Glioblastoma (GBM), the most common malignant brain tumor, is among the most lethal neoplasms, with a median survival of approximately 1 year. Prognosis is poor since GBMs possess a strong migratory and highly invasive potential, making complete surgical resection impossible. Reduced expression of carboxypeptidase E (CPE), a neuropeptide-processing enzyme, in a cell death-resistant glioma cell line and lower CPE expression levels in the cohort of GBM samples of The Cancer Genome Atlas compared to normal brain control specimens prompted us to analyze the function of CPE as a putative tumor suppressor gene. In our samples, CPE was also reduced in GBM compared to normal brain with the strongest loss in cells surrounding hypoxic tumor areas as well as in most glioma cell lines and primary glioma cells. In our cohort of glioma patients, loss of CPE predominantly occurred in glioblastomas and was associated with worse prognosis. In glioma cells, CPE overexpression was significantly reduced, whereas knockdown or inhibition enhanced glioma cell migration and invasion. The decreased migratory potential following CPE overexpression was paralleled by altered cellular morphology, promoting a transition to focal adhesions and associated stress fibers. In contrast to the decreased migration, high CPE levels were associated with higher proliferative rates. As microenvironmental regulation cues, we identified CPE as being downregulated upon hypoxia or glucose deprivation. Our findings indicate an oxygen- and nutrition-dependent anti-migratory, but pro-proliferative role of CPE in gliomas with prognostic impact for patient survival, thereby contributing to the understanding of the "go or grow" hypothesis in gliomas.

Mantle cell lymphoma (MCL) is a mature B-cell lymphoma characterized by poor clinical outcome. Recent studies revealed the importance of B-cell receptor (BCR) signaling in maintaining MCL survival. However, it remains unclear which role MALT1, an essential component of the CARD11-BCL10-MALT1 complex that links BCR signaling to the NF-κB pathway, plays in the biology of MCL. Here we show that a subset of MCLs is addicted to MALT1, as its inhibition by either RNA or pharmacologic interference induced cytotoxicity both in vitro and in vivo. Gene expression profiling following MALT1 inhibition demonstrated that MALT1 controls an MYC-driven gene expression network predominantly through increasing MYC protein stability. Thus, our analyses identify a previously unappreciated regulatory mechanism of MYC expression. Investigating primary mouse splenocytes, we could demonstrate that MALT1-induced MYC regulation is not restricted to MCL, but represents a common mechanism. MYC itself is pivotal for MCL survival because its downregulation and pharmacologic inhibition induced cytotoxicity in all MCL models. Collectively, these results provide a strong mechanistic rationale to investigate the therapeutic efficacy of targeting the MALT1-MYC axis in MCL patients.

Mantle cell lymphoma (MCL) is a mature B-cell lymphoma characterized by poor prognosis. Recent studies revealed the importance of constitutive B-cell receptor (BCR) signaling in maintaining survival of a subset of MCLs. MALT1 is an essential component of the CARD11-BCL10-MALT1 (CBM) complex that links BCR signaling to the nuclear factor kappa-B (NF-κB) pathway. Additionally, MALT1 functions as a protease that cleaves various substrates to promote proliferation and survival. However, its role in the molecular pathogenesis of MCL is unknown. To elucidate the functional role of MALT1 in the biology of MCL, we determined its proteolytic activity in primary MCL cells and in MCL derived cell lines. A large fraction of MCLs displayed constitutive activity of MALT1. This MALT1 activity is driven by constitutive BCR signaling, as we were able to show that RNA interference-mediated knockdown of central components of the BCR cascade abolished MALT1 activity. To gain insights into the functional significance of MALT1 in MCL, we knocked down its expression by different MALT1 shRNAs. Transduction of these shRNAs induced cytotoxicity in models that are characterized by constitutive MALT1 activity, whereas no effect on survival was observable in MCLs without MALT1 activation. To determine if this MALT1 addiction translates into an in vivosetting, we knocked down MALT1 in mouse MCL models and detected a significant inhibition of tumor growth. This indicates that MALT1-activated MCLs are dependent on the function of MALT1. These results were confirmed as pharmacologic inhibition of MALT1 significantly reduced cell viability in MALT1-activated MCLs, implying that MALT1 inhibition might represent a promising therapeutic strategy for MCL patients. To understand which biologic processes are regulated by MALT1 in MCL, we profiled gene expression changes at different time points following MALT1 inhibition. An unbiased gene set enrichment analysis identified various previously described MYC gene expression signatures to be among the top downregulated signatures, suggesting that MALT1 regulates MYC expression. MYC downregulation following MALT1 inhibition or MALT1 knockdown was confirmed at the protein level and various analyses revealed that MALT1 regulates MYC expression posttranslationally by preventing its proteasomal degradation. These results were further confirmed in primary mouse splenocytes, indicating that this novel molecular mechanism of regulating MYC expression is not restricted to MCL. To confirm that MYC is indeed expressed in primary MCLs, we determined MYC expression in 234 primary MCL samples by immunohistochemistry. These analyses revealed that 75 samples (32.1%) displayed an intermediate and 55 samples (23.5%) a high MYC positivity, suggesting that MYC is expressed in a substantial number of primary MCLs. As common alterations such as MYC high-levelamplifications and translocations determined by FISH occurred extremely rarely in our primary MCL samples (1% of samples), it is conceivable that BCR-driven MALT1 signaling is the predominant mechanism of MYC upregulation in MCL. In summary, we report for the first time that a substantial fraction of MCLs is addicted to constitutive MALT1 signaling. Thus, MCLs can be divided based on their MALT1 activation status into two distinct subgroups. We further identified a novel regulatory mechanism of MYC expression by MALT1. Thus, our study provides a strong mechanistic rationale to investigate the therapeutic efficacy in targeting the MALT1-MYC axis in MCL patients. Disclosures Trneny: Janssen Research & Development: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Gilead: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees. Dreyling:Roche: Consultancy, Honoraria, Research Funding, Speakers Bureau. Tzankov:Novartis: Speakers Bureau; Abbott: Speakers Bureau.