Greetings from the Academic Medical Center of Amsterdam (a short presentation)

AMC_Front

Hi everybody! First of all, thanks for reading! Let’s use this first post as a short presentation. My name is Rubén Zapata Pérez and I am a Molecular Biologist. Since I was studying at High School, the scientific career has been my professional goal. Once at University, I decided to become a Doctor, since I wanted to continue my career as a researcher. Now that I am a PhD, and thanks to Fundación Séneca, I am going to develop a postdoctoral project at the Academic Medical Center (AMC) of Amsterdam, under the supervision of Dr. Riekelt Houtkooper (www.facebook.com/Houtkooperlab), the head of the Translational Metabolism group.

In this blog, I will discuss my technical and scientific progress, including descriptions and tutorials of different techniques, as well as announcements of upcoming publications (luckily!) and other activities. However, this blog will not only be focused on technical issues, but also will talk about my experience as a fSéneca fellow, and how scientists develop their careers in a foreign country, in this case The Netherlands.

Tot ziens!

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Exercise training and adipose tissue browning

Gym

In previous posts we have learnt, in a general way, about the different approaches available to either activate our brown fat depots or promote the conversion of white adipocytes into brownish (beige) ones, a process called browning. Among these approaches, we slightly introduced exercise training as a browning enhancer. However, a recently published review shows us that there is still a long way to go before we can completely understand how exercise exerts its effects on adipose tissue. Thus, in this post I will talk about the current knowledge on adipose tissue browning through exercise training.

“Regular exercise training decreases brown adipose tissue content and activity but promotes white adipose tissue browning”

Very little is known about how exercise modulates the switch from white (no expression of uncoupling protein 1 or UCP1) to brown phenotype (expression of UCP1) and most of the evidence available comes from mouse experiments. What is known is that physical activity has several benefits on adipose tissue, such as a reduction in adipocyte size, improvements in adipose tissue inflammation, vascularisation and mitochondrial health and biogenesis. Moreover, recent findings suggest that some factors produced during exercise by the muscle or adipose tissues induce the browning of WAT in an endocrine or paracrine manner. However (and in contrast with what we may expect), regular exercise leads to a reduction in BAT content, and a reduced thermogenic capacity (both in rodents and humans). This can be explained by the increase in core body temperature due to exercise, which makes thermogenesis through BAT activity unnecessary.

Adipocyte Types

Different types of adipocytes

At this point we could think, why does exercise reduces BAT content but, in contrast, induces browning of WAT? It has been suggested that, since regular exercise leads to a reduction in total fat mass (especially in the subcutaneous WAT that surrounds us), the insulation of the body is reduced, and subcutaneous WAT depots must adapt (by generating beige adipocytes) to increase their thermogenic potential. However, although BAT mass reduction and activity with exercise has been studied both in rodents and humans, exercise induced WAT browning has only been detected in mice, and better designed human studies are needed.

Despite of the lack of human studies, the browning of WAT in rodents exposed to regular exercise is a reality. The next question would be, why is this happening? There is a growing consensus that the browning of adipose tissues occurs as an adaptive mechanism to stress (apart from cold exposure, of course). It is known that regular physical exercise induces physiological stressors, such as increased catecholamine (adrenaline and noradrenaline) secretion, increased free radicals or increased secretion of natriuretic peptides from the heart. All these stressors evoke adaptive responses in adipose tissues, promoting the browning process.

Mouse Bike

Finally, it is thought that the browning of WAT could also be related to the process of vascularisation (increase in the number of blood vessels in a tissue) due to regular exercise training. Vascularisation increases tissue oxygenation, blood flow and nutrient delivery, and is one of the mechanisms through which exercise improves health. Since beige adipocytes were shown to share origins with the smooth muscle cells that surround blood vessels (vascular smooth muscle), the increase in vascularisation through exercise training could partly explain the browning process.

As a summary, available data show that exercise training clearly induces the browning of white adipose tissue, at least in rodents, and leads to a reduced content and activity of brown adipose tissue both in mice and humans. However, a valid physiological explanation for this process is yet to be discovered. In any case, the beneficial effects of regular physical activity on adiposity and cardiometabolic health are well-known so, take care of your body and your body will take care of you!

 

Understanding heat production by brown adipose tissue

Heat

As discussed in previous posts, the main role for brown adipose tissue (BAT) is heat generation by means of glucose and fatty acid consumption, thus increasing overall energy expenditure. This increase in metabolic oxidative capacity is the reason why BAT has attracted the attention as a possible therapeutic target for combating obesity, diabetes and other metabolic diseases. But what is the mechanism underlying this heat generation? What happens in brown adipocytes when they are activated by cold? Let’s get scientific for a moment!

The cellular signalling cascade activated by cold exposure

As we know, the physiological (natural) trigger of BAT activation is cold. Upon cold exposure, BAT starts producing heat to maintain body temperature. This heat production relies on the activity of the uncoupling protein 1 (UCP1). But to understand the role of this protein we first have to know what happens to BAT metabolism when this tissue is activated.

When exposed to cold, our sympathetic nervous system induces the release of norepinephrine (noradrenaline) by sympathetic neurons, which is a hormone able to bind to the β3-noradrenergic receptors (β-AR) present on the surface of brown adipocytes. When norepinephrine binds to these receptors, a molecule called cyclic adenosine monophosphate (cAMP) is synthesized by the enzyme adenylyl cyclase. This cAMP activates then another enzyme called protein kinase A (PKA), which main role is to activate or inhibit other proteins through phosphorylation. One of the targets of PKA that is important to us is the nuclear transcription factor CREB. When CREB is phosphorylated by PKA it is able to activate the expression of the UCP1 gene, thus increasing the content of this protein.

Figure1_Post

Signalling cascade activated by norepinephrine in brown adipocytes (modified from Fenzl and Kiefer, 2014)

Apart from phosphorylating CREB, PKA can also promote the release of fatty acids from the triglyceride droplets. This is done by the inactivation of perilipin, which is protecting the lipid droplets from the attack by the hormone-sensitive lipase (HSL). Thus, through PKA phosphorylation, HSL is activated and, thanks to the inactivation of perilipin (also by PKA), it can attack the freely exposed triglycerides, releasing fatty acids.

Energy production in the cell

Hitherto, β3-noradrenergic receptors activation by norepinephrine has led to the release of fatty acids from the lipid droplets, and also to an increase in UCP1 expression. Fatty acids are now transferred into the mitochondria and subjected to the process of β-oxidation. In this process, fatty acid molecules are broken down to generate acetyl-CoA and the cofactors NADH and FADH2. These cofactors are very important for energy generation in the cell, since they are used in the electron transport chain.

Electron transport chain

Electron transport chain in the mitochondrial inner membrane. The proton gradient generated between the mitochondrial matrix and the intermembrane space is used by the ATP synthase to produce ATP (OpenStax College/Wikimedia Commons)

The electron transport chain is composed by a series of complexes in the inner membrane of the mitochondrion. These complexes have the ability to transfer electrons from electron donors (the cofactors NADH and FADH2 already mentioned) to electron acceptors in several steps. This electron transference is coupled to the transfer of protons (H+) by the complexes across the membrane. Thus, protons go from the mitochondrial matrix to the intermembrane space, since mitochondria have two membranes. This proton “pumping” outside the matrix generates a gradient that drives the synthesis of adenosine triphosphate (ATP) by the ATP synthase complex. This ATP is a molecule that stores chemical energy, and is considered the “energy currency of the cell”.

The pathway described above is not exclusive for brown adipocytes, since other cell types also have to produce their own energy using the electron transport chain. However, brown adipocytes have a very important difference in comparison with other cells, and that is the presence of high amounts of the uncoupling protein 1 (UCP1) in their mitochondria. So, is the presence of this UCP1 responsible for heat generation in brown adipocytes? The answer is, yes!

The uncoupling protein 1

UCP1, also known as thermogenin, is a member of the mitochondrial carrier protein family. It is generally accepted that this protein is activated by fatty acids, which makes sense if we remember that BAT activation implies their release from triglycerides. But, why is UCP1 function so important for heat generation? If we take a look to the electron transport chain, it is clear that the proton gradient generated during the electron transfer is used by the ATP synthase for ATP production. However, when UCP1 is present, this protein is able to dissipate the gradient by “pumping” protons again into the mitochondrial matrix. When this happens, the ATP synthase complex reaction is lowered, and the energy generated during the electron transference is dissipated as heat instead of being used for ATP synthesis.

ucp1

Uncoupling protein 1 function. Protons are transfered again from the intermembrane space to the mitochondrial matrix, preventing ATP production by the ATP synthetase

Now we have a better understanding on what the mechanism behind heat generation by brown adipose tissue is. We have also learnt the changes that occur upon cold exposure in this particular tissue. So, from now, everytime you feel cold remember that, if you are not shivering, it is because your brown adipocytes are undergoing all these processes that you now know!

Bibliography:

Cannon B. and Nedergaard J. 2004. Brown adipose tissue: function and physiological significance. Physiol Rev. 84: 277-359

Fenzl A. and Kiefer F.W. 2014. Brown adipose tissue and thermogenesis. Horm Mol Biol Clin Invest. 19(1): 25-37

 

Dietary polyphenols and brown adipose tissue activation

berries-1514741225-621x354

If you already read my previous post, you should know that there are many different ways of activating our brown adipose tissue (BAT) depots, and thus increase our overall energy expenditure or EE (remember that BAT converts energy into heat to help body thermoregulation). This increase in EE has arisen as a novel target for controlling body weight, as an alternative to the obvious strategy of restricting caloric intake. Among the different approaches discussed before, a potential increase of BAT volume and activity has been postulated through the intake of some dietary compounds. Several studies suggest that food-derived components, particularly polyphenols, may play a role in preventing and managing obesity by increasing EE through BAT activation. Based on a recently published review on polyphenol supplementation and BAT activation, in this post I will talk about different dietary sources for these compounds, and about the current knowledge on the effects of their administration. But before getting started…what is a polyphenol?

“the role of polyphenolic compounds on energy expenditure enhancement represent a promising target in the fighting against obesity and its related diseases”

Polyphenols – also known as polyhydroxyphenols – are compounds characterized by the presence of large multiples of phenol units. Depending on the number and characteristic of these phenol structures, polyphenols can have different physical, chemical and biological properties. These compounds are secondary metabolites of plants and are generally involved in the defense against ultraviolet radiation or aggression by pathogens.

Phenol and catechin

Chemical structure of phenol, the structural unit of polyphenols; and catechin, a flavan-3-ol present in several food sources

There are more than 8000 polyphenolic compounds identified in several plant species, but only a handful of them has been linked to an increase in energy expenditure. So, which are the sources for these compounds? What do we have to eat or drink to include them in our diet?

dark-chocolate-and-cocoa-beans-on-a-table

Flavan-3-ols. This group includes the most consumed polyphenols in western populations. The main sources of flavan-3-ols are dark chocolate, green tea, berries, nuts, red wine and grape seeds, which contain the flavan-3-ols known as proanthocyanidins and catechins. A diet rich in flavan-3-ols has been associated with a reversal in BAT dysfunction and stimulation of thermogenesis, together with an increase in EE and fatty acid oxidation stimulation, both in rodents and humans. However, we have to take into consideration that in most of these studies, the supplementation is made at extremely high doses, amply exceeding the mean dietary exposure to these compounds.

Tea

Tea flavan-3-ols. Within the flavan-3-ols, a separate group is that composed by the tea flavan-3-ol monomers. These polyphenols can be found at high levels in green tea. Supplementation with green tea for two weeks has been linked to a reduction in body fat and an increase in EE and BAT protein content in rats with induced obesity. Also, the intake of black tea has been shown to suppress adiposity and promote browning of white adipose tissue (WAT) in mice. Moreover, some studies carried out in humans point out to an increase in BAT density, with the concomitant increase in EE. Unfortunately, tea extracts have also a high content in caffeine and the effect of this molecule on EE was not evaluated in these studies, making the interpretation of results very difficult.

Resveratrol. Probably the best known and most famous phenolic compound over the last years. Resveratrol is a phenolic compound found only in trace amounts in red wine, peanuts, berries, red cabbage and spinach. Although its concentration in food is extremely low compared to other polyphenols, there is a great interest in this compound due to its remarkable effects on energy metabolism in mammals. It has been demonstrated that, in rodents, resveratrol improves glucose homeostasis, reducing the effects of obesity, diabetes and metabolic dysfunction. Also in rodents, it seems that supplementation with resveratrol at high doses reduces weight gain in animals fed a high fat diet, and decreases the size of white adipocytes. Moreover, resveratrol has been linked to an increase in basal EE and improved glucose tolerance, and also to a higher expression of specific thermogenic genes in BAT. Apart from the studies in rodents, there are also some evidences that link resveratrol to an increase in EE and decreased WAT adipocyte size in nonhuman primate models of obesity. However, it still remains to be determined whether resveratrol can exert the same effects in humans, since no changes in body weight have been reported in human trials so far.

Resveratrol

Chemical structure of resveratrol

Other (poly)phenols. Apart from the several studies carried out on flavan-3-ols and resveratrol, there are a few reports that support the role of other polyphenolic compounds in energy expenditure enhancement. Isoflavones, for instance, which are found in leguminous plants (especially in soybean) are known to improve insulin sensitivity, reduce fat mass and promote the browning of white adipocytes. Flavonols, present in berries, onion, broccoli, kale and tomatoes, seem to also have WAT browning effects. Gallic acid, a phenolic acid found in high amounts in red wine, tea and berries has been related to increased expression of thermogenic-related genes in BAT of mice. Finally, curcumine, which is a flavor ingredient of curries, has been shown to stimulate EE, decreasing body weight and fat mass and improving cold tolerance in mice.

In light of the different studies compiled in this review, it is clear that, although the role of polyphenols in the enhancement of energy expenditure is supported by most of the available evidence, further studies are needed to better understand the mechanisms underlying the biological effects of these compounds. Moreover, the bioavailability in their food sources and, especially, their effects on energy expenditure and body weight in humans, still require thorough investigation. However, it is obvious that changes in dietary habits are crucial for preventing and managing obesity and thus, despite the several limitations and discrepancies in literature, the role of polyphenolic compounds on energy expenditure enhancement represents a promising target in the fighting against obesity and its related diseases.

So now you know what to do, at least in part, to prevent obesity and stay healthier through diet. Eat more vegetables and fruits (especially berries), drink tea and (responsibly) red wine…and you can even include some chocolate! It is not a secret that these foods are healthy…but now you know why!

Bibliography:

Mele L. et al. 2017. Dietary (Poly)phenols, brown adipose tissue activation, and energy expenditure: A narrative review. Adv Nutr. 8(5):694-704

 

 

Activating brown adipose tissue. Guide for beginners

In the previous post we learnt a little bit about what adipose tissue is and, more specifically, why brown adipose tissue (BAT) has attracted the attention of the scientific community for its role in cardiometabolic disorders. We also learnt that BAT has a crucial role in energy expenditure, since it is rich in mitochondria (the powerhouse of the cell) and converts a lot of energy into heat. In light of the recent discoveries on the beneficial effects that BAT activation can have, I decided to write a new post focused on the several approaches available for the activation of this important tissue. So…let´s go! I want to burn calories! How can I activate my brown fat depots?

To do so, first we should know where these depots are localized. In adult humans, brown adipose tissue can be found in the supraclavicular and neck regions, but also along the vertebrae, aorta and near the kidneys, as it can be seen in the image below.

BAT Localization

Brown adipose tissue activity assessed by positron emission tomography with 2-deoxy-2-[fluorine-18]fluoro-D-glucose integrated with computed tomography [18F-FDG PET/CT] (van Marken et al. 2009)

Now that we know where we can find it, let´s talk about activation. We can distinguish between two different approaches for activating BAT, the pharmacological and the non-pharmacological one.

Non-pharmacological approaches

The best known approach for BAT activation is cold exposure, which is actually its physiological trigger. Cold exposure leads to the release of norepinephrine, which acts on β3-adrenergic receptors (AR) found at the cell surface of brown adipocytes, activating a signaling cascade that mobilize substrates for fueling BAT thermogenesis. However and unfortunately, studies of cold exposure in lean and obese patients reported that BAT seems less prone to be activated in the latter ones.

Apart from cold exposure, non-pharmacological ways of activating BAT are changes in nutritional habits or an increment in physical activity. In fact, a potential increase of BAT volume and activity has been postulated with dietary compounds such as polyphenols or exercise training. In following posts I will talk a little bit more about polyphenols as BAT activators and how we can change diet to promote BAT function.

Pharmacological approaches

Among the pharmacological methods available for BAT stimulation, the treatment with β3-adrenergic receptor agonists, which mimic the effect of cold exposure by activating the corresponding signaling cascade, has arisen as a promising strategy for cardiometabolic disease treatment through BAT activation. However, the use of these compounds can have undesirable effects, such as an increased heart rate and blood pressure.

An alternative approach on the pharmacological side involves resveratrol, a natural polyphenol present in grapes, peanuts and berries. Dietary supplementation with resveratrol has been related to BAT formation and function through the activation of a family of enzymes known as sirtuins, which influence a wide range of cellular processes such as aging, transcription, apoptosis, inflammation and stress resistance, and which will be further described in future posts.

Captura

Conversion of white adipocytes into brown ones through different strategies (adapted from Aldiss et al. 2017) 

Now that the different ways for activating our brown adipose tissue have been explained (see scheme above), it is up to each of you to decide which one you want to use. In my humble opinion, the easiest way is just to turn down the thermostat a little bit, eat healthier and increase physical exercise. Doing so, your BAT will be more than happy!

Bibliography:

González N., et al. 2017. Regulation of visceral and epicardial adipose tissue for preventing cardiovascular injuries associated to obesity and diabetes. Cardiovasc Diabetol 16(1):44

Blondin D.P. and Carpentier A.C. 2016. The role of BAT in cardiometabolic disorders and aging. Best Pract Res Clin Endocrinol Metab 30(4):497-513

Berbée J.F. et al. 2015. Brown fat activation reduces hypercholesterolaemia and protects from atherosclerosis development. Nat Commun 6:6356

Aldiss P. et al. 2017. Browning the cardiac and peri-vascular adipose tisues to modulate cardiovascular risk. Int J Cardiol 228:265-274

van Marken L. et al. 2009. Cold-activated brown adipose tissue in healthy men. N Engl J Med 360(18):1917

What is brown fat? Implications of this tissue in cardiometabolic disease

Fat_Cells_Scanning

Adipose tissue under a (false) color electron scanning micrography

During the following two years (seems a lot but it is not that much!) I will work on brown adipose tissue (BAT) activation (enhancing) as a way of treating cardiometabolic disease, considering the latter as a group of disorders that include obesity, hypertension, dyslipidaemia, hypercholesterolaemia and glucose intolerance. We will talk about the ways of activating this tissue later but, first of all, we should, at least, know what BAT is and why it is so important for the development of several diseases.

“a great interest has emerged on the potential role of BAT dysfunction in the development of cardiometabolic disorders, and also on the potential of BAT activation to treat them”

Body fat, or adipose tissue, is a loose connective tissue composed mostly by cells called adipocytes. Apart from storing fat, this tissue is a major endocrine organ, as it produces several hormones. Adipose tissue can be divided in white adipose tissue (WAT), which is specialized in the storage of energy, and brown adipose tissue, which is composed by adipocytes with a large number of mitochondria (the power plant of the cell) and small lipid droplets, playing a central role in energy expenditure, since it helps generating body heat. In fact, brown adipocytes convert energy from glucose and fatty acids into heat, contributing to the maintenance of body temperature, maintaining cellular functions and physiological processes, especially in cold environments.

BAT

Differences between white and brown adipocytes

But, why is BAT activation so important and promising for the treatment of cardiometabolic disease?  It is well known that an excess of adiposity is a major risk factor for the development of cardiovascular diseases and the associated metabolic syndrome. The rediscovery of active BAT in adult humans some years ago has revitalized the interest in this tissue for its pathophysiological role. In fact, it has been demonstrated that under excessive fat accumulation, BAT becomes atrophied and inactive, and its cells experience a transition from brown to white-like adipocytes. This loss of activity in BAT has been associated with high cardiovascular risk, as it was studied in some South Asian populations in which a reduced amount of BAT was related with a high incidence of metabolic and cardiovascular disorders. By contrast, the activation of BAT is linked to a reduction in hypercholesterolaemia and the protection against atherosclerosis development.

In light of these discoveries around BAT metabolism and function, a great interest has emerged on the potential role of BAT dysfunction in the development of cardiometabolic disorders, and also on the potential of BAT activation to treat them. The activation of existing BAT, the conversion of white into brown adipocytes (browning) or the preservation of a BAT phenotype are of great interest as new therapeutic targets for combating cardiometabolic disease.

Bibliography:

Takx, R.A., et al. 2016. Supraclavicular brown adipose tissue 18F-FDG uptake and cardiovascular disease. J Nucl Med. 57(8), 1221-1225

Boon, M. R., et al. 2015. High prevalence of cardiovascular disease in South Asians: Central role for brown adipose tissue? Crit Rev Clin Lab Sci. 52(3), 150-157

Berbee, J.F., et al. 2015. Brown fat activation reduces hypercholesterolaemia and protects from atherosclerosis development. Nat Commun. 6:6356

Blondin, D.P., et al. 2016. The role of BAT in cardiometabolic disorders and aging. Best Pract Res Clin Endocrinol Metab. 30(4), 497-513