ENHANCEMENT OF PERTUSSIS-TOXIN-SENSITIVE NA+-DEPENDENT URIDINE TRANSPORTER ACTIVITY IN HL-60 GRANULOCYTES BY N-FORMYLMETHIONYL-LEUCYL-PHENYLALANINE

N-Formyl-Met-Leu-Phe (FMLP), at concentrations as low as 5 nM, caused an increase in intracellular uridine pools in dimethyl sulphoxide (Me2SO)-differentiated HL-60 cells. Intracellular uridine pools were elevated rapidly and reached a maximum within 10 min of exposure to 10 muM FMLP, followed by a gradual decline. This enhancement by FMLP was a consequence of a 3-fold increase in the V(max) of pertussis-toxin-sensitive Na+-dependent uridine transport system, with no change in the apparent K(m). K(m) values of 2.67+/-0.45 and 3.85+/-0.52 muM and V(max) values of 0.046+/-0.017 and 0.125+/-0.020 muM/s were obtained for untreated and FMLP-treated Me2SO-differentiated cells respectively. The effect of FMLP on the Na+-dependent transport of uridine in Me2SO-differentiated HL-60 cells was specific, as the facilitated transport of uridine was unaffected. Furthermore, this phenomenon was not observed in undifferentiated, phorbol 12-myristate 13-acetate (PMA)-differentiated or pertussis-toxin-treated Me2SO-differentiated HL-60 cells. Removal of extracellular Ca2+ with EGTA abolished the FMLP enhancement of uridine transport in a reversible manner, suggesting the involvement of Ca2+. However, the Ca2+ ionophore A23187 only partially mimicked the effect of FMLP. Similarly, with PMA the transport was sub-optimally enhanced, but a full activation was observed in cells treated with both A23187 and PMA. These findings suggest that activation of the Na+-dependent uridine transporter by FMLP in Me2SO-differentiated HL-60 cells involves a pertussis-toxin-sensitive G-protein with a bifurcating signal-transduction pathway.