Sphingolipidome of plasma, liver, and adipose tissues and its association with insulin response to oral glucose testing in Icelandic horses.

Insulin dysregulation (ID) is a determinant of equine metabolic syndrome. Among the sphingolipids, ceramides contribute to the development of ID; however, the cross talk between the liver and adipose tissue (AT) depots and the variation among AT depots in terms of ceramide metabolism are not well understood. We aimed to characterize the sphingolipidome of plasma, liver, and AT (nuchal, NUAT; subcutaneous, SCAT; omental, OMAT; retroperitoneal, RPAT) and their associations with insulin response to oral glucose testing (OGT) in normoinsulinemic and hyperinsulinemic horses. Plasma, liver, and AT samples were collected from 12 Icelandic horses upon euthanasia and analyzed by liquid chromatography-mass spectrometry. Eighty-four targeted compounds were effectively quantified. Comparing the AT depots, greater (false discovery rate, FDR < 0.05) ceramide, dihydroceramide, and sphingomyelin concentrations and lower glucosyl- and galactosyl-ceramides were found in RPAT and OMAT than in NUAT and SCAT. Hyperinsulinemic response to OGT was associated with sphingolipidome alterations primarily in the RPAT and OMAT, whereas the NUAT sphingolipidome did not show signs of ceramide accumulation, which was inconsistent with the previously proposed role of nuchal adiposity in ID. The plasma sphingolipidome was not significantly associated with the liver or AT sphingolipidomes, indicating that plasma profiles are determined by an interplay of various organs. Furthermore, hepatic sphingolipid profiles were not correlated with the profiles of AT depots. Finally, statistically valid partial least square regression models predicting insulin response were found in the plasma (Q2 = 0.58, R2 = 0.98), liver (Q2 = 0.64, R2 = 0.74), and RPAT (Q2 = 0.68, R2 = 0.79) sphingolipidome, but not in the other adipose tissues.

Associations of plasma sphingolipid profiles with insulin response during oral glucose testing in Icelandic horses.

BACKGROUND: Sphingolipids modulate insulin sensitivity in mammals. Increased synthesis of ceramides is linked to decreased insulin sensitivity of tissues. Conversely, activation of the insulin signaling pathway can downregulate ceramide synthesis. Elucidating the association between sphingolipid metabolism and insulin response during oral glucose testing may help explain the pathophysiology of insulin dysregulation in horses. HYPOTHESES: Horses with insulin dysregulation will have a plasma sphingolipid profile characterized by increased ceramide concentrations. The plasma sphingolipid profile will have decreased ceramide concentrations after acute activation of the insulin signaling pathway by oral glucose testing. ANIMALS: Twelve Icelandic horses. METHODS: Horses were subjected to an oral glucose test (0.5 g/kg body weight glucose), with plasma insulin concentrations measured at 0, 30, 60, 120, 180, and 240 minutes postglucose administration. Plasma samples were collected at 0 and 120 minutes for sphingolipid profiling using a liquid chromatography-mass spectrometry-based metabolomics analysis. Eighty-three species of sphingolipids were detected, including 3-ketosphinganines, dihydroceramides, ceramides, dihydrosphingomyelins, sphingomyelins, galatosylceramides, glucosylceramides, lactosylceramides, and ceramide-1-phosphates. RESULTS: Glucose administration did not significantly alter plasma sphingolipid profiles. C22:0-ceramide, C24:1-ceramide, C23:0-ceramide, C16:1-sphingomyelin, C22:0-dihydroceramide, and C24:0-ceramide were positively correlated with the insulin response (area under the curve). CONCLUSION AND CLINICAL IMPORTANCE: Positive correlation between the insulin response and sphingolipid concentrations implies upregulated sphingolipid metabolism in insulin dysregulated horses. A high plasma ceramide concentration can indicate insulin dysregulation in horses.