Characterization of ceramide synthases (Cers) in mammalian cells

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Park, Hyejung
Merrill, Alfred H.
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This thesis describes the characterization of ceramide (Cer) biosynthesis by mammalian cells. The possibility that Cer undergo developmental changes was explored using mouse embryonic stem cells versus embryoid bodies by analysis of the Cer subspecies by liquid chromatography, electrospray ionization-tandem mass spectrometry (LC ESI-MS/MS) and of the transcript levels for enzymes involved in Cer biosynthesis by qRT-PCR. Cer of embroid bodies had higher proportions of very-long-chain fatty acids, which correlated with the relative expression of mRNA for the respective Cer synthases (CerS) and fatty acyl-CoA elongases, as well as changes in the fatty acyl-CoA's of the cells. Therefore, it is clear that Cer subspecies change during embryogenesis, possibly for functionally important reasons. One CerS isoform, CerS2, was studied further because it has the broadest tissue distribution and a remarkable fatty acyl-CoA specificity, utilizing longer acyl-chain CoAs (C20-C26) in vitro. The fatty acid chain selectivity was refined by analysis of the Cer from livers from CerS2 null mice, which displayed very little Cer with fatty acyl chains with 24 + 2 carbons. Another interesting structural variation was discovered in studies of cells treated with fumonisin B1 (FB1), which inhibits CerS. Under these conditions, cells in culture and animals accumulate substantial amounts of a novel sphingoid base that was identified as 1-deoxysphinganine. This compound arises from utilization of L-alanine instead of L-serine by serine palmitoyltransferase (SPT) based on the inability of LYB cells, which lack SPT, to make 1-deoxysphinganine. In the absence of FB1, 1-deoxysphinganine is primarily acylated to 1-deoxydihydroceramides. These are an underappreciated category of bioactive sphingoid bases and "ceramides" that might play important roles in cell regulation and disease. In summary, cells contain a wide variety of Cer subspecies that are determined by changes in expression of CerS, enzymes that produce co-substrates (such as fatty acyl-CoAs), and the types of amino acids utilized by SPT, the initial enzyme of de novo sphingolipid biosynthesis. One can envision how these changes might impact membranes structure as well as signaling by this family of highly bioactive compounds.
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