Lipid bodies in innate immune response to bacterial and parasite infections

Recent Advances on the Role of ATGL in Cancer

The hypoxic state of the tumor microenvironment leads to reprogramming lipid metabolism in tumor cells. Adipose triglyceride lipase, also known as patatin-like phospholipase= domain-containing protein 2 and Adipose triglyceride lipase (ATGL), as an essential lipid metabolism-regulating enzyme in cells, is regulated accordingly under hypoxia induction. However, studies revealed that ATGL exhibits both tumor-promoting and tumor-suppressing effects, which depend on the cancer cell type and the site of tumorigenesis. For example, elevated ATGL expression in breast cancer is accompanied by enhanced fatty acid oxidation (FAO), enhancing cancer cells’ metastatic ability. In prostate cancer, on the other hand, tumor activity tends to be negatively correlated with ATGL expression. This review outlined the regulation of ATGL-mediated lipid metabolism pathways in tumor cells, emphasizing the Hypoxia-inducible factors 1 (HIF-1)/Hypoxia-inducible lipid droplet-associated (HIG-2)/ATGL axis, peroxisome proliferator-activated receptor (PPAR)/G0/G1 switch gene 2 (G0S2)/ATGL axis, and fat-specific protein 27 (FSP-27)/Early growth response protein 1 (EGR-1)/ATGL axis. In the light of recent research on different cancer types, the role of ATGL on tumorigenesis, tumor proliferation, and tumor metastasis was systemically reviewed.

Macrophage: A Cell With Many Faces and Functions in Tuberculosis

Mycobacterium tuberculosis (Mtb) is the causative agent of human tuberculosis (TB) which primarily infects the macrophages. Nearly a quarter of the world's population is infected latently by Mtb. Only around 5%-10% of those infected develop active TB disease, particularly during suppressed host immune conditions or comorbidity such as HIV, hinting toward the heterogeneity of Mtb infection. The aerosolized Mtb first reaches the lungs, and the resident alveolar macrophages (AMs) are among the first cells to encounter the Mtb infection. Evidence suggests that early clearance of Mtb infection is associated with robust innate immune responses in resident macrophages. In addition to lung-resident macrophage subsets, the recruited monocytes and monocyte-derived macrophages (MDMs) have been suggested to have a protective role during Mtb infection. Mtb, by virtue of its unique cell surface lipids and secreted protein effectors, can evade killing by the innate immune cells and preferentially establish a niche within the AMs. Continuous efforts to delineate the determinants of host defense mechanisms have brought to the center stage the crucial role of macrophage phenotypical variations for functional adaptations in TB. The morphological and functional heterogeneity and plasticity of the macrophages aid in confining the dissemination of Mtb. However, during a suppressed or hyperactivated immune state, the Mtb virulence factors can affect macrophage homeostasis which may skew to favor pathogen growth, causing active TB. This mini-review is aimed at summarizing the interplay of Mtb pathomechanisms in the macrophages and the implications of macrophage heterogeneity and plasticity during Mtb infection.

Impact of the Extracellular Vesicles Derived From Trypanosoma cruzi: A Paradox in Host Response and Lipid Metabolism Modulation

Chagas disease is a major public health problem, especially in the South and Central America region. Its incidence is related to poverty and presents a high rate of morbidity and mortality. The pathogenesis of Chagas disease is complex and involves many interactive pathways between the hosts and the Trypanosoma cruzi. Several factors have been implicated in parasite-host interactions, including molecules secreted by infected cells, lipid mediators and most recent, extracellular vesicles (EVs). The EVs of T. cruzi (EVsT) were reported for the first time in the epimastigote forms about 42 years ago. The EVsT are involved in paracrine communication during the infection and can have an important role in the inflammatory modulation and parasite escape mechanism. However, the mechanisms by which EVs employ their pathological effects are not yet understood. The EVsT seem to participate in the activation of macrophages via TLR2 triggering the production of cytokines and a range of other molecules, thus modulating the host immune response which promotes the parasite survival. Moreover, new insights have demonstrated that EVsT induce lipid body formation and PGE₂ synthesis in macrophages. This phenomenon is followed by the inhibition of the synthesis of pro-inflammatory cytokines and antigen presentation, causing decreased parasitic molecules and allowing intracellular parasite survival. Therefore, this mini review aims to discuss the role of the EVs from T. cruzi as well as its involvement in the mechanisms that regulate the host immune response in the lipid metabolism and its significance for the Chagas disease pathophysiology.

Metabolic Alterations in SARS-CoV-2 Infection and Its Implication in Kidney Dysfunction

Clinical strategies focusing on pathogen elimination are expected in an infectious-disease outbreak, such as the severe coronavirus disease 2019 (COVID-19), to avoid organ dysfunction. However, understanding the host response to viral infection is crucial to develop an effective treatment to optimize the patient's conditions. The pathogenic viruses can promote metabolic changes during viral infection, favoring its survival, altering cell phenotype and function, and causing sustained inflammation and tissue injury. Severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2), the etiological agent of COVID-19, provokes systemic and cell metabolic changes and possibly altering lipid and glucose metabolism. Besides severe acute respiratory syndrome (SARS), SARS-CoV-2 can cause acute kidney injury, which has been associated with the severity of the disease. Although it is not clear the mechanisms whereby SARS-CoV-2 induces kidney dysfunction, it is known that the virus presents kidney tropism, namely, podocytes and proximal tubular epithelial cells. Changes in renal cell metabolism and systemic metabolic disorders are important events in kidney injury progression. Here, we explored the metabolism and its interface with SARS-CoV-2 infection and raised the perspective on metabolism disturbances as a critical event to kidney dysfunction in COVID-19.

Quantification of lipid bodies in monocytes from patients with periodontitis

OBJECTIVES

For the first time in the history of periodontics, the production of lipid bodies by monocytes was assessed from blood of patients with periodontitis in comparison to systemically healthy individuals. The purpose of this study was to compare the lipid body frequency within monocytes between healthy patients and those with periodontal disease.

MATERIALS AND METHODS

A total of 30 participants (11 males and 19 females), were divided between orally healthy control subjects (C, n = 16) and periodontitis subjects (P, n = 14), in a cross-sectional study. Both groups were systemically healthy. The following clinical periodontal parameters were assessed: probing depth, clinical attachment level, visible plaque index and gingival bleeding on probing index. Blood samples were collected to obtain monocytes containing lipid bodies, which were analyzed by light microscopy.

RESULTS

The periodontitis group demonstrated a higher corpuscular index than the control group (nonopsonized p = .0296 or opsonized p = .0459; Mann-Whitney). The frequency of monocyte cells containing lipid bodies (basal p = .0147, opsonized p = .0084 or nonopsonized, p = .026; Mann-Whitney) was also higher compared to those observed in healthy individuals.

CONCLUSIONS

The data suggest that periodontitis may contribute to a higher production of lipid bodies. It was also hypothesized that a major production of lipid bodies by monocytes in severe periodontitis, compared to orally healthy subjects, could interfere with the innate immune response or represents a higher reservoir of cholesterol esters within macrophages and a major risk to systemic implications, such as atherosclerosis.

Dengue virus-activated platelets modulate monocyte immunometabolic response through lipid droplet biogenesis and cytokine signaling

Dengue is characterized as one of the most important arthropod-borne human viral diseases, representing a public health problem. Increased activation of immune cells is involved in the progression of infection to severe forms. Recently, our group demonstrated the contribution of platelet-monocyte interaction to inflammatory responses in dengue, adding to evolving evidence that platelets have inflammatory functions and can regulate different aspects of innate immune responses. Furthermore, stimuli-specific-activated platelets can promote phenotypic changes and metabolic reprogramming in monocytes. Thus, this study aimed to evaluate the roles of dengue virus (DENV)-activated platelets on immunometabolic reprogramming of monocytes in vitro, focusing on lipid droplet (LD) biogenesis. We demonstrated that platelets exposed to DENV in vitro form aggregates with monocytes and signal to LD formation and CXCL8/IL-8, IL-10, CCL2, and PGE₂ secretion. Pharmacologic inhibition of LD biogenesis prevents PGE₂ secretion, but not CXCL8/IL-8 release, by platelet-monocyte complexes. In exploring the mechanisms involved, we demonstrated that LD formation in monocytes exposed to DENV-activated platelets is partially dependent on platelet-produced MIF. Additionally, LD formation is higher in monocytes, which have platelets adhered on their surface, suggesting that beyond paracrine signaling, platelet adhesion is an important event in platelet-mediated modulation of lipid metabolism in monocytes. Together, our results demonstrate that activated platelets aggregate with monocytes during DENV infection and signal to LD biogenesis and the secretion of inflammatory mediators, which may contribute to dengue immunopathogenesis.

Involvement of lipids from Leishmania braziliensis promastigotes and amastigotes in macrophage activation

Leishmania are obligate protozoan parasites responsible for substantial public health problems in tropical and subtropical regions around the world, with L. braziliensis being one of the causative agents of American Tegumentary Leishmaniasis. Macrophages, fundamental cells in the innate inflammatory response against Leishmania, constitute a heterogeneous group with multiple activation phenotypes and functions. The outcome of this infection depends largely on the activation status of macrophages, the first line of mammalian defense and the major target cells for parasite replication. The importance of lipids, the major components of cell membranes, goes beyond their basic structural functions. Lipid bioactive molecules have been described in Leishmania spp., and in the recent years the knowledge about the biological relevance of lipids in particular and their relationship with the immune response is expanding. The present work analyzes the biological effects of L. braziliensis lipids from lysed promastigotes (PRO) to mimic rapid modulatory processes that could occur in the initial steps of infection or the effects of lipids from lysed and incubated promastigotes (PROinc), simulating the parasite lipid degradation processes triggered after parasite lysis that might occur in the mammalian host. To perform these studies, lipid profiles of PRO and PROinc were compared with lipids from amastigotes under similar conditions (AMA and AMAinc), and the effect of these lipid extracts were analyzed on the induction of an inflammatory response in murine peritoneal macrophages: LB induction, COX-2, iNOS and Arginase expression, TNF-α, IL-10 and NO production, Arginase activity and M1/M2 markers mRNA induction.

Lipid droplet-dependent fatty acid metabolism controls the immune suppressive phenotype of tumor-associated macrophages

Tumor‐associated macrophages (TAMs) promote tumor growth and metastasis by suppressing tumor immune surveillance. Herein, we provide evidence that the immunosuppressive phenotype of TAMs is controlled by long‐chain fatty acid metabolism, specifically unsaturated fatty acids, here exemplified by oleate. Consequently, en‐route enriched lipid droplets were identified as essential organelles, which represent effective targets for chemical inhibitors to block in vitro polarization of TAMs and tumor growth in vivo. In line, analysis of human tumors revealed that myeloid cells infiltrating colon cancer but not gastric cancer tissue indeed accumulate lipid droplets. Mechanistically, our data indicate that oleate‐induced polarization of myeloid cells depends on the mammalian target of the rapamycin pathway. Thus, our findings reveal an alternative therapeutic strategy by targeting the pro‐tumoral myeloid cells on a metabolic level.

Glycerol-3-phosphate acyltransferases 3 and 4 direct glycerolipid synthesis and affect functionality in activated macrophages

Macrophage classical M1 activation via TLR4 triggers a variety of responses to achieve the elimination of foreign pathogens. During this process, there is also an increase in lipid droplets which contain large quantities of triacylglycerol (TAG) and phospholipid (PL). The functional consequences of this increment in lipid mass are poorly understood. Here, we studied the contribution of glycerolipid synthesis to lipid accumulation, focusing specifically on the first and rate-limiting enzyme of the pathway: glycerol-3-phosphate acyltransferase (GPAT). Using bone marrow-derived macrophages (BMDMs) treated with Kdo2-lipid A, we showed that glycerolipid synthesis is induced during macrophage activation. GPAT4 protein level and GPAT3/GPAT4 enzymatic activity increase during this process, and these two isoforms were required for the accumulation of cell TAG and PL. The phagocytic capacity of Gpat3−/− and Gpat4−/− BMDM was impaired. Additionally, inhibiting fatty acid β-oxidation reduced phagocytosis only partially, suggesting that lipid accumulation is not necessary for the energy requirements for phagocytosis. Finally, Gpat4−/− BMDM expressed and released more pro-inflammatory cytokines and chemokines after macrophage activation, suggesting a role for GPAT4 in suppressing inflammatory responses. Together, these results provide evidence that glycerolipid synthesis directed by GPAT4 is important for the attenuation of the inflammatory response in activated macrophages.

Radiation-induced lung injury: impact on macrophage dysregulation and lipid alteration - a review

Lung cancer continues to be the leading cause of cancer deaths and more than one million lung cancer patients will die every year worldwide. Radiotherapy (RT) plays an important role in lung cancer treatment, but the side effects of RT are pneumonitis and pulmonary fibrosis. RT-induced lung injury causes damage to alveolar-epithelial cells and vascular endothelial cells. Macrophages play an important role in the development of pulmonary fibrosis despite its role in immune response. These injury activated macrophages develop into classically activated M1 macrophage or alternative activated M2 macrophage. It secretes cytokines, interleukins, interferons, and nitric oxide. Several pro-inflammatory lipids and pro-apoptotic proteins cause lipotoxicity such as LDL, FC, DAG, and FFA. The overall findings in this review conclude the importance of macrophages in inducing toxic/inflammatory effects during RT of lung cancer, which is clinically vital to treat the radiation-induced fibrosis.

Lipids From Trypanosoma cruzi Amastigotes of RA and K98 Strains Generate a Pro-inflammatory Response via TLR2/6

Lipids from microorganisms are ligands of Toll like receptors (TLRs) and modulate the innate immune response. Herein, we analyze in vitro the effect of total lipid extracts from Trypanosoma cruzi amastigotes of RA and K98 strains (with polar biological behavior) on the induction of the inflammatory response and the involvement of TLRs in this process. We demonstrated that total lipid extracts from both strains induced lipid body formation, cyclooxygenase-2 expression and TNF-α and nitric oxide release in macrophages, as well as NF-κB activation and IL-8 release in HEK cells specifically through a TLR2/6 dependent pathway. We also evaluated the inflammatory response induced by total lipid extracts obtained from lysed parasites that were overnight incubated to allow the action of parasite hydrolytic enzymes, such as Phospholipase A₁, over endogenous phospholipids. After incubation, these total lipid extracts showed a significantly reduced pro-inflammatory response, which could be attributed to the changes in the content of known bioactive lipid molecules like lysophospholipids and fatty acids, here reported. Moreover, analyses of total fatty acids in each lipid extract were performed by gas chromatography-mass spectrometry. Our results indicate a relevant role of T. cruzi lipids in the induction of a pro-inflammatory response through the TLR2/6 pathway that could contribute to the modulation of the immune response and host survival.

Benzoxazole derivatives suppress lipopolysaccharide-induced mast cell activation

Mast cells are central regulators of allergic inflammation that function by releasing various proallergic inflammatory mediators, including histamine, eicosanoids and proinflammatory cytokines. Occasionally, bacterial infections may initiate or worsen allergic inflammation. A number of studies have indicated that activation of lipoxygenase in mast cells positive regulates allergic inflammatory responses by generating leukotrienes and proinflammatory cytokines. In the present study, the effects of benzoxazole derivatives on the lipopolysaccharide (LPS)‑induced expression of proinflammatory cytokines, production of histamine and surface expression of co‑stimulatory molecules on bone marrow-derived mast cells (BMMCs) were studied. The benzoxazole derivatives significantly reduced the expression of interleukin (IL)‑1β, IL‑6, IL‑13, tumor necrosis factor‑α, perilipin (PLIN) 2, and PLIN3 in BMMCs treated with LPS. Furthermore, histamine production was suppressed in BMMCs treated with LPS, or treated with phorbol-12-myristate-13-acetate/ionomycin. Benzoxazole derivatives marginally affected the surface expression of cluster of differentiation (CD)80 and CD86 on BMMCs in the presence of LPS, although LPS alone did not increase the expression of those proteins. Therefore, benzoxazole derivatives inhibited the secretion of proinflammatory cytokines in mast cells and may be potential candidate anti‑allergic agents to suppress mast cell activation.

Lipid droplet density alters the early innate immune response to viral infection

The cellular localisation of many innate signalling events following viral infection has yet to be elucidated, however there has been a few cases in which membranes of certain cellular organelles have acted as platforms to these events. Of these, lipid droplets (LDs) have recently been identified as signalling platforms for innate TLR7 and 9 signalling. Despite their wide range of similar roles in various metabolic pathways, LDs have been overlooked as potential platforms for antiviral innate signalling events. This study established an in vitro model to evaluate the efficiency of the early innate immune response in cells with reduced LD content to the viral mimics, dsDNA and dsRNA, and Sendai viral infection. Using RT-qPCR, the expression of IFN-β and IFN-λ was quantified following stimulation along with the expression of specific ISGs. Luciferase based assays evaluated the combined expression of ISRE-promoter driven ISGs under IFN-β stimulation. Cellular LD content did not alter the entry of fluorescently labelled viral mimics into cells, but significantly decreased the ability of both Huh-7 and HeLa cells to produce type I and III IFN, as well as downstream ISG expression, indicative of an impeded innate immune response. This observation was also seen during Sendai virus infection of HeLa cells, where both control and LD reduced cells replicated the virus to the same level, but a significantly impaired type I and III IFN response was observed in the LD reduced cells. In addition to altered IFN production, cells with reduced LD content exhibited decreased expression of specific antiviral ISGs: Viperin, IFIT-1 and OAS-1 under IFN-β stimulation; However the overall induction of the ISRE-promoter was not effected. This study implicates a role for LDs in an efficient early innate host response to viral infection and future work will endeavour to determine the precise role these important organelles play in induction of an antiviral response.

The size matters: regulation of lipid storage by lipid droplet dynamics

Adequate energy storage is essential for sustaining healthy life. Lipid droplet (LD) is the subcellular organelle that stores energy in the form of neutral lipids and releases fatty acids under energy deficient conditions. Energy storage capacity of LDs is primarily dependent on the sizes of LDs. Enlargement and growth of LDs is controlled by two molecular pathways: neutral lipid synthesis and atypical LD fusion. Shrinkage of LDs is mediated by the degradation of neutral lipids under energy demanding conditions and is controlled by neutral cytosolic lipases and lysosomal acidic lipases. In this review, we summarize recent progress regarding the regulatory pathways and molecular mechanisms that control the sizes and the energy storage capacity of LDs.

Macrophage form, function, and phenotype in mycobacterial infection: lessons from tuberculosis and other diseases

Macrophages play a central role in mycobacterial pathogenesis. Recent work has highlighted the importance of diverse macrophage types and phenotypes that depend on local environment and developmental origins. In this review, we highlight how distinct macrophage phenotypes may influence disease progression in tuberculosis. In addition, we draw on work investigating specialized macrophage populations important in cancer biology and atherosclerosis in order to suggest new areas of investigation relevant to mycobacterial pathogenesis. Understanding the mechanisms controlling the repertoire of macrophage phenotypes and behaviors during infection may provide opportunities for novel control of disease through modulation of macrophage form and function.

Leptin Regulation of Immune Responses

Leptin is a regulatory hormone with multiple roles in the immune system. We favor the concept that leptin signaling 'licenses' various immune cells to engage in immune responses and/or to differentiate. Leptin is an inflammatory molecule that is capable of activating both adaptive and innate immunity. It can also 'enhance' immune functions, including inflammatory cytokine production in macrophages, granulocyte chemotaxis, and increased Th17 proliferation. Leptin can also 'inhibit' cells; CD4(+) T cells are inhibited from differentiating into regulatory T cells in the presence of elevated leptin, while NK cells can exhibit impaired cytotoxicity under the same circumstances. Consequently, understanding the effect of leptin signaling is important to appreciate various aspects of immune dysregulation observed in malnutrition, obesity, and autoimmunity.

Chronic Insulin Exposure Induces ER Stress and Lipid Body Accumulation in Mast Cells at the Expense of Their Secretory Degranulation Response

Lipid bodies (LB) are reservoirs of precursors to inflammatory lipid mediators in immunocytes, including mast cells. LB numbers are dynamic, increasing dramatically under conditions of immunological challenge. We have previously shown in vitro that insulin-influenced lipogenic pathways induce LB biogenesis in mast cells, with their numbers attaining steatosis-like levels. Here, we demonstrate that in vivo hyperinsulinemia resulting from high fat diet is associated with LB accumulation in murine mast cells and basophils. We characterize the lipidome of purified insulin-induced LB, and the shifts in the whole cell lipid landscape in LB that are associated with their accumulation, in both model (RBL2H3) and primary mast cells. Lipidomic analysis suggests a gain of function associated with LB accumulation, in terms of elevated levels of eicosanoid precursors that translate to enhanced antigen-induced LTC4 release. Loss-of-function in terms of a suppressed degranulation response was also associated with LB accumulation, as were ER reprogramming and ER stress, analogous to observations in the obese hepatocyte and adipocyte. Taken together, these data suggest that chronic insulin elevation drives mast cell LB enrichment in vitro and in vivo, with associated effects on the cellular lipidome, ER status and pro-inflammatory responses.

Prostaglandin E2/leukotriene B4 balance induced by Lutzomyia longipalpis saliva favors Leishmania infantum infection

Background

Eicosanoids and sand fly saliva have a critical role in the Leishmania infection. Here, we evaluated the effect of Lutzomyia longipalpis salivary gland sonicate (SGS) on neutrophil and monocyte recruitment and activation of eicosanoid production in a murine model of inflammation.

Methods

C57BL/6 mice were inoculated intraperitonealy with Lutzomyia longipalpis SGS or Leishmania infantum or both, followed by analyses of cell recruitment, parasite load and eicosanoid production.

Results

Intraperitoneal injection of Lutzomyia longipalpis SGS together with Leishmania infantum induced an early increased parasite viability in monocytes and neutrophils. L. longipalpis SGS increased prostaglandin E₂ (PGE₂), but reduced leukotriene B₄ (LTB₄) production ex vivo in peritoneal leukocytes. In addition, the pharmacological inhibition of cyclooxygenase 2 (COX-2) with NS-398 decreased parasite viability inside macrophages during Leishmania infection in the presence of L. longipalpis SGS arguing that PGE₂ production is associated with diminished parasite killing.

Conclusions

These findings indicate that L. longipalpis SGS is a critical factor driving immune evasion of Leishmania through modulation of PGE₂/LTB₄ axis, which may represent an important mechanism on establishment of the infection.

Role of prostaglandin F2α production in lipid bodies from Leishmania infantum chagasi: insights on virulence

Lipid bodies (LB; lipid droplets) are cytoplasmic organelles involved in lipid metabolism. Mammalian LBs display an important role in host-pathogen interactions, but the role of parasite LBs in biosynthesis of prostaglandin F2α (PGF2α) has not been investigated. We report herein that LBs increased in abundance during development of Leishmania infantum chagasi to a virulent metacyclic stage, as did the expression of PGF2α synthase (PGFS). The amount of parasite LBs and PGF2α were modulated by exogenous arachidonic acid. During macrophage infection, LBs were restricted to parasites inside the parasitophorous vacuoles (PV). We detected PGF2α receptor (FP) on the Leishmania PV surface. The blockage of FP with AL8810, a selective antagonist, hampered Leishmania infection, whereas the irreversible inhibition of cyclooxygenase with aspirin increased the parasite burden. These data demonstrate novel functions for parasite-derived LBs and PGF2α in the cellular metabolism of Leishmania and its evasion of the host immune response.

Mycobacterium leprae intracellular survival relies on cholesterol accumulation in infected macrophages: a potential target for new drugs for leprosy treatment

We recently showed that Mycobacterium leprae (ML) is able to induce lipid droplet formation in infected macrophages. We herein confirm that cholesterol (Cho) is one of the host lipid molecules that accumulate in ML-infected macrophages and investigate the effects of ML on cellular Cho metabolism responsible for its accumulation. The expression levels of LDL receptors (LDL-R, CD36, SRA-1, SR-B1, and LRP-1) and enzymes involved in Cho biosynthesis were investigated by qRT-PCR and/or Western blot and shown to be higher in lepromatous leprosy (LL) tissues when compared to borderline tuberculoid (BT) lesions. Moreover, higher levels of the active form of the sterol regulatory element-binding protein (SREBP) transcriptional factors, key regulators of the biosynthesis and uptake of cellular Cho, were found in LL skin biopsies. Functional in vitro assays confirmed the higher capacity of ML-infected macrophages to synthesize Cho and sequester exogenous LDL-Cho. Notably, Cho colocalized to ML-containing phagosomes, and Cho metabolism impairment, through either de novo synthesis inhibition by statins or depletion of exogenous Cho, decreased intracellular bacterial survival. These findings highlight the importance of metabolic integration between the host and bacteria to leprosy pathophysiology, opening new avenues for novel therapeutic strategies to leprosy.

Are reactive oxygen species always detrimental to pathogens?

Reactive oxygen species (ROS) are deadly weapons used by phagocytes and other cell types, such as lung epithelial cells, against pathogens. ROS can kill pathogens directly by causing oxidative damage to biocompounds or indirectly by stimulating pathogen elimination by various nonoxidative mechanisms, including pattern recognition receptors signaling, autophagy, neutrophil extracellular trap formation, and T-lymphocyte responses. Thus, one should expect that the inhibition of ROS production promote infection. Increasing evidences support that in certain particular infections, antioxidants decrease and prooxidants increase pathogen burden. In this study, we review the classic infections that are controlled by ROS and the cases in which ROS appear as promoters of infection, challenging the paradigm. We discuss the possible mechanisms by which ROS could promote particular infections. These mechanisms are still not completely clear but include the metabolic effects of ROS on pathogen physiology, ROS-induced damage to the immune system, and ROS-induced activation of immune defense mechanisms that are subsequently hijacked by particular pathogens to act against more effective microbicidal mechanisms of the immune system. The effective use of antioxidants as therapeutic agents against certain infections is a realistic possibility that is beginning to be applied against viruses.

Toxoplasma gondii-skeletal muscle cells interaction increases lipid droplet biogenesis and positively modulates the production of IL-12, IFN-g and PGE2

Background

The interest in the mechanisms involved in Toxoplasma gondii lipid acquisition has steadily increased during the past few decades, but it remains not completely understood. Here, we investigated the biogenesis and the fate of lipid droplets (LD) of skeletal muscle cells (SkMC) during their interaction with T. gondii by confocal and electron microscopy. We also evaluated whether infected SkMC modulates the production of prostaglandin E₂ (PGE₂), cytokines interleukin-12 (IL-12) and interferon-gamma (INF-g), and also the cyclooxygenase-2 (COX-2) gene induction.

Methods

Primary culture of skeletal muscle cells were infected with tachyzoites of T. gondii and analysed by confocal microscopy for observation of LD. Ultrastructural cytochemistry was also used for lipid and sarcoplasmatic reticulum (SR) detection. Dosage of cytokines (IL-12 and INF-g) by ELISA technique and enzyme-linked immunoassay (EIA) for PGE₂ measurement were employed. The COX-2 gene expression analysis was performed by real time reverse transcriptase polymerase chain reaction (qRT-PCR).

Results

We demonstrated that T. gondii infection of SkMC leads to increase in LD number and area in a time course dependent manner. Moreover, the ultrastructural analysis demonstrated that SR and LD are in direct contact with parasitophorous vacuole membrane (PVM), within the vacuolar matrix, around it and interacting directly with the membrane of parasite, indicating that LD are recruited and deliver their content inside the parasitophorous vacuole (PV) in T. gondii-infected SkMC. We also observed a positive modulation of the production of IL-12 and IFN-g, increase of COX-2 mRNA levels in the first hour of T. gondii-SkMC interaction and an increase of prostaglandin E₂ (PGE₂) synthesis from 6 h up to 48 h of infection.

Conclusions

Taken together, the close association between SR and LD with PV could represent a source of lipids as well as other nutrients for the parasite survival, and together with the increased levels of IL-12, INF-g and inflammatory indicators PGE₂ and COX-2 might contribute to the establishment and maintenance of chronic phase of the T. gondii infection in muscle cell.

Histopathological findings in immunohistological staining of the granulomatous tissue reaction associated with tuberculosis

Purpose. The histological diagnosis of Mycobacterium tuberculosis (MTB) remains a diagnostic challenge despite different methods. Immunohistochemistry (IHC) not only could confirm granulomatous tissue involvement but also can demonstrate MTB antigen immunolocalization. This study tries to clarify the details of immunohistochemical staining for MTB with pAbBCG. Materials/Methods. Twenty-three confirmed TB granulomatous tissue samples were studied by Ziehl-Neelsen and immunohistochemistry (IHC) staining with pAbBCG. Samples were selected from the archive of the Department of Pathology, National Research Institute of Tuberculosis and Lung Disease, Tehran, Iran. Results. IHC staining was positive in all samples, whereas Ziehl-Neelsen was positive in 9 cases out of 23 (39.1%). Tissue types used were pleural tissue, lymph nodes, and lung tissue. IHC showed positive coarse granular cytoplasmic and round, fragmented bacillary staining. In this study, epithelioid cells clearly showed more positive staining at the periphery of the granuloma rather than the center of granuloma. There is also positive staining in endothelial cells, fibroblasts, plasma cells, lymphocytes, and macrophages outside the granuloma. Conclusion. Considering the criteria of positive immunohistochemical staining of TB granulomatous reactions, this stain not only highlights the presence of mycobacterial antigens for tissue diagnosis, but also could morphologically localize its distribution in different cells.

Mycobacteria and the greasy macrophage: getting fat and frustrated

In the current issue of Infection and Immunity, Caire-Brändli and coworkers (Infect. Immun. 82:476-490, 2014, doi:10.1128/IAI.01196-13) describe a novel cell system for studying mycobacterial interactions with foamy macrophages and provide a magnificent series of electron microscopy-based observations providing major insight into the microbiology and cell biology of these interactions.

Deciphering the contribution of lipid droplets in leprosy: multifunctional organelles with roles in Mycobacterium leprae pathogenesis

Leprosy is an infectious disease caused by Mycobacterium leprae that affects the skin and nerves, presenting a singular clinical picture. Across the leprosy spectrum, lepromatous leprosy (LL) exhibits a classical hallmark: the presence of a collection of M. leprae-infected foamy macrophages/Schwann cells characterised by their high lipid content. The significance of this foamy aspect in mycobacterial infections has garnered renewed attention in leprosy due to the recent observation that the foamy aspect represents cells enriched in lipid droplets (LD) (also known as lipid bodies). Here, we discuss the contemporary view of LD as highly regulated organelles with key functions in M. leprae persistence in the LL end of the spectrum. The modern methods of studying this ancient disease have contributed to recent findings that describe M. leprae-triggered LD biogenesis and recruitment as effective mycobacterial intracellular strategies for acquiring lipids, sheltering and/or dampening the immune response and favouring bacterial survival, likely representing a fundamental aspect of M. leprae pathogenesis. The multifaceted functions attributed to the LD in leprosy may contribute to the development of new strategies for adjunctive anti-leprosy therapies.

Innate immunity stimulates permeability barrier homeostasis

A key function of the skin is to provide a permeability barrier to restrict the movement of water, electrolytes, and other small molecules between the outside environment and the internal milieu. Following disruption of the permeability barrier, there is a rapid restoration of barrier function, and one of the key signals initiating this repair response is a decrease in the concentration of calcium in the outer epidermis. In this issue, Borkowski et al. present evidence showing that activation of Toll receptor 3 by double-stranded RNA may be another pathway for activation of permeability barrier repair. These results provide further evidence for a link between innate immunity and the permeability barrier.

Differential TLR2 downstream signaling regulates lipid metabolism and cytokine production triggered by Mycobacterium bovis BCG infection

The nuclear receptor PPARγ acts as a key modulator of lipid metabolism, inflammation and pathogenesis in BCG-infected macrophages. However, the molecular mechanisms involved in PPARγ expression and functions during infection are not completely understood. Here, we investigate signaling pathways triggered by TLR2, the involvement of co-receptors and lipid rafts in the mechanism of PPARγ expression, lipid body formation and cytokine synthesis in macrophages during BCG infection. BCG induces NF-κB activation and increased PPARγ expression in a TLR2-dependent manner. Furthermore, BCG-triggered increase of lipid body biogenesis was inhibited by the PPARγ antagonist GW9662, but not by the NF-κB inhibitor JSH-23. In contrast, KC/CXCL1 production was largely dependent on NF-κB but not on PPARγ. BCG infection induced increased expression of CD36 in macrophages in vitro. Moreover, CD36 co-immunoprecipitates with TLR2 in BCG-infected macrophages, suggesting its interaction with TLR2 in BCG signaling. Pretreatment with CD36 neutralizing antibodies significantly inhibited PPARγ expression, lipid body formation and PGE2 production induced by BCG. Involvement of CD36 in lipid body formation was further confirmed by decreased BCG-induced lipid body formation in CD36 deficient macrophages. Similarly, CD14 and CD11b/CD18 blockage also inhibited BCG-induced lipid body formation, whereas TNF-α synthesis was not affected. Disruption of rafts recapitulates the latter result, inhibiting lipid body formation, but not TNF-α synthesis in BCG-infected macrophages. In conclusion, our results suggest that CD36-TLR2 cooperation and signaling compartmentalization within rafts, divert host response signaling through PPARγ-dependent and NF-κB-independent pathways, leading to increased macrophage lipid accumulation and down-modulation of macrophage response.

Coordinate stimulation of macrophages by microparticles and TLR ligands induces foam cell formation

Aberrant activation of macrophages in arterial walls by oxidized lipoproteins can lead to atherosclerosis. Oxidized lipoproteins convert macrophages to foam cells through lipid uptake and TLR signaling. To investigate the relative contributions of lipid uptake and TLR signaling in foam cell formation, we established an in vitro assay using liposomes of defined lipid compositions. We found that TLRs signaling through Toll/IL-1R domain-containing adapter inducing IFN-β promoted foam cell formation by inducing both NF-κB signaling and type I IFN production, whereas TLRs that do not induce IFN, like TLR2, did not enhance foam cell formation. Addition of IFN-α to TLR2 activator promoted robust foam cell formation. TLR signaling further required peroxisome proliferator-activated receptor α, as inhibition of peroxisome proliferator-activated receptor α blocked foam cell formation. We then investigated the ability of endogenous microparticles (MP) to contribute to foam cell formation. We found that lipid-containing MP promoted foam cell formation, which was enhanced by TLR stimulation or IFN-α. These MP also stimulated foam cell formation in a human skin model. However, these MP suppressed TNF-α production and T cell activation, showing that foam cell formation can occur by immunosuppressive MP. Taken together, the data reveal novel signaling requirements for foam cell formation and suggest that uptake of distinct types of MP in the context of activation of multiple distinct TLR can induce foam cell formation.

Cytosolic lipid inclusions formed during infection by viral and bacterial pathogens

Lipid inclusions play an important role in several pathological processes. Intracellular bacterial pathogens, such as members of the Mycobacterium and Chlamydia species are able to trigger the formation of lipid-laden foamy macrophages. Lipid droplet accumulation in the host constitutes a reservoir used by the bacilli for long-term persistence. Viruses need lipid droplets as assembly platform. We present the current knowledge about structural, functional and regulatory aspects of lipid inclusions.

The dynamic roles of intracellular lipid droplets: from archaea to mammals

During the past decade, there has been a paradigm shift in our understanding of the roles of intracellular lipid droplets (LDs). New genetic, biochemical and imaging technologies have underpinned these advances, which are revealing much new information about these dynamic organelles. This review takes a comparative approach by examining recent work on LDs across the whole range of biological organisms from archaea and bacteria, through yeast and Drosophila to mammals, including humans. LDs probably evolved originally in microorganisms as temporary stores of excess dietary lipid that was surplus to the immediate requirements of membrane formation/turnover. LDs then acquired roles as long-term carbon stores that enabled organisms to survive episodic lack of nutrients. In multicellular organisms, LDs went on to acquire numerous additional roles including cell- and organism-level lipid homeostasis, protein sequestration, membrane trafficking and signalling. Many pathogens of plants and animals subvert their host LD metabolism as part of their infection process. Finally, malfunctions in LDs and associated proteins are implicated in several degenerative diseases of modern humans, among the most serious of which is the increasingly prevalent constellation of pathologies, such as obesity and insulin resistance, which is associated with metabolic syndrome.

Mechanisms of triglyceride accumulation in activated macrophages

LPS treatment of macrophages induces TG accumulation, which is accentuated by TG-rich lipoproteins or FFA. We defined pathways altered during macrophage activation that contribute to TG accumulation. Glucose uptake increased with activation, accompanied by increased GLUT1. Oxidation of glucose markedly decreased, whereas incorporation of glucose-derived carbon into FA and sterols increased. Macrophage activation also increased uptake of FFA, associated with an increase in CD36. Oxidation of FA was markedly reduced, whereas the incorporation of FA into TGs increased, associated with increased GPAT3 and DGAT2. Additionally, macrophage activation decreased TG lipolysis; however, expression of ATGL or HSL was not altered. Macrophage activation altered gene expression similarly when incubated with exogenous FA or AcLDL. Whereas activation with ligands of TLR2 (zymosan), TLR3 (poly I:C), or TLR4 (LPS) induced alterations in macrophage gene expression, leading to TG accumulation, treatment of macrophages with cytokines had minimal effects. Thus, activation of TLRs leads to accumulation of TG in macrophages by multiple pathways that may have beneficial effects in host defense but could contribute to the accelerated atherosclerosis in chronic infections and inflammatory diseases.

Mycobacterium tuberculosis modulates macrophage lipid-sensing nuclear receptors PPARγ and TR4 for survival

Mycobacterium tuberculosis-macrophage interactions are key to pathogenesis and clearance of these bacteria. Although interactions between M. tuberculosis-associated lipids and TLRs, non-TLRs, and opsonic receptors have been investigated, interactions of these lipids and infected macrophage lipid repertoire with lipid-sensing nuclear receptors expressed in macrophages have not been addressed. In this study, we report that M. tuberculosis-macrophage lipids can interact with host peroxisome proliferator-activated receptor γ and testicular receptor 4 to ensure survival of the pathogen by modulating macrophage function. These two lipid-sensing nuclear receptors create a foamy niche within macrophage by modulating oxidized low-density lipoprotein receptor CD36, phagolysosomal maturation block by induction of IL-10, and a blunted innate response by alternative polarization of the macrophages, which leads to survival of M. tuberculosis. These results also suggest possible heterologous ligands for peroxisome proliferator-activated receptor γ and testicular receptor 4 and are suggestive of adaptive or coevolution of the host and pathogen. Relative mRNA expression levels of these receptors in PBMCs derived from clinical samples convincingly implicate them in tuberculosis susceptibility. These observations expose a novel paradigm in the pathogenesis of M. tuberculosis amenable for pharmacological modulation.

Diverse bacteria promote macrophage foam cell formation via Toll-like receptor-dependent lipid body biosynthesis

AIM

Atherosclerotic lesions contain DNA signatures from a wide variety of bacteria, although little is known of how exposure to these organisms may modulate the accumulation of lipids in macrophages.

METHODS

To address this, a panel of nine bacteria representing those most frequently reported to be present in human atheroma were examined for their potential to promote lipid accumulation in human primary monocytes and murine J774 macrophages.

RESULTS

All bacteria examined, and defined stimulants of Toll-like receptors (TLRs) 2, 3, 4, 5 and 9, induced lipid body formation and cholesterol ester accumulation in a dose-dependent manner. The mechanisms of bacteria-mediated foam cell formation were found to be dependent on TLR2 and/or TLR4 signalling, but independent of lipoprotein oxidation pathways, since lipid accumulation was significantly inhibited by the TLR4 inhibitors polymyxin-B and TAK-242, or the TLR2 and TLR4 inhibitor oxidised palmitoyl-arachidonyl-phosphatidyl-choline, but not by the scavenger receptor blocker polyinosinic acid or the antioxidant butylated hydroxytoluene. A number of genes involved in lipid body biosynthesis, including perilipin-A, stearoyl-coenzyme-A desaturase 1, fatty acid synthase and HMG-CoA reductase were upregulated in response to TLR4 stimulation.

CONCLUSIONS

The bacterial debris observed in human atheroma, which is currently considered to be harmless, may have potential to contribute to disease progression via TLR-dependent lipid body formation in macrophages.

Host cell lipid bodies triggered by Trypanosoma cruzi infection and enhanced by the uptake of apoptotic cells are associated with prostaglandin E₂ generation and increased parasite growth

Lipid bodies (lipid droplets) are lipid-rich organelles with functions in cell metabolism and signaling. Here, we investigate the mechanisms of Trypanosoma cruzi-induced lipid body formation and their contributions to host-parasite interplay. We demonstrate that T. cruzi-induced lipid body formation in macrophages occurs in a Toll-like receptor 2-dependent mechanism and is potentiated by apoptotic cell uptake. Lipid body biogenesis and prostaglandin E₂ (PGE₂) production triggered by apoptotic cell uptake was largely dependent of α(v)β₃ and transforming growth factor-β signaling. T. cruzi-induced lipid bodies act as sites of increased PGE synthesis. Inhibition of lipid body biogenesis by the fatty acid synthase inhibitor C75 reversed the effects of apoptotic cells on lipid body formation, eicosanoid synthesis, and parasite replication. Our findings indicate that lipid bodies are highly regulated organelles during T. cruzi infection with roles in lipid mediator generation by macrophages and are potentially involved in T. cruzi-triggered escape mechanisms.

Lipid body formation during maturation of human mast cells

Lipid droplets, also called lipid bodies (LB) in inflammatory cells, are important cytoplasmic organelles. However, little is known about the molecular characteristics and functions of LBs in human mast cells (MC). Here, we have analyzed the genesis and components of LBs during differentiation of human peripheral blood-derived CD34(+) progenitors into connective tissue-type MCs. In our serum-free culture system, the maturing MCs, derived from 18 different donors, invariably developed triacylglycerol (TG)-rich LBs. Not known heretofore, the MCs transcribe the genes for perilipins (PLIN)1-4, but not PLIN5, and PLIN2 and PLIN3 display different degrees of LB association. Upon MC activation and ensuing degranulation, the LBs were not cosecreted with the cytoplasmic secretory granules. Exogenous arachidonic acid (AA) enhanced LB genesis in Triacsin C-sensitive fashion, and it was found to be preferentially incorporated into the TGs of LBs. The large TG-associated pool of AA in LBs likely is a major precursor for eicosanoid production by MCs. In summary, we demonstrate that cultured human MCs derived from CD34(+) progenitors in peripheral blood provide a new tool to study regulatory mechanisms involving LB functions, with particular emphasis on AA metabolism, eicosanoid biosynthesis, and subsequent release of proinflammatory lipid mediators from these cells.

TLR6-driven lipid droplets in Mycobacterium leprae-infected Schwann cells: immunoinflammatory platforms associated with bacterial persistence

The mechanisms responsible for nerve injury in leprosy need further elucidation. We recently demonstrated that the foamy phenotype of Mycobacterium leprae-infected Schwann cells (SCs) observed in nerves of multibacillary patients results from the capacity of M. leprae to induce and recruit lipid droplets (LDs; also known as lipid bodies) to bacterial-containing phagosomes. In this study, we analyzed the parameters that govern LD biogenesis by M. leprae in SCs and how this contributes to the innate immune response elicited by M. leprae. Our observations indicated that LD formation requires the uptake of live bacteria and depends on host cell cytoskeleton rearrangement and vesicular trafficking. TLR6 deletion, but not TLR2, completely abolished the induction of LDs by M. leprae, as well as inhibited the bacterial uptake in SCs. M. leprae-induced LD biogenesis correlated with increased PGE(2) and IL-10 secretion, as well as reduced IL-12 and NO production in M. leprae-infected SCs. Analysis of nerves from lepromatous leprosy patients showed colocalization of M. leprae, LDs, and cyclooxygenase-2 in SCs, indicating that LDs are sites for PGE(2) synthesis in vivo. LD biogenesis Inhibition by the fatty acid synthase inhibitor C-75 abolished the effect of M. leprae on SC production of immunoinflammatory mediators and enhanced the mycobacterial-killing ability of SCs. Altogether, our data indicated a critical role for TLR6-dependent signaling in M. leprae-SC interactions, favoring phagocytosis and subsequent signaling for induction of LD biogenesis in infected cells. Moreover, our observations reinforced the role of LDs favoring mycobacterial survival and persistence in the nerve. These findings give further support to a critical role for LDs in M. leprae pathogenesis in the nerve.

Lutzomyia longipalpis saliva triggers lipid body formation and prostaglandin E₂ production in murine macrophages

Background

Sand fly saliva contains molecules that modify the host's hemostasis and immune responses. Nevertheless, the role played by this saliva in the induction of key elements of inflammatory responses, such as lipid bodies (LB, also known as lipid droplets) and eicosanoids, has been poorly investigated. LBs are cytoplasmic organelles involved in arachidonic acid metabolism that form eicosanoids in response to inflammatory stimuli. In this study, we assessed the role of salivary gland sonicate (SGS) from Lutzomyia (L.) longipalpis, a Leishmania infantum chagasi vector, in the induction of LBs and eicosanoid production by macrophages in vitro and ex vivo.

Methodology/Principal Findings

Different doses of L. longipalpis SGS were injected into peritoneal cavities of C57BL/6 mice. SGS induced increased macrophage and neutrophil recruitment into the peritoneal cavity at different time points. Sand fly saliva enhanced PGE₂ and LTB₄ production by harvested peritoneal leukocytes after ex vivo stimulation with a calcium ionophore. At three and six hours post-injection, L. longipalpis SGS induced more intense LB staining in macrophages, but not in neutrophils, compared with mice injected with saline. Moreover, macrophages harvested by peritoneal lavage and stimulated with SGS in vitro presented a dose- and time-dependent increase in LB numbers, which was correlated with increased PGE₂ production. Furthermore, COX-2 and PGE-synthase co-localized within the LBs induced by L. longipalpis saliva. PGE₂ production by macrophages induced by SGS was abrogated by treatment with NS-398, a COX-2 inhibitor. Strikingly, SGS triggered ERK-1/2 and PKC-α phosphorylation, and blockage of the ERK-1/2 and PKC-α pathways inhibited the SGS effect on PGE₂ production by macrophages.

Conclusion

In sum, our results show that L. longipalpis saliva induces lipid body formation and PGE₂ production by macrophages ex vivo and in vitro via the ERK-1/2 and PKC-α signaling pathways. This study provides new insights regarding the pharmacological mechanisms whereby L. longipalpis saliva influences the early steps of the host's inflammatory response.

After the injection of saliva into the host's skin by sand flies, a transient erythematous reaction is observed, which is related to an influx of inflammatory cells and the release of various molecules that actively facilitate the blood meal. It is important to understand the specific mechanisms by which sand fly saliva manipulates the host's inflammatory responses. Herein, we report that saliva from Lutzomyia (L.) longipalpis, a widespread Leishmania vector, induces early production of eicosanoids. Intense formation of intracellular organelles called lipid bodies (LBs) was noted within those cells that migrated to the site of saliva injection. In vitro and ex vivo, sand fly saliva was able to induce LB formation and PGE₂ release by macrophages. Interestingly, PGE₂ production induced by L. longipalpis saliva was dependent on intracellular mechanisms involving phosphorylation of signaling proteins such as PKC-α and ERK-1/2 and subsequent activation of cyclooxygenase-2. Thus, this study provides new insights into the pharmacological properties of sand fly saliva and opens new opportunities for intervening with the induction of the host's inflammatory pathways by L. longipalpis bites.

Modulation of lipid droplets by Mycobacterium leprae in Schwann cells: a putative mechanism for host lipid acquisition and bacterial survival in phagosomes

The predilection of Mycobacterium leprae (ML) for Schwann cells (SCs) leads to peripheral neuropathy, a major concern in leprosy. Highly infected SCs in lepromatous leprosy nerves show a foamy, lipid-laden appearance; but the origin and nature of these lipids, as well as their role in leprosy, have remained unclear. The data presented show that ML has a pronounced effect on host-cell lipid homeostasis through regulation of lipid droplet (lipid bodies, LD) biogenesis and intracellular distribution. Electron microscopy and immunohistochemical analysis of lepromatous leprosy nerves for adipose differentiation-related protein expression, a classical LD marker, revealed accumulating LDs in close association to ML in infected SCs. The capacity of ML to induce LD formation was confirmed in in vitro studies with human SCs. Moreover, via confocal and live-cell analysis, it was found that LDs are promptly recruited to bacterial phagosomes and that this process depends on cytoskeletal reorganization and PI3K signalling. ML-induced LD biogenesis and recruitment were found to be independent of TLR2 bacterial sensing. Notably, LD recruitment impairment by cytoskeleton drugs decreased intracellular bacterial survival. Altogether, our data revealed SC lipid accumulation in ML-containing phagosomes, which may represent a fundamental aspect of bacterial pathogenesis in the nerve.

Toll-like receptor-dependent lipid body formation in macrophage foam cell formation

PURPOSE OF REVIEW

The differentiation of macrophages into lipid-laden foam cells is central to the development of atherosclerosis. Traditionally, it has been assumed that the uptake of oxidized low-density lipoprotein by macrophage scavenger receptors is largely responsible for this process. However, in light of recent evidence that these mechanisms may not play as large a role as previously thought, alternative mechanisms of foam cell formation are now being explored.

RECENT FINDINGS

The stimulation of Toll-like receptor (TLR) signalling by bacterial molecules has been shown to promote the accumulation of lipid in macrophages in the form of intracellular inclusions termed 'lipid bodies'. Interactions between TLR-signalling pathways and the liver-X receptor and peroxisome proliferator-activated receptor-γ signalling pathways modulate the formation of lipid bodies in macrophages and thereby cellular accumulation of cholesterol and triglyceride. These pathways appear to involve TLR-mediated regulation of lipid-binding proteins, cellular cholesterol sensors, lipid-body-associated proteins and secreted autocrine factors, but are independent of scavenger receptor or lipoprotein oxidation-dependent pathways.

SUMMARY

TLR stimulation promotes the accumulation of lipid bodies in macrophages and consequently foam cell formation. The pathways responsible for these processes may constitute novel therapeutic targets for atherosclerosis.

Host cholesterol and inflammation as common key regulators of toxoplasmosis and artherosclerosis development

Atherosclerosis and toxoplasmosis are two widely prevalent diseases worldwide. The relationship between these diseases is now being elucidated. Atherosclerosis is a disease with three main components: increased blood lipoprotein/cholesterol and their deposition in the arterial wall, an important Th1-mediated proinflammatory reaction and thrombogenic status. Toxoplasma gondii, in turn, is dependent on host cholesterol for optimal intracellular growth and replication. As a result, host cholesterol will be cleared from the blood, reducing plasma low-density lipoprotein, a crucial atherosclerosis risk factor. On the other hand, T. gondii infection elicits an important Th1 systemic inflammatory response in the host. Therefore, this additional proinflammatory stimulus may impose an enhanced pro-atherogenic environment in the host. As result, the association between these two diseases in one individual could change the course of atherosclerosis. In this review, we demonstrate that the host-parasite relationship is complex and that the outcome of each disease is dependent on the availability of intracellular cholesterol, as well as the intensity of the inflammatory reaction triggered by the parasite. We also discuss the possible clinical implications of these studies.

Lipids from attenuated and virulent Babesia bovis strains induce differential TLR2-mediated macrophage activation

Babesia bovis is an intraerythrocytic apicomplexan protozoa of cattle that causes an acute infection with parasite persistence. Babesiosis limitation depends on macrophages, essential effector cells of the host innate defense, which generate inflammatory cytokines and nitric oxide. Herein, we report quantitative differences in the lipid composition of merozoites from two B. bovis strains with polar behaviour: attenuated R1A and virulent S2P. Accordingly, we observed a distinct inflammatory response induced by the total lipids of R1A (L(A)) and S2P (L(V)) in murine peritoneal macrophages. L(A) and particularly its fractions phosphatidic acid and phosphatidylserine+phosphatidylinositol (PS+PI), produced a strong activation of these cells with lipid body formation, cyclooxygenase-2 expression and pro-inflammatory TNFalpha, IL-6 and KC secretion. Although L(V) did not activate these cells, the corresponding PS+PI fraction induced TNFalpha, IL-6 and KC release. Therefore, these facts might be suggesting the presence of an inhibitor in L(V). Furthermore, the employment of wild type and toll like receptor 2 knockout (TLR2KO) mice allowed us to demonstrate that macrophage activation by the stimulating lipid fractions was mediated through TLR2. Interestingly, only L(A) activated the extracellular signal-regulated kinases 1 and 2 (ERK1/2). Inhibitory studies employing UO126, indicated that the ERK pathway was required for TNFalpha, IL-6 and KC release. In conclusion, the absence of inflammatory response observed with the lipids of S2P virulent strain could constitute an evasion mechanism of the innate immune response enabling parasite establishment in the host.

The role of hypoxia in atherosclerosis

PURPOSE OF REVIEW

It is important to address the factors involved in the progression of atherosclerosis because advanced atherosclerotic lesions are prone to rupture, leading to disability or death. Hypoxic areas are known to be present in human atherosclerotic lesions, and lesion progression is associated with the formation of lipid-loaded macrophages and increased local inflammation. Here we summarize the role of hypoxia in the development of advanced atherosclerotic lesions by promoting lipid accumulation, inflammation, ATP depletion, and angiogenesis.

RECENT FINDINGS

A recent study clearly demonstrated the presence of hypoxia in macrophage-rich regions of advanced human carotid atherosclerotic lesions. We showed that hypoxia increases the formation of lipid droplets in macrophages and promotes increased secretion of inflammatory mediators, and recent evidence indicates that lipid droplets may play a role in mediating the inflammatory response. Hypoxia also promotes lesion progression by exacerbating ATP depletion and lactate accumulation, and the presence of hypoxia in human carotid atherosclerotic lesions correlates with angiogenesis.

SUMMARY

Recent studies indicate that hypoxia may play a key role in the progression to advanced lesions by promoting lipid accumulation, increased inflammation, ATP depletion, and angiogenesis. Further understanding of the effects of hypoxia in atherosclerotic lesions could indicate potential therapeutic targets.

The conserved metalloprotease invadolysin localizes to the surface of lipid droplets

Invadolysin is a metalloprotease conserved in many different organisms, previously shown to be essential in Drosophila with roles in cell division and cell migration. The gene seems to be ubiquitously expressed and four distinct splice variants have been identified in human cells but not in most other species examined. Immunofluorescent detection of human invadolysin in cultured cells reveals the protein to be associated with the surface of lipid droplets. By means of subcellular fractionation, we have independently confirmed the association of invadolysin with lipid droplets. We thus identify invadolysin as the first metalloprotease located on these dynamic organelles. In addition, analysis of larval fat-body morphological appearance and triglyceride levels in the Drosophila invadolysin mutant suggests that invadolysin plays a role in lipid storage or metabolism.