Abstract 269: Stem Cell Factor (SCF) secreted by differentiated tumor cells helps maintaining the cancer stem cell phenotype in colorectal tumors.
Authors: Fatrai Szabolcs , van Schelven Susanne J. , Borel Rinkes Inne H.M. , Kranenburg Onno 
Published: August 2014

Abstract
Colorectal tumors are hierarchically organized: A small cancer stem cell (CSC) pool drives the formation of proliferating and differentiating tumor cells that make up the bulk of the tumor but lack tumor-initiating capacity. CSCs are thought to be derived from normal intestinal stem cells (ISCs) through the acquisition of multiple pro-tumorigenic genetic alterations. The maintenance of ISCs requires ‘niche’ signals that are provided, at least in the small intestine, by neighboring Paneth cells. CSC maintenance may also depend on niche signals although little is known about the nature of such signals and their potential source.
Here we tested the hypothesis that differentiated tumor cells (DTC) could provide CSC-maintaining signals by making use of CSC-enriched colonosphere cultures that were freshly established from human colorectal tumors. We found that DTCs established from multiple colorectal tumors greatly increased the clone-forming capacity of CSCs from their own and from other tumors. This effect was reproduced by DTC-conditioned medium, suggesting the involvement of a secreted factor. By using a cytokine array and three CSC-supporting DTC cultures we identified 24 factors that were secreted by all three DTCs. Analysis of the gene expression profiles of two independent cohorts of primary colorectal tumors showed that only one of these secreted factors, stem cell factor (SCF), was significantly correlated with metastasis formation and poor overall survival. In human colonospheres SCF was mainly produced by DTCs, while the receptor for SCF, c-Kit, was expressed by CSCs. Indeed, recombinant SCF enhanced clone-forming capacity to a similar extent as DTC-conditioned medium. Furthermore, basal and DTC-stimulated clone-forming capacity was strongly reduced in the presence of an SCF-neutralizing antibody or in the presence of the cKit inhibitors Imatinib or ISCK03. qPCR analysis showed that inhibition of cKit caused a strong reduction in the expression of stem cell genes, including LGR5, CD133, OLMF4 and SOX2. In addition, both purified SCF and DTC-conditioned medium induced cKit-dependent expression of EMT and mesenchymal genes, including ZEB1, SNAIL1, fibronectin and vimentin.
Our results identify a critical role for non-tumorigenic DTCs in the support of colorectal CSCs via paracrine SCF-cKit signaling. We conclude that cKit inhibitors that are widely used in the clinic to treat gastrointestinal stromal tumors (GIST) and acute myeloid leukemia (AML), may also have therapeutic value in the treatment of (a subgroup of) colorectal tumors.
Citation Format: Szabolcs Fatrai, Susanne J. van Schelven, Inne H.M. Borel Rinkes, Onno Kranenburg. Stem Cell Factor (SCF) secreted by differentiated tumor cells helps maintaining the cancer stem cell phenotype in colorectal tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 269. doi:10.1158/1538-7445.AM2013-269