Sphingomyelin synthase-related protein SMSr is a phosphatidylethanolamine phospholipase C that promotes nonalcoholic fatty liver disease.

Sphingomyelin synthase (SMS)-related protein (SMSr) is a phosphatidylethanolamine phospholipase C (PE-PLC) that is conserved and ubiquitous in mammals. However, its biological function is still not clear. We previously observed that SMS1 deficiency-mediated glucosylceramide accumulation caused nonalcoholic fatty liver diseases (NAFLD), including nonalcoholic steatohepatitis (NASH) and liver fibrosis. Here, first, we evaluated high-fat diet/fructose-induced NAFLD in Smsr KO and WT mice. Second, we evaluated whether SMSr deficiency can reverse SMS1 deficiency-mediated NAFLD, using Sms1/Sms2 double and Sms1/Sms2/Smsr triple KO mice. We found that SMSr/PE-PLC deficiency attenuated high-fat diet/fructose-induced fatty liver and NASH, and attenuated glucosylceramide accumulation-induced NASH, fibrosis, and tumor formation. Further, we found that SMSr/PE-PLC deficiency reduced the expression of many inflammatory cytokines and fibrosis-related factors, and PE supplementation in vitro or in vivo mimicked the condition of SMSr/PE-PLC deficiency. Furthermore, we demonstrated that SMSr/PE-PLC deficiency or PE supplementation effectively prevented membrane-bound ß-catenin transfer to the nucleus, thereby preventing tumor-related gene expression. Finally, we observed that patients with NASH had higher SMSr protein levels in the liver, lower plasma PE levels, and lower plasma PE/phosphatidylcholine ratios, and that human plasma PE levels are negatively associated with tumor necrosis factor-α and transforming growth factor ß1 levels. In conclusion, SMSr/PE-PLC deficiency causes PE accumulation, which can attenuate fatty liver, NASH, and fibrosis. These results suggest that SMSr/PE-PLC inhibition therapy may mitigate NAFLD.

Effect of Total SMS Activity on LDL Catabolism in Mice.

BACKGROUND: Sphingomyelin (SM) and cholesterol are 2 key lipid partners on cell membranes and on lipoproteins. Many studies have indicated the influence of cholesterol on SM metabolism. This study examined the influence of SM biosynthesis on cholesterol metabolism. METHODS: Inducible global Sms1 KO (knockout)/global Sms2 KO mice were prepared to evaluate the effect of whole-body SM biosynthesis deficiency on lipoprotein metabolism. Tissue cholesterol, SM, ceramide, and glucosylceramide levels were measured. Triglyceride production rate and LDL (low-density lipoprotein) catabolism were measured. Lipid rafts were isolated and LDL receptor mass and function were evaluated. Also, the effects of exogenous sphingolipids on hepatocytes were investigated. RESULTS: We found that total SMS (SM synthase) depletion significantly reduced plasma SM levels. Also, the total deficiency significantly induced plasma cholesterol, apoB (apolipoprotein B), and apoE (apolipoprotein E) levels. Importantly, total SMS deficiency, but not SMS2 deficiency, dramatically decreased LDL receptors in the liver and attenuated LDL uptake through the receptor. Further, we found that total SMS deficiency greatly reduced LDL receptors in the lipid rafts, which contained significantly lower SM and significantly higher glucosylceramide, as well as cholesterol. Furthermore, we treated primary hepatocytes and Huh7 cells (a human hepatoma cell line) with SM, ceramide, or glucosylceramide, and we found that only SM could upregulate LDL receptor levels in a dose-dependent fashion. CONCLUSIONS: Whole-body SM biosynthesis plays an important role in LDL cholesterol catabolism. The total SMS deficiency, but not SMS2 deficiency, reduces LDL uptake and causes LDL cholesterol accumulation in the circulation. Given the fact that serum SM level is a risk factor for cardiovascular diseases, inhibiting SMS2 but not SMS1 should be the desirable approach.

Alterations to Sphingomyelin Metabolism Affect Hemostasis and Thrombosis.

BACKGROUND: Our recent studies suggest that sphingomyelin levels in the plasma membrane influence TF (tissue factor) procoagulant activity. The current study was performed to investigate how alterations to sphingomyelin metabolic pathway would affect TF procoagulant activity and thereby affect hemostatic and thrombotic processes. METHODS: Macrophages and endothelial cells were transfected with specific siRNAs or infected with adenoviral vectors to alter sphingomyelin levels in the membrane. TF activity was measured in factor X activation assay. Saphenous vein incision-induced bleeding and the inferior vena cava ligation-induced flow restriction mouse models were used to evaluate hemostasis and thrombosis, respectively. RESULTS: Overexpression of SMS (sphingomyelin synthase) 1 or SMS2 in human monocyte-derived macrophages suppresses ATP-stimulated TF procoagulant activity, whereas silencing SMS1 or SMS2 increases the basal cell surface TF activity to the same level as of ATP-decrypted TF activity. Consistent with the concept that sphingomyelin metabolism influences TF procoagulant activity, silencing of acid sphingomyelinase or neutral sphingomyelinase 2 or 3 attenuates ATP-induced enhanced TF procoagulant activity in macrophages and endothelial cells. Niemann-Pick disease fibroblasts with a higher concentration of sphingomyelin exhibited lower TF activity compared with wild-type fibroblasts. In vivo studies revealed that LPS+ATP-induced TF activity and thrombin generation were attenuated in ASMase-/- mice, while their levels were increased in SMS2-/- mice. Further studies revealed that acid sphingomyelinase deficiency leads to impaired hemostasis, whereas SMS2 deficiency increases thrombotic risk. CONCLUSIONS: Overall, our data indicate that alterations in sphingomyelin metabolism would influence TF procoagulant activity and affect hemostatic and thrombotic processes.

Atomic force microscopy reveals involvement of the cell envelope in biomechanical properties of sickle erythrocytes.

BACKGROUND: Intracellular hemoglobin polymerization has been supposed to be the major determinant for the elevated rigidity/stiffness of sickle erythrocytes from sickle cell anemia (SCA) patients. However, the contribution of the cell envelope remains unclear. RESULTS: In this study, using atomic force microscopy (AFM), we compared the normal and sickled erythrocyte surfaces for stiffness and topography. AFM detected that sickle cells had a rougher surface and were stiffer than normal erythrocytes and that sickle cell ghosts had a rougher surface (for both outer and inner surfaces) and were thicker than normal ghosts, the latter implying a higher membrane-associated hemoglobin content/layer in the sickle cell envelope. Compared to healthy subjects, the SCA patients had lower plasma lipoprotein levels. AFM further revealed that a mild concentration of methyl-ß-cyclodextrin (MßCD, a putative cholesterol-depleting reagent) could induce an increase in roughness of erythrocytes/ghosts and a decrease in thickness of ghosts for both normal and sickle cells, implying that MßCD can alter the cell envelope from outside (cholesterol in the plasma membrane) to inside (membrane-associated hemoglobin). More importantly, MßCD also caused a more significant decrease in stiffness of sickle cells than that of normal erythrocytes. CONCLUSIONS: The data reveal that besides the cytosolic hemoglobin fibers, the cell envelope containing the membrane-associated hemoglobin also is involved in the biomechanical properties (e.g., stiffness and shape maintenance) of sickle erythrocytes.

Sphingomyelin synthases 1 and 2 exhibit phosphatidylcholine phospholipase C activity.

Many studies have confirmed the enzymatic activity of a mammalian phosphatidylcholine (PC) phospholipase C (PLC) (PC-PLC), which produces diacylglycerol (DAG) and phosphocholine through the hydrolysis of PC in the absence of ceramide. However, the protein(s) responsible for this activity have never yet been identified. Based on the fact that tricyclodecan-9-yl-potassium xanthate can inhibit both PC-PLC and sphingomyelin synthase (SMS) activities, and SMS1 and SMS2 have a conserved catalytic domain that could mediate a nucleophilic attack on the phosphodiester bond of PC, we hypothesized that both SMS1 and SMS2 might have PC-PLC activity. In the present study, we found that purified recombinant SMS1 and SMS2 but not SMS-related protein have PC-PLC activity. Moreover, we prepared liver-specific Sms1/global Sms2 double-KO mice. We found that liver PC-PLC activity was significantly reduced and steady-state levels of PC and DAG in the liver were regulated by the deficiency, in comparison with control mice. Using adenovirus, we expressed Sms1 and Sms2 genes in the liver of the double-KO mice, respectively, and found that expressed SMS1 and SMS2 can hydrolyze PC to produce DAG and phosphocholine. Thus, SMS1 and SMS2 exhibit PC-PLC activity in vitro and in vivo.

Long-term effect of human mini-dystrophin in transgenic mdx mice improves muscle physiological function.

Duchenne muscular dystrophy (DMD) is a lethal genetic muscle disorder caused by recessive mutations in dystrophin gene, affecting 1/3000 males. Gene therapy has been proven to ameliorate dystrophic pathology. To investigate therapeutic benefits from long-term effect of human mini-dystrophin and functional outcomes, transgenic mdx mice (Tg-mdx) containing a single copy of human mini-dystrophin (∆hDys3849) gene, five rods (Rods1-2, Rods22-24), and two hinges (H1 and H4) driven by a truncated creatine-kinase promoter (dMCK) in a recombinant adeno-associated viral vector (rAAV) backbone, were generated and used to determine gene expression and improvement of muscle function. Human mini-dystrophin gene expression was found in a majority of the skeletal muscles, but no expression in cardiac muscle. Dystrophin-associated glycoproteins (DAGs) such as sarcoglycans and nNOS were restored at the sarcolemma and coincided with human mini-dystrophin gene expression at the ages of 6, 10, and 20 months; Morphology of dystrophic muscle expressing the human mini-dystrophin gene was improved and central nuclei were reduced. Myofiber membrane integrity was improved by Evans blue dye test. Improvement in treadmill running and grip force was observed in transgenic mice at 6 months. Tetanic force and specific force of tibialis anterior (TA) muscle were significantly increased at the ages of 6, 10, and 20 months. Pseudohypertrophy was not found in TA muscle at 10 and 20 months when compared with wild-type C57 (WT) group. This study demonstrated that the long-term effects of human mini-dystrophin effectively ameliorated pathology and improved the functions of the dystrophic muscles in the transgenic DMD mouse model.

Sphingolipids and Cholesterol.

Sphingolipids and cholesterol are two lipid partners on cellular membranes where they form specific microdomains, named lipid rafts, which mediate specific cell functions. Sphingomyelin (SM) is one of the major sphingolipids. SM and free cholesterol are also two key lipids on the monolayer of plasma lipoproteins, including chylomicron, very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL), which participate in lipid transport in the circulation. Thus, sphingolipids and cholesterol play a fundamental role in cell membrane structure and blood lipid transport. In this chapter we will discuss the relationship between both lipids, on the cell membrane and in the circulation, as well as the impact of such relationship in the development of metabolic diseases.

Sphingomyelin Synthase Family and Phospholipase Cs.

The sphingomyelin synthase (SMS) gene family has three members: SMS1 and SMS2 have SM synthase activity, while SMS-related protein (SMSr) has no SM synthase activity but has ceramide phosphorylethanolamine (CPE) synthase activity in vitro. Recently, we found that SMS family members are a group of phospholipase Cs (PLC). SMS1 and SMS2 are two phosphatidylcholine (PC)-PLCs and SMSr is a phosphatidylethanolamine (PE)-PLC. SMS family members not only influence SM levels but also influence the levels of diacylglycerol (DAG), PC, PE, and glycosphingolipids, thus influencing cell functions. In this chapter, we will discuss the recent progress in the research field of SMS family and will focus on its impact on metabolic diseases.

The Role of Phospholipid Transfer Protein in the Development of Atherosclerosis.

PURPOSE OF REVIEW: Phospholipid transfer protein (PLTP), a member of lipid transfer protein family, is an important protein involved in lipid metabolism in the circulation. This article reviews recent PLTP research progresses, involving lipoprotein metabolism and atherogenesis. RECENT FINDINGS: PLTP activity influences atherogenic and anti-atherogenic lipoprotein levels. Human serum PLTP activity is a risk factor for human cardiovascular disease and is an independent predictor of all-cause mortality. PLTP deficiency reduces VLDL and LDL levels and attenuates atherosclerosis in mouse models, while PLTP overexpression exerts an opposite effect. Both PLTP deficiency and overexpression result in reduction of HDL which has different size, inflammatory index, and lipid composition. Moreover, although both PLTP deficiency and overexpression reduce cholesterol efflux capacity, but this effect has no impact in macrophage reverse cholesterol transport in mice. Furthermore, PLTP activity is related with metabolic syndrome, thrombosis, and inflammation. PLTP could be target for the treatment of dyslipidemia and atherosclerosis, although some potential off-target effects should be noted.

Sphingomyelin synthase 2 facilitates M2-like macrophage polarization and tumor progression in a mouse model of triple-negative breast cancer.

High infiltration of M2-polarized macrophages in the primary tumor indicates unfavorable prognosis and poor overall survival in the patients with triple-negative breast cancer (TNBC). Thus, reversing M2-polarized tumor-associated macrophages in the tumors has been considered as a potential therapeutic strategy for TNBC. Sphingomyelin synthase 2 (SMS2) is the key enzyme for sphingomyelin production, which plays an important role in plasma membrane integrity and function. In this study we investigated whether SMS2 inhibitor or SMS2 gene knockout could reduce macrophages M2 polarization and tumor progression in a mouse model of TNBC. We showed that SMS2 mRNA expression was linked to immunosuppressive tumor microenvironment and poor prognosis in TNBC patients. The knockout of SMS2 or application of 15w (a specific SMS2 inhibitor) markedly decreased the generation of M2-type macrophages in vitro, and reduced the tumor weight and lung metastatic niche formation in a 4T1-TNBC mouse model. We further demonstrated that the in vivo antitumor efficacy of 15w was accompanied by a multifaceted remodeling of tumor immune environment reflecting not only the suppression of M2-type macrophages but also diminished levels of regulatory T cells and myeloid-derived suppressor cells leading to a dramatically improved infiltration of antitumor CD8+ T lymphocytes. Collectively, our results reveal a novel and important role of SMS2 in the protumorigenic function and may offer a new strategy for macrophage-targeted anticancer therapy.

Airway Resistance Caused by Sphingomyelin Synthase 2 Insufficiency in Response to Cigarette Smoke.

Sphingomyelin synthase is responsible for the production of sphingomyelin (SGM), the second most abundant phospholipid in mammalian plasma, from ceramide, a major sphingolipid. Knowledge of the effects of cigarette smoke on SGM production is limited. In the present study, we examined the effect of chronic cigarette smoke on sphingomyelin synthase (SGMS) activity and evaluated how the deficiency of Sgms2, one of the two isoforms of mammalian SGMS, impacts pulmonary function. Sgms2-knockout and wild-type control mice were exposed to cigarette smoke for 6 months, and pulmonary function testing was performed. SGMS2-dependent signaling was investigated in these mice and in human monocyte-derived macrophages of nonsmokers and human bronchial epithelial (HBE) cells isolated from healthy nonsmokers and subjects with chronic obstructive pulmonary disease (COPD). Chronic cigarette smoke reduces SGMS activity and Sgms2 gene expression in mouse lungs. Sgms2-deficient mice exhibited enhanced airway and tissue resistance after chronic cigarette smoke exposure, but had similar degrees of emphysema, compared with smoke-exposed wild-type mice. Sgms2-/- mice had greater AKT phosphorylation, peribronchial collagen deposition, and protease activity in their lungs after smoke inhalation. Similarly, we identified reduced SGMS2 expression and enhanced phosphorylation of AKT and protease production in HBE cells isolated from subjects with COPD. Selective inhibition of AKT activity or overexpression of SGMS2 reduced the production of several matrix metalloproteinases in HBE cells and monocyte-derived macrophages. Our study demonstrates that smoke-regulated Sgms2 gene expression influences key COPD features in mice, including airway resistance, AKT signaling, and protease production.

Impact of Phospholipid Transfer Protein in Lipid Metabolism and Cardiovascular Diseases.

PLTP plays an important role in lipoprotein metabolism and cardiovascular disease development in humans; however, the mechanisms are still not completely understood. In mouse models, PLTP deficiency reduces cardiovascular disease, while its overexpression induces it. Therefore, we used mouse models to investigate the involved mechanisms. In this chapter, the recent main progresses in the field of PLTP research are summarized, and our focus is on the relationship between PLTP and lipoprotein metabolism, as well as PLTP and cardiovascular diseases.

2-Hydroxy-oleic acid does not activate sphingomyelin synthase activity.

2-Hydroxy-oleic acid (2OHOA) is a potent anticancer drug that induces cancer cell cycle arrest and apoptosis. Previous studies have suggested that 2OHOA's anticancer effect is mediated by SMS activation in cancer cells, including A549 and U118 cells. To confirm this phenomenon, in this study, we treated both A549 and U118 cells with 2OHOA and measured SMS activity. To our surprise, we found neither 2OHOA-mediated SMS activation nor sphingomyelin accumulation in the cells. However, we noted that 2OHOA significantly reduces phosphatidylcholine in these cells. We also did not observe 2OHOA-mediated SMS activation in mouse tissue homogenates. Importantly, 2OHOA inhibited rather than activated recombinant SMS1 (rSMS1) and rSMS2 in a dose-dependent fashion. Intra-gastric treatment of C57BL/6J mice with 2OHOA for 10 days had no effects on liver and small intestine SMS activities and plasma sphingomyelin levels. The treatment inhibited lysophosphatidylcholine acyltransferase (LPCAT) activity, consistent with the aforementioned reduction in plasma phosphatidylcholine. Because total cellular phosphatidylcholine is used as a predictive biomarker for monitoring tumor responses, the previously reported 2OHOA-mediated cancer suppression could be related to this phosphatidylcholine reduction, which may influence cell membrane structure and properties. We conclude that 2OHOA is not a SMS activator and that its anticancer property may be related to an effect on phosphatidylcholine metabolism.

Alkaline ceramidase 2 is essential for the homeostasis of plasma sphingoid bases and their phosphates.

Sphingosine-1-phosphate (S1P) plays important roles in cardiovascular development and immunity. S1P is abundant in plasma because erythrocytes-the major source of S1P-lack any S1P-degrading activity; however, much remains unclear about the source of the plasma S1P precursor, sphingosine (SPH), derived mainly from the hydrolysis of ceramides by the action of ceramidases that are encoded by 5 distinct genes, acid ceramidase 1 ( ASAH1)/ Asah1, ASAH2/ Asah2, alkaline ceramidase 1 ( ACER1)/ Acer1, ACER2/ Acer2, and ACER3/ Acer3, in humans/mice. Previous studies have reported that knocking out Asah1 or Asah2 failed to reduce plasma SPH and S1P levels in mice. In this study, we show that knocking out Acer1 or Acer3 also failed to reduce the blood levels of SPH or S1P in mice. In contrast, knocking out Acer2 from either whole-body or the hematopoietic lineage markedly decreased the blood levels of SPH and S1P in mice. Of interest, knocking out Acer2 from whole-body or the hematopoietic lineage also markedly decreased the levels of dihydrosphingosine (dhSPH) and dihydrosphingosine-1-phosphate (dhS1P) in blood. Taken together, these results suggest that ACER2 plays a key role in the maintenance of high plasma levels of sphingoid base-1-phosphates-S1P and dhS1P-by controlling the generation of sphingoid bases-SPH and dhSPH-in hematopoietic cells.-Li, F., Xu, R., Low, B. E., Lin, C.-L., Garcia-Barros, M., Schrandt, J., Mileva, I., Snider, A., Luo, C. K., Jiang, X.-C., Li, M.-S., Hannun, Y. A., Obeid, L. M., Wiles, M. V., Mao, C. Alkaline ceramidase 2 is essential for the homeostasis of plasma sphingoid bases and their phosphates.

Phospholipid transfer protein: its impact on lipoprotein homeostasis and atherosclerosis.

Phospholipid transfer protein (PLTP) is one of the major modulators of lipoprotein metabolism and atherosclerosis development in humans; however, we still do not quite understand the mechanisms. In mouse models, PLTP overexpression induces atherosclerosis, while its deficiency reduces it. Thus, mouse models were used to explore the mechanisms. In this review, I summarize the major progress made in the PLTP research field and emphasize its impact on lipoprotein metabolism and atherosclerosis, as well as its regulation.

Lysophosphatidylcholine acyltransferase 3 deficiency impairs 3T3L1 cell adipogenesis through activating Wnt/ß-catenin pathway.

Levels of polyunsaturated phosphatidylcholine (PC) influence plasma membrane structure and function. Phosphatidylcholine (PC) is synthesized de novo in the Kennedy pathway and then undergoes extensive deacylation/reacylation remodeling via Lands' cycle (non-Kennedy pathway). The reacylation is catalyzed by lysophosphatidylcholine acyltransferase (LPCAT), which adds a polyunsaturated fatty acid at the sn-2 position. Four LPCAT isoforms have been described to date, among which we found LPCAT3 to be the major isoform in adipose tissue, but its exact role in adipogenesis is unclear. In this study, we aimed to investigate whether LPCAT3 activity affects 3T3L1 cell adipogenic differentiation potential and its underline mechanism. Lentivirus-mediated LPCAT3 shRNA expression stably knocked down LPCAT3 in 3T3L1 preadipocytes and LPCAT3 deficiency dramatically reduced the levels of cellular polyunsaturated PCs. Importantly, we found that this deficiency activated the ß-catenin dependent Wnt signaling pathway, which suppressed the expression of adipogenesis-related genes, thereby inhibiting 3T3L1 preadipocyte differentiation and lipid accumulation. Moreover, three different Wnt/ß-catenin pathway inhibitors reversed the effect of LPCAP3 deficiency, suggesting that Wnt/ß-catenin pathway activation is one of the causes for the observed phenotypes. To the best of our knowledge, we show here for the first time that PC remodeling is an important regulator of adipocyte differentiation.

Circulating PCSK9 levels and 2-hPG are positively correlated in metabolic diseases in a Chinese Han population.

BACKGROUND: Proprotein convertase subtilisin/kexin type 9 (PCSK9), which plays a crucial role in lipoprotein metabolism, has been also regarded as an important marker for atherosclerosis. Available evidence indicated that 2-h postchallenge plasma glucose (2-hPG) could be another biomarker for atherosclerosis. However, currently the association between circulating PCSK9 and 2-hPG remains unclear. Here, we explored this potential link in a Chinese Han population. METHODS: Totally, 600 Chinese Han subjects from Nanjing district, China, were enrolled for the 75-g oral glucose tolerance test (OGTT), and they included normal glucose tolerance (NGT, n = 200), impaired glucose regulation (IGR, n = 200), and type 2 diabetes (T2DM, n = 200). Anthropometric and biochemical determinations such as serum lipid measurements were made. A sandwich ELISA assay was performed to measure serum PCSK9 levels in all subjects. RESULTS: Serum PCSK9 concentrations were higher in IGR group (77.63 ± 28.14 ng/ml) and T2DM group (90.62 ± 39.96 ng/ml) than in NGT group (65.33 ± 32.68 ng/ml), and it was significantly higher in T2DM group than in IGR group (p < 0.01). Serum PCSK9 levels positively correlated with 2-hPG and LDL-C in all subgroups, but presented a positive correlation with fasting blood glucose (FBG) only in T2DM group. Using multiple regression model analysis, we also found that PCSK9 levels closely correlated with 2-hPG in all tested groups. According to multinomial logistic regression analysis, PCSK9 levels positively correlated with T2DM (OR = 1.017[1.010-1.025], p < 0.001) even after adjustment for lipid levels. Moreover, in subjects with normal FBG level, 2-hPG gradually and significantly increased across PCSK9 tertiles (6.68 ± 2.01, 7.48 ± 2.10 and 8.27 ± 2.41 mmol/L, respectively, p < 0.01); however, in subjects with normal 2-hPG levels, no such difference was observed. CONCLUSIONS: PCSK9 levels increase as glucose metabolism deteriorated. Serum PCSK9 levels positively correlated with 2-hPG in patients with metabolic diseases.

Small Intestine but Not Liver Lysophosphatidylcholine Acyltransferase 3 (Lpcat3) Deficiency Has a Dominant Effect on Plasma Lipid Metabolism.

Lysophosphatidylcholine acyltransferase 3 (Lpcat3) is involved in phosphatidylcholine remodeling in the small intestine and liver. We investigated lipid metabolism in inducible intestine-specific and liver-specificLpcat3gene knock-out mice. We producedLpcat3-Flox/villin-Cre-ER(T2)mice, which were treated with tamoxifen (at days 1, 3, 5, and 7), to deleteLpcat3specifically in the small intestine. At day 9 after the treatment, we found that Lpcat3 deficiency in enterocytes significantly reduced polyunsaturated phosphatidylcholines in the enterocyte plasma membrane and reduced Niemann-Pick C1-like 1 (NPC1L1), CD36, ATP-binding cassette transporter 1 (ABCA1), and ABCG8 levels on the membrane, thus significantly reducing lipid absorption, cholesterol secretion through apoB-dependent and apoB-independent pathways, and plasma triglyceride, cholesterol, and phospholipid levels, as well as body weight. Moreover, Lpcat3 deficiency does not cause significant lipid accumulation in the small intestine. We also utilized adenovirus-associated virus-Cre to depleteLpcat3in the liver. We found that liver deficiency only reduces plasma triglyceride levels but not other lipid levels. Furthermore, there is no significant lipid accumulation in the liver. Importantly, small intestine Lpcat3 deficiency has a much bigger effect on plasma lipid levels than that of liver deficiency. Thus, inhibition of small intestine Lpcat3 might constitute a novel approach for treating hyperlipidemia.

Liver serine palmitoyltransferase activity deficiency in early life impairs adherens junctions and promotes tumorigenesis.

Serine palmitoyltransferase is the key enzyme in sphingolipid biosynthesis. Mice lacking serine palmitoyltransferase are embryonic lethal. We prepared liver-specific mice deficient in the serine palmitoyltransferase long chain base subunit 2 gene using an albumin-cyclization recombination approach and found that the deficient mice have severe jaundice. Moreover, the deficiency impairs hepatocyte polarity, attenuates liver regeneration after hepatectomy, and promotes tumorigenesis. Importantly, we show that the deficiency significantly reduces sphingomyelin but not other sphingolipids in hepatocyte plasma membrane; greatly reduces cadherin, the major protein in adherens junctions, on the membrane; and greatly induces cadherin phosphorylation, an indication of its degradation. The deficiency affects cellular distribution of ß-catenin, the central component of the canonical Wnt pathway. Furthermore, such a defect can be partially corrected by sphingomyelin supplementation in vivo and in vitro. CONCLUSION: The plasma membrane sphingomyelin level is one of the key factors in regulating hepatocyte polarity and tumorigenesis. (Hepatology 2016;64:2089-2102).