Externalization and binding of galectin-1 on cell surface of K562 cells upon erythroid differentiation

Galectin 1 (GAL1) is a P-galactoside-binding lectin in- volved in cell cycle progression. GAL1 overexpression is associated with neoplastic transformation and loss of dif- ferentiation. The gene encoding for human GAL1 resides on chromosome 22(ql2; ql3), and its expression is devel- opmentally regulated. Although devoid of signal peptide GAL1 can be externalized from cells by a mechanism in- dependent 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 expres- sion. In undlfferentiated K562 cells, GAL1 was expressed into the cytosol. However, the amount of GAL1 was sur- prisingly weaker in K562 cells than in other leukemia cell lines such as TF-1 or KGla. Treatment of K562 cells with erythropoietin (EPO) or with aphidicolin (APH), an inhibi- tor for DNA polymerase a, induced an erythroid pheno- type and led to the extemalization of cytosolic GAL1 which was then bound to ligands on cell surface in a galactoside- inhibitable fashion. Our results demonstrate that acquisi- tion of an erythroid phenotype is associated with an exter- nalization of GALL 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 ceu-cell or cell-matrix interaction. Moreover, the recip- rocal translocation involving chromosomes 9 and 221(9;22) present in K562 cells might explain the weak expression of GAL1 in K562 leukemia cells.