Sphingosine 1-phosphate protects primary human keratinocytes from apoptosis via nitric oxide formation through the receptor subtype S1P3

Although the lipid mediator sphingosine 1-phosphate (S1P) has been identified to induce cell growth arrest of human keratinocytes, the sphingolipid effectively protects these epidermal cells from apoptosis. The molecular mechanism of the anti-apoptotic action induced by S1P is less characterized. Apart from S1P, endogenously produced nitric oxide (NO^?) has been recognized as a potent modulator of apoptosis in keratinocytes. Therefore, it was of great interest to elucidate whether S1P protects human keratinocytes via a NO^?-dependent signalling pathway. Indeed, S1P induced an activation of endothelial nitric oxide synthase (eNOS) in human keratinocytes leading to an enhanced formation of NO^?. Most interestingly, the cell protective effect of S1P was almost completely abolished in the presence of the eNOS inhibitor L-NAME as well as in eNOS-deficient keratinocytes indicating that the sphingolipid metabolite S1P protects human keratinocytes from apoptosis via eNOS activation and subsequent production of protective amounts of NO^?. It is well established that most of the known actions of S1P are mediated by a family of five specific G protein-coupled receptors. Therefore, the involvement of S1P-receptor subtypes in S1P-mediated eNOS activation has been examined. Indeed, this study clearly shows that the S1P₃ is the exclusive receptor subtype in human keratinocytes which mediates eNOS activation and NO^? formation in response to S1P. In congruence, when the S1P₃ receptor subtype is abrogated, S1P almost completely lost its ability to protect human keratinocytes from apoptosis.