Atorvastatin accelerates clearance of lipoprotein remnants generated by activated brown fat to further reduce hypercholesterolemia and atherosclerosis

Sympathetic Nervous System and Atherosclerosis

Atherosclerosis is characterized by the narrowing of the arterial lumen due to subendothelial lipid accumulation, with hypercholesterolemia being a major risk factor. Despite the recent advances in effective lipid-lowering therapies, atherosclerosis remains the leading cause of mortality globally, highlighting the need for additional therapeutic strategies. Accumulating evidence suggests that the sympathetic nervous system plays an important role in atherosclerosis. In this article, we reviewed the sympathetic innervation in the vasculature, norepinephrine synthesis and metabolism, sympathetic activity measurement, and common signaling pathways of sympathetic activation. The focus of this paper was to review the effectiveness of pharmacological antagonists or agonists of adrenoceptors (α1, α2, β1, β2, and β3) and renal denervation on atherosclerosis. All five types of adrenoceptors are present in arterial blood vessels. α1 blockers inhibit atherosclerosis but increase the risk of heart failure while α2 agonism may protect against atherosclerosis and newer generations of β blockers and β3 agonists are promising therapies against atherosclerosis; however, new randomized controlled trials are warranted to investigate the effectiveness of these therapies in atherosclerosis inhibition and cardiovascular risk reduction in the future. The role of renal denervation in atherosclerosis inhibition in humans is yet to be established.

Triglyceride-Rich Lipoprotein-Mediated Polymer Dots for Multimodal Imaging Interscapular Brown Adipose Tissue Capillaries

Brown adipose tissues (BATs) have been identified as a promising target of metabolism disorders. [18F]FDG-PET (FDG = fluorodeoxyglucose; PET = positron emission tomography) has been predominantly employed for BAT imaging, but its limitations drive the urgent need for novel functional probes combined with multimodal imaging approaches. It has been reported that polymer dots (Pdots) display rapid BAT imaging without additional cold stimulation. However, the mechanism by which Pdots image BAT remains unclear. Here, we made an intensive study of the imaging mechanism and found that Pdots can bind to triglyceride-rich lipoproteins (TRLs). By virtue of their high affinity to TRLs, Pdots selectively accumulate in capillary endothelial cells (ECs) in interscapular brown adipose tissues (iBATs). Compared to poly(styrene-co-maleic anhydride)cumene terminated (PSMAC)-Pdots with a short half-life and polyethylene glycol (PEG)-Pdots with low lipophilicity, naked-Pdots have good lipophilicity, with a half-life of about 30 min and up to 94% uptake in capillary ECs within 5 min, increasing rapidly after acute cold stimulation. These results suggested that the accumulation changes of Pdots in iBAT can reflect iBAT activity sensitively. Based on this mechanism, we further developed a strategy to detect iBAT activity and quantify the TRL uptake in vivo using multimodal Pdots.

Role of thermogenic adipose tissue in lipid metabolism and atherosclerotic cardiovascular disease: lessons from studies in mice and humans

Brown adipocytes within brown adipose tissue (BAT) and beige adipocytes within white adipose tissue dissipate nutritional energy as heat. Studies in mice have shown that activation of thermogenesis in brown and beige adipocytes enhances the lipolytic processing of triglyceride-rich lipoproteins (TRLs) in plasma to supply these adipocytes with fatty acids for oxidation. This process results in formation of TRL remnants that are removed from the circulation through binding of apolipoprotein E (ApoE) on their surface to the low-density lipoprotein receptor (LDLR) on hepatocytes, followed by internalization. Concomitantly, lipolytic processing of circulating TRLs leads to generation of excess surface phospholipids that are transferred to nascent high-density lipoproteins (HDL), increasing their capacity for reverse cholesterol transport. Activation of thermogenic adipocytes thus lowers circulating triglycerides and non-HDL-cholesterol, while it increases HDL-cholesterol. The combined effect is protection from atherosclerosis development, which becomes evident in humanized mouse models with an intact ApoE-LDLR clearance pathway only, and is additive to the effects of classical lipid-lowering drugs including statins and proprotein convertase subtilisin/kexin type 9 inhibitors. A large recent study revealed that the presence of metabolically active BAT in humans is associated with lower triglycerides, higher HDL-cholesterol and lower risk of cardiovascular diseases. This narrative review aims to provide leads for further exploration of thermogenic adipose tissue as a therapeutic target. To this end, we describe the latest knowledge on the role of BAT in lipoprotein metabolism and address, for example, the discovery of the β2-adrenergic receptor as the dominant adrenergic receptor in human thermogenic adipocytes.

Mechanotransduction regulates inflammation responses of epicardial adipocytes in cardiovascular diseases

Adipose tissue is a crucial regulator in maintaining cardiovascular homeostasis by secreting various bioactive products to mediate the physiological function of the cardiovascular system. Accumulating evidence shows that adipose tissue disorders contribute to several kinds of cardiovascular disease (CVD). Furthermore, the adipose tissue would present various biological effects depending on its tissue localization and metabolic statuses, deciding the individual cardiometabolic risk. Crosstalk between adipose and myocardial tissue is involved in the pathophysiological process of arrhythmogenic right ventricular cardiomyopathy (ARVC), cardiac fibrosis, heart failure, and myocardial infarction/atherosclerosis. The abnormal distribution of adipose tissue in the heart might yield direct and/or indirect effects on cardiac function. Moreover, mechanical transduction is critical for adipocytes in differentiation, proliferation, functional maturity, and homeostasis maintenance. Therefore, understanding the features of mechanotransduction pathways in the cellular ontogeny of adipose tissue is vital for underlining the development of adipocytes involved in cardiovascular disorders, which would preliminarily contribute positive implications on a novel therapeutic invention for cardiovascular diseases. In this review, we aim to clarify the role of mechanical stress in cardiac adipocyte homeostasis and its interplay with maintaining cardiac function.

Correlation Analysis of Huayu Tongmai Decoction Intervention and Prognosis Indexes of Patients with Carotid Atherosclerosis

Background

Carotid atherosclerosis (CAS) is a common disease which seriously threatens the health of senile patients. The studies have indicated that traditional Chinese medicine (TCM) may effectively improve the symptom of CAS, while the therapeutic effect of Huayu Tongmai decoction on CAS remains unclear. Thus, this study aimed to explore the correlation between traditional Chinese medicine Huayu Tongmai decoction intervention and prognosis indexes of patients with CAS.

Methods

Ninety CAS patients admitted to Zibo TCM-Integrated Hospital from September 2018 to September 2020 were selected as the research object and randomly divided into the control group and the observation group according to the male-female ratio of 1 : 1. Patients in the control group accepted the atorvastatin intervention, and on this basis, patients in the observation group were further intervened with TCM Huayu Tongmai decoction. Before and after treatment, patients' levels of total cholesterol (TC), triglyceride (TG), and low-density lipoprotein cholesterol (LDL-C) were measured by the enzyme photometric colorimetry; hypersensitive c-reactive protein (hs-CRP) levels were measured by the ELISA method; nitric oxide (NO) levels were measured by the nitrate reductase assay and endothelin-1 (ET-1) levels were measured by radioimmunoassay; and the right and left carotid internal diameter (CAD), intima-media thickness (IMT), and plaque volume were measured by carotid ultrasonography.

Results

The TC, TG, and LDL-C levels significantly decreased in patients compared to those before intervention; compared with the control group, patients who accepted Huayu Tongmai decoction combined with atorvastatin saw more significant improvement in their blood lipid indexes (P < 0.01); after intervention, patients' hs-CRP and ET-1 levels dropped significantly while the NO level rose remarkably, and between the two groups, the improvement in levels of hs-CRP, ET-1, and NO of patients in the observation group was significantly better (P < 0.01); it was concluded from the imaging diagnosis results that compared with using atorvastatin alone, the combined intervention could better improve patients' CAD, IMT, and plaque volume.

Conclusion

Huayu Tongmai decoction can effectively improve patients' blood lipid, reduce inflammatory response, enhance levels of relevant regulatory factors of CAS, and alleviate the symptoms.

Colesevelam enhances the beneficial effects of brown fat activation on hyperlipidaemia and atherosclerosis development

Aims

Brown fat activation accelerates the uptake of cholesterol-enriched remnants by the liver and thereby lowers plasma cholesterol, consequently protecting against atherosclerosis development. Hepatic cholesterol is then converted into bile acids (BAs) that are secreted into the intestine and largely maintained within the enterohepatic circulation. We now aimed to evaluate the effects of prolonged brown fat activation combined with inhibition of intestinal BA reabsorption on plasma cholesterol metabolism and atherosclerosis development.

Methods and results

APOE*3-Leiden.CETP mice with humanized lipoprotein metabolism were treated for 9 weeks with the selective β3-adrenergic receptor (AR) agonist CL316,243 to substantially activate brown fat. Prolonged β3-AR agonism reduced faecal BA excretion (−31%), while markedly increasing plasma levels of total BAs (+258%), cholic acid-derived BAs (+295%), and chenodeoxycholic acid-derived BAs (+217%), and decreasing the expression of hepatic genes involved in BA production. In subsequent experiments, mice were additionally treated with the BA sequestrant Colesevelam to inhibit BA reabsorption. Concomitant intestinal BA sequestration increased faecal BA excretion, normalized plasma BA levels, and reduced hepatic cholesterol. Moreover, concomitant BA sequestration further reduced plasma total cholesterol (−49%) and non-high-density lipoprotein cholesterol (−56%), tended to further attenuate atherosclerotic lesion area (−54%). Concomitant BA sequestration further increased the proportion of lesion-free valves (+34%) and decreased the relative macrophage area within the lesion (−26%), thereby further increasing the plaque stability index (+44%).

Conclusion

BA sequestration prevents the marked accumulation of plasma BAs as induced by prolonged brown fat activation, thereby further improving cholesterol metabolism and reducing atherosclerosis development. These data suggest that combining brown fat activation with BA sequestration is a promising new therapeutic strategy to reduce hyperlipidaemia and cardiovascular diseases.

A simplified procedure to trace triglyceride-rich lipoprotein metabolism in vivo

Glycerol tri[³H]oleate and [¹⁴C]cholesteryl oleate double‐labeled triglyceride‐rich lipoprotein (TRL)‐like particles are a well‐established tool to trace the effect of lipid‐modulating interventions on TRL metabolism. The routine generation of these particles involves sonication of a lipid mixture and subsequent fractionation of resulting particles into populations of different average size through density gradient ultracentrifugation. Here, we describe a simplified and more time‐efficient procedure for preparing TRL‐like particles without the need of fractionation. The simplified procedure shortened the preparation of particles from over 4 h to less than 2 h and generated particles with a higher yield, although with a smaller average size and more heterogeneous size distribution. In C57Bl/6J mice housed at thermoneutrality (30°C), the two preparations showed highly comparable plasma clearance and organ distribution of glycerol tri[³H]oleate‐derived [³H]oleate and [¹⁴C]cholesteryl oleate, as measures of lipolysis and core remnant uptake, respectively. Upon a cold challenge (14°C), plasma clearance was accelerated due to enhanced uptake of glycerol tri[³H]oleate‐derived [³H]oleate by brown adipose tissue. The simplified procedure resulted in a modestly increased particle uptake by the spleen, while uptake by other organs was comparable between the two preparations. In conclusion, the simplified procedure accelerates the preparation of TRL‐like particles for tracing in vivo TRL metabolism. We anticipate that this time‐efficient procedure will be useful for incorporation of PET‐traceable lipids to obtain more insight into human lipoprotein metabolism.

Hsa_circ_0004831 downregulation is partially responsible for atorvastatinalleviated human umbilical vein endothelial cell injuries induced by ox-LDL through targeting the miR-182-5p/CXCL12 axis

BACKGROUND

The dysfunction and injury of human umbilical vein endothelial cells (HUVECs) are key events of atherosclerosis (AS). Atorvastatin (ATV) has been shown to play a protective role on endothelial cells. However, the associated molecular mechanisms remain not fully illustrated.

METHODS

HUVECs were treated with oxidized low-density lipoprotein (ox-LDL) to mimic the pathological conditions of endothelial cell injury in AS. Cell injuries were assessed according to cell viability, cell apoptosis, cycle progression, oxidative stress and inflammatory responses using CCK-8 assay, flow cytometry assay or commercial kits. The expression of hsa_circ_0004831, miR-182-5p, and C-X-C motif chemokine 12 (CXCL12) mRNA was examined using quantitative real-time PCR (qPCR). The expression of CXCL12 protein was quantitated by western blot. The predicted target relationship between miR-182-5p and hsa_circ_0004831 or CXCL12 was verified by pull-down assay, dual-luciferase reporter assay or RIP assay.

RESULTS

The expression of hsa_circ_0004831 was upregulated by ox-LDL but downregulated by ATV in HUVECs. ATV promoted cell viability and cell cycle progression but inhibited apoptosis, oxidative stress and inflammation in ox-LDL-treated HUVECs, while the role of ATV was partially reversed by hsa_circ_0004831 overexpression. MiR-182-5p was targeted by hsa_circ_0004831, and hsa_circ_0004831 overexpression-restored apoptosis, oxidative stress and inflammation were blocked by miR-182-5p restoration. Further, CXCL12 was targeted by miR-182-5p, and miR-182-5p inhibition-stimulated apoptosis, oxidative stress and inflammation were lessened by CXCL12 knockdown.

CONCLUSION

Hsa_circ_0004831-targeted miR-182-5p/CXCL12 regulatory network is one of the pathways by which ATV protects against ox-LDL-induced endothelial injuries.

Beneficial effects of brown fat activation on top of PCSK9 inhibition with alirocumab on dyslipidemia and atherosclerosis development in APOE*3-Leiden.CETP mice

Proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibition, by increasing hepatic low density lipoprotein (LDL) receptor (LDLR) levels, has emerged as a strategy to reduce atherosclerosis by lowering circulating very low density lipoprotein (VLDL)-cholesterol. We hypothesized that the therapeutic effectiveness of PCSK9 inhibition can be increased by accelerating the generation of VLDL remnants, which typically have a high affinity for the LDLR. Therefore, we aimed to investigate whether accelerating lipolytic processing of VLDL by brown fat activation can further lower (V)LDL and reduce atherosclerosis on top of PCSK9 inhibition. APOE*3-Leiden.CETP mice were fed a Western-type diet and treated with the anti-PCSK9 antibody alirocumab or saline. After 2 weeks, both groups of mice were randomized to receive either the selective β3-adrenergic receptor (AR) agonist CL316,243 to activate brown fat or saline for 3 additional weeks to evaluate VLDL clearance or 12 additional weeks to analyze atherosclerosis development. β3-AR agonism and alirocumab combined decreased (V)LDL-cholesterol compared to alirocumab alone, which was explained by an accelerated plasma clearance of VLDL-cholesteryl esters that were mainly taken up by the liver. In addition, the combination promoted the transfer of VLDL-phospholipids to HDL to a higher extent than alirocumab alone, accompanied by higher plasma HDL-cholesterol levels and increased cholesterol efflux capacity. Consequently, combination treatment largely reduced atherosclerotic lesion area compared to vehicle. Together, β3-AR agonism enhances the lipoprotein-modulating effects of alirocumab to further improve dyslipidemia and non-significantly further attenuate atherosclerosis development. Our findings demonstrate that brown fat activation may enhance the therapeutic effects of PCSK9 inhibition in dyslipidemia.

Hypolipidemic and anti-atherosclerogenic effects of aqueous extract of Ipomoea batatas leaves in diet-induced hypercholesterolemic rats

OBJECTIVE

Ipomoea batatas (L.) Lam. is a food plant used in African traditional medicine to treat cardiovascular diseases and related conditions. We assessed the hypolipidemic and anti-atherosclerogenic properties of the aqueous extract of I. batatas leaves in a rat model of diet-induced hypercholesterolemia.

METHODS

Hypercholesterolemia was induced in male Wistar rats by exclusive feeding with a cholesterol-enriched (1%) standard diet for four weeks. Then, rats were treated once daily (per os) with I. batatas extract at doses of 400, 500 and 600 mg/kg or with atorvastatin (2 mg/kg), for four weeks. Following treatment, animals were observed for another four weeks and then sacrificed. Aortas were excised and processed for histopathological studies, and blood glucose level and lipid profile were measured.

RESULTS

Hypercholesterolemic animals experienced a 21.5% faster increase in body weight, significant increases in blood glucose and blood lipids (148.94% triglycerides, 196.97% high-density lipoprotein cholesterol, 773.04% low-density lipoprotein cholesterol, 148.93% very low-density lipoprotein cholesterol and 210.42% total cholesterol), and increases in aorta thickness and atherosclerotic plaque sizes compared to rats fed standard diet. Treatment of hypercholesterolemic rats with the extract mitigated these alterations and restored blood glucose and blood lipid levels to normocholesterolemic values.

CONCLUSION

Our findings suggest that I. batatas leaves have hypolipidemic and anti-atherosclerogenic properties and justify their use in traditional medicine.

Brown adipose tissue activity is modulated in olanzapine-treated young rats by simvastatin

Background

Prescription of second-generation antipsychotic drugs (SGAs) to childhood/adolescent has exponentially increased in recent years, which was associated with the greater risk of significant weight gain and dyslipidemia. Statin is considered a potential preventive and treatment approach for reducing SGA-induced weight gain and dyslipidemia in schizophrenia patients. However, the effect of statin treatment in children and adolescents with SGA-induced dyslipidemia is not clearly demonstrated.

Methods

To investigate the efficacy of statin interventions for reversing SGA-induced dyslipidemia, young Sprague Dawley rats were treated orally with either olanzapine (1.0 mg/kg, t.i.d.), simvastatin (3.0 mg/kg, t.i.d.), olanzapine plus simvastatin (O + S), or vehicle (control) for 5 weeks.

Results

Olanzapine treatment increased weight gain, food intake and feeding efficiency compared to the control, while O + S co-treatment significantly reversed body weight gain but without significant effects on food intake. Moreover, olanzapine treatment induced a slight but significant reduction in body temperature, with a decrease in locomotor activity. Fasting plasma glucose, triglycerides (TG), and total cholesterol (TC) levels were markedly elevated in the olanzapine-only group, whereas O + S co-treatment significantly ameliorated these changes. Pronounced activation of lipogenic gene expression in the liver and down-regulated expression of uncoupling protein-1 (UCP1) and peroxisome-proliferator-activated receptor-γ co-activator-1α (PGC-1α) in brown adipose tissue (BAT) was observed in the olanzapine-only group. Interestingly, these protein changes could be reversed by co-treatment with O + B.

Conclusions

Simvastatin is effective in ameliorating TC and TG elevated by olanzapine. Modulation of BAT activity by statins could be a partial mechanism in reducing metabolic side effects caused by SGAs in child and adolescent patients.

Graphical abstract

Small rodent models of atherosclerosis

The ease of breeding, low cost of maintenance, and relatively short period for developing atherosclerosis make rodents ideal for atherosclerosis research. However, none of the current models accurately model human lipoprotein profile or atherosclerosis progression since each has its advantages and disadvantages. The advent of transgenic technologies much supports animal models' establishment. Notably, two classic transgenic mouse models, apoE-/- and Ldlr-/-, constitute the primary platforms for studying underlying mechanisms and development of pharmaceutical approaches. However, there exist crucial differences between mice and humans, such as the unhumanized lipoprotein profile, and the different plaque progression and characteristics. Among rodents, hamsters and guinea pigs might be the more realistic models in atherosclerosis research based on the similarities in lipoprotein metabolism to humans. Studies involving rat models, a rodent with natural resistance to atherosclerosis, have revealed evidence of atherosclerotic plaques under dietary induction and genetic manipulation by novel technologies, notably CRISPR-Cas9. Ldlr-/- hamster models were established in recent years with severe hyperlipidemia and atherosclerotic lesion formation, which could offer an alternative to classic transgenic mouse models. In this review, we provide an overview of classic and innovative small rodent models in atherosclerosis researches, including mice, rats, hamsters, and guinea pigs, focusing on their lipoprotein metabolism and histopathological changes.

Atorvastatin combined with routine therapy on HIF-1, VEGF concentration and cardiac function in rats with acute myocardial infarction

Effect of atorvastatin combined with routine therapy on the expression of hypoxia inducible factor (HIF-1) and vascular endothelial growth factor (VEGF) in rats with acute myocardial infarction (AMI) and its therapeutic effect were investigated. The rat models of acute myocardial infarction were established and divided into routine therapy, study, model, single drug and control group according to the treatment plan, with 10 cases in each group. Enzyme-linked immunosorbent assay (ELISA) was used to detect the concentration of HIF-1 and VEGF in serum of rats before treatment (T0), and 3 days (T1), 5 days (T2) and 7 days (T3) after treatment, and the cardiac function was measured at the same time. The concentrations of HIF-1 and VEGF in serum of the study group and the routine therapy group after treatment were significantly higher than those before treatment. The concentrations of HIF-1 and VEGF in serum of model group at T0 were significantly lower than those at T3 (P<0.05). After treatment, the concentrations of HIF-1 and VEGF in serum of the study group were significantly higher than those of the routine therapy, the model and the control group (P<0.05). One week after administration, there were significant differences in the left ventricular function among the five groups (P<0.001). The left ventricular function in the study group was better than that in the routine therapy, model and control group. The levels of HIF-1 and VEGF in the serum of rats with myocardial infarction were negatively correlated with LVIDs and LVIDd, and positively correlated with LVEF% and LVFS%. In conclusion, atorvastatin combined with routine therapy can better reduce serum HIF-1 and VEGF levels and improve the left ventricular function in rats than routine therapy.

The role of adipose tissue in cardiovascular health and disease

Accumulating knowledge on the biology and function of the adipose tissue has led to a major shift in our understanding of its role in health and disease. The adipose tissue is now recognized as a crucial regulator of cardiovascular health, mediated by the secretion of several bioactive products, including adipocytokines, microvesicles and gaseous messengers, with a wide range of endocrine and paracrine effects on the cardiovascular system. The adipose tissue function and secretome are tightly controlled by complex homeostatic mechanisms and local cell-cell interactions, which can become dysregulated in obesity. Systemic or local inflammation and insulin resistance lead to a shift in the adipose tissue secretome from anti-inflammatory and anti-atherogenic towards a pro-inflammatory and pro-atherogenic profile. Moreover, the interplay between the adipose tissue and the cardiovascular system is bidirectional, with vascular-derived and heart-derived signals directly affecting adipose tissue biology. In this Review, we summarize the current knowledge of the biology and regional variability of adipose tissue in humans, deciphering the complex molecular mechanisms controlling the crosstalk between the adipose tissue and the cardiovascular system, and their possible clinical translation. In addition, we highlight the latest developments in adipose tissue imaging for cardiovascular risk stratification and discuss how therapeutic targeting of the adipose tissue can improve prevention and treatment of cardiovascular disease.

Chronic stress: a critical risk factor for atherosclerosis

Chronic stress refers to the non-specific systemic reaction that occurs when the body is stimulated by various internal and external negative factors over a long time. The physiological response to chronic stress exposure has long been recognized as a potent modulator in the occurrence of atherosclerosis. Furthermore, research has confirmed the correlation between atherosclerosis and cardiovascular events. Chronic stress is pervasive during negative life events and may lead to the formation of plaque. Several epidemiological studies have shown that chronic stress is an independent risk factor for the development of vascular disease and for increased morbidity and mortality in patients with pre-existing coronary artery disease. One possible mechanism for this process is that chronic stress causes endothelial injury, directly activating macrophages, promoting foam cell formation and generating the formation of atherosclerotic plaque. This mechanism involves numerous variables, including inflammation, signal pathways, lipid metabolism and endothelial function. The mechanism of chronic stress in atherosclerosis should be further investigated to provide a theoretical basis for efforts to eliminate the effect of chronic stress on the cardiocerebral vascular system.

Brown Adipose Tissue Energy Metabolism in Humans

The demonstration of metabolically active brown adipose tissue (BAT) in humans primarily using positron emission tomography coupled to computed tomography (PET/CT) with the glucose tracer 18-fluorodeoxyglucose (18FDG) has renewed the interest of the scientific and medical community in the possible role of BAT as a target for the prevention and treatment of obesity and type 2 diabetes (T2D). Here, we offer a comprehensive review of BAT energy metabolism in humans. Considerable advances in methods to measure BAT energy metabolism, including nonesterified fatty acids (NEFA), chylomicron-triglycerides (TG), oxygen, Krebs cycle rate, and intracellular TG have led to very good quantification of energy substrate metabolism per volume of active BAT in vivo. These studies have also shown that intracellular TG are likely the primary energy source of BAT upon activation by cold. Current estimates of BAT's contribution to energy expenditure range at the lower end of what would be potentially clinically relevant if chronically sustained. Yet, 18FDG PET/CT remains the gold-standard defining method to quantify total BAT volume of activity, used to calculate BAT's total energy expenditure. Unfortunately, BAT glucose metabolism better reflects BAT's insulin sensitivity and blood flow. It is now clear that most glucose taken up by BAT does not fuel mitochondrial oxidative metabolism and that BAT glucose uptake can therefore be disconnected from thermogenesis. Furthermore, BAT thermogenesis is efficiently recruited upon repeated cold exposure, doubling to tripling its total oxidative capacity, with reciprocal reduction of muscle thermogenesis. Recent data suggest that total BAT volume may be much larger than the typically observed 50-150 ml with 18FDG PET/CT. Therefore, the current estimates of total BAT thermogenesis, largely relying on total BAT volume using 18FDG PET/CT, may underestimate the true contribution of BAT to total energy expenditure. Quantification of the contribution of BAT to energy expenditure begs for the development of more integrated whole body in vivo methods.

Brown adipose tissue and lipid metabolism

PURPOSE OF REVIEW

This article explores how the interplay between lipid metabolism and thermogenic adipose tissues enables proper physiological adaptation to cold environments in rodents and humans.

RECENT FINDINGS

Cold exposure triggers systemic changes in lipid metabolism, which increases fatty acid delivery to brown adipose tissue (BAT) by various routes. Next to fatty acids generated intracellularly by de-novo lipogenesis or by lipolysis at lipid droplets, brown adipocytes utilize fatty acids released by white adipose tissue (WAT) for adaptive thermogenesis. WAT-derived fatty acids are internalized directly by BAT, or indirectly after hepatic conversion to very low-density lipoproteins and acylcarnitines. In the postprandial state, chylomicrons hydrolyzed by lipoprotein lipase - activated specifically in thermogenic adipocytes - are the predominant fatty acid source. Cholesterol-enriched chylomicron remnants and HDL generated by intravascular lipolysis in BAT are cleared more rapidly by the liver, explaining the antiatherogenic effects of BAT activation. Notably, increased cholesterol flux and elevated hepatic synthesis of bile acids under cold exposure further promote BAT-dependent thermogenesis.

SUMMARY

Although pathways providing fatty acids for activated BAT have been identified, more research is needed to understand the integration of lipid metabolism in BAT, WAT and liver, and to determine the relevance of BAT for human energy metabolism.