Mitochondrial heat shock protein-90 modulates vascular smooth muscle cell survival and the vascular injury response in vivo.

The healing response of blood vessels from the vascular injury induced by therapeutic interventions is characterized by increased cellularity and tissue remodeling. Frequently, this leads to intimal hyperplasia and lumen narrowing, with significant clinical sequelae. Vascular smooth muscle cells are the primary cell type involved in this process, wherein they express a dedifferentiated phenotype that transiently resembles neoplastic transformation. Recent studies have highlighted the role of mitochondrial proteins, such as the molecular chaperone heat shock protein-90 (Hsp90), in promoting cancer cell survival, which leads to new candidate chemotherapeutic agents for neoplastic disease. Herein, we identify mitochondrial Hsp90 as a key modulator of the vascular injury response. Hsp90 expression is up-regulated in injured arteries and colocalizes with the apoptosis inhibitor, survivin, in vascular smooth muscle cell in vitro and in vivo. By using a proteomic approach, we demonstrate that targeted disruption of mitochondrial Hsp90 chaperone function in vascular smooth muscle cell leads to loss of cytoprotective client proteins (survivin and Akt), induces mitochondrial permeability, and leads to apoptotic cell death. Hsp90 targeting using a cell-permeable peptidomimetic agent resulted in marked attenuation of neointimal lesions in a murine arterial injury model. These findings suggest that mitochondrial Hsp90 chaperone function is an important regulator of intimal hyperplasia and may have implications for molecular strategies that promote the long-term patency of cardiovascular interventions.

Exploiting the mitochondrial unfolded protein response for cancer therapy in mice and human cells.

Fine tuning of the protein folding environment in subcellular organelles, such as mitochondria, is important for adaptive homeostasis and may participate in human diseases, but the regulators of this process are still largely elusive. Here, we have shown that selective targeting of heat shock protein-90 (Hsp90) chaperones in mitochondria of human tumor cells triggered compensatory autophagy and an organelle unfolded protein response (UPR) centered on upregulation of CCAAT enhancer binding protein (C/EBP) transcription factors. In turn, this transcriptional UPR repressed NF-κB-dependent gene expression, enhanced tumor cell apoptosis initiated by death receptor ligation, and inhibited intracranial glioblastoma growth in mice without detectable toxicity. These data reveal what we believe to be a novel role of Hsp90 chaperones in the regulation of the protein-folding environment in mitochondria of tumor cells. Disabling this general adaptive pathway could potentially be used in treatment of genetically heterogeneous human tumors.

Combined in silico and experimental approach for drug design: the binding mode of peptidic and non-peptidic inhibitors to hsp90 N-terminal domain.

Heat shock protein 90 (Hsp90) is a prime target for antitumor therapies. The information obtained by molecular dynamics (MD) simulations is combined with NMR data to provide a cross-validated atomic resolution model of the complementary interactions of heat shock protein 90 with a peptidic (shepherdin) and a non-peptidic (5-aminoimidazole-4-carboxamide-1-ß-d-ribofuranoside, AICAR) inhibitor, showing antiproliferative and proapoptotic activity in multiple tumor cell lines. This approach highlights the relevant role of imidazolic moiety in the interaction of both antagonist molecules. In 5-aminoimidazole-4-carboxamide-1-ß-d-ribofuranoside bound state, one conformation of those present in solution is selected, where imidazolic, H4 and H5 protons have a key role in defining a non-polar region contacting heat shock protein 90 surface. The dynamic equilibrium between N-type and S-type puckered forms of 5-aminoimidazole-4-carboxamide-1-ß-d-ribofuranoside moiety is shown to be functional to inhibitor binding. The first experimental structural data on these inhibitors are presented and discussed as hints for future design of improved molecules.

Preclinical characterization of mitochondria-targeted small molecule hsp90 inhibitors, gamitrinibs, in advanced prostate cancer.

PURPOSE: This study aimed to characterize the preclinical activity of the first class of combinatorial, mitochondria-targeted, small molecule heat shock protein-90 (Hsp90) inhibitors, gamitrinibs, in models of hormone-refractory, drug-resistant, localized, and bone metastatic prostate cancer in vivo. EXPERIMENTAL DESIGN: Mitochondrial permeability transition, apoptosis, and changes in metabolic activity were examined by time-lapse videomicroscopy, multiparametric flow cytometry, MTT, and analysis of isolated mitochondria. Drug-resistant prostate cancer cells were generated by chronic exposure of hormone-refractory PC3 cells to the Hsp90 inhibitor 17-allylaminogeldanamycin (17-AAG). The effect of gamitrinibs on s.c. or intratibial prostate cancer growth was studied in xenograft models. Bone metastatic tumor growth and bone parameters were quantified by micro-computed tomography imaging. RESULTS: In the NCI 60-cell line screening, gamitrinibs were active against all tumor cell types tested, and efficiently killed metastatic, hormone-refractory, and multidrug-resistant prostate cancer cells characterized by overexpression of the ATP binding cassette transporter P-glycoprotein. Mechanistically, gamitrinibs, but not 17-AAG, induced acute mitochondrial dysfunction in prostate cancer cells with loss of organelle membrane potential, release of cytochrome c, and caspase activity, independently of proapoptotic Bcl-2 proteins Bax and Bak. Systemic administration of gamitrinibs to mice was well tolerated, and inhibited s.c. or bone metastatic prostate cancer growth in vivo. CONCLUSIONS: Gamitrinibs have preclinical activity and favorable safety in models of drug-resistant and bone metastatic prostate cancer in vivo.

Global targeting of subcellular heat shock protein-90 networks for therapy of glioblastoma.

Drug discovery for complex and heterogeneous tumors now aims at dismantling global networks of disease maintenance, but the subcellular requirements of this approach are not understood. Here, we simultaneously targeted the multiple subcellular compartments of the molecular chaperone heat shock protein-90 (Hsp90) in a model of glioblastoma, a highly lethal human malignancy in urgent need of fresh therapeutic strategies. Treatment of cultured or patient-derived glioblastoma cells with Shepherdin, a dual peptidomimetic inhibitor of mitochondrial and cytosolic Hsp90, caused irreversible collapse of mitochondria, degradation of Hsp90 client proteins in the cytosol, and tumor cell killing by apoptosis and autophagy. Stereotactic or systemic delivery of Shepherdin was well tolerated and suppressed intracranial glioma growth via inhibition of cell proliferation, induction of apoptosis, and reduction of angiogenesis in vivo. These data show that disabling Hsp90 cancer networks in their multiple subcellular compartments improves strategies for drug discovery and may provide novel molecular therapy for highly recalcitrant human tumors.

Cytoprotective mitochondrial chaperone TRAP-1 as a novel molecular target in localized and metastatic prostate cancer.

Molecular chaperones of the heat shock protein-90 (Hsp90) family promote cell survival, but the molecular requirements of this pathway in tumor progression are not understood. Here, we show that a mitochondria-localized Hsp90 chaperone, tumor necrosis factor receptor-associated protein-1 (TRAP-1), is abundantly and ubiquitously expressed in human high-grade prostatic intraepithelial neoplasia, Gleason grades 3 through 5 prostatic adenocarcinomas, and metastatic prostate cancer, but largely undetectable in normal prostate or benign prostatic hyperplasia in vivo. Prostate lesions formed in genetic models of the disease, including the transgenic adenocarcinoma of the mouse prostate and mice carrying prostate-specific deletion of the phosphatase tensin homolog tumor suppressor (Pten(pc-/-)), also exhibit high levels of TRAP-1. Expression of TRAP-1 in nontransformed prostatic epithelial BPH-1 cells inhibited cell death, whereas silencing of TRAP-1 in androgen-independent PC3 or DU145 prostate cancer cells by small interfering RNA enhanced apoptosis. Targeting TRAP-1 with a novel class of mitochondria-directed Hsp90 inhibitors, ie, Gamitrinibs, caused rapid and complete killing of androgen-dependent or -independent prostate cancer, but not BPH-1 cells, whereas reintroduction of TRAP-1 in BPH-1 cells conferred sensitivity to Gamitrinib-induced cell death. These data identify TRAP-1 as a novel mitochondrial survival factor differentially expressed in localized and metastatic prostate cancer compared with normal prostate. Targeting this pathway with Gamitrinibs could be explored as novel molecular therapy in patients with advanced prostate cancer.

Endogenous tumor suppression mediated by PTEN involves survivin gene silencing.

Endogenous tumor suppression provides a barrier against oncogenesis, but the molecular requirements of this process are not well understood. Here, we show that the dual specificity phosphatase PTEN, a gene almost universally altered in human tumors, silences the expression of survivin, an essential regulator of cell division and apoptosis in cancer. This pathway is independent of p53, involves active repression of survivin gene transcription, and is mediated by direct occupancy of the survivin promoter by FOXO1 and FOXO3a factors. Conditional deletion of PTEN in the mouse prostate causes deregulated induction of survivin before full-blown transformation in vivo, whereas expression of survivin and PTEN is inversely correlated in cancer patients. Therefore, silencing the survivin gene is an essential requirement of endogenous PTEN tumor suppression.

Combinatorial drug design targeting multiple cancer signaling networks controlled by mitochondrial Hsp90.

Although therapeutically targeting a single signaling pathway that drives tumor development and/or progression has been effective for a number of cancers, in many cases this approach has not been successful. Targeting networks of signaling pathways, instead of isolated pathways, may overcome this problem, which is probably due to the extreme heterogeneity of human tumors. However, the possibility that such networks may be spatially arranged in specialized subcellular compartments is not often considered in pathway-oriented drug discovery and may influence the design of new agents. Hsp90 is a chaperone protein that controls the folding of proteins in multiple signaling networks that drive tumor development and progression. Here, we report the synthesis and properties of Gamitrinibs, a class of small molecules designed to selectively target Hsp90 in human tumor mitochondria. Gamitrinibs were shown to accumulate in the mitochondria of human tumor cell lines and to inhibit Hsp90 activity by acting as ATPase antagonists. Unlike Hsp90 antagonists not targeted to mitochondria, Gamitrinibs exhibited a "mitochondriotoxic" mechanism of action, causing rapid tumor cell death and inhibiting the growth of xenografted human tumor cell lines in mice. Importantly, Gamitrinibs were not toxic to normal cells or tissues and did not affect Hsp90 homeostasis in cellular compartments other than mitochondria. Therefore, combinatorial drug design, whereby inhibitors of signaling networks are targeted to specific subcellular compartments, may generate effective anticancer drugs with novel mechanisms of action.

A functional Notch-survivin gene signature in basal breast cancer.

INTRODUCTION: Basal-type, or triple-negative, breast cancer (lacking estrogen receptor, progesterone receptor, and human epidermal growth factor receptor-2 expression) is a high-risk disease for which no molecular therapies are currently available. We studied genetic signatures of basal breast cancer potentially suitable for therapeutic intervention. METHODS: We analyzed protein expression of the Notch-1 intracellular domain and survivin by immunohistochemistry in a series of basal breast cancer patients. A hierarchical clustering and overall survival analysis was carried out on a microarray mRNA database of 232 breast cancer patients. Fifteen published mRNA datasets containing estrogen receptor-negative or estrogen receptor-positive samples were subjected to meta-analysis for co-segregated gene expression. Experiments of plasmid transfection and gene silencing were carried out in estrogen receptor-negative MDA-MB-231 breast cancer cells. RESULTS: The developmental signaling regulator Notch-1 was highly expressed in breast cancer, compared with normal tissue, and was segregated with basal disease. Higher Notch-1 levels correlated with progressively abbreviated overall survival, and with increased expression of survivin, a tumor-associated cell death and mitotic regulator implicated in stem cell viability. Analysis of Pearson's correlation coefficient indicated that Notch-1 and survivin co-segregated in basal breast cancer. Notch-1 stimulation in MDA-MB-231 cells increased survivin expression, whereas silencing Notch reduced survivin levels. CONCLUSIONS: A Notch-1-survivin functional gene signature is a hallmark of basal breast cancer, and may contribute to disease pathogenesis. Antagonists of Notch and survivin currently in the clinic may be tested as novel molecular therapy for these recurrence-prone patients.

Regulation of tumor cell mitochondrial homeostasis by an organelle-specific Hsp90 chaperone network.

Molecular chaperones, especially members of the heat shock protein 90 (Hsp90) family, are thought to promote tumor cell survival, but this function is not well understood. Here, we show that mitochondria of tumor cells, but not most normal tissues, contain Hsp90 and its related molecule, TRAP-1. These chaperones interact with Cyclophilin D, an immunophilin that induces mitochondrial cell death, and antagonize its function via protein folding/refolding mechanisms. Disabling this pathway using novel Hsp90 ATPase antagonists directed to mitochondria causes sudden collapse of mitochondrial function and selective tumor cell death. Therefore, Hsp90-directed chaperones are regulators of mitochondrial integrity, and their organelle-specific antagonists may provide a previously undescribed class of potent anticancer agents.

Small-molecule targeting of heat shock protein 90 chaperone function: rational identification of a new anticancer lead.

Heat shock protein 90 (Hsp90) is a significant target in the development of rational cancer therapy due to its role at the crossroads of multiple signaling pathways associated with cell proliferation and cell viability. Here we present a combined structure- and dynamics-based computational design strategy, taking the flexibility of the receptor and of a lead peptidic antagonist into account explicitly, to identify the nonpeptidic small molecule 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) as a structurally novel inhibitor of Hsp90. The compound is selected to bind the Hsp90 N-terminal domain, mimicking the chemical and conformational properties of the recently described peptidic antagonist of the survivin-Hsp90 complex, shepherdin [Plescia et al. Cancer Cell 2005, 7, 457-468]. Experimental tests show that AICAR binds the Hsp90 N-domain, destabilizes multiple Hsp90 client proteins in vivo, including survivin, and exhibits antiproliferative and proapoptotic activity in multiple tumor cell lines, while not affecting proliferation of normal human fibroblasts. We propose that AICAR represents a viable lead for further development of anticancer drugs with wide therapeutic opportunities.

Antileukemic activity of shepherdin and molecular diversity of hsp90 inhibitors.

BACKGROUND: Heat shock protein 90 (Hsp90) is a molecular chaperone that is involved in signaling pathways for cell proliferation, survival, and cellular adaptation. Inhibitors of Hsp90 are being examined as cancer therapeutic agents, but the molecular mechanism of their anticancer activity is still unclear. We investigated Hsp90 as a therapeutic target for acute myeloid leukemia (AML) by use of the Hsp90 inhibitor shepherdin (a novel peptidyl antagonist of the interaction between Hsp90 and survivin, which is a regulator of cell proliferation and cell viability in cancer). METHODS: We studied protein interactions by molecular dynamics simulations and conducted competition experiments by use of enzyme-linked immunosorbent assay (ELISA). Shepherdin[79-83], a novel variant carrying the survivin sequence from Lys-79 through Gly-83, or its scrambled peptide was made permeable to cells by adding the antennapedia helix III carrier sequence. Apoptosis, Hsp90 client protein expression, and mitochondrial dysfunction were evaluated in AML types (myeloblastic, monocytic, and chronic myelogenous leukemia in blast crisis), patient-derived blasts, and normal mononuclear cells. Effects of shepherdin on tumor growth were evaluated in AML xenograft tumors in mice (n = 6). Organ tissues were examined histologically. RESULTS: Shepherdin[79-83] bound to Hsp90, inhibited formation of the survivin-Hsp90 complex, and competed with ATP binding to Hsp90. Cell-permeable shepherdin[79-83] induced rapid (within 30 minutes) and complete (with concentrations inducing 50% cell death of 24-35 microM) killing of AML types and blasts, but it did not affect normal mononuclear cells. Shepherdin[79-83] made contact with unique residues in the ATP pocket of Hsp90 (Ile-96, Asp-102, and Phe-138), did not increase Hsp70 levels in AML cells, disrupted mitochondrial function within 2 minutes of treatment, and eliminated the expression of Hsp90 client proteins. Shepherdin[79-83] abolished growth of AML xenograft tumors (mean of control group = 1698 mm3 and mean of treated group = 232 mm3; difference = 1466 mm3, 95% confidence interval = 505.8 to 2426; P = .008) without systemic or organ toxicity and inhibited Hsp90 function in vivo. CONCLUSIONS: Shepherdin is a novel Hsp90 inhibitor with a unique mechanism of anticancer activity.

A survivin gene signature predicts aggressive tumor behavior.

Gene signatures that predict aggressive tumor behavior at the earliest stages of disease, ideally before overt tissue abnormalities, are urgently needed. To search for such genes, we generated a transgenic model of survivin, an essential regulator of cell division and apoptosis overexpressed in cancer. Transgenic expression of survivin in the urinary bladder did not cause histologic abnormalities of the urothelium. However, microarray analysis revealed that survivin-expressing bladders exhibited profound changes in gene expression profile affecting extracellular matrix and inflammatory genes. Following exposure to a bladder carcinogen, N-butyl-N-(4-hydroxybutyl) nitrosamine (OH-BBN), survivin transgenic animals exhibited accelerated tumor progression, preferential incidence of tumors as compared with premalignant lesions, and dramatically abbreviated survival. Conversely, transgenic expression of a survivin Thr34-->Ala dominant-negative mutant did not cause changes in gene expression or accelerated tumor progression after OH-BBN treatment. Therefore, survivin expression induces global transcriptional changes in the tissue microenvironment that may promote tumorigenesis. Detection of survivin or its associated gene signature may provide an early biomarker of aggressive tumor behavior before the appearance of tissue abnormalities.

Rational design of shepherdin, a novel anticancer agent.

Anticancer agents that selectively kill tumor cells and spare normal tissues are urgently needed. Here, we engineered a cell-permeable peptidomimetic, shepherdin, modeled on the binding interface between the molecular chaperone Hsp90 and the antiapoptotic and mitotic regulator, survivin. Shepherdin makes extensive contacts with the ATP pocket of Hsp90, destabilizes its client proteins, and induces massive death of tumor cells by apoptotic and nonapoptotic mechanisms. Conversely, shepherdin does not reduce the viability of normal cells, and does not affect colony formation of purified hematopoietic progenitors. Systemic administration of shepherdin in vivo is well tolerated, and inhibits human tumor growth in mice without toxicity. Shepherdin could provide a potent and selective anticancer agent in humans.

Mitochondrial survivin inhibits apoptosis and promotes tumorigenesis.

Evasion of apoptosis is a hallmark of cancer, but the molecular circuitries of this process are not understood. Here we show that survivin, a member of the inhibitor of apoptosis gene family that is overexpressed in cancer, exists in a novel mitochondrial pool in tumor cells. In response to cell death stimulation, mitochondrial survivin is rapidly discharged in the cytosol, where it prevents caspase activation and inhibits apoptosis. Selective targeting of survivin to mitochondria enhances colony formation in soft agar, accelerates tumor growth in immunocompromised animals, and abolishes tumor cell apoptosis in vivo. Therefore, mitochondrial survivin orchestrates a novel pathway of apoptosis inhibition, which contributes to tumor progression.

IL-11 protects human microvascular endothelium from alloinjury in vivo by induction of survivin expression.

IL-11 can reduce tissue injury in animal models of inflammation but the mechanism(s) is unknown. When C.B-17 SCID/beige mice bearing human skin grafts are injected i.p. with human PBMC allogeneic to the donor skin, infiltrating T cells destroy human microvessels by day 21. Intradermal injection of human IL-11 (500 ng/day) delays the time course of graft microvessel loss without reducing the extent of T cell infiltration. Protective actions of IL-11 are most pronounced on day 15. IL-11 has no effect on T cell activation marker, effector molecule, cytokine expression, or endothelial ICAM-1 expression. IL-11 up-regulates the expression of survivin, a cytoprotective protein, in graft keratinocytes and endothelial cells. Topical application of survivin antisense oligonucleotide down-regulates survivin expression in both cell types and largely abrogates the protective effect of IL-11. We conclude that in this human transplant model, IL-11 exerts a cytoprotective rather than anti-inflammatory or immunomodulatory effect mediated through induction of survivin.

Acute ablation of survivin uncovers p53-dependent mitotic checkpoint functions and control of mitochondrial apoptosis.

Survivin is a member of the Inhibitor of Apoptosis gene family that has been implicated in cell division and suppression of apoptosis. Here, we show that preferential ablation of the nuclear pool of survivin by RNA interference produces a mitotic arrest followed by re-entry into the cell cycle and polyploidy. Survivin ablation causes multiple centrosomal defects, aberrant multipolar spindle formation, and chromatin missegregation, and these phenotypes are exacerbated by loss of the cell cycle regulator, p21(Waf1/Cip1) in p21(-/-) cells. The mitotic checkpoint activated by loss of survivin is mediated by induction of p53 and associated with increased expression of its downstream target, p21(Waf1/Cip1). Accordingly, p53(-/-) cells exhibit reduced mitotic arrest and enhanced polyploidy upon survivin ablation as compared with their p53(+/+) counterparts. Partial reduction of the cytosolic pool of survivin by RNA interference sensitizes cells to ultraviolet B-mediated apoptosis and results in enhanced caspase-9 proteolytic cleavage, whereas complete ablation of cytosolic survivin causes loss of mitochondrial membrane potential and spontaneous apoptosis. These data demonstrate that survivin has separable checkpoint functions at multiple phases of mitosis and in the control of mitochondrial-dependent apoptosis.

Regulation of survivin function by Hsp90.

Pathways controlling cell proliferation and cell survival require flexible adaptation to environmental stresses. These mechanisms are frequently exploited in cancer, allowing tumor cells to thrive in unfavorable milieus. Here, we show that Hsp90, a molecular chaperone that is central to the cellular stress response, associates with survivin, an apoptosis inhibitor and essential regulator of mitosis. This interaction involves the ATPase domain of Hsp90 and the survivin baculovirus inhibitor of apoptosis repeat. Global suppression of the Hsp90 chaperone function or targeted Abmediated disruption of the survivin-Hsp90 complex results in proteasomal degradation of survivin, mitochondrial-dependent apoptosis, and cell cycle arrest with mitotic defects. These data link the cellular stress response to an antiapoptotic and mitotic checkpoint maintained by survivin. Targeting the survivin-Hsp90 complex may provide a rational approach for cancer therapy.

Fibronectin protects prostate cancer cells from tumor necrosis factor-alpha-induced apoptosis via the AKT/survivin pathway.

Integrins are cell surface heterodimeric transmembrane receptors that, in addition to mediating cell adhesion to extracellular matrix proteins modulate cell survival. This mechanism may be exploited in cancer where evasion from apoptosis invariably contributes to cellular transformation. The molecular mechanisms responsible for matrix-induced survival signals begin to be elucidated. Here we report that the inhibitor of apoptosis survivin is expressed in vitro in human prostate cell lines with the highest levels present in aggressive prostate cancer cells such as PC3 and LNCaP-LN3 as well as in vivo in prostatic adenocarcinoma. We also show that interference with survivin in PC3 prostate cancer cells using a Cys84--> Ala dominant negative mutant or survivin antisense cDNA causes nuclear fragmentation, hypodiploidy, cleavage of a 32-kDa proform caspase-3 to active caspase-3, and proteolysis of the caspase substrate poly(ADP-ribose) polymerase. We demonstrate that in the aggressive PC3 cell line, adhesion to fibronectin via beta1 integrins results in up-regulation of survivin and protection from apoptosis induced by tumor necrosis factor-alpha (TNF-alpha). In contrast, survivin is not up-regulated by cell adhesion in the non-tumorigenic LNCaP cell line. Dominant negative survivin counteracts the ability of fibronectin to protect cells from undergoing apoptosis, whereas wild-type survivin protects non-adherent cells from TNF-alpha-induced apoptosis. Evidence is provided that expression of beta1A integrin is necessary to protect non-adherent cells transduced with survivin from TNF-alpha-induced apoptosis. In contrast, the beta1C integrin, which contains a variant cytoplasmic domain, is not able to prevent apoptosis induced by TNF-alpha in non-adherent cells transduced with survivin. Finally, we show that regulation of survivin levels by integrins are mediated by protein kinase B/AKT. These findings indicate that survivin is required to maintain a critical anti-apoptotic threshold in prostate cancer cells and identify integrin signaling as a crucial survival pathway against death receptor-mediated apoptosis.

Therapeutic targeting of the survivin pathway in cancer: initiation of mitochondrial apoptosis and suppression of tumor-associated angiogenesis.

PURPOSE: Molecular antagonists of the inhibitor of apoptosis protein survivin have shown promise as novel anticancer strategies for triggering tumor cell apoptosis, dysregulating mitotic progression, and inhibiting tumor growth in preclinical models. However, how survivin couples to the cell death machinery has remained elusive, and the relevant cellular targets of survivin antagonists have not been completely elucidated. EXPERIMENTAL DESIGN: Human umbilical vein and dermal microvascular endothelial cells were infected with replication-deficient adenoviruses encoding survivin (pAd-Survivin), green fluorescent protein (pAd-GFP), or a phosphorylation-defective survivin Thr(34)-->Ala (pAd-T34A) dominant negative mutant. The effect of wild-type or mutant survivin was investigated on capillary network stability, endothelial cell viability, and caspase activation in vitro and on kinetics of tumor growth and development of angiogenesis in a breast cancer xenograft model in vivo. The cell death pathway initiated by survivin targeting was mapped with respect to cytochrome c release, changes in mitochondrial transmembrane potential, and apoptosome requirements using mouse embryonic fibroblasts deficient in Apaf-1 or caspase-9. RESULTS: Adenoviral transduction of endothelial cells with pAd-Survivin inhibited growth factor deprivation- or ceramide-induced apoptosis, reduced caspase-3 and -7 generation, and stabilized three-dimensional capillary networks in vitro. Conversely, expression of pAd-T34A caused apoptosis in umbilical vein and dermal microvascular endothelial cells and resulted in caspase-3 activity. Cell death induced by survivin targeting exhibited the hallmarks of mitochondrial-dependent apoptosis with release of cytochrome c and loss of mitochondrial transmembrane potential and was suppressed in Apaf-1 or caspase-9 knockout mouse embryonic fibroblasts. When injected in human breast cancer xenografts, pAd-T34A inhibited growth of established tumors and triggered tumor cell apoptosis in vivo. This was associated with a approximately 60% reduction in tumor-derived blood vessels by quantitative morphometry of CD31-stained tumor areas, and appearance of endothelial cell apoptosis by internucleosomal DNA fragmentation in vivo. CONCLUSIONS: Survivin functions as a novel upstream regulator of mitochondrial-dependent apoptosis, and molecular targeting of this pathway results in anticancer activity via a dual mechanism of induction of tumor cell apoptosis and suppression of angiogenesis.