reversible recovery mechanism and undergo cell cycle arrest; however, the persistence of the stress insults inevitably leads to cell death.

The evolutionary conserved gene C16orf35 encodes a nucleo-cytoplasmic protein that interacts with p73

C16orf35 is a highly conserved gene positioned upstream of the alpha-globins in humans and other vertebrates. The deduced protein is also highly conserved, it has no defined structural features or domains, and its function is currently unknown. Here we show that the C16orf35 protein has nuclear and cytosolic distribution, and can localize to PML nuclear bodies. The C16orf35 protein was detected in several human transformed cells lines, and studies of transient and stable overexpression indicate that increased levels of C16orf35 inhibit cell proliferation. We also find that C16orf35 interacts with human p73, and represses transcription by TAp73gamma but not by TAp73alpha. This selectivity is not due to differential interaction, since C16orf35 binds both p73 variants. Our data suggest that C16orf35 can modulate differentially the specific activities of selected p73 isoforms.

p38alpha blockade inhibits colorectal cancer growth in vivo by inducing a switch from HIF1alpha- to FoxO-dependent transcription

Colorectal cancer cell (CRC) fate is governed by an intricate network of signaling pathways, some of which are the direct target of DNA mutations, whereas others are functionally deregulated. As a consequence, cells acquire the ability to grow under nutrients and oxygen shortage conditions. We earlier reported that p38alpha activity is necessary for proliferation and survival of CRCs in a cell type-specific manner and regardless of their phenotype and genotype. Here, we show that p38alpha sustains the expression of HIF1alpha target genes encoding for glycolytic rate-limiting enzymes, and that its inhibition causes a drastic decrease in ATP intracellular levels in CRCs. Prolonged inactivation of p38alpha triggers AMPK-dependent nuclear localization of FoxO3A and subsequent activation of its target genes, leading to autophagy, cell cycle arrest and cell death. In vivo, pharmacological blockade of p38alpha inhibits CRC growth in xenografted nude mice and azoxymethane-treated Apc(Min) mice, achieving both a cytostatic and cytotoxic effect, associated with high nuclear expression of FoxO3A and increased expression of its target genes p21 and PTEN. Hence, inhibition of p38alpha affects the aerobic glycolytic metabolism specific of cancer cells and might be taken advantage of as a therapeutic strategy targeted against CRCs.

Signal-dependent control of autophagy-related gene expression

Several tumors arise from deregulated signaling pathways leading to increased proliferation and impairment of differentiation. To bypass endogenous control mechanisms and to survive the environmental stress associated with increased growth, tumor cells acquire a plethora of modifications that ultimately tend to down-regulate the ability to undergo apoptosis and exacerbate prosurvival mechanisms. Autophagy is an evolutionarily conserved mechanism through which cells recycle essential molecular constituents or eliminate damaged organelles under stress conditions imposed by nutrients or growth factors deprivation. As such, autophagy acts as a prosurvival mechanism for cancer cells. However, when overactivated, autophagy could also represent a cell death mechanism acting through self-cannibalization. Therefore, understanding the various signaling pathways that regulate autophagy could be of extreme importance. Indeed, the identification of specific molecular targets amenable to pharmacological manipulation to induce cancer cell self-cannibalization could represent a promising approach to treat apoptosis-resistant tumors.

Modification of Drosophila p53 by SUMO modulates its transactivation and pro-apoptotic functions

Conjugation to SUMO is a reversible post-translational modification that regulates several transcription factors involved in cell proliferation, differentiation, and disease. The p53 tumor suppressor can be modified by SUMO-1 in mammalian cells, but the functional consequences of this modification are unclear. Here, we demonstrate that the Drosophila homolog of human p53 can be efficiently sumoylated in insect cells. We identify two lysine residues involved in SUMO attachment, one at the C terminus, between the DNA binding and oligomerization domains, and one at the N terminus of the protein. We find that sumoylation helps recruit Drosophila p53 to nuclear dot-like structures that can be marked by human PML and the Drosophila homologue of Daxx. We demonstrate that mutation of both sumoylation sites dramatically reduces the transcriptional activity of p53 and its ability to induce apoptosis in transgenic flies, providing in vivo evidence that sumoylation is critical for Drosophila p53 function.

Signal-dependent regulation of gene expression as a target for cancer treatment: inhibiting p38alpha in colorectal tumors

In the last year, several evidences indicated that pharmacological manipulation of relevant signaling pathways could selectively affect gene expression to influence cell fate. These findings render of extreme importance the elucidation of how external stimuli are transduced to mediate chromatin modifications, resulting in a permissive or repressive environment for gene expression. These signaling cascades activate or repress the function of chromatin binding proteins that represent attractive pharmacological targets for human diseases. Actually, the closer the target is to chromatin, the more the transcriptional effect will be selective. Recent studies suggest that pharmacological manipulation of signaling pathways to modulate cell fate is indeed possible and that chromatin-associated kinases could represent an optimal target. The p38 MAPK is the prototype of this class of enzymes and its central role in the transcription process is evolutionary conserved. In this review we will focus on the possibility to inhibit p38alpha in colorectal cancer to arrest tumor progression and induce autophagic cell death.