BTG1 inactivation drives lymphomagenesis and promotes lymphoma dissemination through activation of BCAR1.

Understanding the functional role of mutated genes in cancer is required to translate the findings of cancer genomics into therapeutic improvement. BTG1 is recurrently mutated in the MCD/C5 subtype of diffuse large B-cell lymphoma (DLBCL), which is associated with extranodal dissemination. Here, we provide evidence that Btg1 knock out accelerates the development of a lethal lymphoproliferative disease driven by Bcl2 overexpression. Furthermore, we show that the scaffolding protein BCAR1 is a BTG1 partner. Moreover, after BTG1 deletion or expression of BTG1 mutations observed in patients with DLBCL, the overactivation of the BCAR1-RAC1 pathway confers increased migration ability in vitro and in vivo. These modifications are targetable with the SRC inhibitor dasatinib, which opens novel therapeutic opportunities in BTG1 mutated DLBCL.

Functions of LKB1 in neural crest development: The story unfolds.

Neural crest cells (NCCs) are highly motile, multipotent, embryonic cells that delaminate from the dorsal edges of the neural tube. NCCs follow stereotypical long-range migratory pathways to reach target organs during development, where they give rise to multiple derivatives. The identification of reservoirs of neural crest stem cells that persist to adulthood has recently aroused renewed interest in the biology of NCCs. In this context, several recent studies have demonstrated the essential role of the metabolic kinase LKB1 in NCC establishment. This review surveys how LKB1 governs the formation and maintenance of several neural crest derivatives, including facial bones, melanocytes, Schwann cells, and the enteric nervous system. We also detail the underlying molecular mechanisms that involve downstream effectors of LKB1, in particular the contribution of the AMPK-mTOR signaling pathway to both polarity and metabolic processes. Collectively, these recent discoveries open promising perspectives for new therapeutic applications for the treatment of neural crest disorders.

MEX3A regulates Lgr5+ stem cell maintenance in the developing intestinal epithelium.

Intestinal stem cells (ISCs) fuel the lifelong self-renewal of the intestinal tract and are paramount for epithelial repair. In this context, the Wnt pathway component LGR5 is the most consensual ISC marker to date. Still, the effort to better understand ISC identity and regulation remains a challenge. We have generated a Mex3a knockout mouse model and show that this RNA-binding protein is crucial for the maintenance of the Lgr5+ ISC pool, as its absence disrupts epithelial turnover during postnatal development and stereotypical organoid maturation ex vivo. Transcriptomic profiling of intestinal crypts reveals that Mex3a deletion induces the peroxisome proliferator-activated receptor (PPAR) pathway, along with a decrease in Wnt signalling and loss of the Lgr5+ stem cell signature. Furthermore, we identify PPARγ activity as a molecular intermediate of MEX3A-mediated regulation. We also show that high PPARγ signalling impairs Lgr5+ ISC function, thus uncovering a new layer of post-transcriptional regulation that critically contributes to intestinal homeostasis.

MEX-3 proteins: recent insights on novel post-transcriptional regulators.

RNA-binding proteins of the evolutionarily-conserved MEX-3 family are mediators of post-transcriptional regulation in different organisms. Recent studies highlight their involvement in diverse physiological settings, including the maintenance of a balance between stem cell self-renewal and differentiation. Here, we draw attention to their putative role in tissue homeostasis and disease, particularly cancer.

LKB1 signaling in cephalic neural crest cells is essential for vertebrate head development.

Head development in vertebrates proceeds through a series of elaborate patterning mechanisms and cell-cell interactions involving cephalic neural crest cells (CNCC). These cells undergo extensive migration along stereotypical paths after their separation from the dorsal margins of the neural tube and they give rise to most of the craniofacial skeleton. Here, we report that the silencing of the LKB1 tumor suppressor affects the delamination of pre-migratory CNCC from the neural primordium as well as their polarization and survival, thus resulting in severe facial and brain defects. We further show that LKB1-mediated effects on the development of CNCC involve the sequential activation of the AMP-activated protein kinase (AMPK), the Rho-dependent kinase (ROCK) and the actin-based motor protein myosin II. Collectively, these results establish that the complex morphogenetic processes governing head formation critically depends on the activation of the LKB1 signaling network in CNCC.

The RNA-binding protein Mex3b regulates the spatial organization of the Rap1 pathway.

The four related mammalian MEX-3 RNA-binding proteins are evolutionarily conserved molecules for which the in vivo functions have not yet been fully characterized. Here, we report that male mice deficient for the gene encoding Mex3b are subfertile. Seminiferous tubules of Mex3b-deficient mice are obstructed as a consequence of the disrupted phagocytic capacity of somatic Sertoli cells. In addition, both the formation and the integrity of the blood-testis barrier are compromised owing to mislocalization of N-cadherin and connexin 43 at the surface of Sertoli cells. We further establish that Mex3b acts to regulate the cortical level of activated Rap1, a small G protein controlling phagocytosis and cell-cell interaction, through the activation and transport of Rap1GAP. The active form of Rap1 (Rap1-GTP) is abnormally increased at the membrane cortex and chemically restoring Rap1-GTP to physiological levels rescues the phagocytic and adhesion abilities of Sertoli cells. Overall, these findings implicate Mex3b in the spatial organization of the Rap1 pathway that orchestrates Sertoli cell functions.

The RNA-binding E3 ubiquitin ligase MEX-3C links ubiquitination with MHC-I mRNA degradation.

RNA-binding E3 ubiquitin ligases were recently identified, though their function remains unclear. While studying the regulation of the MHC class I (MHC-I) pathway, we here characterize a novel role for ubiquitin in mRNA degradation. MHC-I molecules provide ligands for both cytotoxic T-lymphocytes as well as natural killer (NK) cells, and play a central role in innate and adaptive immunity. MHC-I cell-surface expression is closely monitored by NK cells, whose killer immunoglobulin-like receptors encode MHC-I-specific activatory and inhibitory receptors, implying that MHC-I expression needs to be tightly regulated. In a functional siRNA ubiquitome screen we identified MEX-3C, a novel RNA-binding ubiquitin E3 ligase, as responsible for the post-transcriptional, allotype-specific regulation of MHC-I. MEX-3C binds the 3'UTR of HLA-A2 mRNA, inducing its RING-dependent degradation. The RING domain of MEX-3C is not required for HLA-A2 cell-surface downregulation, but regulates the degradation of HLA-A2 mRNA. We have therefore uncovered a novel post-transcriptional pathway for regulation of HLA-A allotypes and provide a link between ubiquitination and mRNA degradation.

CDX2 regulation by the RNA-binding protein MEX3A: impact on intestinal differentiation and stemness.

The homeobox transcription factor CDX2 plays a crucial role in intestinal cell fate specification, both during normal development and in tumorigenic processes involving intestinal reprogramming. The CDX2 regulatory network is intricate, but it has not yet been fully uncovered. Through genome-wide screening of a 3D culture system, the RNA-binding protein MEX3A was identified as putatively involved in CDX2 regulation; therefore, its biological relevance was addressed by setting up cell-based assays together with expression studies in murine intestine. We demonstrate here that MEX3A has a repressive function by controlling CDX2 levels in gastric and colorectal cellular models. This is dependent on the interaction with a specific binding determinant present in CDX2 mRNA 3'untranslated region. We have further determined that MEX3A impairs intestinal differentiation and cellular polarization, affects cell cycle progression and promotes increased expression of intestinal stem cell markers, namely LGR5, BMI1 and MSI1. Finally, we show that MEX3A is expressed in mouse intestine, supporting an in vivo context for interaction with CDX2 and modulation of stem cell properties. Therefore, we describe a novel CDX2 post-transcriptional regulatory mechanism, through the RNA-binding protein MEX3A, with a major impact in intestinal differentiation, polarity and stemness, likely contributing to intestinal homeostasis and carcinogenesis.

LKB1 when associated with methylatedERα is a marker of bad prognosis in breast cancer.

Although the presence of nuclear estrogen receptor is widely used to guide breast cancer therapy, less attention has been paid to the receptor cytoplasmic signaling. Recently, we have shown that this pathway is operative in vivo and is activated in aggressive tumors representing a new potential target for breast cancer therapy. Here, we identified LKB1 as a partner of ERα and we explored its potential role in estrogen nongenomic signaling. The associations between LKB1 expression and the actors of this pathway, namely the methylated form of ERα (metERα), Src and PI3K, have been analyzed both in cultured cells and in 154 primary breast tumor samples. We found that LKB1 is a component of the cytoplasmic signaling complex in breast cell lines as well as in primary breast tumors. Moreover, an inverse correlation between the localization of LKB1 in nuclear and cytoplasmic compartments is observed. Importantly, high expression of cytoplasmic LKB1 is an independent marker of poor prognosis, associated with reduced overall survival (OS) and disease free survival (DFS). Conversely, the presence of nuclear LKB1 associates with increased OS and DFS. In conclusion, our results highlight that LKB1 expression in breast cancer appears to have opposite effects depending on its subcellular localization and may be used as a new prognostic biomarker.

A novel inhibitor of the mitochondrial respiratory complex I with uncoupling properties exerts potent antitumor activity.

Cancer cells are highly dependent on bioenergetic processes to support their growth and survival. Disruption of metabolic pathways, particularly by targeting the mitochondrial electron transport chain complexes (ETC-I to V) has become an attractive therapeutic strategy. As a result, the search for clinically effective new respiratory chain inhibitors with minimized adverse effects is a major goal. Here, we characterize a new OXPHOS inhibitor compound called MS-L6, which behaves as an inhibitor of ETC-I, combining inhibition of NADH oxidation and uncoupling effect. MS-L6 is effective on both intact and sub-mitochondrial particles, indicating that its efficacy does not depend on its accumulation within the mitochondria. MS-L6 reduces ATP synthesis and induces a metabolic shift with increased glucose consumption and lactate production in cancer cell lines. MS-L6 either dose-dependently inhibits cell proliferation or induces cell death in a variety of cancer cell lines, including B-cell and T-cell lymphomas as well as pediatric sarcoma. Ectopic expression of Saccharomyces cerevisiae NADH dehydrogenase (NDI-1) partially restores the viability of B-lymphoma cells treated with MS-L6, demonstrating that the inhibition of NADH oxidation is functionally linked to its cytotoxic effect. Furthermore, MS-L6 administration induces robust inhibition of lymphoma tumor growth in two murine xenograft models without toxicity. Thus, our data present MS-L6 as an inhibitor of OXPHOS, with a dual mechanism of action on the respiratory chain and with potent antitumor properties in preclinical models, positioning it as the pioneering member of a promising drug class to be evaluated for cancer therapy. MS-L6 exerts dual mitochondrial effects: ETC-I inhibition and uncoupling of OXPHOS. In cancer cells, MS-L6 inhibited ETC-I at least 5 times more than in isolated rat hepatocytes. These mitochondrial effects lead to energy collapse in cancer cells, resulting in proliferation arrest and cell death. In contrast, hepatocytes which completely and rapidly inactivated this molecule, restored their energy status and survived exposure to MS-L6 without apparent toxicity.

Oxidative stress contributes to aging by enhancing pancreatic angiogenesis and insulin signaling.

JunD, a transcription factor of the AP-1 family, protects cells against oxidative stress. Here, we show that junD(-/-) mice exhibit features of premature aging and shortened life span. They also display persistent hypoglycemia due to enhanced insulin secretion. Consequently, the insulin/IGF-1 signaling pathways are constitutively stimulated, leading to inactivation of FoxO1, a positive regulator of longevity. Hyperinsulinemia most likely results from enhanced pancreatic islet vascularization owing to chronic oxidative stress. Indeed, accumulation of free radicals in beta cells enhances VEGF-A transcription, which in turn increases pancreatic angiogenesis and insulin secretion. Accordingly, long-term treatment with an antioxidant rescues the phenotype of junD(-/-) mice. Indeed, dietary antioxidant supplementation was protective against pancreatic angiogenesis, hyperinsulinemia, and subsequent activation of insulin signaling cascades in peripheral tissues. Taken together, these data establish a pivotal role for oxidative stress in systemic regulation of insulin and define a key role for the JunD protein in longevity.

AMPK inhibition in health and disease.

All living organisms depend on dynamic mechanisms that repeatedly reassess the status of amassed energy, in order to adapt energy supply to demand. The AMP-activated protein kinase (AMPK) alphabetagamma heterotrimer has emerged as an important integrator of signals managing energy balance. Control of AMPK activity involves allosteric AMP and ATP regulation, auto-inhibitory features and phosphorylation of its catalytic (alpha) and regulatory (beta and gamma) subunits. AMPK has a prominent role not only as a peripheral sensor but also in the central nervous system as a multifunctional metabolic regulator. AMPK represents an ideal second messenger for reporting cellular energy state. For this reason, activated AMPK acts as a protective response to energy stress in numerous systems. However, AMPK inhibition also actively participates in the control of whole body energy homeostasis. In this review, we discuss recent findings that support the role and function of AMPK inhibition under physiological and pathological states.

[Preventable cancers: Is it enough to change our behaviour?] / Cancers évitables - Suffit-il de changer nos comportements ?

In France, part of 40 % of preventable cancers can be attributed to lifestyle habits. Epidemiological data show that occupational exposures are a major cause of these cancers. However, despite this evidence, the prevention actions promoted by public authorities are focused on changing individual behaviors. In this article, we seek to understand the reasons of the erasure of the role of socio-environmental factors in cancer prevention discourse.

Title: Cancers évitables - Suffit-il de changer nos comportements ? Abstract: En France, une partie des 40 % de cancers évitables peut être attribuée aux habitudes de vie. Les données épidémiologiques révèlent que les expositions professionnelles et domestiques à des substances cancérogènes sont aussi responsables d'une fraction significative de ces cancers. Pourtant, en dépit de ces évidences, les actions de prévention promues par les pouvoirs publics se focalisent sur le changement des comportements individuels. Dans cet article, nous cherchons à comprendre les raisons de l'effacement de la place des facteurs socio-environnementaux des discours portant sur la prévention des cancers.

The TLR3 L412F polymorphism prevents TLR3-mediated tumor cell death induction in pediatric sarcomas.

Toll-like receptor 3 (TLR3) is a pattern recognition receptor mainly known for its role in innate immune response to infection. Indeed, binding of double-stranded RNA (dsRNA) to TLR3 triggers a pro-inflammatory cascade leading to cytokine release and immune cell activation. Its anti-tumoral potential has emerged progressively, associated with a direct impact on tumor cell death induction and with an indirect action on immune system reactivation. Accordingly, TLR3 agonists are currently being tested in clinical trials for several adult cancers. Meanwhile, TLR3 variants have been linked to auto-immune disorders, and as risk factors of viral infection and cancers. However, aside from neuroblastoma, TLR3 role in childhood cancers has not been evaluated. Here, by integrating public transcriptomic data of pediatric tumors, we unveil that high TLR3 expression is largely associated with a better prognosis in childhood sarcomas. Using osteosarcomas and rhabdomyosarcomas as models, we show that TLR3 efficiently drives tumor cell death in vitro and induces tumor regression in vivo. Interestingly, this anti-tumoral effect was lost in cells expressing the homozygous TLR3 L412F polymorphism, which is enriched in a rhabdomyosarcomas cohort. Thus, our results demonstrate the therapeutic potential associated with the targeting of TLR3 in pediatric sarcomas, but also the need to stratify patients eligible for this clinical approach with respect to the TLR3 variants expressed.

Identification and characterization of human Mex-3 proteins, a novel family of evolutionarily conserved RNA-binding proteins differentially localized to processing bodies.

In Caenorhabditis elegans, the Mex-3 protein is a translational regulator that specifies the posterior blastomere identity in the early embryo and contributes to the maintenance of the germline totipotency. We have now identified a family of four homologous human Mex-3 genes, called hMex-3A to -3D that encode proteins containing two heterogeneous nuclear ribonucleoprotein K homology (KH) domains and one carboxy-terminal RING finger module. The hMex-3 are phosphoproteins that bind RNA through their KH domains and shuttle between the nucleus and the cytoplasm via the CRM1-dependent export pathway. Our analysis further revealed that hMex-3A and hMex-3B, but not hMex-3C, colocalize with both the hDcp1a decapping factor and Argonaute (Ago) proteins in processing bodies (P bodies), recently characterized as centers of mRNA turnover. Taken together, these findings indicate that hMex-3 proteins constitute a novel family of evolutionarily conserved RNA-binding proteins, differentially recruited to P bodies and potentially involved in post-transcriptional regulatory mechanisms.

LKB1 specifies neural crest cell fates through pyruvate-alanine cycling.

Metabolic processes underlying the development of the neural crest, an embryonic population of multipotent migratory cells, are poorly understood. Here, we report that conditional ablation of the Lkb1 tumor suppressor kinase in mouse neural crest stem cells led to intestinal pseudo-obstruction and hind limb paralysis. This phenotype originated from a postnatal degeneration of the enteric nervous ganglia and from a defective differentiation of Schwann cells. Metabolomic profiling revealed that pyruvate-alanine conversion is enhanced in the absence of Lkb1. Mechanistically, inhibition of alanine transaminases restored glial differentiation in an mTOR-dependent manner, while increased alanine level directly inhibited the glial commitment of neural crest cells. Treatment with the metabolic modulator AICAR suppressed mTOR signaling and prevented Schwann cell and enteric defects of Lkb1 mutant mice. These data uncover a link between pyruvate-alanine cycling and the specification of glial cell fate with potential implications in the understanding of the molecular pathogenesis of neural crest diseases.

Dialogue between LKB1 and AMPK: a hot topic at the cellular pole.

Disruption of cell architecture and change of energy metabolism are two traits of malignant cells. Yet, there was scant evidence that these two cancer hallmarks involved perturbations of a common signaling pathway. Enter LKB1, a kinase that is a tumor suppressor and that is an upstream activator of the adenosine monophosphate (AMP)-activated protein kinase (AMPK), a key sensor of cellular energy status. Four studies now reveal that LKB1 signals through AMPK to facilitate the formation of tight junctions and to maintain epithelial polarity. Thus, LKB1 appears to be a novel class of tumor suppressor that acts as an energy-sensing and polarity checkpoint.

[Even the Warburg effect can be oxidized: metabolic cooperation and tumor development]. / Même l'effet Warburg est oxydable - Coopération métabolique et développement tumoral.

During tumor development, malignant cells rewire their metabolism to meet the biosynthetic needs required to increase their biomass and to overcome their microenvironment constraints. The sustained activation of aerobic glycolysis, also called Warburg effect, is one of these adaptative mechanisms. The progresses in this area of research have revealed the flexibility of cancer cells that alternate between glycolytic and oxidative metabolism to cope with their conditions of development while sharing their energetic resources. In this survey, we review these recent breakthroughs and discuss a model that likens tumor to an evolutive metabolic ecosystem. We further emphasize the ensuing therapeutic applications that target metabolic weaknesses of neoplastic cells.