Hydrophobic ligand binding properties of the human lipocalin apolipoprotein M

Unraveling the underlying mechanism of interactions between astaxanthin geometrical isomers and bovine serum albumin

The health benefits of astaxanthin (AST) are related to its geometric isomers. Generally, functional activity is realized by the interactions between active substances and transporters. Hereto, bovine serum albumin (BSA), as a model-binding protein and transporter, is able to recognize and transport isomers of active substances through binding with them. However, differences in the binding mechanism of isomers to BSA may affect the functional activities of isomers through the "binding-transport-activity" chain reaction. Thus, this study sought to elucidate the interactions between AST geometrical isomers and BSA using multi-spectroscopy, surface plasmon resonance and molecular docking. The results showed that Z-AST displayed more interacting amino acid residues and lower thermodynamic parameters than all-E-AST. Meanwhile, the order of binding affinity to BSA was 13Z-AST (1.56 × 10-7 M) > 9Z-AST (2.70 × 10-7 M) > all-E-AST (4.01 × 10-7 M), indicating that Z-AST possessed stronger binding ability to BSA. Moreover, AST isomers were located at the junction between subdomains ⅡA and ⅢA of BSA, and showed the same interaction forces (hydrogen bond and van der Waals force) as well as kinetic processes (slow combination, slow dissociation). These interaction parameters provide valuable insights into their pharmacokinetics in vivo, and it was of great significance to explain the potential differences among AST isomers in functional activities.

Nitric oxide synthase-guided genome mining identifies a cytochrome P450 enzyme for olefin nitration in bacterial specialized metabolism

The biological signaling molecule nitric oxide (NO) has recently emerged as a metabolic precursor for the creation of microbial natural products with diversified structures and biological activities. Within the biosynthetic gene clusters (BGCs) of these compounds, genes associated with NO production pathways have been pinpointed. In this study, we employ a nitric oxide synthase (NOS)-guided genome mining strategy for the targeted discovery of NO-derived bacterial natural products and NO-utilizing biocatalysts. We show that a conserved NOS-containing BGC, distributed across several actinobacterial genomes, is responsible for the biosynthesis of lajollamycin, a unique nitro-tetraene-containing antibiotic whose biosynthetic mechanism remains elusive. Through a combination of in vivo and in vitro studies, we unveil the first cytochrome P450 enzyme capable of catalyzing olefin nitration in natural product biosynthesis. These results not only expand the current knowledge about biosynthetic nitration processes but also offer an efficient way for targeted identification of NO-utilizing metabolic pathways and novel nitrating biocatalysts.

Kidney derived apolipoprotein M and its role in acute kidney injury

Aim: Apolipoprotein M (apoM) is mainly expressed in liver and in proximal tubular epithelial cells in the kidney. In plasma, apoM associates with HDL particles via a retained signal peptide and carries sphingosine-1-phosphate (S1P), a small bioactive lipid. ApoM is undetectable in urine from healthy individuals but lack of megalin receptors in proximal tubuli cells induces loss of apoM into the urine. Besides this, very little is known about kidney-derived apoM. The aim of this study was to address the role of apoM in kidney biology and in acute kidney injury. Methods: A novel kidney-specific human apoM transgenic mouse model (RPTEC-hapoMTG) was generated and subjected to either cisplatin or ischemia/reperfusion injury. Further, a stable transfection of HK-2 cells overexpressing human apoM (HK-2-hapoMTG) was developed to study the pattern of apoM secretion in proximal tubuli cells. Results: Human apoM was present in plasma from RPTEC-hapoMTG mice (mean 0.18 μM), with a significant increase in plasma S1P levels. In vitro apoM was secreted to both the apical (urine) and basolateral (blood) compartment from proximal tubular epithelial cells. However, no differences in kidney injury score was seen between RPTEC-hapoMTG and wild type (WT) mice upon kidney injury. Further, gene expression of inflammatory markers (i.e., IL6, MCP-1) was similar upon ischemia/reperfusion injury. Conclusion: Our study suggests that kidney-derived apoM is secreted to plasma, supporting a role for apoM in sequestering molecules from excretion in urine. However, overexpression of human apoM in the kidney did not protect against acute kidney injury.

The host plant strongly modulates acaricide resistance levels to mitochondrial complex II inhibitors in a multi-resistant field population of Tetranychus urticae

The two-spotted spider mite Tetranychus urticae is a polyphagous pest with an extraordinary ability to develop acaricide resistance. Here, we characterize the resistance mechanisms in a T. urticae population (VR-BE) collected from a Belgian tomato greenhouse, where the grower was unsuccessful in chemically controlling the mite population resulting in crop loss. Upon arrival in the laboratory, the VR-BE population was established both on bean and tomato plants as hosts. Toxicity bioassays on both populations confirmed that the population was highly multi-resistant, recording resistance to 12 out of 13 compounds tested from various mode of action groups. DNA sequencing revealed the presence of multiple target-site resistance mutations, but these could not explain resistance to all compounds. In addition, striking differences in toxicity for six acaricides were observed between the populations on bean and tomato. The highest difference was recorded for the complex II inhibitors cyenopyrafen and cyflumetofen, which were 4.4 and 3.3-fold less toxic for VR-BE mites on tomato versus bean. PBO synergism bioassays suggested increased P450 based detoxification contribute to the host-dependent toxicity. Given the involvement of increased detoxification, we subsequently determined genome-wide gene expression levels of VR-BE on both hosts, in comparison to a reference susceptible population, revealing overexpression of a large set of detoxification genes in VR-BE on both hosts compared to the reference. In addition, a number of mainly detoxification genes with higher expression in VR-BE on tomato compared to bean was identified, including several cytochrome P450s. Together, our work suggests that multi-resistant field populations can accumulate a striking number of target-site resistance mutations. We also show that the host plant can have a profound effect on the P450-associated resistance levels to cyenopyrafen and cyflumetofen.

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

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

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

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

A comparative proteomic analysis to define the influencing factors on gingival crevicular fluid using LC-MS/MS

Gingival crevicular fluid (GCF) is a promising biofluid for disease identification and biomarker searching in periodontology. This study aimed to investigate the possible influencing factors, including tooth site, sex and age, on the normal GCF proteome. Forty periodontal healthy adults were randomly divided into a training group and a testing group. In the training group, GCF samples from 12 adults were analyzed using the iTRAQ 2D LC-MS/MS method. The influencing factors, tooth site (including periodontitis-susceptible and -insusceptible tooth sites), sex and age, and related differential proteins were defined and functionally annotated. The important differential proteins from 28 adults in the testing group were then validated by PRM analysis. An average of approximately 5 differential proteins were found between tooth sites of periodontitis-susceptible and -insusceptible sites. Eighty-five differentially expressed proteins were obtained between sexes in the young group, while only 7 sex-associated proteins were found in the old group. A total of 203 and 235 age-associated proteins were found in the male and female groups, respectively. The differential protein functional annotation showed that sex-related proteins were mainly related to immune function and metabolism, and age-related proteins were primarily associated with inflammation, lipid metabolism and immune function. In the testing group, a total of 4 sex-related proteins and 12 age-related proteins were validated by PRM analysis. SIGNIFICANCE: The influences of tooth site, sex and age in GCF proteomics in periodontal health were firstly analyzed using LC-MS/MS. Tooth site showed a small influence on the GCF proteome. The sex effect was significant in young adults, but its influence in old adults is small. Age is an important impact factor for the GCF proteome. These findings enrich the knowledge about the normal GCF proteome and might benefit future disease analyses.

Dietary Strategies to Improve Cardiovascular Health: Focus on Increasing High-Density Lipoprotein Functionality

Cardiovascular disease is one of the leading causes of morbidity and mortality worldwide, with increasing incidence. A cornerstone of cardiovascular disease prevention is lifestyle modification through dietary changes to influence various risk factors such as obesity, hypertension and diabetes. The effects of diet on cardiovascular health are complex. Some dietary components and metabolites directly affect the composition and structure of high-density lipoproteins (HDL) and increase anti-inflammatory and vasoprotective properties. HDLs are composed of distinct subpopulations of particles of varying size and composition that have several dynamic and context-dependent functions. The identification of potential dietary components that improve HDL functionality is currently an important research goal. One of the best-studied diets for cardiovascular health is the Mediterranean diet, consisting of fish, olive oil, fruits, vegetables, whole grains, legumes/nuts, and moderate consumption of alcohol, most commonly red wine. The Mediterranean diet, especially when supplemented with extra virgin olive oil rich in phenolic compounds, has been shown to markedly improve metrics of HDL functionality and reduce the burden, or even prevent the development of cardiovascular disease. Particularly, the phenolic compounds of extra virgin olive oil seem to exert the significant positive effects on HDL function. Moreover, supplementation of anthocyanins as well as antioxidants such as lycopene or the omega-3 fatty acid eicosapentaenoic acid improve parameters of HDL function. In this review, we aim to highlight recent discoveries on beneficial dietary patterns as well as nutritional components and their effects on cardiovascular health, focusing on HDL function.

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.

Into the Labyrinth of the Lipocalin α1-Acid Glycoprotein

α₁-acid glycoprotein (AGP), also known as Orosomucoid (ORM), belongs to the Lipocalin protein family and it is well-known for being a positive acute-phase protein. AGP is mostly found in plasma, with the liver as main contributor, but it is also expressed in other tissues such as the brain or the adipose tissue. Despite the vast literature on AGP, the physiological functions of the protein remain to be elucidated. A large number of activities mostly related to protection and immune system modulation have been described. Recently created AGP-knockout models have suggested novel physiological roles of AGP, including regulation of metabolism. AGP has an outstanding ability to efficiently bind endogenous and exogenous small molecules that together with the complex and variable glycosylation patterns, determine AGP functions. This review summarizes and discusses the recent findings on AGP structure (including glycans), ligand-binding ability, regulation, and physiological functions of AGP. Moreover, this review explores possible molecular and functional connections between AGP and other members of the Lipocalin protein family.

Fetal High-Density Lipoproteins: Current Knowledge on Particle Metabolism, Composition and Function in Health and Disease

Cholesterol and other lipids carried by lipoproteins play an indispensable role in fetal development. Recent evidence suggests that maternally derived high-density lipoprotein (HDL) differs from fetal HDL with respect to its proteome, size, and function. Compared to the HDL of adults, fetal HDL is the major carrier of cholesterol and has a unique composition that implies other physiological functions. Fetal HDL is enriched in apolipoprotein E, which binds with high affinity to the low-density lipoprotein receptor. Thus, it appears that a primary function of fetal HDL is the transport of cholesterol to tissues as is accomplished by low-density lipoproteins in adults. The fetal HDL-associated bioactive sphingolipid sphingosine-1-phosphate shows strong vasoprotective effects at the fetoplacental vasculature. Moreover, lipoprotein-associated phospholipase A2 carried by fetal-HDL exerts anti-oxidative and athero-protective functions on the fetoplacental endothelium. Notably, the mass and activity of HDL-associated paraoxonase 1 are about 5-fold lower in the fetus, accompanied by an attenuation of anti-oxidative activity of fetal HDL. Cholesteryl ester transfer protein activity is reduced in fetal circulation despite similar amounts of the enzyme in maternal and fetal serum. This review summarizes the current knowledge on fetal HDL as a potential vasoprotective lipoprotein during fetal development. We also provide an overview of whether and how the protective functionalities of HDL are impaired in pregnancy-related syndromes such as pre-eclampsia or gestational diabetes mellitus.

High-Density Lipoprotein Modifications: A Pathological Consequence or Cause of Disease Progression?

High-density lipoprotein (HDL) is well-known for its cardioprotective effects, as it possesses anti-inflammatory, anti-oxidative, anti-thrombotic, and cytoprotective properties. Traditionally, studies and therapeutic approaches have focused on raising HDL cholesterol levels. Recently, it became evident that, not HDL cholesterol, but HDL composition and functionality, is probably a more fruitful target. In disorders, such as chronic kidney disease or cardiovascular diseases, it has been observed that HDL is modified and becomes dysfunctional. There are different modification that can occur, such as serum amyloid, an enrichment and oxidation, carbamylation, and glycation of key proteins. Additionally, the composition of HDL can be affected by changes to enzymes such as cholesterol ester transfer protein (CETP), lecithin-cholesterol acyltransferase (LCAT), and phospholipid transfer protein (PLTP) or by modification to other important components. This review will highlight some main modifications to HDL and discuss whether these modifications are purely a consequential result of pathology or are actually involved in the pathology itself and have a causal role. Therefore, HDL composition may present a molecular target for the amelioration of certain diseases, but more information is needed to determine to what extent HDL modifications play a causal role in disease development.

Proteomics of Non-human Primate Plasma after Partial-body Radiation with Minimal Bone Marrow Sparing

High-dose radiation exposure results in organ-specific sequelae that occurs in a time- and dose-dependent manner. The partial body irradiation with minimal bone marrow sparing model was developed to mimic intentional or accidental radiation exposures in humans where bone marrow sparing is likely and permits the concurrent analysis of coincident short- and long-term damage to organ systems. To help inform on the natural history of the radiation-induced injury of the partial body irradiation model, we quantitatively profiled the plasma proteome of non-human primates following 12 Gy partial body irradiation with 2.5% bone marrow sparing with 6 MV LINAC-derived photons at 0.80 Gy min over a time period of 3 wk. The plasma proteome was analyzed by liquid chromatography-tandem mass spectrometry. A number of trends were identified in the proteomic data including pronounced protein changes as well as protein changes that were consistently upregulated or downregulated at all time points and dose levels interrogated. Pathway and gene ontology analysis were performed; bioinformatic analysis revealed significant pathway and biological process perturbations post high-dose irradiation and shed light on underlying mechanisms of radiation damage. Additionally, proteins were identified that had the greatest potential to serve as biomarkers for radiation exposure.

A conformation-specific ON-switch for controlling CAR T cells with an orally available drug

Molecular ON-switches in which a chemical compound induces protein-protein interactions can allow cellular function to be controlled with small molecules. ON-switches based on clinically applicable compounds and human proteins would greatly facilitate their therapeutic use. Here, we developed an ON-switch system in which the human retinol binding protein 4 (hRBP4) of the lipocalin family interacts with engineered hRBP4 binders in a small molecule-dependent manner. Two different protein scaffolds were engineered to bind to hRBP4 when loaded with the orally available small molecule A1120. The crystal structure of an assembled ON-switch shows that the engineered binder specifically recognizes the conformational changes induced by A1120 in two loop regions of hRBP4. We demonstrate that this conformation-specific ON-switch is highly dependent on the presence of A1120, as demonstrated by an ∼500-fold increase in affinity upon addition of the small molecule drug. Furthermore, the ON-switch successfully regulated the activity of primary human CAR T cells in vitro. We anticipate that lipocalin-based ON-switches have the potential to be broadly applied for the safe pharmacological control of cellular therapeutics.

The role of lipocalin-2 in age-related macular degeneration (AMD)

Lipocalins are a family of secreted adipokines which play important roles in various biological processes. Lipocalin-2 (LCN-2) has been shown to be involved in acute and chronic inflammation. This particular protein is critical in the pathogenesis of several diseases including cancer, diabetes, obesity, and multiple sclerosis. Herein, we discuss the general molecular basis for the involvement of LCN-2 in acute infections and chronic disease progression and also ascertain the probable role of LCN-2 in ocular diseases, particularly in age-related macular degeneration (AMD). We elaborate on the signaling cascades which trigger LCN-2 upregulation in AMD and suggest therapeutic strategies for targeting such pathways.

Apolipoprotein M-bound sphingosine-1-phosphate regulates blood-brain barrier paracellular permeability and transcytosis

The blood-brain barrier (BBB) is formed by the endothelial cells lining cerebral microvessels, but how blood-borne signaling molecules influence permeability is incompletely understood. We here examined how the apolipoprotein M (apoM)-bound sphingosine 1-phosphate (S1P) signaling pathway affects the BBB in different categories of cerebral microvessels using ApoM deficient mice (Apom-/-). We used two-photon microscopy to monitor BBB permeability of sodium fluorescein (376 Da), Alexa Fluor (643 Da), and fluorescent albumin (45 kDA). We show that BBB permeability to small molecules increases in Apom-/- mice. Vesicle-mediated transfer of albumin in arterioles increased 3 to 10-fold in Apom-/- mice, whereas transcytosis in capillaries and venules remained unchanged. The S1P receptor 1 agonist SEW2871 rapidly normalized paracellular BBB permeability in Apom-/- mice, and inhibited transcytosis in penetrating arterioles, but not in pial arterioles. Thus, apoM-bound S1P maintains low paracellular BBB permeability in all cerebral microvessels and low levels of vesicle-mediated transport in penetrating arterioles.

Decoding the human serum interactome of snake-derived antimicrobial peptide Ctn[15-34]: Toward an explanation for unusually long half-life

The inherent propensity to enzymatic degradation of most peptides remains a bottleneck in their therapeutic development. Efficient, early screening methods are necessary for in vitro characterization of the molecular events occurring when peptides get in contact with biological fluids such us plasma. Herein we present an affinity purification/MS approach for mapping peptide serum interactors. We have applied this methodology to identify the serum partners of antibiotic peptide Ctn [15-34], aiming to ascertain the molecular interactions underlying its unusually long half-life (~ 12 h) in human serum. From 42 proteins captured in pull-downs with biotinylated Ctn [15-34] as bait, five are of special interest for their transport/binding properties hence alleged peptide arresting potential. The subset contains two members of the albumin superfamily, two apolipoproteins and a globulin. All five share a binding ability for hydrophobic species, and also bind Ctn [15-34], presumably via its C-terminal hydrophobic section, with affinities in the μM range as shown by surface plasmon resonance. Additionally, our functional enrichment reveals several significant immune-related processes suggesting an immunomodulatory role of Ctn [15-34]. Taken together, this study exemplifies how pharmacoproteomics can be used to analyze bioavailability issues and shed light on the serum interactors ultimately conferring protection to Ctn [15-34] against proteolytic events. SIGNIFICANCE: The affinity purification/MS identification methodology reported here can be viewed as a routine pharmacoproteomic approach to investigate the serum interactome of peptide drugs, identifying proteins affecting bioavailability and thus assisting the peptide drug development process. The specific results described here enlighten the serum stability issues of peptide Ctn [15-34] and ratify its promising future as an anti-infective lead.

OBP fused with cell-penetrating peptides promotes liposomal transduction

Cell-penetrating peptides (CPPs) have been applied as novel transport systems with the ability to facilitate the delivery of peptides, proteins, and oligonucleotides into cells. Herein, we designed different fusion proteins composed by pig odorant binding protein (OBP-I) and three CPPs, namely Tat, pVEC and Pep-1. A new methodology using liposomes as reservoirs and OBP:CPPs as carriers was developed as an advanced system to capture odorant molecules. 1-aminoanthracene (1-AMA) was used as a model molecule to evaluate the transduction ability of OBP:CPPs into the reservoirs. The transduction efficiency was dependent on the initial capacity of OBP:CPPs to bind 1-AMA and on the penetration of liposomes promoted by the CPPs. An encapsulation efficiency of 42% was obtained with OBP:Tat fusion protein. The presence of Tat peptide increased the 1-AMA transduction of 1.3 and 2.5 fold compared with Pep-1 and pVEC, respectively. This work expands the application of OBPs and CPPs on the design of promising capture and delivery systems for textile and cosmetic applications.

Apolipoprotein M mediates sphingosine-1-phosphate efflux from erythrocytes

Sphingosine-1-phosphate (S1P) is a bioactive lipid implicated in e.g. angiogenesis, lymphocyte trafficking, and endothelial barrier function. Erythrocytes are a main source of plasma S1P together with platelets and endothelial cells. Apolipoprotein M (apoM) in HDL carries 70% of plasma S1P, whereas 30% is carried by albumin. The current aim was to investigate the role of apoM in export of S1P from human erythrocytes. Erythrocytes exported S1P more efficiently to HDL than to albumin, particularly when apoM was present in HDL. In contrast, export of sphingosine to HDL was unaffected by the presence of apoM. The specific ability of apoM to promote export of S1P was independent of apoM being bound in HDL particles. Treatment with MK-571, an inhibitor of the ABCC1 transporter, effectively reduced export of S1P from human erythrocytes to apoM, whereas the export was unaffected by inhibitors of ABCB1 or ATPase. Thus, ABCC1 could be involved in export of S1P from erythrocytes to apoM.

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.

HDL-associated ApoM is anti-apoptotic by delivering sphingosine 1-phosphate to S1P1 & S1P3 receptors on vascular endothelium

Background

High-density Lipoprotein (HDL) attenuates endothelial cell apoptosis induced by different cell-death stimuli such as oxidation or growth factor deprivation. HDL is the main plasma carrier of the bioactive lipid sphingosine 1-phosphate (S1P), which it is a signaling molecule that promotes cell survival in response to several apoptotic stimuli. In HDL, S1P is bound to Apolipoprotein M (ApoM), a Lipocalin that is only present in around 5% of the HDL particles. The goal of this study is to characterize ApoM-bound S1P role in endothelial apoptosis protection and the signaling pathways involved.

Methods

Human umbilical vein endothelial cells (HUVEC) cultures were switched to serum/grow factor deprivation medium to induce apoptosis and the effect caused by the addition of ApoM and S1P analyzed.

Results

The addition of HDL^+ApoM or recombinant ApoM-bound S1P promoted cell viability and blocked apoptosis, whereas HDL^-ApoM had no protective effect. Remarkably, S1P exerted a more potent anti-apoptotic effect when carried by ApoM as compared to albumin, or when added as free molecule. Mechanistically, cooperation between S1P1 and S1P3 was required for the HDL/ApoM/S1P-mediated anti-apoptotic ability. Furthermore, AKT and ERK phosphorylation was also necessary to achieve the anti-apoptotic effect of the HDL/ApoM/S1P complex.

Conclusions

Altogether, our results indicate that ApoM and S1P are key elements of the anti-apoptotic activity of HDL and promote optimal endothelial function.

Electronic supplementary material

The online version of this article (doi:10.1186/s12944-017-0429-2) contains supplementary material, which is available to authorized users.

Sphingosine 1-phosphate and its carrier apolipoprotein M in human sepsis and in Escherichia coli sepsis in baboons

Sphingosine 1‐phosphate (S1P) is an important regulator of vascular integrity and immune cell migration, carried in plasma by high‐density lipoprotein (HDL)‐associated apolipoprotein M (apoM) and by albumin. In sepsis, the protein and lipid composition of HDL changes dramatically. The aim of this study was to evaluate changes in S1P and its carrier protein apoM during sepsis. For this purpose, plasma samples from both human sepsis patients and from an experimental Escherichia coli sepsis model in baboons were used. In the human sepsis cohort, previously studied for apoM, plasma demonstrated disease‐severity correlated decreased S1P levels, the profile mimicking that of plasma apoM. In the baboons, a similar disease‐severity dependent decrease in plasma levels of S1P and apoM was observed. In the lethal E. coli baboon sepsis, S1P decreased already within 6–8 hrs, whereas the apoM decrease was seen later at 12–24 hrs. Gel filtration chromatography of plasma from severe human or baboon sepsis on Superose 6 demonstrated an almost complete loss of S1P and apoM in the HDL fractions. S1P plasma concentrations correlated with the platelet count but not with erythrocytes or white blood cells. The liver mRNA levels of apoM and apoA1 decreased strongly upon sepsis induction and after 12 hr both were almost completely lost. In conclusion, during septic challenge, the plasma levels of S1P drop to very low levels. Moreover, the liver synthesis of apoM decreases severely and the plasma levels of apoM are reduced. Possibly, the decrease in S1P contributes to the decreased endothelial barrier function observed in sepsis.

Impaired endothelial barrier function in apolipoprotein M-deficient mice is dependent on sphingosine-1-phosphate receptor 1

Apolipoprotein M (ApoM) transports sphingosine-1-phosphate (S1P) in plasma, and ApoM-deficient mice (Apom(-/-)) have ∼50% reduced plasma S1P levels. There are 5 known S1P receptors, and S1P induces adherens junction formation between endothelial cells through the S1P1 receptor, which in turn suppresses vascular leak. Increased vascular permeability is a hallmark of inflammation. The purpose of this study was to explore the relationships between vascular leakage in ApoM deficiency and S1P1 function in normal physiology and in inflammation. Vascular permeability in the lungs was assessed by accumulation of dextran molecules (70 kDa) and was increased ∼40% in Apom(-/-) mice compared to WT (C57Bl6/j) mice. Reconstitution of plasma ApoM/S1P or treatment with an S1P1 receptor agonist (SEW2871) rapidly reversed the vascular leakage to a level similar to that in WT mice, suggesting that it is caused by decreased plasma levels of S1P and reduced S1P1 stimulation. In a carrageenan-induced model of inflammation, Apom(-/-) mice had increased vascular leakage compared with that in WT mice. Adenoviral overexpression of ApoM in Apom(-/-) mice decreased the vascular leakage compared to adenoviral overexpression of green fluorescent protein. The study suggests that vascular leakage of albumin-sized particles in ApoM deficiency is S1P- and S1P1-dependent and this dependency exacerbates the response to inflammatory stimuli.-Christensen, P. M., Liu, C. H., Swendeman, S. L., Obinata, H., Qvortrup, K., Nielsen, L B., Hla, T., Di Lorenzo, A., Christoffersen, C. Impaired endothelial barrier function in apolipoprotein M-deficient mice is dependent on sphingosine-1-phosphate receptor 1.

Human plasma lipocalins and serum albumin: Plasma alternative carriers?

Lipocalins are an evolutionarily conserved family of proteins that bind and transport a variety of exogenous and endogenous ligands. Lipocalins share a conserved eight anti-parallel β-sheet structure. Among the different lipocalins identified in humans, α-1-acid glycoprotein (AGP), apolipoprotein D (apoD), apolipoprotein M (apoM), α1-microglobulin (α1-m) and retinol-binding protein (RBP) are plasma proteins. In particular, AGP is the most important transporter for basic and neutral drugs, apoD, apoM, and RBP mainly bind endogenous molecules such as progesterone, pregnenolone, bilirubin, sphingosine-1-phosphate, and retinol, while α1-m binds the heme. Human serum albumin (HSA) is a monomeric all-α protein that binds endogenous and exogenous molecules like fatty acids, heme, and acidic drugs. Changes in the plasmatic levels of lipocalins and HSA are responsible for the onset of pathological conditions associated with an altered drug transport and delivery. This, however, does not necessary result in potential adverse effects in patients because many drugs can bind both HSA and lipocalins, and therefore mutual compensatory binding mechanisms can be hypothesized. Here, molecular and clinical aspects of ligand transport by plasma lipocalins and HSA are reviewed, with special attention to their role as alterative carriers in health and disease.

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

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

Protein unfolding allows use of commercial antibodies in an apolipoprotein M sandwich ELISA

apoM is a member of the lipocalin superfamily and circulates in plasma attached to HDL particles. apoM plays a role in cholesterol metabolism and has recently been identified as transporter for the signaling lipid, sphingosine-1-phosphate (S1P), in plasma. S1P is implicated in several inflammatory diseases such as multiple sclerosis and rheumatoid arthritis. The ability to accurately measure apoM is crucial for investigating its biological functions and possible clinical implications. However, reliable commercial methods have been lacking so far. Therefore, we have developed an assay that specifically recognizes human apoM in plasma using commercially available reagents. Commercial apoM antibodies were screened for compatibility in a sandwich ELISA-based assay. One optimal pair of antibodies was chosen, and sample preparation, buffers, and incubation times were optimized to generate a simple and reproducible method. Validation and comparison to a previously described ELISA for apoM confirmed that the assay displays a high degree of sensitivity, specificity, and precision. Our results show that commercially available antibodies can be used to accurately measure human plasma apoM. This method can be implemented in every laboratory and will help promote high quality research.

Structure of HDL: particle subclasses and molecular components

A molecular understanding of high-density lipoprotein (HDL) will allow a more complete grasp of its interactions with key plasma remodelling factors and with cell-surface proteins that mediate HDL assembly and clearance. However, these particles are notoriously heterogeneous in terms of almost every physical, chemical and biological property. Furthermore, HDL particles have not lent themselves to high-resolution structural study through mainstream techniques like nuclear magnetic resonance and X-ray crystallography; investigators have therefore had to use a series of lower resolution methods to derive a general structural understanding of these enigmatic particles. This chapter reviews current knowledge of the composition, structure and heterogeneity of human plasma HDL. The multifaceted composition of the HDL proteome, the multiple major protein isoforms involving translational and posttranslational modifications, the rapidly expanding knowledge of the HDL lipidome, the highly complex world of HDL subclasses and putative models of HDL particle structure are extensively discussed. A brief history of structural studies of both plasma-derived and recombinant forms of HDL is presented with a focus on detailed structural models that have been derived from a range of techniques spanning mass spectrometry to molecular dynamics.

Atheroprotective role of high-density lipoprotein (HDL)-associated sphingosine-1-phosphate (S1P)

Numerous epidemiological studies documented an inverse relationship between plasma high-density lipoprotein (HDL) cholesterol levels and the extent of atherosclerotic disease. However, clinical interventions targeting HDL cholesterol failed to show clinical benefits with respect to cardiovascular risk reduction, suggesting that HDL components distinct from cholesterol may account for anti-atherogenic effects attributed to this lipoprotein. Sphingosine-1-phosphate (S1P)-a lysosphingolipid exerting its biological activity via binding to specific G protein-coupled receptors and regulating a wide array of biological responses in a variety of different organs and tissues including the cardiovascular system-has been identified as an integral constituent of HDL particles. In the present review, we discuss current evidence from epidemiological studies, experimental approaches in vitro, and animal models of atherosclerosis, suggesting that S1P contributes to atheroprotective effects exerted by HDL particles.

Ligand binding-dependent functions of the lipocalin NLaz: an in vivo study in Drosophila

Lipocalins are small extracellular proteins mostly described as lipid carriers. The Drosophila lipocalin NLaz (neural Lazarillo) modulates the IIS pathway and regulates longevity, stress resistance, and behavior. Here, we test whether a native hydrophobic pocket structure is required for NLaz to perform its functions. We use a point mutation altering the binding pocket (NLaz(L130R)) and control mutations outside NLaz binding pocket. Tryptophan fluorescence titration reveals that NLaz(L130R) loses its ability to bind ergosterol and the pheromone 7(z)-tricosene but retains retinoic acid binding. Using site-directed transgenesis in Drosophila, we test the functionality of the ligand binding-altered lipocalin at the organism level. NLaz-dependent life span reduction, oxidative stress and starvation sensitivity, aging markers accumulation, and deficient courtship are rescued by overexpression of NLaz(WT), but not of NLaz(L130R). Transcriptional responses to aging and oxidative stress show a large set of age-responsive genes dependent on the integrity of NLaz binding pocket. Inhibition of IIS activity and modulation of oxidative stress and infection-responsive genes are binding pocket-dependent processes. Control of energy metabolites on starvation appears to be, however, insensitive to the modification of the NLaz binding pocket.

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.

A simple model-free method for direct assessment of fluorescent ligand binding by linear spectral summation

Fluorescent tagged ligands are commonly used to determine binding to proteins. However, bound and free ligand concentrations are not directly determined. Instead the response in a fluorescent ligand titration experiment is considered to be proportional to the extent of binding and, therefore, the maximum value of binding is scaled to the total protein concentration. Here, a simple model-free method is presented to be performed in two steps. In the first step, normalized bound and free spectra of the ligand are determined. In the second step, these spectra are used to fit composite spectra as the sum of individual components or linear spectral summation. Using linear spectral summation, free and bound 1-Anilinonaphthalene-8-Sulfonic Acid (ANS) fluorescent ligand concentrations are directly calculated to determine ANS binding to tear lipocalin (TL), an archetypical ligand binding protein. Error analysis shows that the parameters that determine bound and free ligand concentrations were recovered with high certainty. The linear spectral summation method is feasible when fluorescence intensity is accompanied by a spectral shift upon protein binding. Computer simulations of the experiments of ANS binding to TL indicate that the method is feasible when the fluorescence spectral shift between bound and free forms of the ligand is just 8 nm. Ligands tagged with environmentally sensitive fluorescent dyes, e.g., dansyl chromophore, are particularly suitable for this method.

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.

Characterization of hydrophobic-ligand-binding proteins of Taenia solium that are expressed specifically in the adult stage

Taenia solium, a causative agent of taeniasis and cysticercosis, has evolved a repertoire of lipid uptake mechanisms. Proteome analysis of T. solium excretory-secretory products (TsESP) identified 10 kDa proteins displaying significant sequence identity with cestode hydrophobic-ligand-binding-proteins (HLBPs). Two distinct 362- and 352-bp-long cDNAs encoding 264- and 258-bp-long open reading frames (87 and 85 amino acid polypeptides) were isolated by mining the T. solium expressed sequence tags and a cDNA library screening (TsHLBP1 and TsHLBP2; 94% sequence identity). They clustered into the same clade with those found in Moniezia expansa and Hymenolepis diminuta. Genomic structure analysis revealed that these genes might have originated from a common ancestor. Both the crude TsESP and bacterially expressed recombinant proteins exhibited binding activity toward 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS), which was competitively inhibited by oleic acid. The proteins also bound to cis-parinaric acid (cPnA) and 16-(9-anthroyloxy) palmitic acid (16-AP), but showed no binding activity against 11-[(5-dimethylaminonaphthalene-1-sulfonyl) amino] undecanoic acid (DAUDA) and dansyl-DL-α-aminocaprylic acid (DACA). Unsaturated fatty acids (FAs) showed greater affinity than saturated FAs. The proteins were specifically expressed in adult worms throughout the strobila. The TsHLBPs might be involved in uptake and/or sequestration of hydrophobic molecules provided by their hosts, thus contributing to host-parasite interface interrelationships.

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.

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.

Mouse ApoM displays an unprecedented seven-stranded lipocalin fold: folding decoy or alternative native fold?

Mouse apolipoprotein M (m-apoM) displays a 79% sequence identity to human apolipoprotein M (h-apoM). Both proteins are apolipoproteins associated with high-density lipoproteins, with similar anticipated biological functions. The structure of h-apoM has recently been determined by X-ray crystallography, which revealed that h-apoM displays, as expected, a lipocalin-like fold characterized by an eight-stranded β‑barrel that encloses an internal fatty-acid-binding site. Surprisingly, this is not true for m-apoM. After refolding from inclusion bodies, the crystal structure of m-apoM (reported here at 2.5 Å resolution) displays a novel yet unprecedented seven-stranded β-barrel structure. The fold difference is not caused by a mere deletion of a single β-strand; instead, β-strands E and F are removed and replaced by a single β-strand A' formed from residues from the N-terminus. Molecular dynamics simulations suggest that m-apoM is able to adopt both a seven-stranded barrel structure and an eight-stranded barrel structure in solution, and that both folds are comparably stable. Thermal unfolding simulations identify the position where β-strand exchange occurs as the weak point of the β-barrel. We wonder whether the switch in topology could have a biological function and could facilitate ligand release, since it goes hand in hand with a narrowing of the barrel diameter. Possibly also, the observed conformation represents an on-pathway or off-pathway folding intermediate of apoM. The difference in fold topology is quite remarkable, and the fold promiscuity observed for m-apoM might possibly provide a glimpse at potential cross-points during the evolution of β-barrels.

Regulation of human apolipoprotein m gene expression by orphan and ligand-dependent nuclear receptors

Apolipoprotein M (apoM) plays an important role in the biogenesis and the metabolism of anti-atherogenic HDL particles in plasma and is expressed primarily in the liver and the kidney. We investigated the role of hormone nuclear receptors in apoM gene regulation in hepatic cells. Overexpression via adenovirus-mediated gene transfer and siRNA-mediated gene silencing established that hepatocyte nuclear factor 4 (HNF-4) is an important regulator of apoM gene transcription in hepatic cells. apoM promoter deletion analysis combined with DNA affinity precipitation and chromatin immunoprecipitation assays revealed that HNF-4 binds to a hormone-response element (HRE) in the proximal apoM promoter (nucleotides -33 to -21). Mutagenesis of this HRE decreased basal hepatic apoM promoter activity to 10% of control and abolished the HNF4-mediated transactivation of the apoM promoter. In addition to HNF-4, homodimers of retinoid X receptor and heterodimers of retinoid X receptor with receptors for retinoic acid, thyroid hormone, fibrates (peroxisome proliferator-activated receptor), and oxysterols (liver X receptor) were shown to bind with different affinities to the proximal HRE in vitro and in vivo. Ligands of these receptors strongly induced human apoM gene transcription and apoM promoter activity in HepG2 cells, whereas mutations in the proximal HRE abolished this induction. These findings provide novel insights into the role of apoM in the regulation of HDL by steroid hormones and into the development of novel HDL-based therapies for diseases such as diabetes, obesity, metabolic syndrome, and coronary artery disease that affect a large proportion of the population in Western countries.

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

RATIONALE

Plasma apolipoprotein (apo)M is mainly associated with high-density lipoprotein (HDL). HDL-bound apoM is antiatherogenic in vitro. However, plasma apoM is not associated with coronary heart disease in humans, perhaps because of a positive correlation with plasma low-density lipoprotein (LDL).

OBJECTIVE

We explored putative links between apoM and very-low-density (VLDL)/LDL metabolism and the antiatherogenic potential of apoM in vivo.

METHODS AND RESULTS

Plasma apoM was increased approximately 2.1 and approximately 1.5 fold in mice lacking LDL receptors (Ldlr(-/-)) and expressing dysfunctional LDL receptor-related protein 1 (Lrp1(n2/n2)), respectively, but was unaffected in apoE-deficient (ApoE(-/-)) mice. Thus, pathways controlling catabolism of VLDL and LDL affect plasma apoM. Overexpression (approximately 10-fold) of human apoM increased (50% to 70%) and apoM deficiency decreased ( approximately 25%) plasma VLDL/LDL cholesterol in Ldlr(-/-) mice, whereas apoM did not affect plasma VLDL/LDL in mice with intact LDL receptors. Moreover, plasma clearance of apoM-enriched VLDL/LDL was slower than that of control VLDL/LDL in mice lacking functional LDL receptors and LRP1, suggesting that apoM impairs the catabolism of VLDL/LDL that occurs independently of the LDL receptor and LRP1. ApoM overexpression decreased atherosclerosis in ApoE(-/-) (60%) and cholate/cholesterol-fed wild-type mice (70%). However, in Ldlr(-/-) mice the antiatherogenic effect of apoM was attenuated by its VLDL/LDL-raising effect.

CONCLUSION

The data suggest that defect LDL receptor function leads to increased plasma apoM concentrations, which in turn, impairs the removal of VLDL/LDL from plasma. This mechanism opposes the otherwise antiatherogenic effect of apoM.

An investigation of serum concentration of apoM as a potential MODY3 marker using a novel ELISA

OBJECTIVE

To investigate the fitness of serum apolipoprotein M (apoM) concentration as a marker for maturity-onset diabetes of the young 3 (MODY3).

STUDY DESIGN AND SUBJECTS

This study consisted of two parts. A family study included 71 carriers of the P291fsinsC mutation in hepatocyte nuclear factor-1alpha (HNF-1alpha) from the Finnish Botnia study, 53 of whom were diabetic, and 75 matched family controls. A second, case-control study included 24 MODY3 patients, 17 healthy MODY3 mutation carriers, 11 MODY1 patients, 18 type 2 diabetes patients and 19 healthy control individuals. Subjects in the case-control study were recruited from the Botnia study or the Clinic of Endocrinology, Malmö University Hospital. Serum apoM levels were measured using a novel ELISA based on two monoclonal apoM antibodies.

RESULTS

In the family study, mean serum apoM was 10% lower in female carriers of the P291fsinsC mutation compared to the family controls (P = 0.0058), a difference which remained significant after adjustment for diabetes status. There was no observed difference between groups for men. In the case-control study, no significant difference in apoM concentration was observed between MODY3 and type 2 diabetes patients, neither before nor after adjustment for total cholesterol.

CONCLUSIONS

Female carriers of the P291fsinsC mutation in HNF-1alpha displayed slightly lower apoM serum levels. This difference is too small for apoM to be reliably employed as a biomarker for HNF-1alpha mutation status.

LipocalinPred: a SVM-based method for prediction of lipocalins

BACKGROUND

Functional annotation of rapidly amassing nucleotide and protein sequences presents a challenging task for modern bioinformatics. This is particularly true for protein families sharing extremely low sequence identity, as for lipocalins, a family of proteins with varied functions and great diversity at the sequence level, yet conserved structures.

RESULTS

In the present study we propose a SVM based method for identification of lipocalin protein sequences. The SVM models were trained with the input features generated using amino acid, dipeptide and secondary structure compositions as well as PSSM profiles. The model derived using both PSSM and secondary structure emerged as the best model in the study. Apart from achieving a high prediction accuracy (>90% in leave-one-out), lipocalinpred correctly differentiates closely related fatty acid-binding proteins and triabins as non-lipocalins.

CONCLUSION

The method offers a promising approach as a lipocalin prediction tool, complementing PROSITE, Pfam and homology modelling methods.

Association of apolipoprotein M with high-density lipoprotein kinetics in overweight-obese men

OBJECTIVE

The aim of this study was to investigate associations between plasma apoM concentration and HDL apoA-I and apoA-II kinetics in 60 overweight-obese, insulin resistant men.

METHODS

Plasma apoM concentration was determined using a sandwich ELISA with two monoclonal antibodies (CV<5%). The kinetics of HDL apoA-I and apoA-II were measured using intravenous administration of D(3)-leucine, gas chromatography-mass spectrometry and multi-compartmental modeling.

RESULTS

Plasma apoM was inversely associated with body mass index and positively associated with plasma total cholesterol, LDL cholesterol and HDL cholesterol (p<0.05). There were no associations between plasma apoM and plasma triglyceride, NEFA, insulin, glucose, HOMA score or adiponectin concentrations. Plasma apoM was positively associated with both apoA-I and apoA-II concentrations (r=0.406, p<0.01 and r=0.510, p<0.01, respectively) and negatively associated with HDL apoA-I and apoA-II fractional catabolic rate (FCR) (r=-0.291, p=0.03 and r=-0.291, p=0.026, respectively). No significant associations were observed between plasma apoM and HDL apoA-I and apoA-II production rate. In multivariate regression models, both plasma apoM and triglycerides were significant, independent predictors of HDL apoA-I FCR (adjusted R(2)=16%, p<0.01) and HDL apoA-II FCR (adjusted R(2)=14%, p<0.01).

CONCLUSION

ApoM may be a significant, independent predictor of HDL apoA-I and apoA-II catabolism in overweight-obese, insulin resistant men.