Heterogeneous expression of apolipoprotein-E by human macrophages

High-resolution longitudinal N- and O-glycoprofiling of human monocyte-to-macrophage transition

Protein glycosylation impacts the development and function of innate immune cells. The glycophenotypes and the glycan remodelling associated with the maturation of macrophages from monocytic precursor populations remain incompletely described. Herein, label-free porous graphitised carbon–liquid chromatography–tandem mass spectrometry (PGC-LC-MS/MS) was employed to profile with high resolution the N- and O-glycome associated with human monocyte-to-macrophage transition. Primary blood-derived CD14+ monocytes were differentiated ex vivo in the absence of strong anti- and proinflammatory stimuli using a conventional 7-day granulocyte-macrophage colony-stimulating factor differentiation protocol with longitudinal sampling. Morphology and protein expression monitored by light microscopy and proteomics validated the maturation process. Glycomics demonstrated that monocytes and macrophages display similar N-glycome profiles, comprising predominantly paucimannosidic (Man1-3GlcNAc2Fuc0–1, 22.1–30.8%), oligomannosidic (Man5-9GlcNAc2, 29.8–35.7%) and α2,3/6-sialylated complex-type N-glycans with variable core fucosylation (27.6–39.1%). Glycopeptide analysis validated conjugation of these glycans to human proteins, while quantitative proteomics monitored the glycoenzyme expression levels during macrophage differentiation. Significant interperson glycome variations were observed suggesting a considerable physiology-dependent or heritable heterogeneity of CD14+ monocytes. Only few N-glycome changes correlated with the monocyte-to-macrophage transition across donors including decreased core fucosylation and reduced expression of mannose-terminating (paucimannosidic-/oligomannosidic-type) N-glycans in macrophages, while lectin flow cytometry indicated that more dramatic cell surface glycan remodelling occurs during maturation. The less heterogeneous core 1-rich O-glycome showed a minor decrease in core 2-type O-glycosylation but otherwise remained unchanged with macrophage maturation. This high-resolution glycome map underpinning normal monocyte-to-macrophage transition, the most detailed to date, aids our understanding of the molecular makeup pertaining to two vital innate immune cell types and forms an important reference for future glycoimmunological studies.

The Genetic Variability of APOE in Different Human Populations and Its Implications for Longevity

Human longevity is a complex phenotype resulting from the combinations of context-dependent gene-environment interactions that require analysis as a dynamic process in a cohesive ecological and evolutionary framework. Genome-wide association (GWAS) and whole-genome sequencing (WGS) studies on centenarians pointed toward the inclusion of the apolipoprotein E (APOE) polymorphisms ε2 and ε4, as implicated in the attainment of extreme longevity, which refers to their effect in age-related Alzheimer's disease (AD) and cardiovascular disease (CVD). In this case, the available literature on APOE and its involvement in longevity is described according to an anthropological and population genetics perspective. This aims to highlight the evolutionary history of this gene, how its participation in several biological pathways relates to human longevity, and which evolutionary dynamics may have shaped the distribution of APOE haplotypes across the globe. Its potential adaptive role will be described along with implications for the study of longevity in different human groups. This review also presents an updated overview of the worldwide distribution of APOE alleles based on modern day data from public databases and ancient DNA samples retrieved from literature in the attempt to understand the spatial and temporal frame in which present-day patterns of APOE variation evolved.

The early signaling pathway of live yeast cell derivative in THP-1 monocytes

Live yeast cell derivative (LYCD) is a medicinal yeast extract that has been used in the treatment of burns, wounds and hemorrhoids for over 70 years. It has been shown to enhance the closure of skin wounds in diabetic mice by increasing inflammation, angiogenesis, formation of granulation tissue and epithelial migration. An active fraction of LYCD has been identified as a mixture of peptides ranging in size from 5 kDA to 17 kDA. Despite its widespread use over many years, understanding of the mechanism by which LYCD acts to effect tissue repair responses is very limited. In this study, we have used a human monocyte-derived cell line, THP-1, as a representative of the inflammatory component of the wound healing process. We have identified two of the earliest responses to LYCD as an increase in cytoplasmic free calcium ([Ca2+]i) and the transcripts for c-fos. We have found that the increase in [Ca2+]i is both necessary and sufficient to account for the LYCD-induced elevation of c-fos. Furthermore, we have shown that the signaling pathway by which LYCD elevates [Ca2+]i involves both mobilization of Ca2+ from intracellular stores and influx of Ca2+ from the extracellular medium. Mobilization of store Ca2+ occurs first via activation of phospholipase C and this is followed by influx through activation of store operated calcium channels. These results constitute the first delineation of the early steps of the LYCD signaling pathway.

Cell-specific production, secretion, and function of apolipoprotein E

Apolipoprotein E (apoE) is a 34-kDa glycoprotein that is secreted from many cells throughout the body. ApoE is best known for its role in lipoprotein metabolism. Recent studies underline the association of circulating lipoprotein-associated apoE levels and the development for cardiovascular disease (CVD). Besides its well-established role in pathology of CVD, it is also implicated in neurodegenerative diseases and recent new data on adipose-produced apoE point to a novel metabolic role for apoE in obesity. The regulation of apoE production and secretion is remarkably cell and tissue specific. Here, we summarize recent insights into the differential regulation apoE production and secretion by hepatocytes, monocytes/macrophages, adipocytes, and the central nervous system and relevant variations in apoE biochemistry and function.

Macrophage Apoptosis and Efferocytosis in the Pathogenesis of Atherosclerosis

Macrophage apoptosis and the ability of macrophages to clean up dead cells, a process called efferocytosis, are crucial determinants of atherosclerosis lesion progression and plaque stability. Environmental stressors initiate endoplasmic reticulum (ER) stress and activate the unfolded protein response (UPR). Unresolved ER stress with activation of the UPR initiates apoptosis. Macrophages are resistant to apoptotic stimuli, because of activity of the PI3K/Akt pathway. Macrophages express 3 Akt isoforms, Akt1, Akt2 and Akt3, which are products of distinct but homologous genes. Akt displays isoform-specific effects on atherogenesis, which vary with different vascular cell types. Loss of macrophage Akt2 promotes the anti-inflammatory M2 phenotype and reduces atherosclerosis. However, Akt isoforms are redundant with regard to apoptosis. c-Jun NH₂-terminal kinase (JNK) is a pro-apoptotic effector of the UPR, and the JNK1 isoform opposes anti-apoptotic Akt signaling. Loss of JNK1 in hematopoietic cells protects macrophages from apoptosis and accelerates early atherosclerosis. IκB kinase α (IKKα, a member of the serine/threonine protein kinase family) plays an important role in mTORC2-mediated Akt signaling in macrophages, and IKKα deficiency reduces macrophage survival and suppresses early atherosclerosis. Efferocytosis involves the interaction of receptors, bridging molecules, and apoptotic cell ligands. Scavenger receptor class B type I is a critical mediator of macrophage efferocytosis via the Src/PI3K/Rac1 pathway in atherosclerosis. Agonists that resolve inflammation offer promising therapeutic potential to promote efferocytosis and prevent atherosclerotic clinical events. (Circ J 2016; 80: 2259-2268).

Endogenous apolipoprotein A-I stabilizes ATP-binding cassette transporter A1 and modulates Toll-like receptor 4 signaling in human macrophages

Apolipoprotein A-I (ApoA-I) is the main functional protein component of human high-density lipoproteins. ApoA-I shows various anti-inflammatory and atheroprotective properties toward macrophages; however, endogenous apoA-I expression has not been investigated in macrophages. We have shown that endogenous apoA-I gene is expressed in human macrophages at both mRNA and protein levels. Endogenous ApoA-I is localized in intracellular vesicles and at the external side of the plasma membrane in association with ATP-binding cassette transporter A1 (ABCA1) and lipid rafts in macrophages. We have shown that endogenous ApoA-I stabilizes ABCA1, moreover, down-regulation of ApoA-I by siRNA results in an increase of Toll-like receptor 4 (TLR4) mRNA and membrane surface protein expression, as well as an enhancement of bacterial lipopolysaccharide (LPS)-induced expression of tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), and inducible nitric oxide synthase (NOS2) genes in human macrophages. TNF-α stimulates ApoA-I expression and secretion (1.2±0.2 vs. 4.3±0.9 ng/mg total protein) in macrophages. Obtained results suggest that endogenous ApoA-I has anti-inflammatory properties, presumably due to ABCA1 stabilization in macrophages; these results elucidate the cell type-specific mechanism of the TNF-α-mediated regulation of apoA-I gene expression in monocytes and macrophages.

Macrophage LRP-1 controls plaque cellularity by regulating efferocytosis and Akt activation

OBJECTIVE

The balance between apoptosis susceptibility and efferocytosis of macrophages is central to plaque remodeling and inflammation. LRP-1 and its ligand, apolipoprotein E, have been implicated in efferocytosis and apoptosis in some cell types. We investigated the involvement of the macrophage LRP-1/apolipoprotein E axis in controlling plaque apoptosis and efferocytosis. Method and Results- LRP-1(-/-) macrophages displayed nearly 2-fold more TUNEL positivity compared to wild-type cells in the presence of DMEM alone or with either lipopolysaccharide or oxidized low-density lipoprotein. The survival kinase, phosphorylated Akt, was barely detectable in LRP-1(-/-) cells, causing decreased phosphorylated Bad and increased cleaved caspase-3. Regardless of the apoptotic stimulation and degree of cell death, LRP-1(-/-) macrophages displayed enhanced inflammation with increased IL-1 beta, IL-6, and tumor necrosis factor-alpha expression. Efferocytosis of apoptotic macrophages was reduced by 60% in LRP-1(-/-) vs wild-type macrophages despite increased apolipoprotein E expression by both LRP-1(-/-) phagocytes and wild-type apoptotic cells. Compared to wild-type macrophage lesions, LRP-1(-/-) lesions had 5.7-fold more necrotic core with more dead cells not associated with macrophages.

CONCLUSIONS

Macrophage LRP-1 deficiency increases cell death and inflammation by impairing phosphorylated Akt activation and efferocytosis. Increased apolipoprotein E expression in LRP-1(-/-) macrophages suggests that the LRP-1/apolipoprotein E axis regulates the balance between apoptosis and efferocytosis, thereby preventing necrotic core formation.

Apoptosis induces neuronal apolipoprotein-E synthesis and localization in apoptotic bodies

Neuronal apoptosis is crucial for central nervous system development and also contributes to neurodegenerative disease. Apolipoprotein-E (apoE) regulates brain lipid transport and specific neuronal functions and previous research, investigating non-neuronal cell types, identified an association between apoptosis and increased apoE expression. In the present study we used the human SK-N-SH neuronal cell line to investigate potential changes in apoE expression during apoptosis which occurs as a consequence of extended culture (up to 5 days) without replenishing trophic factors. Standard and real-time PCR analysis indicated a significant 6-fold increase in apoE mRNA after 3 days which was correlated with caspase-3 activation, TUNEL positivity and the formation of apoptotic bodies. ApoE protein levels were low in the absence of apoptosis but increased by 8-fold when apoptosis was induced. Analysis of cellular debris that accumulated in the culture supernatants indicated that apoE levels became progressively concentrated in apoptotic bodies. These data indicate that apoE is up-regulated during neuronal apoptosis and raise the possibility that apoE may play a role in the clearance of apoptotic bodies through apoE-receptor interactions.

Proteomic and biochemical analysis of purified human immunodeficiency virus type 1 produced from infected monocyte-derived macrophages

Human immunodeficiency virus type 1 (HIV-1) infects CD4(+) T lymphocytes and monocytes/macrophages, incorporating host proteins in the process of assembly and budding. Analysis of the host cell proteins incorporated into virions can provide insights into viral biology. We characterized proteins in highly purified HIV-1 virions produced from human monocyte-derived macrophages (MDM), within which virus buds predominantly into intracytoplasmic vesicles, in contrast to the plasmalemmal budding of HIV-1 typically seen with infected T cells. Liquid chromatography-linked tandem mass spectrometry of highly purified virions identified many cellular proteins, including 33 previously described proteins in HIV-1 preparations from other cell types. Proteins involved in many different cellular structures and functions were present, including those from the cytoskeleton, adhesion, signaling, intracellular trafficking, chaperone, metabolic, ubiquitin/proteasomal, and immune response systems. We also identified annexins, annexin-binding proteins, Rab proteins, and other proteins involved in membrane organization, vesicular trafficking, and late endosomal function, as well as apolipoprotein E, which participates in cholesterol transport, immunoregulation, and modulation of cell growth and differentiation. Several tetraspanins, markers of the late endosomal compartment, were also identified. MDM-derived HIV contained 26 of 37 proteins previously found in exosomes, consistent with the idea that HIV uses the late endosome/multivesicular body pathway during virion budding from macrophages.