Identification of mRNA splicing factors as the endothelial receptor for carbohydrate-dependent lung colonization of cancer cells

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.