Publisher Correction: Investigation of brain tissue infiltration by medulloblastoma cells in an ex vivo model.

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TGF-ß Determines the Pro-migratory Potential of bFGF Signaling in Medulloblastoma.

The microenvironment shapes cell behavior and determines metastatic outcomes of tumors. We addressed how microenvironmental cues control tumor cell invasion in pediatric medulloblastoma (MB). We show that bFGF promotes MB tumor cell invasion through FGF receptor (FGFR) in vitro and that blockade of FGFR represses brain tissue infiltration in vivo. TGF-ß regulates pro-migratory bFGF function in a context-dependent manner. Under low bFGF, the non-canonical TGF-ß pathway causes ROCK activation and cortical translocation of ERK1/2, which antagonizes FGFR signaling by inactivating FGFR substrate 2 (FRS2), and promotes a contractile, non-motile phenotype. Under high bFGF, negative-feedback regulation of FRS2 by bFGF-induced ERK1/2 causes repression of the FGFR pathway. Under these conditions, TGF-ß counters inactivation of FRS2 and restores pro-migratory signaling. These findings pinpoint coincidence detection of bFGF and TGF-ß signaling by FRS2 as a mechanism that controls tumor cell invasion. Thus, targeting FRS2 represents an emerging strategy to abrogate aberrant FGFR signaling.

MAP4K4 controlled integrin ß1 activation and c-Met endocytosis are associated with invasive behavior of medulloblastoma cells.

Local tissue infiltration of Medulloblastoma (MB) tumor cells precedes metastatic disease but little is still known about intrinsic regulation of migration and invasion in these cells. We found that MAP4K4, a pro-migratory Ser/Thr kinase, is overexpressed in 30% of primary MB tumors and that increased expression is particularly associated with the frequently metastatic SHH ß subtype. MAP4K4 is a driver of migration and invasion downstream of c-Met, which is transcriptionally up-regulated in SHH MB. Consistently, depletion of MAP4K4 in MB tumor cells restricts HGF-driven matrix invasion in vitro and brain tissue infiltration ex vivo. We show that these pro-migratory functions of MAP4K4 involve the activation of the integrin ß-1 adhesion receptor and are associated with increased endocytic uptake. The consequent enhanced recycling of c-Met caused by MAP4K4 results in the accumulation of activated c-Met in cytosolic vesicles, which is required for sustained signaling and downstream pathway activation. The parallel increase of c-Met and MAP4K4 expression in SHH MB could predict an increased potential of these tumors to infiltrate brain tissue and cause metastatic disease. Molecular targeting of the underlying accelerated endocytosis and receptor recycling could represent a novel approach to block pro-migratory effector functions of MAP4K4 in metastatic cancers.

Investigation of brain tissue infiltration by medulloblastoma cells in an ex vivo model.

Medulloblastoma (MB) is a paediatric cancer of the cerebellum that can develop cerebellar and leptomeningeal metastases. Local brain tissue infiltration, the underlying cause of metastasis and relapse, remains unexplored. We developed a novel approach to investigate tissue infiltration of MB using organotypic cerebellum slice culture (OCSC). We show that cellular and structural components of cerebellar tissue in OCSCs are maintained for up to 30 days ex vivo, and that OCSCs foster tumour growth and cell proliferation. Using cell-based models of sonic hedgehog (SHH) and group 3 (G3) MB, we quantified tumour growth and infiltration and determined the morphological characteristics of the infiltrating cells. We observed basal levels of dissemination occurring in both subgroups with cells migrating either individually or collectively as clusters. Collective cerebellar tissue infiltration of SHH MB cells was further enhanced by EGF but not HGF, demonstrating differential tumour cell responses to microenvironmental cues. We found G3 cells to be hyper proliferative and observed aggressive tumour expansion even in the absence of exogenous growth factors. Our study thus provides unprecedented insights into brain tissue infiltration of SHH and G3 MB cells and reveals the cellular basis of the tumour progressing functions of EGF in SHH MB.

The Ser/Thr kinase MAP4K4 drives c-Met-induced motility and invasiveness in a cell-based model of SHH medulloblastoma.

Medulloblastoma (MB) comprises four molecularly and genetically distinct subgroups of embryonal brain tumors that develop in the cerebellum. MB mostly affects infants and children and is difficult to treat because of frequent dissemination of tumor cells within the leptomeningeal space. A potential promoter of cell dissemination is the c-Met proto-oncogene receptor tyrosine kinase, which is aberrantly expressed in many human tumors including MB. Database analysis showed that c-Met is highly expressed in the sonic hedgehog (SHH) subgroup and in a small subset of Group 3 and Group 4 MB tumors. Using a cell-based three-dimensional cell motility assay combined with live-cell imaging, we investigated whether the c-Met ligand HGF could drive dissemination of MB cells expressing high levels of c-Met, and determined downstream effector mechanisms of this process. We detected variable c-Met expression in different established human MB cell lines, and we found that in lines expressing high c-Met levels, HGF promoted cell dissemination and invasiveness. Specifically, HGF-induced c-Met activation enhanced the capability of the individual cells to migrate in a JNK-dependent manner. Additionally, we identified the Ser/Thr kinase MAP4K4 as a novel driver of c-Met-induced invasive cell dissemination. This increased invasive motility was due to MAP4K4 control of F-actin dynamics in structures required for migration and invasion. Thus, MAP4K4 couples growth factor signaling to actin cytoskeleton regulation in tumor cells, suggesting that MAP4K4 could present a promising novel target to be evaluated for treating growth factor-induced dissemination of MB tumors of different subgroups and of other human cancers.