SPHK1 and SPHK2 are encoded by different genes with some redundancy of function; genetic deletion of both Sphk1 and Sphk2, but not either alone, is embryonic lethal in mice. There are splice variants of each isoform (SphK1a-c and SphK2a, b), distinguished by their N-terminal sequences. SPHK1 and SPHK2 differ in tissue distribution, sub-cellular localisation, biochemical properties and regulation. They regulate discrete pools of S1P. Receptor stimulation induces SPHK1 translocation from the cytoplasm to the plasma membrane. SPHK1 translocation is regulated by phosphorylation/dephosphorylation, specific protein:protein interactions and interaction with specific lipids at the plasma membrane. SPHK1 is a dimeric protein, as confirmed by its crystal structure which forms a positive cluster, between protomers, essential for interaction with anionic phospholipids in the plasma membrane. SPHK2 is localised to the ER or associated with mitochondria or shuttles in/out of the nucleus, regulated by phosphorylation. Intracellular targets of nuclear S1P include the catalytic subunit of telomerase (TERT) and regulators of gene expression including histone deacetylases (HDAC 1/2) and peroxisome proliferator-activated receptor gamma (PPARγ). SPHK2 phosphorylates the pro-drug FTY720 (fingolimod, which is used to treat some forms of multiple sclerosis) to a mimic of S1P and that acts as a functional antagonist of S1P₁ receptors. Inhibitors of SPHK1 and SPHK2 have therapeutic potential in many diseases. Isoform-selective inhibitors are becoming available; some early inhibitors have recognised off-target effects.

MP-A08 is competitive with ATP; other SPHK inhibitors are competitive with sphingosine. ABC294640 (opaganib) has known off-target effects on dihydroceramide desaturase (DEGS1) [8,12]) and induces proteasomal degradation of SPHK1 [8]. ABC294640 is in clinical trials for advanced cholangiocarcinoma, advanced hepatocellular carcinoma and refractory/relapsed multiple myeloma (to view ClinicalTrials.gov list click here).

* Key recommended reading is highlighted with an asterisk

* Adams DR, Pyne S, Pyne NJ. (2016) Sphingosine Kinases: Emerging Structure-Function Insights. Trends Biochem Sci, 41 (5): 395-409. [PMID:27021309]

Lynch KR, Thorpe SB, Santos WL. (2016) Sphingosine kinase inhibitors: a review of patent literature (2006-2015). Expert Opin Ther Pat, 26 (12): 1409-1416. [PMID:27539678]

Marfe G, Mirone G, Shukla A, Di Stefano C. (2015) Sphingosine kinases signalling in carcinogenesis. Mini Rev Med Chem, 15 (4): 300-14. [PMID:25723458]

Neubauer HA, Pitson SM. (2013) Roles, regulation and inhibitors of sphingosine kinase 2. FEBS J, 280 (21): 5317-36. [PMID:23638983]

* Pitman MR, Costabile M, Pitson SM. (2016) Recent advances in the development of sphingosine kinase inhibitors. Cell Signal, 28 (9): 1349-63. [PMID:27297359]

* Powell JA, Pitman MR, Zebol JR, Moretti PAB, Neubauer HA, Davies LT, Lewis AC, Dagley LF, Webb AI, Costabile M et al.. (2019) Kelch-like protein 5-mediated ubiquitination of lysine 183 promotes proteasomal degradation of sphingosine kinase 1. Biochem J, 476 (21): 3211-3226. [PMID:31652307]

* Pulkoski-Gross MJ, Jenkins ML, Truman JP, Salama MF, Clarke CJ, Burke JE, Hannun YA, Obeid LM. (2018) An intrinsic lipid-binding interface controls sphingosine kinase 1 function. J Lipid Res, 59 (3): 462-474. [PMID:29326159]

* Pyne NJ, Adams DR, Pyne S. (2017) Sphingosine Kinase 2 in Autoimmune/Inflammatory Disease and the Development of Sphingosine Kinase 2 Inhibitors. Trends Pharmacol Sci, 38 (7): 581-591. [PMID:28606480]

Pyne S, Adams DR, Pyne NJ. (2016) Sphingosine 1-phosphate and sphingosine kinases in health and disease: Recent advances. Prog Lipid Res, 62: 93-106. [PMID:26970273]

* Pyne S, Adams DR, Pyne NJ. (2020) Sphingosine Kinases as Druggable Targets. Handb Exp Pharmacol, 259: 49-76. [PMID:29460151]

Santos WL, Lynch KR. (2015) Drugging sphingosine kinases. ACS Chem Biol, 10 (1): 225-33. [PMID:25384187]

Truman JP, García-Barros M, Obeid LM, Hannun YA. (2014) Evolving concepts in cancer therapy through targeting sphingolipid metabolism. Biochim Biophys Acta, 1841 (8): 1174-88. [PMID:24384461]

1. Adams DR, Pyne S, Pyne NJ. (2016) Sphingosine Kinases: Emerging Structure-Function Insights. Trends Biochem Sci, 41 (5): 395-409. [PMID:27021309]

2. Adams DR, Tawati S, Berretta G, Rivas PL, Baiget J, Jiang Z, Alsfouk A, Mackay SP, Pyne NJ, Pyne S. (2019) Topographical Mapping of Isoform-Selectivity Determinants for J-Channel-Binding Inhibitors of Sphingosine Kinases 1 and 2. J Med Chem, 62 (7): 3658-3676. [PMID:30889352]

3. Childress ES, Kharel Y, Brown AM, Bevan DR, Lynch KR, Santos WL. (2017) Transforming Sphingosine Kinase 1 Inhibitors into Dual and Sphingosine Kinase 2 Selective Inhibitors: Design, Synthesis, and in Vivo Activity. J Med Chem, 60 (9): 3933-3957. [PMID:28406646]

4. French KJ, Zhuang Y, Maines LW, Gao P, Wang W, Beljanski V, Upson JJ, Green CL, Keller SN, Smith CD. (2010) Pharmacology and antitumor activity of ABC294640, a selective inhibitor of sphingosine kinase-2. J Pharmacol Exp Ther, 333 (1): 129-39. [PMID:20061445]

5. Gao P, Peterson YK, Smith RA, Smith CD. (2012) Characterization of isoenzyme-selective inhibitors of human sphingosine kinases. PLoS ONE, 7 (9): e44543. [PMID:22970244]

6. Lim KG, Sun C, Bittman R, Pyne NJ, Pyne S. (2011) (R)-FTY720 methyl ether is a specific sphingosine kinase 2 inhibitor: Effect on sphingosine kinase 2 expression in HEK 293 cells and actin rearrangement and survival of MCF-7 breast cancer cells. Cell Signal, 23 (10): 1590-5. [PMID:21620961]

7. Loveridge C, Tonelli F, Leclercq T, Lim KG, Long JS, Berdyshev E, Tate RJ, Natarajan V, Pitson SM, Pyne NJ et al.. (2010) The sphingosine kinase 1 inhibitor 2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole induces proteasomal degradation of sphingosine kinase 1 in mammalian cells. J Biol Chem, 285 (50): 38841-52. [PMID:20926375]

8. McNaughton M, Pitman M, Pitson SM, Pyne NJ, Pyne S. (2016) Proteasomal degradation of sphingosine kinase 1 and inhibition of dihydroceramide desaturase by the sphingosine kinase inhibitors, SKi or ABC294640, induces growth arrest in androgen-independent LNCaP-AI prostate cancer cells. Oncotarget, 7 (13): 16663-75. [PMID:26934645]

9. Pitman MR, Powell JA, Coolen C, Moretti PA, Zebol JR, Pham DH, Finnie JW, Don AS, Ebert LM, Bonder CS et al.. (2015) A selective ATP-competitive sphingosine kinase inhibitor demonstrates anti-cancer properties. Oncotarget, 6 (9): 7065-83. [PMID:25788259]

10. Schnute ME, McReynolds MD, Carroll J, Chrencik J, Highkin MK, Iyanar K, Jerome G, Rains JW, Saabye M, Scholten JA et al.. (2017) Discovery of a Potent and Selective Sphingosine Kinase 1 Inhibitor through the Molecular Combination of Chemotype-Distinct Screening Hits. J Med Chem, 60 (6): 2562-2572. [PMID:28231433]

11. Schnute ME, McReynolds MD, Kasten T, Yates M, Jerome G, Rains JW, Hall T, Chrencik J, Kraus M, Cronin CN et al.. (2012) Modulation of cellular S1P levels with a novel, potent and specific inhibitor of sphingosine kinase-1. Biochem J, 444 (1): 79-88. [PMID:22397330]

12. Venant H, Rahmaniyan M, Jones EE, Lu P, Lilly MB, Garrett-Mayer E, Drake RR, Kraveka JM, Smith CD, Voelkel-Johnson C. (2015) The Sphingosine Kinase 2 Inhibitor ABC294640 Reduces the Growth of Prostate Cancer Cells and Results in Accumulation of Dihydroceramides In Vitro and In Vivo. Mol Cancer Ther, 14 (12): 2744-52. [PMID:26494858]

Nigel J Pyne Strathclyde Institute of Pharmacy and Biomedical Science University of Strathclyde 161 Cathedral Street Glasgow G4 0RE, UK Susan Pyne (Past contributor) Strathclyde Institute of Pharmacy and Biomedical Science University of Strathclyde 161 Cathedral Street Glasgow G4 0RE, UK