Loss of SMAD4 From Colorectal Cancer Cells Promotes CCL15 Expression to Recruit CCR1+ Myeloid Cells and Facilitate Liver Metastasis

Loss of the tumor suppressor SMAD4 correlates with progression of colorectal cancer (CRC). In mice, colon tumors that express CCL9 recruit CCR1(+) myeloid cells, which facilitate tumor invasion and metastasis by secreting matrix metalloproteinase 9.We used human CRC cell lines to investigate the ability of SMAD4 to regulate expression of CCL15, a human ortholog of mouse CCL9. We used immunohistochemistry to compare levels of CCL15 and other proteins in 141 samples of human liver metastases.In human CRC cell lines, knockdown of SMAD4 increased CCL15 expression, and overexpression of SMAD4 decreased it. SMAD4 bound directly to the promoter region of the CCL15 gene to negatively regulate its expression; transforming growth factor-β increased binding of SMAD4 to the CCL15 promoter and transcriptional repression. In livers of nude mice, SMAD4-deficient human CRC cells up-regulated CCL15 to recruit CCR1(+) cells and promote metastasis. In human tumor samples, there was a strong inverse correlation between levels of CCL15 and SMAD4; metastases that expressed CCL15 contained 3-fold more CCR1(+) cells than those without CCL15. Patients with CCL15-expressing metastases had significantly shorter times of disease-free survival than those with CCL15-negative metastases. CCR1(+) cells in the metastases expressed the myeloid cell markers CD11b and myeloperoxidase, and also matrix metalloproteinase 9.In human CRC cells, loss of SMAD4 leads to up-regulation of CCL15 expression. Human liver metastases that express CCL15 contain higher numbers CCR1(+) cells; patients with these metastases have shorter times of disease-free survival. Reagents designed to block CCL15 recruitment of CCR1(+) cells could prevent metastasis of CRC to liver.