Lipoproteomics I: mapping of proteins in low-density lipoprotein using two-dimensional gel electrophoresis and mass spectrometry

Human body fluid proteome analysis

The focus of this article is to review the recent advances in proteome analysis of human body fluids, including plasma/serum, urine, cerebrospinal fluid, saliva, bronchoalveolar lavage fluid, synovial fluid, nipple aspirate fluid, tear fluid, and amniotic fluid, as well as its applications to human disease biomarker discovery. We aim to summarize the proteomics technologies currently used for global identification and quantification of body fluid proteins, and elaborate the putative biomarkers discovered for a variety of human diseases through human body fluid proteome (HBFP) analysis. Some critical concerns and perspectives in this emerging field are also discussed. With the advances made in proteomics technologies, the impact of HBFP analysis in the search for clinically relevant disease biomarkers would be realized in the future.

C-terminal interactions of apolipoprotein E4 respond to the postprandial state

Increased triglyceride-rich lipoproteins (TGRLs) in the postprandial state are associated with atherosclerosis. We investigated whether the postprandial state induced structural changes at the apolipoprotein E4 (apoE4) C terminus, its principal lipid binding domain, using electron paramagnetic resonance (EPR) spectroscopy of a site-directed spin label attached to the cysteine of apoE4-W264C. Spin coupling between labels located in the C termini was followed after mixing with preprandial and postprandial human plasma samples. Our results indicate that postprandial plasma triggers a reorganization of the protein such that the dipolar broadening is diminished, indicating a reduction in C-terminal interaction. The loss of spectral broadening was directly correlated with an increase in postprandial plasma triglycerides and was reduced with delipidated plasma. The spin-labeled apoE4 displayed a lipid preference of VLDL > LDL > HDL in the preprandial and postprandial states. The apoE4 shift to VLDL during the postprandial state was accompanied by a loss in spectral broadening of the protein. These findings suggest that apoE4 associated with LDL maintains self-association via its C terminus and that this association is diminished in VLDL-associated protein. Lipolyzed TGRL reflected a depletion of the C-terminal interaction of apoE4. Addition of palmitate to VLDL gave a similar response as lipolyzed TGRL, suggesting that lipolysis products play a major role in reorganizing apoE4 during the postprandial state.

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

PURPOSE OF REVIEW

Apolipoprotein M is a recently described apolipoprotein predominantly associated with high-density lipoprotein, but also found in chylomicrons, very low-density lipoproteins, and low-density lipoprotein. The purpose is to review recent information on the unusual structural properties of apolipoprotein M and its possible role in formation of pre-beta high-density lipoprotein and reverse cholesterol metabolism.

RECENT FINDINGS

Apolipoprotein M is a lipocalin having a coffee filter-like structure with a hydrophobic ligand-binding pocket. Mature apolipoprotein M retains its signal peptide, which serves as a hydrophobic anchor. In mice, silencing of expression in the liver with siRNA led to disappearance of pre-beta high-density lipoprotein and appearance of unusually large high-density lipoproteins. This suggests that apolipoprotein M is important for the formation of pre-beta high-density lipoprotein and reverse cholesterol transport. In accordance with this idea, hepatic overexpression of apolipoprotein M with an adenovirus in low-density lipoprotein-receptor deficient mice led to an approximately 70% reduction of atherosclerosis. In addition to the liver, apolipoprotein M is also expressed in the kidney. Kidney-derived apolipoprotein M binds to megalin, a member of the low-density lipoprotein-receptor family, which interacts with many lipocalins in renal tubuli. Apolipoprotein M is excreted in the urine of mice with a kidney-specific megalin deficiency but not in the urine of normal mice, suggesting megalin-mediated uptake of apolipoprotein M in the tubular epithelium of normal mice.

SUMMARY

Apolipoprotein M is a novel apolipoprotein with unusual structural features that appears to play important roles in high-density lipoprotein metabolism and prevention of atherosclerosis.

Recent advances in blood-related proteomics

Blood is divided in two compartments, namely, plasma and cells. The latter contain red blood cells, leukocytes, and platelets. From a descriptive medical discipline, hematology has evolved towards a pioneering discipline where molecular biology has permitted the development of prognostic and diagnostic indicators for disease. The recent advance in MS and protein separation now allows similar progress in the analysis of proteins. Proteomics offers great promise for the study of proteins in plasma/serum, indeed a number of proteomics databases for plasma/serum have been established. This is a very complex body fluid containing lipids, carbohydrates, amino acids, vitamins, nucleic acids, hormones, and proteins. About 1500 different proteins have recently been identified, and a number of potential new markers of diseases have been characterized. Here, examples of the enormous promise of plasma/serum proteomic analysis for diagnostic/prognostic markers and information on disease mechanism are given. Within the blood are also a large number of different blood cell types that potentially hold similar information. Proteomics of red blood cells, until now, has not improved our knowledge of these cells, in contrast to the major progresses achieved while studying platelets and leukocytes. In the future, proteomics will change several aspects of hematology.

Lipoproteomics II: mapping of proteins in high-density lipoprotein using two-dimensional gel electrophoresis and mass spectrometry

High-density lipoprotein (HDL) is the most abundant lipoprotein particle in the plasma and a negative risk factor of atherosclerosis. By using a proteomic approach it is possible to obtain detailed information about its protein content and protein modifications that may give new information about the physiological roles of HDL. In this study the two subfractions; HDL(2) and HDL(3), were isolated by two-step discontinuous density-gradient ultracentrifugation and the proteins were separated with two-dimensional gel electrophoresis and identified with peptide mass fingerprinting, using matrix-assisted laser desorption/ionisation time of flight mass spectrometry. Identified proteins in HDL were: the dominating apo A-I as six isoforms, four of them with a glycosylation pattern and one of them with retained propeptide, apolipoprotein (apo) A-II, apo A-IV, apo C-I, apo C-II, apo C-III (two isoforms), apo E (five isoforms), the recently discovered apo M (two isoforms), serum amyloid A (two isoforms) and serum amyloid A-IV (six isoforms). Furthermore, alpha-1-antitrypsin was identified in HDL for the first time. Additionally, salivary alpha-amylase was identified as two isoforms in HDL(2), and apo L and a glycosylated apo A-II were identified in HDL(3). Besides confirming the presence of different apolipoproteins, this study indicates new patterns of glycosylated apo A-I and apo A-II. Furthermore, the study reveals new proteins in HDL; alpha-1-antitrypsin and salivary alpha-amylase. Further investigations about these proteins may give new insight into the functional role of HDL in coronary artery diseases.

FGF-2, IL-1β and TGF-β regulate fibroblast expression of S100A8

Growth factors, including fibroblast growth factor-2 (FGF-2) and transforming growth factor-beta (TGF-beta) regulate fibroblast function, differentiation and proliferation. S100A8 and S100A9 are members of the S100 family of Ca2+-binding proteins and are now accepted as markers of inflammation. They are expressed by keratinocytes and inflammatory cells in human/murine wounds and by appropriately activated macrophages, endothelial cells, epithelial cells and keratinocytes in vitro. In this study, regulation and expression of S100A8 and S100A9 were examined in fibroblasts. Endotoxin (LPS), interferon gamma (IFNgamma), tumour-necrosis factor (TNF) and TGF-beta did not induce the S100A8 gene in murine fibroblasts whereas FGF-2 induced mRNA maximally after 12 h. The FGF-2 response was strongly enhanced and prolonged by heparin. Interleukin-1beta (IL-1beta) alone, or in synergy with FGF-2/heparin strongly induced the gene in 3T3 fibroblasts. S100A9 mRNA was not induced under any condition. Induction of S100A8 in the absence of S100A9 was confirmed in primary fibroblasts. S100A8 mRNA induction by FGF-2 and IL-1beta was partially dependent on the mitogen-activated-protein-kinase pathway and dependent on new protein synthesis. FGF-2-responsive elements were distinct from the IL-1beta-responsive elements in the S100A8 gene promoter. FGF-2-/heparin-induced, but not IL-1beta-induced responses were significantly suppressed by TGF-beta, possibly mediated by decreased mRNA stability. S100A8 in activated fibroblasts was mainly intracytoplasmic. Rat dermal wounds contained numerous S100A8-positive fibroblast-like cells 2 and 4 days post injury; numbers declined by 7 days. Up-regulation of S100A8 by FGF-2/IL-1beta, down-regulation by TGF-beta, and its time-dependent expression in wound fibroblasts suggest a role in fibroblast differentiation at sites of inflammation and repair.