The p75 receptor mediates axon growth inhibition through an association with PIR-B

The Nogo receptor and paired immunoglobulin-like receptor B (PIR-B) are receptors for three myelin-derived axon-growth inhibitors, including myelin-associated glycoprotein (MAG). In this study, we report that the p75 receptor is required for the signal transduction of PIR-B, which interacted with p75 upon ligand binding. In addition, p75 was required for activation of Src homology 2-containing protein tyrosine phosphatase (SHP), which is induced by MAG binding to PIR-B. Mice carrying a mutation in the p75 gene showed promotion of axonal regeneration after optic nerve injury. Thus, our results indicate that p75 has a critical role in axon growth inhibition in specific neuronal tracts

A Transgenic Drosophila Model Demonstrates That the Helicobacter pylori CagA Protein Functions as a Eukaryotic Gab Adaptor

Infection with the human gastric pathogen Helicobacter pylori is associated with a spectrum of diseases including gastritis, peptic ulcers, gastric adenocarcinoma, and gastric mucosa–associated lymphoid tissue lymphoma. The cytotoxin-associated gene A (CagA) protein of H. pylori, which is translocated into host cells via a type IV secretion system, is a major risk factor for disease development. Experiments in gastric tissue culture cells have shown that once translocated, CagA activates the phosphatase SHP-2, which is a component of receptor tyrosine kinase (RTK) pathways whose over-activation is associated with cancer formation. Based on CagA's ability to activate SHP-2, it has been proposed that CagA functions as a prokaryotic mimic of the eukaryotic Grb2-associated binder (Gab) adaptor protein, which normally activates SHP-2. We have developed a transgenic Drosophila model to test this hypothesis by investigating whether CagA can function in a well-characterized Gab-dependent process: the specification of photoreceptors cells in the Drosophila eye. We demonstrate that CagA expression is sufficient to rescue photoreceptor development in the absence of the Drosophila Gab homologue, Daughter of Sevenless (DOS). Furthermore, CagA's ability to promote photoreceptor development requires the SHP-2 phosphatase Corkscrew (CSW). These results provide the first demonstration that CagA functions as a Gab protein within the tissue of an organism and provide insight into CagA's oncogenic potential. Since many translocated bacterial proteins target highly conserved eukaryotic cellular processes, such as the RTK signaling pathway, the transgenic Drosophila model should be of general use for testing the in vivo function of bacterial effector proteins and for identifying the host genes through which they function

The mTOR inhibitor, everolimus (RAD001), overcomes resistance to imatinib in quiescent Ph-positive acute lymphoblastic leukemia cells

In Ph-positive (Ph+) leukemia, the quiescent cell state is one of the reasons for resistance to the BCR-ABL-kinase inhibitor, imatinib. In order to examine the mechanisms of resistance due to quiescence and the effect of the mammalian target of rapamycin inhibitor, everolimus, for such a resistant population, we used Ph+ acute lymphoblastic leukemia patient cells serially xenotransplanted into NOD/SCID/IL2rγnull (NOG) mice. Spleen cells from leukemic mice showed a higher percentage of slow-cycling G0 cells in the CD34+CD38− population compared with the CD34+CD38+ and CD34− populations. After ex vivo imatinib treatment, more residual cells were observed in the CD34+CD38− population than in the other populations. Although slow-cycling G0 cells were insensitive to imatinib in spite of BCR-ABL and CrkL dephosphorylation, combination treatment with everolimus induced substantial cell death, including that of the CD34+CD38− population, with p70-S6 K dephosphorylation and decrease of MCL-1 expression. The leukemic non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mouse system with the in vivo combination treatment with imatinib and everolimus showed a decrease of tumor burden including CD34+ cells. These results imply that treatment with everolimus can overcome resistance to imatinib in Ph+ leukemia due to quiescence

mTOR signaling: implications for cancer and anticancer therapy

Mounting evidence links deregulated protein synthesis to tumorigenesis via the translation initiation factor complex eIF4F. Components of this complex are often overexpressed in a large number of cancers and promote malignant transformation in experimental systems. mTOR affects the activity of the eIF4F complex by phosphorylating repressors of the eIF4F complex, the eIF4E binding proteins. The immunosuppressant rapamycin specifically inhibits mTOR activity and retards cancer growth. Importantly, mutations in upstream negative regulators of mTOR cause hamartomas, haemangiomas, and cancers that are sensitive to rapamycin treatment. Such mutations lead to increased eIF4F formation and consequently to enhanced translation initiation and cell growth. Thus, inhibition of translation initiation through targeting the mTOR-signalling pathway is emerging as a promising therapeutic option

GILZ inhibits the mTORC2/AKT pathway in BCR-ABL+ cells

The malignant phenotype of chronic myeloid leukemia (CML) is due to the abnormal tyrosine kinase activity of the BCR-ABL oncoprotein, which signals several downstream cell survival pathways, including phosphoinositide 3-kinase/AKT, signal transducer and activator of transcription 5 and extracellular signal-regulated kinase 1/2. In patients with CML, tyrosine kinase inhibitors (TKIs) are used to suppress the BCR-ABL tyrosine kinase, resulting in impressive response rates. However, resistance can occur, especially in acute-phase CML, through various mechanisms. Here, we show that the glucocorticoid-induced leucine zipper protein (GILZ) modulates imatinib and dasatinib resistance and suppresses tumor growth by inactivating the mammalian target of rapamycin complex-2 (mTORC2)/AKT signaling pathway. In mouse and human models, GILZ binds to mTORC2, but not to mTORC1, inhibiting phosphorylation of AKT (at Ser473) and activating FoxO3a-mediated transcription of the pro-apoptotic protein Bim; these results demonstrate that GILZ is a key inhibitor of the mTORC2 pathway. Furthermore, CD34+ stem cells isolated from relapsing CML patients underwent apoptosis and showed inhibition of mTORC2 after incubation with glucocorticoids and imatinib. Our findings provide new mechanistic insights into the role of mTORC2 in BCR-ABL+ cells and indicate that regulation by GILZ may influence TKI sensitivity

Synergistic growth inhibition by Iressa and Rapamycin is modulated by VHL mutations in renal cell carcinoma

Epidermal growth factor receptor (EGFR) and tumour growth factor alpha (TGFα) are frequently overexpressed in renal cell carcinoma (RCC) yet responses to single-agent EGFR inhibitors are uncommon. Although von Hippel–Lindau (VHL) mutations are predominant, RCC also develops in individuals with tuberous sclerosis (TSC). Tuberous sclerosis mutations activate mammalian target of rapamycin (mTOR) and biochemically resemble VHL alterations. We found that RCC cell lines expressed EGFR mRNA in the near-absence of other ErbB family members. Combined EGFR and mTOR inhibition synergistically impaired growth in a VHL-dependent manner. Iressa blocked ERK1/2 phosphorylation specifically in wt-VHL cells, whereas rapamycin inhibited phospho-RPS6 and 4E-BP1 irrespective of VHL. In contrast, phospho-AKT was resistant to these agents and MYC translation initiation (polysome binding) was similarly unaffected unless AKT was inhibited. Primary RCCs vs cell lines contained similar amounts of phospho-ERK1/2, much higher levels of ErbB-3, less phospho-AKT, and no evidence of phospho-RPS6, suggesting that mTOR activity was reduced. A subset of tumours and cell lines expressed elevated eIF4E in the absence of upstream activation. Despite similar amounts of EGFR mRNA, cell lines (vs tumours) overexpressed EGFR protein. In the paired cell lines, PRC3 and WT8, EGFR protein was elevated post-transcriptionally in the VHL mutant and EGF-stimulated phosphorylation was prolonged. We propose that combined EGFR and mTOR inhibitors may be useful in the subset of RCCs with wt-VHL. However, apparent differences between primary tumours and cell lines require further investigation

Protein tyrosine phosphatases in glioma biology

Gliomas are a diverse group of brain tumors of glial origin. Most are characterized by diffuse infiltrative growth in the surrounding brain. In combination with their refractive nature to chemotherapy this makes it almost impossible to cure patients using combinations of conventional therapeutic strategies. The drastically increased knowledge about the molecular underpinnings of gliomas during the last decade has elicited high expectations for a more rational and effective therapy for these tumors. Most studies on the molecular pathways involved in glioma biology thus far had a strong focus on growth factor receptor protein tyrosine kinase (PTK) and phosphatidylinositol phosphatase signaling pathways. Except for the tumor suppressor PTEN, much less attention has been paid to the PTK counterparts, the protein tyrosine phosphatase (PTP) superfamily, in gliomas. PTPs are instrumental in the reversible phosphorylation of tyrosine residues and have emerged as important regulators of signaling pathways that are linked to various developmental and disease-related processes. Here, we provide an overview of the current knowledge on PTP involvement in gliomagenesis. So far, the data point to the potential implication of receptor-type (RPTPδ, DEP1, RPTPμ, RPTPζ) and intracellular (PTP1B, TCPTP, SHP2, PTPN13) classical PTPs, dual-specific PTPs (MKP-1, VHP, PRL-3, KAP, PTEN) and the CDC25B and CDC25C PTPs in glioma biology. Like PTKs, these PTPs may represent promising targets for the development of novel diagnostic and therapeutic strategies in the treatment of high-grade gliomas