Transdifferentiation of glioblastoma cells into vascular endothelial cells.

Glioblastoma (GBM) is the most malignant brain tumor and is highly resistant to intensive combination therapies and anti-VEGF therapies. To assess the resistance mechanism to anti-VEGF therapy, we examined the vessels of GBMs in tumors that were induced by the transduction of p53(+/-) heterozygous mice with lentiviral vectors containing oncogenes and the marker GFP in the hippocampus of GFAP-Cre recombinase (Cre) mice. We were surprised to observe GFP(+) vascular endothelial cells (ECs). Transplantation of mouse GBM cells revealed that the tumor-derived endothelial cells (TDECs) originated from tumor-initiating cells and did not result from cell fusion of ECs and tumor cells. An in vitro differentiation assay suggested that hypoxia is an important factor in the differentiation of tumor cells to ECs and is independent of VEGF. TDEC formation was not only resistant to an anti-VEGF receptor inhibitor in mouse GBMs but it led to an increase in their frequency. A xenograft model of human GBM spheres from clinical specimens and direct clinical samples from patients with GBM also showed the presence of TDECs. We suggest that the TDEC is an important player in the resistance to anti-VEGF therapy, and hence a potential target for GBM therapy.

Inhibition of GSK-3beta promotes survival and proliferation of megakaryocytic cells through a beta-catenin-independent pathway.

The interaction of thrombopoietin (TPO) with its receptor c-Mpl initiates intracellular signals that are critical for megakaryopoiesis. Previously we and others have shown that TPO activates PI3K and Akt and that this pathway is important for megakaryocyte growth. Here, we investigate the importance of the Akt substrate glycogen synthase kinase (GSK)-3beta in TPO signaling. GSK-3beta is phosphorylated and inhibited by Akt as part of the PI3K pathway. GSK-3beta can also be inhibited by Wnt signaling through a distinct mechanism, leading to reduced phosphorylation and accumulation of the transcription factor beta-catenin. Therefore, we asked if TPO and Wnt3a can both inhibit GSK-3beta in megakaryocytic cells, and if they can act synergistically to promote cell growth. Although both TPO and specific chemical inhibitors of GSK-3beta result in increased survival and proliferation in a megakaryocytic cell line model, treatment with Wnt3a failed to increase cell growth either in the absence or presence of TPO, despite inducing high levels of beta-catenin. Similarly, expression of a constitutively active version of beta-catenin did not increase cell growth either in the absence or presence of TPO, suggesting that the effects of GSK-3beta inhibition downstream of TPO signaling are distinct from those induced by Wnt3a and independent of beta-catenin. The growth promoting effects of TPO are not mediated by either of the two known GSK-3beta targets, cyclin D or HIF-1alpha. We conclude that GSK-3beta is phosphorylated and inhibited by TPO-induced Akt, promoting survival and proliferation in megakaryocytic cells through a pathway that does not involve beta-catenin.