Tumor Microenvironment Program, Cancer Research Center, Burnham Institute for Medical Research, La Jolla, CA 92037, USA.
Cell surfaces of epithelial cancer are covered by complex carbohydrates, whose structures function in malignancy and metastasis. However, the mechanism underlying carbohydrate-dependent cancer metastasis has not been defined. Previously, we identified a carbohydrate-mimicry peptide designated I-peptide, which inhibits carbohydrate-dependent lung colonization of sialyl Lewis X-expressing B16-FTIII-M cells in E/P-selectin doubly-deficient mice. We hypothesized that lung endothelial cells express an unknown carbohydrate receptor, designated as I-peptide receptor (IPR), responsible for lung colonization of B16-FTIII-M cells. Here, we visualized IPR by in vivo biotinylation, which revealed that the major IPR is a group of 35-kDa proteins. IPR proteins isolated by I-peptide affinity chromatography were identified by proteomics as Ser/Arg-rich alternative pre-mRNA splicing factors or Sfrs1, Sfrs2, Sfrs5, and Sfrs7 gene products. Bacterially expressed Sfrs1 protein bound to B16-FTIII-M cells but not to parental B16 cells. Recombinant Sfrs1 protein bound to a series of fucosylated oligosaccharides in glycan array and plate-binding assays. When anti-Sfrs antibodies were injected intravenously into mice, antibodies labeled a subset of lung capillaries. Anti-Sfrs antibodies inhibited homing of I-peptide-displaying phage to the lung colonization of B16-FTIII-M cells in vivo in the mouse. These results strongly suggest that Sfrs proteins are responsible for fucosylated carbohydrate-dependent lung metastasis of epithelial cancers.