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 cell cycle progression. GAL1 overexpression is associated with neoplastic transformation and loss of differentiation. The gene encoding for human GAL1 resides on chromosome 22(q12; q13), and its expression is developmentally regulated. Although devoid of signal peptide GAL1 can be externalized from cells by a mechanism independent of the normal secretory process. We report here on a study of the effects of erythroid differentiation of the human leukemia cell line K562 on GAL1 protein expression. In undifferentiated K562 cells, GAL1 was expressed into the cytosol. However, the amount of GAL1 was surprisingly weaker in K562 cells than in other leukemia cell lines such as TF-1 or KG1a. Treatment of K562 cells with erythropoietin (EPO) or with aphidicolin (APH), an inhibitor for DNA polymerase alpha, induced an erythroid phenotype and led to the externalization of cytosolic GAL1 which was then bound to ligands on cell surface in a galactoside-inhibitable fashion. Our results demonstrate that acquisition of an erythroid phenotype is associated with an externalization of GAL1. The autocrine binding of GAL1 to cell surface ligands of non adherent cells such as K562 suggest that GAL1 is implicated rather in signal transduction than in cell-cell or cell-matrix interaction. Moreover, the reciprocal translocation involving chromosomes 9 and 22 t(9;22) present in K562 cells might explain the weak expression of GAL1 in K562 leukemia cells.