Opposing effects of apolipoprotein m on catabolism of apolipoprotein B-containing lipoproteins and atherosclerosis

Association of apolipoprotein M and sphingosine-1-phosphate with brown adipose tissue after cold exposure in humans

The HDL-associated apolipoprotein M (apoM) and its ligand sphingosine-1-phosphate (S1P) may control energy metabolism. ApoM deficiency in mice is associated with increased vascular permeability, brown adipose tissue (BAT) mass and activity, and protection against obesity. In the current study, we explored the connection between plasma apoM/S1P levels and parameters of BAT as measured via ¹⁸F-FDG PET/CT after cold exposure in humans. Fixed (n = 15) vs personalized (n = 20) short-term cooling protocols decreased and increased apoM (- 8.4%, P = 0.032 vs 15.7%, P < 0.0005) and S1P (- 41.0%, P < 0.0005 vs 19.1%, P < 0.005) plasma levels, respectively. Long-term cooling (n = 44) did not affect plasma apoM or S1P levels. Plasma apoM and S1P did not correlate significantly to BAT volume and activity in the individual studies. However, short-term studies combined, showed that increased changes in plasma apoM correlated with BAT metabolic activity (β: 0.44, 95% CI [0.06-0.81], P = 0.024) after adjusting for study design but not BAT volume (β: 0.39, 95% CI [- 0.01-0.78], P = 0.054). In conclusion, plasma apoM and S1P levels are altered in response to cold exposure and may be linked to changes in BAT metabolic activity but not BAT volume in humans. This contrasts partly with observations in animals and highlights the need for further studies to understand the biological role of apoM/S1P complex in human adipose tissue and lipid metabolism.

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.

The apoM/S1P Complex-A Mediator in Kidney Biology and Disease?

Kidney disease affects more than 10% of the population, can be both acute and chronic, and is linked to other diseases such as cardiovascular disease, diabetes, and sepsis. Despite the detrimental consequences for patients, no good treatment options directly targeting the kidney are available. Thus, a better understanding of the pathology and new treatment modalities are required. Accumulating evidence suggests that the apolipoprotein M/sphingosine-1-phosphate (apoM/S1P) axis is a likely drug target, but significant gaps in our knowledge remain. In this review, we present what has so far been elucidated about the role of apoM in normal kidney biology and describe how changes in the apoM/S1P axis are thought to affect the development of kidney disease. ApoM is primarily produced in the liver and kidneys. From the liver, apoM is secreted into circulation, where it is attached to lipoproteins (primarily HDL). Importantly, apoM is a carrier of the bioactive lipid S1P. S1P acts by binding to five different receptors. Together, apoM/S1P plays a role in several biological mechanisms, such as inflammation, endothelial cell permeability, and lipid turnover. In the kidney, apoM is primarily expressed in the proximal tubular cells. S1P can be produced locally in the kidney, and several of the five S1P receptors are present in the kidney. The functional role of kidney-derived apoM as well as plasma-derived apoM is far from elucidated and will be discussed based on both experimental and clinical studies. In summary, the current studies provide evidence that support a role for the apoM/S1P axis in kidney disease; however, additional pre-clinical and clinical studies are needed to reveal the mechanisms and target potential in the treatment of patients.

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.

Increased plasma apoM levels impair triglyceride turnover in mice

OBJECTIVE

Apolipoprotein M (apoM) is an essential transporter of plasma Sphingosine-1-Phosphate (S1P), typically attached to all lipoprotein classes, but with a majority bound to high density lipoproteins (HDL). ApoM-deficient mice display an increased activity in brown adipose tissue and a concomitant fast turnover of triglycerides. In what manner apoM/S1P affect the triglyceride metabolism is however still unknown and explored in the present study.

METHODS

Triglyceride turnover and potentially associated metabolic pathways were studied in the female human apoM transgenic mouse model (apoM-Tg) with increased plasma apoM and S1P levels. The model was compared with wild type (WT) mice.

RESULTS

ApoM-Tg mice had a reduced plasma triglyceride turnover rate and a lower free fatty acid uptake in subcutaneous adipocytes compared to WT mice. Screening for potential molecular mechanisms furthermore revealed a reduction in plasma lipase activity in apoM-Tg animals. Overexpression of apoM also reduced the plasma levels of fibroblast growth factor 21 (FGF21).

CONCLUSIONS

The study features the significant role of the apoM/S1P axis in maintaining a balanced triglyceride metabolism. Further, it also highlights the risk of inducing dyslipidaemia in patients receiving S1P-analouges and additionlly emphasizes the apoM/S1P axis as a potential therapeutic target in treatment of hypertriglyceridemia.

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.

Apolipoprotein M Gene Polymorphism Rs805297 (C-1065A): Association With Type 2 Diabetes Mellitus and Related Microvascular Complications in South Egypt

BACKGROUND

Apolipoprotein M (ApoM) may have a role in the susceptibility of type 2 diabetes mellitus (T2DM). Polymorphisms in the promoter region of the ApoM gene were found to be significantly associated with diabetes. The aim of this study was to investigate the association of ApoM SNP rs805297 (C-1065A) with the susceptibility of T2DM and related microvascular complications in South Egypt.

METHODS

We conducted a case-control study of 60 T2DM patients and 60 healthy control subjects. Lipid profile, fasting, and 2 hours postprandial glucose and creatinine levels were estimated. Patients were subjected to general and Fundus examinations, and screening for nephropathy by urinary albumin levels. ApoM level was assayed by ELISA. Genotyping of the human ApoM gene polymorphism SNP rs805297 (C-1065A) was done by PCR-restriction fragment length polymorphism followed by sequencing to confirm the polymorphism results.

RESULTS

ApoM was not different between T2DM and the control group (p=0.075) and was negatively correlated with LDL-c (p=0.029). There were significant differences in ApoM genotypes (p=0.001) and allele frequencies (p=0.019) between T2DM and the control group. A significant reduction in FBG, 2hPPG, and HbA1c levels in the heterozygous than the wild genotype in the group with diabetes with no difference in other lab parameters and microvascular complications. The C-allele is associated with lower blood glucose levels and retinopathy. The wild (CC) genotype is considered as a risk factor for developing T2DM in South Egyptians but not CA+AA genotypes.

CONCLUSIONS

In South Egyptians the ApoM polymorphism rs805297 (C-1065A) wild type (CC) was associated with T2DM susceptibility and may have a role in controlling hyperglycemia in these patients. The A-allele is associated with hyperglycemia and diabetic retinopathy.

Apolipoprotein M-A Marker or an Active Player in Type II Diabetes?

Apolipoprotein M (apoM) is a member of the lipocalin superfamily and an important carrier of the small bioactive lipid sphingosine-1-phosphate (S1P). The apoM/S1P complex is attached to all lipoproteins, but exhibits a significant preference for high-density lipoproteins. Although apoM, S1P, and the apoM/S1P complex have been discovered more than a decade earlier, the overall function of the apoM/S1P complex remains controversial. Evidence suggests that the complex plays a role in inflammation and cholesterol metabolism and is important for maintaining a healthy endothelial barrier, regulating the turnover of triglycerides from lipoproteins, and reducing cholesterol accumulation in vessel walls. Recent studies have also addressed the role of apoM and S1P in the development of diabetes and obesity. However, limited evidence is available, and the data published so far deviates. This review discusses the specific elements indicative of the protective or harmful effects of apoM, S1P, and the apoM/S1P complex on type 2 diabetes development. Since drugs targeting the S1P system and its receptors are available and could be potentially used for treating diabetes, this research topic is a pertinent one.

Apolipoprotein M and Risk of Type 2 Diabetes

CONTEXT

Recent studies have discovered a role of apolipoprotein M (apoM) in energy metabolism, and observational analyses in humans suggest an association with type 2 diabetes. The causal relationship remains however elusive.

OBJECTIVE

To investigate whether reduced plasma apoM concentrations are causally linked to increased risk of type 2 diabetes.

DESIGN

Prospective study design analyzed by Mendelian randomization.

SETTING AND PARTICIPANTS

Two cohorts reflecting the Danish general population: the Copenhagen City Heart Study (CCHS, n = 8589) and the Copenhagen General Population Study (CGPS; n = 93 857). Observational analyses included a subset of participants from the CCHS with available plasma apoM (n = 725). Genetic analyses included the complete cohorts (n = 102 446). During a median follow-up of 16 years (CCHS) and 8 years (CGPS), 563 and 2132 participants developed type 2 diabetes.

MAIN OUTCOME MEASURES

Plasma apoM concentration, genetic variants in APOM, and type 2 diabetes.

RESULTS

First, we identified an inverse correlation between plasma apoM and risk of type 2 diabetes in a subset of participants from the CCHS (hazard ratio between highest vs lowest quartile (reference) = 0.32; 95% confidence interval = 0.1-1.01; P for trend = .02). Second, genotyping of specific single nucleotide polymorphisms in APOM further revealed a 10.8% (P = 6.2 × 10-5) reduced plasma apoM concentration in participants with variant rs1266078. Third, a meta-analysis including data from 599 451 individuals showed no association between rs1266078 and risk of type 2 diabetes.

CONCLUSIONS

The present study does not appear to support a causal association between plasma apoM and risk of type 2 diabetes.

NPC1L1 Facilitates Sphingomyelin Absorption and Regulates Diet-Induced Production of VLDL/LDL-associated S1P

Niemann-Pick C1-Like 1 (NPC1L1) is a cholesterol importer and target of ezetimibe, a cholesterol absorption inhibitor used clinically for dyslipidemia. Recent studies demonstrated that NPC1L1 regulates the intestinal absorption of several fat-soluble nutrients, in addition to cholesterol. The study was conducted to reveal new physiological roles of NPC1L1 by identifying novel dietary substrate(s). Very low-density lipoprotein and low-density lipoprotein (VLDL/LDL) are increased in Western diet (WD)-fed mice in an NPC1L1-dependent manner, so we comprehensively analyzed the NPC1L1-dependent VLDL/LDL components. Apolipoprotein M (apoM), a binding protein of sphingosine-1-phosphate (S1P: a lipid mediator), and S1P were NPC1L1-dependently increased in VLDL/LDL by WD feeding. S1P is metabolized from sphingomyelin (SM) and SM is abundant in WD, so we focused on intestinal SM absorption. In vivo studies with Npc1l1 knockout mice and in vitro studies with NPC1L1-overexpressing cells revealed that SM is a physiological substrate of NPC1L1. These results suggest a scenario in which dietary SM is absorbed by NPC1L1 in the intestine, followed by SM conversion to S1P and, after several steps, S1P is exported into the blood as the apoM-bound form in VLDL/LDL. Our findings provide insight into the functions of NPC1L1 for a better understanding of sphingolipids and S1P homeostasis.

Druggable Sphingolipid Pathways: Experimental Models and Clinical Opportunities

Intensive research in the field of sphingolipids has revealed diverse roles in cell biological responses and human health and disease. This immense molecular family is primarily represented by the bioactive molecules ceramide, sphingosine, and sphingosine 1-phosphate (S1P). The flux of sphingolipid metabolism at both the subcellular and extracellular levels provides multiple opportunities for pharmacological intervention. The caveat is that perturbation of any single node of this highly regulated flux may have effects that propagate throughout the metabolic network in a dramatic and sometimes unexpected manner. Beginning with S1P, the receptors for which have thus far been the most clinically tractable pharmacological targets, this review will describe recent advances in therapeutic modulators targeting sphingolipids, their chaperones, transporters, and metabolic enzymes.

Novel Insights into the Role of HDL-Associated Sphingosine-1-Phosphate in Cardiometabolic Diseases

Sphingolipids are key signaling molecules involved in the regulation of cell physiology. These species are found in tissues and in circulation. Although they only constitute a small fraction in lipid composition of circulating lipoproteins, their concentration in plasma and distribution among plasma lipoproteins appears distorted under adverse cardiometabolic conditions such as diabetes mellitus. Sphingosine-1-phosphate (S1P), one of their main representatives, is involved in regulating cardiomyocyte homeostasis in different models of experimental cardiomyopathy. Cardiomyopathy is a common complication of diabetes mellitus and represents a main risk factor for heart failure. Notably, plasma concentration of S1P, particularly high-density lipoprotein (HDL)-bound S1P, may be decreased in patients with diabetes mellitus, and hence, inversely related to cardiac alterations. Despite this, little attention has been given to the circulating levels of either total S1P or HDL-bound S1P as potential biomarkers of diabetic cardiomyopathy. Thus, this review will focus on the potential role of HDL-bound S1P as a circulating biomarker in the diagnosis of main cardiometabolic complications frequently associated with systemic metabolic syndromes with impaired insulin signaling. Given the bioactive nature of these molecules, we also evaluated its potential of HDL-bound S1P-raising strategies for the treatment of cardiometabolic disease.

Apolipoprotein M/sphingosine-1-phosphate: novel effects on lipids, inflammation and kidney biology

PURPOSE OF REVIEW

In 2011, the crystal structure of apolipoprotein M (apoM) and its capacity to bind sphingosine-1-phosphate (S1P) was characterized. Since then, a variety of studies has increased our knowledge on apoM biology and functionality. From being an unknown and hardly significant player in overall metabolism, apoM has gained significant interest.

RECENT FINDINGS

Key discoveries in the last 2 years have indicated that the apoM/S1P complex has important roles in lipid metabolism (affecting triglyceride turnover), inflammation (a marker of severe sepsis and potentially providing anti-inflammatory signaling) and kidney biology (potential to protect against immunoglobulin A nephropathy).

SUMMARY

Several studies suggest a potential for apoM/S1P as biomarkers for inflammation, sepsis and nephropathy. Also, a novel chaperone is characterized and could have potential as a drug for treatment in inflammation and nephropathy.

Plasma apoM and S1P levels are inversely associated with mortality in African Americans with type 2 diabetes mellitus

apoM is a minor HDL apolipoprotein and carrier for sphingosine-1-phosphate (S1P). HDL apoM and S1P concentrations are inversely associated with atherosclerosis progression in rodents. We evaluated associations between plasma concentrations of S1P, plasma concentrations of apoM, and HDL apoM levels with prevalent subclinical atherosclerosis and mortality in the African American-Diabetes Heart Study participants (N = 545). Associations between plasma S1P, plasma apoM, and HDL apoM with subclinical atherosclerosis and mortality were assessed using multivariate parametric, nonparametric, and Cox proportional hazards models. At baseline, participants' median (25th percentile, 75th percentile) age was 55 (49, 62) years old and their coronary artery calcium (CAC) mass score was 26.5 (0.0, 346.5). Plasma S1P, plasma apoM, and HDL apoM were not associated with CAC. After 64 (57.6, 70.3) months of follow-up, 81 deaths were recorded. Higher concentrations of plasma S1P [odds ratio (OR) = 0.14, P = 0.01] and plasma apoM (OR = 0.10, P = 0.02), but not HDL apoM (P = 0.89), were associated with lower mortality after adjusting for age, sex, statin use, CAC, kidney function, and albuminuria. We conclude that plasma S1P and apoM concentrations are inversely and independently associated with mortality, but not CAC, in African Americans with type 2 diabetes after accounting for conventional risk factors.

Potent anti-inflammatory properties of HDL in vascular smooth muscle cells mediated by HDL-S1P and their impairment in coronary artery disease due to lower HDL-S1P: a new aspect of HDL dysfunction and its therapy

Dysfunctional HDL is associated with coronary artery disease (CAD), but its effect on inflammation in vascular smooth muscle cells (VSMCs) in atherosclerosis is unknown. We investigated the effect of healthy human HDL and CAD-HDL on TNF-α-driven inflammation in VSMCs and examined whether HDL-associated sphingosine-1-phosphate (HDL-S1P) could modulate inflammation with the aim of designing novel HDL-based anti-inflammatory strategies. Healthy human HDL, human CAD-HDL, and mouse HDL were isolated by ultracentrifugation, S1P was measured by liquid chromatography-tandem mass spectrometry, and TNF-α-induced inflammation was characterized by gene expression and analysis of NF-κB-dependent signaling. Mechanisms of S1P interference with TNF-α were assessed by S1P receptor antagonists, mouse knockouts, and short interfering RNA. We observed that healthy HDL potently inhibited the induction of TNF-α-stimulated inflammatory genes, such as iNOS (inducible NO synthase) and MMP9 (matrix metalloproteinase 9), a process that was entirely dependent on HDL-S1P, as evidenced by loss-of-function using S1P-less HDL and mimicked by genuine S1P. Inhibition was based on suppression of TNF-α-activated Akt signaling resulting in reduced IkBαSer32 and p65Ser534 NF-κB phosphorylation based on a persistent phosphatase and tensin homolog activation by S1P through the S1P receptor 2. Intriguingly, S1P suppressed inflammation even hours after initial TNF-α stimulation. The anti-inflammatory effect of healthy HDL correlated with HDL-S1P content and was superior to that of CAD-HDL featuring lower HDL-S1P. Nevertheless, therapeutic loading of HDL with S1P completely restored the anti-inflammatory capacity of CAD-HDL and greatly boosted that of both healthy and CAD-HDL. Suppression of inflammation by HDL-S1P defines a novel pathophysiologic characteristic that distinguishes functional from dysfunctional HDL. The anti-inflammatory HDL function can be boosted by S1P-loading and exploited by S1P receptor-targeting to prevent and even turn off ongoing inflammation.-Keul, P., Polzin, A., Kaiser, K., Gräler, M., Dannenberg, L., Daum, G., Heusch, G., Levkau, B. Potent anti-inflammatory properties of HDL in vascular smooth muscle cells mediated by HDL-S1P and their impairment in coronary artery disease due to lower HDL-S1P: a new aspect of HDL dysfunction and its therapy.

Apolipoprotein M in patients with chronic kidney disease

BACKGROUND AND AIMS

Plasma apolipoprotein M (APOM) is bound to HDL-particles and has anti-atherogenic effects. The present study explored whether plasma APOM is reduced in patients with chronic kidney disease (CKD), and associated with cardiovascular disease (CVD). In addition, we tested the hypothesis that the excretion of APOM into the urine is increased in patients with kidney disease.

METHODS

Plasma samples were collected from a cohort of patients with CKD stages 1 to 5D (N = 409) and controls (N = 35). Urine was collected from 47 subjects. Plasma APOM was measured with sandwich ELISA and urine APOM with competitive ELISA.

RESULTS

Plasma APOM levels were reduced in patients with CKD stages 3-5D as compared to patients with CKD stages 1 + 2 and controls (p < 0.01). CKD patients with known CVD displayed even further reduction in plasma APOM levels than CKD patients without known CVD (p < 0.001). Fast-phase liquid chromatography showed that plasma APOM was primarily associated with HDL-cholesterol (HDL-C) across CKD stages. Accordingly, when plasma APOM values were corrected for HDL-C, a significant difference only persisted between patients with CKD stage 3 and stages 1 + 2 (p < 0.05), and the difference between CKD patients with and without known CVD disappeared. Urine APOM/creatinine ratio was not significantly increased in patients with kidney disease.

CONCLUSIONS

The results show that the difference in plasma APOM levels observed between patients with mild and advanced CKD may mainly be due to differences in plasma HDL-C. Whether APOM plays a role in human uremic atherogenesis warrants further experimental studies.

Stable Isotope Kinetic Study of ApoM (Apolipoprotein M)

OBJECTIVE

ApoM (apolipoprotein M) binds primarily to high-density lipoprotein before to be exchanged with apoB (apolipoprotein B)-containing lipoproteins. Low-density lipoprotein (LDL) receptor-mediated clearance of apoB-containing particles could influence plasma apoM kinetics and decrease its antiatherogenic properties. In humans, we aimed to describe the interaction of apoM kinetics with other components of lipid metabolism to better define its potential benefit on atherosclerosis.

APPROACH AND RESULTS

Fourteen male subjects received a primed infusion of ²H₃-leucine for 14 hours, and analyses were performed by liquid chromatography-tandem mass spectrometry from the hourly plasma samples. Fractional catabolic rates and production rates within lipoproteins were calculated using compartmental models. ApoM was found not only in high-density lipoprotein (59%) and LDL (4%) but also in a non-lipoprotein-related compartment (37%). The apoM distribution was heterogeneous within LDL and non-lipoprotein-related compartments according to plasma triglycerides (r=0.86; P<0.001). The relationships between sphingosine-1-phosphate and apoM were confirmed in all compartments (r range, 0.55-0.89; P<0.05). ApoM fractional catabolic rates and production rates were 0.16±0.07 pool/d and 0.14±0.06 mg/kg per day in high-density lipoprotein and 0.56±0.10 pool/d and 0.03±0.01 mg/kg per day in LDL, respectively. Fractional catabolic rates of LDL-apoM and LDL-apoB100 were correlated (r=0.55; P=0.042). Significant correlations were found between triglycerides and production rates of LDL-apoM (r=0.73; P<0.004).

CONCLUSIONS

In humans, LDL kinetics play a key role in apoM turnover. Plasma triglycerides act on both apoM and sphingosine-1-phosphate distributions between lipoproteins. These results confirmed that apoM could be bound to high-density lipoprotein after secretion and then quickly exchanged with a non-lipoprotein-related compartment and to LDL to be slowly catabolized.

Characteristics of lipid metabolism including serum apolipoprotein M levels in patients with primary nephrotic syndrome

Background

Apolipoprotein M (apoM) is a 26-kD apolipoprotein that is mainly expressed in specific cell types, such as human liver parenchymal cells and kidney proximal renal tubular epithelial cells. ApoM can regulate the formation of pre-β-HDL and the reverse cholesterol transport and thus plays an important role in the metabolism of lipids and lipoproteins, meaning that it can affect the development of lipid metabolism disorders. Significantly elevated serum apoM levels are detected in patients with hyperlipidemia. However, few studies have shown how apoM is expressed in primary nephrotic syndrome (PNS), which is often accompanied with hyperlipidemia, and the underlying mechanism is poorly understood. This study was aimed at examining the apoM levels in patients with PNS and at determining the effects of PNS on serum apoM levels in these patients.

Methods

This study included patients with hyperlipidemia (n = 37), the PNS with hyperlipidemia group (n = 62), PNS without hyperlipidemia group (n = 33), and healthy controls (n = 73). The age and body–mass index (BMI) matched among the groups of participants. Their serum apoM concentrations were measured by an enzyme-linked immunosorbent assay. Serum levels of conventional lipids and renal function indices were assessed using an automatic biochemical analyzer. The data were analyzed by means of Pearson’s correlation coefficient (continuous variables) or Student’s t test (mean differences).

Results

The average serum apoM concentrations were higher in the hyperlipidemia group (61.1 ± 23.2 mg/L, P = 0.004) than in the healthy controls (31.6 ± 18.92 mg/L). The serum apoM concentrations were lower in the PNS with hyperlipidemia group (25.1 ± 16.31 mg/L, P = 0.007) and in the PNS without hyperlipidemia group (21.00 ± 17.62 mg/L, P = 0.003) than in the healthy controls. The serum apoM concentrations in the PNS with hyperlipidemia group did not differ significantly from those in the PNS without hyperlipidemia group (P = 0.083). Moreover, serum apoM levels positively correlated with serum high-density lipoprotein cholesterol (HDL-C) and apoA1 levels and negatively correlated with proteinuria in PNS patients (r = 0.458, P = 0.003; r = 0.254, P = 0.022; r = −0.414, P = 0.028).

Conclusion

Serum apoM concentrations are higher in patients with hyperlipidemia than in healthy controls. Low serum apoM levels in patients with PNS are likely caused by PNS.

Effects of apolipoprotein M in uremic atherosclerosis

BACKGROUND AND AIMS

Chronic kidney disease is characterized by uremia and causes premature death, partly due to accelerated atherosclerosis. Apolipoprotein (apo) M is a plasma carrier protein for the lipid sphingosine-1-phosphate (S1P). The Apom-S1P complex associates with HDL, and may contribute to its anti-atherosclerotic effects. The role of Apom/S1P in atherosclerosis is presently controversial and has not been explored in a uremic setting. We aimed to explore whether plasma concentrations of Apom/S1P are altered by uremia and whether Apom overexpression or deficiency affects classical and uremic atherosclerosis.

METHODS

Mild to moderate uremia was induced by subtotal nephrectomy (NX) in 86-92 Apoe-deficient mice that were either Apom-wild type, Apom-deficient, or overexpressed Apom (∼10 fold). The effects of uremia on plasma Apom/S1P and atherosclerosis were evaluated and compared to non-nephrectomized controls.

RESULTS

Uremia increased plasma Apom by ∼25%, but not S1P. Plasma S1P was elevated by ∼300% in mice overexpressing Apom, and decreased by ∼25% in Apom-deficient mice. Apom overexpression augmented aortic root atherosclerosis and plasma cholesterol. In contrast, aortic arch atherosclerosis was unaffected by the Apom genotype. There was no effect of Apom-deficiency or Apom overexpression on uremic atherosclerosis.

CONCLUSIONS

This study highlights the complexity of Apom/S1P in atherosclerosis and challenges the notion that the Apom/S1P complex is anti-atherogenic, at least in Apoe-deficient mice.

A Novel Perspective on the ApoM-S1P Axis, Highlighting the Metabolism of ApoM and Its Role in Liver Fibrosis and Neuroinflammation

Hepatocytes, renal proximal tubule cells as well as the highly specialized endothelium of the blood brain barrier (BBB) express and secrete apolipoprotein M (apoM). ApoM is a typical lipocalin containing a hydrophobic binding pocket predominantly carrying Sphingosine-1-Phosphate (S1P). The small signaling molecule S1P is associated with several physiological as well as pathological pathways whereas the role of apoM is less explored. Hepatic apoM acts as a chaperone to transport S1P through the circulation and kidney derived apoM seems to play a role in S1P recovery to prevent urinal loss. Finally, polarized endothelial cells constituting the lining of the BBB express apoM and secrete the protein to the brain as well as to the blood compartment. The review will provide novel insights on apoM and S1P, and its role in hepatic fibrosis, neuroinflammation and BBB integrity.

ApoB and apoM - New aspects of lipoprotein biology in uremia-induced atherosclerosis

Chronic kidney disease affects as much as 13% of the population, and is associated with a markedly increased risk of developing cardiovascular disease. One of the underlying reasons is accelerated development of atherosclerosis. This can be ascribed both to increased occurrence of traditional cardiovascular risk factors, and to risk factors that may be unique to patients with chronic kidney disease. The latter is reflected in the observation that the current treatment modalities, mainly directed against traditional risk factors, are insufficient to prevent cardiovascular disease in the patient with chronic kidney disease. This review discusses mechanisms accelerating uremic atherosclerosis with a specific focus on the putative roles of apolipoprotein(apo)s B and M that may be particularly important in patients with chronic kidney disease.

High-Density Lipoprotein-Associated Apolipoprotein M Limits Endothelial Inflammation by Delivering Sphingosine-1-Phosphate to the Sphingosine-1-Phosphate Receptor 1

OBJECTIVE

Plasma high-density lipoproteins (HDL) are potent antiatherogenic and anti-inflammatory particles. However, HDL particles are highly heterogenic in composition, and different HDL-mediated functions can be ascribed to different subclasses of HDL. Only a small HDL population contains apolipoprotein M (ApoM), which is the main plasma carrier of the bioactive lipid mediator sphingosine-1-phosphate (S1P). Vascular inflammation is modulated by S1P, but both pro- and anti-inflammatory roles have been ascribed to S1P. The goal of this study is to elucidate the role of ApoM and S1P in endothelial anti-inflammatory events related to HDL.

APPROACH AND RESULTS

Aortic or brain human primary endothelial cells were challenged with tumor necrosis factor-α (TNF-α) as inflammatory stimuli. The presence of recombinant ApoM-bound S1P or ApoM-containing HDL reduced the abundance of adhesion molecules in the cell surface, whereas ApoM and ApoM-lacking HDL did not. Specifically, ApoM-bound S1P decreased vascular adhesion molecule-1 (VCAM-1) and E-selectin surface abundance but not intercellular adhesion molecule-1. Albumin, which is an alternative S1P carrier, was less efficient in inhibiting VCAM-1 than ApoM-bound S1P. The activation of the S1P receptor 1 was sufficient and required to promote anti-inflammation. Moreover, ApoM-bound S1P induced the rearrangement of the expression of S1P-related genes to counteract TNF-α. Functionally, HDL/ApoM/S1P limited monocyte adhesion to the endothelium and maintained endothelial barrier integrity under inflammatory conditions.

CONCLUSIONS

ApoM-bound S1P is a key component of HDL and is responsible for several HDL-associated protective functions in the endothelium, including regulation of adhesion molecule abundance, leukocyte-endothelial adhesion, and endothelial barrier.

Effects of hyperlipidaemia on plasma apolipoprotein M levels in patients with type 2 diabetes mellitus: an independent case-control study

Background

Apolipoprotein M (apoM) is mainly enriched in high-density lipoprotein (HDL) cholesterol and is slightly present in low-density lipoprotein (LDL) cholesterol and very low-density lipoprotein cholesterol. apoM is involved in HDL formation and HDL-mediated reverse cholesterol transport. apoM is also associated with hyperlipidaemia and type 2 diabetes mellitus (T2DM). Significantly high plasma apoM levels are detected in hyperlipidaemia mice with a defective LDL receptor. By contrast, low plasma apoM levels are observed in patients with T2DM, which is often accompanied with hyperlipidaemia. However, the underlying mechanism of this condition is poorly understood. This research aims to examine the changes in apoM levels in patients with hyperlipidaemia and to determine the effects of hyperlipidaemia on plasma apoM levels in patients with T2DM.

Methods

This study included patients with hyperlipidaemia (n = 79), patients with T2DM but without hyperlipidaemia (n = 125), patients with T2DM and hyperlipidaemia (n = 98), and healthy controls (n = 105). Their plasma apoM concentrations were measured with enzyme-linked immunosorbent assay.

Results

The average plasma apoM concentrations were 18 % higher in the hyperlipidaemia group (26.63 ± 10.35 ng/μL) than in the healthy controls (22.61 ± 10.81 ng/μL, P <0.01). The plasma apoM concentrations were lower in the T2DM without hyperlipidaemia group (18.54 ± 10.33 ng/μL, P <0.01) and the T2DM with hyperlipidaemia group (19.83 ± 7.41 ng/μL, P <0.05) than in the healthy controls. Similar to apoA-I (1.29 ± 0.33 g/L vs. 1.28 ± 0.31 g/L, P >0.05), the plasma apoM concentrations in the T2DM with hyperlipidaemia group did not significantly differ from those in the T2DM without hyperlipidaemia group (P >0.05). Multivariate linear regression analysis showed that hyperlipidaemia (β = 5.18, P = 0.007) is an independent promoting factor of plasma apoM levels and diabetes (β = −3.09, P = 0.005) is an inhibiting factor of plasma apoM levels.

Conclusion

Plasma apoM concentrations are higher in patients with hyperlipidaemia than in healthy controls. Low plasma apoM levels in patients with T2DM are likely caused by diabetes but are not induced by hyperlipidaemia.

Altered metabolism of LDL in the arterial wall precedes atherosclerosis regression

RATIONALE

Plasma cholesterol lowering is beneficial in patients with atherosclerosis. However, it is unknown how it affects entry and degradation of low-density lipoprotein (LDL) particles in the lesioned arterial wall.

OBJECTIVE

We studied the effect of lipid-lowering therapy on LDL permeability and degradation of LDL particles in atherosclerotic aortas of mice by measuring the accumulation of iodinated LDL particles in the arterial wall.

METHODS AND RESULTS

Cholesterol-fed, LDL receptor-deficient mice were treated with either an anti-Apob antisense oligonucleotide or a mismatch control antisense oligonucleotide once a week for 1 or 4 weeks before injection with preparations of iodinated LDL particles. The anti-Apob antisense oligonucleotide reduced plasma cholesterol by ≈90%. The aortic LDL permeability and degradation rates of newly entered LDL particles were reduced by ≈50% and ≈85% already after 1 week of treatment despite an unchanged pool size of aortic iodinated LDL particles. In contrast, the size, foam cell content, and aortic pool size of iodinated LDL particles of aortic atherosclerotic plaques were not reduced until after 4 weeks of treatment with the anti-Apob antisense oligonucleotide.

CONCLUSIONS

Improved endothelial barrier function toward the entry of plasma LDL particles and diminished aortic degradation of the newly entered LDL particles precede plaque regression.

Apolipoprotein M in lipid metabolism and cardiometabolic diseases

PURPOSE

This review will address recent findings on apolipoprotein M (apoM) and its ligand sphingosine-1-phosphate (S1P) in lipid metabolism and inflammatory diseases.

RECENT FINDINGS

ApoM's likely role(s) in health and disease has become more diverse after the discovery that apoM functions as a chaperone for S1P. Hence, apoM has recently been implicated in lipid metabolism, diabetes and rheumatoid arthritis through in-vivo, in-vitro and genetic association studies. It remains to be established to which degree such associations with apoM can be attributed to its ability to bind S1P.

SUMMARY

The apoM/S1P axis and its implications in atherosclerosis and lipid metabolism have been thoroughly studied. Owing to the discovery of the apoM/S1P axis, the scope of apoM research has broadened. ApoM and S1P have been implicated in lipid metabolism, that is by modulating HDL particles. Also, the importance in regulating endothelial function is being investigated. Furthermore, both apoM and S1P have been linked to diabetes and glucose and insulin metabolism. Finally, genetic variations in the apoM gene are associated with lipid disturbances, diabetes and rheumatoid arthritis. These findings suggest not only diverse effects of apoM, but also the important question of whether apoM mainly acts as a S1P carrier, if apoM carries other substances with biological effects as well, or whether the apoM protein has effects on its own.

LDL receptor and ApoE are involved in the clearance of ApoM-associated sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P) is a vasoactive lipid mediator that is speculated to be involved in various aspects of atherosclerosis. About 70% of circulating plasma S1P is carried on HDL, and several pleiotropic properties of HDL have been ascribed to S1P. In the previous study with human subjects, however, LDL cholesterol or apoB, but not HDL cholesterol or apoA-I, had a significant positive correlation with the plasma S1P level, suggesting that the metabolic pathway for LDL might have some roles in the metabolism of S1P. In this study, we analyzed the association between LDL receptor, an important protein in the clearance of LDL, and circulating S1P. We observed that in LDL receptor-overexpressing mice, the plasma S1P levels as well as apolipoprotein M (apoM), a carrier of S1P, were decreased and that exogenously administered C17S1P bound to apoM-containing lipoproteins was cleared more rapidly. Unlike the situation in wild-type mice, LDL receptor overexpression in apoE-deficient mice did not reduce the plasma S1P or apoM levels, suggesting that apoE might be a ligand for the LDL receptor during the clearance of these factors. The present findings clarify the novel roles of the LDL receptor and apoE in the clearance of S1P, a multifunctional bioactive phospholipid.

Hepatic apolipoprotein M (apoM) overexpression stimulates formation of larger apoM/sphingosine 1-phosphate-enriched plasma high density lipoprotein

Apolipoprotein M (apoM), a lipocalin family member, preferentially associates with plasma HDL and binds plasma sphingosine 1-phosphate (S1P), a signaling molecule active in immune homeostasis and endothelial barrier function. ApoM overexpression in ABCA1-expressing HEK293 cells stimulated larger nascent HDL formation, compared with cells that did not express apoM; however, the in vivo role of apoM in HDL metabolism remains poorly understood. To test whether hepatic apoM overexpression increases plasma HDL size, we generated hepatocyte-specific apoM transgenic (APOM Tg) mice, which had an ∼3-5-fold increase in plasma apoM levels compared with wild-type mice. Although HDL cholesterol concentrations were similar to wild-type mice, APOM Tg mice had larger plasma HDLs enriched in apoM, cholesteryl ester, lecithin:cholesterol acyltransferase, and S1P. Despite the presence of larger plasma HDLs in APOM Tg mice, in vivo macrophage reverse cholesterol transport capacity was similar to that in wild-type mice. APOM Tg mice had an ∼5-fold increase in plasma S1P, which was predominantly associated with larger plasma HDLs. Primary hepatocytes from APOM Tg mice generated larger nascent HDLs and displayed increased sphingolipid synthesis and S1P secretion. Inhibition of ceramide synthases in hepatocytes increased cellular S1P levels but not S1P secretion, suggesting that apoM is rate-limiting in the export of hepatocyte S1P. Our data indicate that hepatocyte-specific apoM overexpression generates larger nascent HDLs and larger plasma HDLs, which preferentially bind apoM and S1P, and stimulates S1P biosynthesis for secretion. The unique apoM/S1P-enriched plasma HDL may serve to deliver S1P to extrahepatic tissues for atheroprotection and may have other as yet unidentified functions.

Apolipoprotein M promotes mobilization of cellular cholesterol in vivo

OBJECTIVE

The HDL associated apolipoprotein M (apoM) protects against experimental atherosclerosis but the mechanism is unknown. ApoM increases prebeta-HDL formation. We explored whether plasma apoM affects mobilization of cholesterol from peripheral cells in mice.

METHODS AND RESULTS

ApoM-enriched HDL from apoM-transgenic mice increased the in vitro efflux of 3H-cholesterol from macrophages by 24 +/- 3% (p < 0.05) as compared with HDL from wild type (WT) mice, thus confirming previous findings. However, apoM-free HDL was not poorer than that of WT HDL to mobilize 3H-cholesterol. 3H-cholesterol-labeled foam cells were implanted in the peritoneal cavity of apoM-/-, WT and apoM-transgenic mice to assess the mobilization of cholesterol from foam cells in vivo and subsequent excretion into feces. The results showed a statistically non-significant trend towards increased mobilization of cellular cholesterol to plasma with increasing plasma apoM. However, the apoM-genotype did not affect the excretion of 3H-cholesterol in feces. Nevertheless, when apoM-/-, apoM-transgenic and WT mice received a constant intravenous infusion of 13C2-cholesterol/intralipid for 5 h, the rate of enrichment of blood free cholesterol with free 13C2-cholesterol was significantly lower (consistent with an increase in flux of unlabeled free cholesterol into the plasma) in the apoM-transgenic (3.0 +/- 0.9 per thousand/h) as compared to WT (5.7 +/- 0.9 per thousand/h, p < 0.05) and apoM-/- (6.5 +/- 0.6 per thousand/h, p < 0.01) mice.

CONCLUSION

The present data indicate that the plasma apoM levels modulate the ability of plasma to mobilize cellular cholesterol, whereas apoM has no major effect on the excretion of cholesterol into feces.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Apolipoprotein M: bridging HDL and endothelial function

PURPOSE OF REVIEW

The review will address the potential roles of apolipoprotein M (apoM) as a carrier protein and modulator of sphingosine-1-phosphate (S1P) bioactivity.

RECENT FINDINGS

Recombinant apoM can bind small lipids such as retinoic acid, oxidized phospholipids, and S1P. Thus, the effects of apoM may be pleiotrophic. The S1P binding ability of apoM has biological impact. ApoM-bound S1P can activate S1P1 receptors on endothelial cells and deficiency of apoM abolishes the presence of S1P in HDL. In mice, the lack of apoM causes dysfunctional endothelial barrier function in the lungs. In humans, sepsis that is characterized by impaired endothelial function is associated with low plasma apoM.

SUMMARY

Plasma apoM is mainly bound to HDL. The roles of apoM in atherosclerosis and lipoprotein metabolism have been given much attention. New in the field is the discovery of apoM as a chaperone for S1P. S1P is a bioactive lipid with effects on angiogenesis, lymphocyte trafficking, endothelial cell migration, and inflammation. A drug targeting the S1P-system (fingolimod) is now used for treatment of multiple sclerosis. It improves the blood-brain barrier and inhibits migration of lymphocytes into the brain. Further exploration of the apoM/S1P axis may uncover its potential as a biomarker and target for new treatments.

The apolipoprotein m-sphingosine-1-phosphate axis: biological relevance in lipoprotein metabolism, lipid disorders and atherosclerosis

Apolipoprotein M (apoM) is a plasma apolipoprotein that mainly associates with high-density lipoproteins. Hence, most studies on apoM so far have investigated its effect on and association with lipid metabolism and atherosclerosis. The insight into apoM biology recently took a major turn. ApoM was identified as a carrier of the bioactive lipid sphingosine-1-phosphate (S1P). S1P activates five different G-protein-coupled receptors, known as the S1P-receptors 1–5 and, hence, affects a wide range of biological processes, such as lymphocyte trafficking, angiogenesis, wound repair and even virus suppression and cancer. The ability of apoM to bind S1P is due to a lipophilic binding pocket within the lipocalin structure of the apoM molecule. Mice overexpressing apoM have increased plasma S1P concentrations, whereas apoM-deficient mice have decreased S1P levels. ApoM-S1P is able to activate the S1P-receptor-1, affecting the function of endothelial cells, and apoM-deficient mice display impaired endothelial permeability in the lung. This review will focus on the putative biological roles of the new apoM–S1P axis in relation to lipoprotein metabolism, lipid disorders and atherosclerosis.

The plasma concentration of HDL-associated apoM is influenced by LDL receptor-mediated clearance of apoB-containing particles

ApoM is mainly associated with HDL. Nevertheless, we have consistently observed positive correlations of apoM with plasma LDL cholesterol in humans. Moreover, LDL receptor deficiency is associated with increased plasma apoM in mice. Here, we tested the idea that plasma apoM concentrations are affected by the rate of LDL receptor-mediated clearance of apoB-containing particles. We measured apoM in humans each carrying one of three different LDL receptor mutations (n = 9) or the apoB3500 mutation (n = 12). These carriers had increased plasma apoM (1.34 ± 0.13 µM, P = 0.003, and 1.23 ± 0.10 µM, P = 0.02, respectively) as compared with noncarriers (0.93 ± 0.04 µM). When we injected human apoM-containing HDL into Wt (n = 6) or LDL receptor-deficient mice (n = 6), the removal of HDL-associated human apoM was delayed in the LDL receptor-deficient mice. After 2 h, 54 ± 5% versus 90 ± 8% (P < 0.005) of the initial amounts of human apoM remained in the plasma of Wt and LDL receptor-deficient mice, respectively. Finally, we compared the turnover of radio-iodinated LDL and plasma apoM concentrations in 45 normocholesterolemic humans. There was a negative correlation between plasma apoM and the fractional catabolic rate of LDL (r = -0.38, P = 0.009). These data suggest that the plasma clearance of apoM, despite apoM primarily being associated with HDL, is influenced by LDL receptor-mediated clearance of apoB-containing particles.

Apolipoprotein M binds oxidized phospholipids and increases the antioxidant effect of HDL

OBJECTIVE

Oxidation of LDL plays a key role in the development of atherosclerosis. HDL may, in part, protect against atherosclerosis by inhibiting LDL oxidation. Overexpression of HDL-associated apolipoprotein M (apoM) protects mice against atherosclerosis through a not yet clarified mechanism. Being a lipocalin, apoM contains a binding pocket for small lipophilic molecules. Here, we report that apoM likely serves as an antioxidant in HDL by binding oxidized phospholipids, thus enhancing the antioxidant potential of HDL.

METHODS AND RESULTS

HDL was isolated from wild type mice, apoM-deficient mice, and two lines of apoM-Tg mice with ∼2-fold and ∼10-fold increased plasma apoM, respectively. Increasing amounts of HDL-associated apoM were associated with an increase in the resistance of HDL to oxidation with Cu(2+) or 2,2'-azobis 2-methyl-propanimidamide, dihydrochloride (AAPH) and to an increased ability of HDL to protect human LDL against oxidation. Oxidized phospholipids, but not native phospholipids, quenched the intrinsic fluorescence of recombinant human apoM and the quenching could be competed with myristic acid suggesting selective binding of oxidized phospholipid in the lipocalin-binding pocket of apoM.

CONCLUSIONS

The results suggest that apoM can bind oxidized phospholipids and that it increases the antioxidant effect of HDL. This new mechanism may explain at least part of the antiatherogenic potential of apoM.

Endothelium-protective sphingosine-1-phosphate provided by HDL-associated apolipoprotein M

Protection of the endothelium is provided by circulating sphingosine-1-phosphate (S1P), which maintains vascular integrity. We show that HDL-associated S1P is bound specifically to both human and murine apolipoprotein M (apoM). Thus, isolated human ApoM(+) HDL contained S1P, whereas ApoM(-) HDL did not. Moreover, HDL in Apom(-/-) mice contains no S1P, whereas HDL in transgenic mice overexpressing human apoM has an increased S1P content. The 1.7-Å structure of the S1P-human apoM complex reveals that S1P interacts specifically with an amphiphilic pocket in the lipocalin fold of apoM. Human ApoM(+) HDL induced S1P(1) receptor internalization, downstream MAPK and Akt activation, endothelial cell migration, and formation of endothelial adherens junctions, whereas apoM(-) HDL did not. Importantly, lack of S1P in the HDL fraction of Apom(-/-) mice decreased basal endothelial barrier function in lung tissue. Our results demonstrate that apoM, by delivering S1P to the S1P(1) receptor on endothelial cells, is a vasculoprotective constituent of HDL.

Relationship of plasma apolipoprotein M with proprotein convertase subtilisin-kexin type 9 levels in non-diabetic subjects

PURPOSE

Apolipoprotein M (apoM) retards atherosclerosis development in murine models, and may be regulated by pathways involved in LDL metabolism. Proprotein convertase subtilisin-kexin type 9 (PCSK9) plays a key role in LDL receptor processing. We determined the extent to which plasma apoM is related to PCSK9 levels in subjects with varying degrees of obesity.

METHODS

We sought correlations between plasma apoM and PCSK9, measured using recently developed ELISAs, in 79 non-diabetic subjects.

RESULTS

ApoM and PCSK9 levels were both correlated positively with total cholesterol, non-HDL cholesterol, LDL cholesterol and apoB (P < 0.05 to P < 0.001). ApoM correlated positively with PCSK9 in lean individuals (n = 37, r = 0.337, P = 0.041), but not in overweight subjects (n = 32, r = 0.125, P = 0.50) and in obese subjects (n = 10, r = -0.055, P = 0.88).

CONCLUSIONS

The PCSK9 pathway may contribute to plasma apoM regulation in humans. The influence of PCSK9 on circulating apoM appears to be modified by adiposity.

Plasma apolipoprotein M responses to statin and fibrate administration in type 2 diabetes mellitus

PURPOSE

Plasma apolipoprotein M (apoM) is potentially anti-atherogenic, and has been found to be associated positively with plasma total, LDL and HDL cholesterol in humans. ApoM may, therefore, be intricately related to cholesterol metabolism. Here, we determined whether plasma apoM is affected by statin or fibrate administration in patients with diabetes mellitus.

METHODS

Fourteen type 2 diabetic patients participated in a placebo-controlled crossover study which included three 8-week treatment periods with simvastatin (40 mg daily), bezafibrate (400 mg daily), and their combination.

RESULTS

ApoM was decreased by 7% in response to simvastatin (P<0.05 from baseline and placebo), and remained unchanged during bezafibrate and combined simvastatin+bezafibrate administration. Plasma apoM concentrations correlated positively with apoB-containing lipoprotein measures at baseline and during placebo (P<0.02 to P<0.001), but these relationships were lost during all lipid lowering treatment periods.

CONCLUSIONS

This study suggests that, even though plasma apoM is lowered by statins, apoM metabolism is to a considerable extent independent of statin- and fibrate-affected pathways involved in cholesterol homeostasis.