Zotatifin, an eIF4A-Selective Inhibitor, Blocks Tumor Growth in Receptor Tyrosine Kinase Driven Tumors

Oncoprotein expression is controlled at the level of mRNA translation and is regulated by the eukaryotic translation initiation factor 4F (eIF4F) complex. eIF4A, a component of eIF4F, catalyzes the unwinding of secondary structure in the 5'-untranslated region (5'-UTR) of mRNA to facilitate ribosome scanning and translation initiation. Zotatifin (eFT226) is a selective eIF4A inhibitor that increases the affinity between eIF4A and specific polypurine sequence motifs and has been reported to inhibit translation of driver oncogenes in models of lymphoma. Here we report the identification of zotatifin binding motifs in the 5'-UTRs of HER2 and FGFR1/2 Receptor Tyrosine Kinases (RTKs). Dysregulation of HER2 or FGFR1/2 in human cancers leads to activation of the PI3K/AKT and RAS/ERK signaling pathways, thus enhancing eIF4A activity and promoting the translation of select oncogenes that are required for tumor cell growth and survival. In solid tumor models driven by alterations in HER2 or FGFR1/2, downregulation of oncoprotein expression by zotatifin induces sustained pathway-dependent anti-tumor activity resulting in potent inhibition of cell proliferation, induction of apoptosis, and significant in vivo tumor growth inhibition or regression. Sensitivity of RTK-driven tumor models to zotatifin correlated with high basal levels of mTOR activity and elevated translational capacity highlighting the unique circuitry generated by the RTK-driven signaling pathway. This dependency identifies the potential for rational combination strategies aimed at vertical inhibition of the PI3K/AKT/eIF4F pathway. Combination of zotatifin with PI3K or AKT inhibitors was beneficial across RTK-driven cancer models by blocking RTK-driven resistance mechanisms demonstrating the clinical potential of these combination strategies.

mRNA translation is a therapeutic vulnerability necessary for bladder epithelial transformation

Using genetically engineered mouse models, this work demonstrates that protein synthesis is essential for efficient urothelial cancer formation and growth but dispensable for bladder homeostasis. Through a candidate gene analysis for translation regulators implicated in this dependency, we discovered that phosphorylation of the translation initiation factor eIF4E at serine 209 is increased in both murine and human bladder cancer, and this phosphorylation corresponds with an increase in de novo protein synthesis. Employing an eIF4E serine 209 to alanine knock-in mutant mouse model, we show that this single posttranslational modification is critical for bladder cancer initiation and progression, despite having no impact on normal bladder tissue maintenance. Using murine and human models of advanced bladder cancer, we demonstrate that only tumors with high levels of eIF4E phosphorylation are therapeutically vulnerable to eFT508, the first clinical-grade inhibitor of MNK1 and MNK2, the upstream kinases of eIF4E. Our results show that phospho-eIF4E plays an important role in bladder cancer pathogenesis, and targeting its upstream kinases could be an effective therapeutic option for bladder cancer patients with high levels of eIF4E phosphorylation.

Revealing molecular pathways for cancer cell fitness through a genetic screen of the cancer translatome

The major cap-binding protein eukaryotic translation initiation factor 4E (eIF4E), an ancient protein required for translation of all eukaryotic genomes, is a surprising yet potent oncogenic driver. The genetic interactions that maintain the oncogenic activity of this key translation factor remain unknown. In this study, we carry out a genome-wide CRISPRi screen wherein we identify more than 600 genetic interactions that sustain eIF4E oncogenic activity. Our data show that eIF4E controls the translation of Tfeb, a key executer of the autophagy response. This autophagy survival response is triggered by mitochondrial proteotoxic stress, which allows cancer cell survival. Our screen also reveals a functional interaction between eIF4E and a single anti-apoptotic factor, Bcl-xL, in tumor growth. Furthermore, we show that eIF4E and the exon-junction complex (EJC), which is involved in many steps of RNA metabolism, interact to control the migratory properties of cancer cells. Overall, we uncover several cancer-specific vulnerabilities that provide further resolution of the cancer translatome.

Targeting Oncogene mRNA Translation in B-Cell Malignancies with eFT226, a Potent and Selective Inhibitor of eIF4A

The PI3K/AKT/mTOR pathway is often activated in lymphoma through alterations in PI3K, PTEN, and B-cell receptor signaling, leading to dysregulation of eIF4A (through its regulators, eIF4B, eIF4G, and PDCD4) and the eIF4F complex. Activation of eIF4F has a direct role in tumorigenesis due to increased synthesis of oncogenes that are dependent on enhanced eIF4A RNA helicase activity for translation. eFT226, which inhibits translation of specific mRNAs by promoting eIF4A1 binding to 5'-untranslated regions (UTR) containing polypurine and/or G-quadruplex recognition motifs, shows potent antiproliferative activity and significant in vivo efficacy against a panel of diffuse large B-cell lymphoma (DLBCL), and Burkitt lymphoma models with ≤1 mg/kg/week intravenous administration. Evaluation of predictive markers of sensitivity or resistance has shown that activation of eIF4A, mediated by mTOR signaling, correlated with eFT226 sensitivity in in vivo xenograft models. Mutation of PTEN is associated with reduced apoptosis in vitro and diminished efficacy in vivo in response to eFT226. In models evaluated with PTEN loss, AKT was stimulated without a corresponding increase in mTOR activation. AKT activation leads to the degradation of PDCD4, which can alter eIF4F complex formation. The association of eFT226 activity with PTEN/PI3K/mTOR pathway regulation of mRNA translation provides a means to identify patient subsets during clinical development.

Abstract B33: Targeting PI3K/mTOR signaling with potent, selective and orally-available small-molecule inhibitors of eIF4E

Aberrant protein translation plays a role in the pathogenesis of multiple solid tumors and hematologic malignancies. The translation initiation factor eIF4E is essential for the translation of m7G-capped mRNA and is a key point of convergence for several signaling pathways, such as PI3K/mTOR and MAPK, which are intimately involved in tumor cell growth and survival. As such, eIF4E has generated intense interest as a target for anticancer drug discovery. We have designed a series of potent, selective, and orally available m7G cap-competitive inhibitors of eIF4E (eFT-4Ei) with favorable drug-like properties. These inhibitors bind free eIF4E, eIF4E-4EBP and eIF4E-eIF4F complexes within tumor cells. Ribosomal profiling of eIF4E inhibitor-treated tumor cells has identified a subset of translationally regulated target genes that overlap with mTORC1/2 regulated genes, but also include a larger set of unique translationally regulated target mRNAs that are enriched for 5'-TOP, PRTE and CERT sequence elements in their 5'-untranslated regions. eIF4E inhibition results in potent antiproliferative activity and induction of apoptosis in a subset of tumor cell lines. Consistent with this observation, our eIF4E inhibitors show some similarities, yet several important differences from existing mTORC1 or mTORC1/2 dual inhibitors in both cellular and physiologic assays. Finally, significant antitumor efficacy was observed with eIF4E inhibition in both solid tumor and hematologic xenografts in vivo. Taken together, these results highlight the potential of targeting eIF4E as a novel and differentiated therapeutic strategy to treat cancer.

Citation Format: Gregory S. Parker, Ivy N.J. Hung, Jocelyn Staunton, Maria Barrera, Eric Sung, Ana Parra, Craig R. Stumpf, Joan Chen, Peggy A. Thompson, Andreas Nevarez, Christopher J. Wegerski, Jeff Clarine, Samuel Sperry, Alan Xiang, Christian Nilewski, Garrick K. Packard, Kaveri Urklalan, Takasuke Mukaiyama, Theo Michels, Justin T. Ernst, Paul A. Sprengeler, Siegfried H. Reich, Gary G. Chiang, Kevin R. Webster. Targeting PI3K/mTOR signaling with potent, selective and orally-available small-molecule inhibitors of eIF4E [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr B33.

Abstract 1955: Inhibition of eIF4A by eFT226 blocks KRAS mutant tumor growth

Mutations in KRAS are among the most common oncogenic lesions across a variety of human cancers. Activation of KRAS directs signaling via the MAPK and PI3K pathways to promote tumor growth. One outcome of enhanced KRAS signaling is the induction of mRNA translation by eIF4A, eIF4E, and eIF4G, which together comprise the eIF4F complex. eIF4A is an RNA helicase that functions to unwind elements in the 5'-untranslated region (UTR) of mRNAs to facilitate scanning of the 40S ribosomal subunit. eFT226 is a highly potent and selective inhibitor of eIF4A that functions by forming a ternary complex between eIF4A, eFT226 and specific polypurine motifs in the 5'-UTR of select mRNAs, thus blocking ribosome scanning and inhibiting mRNA translation. This polypurine sequence motif is highly enriched in the 5'-UTR of eFT226 target genes, many of which are known proto-oncogenes. Translational profiling revealed KRAS to be a target of eFT226 and 5'-UTR sequence analysis of KRAS mRNA identified the polypurine regulatory motif, which imparts sensitivity to eFT226 mediated inhibition of translation. In cell-based reporter assays used to monitor translation, mutation of the KRAS 5'-UTR polypurine motifs resulted in a 10-fold decrease in sensitivity to eFT226 relative to the wild-type sequence. In a cell panel screen for in vitro apoptosis induction by eFT226, the most sensitive models were enriched for cell lines driven by KRAS mutations. Consistent with these results, treatment with eFT226 decreased KRAS protein levels, repressed downstream MAPK signaling, inhibited cell proliferation, and induced apoptosis in a collection of non-small cell lung, colorectal and pancreatic KRAS mutant cancer cell lines. The ability of eFT226 to block tumor cell growth and induce apoptosis is independent of the specific KRAS mutation present (e.g. G12C/V, G13D, Q61H/L) suggesting that eFT226 could be broadly efficacious in treating tumors with activating KRAS mutations. Treatment of KRAS mutant solid tumor xenografts with eFT226 as a monotherapy significantly inhibited tumor growth. Together, these results highlight the ability of eFT226 to inhibit growth and promote apoptosis in KRAS mutant tumors and support the clinical development of eFT226 in KRAS driven tumors. A clinical trial evaluating eFT226 in patients with solid tumor malignancies has initiated.

Citation Format: Craig R. Stumpf, Vikas K. Goel, Joan Chen, Jocelyn Staunton, Emily M. Santori, Maria Barrera, Haleigh Howard, Kevin R. Webster, Gary G. Chiang, Peggy A. Thompson. Inhibition of eIF4A by eFT226 blocks KRAS mutant tumor growth [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1955.

Abstract 3399: Preclinical evaluation of eFT226, a potent and selective eIF4A inhibitor with anti-tumor activity in FGFR1,2 and HER2 driven cancers

Mutations or amplifications affecting receptor tyrosine kinases (RTKs) activate the RAS/MAPK and PI3K/AKT signaling pathways thereby promoting cancer cell proliferation and survival. Oncoprotein expression is tightly controlled at the level of mRNA translation and is regulated by the eukaryotic translation initiation factor 4F (eIF4F) complex consisting of eIF4A, eIF4E, and eIF4G. eIF4A functions to catalyze the unwinding of secondary structure in the 5'-untranslated region (5'-UTR) of mRNA facilitating ribosome scanning and translation initiation. The activation of oncogenic signaling pathways, including RAS and PI3K, facilitate formation of eIF4F and enhance eIF4A activity promoting the translation of oncogenes with highly structured 5'-UTRs that are required for tumor cell proliferation, survival and metastasis. eFT226 is a selective eIF4A inhibitor that converts eIF4A into a sequence specific translational repressor by increasing the affinity between eIF4A and 5'-UTR polypurine motifs leading to selective downregulation of mRNA translation. The polypurine element is highly enriched in the 5'-UTR of eFT226 target genes, many of which are known oncogenic drivers, including FGFR1,2 and HER2, enabling eFT226 to selectively inhibit dysregulated oncogene expression. Formation of a ternary complex [eIF4A-eFT226-mRNA] blocks ribosome scanning along the 5'-UTR leading to dose dependent inhibition of RTK protein expression. The 5'-UTR sequence dependency of eFT226 translational inhibition was evaluated in cell-based reporter assays demonstrating 10-45-fold greater sensitivity for reporter constructs containing an RTK 5'-UTR compared to a control. In solid tumor cell lines driven by alterations in FGFR1, FGFR2 or HER2, downregulation of RTK expression by eFT226 resulted in decreased MAPK and AKT signaling, potent inhibition of cell proliferation and an induction of apoptosis suggesting that eFT226 could be effective in treating tumor types dependent on these oncogenic drivers. Solid tumor xenograft models harboring FGFR1,2 or HER2 amplifications treated with eFT226 resulted in significant in vivo tumor growth inhibition and regression at well tolerated doses in breast, non-small cell lung and colorectal cancer models. Treatment with eFT226 also decreased RTK protein levels supporting the potential to use these eFT226 target genes as pharmacodynamic markers of target engagement. Further evaluation of predictive markers of sensitivity or resistance showed that RTK tumor models with mTOR mediated activation of eIF4A are most sensitive to eFT226. The association of eFT226 activity in RTK tumor models with mTOR pathway activation provides a means to further enrich for sensitive patient subsets during clinical development. Clinical trials with eFT226 in patients with solid tumor malignancies have initiated.

Citation Format: Peggy A. Thompson, Nathan P. Young, Adina Gerson-Gurwitz, Boreth Eam, Vikas Goel, Craig R. Stumpf, Joan Chen, Gregory S. Parker, Sarah Fish, Maria Barrera, Eric Sung, Jocelyn Staunton, Gary G. Chiang, Kevin R. Webster. Preclinical evaluation of eFT226, a potent and selective eIF4A inhibitor with anti-tumor activity in FGFR1,2 and HER2 driven cancers [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3399.

Design of Development Candidate eFT226, a First in Class Inhibitor of Eukaryotic Initiation Factor 4A RNA Helicase

Dysregulation of protein translation is a key driver for the pathogenesis of many cancers. Eukaryotic initiation factor 4A (eIF4A), an ATP-dependent DEAD-box RNA helicase, is a critical component of the eIF4F complex, which regulates cap-dependent protein synthesis. The flavagline class of natural products (i.e., rocaglamide A) has been shown to inhibit protein synthesis by stabilizing a translation-incompetent complex for select messenger RNAs (mRNAs) with eIF4A. Despite showing promising anticancer phenotypes, the development of flavagline derivatives as therapeutic agents has been hampered because of poor drug-like properties as well as synthetic complexity. A focused effort was undertaken utilizing a ligand-based design strategy to identify a chemotype with optimized physicochemical properties. Also, detailed mechanistic studies were undertaken to further elucidate mRNA sequence selectivity, key regulated target genes, and the associated antitumor phenotype. This work led to the design of eFT226 (Zotatifin), a compound with excellent physicochemical properties and significant antitumor activity that supports clinical development.

Abstract B133: eFT226, a first in class inhibitor of eIF4A1, targets FGFR1/2 and HER2 driven cancers

Background: Mutations or amplifications affecting receptor tyrosine kinases (RTKs) activate the RAS/MAPK and PI3K/AKT signaling pathways thereby promoting cancer cell proliferation and survival. Oncoprotein expression is tightly controlled at the level of mRNA translation and is regulated by the eukaryotic translation initiation factor 4F (eIF4F) complex consisting of eIF4A, eIF4E, and eIF4G. eIF4A functions to catalyze the unwinding of secondary structure in the 5’-untranslated region (5’-UTR) of mRNA facilitating ribosome scanning and translation initiation. eFT226 is a first in class inhibitor that converts eIF4A1 into a sequence specific translational repressor. eFT226 increases the affinity between eIF4A1 and polypurine recognition elements in the 5’-UTR leading to selective downregulation of mRNA translation. The polypurine element is highly enriched in the 5’-UTR of eFT226 target genes, many of which are known oncogenic drivers, including FGFR1/2 and HER2, enabling eFT226 to selectively inhibit dysregulated oncogene expression. Methods: 5’-UTR dependency was evaluated using cell-based luciferase reporter assays. Regulation of protein expression was analyzed by western blot analysis. Antitumor activity was assessed in vitro by proliferation and apoptosis assays. For in vivo experiments, athymic nude or NOD/SCID mice were implanted with subcutaneous xenograft models of FGFR1, FGFR2 or HER2 driven tumors and treated with eFT226 administered Q4D IV. Results: eFT226 inhibits the translation of FGFR1, FGFR2 and HER2 through formation of a sequence dependent ternary complex with eIF4A1 and polypurine elements within the 5’-UTR of mRNA [eFT226-eIF4A1-mRNA]. Formation of this ternary complex blocks ribosome scanning along the 5’-UTR leading to dose dependent inhibition of RTK protein expression. Cells transiently transfected with luciferase reporter constructs containing the 5’-UTR of each RTK resulted in 10-45-fold greater sensitivity to inhibition by eFT226 compared to a control 5’-UTR confirming the 5’-UTR dependency. In solid tumor cell lines driven by alterations in FGFR1, FGFR2 or HER2, downregulation of RTK expression by eFT226 resulted in decreased MAPK and AKT signaling, potent inhibition of cell proliferation and an induction of apoptosis suggesting that eFT226 could be effective in treating tumor types dependent on these oncogenic drivers. Solid tumor xenograft models harboring FGFR1/2 or HER2 amplifications treated with eFT226 resulted in significant in vivo tumor growth inhibition and regression at well tolerated doses in breast, non-small cell lung and colorectal cancer models. Treatment with eFT226 also decreased RTK protein levels supporting the potential to use these eFT226 target genes as pharmacodynamic markers of target engagement. Conclusions: eFT226 is efficacious against tumor models with alterations in FGFR1, FGFR2 and HER2 RTKs. The antitumor response observed in preclinical in vivo models driven by RTK amplifications demonstrates the potential for eFT226 in the treatment of solid tumors with FGFR1/2 or HER2 alterations. Furthermore, this data provides a means to select sensitive patient subsets during clinical development. Clinical trials in patients with solid tumor malignancies are planned.

Citation Format: Peggy A Thompson, Nathan P Young, Craig R Stumpf, Boreth Eam, Vikas K Goel, Joan Chen, Sarah Fish, Gregory S Parker, Adina Gerson-Gurwitz, Maria Barrera, Eric Sung, Jocelyn Staunton, Gary G Chiang, Christopher J Wegerski, Samuel Sperry, Kevin R Webster, Siegfried H Reich. eFT226, a first in class inhibitor of eIF4A1, targets FGFR1/2 and HER2 driven cancers [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B133. doi:10.1158/1535-7163.TARG-19-B133

Abstract LB-068: Tomivosertib (eFT508), a potent and highly selective inhibitor of MNK1 and MNK2, enhances CAR T cell activity through modulating T cell differentiation

Chimeric antigen receptor (CAR) T cells have shown great promise in treating hematopoietic malignancies, such as leukemia and non-Hodgkin’s lymphoma. The efficacy and durability of CAR T cell therapy have been correlated with higher levels of T stem cell memory (TSCM) and T central memory (TCM) populations. Accordingly, various strategies to enrich these CAR T cell populations are of intense interest. It is well-established that the PI3K/mTOR pathway plays an important role in T cell activation and differentiation, and perturbations in this pathway can enhance T cell memory populations. Work from our group has demonstrated that MNK modifies mTOR signaling and T cell differentiation. Here, we demonstrate that the MNK1/2 inhibitor tomivosertib can substantially increase TCM and TSCM populations in both primary murine and human T cells. Tomivosertib treatment of murine OT-I T cells biases T cell differentiation towards a TCM (CD8+ CD44+ CD62L+) population upon SIINFEKL peptide stimulation in vitro without adverse effects on T cell proliferation, interferon-γ production or cytotoxic function. Similar effects are seen in vivo, where tomivosertib treatment also enriches the TCM cell pool in a SIINFEKL vaccine-induced OT-I adoptive T cell transfer model, which results in increased persistence as demonstrated by a higher memory-recall T cell response upon re-challenge. Furthermore, addition of tomivosertib during production of human CAR (anti-CD19 scFv-4-1BB-CD3ζ) T cells leads to a significantly increased population of TSCM (CD8+ CD45RO CD45RA+ CD27+ CD95+) cells. Based on these data, the combination of tomivosertib with CD19-directed CAR T cells was assessed in the CD19+ Pfeiffer DLBCL model in vivo. Daily oral administration of tomivosertib in combination with CD19-directed CAR T cells results in improved efficacy in comparison to either monotherapy alone, consistent with tomivosertib promoting and maintaining TSCM populations in vivo. Tomivosertib is currently in multiple Phase II clinical trials as a monotherapy or in combination with checkpoint inhibitors.

Citation Format: Vikas K. Goel, Rajesh K. Sharma, Jocelyn Staunton, Craig R. Stumpf, Nathan P. Young, Peggy A. Thompson, Gary G. Chiang, Kevin R. Webster. Tomivosertib (eFT508), a potent and highly selective inhibitor of MNK1 and MNK2, enhances CAR T cell activity through modulating T cell differentiation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-068.

Abstract 4142: Physical and functional interactions between MNK and mTOR signaling regulate the activation and differentiation of T cells

The role of the PI3K/mTOR and MAPK signaling pathways in regulating T cell activation and differentiation is well established. It has been shown that mTOR acts as a sensor of the T cell metabolic state, coordinating diverse inputs to determine the balance between effector versus memory cell fates. Similarly, the MAPK interacting kinases, MNK1 and MNK2, are key downstream effector kinases that mediate post-transcriptional gene regulation of critical mediators of the T cell response, including immune checkpoint proteins and cytokines, via the phosphorylation of specific RNA binding proteins such as eIF4E and hnRNPA1. eFT508 is a potent and selective inhibitor of both MNK1 and MNK2, which enhances anti-tumor immune responses by decreasing the expression of immune response modulators including PD-1, PD-L1, LAG3, TIM3 and IL-10. Furthermore, eFT508 treatment boosts memory T cell populations while increasing the effectiveness of cytotoxic T lymphocytes. Phosphoproteomic analysis of the effects of eFT508 on early T cell activation identified novel eFT508 sensitive phosphopeptides, including a subset that overlaps with mTOR-dependent phosphorylation sites. In vitro biochemical analysis has shown that eFT508 is not a mTOR kinase inhibitor and that most of these sites are not direct MNK substrates. These results suggest inhibition of MNK by eFT508 can have significant effects on mTOR signaling. Consistent with these findings, we observe a physical association between MNK and mTOR that is disrupted by either eFT508 or the allosteric mTOR inhibitor rapamycin. Our phosphoproteomic analysis also identified novel MNK-dependent phosphorylation sites within the translation initiation factor eIF4G1 that regulate the ability of mTOR to recognize eIF4G1 as a substrate. In addition, phosphorylation of the mTOR target 4EBP1 is decreased upon treatment of T cells with eFT508, although 4EBP1 itself is not a direct substrate of MNK in vitro. These data are consistent with a role for MNK in regulating mRNA translation, not only through phosphorylation of eIF4E, but also by modulating the activity of mTOR toward specific substrates. Given the role of mTOR in controlling memory T cell differentiation, our results provide a potential mechanistic basis to explain the impact of MNK inhibition on T cell differentiation. Ongoing studies are further characterizing the consequences of eFT508 regulation on MNK/mTOR signaling and T cell differentiation. This work significantly expands our current understanding of eFT508’s mechanism of action as well as its ability to promote a prolonged anti-tumor immune response.

Citation Format: Craig R. Stumpf, Vikas K. Goel, Rajesh K. Sharma, Gary G. Chiang, Peggy A. Thompson, Kevin R. Webster. Physical and functional interactions between MNK and mTOR signaling regulate the activation and differentiation of T cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4142.

Abstract 4343: A focused CRISPR screen to identify synthetic lethal interactions with the novel eIF4A inhibitor eFT226 in KRAS driven NSCLC

Tumor development is often characterized by dysregulated messenger RNA (mRNA) translation of key oncogenic factors that promote increased proliferation, resistance to apoptosis and enhanced metastatic potential. The eukaryotic translation initiation factor 4F (eIF4F) complex, a master regulator of protein synthesis, is comprised of eIF4E (mRNA-cap-binding protein), eIF4A (RNA-helicase), and eIF4G (scaffolding protein), that together orchestrate mRNA recruitment to ribosomal subunits as well as efficient scanning of the mRNA 5’-untranslated region. As a downstream target of growth-promoting signaling cascades, eIF4F also serves as a central node in several important oncogenic pathways including KRAS and PI3K/mTOR. eIF4F subunits are frequently over-expressed in various malignancies; therefore, repressing eIF4F activity has emerged as a promising anti-cancer therapeutic strategy. eFT226 is a potent and selective translational regulator that targets eIF4A1. eFT226 down-regulates the translation of a unique gene set and displays robust anti-tumor activity across multiple models in vitro and in vivo. A functional genomic screen using CRISPR/Cas9 was performed to discover synthetic lethal genetic interactions with eFT226 to enable development of novel drug combination and/or patient selection strategies. The screen was conducted in a panel of non-small cell lung cancer (NSCLC) cell lines driven by oncogenic KRAS and p53 pathway mutations, a tumor subtype with limited therapeutic options. Utilizing a focused guide RNA (gRNA) lentiviral library of ~700 cancer-related targets with diverse roles in growth-factor signaling, metabolism, epigenetics, translation and stress-responses, several genetic perturbations that sensitized tumor cells to eFT226 were uncovered. Interestingly, many of the genes whose loss-of-function enhanced eFT226’s activity fall into distinct classes including an oncogenic signaling pathway commonly activated in many cancers as well as redox homeostasis. Moreover, a number of these targets include tumor suppressor genes that are frequently mutated or inactivated through loss of function in certain cancer types suggesting that specific genetic contexts may dictate eFT226 efficacy. Ongoing studies are aimed at confirming these hits from the primary screen. Collectively, these validated targets will represent genomic vulnerabilities to eFT226 that can aid in the design of drug combination and patient selection strategies for NSCLC.

Citation Format: Nathan P. Young, Craig R. Stumpf, Joan Chen, Gary G. Chiang, Peggy A. Thompson, Kevin R. Webster. A focused CRISPR screen to identify synthetic lethal interactions with the novel eIF4A inhibitor eFT226 in KRAS driven NSCLC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4343.

Abstract 1302: Targeting hormone receptor-dependent cancers with potent, selective and orally-available small molecule inhibitors of eIF4E

The PI3K/mTOR pathway is commonly dysregulated in many hormone receptor-dependent tumors and plays a key role in promoting tumor growth and mediating drug resistance. In particular, PI3K and mTORC1/2 inhibitors have been intensively studied in the treatment of hormone receptor-dependent cancers and have shown benefit in some clinical settings. However, issues such as dose-limiting toxicities and emergent resistance limit the broader utility of these inhibitors. The translation initiation factor eIF4E is essential for the translation of m7G-capped mRNA and is a key point of convergence for both the PI3K/mTOR and MAPK signaling pathways. We have designed a series of potent, selective and orally-available m7G cap-competitive inhibitors of eIF4E (eFT-4Ei) with favorable drug-like properties. These inhibitors bind to eIF4E either as its free form or with eIF4E-4EBP and eIF4F complexes within tumor cells and downregulate hormone receptor-dependent signaling. Ribosomal profiling of eIF4E inhibitor-treated tumor cells identified a subset of translationally regulated target genes that overlap with mTORC1/2 regulated genes, but also a unique set of translationally regulated target mRNAs. Consistent with this observation, our eIF4E inhibitors show some similarities yet several important differences from existing mTORC1 or mTORC1/2 dual inhibitors in both cellular and physiological assays. Finally, significant anti-tumor efficacy was observed with eIF4E inhibition in vitro and in vivo. Taken together, these results highlight the potential for targeting eIF4E as a novel therapeutic strategy to treat hormone-receptor dependent cancers.

Citation Format: Gary G. Chiang, Gregory S. Parker, Ivy N. Hung, Vikas K. Goel, Jocelyn Staunton, Maria Barrera, Eric Sung, Ana Parra, Craig R. Stumpf, Joan Chen, Peggy A. Thompson, Andreas Nevarez, Christopher J. Wegerski, Cody Parker, Jeff Clarine, Samuel Sperry, Alan Xiang, Christian Nilewski, Garrick K. Packard, Kaveri Urkalan, Takasuke Mukaiyama, Theo Michels, Justin T. Ernst, Paul A. Sprengeler, Siegfried H. Reich, Kevin R. Webster. Targeting hormone receptor-dependent cancers with potent, selective and orally-available small molecule inhibitors of eIF4E [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1302.

Abstract 2698: eFT226, a potent and selective inhibitor of eIF4A, is efficacious in preclinical models of lymphoma

Dysregulated messenger RNA (mRNA) translation drives the pathogenesis of multiple hematological malignancies. In lymphoma this includes the upregulation of key driver oncogenes and anti-apoptotic proteins (e.g., MYC, CCND1/3, BCL2 and MCL1) that contain a highly structured 5’-untranslated region (UTR) in their mRNA requiring enhanced eIF4A helicase activity for translation. eIF4A is a component of the eIF4F translation initiation complex and catalyzes the ATP-dependent unwinding of RNA duplexes and facilitates 43S ribosome scanning within the 5’-UTR. The activation of oncogenic signaling pathways, including RAS and PI3K, enhance eIF4A activity through phosphorylation of eIF4B, eIF4G and PDCD4 which facilitates formation of eIF4F and full activation of eIF4A. The PI3K/AKT/mTOR pathway is frequently activated in lymphoma, promoting the translation of oncogenes with complex 5’-UTRs that are required for tumor cell proliferation, survival and metastasis.

eFT226 is a potent and sequence selective eIF4A1 inhibitor that promotes eIF4A1 binding to specific 5’-UTR polypurine and/or G-quadraplex recognition motifs leading to a selective block in ribosome mRNA scanning. The sequence dependency of eFT226 translational inhibition was evaluated in cell-based reporter assays demonstrating >100-fold greater sensitivity for reporter constructs containing a polypurine motif in the 5’-UTR (IC50 ~2 nM). Direct binding studies also confirmed the formation of a stable ternary complex with increased drug residence time between eFT226, eIF4A1 and RNA oligonucleotides containing polypurine motifs. The ability of eFT226 to inhibit MYC or MCL1 expression was found to be dependent on the presence of their respective 5’-UTR supporting a translational regulation mechanism dependent on recognition elements within the 5’-UTR.

eFT226 shows potent anti-proliferative activity (GI50 < 15 nM) against a panel of B-cell lymphoma cell lines. Treatment with eFT226 leads to coordinated inhibition of MYC, CCND1/3, BCL2 or MCL1 protein expression resulting in significant anti-tumor activity. eFT226 has good pharmacokinetic properties and exhibits significant in vivo activity across a panel of diffuse large B cell lymphoma (DLBCL), and Burkitt lymphoma tumor models with ≤1 mg/kg/week IV administration. Further evaluation of predictive markers of sensitivity or resistance has shown that tumors with mTOR mediated activation of eIF4A are most sensitive to eFT226. In addition, tumors with PTEN mutations do not exhibit activated eIF4A and are generally resistant to induction of apoptosis by eFT226, resulting in reduced in vivo efficacy. The association of eFT226 activity with PI3K/mTOR pathway activation and mutational status provides a means to identify patient subsets during clinical development. Clinical trials in patients with lymphoma and other malignancies are planned.

Citation Format: Peggy A. Thompson, Boreth Eam, Nathan P. Young, Sarah Fish, Joan Chen, Maria Barrera, Haleigh Howard, Eric Sung, Ana Parra, Jocelyn Staunton, Gary G. Chiang, Christopher J. Wegerski, Andres Nevarez, Jeff Clarine, Samuel Sperry, Alan Xiang, Chinh Tran, Christian Nilewski, Garrick K. Packard, Theodore Michels, Paul A. Sprengeler, Justin T. Ernst, Siegfried H. Reich, Kevin R. Webster. eFT226, a potent and selective inhibitor of eIF4A, is efficacious in preclinical models of lymphoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2698.

Abstract 5546: eFT508, a potent and highly selective inhibitor of MNK1 and MNK2, regulates T-cell differentiation promoting an antitumor immune response

An effective and durable T-cell response is a cornerstone of current immunotherapies. We show that eFT508, a potent, selective inhibitor of MNK1 and MNK2, establishes a regulatory program that promotes multiple steps in the cancer immunity cycle including expansion of memory T cells and prevention of T-cell exhaustion. Using OT-I and OT-II transgenic systems, we show that eFT508 shifts the distribution of T cells towards a CD62LhighCD44high central memory (CM) phenotype in both CD4 and CD8 T cells upon activation with SIINFEKL peptide in vitro without adverse effects on T-cell proliferation, interferon-γ production or cytotoxic function. Similar effects are seen in vivo, where eFT508 treatment also enriches the CM T-cell pool in a SIINFEKL vaccine-induced OT-I adoptive T-cell transfer model, which results in increased persistence as demonstrated by a higher memory-recall T-cell response upon rechallenge. In addition, the CM bias elicited by eFT508 remains dominant when combined with agonists of co-stimulatory molecules, such as 4-1BB, OX-40 and GITR, or checkpoint inhibitors, such as PD-1, PD-L1 and CTLA-4, suggesting that eFT508 can affect the rate of T-cell differentiation in these combinations. eFT508 treatment also reduces the expression of exhaustion markers such as PD-1, LAG3 and TIM3, leading to increased cytotoxic T-cell function. eFT508 is currently under evaluation as a single agent in two phase 1/2 clinical trials for patients with advanced solid tumors and patients with advanced lymphoma. In addition, a phase 2 study evaluating eFT508, alone or in combination with avelumab, a PD-L1 immune checkpoint inhibitor, in microsatellite stable relapsed or refractory CRC patients is ongoing. The preclinical studies presented here provide further evidence that eFT508 may combine well with additional immunotherapies beyond checkpoint blockade.

Citation Format: Rajesh K. Sharma, Vikas K. Goel, Jocelyn Staunton, Maria Barrera, Ana Parra, Eric Sung, Gary G. Chiang, Kevin R. Webster. eFT508, a potent and highly selective inhibitor of MNK1 and MNK2, regulates T-cell differentiation promoting an antitumor immune response [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5546.

Abstract 3855: Inhibition of MNK by eFT508 reprograms T-cell signaling to promote an antitumor immune response

Mitogen-activated protein (MAP) kinase signaling cascades play a vital role in T-cell activation upon antigen recognition. MNK1 and MNK2 are important downstream effector kinases in the MAPK pathway that largely function in regulating the expression of important signaling molecules, including cytokines and immune checkpoint receptors. MNKs are primarily thought to regulate the expression of select mRNAs, predominantly via post-transcriptional mechanisms involving the phosphorylation of the eukaryotic translation initiation factor eIF4E as well as the RNA binding proteins hnRNPA1 and PSF. eFT508 is a potent and highly selective inhibitor of MNK1 and MNK2 that has been shown to promote antitumor immunity by decreasing the expression of immunosuppressive molecules, such as immune checkpoint receptors. In order to identify key T-cell components that are regulated by MNK phosphorylation and may mediate the effects of eFT508 treatment, we performed an unbiased phosphoproteomic analysis of T cells during the early stages of T-cell receptor-mediated stimulation with and without eFT508 treatment. Primary human T cells were pretreated with eFT508 for two hours prior to stimulation with αCD3/αCD28 for an additional 30 minutes. Protein samples were then prepared for multiplexed phosphoproteomic analysis by mass spectrometry. Consistent with previous proteomic studies using stimulated T cells, a number of phosphopeptides associated with T-cell receptor signaling, among other cellular activities, were detected. Moreover, treatment with eFT508 specifically blocked the phosphorylation of distinct phosphosites on select proteins. The phosphoproteins modulated by eFT508 treatment are involved in important T-cell signaling pathways, cell proliferation and differentiation programs, stress responses, and post-transcriptional and translational gene regulation. Furthermore, there was enrichment for specific sequences surrounding the phosphorylation site in eFT508-sensitive peptides, highlighting a potential mechanism mediating MNK target recognition. Confirmation of MNK-mediated phosphorylation of novel substrates is being conducted in vitro by biochemical analysis of direct phosphorylation of potential substrates by MNK1 or MNK2 and in cellular lysates treated with eFT508 by Western blot analysis using phosphosite-specific antibodies. These findings have significantly expanded our understanding of cell signaling through MNK1 and MNK2 and will help to illuminate potential regulatory programs through which inhibition of MNKs by eFT508 can modulate antitumor immunity.

Citation Format: Craig R. Stumpf, Joan Chen, Vikas K. Goel, Gregory S. Parker, Gary G. Chiang, Peggy A. Thompson, Kevin R. Webster. Inhibition of MNK by eFT508 reprograms T-cell signaling to promote an antitumor immune response [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3855.

Author Correction: Discovery of a selective catalytic p300/CBP inhibitor that targets lineage-specific tumours

In the originally published version of this Letter, the authors Arthur F. Kluge, Michael A. Patane and Ce Wang were inadvertently omitted from the author list. Their affiliations are: I-to-D, Inc., PO Box 6177, Lincoln, Massachusetts 01773, USA (A.F.K.); Mitobridge, Inc. 1030 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA (M.A.P.); and China Novartis Institutes for BioMedical Research, No. 4218 Jinke Road, Zhangjiang Hi-Tech Park, Pudong District, Shanghai 201203, China (C.W.). These authors contributed to the interpretation of results and design of compounds. In addition, author 'Edward A. Kesicki' was misspelled as 'Ed Kesicki'. These errors have been corrected online.

Structure-based Design of Pyridone-Aminal eFT508 Targeting Dysregulated Translation by Selective Mitogen-activated Protein Kinase Interacting Kinases 1 and 2 (MNK1/2) Inhibition

Dysregulated translation of mRNA plays a major role in tumorigenesis. Mitogen-activated protein kinase interacting kinases (MNK)1/2 are key regulators of mRNA translation integrating signals from oncogenic and immune signaling pathways through phosphorylation of eIF4E and other mRNA binding proteins. Modulation of these key effector proteins regulates mRNA, which controls tumor/stromal cell signaling. Compound 23 (eFT508), an exquisitely selective, potent dual MNK1/2 inhibitor, was designed to assess the potential for control of oncogene signaling at the level of mRNA translation. The crystal structure-guided design leverages stereoelectronic interactions unique to MNK culminating in a novel pyridone-aminal structure described for the first time in the kinase literature. Compound 23 has potent in vivo antitumor activity in models of diffuse large cell B-cell lymphoma and solid tumors, suggesting that controlling dysregulated translation has real therapeutic potential. Compound 23 is currently being evaluated in Phase 2 clinical trials in solid tumors and lymphoma. Compound 23 is the first highly selective dual MNK inhibitor targeting dysregulated translation being assessed clinically.

Discovery and Characterization of Novel Nonsubstrate and Substrate NAMPT Inhibitors

Cancer cells are highly reliant on NAD⁺-dependent processes, including glucose metabolism, calcium signaling, DNA repair, and regulation of gene expression. Nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme for NAD⁺ salvage from nicotinamide, has been investigated as a target for anticancer therapy. Known NAMPT inhibitors with potent cell activity are composed of a nitrogen-containing aromatic group, which is phosphoribosylated by the enzyme. Here, we identified two novel types of NAM-competitive NAMPT inhibitors, only one of which contains a modifiable, aromatic nitrogen that could be a phosphoribosyl acceptor. Both types of compound effectively deplete cellular NAD⁺, and subsequently ATP, and produce cell death when NAMPT is inhibited in cultured cells for more than 48 hours. Careful characterization of the kinetics of NAMPT inhibition in vivo allowed us to optimize dosing to produce sufficient NAD⁺ depletion over time that resulted in efficacy in an HCT116 xenograft model. Our data demonstrate that direct phosphoribosylation of competitive inhibitors by the NAMPT enzyme is not required for potent in vitro cellular activity or in vivo antitumor efficacy. Mol Cancer Ther; 16(7); 1236-45. ©2017 AACR.

Abstract PR11: eFT508: An oral, potent and highly selective inhibitor of MNK1 and MNK2, promotes anti-tumor immunity as a monotherapy and in combination with immune checkpoint blockade

Purpose: This study was designed to evaluate the potential of eFT508 to selectively regulate key immune signaling pathways and enhance anti-tumor immunity as a monotherapy or in combination with checkpoint blockade in immunocompetent syngeneic cancer models.

Methods: eFT508 and its effect on mRNA translation, effector protein production, immune cell signaling and tumor infiltrating lymphocytes was evaluated in vitro using normal human T cells and in vivo utilizing immunocompetent syngeneic models. The mechanism of translational regulation of specific target genes was further evaluated in these model systems.

Results: Dysregulated translation of messenger RNA (mRNA) plays a role in the pathogenesis of multiple solid tumors and hematological malignancies. MNK1 and MNK2 integrate signals from several oncogenic and immune signaling pathways (including RAS, p38 and toll-like receptors) by phosphorylating eukaryotic initiation factor 4E (eIF4E) and other key effector proteins including hnRNPA1 and PSF. Phosphorylation of these RNA-binding proteins by MNK1 and MNK2 selectively regulates the stability and translation of a subset of cellular mRNA that control tumor/stromal cell signaling and the tumor microenvironment. eFT508 inhibits both MNK1 and MNK2 through a reversible, ATP-competitive mechanism of action with an IC50 of 2 and 1 nM against MNK1 and MNK2 respectively. eFT508 is highly selective (≥100-fold) for MNK1 and MNK2 relative to over 400 other protein and lipid kinases. Ribosome profiling has demonstrated that inhibition of MNK1 and MNK2 by eFT508 selectively regulates the translational efficiency and mRNA stability of a subset of genes that include inflammatory cytokines/chemokines, regulators of reactive oxygen species (ROS), and effectors of anti-tumor immune response. Given the importance of both RAS signaling and translational control to immune cell function the immunological effect of eFT508 was evaluated in both normal human T cells in vitro and immunocompetent syngeneic cancer models in vivo. eFT508 treatment of normal donor T cells has no deleterious effect on CD3/CD28 activation of IL-2 production, T cell proliferation or on T cell viability. However, eFT508 selectively down regulates the induction of IL-10 and specific immune checkpoint mechanisms. The effect of eFT508 on IL-10 protein production corresponded with reduced mRNA stability. The in vivo antitumor effect of eFT508 was assessed in the CT26 BALB/C syngeneic tumor model. CT26 mouse tumor cell proliferation and survival are insensitive to eFT508 in vitro. In vivo, daily oral treatment with 1 mg/kg eFT508 results in significant anti-tumor activity and establishment of immune memory. In addition, combination of daily oral treatment of 1 mg/kg eFT508 with either anti-PD-1 or anti-PD-L1 monoclonal antibodies increases the number of responder animals and results in synergistic activity that corresponds to the modulation of tumor infiltrating lymphocyte populations.

Conclusions: eFT508 is a selective, orally bioavailable small molecule inhibitor of MNK1 and MNK2 that can decrease the production of key immune checkpoint regulators and immunosuppressive cytokines. This novel mechanism of action triggers anti-tumor immune response in immunocompetent syngeneic animal models as a monotherapy and in combination with established immune checkpoint antibodies. eFT508 is currently under evaluation in two phase I clinical trials for patients with advanced solid tumors and patients with advanced lymphoma respectively. These findings support further clinical evaluation of eFT508 in combination with checkpoint blockade.

This abstract is also being presented as Poster B29.

Citation Format: Kevin R. Webster, Vikas K. Goel, Jocelyn Staunton, Ivy NJ Hung, Gregory S. Parker, Craig R. Stumpf, Jolene Molter, Gary G. Chiang, Christopher J. Wegerski, Samuel Sperry, Joan Chen, Vera Huang, Peggy A. Thompson, Chinh Tran, Justin T. Ernst, Stephen E. Webber, Paul A. Sprengeler, Siegfried H. Reich. eFT508: An oral, potent and highly selective inhibitor of MNK1 and MNK2, promotes anti-tumor immunity as a monotherapy and in combination with immune checkpoint blockade. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr PR11.

The Histone Methyltransferase Inhibitor A-366 Uncovers a Role for G9a/GLP in the Epigenetics of Leukemia

Histone methyltransferases are epigenetic regulators that modify key lysine and arginine residues on histones and are believed to play an important role in cancer development and maintenance. These epigenetic modifications are potentially reversible and as a result this class of enzymes has drawn great interest as potential therapeutic targets of small molecule inhibitors. Previous studies have suggested that the histone lysine methyltransferase G9a (EHMT2) is required to perpetuate malignant phenotypes through multiple mechanisms in a variety of cancer types. To further elucidate the enzymatic role of G9a in cancer, we describe herein the biological activities of a novel peptide-competitive histone methyltransferase inhibitor, A-366, that selectively inhibits G9a and the closely related GLP (EHMT1), but not other histone methyltransferases. A-366 has significantly less cytotoxic effects on the growth of tumor cell lines compared to other known G9a/GLP small molecule inhibitors despite equivalent cellular activity on methylation of H3K9me2. Additionally, the selectivity profile of A-366 has aided in the discovery of a potentially important role for G9a/GLP in maintenance of leukemia. Treatment of various leukemia cell lines in vitro resulted in marked differentiation and morphological changes of these tumor cell lines. Furthermore, treatment of a flank xenograft leukemia model with A-366 resulted in growth inhibition in vivo consistent with the profile of H3K9me2 reduction observed. In summary, A-366 is a novel and highly selective inhibitor of G9a/GLP that has enabled the discovery of a role for G9a/GLP enzymatic activity in the growth and differentiation status of leukemia cells.

Discovery of A-893, A New Cell-Active Benzoxazinone Inhibitor of Lysine Methyltransferase SMYD2

A lack of useful small molecule tools has precluded thorough interrogation of the biological function of SMYD2, a lysine methyltransferase with known tumor-suppressor substrates. Systematic exploration of the structure-activity relationships of a previously known benzoxazinone compound led to the synthesis of A-893, a potent and selective SMYD2 inhibitor (IC50: 2.8 nM). A cocrystal structure reveals the origin of enhanced potency, and effective suppression of p53K370 methylation is observed in a lung carcinoma (A549) cell line.

Abstract 5532: Discovery of A-366, a novel small molecule inhibitor that uncovers an unappreciated role for G9a/GLP in the epigenetics of leukemia

Understanding the roles of epigenetic alterations in cancer development and maintenance holds great promise for cancer prevention, detection, and therapy. Cancer can be considered as a pathogenic state where cellular differentiation is suppressed (i.e. stem cell-like) and aberrant epigenetic patterning is commonly observed in tumors. Histone methyltransferases play a key role in epigenetics by modifying key lysine and arginine residues on histones and thereby influencing biological processes. Previous studies have suggested that the histone lysine methyltransferase G9a (EHMT2) is required to perpetuate malignant phenotypes through over-expression in a variety of cancer types. These reports have shown that pharmacologic inhibition or genetic ablation of G9a leads to retardation of tumor cell growth and cellular invasion in vitro as well as inhibition of metastasis in vivo. To further elucidate the enzymatic role of G9a in cancer, we describe herein the discovery of a novel histone methyltransferase inhibitor, A-366, that selectively inhibits G9a and the closely related GLP (EHMT1). A-366 is a peptide competitive inhibitor of G9a/GLP with in vitro enzymatic IC50 of ∼ 3 nM and cellular activity of ∼ 100 nM and > 100-fold selectivity over other methyltransferases and other non-epigenetic targets. A-366 has significantly less cytotoxic effects on the growth of solid tumor cell lines compared to other known G9a/GLP small molecule inhibitors despite roughly equivalent cellular activity on methylation of H3K9me2. However, the excellent selectivity profile of A-366 has aided in the discovery of an important role for G9a/GLP in lineage maintenance of a subset of leukemias. Treatment of various leukemia cell lines in vitro resulted in marked differentiation and morphological changes of these tumors in the absence of cytotoxicity resulting in cytostasis. Furthermore, treament of MV4;11 flank xenografts with A-366 resulted in growth inhibition in vivo consistent with the profile of H3K9me2 reduction observed. In summary, A-366 is a novel and highly selective peptide-competitive inhibitor of G9a/GLP that has enabled the discovery of a role for G9a/GLP enzymatic activity in the epigenetic maintenance of a subset of leukemia cells.

Citation Format: Jun Guo, Marina Pliushchev, Yupeng He, Debra Ferguson, Sujatha Jagadeeswaran, Andrew Petros, Chaohong Sun, Niru B. Soni, Bailin Shaw, Alla Korepanova, David Maag, Ramzi Sweis, Fritz G. Buchanan, Michael Michaelides, Alex Shoemaker, Chris Tse, Gary G. Chiang, William N. Pappano. Discovery of A-366, a novel small molecule inhibitor that uncovers an unappreciated role for G9a/GLP in the epigenetics of leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5532. doi:10.1158/1538-7445.AM2014-5532

Discovery and development of potent and selective inhibitors of histone methyltransferase g9a

G9a is a histone lysine methyltransferase responsible for the methylation of histone H3 lysine 9. The discovery of A-366 arose from a unique diversity screening hit, which was optimized by incorporation of a propyl-pyrrolidine subunit to occupy the enzyme lysine channel. A-366 is a potent inhibitor of G9a (IC50: 3.3 nM) with greater than 1000-fold selectivity over 21 other methyltransferases.

Comparative metabolic flux profiling of melanoma cell lines: beyond the Warburg effect

Metabolic rewiring is an established hallmark of cancer, but the details of this rewiring at a systems level are not well characterized. Here we acquire this insight in a melanoma cell line panel by tracking metabolic flux using isotopically labeled nutrients. Metabolic profiling and flux balance analysis were used to compare normal melanocytes to melanoma cell lines in both normoxic and hypoxic conditions. All melanoma cells exhibited the Warburg phenomenon; they used more glucose and produced more lactate than melanocytes. Other changes were observed in melanoma cells that are not described by the Warburg phenomenon. Hypoxic conditions increased fermentation of glucose to lactate in both melanocytes and melanoma cells (the Pasteur effect). However, metabolism was not strictly glycolytic, as the tricarboxylic acid (TCA) cycle was functional in all melanoma lines, even under hypoxia. Furthermore, glutamine was also a key nutrient providing a substantial anaplerotic contribution to the TCA cycle. In the WM35 melanoma line glutamine was metabolized in the "reverse" (reductive) direction in the TCA cycle, particularly under hypoxia. This reverse flux allowed the melanoma cells to synthesize fatty acids from glutamine while glucose was primarily converted to lactate. Altogether, this study, which is the first comprehensive comparative analysis of metabolism in melanoma cells, provides a foundation for targeting metabolism for therapeutic benefit in melanoma.

Effective inhibition of melanoma by BI-69A11 is mediated by dual targeting of the AKT and NF-κB pathways

In melanoma, the activation of pro-survival signaling pathways, such as the AKT and NF-κB pathways, is critical for tumor growth. We have recently reported that the AKT inhibitor BI-69A11 causes efficient inhibition of melanoma growth. Here, we show that in addition to its AKT inhibitory activity, BI-69A11 also targets the NF-κB pathway. In melanoma cell lines, BI-69A11 inhibited TNF-α-stimulated IKKα/β and IκB phosphorylation as well as NF-κB reporter gene expression. Furthermore, the effective inhibition of melanoma growth by BI-69A11 was attenuated upon NF-κB activation. Mechanistically, reduced NF-κB signaling by BI-69-A11 is mediated by the inhibition of sphingosine kinase 1, identified in a screen of 315 kinases. Significantly, we demonstrate that BI-69A11 is well tolerated and orally active against UACC 903 and SW1 melanoma xenografts. Our results demonstrate that BI-69A11 inhibits both the AKT and the NF-κB pathways and that the dual targeting of these pathways may be efficacious as a therapeutic strategy in melanoma.

Control of mTORC1 signaling by the Opitz syndrome protein MID1

Mutations in the MID1 gene are causally linked to X-linked Opitz BBB/G syndrome (OS), a congenital disorder that primarily affects the formation of diverse ventral midline structures. The MID1 protein has been shown to function as an E3 ligase targeting the catalytic subunit of protein phosphatase 2A (PP2A-C) for ubiquitin-mediated degradation. However, the molecular pathways downstream of the MID1/PP2A axis that are dysregulated in OS and that translate dysfunctional MID1 and elevated levels of PP2A-C into the OS phenotype are poorly understood. Here, we show that perturbations in MID1/PP2A affect mTORC1 signaling. Increased PP2A levels, resulting from proteasome inhibition or depletion of MID1, lead to disruption of the mTOR/Raptor complex and down-regulated mTORC1 signaling. Congruously, cells derived from OS patients that carry MID1 mutations exhibit decreased mTORC1 formation, S6K1 phosphorylation, cell size, and cap-dependent translation, all of which is rescued by expression of wild-type MID1 or an activated mTOR allele. Our findings define mTORC1 signaling as a downstream pathway regulated by the MID1/PP2A axis, suggesting that mTORC1 plays a key role in OS pathogenesis.

Tuberous sclerosis complex 1: an epithelial tumor suppressor essential to prevent spontaneous prostate cancer in aged mice

The phosphoinositide 3-kinase (PI3K) pathway regulates mammalian cell growth, survival, and motility and plays a major pathogenetic role in human prostate cancer (PCa). However, the oncogenic contributions downstream of the PI3K pathway made by mammalian target of rapamycin complex 1 (mTORC1)-mediated cell growth signal transduction in PCa have yet to be elucidated in detail. Here, we engineered constitutive mTORC1 activation in prostate epithelium by a conditional genetic deletion of tuberous sclerosis complex 1 (Tsc1), a potent negative regulator of mTORC1 signaling. Epithelial inactivation was not immediately tumorigenic, but Tsc1-deficient mice developed prostatic intraepithelial neoplasia (mPIN) in lateral and anterior prostates by 6 months of age, with increasing disease penetrance over time. Lateral prostate lesions in 16- to 22-month-old mutant mice progressed to two types of more advanced lesions, adenomatous gland forming lesion (Type 1) and atypical glands embedded in massively expanded reactive stroma (Type 2). Both Type 1 and Type 2 lesions contained multiple foci of microinvasive carcinoma. Epithelial neoplastic and atypical stromal lesions persisted despite 4 weeks of RAD001 chemotherapy. Rapalogue resistance was not due to AKT or extracellular signal-regulated kinase 1/2 activation. Expression of the homeobox gene Nkx3.1 was lost in Tsc1-deficient mPIN, and it cooperated with TSC1 loss in mPIN initiation in doubly mutant Tsc1:Nkx3.1 prostatic epithelial knockout mice. Thus, TSC1 inactivation distal to PI3K and AKT activation is sufficient to activate a molecular signaling cascade producing prostatic neoplasia and focal carcinogenesis.

Mammalian target of rapamycin activator RHEB is frequently overexpressed in human carcinomas and is critical and sufficient for skin epithelial carcinogenesis

Small GTPase Ras homologue enriched in brain (RHEB) binds and activates the key metabolic regulator mTORC1, which has an important role in cancer cells, but the role of RHEB in cancer pathogenesis has not been shown. By performing a meta-analysis of published cancer cytogenetic and transcriptome databases, we defined a gain of chromosome 7q36.1-q36.3 containing the RHEB locus, an overexpression of RHEB mRNA in several different carcinoma histotypes, and an association between RHEB upregulation and poor prognosis in breast and head and neck cancers. To model gain of function in epithelial malignancy, we targeted Rheb expression to murine basal keratinocytes of transgenic mice at levels similar to those that occur in human squamous cancer cell lines. Juvenile transgenic epidermis displayed constitutive mTORC1 pathway activation, elevated cyclin D1 protein, and diffuse skin hyperplasia. Skin tumors subsequently developed with concomitant stromal angio-inflammatory foci, evidencing induction of an epidermal hypoxia-inducible factor-1 transcriptional program, and paracrine feed-forward activation of the interleukin-6-signal transducer and activator of transcription 3 pathway. Rheb-induced tumor persistence and neoplastic molecular alterations were mTORC1 dependent. Rheb markedly sensitized transgenic epidermis to squamous carcinoma induction following a single dose of Ras-activating carcinogen 7,12-dimethylbenz(a)anthracene. Our findings offer direct evidence that RHEB facilitates multistage carcinogenesis through induction of multiple oncogenic mechanisms, perhaps contributing to the poor prognosis of patients with cancers overexpressing RHEB.

Abstract 69: Functional metabolomic profiling of human melanocyte and melanoma cells during hypoxic adaptation

Adaptation to hypoxia is an important part of the cellular progression from normal melanocyte to malignant melanoma. This adaptation can be comprised of short-term remodeling of the cellular metabolic network and/or the permanent, gene-driven development of a new metabolic program. In either case, low oxygen conditions require a unique metabolic program for continued survival and proliferation. However, beyond a general shift from respiration to glycolysis, the details of this permissive metabolic program in melanoma are generally unknown. In order to better understand the changes to both the metabolome and metabolic flux in this system, we applied functional metabolomic profiling to seven human melanoma cell lines and to two human melanocyte cell lines under both hypoxic and normoxic conditions. The melanoma panel consists of four cell lines driven by B-raf mutations (WM35, UACC903, WM793 and LU1205), two containing N-ras mutations (MEL501 and WM1346) and one p53 mutant (MeWo). These cells, along with the two melanocyte lines, were grown in media containing a 1:1 mixture of natural and [U-13C]-labeled glucose for 24 hours in either 1% oxygen or atmospheric conditions. We then employed GCMS, NMR and biochemical experiments to map flux through five metabolic hubs and quantify the abundance of 28 key metabolites for each cell line and condition. These maps of central carbon metabolism encompass glycolysis, the pentose phosphate pathway, fatty acid biosynthesis, the glutamate-glutamine-proline network, and glycine/serine de novo synthesis, along with a number of other important metabolites. Analyzing these data with unbiased hierarchical clustering and Ingenuity Pathway Analysis, we discovered a metabolite-derived signature for hypoxic adaptation in melanocytes. Furthermore, we found that certain hypoxic responses are unique to either the B-raf or N-ras of the cell line subset. Interestingly, the hypoxic response of the mutant p53 cell line is more like that of the B-raf mutant cell lines than the B-raf and N-ras sets are to each other. The information gained from this functional metabolomic profiling advances our ability to identify hypoxia-adapted melanomas and develop metabolism-based therapeutic interventions.

This work was supported by P01 CA128814-01 from the National Cancer Institute.

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 69.

Divergent S phase checkpoint activation arising from prereplicative complex deficiency controls cell survival

The DNA replication machinery plays additional roles in S phase checkpoint control, although the identities of the replication proteins involved in checkpoint activation remain elusive. Here, we report that depletion of the prereplicative complex (pre-RC) protein Cdc6 causes human nontransformed diploid cells to arrest nonlethally in G1-G1/S and S phase, whereas multiple cancer cell lines undergo G1-G1/S arrest and cell death. These divergent phenotypes are dependent on the activation, or lack thereof, of an ataxia telangiectasia and Rad3-related (ATR)-dependent S phase checkpoint that inhibits replication fork progression. Although pre-RC deficiency induces chromatin structural alterations in both nontransformed and cancer cells that normally lead to ATR checkpoint activation, the sensor mechanisms in cancer cells seem to be compromised such that higher levels of DNA replication stress/damage are required to trigger checkpoint response. Our results suggest that therapy-induced disruption of pre-RC function might exert selective cytotoxic effects on tumor cells in human patients.

Targeting the mTOR signaling network in cancer

The mammalian target of rapamycin (mTOR) is an unconventional protein kinase that is centrally involved in the control of cancer cell metabolism, growth and proliferation. The mTOR pathway has attracted broad scientific and clinical interest, particularly in light of the ongoing clinical cancer trials with mTOR inhibitors. The mixed clinical results to date reflect the complexity of both cancer as a disease target, and the mTOR signaling network, which contains two functionally distinct mTOR complexes, parallel regulatory pathways, and feedback loops that contribute to the variable cellular responses to the current inhibitors. In this review, we discuss the regulatory pathways that govern mTOR activity, and highlight clinical results obtained with the first generation of mTOR inhibitors to reach the oncology clinics.

The phosphatidylinositol 3-kinase inhibitor, PX-866, is a potent inhibitor of cancer cell motility and growth in three-dimensional cultures

The phosphatidylinositol 3-kinase (PI3K) pathway is activated in many human tumors and mediates processes such as cell proliferation, survival, adhesion, and motility. The natural product, wortmannin, has been widely used to study the functional consequences of PI3K inhibition in both normal and transformed cells in culture but is not a suitable cancer chemotherapeutic agent due to stability and toxicity issues. PX-866, an improved wortmannin analogue, displays significant antitumor activity in xenograft models. Here, we directly compare PX-866 and wortmannin in human cancer cell lines cultured in monolayer or as three-dimensional spheroids. Both PI3K inhibitors failed to inhibit monolayer cell growth at concentrations up to 100 nmol/L but strongly suppressed spheroid growth at low nanomolar concentrations, with PX-866 showing greater potency than wortmannin. Relative to wortmannin, PX-866 treatment results in a more sustained loss of Akt phosphorylation, suggesting that the increased potency of PX-866 is related to a more durable inhibition of PI3K signaling. PX-866 and wortmannin both inhibit spheroid growth without causing cytotoxicity, similar to known cytostatic agents, such as rapamycin. PX-866 also inhibits cancer cell motility at subnanomolar concentrations. These findings suggest that the antitumor activities of PX-866 stem from prolonged inhibition of the PI3K pathway and inhibition of cell motility. In addition, we propose that the use of three-dimensional tumor models is more predictive of in vivo growth inhibition by PI3K inhibitors in cancer cell lines lacking phosphatase and tensin homologue activity or expression.

The FRB Domain of mTOR: NMR Solution Structure and Inhibitor Design†,‡

The mammalian target of rapamycin (mTOR) is a protein that is intricately involved in signaling pathways controlling cell growth. Rapamycin is a natural product that binds and inhibits mTOR function by interacting with its FKBP-rapamycin-binding (FRB) domain. Here we report on the NMR solution structure of FRB and on further studies aimed at the identification and characterization of novel ligands that target the rapamycin binding pocket. The biological activity of the ligands, and that of rapamycin in the absence of FKBP12, was investigated by assaying the kinase activity of mTOR. While we found that rapamycin binds the FRB domain and inhibits the kinase activity of mTOR even in the absence of FKBP12 (in the low micromolar range), our most potent ligands bind to FRB with similar binding affinity but inhibit the kinase activity of mTOR at much higher concentrations. However, we have also identified one low-affinity compound that is also capable of inhibiting mTOR. Hence, we have identified compounds that can directly mimic rapamycin or can dissociate the FRB binding from the inhibition of the catalytic activity of mTOR. As such, these ligands could be useful in deciphering the complex regulation of mTOR in the cell and in validating the FRB domain as a possible target for the development of novel therapeutic compounds.

Genotoxic stress targets human Chk1 for degradation by the ubiquitin-proteasome pathway

The Chk1 kinase is a major effector of S phase checkpoint signaling during the cellular response to genotoxic stress. Here, we report that replicative stress induces the polyubiquitination and degradation of Chk1 in human cells. This response is triggered by phosphorylation of Chk1 at Ser-345, a known target site for the upstream activating kinase ATR. The ubiquitination of Chk1 is mediated by E3 ligase complexes containing Cul1 or Cul4A. Treatment of cells with the anticancer agent camptothecin (CPT) triggers Chk1 destruction, which blocks recovery from drug-induced S phase arrest and leads to cell death. These findings indicate that ATR-dependent phosphorylation of Chk1 delivers a signal that both activates Chk1 and marks this protein for proteolytic degradation. Proteolysis of activated Chk1 may promote checkpoint termination under normal conditions, and may play an important role in the cytotoxic effects of CPT and related anticancer drugs.

Isoform-specific phosphoinositide 3-kinase inhibitors from an arylmorpholine scaffold

Phosphoinositide 3-kinases (PI3-Ks) are an ubiquitous class of signaling enzymes that regulate diverse cellular processes including growth, differentiation, and motility. Physiological roles of PI3-Ks have traditionally been assigned using two pharmacological inhibitors, LY294002 and wortmannin. Although these compounds are broadly specific for the PI3-K family, they show little selectivity among family members, and the development of isoform-specific inhibitors of these enzymes has been long anticipated. Herein, we prepare compounds from two classes of arylmorpholine PI3-K inhibitors and characterize their specificity against a comprehensive panel of targets within the PI3-K family. We identify multiplex inhibitors that potently inhibit distinct subsets of PI3-K isoforms, including the first selective inhibitor of p110beta/p110delta (IC(50) p110beta=0.13 microM, p110delta=0.63 microM). We also identify trends that suggest certain PI3-K isoforms may be more sensitive to potent inhibition by arylmorpholines, thereby guiding future drug design based on this pharmacophore.

Phosphorylation of mammalian target of rapamycin (mTOR) at Ser-2448 is mediated by p70S6 kinase

The mammalian target of rapamycin (mTOR) coordinates cell growth with the growth factor and nutrient/energy status of the cell. The phosphatidylinositol 3-kinase-AKT pathway is centrally involved in the transmission of mitogenic signals to mTOR. Previous studies have shown that mTOR is a direct substrate for the AKT kinase and identified Ser-2448 as the AKT target site in mTOR. In this study, we demonstrate that rapamycin, a specific inhibitor of mTOR function, blocks serum-stimulated Ser-2448 phosphorylation and that this drug effect is not explained by the inhibition of AKT. Furthermore, the phosphorylation of Ser-2448 was dependent on mTOR kinase activity, suggesting that mTOR itself or a protein kinase downstream from mTOR was responsible for the modification of Ser-2448. Here we show that p70S6 kinase phosphorylates mTOR at Ser-2448 in vitro and that ectopic expression of rapamycin-resistant p70S6 kinase restores Ser-2448 phosphorylation in rapamycin-treated cells. In addition, we show that cellular amino acid status, which modulates p70S6 kinase (S6K1) activity via the TSC/Rheb pathway, regulates Ser-2448 phosphorylation. Finally, small interfering RNA-mediated depletion of p70S6 kinase reduces Ser-2448 phosphorylation in cells. Taken together, these results suggest that p70S6 kinase is a major effector of mTOR phosphorylation at Ser-2448 in response to both mitogen- and nutrient-derived stimuli.

Determination of the catalytic activities of mTOR and other members of the phosphoinositide-3-kinase-related kinase family

Members of the phosphoinositide-3-kinase-related kinase (PIKK) family, which includes mTOR, ATM, ATR, and hSMG-1, play important roles in regulating the cellular response to environmental stimuli. Despite the similarity of their catalytic domain to that of phosphoinositide-3-kinase, these extremely large (>250 kDa) polypeptides function as serine/threonine protein kinases. The catalytic activities of these PIKK family members can now be measured in immune-complex kinase assays. This assay involves isolation of the kinase by immunoprecipitation and the in vitro phosphorylation of a specific substrate in the presence of radio-labeled ATP. Here we describe, in detail, the determination of PIKK catalytic activity with a standardized immune-complex kinase assay protocol.

Synthesis and biological evaluation of synthetic viridins derived from C(20)-heteroalkylation of the steroidal PI-3-kinase inhibitor wortmannin

A series of viridin analogs was prepared from wortmannin by nucleophilic ring opening at C(20) and evaluated against the signaling kinases PI-3-kinase and mTOR. Several subnanomolar enzyme inhibitors with orders of magnitude selectivity for PI-3-kinase and strong cytotoxic activity against four cancer cell lines were identified. Among the ten most promising derivatives, six demonstrated lower liver toxicity and greater promise for inhibition of tumor cell growth than the lead structure wortmannin.

Differential effects of rapamycin on mammalian target of rapamycin signaling functions in mammalian cells

Rapamycin and its analogues have shown promising anticancer activities in preclinical and clinical studies. However, the mechanism whereby rapamycin inhibits signaling through the mammalian target of rapamycin (mTOR) remains poorly understood. Here, we show that the FKBP12/rapamycin complex is an essentially irreversible inhibitor of mTOR kinase activity in vitro. However, we observe no suppression of mTOR catalytic activity after immunoprecipitation from rapamycin-treated cells. These results suggest either that rapamycin acts as a reversible kinase inhibitor in intact cells or that the cellular effects of rapamycin are not mediated through global suppression in mTOR kinase activity. To better understand the cellular pharmacology of rapamycin, we compared the individual and combined effects of rapamycin and kinase-inactive mTOR expression on a panel of mTOR-dependent cellular responses. These studies identified glycolytic activity, amino acid transporter trafficking, and Akt kinase activity as novel, mTOR-modulated functions in mammalian cells. Whereas kinase-inactive mTOR did not enhance the decreases in cell size and glycolysis induced by rapamycin, expression of this mTOR mutant significantly enhanced the inhibitory effects of rapamycin on cell proliferation, 4EBP1 phosphorylation, and Akt activity. Unexpectedly, amino acid transporter trafficking was perturbed by kinase-inactive mTOR but not by rapamycin, indicating that this process is rapamycin insensitive. These results indicate that rapamycin exerts variable inhibitory actions on mTOR signaling functions and suggest that direct inhibitors of the mTOR kinase domain will display substantially broader anticancer activities than rapamycin.

A simple method for enriching populations of transfected CHO cells for cells of higher specific productivity

To establish a simple and rapid method for the screening of stable recombinant Chinese hamster ovary (CHO) cell lines, we have developed a cell surface labeling technique using fluorescently tagged antibodies that bind to secreted target proteins at low temperature. Using fluorescence intensity as the sole criterion for selection of cells, we are able to enrich populations of highly productive cells using preparative flow cytometry sorting. Reiterative sorting based on selection of cells having the highest fluorescence intensity of cell surface labeled protein results in dramatic increases in specific cellular productivity. Using lymphotoxin-beta receptor IgG fusion protein as a model system, we have demonstrated a greater than 20-fold increase in specific productivity (0.49-11.5 pg cell(-1) day(-1)) (pcd) without the use of methotrexate (MTX)-mediated selection or amplification. In addition, the flow cytometry used to enrich for and clone high producer cell lines has reduced development time by more than 50% and the number of screening assays by more than 10-fold. When a transfected population of CHO cells expressing a humanized version of the murine monoclonal antibody (mAb) AQC2 directed against human alpha 1 beta 1 integrin was subjected to the same treatment, a 25-fold improvement in specific productivity (0.3-8.0 pcd) was observed. Furthermore, similar application of this technique to MTX-amplified clones resulted in up to 120-fold overall improvement in specific productivity (up to 42 pcd). Greater than 20 examples are also presented to demonstrate the robustness and performance of this technique.

Regulation of hypoxia-inducible factor 1alpha expression and function by the mammalian target of rapamycin

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor containing an inducibly expressed HIF-1alpha subunit and a constititutively expressed HIF-1beta subunit. Under hypoxic conditions, the HIF-1alpha subunit accumulates due to a decrease in the rate of proteolytic degradation, and the resulting HIF-1alpha-HIF-1beta heterodimers undergo posttranslational modifications that promote transactivation. Recent studies suggest that amplified signaling through phosphoinositide 3-kinase, and its downstream target, mTOR, enhances HIF-1-dependent gene expression in certain cell types. In the present study, we have explored further the linkage between mTOR and HIF-1 in PC-3 prostate cancer cells treated with hypoxia or the hypoxia mimetic agent, CoCl(2). Pretreatment of PC-3 cells with the mTOR inhibitor, rapamycin, inhibited both the accumulation of HIF-1alpha and HIF-1-dependent transcription induced by hypoxia or CoCl(2). Transfection of these cells with wild-type mTOR enhanced HIF-1 activation by hypoxia or CoCl(2), while expression of a rapamycin-resistant mTOR mutant rendered both HIF-1alpha stabilization and HIF-1 transactivating function refractory to inhibition by rapamycin. Studies with GAL4-HIF-1alpha fusion proteins pinpointed the oxygen-dependent degradation domain as a critical target for the rapamycin-sensitive, mTOR-dependent signaling pathway leading to HIF-1alpha stabilization by CoCl(2). These studies position mTOR as an upstream activator of HIF-1 function in cancer cells and suggest that the antitumor activity of rapamycin is mediated, in part, through the inhibition of cellular responses to hypoxic stress.

Specific dephosphorylation of the Lck tyrosine protein kinase at Tyr-394 by the SHP-1 protein-tyrosine phosphatase

The protein-tyrosine phosphatase SHP-1 has been shown to be a negative regulator of multiple signaling pathways in hematopoietic cells. In this study, we demonstrate that SHP-1 dephosphorylates the lymphoid-specific Src family kinase Lck at Tyr-394 when both are transiently co-expressed in nonlymphoid cells. We also demonstrate that a GST-SHP-1 fusion protein specifically dephosphorylates Lck at Tyr-394 in vitro. Because phosphorylation of Tyr-394 activates Lck, the fact that SHP-1 specifically dephosphorylates this site suggests that SHP-1 is a negative regulator of Lck. The failure of SHP-1 to inactivate Lck may contribute to some of the lymphoid abnormalities observed in motheaten mice.

Phosphorylation of a Src kinase at the autophosphorylation site in the absence of Src kinase activity

Exposure of cells to oxidants increases the phosphorylation of the Src family tyrosine protein kinase Lck at Tyr-394, a conserved residue in the activation loop of the catalytic domain. Kinase-deficient Lck expressed in fibroblasts that do not express any endogenous Lck has been shown to be phosphorylated at Tyr-394 following H(2)O(2) treatment to an extent indistinguishable from that seen with wild type Lck. This finding indicates that a kinase other than Lck itself is capable of phosphorylating Tyr-394. Because fibroblasts express other Src family members, it remained to be determined whether the phosphorylation of Tyr-394 was carried out by another Src family kinase or by an unrelated tyrosine protein kinase. We examined here whether Tyr-394 in kinase-deficient Lck was phosphorylated following exposure of cells devoid of endogenous Src family kinase activity to H(2)O(2). Strikingly, treatment of such cells with H(2)O(2) led to the phosphorylation of Tyr-394 to an extent identical to that seen with wild type Lck, demonstrating that Src family kinases are not required for H(2)O(2)-induced phosphorylation of Lck. Furthermore, this efficient phosphorylation of Lck at Tyr-394 in non-lymphoid cells suggests the existence of an ubiquitous activator of Src family kinases.

Bone marrow stromal cell-mediated gene therapy for hemophilia A: in vitro expression of human factor VIII with high biological activity requires the inclusion of the proteolytic site at amino acid 1648

To evaluate the potential of the ex vivo bone marrow stromal cell (BMSC) system as a gene therapy for hemophilia A, we studied the in vitro expression of human factor VIII (hFVIII) in canine BMSCs following transfection with plasmid vectors and transduction with retroviral vectors. Vectors were composed of B domain-deleted forms of hFVIII that either retain or delete the proteolytic site at amino acid 1648. On transfection of BMSCs, vectors supported expression and secretion of similar levels of up to 386 mU/10(6) cells/24 hr, even though only 3-9% of the cells expressed hFVIII while 42-48% of transfected cells harbored plasmid vector. Much higher percentages (approximately 70%) of cells expressing hFVIII were achieved when BMSCs were transduced by retroviral vectors, resulting in expression and secretion as high as 1000-4000 mU/10(6) cells/24 hr. Western analysis demonstrated that the B domain-deleted forms possessing the proteolytic site were secreted predominantly as heavy and light chain heterodimers that resemble native forms found in plasma. In contrast, the hFVIII lacking the proteolytic site was expressed mostly as unprocessed, single heavy-light chains. Both hFVIII forms were correctly cleaved and activated by thrombin. The proteolyzed hFVIII form possessed > or = 93% normal biological activity while the unproteolyzed form possessed consistently less than 55% normal biological activity and was therefore considered less suitable for therapeutic application. These results demonstrate that the BMSC system has potential utility in gene therapy for hemophilia A and stress the importance of selecting the appropriate hFVIII structure for prospective clinical use.

Retroviral vector-modified bone marrow stromal cells secrete biologically active factor IX in vitro and transiently deliver therapeutic levels of human factor IX to the plasma of dogs after reinfusion

Canine bone marrow stromal cells (BMSCs), transduced ex vivo with retroviral vectors, expressed and secreted biologically active human and canine coagulation factor IX (hFIX and cFIX) in vitro, and on autologous reinfusion expressed hFIX into the circulation of normal (nonhemophiliac) dogs. Human FIX, when expressed in vitro by BMSCs of two dogs at 1.22 and 1.39 microg/10(6) cells/24 hr in medium supplemented with vitamin K, respectively, exhibited 28.1 and 27.3% normal biological activity as determined on the basis of a one-stage clotting assay. BMSCs of two additional dogs expressed 1.54 and 4.81 microg of cFIX/10(6) cells/24 hr in vitamin K-supplemented medium and the expressed cFIX possessed 58.4 and 32.9% normal activity, respectively. Between 2.33 and 3.35 x 10(8) transduced BMSCs, expressing 1.22 and 2.61 microg of hFIX/10(6) cells/24 hr or 3.24 and 7.82 microg of cFIX/10(6) cells/24 hr were reintroduced into the four donor dogs by intravenous infusion. Human FIX was detected in plasma for 7 or 12 days after BMSC reinfusion, with peak levels of 85.8 and 233.0 ng/ml observed at 2 days. Canine anti-hFIX antibodies, which were detected as early as 2-4 days after reinfusion of BMSCs expressing hFIX, may have masked potentially longer duration expression in vivo. Peak plasma levels of hFIX represented 2.1 and 5.8% normal human hFIX levels. When adjusted for percent normal one-stage clotting activity determined in vitro, these levels represented 0.6 and 1.6% normal human hFIX activity levels. Thus, we have demonstrated that retroviral vector-modified BMSCs can deliver human therapeutic levels of hFIX to the circulation of dogs.

Tyrosine phosphorylation of RNA polymerase II carboxyl-terminal domain by the Abl-related gene product

The largest subunit of RNA polymerase II contains a C-terminal repeated domain (CTD) that is the site of phosphorylation by serine (threonine) and tyrosine kinases. Phosphorylation of the CTD is correlated with transcription elongation. A number of different kinases have previously been shown to phosphorylate the CTD; among them is a nuclear tyrosine kinase encoded by the c-abl proto-oncogene. The processive and high stoichiometric phosphorylation of RNA polymerase II by c-Abl requires the tyrosine kinase, the SH2 domain, and a CTD-interacting domain (CTD-ID) in the Abl protein. The physiological tyrosine phosphorylation of RNA polymerase II by c-Abl in DNA damage response has previously been demonstrated. Basal tyrosine phosphorylation of RNA polymerase II, however, is observed in cells derived from abl-deficient mice, indicating the existence of other CTD tyrosine kinases. In this report, we show that the tyrosine kinase encoded by an Abl-related gene (Arg) also phosphorylates the CTD in vitro and in transfected cells. The SH2 and kinase domain of Arg are 95% identical to that of c-Abl. However, these two proteins share only 29% identity in the large C-terminal region. Interestingly, a CTD-ID is also found in the C-terminal region of Arg. Mapping studies and sequence analysis have led to the identification of the CTD-ID that is highly conserved among the divergent C-terminal regions of Abl and Arg. These results indicate that tyrosine phosphorylation of RNA polymerase II CTD could be catalyzed by either c-Abl or Arg kinase.