Apolipoprotein M

Cholesterol transport and beyond: Illuminating the versatile functions of HDL apolipoproteins through structural insights and functional implications

High-density lipoproteins (HDLs) play a vital role in lipid metabolism and cardiovascular health, as they are intricately involved in cholesterol transport and inflammation modulation. The proteome of HDL particles is indeed complex and distinct from other components in the bloodstream. Proteomics studies have identified nearly 285 different proteins associated with HDL; however, this review focuses more on the 15 or so traditionally named "apo" lipoproteins. Important lipid metabolizing enzymes closely working with the apolipoproteins are also discussed. Apolipoproteins stand out for their integral role in HDL stability, structure, function, and metabolism. The unique structure and functions of each apolipoprotein influence important processes such as inflammation regulation and lipid metabolism. These interactions also shape the stability and performance of HDL particles. HDLs apolipoproteins have multifaceted roles beyond cardiovascular diseases (CVDs) and are involved in various physiological processes and disease states. Therefore, a detailed exploration of these apolipoproteins can offer valuable insights into potential diagnostic markers and therapeutic targets. This comprehensive review article aims to provide an in-depth understanding of HDL apolipoproteins, highlighting their distinct structures, functions, and contributions to various physiological processes. Exploiting this knowledge holds great potential for improving HDL function, enhancing cholesterol efflux, and modulating inflammatory processes, ultimately benefiting individuals by limiting the risks associated with CVDs and other inflammation-based pathologies. Understanding the nature of all 15 apolipoproteins expands our knowledge of HDL metabolism, sheds light on their pathological implications, and paves the way for advancements in the diagnosis, prevention, and treatment of lipid and inflammatory-related disorders.

Apolipoprotein M promotes the anti-inflammatory effect of high-density lipoprotein by binding to scavenger receptor BI

Background

Inflammation participates pivotally in the pathogenesis of atherosclerosis. Apolipoprotein M (apoM) is a high-density lipoprotein (HDL)-associated plasma protein that affects HDL metabolism and shows various anti-inflammatory functions in atherosclerosis. In this study, we aim to determine whether apoM is expressed in peripheral blood mononuclear cells (PBMCs) and promoted the anti-inflammatory effect of HDL by combing with scavenger receptor BI (SR-BI).

Methods

The expression of apoM in PBMCs is detected by a confocal fluorescence microscope and flow cytometry. The interactions between apoM and SR-BI are detected with co-immunoprecipitation. The multiplexed Luminex xMAP assay detects the inflammatory factors induced by apoM⁺ HDL and apoM^– HDL in inflammatory cell model.

Results

ApoM is expressed on CD14⁺ monocytes, CD3⁺ T cells, and CD19⁺ B cells, CD16⁺ and CD56⁺ NK cells. CD14⁺ monocytes have the highest ratio of apoM⁺ cells. ApoM⁺ HDL, apoM^– HDL, and recombinant apoM protein could be co-precipitated with SR-BI on the surface of human THP-1 monocytic leukemia cells. In vitro, apoM⁺ HDL induces significantly less expression of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) and IL-1β than apoM^– HDL.

Conclusions

ApoM was expressed on all PBMCs. ApoM interacted with SR-BI on THP-1. ApoM⁺ HDL has a more significant anti-inflammatory effect than apoM^– HDL.

Apolipoprotein M, identified as a novel hepatitis C virus (HCV) particle associated protein, contributes to HCV assembly and interacts with E2 protein

Hepatitis C virus (HCV) infection is a major cause of chronic liver diseases such as steatosis, cirrhosis, and hepatocellular carcinoma. HCV particles have been found to associate with apolipoproteins, and apolipoproteins not only participate in the HCV life cycle, but also help HCV escape recognition by the host immune system, which pose challenges for the development of both HCV treatments and vaccines. However, no study has reported on the comprehensive identification of apolipoprotein associations with HCV particles. In the present study, we performed proteome analysis by affinity purification coupled with mass spectrometry (AP-MS) to comprehensively identify the apolipoprotein associations with HCV particles, and ApoM was first identified by AP-MS besides the previously reported ApoE, ApoB, ApoA-I and ApoC-I. Additionally, three assays further confirmed that ApoM was a novel virus particle associated protein. We also showed that ApoM was required for HCV production, especially for the assembly/release step of HCV life cycle. Furthermore, ApoM interacted with the HCV E2 protein. Finally, HCV infection reduced ApoM expression both in vitro and in vivo. Collectively, our study demonstrates that ApoM, identified as a novel HCV particle associated protein, contributes to HCV assembly/release and interacts with HCV E2 protein. It provides new insights on how HCV and the host apolipoproteins are reciprocally influenced and lays a basis for research in developing innovative antiviral strategies.

A cross-talk between leptin and 17β-estradiol in vitellogenin synthesis in rainbow trout Oncorhynchus mykiss liver

The existence of nutritional and energy reserves is fundamental for fish female fertility, so that the existence of a correlation between metabolic reserves and reproductive capacity is suggested. Leptin regulates body weight and energy homeostasis. Estradiol induces the synthesis of vitellogenin, a phospholipoglycoprotein produced by the liver and taken up by the growing oocytes. The objective of this study was to investigate the possible existence of a crosstalk between 17β-estradiol (E₂) and leptin in the modulation of E₂-induced vtg in the rainbow trout Oncorhynchus mykiss. Liver slices were incubated with recombinant trout leptin (rt-lep) at three different concentrations (1-10-100 ng/ml). rt-lep brought about the decrease of E₂-induced vtg secretion in the medium and the down-regulation of vtg mRNA expression. Moreover, rt-lep stimulated the lipase activity and diminished the liver fatty acid content. The combined employment of signal transduction inhibitors and the analysis of signal transduction phosphorylated factors revealed that rt-lep effect on E₂-induced vtg occurred through the activation of phosphodiesterase, protein kinase C, MAP kinases, and protein kinase A. In conclusion, our study suggests that leptin influences E₂-induced vtg synthesis in the rainbow trout Oncorhynchus mykiss by modifying both the protein and the lipid components.

Apolipoprotein M Serum Levels Correlate with IgA Vasculitis and IgA Vasculitis Nephritis

Objective

IgA vasculitis (lgAV) is the most frequent vessel vasculitis in children, and the prognosis is related to the children's age and degree of nephritis. This study is aimed at investigating serum apolipoprotein M (apoM) levels in patients with lgAV patients and at evaluating the association between apoM and disease severity.

Methods

A total of 109 lgAV patients and 76 age- and sex-matched healthy controls were included. The age and gender of the study participants were matched. ApoM levels were measured by an enzyme-linked immunosorbent assay. Additionally, the serum levels of lipids, apolipoproteins, kidney biochemical profiles, immunoglobulins (IgA, IgG, IgM, and IgE), and the complements (C3 and C4) were assessed using an automatic biochemical analyzer.

Results

ApoM was increased significantly in lgAV patients compared to healthy controls. ApoM, meanwhile, was lower in patients with nephritis than in those without nephritis. The apoM levels were higher in classes I and II IgA vasculitis nephritis (lgAVN) patients than in classes III and IV. Besides, the apoM serum level < 24.81 mg/L was an independent predictive factor for lgAVN and can be independently associated with the presence of nephritis in lgAV patients. Meanwhile, the serum apoM concentration negatively correlated with the ISKDC grading score in lgAVN patients.

Conclusions

Serum apoM was elevated in lgAV patients and decreased gradually with the ISKDC grading score. ApoM (OR = 0.32, 95%CI = 0.12-0.85, p = 0.023) was identified as a protective factor for nephritis in all lgAV patients.

Molecular basis of degenerative spinal disorders from a proteomic perspective (Review)

Intervertebral disc degeneration (IDD) and ligamentum flavum hypertrophy (LFH) are major causes of degenerative spinal disorders. Comparative and proteomic analysis was used to identify differentially expressed proteins (DEPs) in IDD and LFH discs compared with normal discs. Subsequent gene ontology term enrichment analysis and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of the DEPs in human IDD discs or LFH samples were performed to identify the biological processes and signaling pathways involved in IDD and LFH. The PI3K-AKT signaling pathway, advanced glycation endproducts-receptor for advanced glycation endproducts signaling pathway, p53 signaling pathway, and transforming growth factor-b signaling pathway were activated in disc degeneration. This review summarizes the recently identified DEPs, including prolargin, fibronectin 1, cartilage intermediate layer protein, cartilage oligomeric matrix protein, and collagen types I, II and IV, and their pathophysiological roles in degenerative spinal disorders, and may provide a deeper understanding of the pathological processes of human generative spinal disorders. The present review aimed to summarize significantly changed proteins in degenerative spinal disorders and provide a deeper understanding to prevent these diseases.

Niacin regulates apolipoprotein M expression via liver X receptor‑α

Niacin is currently the most effective drug that increases HDL-C levels. Apolipoprotein M (ApoM) in humans is mainly found in plasma high-density lipoprotein (HDL). Little is known about the role played by niacin in ApoM expression. In this study, the effects of niacin on ApoM expression were assessed as well as the associated mechanism. Human liver cancer cell line HepG2 was treated with niacin alone or with liver X receptor-α (LXRα) inhibitor at multiple concentrations. The mRNA and protein expression of ApoM were assessed by qRT-PCR and western blotting. Specific LXRα shRNA was transfected into HepG2 cells to further evaluate the regulatory effects of LXRα on ApoM. An in vivo model was also established to investigate the LXRα inhibitor on the mouse ApoM levels. The comparisons among groups were evaluated using one-way ANOVA and Student-Newman-Keuls test. It was revealed that in HepG2 cells, niacin dose-dependently increased ApoM gene and protein expression levels. Niacin-induced upregulation of ApoM was attenuated by an LXRα inhibitor or LXRα shRNA, indicating that LXRα mediated this effect. Moreover, niacin treatment resulted in increased LXRα mRNA levels, in vivo and in vitro; niacin treatment resulted in increased ApoM gene and protein expression levels in mice. In conclusion, niacin upregulates ApoM expression by increasing LXRα expression in vivo and in vitro.

Apolipoprotein M increases the expression of vitamin D receptor mRNA in colorectal cancer cells detected with duplex fluorescence reverse transcription-quantitative polymerase chain reaction

Apolipoprotein M (ApoM) and the vitamin D receptor (VDR) are apolipoproteins predominantly presenting in high-density lipoprotein (HDL) and a karyophilic protein belonging to the steroid‑thyroid receptor superfamily, respectively. Previous studies have demonstrated that ApoM and VDR are associated with cholesterol metabolism, immune and colorectal cancer regulation. In order to investigate whether ApoM affected the expression of VDR in colorectal cancer cells, a single‑tube duplex fluorescence reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) system was developed to simultaneously detect the mRNA levels of VDR and GAPDH in HT‑29 cells overexpressing ApoM. The results demonstrated that the amplification products were confirmed as the specific fragment of VDR/GAPDH using the DNA sequencing instrument. The sensitivity, linear range, correlation coefficient, amplification efficiency, intra‑assay and inter‑assay coefficients of variation were 40 copies/µl, 4.00x101‑4.00x105 copies/µl, 0.999, 92.42%, 0.09‑0.34% and 0.32‑0.65% for VDR, and 40 copies/µl, 4.00x101‑4.00x105 copies/µl, 0.999, 98.07%, 0.19‑0.43% and 0.40‑0.75% for GAPDH, respectively. The results indicated that the expression of VDR mRNA was significantly higher in HT‑29 cells overexpressing ApoM, compared with the negative control group (P<0.05). In conclusion, the current study successfully developed the single‑tube duplex RT‑qPCR to simultaneously detect VDR and GAPDH expression in colorectal cancer cells. The methodology results demonstrated that the duplex RT‑qPCR system with high sensitivity and specificity could ensure the objectivity and credibility of the detection. The present study confirmed that ApoM significantly increased the expression of VDR in HT‑29 cells. In addition, it was hypothesized that ApoM may be involved in antineoplastic activity via the upregulation of VDR expression, which may provide novel directions for the investigation of ApoM in cancer.

The Effect of apoM Polymorphism Associated with HDL Metabolism on Obese Korean Adults

BACKGROUND

Apolipoprotein M (apoM) is a recently identified apolipoprotein associated with high-density lipoprotein (HDL) in coronary artery disease (CAD), but the association between apoM polymorphism and obesity has not been reported.

AIM

To investigate the association between apoM polymorphism and obesity prevalence in 584 Korean adults.

METHODS

A total of 584 individuals aged between 30 and 80 years were recruited from Yonsei Medical Center in Seoul, Korea, and divided into obese (OB; body mass index, BMI ≥25) and nonobese (non-OB; BMI <25) groups. Anthropometric variables, lipid profiles, insulin-resistant profiles, reverse cholesterol transport (RCT) enzymes, HDL subfraction, and apoM polymorphism were determined.

RESULTS

In OB with T-855C polymorphism, TT genotype carriers significantly showed 6.2% higher diastolic blood pressure (DBP), 1.3% lower amount of HDL2b subfraction, and 19.7% higher lecithin-cholesterol acyltransferase (LCAT) mass than TC+CC carriers. OB subjects with the T allele of T-778C polymorphism significantly demonstrated 43% higher plasma insulin, 17.7% higher total cholesterol, 26.7% higher triglyceride, 40.7% higher leptin, 1.6% lower HDL2b, and 12.6% higher LCAT mass than those with the C allele. These results were reversed in non-OB with T-778C polymorphism regarding HDL subfractions and RCT enzymes.

CONCLUSION

apoM T-855C and T-778C polymorphisms were found to be associated with obesity by regulating HDL metabolism, and the T alleles of apoM T-778C were shown to be more strongly correlated.

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

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

High apolipoprotein M serum levels correlate with chronic obstructive pulmonary disease

Background

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

Methods

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

Results

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

Conclusions

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

Electronic supplementary material

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

Propofol Attenuates Lipopolysaccharide-Induced Monocyte Chemoattractant Protein-1 Production Through Enhancing apoM and foxa2 Expression in HepG2 Cells

Monocyte chemoattractant protein-1 (MCP-1) is a cytokine that mediates the influx of cells to sites of inflammation. Our group recently reported that propofol exerted an anti-inflammatory effect and could inhibit lipopolysaccharide (LPS)-induced production of pro-inflammatory cytokines. However, the effect and possible mechanisms of propofol on MCP-1 expression remain unclear. LPS-stimulated HepG2 cells were treated with 50 μM propofol for 0, 6, 12, and 24 h, respectively. The transcript and protein levels were measured by real-time quantitative PCR and Western blot analyses, respectively. We found that propofol markedly decreased both MCP-1 messenger RNA (mRNA) and protein levels in LPS-stimulated HepG2 cells in a time-dependent manner. Expression of apolipoprotein M (apoM) and forkhead box protein A2 (foxa2) was increased by propofol treatment in HepG2 cells. In addition, the inhibitory effect of propofol on MCP-1 expression was significantly abolished by small interfering RNA against apoM and foxa2 in LPS-stimulated HepG2 cells. Propofol attenuates LPS-induced MCP-1 production through enhancing apoM and foxa2 expression in HepG2 cells.

Meta-Analysis on the Correlation Between APOM rs805296 Polymorphism and Risk of Coronary Artery Disease

BACKGROUND

The present meta-analysis aimed to summarize the inconsistent findings on the association of apolipoprotein M gene (ApoM) rs805296 polymorphism with the risk of coronary artery disease (CAD), and to obtain a more authentic result about this topic.

MATERIAL/METHODS

A total of 7 available articles were identified through electronic databases--PubMed, EMBASE, and Chinese National Knowledge Infrastructure (CNKI)--and their useful data were carefully extracted. The relationship between ApoM rs805296 polymorphism and CAD risk was assessed by odds ratios (ORs) and corresponding 95% confidence intervals (95% CIs), which were calculated using the fixed- or random-effects model, according to the degree of heterogeneity. Hardy-Weinberg equilibrium test, sensitivity test, and publication bias examination were also performed in this meta-analysis.

RESULTS

According to the pooled results, ApoM rs805296 polymorphism conferred an increased risk of CAD under all the genetic contrasts: CC versus TT, CC + TC versus TT, CC versus TT+TC, C versus T, and TC versus TT (OR=2.13, 95% CI=1.16-3.91; OR=1.80, 95% CI=1.50-2.17; OR=1.91, 95% CI=1.04-3.51; OR=1.72, 95% CI=1.45-2.04; OR=1.78, 95% CI=1.47-2.15).

CONCLUSIONS

ApoM rs805296 polymorphism may be a risk factor for developing CAD.

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

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

ApoM Suppresses TNF-α-Induced Expression of ICAM-1 and VCAM-1 Through Inhibiting the Activity of NF-κB

To explore the anti-inflammatory effect of apolipoprotein M (apoM) on regulation of tumor necrosis factor-α (TNF-α)-induced expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) and further investigate the molecular mechanism of apoM in this process. We found that TNF-α could decrease expression of apoM and inhibitor of NF-κB-α (IκBα) in HepG2 cells. Overexpression of apoM caused a significant decrease of ICAM-1 and VCAM-1 expression, while it caused a significant increase of IκBα expression in HepG2 cells. Furthermore, the treatment with TNF-α could increase ICAM-1 and VCAM-1 expression, decrease IκBα protein expression, and increase nuclear factor-κB (NF-κB) activity, and these effects were markedly enhanced by small interfering RNA (siRNA)-mediated silencing of apoM in HepG2 cells. Our findings demonstrated that apoM suppressed TNF-α-induced expression of ICAM-1 and VCAM-1 through inhibiting the activity of NF-κB.

Apolipoprotein M regulates the orphan nuclear receptor LRH-1 gene expression through binding to its promoter region in HepG2 cells

Apolipoprotein M (ApoM) is predominantly located in the high-density lipoprotein in human plasma. It has been demonstrated that ApoM expression could be regulated by several crucial nuclear receptors that are involved in the bile acid metabolism. In the present study, by combining gene-silencing experiments, overexpression studies, and chromatin immunoprecipitation assays, we showed that ApoM positively regulated liver receptor homolog-1 (LRH-1) gene expression via direct binding to an LRH-1 promoter region (nucleotides -406/ -197). In addition, we investigated the effects of farnesoid X receptor agonist GW4064 on hepatic ApoM expression in vitro. In HepG2 cell cultures, both mRNA and protein levels of ApoM and LRH-1 were decreased in a time-dependent manner in the presence of 1 μM GW4064, and the inhibition effect was gradually attenuated after 24 hours. In conclusion, our findings present supportive evidence that ApoM is a regulator of human LRH-1 transcription, and further reveal the importance of ApoM as a critical regulator of bile acids metabolism.

Comparative analysis of serum proteomes: Identification of proteins associated with sciatica due to lumbar intervertebral disc herniation

Lumbar intervertebral disc herniation (LDH) is one of the most common orthopedic conditions that can cause lower back pain and sciatica. However, the pathogenesis of LDH is poorly understood. The aim of the present study was to use proteomic analysis of blood samples to establish whether there are serum proteins associated with LDH, which may be useful in elucidating LDH pathogenesis. The ultimate aim was to develop a simple technique for the diagnosis of LDH based on the blood samples of patients with sciatica. The study used comparative analysis of serum proteomes associated with sciatica due to LDH. A total of 30 LDH patients with sciatica, receiving treatment between August and December 2007, were selected as the experimental group (or LDH group). A total of 2 ml of blood was obtained from each of the 30 patients in the LDH group and from 30 healthy volunteers, who constituted the control group. Two-dimensional electrophoresis of the blood samples was conducted, distinct protein spots were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and proteins associated with LDH were detected. An enzyme-linked immunosorbent assay (ELISA) was developed to screen for the LDH proteins and was tested on the sera of a second test and control group that included 10 patients with LDH and 10 healthy subjects, respectively. Based on signal intensity, the expression levels of 6 proteins on the dielectrophoretogram were found to be significantly associated with LDH. The identities of the LDH proteins were upregulated apolipoprotein-L1 (APO-L1) and two types of serum albumin precursors, and downregulated apolipoprotein M (APO-M), tetranectin (TN) and immunoglobulin light chain (IGL). Further ELISA experiments confirmed that there were increased serum levels of 4 out of the 6 proteins in patients with sciatica due to LDH, which was statistically different compared to the healthy subjects. In conclusion, these results suggest that serum APO-L1, TN, APO-M and IGL may serve as LDH biomarkers.

Urinary apolipoprotein M could be used as a biomarker of acute renal injury: an ischemia-reperfusion injury model of kidney in rat

BACKGROUND

It has been well documented that apolipoprotein M (apoM) is principally expressed in hepatocytes as well as renal tubular epithelial cells. The importance of apoM in the kidney is unknown. In the present study we examined urinary any apoM after short-term ischemia-reperfusion injury (IRI) of kidney in a rat model.

METHODS

The kidneys of 11 male Sprague-Dawley rats were rendered ischemic for 45 minutes followed by different intervals of reperfusion. Serum and urine apoM concentrations were determined using a dot-blot analysis with specific rabbit anti-human apoM antibodies that cross-react with rat apoM. Serum concentrations of blood urea nitrogen (BUN) and creatinine (Cr) were determined using standard clinical automated analyses.

RESULTS

BUN was significantly elevated after 45 minutes of ischemia followed by 24 hours of reperfusion; serum Cr concentrations were also significantly increased at 6 and 24 hours of reperfusion. Interestingly, similar to BUN and Cr, serum apoM concentrations were significantly increased after ischemia for 45 minutes alone and after 2 hours of reperfusion. Urinary apoM concentrations were obviously increased after 2 h as well as 6 hours of reperfusion.

CONCLUSION

apoM showed characteristics of an acute-phase reactive protein; its occurrence in urine may be considered to be a biomarker of acute renal injury.

Dihydrocapsaicin down-regulates apoM expression through inhibiting Foxa2 expression and enhancing LXRα expression in HepG2 cells

Background

Apolipoprotein M (apoM), as a novel apolipoprotein which is mainly expressed in liver and kidney tissues, is associated with development and progression of atherosclerosis and diabetes. Our group have recently shown that Dihydrocapsaicin(DHC)can significantly decrease atherosclerotic plaque formation in apoE−/− mice. However, the effect and possible mechanism of DHC on apoM expression remain unclear.

Methods

HepG2 cells were treated with 0 μM, 25 μM, 50 μM and 100 μM DHC for 24 h or were treated with 100 μM DHC for 0, 6, 12, and 24 h, respectively. The mRNA levels and protein levels were measured by real-time quantitative PCR and western blot analysis, respectively.

Results

We found that DHC markedly decreased expression of apoM at both mRNA and protein level in HepG2 cells in a dose-dependent and time-dependent manner. Expression of Foxa2 was decreased while expression of LXRα was increased by DHC treatment in HepG2 cells. In addittion, overexpression of Foxa2 markedly compensated the inhibition effect induced by DHC on apoM expression. LXRα small interfering RNA significantly abolished the inhibition effect which induced by DHC on apoM expression. The liver of C57BL/6 mice treated with DHC had significantly lower expression of apoM. Furthermore, the liver had lower expression of Foxa2 while had higher expression of LXRα.

Conclusions

DHC could down-regulate apoM expression through inhibiting Foxa2 expression and enhancing LXRα expression in HepG2 cells.

Apolipoprotein M T-778C polymorphism is associated with serum lipid levels and the risk of coronary artery disease in the Chinese population: a meta-analysis

BACKGROUND

The apolipoprotein M (APOM) T-778C gene polymorphism has been associated with serum lipid levels and the risk of coronary artery disease (CAD), but the results are inconclusive. The purpose of this meta-analysis was to detect the association between the APOM T-778C polymorphism and serum lipid levels and the risk of CAD in the Chinese population.

METHODS

Databases of MEDLINE, EMBASE, the Cochrane Library and CNKI were systematically searched. Data were extracted using standardized methods. The association was assessed by mean difference (MD) with 95% confidence intervals (CI) or odds ratio (OR) with 95% CI.

RESULTS

Ten studies with 4,413 patients were included in this meta-analysis. Pooled effects indicated that CT+CC group had higher levels of total cholesterol (TC) (MD:-0.36, 95% CI: -0.53 - -0.19, P < 0.0001) and low-density lipoprotein cholesterol (LDL-C) (MD: -0.08, 95% CI: -0.16 - -0.01, P = 0.03) than TT group. There was no difference in the levels of triglyceride (MD: 0.06, 95% CI: -0.04 - 0.15, P = 0.22) and high-density lipoprotein cholesterol (MD: 0.00, 95% CI: -0.03-0.03, P = 0.93) between TT and CT+CC groups. Pooled effects showed that CAD group had higher CT+CC genotype frequency than control group (OR: 1.97, 95% CI: 1.62-2.39, P < 0.00001; heterogeneity test x(2) = 2.96, P = 0.71, I(2) = 0%).

CONCLUSIONS

The results of the current meta-analysis show that the CT+CC group has higher levels of TC and LDL-C than the TT group. Moreover, there is also a prominent association between APOM T-778C polymorphism and the risk of CAD in the Chinese population, the CT+CC genotype is associated with increased risk of CAD.

Expression levels of obesity-related genes are associated with weight change in kidney transplant recipients

Background

The aim of this study was to investigate the association of gene expression profiles in subcutaneous adipose tissue with weight change in kidney transplant recipients and to gain insights into the underlying mechanisms of weight gain.

Methodology/Principal Findings

A secondary data analysis was done on a subgroup (n = 26) of existing clinical and gene expression data from a larger prospective longitudinal study examining factors contributing to weight gain in transplant recipients. Measurements taken included adipose tissue gene expression profiles at time of transplant, baseline and six-month weight, and demographic data. Using multivariate linear regression analysis controlled for race and gender, expression levels of 1553 genes were significantly (p<0.05) associated with weight change. Functional analysis using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes classifications identified metabolic pathways that were enriched in this dataset. Furthermore, GeneIndexer literature mining analysis identified a subset of genes that are highly associated with obesity in the literature and Ingenuity pathway analysis revealed several significant gene networks associated with metabolism and endocrine function. Polymorphisms in several of these genes have previously been linked to obesity.

Conclusions/Significance

We have successfully identified a set of molecular pathways that taken together may provide insights into the mechanisms of weight gain in kidney transplant recipients. Future work will be done to determine how these pathways may contribute to weight gain.

Dihydrotestosterone regulating apolipoprotein M expression mediates via protein kinase C in HepG2 cells

Background

Administration of androgens decreases plasma concentrations of high-density lipid cholesterol (HDL-C). However, the mechanisms by which androgens mediate lipid metabolism remain unknown. This present study used HepG2 cell cultures and ovariectomized C57BL/6 J mice to determine whether apolipoprotein M (ApoM), a constituent of HDL, was affected by dihydrotestosterone (DHT).

Methods

HepG2 cells were cultured in the presence of either DHT, agonist of protein kinase C (PKC), phorbol-12-myristate-13-acetate (PMA), blocker of androgen receptor flutamide together with different concentrations of DHT, or DHT together with staurosporine at different concentrations for 24 hrs. Ovariectomized C57BL/6 J mice were treated with DHT or vehicle for 7d or 14d and the levels of plasma ApoM and livers ApoM mRNA were measured. The mRNA levels of ApoM, ApoAI were determined by real-time RT-PCR. ApoM and ApoAI were determined by western blotting analysis.

Results

Addition of DHT to cell culture medium selectively down-regulated ApoM mRNA expression and ApoM secretion in a dose-dependent manner. At 10 nM DHT, the ApoM mRNA levels were about 20% lower than in untreated cells and about 40% lower at 1000 nM DHT than in the control cells. The secretion of ApoM into the medium was reduced to a similar extent. The inhibitory effect of DHT on ApoM secretion was not blocked by the classical androgen receptor blocker flutamide but by an antagonist of PKC, Staurosporine. Agonist of PKC, PMA, also reduced ApoM. At 0.5 μM PMA, the ApoM mRNA levels and the secretion of ApoM into the medium were about 30% lower than in the control cells. The mRNA expression levels and secretion of another HDL-associated apolipoprotein AI (ApoAI) were not affected by DHT. The levels of plasma ApoM and liver ApoM mRNA of DHT-treated C57BL/6 J mice were lower than those of vehicle-treated mice.

Conclusions

DHT directly and selectively down-regulated the level of ApoM mRNA and the secretion of ApoM by protein kinase C but independently of the classical androgen receptor.

Expression and localization of apolipoprotein M in human colorectal tissues

Background

It has been well documented that apolipoprotein M (apoM) is principally expressed in the liver and kidney. However we found that there was weak apoM expression in other tissues or organs too, which could not be ignored. In the present study, we therefore examined apoM expression in human colorectal tissues including cancer tissues, cancer adjacent normal tissues, polyp tissues and normal mucosa as well as inflammatory mucosa.

Methods

Tissue samples were collected from patients who underwent surgical resection or endoscopic examination. ApoM mRNA levels were determined by the real-time RT-PCR and apoM protein mass were examined by the immunohistochemistry.

Results

ApoM protein can be detected in all colorectal tissues. However, apoM protein mass were significantly lower in the cancer tissues than its matched adjacent normal tissues, polyp tissues, normal mucosa and inflammatory mucosa. In parallel, apoM mRNA levels in the colorectal cancer tissues (0.0536 ± 0.0131) were also significantly lower than those in their adjacent normal tissues (0.1907 ± 0.0563) (P = 0.033). Interestingly, apoM mRNA levels in colorectal cancer tissues were statistic significant higher in the patients with lymph node metastasis than the patients without lymph node metastasis (P = 0.008). Patients under Dukes' C and D stages had much higher apoM mRNA levels than patients under Dukes' A and B stages (P = 0.034).

Conclusion

It is concluded that apoM could also be expressed in human colorectal tissues besides liver and kidney. ApoM mRNA levels in the colorectal cancer tissues were significantly increased in the patients with lymph node metastasis. Whether increased apoM expression in the patients with lymph node metastasis being related to patients' prognosis and the physiopathological importance of apoM expression in colorectal tissues need further investigation.

Increase in serum haptoglobin and apolipoprotein M in patients with obstructive sleep apnoea

Obstructive sleep apnoea (OSA) is a common syndrome, which is associated with a number of medical problems that impact morbidity and mortality. Although the precise mechanisms that underlie these associations are not fully understood, previous studies have suggested that patients with OSA show elevations of several biomarkers that are associated with cardiovascular risk. This study was undertaken to identify serum proteins associated with OSA by using a proteomics technique and to examine changes in identified protein levels after continuous positive airway pressure treatment. The study participants consisted of 40 male patients (aged 40-49 years) with severe OSA and 34 male control subjects matched for age and body mass index. All subjects underwent polysomnography. Using a proteomics approach, we identified nine proteins that were differentially expressed in patients with severe OSA and controls. Three of these nine proteins, haptoglobin, paraoxonase-1 and apolipoprotein M, were quantified by using enzyme-linked immunosorbent assays, kinetic assays and by using Western blotting. Multiple regression analysis showed that haptoglobin and apolipoprotein M levels are independently related to apnoea-hypopnoea index (P < 0.01). A further study is required to determine the nature of associations between these identified proteins and OSA in a large population.

Molecular characterization, expression profile and association analysis with fat deposition traits of the porcine APOM gene

Apolipoprotein M (APOM), a novel apolipoprotein presented mostly in high-density lipoprotein (HDL) in plasma, is involved in lipid and lipoprotein metabolism. Through comparative mapping, we have mapped this gene to SSC7 p1.1 in which many QTLs affecting fat deposition traits have been reported. As a candidate gene for fat deposition traits, in this study, we obtained the 742-bp mRNA sequence of porcine APOM including the full coding region and encoding a protein of 188 amino acids. The sequence was deposited into the GenBank under the accession no. DQ329240. Semi-quantitative RT-PCR results showed that the porcine APOM gene is expressed predominantly in liver and kidney tissue. The genomic sequence of this gene which contains six exons and five introns, is 3,621 bp in length (DQ272488). Bioinformatic analysis of the 5' regulatory region has revealed that classical TATA-box element and species conserved Hepatocyte nuclear factor-1a (HNF-1alpha) biding site were represented in this region. A G2289C single nucleotide polymorphism (SNP) in the intron 2 of porcine APOM gene detected as an Eco130I PCR-restriction fragment length polymorphism (PCR-RFLP) showed allele frequency differences among three purebreds. Association of the genotypes with fat deposition traits showed that different genotypes of porcine APOM gene were significantly associated with leaf fat weight (P < 0.05), backfat thickness at shoulder (P < 0.05), backfat thickness at thorax-waist (P < 0.05), backfat thickness at buttock (P < 0.01) and average backfat thickness over shoulder, thorax-waist and buttock (P < 0.01).

The periodontal pathogen Porphyromonas gingivalis cleaves apoB-100 and increases the expression of apoM in LDL in whole blood leading to cell proliferation

OBJECTIVE

Several studies support an association between periodontal disease and atherosclerosis with a crucial role for the pathogen Porphyromonas gingivalis. This study aims at investigating the proteolytic and oxidative activity of P. gingivalis on LDL in a whole blood system using a proteomic approach and analysing the effects of P. gingivalis-modified LDL on cell proliferation.

METHODS

The cellular effects of P. gingivalis in human whole blood were assessed using lumi-aggregometry analysing reactive oxygen species production and aggregation. Blood was incubated for 30 min with P. gingivalis, whereafter LDL was isolated and a proteomic approach was applied to examine protein expression. LDL-oxidation was determined by analysing the formation of protein carbonyls. The effects of P. gingivalis-modified LDL on fibroblast proliferation were studied using the MTS assay.

RESULTS

Incubation of whole blood with P. gingivalis caused an extensive aggregation and ROS production, indicating platelet and leucocyte activation. LDL prepared from bacteria-exposed blood showed an increased protein oxidation, elevated levels of apoM and formation of two apoB-100 N-terminal fragments. Porphyromonas gingivalis-modified LDL markedly increased the growth of fibroblasts. Inhibition of gingipain R suppressed the modification of LDL by P. gingivalis.

CONCLUSIONS

The ability of P. gingivalis to change the protein expression and proliferative capacity of LDL may represent a crucial event in periodontitis-associated atherosclerosis.

Liver X receptor agonist downregulates hepatic apoM expression in vivo and in vitro

It has been demonstrated that apolipoprotein M (apoM), a recently discovered HDL apolipoprotein, has antiatherosclerotic properties, which may be mediated by the enhancement of reversed cholesterol transportation and/or hepatic cholesterol catabolism. The detailed mechanisms are unknown yet. Liver X receptor (LXR) belongs to the nuclear receptor superfamily and is a ligand-activated transcription factor involved in the regulation of lipid metabolism and inflammation. Activation of LXR in the cell cultures results in an enhancement of cholesterol efflux to apoAI. In the present study, we investigated effects of the LXR agonist, T0901317 on hepatic apoM expression in vivo and in vitro. Serum apoM levels in mice given T0901317 at 10 mg or 100 mg/kg for 7 days were reduced by 12-17% (P<0.05). In HepG2 cell cultures, apoM mRNA levels were significantly lower in presence of 25 microM T0901317 (37.1%) than in control cells (P<0.001). A similar reduction was found by the addition of 9-cis retinoic acid (RA). Twenty-five micromolar T0901317 together with 100 nM RA decreased apoM mRNA expression by 65% (P<0.001). Thus, the LXR agonist T0901317 significantly downregulates apoM mRNA expression in vivo and in vitro, which indicates that apoM is another novel target gene regulated by the LXR. The combination of RA and T0901317 showed additive effects, which suggests that apoM expression can be modulated by LXR/RXR pathway.

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

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

Mapping of the serum proteome of hepatocellular carcinoma induced by targeted overexpression of epidermal growth factor to liver cells of transgenic mice

Epidermal growth factor (EGF) is an important mitogen for hepatocytes. Its targeted overexpression induced hepatocellular carcinomas (HCC), as recently reported by us (Borlak et al. 2005). Early detection of disease is essential for successful therapy and overall survival. Here we describe our efforts in identifying serum biomarkers of liver cancer in a transgenic disease model that mimics effectively the consequence of exaggerated EGF signaling. We report a reference 2-DE map of mouse serum proteins. About 180 proteins were detected per gel and 130 proteins were identified by 2-DE-MALDI-MS analysis. We compared serum proteins of healthy nontransgenic and HCC tumor-bearing mice and identified 25 regulated proteins, of which n = 7 reached statistical significance ( p < 0.05). Furthermore, we identified several fragments of fibrinogens and of the alpha-2-macroglobulin to be disease-associated. We also found immunoglobulins to be repressed or absent in serum samples of tumor-bearing mice, and this included, among others, the Ig K and L class. In contrast, amyloid component P and apolipoprotein M were highly significantly increased by 10- and 8-fold, respectively, in serum samples of HCC mice. Overall, our findings provide a rationale for further clinical evaluation of the herein identified biomarker candidates.

Proteomic analysis of human very low-density lipoprotein by two-dimensional gel electrophoresis and MALDI-TOF/TOF

Biochemical studies of lipoproteins have shed light on their composition, highly contributing to the comprehension of their function. Due to the complexity of their structure, however, an in-depth structural analysis, in terms of components and PTMs, may still unravel important players in physiological and pathological processes of lipid metabolism. In this study, we performed a protein map of very low-density lipoprotein (VLDL) using a 2-DE MALDI-TOF/TOF proteomic approach. Several VLDL-associated apolipoproteins were identified, including five isoforms of apoE, three isoforms of apoC-IV, and one isoform each of apoC-III, apoM, apoA-I, and apoA-IV. Notably, we also identified seven isoforms of apoL-I and two isoforms of prenylcysteine lyase as new VLDL-associated proteins. Furthermore, we were able to identify PTM of apoE, which was found to be differently O-glycosylated at Thr212 residue, and PTM of apoL-I which we described, for the first time, to be phosphorylated at Ser296. While the physiological relevance of our finding remains to be assessed, we believe that our results will be useful as reference for future studies of VLDL structure in specific physiopathological conditions.

Apolipoprotein M likely extends its anti-atherogenesis via anti-inflammation

Apolipoprotein M (apoM), a novel human apolipoprotein recently discovered, predominantly presents in high density lipoprotein (HDL) in plasma, exclusively expressed in liver and in kidney. The present data demonstrated apoM protects against atherosclerosis (AS) primarily via partaking in prebeta-HDL formation and promoting cholesterol efflux to HDL. However, this lipid-metabolism-associated pathway seems unlikely responsible for all atheroprotective effects of apoM. Notably, the human apoM gene is just located in the major histocompatibility complex class III region (MHC-III) on chromosome 6, many genes in this region are related to the immune and inflammatory response. Furthermore, apoM has been documented to link with some inflammatory factors including platelet activating factor (PAF) and leptin. These evidences indicate that apoM may be involved in inflammatory activities in vivo and the potential immuno- and inflam-reactive property of apoM may contribute to the anti-inflammatory function of HDL, as generally acknowledged as an important atheroprotective mechanism of HDL.

Characterization of apolipoprotein M isoforms in low-density lipoprotein

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

Effects of Ischemia-Reperfusion Injury on Apolipoprotein M Expression in the Liver

The present study investigated the expression pattern of apolipoprotein M (apoM) mRNA in a rat model of hepatic ischemia-reperfusion injury (IRI). Animals were ischemic for 1 hour followed by various reperfusion times. As expected, serum alanine aminotransferase levels were significantly increased under IRI, which indicated the severity of liver injury. Hepatic mRNA levels of HSP70, which is the most common characterized protein within the family of heat-shock proteins (HSP), were significantly increased after 0.5 to 3 hours of IRI. Plasma C-reactive protein, high-density lipoprotein-cholesterol, and lipoprotein (a) levels were significantly increased after 1-hour ischemia followed by 0.5 to 3 hours of reperfusion. Interestingly, similar to HSP70, apoM mRNA levels in the liver were gradually increased after 0.5 to 3 hours of IRI, whereas it returned to a lower level after 6 or 24 hours of IRI, which indicated that hepatic apoM expression was significantly influenced by the acute phase of IRI. However, plasma apoM levels were not increased in parallel, even slightly decreasing after 0.5 or 1 hour of IRI. We concluded that apoM mRNA expression pattern, like HSP70, in the liver showed rapid, significant changes during hepatic local IRI.

Suppression of apolipoprotein M expression and secretion in alloxan-diabetic mouse: Partial reversal by insulin

We previously reported that apolipoprotein M (apoM) expression is reduced in ob/ob mice. Whether such a reduction is specific for this model or is a more general phenomenon in diabetes models is not known. In the present study, we therefore investigated apoM expression and secretion in NMRI mice rendered diabetes through administration of alloxan (120 mg/kg). Plasma glucose levels were markedly increased and plasma insulin levels markedly reduced at 3 days after alloxan. At the same time, that plasma apoM concentrations were decreased by 70%, apoM mRNA levels in liver was decreased by 40%, and apoM mRNA in kidney was decreased by 20% in alloxan-treated mice compared to saline-injected controls. Furthermore we found also that daily sc administration of insulin (5 IU/kg per day) increased plasma apoM levels, and apoM mRNA levels in liver and kidney. We therefore conclude that apoM is reduced in this diabetes model and that exogenous insulin administrations partially reverses the abnormal apoM expression. Based on these results, we suggest that insulin regulates apoM synthesis in vivo and, therefore, that the reduction of apoM expression is a general phenomenon in diabetes models.

Comparative ligand-binding analysis of ten human lipocalins

At least ten different lipocalins occur in the human body: retinol-binding protein (RBP), alpha1-acid glycoprotein, alpha1-microglobulin, apolipoprotein D, beta-trace protein, complement component 8gamma, glycodelin, neutrophil gelatinase-associated lipocalin, odorant-binding protein, and tear lipocalin. Although many of these lipocalins seem to play an important physiological role, their precise biological function is not always clear. Especially the interpretation of their diverse ligand-binding activities has been hampered by the fact that the natural lipocalins were prepared from different sources and with varying purity. Here we present a generic expression and purification strategy for the recombinant lipocalins, which is based on secretion into the periplasm of E. coli, where disulphide bonds are readily formed, followed by affinity purification via the Strep-tag II and gel filtration. The ten human lipocalins were successfully prepared and their ligand-binding activities were compared via fluorescence titration with a set of typical ligands: retinol, retinoic acid (RA), 11-(5-(dimethylamino)-1-naphthalene-sulfonylamino)undecanoic acid (DAUDA), and 8-anilino-1-naphtalene-sulfonic acid (ANS). As result, merely two lipocalins, RBP and beta-trace, revealed high affinities both for retinol and for RA, which probably reflects a specialized physiological function in retinoid complexation. Surprisingly, the strongest retinol affinity was detected for apolipoprotein D, whereas this lipocalin exhibits much weaker binding activity for retinoic acid. Binding studies with the two spectroscopic probes DAUDA and ANS revealed mixed patterns, which demonstrates that the affinity for lipophilic substances varies considerably among human lipocalins. Notably, RBP with its perfectly moulded retinol-binding site did not show any detectable binding activity for both compounds. Hence, our recombinant expression and purification system should be useful for further structural and functional studies of lipocalins from human origin and beyond.

Leptin inhibits apolipoprotein M transcription and secretion in human hepatoma cell line, HepG2 cells

Apolipoprotein M (apoM) is a novel apolipoprotein presented mostly in high-density lipoprotein (HDL) in human plasma. Previously we have reported that both leptin and leptin receptor are essential for apoM expression in vivo. The expression of apoM is lower in the leptin deficient (ob/ob) mouse and leptin receptor deficient (db/db) mouse than in the normal mouse. In the present study, however, we demonstrated that supra-physiological concentrations of recombinant leptin significantly inhibited apoM transcription and secretion in the human hepatoma cell line, HepG2 cells. Both Northern blotting and real-time RT-PCR were applied into the analyses of apoM mRNA levels, and compatible data were obtained. The inhibitory effect of leptin on apoM mRNA levels in HepG2 cells is dose dependent, i.e. 100 ng/mL of leptin decreased apoM mRNA levels by 30%, and 500 ng/mL of leptin decreased apoM mRNA levels about 50%. Even at a physiological concentration of leptin (10 ng/mL), apoM expression was decreased, and in parallel, the secretion of apoM into the medium was also decreased. Furthermore, we examined apoAI, apoB and apoE by Northern blotting analyses. The results demonstrated that leptin does not significantly influence the expressions of apoAI, apoB and apoE in HepC2 cells, suggesting that leptin has a specific regulatory effect on hepatic apoM transcription and secretion in vitro. The mechanism on the contradictory effects of leptin on apoM expression in vivo and in vitro needs further investigation.