Dynamin II function is required for EGF-mediated Stat3 activation but not Erk1/2 phosphorylation

Shutting the gate: targeting endocytosis in acute leukemia

Endocytosis entails selective packaging of cell surface cargos in cytoplasmic vesicles, thereby controlling key intrinsic cellular processes as well as the response of normal and malignant cells to their microenvironment. The purpose of this review is to outline the latest advances in the development of endocytosis-targeting therapeutic strategies in hematological malignancies.

Targeting Glioma Stem Cells by Functional Inhibition of Dynamin 2: A Novel Treatment Strategy for Glioblastoma

Glioma stem cells (GSCs) play major roles in drug resistance, tumour maintenance and recurrence of glioblastoma. We investigated inhibition of the GTPase dynamin 2 as a therapy for glioblastoma. Glioma cell lines and patient-derived GSCs were treated with dynamin inhibitors, Dynole 34-2 and CyDyn 4-36. We studied about cell viability, and GSC neurosphere formation in vitro and orthotopic tumour growth in vivo. Dynamin inhibition reduced glioblastoma cell line viability and suppressed neurosphere formation and migration of GSCs. Tumour growth was reduced by CyDyn 4-36 treatment. Dynamin 2 inhibition therefore represents a novel approach for stem cell-directed Glioblastoma therapy.

Hepatic Stellate Cell Selective Disruption of Dynamin-2 GTPase Increases Murine Fibrogenesis through Up-Regulation of Sphingosine-1 Phosphate-Induced Cell Migration

Dynamin-2 (Dyn2) is implicated in endocytosis of receptor tyrosine kinases, which contribute to hepatic stellate cell (HSC) activation and liver fibrosis. A point mutation converting lysine 44 of Dyn2 to alanine (Dyn2K44A) disrupts its GTPase activity. We hypothesized that Dyn2K44A expression in HSCs would decrease HSC activation and fibrogenesis in vivo by disrupting receptor tyrosine kinase endocytosis and signaling. Dyn2K44A^fl/fl mice were crossed with Collagen1-Cre (Col1^Cre) mice to generate offspring with HSC selective expression of Dyn2K44A (Col1^Cre/Dyn2K44A^fl/fl). Contrary to our hypothesis, Col1^Cre/Dyn2K44A^fl/fl mice showed increased hepatic fibrosis in response to liver injury. To elucidate mechanisms, we conducted in vitro experiments with HSCs infected with adenoviral vectors encoding LacZ, Dyn2K44A, or Dyn2WT. HSC-expressing Dyn2K44A displayed increased mRNA and protein levels of sphingosine kinase-1 (SK1), an enzyme previously implicated in the pathogenesis of fibrosis. To study the functional effects of Dyn2K44A regulation of SK1, we examined effects of AKT signaling and migration in HSCs. Dyn2K44A promoted both AKT phosphorylation and HSC migration in an SK1-dependent manner. Genetic disruption of Dyn2 GTPase activity selectively in HSC enhances fibrogenesis, driven at least in part through up-regulation of the SK1 pathway and cell migration in HSCs.

Efficacy of aerosol therapy of lung cancer correlates with EGFR paralysis induced by AvidinOX-anchored biotinylated Cetuximab

Lung cancer, as well as lung metastases from distal primary tumors, could benefit from aerosol treatment. Unfortunately, because of lung physiology, clearance of nebulized drugs is fast, paralleled by unwanted systemic exposure. Here we report that nebulized AvidinOX can act as an artificial receptor for biotinylated drugs. In nude and SCID mice with advanced human KRAS-mutated A549 metastatic lung cancer, pre-nebulization with AvidinOX enables biotinylated Cetuximab to control tumor growth at a dose lower than 1/25,000 the intravenous effective dose. This result correlates with a striking, specific and unpredictable effect of AvidinOX-anchored biotinylated Cetuximab, as well as Panitumumab, observed on a panel of tumor cell lines, leading to inhibition of dimerization and signalling, blockade of endocytosis, induction of massive lysosomal degradation and abrogation of nuclear translocation of EGFR. Excellent tolerability, together with availability of pharmaceutical-grade AvidinOX and antibodies, will allow rapid clinical translation of the proposed therapy.

EGFR signaling patterns are regulated by its different ligands

EGF receptor (EGFR) and its signaling have been investigated for many years, but how its different ligands regulate signaling has not been thoroughly explored. When investigating EGFR activation and downstream signaling in HeLa cells using a panel of ligands, we found a ligand-dependent differential activation of EGFR and the signaling pathways Akt, PLCγ and STAT with HB-EGF and BTC being the most potent ligands. All the tested ligands induced full activation of Erk signaling at 1 nM, whereas only HB-EGF and partly BTC and EGF induced strong activation of Akt, STAT3 and PLCγ at this concentration. Interestingly, we also found that the high activation potencies of HB-EGF and BTC could only partially be explained by their binding affinities, and are therefore likely to be regulated by other mechanisms. We thus suggest that the signaling pathways initiated from the EGFR vary depending on the ligands bound in a cell specific manner.

Dynamin inhibitors impair endocytosis and mitogenic signaling of PDGF

Platelet-derived growth factor (PDGF) isoforms regulate cell proliferation, migration and differentiation both in embryonic development and adult tissue remodeling. At the cellular level, growth-factor signaling is often modulated by endocytosis. Despite important functions of PDGF, its endocytosis remains poorly studied, mainly for lack of tools to track internalized ligand by microscopy. Here, we developed such a tool and quantitatively analyzed internalization and endosomal trafficking of PDGF-BB in human fibroblasts. We further show that PDGF can be internalized in the presence of dynamin inhibitors, arguing that both dynamin-dependent and dynamin-independent pathways can mediate PDGF uptake. Although these routes operate with somewhat different kinetics, they both ultimately lead to lysosomal degradation of PDGF. Although acute inhibition of dynamin activity only moderately affects PDGF endocytosis, it specifically decreases downstream signaling of PDGF via signal transducer and activator of transcription 3 (STAT3). This correlates with reduced expression of MYC and impaired cell entry into S-phase, indicating that dynamin activity is required for PDGF-induced mitogenesis. Our data support a general view that the components governing endocytic trafficking may selectively regulate certain signaling effectors activated by a growth factor.

Essential role for endocytosis in the growth factor-stimulated activation of ERK1/2 in endothelial cells

Vascular endothelial growth factor (VEGF) stimulates angiogenesis by binding to VEGF receptor 2 (VEGFR2) on endothelial cells (ECs). Downstream activation of the extracellular related kinases 1/2 (ERK1/2) is important for angiogenesis to proceed. Receptor internalization has been implicated in VEGFR2 signaling, but its role in the activation of ERK1/2 is unclear. To explore this question we utilized pitstop and dynasore, two small molecule inhibitors of endocytosis. First, we confirmed that both inhibitors block the internalization of VEGFR2 in ECs. We then stimulated ECs with VEGF in the presence and absence of the inhibitors and examined VEGFR2 signaling to ERK1/2. Activation of VEGFR2 and C-Raf still occurred in the presence of the inhibitors, whereas the activation of MEK1/2 and ERK1/2 was abrogated. Therefore, although internalization is not required for activation of either VEGFR2 or C-Raf in ECs stimulated with VEGF, internalization is necessary to activate the more distal kinases in the cascade. Importantly, inhibition of internalization also prevented activation of ERK1/2 when ECs were stimulated with other pro-angiogenic growth factors, namely fibroblast growth factor 2 and hepatocyte growth factor. In contrast, the same inhibitors did not block ERK1/2 activation in fibroblasts or cancer cells stimulated with growth factors. Finally, we show that these small molecule inhibitors of endocytosis block angiogenesis in vitro and in vivo. Therefore, receptor internalization may be a generic requirement for pro-angiogenic growth factors to activate ERK1/2 signaling in human ECs, and targeting receptor trafficking may present a therapeutic opportunity to block tumor angiogenesis.

Spatial regulation of epidermal growth factor receptor signaling by endocytosis

Signaling by cell surface receptors appears to be relatively straight-forward: ligand binds to the extracellular domain of the receptor and biochemical changes are communicated into the cell. However, this process is more complex than it first seems due to the various mechanisms that regulate signaling. In order to effectively target these receptors for pharmacological purposes, a more complete understanding of how their signaling is regulated is needed. Here, how the endocytic pathway regulates receptor signaling is discussed, using the epidermal growth factor receptor (EGFR) as a model. In particular, the spatial regulation of signaling is examined. Areas of discussion include: how endocytic trafficking affects biology/pathology, varying approaches for studying the relationship between receptor endocytosis and signaling, and developments in how the endocytic pathway controls EGFR:effector communication and EGFR-mediated cell biology.