EXTERNALIZATION AND BINDING OF GALECTIN-1 ON CELL-SURFACE OF K562 CELLS UPON ERYTHROID-DIFFERENTIATION

Galectin 1 (GAL1) is a beta-galactoside-binding lectin involved in cel l cycle progression, GAL1 overexpression is associated with neoplastic transformation and loss of differentiation, The gene encoding for hum an GAL1 resides on chromosome 22(q12; q13), and its expression is deve lopmentally regulated, Although devoid of signal peptide GAL1 can be e xternalized from cells by a mechanism independent of the normal secret ory process, We report here on a study of the effects of erythroid dif ferentiation of the human leukemia cell line K562 on GAL1 protein expr ession, In undifferentiated K562 cells, GAL1 was expressed into the cy tosol, However, the amount of GAL1 was surprisingly weaker in K562 cel ls than in other leukemia cell lines such as TF-1 or KG1a, Treatment o f K562 cells with erythropoietin (EPO) or with aphidicolin (APH), an i nhibitor for DNA polymerase a, induced an erythroid phenotype and led to the externalization of cytosolic GAL1 which was then bound to ligan ds on cell surface in a galactoside-inhibitable fashion, Our results d emonstrate that acquisition of an erythroid phenotype is associated wi th an externalization of GAL1, The autocrine binding of GAL1 to cell s urface ligands of non adherent cells such as K562 suggest that GAL1 is implicated rather in signal transduction than in cell-cell or cell-ma trix interaction, Moreover, the reciprocal translocation involving chr omosomes 9 and 22 t(9;22) present in K562 cells might explain the weak expression of GAL1 in K562 leukemia cells.