Regorafenib and glioblastoma: a literature review of preclinical studies, molecular mechanisms and clinical effectiveness.

Glioblastoma IDH wild type (GBM) is a very aggressive brain tumour, characterised by an infiltrative growth pattern and by a prominent neoangiogenesis. Its prognosis is unfortunately dismal, and the median overall survival of GBM patients is short (15 months). Clinical management is based on bulk tumour removal and standard chemoradiation with the alkylating drug temozolomide, but the tumour invariably recurs leading to patient's death. Clinical options for GBM patients remained unaltered for almost two decades until the encouraging results obtained by the phase II REGOMA trial allowed the introduction of the multikinase inhibitor regorafenib as a preferred regimen in relapsed GBM treatment by the National Comprehensive Cancer Network (NCCN) 2020 Guideline. Regorafenib, a sorafenib derivative, targets kinases associated with angiogenesis (VEGFR 1-3), as well as oncogenesis (c-KIT, RET, FGFR) and stromal kinases (FGFR, PDGFR-b). It was already approved for metastatic colorectal cancers and hepatocellular carcinomas. The aim of the present review is to focus on both the molecular and clinical knowledge collected in these first three years of regorafenib use in GBM.

Telomerase inhibition in malignant gliomas: a systematic review.

Glioblastoma (GBM) is the most frequent adult malignant brain tumour and despite different therapeutic efforts, the median overall survival still ranges from 14 to 18 months. Thus, new therapeutic strategies are urgently needed. However, the identification of cancer-specific targets is particularly challenging in GBM, due to the high heterogeneity of this tumour in terms of histopathological, molecular, genetic and epigenetic features. Telomerase reactivation is a hallmark of malignant glioma. An activating mutation of the hTERT gene, encoding for the active subunit of telomerase, is one of the molecular criteria to establish a diagnosis of GBM, IDH-wildtype, in the 2021 WHO classification of central nervous system tumours. Telomerase inhibition therefore represents, at least theoretically, a promising strategy for GBM therapy: pharmacological compounds, as well as direct gene expression modulation therapies, have been successfully employed in in vitro and in vivo settings. Unfortunately, the clinical applications of telomerase inhibition in GBM are currently scarce. The aim of the present systematic review is to provide an up-to-date report on the studies investigating telomerase inhibition as a therapeutic strategy for malignant glioma in order to foster the future translational and clinical research on this topic.

Gene expression profiling of hypoxic response in different models of senescent endothelial cells.

Endothelial cells senescence is a physiological process affecting vascular integrity. It can contribute to heart and arterial stiffening and remodeling, impaired angiogenesis, defective vascular repair, and with an increasing prevalence of atherosclerosis. Drugs used as antineoplastic therapies, targeting tumor as well as endothelial cells, can also trigger endothelial cells senescence. We demonstrated that a short pulse of axitinib, a specific inhibitor of vascular endothelial growth factor receptors, induces cell senescence of endothelial cells. Here, we performed a high-throughput gene expression analysis to characterize the response of proliferating versus senescent endothelial cells to hypoxia, the main trigger of neo-angiogenetic phenomena in tumors. We compared the response to hypoxia of replicative senescent cells, with that of axitinib or of DNA damage-induced senescence. Overall, we enlightened common and specific responses to different senescence inducers and changes in the Senescent Associated Secretory Phenotype.

Glioblastoma Cells Do Not Affect Axitinib-Dependent Senescence of HUVECs in a Transwell Coculture Model.

Axitinib is an orally available inhibitor of tyrosine kinases, with high specificity for vascular endothelial growth factor receptors (VEGFRs) 1, 2, and 3. It is approved for the treatment of advanced renal cell carcinoma and is in phase II clinical trials for recurrent glioblastoma (GBM). GBM is a brain tumor peculiar in its ability to induce neoangiogenesis. Since both GBM tumor cells and endothelial cells of tumor vasculature express VEGFRs, Axitinib exerts its inhibitory action on both tumor and endothelial cells. We and others previously demonstrated that Axitinib triggers cellular senescence. In particular, Axitinib-dependent senescence of HUVECs (human umbilical vein endothelial cells) is accompanied by intracellular reactive oxygen species(ROS) increase and early ataxia telangiectasia mutated(ATM) activation. Here we wondered if the presence of glioblastoma tumor cells could affect the HUVEC senescence upon Axitinib exposure. To address this issue, we cocultured HUVECs together with GBM tumor cells in transwell plates. HUVEC senescence did not result in being affected by GBM cells, neither in terms of ß galactosidase activity nor of proliferation index or ATM phosphorylation. Conversely, Axitinib modulation of HUVEC gene expression was altered by cocultured GBM cells. These data demonstrate that the GBM secretome modifies HUVECs' transcriptomic profile upon Axitinib exposure, but does not prevent drug-induced senescence.

Glioblastoma endothelium drives bevacizumab-induced infiltrative growth via modulation of PLXDC1.

Bevacizumab, a VEGF-targeting monoclonal antibody, may trigger an infiltrative growth pattern in glioblastoma. We investigated this pattern using both a human specimen and rat models. In the human specimen, a substantial fraction of infiltrating tumor cells were located along perivascular spaces in close relationship with endothelial cells. Brain xenografts of U87MG cells treated with bevacizumab were smaller than controls (p = 0.0055; Student t-test), however, bands of tumor cells spread through the brain farther than controls (p < 0.001; Student t-test). Infiltrating tumor Cells exhibited tropism for vascular structures and propensity to form tubules and niches with endothelial cells. Molecularly, bevacizumab triggered an epithelial to mesenchymal transition with over-expression of the receptor Plexin Domain Containing 1 (PLXDC1). These results were validated using brain xenografts of patient-derived glioma stem-like cells. Enforced expression of PLXDC1 in U87MG cells promoted brain infiltration along perivascular spaces. Importantly, PLXDC1 inhibition prevented perivascular infiltration and significantly increased the survival of bevacizumab-treated rats. Our study indicates that bevacizumab-induced brain infiltration is driven by vascular endothelium and depends on PLXDC1 activation of tumor cells.

Mesenchymal stromal cells primed with Paclitaxel attract and kill leukaemia cells, inhibit angiogenesis and improve survival of leukaemia-bearing mice.

Current leukaemia therapy focuses on increasing chemotherapy efficacy. Mesenchymal stromal cells (MSCs) have been proposed for carrying and delivery drugs to improve killing of cancer cells. We have shown that MSCs loaded with Paclitaxel (PTX) acquire a potent anti-tumour activity. We investigated the effect of human MSCs (hMSCs) and mouse SR4987 loaded with PTX (hMSCsPTX and SR4987PTX) on MOLT-4 and L1210, two leukaemia cell (LCs) lines of human and mouse origin, respectively. SR4987PTX and hMSCsPTX showed strong anti-LC activity. hMSCsPTX, co-injected with MOLT-4 cells or intra-tumour injected into established subcutaneous MOLT-4 nodules, strongly inhibited growth and angiogenesis. In BDF1-mice-bearing L1210, the intraperitoneal administration of SR4987PTX doubled mouse survival time. In vitro, both hMSCs and hMSCsPTX released chemotactic factors, bound and formed rosettes with LCs. In ultrastructural analysis of rosettes, hMSCsPTX showed no morphological alterations while the attached LCs were apoptotic and necrotic. hMSCs and hMSCsPTX released molecules that reduced LC adhesion to microvascular endothelium (hMECs) and down-modulated ICAM1 and VCAM1 on hMECs. Priming hMSCs with PTX is a simple procedure that does not require any genetic cell manipulation. Once the effectiveness of hMSCsPTX on established cancers in mice is proven, this procedure could be proposed for leukaemia therapy in humans.

5-Aminolevulinic Acid (5-ALA)-Induced Protoporphyrin IX Fluorescence by Glioma Cells-A Fluorescence Microscopy Clinical Study.

5-aminolevulinic acid (5-ALA)-induced PpIX fluorescence is used by neurosurgeons to identify the tumor cells of high-grade gliomas during operation. However, the issue of whether 5-ALA-induced PpIX fluorescence consistently stains all the tumor cells is still debated. Here, we assessed the cytoplasmatic signal of 5-ALA by fluorescence microscopy in a series of human gliomas. As tumor markers, we used antibodies against collapsin response-mediated protein 5 (CRMP5), alpha thalassemia/mental retardation syndrome X-linked (ATRX), and anti-isocitrate dehydrogenase 1 (IDH1). In grade III-IV gliomas, the signal induced by 5-ALA was detected in 32.7-75.5 percent of CRMP5-expressing tumor cells. In low-grade gliomas (WHO grade II), the CRMP5-expressing tumor cells did not fluoresce following 5-ALA. Immunofluorescence with antibodies that stain various components of the blood-brain barrier (BBB) suggested that 5-ALA does not cross the un-breached BBB, in spite of its small dimension. To conclude, 5-ALA-induced PpIX fluorescence has an established role in high-grade glioma surgery, but it has limited usefulness in surgery for low-grade glioma, especially when the BBB is preserved.

Characterization of Glioblastoma Cells Response to Regorafenib.

Glioblastoma (GBM), the most malignant primary brain tumor in adults. Although not frequent, it has a relevant social impact because the peak incidence coincides with the age of professional maturity. A number of novel treatments have been proposed, yet clinical trials have been disappointing. Recently, a phase II clinical trial (REGOMA) demonstrated that the multikinase inhibitor regorafenib significantly increased the median overall survival (OS) of GBM patients when compared to lomustine-treated patients. On this basis, the National Comprehensive Cancer Network (NCCN) 2020 Guidelines included regorafenib as a preferred regimen in relapsed GBM treatment. Despite the use in GBM patients' therapy, little is known about the molecular mechanisms governing regorafenib effectiveness on the GBM tumor. Here we report an in vitro characterization of GBM tumor cells' response to regorafenib, performed both on cell lines and on patient-derived glioma stem cells (GSCs). Overall, regorafenib significantly reduced cell growth of 2D tumor cell cultures and of 3D tumor spheroids. Strikingly, this effect was accompanied by transcriptional regulation of epithelial to mesenchymal transition (EMT) genes and by an increased ability of surviving tumor cells to invade the surrounding matrix. Taken together, our data suggest that regorafenib limits cell growth, however, it might induce an invasive phenotype.

Transport of cationic liposomes in a human blood brain barrier model: Role of the stereochemistry of the gemini amphiphile on liposome biological features.

HYPOTHESIS: The positive charge on liposome surface is known to promote the crossing of the Blood brain barrier (BBB). However, when diastereomeric cationic gemini amphiphiles are among lipid membrane components, also the stereochemistry may affect the permeability of the vesicle across the BBB. EXPERIMENTS: Liposomes featuring cationic diasteromeric gemini amphiphiles were formulated, characterized, and their interaction with cell culture models of BBB investigated. FINDINGS: Liposomes featuring the gemini amphiphiles were internalized in a monolayer of brain microvascular endothelial cells derived from human induced pluripotent stem cells (hiPSC) through an energy dependent transport, internalization involving both clathrin- and caveolae-mediated endocytosis. On the same formulations, the permeability was also evaluated across a human derived in vitro BBB transport model. The permeability of liposomes featuring the gemini amphiphiles was significantly higher compared to that of neutral liposomes (DPPC/Cholesterol), that were not able to cross BBB. Most importantly, the permeability was influenced by the stereochemistry of the gemini and pegylation of these formulations did not result in a drastic reduction of the crossing ability. The in vitro iPSC-derived BBB models used in this work represent an important advancement in the drug discovery research of novel brain delivery strategies and therapeutics for central nervous system diseases.

ATM Kinase Dead: From Ataxia Telangiectasia Syndrome to Cancer.

ATM is one of the principal players of the DNA damage response. This protein exerts its role in DNA repair during cell cycle replication, oxidative stress, and DNA damage from endogenous events or exogenous agents. When is activated, ATM phosphorylates multiple substrates that participate in DNA repair, through its phosphoinositide 3-kinase like domain at the 3'end of the protein. The absence of ATM is the cause of a rare autosomal recessive disorder called Ataxia Telangiectasia characterized by cerebellar degeneration, telangiectasia, immunodeficiency, cancer susceptibility, and radiation sensitivity. There is a correlation between the severity of the phenotype and the mutations, depending on the residual activity of the protein. The analysis of patient mutations and mouse models revealed that the presence of inactive ATM, named ATM kinase-dead, is more cancer prone and lethal than its absence. ATM mutations fall into the whole gene sequence, and it is very difficult to predict the resulting effects, except for some frequent mutations. In this regard, is necessary to characterize the mutated protein to assess if it is stable and maintains some residual kinase activity. Moreover, the whole-genome sequencing of cancer patients with somatic or germline mutations has highlighted a high percentage of ATM mutations in the phosphoinositide 3-kinase domain, mostly in cancer cells resistant to classical therapy. The relevant differences between the complete absence of ATM and the presence of the inactive form in in vitro and in vivo models need to be explored in more detail to predict cancer predisposition of A-T patients and to discover new therapies for ATM-associated cancer cells. In this review, we summarize the multiple discoveries from humans and mouse models on ATM mutations, focusing into the inactive versus null ATM.

Cancer Response to Therapy-Induced Senescence: A Matter of Dose and Timing.

Cellular senescence participates to fundamental processes like tissue remodeling in embryo development, wound healing and inhibition of preneoplastic cell growth. Most senescent cells display common hallmarks, among which the most characteristic is a permanent (or long lasting) arrest of cell division. However, upon senescence, different cell types acquire distinct phenotypes, which also depend on the specific inducing stimuli. Senescent cells are metabolically active and secrete a collection of growth factors, cytokines, proteases, and matrix-remodeling proteins collectively defined as senescence-associated secretory phenotype, SASP. Through SASP, senescent cells modify their microenvironment and engage in a dynamic dialog with neighbor cells. Senescence of neoplastic cells, at least temporarily, reduces tumor expansion, but SASP of senescent cancer cells as well as SASP of senescent stromal cells in the tumor microenvironment may promote the growth of more aggressive cancer subclones. Here, we will review recent data on the mechanisms and the consequences of cancer-therapy induced senescence, enlightening the potentiality and the risk of senescence inducing treatments.

Dilation of Brain Veins and Perivascular Infiltration by Glioblastoma Cells in an In Vivo Assay of Early Tumor Angiogenesis.

The cranial window (CW) technique provides a simple and low-cost method to assess tumor angiogenesis in the brain. The CW combined with histology using selective markers for tumor and endothelial cells can allow a sensitive monitoring of novel antiangiogenesis therapies in preclinical models. The CW was established in cyclosporine immunosuppressed rats that were stereotactically grafted with fluorescent U87MG glioblastoma cells. One to 3 weeks after grafting, brain vasculature was visualized in vivo and assessed by immunofluorescence microscopy using antibodies against endothelial and smooth-muscle cells and blood brain barrier. At 1-2 weeks after grafting, the CW reliably detected the hypertrophy of venous-venous anastomoses and cortical veins. These structures increased highly significantly their pregrafting diameter. Arterialized veins and hemorrhages were seen by three weeks after grafting. Immunofluorescence microscopy showed significant branching and dilation of microvessels, particularly those surrounded by tumor cells. Mechanistically, these changes lead to loss of vascular resistance, increased venous outflow, and opening of venous-venous anastomoses on the cortical surface. Data from the present study, namely, the hypertrophy of cortical venous-venous anastomoses, microvessel branching, and dilation of the microvessels surrounded by tumor cells, indicate the power of this in vivo model for the sensitive monitoring of early tumor angiogenesis.

Brain Invasion along Perivascular Spaces by Glioma Cells: Relationship with Blood-Brain Barrier.

The question whether perivascular glioma cells invading the brain far from the tumor bulk may disrupt the blood-brain barrier (BBB) represents a crucial issue because under this condition tumor cells would be no more protected from the reach of chemotherapeutic drugs. A recent in vivo study that used human xenolines, demonstrated that single glioma cells migrating away from the tumor bulk are sufficient to breach the BBB. Here, we used brain xenografts of patient-derived glioma stem-like cells (GSCs) to show by immunostaining that in spite of massive perivascular invasion, BBB integrity was preserved in the majority of vessels located outside the tumor bulk. Interestingly, the tumor cells that invaded the brain for the longest distances traveled along vessels with retained BBB integrity. In surgical specimens of malignant glioma, the area of brain invasion showed several vessels with preserved BBB that were surrounded by tumor cells. On transmission electron microscopy, the cell inter-junctions and basal lamina of the brain endothelium were preserved even in conditions in which the tumor cells lay adjacently to blood vessels. In conclusion, BBB integrity associates with extensive perivascular invasion of glioma cells.

Axitinib exposure triggers endothelial cells senescence through ROS accumulation and ATM activation.

Inhibitors of Vascular Endothelial Growth Factor target both tumor vasculature and cancer cells that have hijacked VEGF Receptors (VEGFRs) signaling for tumor growth-promoting activities. It is important to get precise insight in the specificity of cell responses to these antiangiogenic drugs to maximize their efficiency and minimize off-target systemic toxicity. Here we report that Axitinib, an inhibitor of VEGFRs currently in use as a second line treatment for advanced renal cell carcinoma, promotes senescence of human endothelial cells in vitro. A one-hour pulse of Axitinib is sufficient for triggering cell senescence. Mechanistically, this requires oxidative stress-dependent activation of the Ataxia Telangiectasia Mutated (ATM) kinase. Axitinib-mediated senescence promoting action is prevented by short-term treatment with antioxidants or ATM inhibitors, which conversely fail to prevent senescence induced by the DNA-damaging drug doxorubicin. Coherently, induction of oxidative stress-related genes distinguishes the response of endothelial cells to Axitinib from that to doxorubicin. Importantly, an Axitinib pulse causes cell senescence in glioblastoma cells. However, neither antioxidants nor ATM inhibitors can reverse this phenotype. Thus, antioxidants may selectively protect endothelial cells from Axitinib by decreasing systemic toxicity and maintaining a functional vascularization necessary for efficient delivery of chemotherapeutic drugs within the tumor mass.

Inhibition of telomerase in the endothelial cells disrupts tumor angiogenesis in glioblastoma xenografts.

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.

Human mesenchymal stromal cells inhibit tumor growth in orthotopic glioblastoma xenografts.

BACKGROUND: Mesenchymal stem/stromal cells (MSCs) represent an attractive tool for cell-based cancer therapy mainly because of their ability to migrate to tumors and to release bioactive molecules. However, the impact of MSCs on tumor growth has not been fully established. We previously demonstrated that murine MSCs show a strong tropism towards glioblastoma (GBM) brain xenografts and that these cells are able to uptake and release the chemotherapeutic drug paclitaxel (PTX), maintaining their tropism towards the tumor. Here, we address the therapy-relevant issue of using MSCs from human donors (hMSCs) for local or systemic administration in orthotopic GBM models, including xenografts of patient-derived glioma stem cells (GSCs). METHODS: U87MG or GSC1 cells expressing the green fluorescent protein (GFP) were grafted onto the striatum of immunosuppressed rats. Adipose hMSCs (Ad-hMSCs), fluorescently labeled with the mCherry protein, were inoculated adjacent to or into the tumor. In rats bearing U87MG xenografts, systemic injections of Ad-hMSCs or bone marrow (BM)-hMSCs were done via the femoral vein or carotid artery. In each experiment, either PTX-loaded or unloaded hMSCs were used. To characterize the effects of hMSCs on tumor growth, we analyzed survival, tumor volume, tumor cell proliferation, and microvascular density. RESULTS: Overall, the AD-hMSCs showed remarkable tropism towards the tumor. Intracerebral injection of Ad-hMSCs significantly improved the survival of rats with U87MG xenografts. This effect was associated with a reduction in tumor growth, tumor cell proliferation, and microvascular density. In GSC1 xenografts, intratumoral injection of Ad-hMSCs depleted the tumor cell population and induced migration of resident microglial cells. Overall, PTX loading did not significantly enhance the antitumor potential of hMSCs. Systemically injected Ad- and BM-hMSCs homed to tumor xenografts. The efficiency of hMSC homing ranged between 0.02 and 0.5% of the injected cells, depending both on the route of cell injection and on the source from which the hMSCs were derived. Importantly, systemically injected PTX-loaded hMSCs that homed to the xenograft induced cytotoxic damage to the surrounding tumor cells. CONCLUSIONS: hMSCs have a therapeutic potential in GBM brain xenografts which is also expressed against the GSC population. In this context, PTX loading of hMSCs seems to play a minor role.

Establishing tumor cell lines from aggressive telomerase-positive chordomas of the skull base. Technical note.

Permanent cell cultures are invaluable tools for understanding the biological characteristics of tumors. In the present study the authors report on the establishment of permanent human cell lines from three cases of aggressive chordomas of the clival region. All of the parental tumors showed telomerase activity. Cultured chordoma cells had a doubling time of 5 to 7 days and grew as a monolayer of cells that retained both the immunophenotype and the p53 status of the parental tumor. In vitro, chordoma cells overexpressed telomerase, supporting the hypothesis that this enzyme is required for the immortalization process.

Telomerase inhibition impairs tumor growth in glioblastoma xenografts.

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.

Influence of local environment on the differentiation of neural stem cells engrafted onto the injured spinal cord.

OBJECTIVES: In vitro, neural stem cells (NSCs) proliferate as undifferentiated spheroids and differentiate into neurons, astrocytes and oligodendrocytes. These features make NSCs suitable for spinal cord (SC) reconstruction. However, in vivo experiments have demonstrated that in the injured SC transplanted NSCs either remain undifferentiated or differentiate into the astrocytic phenotype. The microenvironment of the injured SC is believed to play a crucial role in driving the differentiation of the engrafted NSCs. Here, we tested the hypothesis that inflammatory cytokines (ICs) may be involved in the restricted differentiation of NSCs after grafting onto the injured SC. METHODS: As the first step, we used immunohistochemistry to analyse the expression of tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta and interferon (IFN)-gamma in the normal SC of mice and following traumatic injury. Then, we investigated whether a combination of TNF-alpha, IL-1beta and IFN-gamma may affect the phenotype of murine NSCs in vitro. RESULTS: We found that TNF-alpha, IL-1beta and IFN-gamma, which are absent in the normal SC, are all expressed in the injured SC and the expression of these cytokines follows a timely tuned fashion with IFN-gamma being detectable as long as 4 weeks after injury. In culture, exposure of proliferating NSCs to a combination of TNF-alpha, IL-1beta and IFN-gamma was per se sufficient to induce the astrocytic differentiation of these cells even in the absence of serum. CONCLUSIONS: In the traumatically injured SC, differentiation of engrafted NSCs is restricted towards the astrocytic lineage because of the inflammatory environment. ICs are likely to play a major role in differentiation of NSCs in the in vivo conditions.

Glioblastoma induces vascular endothelial cells to express telomerase in vitro.

Angiogenesis is essential for the growth of solid tumors. We have observed previously that the vascular endothelial cells of astrocytic brain tumors express human telomerase reverse transcriptase (hTERT) mRNA, suggesting a role for telomerase in the angiogenesis of these neoplasms. Here, we used an in vitro model to demonstrate that the telomerase machinery might be trans-activated in primary endothelial cells by glioblastoma tumor cells. We found that glioblastoma cells in vitro do induce hTERT mRNA and hTERT protein expression, as well as telomerase enzyme activity in the endothelial cells, and that this phenomenon is mediated by diffusible factor(s). These results provide strong evidence of the involvement of telomerase in tumor angiogenesis and will stimulate research on antitelomerase drugs for treatment of malignant brain gliomas.