Lipin-1 contributes to modified low-density lipoprotein-elicited macrophage pro-inflammatory responses

Role of lipins in cardiovascular diseases

Lipin family members in mammals include lipins 1, 2, and 3. Lipin family proteins play a crucial role in lipid metabolism due to their bifunctionality as both transcriptional coregulators and phosphatidate phosphatase (PAP) enzymes. In this review, we discuss the structural features, expression patterns, and pathophysiologic functions of lipins, emphasizing their direct as well as indirect roles in cardiovascular diseases (CVDs). Elucidating the regulation of lipins facilitates a deeper understanding of the roles of lipins in the processes underlying CVDs. The activity of lipins is modulated at various levels, e.g., in the form of the transcription of genes, post-translational modifications, and subcellular protein localization. Because lipin characteristics are undergoing progressive clarification, further research is necessitated to then actuate the investigation of lipins as viable therapeutic targets in CVDs.

Myeloid-associated lipin-1 transcriptional co-regulatory activity is atheroprotective

BACKGROUND AND AIMS

Atherosclerosis is the most prominent underlying cause of cardiovascular disease (CVD). It is initiated by cholesterol deposition in the arterial intima, which causes macrophage recruitment and proinflammatory responses that promote plaque growth, necrotic core formation, and plaque rupture. Lipin-1 is a phosphatidic acid phosphohydrolase for glycerolipid synthesis. We have shown that lipin-1 phosphatase activity promotes macrophage pro-inflammatory responses when stimulated with modified low-density lipoprotein (modLDL) and accelerates atherosclerosis. Lipin-1 also independently acts as a transcriptional co-regulator where it enhances the expression of genes involved in β-oxidation. In hepatocytes and adipocytes, lipin-1 augments the activity of transcription factors such as peroxisome proliferator-activated receptor (PPARs). PPARs control the expression of anti-inflammatory genes in macrophages and slow or reduce atherosclerotic progression. Therefore, we hypothesize myeloid-derived lipin-1 transcriptional co-regulatory activity reduces atherosclerosis.

METHODS

We used myeloid-derived lipin-1 knockout (lipin-1mKO) and littermate control mice and AAV8-PCSK9 along with high-fat diet to elicit atherosclerosis.

RESULTS

Lipin-1mKO mice had larger aortic root plaques than littermate control mice after 8 and 12 weeks of a high-fat diet. Lipin-1mKO mice also had increased serum proinflammatory cytokine concentrations, reduced apoptosis in plaques, and larger necrotic cores in the plaques compared to control mice.

CONCLUSIONS

Combined, the data suggest lipin-1 transcriptional co-regulatory activity in myeloid cells is atheroprotective.

Lipin-1, a Versatile Regulator of Lipid Homeostasis, Is a Potential Target for Fighting Cancer

The rewiring of lipid metabolism is a major adaptation observed in cancer, and it is generally associated with the increased aggressiveness of cancer cells. Targeting lipid metabolism is therefore an appealing therapeutic strategy, but it requires a better understanding of the specific roles played by the main enzymes involved in lipid biosynthesis. Lipin-1 is a central regulator of lipid homeostasis, acting either as an enzyme or as a co-regulator of transcription. In spite of its important functions it is only recently that several groups have highlighted its role in cancer. Here, we will review the most recent research describing the role of lipin-1 in tumor progression when expressed by cancer cells or cells of the tumor microenvironment. The interest of its inhibition as an adjuvant therapy to amplify the effects of anti-cancer therapies will be also illustrated.

Macrophage-Associated Lipin-1 Promotes β-Oxidation in Response to Proresolving Stimuli

Macrophages reprogram their metabolism to promote appropriate responses. Proresolving macrophages primarily use fatty acid oxidation as an energy source. Metabolites generated during the catabolism of fatty acids aid in the resolution of inflammation and tissue repair, but the regulatory mechanisms that control lipid metabolism in macrophages are not fully elucidated. Lipin-1, a phosphatidic acid phosphatase that has transcriptional coregulator activity, regulates lipid metabolism in a variety of cells. In this current study, we show that lipin-1 is required for increased oxidative phosphorylation in IL-4 stimulated mouse (Mus musculus) macrophages. We also show that the transcriptional coregulatory function of lipin-1 is required for β-oxidation in response to palmitate (free fatty acid) and apoptotic cell (human) stimulation. Mouse bone marrow-derived macrophages lacking lipin-1 have a reduction in critical TCA cycle metabolites following IL-4 stimulation, suggesting a break in the TCA cycle that is supportive of lipid synthesis rather than lipid catabolism. Together, our data demonstrate that lipin-1 regulates cellular metabolism in macrophages in response to proresolving stimuli and highlights the importance of aligning macrophage metabolism with macrophage phenotype.

Interface of Phospholipase Activity, Immune Cell Function, and Atherosclerosis

Phospholipases are a family of lipid-altering enzymes that can either reduce or increase bioactive lipid levels. Bioactive lipids elicit signaling responses, activate transcription factors, promote G-coupled-protein activity, and modulate membrane fluidity, which mediates cellular function. Phospholipases and the bioactive lipids they produce are important regulators of immune cell activity, dictating both pro-inflammatory and pro-resolving activity. During atherosclerosis, pro-inflammatory and pro-resolving activities govern atherosclerosis progression and regression, respectively. This review will look at the interface of phospholipase activity, immune cell function, and atherosclerosis.

Lipin-1 Contributes to IL-4 Mediated Macrophage Polarization

Macrophage responses contribute to a diverse array of pathologies ranging from infectious disease to sterile inflammation. Polarization of macrophages determines their cellular function within biological processes. Lipin-1 is a phosphatidic acid phosphatase in which its enzymatic activity contributes to macrophage pro-inflammatory responses. Lipin-1 also possesses transcriptional co-regulator activity and whether this activity is required for macrophage polarization is unknown. Using mice that lack only lipin-1 enzymatic activity or both enzymatic and transcriptional coregulator activities from myeloid cells, we investigated the contribution of lipin-1 transcriptional co-regulator function toward macrophage wound healing polarization. Macrophages lacking both lipin-1 activities did not elicit IL-4 mediated gene expression to levels seen in either wild-type or lipin-1 enzymatically deficient macrophages. Furthermore, mice lacking myeloid-associated lipin-1 have impaired full thickness excisional wound healing compared to wild-type mice or mice only lacking lipin-1 enzymatic activity from myeloid cell. Our study provides evidence that lipin-1 transcriptional co-regulatory activity contributes to macrophage polarization and influences wound healing in vivo.

UVB-dependent inhibition of lipin-1 protects against proinflammatory responses in human keratinocytes

Lipin-1 is an Mg²⁺-dependent phosphatidate phosphatase (PAP1) that catalyzes a critical step in the synthesis of glycerophospholipids and is also a cotranscriptional regulator. The role of lipin-1 in the regulation of inflammatory responses has been extensively studied in various cell types but not in skin cells. In the present study, the function of lipin-1 in UVB-induced proinflammatory responses was assessed in normal human epidermal keratinocytes (NHEKs). UVB radiation downregulated lipin-1 expression. Lipin-1 inhibition was mediated by UVB-dependent sterol-response element binding protein-1 (SREBP-1) inhibition. The UVB-dependent inhibition of lipin-1 and SREBP-1 was mediated by AMPK activation. UVB-induced activation of JNK was dependent on AMPK activation and mediated lipin-1 inhibition. Prevention of UVB-mediated lipin-1 repression by introducing a lipin-1 expression vector stimulated IL-6 and IL-8 production, suggesting that lipin-1 inhibition attenuates UVB-induced IL-6 and IL-8 production. The downregulation of lipin-1 ameliorated UVB-induced NF-ĸB phosphorylation, which might be attributed to the suppression of UVB-induced accumulation of free fatty acids (FFAs). Pharmacological inhibition of PAP1 with propranolol suppressed UVB-induced production of IL-6 and IL-8 in NHEKs and reconstituted human skin models. Taken together, lipin-1 is downregulated by exposure to UVB radiation, which confers protection against UVB-induced proinflammatory responses; therefore, the inhibition of lipin-1 is a potential strategy for photoaging.

Reduced production and activity of an enzyme in skin cells helps protect them from damage caused by exposure to ultra-violet light. Minjung Chae and colleagues at the Amorepacific Corporation in Yongin, South Korea, identified an anti-inflammatory effect caused by the reduction in expression of the enzyme lipin-1 when skin cells are exposed to UVB radiation. These ultra-violet rays are associated with aging and increased risk of skin cancer. Lipin-1 is involved in making glycerophospholipid molecules, which are key components of the membranes surrounding and inside cells. Identifying the enzyme’s significance for inflammation in skin cells extends previous similar findings with other cell types. The research also uncovered aspects of the molecular mechanisms mediating the skin cell response. Inhibiting lipin-1 activity might reduce the damage sunlight causes to skin.

miR-532-3p-CSF2RA Axis as a Key Regulator of Vulnerable Atherosclerotic Plaque Formation

BACKGROUND

The most dangerous atherosclerotic plaques, referred to as "vulnerable," are most likely to trigger acute atherothrombotic events such as myocardial infarction (heart attack) and stroke. Our goal was to uncover the molecular drivers of vulnerable plaque formation.

METHODS

To elucidate the functional gene modules that drive vulnerable plaque formation, we performed a weighted gene coexpression network analysis integrated with a protein-protein interaction network analysis in human atherosclerotic carotid samples, which identified the candidate gene granulocyte-macrophage colony-stimulating factor 2 (GM-CSF) receptor alpha subunit (CSF2RA). Follow-up in vitro experiments were performed to elucidate the regulatory relationship between CSF2RA and the microRNA miR-532-3p as well as modifiers of macrophagic miR-532-3p-CSF2RA axis expression. Microarray and quantitative reverse transcription polymerase chain reaction (qRT-PCR) studies elucidated the effect of statins on carotid miR-532-3p-CSF2RA axis expression in patients with carotid atherosclerotic disease. Apoe^-/-, Ldlr^-/-, and Csf2ra mutant Apoe^-/- mouse models of atherosclerosis were employed to assess the effects of agomiR-532-3p therapy in vivo.

RESULTS

The integrated weighted gene coexpression network analysis/protein-protein interaction network analysis revealed that the macrophagic GM-CSF receptor CSF2RA is significantly upregulated in macrophage-rich vulnerable plaques. Follow-up analysis identified the miR-532-3p-CSF2RA axis, as miR-532-3p downregulates CSF2RA via binding to CSF2RA's 3'UTR. Macrophagic miR-532-3p-CSF2RA dysregulation was enhanced via modified low-density lipoprotein or tumor necrosis factor α exposure in vitro. Moreover, this miR-532-3p-CSF2RA dysregulation was observed in human vulnerable plaques and Apoe^-/- mouse plaques, effects rescued by statin therapy. In vivo, agomiR-532-3p therapy suppressed murine plaque formation and promoted plaque stabilization in a Csf2ra-dependent manner.

CONCLUSION

Macrophagic miR-532-3p-CSF2RA axis dysregulation is a key driver in vulnerable plaque formation.

Casein kinase II-mediated phosphorylation of lipin 1β phosphatidate phosphatase at Ser-285 and Ser-287 regulates its interaction with 14-3-3β protein

The mammalian lipin 1 phosphatidate phosphatase is a key regulatory enzyme in lipid metabolism. By catalyzing phosphatidate dephosphorylation, which produces diacylglycerol, the enzyme plays a major role in the synthesis of triacylglycerol and membrane phospholipids. The importance of lipin 1 to lipid metabolism is exemplified by cellular defects and lipid-based diseases associated with its loss or overexpression. Phosphorylation of lipin 1 governs whether it is associated with the cytoplasm apart from its substrate or with the endoplasmic reticulum membrane where its enzyme reaction occurs. Lipin 1β is phosphorylated on multiple sites, but less than 10% of them are ascribed to a specific protein kinase. Here, we demonstrate that lipin 1β is a bona fide substrate for casein kinase II (CKII), a protein kinase that is essential to viability and cell cycle progression. Phosphoamino acid analysis and phosphopeptide mapping revealed that lipin 1β is phosphorylated by CKII on multiple serine and threonine residues, with the former being major sites. Mutational analysis of lipin 1β and its peptides indicated that Ser-285 and Ser-287 are both phosphorylated by CKII. Substitutions of Ser-285 and Ser-287 with nonphosphorylatable alanine attenuated the interaction of lipin 1β with 14-3-3β protein, a regulatory hub that facilitates the cytoplasmic localization of phosphorylated lipin 1. These findings advance our understanding of how phosphorylation of lipin 1β phosphatidate phosphatase regulates its interaction with 14-3-3β protein and intracellular localization and uncover a mechanism by which CKII regulates cellular physiology.

Macrophage-Associated Lipin-1 Enzymatic Activity Contributes to Modified Low-Density Lipoprotein-Induced Proinflammatory Signaling and Atherosclerosis

OBJECTIVE

Macrophage proinflammatory responses induced by modified low-density lipoproteins (modLDL) contribute to atherosclerotic progression. How modLDL causes macrophages to become proinflammatory is still enigmatic. Macrophage foam cell formation induced by modLDL requires glycerolipid synthesis. Lipin-1, a key enzyme in the glycerolipid synthesis pathway, contributes to modLDL-elicited macrophage proinflammatory responses in vitro. The objective of this study was to determine whether macrophage-associated lipin-1 contributes to atherogenesis and to assess its role in modLDL-mediated signaling in macrophages.

APPROACH AND RESULTS

We developed mice lacking lipin-1 in myeloid-derived cells and used adeno-associated viral vector 8 expressing the gain-of-function mutation of mouse proprotein convertase subtilisin/kexin type 9 (adeno-associated viral vector 8-proprotein convertase subtilisin/kexin type 9) to induce hypercholesterolemia and plaque formation. Mice lacking myeloid-associated lipin-1 had reduced atherosclerotic burden compared with control mice despite similar plasma lipid levels. Stimulation of bone marrow-derived macrophages with modLDL activated a persistent protein kinase Cα/βII-extracellular receptor kinase1/2-jun proto-oncogene signaling cascade that contributed to macrophage proinflammatory responses that was dependent on lipin-1 enzymatic activity.

CONCLUSIONS

Our data demonstrate that macrophage-associated lipin-1 is atherogenic, likely through persistent activation of a protein kinase Cα/βII-extracellular receptor kinase1/2-jun proto-oncogene signaling cascade that contributes to foam cell proinflammatory responses. Taken together, these results suggest that modLDL-induced foam cell formation and modLDL-induced macrophage proinflammatory responses are not independent consequences of modLDL stimulation but rather are both directly influenced by enhanced lipid synthesis.

EphA2 Expression Regulates Inflammation and Fibroproliferative Remodeling in Atherosclerosis

BACKGROUND

Atherosclerotic plaque formation results from chronic inflammation and fibroproliferative remodeling in the vascular wall. We previously demonstrated that both human and mouse atherosclerotic plaques show elevated expression of EphA2, a guidance molecule involved in cell-cell interactions and tumorigenesis.

METHODS

Here, we assessed the role of EphA2 in atherosclerosis by deleting EphA2 in a mouse model of atherosclerosis (Apoe^-/-) and by assessing EphA2 function in multiple vascular cell culture models. After 8 to 16 weeks on a Western diet, male and female mice were assessed for atherosclerotic burden in the large vessels, and plasma lipid levels were analyzed.

RESULTS

Despite enhanced weight gain and plasma lipid levels compared with Apoe^-/- controls, EphA2^-/-Apoe^-/- knockout mice show diminished atherosclerotic plaque formation, characterized by reduced proinflammatory gene expression and plaque macrophage content. Although plaque macrophages express EphA2, EphA2 deletion does not affect macrophage phenotype, inflammatory responses, and lipid uptake, and bone marrow chimeras suggest that hematopoietic EphA2 deletion does not affect plaque formation. In contrast, endothelial EphA2 knockdown significantly reduces monocyte firm adhesion under flow. In addition, EphA2^-/-Apoe^-/- mice show reduced progression to advanced atherosclerotic plaques with diminished smooth muscle and collagen content. Consistent with this phenotype, EphA2 shows enhanced expression after smooth muscle transition to a synthetic phenotype, and EphA2 depletion reduces smooth muscle proliferation, mitogenic signaling, and extracellular matrix deposition both in atherosclerotic plaques and in vascular smooth muscle cells in culture.

CONCLUSIONS

Together, these data identify a novel role for EphA2 in atherosclerosis, regulating both plaque inflammation and progression to advanced atherosclerotic lesions. Cell culture studies suggest that endothelial EphA2 contributes to atherosclerotic inflammation by promoting monocyte firm adhesion, whereas smooth muscle EphA2 expression may regulate the progression to advanced atherosclerosis by regulating smooth muscle proliferation and extracellular matrix deposition.

Lipin-2 regulates NLRP3 inflammasome by affecting P2X7 receptor activation

Lordén et al. show that the phosphatidic acid phosphatase lipin-2 is a key regulator of the cellular machinery that generates IL-1β in macrophages. This work provides a molecular explanation for the development of the autoinflammatory disease known as Majeed syndrome.

Mutations in human LPIN2 produce a disease known as Majeed syndrome, the clinical manifestations of which are ameliorated by strategies that block IL-1β or its receptor. However the role of lipin-2 during IL-1β production remains elusive. We show here that lipin-2 controls excessive IL-1β formation in primary human and mouse macrophages by several mechanisms, including activation of the inflammasome NLRP3. Lipin-2 regulates MAPK activation, which mediates synthesis of pro–IL-1β during inflammasome priming. Lipin-2 also inhibits the activation and sensitization of the purinergic receptor P2X7 and K⁺ efflux, apoptosis-associated speck-like protein with a CARD domain oligomerization, and caspase-1 processing, key events during inflammasome activation. Reduced levels of lipin-2 in macrophages lead to a decrease in cellular cholesterol levels. In fact, restoration of cholesterol concentrations in cells lacking lipin-2 decreases ion currents through the P2X7 receptor, and downstream events that drive IL-1β production. Furthermore, lipin-2–deficient mice exhibit increased sensitivity to high lipopolysaccharide doses. Collectively, our results unveil lipin-2 as a critical player in the negative regulation of NLRP3 inflammasome.

Myeloid Cell-Specific Lipin-1 Deficiency Stimulates Endocrine Adiponectin-FGF15 Axis and Ameliorates Ethanol-Induced Liver Injury in Mice

Lipin-1 is a phosphatidate phosphohydrolase (PAP) required for the generation of diacylglycerol during glycerolipid synthesis, and exhibits dual functions in the regulation of lipid metabolism. Lipin-1 has been implicated in the pathogenesis of alcoholic liver disease (ALD). In the present study, we assessed lipin-1 function in myeloid cells in ALD using a myeloid cell-specific lipin-1 knockout (mLipin-1KO) mouse model. Utilizing the Gao-binge ethanol feeding protocol, matched mLipin-1KO mice and littermate loxP control (WT) mice were pair-fed with either an ethanol-containing diet or an ethanol-free diet (control). Surprisingly, deletion of lipin-1 in myeloid cells dramatically attenuated liver inflammatory responses and ameliorated liver injury that would normally occur following the ethanol feeding protocol, but slightly exacerbated the ethanol-induced steatosis in mice. Mechanistically, myeloid cell-specific lipin-1 deficiency concomitantly increased the fat-derived adiponectin and ileum-derived fibroblast growth factor (FGF) 15. In concordance with concerted elevation of circulating adiponectin and FGF15, myeloid cell-specific lipin-1 deficiency diminished hepatic nuclear factor kappa B (NF-κB) activity, limited liver inflammatory responses, normalized serum levels of bile acids, and protected mice from liver damage after ethanol challenge. Our novel data demonstrate that myeloid cell-specific deletion of lipin-1 ameliorated inflammation and alcoholic hepatitis in mice via activation of endocrine adiponectin-FGF15 signaling.