Abstract 479: Targeting tumor initiating cells inhibits tumor growth and serial transplantation ability in soft-tissue sarcomas

Tumors contain heterogeneous cell populations. There is a subpopulation of cells enriched for tumor initiating potential in sarcomas, which excludes Hoechst dye, and resides in the “side population” when subjected to flow cytometry (SP cells). These cells possess multi-potent differentiation potential, and as such they behave as “cancer stem cells” (CSCs), while the remainder of tumor cells act as “transient amplifying” cells. Because traditional cancer therapies may not target these tumor initiating cells, the persistence of SP cells could be responsible for relapse or a failed response to therapy. To detect signaling pathways that are differentially regulated in SP cells versus the remainder (non-SP) of the cells, we used gene profiling by microarray to compare their differences. RNA expressions were compared between these two cell populations from six malignant fibrous histiocytoma (MFH) samples using microarray. Differentially regulated genes were then analyzed by compiling a list of genes that showed a fold change greater than 1.25 in at least five of the six samples all in the same direction, and identifying if the list is enriched for genes involved in common molecular pathways, using Genespring® analysis tool. Two of the differentially regulated pathways detected were the Hedgehog signaling pathway and the Notch signaling pathway. Differential target gene expression for both pathways was verified using quantitative PCR. Triparanol (an agent that inhibits hedgehog signaling) and DAPT (an agent that inhibits Notch signaling) were used to treat eight primary MFH xenografts established in NOD-SCID mice. The xenografts produced visible tumors six weeks after subcutaneous transplantation, after which the mice were treated with one of the agents or a carrier as a control. At the end of the treatment, the tumors were harvested; their growth and SP% were assessed and compared; and the cells harvested from the treated xenografts were again implanted into mice to study the rate of re-growth upon secondary transplantation. Both triparanol and the Notch blocker DAPT treatment suppressed these pathways in tumor cells, depleted the abundance of SP cells, and reduced tumour growth. Intriguingly, treatment substantially inhibited the tumour-initiating potential of the treated sarcoma cells upon secondary transplantation. The data provides support that SP cells act as tumour initiating cells in sarcomas and shows that targeting the SP (in this case by targeting the Hh and Notch pathways) is an enticing approach for sarcoma therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 479. doi:10.1158/1538-7445.AM2011-479