Don Benjamin

Rapamycin passes the torch: a new generation of mTOR inhibitors

Mammalian target of rapamycin (mTOR) is an atypical protein kinase that controls growth and metabolism in response to nutrients, growth factors and cellular energy levels, and it is frequently dysregulated in cancer and metabolic disorders. Rapamycin is an allosteric inhibitor of mTOR, and was approved as an immuno-suppressant in 1999. In recent years, interest has focused on its potential as an anticancer drug. However, the performance of rapamycin and its analogues (rapalogues) has been undistinguished despite isolated successes in subsets of cancer, suggesting that the full therapeutic potential of targeting mTOR has yet to be exploited. A new generation of ATP-competitive inhibitors that directly target the mTOR catalytic site display potent and comprehensive mTOR inhibition and are in early clinical trials.

BRF1 Protein Turnover and mRNA Decay Activity Are Regulated by Protein Kinase B at the Same Phosphorylation Sites

ABSTRACT BRF1 posttranscriptionally regulates mRNA levels by targeting ARE-bearing transcripts to the decay machinery. We previously showed that protein kinase B (PKB) phosphorylates BRF1 at Ser92, resulting in binding to 14-3-3 and impairment of mRNA decay activity. Here we identify an additional regulatory site at Ser203 that cooperates in vivo with Ser92. In vitro kinase labeling and wortmannin sensitivity indicate that Ser203 phosphorylation is also performed by PKB. Mutation of both serines to alanine uncouples BRF1 from PKB regulation, leading to constitutive mRNA decay even in the presence of stabilizing signals. BRF1 protein is labile because of proteasomal degradation (half-life, <3 h) but becomes stabilized upon phosphorylation and is less stable in PKBα−/− cells. Surprisingly, phosphorylation-dependent protein stability is also regulated by Ser92 and Ser203, with parallel phosphorylation required at these sites. Phosphorylation-dependent binding to 14-3-3 is abolished only when both sites are mutated. Cell compartment fractionation experiments support a model in which binding to 14-3-3 sequesters BRF1 through relocalization and prevents it from executing its mRNA decay activity, as well as from proteasomal degradation, thereby maintaining high BRF1 protein levels that are required to reinstate decay upon dissipation of the stabilizing signal.

mRNA stability and cancer: an emerging link?

Many oncogenes, growth factor, cytokine and cell-cycle genes are regulated post-transcriptionally. The major mechanism is by controlling the rate of mRNA turnover for transcripts bearing destabilising cis-elements. To date, only a handful of regulatory factors have been identified that appear to control a large pool of target mRNAs, suggesting that a slight perturbation in the control mechanism may generate wide-ranging effects that could contribute to the development of a complex disorder such as cancer. In support of this view, mRNA turnover responds to signalling pathways that are often overactive in cancer, suggesting a post-transcriptional component in addition to the well-recognised transcriptional aspect of oncogenesis. Here the authors review examples of deregulated post-transcriptional control in oncogenesis, discuss post-transcriptionally regulated transcripts of oncologic significance, and consider the key role of signalling pathways in linking both processes and as an enticing therapeutic prospect.

Genome-wide shRNA screen reveals increased mitochondrial dependence upon mTORC2 addiction

Release from growth factor dependence and acquisition of signalling pathway addiction are critical steps in oncogenesis. To identify genes required on mammalian target of rapamycin (mTOR) addiction, we performed a genome-wide short hairpin RNA screen on a v-H-ras-transformed Pten-deficient cell line that displayed two alternative growth modes, interleukin (IL)-3-independent/mTOR-addicted proliferation (transformed growth mode) and IL-3-dependent/mTOR-non-addicted proliferation (normal growth mode). We screened for genes required only in the absence of IL-3 and thus specifically for the transformed growth mode. The top 800 hits from this conditional lethal screen were analyzed in silico and 235 hits were subsequently rescreened in two additional Pten-deficient cell lines to generate a core set of 47 genes. Hits included genes encoding mTOR and the mTOR complex 2 (mTORC2) component rictor and several genes encoding mitochondrial functions including components of the respiratory chain, adenosine triphosphate synthase, the mitochondrial ribosome and mitochondrial fission factor. Small interfering RNA knockdown against a sizeable fraction of these genes triggered apoptosis in human cancer cell lines but not in normal fibroblasts. We conclude that mTORC2-addicted cells require mitochondrial functions that may be novel drug targets in human cancer.

TSC on the peroxisome controls mTORC1.

mTOR is a central controller that integrates many inputs to regulate cell growth and ensure cellular homeostasis. The mTORC1 inhibitor TSC (tuberous sclerosis complex) on the peroxisome is found to inhibit mTORC1 in response to endogenous reactive oxygen species. Thus, mTOR may avoid confounding different inputs by sensing them at different cellular locations.

Syrosingopine sensitizes cancer cells to killing by metformin

Synthetic lethality between the clinically approved noncancer drugs metformin and syrosingopine specifically kills cancer cells. We report that the anticancer activity of the widely used diabetic drug metformin is strongly potentiated by syrosingopine. Synthetic lethality elicited by combining the two drugs is synergistic and specific to transformed cells. This effect is unrelated to syrosingopine’s known role as an inhibitor of the vesicular monoamine transporters. Syrosingopine binds to the glycolytic enzyme α-enolase in vitro, and the expression of the γ-enolase isoform correlates with nonresponsiveness to the drug combination. Syrosingopine sensitized cancer cells to metformin and its more potent derivative phenformin far below the individual toxic threshold of each compound. Thus, combining syrosingopine and codrugs is a promising therapeutic strategy for clinical application for the treatment of cancer.

Abstract 2197: A robotic genome-scale shRNA screen identifies metabolic drug targets and a glioblastoma gene signature

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Proteins encoded by oncogenes represent suitable drug targets and several have been validated clinically. However, certain non-oncogenes, which are downstream effectors of oncogenes, may be similarly useful as potential targets of therapy. We have developed a high-throughput strategy using a shRNA genomic library and tumor cells addicted to the mTOR pathway to identify genes (oncogenes or non-oncogenes) which control the oncogenic phenotype. mTOR addicted cells were generated from IL-3 dependent cells following frame-shift mutagenesis and selection for IL-3 independence. mTOR addicted cells obtained from this selection carried frame-shift mutations in the PTEN gene. mTOR addiction was revealed by sensitivity to rapamycin and confirmed biochemically by examining phosphorylation of the mTOR targets S6K and PKB. Interestingly (and of importance to our shRNA-based screening strategy), addition of IL-3 rescued cells from mTOR addiction, as this growth factor antagonized the apoptotic effect of rapamycin. Using robotics (BioMek-NX) and a genomic retroviral shRNA library we evaluated 14’000 genes for a potential role in maintaining mTOR addicted growth. Our strategy was to screen cells in the presence and absence of IL-3 to identify shRNAs that antagonize growth of cells in the absence of IL-3. We identified around 300 genes required for mTOR addicted growth. This set of genes was enriched in highly significant manner for genes known to be involved in cancer, providing proof of concept for the strategy. Examples of identified genes include ras, raf, PKB and mTOR. Also, genes with no cancer link and genes with unknown function were identified. Use of Ingenuity software allowed to place many of the identified genes into functionally distinct groups or pathways. In addition numerous hits affecting metabolic functions, redox functions, and ROS generating systems were identified including the cytoplasmic NADPH complex and complex I-IV of the respiratory chain. Combining the genes et with an Affymetrix gene overexpression analysis in the same cells and analysing by Oncomine software, we identified a glioblastoma gene signature consisting of 13 genes (p-value 2.4E-5) containing 3 oncogenes and several signaling elements. Downregulation of several of these genes in human cancer lines induced apoptosis. Some of he genes identified represent novel potential drug targets. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2197.