EFFECT OF EXTRACELLULAR ATP ON BREAST-TUMOR CELL-GROWTH, IMPLICATION OF INTRACELLULAR CALCIUM

We studied the effects of purine nucleotides and particularly adenosin e triphosphate (ATP) in two (one hormonosensitive, MCF7 and one hormon oinsensitive, MDA-MB 231) human breast tumor cell lines. As described in other cells, we observed that purine nucleotides produced transient elevations in intracellular calcium ions, [Ca2+](i), in both types of cells as determined from Indo-1 fluorescence of loaded cells. In the absence of external calcium the [Ca2+](i) transients consisted of sing le narrow peaks while an extension of peak duration along with a bipha sic appearance were observed in the presence of extemal calcium. The p otency of different purine nucleotides in elevating [Ca2+](i) was ATP > ADP much greater than AMP > adenosine (which was inefficient) provin g the presence of P2 purinergic receptor subtypes. Suramin, a compound known to compete with ATP for its binding sites, nearly abolished the effect of ATP on [Ca2+](i) increase while verapamil, a calcium channe l blocker, was unable to abolish such an ATP-induced [Ca2+](i) increas e. The concentrations of ATP required to increase [Ca2+](i) ranged fro m 10(-7) M to 10(-3) M, the maximal effect being obtained with 10(-4) M ATP. At this latter concentration, ATP induced cell growth inhibitio n which was dose-independent as triggered only when maximal elevation of [Ca2+](i) was attained. This ATP concentration also induced maximal apoptotic features in both types of cells. Together, our results high lighted an 'all or none' effect of ATP on breast tumor cell growth med iated by its effect on [Ca2+](i) liberation from intracellular stores, the first rise of [Ca2+](i) being further amplified by an influx of c alcium from extracellular space. The attainment of sufficient [Ca2+](i ) level then triggers cellular events.