The role of the Hedgehog (Hh) signaling pathway in development is well established. Several recent studies independently implicate that the Hh pathway is active in cancer cells and regulates their malignant behavior. Signaling via the Hh pathway culminates in the nuclear translocation of the transcription factor, Gli1, which regulates the expression of target genes. Deregulation of the Hh pathway has been reported in breast cancer. We have discovered that the oncoprotein, Osteopontin (OPN) is transcriptionally upregulated by the Hh pathway. Our previous studies have demonstrated that expression of OPN is essential to the tumor-forming ability of breast cancer cells. Treatment of breast cancer cells with the Hh ligands, Shh and Ihh boosts signaling via the Hh pathway and upregulates the promoter activity of OPN. This is complemented by the effect of cyclopamine, an inhibitor of the Hh pathway. Treatment of breast cancer cells using cyclopamine blocked the nuclear translocation of Gli1 and markedly decreased OPN expression. Treatment with cyclopamine also caused a significant reduction in the reporter activity from the OPN promoter, suggesting that the Hh pathway transcriptionally regulates OPN. Using serial deletions of the OPN promoter and chromatin immunoprecipitation assays we established that OPN is a target of Hh signaling. Stable knockdown of Gli1 expression in breast cancer cells significantly reduced OPN expression concomitant with decreased attributes of malignant behavior as assessed in vitro by proliferation, invasion, migration and motility. In vivo, the Gli1-knocked-down cells displayed reduced tumorigenicity and metastases, suggesting that the Hh pathway-mediated OPN expression plays a functionally determinative role in the malignant behavior of breast cancer. We acknowledge grant support from the Department of Defense-Breast Cancer Research Program (07-1-0400).

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3085.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO