Instruct: Given a scientific claim, retrieve relevant documents that support or refute the claim.
Query: The M2-phenotype in brown adipose tissue macrophages increases brown adipose tissue thermogenic activity.
All homeotherms use thermogenesis to maintain their core body temperature, ensuring that cellular functions and physiological processes can continue in cold environments. In the prevailing model of thermogenesis, when the hypothalamus senses cold temperatures it triggers sympathetic discharge, resulting in the release of noradrenaline in brown adipose tissue and white adipose tissue. Acting via the β(3)-adrenergic receptors, noradrenaline induces lipolysis in white adipocytes, whereas it stimulates the expression of thermogenic genes, such as PPAR-γ coactivator 1a (Ppargc1a), uncoupling protein 1 (Ucp1) and acyl-CoA synthetase long-chain family member 1 (Acsl1), in brown adipocytes. However, the precise nature of all the cell types involved in this efferent loop is not well established. Here we report in mice an unexpected requirement for the interleukin-4 (IL-4)-stimulated program of alternative macrophage activation in adaptive thermogenesis. Exposure to cold temperature rapidly promoted alternative activation of adipose tissue macrophages, which secrete catecholamines to induce thermogenic gene expression in brown adipose tissue and lipolysis in white adipose tissue. Absence of alternatively activated macrophages impaired metabolic adaptations to cold, whereas administration of IL-4 increased thermogenic gene expression, fatty acid mobilization and energy expenditure, all in a macrophage-dependent manner. Thus, we have discovered a role for alternatively activated macrophages in the orchestration of an important mammalian stress response, the response to cold.
The interest in brown adipose tissue (BAT) as a target to combat metabolic disease has recently been renewed with the discovery of functional BAT in humans. In rodents, BAT can be activated by bile acids, which activate type 2 iodothyronine deiodinase (D2) in BAT via the G-coupled protein receptor TGR5, resulting in increased oxygen consumption and energy expenditure. Here we examined the effects of oral supplementation of the bile acid chenodeoxycholic acid (CDCA) on human BAT activity. Treatment of 12 healthy female subjects with CDCA for 2 days resulted in increased BAT activity. Whole-body energy expenditure was also increased upon CDCA treatment. In vitro treatment of primary human brown adipocytes derived with CDCA or specific TGR5 agonists increased mitochondrial uncoupling and D2 expression, an effect that was absent in human primary white adipocytes. These findings identify bile acids as a target to activate BAT in humans.
Macrophage-derived foam cells play important roles in the progression of atherosclerosis. We reported previously that ERK1/2-dependent granulocyte/macrophage colony-stimulating factor (GM-CSF) expression, leading to p38 MAPK/ Akt signaling, is important for oxidized low density lipoprotein (Ox-LDL)-induced macrophage proliferation. Here, we investigated whether activation of AMP-activated protein kinase (AMPK) could suppress macrophage proliferation. Ox-LDL-induced proliferation of mouse peritoneal macrophages was assessed by [(3)H]thymidine incorporation and cell counting assays. The proliferation was significantly inhibited by the AMPK activator 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) and restored by dominant-negative AMPKalpha1, suggesting that AMPK activation suppressed macrophage proliferation. AICAR partially suppressed Ox-LDL-induced ERK1/2 phosphorylation and GM-CSF expression, suggesting that another mechanism is also involved in the AICAR-mediated suppression of macrophage proliferation. AICAR suppressed GM-CSF-induced macrophage proliferation without suppressing p38 MAPK/Akt signaling. GM-CSF suppressed p53 phosphorylation and expression and induced Rb phosphorylation. Overexpression of p53 or p27(kip) suppressed GM-CSF-induced macrophage proliferation. AICAR induced cell cycle arrest, increased p53 phosphorylation and expression, and suppressed GM-CSF-induced Rb phosphorylation via AMPK activation. Moreover, AICAR induced p21(cip) and p27(kip) expression via AMPK activation, and small interfering RNA (siRNA) of p21(cip) and p27(kip) restored AICAR-mediated suppression of macrophage proliferation. In conclusion, AMPK activation suppressed Ox-LDL-induced macrophage proliferation by suppressing GM-CSF expression and inducing cell cycle arrest. These effects of AMPK activation may represent therapeutic targets for atherosclerosis.
The object of this study was to assess the role of brown adipose tissue (BAT) and the sympathetic nervous system in the rise in heat production associated with endotoxin-induced fever. Oxygen consumption (VO2) was found to be significantly increased (28%) over a 4-h period after two doses of endotoxin (Escherichia coli lipopolysaccharide, 0.3 mg/100 g body wt) given 24 h apart. Injection of a mixed beta-adrenoceptor antagonist (propranolol) reduced VO2 by 14% in endotoxin-treated rats, whereas the selective beta 1- (atenolol) or beta 2- (ICI 118551) antagonists suppressed VO2 by 10%. These drugs did not affect VO2 in control animals. BAT thermogenic activity assessed from measurements of in vitro mitochondrial guanosine 5'-diphosphate (GDP) binding was elevated by 54% in interscapular BAT and by 171% in other BAT depots. Surgical denervation of one lobe of the interscapular depot prevented these responses. Endotoxin failed to stimulate GDP binding in rats fed protein-deficient diets. This may have been because BAT thermogenic activity was already elevated in control rats fed these diets or because endotoxin caused a marked suppression of food intake in the protein-deficient animals. The results indicate that sympathetic activation of BAT is involved in the thermogenic responses to endotoxin and that these can be modified by dietary manipulation.
The genetically obese (ob/ob) mouse exhibits defective thermoregulatory responses to cold exposure. Pathophysiological explanations for this phenomenon have focused on abnormalities in intracellular metabolism or insensitivity of peripheral tissues to the thermogenic effects of catecholamines. Because the sympathetic nervous system (SNS) is subject to feedback regulation, a peripheral impairment in thermogenesis should be associated with a compensatory increase in SNS activity. To examine SNS activity in the ob/ob mouse, norepinephrine (NE) turnover was measured in heart and interscapular brown adipose tissue (IBAT) of ob/ob and lean mice. The results from studies utilizing radiolabeled NE or inhibition of NE biosynthesis with alpha-methyl-p-tyrosine to measure NE turnover demonstrated reductions in SNS activity of 33-56% in heart and of 45-73% in IBAT in ob/ob mice at ambient temperature (22 degrees C) compared with measurements in lean controls. During cold exposure (4 degrees C) NE turnover increased in heart and IBAT to a similar extent in both ob/ob and lean mice, but NE turnover rates in heart, and probably in IBAT as well, remained lower in the obese mice than in the lean despite the gradual development of hypothermia in the ob/ob mice during this period. Administration of naltrexone, a long-acting opiate antagonist, failed to reverse the suppression of SNS activity observed in the ob/ob mice. These data indicate that diminished SNS activity in ob/ob mice may be an additional factor contributing to the defective thermogenesis characteristic of these animals.
The tumor microenvironment (TME) plays a prominent role in the growth of tumor cells. As the major inflammatory component of the TME, M2d macrophages are educated by the TME such that they adopt an immunosuppressive role that promotes tumor metastasis and progression. Fra-1 forms activator protein-1 heterodimers with Jun partners and drives gene transcription. Fra-1 is thought to drastically induce tumorigenesis and progression. However, the functional role of Fra-1 in the generation of M2d macrophages is poorly understood to date. Here, we demonstrate that 4T1 mammary carcinoma cells, when co-cultured with RAW264.7 macrophage cells, skew the RAW264.7 macrophage cell differentiation into M2d macrophages. The 4T1 cells stimulate de novo overexpression of Fra-1 in RAW264.7 cells, and then Fra-1 binds to the interleukin 6 (IL-6) promoter to increase the production of the cytokine IL-6 in RAW264.7 cells. IL-6 acts in an autocrine fashion to skew RAW264.7 macrophage cell differentiation into M2d macrophages. These findings open new insights into how to reverse M2d macrophage-induced immune tolerance to improve the efficacy of immunotherapeutic approaches.
Uncoupling protein 1 (UCP1) is highly expressed in brown adipose tissue, where it generates heat by uncoupling electron transport from ATP production. UCP1 is also found outside classical brown adipose tissue depots, in adipocytes that are termed 'brite' (brown-in-white) or 'beige'. In humans, the presence of brite or beige (brite/beige) adipocytes is correlated with a lean, metabolically healthy phenotype, but whether a causal relationship exists is not clear. Here we report that human brite/beige adipocyte progenitors proliferate in response to pro-angiogenic factors, in association with expanding capillary networks. Adipocytes formed from these progenitors transform in response to adenylate cyclase activation from being UCP1 negative to being UCP1 positive, which is a defining feature of the beige/brite phenotype, while displaying uncoupled respiration. When implanted into normal chow-fed, or into high-fat diet (HFD)-fed, glucose-intolerant NOD-scid IL2rg(null) (NSG) mice, brite/beige adipocytes activated in vitro enhance systemic glucose tolerance. These adipocytes express neuroendocrine and secreted factors, including the pro-protein convertase PCSK1, which is strongly associated with human obesity. Pro-angiogenic conditions therefore drive the proliferation of human beige/brite adipocyte progenitors, and activated beige/brite adipocytes can affect systemic glucose homeostasis, potentially through a neuroendocrine mechanism.
Macrophages are distributed in tissues throughout the body and contribute to both homeostasis and disease. Recently, it has become evident that most adult tissue macrophages originate during embryonic development and not from circulating monocytes. Each tissue has its own composition of embryonically derived and adult-derived macrophages, but it is unclear whether macrophages of distinct origins are functionally interchangeable or have unique roles at steady state. This new understanding also prompts reconsideration of the function of circulating monocytes. Classical Ly6c(hi) monocytes patrol the extravascular space in resting organs, and Ly6c(lo) nonclassical monocytes patrol the vasculature. Inflammation triggers monocytes to differentiate into macrophages, but whether resident and newly recruited macrophages possess similar functions during inflammation is unclear. Here, we define the tools used for identifying the complex origin of tissue macrophages and discuss the relative contributions of tissue niche versus ontological origin to the regulation of macrophage functions during steady state and inflammation.
Uncoupling protein 1 (UCP1) plays a central role in nonshivering thermogenesis in brown fat; however, its role in beige fat remains unclear. Here we report a robust UCP1-independent thermogenic mechanism in beige fat that involves enhanced ATP-dependent Ca2+ cycling by sarco/endoplasmic reticulum Ca2+-ATPase 2b (SERCA2b) and ryanodine receptor 2 (RyR2). Inhibition of SERCA2b impairs UCP1-independent beige fat thermogenesis in humans and mice as well as in pigs, a species that lacks a functional UCP1 protein. Conversely, enhanced Ca2+ cycling by activation of α1- and/or β3-adrenergic receptors or the SERCA2b-RyR2 pathway stimulates UCP1-independent thermogenesis in beige adipocytes. In the absence of UCP1, beige fat dynamically expends glucose through enhanced glycolysis, tricarboxylic acid metabolism and pyruvate dehydrogenase activity for ATP-dependent thermogenesis through the SERCA2b pathway; beige fat thereby functions as a 'glucose sink' and improves glucose tolerance independently of body weight loss. Our study uncovers a noncanonical thermogenic mechanism through which beige fat controls whole-body energy homeostasis via Ca2+ cycling.
Maximum activities of some key enzymes of metabolism were studied in elicited (inflammatory) macrophages of the mouse and lymph-node lymphocytes of the rat. The activity of hexokinase in the macrophage is very high, as high as that in any other major tissue of the body, and higher than that of phosphorylase or 6-phosphofructokinase, suggesting that glucose is a more important fuel than glycogen and that the pentose phosphate pathway is also important in these cells. The latter suggestion is supported by the high activities of both glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. However, the rate of glucose utilization by 'resting' macrophages incubated in vitro is less than the 10% of the activity of 6-phosphofructokinase: this suggests that the rate of glycolysis is increased dramatically during phagocytosis or increased secretory activity. The macrophages possess higher activities of citrate synthase and oxoglutarate dehydrogenase than do lymphocytes, suggesting that the tricarboxylic acid cycle may be important in energy generation in these cells. The activity of 3-oxoacid CoA-transferase is higher in the macrophage, but that of 3-hydroxybutyrate dehydrogenase is very much lower than those in the lymphocytes. The activity of carnitine palmitoyltransferase is higher in macrophages, suggesting that fatty acids as well as acetoacetate could provide acetyl-CoA as substrate for the tricarboxylic acid cycle. No detectable rate of acetoacetate or 3-hydroxybutyrate utilization was observed during incubation of resting macrophages, but that of oleate was 1.0 nmol/h per mg of protein or about 2.2% of the activity of palmitoyltransferase. The activity of glutaminase is about 4-fold higher in macrophages than in lymphocytes, which suggests that the rate of glutamine utilization could be very high. The rate of utilization of glutamine by resting incubated macrophages was similar to that reported for rat lymphocytes, but was considerably lower than the activity of glutaminase.
Macrophages play a relevant role in innate and adaptive immunity depending on the balance of the stimuli received. From an analytical and functional point of view, macrophage stimulation can be segregated into three main modes, as follows: innate, classic, and alternative pathways. These differential activations result in the expression of specific sets of genes involved in the release of pro- or anti-inflammatory stimuli. In the present work, we have analyzed whether specific metabolic patterns depend on the signaling pathway activated. A [1,2-(13)C(2)]glucose tracer-based metabolomics approach has been used to characterize the metabolic flux distributions in macrophages stimulated through the classic, innate, and alternative pathways. Using this methodology combined with mass isotopomer distribution analysis of the new formed metabolites, the data show that activated macrophages are essentially glycolytic cells, and a clear cutoff between the classic/innate activation and the alternative pathway exists. Interestingly, macrophage activation through LPS/IFN-gamma or TLR-2, -3, -4, and -9 results in similar flux distribution patterns regardless of the pathway activated. However, stimulation through the alternative pathway has minor metabolic effects. The molecular basis of the differences between these two types of behavior involves a switch in the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) from the liver type-PFK2 to the more active ubiquitous PFK2 isoenzyme, which responds to Hif-1alpha activation and increases fructose-2,6-bisphosphate concentration and the glycolytic flux. However, using macrophages targeted for Hif-1alpha, the switch of PFK2 isoenzymes still occurs in LPS/IFN-gamma-activated macrophages, suggesting that this pathway regulates ubiquitous PFK2 expression through Hif-1alpha-independent mechanisms.
Macrophages play a pivotal role in innate and acquired immune responses to Schistosoma mansoni. Classical (M1) or alternative (M2) activation states of these cells further delineate their roles in tissue damage through innate immunity or fibrotic remodeling, respectively. In the present study, we addressed the following question: Does systemic Th2-type cytokine polarization evoked by S. mansoni affect macrophage differentiation and activation? To this end, we analyzed bone marrow-derived macrophages from mice with S. mansoni egg-induced pulmonary granulomas and unchallenged (or naïve) mice to determine their activation state and their response to specific TLR agonists, including S. mansoni egg antigens. Unlike naïve macrophages, macrophages from Th2-polarized mice constitutively expressed significantly higher "found in inflammatory zone-1" (FIZZ1) and ST2 (M2 markers) and significantly lower NO synthase 2, CCL3, MIP-2, TNF-alpha, and IL-12 (M1 markers). Also, compared with naïve macrophages, Th2-polarized macrophages exhibited enhanced responses to the presence of specific TLR agonists, which consistently induced significantly higher levels of gene and protein levels for M2 and M1 markers in these cells. Together, these data show that signals received by bone marrow precursors during S. mansoni egg-induced granuloma responses dynamically alter the development of macrophages and enhance the TLR responsiveness of these cells, which may ultimately have a significant effect on the pulmonary granulomatous response.
Despite accumulating evidence suggesting local self-maintenance of tissue macrophages in the steady state, the dogma remains that tissue macrophages derive from monocytes. Using parabiosis and fate-mapping approaches, we confirmed that monocytes do not show significant contribution to tissue macrophages in the steady state. Similarly, we found that after depletion of lung macrophages, the majority of repopulation occurred by stochastic cellular proliferation in situ in a macrophage colony-stimulating factor (M-Csf)- and granulocyte macrophage (GM)-CSF-dependent manner but independently of interleukin-4. We also found that after bone marrow transplantation, host macrophages retained the capacity to expand when the development of donor macrophages was compromised. Expansion of host macrophages was functional and prevented the development of alveolar proteinosis in mice transplanted with GM-Csf-receptor-deficient progenitors. Collectively, these results indicate that tissue-resident macrophages and circulating monocytes should be classified as mononuclear phagocyte lineages that are independently maintained in the steady state.
Until recently, the mechanism of adaptive thermogenesis was ascribed to the expression of uncoupling protein 1 (UCP1) in brown and beige adipocytes. UCP1 is known to catalyze a proton leak of the inner mitochondrial membrane, resulting in uncoupled oxidative metabolism with no production of adenosine triphosphate and increased energy expenditure. Thus increasing brown and beige adipose tissue with augmented UCP1 expression is a viable target for obesity-related disorders. Recent work demonstrates an UCP1-independent pathway to uncouple mitochondrial respiration. A secreted enzyme, PM20D1, enriched in UCP1+ adipocytes, exhibits catalytic and hydrolytic activity to reversibly form N-acyl amino acids. N-acyl amino acids act as endogenous uncouplers of mitochondrial respiration at physiological concentrations. Administration of PM20D1 or its products, N-acyl amino acids, to diet-induced obese mice improves glucose tolerance by increasing energy expenditure. In short-term studies, treated animals exhibit no toxicity while experiencing 10% weight loss primarily of adipose tissue. Further study of this metabolic pathway may identify novel therapies for diabesity, the disease state associated with diabetes and obesity.
During the inflammatory response that drives atherogenesis, macrophages accumulate progressively in the expanding arterial wall. The observation that circulating monocytes give rise to lesional macrophages has reinforced the concept that monocyte infiltration dictates macrophage buildup. Recent work has indicated, however, that macrophage accumulation does not depend on monocyte recruitment in some inflammatory contexts. We therefore revisited the mechanism underlying macrophage accumulation in atherosclerosis. In murine atherosclerotic lesions, we found that macrophages turn over rapidly, after 4 weeks. Replenishment of macrophages in these experimental atheromata depends predominantly on local macrophage proliferation rather than monocyte influx. The microenvironment orchestrates macrophage proliferation through the involvement of scavenger receptor A (SR-A). Our study reveals macrophage proliferation as a key event in atherosclerosis and identifies macrophage self-renewal as a therapeutic target for cardiovascular disease.
The rising incidence of obesity and associated metabolic diseases has increased the urgency in understanding all aspects of adipose tissue biology. This includes the function of adipocytes, how adipose tissue expands in obesity, and how expanded adipose tissues in adults can impact physiology. Here, we highlight the growing appreciation for the importance of de novo adipocyte differentiation to adipose tissue expansion in adult humans and animals. We detail recent efforts to identify adipose precursor populations that contribute to the physiological postnatal recruitment of white, brown, and beige adipocytes in mice, and summarize new data that reveal the complexity of adipose tissue development in vivo.
There is persuasive clinical and experimental evidence that macrophages promote cancer initiation and malignant progression. During tumor initiation, they create an inflammatory environment that is mutagenic and promotes growth. As tumors progress to malignancy, macrophages stimulate angiogenesis, enhance tumor cell migration and invasion, and suppress antitumor immunity. At metastatic sites, macrophages prepare the target tissue for arrival of tumor cells, and then a different subpopulation of macrophages promotes tumor cell extravasation, survival, and subsequent growth. Specialized subpopulations of macrophages may represent important new therapeutic targets.
Uncoupling protein 1 (Ucp1), which is localized in the mitochondrial inner membrane of mammalian brown adipose tissue (BAT), generates heat by uncoupling oxidative phosphorylation. Upon cold exposure or nutritional abundance, sympathetic neurons stimulate BAT to express Ucp1 to induce energy dissipation and thermogenesis. Accordingly, increased Ucp1 expression reduces obesity in mice and is correlated with leanness in humans. Despite this significance, there is currently a limited understanding of how Ucp1 expression is physiologically regulated at the molecular level. Here, we describe the involvement of Sestrin2 and reactive oxygen species (ROS) in regulation of Ucp1 expression. Transgenic overexpression of Sestrin2 in adipose tissues inhibited both basal and cold-induced Ucp1 expression in interscapular BAT, culminating in decreased thermogenesis and increased fat accumulation. Endogenous Sestrin2 is also important for suppressing Ucp1 expression because BAT from Sestrin2(-/-) mice exhibited a highly elevated level of Ucp1 expression. The redox-inactive mutant of Sestrin2 was incapable of regulating Ucp1 expression, suggesting that Sestrin2 inhibits Ucp1 expression primarily through reducing ROS accumulation. Consistently, ROS-suppressing antioxidant chemicals, such as butylated hydroxyanisole and N-acetylcysteine, inhibited cold- or cAMP-induced Ucp1 expression as well. p38 MAPK, a signaling mediator required for cAMP-induced Ucp1 expression, was inhibited by either Sestrin2 overexpression or antioxidant treatments. Taken together, these results suggest that Sestrin2 and antioxidants inhibit Ucp1 expression through suppressing ROS-mediated p38 MAPK activation, implying a critical role of ROS in proper BAT metabolism.
RATIONALE Idiopathic pulmonary fibrosis (IPF) is a devastating disease. Antiinflammatory therapies, including corticosteroids, are of no benefit. The role of monocytes and macrophages is therefore controversial. OBJECTIVES To define the role of monocytes and macrophages during lung fibrogenesis and resolution, and explore the phenotype of the cells involved. METHODS We used multiple in vivo depletional strategies, backed up by adoptive transfer techniques. Further studies were performed on samples from patients with IPF. MEASUREMENTS AND MAIN RESULTS Depletion of lung macrophages during fibrogenesis reduced pulmonary fibrosis as measured by lung collagen (P = 0.0079); fibrosis score (P = 0.0051); and quantitative polymerase chain reaction for surrogate markers of fibrosis Col1 (P = 0.0083) and a-smooth muscle actin (P = 0.0349). There was an associated reduction in markers of the profibrotic alternative macrophage activation phenotype, Ym1 (P = 0.0179), and Arginase 1. The alternative macrophage marker CD163 was expressed on lung macrophages from patients with IPF. Depletion of Ly6Chi circulating monocytes reduced pulmonary fibrosis (P = 0.0052) and the number of Ym1- positive alternatively activated lung macrophages (P = 0.0310). Their adoptive transfer during fibrogenesis exacerbated fibrosis (P = 0.0304); however, adoptively transferred CD45.1 Ly6Chi cells were not found in the lungs of recipient CD45.2 mice. CONCLUSIONS We demonstrate the importance of circulating monocytes and lung macrophages during pulmonary fibrosis, and emphasize the importance of the alternatively activated macrophage phenotype. We show that Ly6Chi monocytes facilitate the progression of pulmonary fibrosis, but are not obviously engrafted into lungs thereafter. Finally, we provide empirical data to suggest that macrophages may have a resolution-promoting role during the reversible phase of bleomycin-induced pulmonary fibrosis.
Microbe-macrophage interactions play a central role in the pathogenesis of many infections. The ability of some bacterial pathogens to induce macrophage apoptosis has been suggested to contribute to their ability to elude innate immune responses and successfully colonize the host. Here, we provide evidence that activation of liver X receptors (LXRs) and retinoid X receptors (RXRs) inhibits apoptotic responses of macrophages to macrophage colony-stimulating factor (M-CSF) withdrawal and several inducers of apoptosis. In addition, combined activation of LXR and RXR protected macrophages from apoptosis caused by infection with Bacillus anthracis, Escherichia coli, and Salmonella typhimurium. Expression-profiling studies demonstrated that LXR and RXR agonists induced the expression of antiapoptotic regulators, including AIM/CT2, Bcl-X(L), and Birc1a. Conversely, LXR and RXR agonists inhibited expression of proapoptotic regulators and effectors, including caspases 1, 4/11, 7, and 12; Fas ligand; and Dnase1l3. The combination of LXR and RXR agonists was more effective than either agonist alone at inhibiting apoptosis in response to various inducers of apoptosis, and it acted synergistically to induce expression of AIM/CT2. Inhibition of AIM/CT2 expression in response to LXR/RXR agonists partially reversed their antiapoptotic effects. These findings reveal unexpected roles of LXRs and RXRs in the control of macrophage survival and raise the possibility that LXR/RXR agonists may be exploited to enhance innate immunity to bacterial pathogens that induce apoptotic programs as a strategy for evading host responses.
Cancer-associated cachexia (CAC) is a wasting syndrome characterized by systemic inflammation, body weight loss, atrophy of white adipose tissue (WAT) and skeletal muscle. Limited therapeutic options are available and the underlying mechanisms are poorly defined. Here we show that a phenotypic switch from WAT to brown fat, a phenomenon termed WAT browning, takes place in the initial stages of CAC, before skeletal muscle atrophy. WAT browning is associated with increased expression of uncoupling protein 1 (UCP1), which uncouples mitochondrial respiration toward thermogenesis instead of ATP synthesis, leading to increased lipid mobilization and energy expenditure in cachectic mice. Chronic inflammation and the cytokine interleukin-6 increase UCP1 expression in WAT, and treatments that reduce inflammation or β-adrenergic blockade reduce WAT browning and ameliorate the severity of cachexia. Importantly, UCP1 staining is observed in WAT from CAC patients. Thus, inhibition of WAT browning represents a promising approach to ameliorate cachexia in cancer patients.
Macrophages and dendritic cells (DCs) are key components of cellular immunity and are thought to originate and renew from hematopoietic stem cells (HSCs). However, some macrophages develop in the embryo before the appearance of definitive HSCs. We thus reinvestigated macrophage development. We found that the transcription factor Myb was required for development of HSCs and all CD11b(high) monocytes and macrophages, but was dispensable for yolk sac (YS) macrophages and for the development of YS-derived F4/80(bright) macrophages in several tissues, such as liver Kupffer cells, epidermal Langerhans cells, and microglia--cell populations that all can persist in adult mice independently of HSCs. These results define a lineage of tissue macrophages that derive from the YS and are genetically distinct from HSC progeny.
CCAAT/enhancer-binding proteins, C/EBPalpha and C/EBPbeta, are required for fat cell differentiation and maturation. Previous studies showed that replacement of C/EBPalpha with C/EBPbeta, generating the beta/beta alleles in the mouse genome, prevents lipid accumulation in white adipose tissue (WAT). In this study, beta/beta mice lived longer and had higher energy expenditure than their control littermates due to increased WAT energy oxidation. The WAT of beta/beta mice was enriched with metabolically active, thermogenic mitochondria known for energy burning. The beta/beta allele exerted its effect through the elevated expression of the G protein alpha stimulatory subunit (Galphas) in WAT. Galphas, when overexpressed in fat-laden 3T3-L1 cells, stimulated mitochondrial biogenesis similar to that seen in the WAT of beta/beta mice, and effectively diminished the stored lipid pool.
Tumor-associated macrophages and high levels of cyclooxygenase-2 (COX-2) are associated with poor prognosis in breast cancer patients, but their potential interdependence has not been evaluated. The objective of this study was to determine whether macrophages regulate COX-2 expression in breast cancer cells. For this purpose, THP-1 cells were cocultured with HCC1954 breast cancer cells. Coculture led to increased COX-2 expression in the HCC1954 cells and elevated prostaglandin E(2) levels in conditioned media. Similar results were observed when THP-1 cells were incubated with HCC1937 breast cancer cells or when human monocyte-derived macrophages were cocultured with HCC1954 cells. Coculture triggered production of reactive oxygen species (ROS) in HCC1954 cells. COX-2 induction was blocked in cells preincubated with an reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor or by silencing p67PHOX, a subunit of NADPH oxidase. ROS production triggered activation of Src and mitogen-activated protein kinases (MAPKs). Blocking Src or MAPK activities or antagonizing the activator protein-1 (AP-1) transcription factor attenuated COX-2 induction in HCC1954 cells. Coculture caused rapid induction of interleukin-1β (IL-1β) in both breast cancer cells and macrophages. Increased IL-1β expression was blocked by an interleukin-1 receptor antagonist (IL-1Ra), suggesting autocrine and paracrine effects. Importantly, macrophage-induced COX-2 expression was blocked in HCC1954 cells preincubated with IL-1Ra or anti-IL-1β IgG. Together, these results indicate that macrophage-mediated induction of COX-2 in breast cancer cells is a consequence of IL-1β-mediated stimulation of ROS→Src→MAPK→AP-1 signaling. IL-1β-dependent induction of COX-2 in breast cancer cells provides a mechanism whereby macrophages contribute to tumor progression and potential therapeutic targets in breast cancer.
Mononuclear phagocytes, including monocytes, macrophages, and dendritic cells, contribute to tissue integrity as well as to innate and adaptive immune defense. Emerging evidence for labor division indicates that manipulation of these cells could bear therapeutic potential. However, specific ontogenies of individual populations and the overall functional organization of this cellular network are not well defined. Here we report a fate-mapping study of the murine monocyte and macrophage compartment taking advantage of constitutive and conditional CX(3)CR1 promoter-driven Cre recombinase expression. We have demonstrated that major tissue-resident macrophage populations, including liver Kupffer cells and lung alveolar, splenic, and peritoneal macrophages, are established prior to birth and maintain themselves subsequently during adulthood independent of replenishment by blood monocytes. Furthermore, we have established that short-lived Ly6C(+) monocytes constitute obligatory steady-state precursors of blood-resident Ly6C(-) cells and that the abundance of Ly6C(+) blood monocytes dynamically controls the circulation lifespan of their progeny.