Enrico De Smaele

Induction of gadd45beta by NF-kappaB downregulates pro-apoptotic JNK signalling.

In addition to coordinating immune and inflammatory responses, NF-κB/Rel transcription factors control cell survival. Normally, NF-κB dimers are sequestered in the cytoplasm by binding to inhibitory IκB proteins, and can be activated rapidly by signals that induce the sequential phosphorylation and proteolysis of IκBs. Activation of NF-κB antagonizes apoptosis or programmed cell death by numerous triggers, including the ligand engagement of ‘death receptors’ such as tumour-necrosis factor (TNF) receptor. The anti-apoptotic activity of NF-κB is also crucial to oncogenesis and to chemo- and radio-resistance in cancer. Cytoprotection by NF-κB involves the activation of pro-survival genes; however, its basis remains poorly understood. Here we report that NF-κB complexes downregulate the c-Jun amino-terminal kinase (JNK) cascade, thus establishing a link between the NF-κB and the JNK pathways. This link involves the transcriptional upregulation of gadd45β/myd118 (ref. 4), which downregulates JNK signalling induced by the TNF receptor (TNF-R). This NF-κB-dependent inhibition of the JNK pathway is central to the control of cell death. Our findings define a protective mechanism that is mediated by NF-κB complexes and establish a role for the persistent activation of JNK in the apoptotic response to TNF-α.

Ferritin Heavy Chain Upregulation by NF-κB Inhibits TNFα-Induced Apoptosis by Suppressing Reactive Oxygen Species

Abstract During inflammation, NF-κB transcription factors antagonize apoptosis induced by tumor necrosis factor (TNF)α. This antiapoptotic activity of NF-κB involves suppressing the accumulation of reactive oxygen species (ROS) and controlling the activation of the c-Jun N-terminal kinase (JNK) cascade. However, the mechanism(s) by which NF-κB inhibits ROS accumulation is unclear. We identify ferritin heavy chain (FHC)—the primary iron storage factor—as an essential mediator of the antioxidant and protective activities of NF-κB. FHC is induced downstream of NF-κB and is required to prevent sustained JNK activation and, thereby, apoptosis triggered by TNFα. FHC-mediated inhibition of JNK signaling depends on suppressing ROS accumulation and is achieved through iron sequestration. These findings establish a basis for the NF-κB-mediated control of ROS induction and identify a mechanism by which NF-κB suppresses proapoptotic JNK signaling. Our results suggest modulation of FHC or, more broadly, of iron metabolism as a potential approach for anti-inflammatory therapy.

Gadd45|[beta]| mediates the NF-|[kappa]|B suppression of JNK signalling by targeting MKK7/JNKK2

NF-κB/Rel transcription factors control apoptosis, also known as programmed cell death. This control is crucial for oncogenesis, cancer chemo-resistance and for antagonizing tumour necrosis factor α (TNFα)-induced killing. With regard to TNFα, the anti-apoptotic activity of NF-κB involves suppression of the c-Jun N-terminal kinase (JNK) cascade. Using an unbiased screen, we have previously identified Gadd45β/Myd118, a member of the Gadd45 family of inducible factors, as a pivotal mediator of this suppressive activity of NF-κB. However, the mechanisms by which Gadd45β inhibits JNK signalling are not understood. Here, we identify MKK7/JNKK2 — a specific and essential activator of JNK — as a target of Gadd45β, and in fact, of NF-κB itself. Gadd45β binds to MKK7 directly and blocks its catalytic activity, thereby providing a molecular link between the NF-κB and JNK pathways. Importantly, Gadd45β is required to antagonize TNFα-induced cytotoxicity, and peptides disrupting the Gadd45β/MKK7 interaction hinder the ability of Gadd45β, as well as of NF-κB, to suppress this cytotoxicity. These findings establish a basis for the NF-κB control of JNK activation and identify MKK7 as a potential target for anti-inflammatory and anti-cancer therapy.

Concerted microRNA control of Hedgehog signalling in cerebellar neuronal progenitor and tumour cells

MicroRNAs (miRNA) are crucial post‐transcriptional regulators of gene expression and control cell differentiation and proliferation. However, little is known about their targeting of specific developmental pathways. Hedgehog (Hh) signalling controls cerebellar granule cell progenitor development and a subversion of this pathway leads to neoplastic transformation into medulloblastoma (MB). Using a miRNA high‐throughput profile screening, we identify here a downregulated miRNA signature in human MBs with high Hh signalling. Specifically, we identify miR‐125b and miR‐326 as suppressors of the pathway activator Smoothened together with miR‐324‐5p, which also targets the downstream transcription factor Gli1. Downregulation of these miRNAs allows high levels of Hh‐dependent gene expression leading to tumour cell proliferation. Interestingly, the downregulation of miR‐324‐5p is genetically determined by MB‐associated deletion of chromosome 17p. We also report that whereas miRNA expression is downregulated in cerebellar neuronal progenitors, it increases alongside differentiation, thereby allowing cell maturation and growth inhibition. These findings identify a novel regulatory circuitry of the Hh signalling and suggest that misregulation of specific miRNAs, leading to its aberrant activation, sustain cancer development.

MicroRNA profiling in human medulloblastoma

Medulloblastoma is an aggressive brain malignancy with high incidence in childhood. Current treatment approaches have limited efficacy and severe side effects. Therefore, new risk‐adapted therapeutic strategies based on molecular classification are required. MicroRNA expression analysis has emerged as a powerful tool to identify candidate molecules playing an important role in a large number of malignancies. However, no data are yet available on human primary medulloblastomas. A high throughput microRNA expression profiles was performed in human primary medulloblastoma specimens to investigate microRNA involvement in medulloblastoma carcinogenesis. We identified specific microRNA expression patterns which distinguish medulloblastoma differing in histotypes (anaplastic, classic and desmoplastic), in molecular features (ErbB2 or c‐Myc overexpressing tumors) and in disease‐risk stratification. MicroRNAs expression profile clearly differentiates medulloblastoma from either adult or fetal normal cerebellar tissues. Only a few microRNAs displayed upregulated expression, while most of them were downregulated in tumor samples, suggesting a tumor growth‐inhibitory function. This property has been addressed for miR‐9 and miR‐125a, whose rescued expression promoted medulloblastoma cell growth arrest and apoptosis while targeting the proproliferative truncated TrkC isoform. In conclusion, misregulated microRNA expression profiles characterize human medulloblastomas, and may provide potential targets for novel therapeutic strategies.

Numb is a suppressor of Hedgehog signalling and targets Gli1 for Itch-dependent ubiquitination

The developmental protein Numb is a major determinant of binary cell fates. It is also required for the differentiation of cerebellar granule cell progenitors (GCPs) at a stage of development responsive to the morphogenic glycoprotein Hedehog. Hedgehog signalling is crucial for the physiological maintenance and self-renewal of neural stem cells and its deregulation is responsible for their progression towards tumorigenesis. The mechanisms that inhibit this pathway during the differentiation stage are poorly understood. Here, we identify Numb as a Hedgehog-pathway inhibitor that is downregulated in early GCPs and GCP-derived cancer cells. We demonstrate that the Hedgehog transcription factor Gli1 is targeted by Numb for Itch-dependent ubiquitination, which suppresses Hedgehog signals, thus arresting growth and promoting cell differentiation. This novel Numb-dependent regulatory loop may limit the extent and duration of Hedgehog signalling during neural-progenitor differentiation, and its subversion may be a relevant event in brain tumorigenesis.

Histone deacetylase and Cullin3-REN(KCTD11) ubiquitin ligase interplay regulates Hedgehog signalling through Gli acetylation.

Hedgehog signalling is crucial for development and is deregulated in several tumours, including medulloblastoma. Regulation of the transcriptional activity of Gli (glioma-associated oncogene) proteins, effectors of the Hedgehog pathway, is poorly understood. We show here that Gli1 and Gli2 are acetylated proteins and that their HDAC-mediated deacetylation promotes transcriptional activation and sustains a positive autoregulatory loop through Hedgehog-induced upregulation of HDAC1. This mechanism is turned off by HDAC1 degradation through an E3 ubiquitin ligase complex formed by Cullin3 and REN, a Gli antagonist lost in human medulloblastoma. Whereas high HDAC1 and low REN expression in neural progenitors and medulloblastomas correlates with active Hedgehog signalling, loss of HDAC activity suppresses Hedgehog-dependent growth of neural progenitors and tumour cells. Consistent with this, abrogation of Gli1 acetylation enhances cellular proliferation and transformation. These data identify an integrated HDAC- and ubiquitin-mediated circuitry, where acetylation of Gli proteins functions as an unexpected key transcriptional checkpoint of Hedgehog signalling.

Hedgehog controls neural stem cells through p53-independent regulation of Nanog

Hedgehog (Hh) pathway has a pivotal function in development and tumorigenesis, processes sustained by stem cells (SCs). The transcription factor Nanog controls stemness acting as a key determinant of both embryonic SC self‐renewal and differentiated somatic cells reprogramming to pluripotency, in concert with the loss of the oncosuppressor p53. How Nanog is regulated by microenvironmental signals in postnatal SC niches has been poorly investigated. Here, we show that Nanog is highly expressed in SCs from postnatal cerebellum and medulloblastoma, and acts as a critical mediator of Hh‐driven self‐renewal. Indeed, the downstream effectors of Hh activity, Gli1 and Gli2, bind to Nanog‐specific cis‐regulatory sequences both in mouse and human SCs. Loss of p53, a key event promoting cell stemness, activates Hh signalling, thereby contributing to Nanog upregulation. Conversely, Hh downregulates p53 but does not require p53 to control Nanog. Our data reveal a mechanism for the function of Hh in the control of stemness that represents a crucial component of an integrated circuitry determining cell fate decision and involved in the maintenance of cancer SCs.

MicroRNAs as biomarkers for CNS cancer and other disorders

The use of miRNAs as biomarkers has gained growing interest in the last few years. Their role in regulating a great variety of targets and, as a consequence, multiple pathways, makes their use in diagnostics a powerful tool to be exploited for early detection of disease, risk assessment and prognosis and for the design of innovative therapeutic strategies. While still not fully validated, profiling of blood cells, exosomes or body fluid miRNAs would represent a tremendous and promising advance in non-invasive diagnostics of CNS disorders. A major challenge is represented by technological aspects of miRNA detection and discovery aiming to genome-wide high throughput, sensitive and accurate analysis. Although there is much to be learned in the field, this review will highlight the potential role of miRNA as a new class of biomarkers in several CNS disorders, including neurodegenerative diseases such as Alzheimer, Huntington and Parkinson diseases, schizophrenia and autism as well as different types of cancer (e.g. gliomas and medulloblastomas).

Insights into the structural basis of the GADD45β-mediated inactivation of the JNK kinase, MKK7/JNKK2

NF-κB/Rel factors control programmed cell death (PCD), and this control is crucial to oncogenesis, cancer chemoresistance, and antagonism of tumor necrosis factor (TNF) α-induced killing. With TNFα, NF-κB-mediated protection involves suppression of the c-Jun-N-terminal kinase (JNK) cascade, and we have identified Gadd45β, a member of the Gadd45 family, as a pivotal effector of this activity of NF-κB. Inhibition of TNFα-induced JNK signaling by Gadd45β depends on direct targeting of the JNK kinase, MKK7/JNKK2. The mechanism by which Gadd45β blunts MKK7, however, is unknown. Here we show that Gadd45β is a structured protein with a predicted four-stranded β-sheet core, five α-helices, and two acidic loops. Association of Gadd45β with MKK7 involves a network of interactions mediated by its putative helices α3 and α4 and loops 1 and 2. Whereas α3 appears to primarily mediate docking to MKK7, loop 1 and α4-loop 2 seemingly afford kinase inactivation by engaging the ATP-binding site and causing conformational changes that impede catalytic function. These data provide a basis for Gadd45β-mediated blockade of MKK7, and ultimately, TNFα-induced PCD. They also have important implications for treatment of widespread diseases.