Apolipoprotein M--a novel player in high-density lipoprotein metabolism and atherosclerosis

Plasma apoM Levels and Progression to Kidney Dysfunction in Patients With Type 1 Diabetes

Apolipoprotein M (apoM), primarily carried by HDL, has been associated with several conditions, including cardiovascular disease and diabetic nephropathy. This study proposes to examine whether plasma apoM levels are associated with the development of diabetic kidney disease, assessed as progression to macroalbuminuria (MA) and chronic kidney disease (CKD). Plasma apoM was measured using an enzyme immunoassay in 386 subjects from the Diabetes Control and Complications Trial (DCCT)/Epidemiology of Diabetes Interventions and Complications (EDIC) cohort at DCCT entry and closeout and the concentrations used to determine the association with risk of progression to kidney dysfunction from the time of measurement through 18 years of EDIC follow-up. apoM levels, at DCCT baseline, were higher in patients who developed CKD than in those who retained normal renal function. At DCCT closeout, participants who progressed to MA, CKD, or both MA and CKD also had significantly higher apoM levels than those who remained normal, and increased levels of apoM were associated with increased risk of progression to both MA (risk ratio [RR] 1.30 [95% CI 1.01, 1.66]) and CKD (RR 1.69 [95% CI 1.18, 2.44]). Our results strongly suggest that alterations in apoM and therefore in the composition and function of HDL in type 1 diabetes are present early in the disease process and are associated with the development of nephropathy.

Sphingosine 1-phosphate receptor-targeted therapeutics in rheumatic diseases

Sphingosine 1-phosphate (S1P), which acts via G protein-coupled S1P receptors (S1PRs), is a bioactive lipid essential for vascular integrity and lymphocyte trafficking. The S1P-S1PR signalling axis is a key component of the inflammatory response in autoimmune rheumatic diseases. Several drugs that target S1PRs have been approved for the treatment of multiple sclerosis and inflammatory bowel disease and are under clinical testing for patients with systemic lupus erythematosus (SLE). Preclinical studies support the hypothesis that targeting the S1P-S1PR axis would be beneficial to patients with SLE, rheumatoid arthritis (RA) and systemic sclerosis (SSc) by reducing pathological inflammation. Whereas most preclinical research and development efforts are focused on reducing lymphocyte trafficking, protective effects of circulating S1P on endothelial S1PRs, which maintain the vascular barrier and enable blood circulation while dampening leukocyte extravasation, have been largely overlooked. In this Review, we take a holistic view of S1P-S1PR signalling in lymphocyte and vascular pathobiology. We focus on the potential of S1PR modulators for the treatment of SLE, RA and SSc and summarize the rationale, pathobiology and evidence from preclinical models and clinical studies. Improved understanding of S1P pathobiology in autoimmune rheumatic diseases and S1PR therapeutic modulation is anticipated to lead to efficacious and safer management of these diseases.

The gill transcriptome reveals unique antimicrobial features that protect Nibea albiflora from Cryptocaryon irritans infection

Cryptocaryonosis is the greatest threat to most teleost species among all parasitic diseases, causing mass loss to the marine aquaculture industry. Epidemiological investigation of teleost susceptibility to Cryptocaryon irritans infection revealed that yellow drum (Nibea albiflora) is highly resistant. In order to further understand the activation of the immune system in the gill, which is one of the main mucosal-associated lymphoid tissues and a target of parasites, transcriptome analysis of the yellow drum gill was performed. Gill samples were collected from fish challenged after 24 hr and 72 hr with theronts at a median death rate (2050 theronts per gram fish). Gene expression profiles showed that TLR5 was the only receptor that activated the downstream immune response. The infection activated complement cascade through alternative pathway and increased the expression of C5a anaphylatoxin chemotactic receptor 1. In addition, possible antimicrobial molecules, including lipoprotein and haptoglobin, which are responsible for trypanolysis in humans, were among the top significantly upregulated genes at 24 hr. After 72 hr, the expression of secreted immunoglobulin T-related genes was induced. These results suggested a rapid innate and adaptive immune response at the mucosal level. In conclusion, the results provide new perspectives on mucosal immune resistance in yellow drum against cryptocaryonosis and provide the possibility of mining resistance genes for future therapy.

Interaction Between Apolipoprotein M Gene Single-Nucleotide Polymorphisms and Obesity and its Effect on Type 2 Diabetes Mellitus Susceptibility

This study investigated the correlation of four single nucleotide polymorphisms (SNPs) in Apolipoprotein M (ApoM) with the risk of type 2 diabetes mellitus (T2DM) and effects of the interactions of this gene and obesity. The effects of SNP and obesity interaction on T2DM was examined by generalized multifactor dimensionality reduction (GMDR) combined with the logistic regression model. T2DM patient-control haplotype was analyzed in silico using the haplotype analysis algorithm SHEsis. The rs805296-C allele or 724-del allele indicted high risk of T2DM. The incidence of T2DM in individuals with rs805296-C allele polymorphism (TC + CC) was higher than those without (TT), adjusted OR (95%CI) = 1.29 (1.10–1.66) (p < 0.001). Moreover, the individuals with 724-delallele have a higher risk of T2DM compared to those with 724-ins variants, adjusted OR (95%CI) = 1.66 (1.40–2.06), p < 0.001. GMDR analysis suggested that the interaction model composed of the two factors, rs805296 and obesity, was the best model with statistical significance (P value from sign test [P_sign]=0.0107). The T2DM risk in obese individuals having TC or CC genotype was higher than non-obese individuals with TT genotype (OR = 2.38, 95% CI = 1.58–3.53). Haplotype analysis suggests that rs805297-C and rs9404941-C alleles haplotype indicate high risk of T2DM, OR (95%CI) = 1.62 (1.29–2.16), p < 0.001. Our results suggested that rs805296 and 724-del minor allele of ApoM gene, interaction of rs805296 and obesity, rs805297-C and rs9404941-C alleles haplotype were indicators of high T2DM risk.

[Screening and verification of plasma biomarkers for stable angina pectoris: a differential proteomic analysis]

OBJECTIVE

To compare and analyze the differentially expressed plasma proteome between patients with stable angina pectoris (SAP) and healthy donors to identify the biomarkers for early diagnosis of SAP.

METHODS

Plasma samples from 60 patients with SAP and 60 healthy controls were collected. Twenty samples (100 mL each) randomly selected from each group were pooled and after removing high-abundance proteins from the pooled plasma, two-dimensional gel electrophoresis (2DE) was performed to isolate the total proteins. The protein spots with more than 2 fold changes were selected after 2D analysis using software, and the differentially expressed proteins were identified by MALDI TOF/TOF mass spectrometer. ELISA was performed to detect hemoglobin subunit delta (HBD) levels in 40 randomly selected samples from each group for verification of the results of 2DE.

RESULTS

A total of 7 differentially expressed proteins were found in plasma samples from patients with SAP, including 3 up regulated proteins (serum albumin, hemoglobin subunit alpha and hemoglobin subunit delta,) and 4 down?regulated ones (apolipoprotein L1, apolipoprotein C3, apolipoprotein E and complement C4B). ELISA results showed that HBD level was increased in SAP plasma, which was consistent with the results of 2DE.

CONCLUSION

Patients with SAP have different plasma protein profiles from those of healthy controls, and HBD may serve as a potential specific biomarker for early diagnosis of SAP.

A Shift in ApoM/S1P Between HDL-Particles in Women With Type 1 Diabetes Mellitus Is Associated With Impaired Anti-Inflammatory Effects of the ApoM/S1P Complex

Objective—

Type 1 diabetes mellitus (T1D) patients have an increased risk of cardiovascular disease despite high levels of high-density lipoproteins (HDL). Apolipoprotein M (apoM) and its ligand sphingosine 1-phospate (S1P) exert many of the anti-inflammatory effects of HDL. We investigated whether apoM and S1P are altered in T1D and whether apoM and S1P are important for HDL functionality in T1D.

Approach and Results—

ApoM and S1P were quantified in plasma from 42 healthy controls and 89 T1D patients. HDL was isolated from plasma and separated into dense, medium-dense, and light HDL by ultracentrifugation. Primary human aortic endothelial cells were challenged with tumor necrosis factor-α in the presence or absence of isolated HDL. Proinflammatory adhesion molecules E-selectin and vascular cellular adhesion molecule-1 were quantified by flow cytometry. Activation of the S1P 1 - receptor was evaluated by analyzing downstream signaling targets and receptor internalization. There were no differences in plasma levels of apoM and S1P between controls and T1D patients, but the apoM/S1P complexes were shifted from dense to light HDL particles in T1D. ApoM/S1P in light HDL particles from women were less efficient in inhibiting expression of vascular cellular adhesion molecule-1 than apoM/S1P in denser particles. The light HDL particles were unable to activate Akt, whereas all HDL subfractions were equally efficient in activating Erk and receptor internalization.

Conclusions—

ApoM/S1P in light HDL particles were inefficient in inhibiting tumor necrosis factor-α–induced vascular cellular adhesion molecule-1 expression in contrast to apoM/S1P in denser HDL particles. T1D patients have a higher proportion of light particles and hence more dysfunctional HDL, which could contribute to the increased cardiovascular disease risk associated with T1D.

Implications of cerebrovascular ATP-binding cassette transporter G1 (ABCG1) and apolipoprotein M in cholesterol transport at the blood-brain barrier

Impaired cholesterol/lipoprotein metabolism is linked to neurodegenerative diseases such as Alzheimer's disease (AD). Cerebral cholesterol homeostasis is maintained by the highly efficient blood-brain barrier (BBB) and flux of the oxysterols 24(S)-hydroxycholesterol and 27-hydroxycholesterol, potent liver-X-receptor (LXR) activators. HDL and their apolipoproteins are crucial for cerebral lipid transfer, and loss of ATP binding cassette transporters (ABC)G1 and G4 results in toxic accumulation of oxysterols in the brain. The HDL-associated apolipoprotein (apo)M is positively correlated with pre-β HDL formation in plasma; its presence and function in the brain was thus far unknown. Using an in vitro model of the BBB, we examined expression, regulation, and functions of ABCG1, ABCG4, and apoM in primary porcine brain capillary endothelial cells (pBCEC). RT Q-PCR analyses and immunoblotting revealed that in addition to ABCA1 and scavenger receptor, class B, type I (SR-BI), pBCEC express high levels of ABCG1, which was up-regulated by LXR activation. Immunofluorescent staining, site-specific biotinylation and immunoprecipitation revealed that ABCG1 is localized both to early and late endosomes and on apical and basolateral plasma membranes. Using siRNA interference to silence ABCG1 (by 50%) reduced HDL-mediated [³H]-cholesterol efflux (by 50%) but did not reduce [³H]-24(S)-hydroxycholesterol efflux. In addition to apoA-I, pBCEC express and secrete apoM mainly to the basolateral (brain) compartment. HDL enhanced expression and secretion of apoM by pBCEC, apoM-enriched HDL promoted cellular cholesterol efflux more efficiently than apoM-free HDL, while apoM-silencing diminished cellular cholesterol release. We suggest that ABCG1 and apoM are centrally involved in regulation of cholesterol metabolism/turnover at the BBB.

Correlation analysis between ApoM gene-promoter polymorphisms and coronary heart disease

Objectives:

Apolipoprotein M (ApoM), a 25-kDa plasma protein belonging to the lipocalin protein family, is predominantly associated with high-density lipoprotein cholesterol (HDL-C). Studies have suggested ApoM to be important for the formation of pre-β-HDL and to increase cholesterol efflux from macrophage foam cells. The aim of this study was to explore the association of single-nucleotide polymorphisms(SNPs) in the ApoM promoter with coronary atherosclerotic disease (CAD), and the contribution of latent factors.

Methods:

ApoM was measured in samples from two separate case–control studies, of whom 88 patients developed CAD and 88 were controls. Whole-blood samples from subjects were genotyped by PCR-restriction fragment length polymorphism (PCR-RFLP). Luciferase activities were measured for HepG2 cells with two SNPs, rs805296 (T-778C) and rs940494 (T-855C), and after interfering with or overexpressing the predicted transcription factors. The ability of the SNPs to combine with nucleoproteins was analysed by electophoretic mobility shift assay (EMSA).

Results:

Mean plasma ApoM concentrations in the CAD and non-CAD groups were 9.58 ± 4.30 and 12.22 ± 6.59 μg/ ml, respectively. Correlation studies of ApoM concentrations with several analytes showed a marked positive correlation with HDL-C, fasting plasma glucose and triglyceride levels. The CC genotype showed lower luciferase activities compared to the TC and TT genotypes. The ApoM-855 mutant-typecould bind to the AP-2α. Interference and overexpression of AP-2 increased and decreased luciferase activities of the wild and mutant types to different degrees.

Conclusion::

ApoM may be a biomarker of CAD. ApoM- 855 T→C substitution provides binding sites for AP-2α and reduces ApoM transcription activity.

iTRAQ-based proteomic analysis of plasma reveals abnormalities in lipid metabolism proteins in chronic kidney disease-related atherosclerosis

Patients with chronic kidney disease (CKD) have a considerably higher risk of death due to cardiovascular causes. Using an iTRAQ MS/MS approach, we investigated the alterations in plasma protein accumulation in patients with CKD and classical cardiovascular disease (CVD) without CKD. The proteomic analysis led to the identification of 130 differentially expressed proteins among CVD and CKD patients and healthy volunteers. Bioinformatics analysis revealed that 29 differentially expressed proteins were involved in lipid metabolism and atherosclerosis, 20 of which were apolipoproteins and constituents of high-density lipoprotein (HDL) and low-density lipoprotein (LDL). Although dyslipidemia is common in CKD patients, we found that significant changes in apolipoproteins were not strictly associated with changes in plasma lipid levels. A lack of correlation between apoB and LDL concentration and an inverse relationship of some proteins with the HDL level were revealed. An increased level of apolipoprotein AIV, adiponectin, or apolipoprotein C, despite their anti-atherogenic properties, was not associated with a decrease in cardiovascular event risk in CKD patients. The presence of the distinctive pattern of apolipoproteins demonstrated in this study may suggest that lipid abnormalities in CKD are characterized by more qualitative abnormalities and may be related to HDL function rather than HDL deficiency.

A single-nucleotide polymorphism C-724 /del in the proter region of the apolipoprotein M gene is associated with type 2 diabetes mellitus

BACKGROUND

Apolipoprotein M (apoM) was the carrier of the biologically active lipid mediator sphingosine-1-phospate in high density lipoprotein cholesterol (HDL-C) and played a critical role in formation and maturation of prebeta-HDL-C particles. The plasma apoM levels were decreased obviously in patients with type 2 diabetes mellitus (T2DM). A new single-nucleotide polymorphism (SNP) C-724del in apoM promoter was associated with a higher risk for coronary artery diseases (CAD) and myocardial infarction, could reduce promoter activities and apoM expression in vitro. The primary aim of the present case-controls study was to investigate the effect of apoM SNP C-724del on apoM expression in vivo and its association with T2DM susceptibility in an eastern Han Chinese cohort.

METHODS

Two hundred and fifty-nine T2DM patients and seventy-six healthy controls were included in this study. Amplifying DNA of apoM proximal promoter region including SNP C-724del by Real-Time Polymerase Chain Reaction (RT-PCR) and amplicons sequencing. The plasma apoM concentrations were assayed by enzyme linked immunosorbentassay (ELISA).

RESULTS

Four polymorphic sites, rs805297 (C-1065A), rs9404941 (T-855C), rs805296 (T-778C), C-724del were confirmed. rs805297 (C-1065A) and rs9404941 (T-855C) showed no statistical difference in allele frequencies and genotype distributions between T2DM patients and healthy controls just as previous studies. It's worth noting that the difference of rs805296 (T-778C) between these two groups was not found in this study. In SNP C-724del, the frequency of del allele and mutant genotypes (del/del, C/del) were higher in T2DM patients compared with healthy controls (p = 0.035; P = 0.040, respectively), while the plasma apoM levels of C-724del mutant allele carriers compared with the wide-type homozygotes carriers were not statistically different in T2DM patients (18.20 ± 8.53 ng/uL vs 20.44 ± 10.21 ng/uL, P = 0.245).

CONCLUSION

The polymorphism C-724del in the promoter region of the apoM gene could confer the risk of T2DM among eastern Han Chinese. Unfortunately, the lowing of plasma apoM levels of C-724del mutant allele carriers compared with the wide-type homozygotes carriers in T2DM patients was not statistically different in present study, so further researchs were needed by enlarging the sample.

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.

Decreased Splenic CD4(+) T-Lymphocytes in Apolipoprotein M Gene Deficient Mice

Spleen T-lymphocytes, especially CD4(+) T-cells, have been demonstrated to be involved in broad immunomodulation and host-defense activity in vivo. Apolipoprotein M gene (apoM) may have an important role in the regulation of immunoprocess and inflammation, which could be hypothesized to the apoM containing sphingosine-1-phosphate (S1P). In the present study we demonstrate that the splenic CD4(+) T-lymphocytes were obviously decreased in the apoM gene deficient (apoM(-/-)) mice compared to the wild type (apoM(+/+)). Moreover, these mice were treated with lipopolysaccharide (LPS) and it was found that even more pronounced decreasing CD4(+) T-lymphocytes occurred in the spleen compared to the apoM(+/+) mice. The similar phenomena were found in the ratio of CD4(+)/CD8(+) T-lymphocytes. After administration of LPS, the hepatic mRNA levels of tumor necrosis factor-α (TNF-α) and monocyte chemotactic protein-1 (MCP-1) were markedly increased; however, there were no statistical differences observed between apoM(+/+) mice and apoM(-/-) mice. The present study demonstrated that apoM might facilitate the maintenance of CD4(+) T-lymphocytes or could modify the T-lymphocytes subgroups in murine spleen, which may further explore the importance of apoM in the regulation of the host immunomodulation, although the detailed mechanism needs continuing investigation.

Advances in the Study of the Antiatherogenic Function and Novel Therapies for HDL

The hypothesis that raising high-density lipoprotein cholesterol (HDL-C) levels could improve the risk for cardiovascular disease (CVD) is facing challenges. There is multitudinous clear clinical evidence that the latest failures of HDL-C-raising drugs show no clear association with risks for CVD. At the genetic level, recent research indicates that steady-state HDL-C concentrations may provide limited information regarding the potential antiatherogenic functions of HDL. It is evident that the newer strategies may replace therapeutic approaches to simply raise plasma HDL-C levels. There is an urgent need to identify an efficient biomarker that accurately predicts the increased risk of atherosclerosis (AS) in patients and that may be used for exploring newer therapeutic targets. Studies from recent decades show that the composition, structure and function of circulating HDL are closely associated with high cardiovascular risk. A vast amount of data demonstrates that the most important mechanism through which HDL antagonizes AS involves the reverse cholesterol transport (RCT) process. Clinical trials of drugs that specifically target HDL have so far proven disappointing, so it is necessary to carry out review on the HDL therapeutics.

A systematic analysis of human lipocalin family and its expression in esophageal carcinoma

The lipocalin proteins (lipocalins) are a large family of small proteins characterized by low sequence similarity and highly conserved crystal structures. Lipocalins have been found to play important roles in many human diseases. For this reason, a systemic analysis of the molecular properties of human lipocalins is essential. In this study, human lipocalins were found to contain four structurally conserved regions (SCRs) and could be divided into two subgroups. A human lipocalin protein-protein interaction network (PPIN) was constructed and integrated with their expression data in esophageal carcinoma. Many lipocalins showed obvious co-expression patterns in esophageal carcinoma. Their subcellular distributions also suggested these lipocalins may transfer signals from the extracellular space to the nucleus using the pathway-like paths. These analyses also expanded our knowledge about this human ancient protein family in the background of esophageal carcinoma.

The miRNAome of the postpartum dairy cow liver in negative energy balance

Background

Negative energy balance (NEB) is an altered metabolic state in high yielding cows that occurs during the first few weeks postpartum when energy demands for lactation and maintenance exceed the energy supply from dietary intake. NEB can, in turn, lead to metabolic disorders and to reduced fertility. Alterations in the expression of more than 700 hepatic genes have previously been reported in a study of NEB in postpartum dairy cows. miRNAs (microRNA) are known to mediate many alterations in gene expression post transcriptionally. To study the hepatic miRNA content of postpartum dairy cows, including their overall abundance and differential expression, in mild NEB (MNEB) and severe NEB (SNEB), short read RNA sequencing was carried out. To identify putative targets of differentially expressed miRNAs among differentially expressed hepatic genes reported previously in dairy cows in SNEB computational target identification was employed.

Results

Our results indicate that the dairy cow liver expresses 53 miRNAs at a lower threshold of 10 reads per million. Of these, 10 miRNAs accounted for greater than 95% of the miRNAome (miRNA content). Of the highly expressed miRNAs, miR-122 constitutes 75% followed by miR-192 and miR-3596. Five out of thirteen let-7 miRNA family members are also among the highly expressed miRNAs. miR-143, down-regulated in SNEB, was found to have 4 putative up-regulated gene targets associated with SNEB including LRP2 (low density lipoprotein receptor-related protein 2), involved in lipid metabolism and up-regulated in SNEB.

Conclusions

This is the first liver miRNA-seq profiling study of moderate yielding dairy cows in the early postpartum period. Tissue specific miR-122 and liver enriched miR-192 are two of the most abundant miRNAs in the postpartum dairy cow liver. miR-143 is significantly down-regulated in SNEB and putative targets of miRNA-143 which are up-regulated in SNEB, include a gene involved in lipid metabolism.

Decreased activities of apolipoprotein m promoter are associated with the susceptibility to coronary artery diseases

The present study investigated the correlation among genetic polymorphisms of the proximal promoter region of apolipoprotein M (apoM) gene, the polymorphisms in relation to apoM expressions and the susceptibility to coronary artery diseases (CAD) in a Han Chinese population. Four common polymorphic sites, i.e., T-1628G, C-1065A, T-855C and T-778C, were confirmed, and a new deletion mutation C-724del was found, in 206 CAD patients and 209 non-CAD patients using direct DNA sequencing analyses. Occurrences of alleles T-1628G, T-855C and C-724del were significantly higher in CAD patients compared to non-CAD patients. Moreover we examined all these polymorphisms in relation to apoM expression by applying luciferase reporter assay. It demonstrated that constructs -855C and 724del showed obvious decreased luciferase activities, i.e., (0.93±0.15 vs. 2.11±0.15; P=0.012) and (1.13±0.25 vs. 2.11±0.15; P=0.009) respectively, which indicates these two polymorphisms could confer decreased apoM expressions. Meanwhile the occurrences of these two SNP were also significantly higher in the CAD patients than in non-CAD patients. It is therefore reasonable to speculate that down-regulated apoM expressions in relation to these polymorphisms may affect HDL and cholesterol metabolism in vivo and further influence the susceptibility to CAD, although the underlying mechanisms need further investigation.

Enchanced levels of apolipoprotein M during HBV infection feedback suppresses HBV replication

Background

Chronic liver diseases can interfere with hepatic metabolism of lipoproteins, apolipoproteins. Hepatitis B virus (HBV) is a major etiological agent causing acute and chronic liver diseases. Apolipoprotein M (ApoM) is a high-density lipoprotein (HDL) apolipoprotein and exclusively expressed in the liver parenchyma cells and in the tubular cells of the kidney. This study was to determine the correlation between HBV infection and ApoM expression.

Materials and methods

Serum ApoM levels in patients with HBV infection and in healthy individuals were measured by ELISA, ApoM mRNA expression were determined by RT-PCR, and the expression of S and E proteins of HBV, as well as the synthesis of viral DNA were measured by ELISA and real-time PCR.

Results

The levels of serum ApoM was significantly elevated in patients as compared to healthy individuals (P < 0.001), ApoM promoter activity, mRNA and protein expression were all stimulated in cells transfected with infectious HBV clone. In addition, ApoM decreases the expression of S and E proteins of HBV and the synthesis of viral DNA.

Conclusion

Raised ApoM levels in HBV infection may in turn suppress HBV replication, one of the protective mechanisms of nature.

Apolipoprotein M gene (APOM) polymorphism modifies metabolic and disease traits in type 2 diabetes

This study aimed at substantiating the associations of the apolipoproein M gene (APOM) with type 2 diabetes (T2D) as well as with metabolic traits in Hong Kong Chinese. In addition, APOM gene function was further characterized to elucidate its activity in cholesterol metabolism. Seventeen APOM SNPs documented in the NCBI database were genotyped. Five SNPs were confirmed in our study cohort of 1234 T2D and 606 control participants. Three of the five SNPs rs707921(C+1871A), rs707922(G+1837T) and rs805264(G+203A) were in linkage disequilibrium (LD). We chose rs707922 to tag this LD region for down stream association analyses and characterized the function of this SNP at molecular level. No association between APOM and T2D susceptibility was detected in our Hong Kong Chinese cohort. Interestingly, the C allele of rs805297 was significantly associated with T2D duration of longer than 10 years (OR = 1.245, p = 0.015). The rs707922 TT genotype was significantly associated with elevated plasma total- and LDL- cholesterol levels (p = 0.006 and p = 0.009, respectively) in T2D patients. Molecular analyses of rs707922 lead to the discoveries of a novel transcript APOM5 as well as the cryptic nature of exon 5 of the gene. Ectopic expression of APOM5 transcript confirmed rs707922 allele-dependent activity of the transcript in modifying cholesterol homeostasis in vitro. In conclusion, the results here did not support APOM as a T2D susceptibility gene in Hong Kong Chinese. However, in T2D patients, a subset of APOM SNPs was associated with disease duration and metabolic traits. Further molecular analysis proved the functional activity of rs707922 in APOM expression and in regulation of cellular cholesterol content.

Regulation of cholesterol homeostasis by liver X receptors

Cellular cholesterol levels reflect a balance between uptake, efflux, and endogenous synthesis. The sterol-responsive transcription factors liver X receptors (LXRalpha and LXRbeta) help maintain cholesterol homeostasis, not only through promotion of cholesterol efflux from peripheral tissues but also through suppression of de novo synthesis and exogenous cholesterol uptake. In this review, we summarize the important role of LXRs in regulating expression of key members that keep cholesterol levels in balance.

Apolipoprotein M: Progress in understanding its regulation and metabolic functions

ApoM is a novel apolipoprotein mainly present in high-density lipoprotein (HDL). It belongs to the lipocalin protein superfamily and may bind a small but so far unknown lipophilic ligand. It is secreted without cleavage of its hydrophobic signal peptide, which probably anchors apoM in the phospholipid moiety of plasma lipoproteins. Recent studies suggest that apoM may affect HDL metabolism and have anti-atherogenic functions. The subfraction of human HDL that contains apoM therefore protects LDL from oxidation and mediates cholesterol efflux more efficiently then HDL without apoM. In addition to hepatocytes, apoM is highly expressed in kidney proximal tubule cells. Recent data suggest that apoM is secreted into the pre-urine from the tubule cells but is normally taken up again in a megalin-dependent fashion. Further studies of mice with genetically modified apoM expression will be essential to unravel the potential roles of apoM in lipoprotein metabolism, atherosclerosis and kidney biology.

Evaluation of Apolipoprotein M Serum Concentration as a Biomarker of HNF-1alpha MODY

Apolipoprotein M (apoM) is a 26-kDa protein expressed mainly in the liver and kidneys. It is present predominantly in high-density lipoproteins (HDL). ApoM expression is influenced by the hepatocyte nuclear factor-1alpha (HNF-1alpha), which is a transcription factor associated with the pathogenesis of MODY. Some earlier data suggested that apoM levels were lower in the serum of HNF-1alpha MODY subjects, than in that of other diabetics and healthy controls. The aim of this study was to evaluate apoM as a biomarker for HNF-1alpha MODY. We included in this study 48 HNF-1alpha mutation carriers (40 diabetic patients and 8 subjects with normal glucose levels in the fasted state) from the Polish Nationwide Registry of MODY. In addition, we examined 55 T2DM patients and 55 apparently healthy volunteers who had normal fasting glucose levels. ApoM was measured by the sandwich dot-blot technique with recombinant apoM (Abnova) as a protein standard, mouse anti-human apoM monoclonal primary antibody and rat anti-mouse HRP-conjugated secondary antibody (BD Biosciences). Mean apoM level in the MODY group was 13.6 mug/ml, SD 1.9 (13.5 mug/ml, SD 1.7 in diabetic subjects and 13.9 mug/ml, SD 2.0 in non-diabetic mutation carriers respectively). In the T2DM group, mean apoM level was 13.7 mug/ml, SD 2.1, while it reached 13.8 mug/ml, SD 2.0 in healthy controls. There was no difference between apoM serum concentrations in all the study groups. In summary, our study showed no association between HNF-1alpha mutations resulting in MODY phenotype and apoM levels. Thus, we cannot confirm the clinical usefulness of apoM as a biomarker of HNF-1alpha MODY.

Effect of apolipoprotein M on high density lipoprotein metabolism and atherosclerosis in low density lipoprotein receptor knock-out mice

To investigate the role of apoM in high density lipoprotein (HDL) metabolism and atherogenesis, we generated human apoM transgenic (apoM-Tg) and apoM-deficient (apoM(-/-)) mice. Plasma apoM was predominantly associated with 10-12-nm alpha-migrating HDL particles. Human apoM overexpression (11-fold) increased plasma cholesterol concentration by 13-22%, whereas apoM deficiency decreased it by 17-21%. The size and charge of apoA-I-containing HDL in plasma were not changed in apoM-Tg or apoM(-/-) mice. However, in plasma incubated at 37 degrees C, lecithin:cholesterol acyltransferase-dependent conversion of alpha- to pre-alpha-migrating HDL was delayed in apoM-Tg mice. Moreover, lecithin: cholesterol acyltransferase-independent generation of pre-beta-migrating apoA-I-containing particles in plasma was increased in apoM-Tg mice (4.2 +/- 1.1%, p = 0.06) and decreased in apoM(-/-) mice (0.5 +/- 0.3%, p = 0.03) versus controls (1.8 +/- 0.05%). In the setting of low density lipoprotein receptor deficiency, apoM-Tg mice with approximately 2-fold increased plasma apoM concentrations developed smaller atherosclerotic lesions than controls. The effect of apoM on atherosclerosis may be facilitated by enzymatic modulation of plasma HDL particles, increased cholesterol efflux from foam cells, and an antioxidative effect of apoM-containing HDL.

Regulation of anti-atherogenic apolipoprotein M gene expression by the orphan nuclear receptor LRH-1

The orphan nuclear receptor liver receptor homolog-1 (LRH-1, NR5A2) has been reported to play a crucial role in early development, in the control of the hepatic inflammatory response, in intestinal cell crypt renewal as well as in bile acid biosynthesis and reverse cholesterol transport (RCT). Here, we report the identification of apolipoprotein M (APOM) as a novel target gene for LRH-1. Using gene-silencing experiments, adenovirus-mediated overexpression, transient transfection, and chromatin immunoprecipitation (ChIP) assays, it is shown that LRH-1 directly regulates human and mouse APOM transcription by binding to an LRH-1 response element located in the proximal APOM promoter region. In addition, we demonstrate that bile acids suppress APOM expression in a SHP-dependent manner in vitro and in vivo by inhibiting LRH-1 transcriptional activity on the APOM promoter as demonstrated by in vivo ChIP assay. Taken together, our results demonstrate that LRH-1 is a novel regulator of APOM transcription and further extend the role of this orphan nuclear receptor in lipoprotein metabolism and cholesterol homeostasis.

Characterization of apolipoprotein M isoforms in low-density lipoprotein

Apo M is a recently discovered human lipoprotein thought to be involved in the metabolism of lipids and lipoprotein particles. Here, a proteomic approach was applied to examine the glycosylation pattern of apo M in human LDL. We treated LDL proteins with N-glycosidase or neuraminidase, studied mobility shifts of Apo M by two-dimensional gel electrophoresis, and different isoforms were then identified with mass spectrometry. This way, we demonstrated the presence of five isoforms of apo M in LDL: three that are both N-glycosylated and sialylated, one that is N-glycosylated but not sialylated, and one that is neither N-glycosylated nor sialylated. As judged from the examination of LDL from 20 healthy human subjects, the three N-glycosylated and sialylated forms are most abundant (80-100% of the total apo M in LDL) whereas the unsialylated and unglycosylated variants constitute at most 20%. Comparative analysis showed that the same five isoforms of apo M are also present in HDL. Further studies aiming at elucidating the role of apo M in health and disease will have to take this polymorphism of apo M proteins into account.