A boost of BMP4 accelerates the commitment of human embryonic stem cells to the endothelial lineage

Embryoid bodies (EBs) generated during differentiation of human embryonic stem cells (hESCs) contain vascular-like structures, suggesting that commitment of mesoderm progenitors into endothelial cells occurs spontaneously. We showed that bone morphogenetic protein 4 (BMP4), an inducer of mesoderm, accelerates the peak expression of CD133/kinase insert domain-containing receptor (KDR) and CD144/KDR. Because the CD133(+)KDR(+) population could represent endothelial progenitors, we sorted them at day 7 and cultured them in endothelial medium. These cells were, however, unable to differentiate into endothelial cells. Under standard conditions, the CD144(+)KDR(+) population represents up to 10% of the total cells at day 12. In culture, these cells, if sorted, give rise to a homogeneous population with a morphology typical of endothelial cells and express endothelial markers. These endothelial cells derived from the day 12 sorted population were functional, as assessed by different in vitro assays. When EBs were stimulated by BMP4, the CD144(+)KDR(+) peak was shifted to day 7. Most of these cells, however, were CD31(-), becoming CD31(+) in culture. They then expressed von Willebrand factor and were functional. This suggests that, initially, the BMP4-boosted day 7, CD144(+)KDR(+)CD31(-) population represents immature endothelial cells that differentiate into mature endothelial cells in culture. The expression of OCT3/4, a marker of immaturity for hESCs decreases during EB differentiation, decreasing faster following BMP4 induction. We also show that BMP4 inhibits the global expression of GATA2 and RUNX1, two transcription factors involved in hemangioblast formation, at day 7 and day 12.

Characterization of human PGD blastocysts with unbalanced chromosomal translocations and human embryonic stem cell line derivation?

Novel embryonic stem cell lines derived from embryos carrying structural chromosomal abnormalities obtained after preimplantation genetic diagnosis (PGD) are of interest to study in terms of the influence of abnormalities on further development. A total of 22 unbalanced blastocysts obtained after PGD were analysed for structural chromosomal defects. Morphological description and chromosomal status of these blastocysts was established and they were used to derive human embryonic stem cell (ESC) lines. An outgrowth of cells was observed for six blastocysts (6/22; 27%). For two blastocysts, the exact morphology was unknown since they were at early stage, and for four blastocysts, the inner cell mass was clearly visible. Fifteen blastocysts carried an unbalanced chromosomal defect linked to a reciprocal translocation, resulting in a positive outgrowth of cells for five blastocysts. One human ESC line was obtained from a blastocyst carrying a partial chromosome-21 monosomy and a partial chromosome-1 trisomy. Six blastocysts carried an unbalanced chromosomal defect linked to a Robertsonian translocation, and one showed a positive outgrowth of cells. One blastocyst carried an unbalanced chromosomal defect linked to an insertion and no outgrowth was observed. The efficiency of deriving human ESC lines with constitutional chromosomal disorders was low and probably depends on the initial morphological aspect of the blastocysts and/or the type of the chromosomal disorders.

Telomere Shortening and Associated Chromosomal Instability in Peripheral Blood Lymphocytes of Patients With Hodgkin’s Lymphoma Prior to Any Treatment Are Predictive of Second Cancers

PURPOSE

To investigate a potential link between telomere length, chromosomal instability, and the advent of a second cancer (SC) in patients with Hodgkin's lymphoma (HL), who are known to be at risk for SCs. This study was premised on the finding that telomere dysfunction and DNA repair pathways were related to many pathologic conditions.

METHODS AND MATERIALS

Three cohorts of patients with HL were studied: 73 who were prospectively followed >5 years after diagnosis (prospective HL cohort), 28 who developed a SC (SC HL cohort), and 18 long-term survivors with no evidence of disease or complication since their initial treatment (NED HL cohort). Telomere length was analyzed by a telomeric restriction fragment assay in peripheral blood lymphocytes. Thirty healthy donors and 70 patients with a newly diagnosed solid tumor were the control population.

RESULTS

Compared with controls, patients from the prospective HL cohort, before any treatment, showed age-independent shorter telomeres (mean, 8.3 vs. 11.7 kb in healthy donors; <6 kb in 18% in HL patients), increased spontaneous chromosomal abnormalities, and increased in vitro radiation sensitivity (p < 10(-4) each). After treatment, telomere shortening was associated with cytogenetic profiles characterized by the persistence of complex chromosomal rearrangement and clonal aberrations. Moreover, the two cases of SC in the prospective HL patients had short telomeres and CCR initially. In addition, the SC HL cohort was characterized by markedly short telomeres (6.6 vs. 9.7 kb in the NED HL cohort), the presence of complex chromosome rearrangements, and increased in vitro radiation sensitivity.

CONCLUSIONS

An intimate relationship between pre-treatment telomere shortening, chromosomal instability, radiation sensitivity and occurrence of SC was found in HL patients.

The SCL relative LYL-1 is required for fetal and adult hematopoietic stem cell function and B-cell differentiation

Hematopoietic stem cells (HSCs) arise, self-renew, or give rise to all hematopoietic lineages through the effects of transcription factors activated by signaling cascades. Lyl-1 encodes a transcription factor containing a basic helix-hoop-helix (bHLH) motif closely related to scl/tal, which controls numerous decisions in embryonic and adult hematopoiesis. We report here that Lyl-1 null mice are viable and display normal blood cell counts, except for a reduced number of B cells resulting from a partial block after the pro-B stage. Nevertheless, the deletion of Lyl-1 results in a diminution in the frequency of immature progenitors (Lin–, CD34–, sca-1+, c-kit+ [LSK], and LSK-side population [LSK-SP]) and in S12 colony-forming unit (CFU-S12) and long-term culture-initiating cell (LTC-IC) content in embryonic day 14 fetal liver (E14 FL) and adult bone marrow (BM). More important, Lyl-1–/– E14 FL cells and BM are severely impaired in their competitive reconstituting abilities, especially with respect to B and T lineage reconstitution. Thus, ablation of Lyl-1 quantitatively and functionally affects HSCs, a cell population that transcribes Lyl-1 more actively than their differentiated progenies. Our results demonstrate for the first time that Lyl-1 functions are important for HSC properties and B-cell differentiation and that they are largely distinct from scl functions.

Constitutive activation of STAT5 and Bcl-xL overexpression can induce endogenous erythroid colony formation in human primary cells

The biologic hallmark of polycythemia vera (PV) is the formation of endogenous erythroid colonies (EECs) with an erythropoietin-independent differentiation. Recently, it has been shown that an activating mutation of JAK2 (V617F) was at the origin of PV. In this work, we studied whether the STAT5/Bcl-xL pathway could be responsible for EEC formation. A constitutively active form of STAT5 was transduced into human erythroid progenitors and induced an erythropoietin-independent terminal differentiation and EEC formation. Furthermore, Bcl-xL overexpression in erythroid progenitors was also able to induce erythroid colonies despite the absence of erythropoietin. Conversely, siRNA-mediated STAT5 and Bcl-xL knock-down in human erythroid progenitors inhibited colony-forming unit-erythroid (CFU-E) formation in the presence of Epo. Altogether, these results demonstrate that a sustained level of the sole Bcl-xL is capable of giving rise to Epo-independent erythroid colony formation and suggest that, in PV patients, JAK2(V617F) may induce EEC via the STAT5/Bcl-xL pathway.

Notch/Delta4 interaction in human embryonic liver CD34+ CD38- cells: positive influence on BFU-E production and LTC-IC potential maintenance

We investigated whether Notch signaling pathways have a role in human developmental hematopoiesis. In situ histochemistry analysis revealed that Notch1, 2, and 4 and Notch ligand (Delta1-4, and Jagged1) proteins were not expressed in the yolk sac blood islands, the para-aortic splanchnopleure, the hematopoietic aortic clusters, and at the early stages of embryonic liver hematopoiesis. Notch1-2, and Delta4 were eventually detected in the embryonic liver, from 34 until 38 days postconception. Fluorescence-activated cell sorter analysis showed that first-trimester embryonic liver CD34(+)CD38(low) cells expressed both Notch1 and Notch2. When these cells were cultured on S17 stroma stably expressing Delta4, a 2.6-fold increase in BFU-E number was observed at day 7, as compared with cultures with control stroma, and this effect was maintained for 2 weeks. Importantly, exposure of these cells to Delta4 under these conditions maintained the original frequency and quality of long-term culture-initiating cells (LTC-ICs), while control cultures quickly resulted in the extinction of this LTC-IC potential. Furthermore, short-term exposure of embryonic liver adherent cells to erythropoietin resulted in a dose-dependent increase in Delta4 expression, almost doubling the expression observed with untreated stroma. This suggests that Delta4 has a role in the regulation of hematopoiesis after a hypoxic stress in the fetus.

Microarray Analysis of LIF/Stat3 Transcriptional Targets in Embryonic Stem Cells

Mouse embryonic stem (ES) cells can be propagated in vitro while retaining their properties of pluripotency and self-renewal under the continuous presence of leukemia inhibitor factor (LIF). An essential role has been attributed to subsequent activation of the Stat3 transcription factor in mediating LIF self-renewal response. To date, however, downstream target genes of Stat3 in ES cells are still unknown. To isolate these genes, we performed a microarray-based kinetic comparison of LIF-stimulated (undifferentiated) ES cells versus ES cells induced to differentiate by shutting down Stat3 activity through either LIF deprivation or, more specifically, expression of a Stat3 dominant-negative mutant. In each case, we chose the earliest time at which ES cells lose their self-renewal properties, as illustrated by a decrease in the number of embryoid bodies and blast cell colony formation as well as germ layer marker expression. Comparison of the two independent approaches revealed similarly regulated genes that are likely to be involved in the Stat3 effects on ES cell self-renewal. For instance, upregulation of growth factors such as the transforming growth factor-beta relative Lefty1 or transcriptional regulators such as Id1 and Id2 and down-regulation of the groucho-like protein Aes1 (grg5) were found. Promoter analysis of the aes1 gene revealed three functional Stat3 consensus sites, as shown by luciferase assays. Furthermore, chromatin immunoprecipitation experiment demonstrated that Stat3 is recruited to the promoter of aes1 in ES cells. These data demonstrated that the aes1 gene is a direct transcriptional target of Stat3 in ES cells.

[Hope and limits in cell therapies]

The hematopoietic system is one of the best characterized human cellular system in which a multipotent adult stem cell is at the origin of all the cells of a tissue. These last years, the use of stem cells has given rise to many hopes in regenerative medicine, especially in diseases without efficient therapies. However the hematopoietic system in which stem cell transplantation has entered in clinical practice for many years has also shown the limits of these approaches. Especially the in vitro manipulation of hematopoietic stem cells remains a challenge which requires more fundamental knowledge on the biology of stem cells including self renewal and homing. Knowledge for other tissue system is even more preliminary but the same experimental strategy used for hematopoietic stem cell can be translated and may accelerate their use in cell therapies. Characterization of human adult pluripotent stem cells and the generation of human ES cells capable to differentiate towards several tissues have led to new hopes but the road to their use in therapies may be long and will require a lot of investment in basic biology.

A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera

Myeloproliferative disorders are clonal haematopoietic stem cell malignancies characterized by independency or hypersensitivity of haematopoietic progenitors to numerous cytokines. The molecular basis of most myeloproliferative disorders is unknown. On the basis of the model of chronic myeloid leukaemia, it is expected that a constitutive tyrosine kinase activity could be at the origin of these diseases. Polycythaemia vera is an acquired myeloproliferative disorder, characterized by the presence of polycythaemia diversely associated with thrombocytosis, leukocytosis and splenomegaly. Polycythaemia vera progenitors are hypersensitive to erythropoietin and other cytokines. Here, we describe a clonal and recurrent mutation in the JH2 pseudo-kinase domain of the Janus kinase 2 (JAK2) gene in most (> 80%) polycythaemia vera patients. The mutation, a valine-to-phenylalanine substitution at amino acid position 617, leads to constitutive tyrosine phosphorylation activity that promotes cytokine hypersensitivity and induces erythrocytosis in a mouse model. As this mutation is also found in other myeloproliferative disorders, this unique mutation will permit a new molecular classification of these disorders and novel therapeutical approaches.

High Level of Glutathione-S-Transferase π Expression in Mantle Cell Lymphomas

PURPOSE

Prognosis of mantle cell lymphoma (MCL) remains poor. Patients who achieve a response to first line therapy usually relapse, and the probability of cure remains low. Glutathione-S-transferase pi (GST-pi) overexpression has been associated with alkylating agents and anthracycline resistance. GST-pi gene is located in 11q13 and is coamplified along with CCND1 gene in some human solid tumors.

EXPERIMENTAL DESIGN

We performed immunohistochemical analysis of GST-pi expression in 24 consecutive MCLs, 12 follicular lymphomas (FLs), and 69 diffuse large B-cell lymphomas (DLBCLs). Cases were classified in three groups: high GST-pi expression (> 50% of cells were stained), moderate (5 to 50% cells were stained), or absent (< 5% cells were stained). GST-pi and CCND1 mRNA levels were also assessed by real-time reverse transcription-PCR analysis.

RESULTS

All MCLs exhibit high GST-pi protein expression, compared with 29% of the DLBCLs and none of the FLs. MCLs expressed high levels of GST-pi and CCND1 mRNAs compared with DLBCLs and FLs. There was a strong relation between GST-pi and CCND1 mRNAs transcript levels in MCLs but not in DLBCLs. In conclusion, protein and mRNA GST-pi expression is high in MCL compared with FL and DLBCL.

CONCLUSIONS

Overexpression of CCND1 in MCL is associated with a transcriptional up-regulation of the GST-pi gene. Our results suggest that the glutathione system could play a role in drug resistance in MCL.

High vaccination efficiency of low-affinity epitopes in antitumor immunotherapy

Most of the human tumor-associated antigens (TAAs) characterized thus far are derived from nonmutated "self"-proteins. Numerous strategies have been developed to break tolerance to TAAs, combining various forms of antigens with different vectors and adjuvants. However, no study has yet determined how to select epitopes within a given TAA to induce the highest antitumor effector response. We addressed this question by evaluating in HLA-A*0201-transgenic HHD mice the antitumor vaccination efficacy of high- and low-affinity epitopes from the naturally expressed murine telomerase reverse transcriptase (mTERT). Immunity against low-affinity epitopes was induced with heteroclitical variants. We show here that the CTL repertoire against high-affinity epitopes is partially tolerized, while that against low-affinity epitopes is composed of frequent CTLs with high avidity. The high-affinity p797 and p545 mTERT epitopes are not able to protect mice from a lethal challenge with the mTERT-expressing EL4-HHD tumor. In contrast, mice developing CTL responses against the p572 and p988 low-affinity epitopes exhibit potent antitumor immunity and no sign of autoimmune reactivity against TERT-expressing normal tissues. Our results strongly argue for new TAA epitope selection and modification strategies in antitumor immunotherapy applications in humans.

High vaccination efficiency of low-affinity epitopes in antitumor immunotherapy

Most of the human tumor-associated antigens (TAAs) characterized thus far are derived from nonmutated "self"-proteins. Numerous strategies have been developed to break tolerance to TAAs, combining various forms of antigens with different vectors and adjuvants. However, no study has yet determined how to select epitopes within a given TAA to induce the highest antitumor effector response. We addressed this question by evaluating in HLA-A*0201-transgenic HHD mice the antitumor vaccination efficacy of high- and low-affinity epitopes from the naturally expressed murine telomerase reverse transcriptase (mTERT). Immunity against low-affinity epitopes was induced with heteroclitical variants. We show here that the CTL repertoire against high-affinity epitopes is partially tolerized, while that against low-affinity epitopes is composed of frequent CTLs with high avidity. The high-affinity p797 and p545 mTERT epitopes are not able to protect mice from a lethal challenge with the mTERT-expressing EL4-HHD tumor. In contrast, mice developing CTL responses against the p572 and p988 low-affinity epitopes exhibit potent antitumor immunity and no sign of autoimmune reactivity against TERT-expressing normal tissues. Our results strongly argue for new TAA epitope selection and modification strategies in antitumor immunotherapy applications in humans.

Membrane-bound Delta-4 Notch ligand reduces the proliferative activity of primitive human hematopoietic CD34+CD38low cells while maintaining their LTC-IC potential

To examine the role of the Notch ligand Delta-4 on hematopoietic stem cells, human CD34+CD38low cord blood cells were cocultured on S17 cells transduced with transmembrane Delta-4 (mbD4/S17) or an empty vector (C/S17). By the end of a 3-week culture, mbD4/S17 induced a 25-fold reduction in nucleated cell production, as compared to C/S17, by maintaining a higher proportion of cells in G0/G1 phase. A specific retention of a high proportion of CD34+ cells throughout the culture was observed with mbD4/S17, contrary to C/S17. Although mbD4/S17 promoted expansion of cells with the phenotype of committed lymphoid precursors (CD34+CD7+CD45RA+), these cells still retained their myeloid differentiation potential. mbD4/S17 maintained a higher LTC-IC frequency in output CD34+ cells, compared to C/S17, as in the subsets of cells having completed the same number of divisions on mbD4/S17. A Delta4-Fc protein (extracellular part of human Delta4 fused to Fc human IgG1 portion), immobilized on plastic, also reduced cell production and retained the LTC-IC potential. Transplantation of cells grown on mbD4/S17 into NOD/SCID mice showed no significant enhancement of the long-term repopulating ability. Thus, Delta4 appears to inhibit hematopoietic stem cell proliferation, in association with the maintenance of short-term lymphoid and myeloid repopulation capacity.

Retinoic acid receptor α and retinoid-X receptor-specific agonists synergistically target telomerase expression and induce tumor cell death

Retinoids modulate growth and differentiation of cancer cells through activation of gene transcription via the nuclear retinoic-acid receptors (RAR) and retinoid-X receptors (RXR). Their use in differentiation therapy of acute promyelocytic leukemia (APL) represents a model concept for reprogramming cancer cells. However, they also regulate antiproliferative genes whose functions do not mechanistically concur to this program. Recently, we have shown that, independently of maturation, a long-term all-trans retinoic acid (ATRA) treatment of the maturation-resistant APL cell line (NB4-LR1) represses telomerase (hTERT), leading to telomere shortening and death. Using retinoid-receptor-specific agonists, we demonstrate herein that cross-talk between RARalpha and RXR dual-liganded to their respective agonists resulted in strong synergistic downregulation of hTERT and subsequent cell death. Importantly, unlike ATRA, this synergy was obtained at very low agonist concentrations and occurred in other ATRA maturation-resistant APL cells. These findings provide the first demonstration that dual-liganded RXR and RARalpha signaling should allow efficient targeting of telomerase in differentiation-resistant tumor cells. Such a combination therapy might hold promise in clinic to avoid side effects of ATRA whose administration can indiscriminately activate all RARs. Given the tissue-specific expression of RARs, a tissue-selective therapy targeting telomerase in tumor cells by synthetic agonists can be envisioned.

FLT3 and MLL intragenic abnormalities in AML reflect a common category of genotoxic stress

MLL rearrangements in acute myeloid leukemia (AML) include translocations and intragenic abnormalities such as internal duplication and breakage induced by topoisomerase II inhibitors. In adult AML, FLT3 internal tandem duplications (ITDs) are more common in cases with MLL intragenic abnormalities (33%) than those with MLL translocation (8%). Mutation/deletion involving FLT3 D835 are found in more than 20% of cases with MLL intragenic abnormalities compared with 10% of AML with MLL translocation and 5% of adult AML with normal MLL status. Real-time quantification of FLT3 in 141 cases of AML showed that all cases with FLT3 D835 express high level transcripts, whereas FLT3-ITD AML can be divided into cases with high-level FLT3 expression, which belong essentially to the monocytic lineage, and those with relatively low-level expression, which predominantly demonstrate PML-RARA and DEK-CAN. FLT3 abnormalities in CBF leukemias with AML1-ETO or CBFbeta-MYH11 were virtually restricted to cases with variant CBFbeta-MYH11 fusion transcripts and/or atypical morphology. These data suggest that the FLT3 and MLL loci demonstrate similar susceptibility to agents that modify chromatin configuration, including topoisomerase II inhibitors and abnormalities involving PML and DEK, with consequent errors in DNA repair. Variant CBFbeta-MYH11 fusions and bcr3 PML-RARA may also be initiated by similar mechanisms.

Blastoid mantle cell lymphoma: evidence for nonrandom cytogenetic abnormalities additional to t(11;14) and generation of a mouse model

Mantle cell lymphoma (MCL) is characterized by the t(11;14)(q13;q32), which is associated with cyclin D1 hyperexpression and a poor prognosis. MCL cases have been shown to progress to a more aggressive disease but the molecular events responsible of this phenomenon have not been determined. We have established two cell lines from the pleural effusions of two patients with MCL that we have used for further cytogenetic characterization to better define the incidence and nature of secondary chromosome abnormalities using multicolor fluorescence in situ hybridization, whole chromosome paint, and specific probes. Both cell lines grew independently without growth factors. Using CCND1/IGH-specific probes, patient UPN1 was found to have a masked t(11;14). Numerous and complex chromosomal abnormalities were found in both cell lines affecting chromosomes 2, 8, 13, 18, 22, X, and Y. These abnormalities included 8p losses, suggesting the presence of an anti-oncogene in this region, rearrangements of 8q24, MYC gene, and translocations involving 8, X, and Y chromosomes, which might be significant in the pathogenesis of MCL progression. The use of the cell lines (UPN1) allowed us to generate a mouse model of human MCL, mimicking a disseminated lymphoma and leading to the death of the animals in 4 weeks. This blastoid MCL model could be of major interest to determine molecular events involved in MCL progression, allowing isolation of involved genes and their functional characterization, and to study the effects of new chemotherapy regimens in mouse models.

Telomere dysfunction and telomerase reactivation in human leukemia cell lines after telomerase inhibition by the expression of a dominant-negative hTERT mutant

As activation of telomerase represents a key step in the malignant transformation process, experimental models to develop anti-telomerase drugs provide a rational basis for anticancer strategies. We analysed the short and long-term efficacy of a stably expressed dominant-negative mutant (DN) of the telomerase catalytic unit (hTERT) in UT-7 and U937 human leukemia cell lines by using an IRES-e-GFP retrovirus. As expected, telomerase inactivation resulted in drastic telomere shortening, cytogenetic instability and cell growth inhibition in all e-GFP positive DN clones after 15-35 days of culture. However, despite this initial response, 50% of e-GFP positive DN clones with short telomeres escaped from crisis after 35 days of culture and recovered a proliferation rate similar to the control cells. This rescue was associated with a telomerase reactivation inducing telomere lengthening. We identified two pathways, one involving the loss of the DN transgene expression and the other the transcriptional up-regulation of endogenous hTERT with persistence of the DN transgene expression. Although this second mechanism appears to be a very rare event (one clone), these findings suggest that genomic instability induced by short telomeres after telomerase inhibition might enhance the probability of activation or selection of telomere maintenance mechanisms dependent on hTERT transcription.

Stromal cells retard the differentiation of CD34(+)CD38(low/neg) human primitive progenitors exposed to cytokines independent of their mitotic history

Stem cell proliferation induced by potent cytokines usually leads to a loss of primitive potential through differentiation. In this study, the ability of cytokines and murine MS5 stromal cells to independently regulate the proliferation and long-term culture-initiating cell (LTC-IC) activity of primitive CD34(+)CD38(low/neg) human bone marrow cells was evaluated. To compare populations with identical proliferation histories, cells were labeled with carboxy fluorescein diacetate succinimidyl ester, and LTC-IC activity was assessed 4 days later in cells that had accomplished the same number of divisions with or without MS5 cells. MS5 cells counteracted dramatically the loss of LTC-IC activity observed in the presence of cytokines alone. Thus, in the presence of MS5 cells, means of 1233 (n = 5) and 355 (n = 9) LTC-IC-derived colony-forming cells (CFCs) were generated by 1000 cells that performed 3 and 4 divisions respectively, whereas 311 (n = 5) and 64 (n = 5) CFCs were generated by 1000 cells cultured without MS5 cells. Interestingly, MS5 cells had no detectable effect on the LTC-IC activity of cells that divided only twice in 4 days-1606 CFCs (n = 6) and 1993 (n = 6) CFCs, respectively, without and with MS5 cells-and a 48 additional hours of coculture were necessary to unmask changes in the LTC-IC activity mediated by stromal cells. These results indicate that cytokines and stroma-derived signals can regulate independently the proliferation and differentiation of primitive cells and that these stroma-derived extracellular factors act directly on their target cells.

Combined effects of radiotherapy and angiostatin gene therapy in glioma tumor model

The objective of the present study was to evaluate the antitumor effect of a defective adenovirus expressing a secretable angiostatin-like molecule (AdK3) in combination with radiotherapy in rat C6 gliomas s.c. preestablished into athymic mice. In vitro, the combination regimen was significantly (P < 0.001) more cytotoxic for human microcapillary endothelial cells than either treatment alone, whereas survival of C6 glioma cells was not affected in the conditions used. Radiotherapy and AdK3 gene delivery was then studied on well established C6 xenografts (165 +/- 70 mm(3)). In these tumors, AdK3 intratumoral injections had only a marginal effect. Interestingly, when experimental radiotherapy was added, significantly higher (P < 0.005), and possibly synergistic, antitumoral effects were observed that tightly correlated a marked decrease of intratumoral vascularization. The combination of radiotherapy and AdK3 intratumoral injections also revealed a significant (P < 0.05) inhibition of tumor growth as compared with either treatment alone for larger tumors (467 +/- 120 mm(3)). Altogether, these data emphasize the potential of combining a destructive strategy directed against the tumor cells with an anti-angiogenic approach to fight cancer.