Introduction: In the treatment of colorectal cancers, the control of liver metastasis is an important issue. Here we present our data showing the importance of tumor microenvironment and premetastatic niche in the process of colorectal liver metastasis and discuss possibility of novel molecular targeting therapy to inhibit liver metastasis.
Methods: 1) We implanted the highly metastatic human colon cancer TK-4 orthotopically into the cecal walls of nude mice, followed by twice-daily administration of TSU68, an antiangiogenic receptor tyrosine kinase inhibitor, or vehicle. Gene expression profile in premetastatic liver was analyzed using microarrays. 2) Cancer tissues were collected from FFPE samples of human primary colorectal cancer and colorectal liver metastasis using laser-capture microdissection methods. Total RNA was extracted and expression profile of microRNA was analyzed by TaqMan microRNA Array.
Results: 1) Five weeks of treatment with TSU68 significantly inhibited liver metastasis compared with the control group (P < 0.001). mRNA levels for the chemokine CXCL1 were significantly increased in premetastatic liver but not in lung of tumorbearing mice compared with non–tumor-bearing mice. CXCR2 expression was detected predominantly in orthotopic tumors compared with ectopic tumors. The amount of IL-12 p40 subunit protein in the portal vein was elevated in tumor-bearing mice. Blockade of both CXCR2 and IL-12 p40 with a neutralizing antibody and TSU68 treatment significantly inhibited liver metastasis. 2) miR-122 was the best upregulated in colorectal liver metastasis compared with primary tumor. Immunohistochemical analysis revealed that the expression levels of cationic amino acid transporter 1 (CAT1) protein which has been reported as a negative target gene of miR-122 were lower in liver metastases than primary tumors. Expression levels of CAT1 in 132 primary tumors were correlated with the existence of synchronous liver metastasis. In the analysis of 121 colon cancer patients without synchronous liver metastasis, patients with CAT1-low colon cancer had significantly shorter liver metastasis-free survival but not overall survival.
Conclusions: Tumor microenvironment in the primary site plays important roles in cancer metastasis by forming premetastatic niche in the target organ through the CXCL1/CXCR2 axis. Overexpression of miR-122 and concomitant suppression of CAT1 in primary tumor appears to be involved in the process of colorectal liver metastasis.