Apolipoprotein D overexpression alters hepatic prostaglandin and omega fatty acid metabolism during the development of a non-inflammatory hepatic steatosis

Astrocyte-derived apolipoprotein D is required for neuronal survival in Parkinson's disease

Apolipoprotein D (ApoD), a lipocalin transporter of small hydrophobic molecules, plays an essential role in several neurodegenerative diseases. It was reported that increased immunostaining for ApoD of glial cells surrounding dopaminergic (DAergic) neurons was observed in the brains of Parkinson's disease (PD) patients. Although preliminary findings supported the role of ApoD in neuroprotection, its derivation and effects on the degeneration of nigral DAergic neurons are largely unknown. In the present study, we observed that ApoD levels released from astrocytes were increased in PD models both in vivo and in vitro. When co-cultured with astrocytes, due to the increased release of astrocytic ApoD, the survival rate of primary cultured ventral midbrain (VM) neurons was significantly increased with 1-methyl-4-phenylpyridillium ion (MPP+) treatment. Increased levels of TAp73 and its phosphorylation at Tyr99 in astrocytes were required for the increased ApoD levels and its release. Conditional knockdown of TAp73 in the nigral astrocytes in vivo could aggravate the neurodegeneration in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated PD mice. Our findings reported that astrocyte-derived ApoD was essential for DAergic neuronal survival in PD models, might provide new therapeutic targets for PD.

Apolipoprotein D in Oxidative Stress and Inflammation

Apolipoprotein D (ApoD) is lipocalin able to bind hydrophobic ligands. The APOD gene is upregulated in a number of pathologies, including Alzheimer's disease, Parkinson's disease, cancer, and hypothyroidism. Upregulation of ApoD is linked to decreased oxidative stress and inflammation in several models, including humans, mice, Drosophila melanogaster and plants. Studies suggest that the mechanism through which ApoD modulates oxidative stress and regulate inflammation is via its capacity to bind arachidonic acid (ARA). This polyunsaturated omega-6 fatty acid can be metabolised to generate large variety of pro-inflammatory mediators. ApoD serves as a sequester, blocking and/or altering arachidonic metabolism. In recent studies of diet-induced obesity, ApoD has been shown to modulate lipid mediators derived from ARA, but also from eicosapentaenoic acid and docosahexaenoic acid in an anti-inflammatory way. High levels of ApoD have also been linked to better metabolic health and inflammatory state in the round ligament of morbidly obese women. Since ApoD expression is upregulated in numerous diseases, it might serve as a therapeutic agent against pathologies aggravated by OS and inflammation such as many obesity comorbidities. This review will present the most recent findings underlying the central role of ApoD in the modulation of both OS and inflammation.

Apolipoprotein D modulates lipid mediators and osteopontin in an anti-inflammatory direction

BACKGROUND

HDL has been proposed to possess anti-inflammatory properties; however, the detail mechanisms have not been fully elucidated.

METHODS

We investigated the roles of Apolipoprotein D (ApoD) in the pathogenesis of inflammation in the mouse model of diet-induced obesity and that of lipopolysaccharide-induced sepsis and the in vitro experiments. Furthermore, we analyzed serum ApoD levels in human subjects.

RESULTS

The overexpression of human ApoD decreased the plasma IL-6 and TNF-a levels in both mice models. Lipidomics analyses demonstrated association of ApoD with increase of arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid, as well as of their metabolites, and of the anti-inflammatory molecule sphingosine 1-phosphate, and decrease of proinflammatory lysophosphatidic acids and lysophosphatidylinositol. ApoD-containing lipoproteins might directly bind eicosapentaenoic acid and docosahexaenoic acid. The modulations of the lysophosphatidic acid and sphingosine 1-phosphate levels resulted from the suppression of autotaxin expression and elevation of apolipoprotein M (ApoM), respectively. Moreover, ApoD negatively regulated osteopontin, a proinflammatory adipokine. The activation of PPARg by ApoD might suppress autotaxin and osteopontin. Serum ApoD levels were negatively correlated with the serum osteopontin and autotaxin levels and, positively with serum ApoM levels.

CONCLUSION

ApoD is an anti-inflammatory apolipoprotein, which modulates lipid mediators and osteopontin in an anti-inflammatory direction.

The Lipocalin Apolipoprotein D Functional Portrait: A Systematic Review

Apolipoprotein D is a chordate gene early originated in the Lipocalin protein family. Among other features, regulation of its expression in a wide variety of disease conditions in humans, as apparently unrelated as neurodegeneration or breast cancer, have called for attention on this gene. Also, its presence in different tissues, from blood to brain, and different subcellular locations, from HDL lipoparticles to the interior of lysosomes or the surface of extracellular vesicles, poses an interesting challenge in deciphering its physiological function: Is ApoD a moonlighting protein, serving different roles in different cellular compartments, tissues, or organisms? Or does it have a unique biochemical mechanism of action that accounts for such apparently diverse roles in different physiological situations? To answer these questions, we have performed a systematic review of all primary publications where ApoD properties have been investigated in chordates. We conclude that ApoD ligand binding in the Lipocalin pocket, combined with an antioxidant activity performed at the rim of the pocket are properties sufficient to explain ApoD association with different lipid-based structures, where its physiological function is better described as lipid-management than by long-range lipid-transport. Controlling the redox state of these lipid structures in particular subcellular locations or extracellular structures, ApoD is able to modulate an enormous array of apparently diverse processes in the organism, both in health and disease. The new picture emerging from these data should help to put the physiological role of ApoD in new contexts and to inspire well-focused future research.

Adipose-Derived Lipid-Binding Proteins: The Good, the Bad and the Metabolic Diseases

Adipose tissue releases a large range of bioactive factors called adipokines, many of which are involved in inflammation, glucose homeostasis and lipid metabolism. Under pathological conditions such as obesity, most of the adipokines are upregulated and considered as deleterious, due to their pro-inflammatory, pro-atherosclerotic or pro-diabetic properties, while only a few are downregulated and would be designated as beneficial adipokines, thanks to their counteracting properties against the onset of comorbidities. This review focuses on six adipose-derived lipid-binding proteins that have emerged as key factors in the development of obesity and diabetes: Retinol binding protein 4 (RBP4), Fatty acid binding protein 4 (FABP4), Apolipoprotein D (APOD), Lipocalin-2 (LCN2), Lipocalin-14 (LCN14) and Apolipoprotein M (APOM). These proteins share structural homology and capacity to bind small hydrophobic molecules but display opposite effects on glucose and lipid metabolism. RBP4 and FABP4 are positively associated with metabolic syndrome, while APOD and LCN2 are ubiquitously expressed proteins with deleterious or beneficial effects, depending on their anatomical site of expression. LCN14 and APOM have been recently identified as adipokines associated with healthy metabolism. Recent findings on these lipid-binding proteins exhibiting detrimental or protective roles in human and murine metabolism and their involvement in metabolic diseases are also discussed.

Cerebral Apolipoprotein D Exits the Brain and Accumulates in Peripheral Tissues

Apolipoprotein D (ApoD) is a secreted lipocalin associated with neuroprotection and lipid metabolism. In rodent, the bulk of its expression occurs in the central nervous system. Despite this, ApoD has profound effects in peripheral tissues, indicating that neural ApoD may reach peripheral organs. We endeavor to determine if cerebral ApoD can reach the circulation and accumulate in peripheral tissues. Three hours was necessary for over 40% of all the radiolabeled human ApoD (hApoD), injected bilaterally, to exit the central nervous system (CNS). Once in circulation, hApoD accumulates mostly in the kidneys/urine, liver, and muscles. Accumulation specificity of hApoD in these tissues was strongly correlated with the expression of lowly glycosylated basigin (BSG, CD147). hApoD was observed to pass through bEnd.3 blood brain barrier endothelial cells monolayers. However, cyclophilin A did not impact hApoD internalization rates in bEnd.3, indicating that ApoD exit from the brain is either independent of BSG or relies on additional cell types. Overall, our data showed that ApoD can quickly and efficiently exit the CNS and reach the liver and kidneys/urine, organs linked to the recycling and excretion of lipids and toxins. This indicated that cerebral overexpression during neurodegenerative episodes may serve to evacuate neurotoxic ApoD ligands from the CNS.

Apolipoprotein D

ApoD is a 25 to 30 kDa glycosylated protein, member of the lipocalin superfamily. As a transporter of several small hydrophobic molecules, its known biological functions are mostly associated to lipid metabolism and neuroprotection. ApoD is a multi-ligand, multi-function protein that is involved lipid trafficking, food intake, inflammation, antioxidative response and development and in different types of cancers. An important aspect of ApoD's role in lipid metabolism appears to involve the transport of arachidonic acid, and the modulation of eicosanoid production and delivery in metabolic tissues. ApoD expression in metabolic tissues has been associated positively and negatively with insulin sensitivity and glucose homeostasis in a tissue dependent manner. ApoD levels rise considerably in association with aging and neuropathologies such as Alzheimer's disease, stroke, meningoencephalitis, moto-neuron disease, multiple sclerosis, schizophrenia and Parkinson's disease. ApoD is also modulated in several animal models of nervous system injury/pathology.