HER2 signaling regulates different angiogenic factors through different pathways in breast cancer cells Free

2035

The HER family of receptors, including EGFR (HER1), HER2, HER3, and HER4 are important mediators of cell proliferation, survival, and angiogensis. HER2 overexpression occurs in about 20-30% of breast cancers and is associated with poor prognosis. HER2 signaling has been linked to expression of vascular endothelial growth factor (VEGF), an angiogenic factor. However, effects of HER2 signaling on other angiogenic factors such as interleukin-8 (IL-8) and basic fibroblast growth factor (bFGF) and antiangiogenic factor such as thrombspondin-1 (TSP-1) remain to be elucidated. In this study, we have investigated the effects of activation and inhibition of HER2 signaling on several breast cancer cell lines that overexpress HER2 exogenously and endogenously. Reexpression of the HER2 oncogene in MCF-7 and T47D cells that express low levels of HER2 resulted in elevated expression of VEGF and IL-8, decreased expression of TSP-1, and no significant change of bFGF levels on RT-PCR, ELISA, or Western blot analysis. Reexpression of HER2 was associated with elevated PI3K-AKT activity. Inhibition of HER2 function in BT474 cells that overexpress HER2 with its specific antibody trastuzumab (Herceptinâ) led to a decrease in VEGF and IL-8 expression and to an increase in TSP-1 expression. Inhibition of HER2 with a specific small interference RNA (SiRNA) resulted in similar effects to trastuzumab. Inhibition of HER2 with trastuzumab or HER2 SiRNA did not affect bFGF expression. To further elucidate the pathways by which HER2 signaling regulates the expression of VEGF, IL-8, and TSP-1, different signaling inhibitors were employed. Inhibition of the PI3K-AKT pathway with LY294002 downregulated IL-8 and VEGF expression. Inhibition of HER2 with trastuzumab and SiRNA inhibited PI3K-AKT activity. However, LY294002 had no effect on TSP-1 expression, suggesting TSP-1 production, unlike IL-8 and VEGF, did not depend on the PI3K-AKT pathway. Instead, the p38 MAPK specific inhibitor SB203580 inhibited the TSP-1 expression. On the other hand, overexpression of p38 MAPK increased TSP-1 expression. Trastuzumab transiently activated p38 MAPK and was able to reverse the TSP-1 inhibition induced by SB203580. Consequently, HER2 signaling is able to upregulate pro-angiogenic factors VEGF and IL-8 via the PI3K-AKT pathway and to downregulate anti-angiogenic factor TSP-1 via the p38 MAPK pathway. Anti-HER2 antibody trastuzumab is a potent inhibitor of angiogenesis in breast cancer by affecting different angiogenic factors simultaneously.