The associations between individual plasma SFAs, serine palmitoyl-transferase long-chain base subunit 3 gene rs680379 polymorphism, and type 2 diabetes among Chinese adults.

BACKGROUND: Several individual studies have shown that circulating levels of odd-chain SFAs and very-long-chain SFAs (VLSFAs) may have beneficial effects, but the results are mixed. While the dietary and metabolic factors that may influence VLSFAs are not well-known, a previous study observed associations of VLSFA concentrations with variants in serine palmitoyl-transferase long-chain base subunit 3 (SPTLC3) gene. OBJECTIVES: We investigated the associations of individual plasma SFAs and SPTLC3 gene rs680379 polymorphism with metabolic risk factors and type 2 diabetes (T2D). METHODS: We measured plasma SFAs using gas chromatography among 898 T2D cases and 1618 matched controls, and genotyped the SPTLC3 gene rs680379 polymorphism using the MassArray System among 1178 T2D cases and 1907 matched controls. Conditional logistic regression was used to estimate ORs and 95% CIs. RESULTS: We found that plasma odd-chain SFAs and VLSFAs were correlated with favorable blood lipids and insulin resistance marker profiles. After multivariable adjustment, pentadecanoic acid (15:0) was inversely associated with the odds of T2D (OR per 1 SD difference: 0.63; 95% CI: 0.57, 0.70), as were measurements of 3 individual VLSFAs [arachidic acid (20:0), behenic acid (22:0), and lignoceric acid (24:0)], with ORs ranging from 0.60 to 0.72 (95% CIs ranging between 0.52 and 0.79). The associations between 3 individual VLSFAs and T2D were attenuated after further adjustment for triglycerides. Meanwhile, compared with the rs680379 GG genotype carriers, the ORs of T2D for the GA and AA genotype carriers were 0.81 (95% CI: 0.68-0.97) and 0.76 (95% CI: 0.61-0.96), respectively. CONCLUSIONS: Plasma 15:0 and VLSFAs were inversely associated with T2D. Meanwhile, compared with the rs680379 GG genotype carriers, subjects with GA and AA genotypes were associated with decreased odds of T2D. More investigations are warranted to confirm our findings.

Clinical and metabolic consequences of L-serine supplementation in hereditary sensory and autonomic neuropathy type 1C.

Hereditary sensory neuropathy type 1 (HSAN1) may be the first genetic neuropathy amenable to a specific mechanism-based treatment, as L-serine supplementation can be used to lower the neurotoxic levels of 1-deoxysphingolipids (1-deoxySL) that cause the neurodegeneration. The treatment is so far untested in HSAN1C caused by variants in the serine palmitoyl transferase subunit 2 (SPTLC2) gene. The aim of this study was to establish whether oral L-serine lowers 1-deoxySL in a patient with HSAN1C, to perform a dose escalation to find the minimal effective dose, and to assess the safety profile and global metabolic effects of the treatment. Our patient underwent a 52-wk treatment in which the L-serine dose was titrated up to 400 mg/kg/day. She was followed up by repeated clinical examination, nerve conduction testing, and skin biopsies to document effects on small nerve fibers. Serum was assayed for 1-deoxySL and metabolomics analysis of 111 metabolites. We found a robust lowering of 1-deoxySL, which correlated in a near-linear fashion with increased serum L-serine levels. Metabolomics analysis showed a modest elevation in glycine and a marked reduction in the level of cytosine, whereas most of the other assayed metabolites did not change. There were no direct side effects from the treatment, but the patient developed a transitory toe ulceration during the course of the study. The Charcot-Marie-Tooth neuropathy score increased by 1 point. We conclude that oral supplementation of L-serine decreases 1-deoxySL in HSAN1C without major global effects on metabolism. L-serine is therefore a potential treatment for HSAN1C.

HSAN1 mutations in serine palmitoyltransferase reveal a close structure-function-phenotype relationship.

Hereditary sensory and autonomic neuropathy type 1 (HSAN1) is a rare autosomal dominant inherited peripheral neuropathy caused by mutations in the SPTLC1 and SPTLC2 subunits of serine palmitoyltransferase (SPT). The mutations induce a permanent shift in the substrate preference from L-serine to L-alanine, which results in the pathological formation of atypical and neurotoxic 1-deoxy-sphingolipids (1-deoxySL). Here we compared the enzymatic properties of 11 SPTLC1 and six SPTLC2 mutants using a uniform isotope labelling approach. In total, eight SPT mutants (STPLC1p.C133W, p.C133Y, p.S331F, p.S331Y and SPTLC2p.A182P, p.G382V, p.S384F, p.I504F) were associated with increased 1-deoxySL synthesis. Despite earlier reports, canonical activity with l-serine was not reduced in any of the investigated SPT mutants. Three variants (SPTLC1p.S331F/Y and SPTLC2p.I505Y) showed an increased canonical activity and increased formation of C20 sphingoid bases. These three mutations are associated with an exceptionally severe HSAN1 phenotype, and increased C20 sphingosine levels were also confirmed in plasma of patients. A principal component analysis of the analysed sphingoid bases clustered the mutations into three separate entities. Each cluster was related to a distinct clinical outcome (no, mild and severe HSAN1 phenotype). A homology model based on the protein structure of the prokaryotic SPT recapitulated the same grouping on a structural level. Mutations associated with the mild form clustered around the active site, whereas mutations associated with the severe form were located on the surface of the protein. In conclusion, we showed that HSAN1 mutations in SPT have distinct biochemical properties, which allowed for the prediction of the clinical symptoms on the basis of the plasma sphingoid base profile.

Plasma sphingolipids are biomarkers of metabolic syndrome in non-human primates maintained on a Western-style diet.

BACKGROUND: The intake of a Western diet enriched in animal fat has been shown to be a major risk factor for Type 2 diabetes and obesity. Previous rodent studies have indicated that these conditions may be triggered by the accumulation of the sphingolipid ceramide in insulin-sensitive tissues. However, data are lacking in this regard from both humans and non-human primates. OBJECTIVE: Here we have investigated the relationship between plasma ceramides and metabolic syndrome in Rhesus macaques fed a high-fat and high-fructose (HFFD) 'western' diet. METHODS: We investigated this relationship in cohorts of monkeys fed a HFFD for a period of 8 months to 5 years. Animals were classified as control, pre-diabetic or diabetic based on fasting plasma parameters and insulin sensitivity. RESULTS: HFFD treatment produced significant increases in body weight and body fat and also resulted in a decline in insulin sensitivity. In parallel to the reduction in insulin sensitivity, significant increases in both plasma ceramide and dihydroceramide levels were observed, which further increased as animals progressed to the diabetic state. Plasma levels of the rare sphingolipid C18:0 deoxysphinganine, a marker of increased metabolic flux through serine palmitoyl transferase (SPT), were also elevated in both pre- and diabetic animals. Furthermore, plasma serine levels were significantly elevated in diabetic monkeys, which may indicate a shift in SPT substrate selectivity from serine to alanine or glycine. In contrast, branch chain amino acids were unchanged in pre-diabetic non-human primates, and only plasma valine levels were elevated in diabetic animals. CONCLUSION: Together, these data indicate that HFFD induces de novo synthesis of ceramides in non-human primates, and that increased production of plasma ceramides is significantly correlated with the decline in insulin sensitivity.

Plasma deoxysphingolipids: a novel class of biomarkers for the metabolic syndrome?

AIMS/HYPOTHESIS: Sphingolipid synthesis is typically initiated by the conjugation of L-serine and palmitoyl-CoA, a reaction catalysed by serine palmitoyltransferase (SPT). SPT can also metabolise other acyl-CoAs (C(12) to C(18)) and other amino acids such as L-alanine and glycine, giving rise to a spectrum of atypical sphingolipids. Here, we aimed to identify changes in plasma levels of these atypical sphingolipids to explore their potential as biomarkers in the metabolic syndrome and diabetes. METHODS: We compared the plasma profiles of ten sphingoid bases in healthy individuals with those of patients with the metabolic syndrome but not diabetes, and diabetic patients (n = 25 per group). The results were verified in a streptozotocin (STZ) rat model. Univariate and multivariate statistical analyses were used. RESULTS: Deoxysphingolipids (dSLs) were significantly elevated (p = 5 × 10⁻6) in patients with the metabolic syndrome (0.11 ± 0.04 µmol/l) compared with controls (0.06 ± 0.02 µmol/l) but did not differ between the metabolic syndrome and diabetes groups. Levels of C(16)-sphingosine-based sphingolipids were significantly lowered in diabetic patients but not in patients with the metabolic syndrome but without diabetes (p = 0.008). Significantly elevated dSL levels were also found in the plasma and liver of STZ rats. A principal component analysis revealed a similar or even closer association of dSLs with diabetes and the metabolic syndrome in comparison with the established biomarkers. CONCLUSIONS/INTERPRETATION: We showed that dSLs are significantly elevated in patients with type 2 diabetes mellitus and non-diabetic metabolic syndrome compared with healthy controls. They may, therefore, be useful novel biomarkers to improve risk prediction and therapy monitoring in these patients.

The SPTLC3 subunit of serine palmitoyltransferase generates short chain sphingoid bases.

The enzyme serine palmitoyltransferase (SPT) catalyzes the rate-limiting step in the de novo synthesis of sphingolipids. Previously the mammalian SPT was described as a heterodimer composed of two subunits, SPTLC1 and SPTLC2. Recently we identified a novel third SPT subunit (SPTLC3). SPTLC3 shows about 68% identity to SPTLC2 and also includes a pyridoxal phosphate consensus motif. Here we report that the overexpression of SPTLC3 in HEK293 cells leads to the formation of two new sphingoid base metabolites, namely C(16)-sphinganine and C(16)-sphingosine. SPTLC3-expressing cells have higher in vitro SPT activities with lauryl- and myristoyl-CoA than SPTLC2-expressing cells, and SPTLC3 mRNA expression levels correlate closely with the C(16)-sphinganine synthesis rates in various human and murine cell lines. Approximately 15% of the total sphingolipids in human plasma contain a C(16) backbone and are found in the high density and low density but not the very low density lipoprotein fraction. In conclusion, we show that the SPTLC3 subunit generates C(16)-sphingoid bases and that sphingolipids with a C(16) backbone constitute a significant proportion of human plasma sphingolipids.