Inhibition of atherosclerosis by the serine palmitoyl transferase inhibitor myriocin is associated with reduced plasma glycosphingolipid concentration

Lipid oversupply, selective insulin resistance, and lipotoxicity: molecular mechanisms

The accumulation of fat in tissues not suited for lipid storage has deleterious consequences on organ function, leading to cellular damage that underlies diabetes, heart disease, and hypertension. To combat these lipotoxic events, several therapeutics improve insulin sensitivity and/or ameliorate features of metabolic disease by limiting the inappropriate deposition of fat in peripheral tissues (i.e. thiazolidinediones, metformin, and statins). Recent advances in genomics and lipidomics have accelerated progress towards understanding the pathogenic events associated with the excessive production, underutilization, or inefficient storage of fat. Herein we review studies applying pharmacological or genetic strategies to manipulate the expression or activity of enzymes controlling lipid deposition, in order to gain a clearer understanding of the molecular mechanisms by which fatty acids contribute to metabolic disease.

Cell polarity factor Par3 binds SPTLC1 and modulates monocyte serine palmitoyltransferase activity and chemotaxis

Elevated sphingolipids have been associated with increased cardiovascular disease. Conversely, atherosclerosis is reduced in mice by blocking de novo synthesis of sphingolipids catalyzed by serine palmitoyltransferase (SPT). The SPT enzyme is composed of the SPTLC1 and -2 subunits, and here we describe a novel protein-protein interaction between SPTLC1 and the PDZ protein Par3 (partitioning defective protein 3). Mammalian SPTLC1 orthologs have a highly conserved C terminus that conforms to a type II PDZ protein interaction motif, and by screening PDZ domain protein arrays with an SPTLC1 C-terminal peptide, we found it bound the third PDZ domain of Par3. Overlay and immunoprecipitation assays confirmed this interaction and indicate Par3 is able to associate with the SPTLC1/2 holoenzyme by binding the C-terminal SPTLC1 PDZ motif. The physiologic existence of the SPTLC1/2-Par3 complex was detected in mouse liver and macrophages, and short interfering RNA inhibition of Par3 in human THP-1 monocytes significantly reduced SPT activity and de novo ceramide synthesis by nearly 40%. Given monocyte recruitment into inflamed vessels is thought to promote atherosclerosis, and because Par3 and sphingolipids have been associated with polarized cell migration, we tested whether the ability of THP-1 monocytes to migrate toward MCP-1 (monocyte chemoattractant protein 1) depended upon Par3 and SPTLC1 expression. Knockdown of Par3 significantly reduced MCP1-induced chemotaxis of THP-1 monocytes, as did knockdown of SPTLC1, and this Par3 effect depended upon SPT activity and was blunted by ceramide treatment. In conclusion, protein arrays were used to identify a novel SPTLC1-Par3 interaction that associates with increased monocyte serine palmitoyltransferase activity and chemotaxis toward inflammatory signals.

Serine palmitoyltransferase (SPT) deficient mice absorb less cholesterol

Serine palmitoyltransferase (SPT) is the key enzyme for the biosynthesis of sphingolipids. It has been reported that oral administration of myriocin (an SPT inhibitor) decreases plasma sphingomyelin (SM) and cholesterol levels, and reduces atherosclerosis in apoE knockout (KO) mice. We studied cholesterol absorption in myriocin-treated WT or apoE KO animals and found that, after myriocin treatment, the mice absorbed significantly less cholesterol than controls, with no observable pathological changes in the small intestine. More importantly, we found that heterozygous Sptlc1 (a subunit of SPT) KO mice also absorbed significantly less cholesterol than controls. To understand the mechanism, we measured protein levels of Niemann-Pick C1-like 1 (NPC1L1), ABCG5, and ABCA1, three key factors involved in intestinal cholesterol absorption. We found that NPC1L1 and ABCA1 were decreased, whereas ABCG5 was increased in the SPT deficient small intestine. SM levels on the apical membrane were also measured and they were significantly decreased in SPT deficient mice, compared with controls. In conclusion, SPT deficiency might reduce intestinal cholesterol absorption by altering NPC1L1 and ABCG5 protein levels in the apical membranes of enterocytes through lowering apical membrane SM levels. This may be also true for ABCA1 which locates on basal membrane of enterocytes. Manipulation of SPT activity could thus provide a novel alternative treatment for dyslipidemia.

Ceramide signaling in cancer and stem cells

Most of the previous work on the sphingolipid ceramide has been devoted to its function as an apoptosis inducer. Recent studies, however, have shown that in stem cells, ceramide has additional nonapoptotic functions. In this article, ceramide signaling will be reviewed in light of 'systems interface biology': as an interconnection of sphingolipid metabolism, membrane biophysics and cell signaling. The focus will be on the metabolic interconversion of ceramide and sphingomyelin or sphingosine-1-phosphate. Lipid rafts and sphingolipid-induced protein scaffolds will be discussed as a membrane interface for lipid-controlled cell signaling. Ceramide/sphingomyelin and ceramide/sphingosine-1-phosphate-interdependent cell-signaling pathways are significant for the regulation of cell polarity, apoptosis and/or proliferation, and as novel pharmacologic targets in cancer and stem cells.

Sphingolipids, insulin resistance, and metabolic disease: new insights from in vivo manipulation of sphingolipid metabolism

Obesity and dyslipidemia are risk factors for metabolic disorders including diabetes and cardiovascular disease. Sphingolipids such as ceramide and glucosylceramides, while being a relatively minor component of the lipid milieu in most tissues, may be among the most pathogenic lipids in the onset of the sequelae associated with excess adiposity. Circulating factors associated with obesity (e.g., saturated fatty acids, inflammatory cytokines) selectively induce enzymes that promote sphingolipid synthesis, and lipidomic profiling reveals relationships between tissue sphingolipid levels and certain metabolic diseases. Moreover, studies in cultured cells and isolated tissues implicate sphingolipids in certain cellular events associated with diabetes and cardiovascular disease, including insulin resistance, pancreatic beta-cell failure, cardiomyopathy, and vascular dysfunction. However, definitive evidence that sphingolipids contribute to insulin resistance, diabetes, and atherosclerosis has come only recently, as researchers have found that pharmacological inhibition or genetic ablation of enzymes controlling sphingolipid synthesis in rodents ameliorates each of these conditions. Herein we will review the role of ceramide and other sphingolipid metabolites in insulin resistance, beta-cell failure, cardiomyopathy, and vascular dysfunction, focusing on these in vivo studies that identify enzymes controlling sphingolipid metabolism as therapeutic targets for combating metabolic disease.

SPTLC1 binds ABCA1 to negatively regulate trafficking and cholesterol efflux activity of the transporter

ABCA1 transport of cholesterol and phospholipids to nascent HDL particles plays a central role in lipoprotein metabolism and macrophage cholesterol homeostasis. ABCA1 activity is regulated both at the transcriptional level and at the post-translational level. To explore mechanisms involved in the post-translational regulation of the transporter, we have used affinity purification and mass spectrometry to identify proteins that bind ABCA1 and influence its activity. Previously, we demonstrated that an interaction between beta1-syntrophin stimulated ABCA1 activity, at least in part, be slowing the degradation of the transporter. This work demonstrates that one subunit of the serine palmitoyltransferase enzyme, SPTLC1, but not subunit 2 (SPTLC2), is copurified with ABCA1 and negatively regulates its function. In human THP-I macrophages and in mouse liver, the ABCA1-SPTLC1 complex was detected by co-immunoprecipitation, demonstrating that the interaction occurs in cellular settings where ABCA1 activity is critical for HDL genesis. Pharmacologic inhibition of SPTLC1 with myriocin, which resulted in the disruption of the SPTLC1-ABCA1 complex, and siRNA knockdown of SPTLC1 expression both stimulated ABCA1 efflux by nearly 60% ( p < 0.05). In contrast, dominant-negative mutants of SPTLC1 inhibited ABCA1 efflux, indicating that a reduced level of sphingomyelin synthesis could not explain the effect of myriocin on ABCA1 activity. In 293 cells, the SPTLC1 inhibition of ABCA1 activity led to the blockade of the exit of ABCA1 from the endoplasmic reticulum. In contrast, myriocin treatment of macrophages increased the level of cell surface ABCA1. In composite, these results indicate that the physical interaction of ABCA1 and SPTLC1 results in reduction of ABCA1 activity and that inhibition of this interaction produces enhanced cholesterol efflux.

Reduction of plasma glycosphingolipid levels has no impact on atherosclerosis in apolipoprotein E-null mice

Glycosphingolipids (GSLs) have been implicated as potential atherogenic lipids. Studies in apolipoprotein E-null (apoE(-/-)) mice indicate that exacerbated tissue GSL accumulation resulting from alpha-galactosidase deficiency promotes atherosclerosis, whereas the serine palmitoyl transferase inhibitor myriocin (which reduces plasma and tissue levels of several sphingolipids, including sphingomyelin, ceramide, sphingosine-1-phosphate, and GSLs) inhibits atherosclerosis. It is not clear whether GSL synthesis inhibition per se has an impact on atherosclerosis. To address this issue, apoE(-/-) mice maintained on a high-fat diet were treated with a potent glucosylceramide synthesis inhibitor, d-threo-1-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidino-propanol (EtDO-P4), 10 mg/kg/day for 94 days, and lesion development was compared in mice that were treated with vehicle only. EtDO-P4 reduced plasma GSL concentration by approximately 50% but did not affect cholesterol or triglyceride levels. Assessment of atherosclerotic lesions at four different sites indicated that EtDO-P4 had no significant impact on lesion area. Thus, despite the previously observed positive correlations between plasma and aortic GSL concentrations and the development of atherosclerosis, and the in vitro evidence implying that GSLs may be pro-atherogenic, our current data indicate that inhibition of GSL synthesis does not inhibit atherosclerosis in vivo.