Chemotactic cytokines (chemokines) can help regulate tumor cell invasion and metastasis. Here, we show that chemokine 25 (CCL25) and its cognate receptor chemokine receptor 9 (CCR9) inhibit colorectal cancer (CRC)invasion and metastasis. We found that CCR9 protein expression levels were highest in colon adenomas andprogressively decreased in invasive and metastatic CRCs. CCR9 was expressed in both primary tumor cellcultures and colon-cancer-initiating cell (CCIC) lines derived from early-stage CRCs but not from metastaticCRC. CCL25 stimulated cell proliferation by activating AKT signaling. In vivo, systemically injected CCR9+early-stage CCICs led to the formation of orthotopic gastrointestinal xenograft tumors. Blocking CCR9 signaling inhibited CRC tumor formation in the native gastrointestinal CCL25+ microenvironment, while increasingextraintestinal tumor incidence. NOTCH signaling, which promotes CRC metastasis, increased extraintestinal tumor frequency by stimulating CCR9 proteasomal degradation. Overall, these data indicate that CCL25 andCCR9 regulate CRC progression and invasion and further demonstrate an appropriate in vivo experimentalsystem to study CRC progression in the native colon microenvironment.
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