A computational model of adipose tissue metabolism: evidence for intracellular compartmentation and differential activation of lipases

A Lifshitz-Slyozov type model for adipocyte size dynamics: limit from Becker-Döring system and numerical simulation

Biological data show that the size distribution of adipose cells follows a bimodal distribution. In this work, we introduce a Lifshitz-Slyozov type model, based on a transport partial differential equation, for the dynamics of the size distribution of adipose cells. We prove a new convergence result from the related Becker-Döring model, a system composed of several ordinary differential equations, toward mild solutions of the Lifshitz-Slyozov model using distribution tail techniques. Then, this result allows us to propose a new advective-diffusive model, the second-order diffusive Lifshitz-Slyozov model, which is expected to better fit the experimental data. Numerical simulations of the solutions to the diffusive Lifshitz-Slyozov model are performed using a well-balanced scheme and compared to solutions to the transport model. Those simulations show that both bimodal and unimodal profiles can be reached asymptotically depending on several parameters. We put in evidence that the asymptotic profile for the second-order system does not depend on initial conditions, unlike for the transport Lifshitz-Slyozov model.

The Potential Roles of Transacylation in Intracellular Lipolysis and Related Qssa Approximations

Fatty acids (FAs) are crucial energy metabolites, signalling molecules, and membrane building blocks for a wide range of organisms. Adipose triglyceride lipase (ATGL) is the first and presumingly most crucial regulator of FA release from triacylglycerols (TGs) stored within cytosolic lipid droplets. However, besides the function of releasing FAs by hydrolysing TGs into diacylglycerols (DGs), ATGL also promotes the transacylation reaction of two DG molecules into one TG and one monoacylglycerol molecule. To date, it is unknown whether DG transacylation is a coincidental byproduct of ATGL-mediated lipolysis or whether it is physiologically relevant. Experimental evidence is scarce since both, hydrolysis and transacylation, rely on the same active site of ATGL and always occur in parallel in an ensemble of molecules. This paper illustrates the potential roles of transacylation. It shows that, depending on the kinetic parameters but also on the state of the hydrolytic machinery, transacylation can increase or decrease downstream products up to 80% respectively 30%. We provide an extensive asymptotic analysis including quasi-steady-state approximations (QSSA) with higher order correction terms and provide numerical simulation. We also argue that when assessing the validity of QSSAs one should include parameter sensitivity derivatives. Our results suggest that the transacylation function of ATGL is of biological relevance by providing feedback options and altogether stability to the lipolytic machinery in adipocytes.

A systems biology analysis of lipolysis and fatty acid release from adipocytes in vitro and from adipose tissue in vivo

Lipolysis and the release of fatty acids to supply energy fuel to other organs, such as between meals, during exercise, and starvation, are fundamental functions of the adipose tissue. The intracellular lipolytic pathway in adipocytes is activated by adrenaline and noradrenaline, and inhibited by insulin. Circulating fatty acids are elevated in type 2 diabetic individuals. The mechanisms behind this elevation are not fully known, and to increase the knowledge a link between the systemic circulation and intracellular lipolysis is key. However, data on lipolysis and knowledge from in vitro systems have not been linked to corresponding in vivo data and knowledge in vivo. Here, we use mathematical modelling to provide such a link. We examine mechanisms of insulin action by combining in vivo and in vitro data into an integrated mathematical model that can explain all data. Furthermore, the model can describe independent data not used for training the model. We show the usefulness of the model by simulating new and more challenging experimental setups in silico, e.g. the extracellular concentration of fatty acids during an insulin clamp, and the difference in such simulations between individuals with and without type 2 diabetes. Our work provides a new platform for model-based analysis of adipose tissue lipolysis, under both non-diabetic and type 2 diabetic conditions.

Boundary conditions at a thin membrane that generate non-Markovian normal diffusion

We show that some boundary conditions assumed at a thin membrane may result in normal diffusion not being the stochastic Markov process. We consider boundary conditions defined in terms of the Laplace transform in which there is a linear combination of probabilities and probability fluxes defined on both membrane surfaces. The coefficients of the combination may depend on the Laplace transform parameter. Such boundary conditions are most commonly used when considering diffusion in a membrane system unless collective or nonlocal processes in particles diffusion occur. We find Bachelier-Smoluchowski-Chapmann-Kolmogorov (BSCK) equation in terms of the Laplace transform and we derive the criterion to check whether the boundary conditions lead to fundamental solutions of diffusion equation satisfying this equation. If the BSCK equation is not met, then the Markov property is broken. When a probability flux is continuous at the membrane, the general forms of the boundary conditions for which the fundamental solutions meet the BSCK equation are derived. A measure of broken of semi-group property is also proposed. The relation of this measure to the non-Markovian property measure is discussed.

A computational model of postprandial adipose tissue lipid metabolism derived using human arteriovenous stable isotope tracer data

Given the association of disturbances in non-esterified fatty acid (NEFA) metabolism with the development of Type 2 Diabetes and Non-Alcoholic Fatty Liver Disease, computational models of glucose-insulin dynamics have been extended to account for the interplay with NEFA. In this study, we use arteriovenous measurement across the subcutaneous adipose tissue during a mixed meal challenge test to evaluate the performance and underlying assumptions of three existing models of adipose tissue metabolism and construct a new, refined model of adipose tissue metabolism. Our model introduces new terms, explicitly accounting for the conversion of glucose to glyceraldehye-3-phosphate, the postprandial influx of glycerol into the adipose tissue, and several physiologically relevant delays in insulin signalling in order to better describe the measured adipose tissues fluxes. We then applied our refined model to human adipose tissue flux data collected before and after a diet intervention as part of the Yoyo study, to quantify the effects of caloric restriction on postprandial adipose tissue metabolism. Significant increases were observed in the model parameters describing the rate of uptake and release of both glycerol and NEFA. Additionally, decreases in the model's delay in insulin signalling parameters indicates there is an improvement in adipose tissue insulin sensitivity following caloric restriction.

Model of anomalous diffusion-absorption process in a system consisting of two different media separated by a thin membrane

We present the model of a diffusion-absorption process in a system which consists of two media separated by a thin partially permeable membrane. The kind of diffusion as well as the parameters of the process may be different in both media. Based on a simple model of a particle's random walk in a membrane system we derive the Green's functions, then we find the boundary conditions at the membrane. One of the boundary conditions is rather complicated and takes a relatively simple form in terms of the Laplace transform. Assuming that particles diffuse independently of one another, the obtained boundary conditions can be used to solve differential or differential-integral equations describing the processes in multilayered systems for any initial condition. We consider normal diffusion, subdiffusion, and slow subdiffusion processes, and we also suggest how superdiffusion could be included in this model. The presented method provides the functions in terms of the Laplace transform and some useful methods of calculation of the inverse Laplace transform are shown.

Obesity and the dysregulation of fatty acid metabolism: implications for healthy aging

The population of the world is aging. In 2010, an estimated 524 million people were aged 65 years or older representing eight percent of the global population. By 2050, this number is expected to nearly triple to approximately 1.5 billion, 16 percent of the world's population. Although people are living longer, the quality of their lives are often compromised due to ill-health. Areas covered: Of the conditions which compromise health as we age, obesity is at the forefront. Over half of the global older population were overweight or obese in 2010, significantly increasing the risk of a range of metabolic diseases. Although, it is well recognised excessive calorie intake is a fundamental driver of adipose tissue dysfunction, the relationship between obesity; intrinsic aging; and fat metabolism is less understood. In this review we discuss the intersection between obesity, aging and the factors which contribute to the dysregulation of whole-body fat metabolism. Expert commentary: Being obese disrupts an array of physiological systems and there is significant crosstalk among these. Moreover it is imperative to acknowledge the contribution intrinsic aging makes to the dysregulation of these systems and the onset of disease.

The nuclear retinoid-related orphan receptor-α regulates adipose tissue glyceroneogenesis in addition to hepatic gluconeogenesis

Circadian rhythms have an essential role in feeding behavior and metabolism. RORα is a nuclear receptor involved in the interface of the circadian system and metabolism. The adipocyte glyceroneogenesis pathway derives free fatty acids (FFA) liberated by lipolysis to reesterification into triglycerides, thus regulating FFA homeostasis and fat mass. Glyceroneogenesis shares with hepatic gluconeogenesis the key enzyme phosphoenolpyruvate carboxykinase c (PEPCKc), whose gene is a RORα target in the liver. RORα-deficient mice (staggerer, ROR(sg/sg)) have been shown to exhibit a lean phenotype and fasting hypoglycemia for unsolved reasons. In the present study, we investigated whether adipocyte glyceroneogenesis might also be a target pathway of RORα, and we further evaluated the role of RORα in hepatocyte gluconeogenesis. In vivo investigations comparing ROR(sg/sg) mice with their wild-type (WT) littermates under fasting conditions demonstrated that, in the absence of RORα, the release of FFA into the bloodstream was altered and the rise in glycemia in response to pyruvate reduced. The functional analysis of each pathway, performed in adipose tissue or liver explants, confirmed the impairment of adipocyte glyceroneogenesis and liver gluconeogenesis in the ROR(sg/sg) mice; these reductions of FFA reesterification or glucose production were associated with decreases in PEPCKc mRNA and protein levels. Treatment of explants with RORα agonist or antagonist enhanced or inhibited these pathways, respectively, in tissues isolated from WT but not ROR(sg/sg) mice. Our results indicated that both adipocyte glyceroneogenesis and hepatocyte gluconeogenesis were regulated by RORα. This study demonstrates the physiological function of RORα in regulating both glucose and FFA homeostasis.

Systems biology of adipose tissue metabolism: regulation of growth, signaling and inflammation

Adipose tissue (AT) depots actively regulate whole body energy homeostasis by orchestrating complex communications with other physiological systems as well as within the tissue. Adipocytes readily respond to hormonal and nutritional inputs to store excess nutrients as intracellular lipids or mobilize the stored fat for utilization. Co-ordinated regulation of metabolic pathways balancing uptake, esterification, and hydrolysis of lipids is accomplished through positive and negative feedback interactions of regulatory hubs comprising several pleiotropic protein kinases and nuclear receptors. Metabolic regulation in adipocytes encompasses biogenesis and remodeling of uniquely large lipid droplets (LDs). The regulatory hubs also function as energy and nutrient sensors, and integrate metabolic regulation with intercellular signaling. Over-nutrition causes hypertrophic expansion of adipocytes, which, through incompletely understood mechanisms, initiates a cascade of metabolic and signaling events leading to tissue remodeling and immune cell recruitment. Macrophage activation and polarization toward a pro-inflammatory phenotype drives a self-reinforcing cycle of pro-inflammatory signals in the AT, establishing an inflammatory state. Sustained inflammation accelerates lipolysis and elevates free fatty acids in circulation, which robustly correlates with development of obesity-related diseases. The adipose regulatory network coupling metabolism, growth, and signaling of multiple cell types is exceedingly complex. While components of the regulatory network have been individually studied in exquisite detail, systems approaches have rarely been utilized to comprehensively assess the relative engagements of the components. Thus, need and opportunity exist to develop quantitative models of metabolic and signaling networks to achieve a more complete understanding of AT biology in both health and disease.

Effects of green tea extracts on non-shivering thermogenesis during mild cold exposure in young men

The effects of epigallocatechin-3-gallate (EGCG) and caffeine on non-shivering thermogenesis (NST) during cold exposure is unknown. The purpose of the present study was to quantify the effects of co-ingesting EGCG and caffeine on the thermogenic responses of a 3 h cold exposure. A total of eight healthy males were exposed to mild cold, using a liquid-conditioned suit perfused with 158C water, on two occasions and consumed a placebo or an extract of 1600 mg of EGCG and 600 mg of caffeine (Green tea). Thermic, metabolic and electromyographic measurements were monitored at baseline and during the cold exposure. Results showed that the AUC of shivering intensity over the cold exposure period was reduced by approximately 20% in the Green tea (266 (SEM 6)% maximal voluntary contraction (MVC) x min) compared with the Placebo (332 (SEM 69)%MVC x min) (P=0·01) treatments. In contrast, the total AUC for energy expenditure (EE) was approximately 10% higher in the Green tea (23·5 (SEM 1·4) kJ/kg x 180 min) compared with the Placebo (327 (SEM 74) kJ/kg 180 min) (P=0·007) treatments. The decrease in shivering activity combined with an increase in EE, following the ingestion of EGCG and caffeine during the cold exposure, indicates that NST pathways can be significantly stimulated in adult human subjects. The present study provides an experimental approach for human investigations into the potential role of diet and bioactive food ingredients in modulating NST during cold exposure. Stimulating NST pathways in such a manner may also provide important targets in the search of targets for the management of obesity and diabetes.

Protocol for the measurement of fatty acid and glycerol turnover in vivo in baboons

Recognition of the strength of nonhuman primate models in investigating metabolic disorders has resulted in an expanded need for in vivo research techniques. We studied adipose metabolism in 10 baboons (13.0 ± 4.2 years old, 29.5 ± 5.5 kg). Part 1 evaluated the effect of different sedatives on the rate of appearance of plasma free fatty acids (RaFFA), assessed using ¹³C₄-labeled palmitate infusion (7 µmol/kg/min). Animals, were studied with no sedation, with complete isoflurane sedation, and with minimal midazolam infusion (0.04 mg/kg/h), with the last scheme allowing for the most consistent values and animals that were visually more calm. In Part 2, RaFFA and RaGlycerol (D₅-glycerol, 5 mg/kg lean body mass/h) were measured. From midnight to 0300, flux fell and came to a steady state between 0500 and 0700 h (RaFFA, 39.4 ± 29.8 μmol/kg fat mass/min; and RaGlycerol, 26.9 ± 7.3 μmol/kg/min). The RaFFA-to-RaGlycerol ratio was 1.5 ± 0.8 (49% reesterification). The decline in turnover throughout the night reflects natural circadian processes and was mirrored by reductions in FFA and glycerol to 0.62 and ± 0.14 and 0.16 and ± 0.03 mmol/l, respectively. The concurrent changes in both FFA and glycerol kinetics indicate physiologic validity of the method. These techniques will support needed research to determine mechanisms by which treatments act upon the adipocyte in vivo.

Adipose tissue engineering for soft tissue regeneration

Current treatment modalities for soft tissue defects caused by various pathologies and trauma include autologous grafting and commercially available fillers. However, these treatment methods present a number of challenges and limitations, such as donor-site morbidity and volume loss over time. As such, improved therapeutic modalities need to be developed. Tissue engineering techniques offer novel solutions to these problems through development of bioactive tissue constructs that can regenerate adipose tissue in both structure and function. Recently, a number of studies have been designed to explore various methods to engineer human adipose tissue. This review will focus on these developments in the area of adipose tissue engineering for soft tissue replacement. The physiology of adipose tissue and current surgical therapies used to replace lost tissue volume, specifically in breast tissue, are introduced, and current biomaterials, cell sources, and tissue culture strategies are discussed. We discuss future areas of study in adipose tissue engineering.

Regulation of Adipose Tissue Metabolism in Humans: Analysis of Responses to the Hyperinsulinemic-Euglycemic Clamp Experiment

The suppression of lipolysis is one of the key metabolic responses of the adipose tissue during hyperinsulinemia. The failure to respond and resulting increase in plasma fatty acids could contribute to the development of insulin resistance and perturbations in the fuel homeostasis in the whole body. In this study, a mechanistic, computational model of adipose tissue metabolism in vivo has been enhanced to simulate the physiological responses during hyperinsulinemic-euglycemic clamp experiment in humans. The model incorporates metabolic intermediates and pathways that are important in the fed state. In addition, it takes into account the heterogeneity of triose phosphate pools (glycolytic vs. glyceroneogenic), within the adipose tissue. The model can simulate not only steady-state responses at different insulin levels, but also concentration dynamics of major metabolites in the adipose tissue venous blood in accord with the in vivo data. Simulations indicate that (1) regulation of lipoprotein lipase (LPL) reaction is important when the intracellular lipolysis is suppressed by insulin; (2) intracellular diglyceride levels can affect the regulatory mechanisms; and (3) glyceroneogenesis is the dominant pathway for glycerol-3-phosphate synthesis even in the presence of increased glucose uptake by the adipose tissue. Reduced redox and increased phosphorylation states provide a favorable milieu for glyceroneogenesis in response to insulin. A parameter sensitivity analysis predicts that insulin-stimulated glucose uptake would be more severely affected by impairment of GLUT4 translocation and glycolysis than by impairment of glycogen synthesis and pyruvate oxidation. Finally, simulations predict metabolic responses to altered expression of phosphoenolpyruvate carboxykinase (PEP-CK). Specifically, the increase in the rate of re-esterification of fatty acids observed experimentally with the overexpression of PEPCK in the adipose tissue would be accompanied by the up-regulation of acyl Co-A synthase.

Adipogenic differentiation of human adipose-derived stem cells on 3D silk scaffolds

Current treatment modalities for soft tissue defects due to various pathologies and trauma include autologous grafting and the use of commercially available fillers. However, these treatment methods are associated with a number of limitations, such as donor site morbidity and volume loss over time. As such, improved therapeutic options are needed. Tissue engineering techniques offer novel solutions to these problems through development of bioactive tissue constructs that can regenerate adipose tissue with an appropriate structure and function. The recent advances in the derivation and characterization of hASCs have led to numerous studies of soft tissue reconstruction. In this chapter, we discuss methods in which our laboratory has used hASCs and silk scaffolds for adipose tissue engineering. The use of naturally occurring and clinically acceptable materials such as silk protein for tissue-engineering applications poses advantages with respect to biocompatibility and mechanical and biological properties.

Confidence from uncertainty--a multi-target drug screening method from robust control theory

Background

Robustness is a recognized feature of biological systems that evolved as a defence to environmental variability. Complex diseases such as diabetes, cancer, bacterial and viral infections, exploit the same mechanisms that allow for robust behaviour in healthy conditions to ensure their own continuance. Single drug therapies, while generally potent regulators of their specific protein/gene targets, often fail to counter the robustness of the disease in question. Multi-drug therapies offer a powerful means to restore disrupted biological networks, by targeting the subsystem of interest while preventing the diseased network from reconciling through available, redundant mechanisms. Modelling techniques are needed to manage the high number of combinatorial possibilities arising in multi-drug therapeutic design, and identify synergistic targets that are robust to system uncertainty.

Results

We present the application of a method from robust control theory, Structured Singular Value or μ- analysis, to identify highly effective multi-drug therapies by using robustness in the face of uncertainty as a new means of target discrimination. We illustrate the method by means of a case study of a negative feedback network motif subject to parametric uncertainty.

Conclusions

The paper contributes to the development of effective methods for drug screening in the context of network modelling affected by parametric uncertainty. The results have wide applicability for the analysis of different sources of uncertainty like noise experienced in the data, neglected dynamics, or intrinsic biological variability.

Robust signaling networks of the adipose secretome

Type 2 diabetes is a prototypical complex systems disease that has a strong hereditary component and etiologic links with a sedentary lifestyle, overeating and obesity. Adipose tissue has been shown to be a central driver of type 2 diabetes progression, establishing and maintaining a chronic state of low-level inflammation. The number and diversity of identified endocrine factors from adipose tissue (adipokines) is growing rapidly. Here, I argue that a systems biology approach to understanding the robust multi-level signaling networks established by the adipose secretome will be crucial for developing efficient type 2 diabetes treatment. Recent advances in whole-genome association studies, global molecular profiling and quantitative modeling are currently fueling the emergence of this novel research strategy.

Reassessing triglyceride synthesis in adipose tissue

The synthesis and breakdown of triglycerides in adipose tissue and muscle is a crucial element of energy metabolism because it ensures that adequate fuel is available during starvation. Triglyceride turnover determines the availability of fatty acids for utilization by mammalian tissues, and any dysfunction in this process can lead to alterations in glucose metabolism, insulin resistance and type 2 diabetes. Our understanding of the reactions involved in triglyceride synthesis is currently being reassessed, primarily because of the recently identified role that re-esterification of fatty acids plays in triglyceride deposition and, thus, in controlling fatty-acid availability. Here, we review recent information on triglyceride synthesis and introduce the pathway of glyceroneogenesis as an important and highly regulated source of glyceride-glycerol in adipose tissue.