EXPRESSION OF PSA-N-CAM IN HUMAN NEUROBLASTOMA-CELLS INDUCED TO NEURONAL DIFFERENTIATION BY RETINOIC ACID

The neural cell adhesion molecule (N-CAM) plays a significant role in the development of the nervous system. Three different isoforms of the molecule have been described, with molecular masses of 180, 140 and 1 20 kDa, whose differential expression in neurons seems to be related t o their state of differentiation. We took advantage of the use of the human neuroblastoma cell line LAN-5, which can be differentiated in vi tro by retinoic acid (RA) into neuronal cells, for studying the expres sion of N-CAM isoforms, and their polysialic acid (PSA) content, at th e protein and mRNA levels. Anti-N-CAM polyclonal antibodies recognizin g all the N-CAM isoforms and a monoclonal antibody recognizing PSA wer e used in Western blot experiments with extracts from undifferentiated and RA-differentiated cells. We found that undifferentiated cells exp ress very little of the 180 kDa N-CAM isoform and a large amount of th e 140 kDa isoform. A 4-fold increase in the expression of the 180 kDa N-CAM isoform was obtained when LAN-5 cells were differentiated by RA for 8 days, whereas a 1.8-fold increase in the expression of the 140 k Da N-CAM isoform was observed upon differentiation. Similarly, the lev els of the 7.4 kb mRNA coding for N-CAM 180 kDa, determined by Norther n blot analysis, were barely detectable in undifferentiated cells, and showed a 3.8-fold increase upon differentiation. By contrast, only a 1.3-fold increase in the 6.7 kb mRNA, coding for the 140 kDa N-CAM iso form, was observed. N-CAM was always found in its polysialylated form in both undifferentiated and RA-differentiated cells. This indicates t hat, in LAN-5 cells, the expression and activity of the polysialyltran sferase enzyme precedes the acquisition of a neuronal phenotype.