Sphingolipids such as ceramides (Cers) play important roles in cell proliferation, apoptosis, and cell cycle regulation. An increased Cer level is linked to the cytotoxic effects of several chemotherapeutics. Various selective cyclooxygenase-2 (COX-2) inhibitors induce anti-proliferative effects in tumor cells. We addressed the possible interaction of the selective COX-2 inhibitors, coxibs, with the sphingolipid pathway as an explanation of their anti-proliferative effects. Sphingolipids were

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... asured using liquid chromatography tandem mass spectrometry. Treatment of various cancer cell lines with celecoxib significantly increased sphinganine, C 16:0 -, C 24:0 -, C 24:1 -dihydroceramide (dhCer) and led to a depletion of C 24:0 -, C 24:1 -Cer in a time-and concentrationdependent manner, whereas other coxibs had no effect. Using 13 C, 15 N-labeled L-serine, we demonstrated that the augmented dhCers after celecoxib treatment originate from de novo synthesis. Celecoxib inhibited the dihydroceramide desaturase (DEGS) in vivo with an IC 50 of 78.9 6 1.5 mM and increased total Cer level about 2-fold, indicating an activation of sphingolipid biosynthesis. Interestingly, inhibition of the sphingolipid biosynthesis by specific inhibitors of L-serine palmitoyltransferase diminished the anti-proliferative potency of celecoxib. In conclusion, induction of de novo synthesis of sphingolipids and inhibition of DEGS contribute to the anti-proliferative effects of celecoxib.-Schiffmann, S.The selective COX-2 inhibitor celecoxib modulates sphingolipid synthesis. J. Lipid Res. 2009. 50: 32-40. Supplementary key words cancer • cyclooxygenase-2 • apoptosis • dihydroceramide desaturase • ceramide • dihydroceramide Sphingolipids constitute an essential component of the eucaryotic plasma membrane and are also utilized as an important second messenger in a variety of cellular events, including cell senescence, cellular differentiation, apoptosis, and proliferation (1-3). Endogenous sphingolipid lev-els can be controlled by activation of sphingomyelinases (SMases) and de novo synthesis as well as by specific degradation mechanisms (e.g., ceramidases, lyases). The sphingolipid biosynthesis commences in the endoplasmic reticulum with the condensation of palmitoyl-CoA and L-serine by L-serine palmitoyltransferase (L-SPT). The intermediate 3-ketosphinganine is rapidly converted into sphinganine (dhSph) by 3-ketosphinganine reductase. Acyl-CoA thioesters of variable chain lengths are then attached to dhSph by chain-length-specific (dihydro)ceramide synthases (CerSs) (4). Dihydroceramide desaturase (DEGS) introduces a 4, 5-trans double bond in dihydroceramides (dhCers), resulting in formation of the final product, Cer. Via sphingosine (Sph), Cer is metabolized by the enzymes ceramidase and sphingosine kinase to sphingosine-1-phosphate (Sph1P)