Regulation of TFEB and V-ATPases by mTORC1.

Mammalian target of rapamycin (mTOR) complex 1 (mTORC1) is an important, highly conserved, regulator of cell growth. Ancient among the signals that regulate mTORC1 are nutrients. Amino acids direct mTORC1 to the surface of the late endosome/lysosome, where mTORC1 becomes receptive to other inputs. However, the interplay between endosomes and mTORC1 is poorly understood. Here, we report the discovery of a network that links mTORC1 to a critical component of the late endosome/lysosome, the V-ATPase. In an unbiased screen, we found that mTORC1 regulated the expression of, among other lysosomal genes, the V-ATPases. mTORC1 regulates V-ATPase expression both in cells and in mice. V-ATPase regulation by mTORC1 involves a transcription factor translocated in renal cancer, TFEB. TFEB is required for the expression of a large subset of mTORC1 responsive genes. mTORC1 coordinately regulates TFEB phosphorylation and nuclear localization and in a manner dependent on both TFEB and V-ATPases, mTORC1 promotes endocytosis. These data uncover a regulatory network linking an oncogenic transcription factor that is a master regulator of lysosomal biogenesis, TFEB, to mTORC1 and endocytosis.

Imatinib mesylate efficiently achieves therapeutic intratumor concentrations in vivo but has limited activity in a xenograft model of small cell lung cancer.

PURPOSE: Despite recent advances in cancer therapy, long-term survival in small cell lung cancer (SCLC) remains uncommon, underscoring the need for novel therapeutic approaches. Previous studies have identified constitutive expression of the receptor tyrosine kinase, c-Kit, and its ligand, stem cell factor, in a substantial proportion of SCLC specimens. The purpose of this study was to determine whether imatinib mesylate, an inhibitor of c-Kit, could achieve therapeutic concentrations in tumors and in brain (a frequent site of SCLC metastasis) and interfere with SCLC tumor growth in vivo. EXPERIMENTAL DESIGN: Human SCLC tumor cell lines with constitutive c-kit expression and tyrosine phosphorylation (NCI-H209, NCI-H526, and NCI-H1607) were used to establish SCLC tumor xenografts in NCr nude (nu/nu)-immunodeficient mice. SCLC tumor-bearing mice were randomly assigned to imatinib or control (water) administered twice a day by oral gavage. Imatinib concentrations in plasma, brain, and tumor were quantitated and correlated with tumor response. RESULTS: Therapeutic concentrations of imatinib were achieved in plasma and tumor xenografts but not in the brain. Imatinib blocked the constitutive activation of c-kit in SCLC tumor cell lines in vitro but had a negligible effect on SCLC xenograft growth in vivo. CONCLUSIONS: Orally administered imatinib rapidly reaches therapeutic concentrations in SCLC xenografts, suggesting the feasibility of combining imatinib with other novel or traditional chemotherapeutic agents in SCLC or other solid tumors. The c-Kit signaling pathway does not appear to play a critical role in SCLC proliferation and viability in vivo, however, suggesting that imatinib is unlikely to be effective as monotherapy for SCLC.

High-throughput simultaneous screen and counterscreen identifies homoharringtonine as synthetic lethal with von Hippel-Lindau loss in renal cell carcinoma.

Renal cell carcinoma (RCC) accounts for 85% of primary renal neoplasms, and is rarely curable when metastatic. Approximately 70% of RCCs are clear-cell type (ccRCC), and in >80% the von Hippel-Lindau (VHL) gene is mutated or silenced. We developed a novel, high-content, screening strategy for the identification of small molecules that are synthetic lethal with genes mutated in cancer. In this strategy, the screen and counterscreen are conducted simultaneously by differentially labeling mutant and reconstituted isogenic tumor cell line pairs with different fluorochromes and using a highly sensitive high-throughput imaging-based platform. This approach minimizes confounding factors from sequential screening, and more accurately replicates the in vivo cancer setting where cancer cells are adjacent to normal cells. A screen of ~12,800 small molecules identified homoharringtonine (HHT), an FDA-approved drug for treating chronic myeloid leukemia, as a VHL-synthetic lethal agent in ccRCC. HHT induced apoptosis in VHL-mutant, but not VHL-reconstituted, ccRCC cells, and inhibited tumor growth in 30% of VHL-mutant patient-derived ccRCC tumorgraft lines tested. Building on a novel screening strategy and utilizing a validated RCC tumorgraft model recapitulating the genetics and drug responsiveness of human RCC, these studies identify HHT as a potential therapeutic agent for a subset of VHL-deficient ccRCCs.

REDD1/DDIT4-independent mTORC1 inhibition and apoptosis by glucocorticoids in thymocytes.

UNLABELLED: Glucocorticoids induce apoptosis in lymphocytes and are commonly used to treat hematologic malignancies. However, they are also associated with significant adverse effects and their molecular mechanism of action is not fully understood. Glucocorticoid treatment induces expression of the mTORC1 inhibitor Regulated in Development and DNA Damage Response 1 (REDD1), also known as DNA-Damage Inducible Transcript 4 (DDIT4), and mTORC1 inhibition may distinguish glucocorticoid-sensitive from glucocorticoid-resistant acute lymphoblastic leukemia (ALL). Interestingly, REDD1 induction was impaired in glucocorticoid-resistant ALL cells and inhibition of mTORC1 using rapamycin restored glucocorticoid sensitivity. These data suggest that REDD1 may be essential for the response of ALL cells to glucocorticoids. To further investigate the role of REDD1, we evaluated the effects of glucocorticoids on primary thymocytes from wild-type and REDD1-deficient mice. Glucocorticoid-mediated apoptosis was blocked by a glucocorticoid receptor antagonist and by an inhibitor of transcription, which interfered with REDD1 induction and mTORC1 inhibition. However, REDD1 ablation had no effect on glucocorticoid-induced mTORC1 inhibition and apoptosis in thymocytes ex vivo. Overall, these data not only demonstrate the contextual differences of downstream signaling following glucocorticoid treatment but also provide a better mechanistic understanding of the role of REDD1. IMPLICATIONS: These molecular findings underlying glucocorticoid action and the role of REDD1 are fundamental for the design of novel, more efficacious, and less toxic analogs. Mol Cancer Res; 12(6); 867-77. ©2014 AACR.

A validated tumorgraft model reveals activity of dovitinib against renal cell carcinoma.

Most anticancer drugs entering clinical trials fail to achieve approval from the U.S. Food and Drug Administration. Drug development is hampered by the lack of preclinical models with therapeutic predictive value. Herein, we report the development and validation of a tumorgraft model of renal cell carcinoma (RCC) and its application to the evaluation of an experimental drug. Tumor samples from 94 patients were implanted in the kidneys of mice without additives or disaggregation. Tumors from 35 of these patients formed tumorgrafts, and 16 stable lines were established. Samples from metastatic sites engrafted at higher frequency than those from primary tumors, and stable engraftment of primary tumors in mice correlated with decreased patient survival. Tumorgrafts retained the histology, gene expression, DNA copy number alterations, and more than 90% of the protein-coding gene mutations of the corresponding tumors. As determined by the induction of hypercalcemia in tumorgraft-bearing mice, tumorgrafts retained the ability to induce paraneoplastic syndromes. In studies simulating drug exposures in patients, RCC tumorgraft growth was inhibited by sunitinib and sirolimus (the active metabolite of temsirolimus in humans), but not by erlotinib, which was used as a control. Dovitinib, a drug in clinical development, showed greater activity than sunitinib and sirolimus. The routine incorporation of models recapitulating the molecular genetics and drug sensitivities of human tumors into preclinical programs has the potential to improve oncology drug development.

Cell-type-dependent regulation of mTORC1 by REDD1 and the tumor suppressors TSC1/TSC2 and LKB1 in response to hypoxia.

mTORC1 is a critical regulator of cell growth that integrates multiple signals and is deregulated in cancer. We previously reported that mTORC1 regulation by hypoxia involves Redd1 and the Tsc1/Tsc2 complex. Here we show that Redd1 induction by hypoxia is tissue dependent and that hypoxia signals are relayed to mTORC1 through different pathways in a tissue-specific manner. In the liver, Redd1 induction is restricted to the centrilobular area, and in primary hepatocytes, mTORC1 inhibition by hypoxia is independent of Redd1. Furthermore, Tsc1/Tsc2 and Arnt (Hif-1ß) are similarly dispensable. Hypoxia signaling in hepatocytes involves Lkb1, AMP-activated protein kinase (AMPK), and raptor. Differences in signal relay extend beyond hypoxia and involve AMPK signaling. AMPK activation (using 5-aminoimidazole-4-carboxamide riboside [AICAR]) induces raptor phosphorylation and inhibits mTORC1 in both mouse embryo fibroblasts (MEFs) and hepatocytes, but whereas mTORC1 inhibition is Tsc1/Tsc2 dependent in MEFs, it is independent in hepatocytes. In liver cells, raptor phosphorylation is essential for both AMPK and hypoxia signaling. Thus, context-specific signals are required for raptor phosphorylation-induced mTORC1 inhibition. Our data illustrate a heretofore unappreciated topological complexity in mTORC1 regulation. Interestingly, topological differences in mTORC1 regulation by the tumor suppressor proteins Lkb1 and Tsc1/Tsc2 may underlie their tissue specificity of tumor suppressor action.

Interplay between pVHL and mTORC1 pathways in clear-cell renal cell carcinoma.

mTOR complex 1 (mTORC1) is implicated in cell growth control and is extensively regulated. We previously reported that in response to hypoxia, mTORC1 is inhibited by the protein regulated in development and DNA damage response 1 (REDD1). REDD1 is upregulated by hypoxia-inducible factor (HIF)-1, and forced REDD1 expression is sufficient to inhibit mTORC1. REDD1-induced mTORC1 inhibition is dependent on a protein complex formed by the tuberous sclerosis complex (TSC)1 and 2 (TSC2) proteins. In clear-cell renal cell carcinoma (ccRCC), the von Hippel-Lindau (VHL) gene is frequently inactivated leading to constitutive activation of HIF-2 and/or HIF-1, which may be expected to upregulate REDD1 and inhibit mTORC1. However, mTORC1 is frequently activated in ccRCC, and mTORC1 inhibitors are effective against this tumor type; a paradox herein examined. REDD1 was upregulated in VHL-deficient ccRCC by in silico microarray analyses, as well as by quantitative real-time PCR, Western blot, and immunohistochemistry. Vhl disruption in a mouse model was sufficient to induce Redd1. Using ccRCC-derived cell lines, we show that REDD1 upregulation in tumors is VHL dependent and that both HIF-1 and HIF-2 are, in a cell-type-dependent manner, recruited to, and essential for, REDD1 induction. Interestingly, whereas mTORC1 is responsive to REDD1 in some tumors, strategies have evolved in others, such as mutations disrupting TSC1, to subvert mTORC1 inhibition by REDD1. Sequencing analyses of 77 ccRCCs for mutations in TSC1, TSC2, and REDD1, using PTEN as a reference, implicate the TSC1 gene, and possibly REDD1, as tumor suppressors in sporadic ccRCC. Understanding how ccRCCs become refractory to REDD1-induced mTORC1 inhibition should shed light into the development of ccRCC and may aid in patient selection for molecular-targeted therapies.

PD166326, a novel tyrosine kinase inhibitor, has greater antileukemic activity than imatinib mesylate in a murine model of chronic myeloid leukemia.

Imatinib mesylate is highly effective in newly diagnosed chronic myeloid leukemia (CML), but BCR/ABL (breakpoint cluster region/abelson murine leukemia)-positive progenitors persist in most patients with CML treated with imatinib mesylate, indicating the need for novel therapeutic approaches. In this study, we have used the murine CML-like myeloproliferative disorder as a platform to characterize the pharmacokinetic, signal transduction, and antileukemic properties of PD166326, one of the most potent members of the pyridopyrimidine class of protein tyrosine kinase inhibitors. In mice with the CML-like disease, PD166326 rapidly inhibited Bcr/Abl kinase activity after a single oral dose and demonstrated marked antileukemic activity in vivo. Seventy percent of PD166326-treated mice achieved a white blood cell (WBC) count less than 20.0 x 10(9)/L (20,000/microL) at necropsy, compared with only 8% of imatinib mesylate-treated animals. Further, two thirds of PD166326-treated animals had complete resolution of splenomegaly, compared with none of the imatinib mesylate-treated animals. Consistent with its more potent antileukemic effect in vivo, PD166326 was also superior to imatinib mesylate in inhibiting the constitutive tyrosine phosphorylation of numerous leukemia-cell proteins, including the src family member Lyn. PD166326 also prolonged the survival of mice with imatinib mesylate-resistant CML induced by the Bcr/Abl mutants P210/H396P and P210/M351T. Altogether, these findings demonstrate the potential of more potent Bcr/Abl inhibitors to provide more effective antileukemic activity. Clinical development of PD166326 or a related analog may lead to more effective drugs for the treatment of de novo and imatinib mesylate-resistant CML.

The CNS is a sanctuary for leukemic cells in mice receiving imatinib mesylate for Bcr/Abl-induced leukemia.

The chronic myelogenous leukemia (CML)-like myeloproliferative disorder observed in the BCR/ABL murine bone marrow transduction and transplantation model shares several features with the human disease, including a high response rate to the tyrosine kinase inhibitor imatinib mesylate (STI571). To study the impact of chronic imatinib mesylate treatment on the CML-like illness, mice were maintained on therapeutic doses of this drug and serially monitored. Unexpectedly, despite excellent systemic control of the CML-like illness, many of the mice developed progressive neurologic deficits after 2 to 4 months of imatinib mesylate therapy because of central nervous system (CNS) leukemia. Analysis of imatinib mesylate cerebral spinal fluid concentrations revealed levels 155- fold lower than in plasma. Thus, in the mouse, the limited ability of imatinib mesylate to cross the blood-brain barrier allowed the CNS to become a sanctuary for Bcr/Abl-induced leukemia. This model will be a useful tool for the future study of novel anti-CML drugs and in better defining the mechanisms for limited imatinib mesylate penetration into the CNS.