P2-02-01: A Novel Inflammatory Breast Cancer-Specific Oncogene, Tazarotene-Induced Gene 1, Promotes Tumorigenicity and Invasiveness through the Receptor Tyrosine Kinase Axl.
Authors: Wang X , Saso H , Iwamoto T , Pusztai L , Gong Y , Woodward WA , Reuben JM , Hortobagyi GN , Ueno NT 
Published: June 2012

Abstract
Background: Inflammatory breast cancer (IBC) is the most lethal and aggressive form of breast cancer and is highly metastatic. The prognosis of patients with IBC is poor, and effective standard therapies for IBC are limited because the molecular mechanisms underlying the pathogenesis of IBC remain unknown. We recently found that tazarotene-induced gene 1 (TIG1) expression is significantly higher in IBC cell lines than in non-IBC cell lines. In both IBC and non-IBC data sets, estrogen receptor-negative/HER2−negative samples had significantly higher expression of TIG1 than did other clinical subtypes (estrogen receptor-positive/HER2−negative and HER2−positive). Therefore, we hypothesized that TIG1 plays an important role in the malignant process of IBC. In these studies, we determined the biological function of TIG1 in IBC cells and elucidated the molecular mechanism by which TIG1 regulates the invasiveness of IBC cells.
Methods: TIG1 expression in SUM149 and KPL-4 IBC cells was stably knocked down, and the effects of this knockdown on in vitro cell proliferation, migration, and invasion were analyzed. The effects of restoring TIG1 expression on TIG1-silencing IBC cells were also examined. To determine the tumorigenic activity of TIG1 in vivo, TIG1 stable-knockdown SUM149 cells and control shRNA-transfected cells were implanted into the mammary fat pads of athymic nude mice, and tumor growth was monitored. The receptor tyrosine kinase Axl, a potential functional partner of TIG1, was identified using DNA microarray analysis. The interaction between TIG1 and Axl in IBC cells was examined using immunoprecipitation and confocal microscopy assays. The signaling pathway in IBC cells in which TIG1 participates was also investigated.
Results: Knockdown of TIG1 expression in IBC cells reduced their proliferation, migration, and invasion in vitro. Also, silencing of TIG1 dramatically inhibited IBC tumor growth in a xenograft model. Moreover, restoring TIG1 expression rescued the proliferation, motility, and invasiveness of TIG1-silenced IBC cells. Most importantly, we identified Axl as a functional partner of TIG1 by showing that TIG1 interacted with and stabilized Axl in IBC cells. TIG1 regulated the invasiveness of IBC cells through mediation of the Axl signaling pathway. In SUM149 cells, TIG1 depletion decreased Axl expression, which led to downregulation of expression of matrix metalloproteinase-9, a molecule required for Axl-mediated invasion, and inactivation of nuclear factor-kB, ultimately leading to decreased invasiveness of IBC cells.
Conclusion: Our results identified TIG1 as an oncogenic gene that contributes to the tumorigenic and metastatic properties of IBC. Our data also linked TIG1 with the key tumorigenic gene Axl in IBC cells. Further studies designed to establish TIG1 as a therapeutic target in the treatment of patients with IBC are under way.
Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-02-01.