Host tissue damage signal ATP promotes non-directional migration and negatively regulates Toll-like receptor signaling in human monocytes

The activation of Toll-like receptors (TLRs) by lipopolysaccharide or other ligands evokes a proinflammatory immune response, which is not only capable of clearing invading pathogens but can also inflict damage to host tissues. It is therefore important to prevent an overshoot of the TLR-induced response where necessary, and here we show that extracellular ATP is capable of doing this in human monocytes. Using reverse transcription-PCR, we showed that monocytes express P2Y(1), P2Y(2), P2Y(4), P2Y(11), and P2Y(13) receptors, as well as several P2X receptors. To elucidate the function of these receptors, we first studied Ca2+ signaling in single cells. ATP or UTP induced a biphasic increase in cytosolic Ca2+, which corresponded to internal Ca2+ release followed by activation of store- operated Ca2+ entry. The evoked Ca2+ signals stimulated Ca2+-activated K+ channels, producing transient membrane hyperpolarization. In addition, ATP promoted cytoskeleton reorganization and cell migration; however, unlike chemoattractants, the migration was non-directional and further analysis showed that ATP did not activate Akt, essential for sensing gradients. When TLR2, TLR4, or TLR2/6 were stimulated with their respective ligands, ATP gamma S profoundly inhibited secretion of proinflammatory cytokines (tumor necrosis factor-alpha and monocyte chemoattractant protein-1) but increased the production of interleukin-10, an anti-inflammatory cytokine. In radioimmune assays, we found that ATP ( or ATP gamma S) strongly increased cAMP levels, and, moreover, the TLR- response was inhibited by forskolin, whereas UTP neither increased cAMP nor inhibited the TLR- response. Thus, our data suggest that ATP promotes non-directional migration and, importantly, acts as a host tissue damage signal via the Gs protein-coupled P2Y(11) receptor and increased cAMP to negatively regulate TLR signaling.