Distinct mono- and dinucleotide-specific P2Y receptors in A549 lung epithelial cells: Different control of arachidonic acid release and nitric oxide synthase expression

P2Y nucleotide receptors activated by mono- and dinucleotides have already been found in lung tissue. Here, we compare effects of dinucleotides and mononucleotides on arachidonic acid release, intracellular calcium mobilization, and inducible nitric oxide synthase (iNOS) expression in the alveolar lung cell line A549. Both types of nucleotides were effective. Diadenosine polyphosphates (ApnA, n = 2 to 5) increased arachidonic acid release and raised intracellular calcium concentration ([Ca2+]i), albeit with lower potency than mononucleotides (ATP, UTP, UDP). Among the dinucleotides only diadenosine tetraphosphate (Ap4A) was a potent agonist. Arachidonic acid release induced by Ap4A was almost completely abolished in the presence of the P2 receptor antagonists suramin and Reactive blue 2, whereas arachidonic acid release evoked by ATP, UTP or UDP was hardly reduced by these antagonists. Both, the mononucleotides ATP and UDP and the dinucleotide Ap4A induced the expression of iNOS in the cytoplasm around the nucleus, similar to the expression of iNOS evoked by lipopolysaccharide. iNOS is barely detectable in unstimulated cells. Suramin selectively blocked the capacity of Ap4A to induce iNOS, but not that of ATP or UDP. Thus, we find the same pharmacology for nucleotide-induced arachidonic acid release and iNOS expression. Therefore, we suggest that a distinct P2Y receptor subtype specifically activated by Ap4A exists in A549 cells, which is sensitive to the antagonist suramin, in contrast to other P2Y receptor subtypes activated by mononucleotides which are suramin-insensitive. Distinct P2Y receptors activated by mononucleotides or by Ap4A could play a role in inflammatory conditions by affecting the release of arachidonic acid and the expression of iNOS. Therefore, these receptors present a promising target in inflammatory diseases.