Sphingosine 1-phosphate restrains insulin-mediated keratinocyte proliferation via inhibition of Akt through the S1P2 receptor subtype

The balance between keratinocyte proliferation and differentiation plays a decisive role for skin formation and development. Among the well-characterized biological mediators, insulin and sphingosine 1-phosphate (S1P) have been identified as major regulators of keratinocyte growth and differentiation. Insulin induces proliferation of keratinocytes, whereas S1P inhibits keratinocyte growth and initiates keratinocyte differentiation. However, it is not clear which S1P receptor subtype and downstream signaling pathways are involved in the antiproliferative action of S1P. In this study, we present evidence that S1P inhibits insulin-mediated keratinocyte growth via the activation of protein kinase C (PKC) followed by a subsequent dephosphorylation of Akt. The inhibition of insulin-mediated Akt activity by S1P is completely abolished in the presence of PKC delta siRNA indicating that this isozyme is selectively potent at causing dephosphorylation of Akt and modifying keratinocyte proliferation. Further experiments by downregulation of S1P receptor subtypes and the use of specific receptor agonists/antagonists clearly indicated that the S1P(2) receptor is dominantly involved in the S1P-induced dephosphorylation of Akt and keratinocyte growth arrest. This is of great clinical interest, as the immunomodulator FTY720, after being phosphorylated by sphingosine kinase, activates all of the five S1P receptors except S1P(2) and therefore fails to inhibit keratinocyte proliferation.