Phospholipids as signalling intermediaries
Sphingosine-1-phosphate (S1P) is an example of lipid messengers with both intracellular and extracellular functions. Intracellularly S1P regulates proliferation and survival; extracellularly S1P is a ligand for EDG1 (also known as S1P1). Activation of sphingosine kinase (SPHK), the enzyme that catalyzes the phosphorylation of sphingosine, increases cellular levels of S1P. Inhibitors of SPHK block formation of S1P and inhibit cellular proliferation induced by a variety of factors, including as an example platelet-derived growth factor (PDGF) and PMA. In a study using endothelial cells it was demonstrated that S1P induces activation of alpha-v and B3 integrins via RhoA. S1P also activates Akt via Gi and PI3K. The activated Akt phosphorlyates the Edg1 receptor on threonine 236 leading to the activation of Rac1 and subsequent signals leading to actin assembly, chemotaxis and lamellipodia formation. Edg1 stimulation also leads to the activation of the ERK signaling cascade resulting in anti-apoptotic reversal, proliferation and cell survival.
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