Cell adhesion modulates 5-HT(1D) and P2Y receptor signal trafficking differentially in LTK-8 cells

In this study, we investigated adhesion-induced changes in cellular responses to serotonin 5-HT(1D) and purinergic P2Y receptor stimulation. We demonstrated that detachment of LTK-8 cells increased 5-HT(1D) receptor-mediated intracellular Ca(2+) and extracellular signal regulated kinase (ERK) phosphorylation responses without affecting the adenylate cyclase response. Additionally, detachment enabled 5-HT(1D) receptor stimulation to inhibit P2Y receptor-induced [Ca(2+)](i) mobilization. Such a cross talk between the two receptor systems was not observed in attached cells. P2Y receptor-induced Ca(2+) response was insensitive to adhesion state of the cells, while ERK phosphorylation response was enhanced upon detachment. Integrity of the actin cytoskeleton did not appear to play a role in adhesion sensitivity of 5-HT(1D)-mediated responses, as treatment of attached cells with cytochalasin D did not mimic detachment-induced effects. Effects of detachment were reversed immediately after re-attachment of the suspended cells on poly-l-lysine coated cover slips, suggesting that the involvement of integrins or focal adhesion complexes is unlikely. Taken collectively, our results demonstrate that not only cellular responses induced by different G protein-coupled receptors, but also different responses induced by a particular G protein-coupled receptor, can be affected differentially by the adhesion status of cells. This suggests an important role for cell adhesion in controlling the coupling of a single G protein-coupled receptor to different intracellular responses.