Transforming activity of MECT1-MAML2 fusion oncoprotein is mediated by constitutive CREB activation

CREB--a real culprit in oncogenesis

The cAMP response element-binding protein (CREB) is a stimulus-induced transcription factor that responds rapidly to phosphorylation and/or coactivator activation. Regulated activation of CREB has a significant impact on cellular growth, proliferation and survival. To overturn the cellular control of these processes, tumor cells have developed various mechanisms to achieve constitutive activation of CREB, including gene amplification, chromosome translocation, interaction with viral oncoproteins, and inactivation of tumor suppressor genes. These mechanisms converge on the phosphorylation of CREB and/or the activation of transducer of regulated CREB activity (TORC) coactivators to effect uncontrolled proliferation of cells. This minireview summarizes the different lines of existing evidence that support a direct role of CREB in oncogenesis.

Frequent fusion of the CRTC1 and MAML2 genes in clear cell variants of cutaneous hidradenomas

Fusion of the CREB regulated transcription coactivator CRTC1 (a.k.a. MECT1, TORC1, or WAMTP1) to the Notch coactivator MAML2 is a characteristic feature of low-grade mucoepidermoid carcinomas of salivary and bronchial glands. The CRTC1-MAML2 fusion protein acts by inducing transcription of cAMP/CREB target genes, and this activity is crucial for the transforming properties of the protein. Here we show that the CRTC1-MAML2 gene fusion is also frequent in benign hidradenomas of the skin. FISH and RT-PCR analyses revealed that hidradenomas are genetically heterogeneous, and that 10 of the 20 tumors analyzed (50%) contained the CRTC1-MAML2 gene fusion and expressed the resulting fusion transcript. Immunohistochemical analysis demonstrated expression of the fusion protein in the majority of tumor cells, including clear cells, poroid cells, and cells with epidermoid and ductal differentiation. In addition, we could show that all fusion-positive tumors were morphologically distinguished by the presence of more or less abundant areas of clear cells whereas all fusion-negative tumors lacked clear cells. Our findings thus demonstrate that the CRTC1-MAML2 gene fusion is frequent in hidradenomas and is associated with clear cell variants of this tumor. Taken together, the present and previous observations indicate that the CRTC1-MAML2 fusion is etiologically linked to benign and low-grade malignant tumors originating from diverse exocrine glands rather than being linked to a separate tumor entity.

A genomic screen for activators of the antioxidant response element

The antioxidant response element (ARE) is a cis-acting regulatory enhancer element found in the 5' flanking region of many phase II detoxification enzymes. Up-regulation of ARE-dependent target genes is known to have neuroprotective effects; yet, the mechanism of activation is largely unknown. By screening an arrayed collection of approximately 15,000 full-length expression cDNAs in the human neuroblastoma cell line IMR-32 with an ARE-luciferase reporter, we have identified several cDNAs not previously associated with ARE activation. A subset of cDNAs, encoding sequestosome 1 (SQSTM1) and dipeptidylpeptidase 3 (DPP3), activated the ARE in primary mouse-derived cortical neurons. Overexpression of SQSTM1 and DPP3 in IMR-32 cells stimulated NF-E2-related factor 2 (NRF2) nuclear translocation and led to increased levels of NAD(P)H:quinone oxidoreductase 1, a protein which is transcriptionally regulated by the ARE. When transfected into IMR-32 neuroblastoma cells that were depleted of transcription factor NRF2 by RNA interference, SQSTM1 and DPP3 were unable to activate the ARE or induce NAD(P)H:quinone oxidoreductase 1 expression, indicating that the ARE activation upon ectopic expression of these cDNAs is mediated by NRF2. Studies with pharmacological inhibitors indicated that 1-phosphatidylinositol 3-kinase and protein kinase C signaling are essential for activity. Overexpression of these cDNAs conferred partial resistance to hydrogen peroxide or rotenone-induced toxicity, consistent with the induction of antioxidant and phase II detoxification enzymes, which can protect from oxidative stress. This work and other such studies may provide mechanisms for activating the ARE in the absence of general oxidative stress and a yet-unexploited therapeutic approach to degenerative diseases and aging.

Requirement of TORC1 for late-phase long-term potentiation in the hippocampus

Late-phase long-term potentiation (L-LTP) and long-term memory depend on the transcription of mRNA of CRE-driven genes and synthesis of proteins. However, how synaptic signals propagate to the nucleus is unclear. Here we report that the CREB coactivator TORC1 (transducer of regulated CREB activity 1) undergoes neuronal activity-induced translocation from the cytoplasm to the nucleus, a process required for CRE-dependent gene expression and L-LTP. Overexpressing a dominant-negative form of TORC1 or down-regulating TORC1 expression prevented activity-dependent transcription of CREB target genes in cultured hippocampal neurons, while overexpressing a wild-type form of TORC1 facilitated basal and activity-induced transcription of CREB target genes. Furthermore, overexpressing the dominant-negative form of TORC1 suppressed the maintenance of L-LTP without affecting early-phase LTP, while overexpressing the wild-type form of TORC1 facilitated the induction of L-LTP in hippocampal slices. Our results indicate that TORC1 is essential for CRE-driven gene expression and maintenance of long-term synaptic potentiation.

Silencing the constitutive active transcription factor CREB by the LKB1-SIK signaling cascade

Cyclic AMP responsive element (CRE)-binding protein (CREB) is known to activate transcription when its Ser133 is phosphorylated. Two independent investigations have suggested the presence of Ser133-independent activation. One study identified a kinase, salt-inducible kinase (SIK), which repressed CREB; the other isolated a novel CREB-specific coactivator, transducer of regulated CREB activity (TORC), which upregulated CREB activity. These two opposing signals are connected by the fact that SIK phosphorylates TORC and induces its nuclear export. Because LKB1 has been reported to be an upstream kinase of SIK, we used LKB1-defective HeLa cells to further elucidate TORC-dependent CREB activation. In the absence of LKB1, SIK was unable to phosphorylate TORC, which led to constitutive activation of CRE activity. Overexpression of LKB1 in HeLa cells improved the CRE-dependent transcription in a regulated manner. The inactivation of kinase cascades by 10 nm staurosporine in LKB1-positive HEK293 cells also induced unregulated, constitutively activated, CRE activity. Treatment with staurosporine completely inhibited SIK kinase activity without any significant effect on the phosphorylation level at the LKB1-phosphorylatable site in SIK or the activity of AMPK, another target of LKB1. Constitutive activation of CREB in LKB1-defective cells or in staurosporine-treated cells was not accompanied by CREB phosphorylation at Ser133. The results suggest that LKB1 and its downstream SIK play an important role in silencing CREB activity via the phosphorylation of TORC, and such silencing may be indispensable for the regulated activation of CREB.

Molecular classification of mucoepidermoid carcinomas-prognostic significance of the MECT1-MAML2 fusion oncogene

Mucoepidermoid carcinomas (MECs) of the salivary and bronchial glands are characterized by a recurrent t(11;19)(q21;p13) translocation resulting in a MECT1-MAML2 fusion in which the CREB-binding domain of the CREB coactivator MECT1 (also known as CRTC1, TORC1 or WAMTP1) is fused to the transactivation domain of the Notch coactivator MAML2. To gain further insights into the molecular pathogenesis of MECs, we cytogenetically and molecularly characterized a series of 29 MECs. A t(11;19) and/or an MECT1-MAML2 fusion was detected in more than 55% of the tumors. Several cases with cryptic rearrangements that resulted in gene fusions were detected. In fusion-negative MECs, the most common aberration was a single or multiple trisomies. Western blot and immunohistochemical studies demonstrated that the MECT1-MAML2 fusion protein was expressed in all MEC-specific cell types. In addition, cotransfection experiments showed that the fusion protein colocalized with CREB in homogeneously distributed nuclear granules. Analyses of potential downstream targets of the fusion revealed differential expression of the cAMP/CREB (FLT1 and NR4A2) and Notch (HES1 and HES5) target genes in fusion-positive and fusion-negative MECs. Moreover, clinical follow-up studies revealed that fusion-positive patients had a significantly lower risk of local recurrence, metastases, or tumor-related death compared to fusion-negative patients (P = 0.0012). When considering tumor-related deaths only, the estimated median survival for fusion-positive patients was greater than 10 years compared to 1.6 years for fusion-negative patients. These findings suggest that molecularly classifying MECs on the basis of an MECT1-MAML2 fusion is histopathologically and clinically relevant and that the fusion is a useful marker in predicting the biological behavior of MECs.

Sustained expression of Mect1-Maml2 is essential for tumor cell growth in salivary gland cancers carrying the t(11;19) translocation

Mucoepidermoid (MEC) salivary gland tumors arise from a t(11;19) rearrangement which generates a fusion oncogene, Mect1-Maml2, that functions to activate CREB-responsive target genes. To determine if sustained expression of Mect1-Maml2 is required for tumor cell growth, we first showed that ectopic expression of Mect1-Maml2 in rat epithelial RK3E cells is tumorigenic in vivo in nude mice and that excised xenografts continue to express the fusion oncogene. We then generated a hairpin RNAi vector that selectively suppressed the fusion peptide and showed that ectopic expression in either parotid or pulmonary MEC tumor cell lines containing the t(11;19) rearrangement resulted in at least 90% colony growth inhibition. In contrast, single nucleotide changes within this RNAi sequence abolished the ability to suppress Mect1-Maml2 protein and abolished all growth inhibition of these MEC tumor lines. In addition, the RNAi-specific vector had no effect on colony growth of non-MEC tumors including a lung tumor or two other salivary gland cell lines that do not express Mect1-Maml2. We also generated a mutant Mect1-Maml2 expression plasmid that carried silent nucleotide changes within the RNAi target sequence and observed that co-transfection of this mutant, but not wild-type Mect1-Maml2, could partially rescue RNAi growth inhibition in the MEC tumor line. The recent detection of acquired fusion oncogenes in epithelial solid tumors has suggested new possibilities for the diagnosis and therapy of these cancers. Our data show that the 'gain-of-function' activity from aberrant Mect1-Maml2 expression is a candidate therapeutic target for this group of malignant salivary gland tumors.

Conditional knockout mice reveal distinct functions for the global transcriptional coactivators CBP and p300 in T-cell development

The global transcriptional coactivators CREB-binding protein (CBP) and the closely related p300 interact with over 312 proteins, making them among the most heavily connected hubs in the known mammalian protein-protein interactome. It is largely uncertain, however, if these interactions are important in specific cell lineages of adult animals, as homozygous null mutations in either CBP or p300 result in early embryonic lethality in mice. Here we describe a Cre/LoxP conditional p300 null allele (p300flox) that allows for the temporal and tissue-specific inactivation of p300. We used mice carrying p300flox and a CBP conditional knockout allele (CBPflox) in conjunction with an Lck-Cre transgene to delete CBP and p300 starting at the CD4- CD8- double-negative thymocyte stage of T-cell development. Loss of either p300 or CBP led to a decrease in CD4+ CD8+ double-positive thymocytes, but an increase in the percentage of CD8+ single-positive thymocytes seen in CBP mutant mice was not observed in p300 mutants. T cells completely lacking both CBP and p300 did not develop normally and were nonexistent or very rare in the periphery, however. T cells lacking CBP or p300 had reduced tumor necrosis factor alpha gene expression in response to phorbol ester and ionophore, while signal-responsive gene expression in CBP- or p300-deficient macrophages was largely intact. Thus, CBP and p300 each supply a surprising degree of redundant coactivation capacity in T cells and macrophages, although each gene has also unique properties in thymocyte development.

The kinase LKB1 mediates glucose homeostasis in liver and therapeutic effects of metformin

The Peutz-Jegher syndrome tumor-suppressor gene encodes a protein-threonine kinase, LKB1, which phosphorylates and activates AMPK [adenosine monophosphate (AMP)-activated protein kinase]. The deletion of LKB1 in the liver of adult mice resulted in a nearly complete loss of AMPK activity. Loss of LKB1 function resulted in hyperglycemia with increased gluconeogenic and lipogenic gene expression. In LKB1-deficient livers, TORC2, a transcriptional coactivator of CREB (cAMP response element-binding protein), was dephosphorylated and entered the nucleus, driving the expression of peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha), which in turn drives gluconeogenesis. Adenoviral small hairpin RNA (shRNA) for TORC2 reduced PGC-1alpha expression and normalized blood glucose levels in mice with deleted liver LKB1, indicating that TORC2 is a critical target of LKB1/AMPK signals in the regulation of gluconeogenesis. Finally, we show that metformin, one of the most widely prescribed type 2 diabetes therapeutics, requires LKB1 in the liver to lower blood glucose levels.