SK1-I decreases S1P and increases ceramide levels and S1P protects against SK1-I–induced apoptosis. (A) U937 cells were treated without or with 20 μM SK1-I for 6 hours. Lipids were extracted and sphingosine (So), sphinganine (Sa), sphingosine-1-phosphate (SoP), sphinganine-1-phosphate (SaP), and ceramide species were determined by ESI-MS/MS. Data are means of triplicate determinations and are expressed as picomole of lipid per microgram of DNA. Numbers indicate fatty acid chain length followed by the number of double bonds. C16DH indicates C16-dihydroceramide. (B) U937 cells were treated without or with SK1-I (10, 15, 20 μM) in the absence and presence of S1P (1, 10 μM) for 4 hours. Cells were stained with annexin V/PI and apoptosis was determined by flow cytometry. *P ≤ .01.