MET signaling in colon cancer stem-like cells blunts the therapeutic response to EGFR inhibitors.

Metastatic colorectal cancer remains largely incurable, although in a subset of patients survival is prolonged by new targeting agents such as anti-EGFR antibodies. This disease is believed to be supported by a subpopulation of stem-like cells termed colon cancer-initiating cell (CC-ICs), which may also confer therapeutic resistance. However, how CC-ICs respond to EGFR inhibition has not been fully characterized. To explore this question, we systematically generated CC-ICs through spheroid cultures of patient-derived xenografts of metastatic colorectal cancer. These cultures, termed "xenospheres", were capable of long-term self-propagation in vitro and phenocopied the original patient tumors in vivo, thus operationally defining CC-ICs . Xenosphere CC-ICs retained the genetic determinants for EGFR therapeutic response in vitro and in xenografts: like the original tumors, xenospheres harboring a mutated KRAS gene were resistant to EGFR therapy, while those harboring wild-type RAS pathway genes (RASwt) were sensitive. Notably, the effects of EGFR inhibition in sensitive CC-ICs could be counteracted by cytokines secreted by cancer-associated fibroblasts. In particular, we found the MET receptor ligand HGF was especially active in supporting in vitro CC-IC proliferation and resistance to EGFR inhibition. Ectopic production of human HGF in CC-IC xenografts rendered the xenografts susceptible to MET inhibition, which sensitized the response to EGFR therapy. By showing that RASwt CC-ICs rely on both EGFR and MET signaling, our results offer a strong preclinical proof of concept for concurrent targeting of these two pathways in the clinical setting.