Although as always I will try my best to make things as understandable as possible, I can already tell you, if you are not familiar with physiology, you will have to be attentive in order to follow. The reward of reading this series can be enormous, as it will allow you to form a better understanding of why certain fat loss preparations work and others don't and allow you to make better choices with regards to diet and supplementation.
I suggest therefore that you grab a pen and paper as you read, and try to schematically represent what is outlined in this article. It involves a great deal of different enzymes and proteins and their abbreviations. They will not only be important in understanding this article, but also those that follow. So this is no excess luxury. When rereading the article, with your drawing, you will have much greater insight in how these processes work.
The first article is related to lipolysis and non-shivering thermogenisis. This is essential to make you understand the difference between white and brown adipose tissue, and still see the similarity in how they operate. To not complicate things too much, I will stick to discussing only the local effects on fat cells and their adjacent nerves in this first instalment.
Brown Adipose Tissue & White Adipose Tissue
There are two main types of fat deposits in the body. White adipose tissue is the unsightly fat we are all aiming to get rid off. All subcutaneous fat is usually white adipose tissue (WAT). The main function of white adipose tissue is the storage of energy, mainly fatty acids, in the form of tricglycerides. Triglycerides are three fatty acids, esterified to a glycerol backbone. In this form it is hard for them to escape the cell, and thus an ideal manner to be stocked.
In times of starvation, this white adipose tissue can then produce fatty acids for combustion in the liver and muscles (since fatty acids contain twice as much energy as glucose) which leaves the remaining glucose on your low calorie diet for the organs that cannot function without it, namely the brain and the sexual organs. Under stimulation of cathecholamines (adrenaline, noradrenaline) WAT will initiate a cascade that releases fatty acids from their glycerol backbone, allowing them to enter circulation when needed where they can be transported to the cells in need of energy.
Brown adipose tissue (BAT) on the other hand serves thermogenesis as its main purpose. Thermogenesis is the production of heat. BAT functions mainly in a similar manner as WAT, but it contains more stable Fatty acid Binding proteins (FABP) that keep the released fatty acids inside the cell. Unlike WAT, BAT is also metabolically active, meaning it can use energy itself.
In this case it will use its fatty acids and burn them, but at the same time 'uncouple' the production of ATP (the main form of energy in the body) from the combustion of its fatty acids. This makes the production of energy less efficient and leads to the production of heat instead. Normally heat is a by-product of energy production, in this case the cell deliberately makes its production less efficient to produce more heat, creating thermogenesis. It is therefore theorized that BAT played an important role in the survival of several mammal species by being able to produce extra heat in times of cold.
Making the distinction between WAT and BAT is extremely important to follow later follow-up articles on fat loss supplementation. Mainly because of the differences involved in their ways of contribution to fat loss. For instance understanding that when BAT takes up fat, it can be a good thing, since it can do so under stimulation of cathecholamines and thus absorb fat released from WAT and burn it, while uptake of fat by WAT generally leads to fat gain.
The regulation of the two also occurs in a slightly different manner. As we will see BAT is deficient in the beta2 adrenergic receptor and rich in the beta3 adrenergic receptor, while WAT is rich in beta2 adrenergic receptors, but has relatively few beta3 receptors. We will cover the significance of this in following paragraphs.
The functioning of adipose tissue is regulated by adrenoreceptors and also mediated by other receptors, such as the adenosine receptors. The main effector of the adrenoreceptors is norepinephrine or noradrenaline (NE). In this article we will discuss the effect on fat cells, so I will limit myself to the actions on the fat cell and the adjacent nerve. The nerve release NE which then binds to the adrenoreceptors. NE is a ligand for all adrenoreceptors.
We will distinguish three main categories:
1. The beta adrenoreceptors (BAR):
There are three BAR's, numbered 1 to 3. The B2AR is most commonly expressed in most cell types. With regards to adipose tissue we will see that it is however absent in BAT (8). BAT is mainly regulated by the B3AR. The B3AR is unique to adipose tissue and is highly expressed in BAT, and to a much lesser extent in WAT.
So we can already distinguish that the B3AR will predominantly relate to BAT, while the B2AR will predominantly relate to WAT. There is also a B1AR, which is of lesser importance. It most likely mediated the adrenergic response under normal conditions, because a lot of the processes mediated by the other two will continue in animals that do not express those receptors. BAR's increase lipolysis and thermogenesis and are the main players in these processes.
The way the BAR's work is as follows: The receptors are located on the outside of a cell. When a suited ligand (NE for example) binds to it, it activates a G-protein located on the inside of the cell membrane. A G-protein has three subunits, one of them, the alpha subunit, is attached to Guanosine Di-Phosphate (GDP). When the receptor activates the G-protein, the alpha subunit releases its GDP and binds GTP (guanosine Tri-phosphate). This causes the subunit to break away and activate an enzyme called adenylate Cyclase (AC, also called Adenylyl cyclase).
The function of this enzyme is to turn the cell's ATP into cyclic AMP (cAMP). cAMP is called the second messenger, it carries out the function of the first messenger (in this case NE) inside the cell. cAMP does a number of things, including the activation of exchange proteins and cation channels, but most important to us in this discussion is its phosphorylation of Protein Kinase A (PKA). Phosphorylation is the process of adding a phosphate ion to the protein, thereby either activating or inactivating it. In this it activates PKA. The further functions and relevance of PKA will be discussed at a later point.
2. The alpha1 adrenoreceptor (A1AR):
The alpha1 receptor is also a lipolytic receptor but works in a different manner. It uses Calcium (Ca2+) as a second messenger instead of cAMP. It's also considerably less lipolytic than the BAR's, at most 1/10th of the activity. Most likely to maintain some basal functions of the fat cell under less lipolytic conditions.
Like BAR's the A1AR activates a G-protein, this G-protein then breaks down Phosphatodylinositol-4, 5-biphosphate (PIP2) into 1,4,5-inositol Triphosphate (IP3) and DiacylGlycerides (DG's). IP3 then mediates the release of Ca2+ from intracellular stores allowing it to act as a second messenger. The DG's lead to phosphorylation and activation of Protein Kinase C (PKC). Ca2+ and PKC are the main effectors of A1AR activity in adipocytes.
Sometimes BAR's and A1AR's fulfil similar functions. When this occurs, the effector is most likely Src. This protein is activated through PKC (4) and Ca2+, but also via PKA (5). Although the mediation via PKA should theoretically (not verified) involve some form of tyrosine kinase.
3. The alpha2 adrenoreceptor (A2AR):
The alpha2 receptor is, in contrast to the other two types, an anti-lipolytic receptor. It inhibits fat loss, at least locally. It can contribute to fat loss centrally, via actions on the brain, but since we are only discussing the fat cell and its adjacent nerves, we will consider the A2AR an anti-lipolytic receptor.
It's also anti-thermogenic, but I will not keep repeating that, since lipolysis is essential for BAT thermogenesis, one automatically implies the other. Here too the mechanism is related to the activation of a G-protein, like with the BAR's. It will also act on adenylate cyclase, but instead of activating it, it will shut down adenylate cyclase activity, thus reducing the activity of the BAR's.
The Adenosine Receptors
The adenosine receptors are also anti-lipolytic receptors, that reduce adenylate cyclase activity and inhibit cAMP accumulation. Adenosine is sort of a feedback mechanism. Its released from the fat cell itself when energy is low. When a high amount of ATP is used to form cAMP, then the ATP:AMP ratio will be low, signifying low energy.
A second factor in the release of adenosine may be the A1AR. Using Ca2+ as a second messenger it can increases levels and activity of phosphodiesterases (PDE). PDE cause the breakdown of cAMP to AMP further reducing the energy ratio and causing more release of adenosine from the fat cell.
Adenosine, once released attaches to its receptor and further inhibits cAMP accumulation. This is further validated by evidence that A1AR activity increases blood flow to the cell, but that this blood flow is not mediated by glycerol release. The only other factor I can think of that would increase blood flow in such a manner would be adenosine. This could explain why this receptor is only slightly lipolytic, in contrats to the BAR's.
Location Of Adrenoreceptors & Adenosine Receptors
All these receptors are expressed on the cell surface, obviously, of the fat cell and mediate their activity, lipolytic or anti-lipolytic, inside the cell via the use of second messengers. But these receptors are also located elsewhere in the fat tissue. For starters, both the A2AR and the adenosine receptor are present on the end-terminal of adjacent nerves.
This serves as a negative feedback signal. When NE is released from the nerve, it will bind to these inhibitory receptors on the nerve itself and stimulate the reuptake of NE. This causes less available NE and thus lowered lipolysis. These findings demonstrate that at least at a local level, the blockade of A2AR and adenosine receptors is a valid and versatile means of increasing lipolysis.
BAR's, and specifically the B2AR is also highly expressed in the vascular beds of the fat tissue, where it regulates blood flow. NE can both cause vasoconstriction and vasodilation in cardiovascular tissue, making it the prime regulator of blood flow to several organs and the distribution of fatty acids throughout the body. This further emphasizes the importance of stimulating BAR's to enhance lipolysis.
Regulation Of Lipolysis
When we say the A1AR is lipolytic, this deserves some nuance. Lipolysis is the process of releasing fatty acids from their glycerol backbone. While the A1AR supports fat loss in several fashions, it is not lipolytic per se. Lipolysis is namely regulated by Protein Kinase A, which is only activated via cAMP, and thus BAR's.
Protein Kinase A regulates lipolysis in a dual fashion. First it phosphorylates and activates Hormone sensitive Lipase (HSL). HSL initiates a three step catalytic process that releases a fatty acid from the triglyceride molecule in each step, yielding three Free Fatty acids (FFA) and glycerol.
Glycerol is free to flow out of the cell and since it is highly hygroscopic (attracts water) it will improve blood flow to the cell. If the FFA's can be transported out of the cell, as is often the case in WAT, it will therefore facilitate their systemic uptake.
However it seems that triacylglycerol (TAG) is quite resistant to HSL, because it is surrounded by perilipin (1). That is the second fashion in which PKA will improve lipolysis, namely by phosphorylating and deactivating perilipin, freeing up the TAG that is now more susceptible to breakdown by HSL.
Lipolysis in WAT is the primary goal, since now we have FFA's that can be transported out of the cell and used systemically to be combusted for energy. This causes a reduction in WAT size and this is what we are aiming for, to lose that ugly fat.
Understandably these processes do not occur when you are taking in a lot of food since then there will never be a call for fat tissue to release FFA's. And likewise, if you manage to stimulate the release of FFA's but you are eating too much, they simply will not be burned and be re-esterified.
Lipolysis Leads To Thermogenisis In BAT
As we discussed earlier BAT is metabolically active and induces mitochondrial uncoupling to produce heat. Since the body still needs the same amount of energy, a reduction of metabolic efficiency leads to a greater need in calories.
When thermogenesis occurs, it is therefore beneficial to fat loss since you burn more fat for a given amount of food taken in. Mitochondrial uncoupling is induced by an uncoupling protein, UCP1 (also called thermogenin).
So in BAT, lipolysis is not the final step, but increased expression and activation of UCP1 is. Expression of UCP1 is regulated by both BAR's and A1AR.
This suggests that this occurs through Src. Src is capable of activating two (5,6) of the three mitogen activated protein kinasas (MAPK) as well, and since one of them (p38 MAPK) has been named in expression of UCP1 (6) it is likely that it occurs through this Kinase.
However A1AR mediated increase in UCP1 expression is not completely blunted by inhibition of Src, suggesting a direct mediation by PKC as well. However to what extent is not known. Since both A1AR and BAR's were found to be equipotent in their induction of UCP1 expression its likely that Src is the main regulator.
In any case, the final step is the phosphorylation and activation of CREB (7), which directly increases UCP1 mRNA in the cell. It also produces ICER, a negative regulator of CREB, as a negative feedback signal. With this increased expression of UCP1, possibility of activation is insured. The actual activation of UCP1 however occurs under the influence of lipolysis, or rather the presence of Fatty acids.
Some FFA's are taken up by Fatty acid binding proteins (FABP). BAT expresses more stable FABP than WAT does, so most of the FFA's end up bound and remain in the cell. Even though BAT can produce FFA's, it does not do so to a great extent. Instead the FA content in the cell leads to activation of UCP1 and results in the burning of the fat and the production of heat.
Why Is This Important?
Many may reason that its only using its own FA's and thus does not contribute to fat loss. But that is not the case. In BAT, NE stimulation via a cAMP dependent manner, increases Lipoprotein Lipase (LPL) expression, which leads to the uptake of FA's and triglycerides from the blood.
In contrast, NE will lead to downregulation of LPL in WAT. So the same stimulus that causes the release of FFA's from WAT, causes the uptake of fats into BAT for burning. This makes BAT a useful partner is the fat burning process, but in a totally different manner than WAT. We don't really want BAT to atrophy, on the contrary, we want it to take up as much fat as it can during the diet.
Thermogenesis can also occur in other tissues, to a much lesser extent. Including in WAT. However this thermogenesis is not dependent on UCP1 (3), since UCP1 is only expressed in BAT (2).
This is a lot of information to process all at once. No doubt the different cascades and the involvement of all these enzymes and proteins have confused you somewhat. I hope you took my advice and drew it out schematically. When you reread the article with the drawing next to you, you will be able to grasp all the concepts discussed here more clearly.
All of this will be of extreme importance in understanding the following articles in these series, so I do hope you take the time to let these things sink in. It will be worth your while in understanding fat loss, and the best mechanisms to manipulate it in the end so you can achieve maximum fat loss in a minimum amount of time, with little or no loss of muscle.
In any case we have already learned various ways of manipulation that will lead to increased fat loss. The stimulation of the beta adrenoreceptors and of the alpha1 adrenoreceptor, and all their downstream targets. The blockade (at least locally) of the alpha2 adrenoreceptor and the adenosine receptors and of course the reduction of PDE produced by the A1AR.
- Souza SC, Muliro KV, Liscum L, Lien P, Yamamoto MT, Schaffer JE, Dallal GE, Wang X, Kraemer FB, Obin M, Greenberg AS. Modulation of hormone-sensitive lipase and protein kinase A-mediated lipolysis by perilipin A in an adenoviral reconstituted system. J Biol Chem. 2002 Mar 8;277(10):8267-72. Epub 2001 Dec 20.
- Vidal-Puig A, Solanes G, Grujic D, Flier JS, Lowell BB. UCP3: an uncoupling protein homologue expressed preferentially and abundantly in skeletal muscle and brown adipose tissue. Biochem Biophys Res Commun. 1997 Jun 9;235(1):79-82.
- Granneman JG, Burnazi M, Zhu Z, Schwamb LA. White adipose tissue contributes to UCP1-independent thermogenesis. Am J Physiol Endocrinol Metab. 2003 Dec;285(6):E1230-6. Epub 2003 Sep 03.
- Robin P, Boulven I, Desmyter C, Harbon S, Leiber D. ET-1 stimulates ERK signaling pathway through sequential activation of PKC and Src in rat myometrial cells. Am J Physiol Cell Physiol. 2002 Jul;283(1):C251-60.
- Lindquist JM, Fredriksson JM, Rehnmark S, Cannon B, Nedergaard J. b3 and a1 adrenergic Erk1/2 activation is Src but not Gi-mediated in brown adipocytes.J Biol Chem 275: 22670-22677, 2000.
- Cao W, Medvedev AV, Daniel KW, Collins S. B-adrenergic activation of p38 MAP kinase in adipocytes, cAMP induction of the uncoupling protein 1 (UCP1) gene requires p38 MAP kinase. J Biol Chem 176: 27077-27082, 2001
- Thonberg H, Nedergaard J, Cannon B. A novel pathway for adrenergic stimulation of cAMP-response-element binding protein (CREB) phosphorylation : mediation via alpha1 adrenoreceptors and protein kinase C activation. Biochem J 364:73-79, 2002
- Bengtsson T, Nedergaard J, Cannon B. Differential regulation of b3-adrenoreceptor gene expression subtypes in brown adipocytes. Biochem J 347: 643-651, 2000.