Background:We previously reported that sphingosine 1-phosphate receptor 4 (S1P4) is expressed and stimulates the ERK-1/2 pathway via a human epidermal growth factor receptor 2 (HER2)-dependent mechanism in oestrogen receptor-negative (ER−) MDA-MB-453 breast cancer cells.Methods:Clinical relevance of S1P4 and sphingosine kinase 1 (SK1, which catalyses the formation of S1P) was assessed in a cohort of 140 ER− breast tumours by immunohistochemistry (IHC) and the weighted histoscore method. Additional evidence for a functional interaction between S1P4 and SK1 and between HER2 and SK1 was obtained using MDA-MB-453 cells.Results:High S1P4 expression is associated with shorter disease-free (P=0.014) and disease-specific survival (P=0.004), and was independent on multivariate analysis. In addition, patients with tumours that contain high and low levels of SK1 and S1P4, respectively, have a significantly shorter disease-free survival (P=0.043) and disease-specific survival (P=0.033) compared with patients whose tumours contain both low S1P4 and SK1 levels. In addition, high tumour expression of SK1 was significantly associated with shorter disease-specific survival (P=0.0001) in patients with HER2-positive tumours. Treatment of MDA-MB-453 cells with the SK1 inhibitor, SKi (2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole) reduced the basal and S1P/S1P4-induced activation of ERK-1/2 and altered HER2 trafficking in these cells.Conclusion:These findings highlight an important role for S1P4 and SK1 in ER− breast cancer progression.

Although the ability of bioactive lipid sphingosine-1-phosphate (S1P) to positively regulate anti-apoptotic/pro-survival responses by binding to S1P1 is well known, the molecular mechanisms remain unclear. Here we demonstrate that expression of S1P1 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 S1P1-mediated suppression of Bim accumulation, inhibition of caspase-3 cleavage was unaffected. Instead S1P1-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 S1P1 regulation of Bim. However, S1P1 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 S1P1 in disease, analysis of tissue microarrays from ER+ breast cancer patients revealed a significant correlation between S1P1 expression and tumour cell survival. In these tumours, S1P1 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 S1P1 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.

Sphingosine kinase is an enzyme that catalyses the phosphorylation of sphingosine to form sphingosine 1-phosphate. Sphingosine 1-phosphate is a bioactive lipid, which has been shown to have an important role in promoting the survival, growth and invasiveness of cancer cells. Sphingosine 1-phosphate binds to five different plasma membrane sphingosine 1-phosphate receptors (S1P 1-5 ) and can regulate intracellular target proteins. We have used immunohistochemical analysis to determine the concurrent expression levels of sphingosine kinase 1 or S1P receptors and other signaling proteins in estrogen receptor-positive breast cancer tumors and have then assessed the impact of these combinations on clinical outcome. This approach has enabled identification of (i) novel biomarkers and (ii) several spatially controlled associations between either sphingosine kinase 1 or S1P 1-3 and other signaling proteins whose combination affect prognosis. For instance, the translocation of sphingosine kinase 1 to the plasma membrane has been shown to be a critical determinant in cancer progression. However, our findings identify an additional novel role for the nuclear localization of sphingosine kinase 1 combined with either ERK-1/2 or SFK or LYN or AKT or NF-jB, which significantly shortens disease-specific survival and/or recurrence. We also demonstrate that nuclear S1P 2 receptor and c-SRC are associated with improved prognosis and this is linked with a reduction in the nuclear localization of sphingosine kinase 1. These findings identify potential novel biomarker associations, which might serve as new targets for drug intervention designed to improve treatment of estrogen receptorpositive breast cancer.

This version is available at https://strathprints.strath.ac.uk/49243/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output. The lipid sphingosine 1-phosphate (S1P) is a key regulator of cell growth, survival, invasion, lymphocyte trafficking, vascular integrity and cytokine production, and plays a central role in inflammatory disease and cancer. S1P is formed by phosphorylation of sphingosine, catalysed by sphingosine kinases 1 and 2 (SK1 and SK2) that differ in their biochemical properties, sub-cellular localization, and function (Pyne and Pyne, 2011). S1P is cleaved by S1P lyase to produce trans-2-hexadecenal and phosphoethanolamine. S1P can also be dephosphorylated by S1P phosphatases to recycle into sphingolipids (Pyne and Pyne, 2011). S1P is an agonist of S1P-specific G-protein coupled receptors, termed S1P 1-5 , and also binds to intracellular protein targets, such as histone deacetylase 1 and 2 (HDAC1/2, which regulate gene expression) (for review see