A new strategy for identifying polysialylated proteins reveals they are secreted from cancer cells as soluble proteins and as part of extracellular vesicles

Polysialic acid (polySia) is a long homopolymer consisting of 2,8-linked sialic acid with tightly regulated expression in humans. In healthy adults, it occurs on cell surface glycoproteins in neuronal, reproductive, and immune tissues; however, it is aberrantly present in many cancers and its overexpression correlates with significantly increased metastasis and poor prognosis. Prompted by the observation that the MCF-7 breast cancer cell line contains only intracellular polySia, we investigated the secretion of polySia from MCF-7 cells. PolySia was found predominantly on soluble proteins in MCF-7 conditioned media, but also on extracellular vesicles (EVs), secreted from the cells. Since MCF-7 cells do not express known polysialylated proteins, we developed a robust method for purifying polysialylated proteins that uses a metabolic labelling strategy to introduce a bioorthogonal functionality into polySia. Using this method we identified three previously unknown polysialylated proteins, and found that two of these proteins - AGR2 and QSOX2 - were secreted from MCF-7 cells. We confirmed that QSOX2 found in EV-depleted MCF-7 cell conditioned media was polysialylated. Herein we report the secretion of polysialic acid on both soluble and EV-associated proteins from MCF-7 cancer cells and introduce a new method to efficiently identify polysialylated proteins. These findings have exciting implications for understanding the roles of polySia in cancer progression and metastasis and for identifying new cancer biomarkers.