Inverse Correlation of NM23 Expression with Lysophosphatidic Acid Receptor EDG2/lpa1 Expression of Human Leukemia Cells during Myeloid Differentiation Induced by All-Trans Retinoic Acid

Overexpression of the nm23 gene is found in many hematological malignancies, and predicts poor treatment outcome. Because the amount of intracellular NM23-H1 protein is inversely correlated with differentiation and exogenous over expression of the nm23-H1 in myeloid leukemia cells reduced the sensitivity to induction of myeloid differentiation, nm23-overexpression is considered to function as a differentiation-suppressor. However, the molecular mechanism of this phenomenon is unknown. Recently, lysophosphatidic acid (LPA) receptor EDG2/lpa1 was identified as a gene down-regulated by nm23-H1 and associated with cell motility/metastasis suppressor functions in breast cancer cells. Although the clinical significance of nm23-H1 overexpression in leukemia is completely opposite to that in breast cancer, there might be a common molecular mechanism between the metastasis-suppressing activity and differentiation-inhibiting activity of nm23-H1 overexpression, because many functional characteristics of macrophages/neutrophils are similar to those of motile and metastatic tumor cells. Here, we examined the EDG2/lpa1 expression level and its association with NM23 expression level and myeloid differentiation of leukemia cells. EDG2 and NM23 proteins were expressed at different levels in all leukemia cells tested (HL-60, NB4, THP-1, U937, K562, HEL, BALM-1, BALM-3, MOLT4, Jurkat). During all-trans retinoic acid (ATRA)-induced myeloid differentiation of HL-60 cells, the NM23 expression decreased and EDG2 expression inversely increased. Similar effects of ATRA were observed in myeloid differentiation of NB4 and THP-1 cells. NM23 expression levels and EDG2 expression levels modulated by ATRA treatment were inversely correlated (Spearman’s correlation coefficient, rs = −0.7538, p = 0.0237). However, these correlations were not found in the erythroid differentiation of ATRA-treated HEL and K562 cells. Cell motility is required for myeloid differentiation; therefore, there might be a common mechanism, the inhibition of cell motility thorough EDG2 down-regulation by NM23-H1 overexpression, between differentiation-inhibiting activity in leukemia cells and motility-inhibiting activity in breast cancer cells. The discrepancy in the clinical significance of NM23-H1 overexpression between leukemia and breast cancer might be resolved.