Figure 5

Cellular regulation of Sphingosine-1-Phosphate (S1P) and Prohibitin. A. Sphingosine (Sph) – S1P rheostat; showing the two enzymes that control it, sphingosine kinase and S1P phosphatase (SPPase); and regulation pathways of S1P and prohibitin in vertebrate cells. B. Model of S1P and prohibitin roles in cnidarian – algal symbiosis: (1) SPPase is down regulated by an unknown algal-induced mechanism, causing an increase of the levels of S1P over sphingosine; (2) the lower levels of sphingosine produce a suppression of sphingolipid-induced apoptosis; (3) higher levels of S1P enhance the activation of NF-κB, which inhibits apoptosis and increases cell survival; (4) S1P activates the ERK/MAPK pathway, which causes an expedition of G1/S cell cycle that results in cell proliferation; (5) Prohibitin is down-regulated by an unknown algal-induced mechanism, causing a suppression of apoptosis and a release of the blockage of E2F transcription factors. The latter enhances the expedition of G1/S cell cycle resulting in cell proliferation.