Adaptive Transcriptional Responses by CRTC Coactivators in Cancer.

Adaptive stress signaling networks directly influence tumor development and progression. These pathways mediate responses that allow cancer cells to cope with both tumor cell-intrinsic and cell-extrinsic insults and develop acquired resistance to therapeutic interventions. This is mediated in part by constant oncogenic rewiring at the transcriptional level by integration of extracellular cues that promote cell survival and malignant transformation. The cAMP-regulated transcriptional coactivators (CRTCs) are a newly discovered family of intracellular signaling integrators that serve as the conduit to the basic transcriptional machinery to regulate a host of adaptive response genes. Thus, somatic alterations that lead to CRTC activation are emerging as key driver events in the development and progression of many tumor subtypes.

The CREB coactivator CRTC2 promotes oncogenesis in LKB1-mutant non-small cell lung cancer.

The LKB1 tumor suppressor is often mutationally inactivated in non-small cell lung cancer (NSCLC). LKB1 phosphorylates and activates members of the AMPK family of Ser/Thr kinases. Within this family, the salt-inducible kinases (SIKs) modulate gene expression in part via the inhibitory phosphorylation of the CRTCs, coactivators for CREB (cAMP response element-binding protein). The loss of LKB1 causes SIK inactivation and the induction of the CRTCs, leading to the up-regulation of CREB target genes. We identified CRTC2 as a critical factor in LKB1-deficient NSCLC. CRTC2 is unphosphorylated and therefore constitutively activated in LKB1-mutant NSCLC, where it promotes tumor growth, in part via the induction of the inhibitor of DNA binding 1 (ID1), a bona fide CREB target gene. As ID1 expression is up-regulated and confers poor prognosis in LKB1-deficient NSCLC, our results suggest that small molecules that inhibit CRTC2 and ID1 activity may provide therapeutic benefit to individuals with NSCLC.

USP15 regulates SMURF2 kinetics through C-lobe mediated deubiquitination.

Ubiquitin modification of the TGF-ß pathway components is emerging as a key mechanism of TGF-ß pathway regulation. To limit TGF-ß responses, TGF-ß signaling is regulated through a negative feedback loop whereby the E3 ligase SMURF2 targets the TGF-ß receptor (TßR) complex for ubiquitin-mediated degradation. Counteracting this process, a number of deubiquitinating (DUBs) enzymes have recently been identified that deubiquitinate and stabilize the TßR. However the precise mechanism by which these DUBs act on TßR function remains poorly defined. Here, we demonstrate that apart from targeting the TßR complex directly, USP15 also deubiquitinates SMURF2 resulting in enhanced TßR stability and downstream pathway activation. Through proteomic analysis, we show that USP15 modulates the ubiquitination of Lys734, a residue required for SMURF2 catalytic activity. Our results show that SMURF2 is a critical target of USP15 in the TGF-ß pathway and may also explain how USP15 and SMURF2 target multiple complementary protein complexes in other pathways.

Active CREB1 promotes a malignant TGFß2 autocrine loop in glioblastoma.

UNLABELLED: In advanced cancer, including glioblastoma, the TGFß pathway acts as an oncogenic factor. Some tumors exhibit aberrantly high TGFß activity, and the mechanisms underlying this phenomenon are not well understood. We have observed that TGFß can induce TGFß2, generating an autocrine loop leading to aberrantly high levels of TGFß2. We identified cAMP-responsive element-binding protein 1 (CREB1) as the critical mediator of the induction of TGFß2 by TGFß. CREB1 binds to the TGFB2 gene promoter in cooperation with SMAD3 and is required for TGFß to activate transcription. Moreover, the PI3K-AKT and RSK pathways regulate the TGFß2 autocrine loop through CREB1. The levels of CREB1 and active phosphorylated CREB1 correlate with TGFß2 in glioblastoma. In addition, using patient-derived in vivo models of glioblastoma, we found that CREB1 levels determine the expression of TGFß2. Our results show that CREB1 can be considered a biomarker to stratify patients for anti-TGFß treatments and a therapeutic target in glioblastoma. SIGNIFICANCE: TGFß is considered a promising therapeutic target, and several clinical trials using TGFß inhibitors are generating encouraging results. Here, we discerned the molecular mechanisms responsible for the aberrantly high levels of TGFß2 found in certain tumors, and we propose biomarkers to predict the clinical response to anti-TGFß therapies.

USP15 stabilizes TGF-ß receptor I and promotes oncogenesis through the activation of TGF-ß signaling in glioblastoma.

In advanced cancer, including glioblastoma, the transforming growth factor ß (TGF-ß) pathway acts as an oncogenic factor and is considered to be a therapeutic target. Using a functional RNAi screen, we identified the deubiquitinating enzyme ubiquitin-specific peptidase 15 (USP15) as a key component of the TGF-ß signaling pathway. USP15 binds to the SMAD7-SMAD specific E3 ubiquitin protein ligase 2 (SMURF2) complex and deubiquitinates and stabilizes type I TGF-ß receptor (TßR-I), leading to an enhanced TGF-ß signal. High expression of USP15 correlates with high TGF-ß activity, and the USP15 gene is found amplified in glioblastoma, breast and ovarian cancer. USP15 amplification confers poor prognosis in individuals with glioblastoma. Downregulation or inhibition of USP15 in a patient-derived orthotopic mouse model of glioblastoma decreases TGF-ß activity. Moreover, depletion of USP15 decreases the oncogenic capacity of patient-derived glioma-initiating cells due to the repression of TGF-ß signaling. Our results show that USP15 regulates the TGF-ß pathway and is a key factor in glioblastoma pathogenesis.

TGF-ß Receptor Inhibitors Target the CD44(high)/Id1(high) Glioma-Initiating Cell Population in Human Glioblastoma.

Glioma-initiating cells (GICs), also called glioma stem cells, are responsible for tumor initiation, relapse, and therapeutic resistance. Here, we show that TGF-ß inhibitors, currently under clinical development, target the GIC compartment in human glioblastoma (GBM) patients. Using patient-derived specimens, we have determined the gene responses to TGF-ß inhibition, which include inhibitors of DNA-binding protein (Id)-1 and -3 transcription factors. We have identified a cell population enriched for GICs that expresses high levels of CD44 and Id1 and tend to be located in a perivascular niche. The inhibition of the TGF-ß pathway decreases the CD44(high)/Id1(high) GIC population through the repression of Id1 and Id3 levels, therefore inhibiting the capacity of cells to initiate tumors. High CD44 and Id1 levels confer poor prognosis in GBM patients.