The hydrolipidic ratio in age-related maturation of adipose tissues

Opportunities and challenges in three-dimensional brown adipogenesis of stem cells

The formation of brown adipose tissue (BAT) via brown adipogenesis has become a notable process due to its ability to expend energy as heat with implications in the treatment of metabolic disorders and obesity. With the advent of complexity within white adipose tissue (WAT) along with inducible brown adipocytes (also known as brite and beige), there has been a surge in deciphering adipocyte biology as well as in vivo adipogenic microenvironments. A therapeutic outcome would benefit from understanding early events in brown adipogenesis, which can be accomplished by studying cellular differentiation. Pluripotent stem cells are an efficient model for differentiation and have been directed towards both white adipogenic and brown adipogenic lineages. The stem cell microenvironment greatly contributes to terminal cell fate and as such, has been mimicked extensively by various polymers including those that can form 3D hydrogel constructs capable of biochemical and/or mechanical modifications and modulations. Using bioengineering approaches towards the creation of 3D cell culture arrangements is more beneficial than traditional 2D culture in that it better recapitulates the native tissue biochemically and biomechanically. In addition, such an approach could potentially protect the tissue formed from necrosis and allow for more efficient implantation. In this review, we highlight the promise of brown adipocytes with a focus on brown adipogenic differentiation of stem cells using bioengineering approaches, along with potential challenges and opportunities that arise when considering the energy expenditure of BAT for prospective therapeutics.

Molecular imaging of brown adipose tissue in health and disease

PURPOSE

Brown adipose tissue (BAT) has transformed from an interfering tissue in oncological (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to an independent imaging research field. This review takes the perspective from the imaging methodology on which human BAT research has come to rely on heavily.

METHODS

This review analyses relevant PubMed-indexed publications that discuss molecular imaging methods of BAT. In addition, reported links between BAT and human diseases such as obesity are discussed, and the possibilities for imaging in these fields are highlighted. Radiopharmaceuticals aiming at several different biological mechanisms of BAT are discussed and evaluated.

RESULTS

Prospective, dedicated studies allow visualization of BAT function in a high percentage of human subjects. BAT dysfunction has been implicated in obesity, linked with diabetes and associated with cachexia and atherosclerosis. Presently, (18)F-FDG PET/CT is the most useful tool for evaluating therapies aiming at BAT activity. In addition to (18)F-FDG, other radiopharmaceuticals such as (99m)Tc-sestamibi, (123)I-metaiodobenzylguanidine (MIBG), (18)F-fluorodopa and (18)F-14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid (FTHA) may have a potential for visualizing other aspects of BAT activity. MRI methods are under continuous development and provide the prospect of functional imaging without ionizing radiation.

CONCLUSION

Molecular imaging of BAT can be used to quantitatively assess different aspects of BAT metabolic activity.

Regulation of brown adipose tissue development and white fat reduction by L-arginine

PURPOSE OF REVIEW

Brown adipose tissue (BAT), which is present in humans, plays an important role in oxidation of fatty acids and glucose. The purpose of this review is to highlight an important role for L-arginine in regulating BAT growth and development, thereby reducing obesity in mammals.

RECENT FINDINGS

Dietary supplementation with L-arginine reduces white adipose tissue in genetically or diet-induced obese rats, obese pregnant sheep, and obese humans with type II diabetes. L-arginine treatment enhances BAT growth in both fetuses and postnatal animals. At molecular and cellular levels, L-arginine stimulates expression of peroxisome proliferator-activated receptor-γ coactivator 1 (the master regulator of mitochondrial biogenesis), nitric oxide synthase, heme oxygenase, and adenosine monophosphate-activated protein kinase. At the whole body level, L-arginine increases blood flow to insulin-sensitive tissues, adipose tissue lipolysis, and the catabolism of glucose and fatty acids, but inhibits fatty acid synthesis and ameliorates oxidative stress, thereby improving metabolic profile.

SUMMARY

L-arginine increases mammalian BAT growth and development via mechanisms involving gene expression, nitric oxide signaling, and protein synthesis. This enhances the oxidation of energy substrates and, thus, reduces white fat accretion in the body. L-arginine holds great promise in preventing and treating obesity in humans.

Ambient particulate air pollution induces oxidative stress and alterations of mitochondria and gene expression in brown and white adipose tissues

Background

Prior studies have demonstrated a link between air pollution and metabolic diseases such as type II diabetes. Changes in adipose tissue and its mitochondrial content/function are closely associated with the development of insulin resistance and attendant metabolic complications. We investigated changes in adipose tissue structure and function in brown and white adipose depots in response to chronic ambient air pollutant exposure in a rodent model.

Methods

Male ApoE knockout (ApoE^-/-) mice inhaled concentrated fine ambient PM (PM < 2.5 μm in aerodynamic diameter; PM_2.5) or filtered air (FA) for 6 hours/day, 5 days/week, for 2 months. We examined superoxide production by dihydroethidium staining; inflammatory responses by immunohistochemistry; and changes in white and brown adipocyte-specific gene profiles by real-time PCR and mitochondria by transmission electron microscopy in response to PM_2.5 exposure in different adipose depots of ApoE^-/- mice to understand responses to chronic inhalational stimuli.

Results

Exposure to PM_2.5 induced an increase in the production of reactive oxygen species (ROS) in brown adipose depots. Additionally, exposure to PM_2.5 decreased expression of uncoupling protein 1 in brown adipose tissue as measured by immunohistochemistry and Western blot. Mitochondrial number was significantly reduced in white (WAT) and brown adipose tissues (BAT), while mitochondrial size was also reduced in BAT. In BAT, PM_2.5 exposure down-regulated brown adipocyte-specific genes, while white adipocyte-specific genes were differentially up-regulated.

Conclusions

PM_2.5 exposure triggers oxidative stress in BAT, and results in key alterations in mitochondrial gene expression and mitochondrial alterations that are pronounced in BAT. We postulate that exposure to PM_2.5 may induce imbalance between white and brown adipose tissue functionality and thereby predispose to metabolic dysfunction.

In vivo brown adipose tissue detection and characterization using water-lipid intermolecular zero-quantum coherences

Brown adipose tissue and white adipose tissue depots are noninvasively characterized in vitro and in vivo in healthy and obese mice using intermolecular zero-quantum coherence transitions between lipid and water spins. Intermolecular zero-quantum coherences enable selective detection of spatial correlation between water and lipid spins and thereby the hydration of fatty deposits with subvoxel resolution. At about a 100 mm distance scale, the major observed peaks are between water, methylene protons at 1.3 ppm, and olefinic protons at 5.3 ppm. Our in vitro results show that the methylene-olefinic intermolecular zero-quantum coherence signal is strong both in brown and white adipose tissues, but that the water-methylene intermolecular zero-quantum coherence signal is characteristic only of brown adipose tissue. In vivo, the ratio of these peaks is substantially higher in lean or young mice than in old or obese mice.