Id-1 activation of PI3K/Akt signaling pathway and its significance in promoting tumorigenicity and metastasis of human esophageal cancer cells

Abstract

Id-1 is a helix-loop-helix protein that is overexpressed in many types of cancer including esophageal squamous cell carcinoma (ESCC). We previously reported that ectopic Id-1 expression activates the PI3K/Akt signaling pathway in human esophageal cancer cells. In this study, we confirmed a positive correlation between Id-1 expression and phospho-Akt in ESCC cell lines, as well as in ESCC tumor tissues from patients. To investigate the significance of Id-1 in esophageal cancer progression, we also conducted in vivo experiments to compare the tumorigenic and metastatic potential of an Id-1-overexpressing ESCC cell line in nude mice with that of the control cell line expressing the empty vector. We found that the Id-1 overexpressing cancer cells produced significantly larger tumors when inoculated subcutaneously into the flank of nude mice, and pronounced lung metastasis when injected intravenously into the tail vein. The tumor xenografts of esophageal cancer cells with stable ectopic Id-1 expression showed elevated Ki-67 proliferation index and increased angiogenesis, as well as reduced apoptosis. Treatment with the PI3K inhibitor LY294002 attenuated the tumor promotion effects of Id-1, indicating that the effects were mediated by the PI3K/Akt signaling pathway. In addition, our in vitro experiments showed that ectopic Id-1 expression altered the expression levels of markers associated with epithelial-mesenchymal transition (EMT), and enhanced the migration ability of esophageal cancer cells. The Id-1-overexpressing ESCC cells also exhibited increased invasive potential which was in part due to PI3K/Akt-dependent modulation of matrix metalloproteinase-9 (MMP-9) expression. In conclusion, our results provide the first evidence that Id-1 promotes tumorigenicity and metastasis of human esophageal cancer in vivo, and that the PI3K inhibitor LY294002 can attenuate these effects. [This study is supported by The University of Hong Kong CRCG Small Project Funding Programme (Project No. 200807176012) and by a Collaborative Research Fund from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKUST2/06C)]