CD133+CXCR4+ Cancer Initiating Cells Determine Metastatic Activity In Human Colon Cancer

Colorectal cancer (CRC) is one of common tumors and can occur in any site of colon and rectum. The incidence of CRC has been increasing year by year, and it is among the three leading causes of cancer-related death worldwide. Nearly 30-40% of patients with early stage of colorectal cancer develop liver metastases synchronously or metachronously, and in advanced disease the mortality of CRC is principally attributable to the development of hepatic metastases.Since the concept of cancer stem cell (CSC) has been put forward, emerging researches indicated that CSCs are the resources of tumor formation, metastasis and recurrence. Recently, CSCs may represent a heterogeneous population consisting of two forms of CSCs in tumor progression-stationary CSC and migrating CSC. And the latter one is a small subpopulation which combines the two most decisive traits, stemness and mobility and consequently plays an important role in malignant progression. Thus, we hypothesize that there also exists such a subpopulation in colorectal cancer, which contributes to hepatic metastasis of CRC.Recent studies have highlighted the role of chemokines in cancer metastasis, which based on the signaling/homing theory that target organs produce and release specific chemokines and attract nearby or distant cancer cells bearing corresponding receptors. Emerging studies suggest that SDF-1/CXCR4 axis plays a key role in tumor invasiveness leading to local progression and tumor metastasis. In this study, we will explore whether CXCR4 expression is associated with MCSCs.The acquisition of mesenchymal phenotype by epithelial cells, known as epithelial–mesenchymal transition (EMT), is a key process that is required during embryonic development. More recently, EMT is associated with cancer cell migration and metastasis, because cancer cells could acquire more aggressive phenotype via EMT, indicating that EMT is a crucial event in malignancy. Emerging studies have reported the correlation between cancer stem cell and EMT. We hypothesize that EMT plays an essential role in endowing migratory CSC with metastatic capacity in CRC.Based on the above analyses, the study consists of two parts: first, the role of CD133+CXCR4+ tumor cells in the hepatic metastasis of CRC as MCSCs; secondly, EMT is the potential mechanism underlying hepatic metastasis by CD133+CXCR4+ subpopulation in CSCs.PartⅠThe role of CD133+CXCR4+ tumor cells in the hepatic metastasis of CRC as MCSCsWe first examined the content of CD133+ and CD133+CXCR4+ cells in primary colorectal cancers and metastatic liver cancers by flow cytometry. The results demonstrated the enrichment of the two kinds of cells in metastatic liver tumor tissues, with the enrichment of CD133+CXCR4+ subpopulation to a higher extent, which indicated that they might play an important role in hepatic metastasis of colorectal cancer as CSCs and MCSCs respectively. Next, for further in vivo detection, we used human colon cancer cell line HCT116 and experimental tail vein metastatic assays, which indicated that only CD133+CXCR4+ subpopulation could drive cancer metastases in nude mice. Furthermore, clonogenic and tumorigenic assays results suggested that CD133+CXCR4+ subpopulation did not possess higher stem properties than CD133+CXCR4- ones, indicating that other mechanisms endowed this subtype of cells with metastatic capacity.PartⅡEMT is the potential mechanism underlying hepatic metastasis by CD133+CXCR4+ subpopulation in CSCsReal time RT-PCR was performed to detect EMT-related genes expression and transwell migration and invasion assays were employed to compare the migratory and invasive capacities between CD133+CXCR4- and CD133+CXCR4+ cells. The results showed that compared with CD133+CXCR4- cells, CD133+CXCR4+cells have undergone EMT. Next, we also found that SDF-1 could further induce EMT in CD133+CXCR4+ cells, which could not be observed in CD133+CXCR4- cells. At last, nude mouse hepatic metastases assay showed that blockade of SDF-1/CXCR4 axis with CXCR4 inhibitor- AMD3100 could suppress hepatic metastasis of colon cancer, which might provide new direction for exploring effective treatment of colon cancer metastasis.Above all, the following conclusion could be drawn:1. There exist two subpopulations in colon cancer CSC, namely, SCSC (CD133+CXCR4- cell) and MCSC (CD133+CXCR4+ cell), and CD133+ CXCR4+ cells play the key role in hepatic metastasis of colon cancer. 2. EMT is involved in hepatic metastasis of colon cancer caused by CD133+CXCR4+ subpopulation and blockade of SDF-1/CXCR4 axis could suppress hepatic metastasis of colon cancer.