Enhanced sialylation of recombinant erythropoietin in genetically engineered Chinese-hamster ovary cells

Sialic acid, the terminal sugar in N-linked complex glycans, is usually found in glycoproteins and plays a major role in determining the circulatory lifespan of glycoproteins. In the present study we attempted to enhance the sialylation of recombinant EPO (erythropoietin) in CHO (Chinese-hamster ovary) cells. To enhance EPO sialylation, we introduced human alpha 2,3-ST (alpha 2,3-sialyltransferase) and CMP-SAS (CMP-sialic acid synthase) into recombinant human EPO-producing CHO cells. The sialylation of EPO was increased by the expression of alpha 2,3-ST alone. Although the co-expression of alpha 2,3-ST and CMP-SAS did not further increase sialylation, an increase in the intracellular pool of CMP-sialic acid was noted. On the basis of these observations, it was postulated that the transport capacity of CMP-sialic acid into the Golgi lumen was limited, thereby causing the reduced availability of CMP-sialic acid substrate for sialylation. Therefore, we co-expressed human alpha 2,3-ST and CMP-SAS, as well as overexpress Chinese hamster CMP-sialic acid transporter (CMP-SAT) in CHO cells, which produced recombinant human EPO. When alpha 2,3-ST, CMP-SAS, and CMP-SAT were overexpressed in CHO cells, there was a corresponding increase in siallylation compared with the co-expression of alpha 2,3-ST and CMP-SAS. The present study provides a useful strategy for enhancing the sialylation of therapeutic glycoproteins produced in CHO cells.