Xiaobo Zhang

Massachusetts General Hospital, Boston, MA, United States

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Publications (4)48.99 Total impact

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    ABSTRACT: The induction of vascular endothelial growth factor (VEGF) is an essential feature of tumor angiogenesis. Hypoxia is a potent stimulator of VEGF expression, and hypoxia-inducible factor-1 (HIF-1) is considered to be critical for this induction. However, we have previously demonstrated that induction of VEGF by hypoxia was preserved when HIF-1alpha was silenced. We sought to better define the molecular basis of this HIF-1-independent regulation. In colon cancer cells, hypoxia stimulated multiple K-ras effector pathways including phosphatidylinositol 3-kinase. VEGF promoter deletion studies identified a novel promoter region between -418 and -223 bp that was responsive to hypoxia in a PI3K/Rho/ROCK-dependent manner. Electrophoretic mobility shift assays identified a fragment between -300 and -251 bp that demonstrated a unique shift only in hypoxic conditions. Inhibition of PI3K or ROCK blocked the formation of this complex. A binding site for c-Myc, a target of ROCK, was identified at -271 bp. A role for c-Myc in the hypoxic induction of VEGF was demonstrated by site-directed mutagenesis of the VEGF promoter and silencing of c-Myc by small interfering RNA. Collectively, these findings suggest an alternative mechanism for the hypoxic induction of VEGF in colon cancer that does not depend upon HIF-1alpha but instead requires the activation of PI3K/Rho/ROCK and c-Myc.
    Journal of Biological Chemistry 06/2006; 281(20):13957-63. · 4.65 Impact Factor
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    ABSTRACT: Hypoxia inducible factor-1 (HIF-1) is considered a crucial mediator of the cellular response to hypoxia through its regulation of genes that control angiogenesis. It represents an attractive therapeutic target in colon cancer, one of the few tumor types that shows a clinical response to antiangiogenic therapy. But it is unclear whether inhibition of HIF-1 alone is sufficient to block tumor angiogenesis. In HIF-1alpha knockdown DLD-1 colon cancer cells (DLD-1(HIF-kd)), the hypoxic induction of vascular endothelial growth factor (VEGF) was only partially blocked. Xenografts remained highly vascularized with microvessel densities identical to DLD-1 tumors that had wild-type HIF-1alpha (DLD-1(HIF-wt)). In addition to the preserved expression of VEGF, the proangiogenic cytokine interleukin (IL)-8 was induced by hypoxia in DLD-1(HIF-kd) but not DLD-1(HIF-wt) cells. This induction was mediated by the production of hydrogen peroxide and subsequent activation of NF-kappaB. Furthermore, the KRAS oncogene, which is commonly mutated in colon cancer, enhanced the hypoxic induction of IL-8. A neutralizing antibody to IL-8 substantially inhibited angiogenesis and tumor growth in DLD-1(HIF-kd) but not DLD-1(HIF-wt) xenografts, verifying the functional significance of this IL-8 response. Thus, compensatory pathways can be activated to preserve the tumor angiogenic response, and strategies that inhibit HIF-1alpha may be most effective when IL-8 is simultaneously targeted.
    Nature Medicine 10/2005; 11(9):992-7. · 22.86 Impact Factor
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    ABSTRACT: Two key genetic events underlying the development of colon cancer are activation of the K-ras and Wnt signaling pathways. We have previously shown that these 2 pathways can cooperate to regulate vascular endothelial growth factor (VEGF) gene expression. The goal of this study was to define the molecular basis for this interaction. The effects of K-ras(Val12) on VEGF and T-cell factor 4 (TCF-4) promoter activity, nuclear levels of beta-catenin and beta-catenin/TCF-4 complexes, glycogen synthase kinase 3beta (GSK-3beta) phosphorylation, and GSK-3beta kinase activity were measured. LY294002 and PD98059 were used to define the role of specific ras effector pathways. Oncogenic K-ras up-regulated the activity of the VEGF promoter, and selective mutagenesis of TCF-4 binding sites significantly blocked this induction. K-ras(Val12) also induced the activity of a heterologous TCF-4 reporter construct in Caco-2 and HeLa cells. LY294002 and dominant negative phosphatidylinositol 3-kinase nearly completely blocked this induction. K-ras(Val12) increased the stability of beta-catenin, the levels of nuclear beta-catenin, and the formation of nuclear beta-catenin/TCF-4 complexes, and these effects were also blocked by LY294002. Finally, K-ras(Val12) inhibited the kinase activity of total cellular GSK-3beta and GSK-3beta complexed with Axin. This effect was not mediated through phosphorylation at serine 9 but did depend on phosphatidylinositol 3-kinase. Our results suggest a unique cooperative interaction between 2 critical oncogenic pathways in colorectal tumorigenesis and highlight the pivotal role of GSK-3beta.
    Gastroenterology 07/2005; 128(7):1907-18. · 12.82 Impact Factor
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    ABSTRACT: The induction of vascular endothelial growth factor (VEGF) is an essential feature of tumor angiogenesis, and the hypoxia-inducible factor-1 (HIF-1) transcription factor is known to be a key mediator of this process. In colon cancer, the frequently mutated K-ras oncogene also can regulate VEGF expression, but the role that K-ras may play in hypoxia is unknown. Hypoxia induced VEGF promoter activity, mRNA, and protein levels in colon cancer cells. Although HIF-1alpha was induced by hypoxia, VEGF reporter constructs with selectively mutated hypoxia-response elements remained responsive to hypoxia. In addition, "knockdown" of HIF-1alpha by RNA interference only minimally inhibited the hypoxic induction of VEGF. A region of the VEGF promoter between -420 and -90 bp mediated this HIF-independent induction by hypoxia. The introduction of K-ras(Val12) augmented the hypoxic induction of VEGF, and this was observed in wild-type and HIF-1alpha knockdown colon cancer cells. Thus, VEGF may be induced by hypoxia through HIF-dependent and HIF-independent pathways, and K-ras also can induce VEGF in hypoxia independent of HIF-1. These findings suggest the existence of multiple mechanisms regulating the hypoxic induction of VEGF in colon cancer.
    Cancer Research 04/2004; 64(5):1765-72. · 8.65 Impact Factor