Gene transfer of hepatocyte growth factor by electroporation reduces bleomycin-induced lung fibrosis

Abnormal alveolar wound repair contributes to the development of pulmonary fibrosis after lung injury. Hepatocyte growth factor (HGF) is a potent mitogenic factor for alveolar epithelial cells and may therefore improve alveolar epithelial repair in vitro and in vivo. We hypothesized that HGF could increase alveolar epithelial repair in vitro and improve pulmonary fibrosis in vivo. Alveolar wound repair in vitro was determined using an epithelial wound repair model with HGF-transfected A549 alveolar epithelial cells. Electroporation-mediated, nonviral gene transfer of HGF in vivo was performed 7 days after bleomycin-induced lung injury in the rat. Alveolar epithelial repair in vitro was increased after transfection of wounded epithelial monolayers with a plasmid encoding human HGF, pCikhHGF [human HGF (hHGF) gene expressed from the cytomegalovirus (CMV) immediate-early promoter and enhancer] compared with medium control. Electroporation-mediated in vivo HGF gene transfer using pCikhHGF 7 days after intratracheal bleomycin reduced pulmonary fibrosis as assessed by histology and hydroxyproline determination 14 days after bleomycin compared with controls treated with the same vector not containing the HGF sequence (pCik). Lung epithelial cell proliferation was increased and apoptosis reduced in hHGF-treated lungs compared with controls, suggesting increased alveolar epithelial repair in vivo. In addition, profibrotic transforming growth factor-beta1 (TGF-beta1) was decreased in hHGF-treated lungs, indicating an involvement of TGF-beta1 in hHGF-induced reduction of lung fibrosis. In conclusion, electroporation-mediated gene transfer of hHGF decreases bleomycin-induced pulmonary fibrosis, possibly by increasing alveolar epithelial cell proliferation and reducing apoptosis, resulting in improved alveolar wound repair.

Glucocorticosteroid-induced spinal osteoporosis: scientific update on pathophysiology and treatment

Glucocorticosteroid-induced spinal osteoporosis (GIOP) is the most frequent of all secondary types of osteoporosis. The understanding of the pathophysiology of glucocorticoid (GC) induced bone loss is of crucial importance for appropriate treatment and prevention of debilitating fractures that occur predominantly in the spine. GIOP results from depressed bone formation due to lower activity and higher death rate of osteoblasts on the one hand, and from increase bone resorption due to prolonged lifespan of osteoclasts on the other. In addition, calcium/phosphate metabolism may be disturbed through GC effects on gut, kidney, parathyroid glands and gonads. Therefore, therapeutic agents aim at restoring balanced bone cell activity by directly decreasing apoptosis rate of osteoblasts (e.g., cyclical parathyroid hormone) or by increasing apoptosis rate of osteoclasts (e.g., bisphosphonates). Other therapeutical efforts aim at maintaining/restoring calcium/phosphate homeostasis: improving intestinal calcium absorption (using calcium supplementation, vitamin D and derivates) and avoiding increased urinary calcium loss (using thiazides) prevent or counteract a secondary hyperparthyroidism. Bisphosphonates, particularly the aminobisphosphonates risedronate and alendronate, have been shown to protect patients on GCs from (further) bone loss to reduce vertebral fracture risk. Calcitonin may be of interest in situation where bisphosphonates are contraindicated or not applicable and in cases where acute pain due to vertebral fracture has to be manage. The intermittent administration of 1-34-parathormone may be an appealing treatment alternative, based on its documented anabolic effects on bone resulting from the reduction of osteoblastic apoptosis. Calcium and vitamin D should be a systematic adjunctive measure to any drug treatment for GIOP. Based on currently available evidence, fluoride, androgens, estrogens (opposed or unopposed) cannot be recommended for the prevention and treatment of GIOP. However, substitution of gonadal hormones may be indicated if GC-induced hypogonadism is present and leads to clinical symptoms. Data using the SERM raloxifene to treat or prevent GIOP are lacking, as are data using the promising bone anabolic agent strontium ranelate. Kyphoplasty performed in appropriately selected osteoporotic patients with painful vertebral fractures is a promising addition to current medical treatment.

[The study of apoptosis factors released from laser injured cartilage inducing apoptosis of chondrocyte]

OBJECTIVE: To explore the role of pro-apoptotic signals following tissue injury and how these may promote a progression of further cell death. METHODS: Laser treated porcine articular cartilage disks were maintained in culture media. The collected media at various time periods (3, 6, 9, 12, 24 and 48 h), was called treated conditioned media (TCM). Non-laser treated cartilage disks were used to create control conditioned media (CCM). Each disk was subsequently maintained for 28 days and used in confocal microscopic assessment to document the progression of the damaged area. Isolated porcine chondrocytes were cultured in monolayer, and were exposed to TCM, CCM or normal culture medium (NM). As a positive inducer of apoptosis, the monolayer cells were exposed to UV radiation for 10 min and cultured in NM. Following 24 h exposure, the cells were harvested and stained with the appropriate combination of fluorescent dyes and processed via flow cytometry. RESULTS: All cultured cells exposed to TCM displayed a caspase-3 positive subpopulation, a loss of CMXRos, and with a reduced or lost NO signal. CCM exposure signals were comparable to the NM treatments with all having retained CMXRos, NO and without evidence of caspase-3 activity. UV treatment also induced a reduction in NO, but both CMXRos and caspase-3 positive, representing an earlier stage of apoptosis and suggesting that the mode of cell death via UV and TCM exposure are via different processes. The investigation of a dose (100%, 50%, 25% and 12.5%) and time (0.5, 1, 3, 9, 12 h) response to TCM exhibited that all treatments observed an increase in caspase-3 positive cells and a reduction in NO and CMXRos. CONCLUSION: The usefulness of FCM can be used in the study of cell viability and apoptosis. Such a system may be useful in the study of mechanisms of disease such as osteoarthritis, thus may be of practical use for the pharmaceutical industry for screening associated drugs.

Inhibition of ErbB2/neuregulin signaling augments paclitaxel-induced cardiotoxicity in adult ventricular myocytes

Paclitaxel (Taxol) has been successfully combined with the monoclonal antibody trastuzumab (Herceptin) in the treatment of ErbB2 overexpressing cancers. However, this combination therapy showed an unexpected synergistic increase in cardiac dysfunction. We have studied the mechanisms of paclitaxel/anti-ErbB2 cardiotoxicity in adult rat ventricular myocytes (ARVM). Myofibrillar organization was assessed by immunofluorescence microscopy and cell viability was tested by the TUNEL-, LDH- and MTT-assay. Oxidative stress was measured by DCF-fluorescence and myocyte contractile function by video edge-detection and fura-2 fluorescence. Treatment of ARVM with paclitaxel or antibodies to ErbB2 caused a significant increase in myofilament degradation, similarly as observed with an inhibitor of MAPK-signaling, but not apoptosis, necrosis or changes in mitochondrial activity. Paclitaxel-treatment and anti-ErbB2 reduced Erk1/2 phosphorylation. Paclitaxel increased diastolic calcium, shortened relaxation time and reduced fractional shortening in combination with anti-ErbB2. A minor increase in oxidative stress by paclitaxel or anti-ErbB2 was found. We conclude, that concomitant inhibition of ErbB2 receptors and paclitaxel treatment has an additive worsening effect on adult cardiomyocytes, mainly discernible in changes of myofibrillar structure and function, but in the absence of cell death. A potential mechanism is the modulation of the MAPK/Erk1/2 signaling by both drugs.

JNK is activated but does not mediate hippocampal neuronal apoptosis in experimental neonatal pneumococcal meningitis

Pneumococcal meningitis is associated with caspase 3-dependent apoptosis of recently post-mitotic immature neurons in the dentate gyrus of the hippocampus. The death of these cells is implicated in the learning and memory deficits in patients surviving the disease. The stress-activated protein kinase c-Jun N-terminal kinase (JNK) has been shown to be an important mediator of caspase 3-dependent neuronal apoptosis. However, whether JNK is involved in hippocampal apoptosis caused by pneumococcal meningitis has so far not been investigated. Here we show in a neonatal rat model of pneumococcal meningitis that JNK3 but not JNK1 or JNK2 is activated in the hippocampus during the acute phase of infection. At the cellular level, JNK3 activation was accompanied in the dentate gyrus by markedly increased phosphorylation of its major downstream target c-Jun in early immature (Hu-positive) neurons, but not in migrating (doublecortin-positive) neurons, the cells that do undergo apoptosis. These findings suggested that JNK may not be involved in pneumococcal meningitis-induced hippocampal apoptosis. Indeed, although intracerebroventricular administration of D-JNKI-1 or AS601245 (two highly specific JNK inhibitors) inhibited c-Jun phosphorylation and protein expression in the hippocampus, hippocampal apoptosis was unaffected. Collectively, these results demonstrate that JNK does not mediate hippocampal apoptosis in pneumococcal meningitis, and that JNK may be involved in processes unrelated to apoptosis in this disease.

Lung epithelial apoptosis in influenza virus pneumonia: the role of macrophage-expressed TNF-related apoptosis-inducing ligand

Mononuclear phagocytes have been attributed a crucial role in the host defense toward influenza virus (IV), but their contribution to influenza-induced lung failure is incompletely understood. We demonstrate for the first time that lung-recruited "exudate" macrophages significantly contribute to alveolar epithelial cell (AEC) apoptosis by the release of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in a murine model of influenza-induced pneumonia. Using CC-chemokine receptor 2-deficient (CCR2(-/-)) mice characterized by defective inflammatory macrophage recruitment, and blocking anti-CCR2 antibodies, we show that exudate macrophage accumulation in the lungs of influenza-infected mice is associated with pronounced AEC apoptosis and increased lung leakage and mortality. Among several proapoptotic mediators analyzed, TRAIL messenger RNA was found to be markedly up-regulated in alveolar exudate macrophages as compared with peripheral blood monocytes. Moreover, among the different alveolar-recruited leukocyte subsets, TRAIL protein was predominantly expressed on macrophages. Finally, abrogation of TRAIL signaling in exudate macrophages resulted in significantly reduced AEC apoptosis, attenuated lung leakage, and increased survival upon IV infection. Collectively, these findings demonstrate a key role for exudate macrophages in the induction of alveolar leakage and mortality in IV pneumonia. Epithelial cell apoptosis induced by TRAIL-expressing macrophages is identified as a major underlying mechanism.

Hyperbaric oxygen induces apoptosis via a mitochondrial mechanism

During therapeutic hyperbaric oxygenation lymphocytes are exposed to high partial pressures of oxygen. This study aimed to analyze the mechanism of apoptosis induction by hyperbaric oxygen. For intervals of 0.5-4 h Jurkat-T-cells were exposed to ambient air or oxygen atmospheres at 1-3 absolute atmospheres. Apoptosis was analyzed by phosphatidylserine externalization, caspase-3 activation and DNA-fragmentation using flow cytometry. Apoptosis was already induced after 30 min of hyperbaric oxygenation (HBO, P < 0.05). The death receptor Fas was downregulated. Inhibition of caspase-9 but not caspase-8 blocked apoptosis induction by HBO. Hyperbaric oxygen caused a loss of mitochondrial membrane potential and caspase-9 induction. The mitochondrial pro-survival protein Bcl-2 was upregulated, and antagonizing Bcl-2 function potentiated apoptosis induction by HBO. In conclusion, a single exposure to hyperbaric oxygenation induces lymphocyte apoptosis by a mitochondrial and not a Fas-related mechanism. Regulation of Fas and Bcl-2 may be regarded as protective measures of the cell in response to hyperbaric oxygen.

SFRP-4 abrogates Wnt-3a-induced beta-catenin and Akt/PKB signalling and reverses a Wnt-3a-imposed inhibition of in vitro mammary differentiation

ABSTRACT: BACKGROUND: Conserved Wnt ligands are critical for signalling during development; however, various factors modulate their activity. Among these factors are the Secreted Frizzled-Related Proteins (SFRP). We previously isolated the SFRP-4 gene from an involuting rat mammary gland and later showed that transgenic mice inappropriately expressing SFRP-4 during lactation exhibited a high level of apoptosis with reduced survival of progeny. RESULTS: In order to address the questions related to the mechanism of Wnt signalling and its inhibition by SFRP-4 which we report here, we employed partially-purified Wnt-3a in a co-culture model system. Ectopic expression of SFRP-4 was accomplished by infection with a pBabepuro construct. The co-cultures comprised Line 31E mouse mammary secretory epithelial cells and Line 30F, undifferentiated, fibroblast-like mouse mammary cells. In vitro differentiation of such co-cultures can be demonstrated by induction of the beta-casein gene in response to lactogenic hormones.We show here that treatment of cells with partially-purified Wnt-3a initiates Dvl-3, Akt/PKB and GSK-3beta hyperphosphorylation and beta-catenin activation. Furthermore, while up-regulating the cyclin D1 and connexin-43 genes and elevating transepithelial resistance of Line 31E cell monolayers, Wnt-3a treatment abrogates differentiation of co-cultures in response to the lactogenic hormones prolactin, insulin and glucocorticoid. Cells which express SFRP-4, however, are largely unaffected by Wnt-3a stimulation. Since a physical association between Wnt-3a and SFRP-4 could be demonstrated with immunoprecipitation/Western blotting experiments, this interaction, presumably owing to the Frizzled homology region typical of all SFRPs, explains the refractory response to Wnt-3a which was observed. CONCLUSION: This study demonstrates that Wnt-3a treatment activates the Wnt signalling pathway and interferes with in vitro differentiation of mammary co-cultures to beta-casein production in response to lactogenic hormones. Similarly, in another measure of differentiation, following Wnt-3a treatment mammary epithelial cells could be shown to up-regulate the cyclin D1 and connexin-43 genes while phenotypically they show increased transepithelial resistance across the cell monolayer. All these behavioural changes can be blocked in mammary epithelial cells expressing SFRP-4. Thus, our data illustrate in an in vitro model a mechanism by which SFRP-4 can modulate a differentiation response to Wnt-3a.

Death receptor 5 mediated-apoptosis contributes to cholestatic liver disease

Chronic cholestasis often results in premature death from liver failure with fibrosis; however, the molecular mechanisms contributing to biliary cirrhosis are not demonstrated. In this article, we show that the death signal mediated by TNF-related apoptosis-inducing ligand (TRAIL) receptor 2/death receptor 5 (DR5) may be a key regulator of cholestatic liver injury. Agonistic anti-DR5 monoclonal antibody treatment triggered cholangiocyte apoptosis, and subsequently induced cholangitis and cholestatic liver injury in a mouse strain-specific manner. TRAIL- or DR5-deficient mice were relatively resistant to common bile duct ligation-induced cholestasis, and common bile duct ligation augmented DR5 expression on cholangiocytes, sensitizing mice to DR5-mediated cholangitis. Notably, anti-DR5 monoclonal antibody-induced cholangitis exhibited the typical histological appearance, reminiscent of human primary sclerosing cholangitis. Human cholangiocytes constitutively expressed DR5, and TRAIL expression and apoptosis were significantly elevated in cholangiocytes of human primary sclerosing cholangitis and primary biliary cirrhosis patients. Thus, TRAIL/DR5-mediated apoptosis may substantially contribute to chronic cholestatic disease, particularly primary sclerosing cholangitis.

BIOCHEMICAL AND CELLULAR MECHANISMS OF RETINA AND RETINAL PIGMENT EPITHELIUM APOPTOSIS

Oxidative stress, intense light exposure and oxygen imbalances such as hypoxic or hyperoxic conditions perturb mitochondria, nuclear function and further lead to cellular damage of retina and retinal pigment epithelial (RPE) cells. Our major aim is to understand the various biochemical and proteomic events that occur during the progression of retina and RPE cell death. The comprehensive objectives of this dissertation are to understand the functional aspects of protein expression, posttranslational modifications, protein or lipid binding changes, phenotypic, morphological alterations and their regulation during the retina and RPE apoptosis under oxidative stress. The entire study is divided into four chapters Chapter 1 contains introduction and background on apoptotic signaling in retina and RPE cells. In chapter 2, we demonstrated that the oxidative stress biomarker prohibitin shuttles between mitochondria and nucleus as an anti-apoptotic molecule and acts as a transcriptional regulator by altering its lipid binding affinity and by posttranslational modifications during oxidative damage to the retina and RPE. In chapter 3, we demonstrated that oxidative and photo-oxidative stress induced nitric oxide regulates the RPE apoptosis by altering serine/threonine protein phosphatase 2A (PP2A) catalytic subunit, vimentin phosphorylation and Bcl xL expression regulation in the RPE cells in vitro. In chapter 4, we further analyzed the differential expression of prohibitin in the retina and RPE during oxidative stress, diabetic retinopathy (DR) and age-related macular degeneration (AMD) condition. Our analysis of postmortem retinas reveals that prohibitin is significantly increased in aged and AMD retina, and decreased in retinas of human diabetic retinopathy and RPE of AMD. Our study demonstrates that prohibitin levels determine the apoptotic signaling in the retina and RPE during retinal degenerative disease progression.

Natural killer T cell dysfunction in CD39-null mice protects against concanavalin A-induced hepatitis

Concanavalin A (Con A)-induced injury is an established natural killer T (NKT) cell-mediated model of inflammation that has been used in studies of immune liver disease. Extracellular nucleotides, such as adenosine triphosphate, are released by Con A-stimulated cells and bind to specific purinergic type 2 receptors to modulate immune activation responses. Levels of extracellular nucleotides are in turn closely regulated by ectonucleotidases, such as CD39/NTPDase1. Effects of extracellular nucleotides and CD39 on NKT cell activation and upon hepatic inflammation have been largely unexplored to date. Here, we show that NKT cells express both CD39 and CD73/ecto-5'-nucleotidase and can therefore generate adenosine from extracellular nucleotides, whereas natural killer cells do not express CD73. In vivo, mice null for CD39 are protected from Con A-induced liver injury and show substantively lower serum levels of interleukin-4 and interferon-gamma when compared with matched wild-type mice. Numbers of hepatic NKT cells are significantly decreased in CD39 null mice after Con A administration. Hepatic NKT cells express most P2X and P2Y receptors; exceptions include P2X3 and P2Y11. Heightened levels of apoptosis of CD39 null NKT cells in vivo and in vitro appear to be driven by unimpeded activation of the P2X7 receptor. CONCLUSION: CD39 and CD73 are novel phenotypic markers of NKT cells. Deletion of CD39 modulates nucleotide-mediated cytokine production by, and limits apoptosis of, hepatic NKT cells providing protection against Con A-induced hepatitis. This study illustrates a further role for purinergic signaling in NKT-mediated mechanisms that result in liver immune injury.

Soluble TNF-alpha but not transmembrane TNF-alpha sensitizes T cells for enhanced activation-induced cell death

In addition to its proinflammatory effects, TNF-alpha exhibits immunosuppression. Here, we compared the capacities of transmembrane TNF-alpha (tmTNF) and soluble TNF-alpha (sTNF) in regulating expansion of activated T cells by apoptosis. Splenic CD4(+) T cells from wtTNF, TNF-alpha-deficient (TNF(-/-)) and TNF(-/-) mice expressing a non-cleavable mutant tmTNF showed comparable proliferation rates upon TCR-mediated stimulation. Activation-induced cell death (AICD), however, was significantly attenuated in tmTNF and TNF(-/-), compared with wtTNF CD4(+) T cells. Addition of sTNF during initial priming was sufficient to enhance susceptibility to AICD in tmTNF and TNF(-/-) CD4(+) T cells to levels seen in wtTNF CD4(+) T cells, whereas addition of sTNF only during restimulation failed to enhance AICD. sTNF-induced, enhanced susceptibility to AICD was dependent on both TNF receptors. The reduced susceptibility of tmTNF CD4(+) T cells for AICD was also evident in an in vivo model of adoptively transferred CD4(+) T-cell-mediated colonic inflammation. Hence, the presence of sTNF during T-cell priming may represent an important mechanism to sensitize activated T cells for apoptosis, thereby attenuating the extent and duration of T-cell reactivities and subsequent T-cell-mediated, excessive inflammation.

Distinct requirements for activation-induced cell surface expression of preformed Fas/CD95 ligand and cytolytic granule markers in T cells

Fas (CD95/Apo-1) ligand is a potent inducer of apoptosis and one of the major killing effector mechanisms of cytotoxic T cells. Thus, Fas ligand activity has to be tightly regulated, involving various transcriptional and post-transcriptional processes. For example, preformed Fas ligand is stored in secretory lysosomes of activated T cells, and rapidly released by degranulation upon reactivation. In this study, we analyzed the minimal requirements for activation-induced degranulation of Fas ligand. T cell receptor activation can be mimicked by calcium ionophore and phorbol ester. Unexpectedly, we found that stimulation with phorbol ester alone is sufficient to trigger Fas ligand release, whereas calcium ionophore is neither sufficient nor necessary. The relevance of this process was confirmed in primary CD4(+) and CD8(+) T cells and NK cells. Although the activation of protein kinase(s) was absolutely required for Fas ligand degranulation, protein kinase C or A were not involved. Previous reports have shown that preformed Fas ligand co-localizes with other markers of cytolytic granules. We found, however, that the activation-induced degranulation of Fas ligand has distinct requirements and involves different mechanisms than those of the granule markers CD63 and CD107a/Lamp-1. We conclude that activation-induced degranulation of Fas ligand in cytotoxic lymphocytes is differently regulated than other classical cytotoxic granule proteins.

Serum metabolomics reveals irreversible inhibition of fatty acid beta-oxidation through the suppression of PPARalpha activation as a contributing mechanism of acetaminophen-induced hepatotoxicity

Metabolic bioactivation, glutathione depletion, and covalent binding are the early hallmark events after acetaminophen (APAP) overdose. However, the subsequent metabolic consequences contributing to APAP-induced hepatic necrosis and apoptosis have not been fully elucidated. In this study, serum metabolomes of control and APAP-treated wild-type and Cyp2e1-null mice were examined by liquid chromatography-mass spectrometry (LC-MS) and multivariate data analysis. A dose-response study showed that the accumulation of long-chain acylcarnitines in serum contributes to the separation of wild-type mice undergoing APAP-induced hepatotoxicity from other mouse groups in a multivariate model. This observation, in conjunction with the increase of triglycerides and free fatty acids in the serum of APAP-treated wild-type mice, suggested that APAP treatment can disrupt fatty acid beta-oxidation. A time-course study further indicated that both wild-type and Cyp2e1-null mice had their serum acylcarnitine levels markedly elevated within the early hours of APAP treatment. While remaining high in wild-type mice, serum acylcarnitine levels gradually returned to normal in Cyp2e1-null mice at the end of the 24 h treatment. Distinct from serum aminotransferase activity and hepatic glutathione levels, the pattern of serum acylcarnitine accumulation suggested that acylcarnitines can function as complementary biomarkers for monitoring the APAP-induced hepatotoxicity. An essential role for peroxisome proliferator-activated receptor alpha (PPARalpha) in the regulation of serum acylcarnitine levels was established by comparing the metabolomic responses of wild-type and Ppara-null mice to a fasting challenge. The upregulation of PPARalpha activity following APAP treatment was transient in wild-type mice but was much more prolonged in Cyp2e1-null mice. Overall, serum metabolomics of APAP-induced hepatotoxicity revealed that the CYP2E1-mediated metabolic activation and oxidative stress following APAP treatment can cause irreversible inhibition of fatty acid oxidation, potentially through suppression of PPARalpha-regulated pathways.

Switching of vascular phenotypes within a murine breast cancer model induced by angiopoietin-2

Sustained growth of solid tumours can rely on both the formation of new and the co-option of existing blood vessels. Current models suggest that binding of angiopoietin-2 (Ang-2) to its endothelial Tie2 receptor prevents receptor phosphorylation, destabilizes blood vessels, and promotes vascular permeability. In contrast, binding of angiopoietin-1 (Ang-1) induces Tie2 receptor activation and supports the formation of mature blood vessels covered by pericytes. Despite the intense research to decipher the role of angiopoietins during physiological neovascularization and tumour angiogenesis, a mechanistic understanding of angiopoietin function on vascular integrity and remodelling is still incomplete. We therefore assessed the vascular morphology of two mouse mammary carcinoma xenotransplants (M6378 and M6363) which differ in their natural angiopoietin expression. M6378 displayed Ang-1 in tumour cells but no Ang-2 in tumour endothelial cells in vivo. In contrast, M6363 tumours expressed Ang-2 in the tumour vasculature, whereas no Ang-1 expression was present in tumour cells. We stably transfected M6378 mouse mammary carcinoma cells with human Ang-1 or Ang-2 and investigated the consequences on the host vasculature, including ultrastructural morphology. Interestingly, M6378/Ang-2 and M6363 tumours displayed a similar vascular morphology, with intratumoural haemorrhage and non-functional and abnormal blood vessels. Pericyte loss was prominent in these tumours and was accompanied by increased endothelial cell apoptosis. Thus, overexpression of Ang-2 converted the vascular phenotype of M6378 tumours into a phenotype similar to M6363 tumours. Our results support the hypothesis that Ang-1/Tie2 signalling is essential for vessel stabilization and endothelial cell/pericyte interaction, and suggest that Ang-2 is able to induce a switch of vascular phenotypes within tumours.