Neuroblastoma specific effects of DR-nm23 and its mutant forms on differentiation and apoptosis

DR-nm23 belongs to a gene family which includes nm23-H1, originally identif ied as a candidate metastasis suppressor gene. Nm23 genes are expressed in different tumor types where their levels have been alternatively associated with reduced or increased metastatic potential. Nm23-H1, -H2, DR-nm23 and nm23-H4 all possess NDP kinase activity. Overexpression of DR-nm23 inhibits differentiation and promotes apoptosis in hematopoietic cells, By contrast , it induces morphological and biochemical changes associated with neural d ifferentiation in neuroblastoma cells. In this study, we show that mutation s in the catalytic domain and in the serine 61 phosphorylation site, possib ly required for protein-protein interactions, impair the ability of DR-nm23 to induce neural differentiation. Moreover, neuroblastoma cells overexpres sing wild-type or mutant DR-nm23 are less sensitive to apoptosis triggered by serum withdrawal. By subcellular fractionation, wild type and mutant DR- nm23 localize in the cytoplasm and prevalently in the mitochondrial fractio n. In coimmunoprecipitation experiments, wild-type DR-nm23 binds other memb ers of nm23 family, but mutations in the catalytic and in the RGD domains a nd in serine 61 inhibit the formation of hetero-multimers. Thus, the integr ity of the NDP kinase activity and the presence of a serine residue in posi tion 61 seem essential for the ability of DR-nm23 to trigger differentiatio n and to bind other Nm23 proteins, but not for the anti-apoptotic effect in neuroblastoma cells. These studies underline the tissue specificity of the biological effects induced by DR-nm23 expression.