Regulation of cell survival by sphingosine-1-phosphate receptor S1P1 via reciprocal ERK-dependent suppression of Bim and PI-3-kinase/protein kinase C-mediated upregulation of Mcl-1

<jats:title>Abstract</jats:title><jats:p>Although the ability of bioactive lipid sphingosine-1-phosphate (S1P) to positively regulate anti-apoptotic/pro-survival responses by binding to S1P<jats:sub>1</jats:sub> is well known, the molecular mechanisms remain unclear. Here we demonstrate that expression of S1P<jats:sub>1</jats:sub> renders CCL39 lung fibroblasts resistant to apoptosis following growth factor withdrawal. Resistance to apoptosis was associated with attenuated accumulation of pro-apoptotic BH3-only protein Bim. However, although blockade of extracellular signal-regulated kinase (ERK) activation could reverse S1P<jats:sub>1</jats:sub>-mediated suppression of Bim accumulation, inhibition of caspase-3 cleavage was unaffected. Instead S1P<jats:sub>1</jats:sub>-mediated inhibition of caspase-3 cleavage was reversed by inhibition of phosphatidylinositol-3-kinase (PI3K) and protein kinase C (PKC), which had no effect on S1P<jats:sub>1</jats:sub> regulation of Bim. However, S1P<jats:sub>1</jats:sub> suppression of caspase-3 was associated with increased expression of anti-apoptotic protein Mcl-1, the expression of which was also reduced by inhibition of PI3K and PKC. A role for the induction of Mcl-1 in regulating endogenous S1P receptor-dependent pro-survival responses in human umbilical vein endothelial cells was confirmed using S1P receptor agonist FTY720-phosphate (FTY720P). FTY720P induced a transient accumulation of Mcl-1 that was associated with a delayed onset of caspase-3 cleavage following growth factor withdrawal, whereas Mcl-1 knockdown was sufficient to enhance caspase-3 cleavage even in the presence of FTY720P. Consistent with a pro-survival role of S1P<jats:sub>1</jats:sub> in disease, analysis of tissue microarrays from ER<jats:sup>+</jats:sup> breast cancer patients revealed a significant correlation between S1P<jats:sub>1</jats:sub> expression and tumour cell survival. In these tumours, S1P<jats:sub>1</jats:sub> expression and cancer cell survival were correlated with increased activation of ERK, but not the PI3K/PKB pathway. In summary, pro-survival/anti-apoptotic signalling from S1P<jats:sub>1</jats:sub> is intimately linked to its ability to promote the accumulation of pro-survival protein Mcl-1 and downregulation of pro-apoptotic BH3-only protein Bim via distinct signalling pathways. However, the functional importance of each pathway is dependent on the specific cellular context.</jats:p>