Sphingosine-1-phosphate induced mTOR-activation is mediated by the E3-ubiquitin ligase PAM

The signaling pathways that are regulated by sphingosine-1-phosphate (S1P) and mammalian target of rapamycin (mTOR) modulate cell growth, mitogenesis and apoptosis in various cell types and are of major interest for the development of new cancer therapeutics. Previous reports show that S1P can cross-activate the mTOR pathway although the mechanisms that connect both pathways are still unknown. We found that S1P-treatment activates mTOR in several cancer cell lines and primary cells. The activation was independent of ERK, Akt and PI3-kinase, but instead was mediated by the E3 ubiquitin ligase Protein Associated with Myc (PAM). Increased intracellular PAM concentrations facilitated S1P- and insulin-induced mTOR activation as well as p70S6K and 4EBP1 phosphorylation while genetic deletion of PAM decreased S1P- and insulin-induced mTOR activation. PAM activated by facilitating the GDP/GTP-exchange of Rheb which is an activator of mTOR. In conclusion we show that PAM is a novel regulator of the mTOR pathway and that PAM may directly activate Rheb as a guanosine exchange factor (GEF).