The infiltration of glioma cells into adjacent tissue is one of the major obstacles in the therapeutic management of malignant brain tumours, in most cases precluding complete surgical resection. Consequently, malignant glioma patients almost invariably experience tumour recurrences. Within the brain, glioma cells migrate rapidly either amoeboidly or mesenchymally to invade surrounding structures, in dependence on the extracellular environment. In addition, radiotherapy, frequently applied as adjuvant therapeutic modality, may enhance tumour cell mobility. Here, we show that the receptor tyrosine kinase Mer (MerTK) is overexpressed in glioblastoma multiforme (GBM) and that this is accompanied with increased invasive potential. MerTK expression is maintained in primary GBM-derived tumour spheres under stem cell culture conditions but diminishes significantly in serum-containing cultures with concomitant downregulation of Nestin and Sox2. Depletion of MerTK disrupts the rounded morphology of glioma cells and decreases their invasive capacity. Furthermore, the expression and phosphorylation of myosin light chain 2 are strongly associated with MerTK activity, indicating that the effect of MerTK on glioma cell invasion is mediated by actomyosin contractility. Finally, DNA damage robustly triggers the upregulation and phosphorylation of MerTK, which protects cells from apoptosis. This effect is strongly impaired upon MerTK depletion or overexpression of an inactive MerTK mutant. Collectively, our data suggests that MerTK is a novel therapeutic target in the treatment of the malignant gliomas.

Kaposi’s sarcoma (KS) is an angioproliferative and invasive tumor caused by Kaposi’s sarcoma-associated herpesvirus (KSHV). The cellular origin of KS tumor cells remains contentious. Recently, evidence has accrued indicating that KS may arise from KSHV-infected mesenchymal stem cells (MSCs) through mesenchymal-to-endothelial transition (MEndT), but the transformation process has been largely unknown. In this study, we investigated the KSHV-mediated MEndT process and found that KSHV infection rendered MSCs incomplete endothelial lineage differentiation and formed hybrid mesenchymal/endothelial (M/E) state cells characterized by simultaneous expression of mesenchymal markers PDGFRA/Nestin and endothelial markers PDPN/CD31. The hybrid M/E cells have acquired high tumorigenic properties in vitro and the potential to form KS-like tumors after transplanted in mice under renal capsules. These results faithfully recapitulate Kaposi’s sarcoma where proliferating KS spindle-shaped cells and the cells that line KS-specific aberrant vessels were also found to exhibit a hybrid M/E state. Furthermore, the genetic analysis identified KSHV-encoded FLICE inhibitory protein (vFLIP) as a crucial regulator controlling KSHV-induced MEndT and generating hybrid M/E state cells for tumorigenesis. Overall, KSHV-mediated MEndT that transforms MSCs to tumorigenic hybrid M/E state cells driven by vFLIP is an essential event in Kaposi’s sarcomagenesis.Author SummaryKaposi’s sarcoma manifests as multifocal lesions with spindle cell proliferation, intense angiogenesis, and erythrocyte extravasation. Although the origin and true malignant nature of KS remains contentious, it is established that KSHV infection with concomitant viral oncogene expression in normal cell progenitors causes KS. The mechanism of KSHV oncogenesis could be revealed through reproduction of KS by infection of normal cells. This study reports that the KSHV infection of mesenchymal stem cells initiates mesenchymal-to-endothelial transition (MEndT) that generates mesenchymal/endothelial (M/E) hybrid state cells. The hybrid M/E cells acquired high tumorigenic properties, including tumor initiation, angiogenesis, migration, and the potential to form KS-like tumors after transplanted in mice. This finding faithfully recapitulates Kaposi’s sarcoma where proliferating KS spindle cells and the cells that line KS-specific aberrant vessels are also found to exhibit the hybrid M/E state. We also found that KSHV-encoded viral FLICE inhibitory protein (vFLIP) plays a crucial role in promoting MEndT and the generation of M/E state cells. These results provide a new layer of evidence for MSCs being the cell source of KS spindle cells and reveal novel insight into KS pathogenesis and viral tumorigenesis.