Extracellular ATP causes of loss of L-selectin from human lymphocytes via occupancy of P2Z purinocepters.
Lymphocytes from normal subjects or patients with chronic lymphocytic leukemia are known to possess receptors for extracellular ATP termed P2Z purinoceptors whose physiological role is undefined. Addition of extracellular ATP (50-500 microM) to both normal and leukemic lymphocytes caused loss of binding of monoclonal antibodies to L-selectin (CD62L) on the cell surface. UTP, ADP, and adenosine (all at 500 microM) had no effect on L-selectin expression. Several features of the ATP-induced loss of L selectin indicate that this effect is mediated by lymphocyte P2Z purinoceptors. First the loss was attenuated in isotonic NaCl medium compared to 150 mM KCl medium. Second the loss of L-selectin was immediately halted by addition of Mg2+ ions in molar excess of ATP. The most potent nucleotide causing L-selectin loss was benzoylbenzoic ATP (> 10 microM) which is also the most potent agonist for the P2Z purinoceptor. Finally preincubation of lymphocytes with oxidized ATP, an irreversible inhibitor of P2Z purinoceptors, also inhibited ATP induced loss of L-selectin. Extracellular ATP is known to open an ion channel associated with the P2Z purinoceptor on B-lymphocytes which allows influx of Ca2+. However, ATP-induced loss of L-selectin did not require extracellular Ca2+. Moreover addition of the calcium ionophore, ionomycin, had minimal effect on L-selectin expression. Staurosporine (500 nM), an inhibitor of protein kinase C, inhibited only 10% of ATP induced loss of L-selectin but completely inhibited the loss of L-selectin caused by 50 nM PMA. Thus extracellular ATP interacts with lymphocyte P2Z purinoceptors which leads to shedding of L-selectin via a pathway which requires neither Ca2+ influx nor activation of protein kinase C. ATP may have a physiological role in the loss of L-selectin which occurs during the interactions of lymphocytes with other cells.