531 KIT Signaling Promotes Tumor Growth in a Subset of Human Colon Cancers

Abstract

Background & Aims: The role of receptor tyrosine kinases (RTKs) in the regulation of colon cancer growth is unclear even though an RTK inhibitor has been approved for metastatic colon cancer. Previously, we found that the RTK KIT (CD117) marks a subset of crypt base goblet cells that contributes to the stem cell niche in normal murine colon. In human colon, KIT expression has been reported in colon cancer cell lines. We sought to assess the role of KIT in human colon cancers and hypothesized that KIT promotes colon cancer growth. Methods: We performed immunohistochemistry on an array of 316 colon tumor sections to assess the expression of KIT and its ligand, KITLG, in human colon cancers. We generated stable, lentivirally-transduced shRNA knockdowns of KIT and KITLG in colon cancer cell lines and xenografts, and conducted bulk and single-cell gene expression analysis, immunostaining, flow cytometry, and functional assays to assess the roles of KIT in tumor growth in-vitro and in-vivo. We treated KIT+ colon cancers with the KIT inhibitor imatinib and tracked in vivo growth using bioluminescence. We assessed differences in clonogenicity and tumorigenicity using limiting dilution analysis, and queried associated gene expression changes using quantitative RT-PCR. Results: KIT and KITLG are heterogeneously expressed by >50% of our tumor sections. KIT and KITLG immunoreactivity was detected in tumor cells, tumor stroma, or both. Stable lentiviral knockdown of KIT decreased tumor growth in vivo by >60%, increased expression of enterocyte markers (e.g. KRT20, CEACAM1), decreased expression of cycling genes (e.g. MKI67, TOP2A), and unexpectedly increased expression of some stem cell-associated genes (e.g. LGR5, CD44). Interestingly, no activating KIT mutations were detected in the KIT+ cell lines and xenografts, suggesting that KIT signaling occurs through its ligand KITLG. Knockdown of KITLG decreased tumor growth, and single-cell gene expression analysis of tumorigenic (CD44+) cells suggested that KIT may promote growth via KITLG autocrine and/or paracrine signaling. The RTK inhibitor imatinib also inhibited the growth of KIT+ colon cancer xenografts in vitro and in vivo. Finally, KIT-expression enriched for tumorigenic cells. Conclusions: KIT signaling in tumorigenic cells promotes growth of a subset of colon cancers through its ligand KITLG in an autocrine or paracrine manner. KIT+ tumor cells in the tumorigenic CD44+ compartment may be more tumorigenic than KITcells, although both populations can generate tumors. These data suggest that patients with KIT-expressing colon tumors may benefit from KIT inhibition by RTK inhibitors that target KIT.