Reciprocal Interactions between Tumor-Associated Macrophages and CD44-Positive Cancer Cells via Osteopontin/CD44 Promote Tumorigenicity in Colorectal Cancer

CD44 is of functional importance for tumor initiation and progression in colorectal cancer, but how this molecule benefits cancer cells from the tumor microenvironment, especially tumor-associated macrophages (TAM), remains poorly defined. In vivo tumorigenic assays were conducted to assess the role of murine TAMs in the tumorigenesis of human colorectal cancer cells. Both in vitro and in vivo osteopontin (OPN) expression levels in TAMs were examined by immunohistochemistry, quantitative PCR, and Western blotting. Soft agar colony formation assays were used to estimate the clonogenicity of colorectal cancer cells that had received different treatments. The relationships between the expression levels of OPN, CD44v6, and CD68 and clinical prognosis were evaluated by tissue microarray analysis. We found that macrophages, when coinjected or cocultured with CD44-positive colorectal cancer cells, were able to produce higher levels of OPN, which in turn facilitated the tumorigenicity and clonogenicity of the colorectal cancer cells. The knockdown of CD44 or treatment with blocking antibodies to CD44 attenuated OPN secretion. OPN, through binding to its receptor CD44, activated c-jun-NH(2)-kinase signaling and promoted the clonogenicity of colorectal cancer cells. Moreover, tissue microarray data have shown that OPN expression, in combination with CD44v6, has a negative correlation with colorectal cancer patient survival. These results suggest that the OPN-CD44 interaction is important for colorectal cancer progression and could serve as a potential therapeutic target for the treatment of colorectal cancer.