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The c-MYC oncoprotein is a DNA binding transcription factor that enhances the expression of many active genes. c-MYC transcriptional signatures vary according to the transcriptional program defined in each cell type during differentiation. Little is known on the involvement of c-MYC in regulation of gene expression programs that are induced by extracellular cues such as a changing microenvironment. Here we demonstrate that inhibition of c-MYC in glioblastoma multiforme cells blunts hypoxia-dependent glycolytic reprogramming and mitochondria fragmentation in hypoxia. This happens because c-MYC inhibition alters the cell transcriptional response to hypoxia and finely tunes the expression of a subset of Hypoxia Inducible Factor 1-regulated genes. We also show that genes whose expression in hypoxia is affected by c-MYC inhibition are able to distinguish the Proneural subtype of glioblastoma multiforme, thus potentially providing a molecular signature for this class of tumors that are the least tractable among glioblastomas., Competing Interests: The authors declare no conflicts of interest.

Many studies have elucidated the important role played by the tumor microenvironment in cancer evolution. In particular the formation of hypoxic areas within the expanding mass of a solid tumor and the consequent induction of an angiogenic switch are crucial steps that shape tumor progression. Focusing on glioblastoma multiforme (GBM), the most common and lethal brain cancer in the adult, I will review recent data that show how the microenvironment regulates crucial functions of glioblastoma stem cells (GSCs) which in turn affect the angiogenic process.

Glioblastoma multiforme (GBM) is characterized by extensive angiogenesis that is mostly orchestrated by the hypoxia inducible factor HIF-1. Deregulation of HIF-1 is believed to contribute to cancer initiation and progression. However, instances have been described in which loss of HIF-1 leads to more aggressive tumors. Here we investigated the consequences of downregulating HIF-1 function in the human GBM cell line TB10, both on cell proliferation in vitro and on tumor growth in vivo. RNA interference targeting the O2-regulated HIF-1alpha subunit efficiently reduced HIF-1alpha expression and transcriptional induction of HIF-1-responsive genes without affecting cell growth. Thus, singularly grown wild-type and HIF-1alpha-inhibited GBM cell populations did not significantly differ in proliferation rate. However, when the two populations were co-cultured, wild-type cells overgrew the HIF-1alpha-inhibited cells. Subcutaneous grafting in nude mice of wild-type and HIF-1alpha-inhibited GBM cells lead to comparable tumor formation and growth. Interestingly, cografting of wt and HIF-1alpha- inhibited GBM cells in nude mice resulted in more aggressive tumors, both in terms of tumor appearance and tumor growth. This suggests that cellular populations that differ in their ability to mount a response to hypoxia may compete in vitro but cooperate in vivo resulting in increased tumor aggressiveness.

Tumor angiogenesis is a complex process that involves a series of interactions between tumor cells and endothelial cells (ECs). In vitro, glioblastoma multiforme (GBM) cells are known to induce an increase in proliferation, migration and tube formation by the ECs. We have previously shown that in human GBM specimens the proliferating ECs of the tumor vasculature express the catalytic component of telomerase, hTERT, and that telomerase can be upregulated in human ECs by exposing these cells to GBM in vitro. Here, we developed a controlled in vivo assay of tumor angiogenesis in which primary human umbilical vascular endothelial cells (HUVECs) were subcutaneously grafted with or without human GBM cells in immunocompromised mice as Matrigel implants. We found that primary HUVECs did not survive in Matrigel implants, and that telomerase upregulation had little effect on HUVEC survival. In the presence of GBM cells, however, the grafted HUVECs not only survived in Matrigel implants but developed tubule structures that integrated with murine microvessels. Telomerase upregulation in HUVECs enhanced such effect. More importantly, inhibition of telomerase in HUVECs completely abolished tubule formation and greatly reduced survival of these cells in the tumor xenografts. Our data demonstrate that telomerase upregulation by the ECs is a key requisite for GBM tumor angiogenesis., ((c) 2007 Wiley-Liss, Inc.)

Telomerase is a specialized DNA polymerase that is required to replicate the ends of linear chromosomes, the telomeres. The majority of human cancers express high levels of telomerase activity that is permissive for tumor growth because it provides cells with an extended proliferative potential. Additionally, telomerase exerts cell growth promoting functions and favors cell survival. Human glioblastoma multiforme (GBM) cells express high level of telomerase activity owing to the overexpression of human telomerase reverse transcriptase (hTERT), the limiting subunit of the enzyme. Here we used retroviral mediated RNA interference to dampen down telomerase activity in two distinct human GBM cell lines, U87MG and TB10. Substantial decrease of hTERT mRNA and telomerase activity had only minimal effects on telomere length maintenance, cell growth and survival in vitro. On the contrary, development of tumors upon subcutaneously grafting of U87MG and TB10 cells and intracranial implantation of U87MG cells in nude athymic mice was strongly reduced by telomerase inhibition.