Phosphatidylinositol 3-kinase-dependent extracellular calcium influx is essential for CX(3)CR1-mediated activation of the mitogen-activated protein kinase cascade

Fractalkine, the first member of the CX3C chemokine family, induces leukocy te chemotaxis through activation of its high affinity receptor, CX(3)CR1. L ike other chemokine receptors, CX(3)CR1 is coupled to a pertussis toxin-sen sitive heterotrimeric G(i) protein, which is necessary for rapid rise in th e concentration of intracellular calcium. Using a Chinese hamster ovary cel l line stably transfected with the CX(3)CR1 receptor, we show that the sour ce of calcium mobilized by fractalkine stimulation is the extracellular poo l. Calcium influx is blocked by extracellular calcium chelators, as well as by divalent heavy metals such as Ni2+, Co2+, and Cd2+ without affecting th e integrity of intracellular stores. Remarkably, selective phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin and LY294002, abolish the wave extr acellular calcium, suggesting that an active PI3K is necessary for this eve nt. The influx of extracellular calcium is in turn required to trigger the activation of the p42/44 mitogen-activated protein/extracellular signal-reg ulated kinase pathway, but is not necessary for other signals downstream to PI3K, such as phosphorylation of Akt. The potential role of this signaling cascade in fractalkine-mediated chemotaxis is discussed.