Nucleoside transporter expression and activity is regulated during granulocytic differentiation of NB4 cells in response to all-trans-retinoic acid

NB4 cells express multiple nucleoside transporters (NTs), including: hENT1 (es), and hENT2 (ei), and the CNT subtype referred to as, csg; a concentrative sensitive guanosine specific transporter. csg activity is a distinguishing feature of the NB4 cell line and its presence suggests a particular requirement of these cells for guanosine salvage. Proliferation and differentiation pathways determine, in part, the number of NTs in cells and tissues. In this study, all-trans-retinoic acid (ATRA)-induced granulocytic differentiation of NB4 cells resulted in biphasic changes in guanosine transport. Transient increases in csg and es activity, the result of an increase in Vmax (pmol/μl s) of both transporter systems, served as early markers of differentiation while expression of a fully differentiated phenotype was accompanied by a selective loss of csg activity and the return of es activity to that of proliferating cells. Intracellular incorporation of [3H]-guanosine decreased as cells matured despite increased transport rates and suggested a reduced intracellular requirement of NB4-granulocytes compared to their proliferating counterparts. Whether a loss of csg activity could serve to assess clinical response to differentiation therapies is not known. Nitrobenzylthioinosine (NBMPR) binding sites within nuclear membrane (NM) preparations, suggested the presence of functional intracellular NTs. An increase in plasma membrane (PM) associated transporters coincided with the early increase in guanosine transport and a decrease in NBMPR binding to NM fractions and suggests that intracellular NTs may serve as a reserve pool for translocation to the (PM) when additional transport capacity is required. The modulation of transporters during differentiation could potentially regulate drug bioavailability and cytotoxicity and should be evaluated prior to combining differentiating agents with traditional nucleoside analogs in the treatment of APL.