A HUMAN STX CDNA CONFERS POLYSIALIC ACID EXPRESSION IN MAMMALIAN-CELLS

Polysialic acid, or PSA, is a term used to refer to linear homopolymer s of alpha(2,8)-sialic acid residues displayed at the surface of some mammalian cells. PSA is typically linked to the neural cell adhesion m olecule N-CAM, where it can modulate the homotypic adhesive properties of this polypeptide. PSA expression is developmentally regulated, pre sumably through mechanisms involving regulated expression of sialyltra nsferases involved in PSA biosynthesis. Several different sialyltransf erase sequences have been implicated in PSA expression, although the p recise roles of these enzymes in this context remain unclear. One such sequence, termed STX, maintains approximately 59% amino acid sequence identity with another sialyltransferase (PST-1, from hamster; PST, hu man) that is known to participate in PSA expression. While a murine ST X fusion protein can catalyze the synthesis of a single alpha(2,8)-sia lic acid linkage in vitro, the ability of STX to participate in PSA ex pression in vivo has not been demonstrated. We show here that STX tran scripts are present in a PSA-positive, N-CAM-positive human small cell carcinoma line (NCI H69/F3), but are absent in a variant of this line (NCI-H69/E2) selected to be PSA-negative and N-CAM-positive. To funct ionally confirm this correlation, we have cloned a human cDNA encoding the human STX sequence,and show, transfection studies, that human STX can restore PSA expression when expressed in the PSA-negative, NCAM-p ositive small cell carcinoma variant. We furthermore show that STX can confer PSA expression when expressed in a PSA-negative, N-CAM-positiv e murine cell line (NIH-3T3 cells), or when expressed in PSA-negative, N-CAM-negative COS-7 cells. These observations imply that STX, like P ST-1/PST, can determine PSA expression in vivo. When considered togeth er with the correlation between STX expression and PSA expression in v ivo in the brain, these results suggest a regulatory role for STX in P SA expression in the developing central nervous system and small cell lung carcinoma.