Sphingosine kinase-mediated calcium signaling by muscarinic acetylcholine receptors

Based on the finding that G protein-coupled receptors (GPCRs) can induce Ca2+ mobilization, apparently independent of the phospholipase C (Pl,C)/inositol-1,4,5-trisphosphate (IP3) pathway, we investigated whether sphingosine kinase, which generates sphingosine-l-phosphate (SPP), is involved in calcium signaling by mAChR and other GPCRs. Inhibition of sphingosine kinase by DL-threo-dihydrosphingosine and N,N-dimethylsphingosine markedly inhibited [Ca2+](i) increases elicited by M-2 and M-3 mAChRs in HEK-293 cells without affecting PLC activation. Activation of M-2 and M-3 mAChR rapidly and transiently stimulated production of SPP. Furthermore, microinjection of SPP into HEK-293 cells induced rapid and transient Ca2+ mobilization. Pretreatment of HEK-293 cells with the calcium chelator BAPTA/AM fully blocked mAChR-induced SPP production. On the other hand, incubation of HEK-293 cells with calcium ionophores activated SPP production. Similar findings were obtained for formyl peptide and P2Y(2) purinergic receptors in HL-60 cells. On the basis of these studies we propose, that following initial IP3 production by receptor-mediated PLC activation, a local discrete increase in [Ca2+](i) induces sphingosine kinase stimulation, which ultimately leads to full calcium mobilization. Thus, sphingosine kinase activation most likely represents an amplification system for calcium signaling by mAChRs and other GPCRs. (C) 2001 Elsevier Science Inc. AH rights reserved.