Induction of the differentiation of HL-60 cells by phorbol 12-myristate 13-acetate activates a Na+-dependent uridine-transport system. Involvement of protein kinase C

Abstract

The Na+-dependent transport and facilitated diffusion of uridine were measured after differentiation of HL-60 leukaemia cells along the monocytic pathway by phorbol 12-myristate 13-acetate (PMA). PMA (200 ng/ml) caused a marked increase in Na+-dependent uridine transport within 48 h of exposure that was attributable to an increase in transport affinity (apparent K(m) values of 1.15 ± 0.22 and 44 ± 4.4 μM for PMA-induced and uninduced cells respectively), with no change in V(max.) (0.15 ± 0.02 and 0.13 ± 0.01 pmol/s per μl of cell water for PMA-induced and uninduced cells respectively). A corresponding rapid decrease in both the rate of facilitated diffusion and the formation of uracil nucleotides occurred in PMA-induced cells. As a consequence of these changes, intracellular pools of uridine 3-4-fold greater than those in the medium were generated. A similar increase in Na+-dependent transport of adenosine, inosine, guanosine, thymidine and cytidine (K(m) values of 1-4 μM) was observed. The effects of PMA on the activation of the Na+-dependent uridine transporter were inhibited by staurosporine, suggesting the involvement of protein kinase C. The findings indicate that a change in the balance of the cellular mechanisms employed for nucleoside transport occurs during the monocytic differentiation of HL-60 leukaemia cells.