Exosomes Transfer ST6GAL1-mediated Therapeutic Resistance in Rectal Cancer Cells.

BACKGROUND/AIM: Locally advanced Rectal cancer (RC) is treated with neoadjuvant chemoradiation, but treatment resistance is common. We have previously shown that ST6GAL1 causes RC chemoradiation resistance. Exosomes are small particles that can transfer proteins between cells. We hypothesized that exosomes transfer ST6GAL1 between RC cells, spreading treatment resistance. MATERIALS AND METHODS: We characterized exosomes isolated from multiple ST6GAL1-expressing colorectal cancer (CRC) cell lines. Treatment response was assessed in ST6GAL1-knockdown (KD) cells treated with these exosomes. Single cell RNA sequencing (scRNA-seq) and flow cytometry for surface sialylation were performed on CRC organoids to compare presence of ST6GAL1 RNA and protein activity. RESULTS: Exosomes from multiple CRC cell lines contained ST6GAL1 protein and ST6GAL1 KD cells treated with these exosomes demonstrated transfer of ST6GAL1 with increase in protein in treated cells, but not mRNA, and the protein localized to the Golgi complex. In addition, treated cells demonstrated increased resistance to chemoradiation-induced apoptosis (p=0.02, n=4) and increased colony formation after treatment (p=0.01, n=4). Single cell sequencing revealed that only 16 percent of cells have ST6GAL1 mRNA but 86 percent have evidence of ST6GAL1 protein activity. CONCLUSION: These findings demonstrate that ST6GAL1-containing rectal cancer exosomes transfer ST6GAL1 between cells causing treatment resistance.