Gianluca Canettieri

The CREB Coactivator TORC2 Functions as a Calcium- and cAMP-Sensitive Coincidence Detector

Elevations in circulating glucose and gut hormones during feeding promote pancreatic islet cell viability in part via the calcium- and cAMP-dependent activation of the transcription factor CREB. Here, we describe a signaling module that mediates the synergistic effects of these pathways on cellular gene expression by stimulating the dephosphorylation and nuclear entry of TORC2, a CREB coactivator. This module consists of the calcium-regulated phosphatase calcineurin and the Ser/Thr kinase SIK2, both of which associate with TORC2. Under resting conditions, TORC2 is sequestered in the cytoplasm via a phosphorylation-dependent interaction with 14-3-3 proteins. Triggering of the calcium and cAMP second messenger pathways by glucose and gut hormones disrupts TORC2:14-3-3 complexes via complementary effects on TORC2 dephosphorylation; calcium influx increases calcineurin activity, whereas cAMP inhibits SIK2 kinase activity. Our results illustrate how a phosphatase/kinase module connects two signaling pathways in response to nutrient and hormonal cues.

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.

Attenuation of a phosphorylation-dependent activator by an HDAC-PP1 complex

The second messenger cAMP stimulates transcription with burst-attenuation kinetics that mirror the PKA-dependent phosphorylation and subsequent protein phosphatase 1 (PP1)–mediated dephosphorylation of the cAMP responsive element binding protein (CREB) at Ser133. Phosphorylation of Ser133 promotes recruitment of the co-activator histone acetylase (HAT) paralogs CBP and P300, which in turn stimulate acetylation of promoter-bound histones during the burst phase. Remarkably, histone deacetylase (HDAC) inhibitors seem to potentiate CREB activity by prolonging Ser133 phosphorylation in response to cAMP stimulus, suggesting a potential role for HDAC complexes in silencing CREB activity. Here we show that HDAC1 associates with and blocks Ser133 phosphorylation of CREB during pre-stimulus and attenuation phases of the cAMP response. HDAC1 promotes Ser133 dephosphorylation via a stable interaction with PP1, which we detected in co-immunoprecipitation and co-purification studies. These results illustrate a novel mechanism by which signaling and chromatin-modifying activities act coordinately to repress the activity of a phosphorylation-dependent activator.

Distinct effects of cAMP and mitogenic signals on CREB-binding protein recruitment impart specificity to target gene activation via CREB

Ser-133 phosphorylation of the cAMP-responsive element-binding protein (CREB) is sufficient to induce cellular gene expression in response to cAMP, but additional promoter-bound factors are required for target gene activation by CREB in response to mitogen/stress signals. To compare the relative effects of different signals on recruitment of the coactivator CREB-binding protein (CBP) to CREB in living cells, we developed a fluorescence resonance energy transfer (FRET) assay. cAMP promoted the interaction of CREB with CBP in a phosphorylation-dependent manner by FRET analysis, but mitogen/stress signals were far less effective in stimulating complex formation even though they induced comparable levels of Ser-133 phosphorylation. cAMP and non-cAMP stimuli were comparably active in promoting this interaction in the cytosol; the formation of CREB⋅CBP complexes in response to non-cAMP signals was specifically inhibited in the nucleus. Non-cAMP signals had no effect on intrinsic CREB- or CBP-binding activities by Far Western blot assay, thereby supporting the presence of a distinct CREB⋅CBP antagonist. Our studies indicate that the relative effects of cAMP and mitogen/stress signals on CREB⋅CBP complex formation impart selectivity to gene activation through CREB phosphorylated at Ser-133.

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.

Numb activates the E3 ligase Itch to control Gli1 function through a novel degradation signal

Hedgehog pathway regulates tissue patterning and cell proliferation. Gli1 transcription factor is the major effector of Hedgehog signaling and its deregulation is often associated to medulloblastoma formation. Proteolytic processes represent a critical mechanism by which this pathway is turned off. Here, we characterize the regulation of an ubiquitin-mediated mechanism of Gli1 degradation, promoted by the coordinated action of the E3 ligase Itch and the adaptor protein Numb. We show that Numb activates the catalytic activity of Itch, releasing it from an inhibitory intramolecular interaction between its homologous to E6-AP C-terminus and WW domains. The consequent activation of Itch, together with the recruitment of Gli1 through direct binding with Numb, allows Gli1 to enter into the complex, resulting in Gli1 ubiquitination and degradation. This process is mediated by a novel Itch-dependent degron, composed of a combination of two PPXYs and a phospho-serine/proline motifs, localized in Gli1 C-terminal region, indicating the role of two different WW docking sites in Gli1 ubiquitination. Remarkably, Gli1 protein mutated in these modules is no longer regulated by Itch and Numb, and determines enhanced Gli1-dependent medulloblastoma growth, migration and invasion abilities, as well as in vitro transforming activity. Our data reveal a novel mechanism of regulation of Gli1 stability and function, which influences Hedgehog/Gli1 oncogenic potential.

Gli1/DNA interaction is a druggable target for Hedgehog‐dependent tumors

Hedgehog signaling is essential for tissue development and stemness, and its deregulation has been observed in many tumors. Aberrant activation of Hedgehog signaling is the result of genetic mutations of pathway components or other Smo‐dependent or independent mechanisms, all triggering the downstream effector Gli1. For this reason, understanding the poorly elucidated mechanism of Gli1‐mediated transcription allows to identify novel molecules blocking the pathway at a downstream level, representing a critical goal in tumor biology. Here, we clarify the structural requirements of the pathway effector Gli1 for binding to DNA and identify Glabrescione B as the first small molecule binding to Gli1 zinc finger and impairing Gli1 activity by interfering with its interaction with DNA. Remarkably, as a consequence of its robust inhibitory effect on Gli1 activity, Glabrescione B inhibited the growth of Hedgehog‐dependent tumor cells in vitro and in vivo as well as the self‐renewal ability and clonogenicity of tumor‐derived stem cells. The identification of the structural requirements of Gli1/DNA interaction highlights their relevance for pharmacologic interference of Gli signaling.

PCAF ubiquitin ligase activity inhibits Hedgehog/Gli1 signaling in p53-dependent response to genotoxic stress

The Hedgehog (Hh) signaling regulates tissue development, and its aberrant activation is a leading cause of malignancies, including medulloblastoma (Mb). Hh-dependent tumorigenesis often occurs in synergy with other mechanisms, such as loss of p53, the master regulator of the DNA damage response. To date, little is known about mechanisms connecting DNA-damaging events to morphogen-dependent processes. Here, we show that genotoxic stress triggers a cascade of signals, culminating with inhibition of the activity of Gli1, the final transcriptional effector of Hh signaling. This inhibition is dependent on the p53-mediated elevation of the acetyltransferase p300/CBP-associated factor (PCAF). Notably, we identify PCAF as a novel E3 ubiquitin ligase of Gli1. Indeed PCAF, but not a mutant with a deletion of its ubiquitination domain, represses Hh signaling in response to DNA damage by promoting Gli1 ubiquitination and its proteasome-dependent degradation. Restoring Gli1 levels rescues the growth arrest and apoptosis effect triggered by genotoxic drugs. Consistently, DNA-damaging agents fail to inhibit Gli1 activity in the absence of either p53 or PCAF. Finally, Mb samples from p53-null mice display low levels of PCAF and upregulation of Gli1 in vivo, suggesting PCAF as potential therapeutic target in Hh-dependent tumors. Together, our data define a mechanism of inactivation of a morphogenic signaling in response to genotoxic stress and unveil a p53/PCAF/Gli1 circuitry centered on PCAF that limits Gli1-enhanced mitogenic and prosurvival response.

The coactivator CRTC1 promotes cell proliferation and transformation via AP-1

Regulation of gene expression in response to mitogenic stimuli is a critical aspect underlying many forms of human cancers. The AP-1 complex mediates the transcriptional response to mitogens, and its deregulation causes developmental defects and tumors. We report that the coactivator CRTC1 cyclic AMP response element-binding protein (CREB)-regulated transcription coactivator 1 is a potent and indispensable modulator of AP-1 function. After exposure of cells to the AP-1 agonist 12-O-tetradecanoylphorbol-13-acetate (TPA), CRTC1 is recruited to AP-1 target gene promoters and associates with c-Jun and c-Fos to activate transcription. CRTC1 consistently synergizes with the proto-oncogene c-Jun to promote cellular growth, whereas AP-1–dependent proliferation is abrogated in CRTC1-deficient cells. Remarkably, we demonstrate that CRTC1-Maml2 oncoprotein, which causes mucoepidermoid carcinomas, binds and activates both c-Jun and c-Fos. Consequently, ablation of AP-1 function disrupts the cellular transformation and proliferation mediated by this oncogene. Together, these data illustrate a novel mechanism required to couple mitogenic signals to the AP-1 gene regulatory program.

Acetylation controls Notch3 stability and function in T-cell leukemia

Post-translational modifications of Notch3 and their functional role with respect to Notch3 overexpression in T-cell leukemia are still poorly understood. We identify here a specific novel property of Notch3 that is acetylated and deacetylated at lysines 1692 and 1731 by p300 and HDAC1, respectively, a balance impaired by HDAC inhibitors (HDACi) that favor hyperacetylation. By using HDACi and a non-acetylatable Notch3 mutant carrying K/R1692−1731 mutations in the intracellular domain, we show that Notch3 acetylation primes ubiquitination and proteasomal-mediated degradation of the protein. As a consequence, Notch3 protein expression and its transcriptional activity are decreased both in vitro and in vivo in Notch3 transgenic (tg) mice, thus impairing downstream signaling upon target genes. Consistently, Notch3-induced T-cell proliferation is inhibited by HDACi, whereas it is enhanced by the non-acetylatable Notch3-K/R1692−1731 mutant. Finally, HDACi-induced Notch3 hyperacetylation prevents in vivo growth of T-cell leukemia/lymphoma in Notch3 tg mice. Together, our findings suggest a novel level of Notch signaling control in which Notch3 acetylation/deacetylation process represents a key regulatory switch, thus representing a suitable druggable target for Notch3-sustained T-cell acute lymphoblastic leukemia therapy.