The SWI/SNF Protein PBRM1 Restrains VHL-Loss-Driven Clear Cell Renal Cell Carcinoma.

PBRM1 is the second most commonly mutated gene after VHL in clear cell renal cell carcinoma (ccRCC). However, the biological consequences of PBRM1 mutations for kidney tumorigenesis are unknown. Here, we find that kidney-specific deletion of Vhl and Pbrm1, but not either gene alone, results in bilateral, multifocal, transplantable clear cell kidney cancers. PBRM1 loss amplified the transcriptional outputs of HIF1 and STAT3 incurred by Vhl deficiency. Analysis of mouse and human ccRCC revealed convergence on mTOR activation, representing the third driver event after genetic inactivation of VHL and PBRM1. Our study reports a physiological preclinical ccRCC mouse model that recapitulates somatic mutations in human ccRCC and provides mechanistic and therapeutic insights into PBRM1 mutated subtypes of human ccRCC.

Mechanistically distinct cancer-associated mTOR activation clusters predict sensitivity to rapamycin.

Genomic studies have linked mTORC1 pathway-activating mutations with exceptional response to treatment with allosteric inhibitors of mTORC1 called rapalogs. Rapalogs are approved for selected cancer types, including kidney and breast cancers. Here, we used sequencing data from 22 human kidney cancer cases to identify the activating mechanisms conferred by mTOR mutations observed in human cancers and advance precision therapeutics. mTOR mutations that clustered in focal adhesion kinase targeting domain (FAT) and kinase domains enhanced mTORC1 kinase activity, decreased nutrient reliance, and increased cell size. We identified 3 distinct mechanisms of hyperactivation, including reduced binding to DEP domain-containing MTOR-interacting protein (DEPTOR), resistance to regulatory associated protein of mTOR-mediated (RAPTOR-mediated) suppression, and altered kinase kinetics. Of the 28 mTOR double mutants, activating mutations could be divided into 6 complementation groups, resulting in synergistic Rag- and Ras homolog enriched in brain-independent (RHEB-independent) mTORC1 activation. mTOR mutants were resistant to DNA damage-inducible transcript 1-mediated (REDD1-mediated) inhibition, confirming that activating mutations can bypass the negative feedback pathway formed between HIF1 and mTORC1 in the absence of von Hippel-Lindau (VHL) tumor suppressor expression. Moreover, VHL-deficient cells that expressed activating mTOR mutants grew tumors that were sensitive to rapamycin treatment. These data may explain the high incidence of mTOR mutations observed in clear cell kidney cancer, where VHL loss and HIF activation is pathognomonic. Our study provides mechanistic and therapeutic insights concerning mTOR mutations in human diseases.

Proteasome inhibitors evoke latent tumor suppression programs in pro-B MLL leukemias through MLL-AF4.

Chromosomal translocations disrupting MLL generate MLL-fusion proteins that induce aggressive leukemias. Unexpectedly, MLL-fusion proteins are rarely observed at high levels, suggesting excessive MLL-fusions may be incompatible with a malignant phenotype. Here, we used clinical proteasome inhibitors, bortezomib and carfilzomib, to reduce the turnover of endogenous MLL-fusions and discovered that accumulated MLL-fusions induce latent, context-dependent tumor suppression programs. Specifically, in MLL pro-B lymphoid, but not myeloid, leukemias, proteasome inhibition triggers apoptosis and cell cycle arrest involving activation cleavage of BID by caspase-8 and upregulation of p27, respectively. Furthermore, proteasome inhibition conferred preliminary benefit to patients with MLL-AF4 leukemia. Hence, feasible strategies to treat cancer-type and oncogene-specific cancers can be improvised through harnessing inherent tumor suppression properties of individual oncogenic fusions.

Tumor genetic analyses of patients with metastatic renal cell carcinoma and extended benefit from mTOR inhibitor therapy.

PURPOSE: Rapalogs are allosteric mTOR inhibitors and approved agents for advanced kidney cancer. Reports of clonal heterogeneity in this disease challenge the concept of targeted monotherapy, yet a small subset of patients derives extended benefit. Our aim was to analyze such outliers and explore the genomic background of extreme rapalog sensitivity in the context of intratumor heterogeneity. EXPERIMENTAL DESIGN: We analyzed archived tumor tissue of 5 patients with renal cell carcinoma, who previously achieved durable disease control with rapalogs (median duration, 28 months). DNA was extracted from spatially separate areas of primary tumors and metastases. Custom target capture and ultradeep sequencing was used to identify alterations across 230 target genes. Whole-exome sequence analysis was added to investigate genes beyond this original target list. RESULTS: Five long-term responders contributed 14 specimens to explore clonal heterogeneity. Genomic alterations with activating effect on mTOR signaling were detected in 11 of 14 specimens, offering plausible explanation for exceptional treatment response through alterations in two genes (TSC1 and MTOR). In two subjects, distinct yet functionally convergent alterations activated the mTOR pathway in spatially separate sites. In 1 patient, concurrent genomic events occurred in two separate pathway components across different tumor regions. CONCLUSIONS: Analysis of outlier cases can facilitate identification of potential biomarkers for targeted agents, and we implicate two genes as candidates for further study in this class of drugs. The previously reported phenomenon of clonal convergence can occur within a targetable pathway which might have implications for biomarker development beyond this disease and this class of agents.

A clear picture of renal cell carcinoma.

Two recent studies describe the largest molecular profiling analyses to date of clear-cell renal cell carcinoma (ccRCC) and report remarkably similar findings. The recurrent pathway alterations identified in these studies open new avenues for therapeutic advances in this chemotherapy- and radiation-resistant disease.

Upregulation of Twist-1 by NF-kappaB blocks cytotoxicity induced by chemotherapeutic drugs.

NF-kappaB/Rel transcription factors are central to controlling programmed cell death (PCD). Activation of NF-kappaB blocks PCD induced by numerous triggers, including ligand engagement of tumor necrosis factor receptor (TNF-R) family receptors. The protective activity of NF-kappaB is also crucial for oncogenesis and cancer chemoresistance. Downstream of TNF-Rs, this activity of NF-kappaB has been linked to the suppression of reactive oxygen species and the c-Jun-N-terminal-kinase (JNK) cascade. The mechanism by which NF-kappaB inhibits PCD triggered by chemotherapeutic drugs, however, remains poorly understood. To understand this mechanism, we sought to identify unrecognized protective genes that are regulated by NF-kappaB. Using an unbiased screen, we identified the basic-helix-loop-helix factor Twist-1 as a new mediator of the protective function of NF-kappaB. Twist-1 is an evolutionarily conserved target of NF-kappaB, blocks PCD induced by chemotherapeutic drugs and TNF-alpha in NF-kappaB-deficient cells, and is essential to counter this PCD in cancer cells. The protective activity of Twist-1 seemingly halts PCD independently of interference with cytotoxic JNK, p53, and p19(ARF) signaling, suggesting that it mediates a novel protective mechanism activated by NF-kappaB. Indeed, our data indicate that this activity involves a control of inhibitory Bcl-2 phosphorylation. The data also suggest that Twist-1 and -2 play an important role in NF-kappaB-dependent chemoresistance.

A method for isolating prosurvival targets of NF-kappaB/Rel transcription factors.

NF-KappaB/Rel transcription factors are critical regulators of immunity, inflammation, development, and cell survival. Activation of NF-KB inhibits programmed cell death (PCD) triggered by tumor necrosis factor alpha (TNFalpha) and several other stimuli. The prosurvival activity of NF-KB is also crucial to lymphopoiesis, neuroprotection, tumorigenesis, and cancer chemoresistance. The characterization of the downstream targets that mediate the prosurvival activity of NF-KB is therefore a topic of intense investigation. Early screens aimed at identifying these genes were mainly based on expression criteria and so were poised to only isolate genes already known to have protective effects. Here, we describe a new method for the identification of these genes, whereby expression libraries are screened for their ability to halt PCD in NF-KB-deficient cells. This complementation approach provides substantial advantages over other approaches, as it enables functional assessment of isolated genes without any preconceived notion about their sequence or presumed role. Expression libraries are generated from cells that are resistant to TNFalpha-induced cytotoxicity and are then enriched in prosurvival genes upon selection with TNFa in NF-kappaB/RelA-null cells, which are highly susceptible instead to this cytotoxicity. Upon enrichment, libraries are screened through a randomized two-step approach, whereby cDNAs are first tested for cytoprotective function and then for differential expression in NF-kappaB-proficient and NF-KappaB-deficient cells.

Oxygen JNKies: phosphatases overdose on ROS.

Proinflammatory cytokine TNFalpha triggers cell death by inducing reactive oxygen species (ROS). These then inflict cytotoxicity through downstream activation of the JNK MAPK cascade. Yet the mechanisms by which ROS trigger JNK signaling have remained elusive. In a recent issue of Cell, Kamata et al. now provide one such mechanism.

In the Crosshairs: NF-κB Targets the JNK Signaling Cascade.

NF-κB/Rel transcription factors are well-known for their roles in the regulation of inflammation and immunity. NF-κB also blocks programmed cell death (PCD) or apoptosis triggered by proinflammatory cytokine, tumor necrosis factor (TNF)α. Through transcriptional induction of distinct subsets of cyto-protective target genes, NF-κB inhibits the execution of apoptosis activated by this cytokine. This protective action is mediated, in part, by factors (such as A20, GADD45ß, and XIAP) that downregulate the pro-apoptotic c-Jun-N-terminal (JNK) pathway. A suppression of reactive oxygen species (ROS), which are themselves major cell death-inducing elements activated by TNFα, is an additional protective function recently ascribed to NF-κB. This function of NF-κB involves an induction of mitochondrial anti-oxidant enzyme, manganese superoxide dismutase (Mn-SOD), and a control of cellular iron availability through upregulation of Ferritin heavy chain - one of two subunits of Ferritin, the major iron storage protein complex of the cell. An emerging view of NF-κB is that, while integrated, its actions in immunity and in promoting cell survival are executed through upregulation of distinct subsets of target genes. Thus, these inducible blockers of apoptosis may provide potential new targets to inhibit specific functions of NF-κB. In the future, this might allow for a better treatment of complex human diseases involving dysregulated NF-κB activity, including chronic inflammatory conditions and cancer.

NF-kappaB and JNK: an intricate affair.

NF-kappaB/Rel transcription factors block apoptosis or programmed cell death (PCD) induced by tumor necrosis factor (TNF) alpha. The antiapoptotic activity of NF-kappaB is also crucial for immunity, lymphocyte development, tumorigenesis, and cancer chemoresistance. With respect to TNFalpha, the NF-kappaB-mediated suppression of apoptosis involves inhibition of the c-Jun-N-terminal kinase (JNK) cascade. This inhibitory activity of NF-kappaB depends upon transcriptional upregulation of blockers of the JNK cascade such as the caspase inhibitor XIAP, the zinc-finger protein A20, and the inhibitor of the MKK7/JNKK2 kinase Gadd45beta/Myd118. Moreover, NF-kappaB blunts accumulation of reactive oxygen species (ROS) induced by TNFalpha, and this antioxidant effect of NF-kappaB is also critical for inhibition of TNFalpha-induced JNK activation. Suppression of ROS by NF-kappaB is mediated by Ferritin heavy chain (FHC)--the primary iron-storage mechanism in cells--and possibly, by the mitochondrial enzyme Mn++ superoxide dismutase (Mn-SOD). Thus, induction of FHC and Mn-SOD represents an additional, indirect means by which NF-kappaB controls proapoptotic JNK signaling. These findings identify potential new targets for anti-inflammatory and anti-cancer therapy.

Ferritin heavy chain upregulation by NF-kappaB inhibits TNFalpha-induced apoptosis by suppressing reactive oxygen species.

During inflammation, NF-kappaB transcription factors antagonize apoptosis induced by tumor necrosis factor (TNF)alpha. This antiapoptotic activity of NF-kappaB 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-kappaB 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-kappaB. FHC is induced downstream of NF-kappaB and is required to prevent sustained JNK activation and, thereby, apoptosis triggered by TNFalpha. 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-kappaB-mediated control of ROS induction and identify a mechanism by which NF-kappaB suppresses proapoptotic JNK signaling. Our results suggest modulation of FHC or, more broadly, of iron metabolism as a potential approach for anti-inflammatory therapy.

Linking JNK signaling to NF-kappaB: a key to survival.

In addition to marshalling immune and inflammatory responses, transcription factors of the NF-kappaB family control cell survival. This control is crucial to a wide range of biological processes, including B and T lymphopoiesis, adaptive immunity, oncogenesis and cancer chemoresistance. During an inflammatory response, NF-kappaB activation antagonizes apoptosis induced by tumor necrosis factor (TNF)-alpha, a protective activity that involves suppression of the Jun N-terminal kinase (JNK) cascade. This suppression can involve upregulation of the Gadd45-family member Gadd45beta/Myd118, which associates with the JNK kinase MKK7/JNKK2 and blocks its catalytic activity. Upregulation of XIAP, A20 and blockers of reactive oxygen species (ROS) appear to be important additional means by which NF-kappaB blunts JNK signaling. These recent findings might open up entirely new avenues for therapeutic intervention in chronic inflammatory diseases and certain cancers; indeed, the Gadd45beta-MKK7 interaction might be a key target for such intervention.

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

NF-kappa B/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 alpha (TNFalpha)-induced killing. With regard to TNFalpha, the anti-apoptotic activity of NF-kappa B involves suppression of the c-Jun N-terminal kinase (JNK) cascade. Using an unbiased screen, we have previously identified Gadd45 beta/Myd118, a member of the Gadd45 family of inducible factors, as a pivotal mediator of this suppressive activity of NF-kappa B. However, the mechanisms by which Gadd45 beta inhibits JNK signalling are not understood. Here, we identify MKK7/JNKK2--a specific and essential activator of JNK--as a target of Gadd45 beta, and in fact, of NF-kappa B itself. Gadd45 beta binds to MKK7 directly and blocks its catalytic activity, thereby providing a molecular link between the NF-kappa B and JNK pathways. Importantly, Gadd45 beta is required to antagonize TNFalpha-induced cytotoxicity, and peptides disrupting the Gadd45 beta/MKK7 interaction hinder the ability of Gadd45 beta, as well as of NF-kappa B, to suppress this cytotoxicity. These findings establish a basis for the NF-kappa B control of JNK activation and identify MKK7 as a potential target for anti-inflammatory and anti-cancer therapy.

Modulation of integrins and integrin signaling molecules in the pressure-loaded murine ventricle.

Integrins are heterodimeric cell-surface receptors that link the extracellular matrix and the intracellular cytoskeleton and function as mechanotransducers. Signaling through integrins is important for cell growth, migration, and survival. Extracellular matrix is altered in the myocardium during hypertrophic induction and the transition to heart failure. The role of integrins in this process is poorly understood. Recently, integrin subunits have been identified that are dominantly expressed in striated muscle. We tested the hypothesis that since integrins are mechanotransducers, their expression and signaling would be modulated with murine left ventricular hemodynamic loading. The acute and chronic effects of pressure overload on changes in the expression of integrins, as well as related integrin-mediated signaling events were studied. Acute pressure loading increased phosphorylation of focal adhesion kinase, p42 and p44 extracellular signal-regulated kinase. Chronic loading: (1) increased expression of alpha1, alpha5, and beta1 integrin transcripts and (2) increased protein expression of integrin subunits which are dominantly expressed in striated muscle (alpha7 and beta1D) both by western blotting and immunofluorescent microscopy. These results show that adaptive responses of the myocardium to pressure overload include acute modulation of integrin-related signaling molecules and more chronic changes effect expression of integrin subunits, including ones dominantly expressed in muscle.