EFFECT OF CELLULAR-DIFFERENTIATION ON NUCLEOSIDE TRANSPORT IN HUMAN NEUROBLASTOMA-CELLS

The nucleoside transport characteristics of undifferentiated and diffe rentiated LA-N-2 human neuroblastoma cells were compared through measu rement of the cellular accumulation of [H-3]formycin B in the absence and presence of specific nucleoside transport blockers such as dipyrid amole and nitrobenzylthioinosine (NBMPR). [H-3]NBMPR was also used as a high affinity probe to obtain an estimate of the number of NBMPR-sen sitive nucleoside transport proteins. Undifferentiated LA-N-2 cells ac cumulated [H-3]formycin B (25 mu M) via a NBMPR/dipyridamole sensitive , Na+-independent, nucleoside transport system (V-i=1.52 pmol/mu l/s; maximum intracellular concentration=45 pmol/mu l cell water). The undi fferentiated cells also had a high density of site-specific [H-3]NBMPR binding sites (135,000 sites/cell; K-D=0.4 nM). When cell differentia tion was induced by exposure to a serum-free defined medium, the initi al rate of transporter-mediated [H-3]formycin B uptake increased to 1. 92 pmol/mu l/s, and the steady-state intracellular concentration of [H -3]formycin B also increased significantly to 73 pmol/mu l. However, t here was no concomitant change in the number of [H-3]NBMPR binding sit es, and the additional uptake was not Na+-dependent. This enhanced upt ake in the differentiated cells appeared to be due, in part, to an inc reased functional expression of a NBMPR-resistant form of facilitated nucleoside transporter. Approximately 18% of the transporter-mediated uptake in the differentiated cells was resistant to inhibition by NBMP R at concentrations that blocked transport completely in the undiffere ntiated cells. This cell model may prove useful for basic studies on r egulation of nucleoside transporter subtype expression in neural tissu es, and for evaluation of the efficacy and potential host toxicity of cytotoxic nucleoside analogues (+/-specific transport blockers) in the treatment of neuroblastoma.