Apolipoprotein M bound sphingosine 1-phosphate suppresses NETosis through activating S1P1 and S1P4

The pleiotropic effects of high-density lipoprotein (HDL), including its protective properties against sepsis, are attributed to the sphingosine 1-phosphate and apolipoprotein M (ApoM) that are carried on the lipoproteins. In this study, we attempted to elucidate the possible mechanisms underlying the sepsis coagulopathic state by considering the modulation of NETosis. Our results revealed that in a lipopolysaccharide-induced sepsis mouse model, the levels of NETosis markers, such as plasma DNA and histone, were elevated in ApoM-knockout (KO) mice and attenuated in ApoM-overexpressing mice. In ApoM-KO mice, the survival rate decreased and the occurrence rates of coagulopathy and organ injury increased following the administration of histone. Treatment with a conditioned medium of ApoM-overexpressing cells attenuated the observed NETosis in HL-60S cells that differentiated into neutrophils and were inhibited through the suppression of S1P1 or S1P4. The attenuation of PKCδ and PKCα/β by S1P1 and S1P4 activation may also be involved. In ApoM-overexpressing mice, coagulopathy and organ injuries were attenuated following an injection of histone; these effects were partially inhibited by S1P1, 3, S1P4, or S1P1 antagonists. Furthermore, the exogenous administration of ApoM protected ApoM-KO mice that were challenged with histone from developing NETosis. In conclusion, the ApoM/S1P axis protects against NETosis through the attenuation of PKC activation by S1P1 and S1P4. The development of drugs targeting the ApoM/S1P axis may be beneficial for the treatment of pathological conditions involving uncontrolled NETosis, such as sepsis.

Plasma apolipoprotein M predicts overall survival in metastatic breast cancer patients

PURPOSE

Apolipoprotein M (APOM) is a plasma apolipoprotein closely involved with lipid metabolism and inflammation. In vitro studies suggest that APOM may also have a tumor-suppressive role in breast cancer. In the present study, we aimed to evaluate the impact of plasma APOM levels on the prognosis of breast cancer patients.

METHODS

We measured APOM levels using an enzyme-linked immunosorbent assay in 75 patients with ER-positive/HER2-negative metastatic breast cancer. The endpoint was overall survival (OS) at 24 months.

RESULTS

During the 24-month follow-up period, 34.7% of the patients died. Baseline APOM levels were significantly reduced in patients who deceased during follow-up compared to survivors (42.7 ± 14.5 µg/mL versus 52.2 ± 13.8 µg/mL; P = 0.003). Cox regression analysis showed a hazard ratio of 0.30 [95% confidence interval 0.15-0.61]; P < 0.001 per doubling of APOM levels. Correction for age, C-reactive protein, menopausal state, histology of the primary tumor, metastatic site, number of metastases, endocrine resistance, scheduled therapy line, and kind of scheduled therapy indicated that circulating APOM predicted OS independently of these parameters (HRper doubling = 0.23 [0.09-0.56; P = 0.001).

CONCLUSIONS

Our study suggests that circulating APOM is significantly linked with reduced mortality in metastatic breast cancer patients.

Apolipoprotein M supports S1P production and conservation and mediates prolonged Akt activation via S1PR1 and S1PR3

Sphingosine 1-phosphate (S1P) is one of the lipid mediators involved in diverse physiological functions. S1P circulates in blood and lymph bound to carrier proteins. Three S1P carrier proteins have been reported, albumin, apolipoprotein M (ApoM) and apolipoprotein A4 (ApoA4). The carrier-bound S1P exerts its functions via specific S1P receptors (S1PR1-5) on target cells. Previous studies showed several differences in physiological functions between albumin-bound S1P and ApoM-bound S1P. However, molecular mechanisms underlying the carrier-dependent differences have not been clarified. In addition, ApoA4 is a recently identified S1P carrier protein, and its functional differences from albumin and ApoM have not been addressed. Here, we compared the three carrier proteins in the processes of S1P degradation, release from S1P-producing cells and receptor activation. ApoM retained S1P more stable than albumin and ApoA4 in the cell culture medium when compared in the equimolar amounts. ApoM facilitated theS1P release from endothelial cells most efficiently. Furthermore, ApoM-bound S1P showed a tendency to induce prolonged activation of Akt via S1PR1 and S1PR3. These results suggest that the carrier-dependent functional differences of S1P are partly ascribed to the differences in the S1P stability, S1P-releasing efficiency and signaling duration.

Apolipoprotein M/sphingosine 1-phosphate protects against diabetic nephropathy

Diabetic nephropathy remains a common cause of end-stage renal failure and its associated mortality around the world. Sphingosine 1-phosphate (S1P) is a multifunctional lipid mediator and binds to HDL via apolipoprotein M (ApoM). Since HDL has been reported to be epidemiologically associated with kidney disease, we attempted to investigate the involvement of the ApoM/S1P axis in the pathogenesis/progression of diabetic nephropathy. In type 2 diabetic patients, the serum ApoM levels were inversely correlated with the clinical stage of diabetic nephropathy. The decline in the eGFR over a 5-year observation period proceeded more rapidly in subjects with lower serum ApoM levels. In a mouse model of streptozotocin-induced diabetes, deletion of ApoM deteriorated the phenotypes of diabetic nephropathy: the urinary albumin and plasma creatinine levels increased, the kidneys enlarged, and renal fibrosis and thickening of the basement membrane progressed. On the other hand, overexpression of ApoM ameliorated these phenotypes. These protective effects of ApoM were partially inhibited by treatment with VPC23019, an antagonist of S1P1 and S1P3, but not by treatment with JTE013, an antagonist of S1P2. ApoM/S1P axis attenuated activation of the Smad3 pathway, while augmented eNOS phosphorylation through the S1P1 pathway. Moreover, ApoM/S1P increased the SIRT1 protein levels and enhanced mitochondrial functions by increasing the S1P content of the cell membrane, which might cause selective activation of S1P1. ApoM might be a useful biomarker for predicting the progression of diabetic nephropathy, and the ApoM/S1P-S1P1 axis might serve as a novel therapeutic target for preventing the development/progression of diabetic nephropathy.

Apolipoprotein D modulates lipid mediators and osteopontin in an anti-inflammatory direction

BACKGROUND

HDL has been proposed to possess anti-inflammatory properties; however, the detail mechanisms have not been fully elucidated.

METHODS

We investigated the roles of Apolipoprotein D (ApoD) in the pathogenesis of inflammation in the mouse model of diet-induced obesity and that of lipopolysaccharide-induced sepsis and the in vitro experiments. Furthermore, we analyzed serum ApoD levels in human subjects.

RESULTS

The overexpression of human ApoD decreased the plasma IL-6 and TNF-a levels in both mice models. Lipidomics analyses demonstrated association of ApoD with increase of arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid, as well as of their metabolites, and of the anti-inflammatory molecule sphingosine 1-phosphate, and decrease of proinflammatory lysophosphatidic acids and lysophosphatidylinositol. ApoD-containing lipoproteins might directly bind eicosapentaenoic acid and docosahexaenoic acid. The modulations of the lysophosphatidic acid and sphingosine 1-phosphate levels resulted from the suppression of autotaxin expression and elevation of apolipoprotein M (ApoM), respectively. Moreover, ApoD negatively regulated osteopontin, a proinflammatory adipokine. The activation of PPARg by ApoD might suppress autotaxin and osteopontin. Serum ApoD levels were negatively correlated with the serum osteopontin and autotaxin levels and, positively with serum ApoM levels.

CONCLUSION

ApoD is an anti-inflammatory apolipoprotein, which modulates lipid mediators and osteopontin in an anti-inflammatory direction.

Association between plasma apolipoprotein M and cardiac autonomic neuropathy in type 1 diabetes

AIM

Diabetes may lead to severe complications e.g. cardiac autonomic neuropathy (CAN) characterized by an increased risk of cardiovascular mortality. CAN is diagnosed by a decreased heart rate viability (HRV). Sphingosine-1-Phosphate (S1P) carried by the HDL-associated apolipoprotein M (apoM) is linked to a reduction in the heart rate, and treatment with an S1P-agonist increases HRV. The present study aimed to investigate if plasma apoM was associated with an increased risk of CAN.

METHODS

The study includes 278 individuals with Type 1 Diabetes recruited from Steno Diabetes Center in Copenhagen from 2010 to 2012.

RESULTS

A change of 0.1 µM plasma apoM was associated with the diagnosis of CAN (Odds ratio: 1.11 (1.02; 1.21), p = 0.013). ApoM plasma levels were also positively associated with CAN when adjusted for age and gender (Odds ratio: 1.11 (1.02; 1.21), p = 0.013) as well as lipids, beta-blockers, blood pressure, and alcohol (Odds ratio: 1.14 (1.04; 1.26), p = 0.005) and Hbga1c and time with diabetes (Odds ratio: 1.13 (1.02; 1.25), p = 0.01). Plasma apoM was also associated with a significantly lower SDNN as well as high frequency power in all adjusted models.

CONCLUSION

Increased plasma apoM was associated with an increased risk of CAN as well as a significant reduction in HRV indices. This could represent changes in parasympathetic activity, but, further studies are needed to also explore additional molecular alterations behind such observations.

The effects and possible mechanism of action of apolipoprotein M on the growth of breast cancer cells

BACKGROUND

To investigate the effects and mechanism of action of apolipoprotein M (ApoM) on the growth of breast cancer (BC) cells.

METHODS AND RESULTS

Bioinformatics, cell experiments and animal experiments were used to verify the effect of ApoM on breast cancer cell lines and breast tumor growth in vivo. ApoM expression was significantly reduced in BC tissues, and patients with lower ApoM mRNA expression had a poorer prognosis (P < 0.0001). Besides, ApoM can partially inhibit the proliferative, migratory and invasive processes of BC cells. In vivo, the difference between ApoM-OE and NC groups was no significant. The level of vitamin D receptor (VDR) protein in MDA-MB-231 cells was increased by overexpression of ApoM (P < 0.05), while in MCF-7 cells, VDR levels decreased (P < 0.05).

CONCLUSIONS

ApoM can partially inhibit the growth of BC cells. VDR may play a role, but is not the main pathway.

Apolipoprotein M and its impact on endothelial dysfunction and inflammation in the cardiovascular system

Apolipoprotein M (apoM) is a member of the lipocalin superfamily and is predominantly associated with high-density lipoprotein (HDL). It was found that apoM is the chaperon to the bioactive sphingolipid, sphingosine-1-phosphate (S1P). Several studies have since contributed to expand the knowledge on apoM, S1P, and the apoM/S1P-complex in cardiovascular diseases. For instance, the HDL-bound apoM/S1P complex serves as a bridge between HDL and endothelial cells, maintaining a healthy endothelial barrier. Evidence indicates, however, that the apoM/S1P complex may has both protective and harmful effects on the cardiovascular system, which suggests the need for more research to understand the interplay between these molecules. This review aims to shed light on the most recent findings on apoM/S1P-signaling and its impact on endothelial dysfunction, inflammation, and cardiovascular diseases. Finally, it will be discussed whether drugs that target apoM and/or S1P-signaling may be beneficial to patients with cardiovascular and inflammatory diseases.

Regulation of the apolipoprotein M signaling pathway: a review

Apolipoprotein M (apoM), an apolipoprotein predominantly associated with high-density lipoprotein (HDL), is considered a mediator of the numerous roles of HDL, including reverse cholesterol transport, anti-atherosclerotic, anti-inflammatory and anti-oxidant, and mediates pre-β-HDL formation. ApoM expression is known to be regulated by a variety of in vivo and in vitro factors. The transcription factors farnesoid X receptor, small heterodimer partner, liver receptor homolog-1, and liver X receptor comprise the signaling cascade network that regulates the expression and secretion of apoM. Moreover, hepatocyte nuclear factor-1α and c-Jun/JunB have been demonstrated to exert opposing regulatory effects on apoM through competitive binding to the same sites in the proximal region of the apoM gene. Furthermore, as a carrier and modulator of sphingosine 1-phosphate (S1P), apoM binds to S1P within its hydrophobic-binding pocket. The apoM/S1P axis has been discovered to play a crucial role in the apoM signaling pathway through its ability to regulate glucose and lipid metabolism, vascular barrier homeostasis, inflammatory response and other pathological and physiological processes. Using the findings of previous studies, the present review aimed to summarize the regulation of apoM expression by various factors and its role in different physiological and pathological conditions, and provide a new perspective for the further treatment of these diseases.

Glycation of HDL Polymerizes Apolipoprotein M and Attenuates Its Capacity to Bind to Sphingosine 1-Phosphate

Aim: Recently, it has been established that most of the pleiotropic effects of high-density lipoprotein (HDL) are attributed to sphingosine 1-phosphate (S1P), which rides on HDL via apolipoprotein M (ApoM). In subjects with diabetes mellitus, both the pleiotropic effects of HDL and its role in reverse cholesterol transport are reported to be impaired. To elucidate the mechanisms underlying the impaired pleiotropic effects of HDL in subjects with diabetes, from the aspects of S1P and ApoM.

Methods: The incubation of HDL in a high-glucose condition resulted in the dimerization of ApoM. Moreover, the treatment of HDL with methylglyoxal resulted in the modulation of the ApoM structure, as suggested by the results of western blot analysis, isoelectric focusing electrophoresis, and two-dimensional gel electrophoresis, which was reversed by treatment with anti-glycation reagents.

Results: The glycation of HDL resulted in impaired binding of the glycated HDL to S1P, and the S1P on glycated HDL degraded faster. In the case of human subjects, on the other hand, although both the serum ApoM levels and the ApoM content in HDL were lower in subjects with diabetes, we did not observe the polymerization of ApoM.

Conclusions: Modulation of the quantity and quality of ApoM might explain, at least in part, the impaired functions of HDL in subjects with diabetes mellitus. ApoM might be a useful target for laboratory testing and/or the treatment of diabetes mellitus.

Apolipoprotein M promotes growth and inhibits apoptosis of colorectal cancer cells through upregulation of ribosomal protein S27a

Colorectal cancer (CRC) is one of the frequent malignant tumors and has a high mortality-to-incidence ratio. Apolipoprotein M (ApoM), a lipoprotein superfamily member, is primarily bound to high-density lipoprotein (HDL) particles. Our previous studies opined that ApoM crucially modulates CRC progression, but its role in CRC has not been elucidated. Here, lentivirus infection technology was used to overexpress ApoM in Caco-2 cells. Cell growth, apoptosis as well as clone formation assays were performed to explore the biological influences of ApoM in Caco-2 cells. Differentially expressed genes were analyzed via GeneChip microarrays and Quantitative real-time PCR (qPCR) along with Western blotting were applied to verify the results. Ribosomal protein S27a (RPS27A) expression in CRC and tumor-adjacent tissues was detected by qPCR, and its correlation with clinicopathologic characteristics was explored. Our results showed that ApoM overexpression could promote Caco-2 cell proliferation and inhibit apoptosis. The microarray evaluation uncovered 2671 genes, which were differentially expressed, including RPS27A. The qPCR as well as the Western blotting data showed that ApoM overexpression significantly increased the expression of RPS27A. Moreover, RPS27A expression was remarkably higher in CRC tissues in contrast with the tumor-adjacent tissues and was positively correlated with the ApoM level in tumor tissues, and higher RPS27A expression was associated with smaller tumors and lower T stage. Functional recovery experiments indicated that knockdown of RPS27A counteracted the apoptosis inhibition and clone formation promotion induced by ApoM overexpression in Caco-2 cells. In conclusion, ApoM promotes CRC cell growth and inhibits apoptosis through upregulation of RPS27A.

Comprehensive lipidomics in apoM-/- mice reveals an overall state of metabolic distress and attenuated hepatic lipid secretion into the circulation

Apolipoprotein M (apoM) participates in both high-density lipoprotein and cholesterol metabolism. Little is known about how apoM affects lipid composition of the liver and serum. In this study, we systemically investigated the effects of apoM on liver and plasma lipidomes and how apoM participates in lipid cycling, via apoM knockout in mice and the human SMMC-7721 cell line. We used integrated mass spectrometry-based lipidomics approaches to semiquantify more than 600 lipid species from various lipid classes, which include free fatty acids, glycerolipids, phospholipids, sphingolipids, glycosphingolipids, cholesterol, and cholesteryl esters (CEs), in apoM^-/- mouse. Hepatic accumulation of neutral lipids, including CEs, triacylglycerols, and diacylglycerols, was observed in apoM^-/- mice; while serum lipidomic analyses showed that, in contrast to the liver, the overall levels of CEs and saturated/monounsaturated fatty acids were markedly diminished. Furthermore, the level of ApoB-100 was dramatically increased in the liver, whereas significant reductions in both ApoB-100 and low-density lipoprotein (LDL) cholesterol were observed in the serum of apoM^-/- mice, which indicated attenuated hepatic LDL secretion into the circulation. Lipid profiles and proinflammatory cytokine levels indicated that apoM^-/- leads to hepatic steatosis and an overall state of metabolic distress. Taken together, these results revealed that apoM knockout leads to hepatic steatosis, impaired lipid secretion, and an overall state of metabolic distress.

Berberine reduces gut-vascular barrier permeability via modulation of ApoM/S1P pathway in a model of polymicrobial sepsis

AIMS

The hyperpermeability of gut-vascular barrier (GVB) plays a role in gut-derived sepsis. The goal of this study was to evaluate if berberine might improve hepatic apolipoprotein M (ApoM) generation and raise plasma ApoM level to protect the compromised GVB.

MATERIALS AND METHODS

The compromised GVB was induced by sepsis. Hepatic ApoM mRNA and phosphoenolpyruvate carboxykinase (PEPCK) mRNA and plasma ApoM level were assayed by qRT-PCR and ELISA, respectively. The permeability of intestinal capillary in vivo and of rat intestinal microvascular endothelial cells (RIMECs) in vitro was assayed by FITC-dextran. The blood glucose was detected by a glucometer. Plasma insulin, TNF-α and IL-1β were assayed by ELISA. The plasmalemma vesicle-associated protein-1 (PV1), β-catenin and occludin in RIMECs were assayed by Western blot.

KEY FINDINGS

Sepsis decreased hepatic ApoM mRNA and plasma ApoM level, but raised hepatic PEPCK mRNA and plasma glucose, insulin, TNF-α, and IL-1β levels. The increased vascular endothelial permeability was abrogated by recombinant rat ApoM in vivo or ApoM-bound S1P in vitro. ApoM-bound S1P decreased PV1 but increased occludin and β-catenin expression in LPS-treated RIMECs. Berberine in a dose-dependent manner raised hepatic ApoM mRNA and plasma ApoM level, but decreased septic hyperglycemia, insulin resistance and plasma TNF-α and IL-1β levels. Berberine reduced sepsis-induced PEPCK and TLR4 mRNA overexpression in the liver.

SIGNIFICANCE

This study demonstrated berberine inhibited TLR4-mediated hyperglycemia, insulin resistance and proinflammatory molecule production, thereby increasing ApoM gene expression and plasma ApoM. Berberine protected the damaged GVB via modulation of ApoM/S1P pathway.

Apolipoprotein M overexpression through adeno-associated virus gene transfer improves insulin secretion and insulin sensitivity in Goto-Kakizaki rats

AIMS/OBJECTIVE

The development of type 2 diabetes is a result of insulin resistance in various tissues, including skeletal muscle and liver. Apolipoprotein M (ApoM) plays an important role in the function of high-density lipoprotein, and also affects hepatic lipid and glucose metabolism. In this study, we aimed to investigate whether ApoM overexpression modulates glucose metabolism and improves insulin sensitivity.

MATERIALS AND METHODS

The Goto-Kakizaki (GK) rats were transfected with adeno-associated virus (AAV) encoding rat ApoM gene or control blank. The oral glucose tolerance test (OGTT) and hyperinsulinemic-euglycemic clamp (HEC) experiment were used to assess the insulin sensitivity of GK rats.

RESULTS

The results show that ApoM messenger ribonucleic acid and protein were significantly overexpressed in the pancreatic tissues. Overexpression of ApoM decreased fasting blood glucose and random blood glucose, improved glucose tolerance, and increased bodyweight and insulin levels in GK rats. The glucose infusion rate of rats in the AAV encoding rat ApoM gene group during HEC test was 1.04-, 1.23- and 1.95-fold higher than that in the AAV control blank group at 1-3 weeks after injection of AAV, respectively. A Wes-ProteinSimple assay and quantification was carried out to assess phosphorylated protein kinase B/protein kinase B protein levels in the muscle tissues of ApoM-overexpressing GK rats, and they were found to be higher than those of the control group at the seventh week after AAV injection.

CONCLUSIONS

ApoM overexpression through adeno-associated virus gene transfer might improve insulin secretion and insulin sensitivity in GK rats.

Apolipoprotein M: Research Progress and Clinical Perspective

Apolipoprotein M (apoM) was first identified and characterized to the apolipoprotein family in 1999. Human apoM gene is located in a highly conserved segment in the major histocompatibility complex (MHC) class III locus on chromosome 6 and codes for an about 23 kDa protein that structurally belongs to the lipocalin superfamily. ApoM is selectively expressed in hepatocytes and in the tubular epithelium of kidney. In human plasma, apoM is mainly confined to the high-density lipoprotein (HDL) particles, but it may also occur in other lipoprotein classes, such as in the triglyceride-rich particles after fat intake. It has been demonstrated that apoM is critical for the formation of HDL, notably pre-beta HDL1. The antiatherogenic function of HDL is well established, and its ability to promote cholesterol efflux from foam cells in the atherosclerotic lesions is generally regarded as one of the key mechanisms behind this protective function. However, HDL could also display a variety of properties that may affect the complex atherosclerotic processes by other mechanisms, thus being involved in processes related to antioxidant defense, immune system, and systemic effects in septicemia, which may be partly contributed via its apolipoproteins and/or phospholipids. Moreover, it has been demonstrated that apoM functions as a natural carrier of sphingosin-1-phosphate (S1P) in vivo which may be related to its antiatherosclerotic and protective effects on endothelial cell barrier and anti-inflammatory properties. These may also provide a link between the diverse effects of HDL.

Insulin Resistance in Apolipoprotein M Knockout Mice is Mediated by the Protein Kinase Akt Signaling Pathway

BACKGROUND

Previous clinical studies have suggested that apolipoprotein M (apoM) is involved in glucose metabolism and plays a causative role in insulin sensitivity.

OBJECTIVE

The potential mechanism of apoM on modulating glucose homeostasis is explored and differentially expressed genes are analyzed by employing ApoM deficient (ApoM-/- ) and wild type (WT) mice.

METHODS

The metabolism of glucose in the hepatic tissues of high-fat diet ApoM-/- and WT mice was measured by a glycomics approach. Bioinformatic analysis was applied for analyzing the levels of differentially expressed mRNAs in the liver tissues of these mice. The insulin sensitivity of ApoM-/- and WT mice was compared using the insulin tolerance test and the phosphorylation levels of protein kinase Akt (AKT) and insulin stimulation in different tissues were examined by Western blot.

RESULTS

The majority of the hepatic glucose metabolites exhibited lower concentration levels in the ApoM-/- mice compared with those of the WT mice. Gene Ontology (GO) classification and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that ApoM deficiency affected the genes associated with the metabolism of glucose. The insulin tolerance test suggested that insulin sensitivity was impaired in ApoM-/- mice. The phosphorylation levels of AKT in muscle and adipose tissues of ApoM-/- mice were significantly diminished in response to insulin stimulation compared with those noted in WT mice.

CONCLUSION

ApoM deficiency led to the disorders of glucose metabolism and altered genes related to glucose metabolism in mice liver. In vivo data indicated that apoM might augment insulin sensitivity by AKT-dependent mechanism.

The role of sphingosine 1-phosphate receptors on retinal pigment epithelial cells barrier function and angiogenic effects

Sphingosine-1-phosphate (S1P) is a lysophospholipid mediator, promoting angiogenesis and inflammation via interactions with its receptors (S1P1-5), but the receptors and signaling pathways responsible for the progression of choroidal neovascularization (CNV) remain unknown. We investigated the roles of S1P/S1P receptors in RPE cells. ARPE-19 cells were treated with S1P dissolved in carrier proteins of albumin or apolipoprotein M (ApoM). The mRNA expression levels of interleukin-8 (IL-8), C-C motif chemokine ligand 2 (CCL2), and vascular endothelial growth factor (VEGF) were evaluated using quantitative real-time polymerase chain reaction. The protein level of hypoxia-inducible factor (HIF)-1α was assessed via enzyme-linked immunosorbent assay. HIF transcriptional activity was evaluated with a dual-reporter luciferase assay. Cellular barrier integrity was evaluated using transepithelial electrical resistance and the FITC-dextran permeability assay. The suppressive effect of an S1P antagonist on CNV progression was investigated with a laser-induced CNV model in mice. The increase in expression of IL-8, CCL2, and VEGF due to albumin-bound S1P was significantly mitigated by an S1P2 antagonist. The expression of HIF-1α significantly decreased with inhibition of S1P2 and S1P3. In addition, albumin-bound S1P disrupted the barrier integrity of retinal pigment epithelial cells via S1P2, whereas integrity was strengthened by ApoM-bound S1P. CNV lesions were significantly reduced in the mouse model with intravitreal injection of S1P2 antagonist. This study demonstrated that S1P significantly promotes angiogenesis, inflammation, and barrier integrity, which was attenuated by inhibition of S1P2 or S1P3, suggesting that regulation of S1P2 and S1P3 is a novel therapeutic target for CNV.

Low plasma concentrations of apolipoprotein M are associated with disease activity and endothelial dysfunction in systemic lupus erythematosus

BACKGROUND

Apolipoprotein M (apoM) is a 25-kDa apolipoprotein present in 5% of high-density lipoprotein (HDL) particles. It is suggested to be anti-atherogenic and to play a key role in sustaining endothelial barrier integrity. SLE patients have increased cardiovascular disease risk, and we aimed to investigate if apoM levels reflect endothelial function in SLE. Since apoM plasma levels decrease during inflammatory conditions, our aim was also to determine the impact of SLE disease activity on apoM plasma levels.

METHODS

Plasma concentrations of apoM were measured by ELISA in two patient groups with systemic lupus erythematosus (SLE) and in 79 healthy control individuals. In patient group I (n = 84), evaluation time points were selected with the objective to include a wide range of clinical and laboratory variables reflecting disease activity which was measured as SLEDAI. In patient group II consisting of 140 consecutive patients, endothelial function was measured by a finger plethysmograph. A low Reactive Hyperemia Index (RHI) value indicates endothelial dysfunction.

RESULTS

SLE patients had decreased levels of apoM compared to healthy controls (p < 0.01), with apoM levels correlating inversely with SLEDAI (r = - 0.31, p < 0.01) as well as with levels of CRP (r = - 0.26, p = 0.02) and positively with levels of C3 (r = 0.29, p < 0.01). ApoM levels were particularly low in patients with active disease from the kidney and skin and in patients with leukopenia or positive anti-dsDNA antibody test (p < 0.05). ApoM levels correlated with RHI values in young SLE patients (r = 0.32, p = 0.01), consistent with the important role of apoM in regulating endothelial integrity.

CONCLUSIONS

ApoM levels may be regulated by SLE-related inflammatory processes and could be a marker of disease activity and endothelial dysfunction, in particular in young SLE patients. Further studies are needed to investigate the predictive value of apoM in the development of a cardiovascular disease.

Apolipoprotein M promotes proliferation and invasion in non-small cell lung cancers via upregulating S1PR1 and activating the ERK1/2 and PI3K/AKT signaling pathways

Apolipoprotein M (ApoM) is a sphingosine 1-phosphate (S1P) carrier involved in the regulation of S1P. Signaling pathways involving sphingosine kinases (SphKs) and S1P-S1P receptors (S1PRs) play important roles in the oncogenesis of multiple cancers including non-small cell lung cancer (NSCLC). In the present study we have clarified the potential roles of ApoM on the oncogenesis process of NSCLC cells. We detected the ApoM expression in NSCLC tissues and further analyzed its clinical significance. Moreover, we determined effects of ApoM overexpression on tumor cellular behaviours of NSCLC in vitro and in vivo. Our results demonstrated that ApoM protein mass were clearly higher in the NSCLC tissues than in non-NSCLS tissues. Overexpression of ApoM could promote NSCLC cell proliferation and invasion in vitro and tumor growth in vivo, which might be via upregulating S1PR1 and activating the ERK1/2 and PI3K/AKT signaling pathways. It is concluded that up-regulation of ApoM in NSCLC might be associated with the tumor induced inflammation and tumor microenvironment as well as promoting oncogenesis of NSCLC. Further study needs to elucidate the underlying mechanisms.

Deletion of ApoM gene induces apoptosis in mouse kidney via mitochondrial and endoplasmic reticulum stress pathways

Apolipoprotein M (ApoM) is involved in lipid metabolism, and especially is involved in reverse cholesterol transport. However, the relationship between ApoM and apoptosis has been rarely reported. This study aimed to investigate the effect of ApoM on apoptosis using an ApoM gene-deficient mice (ApoM^-/-) model and a mouse mesangial cell model with suppressed ApoM gene expression. First, we observed by transmission electron microscopy that mitochondrial damage and endoplasmic reticulum stress were abnormally altered in the kidneys of ApoM^-/- mice compared with wild-type mice, showing mitochondrial swelling, vacuolization, myeloid changes, and expansion of the rough endoplasmic reticulum. At the molecular level, the expression of pro-apoptotic related proteins such as AIF, Bax, chop, clever-caspase 3, clever-caspase 7, clever-caspase 9, and clever-caspase 12 increased, and the expression of anti-apoptotic protein Bcl-2 decreased. Secondly, by interfering with the expression of the ApoM gene in mouse mesangial cells, we found that, compared with the control group (NC-si), the cells of the experimental group (siApoM) showed decreased cell viability, nuclear chromatin condensation, nuclear lysis, and an increased proportion of early apoptotic cells. The results in cells at the molecular level were consistent with those at the tissue level. These data indicated that the deletion of the ApoM gene led to upregulation of apoptosis in mouse kidney tissues and mesangial cells through the mitochondrial and endoplasmic reticulum pathways.

Increased apolipoprotein M induced by lack of scavenger receptor BI is not activated via HDL-mediated cholesterol uptake in hepatocytes

Background

Scavenger receptor BI (SR-BI) is a classic high-density lipoprotein (HDL) receptor, which mediates selective lipid uptake from HDL cholesterol esters (HDL-C). Apolipoprotein M (ApoM), as a component of HDL particles, could influence preβ-HDL formation and cholesterol efflux. The aim of this study was to determine whether SR-BI deficiency influenced the expression of ApoM.

Methods

Blood samples and liver tissues were collected from SR-BI gene knockout mice, and serum lipid parameters, including total cholesterol (TC), triglyceride (TG), high and low-density lipoprotein cholesterol (HDL-C and LDL-C) and ApoM were measured. Hepatic ApoM and ApoAI mRNA levels were also determined. In addition, BLT-1, an inhibitor of SR-BI, was added to HepG2 cells cultured with cholesterol and HDL, under serum or serum-free conditions. The mRNA and protein expression levels of ApoM were detected by RT-PCR and western blot.

Results

We found that increased serum ApoM protein levels corresponded with high hepatic ApoM mRNA levels in both male and female SR-BI^−/− mice. Besides, serum TC and HDL-C were also significantly increased. Treatment of HepG2 hepatoma cells with SR-BI specific inhibitor, BLT-1, could up-regulate ApoM expression in serum-containing medium but not in serum-free medium, even in the presence of HDL-C and cholesterol.

Conclusions

Results suggested that SR-BI deficiency promoted ApoM expression, but the increased ApoM might be independent from HDL-mediated cholesterol uptake in hepatocytes.

Sphingosine 1-Phosphate and Atherosclerosis

Sphingosine 1-phosphate (S1P) is a potent lipid mediator that works on five kinds of S1P receptors located on the cell membrane. In the circulation, S1P is distributed to HDL, followed by albumin. Since S1P and HDL share several bioactivities, S1P is believed to be responsible for the pleiotropic effects of HDL. Plasma S1P levels are reportedly lower in subjects with coronary artery disease, suggesting that S1P might be deeply involved in the pathogenesis of atherosclerosis. In basic experiments, however, S1P appears to possess both pro-atherosclerotic and anti-atherosclerotic properties; for example, S1P possesses anti-apoptosis, anti-inflammation, and vaso-relaxation properties and maintains the barrier function of endothelial cells, while S1P also promotes the egress and activation of lymphocytes and exhibits pro-thrombotic properties. Recently, the mechanism for the biased distribution of S1P on HDL has been elucidated; apolipoprotein M (apoM) carries S1P on HDL. ApoM is also a modulator of S1P, and the metabolism of apoM-containing lipoproteins largely affects the plasma S1P level. Moreover, apoM modulates the biological properties of S1P. S1P bound to albumin exerts both beneficial and harmful effects in the pathogenesis of atherosclerosis, while S1P bound to apoM strengthens anti-atherosclerotic properties and might weaken the pro-atherosclerotic properties of S1P. Although the detailed mechanisms remain to be elucidated, apoM and S1P might be novel targets for the alleviation of atherosclerotic diseases in the future.

17β-estradiol regulates the expression of apolipoprotein M through estrogen receptor α-specific binding motif in its promoter

Background

We have previously demonstrated that estrogen could significantly enhance expression of apolipoprotein M (apoM), whereas the molecular basis of its mechanism is not fully elucidated yet. To further investigate the mechanism behind the estrogen induced up-regulation of apoM expression.

Results

Our results demonstrated either free 17β-estradiol (E2) or membrane-impermeable bovine serum albumin-conjugated E2 (E2-BSA) could modulate human apoM gene expression via the estrogen receptor alpha (ER-α) pathway in the HepG2 cells. Moreover, experiments with the luciferase activity analysis of truncated apoM promoters could demonstrate that a regulatory region (from-1580 to −1575 bp (−GGTCA-)) upstream of the transcriptional start site of apoM gene was essential for the basal transcriptional activity that regulated by the ER-α. With the applications of an electrophoresis mobility shift assay and a chromatin immunoprecipitation assay, we could successfully identify a specific ER-α binding element in the apoM promoter region.

Conculsion

In summary, the present study indicates that 17β-estradiol induced up-regulation of apoM in HepG2 cells is through an ER-α-dependent pathway involving ER-α binding element in the promoter of the apoM gene.

Vehicle-dependent Effects of Sphingosine 1-phosphate on Plasminogen Activator Inhibitor-1 Expression

Aim: Sphingosine 1-phosphate (S1P) has been suggested to be a positive regulator of plasminogen activator inhibitor 1 (PAI-1) in adipocytes, while some studies are not consistent with this prothrombotic property of S1P. Since S1P is bound to apolipoprotein M (apoM) on HDL or to albumin in plasma, we compared the properties of these two forms on the PAI-1 induction.

Methods: We investigated the associations of S1P, apoM, and PAI-1 concentrations in the plasma of normal coronary artery (NCA), stable angina pectoris (SAP), and acute coronary syndrome (ACS) subjects (n = 32, 71, and 38, respectively). Then, we compared the effects of S1P with various vehicles on the PAI-1 expression in 3T3L1 adipocytes. We also investigated the modulation of the PAI-1 levels in mice infected with adenovirus coding apoM.

Results: Among ACS subjects, the PAI-1 level was positively correlated with the S1P level, but not the apoM level. In adipocytes, S1P bound to an apoM-rich vehicle induced PAI-1 expression to a lesser extent than the control vehicle, while S1P bound to an apoM-depleted vehicle induced PAI-1 expression to a greater extent than the control vehicle in 3T3L1 adipocytes. Additionally, apoM overexpression in mice failed to modulate the plasma PAI-1 level and the adipose PAI-1 expression level. S1P bound to albumin increased PAI-1 expression through the S1P receptor 2-Rho/ROCK-NFκB pathway.

Conclusion: S1P bound to albumin, but not to apoM, induces PAI-1 expression in adipocytes, indicating that S1P can exert different properties on the pathogenesis of vascular diseases, depending on its vehicle.

ApoM/HDL-C and apoM/apoA-I ratios are indicators of diabetic nephropathy in healthy controls and type 2 diabetes mellitus

BACKGROUND

Apolipoprotein M (apoM) concentrations were decreased in type 2 diabetes mellitus (T2DM). ApoM was selectively expressed in renal tubular epithelial cells. We investigated the changes in plasma apoM concentrations in diabetic nephropathy (DN) patients and the potential of apoM as a biomarker of DN.

METHODS

A total of 96 DN patients and 100 age- and sex-matched diabetic non-nephropathy (non-DN) patients and 110 healthy controls were included. All T2DM patients were divided into 3 groups according to urinary albumin excretion: normoalbuminuria (n=100), microalbuminuria (n=50) and macroalbuminuria (n=46). Plasma apoM concentrations were measured by enzyme-linked immunosorbent assay.

RESULTS

DN Patients had higher plasma apoM concentrations than those in non-DN patients (22.23±11.69 vs. 18.96±7.85ng/μl, P<0.05). In addition, microalbuminuria group showed higher plasma apoM concentrations than those in normoalbuminuria group (22.67±11.40 vs. 18.96±7.85ng/μl, P<0.05). The areas under curve (AUC) of apoM using a receiver-operating characteristic (ROC) curve analysis showed that plasma apoM concentrations were not indicators for identification of DN from healthy people (AUC=0.478, P=0.585) and from T2DM (AUC=0.563, P=0.125). DN patients had higher ratios of apoM/HDL-C and apoM/apoA1 than those in healthy controls and in non-DN patients. ApoM/HDL-C and apoM/apoA1 ratios could be used as indicators for identification of DN from healthy people (AUC=0.597, P=0.016; AUC=0.665, P=0.000, respectively) and from T2DM (AUC=0.580, P=0.050; AUC=0.601, P=0.015, respectively).

CONCLUSIONS

ApoM/HDL-C and apoM/apoA1 ratios could be used as indicators for identification of DN from healthy people and from T2DM patients.

High-density lipoprotein-associated sphingosine-1-phosphate activity in heterozygous familial hypercholesterolaemia

BACKGROUND

Patients with heterozygous familial hypercholesterolaemia (FH) suffer from high plasma cholesterol and an environment of increased oxidative stress. We examined its potential effects on high-density lipoprotein (HDL)-associated sphingosine-1-phosphate (S1P) content (HDL-S1P) and HDL-mediated protection against oxidative stress, both with and without statin treatment.

MATERIALS AND METHODS

In a case-control study, HDL was isolated from 12 FH patients with and without statin treatment and from 12 healthy controls. The HDL-S1P content and the capacity of HDL to protect cardiomyocytes against oxidative stress in vitro were measured.

RESULTS

HDL-associated S1P was significantly correlated with cell protection, but not with HDL-cholesterol or apolipoprotein AI. The latter did not correlate with HDL-mediated cell protection. Neither the HDL-S1P content nor HDL protective capacity differed between nontreated FH patients and controls. The relative amounts of apolipoprotein AI and apolipoprotein M were similar between controls and FH patients. Statin treatment had no effect on any of these measures.

CONCLUSIONS

The FH environment is not detrimental to HDL-S1P content or HDL-S1P-mediated cell protection. Statin treatment does not modulate HDL function in this regard.

Positive association between serum apolipoprotein M levels and hepatitis B virus DNA load in HBeAg-negative chronic hepatitis B

BACKGROUND

Hepatitis virus B (HBV) has infected millions of people worldwide. Notably, such infections can be associated with hepatic complications. Levels of apolipoprotein M (apoM), a component of high-density lipoprotein (HDL), are known to be significantly elevated in patients with chronic hepatitis B (CHB). The aim of this study was to investigate the relationship between HBV DNA load in serum and serum apoM levels in patients with CHB.

METHODS

A total of 73 HBeAg-negative CHB patients, 50 HBeAg-positive CHB patients, and 79 non-CHB controls were included in the study cohort. The age and body mass index (BMI) of the study participants were matched. Serum levels of apoM and the HBV antigens HBsAg and HBeAg were measured by enzyme-linked immunosorbent assay (ELISA) analysis. Serum levels of alanine aminotransferase (ALT), aspartate transaminase (AST), cholesterol, and triglycerides (TG) were assessed using an automatic biochemical analyzer. Serum HBV DNA levels were quantified by real-time PCR analysis. Data were analyzed by Spearman's rank correlation coefficient, Pearson correlation coefficient, and multivariate linear regression model (continuous variables), or Student's t-test (mean differences).

RESULTS

Both the HBeAg-negative CHB and HBeAg-positive CHB patient groups exhibited elevated serum levels of apoM. Moreover, serum apoM levels were positively correlated with serum HBV DNA levels in HBeAg-negative CHB patients (r = 0.394, p < 0.001). Conversely, there was no significant relationship between apoM and HBV DNA levels in the HBeAg-positive CHB group (r = 0.197, p = 0.170). The median log copies/mL value for HBV DNA (4.00) was considered the cutoff point for the HBeAg-negative CHB group. Notably, a significant number of patients with HBV DNA levels above the cutoff point also had higher serum apoM levels (63.38 ± 29.84 vs. 41.41 ± 21.84; p = 0.001).

CONCLUSIONS

Our findings reveal that the correlation between serum apoM levels and viral loads may depend on HBeAg status, as serum apoM levels were positively correlated with HBV DNA levels in HBeAg-negative CHB patients. These results suggest that HBeAg may play a role in apoM-related lipid metabolism and anti-inflammatory functions in hepatitis B patients. Thus, our findings may facilitate the clinical management of HBV infection.

Familial hypercholesterolaemia: cholesterol efflux and coronary disease

BACKGROUND

Coronary heart disease (CHD) risk inversely associates with levels of high-density lipoprotein cholesterol (HDL-C). The protective effect of HDL is thought to depend on its functionality, such as its ability to induce cholesterol efflux.

MATERIALS AND METHODS

We compared plasma cholesterol efflux capacity between male familial hypercholesterolaemia (FH) patients with and without CHD relative to their non-FH brothers, and examined HDL constituents including sphingosine-1-phosphate (S1P) and its carrier apolipoprotein M (apoM).

RESULTS

Seven FH patients were asymptomatic and six had experienced a cardiac event at a mean age of 39 years. Compared to their non-FH brothers, cholesterol efflux from macrophages to plasma from the FH patients without CHD was 16 ± 22% (mean ± SD) higher and to plasma from the FH patients with CHD was 7 ± 8% lower (P = 0·03, CHD vs. non-CHD). Compared to their non-FH brothers, FH patients without CHD displayed significantly higher levels of HDL-cholesterol, HDL-S1P and apoM, while FH patients with CHD displayed lower levels than their non-FH brothers.

CONCLUSIONS

A higher plasma cholesterol efflux capacity and higher S1P and apoM content of HDL in asymptomatic FH patients may play a role in their apparent protection from premature CHD.

Identification of polymorphisms in Apolipoprotein M gene and their relationship with risk of recurrent venous thromboembolism

Apolipoprotein M (ApoM) plasma levels have been reported to be associated with risk of venous thromboembolism (VTE) recurrence. However, the role of genetic alterations in the ApoM gene in VTE recurrence remains unknown. The aim of this study was to identify genetic aberrations in ApoM gene in VTE recurrence and their role in prediction of VTE recurrence in a prospective follow-up study of 1465 VTE patients. During follow-up, 156 (10.6 %) patients had VTE recurrence. First screening of whole ApoM gene was performed by Sanger's sequencing in selected age and sex matched non-recurrent and recurrent patients (n=95). In total six polymorphisms were identified and two polymorphisms (rs805297 and rs9404941) with minor allele frequency (MAF) ≥5 % were further genotyped in the whole cohort by Taqman PCR. ApoM rs805297 polymorphism was significantly associated with higher risk of VTE recurrence in males but not in females on both univariate (p= 0.038, hazard ratio = 1.72, confidence interval = 1.03-2.88) and on multivariate analysis adjusted with mild and severe thrombophilia, family history, location and acquired risk factors for VTE. However, ApoM rs9404941 polymorphism showed no significant association with risk of VTE recurrence in all patients as well as in different gender groups. Moreover, ApoM rs805297 and rs9404941 polymorphisms were not associated with the ApoM plasma levels. In conclusion, for the first time we have sequenced whole ApoM gene in VTE and identified six polymorphisms. ApoM rs805297 was significantly associated with higher risk of VTE recurrence in male but not in female patients.

Serum high-density lipoprotein correlates with serum apolipoprotein M and A5 in obstructive sleep apnea hypopnea syndrome

Purpose

The purpose of this study was to investigate the correlation between serum levels of serum apolipoprotein M (ApoM), A5 (ApoA5), and high-density lipoprotein (HDL) in patients with obstructive sleep apnea hypopnea syndrome (OSAHS) and study the effects of nasal continuous positive airway pressure treatment on these serum biomarkers.

Methods

Thirty OSAHS patients and 15 non-OSAHS probands as control were selected for the study. Serum HDL, ApoM, and ApoA5 levels in two groups were detected; differences and association among them were analyzed. Patients with moderate and severe OSAHS underwent 3-month auto-continuous positive airway pressure treatment, and a comparative study was conducted to investigate the changes in blood lipids, serum ApoM, and ApoA5.

Results

In comparison to the control group, the HDL, ApoM, and ApoA5 serum levels were lower (P < 0.05). HDL was positively correlated to ApoM and ApoA5 (P < 0.001), and ApoM was positively correlated to ApoA5 (r = 0.536, P < 0.001). HDL, ApoM, and ApoA5 were significantly increased in the patients of moderate and severe OSAHS after auto-continuous positive airway pressure treatment for 3 months (P < 0.05).

Conclusions

The HDL level was significantly lower in OSAHS patients. The decrease in serum ApoM and ApoA5 in OSAHS patients was correlated to the severity of OSAHS and HDL levels. Auto-continuous positive airway pressure treatment increased serum levels of ApoM, ApoA5, and HDL in OSAHS patients.

High apolipoprotein M serum levels correlate with chronic obstructive pulmonary disease

Background

Recently, variations in a component of high-density lipoprotein (HDL), namely apolipoprotein M (apoM), were found to be associated with chronic obstructive pulmonary disease (COPD). The aim of this study was to evaluate the association between apoM and COPD severity. Factors associated with apoM, COPD, or coronary artery disease (CAD) were also assessed.

Methods

A total of 110 COPD patients and 110 age- and sex-matched non-COPD controls were included. Among them, thirty COPD patients and seven non-COPD controls had CAD. ApoM and pentraxin-3 levels were measured by ELISA. Additionally, the levels of high-sensitivity C-reactive protein (hs-CRP), cholesterol, and triglyceride were assessed using an automatic biochemical analyzer.

Results

Serum apoM levels increased gradually with COPD severity, with the most prominent apoM elevation observed in very severe COPD cases. In addition, ApoM was correlated with percent-predicted forced expiratory volume in one second (% predicted FEV1) (r = −0.38, P < 0.001), low-density lipoprotein cholesterol (LDL-C) (r = 0.23, P < 0.017), and hs-CRP (r = 0.24, P = 0.01) in COPD patients. Furthermore, apoM was shown to be a risk factor for COPD onset (OR = 1.095, 95 % CI = 1.034–1.160, P = 0.002), but not associated with CAD in COPD patients.

Conclusions

Serum apoM was elevated in COPD patients and increased gradually with COPD severity. However, there was no association between apoM and CAD development in COPD patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12944-016-0228-1) contains supplementary material, which is available to authorized users.

Uncleaved ApoM signal peptide is required for formation of large ApoM/sphingosine 1-phosphate (S1P)-enriched HDL particles

Apolipoprotein M (apoM), a plasma sphingosine 1-phosphate (S1P) carrier, associates with plasma HDL via its uncleaved signal peptide. Hepatocyte-specific apoM overexpression in mice stimulates formation of both larger nascent HDL in hepatocytes and larger mature apoM/S1P-enriched HDL particles in plasma by enhancing hepatic S1P synthesis and secretion. Mutagenesis of apoM glutamine 22 to alanine (apoM(Q22A)) introduces a functional signal peptidase cleavage site. Expression of apoM(Q22A) in ABCA1-expressing HEK293 cells resulted in the formation of smaller nascent HDL particles compared with wild type apoM (apoM(WT)). When apoM(Q22A) was expressed in vivo, using recombinant adenoviruses, smaller plasma HDL particles and decreased plasma S1P and apoM were observed relative to expression of apoM(WT). Hepatocytes isolated from both apoM(WT)- and apoM(Q22A)-expressing mice displayed an equivalent increase in cellular levels of S1P, relative to LacZ controls; however, relative to apoM(WT), apoM(Q22A) hepatocytes displayed more rapid apoM and S1P secretion but minimal apoM(Q22A) bound to nascent lipoproteins. Pharmacologic inhibition of ceramide synthesis increased cellular sphingosine and S1P but not medium S1P in both apoM(WT) and apoM(Q22A) hepatocytes. We conclude that apoM secretion is rate-limiting for hepatocyte S1P secretion and that its uncleaved signal peptide delays apoM trafficking out of the cell, promoting formation of larger nascent apoM- and S1P-enriched HDL particles that are probably precursors of larger apoM/S1P-enriched plasma HDL.

The association between apolipoprotein M and insulin resistance varies with country of birth

BACKGROUND AND AIMS

Risk of type 2 diabetes mellitus (T2DM) differs according to ethnicity. Levels of apolipoprotein M (ApoM) have been shown to be decreased in T2DM. However, its role in different ethnicities is not known. We examined the differences in plasma ApoM levels in Swedish residents born in Iraq (Iraqis) and Sweden (Swedes) in relation to T2DM and insulin resistance (IR).

METHODS AND RESULTS

Iraqis and Swedes, aged 45-65 years residing in Rosengård area of Malmö were randomly selected from census records and underwent an oral glucose tolerance test. Plasma levels of ApoM were quantified in 162 participants (Iraqis, n = 91; Swedes, n = 71) by a sandwich ELISA method. Age-, sex-, and body mass index (BMI) adjusted plasma ApoM levels differed by country of birth, with Swedes having 18% higher levels compared to Iraqis (p = 0.001). ApoM levels (mean ± SD) were significantly decreased in Swedes with T2DM (0.73 ± 0.18) compared to those with normal glucose tolerance (NGT) (0.89 ± 0.24; p = 0.03). By contrast, no significant difference in ApoM levels was found between Iraqis with T2DM (0.70 ± 0.17) and those with NGT (0.73 ± 0.13; p = 0.41). In multivariate linear regression analysis with an interaction term between IR and country of birth, low ApoM levels remained significantly associated with IR in Swedes (p = 0.008), independently of age, sex, BMI, family history of diabetes, HDL, LDL, and triglycerides, but not in Iraqis (p = 0.35).

CONCLUSION

Our results show that ApoM levels differ according to country of birth and are associated with IR and T2DM only in Swedes.

Induction of insulin secretion by apolipoprotein M, a carrier for sphingosine 1-phosphate

BACKGROUNDS

High-density lipoprotein (HDL) has been proposed to enhance β-cell functions. Clinical studies have suggested that apolipoprotein M (apoM), which rides mainly on HDL, is involved in diabetes; however, the underlying mechanism has not yet been elucidated. Recently, apoM was shown to be a carrier for sphingosine 1-phosphate (S1P), a bioactive lipid mediator. In the present study, we investigated the modulation of insulin secretion by apoM through the action of S1P.

METHODS AND RESULTS

We overexpressed apoM in the livers of C57BL6 mice using adenovirus gene transfer and found that the blood glucose levels under ad libitum feeding conditions were lower in the apoM-overexpressing mice. While an insulin tolerance test revealed that insulin sensitivity was not significantly affected, a glucose tolerance test revealed that apoM-overexpressing mice had a better glucose tolerance because of enhanced insulin secretion, a phenomenon that was reversed by treatment with VPC 23019, an antagonist against S1P1 and S1P3 receptor. In vitro experiments with MIN6 cells also revealed that apoM-containing lipoproteins enhanced insulin secretion, which was again inhibited by VPC 23019. ApoM retarded the degradation of S1P, and an increase in Pdx1 expression, the attenuation of endoreticulum stress, and the phosphorylation of Akt, AmpK, and Erk were observed as possible underlying mechanisms for the effect of S1P, maintained at a high concentration by apoM, on the increase in insulin secretion.

CONCLUSIONS

ApoM augmented insulin secretion by maintaining the S1P concentration under both in vivo and in vitro conditions.

Modulation of sphingosine-1-phosphate and apolipoprotein M levels in the plasma, liver and kidneys in streptozotocin-induced diabetic mice

AIMS/INTRODUCTION

Sphingosine-1-phosphate (S1P), a multifunctional bioactive lipid mediator, is involved in various diseases. Apolipoprotein M (ApoM) carries S1P on high-density lipoprotein and modulates S1P metabolism to increase the total S1P mass in the body. Both S1P and ApoM are involved in diabetes.

MATERIALS AND METHODS

The present study examined the modulation of S1P and ApoM levels in the plasma, liver and kidneys in streptozotocin-induced diabetes (STZ) mice, and the effects of insulin on the S1P and ApoM levels in the plasma and liver in STZ mice and normal mice. We also examined the effects of insulin and glucose on the ApoM levels in HepG2 cells.

RESULTS

In STZ mice, both the plasma S1P and ApoM levels were higher than those in control mice. The hepatic S1P and ApoM contents were also elevated. The hepatic S1P and ApoM contents were reduced by insulin treatment, whereas high-dose insulin decreased the plasma S1P and ApoM levels. In mice without streptozotocin treatment, the administration of insulin decreased the plasma S1P and ApoM levels, and the hepatic content of ApoM, whereas the hepatic level of S1P was not altered. Treatment with insulin and incubation under a low glucose level decreased the ApoM levels in HepG2 cells. Regarding the kidney, the renal levels of S1P and ApoM were increased in STZ mice, and insulin treatment partially restored this increment.

CONCLUSIONS

In STZ mice, the levels of S1P and ApoM in the plasma, liver, and kidneys were increased. Insulin treatment somehow reversed this modulation in STZ mice.

Palmitic acid suppresses apolipoprotein M gene expression via the pathway of PPARβ/δ in HepG2 cells

It has been demonstrated that apolipoprotein M (APOM) is a vasculoprotective constituent of high density lipoprotein (HDL), which could be related to the anti-atherosclerotic property of HDL. Investigation of regulation of APOM expression is of important for further exploring its pathophysiological function in vivo. Our previous studies indicated that expression of APOM could be regulated by platelet activating factor (PAF), transforming growth factors (TGF), insulin-like growth factor (IGF), leptin, hyperglycemia and etc., in vivo and/or in vitro. In the present study, we demonstrated that palmitic acid could significantly inhibit APOM gene expression in HepG2 cells. Further study indicated neither PI-3 kinase (PI3K) inhibitor LY294002 nor protein kinase C (PKC) inhibitor GFX could abolish palmitic acid induced down-regulation of APOM expression. In contrast, the peroxisome proliferator-activated receptor beta/delta (PPARβ/δ) antagonist GSK3787 could totally reverse the palmitic acid-induced down-regulation of APOM expression, which clearly demonstrates that down-regulation of APOM expression induced by palmitic acid is mediated via the PPARβ/δ pathway.

Rosiglitazone enhances apolipoprotein M (Apom) expression in rat's liver

Apolipoprotein M (APOM) has been suggested as a vasculoprotective constituent of high density lipoprotein (HDL), which plays a crucial role behind the mechanism of HDL-mediated anti-atherosclerosis. Previous studies demonstrated that insulin resistance could associate with decreased APOM expressions. In agreement with our previous reports, here, we further confirmed that the insulin sensitivity was also reduced in rats treated with high concentrations of glucose; such effect could be reversed by administration of rosiglitazone, a peroxisome proliferator-activated receptor-γ (PPARγ). The present study shows that Apom expression is significantly affected by either rosiglitazone or hyperglycemia alone without cross interaction with each other, which indicates that the pathway of Apom expression regulating by hyperglycemia might be differed from that by rosiglitazone. Further study indicated that hyperglycemia could significantly inhibit mRNA levels of Lxrb (P=0.0002), small heterodimer partner 1 (Shp1) (P<0.0001), liver receptor homologue-1 (Lrh1) (P=0.0012), ATP-binding cassette transporter 1 (Abca1) (P=0.0012) and Pparb/d (P=0.0043). Two-way ANOVA analysis demonstrated that the interactions between rosiglitazone and infusion of 25% glucose solution on Shp1 (P=0.0054) and Abca1 (4E, P=0.0004) mRNA expression was statistically significant. It is concluded that rosiglitazone could increase Apom expression, of which the detailed mechanism needs to be further investigated. The downregulation of Apom by hyperglycemia might be mainly through decreasing expression of Pparg and followed by inhibiting Lxrb in rats.