Sphingosine 1-phosphate receptors and sphingosine kinase 1 : novel biomarkers for clinical prognosis in breast, prostate, and hematological cancers

There is substantial evidence for a role in cancer of the bioactive lipid sphingosine 1-phosphate (S1P), the enzyme sphingosine kinase 1 (that catalyses S1P formation) and S1P-specific G protein-coupled receptors. This perspective highlights recent findings demonstrating that sphingosine kinase 1 and S1P receptors are new important biomarkers for detection of early cancer and progression to aggressive cancer. The impact of the sub-cellular distribution of S1P metabolizing enzymes and S1P receptors and their spatial functional interaction with oncogenes is considered with respect to prognostic outcome. These findings suggest that S1P, in addition to being a biomarker of clinical prognosis, might also be a new therapeutic target for intervention in cancer

Identification of novel biomarkers for clinical prognosis in breast cancer

Sphingosine kinase/sphingosine 1-phosphate (SK/S1P) signalling interacts with major cellular pathways controlling cell proliferation, migration, survival, and resistance to chemotherapeutics. Moreover, extensive research has shown that the SK/S1P signalling is up-regulated in numerous human cancers (e.g. stomach cancer, colon, rectal, glioblastoma, ovarian, renal, lung and breast) making S1P signalling an important candidate as a biomarker and a key player in promoting cancer progression. Several inhibitors of SK/S1P signalling pathway have been identified and have shown to inhibit cancer cell survival and resistance to chemo- and radio-therapies. In this study, human breast cancer tissue microarrays at various tissue histological grades of ERα negative breast tumours were analysed for the expression of S1P signalling proteins (e.g. SKs and S1P₁₋₅ receptors) to identify the impact of expression level of these proteins on clinical outcomes. High SK1 and S1P₄ receptor tumour expression is associated with poor cancer prognosis in ERα negative breast cancer patients. Moreover, high SK1 and S1P₄ receptor expression in these tumours was also associated with cancer recurrence and this was dependent on the HER2 receptor expression. Indeed, the SK1/2 dual inhibitor SKi (2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole) abrogated the S1Pstimulated ERK-1/2 activation in ERα-/HER2+ MDA-MB-453 cells suggesting that SK1 activity is required for the S1P₄/HER2-mediated ERK-1/2 activation that is known to promote cancer progression. In addition, a diverse array of kinases and transcription factors e.g. c-RAF-1, ERK-1/2, AKT, LYN, SRC family kinases (SFKs) and NFκB (p50 RelA) were analysed in combination with SK1 and S1P receptors to discover novel prognostic interactions that drive cancer progression in ERα positive breast cancer patients. High tumour SK1 expression in combination with high expression of either S1P₁ receptor or S1P₃ receptor or phosphorylated ERK-1/2 or phosphorylated AKT or phosphorylated NFκB or phosphorylated RAF-1 or Y416 phosphorylated SFK or LYN is associated with shorter disease-specific patient survival and disease-free cancer recurrence. Similarly, high S1P₁ receptor tumour expression in combination with high expression of either Y216 phosphorylated SRC or c-RAF-1 or ERK-1/2 or AKT kinase is associated with shorter disease-specific patient survival and disease-free cancer recurrence. High S1P₂ receptor tumour expression is associated with prolonged patient survival and this is enhanced in combination with high expression of c-SRC and Y416 phosphorylated SFK in ERα positive breast cancer tumours. Finally, high tumour S1P3 receptor expression in combination with high expression of LYN or c-RAF-1 kinases is associated with shorter disease-specific patient survival and disease-free cancer recurrence. Lastly, a new signalling pathway involving SK2, Y416 phosphorylated c-SRC, S1P₄ receptor and S1P₂ receptor was identified using pharmacological agents/gene silencing in ERα negative MDA-MB-231 breast cancer cells. In this pathway, SK2 possibly through 'inside out' S1P signalling activates the S1P₄ receptor that promotes cellular growth by preventing the nuclear accumulation of S1P₂ receptor. Moreover, SK2 activity also prevents the accumulation of Y416 phosphorylated c-SRC into the nucleus that might be crucial for tumour growth. Thus, this study shows that the high tumour expression of S1P signalling proteins is associated with poor disease prognosis in both ERα positive and ERα negative breast cancer patients. However, cancer progression is mediated by distinct set of S1P signalling proteins in different types of breast cancer. Hence, different treatment regiments including SK inhibitors and S1P receptor antagonist must be employed in treatment of ERα positive and ERα negative breast cancer patients.Sphingosine kinase/sphingosine 1-phosphate (SK/S1P) signalling interacts with major cellular pathways controlling cell proliferation, migration, survival, and resistance to chemotherapeutics. Moreover, extensive research has shown that the SK/S1P signalling is up-regulated in numerous human cancers (e.g. stomach cancer, colon, rectal, glioblastoma, ovarian, renal, lung and breast) making S1P signalling an important candidate as a biomarker and a key player in promoting cancer progression. Several inhibitors of SK/S1P signalling pathway have been identified and have shown to inhibit cancer cell survival and resistance to chemo- and radio-therapies. In this study, human breast cancer tissue microarrays at various tissue histological grades of ERα negative breast tumours were analysed for the expression of S1P signalling proteins (e.g. SKs and S1P₁₋₅ receptors) to identify the impact of expression level of these proteins on clinical outcomes. High SK1 and S1P₄ receptor tumour expression is associated with poor cancer prognosis in ERα negative breast cancer patients. Moreover, high SK1 and S1P₄ receptor expression in these tumours was also associated with cancer recurrence and this was dependent on the HER2 receptor expression. Indeed, the SK1/2 dual inhibitor SKi (2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole) abrogated the S1Pstimulated ERK-1/2 activation in ERα-/HER2+ MDA-MB-453 cells suggesting that SK1 activity is required for the S1P₄/HER2-mediated ERK-1/2 activation that is known to promote cancer progression. In addition, a diverse array of kinases and transcription factors e.g. c-RAF-1, ERK-1/2, AKT, LYN, SRC family kinases (SFKs) and NFκB (p50 RelA) were analysed in combination with SK1 and S1P receptors to discover novel prognostic interactions that drive cancer progression in ERα positive breast cancer patients. High tumour SK1 expression in combination with high expression of either S1P₁ receptor or S1P₃ receptor or phosphorylated ERK-1/2 or phosphorylated AKT or phosphorylated NFκB or phosphorylated RAF-1 or Y416 phosphorylated SFK or LYN is associated with shorter disease-specific patient survival and disease-free cancer recurrence. Similarly, high S1P₁ receptor tumour expression in combination with high expression of either Y216 phosphorylated SRC or c-RAF-1 or ERK-1/2 or AKT kinase is associated with shorter disease-specific patient survival and disease-free cancer recurrence. High S1P₂ receptor tumour expression is associated with prolonged patient survival and this is enhanced in combination with high expression of c-SRC and Y416 phosphorylated SFK in ERα positive breast cancer tumours. Finally, high tumour S1P3 receptor expression in combination with high expression of LYN or c-RAF-1 kinases is associated with shorter disease-specific patient survival and disease-free cancer recurrence. Lastly, a new signalling pathway involving SK2, Y416 phosphorylated c-SRC, S1P₄ receptor and S1P₂ receptor was identified using pharmacological agents/gene silencing in ERα negative MDA-MB-231 breast cancer cells. In this pathway, SK2 possibly through 'inside out' S1P signalling activates the S1P₄ receptor that promotes cellular growth by preventing the nuclear accumulation of S1P₂ receptor. Moreover, SK2 activity also prevents the accumulation of Y416 phosphorylated c-SRC into the nucleus that might be crucial for tumour growth. Thus, this study shows that the high tumour expression of S1P signalling proteins is associated with poor disease prognosis in both ERα positive and ERα negative breast cancer patients. However, cancer progression is mediated by distinct set of S1P signalling proteins in different types of breast cancer. Hence, different treatment regiments including SK inhibitors and S1P receptor antagonist must be employed in treatment of ERα positive and ERα negative breast cancer patients

The role of sphingosine 1-phosphate in inflammation and cancer

The enzymes that catalyze formation of the bioactive sphingolipid, sphingosine 1-phosphate, sphingosine kinase 1 and 2, are predictive markers in inflammatory diseases and cancer as evidenced by data from patients, knockout mice and the use of available molecular and chemical inhibitors. Thus, there is a compelling case for therapeutic targeting of sphingosine kinase. In addition, there are several examples of functional interaction between sphingosine 1-phosphate receptors and sphingosine kinase 1 that can drive malicious amplification loops that promote cancer cell growth. These novel aspects of sphingosine 1-phosphate pathobiology are reviewed herein

Sphingosine kinase 2 prevents the nuclear translocation of sphingosine 1-phosphate receptor-2 and tyrosine 416 phosphorylated c-Src and increases estrogen receptor negative MDA-MB-231 breast cancer cell growth : the role of sphingosine 1-phosphate receptor-4

We demonstrate that pre-treatment of estrogen receptor negative MDA-MB-231 breast cancer cells containing ectopically expressed HA-tagged sphingosine 1-phosphate receptor-2 (S1P2) with the sphingosine kinase 1/2 inhibitor SKi (2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole) or the sphingosine kinase 2 selective inhibitor (R)-FTY720 methyl ether (ROMe) or sphingosine kinase 2 siRNA induced the translocation of HA-tagged S1P2 and Y416 phosphorylated c-Src to the nucleus of these cells. This is associated with reduced growth of HA-tagged S1P2 over-expressing MDA-MB-231 cells. Treatment of HA-S1P2 over-expressing MDA-MB-231 cells with the sphingosine 1-phosphate receptor-4 (S1P4) antagonist CYM50367 or with S1P4 siRNA also promoted nuclear translocation of HA-tagged S1P2. These findings identify for the first time a signaling pathway in which sphingosine 1-phosphate formed by sphingosine kinase 2 binds to S1P4 to prevent nuclear translocation of S1P2 and thereby promote the growth of estrogen receptor negative breast cancer cells