Tuesday, 24 March 2015

Crossing thresholds may permanently change you




My ranting and raving about melanotan 2 has inevitably made a few people ask me if the darkening in skin tone was long lasting or permanent. The darkening in skin tone was certainly "long-lasting" appearing to take about 9 months for me to fade back to my pale self, however there has been atleast one permanent side affect.......... I developed new freckles.

I cant say ive noticed any new freckles on my face however, just mostly on my body.

If you read other peoples personal accounts of melanotan 2, weight control doesnt seem to be listed all that often. This makes me think I may of been somewhat a super-responder to MT2. Probably because im both weight reduced .....and my obvious mc1r mutation which makes me tan resistant could indicate I have slight mutations in mc4r and mc5r aswell.

Another thing I noticed when injecting MT2 was that I usually always did the left side of my stomach and after sometime the left side appeared more deflated than the right side, making me think that MT2 might have local effects on adipocytes at the injection site. It appears there are mc5r's directly on adipocytes and that may have accounted for this effect.

Whats curious though is the permanent new freckles.

MT2 is a stronger analog of alpha-MSH, which activates melanocortin receptors, but MT2 also has a substantially longer half life.  oh and I forgot to mention, while on MT2, existing freckles went extremely dark almost to the point of black.

This was all sounding eerily familiar, and I suddenly got the idea that maybe alpha-MSH is to melanocytes as insulin is to adipocytes.

This notion is further supported by the finding that alpha-MSH actually can cause skin cells to differentiate into melanocytes, exactly like how insulin can cause pre-adipocytes to differentiate into full adipocytes.

Skin cells do not normally produce melanin, but they will do once they differentiate into melanocytes. Exactly like how pre-adipocytes do not store triglyceride,  until they differentiate into full adipocytes. BTW this explains where the new freckles comes from with MT2 use. I expect either existing stem cells and/or skin cells each have their own thresholds that must be appeased before they will differentiate into melanocytes, and my normal levels of alpha-MSH was too low to activate them

But once the super stimulus of MT2 is used, the threshold is passed, and additional cells morph into melancoytes = new freckles.  Exactly how like super levels of insulin and glucose causes pre-adipocytes to morph into full adipocytes and make you gain weight.

Even further! as I have said, the new freckles appear to be permanent, again exactly like how new fat cells in obesity appear to be permanent.

And lastly, as mentioned, MT2 caused existing freckles to become extremely dark, indicating that MT2 turbo-charged the production of melanin within these melanocytes.  Well... Lo and behold, this is very similar to what insulin does in adipocytes, it turbo-charges the production and accumulation of triglyceride.......

So,,,,, alpha MSH can cause cells to differentiate into melanocytes and cause over-production of the primary thing melanocytes are suppose to make.......melanin

And....insulin can cause cells to differentiate into adipcytes and cause them to over-produce the primary thing they are suppose to make........triglyceride.

What a coincidence..............


Anway,    there are other areas in biology where crossing thresholds seems to produce irreversible changes. Females that take exogenous androgens ( steroids ) become "permanently" masculinized. .  And its possible that satellite cells permanently morphing into myocytes may account for the phenomenon of "muscle memory" that most bodybuilders swear by.

In both of those cases we are again relying on certain hormones to cross threshold levels.










Monday, 23 March 2015

Fat cell insulin sensitivity

TL:DR at bottom

Is insulin resistance due to a graded loss of insulin response at the individual cellular level or does it reflect changes in the fraction of cells responding to insulin?.......

.... is the crux of a new paper(1) concerned with the insulin sensitivity of fat cells. We are all very use to the idea that the insulin sensitivity of a fat cell varies with its size such that large overstuffed fat cells are insulin resistant (IR) and small fat cells are insulin sensitive (IS). So, with the above question in mind,  it seems obvious that we should chose the former .... I.E. "its a graded response, varying with the size of the adipocyte"....... HOWEVER, this infact may not be true.

Or atleast, only partially true..

The first caveat to note, is that they are using GLUT4 translocation as the surrogate for the IS of the fat cell.

Systemic insulin responses, such as glucose clearance, represent the integrated GLUT4 translocations of all responding muscle and adipose cells

So the more GLUT4 translocation we get in a fat cell, the more IS it is deemed to be. Previously, the authors had discovered that human adipose cells cultured in vitro WITHOUT the hosts serum retain the IS status of their host. This would imply that circulating factors in host serum (i.e. hormone levels etc) may not play as important role in the IS of the individual as previously thought, and instead suggests that the IS of a fat cell is an intrinsic property of that cell.

Another interesting point is that, when adipose cells are isolated and tested individually for their IS, there is marked heterogeneity between cells in their responses.(2) That is, each individual adipocyte will display a unique and highly reproducible response.  As the authors of (2) say...

These data highlight that the response of a cell population to insulin is underpinned by extensive heterogeneity at the single cell level. This heterogeneity is pre-programmed within each cell and is not the result of intracellular stochastic 

Back to (1), they sought to ask the question and test, is the IS status of a host reflected in the IS status of all his individual fat cells. For example, if someone is 50% overall IR, is that because all his individual cells are 50% IR?

changes in adipose function are usually studied at the whole organ or population level and it is naturally assumed that any defect will be due to a similar disruption in all cells(2)


Testing and Data from (1)

The tests performed by (1) were to isolate 8 individual fat cells from the abdominal subcutaneous depot of 19 different subjects,  all of whom showed a very wide variation in their whole body IS status.

The cells were incubated in the absence ( basal ) or presence of 0.1 IU/ml insulin (maximal stimulation) and GLUT4 translocation markers were assessed.

The graph is plotted with the most IR subject on the left and the most IS subject on the right. And the graph proceeds from left to right in terms of the IS of the subjects...



Each blue dot represents an individual fat cell from 1 person in the basal state, and the red dot next to it is one of the individual fat cells in the insulin stimulated state. As you can see, as you move from left to right, more and more red dots jump above the dotted line, indicating that as you move from left to right, more and more cells respond to the insulin.  Red dots that stayed below the dotted line means they didnt respond to the insulin at all.

Interestingly, the major difference between cells from insulin-resistant and insulin-sensitive is not the individual cell response amplitude, but rather the number of cells that exhibit a 3–4 fold response. Simultaneously, in almost every subject, we observed some cells that do not exhibit any insulin response at all.

What this seems to suggest, is that the difference between IS and IR individuals is not how much the cells respond to insulin, but rather the number of cells that respond.  I.E. Insulin resistant people have a much lower number of cells that respond to insulin.  I found this very counter-intuitive and novel.



Infact, if you plot all the data on a BeeSwarm style graph...




....you get a graph that looks like a bimodal function. I.E. 2 distinct states of normal distributions. Based on this the authors propose that fat cells exists in 2 distinct states, "basal" and "insulin stimulated".

Thus, GLUT4 translocation in individual adipose cells isolated from human subjects is better characterized as an all-or-none (bimodal) response than a graded response. Our single cell analysis of human adipose cells reveals a bimodal distribution for each of two characteristic properties of GSV, mobility and fusion, affected by insulin. This invariance in the underlying distributions, refractory and insulin-stimulated, with a clearly determinable fraction of cells populating these two states in each subject, is consistent with a threshold or switch-like transformation from the basal to the stimulated state upon addition of insulin

Threshold type triggering systems exists everywhere in biology, and it wouldnt surprise me if something similar exists for how adipose cells respond to insulin.  I (speculatively) envisage that each individual adipose cell probably has a threshold insulin responding level,  if it sees insulin below that threshold concentration, no response is initiated. However as soon as it sees insulin above the threshold concentration, it immediately enters the "insulin stimulated" state and simultaneously fires its entire GLUT4 cache.  This threshold level is probably changeable depending on hormonal inputs into the adipose cell.

Infact there is some evidence that the insulin sensitivity is dependent on the number on insulin receptors on it........ This quote from ref(2) suggests insulin sensitivity of a fat cell is dependent on receptor count.

"previous studies have shown that changes in insulin receptor number can change the sensitivity but not the maximum responsiveness of insulin action(3) ."


Even more intriguing from (1), the authors ruled out that fat cell size was the deciding factor in whether or not a fat cell responded to 0.1 IU/ml insulin, as no correlation was observed between fat cell size and GLUT4 translocation.


Findings from study ref (2).

The authors in ref (2) worked on groups adipocytes but performed atleast 1 study involving single adipocytes,  (3T3-L1 Adipocytes), but they still found evidence of extensive heterogeneity.

From fig 5a. , the response of the group of fat cells to different insulin doses was a graded response.

They noted that at the lowest insulin dose, only 30% of cells from the group displayed any detectable response, which is consistent with the idea that each cell has a threshold insulin trigger level that must be passed before it will show any insulin response.

However, somewhat contradictory to ref(1), they found fat cells that responded in both a graded response manner and a bimodal response...

I emailed the authors of ref(1) about this seeming contradiction, ( because (1) declared fat cells as being bimodal while (2) found evidence for graded responses aswell as bimodal)   and specifically asked how could they determine the presence or absence of a graded responses if they only tested with 1 different insulin dose. They have yet to email me back.

Anyway......

Three important conclusions from ref(2) are....


  1. These data imply that the cellular response to a specific dose of insulin is an intrinsic property of each cell.
  2. One possibility is that this represents discrete subpopulations of adipocytes that possess intrinsic differences in insulin sensitivity and responsiveness.
  3. changes in adipose function may rather depict changes in the relative abundance of subpopulations of adipocytes that comprise the organ.


Currently, adipocytes are divided into 3 subpopulations, (white, beige, and brown.) What seems likely is that even amoung white adipocytes they can be divided into even further subpopulations that differ in insulin responsiveness and sensitivity.

Personally, what *I* think is happening is that each individual cell has its own level of epigenetic expression of its gene's ( epigenetic = histone acetylation and DNA methylation ) that together makeup the phenotype of that cell. A large fat cell has increased histone acetylation on PPARgamma, and its possible epigenetic expression of that aswell as other gene's will determine its insulin responsiveness. 



Are small fat cells really more insulin sensitive?

Ref(4) found an enhanced number of small fat cells in insulin resistant subjects compared to insulin sensitive ones. This is not what you would expect if you just bluntly thought small fat cells = insulin sensitive.

Here's the key quote from the abstract...

The real-time PCR results showed two- to threefold lower expression of genes encoding markers of adipose cell differentiation (peroxisome proliferator-activated receptor gamma1 [PPARgamma1], PPARgamma2, GLUT4, adiponectin, sterol receptor element binding protein 1c) in insulin-resistant compared with insulin-sensitive individuals.

Although the authors of ref(4) attribute this to"impaired" adipogenesis,  I dont agree. The lowered expression of these adipose gene's *is* what makes the fat cells smaller and probably also more insulin resistant. But this is not a "defect" , but rather it is a reflection of their genetic inheritance.

We know insulin resistance has a genetic component and runs in families. I reckon what is happening is that offspring of insulin resistant parents are simply inheriting pre-adipocyte pools that have lower levels of histone acetylation of those adipogenic gene's in them. (particularly PPARgamma)

Remember from ref(5),  the histone acetylation ( H3 anyway ) of an adipocyte is maintained even in the de-differentiated state. This implies the pre-adipocyte already contains a blueprint for exactly what type of fat cell it should become ( large vs small etc ), if and when it ever gets the signal to turn into a  fat cell.

So.... as this normal person starts to eat obesogenic food ( processed carbs ) , morphological changes to the intestine and pancreas takes place, causing hyperinsulinemia and hyperglycemia, which in turn start to activate all his pre-adipocytes and turn them into mature fat cells. These new fat cells start to store their own fat  ( thats what fat cells do, calories are irrelevant, exactly like how myocytes are going to build myofibrils even if you keep protein intake low.. ) , and the person overall gets "fatter". No doubt the insulin also causes existing adipocytes to hypertrophy.

Depending on the genetics and epigenetics of the pre-adipocyte pools you inherited from your parents, these pre-adipocytes could turn into small insulin resistant fat cells, in which case youll only gain *some* weight.and as your insulin levels continue to increase, putting increased insulin sensitivity demands on your body, these fat cells cant fulfill that demand for increased insulin sensitivity, and youll been seen as "insulin resistant" and labelled diabetic ..........Or they could turn into large insulin sensitive fat cells, in which case youll get super obese. I also expect numbers pre-adipocytes in pools differs between individuals, which would further contribute to your propensity to get extremely fat.

So when a diabetes researcher harvests fat tissue from an obese insulin resistant subject and observes them under a microscope and sees lots of small fat cells and concludes "defect in adipogenesis!" . I say NO.

I say, your just observing the genetically inherited destiny.of his cells.



TL:DR conclusions


  • (1) concluded that fat cells respond to insulin ( GLUT4 translocation ) in an all or nothing manner.
  • It appears fat cells have an insulin threshold trigger level, if they see insulin below the threshold, the response is zero, but if they see insulin above that level, they immediately respond. 
  • contrary to (1),    (2) found evidence that the response of individual fat cells to insulin above the threshold can be both graded, and bimodal. 
  • graded responding fat cells will respond stronger as insulin doses increase, up to maximum dose, while bimodal responding fat cells always give the same response regardless of insulin dose.
  • (1) found that some fat cells will not show any response to insulin even at maximal doses.
  • the difference between insulin sensitive and insulin resistant individuals appears to be accounted for entirely by the number of fat cells that just stop completely responding to insulin. That is, as you progress from insulin sensitive to insulin resistant, more and more of your fat cells will completely ignore insulin
  • this defect is preserved in culture even without host serum, indicating that the response to insulin is an intrinsic property of each individual fat cell.
  • (4) found that insulin resistant individuals have a greater proportion of smaller fat cells with reduced expression of adipogenic gene's, countering the idea that smaller fat cells are always more insulin sensitive.





Refs

1. Human Adipose Cells In Vitro Are Either Refractory or Responsive to Insulin, Reflecting Host Metabolic State - link

2. Novel systems for dynamically assessing insulin action in live cells reveals heterogeneity in the insulin response. - link

3. Mechanisms of insulin resistance in obesity and noninsulin-dependent (type II) diabetes. - link

4. Enhanced proportion of small adipose cells in insulin-resistant vs insulin-sensitive obese individuals implicates impaired adipogenesis. - link

5. Heritability of fat accumulation in white adipocytes. - link







Friday, 27 February 2015

Fat makes you ..... fat ?!

One of the reasons it is hard to dissociate calorie balance to weight is because your intestine secrets a large number of hormones upon food ingestion and the magnitude of the hormone response is proportional to the calorie intake.

I was doing some research on Orlistat which is one of the few drugs approved for obesity treatment. Its suppose to make you lose weight because you "absorb" less calories, and the unabsorbed fat comes out the other end, sometimes violently.  Orlistat produces only mild weight loss after 1 year of use, ~2.89kg, which essentially makes it useless, but its the mechanism of weight loss that im interested in.

This paper looks at the effect of orlistat on GIP secretion. Ive talked alot about GIP before, its an anabolic hormone secreted from the intestine in response to both fat and carbohydrate digestion, but remains almost unchanged in response to protein.  A look back at this post and you see a graph of GIP in response to nutrient ingestion.

Heres some quotes from the orlistat paper....

Our results demonstrate that in nonobese healthy subjects orlistat accelerates gastric emptying of an orally ingested ∼500 kcal solid-liquid mixed meal and attenuates plasma GIP response, while it does not appreciably alter the plasma responses of CCK, GLP-1, and PP compared with control. 
GIP is secreted from enteroendocrine cells of the K type in the upper gut in response to nutrients in a load-dependent manner, fat being the most potent secretagague in humans
There are functional GIP receptors on adipocytes, which have insulin mimetic properties such as uptake of glucose (37), fatty acid synthesis, upregulation of lipoprotein lipase synthesis, and reduction in glucagon-induced lipolysis
Thus GIP acting on its specific receptors and via insulin secretion promotes fat accumulation in adipocytes, obesity, and thus insulin resistance. GIP receptor knockout mice who are fed a high-fat diet are resistant to obesity



So im thinking, orlistat most likely causes weight loss because GIP secretion is reduced. The problem ofcourse is its hard to disentangle this from the idea that the weight loss is caused by reduced calorie absorption because GIP is secreted in response to free fatty acid absorption into the enterocyte.

Anyway, Im not trying to put forward the idea that "fat is fattening", I dont have data on how potent GIP is on adipocytes compared to insulin. But if you look at the data solely on GIP, its clearly a "fattening" hormone. and it *is* secreted primarily in response to fat.  ( GIP also is important for bone density, i.e. it increases bone mass, so blocking it completely might not be a good idea )

intriguingly it was also reported in the "accelerated glucose absorption" post that obese people exhibit enhanced GIP secretion in response to nutrients. A great example of how "a calorie is not a calorie".

I intend to experiment with orlistat on LC when I get the chance. Hopefully the Steatorrhea wont be too bad...





Friday, 6 February 2015

the "master" gene

I was reading through JJ's tweets other day and came across this




despite it being quite disturbing, something out of a horror movie, reading the ensuing discussion was fascinating. This article explains what is going on.

Basically, there is a gene they refer to as "eyeless" and when activated, this gene sets off cascade of signalling that results in the growth of an eye at that location.

The reason I found this so interesting is because we have also seen here on this blog another candidate for a "master" gene. PPAR-γ.    Just as activating "eyeless" caused eye growth in strange places of the body, substantial evidence indicates that activating PPAR-γ causes adipocyte appearance.

My guess is, it probably didn't how much the fly ate and exercised, once the eyeless gene was on, the eye WAS GOING TO GROW, regardless.

At this point im fairly sure that the cause of obesity is refined carbs promoting hyperglycemia and hyperinsulinemia.  These 2 in turn promote epigenetic changes to adipose tissue aswell as the recruitment of adipose tissue progenitor cells ( preadipocytes ) Once a preadipocyte turns into a mature adipocyte, its going to store (alot) fat, everything else is irrelevant.

I also found this interesting post on t-nation, where he explains that people who respond poorly to resistance training do so because of genetics and poor recruitment of muscle progenitors ( satellite cells ).  This could potentially be similar to the reason some people are resistant to obesity, i.e. failure to recruit progenitor cells in adipose tissue.











Tuesday, 9 December 2014

lipophilia and storage vs oxidation



You see this slide in most of Taubes's lectures, and I think what Julius was observing and trying to report can easily be explained by observing what happens when a pre-adipcoyte turns into an adipocyte.

There is a series of papers from the 1970's  looking at what changes are taking place in the conversion of pre to mature adipocyte.

When cells of the established preadipose line 3T3-L1 enter a resting state, they accumulate triglyceride and convert to adipose cells. The adipose conversion is brought about by a large increase in the rate of triglyceride synthesis,
 If 3T3-L1 cells incorporate bromodeoxyuridine during growth, triglyceride synthesis does not increase when the cells reach a stationary state, and triglycerides do not accumulate.
As would be expected from their known actions on tissue adipose cells, lipogenic and lipolytic hormones and drugs affect the rate of synthesis and accumulation of triglyceride by 3T3-L1 cells
Glycerophosphate acyltransferase activity rises sharply during the conversion and reaches a level of 80 times higher than that of another 3T3 subline in which practically no adipose conversion takes place (3T3-C2).(link)
There is alot revealing information here, remember this picture?



The underlying message here is that, a fat cell accumulates fat because there is a change in the internal mechanics and gene expression of that cell. If calorie availability itself was sufficient to drive increased triglyceride accumulation, how would you explain why pre-adipocytes store virtually no fat?

In the same sense, you have to ask yourself, is calorie availability sufficient to turn a small adipocyte ( thin person ) into a large adipocyte ( fat person ) ? Or does this change also require further increases in the trig synthesis rates of the adipocyte?

If you knockout the ASP receptor ( acylation-stimulating protein receptor ), this reduces trig synthesis in adipose tissue, the mice eat 60% more than controls, but weigh the same.  Also, they do not have increased energy expenditure because oxygen consumption was identical to controls, instead what happened was they had massively increased fat oxidation rates.

This seems to suggest that if you reduce the hormonal signals that drive the trig synthesis pathway's ( low-carb anyone? ), the increased FFA availability automatically drives increased fat oxidation. Further, it would appear fat oxidation is a slave to the driving force of storage. A reduced drive to store fat, promotes increased fat oxidation.

In this context, it appears wrong to say "if you dont burn it, you will store it"

but rather it looks like...  "if you dont store it, you will burn it"

Breaking down the Julius Bauer comment above, we have some explanations from what he was trying to describe...

The abnormal lipophilic tissue seizes on foodstuffs -  The foodstuffs flow into the adipocyte, but are seized/trapped due to the aggressive levels of trig synthesis inherent in adipocytes.

even in the case of undernutrition - the drive to store nutrients can override the need for fuel burning and sequester fuels away from oxidation.

it maintains its stock -  as we all know, adipocytes are extremely good at resisting size changes. decreases in adipocyte sizes encourage increased trig synthesis responses.

and may increase it independent of the needs of the organism - hormonal changes that drive increased trig synthesis pathways in adipocytes cause fat growth, whether that fat growth is needed or not is irrelevant.






Wednesday, 26 November 2014

Fucked up glucose digestion in obesity

very quickly...

Accelerated intestinal glucose absorption in morbidly obese humans – relationship to glucose transporters, incretin hormones and glycaemia

Another study showing that obese people have rapid and increased glucose absorption from the duodenum due to increased glucose transporters. This rapid glucose absorption facilitates hyperglycemia and hyperinsulinemia, because as we know, faster digesting carbs spike blood sugar and insulin more aggressively.



Although it has long been thought that the hyperglycemia and hyperinsulinemia in obese people is due to insulin resistance, the authors here question this, and speculate that rapid glucose absorption could instead easily account for this.

Further there is an imbalance of postprandial incretin hormones, obese people exhibit reduced GLP-1 secretion in response to carbs, ( which is also seen in the graphs here ), the authors here mention that reduced GLP-1 secretion allows glucagon secretion to be enhanced in the postprandial state, which is very inconvenient when combined with the hyperglycemia from the food ingested, because the job of glucagon is to raise blood sugar ( and its already raised from the food )  The combination of enhanced glucagon levels AND the carbs results in even HIGHER blood glucose, and you need even HIGHER insulin to deal with it. Something of a vicious cycle.

We know way back from the powdered carbs study that there is something about the digestibility of carbohydrates that can SERIOUSLY enhance how fattening they are. Indeed it seems to be refinement of carbs that makes them more easily and quickly digested that is the devil. The obesity epidemic has risen in parallel with refined carb consumption.

How hard would it be to believe that...

refined carbs -> morphological changes to intestine** -> hyperglycemia + hyperinsulinemia in response to carbs -> adipogenesis + histone acetylation in fat tissue -> elevated fat mass setpoint -> obesity + resistance to weight loss

There is already some evidence hyperglycemia can cause chromatin remodeling to DNA.

**to the best of my knowledge, this has never been investigated, so we dont know if its true, or false. I.E. if the refinement of dietary carbs can directly cause elevated glucose transporters in the duodenum.










Saturday, 22 November 2014

Why does glucose make fat?

I want to come back to this question at the end of the post, after we have examined some research results.

For those not aware a "pre-adipcoyte" is just a cell that has the potential to turn into a proper adipocyte, and is not really an "adipocyte" in the pure sense, it also does not store much fat.

Here's some pictures to do the talking, ( stolen from google )





Something we have to ask ourselves, is, why is the adipocyte storing large amounts of fat, and why is the "pre-adipocyte" not storing hardly any fat? Something to do with calories and energy balance? The adipocytes eat more than they burn, and are in positive energy balance, meanwhile the pre-adipocytes are in perfect energy balance,  they eat as much as they burn and stay slim. Calories in, calories out, second law of thermodynamics. Matter cannot be created or destroyed, only transferred.

Is any of this making any sense yet?

Dont dare ask WHY the adipocyte is eating more than it burns. Thats IRRELEVANT................or it might have something to do with its poor impulse control and lack of willpower when faced with tasty food. mmmmmmm cake.

Meanwhile, back in the lab, I came across this paper recently, which looks at epigenetics of the PPARγ gene. A question that has bugged me and should hopefully of bugged anyone else in obesity research is, why are obese people always drawn to regain weight after weight loss. Somehow it seems that the fat tissue is slowly and surely sucking up fat to regain its original mass. I think we've discussed to death on this blog the possible causes of this. Adipocyte hyperplasia and low leptin being the leading culprits. Aswell as catecholamine resistance, particularly in the subcutaneous depot.

However, I think the answer to the question of why fat tends to return to its original size lies in the answer to the question of how it got to that size in the first place.

1. What determines how much fat a fat cell stores?
2. What turns a pre-adipocyte into a mature adipocyte? ( Adipocyte hyperplasia )

Actually we have already seen the answer to the first question, back in this post. The idea put forward was that the histone H3 acetylation of the PPAR-gamma promoter region increased the transcriptional activity of this gene, and thus resulted in increased fat accumulation in the adipocyte.

I Advise you to watch this video which easily and quickly explains what all this histone H3 acetylation stuff is about



So in short, The histone H3 acetylation uncoils the DNA and allows the PPARγ gene to be read, with increasing permissiveness. The next question is,

Why would increased PPARγ lead to increased fat storage?

While I dont have hard conclusive evidence of this, my guess would be because PPARγ targets the gene transcription of proteins involved in the formation and maintenance of lipid droplets. If you want to build a huge single lipid droplet in the middle of the adipocyte ( which is the defining feature of the adipose ), you need proteins to do that. Fatty acids and triglycerides dont just magically like to clump together in large solid balls.

As an example, PPARγ targets and expresses perilipin1 , which is a lipid droplet protein involved in whole body energy balance. One of the crucial functions of perilipin1 is to coat the lipid droplet surface and stop hormone-sensitive lipase entering the droplet and chopping up triglycerides into fatty acids. (lipolysis )

Infact atleast one team are looking to make an inhibitor of perilipin1 to treat obesity. It has also been discovered that the weight loss associated with anti-retroviral drugs is due to their actions in degrading perilipin1. FSP27, is another lipid droplet protein controlled by PPARγ. The function of FSP27 is to make small lipid droplets fuse together into larger ones.

So, in essence, higher  PPARγ -> more lipid droplet proteins being manufactured and floating around -> increased ability to build large lipid droplets.

I would propose that  PPARγ IS the "vacuum" that is sucking up fat from the blood and causing it to be stored and maintained in the large central lipid droplets of adipocytes.

There is *some* evidence for this, because forced expression of PPAR gamma in fibroblasts and myoblasts  is sufficient to differentiate these cells into adipocytes. Its almost as if PPARγ is itself entirely responsible the adipocyte phenotype. Basically, once PPARγ becomes active in a cell, that cell BECOMES an adipocyte. And with that, this is a good time to move on to the second question....




What turns a pre-adipocyte into a mature adipocyte?

Again you can read pubmed[22991504]  for an in depth description of the very complex multi-step process that is adipocyte differentiation,  But basically....

the transcriptional activation of PPARγ during adipogenesis correlates with an epigenetic switch at the PPARγ gene. For instance, adipocyte differentiation is associated with a strong increase in levels of histone activation marks at the two PPARγ promoters.

in essence, PPARγ is not expressed in pre-adipocytes, then, modifications to the chromatin and promoter regions causes the DNA that codes for PPARγ to unwind from the nucleosome, this allows access by RNA polymerase II to start transcribing PPARγ.   PPARγ itself then starts off a cascade that involves unwinding the DNA in its gene target regions. 

An important ingredient in adipogenic differentiation media is high glucose and insulin.  For some reason, ( and this is what the title of the post refers to ) pre-adipocytes regard high glucose and high insulin levels as a signal to epigenetically modify the DNA to expose the PPARγ promoter region, to start transcribing this gene, and ultimately become an adipocyte.  There are other key ingredients in adipogenic media, indeed glucose and insulin exclusively may not be sufficient. (shrug, glucose and insulin appears to be enough to make me fat in vivo )

Whats the real reason sugary drinks make you fat? It may because,  the hyperglycemia and insulin these drinks promote causes epigenetic changes to DNA in cells that ultimately result in increased and sustained expression of PPARγ.  There is already evidence out there that diet and hyperglycemia cause chromatin remodeling to DNA. 

Is histone acetylation of PPARγ the reason obese people have elevated fat mass set point?

Its important to point out PPARγ is regulated both at the nutritional and hormonal level, aswell as at the genetic level by chromatin modelling acetylation/methylation. The ligands for PPARγ are fatty acids and prostaglandins. Further, PPARγ is strongly upregulated by insulin. 

A curious study from 1997 found increased mRNA of PPARγ in adipose tissue of obese people ( 14.25 obese vs 9.9 lean ), whats more, the increased mRNA of  PPARγ  positively correlated with the BMI of the subject. The fatter you are, the more likely your  PPARγ is to be higher. Given what we have learned above, that would be expected.

But the most curious part of the the study was that they made the obese people lose 10% bodyweight on a 800 calorie diet, and  PPARγ decreased by 25%. This was then followed by a 4 week intervention of weight maintenance, during this time, PPARγ increased back to pre-treatment levels! Isnt that funny? Suddenly my brain is flooded with images and notions of weight regain following weight loss. Its entirely possible these obese people, like pretty much the vast majority of obese people, have chromatin modifications to their  PPARγ promoter regions thus encouraging increased basal levels of PPARγ.

Going on a diet temporarily suppresses PPARγ, because as discussed, PPARγ is also regulated by nutrition and insulin. But returning to normal eating patterns would see metabolic hormones return to normal. The PPARγ levels return to normal, and their weight will probably return to normal. ( normal being the pre-treatment obese state ).

Adipocyte dedifferentiation.

As eluded to in the fat cell size regulation post.... it would appear adipocytes are actually capable of de-differentiating back into pre-cursor cells, losing their lipid content in the process. Indeed they appear to take on stem cell properties.

As such it may not be necessary  to cause any kind of apoptosis to reduce obesity, instead it may only be necessary to silence the PPARγ gene in the adipocyte. Through either DNA methylation or de-acetylation of the histone H3, , or some other complex restructuring of the chromatin, . Once the PPAR gene is silenced, lipid droplet proteins will no longer be manufactured, the adipocyte will have no way to build the large lipid droplet and lipid stores should exhibit a net flow out of the cell.

Whether this can happen in vivo in humans to produce practical weight loss is anyones guess however, and this subject would require another post.