Protons (81) Crown like structures

Hyperinsulinaemia, while being unable to suppress basal lipolysis, is still able to facilitate uptake and storage of fatty acids within adipocytes. In this study the rats which were fed an high polyunsaturate diet were fatter and more hyperinsulinaemic than those fed higher saturate diet, all other aspects of the diets being constant.

Diet fat composition alters membrane phospholipid composition, insulin binding, and glucose metabolism in adipocytes from control and diabetic animals

But when you take adipocytes out of the intact rat and ask how well they processed glucose, the high PUFA group adipocytes were *more* insulin sensitive.

The metabolic milieux renders the live rats insulin resistant (elevated FFAs from increased basal lipolysis) while the individual adipocytes extracted from the rats are pathologically insulin sensitive. So the rats become fat and insulin resistant at the macroscopic level but retain insulin sensitivity at the adipocyte level when supplied FFAs are lowered to the tissue culture levels used.

Aside: There is now an on-line calculator which converts ng/ml of insulin to pmol/l insulin. Anyone who has had to convert grams to moles and then get the decimal point correct when converting to picomoles will understand.  Happy happy happy. 100ng/ml is 225pmol/l, a mild post prandial value. 1000ng/ml is 2250pmol/l, aggressive hyperinsulinaemic clamp levels. Most "ordinary" high insulin clamps use the highest post prandial levels of around 1000pmol/l, ie a bit less than half  highest values on the graph above. The above graph is close to physiology. End aside with happy dance.

To me this sets the scene that elevated plasma insulin, combined with enhanced insulin sensitivity at the adipocyte level, can successfully repackage lipids from basal lipolysis back in to adipocytes. That's Carpentier's idea in this paper, as in the last posts.

Increased postprandial nonesterified fatty acid efflux from adipose tissue in prediabetes is offset by enhanced dietary fatty acid adipose trapping

It is also quite possible to break adipocytes by doing this. We have two opposing processes. Enhanced translocation of glucose and fatty acids in to adipocytes under the failure to correctly resist insulin's storage signal, due to inadequate (but far from zero) ROS generation by linoleic acid. The second is the ability of distended adipocytes to release FFAs irrespective of insulin's action by enhanced basal lipolysis. This limits adipocyte size and supplies a competing substrate for insulin sensitive cells which requires the rejection of a certain amounts of glucose (ie insulin resistance) in proportion to the FFAs available from basal lipolysis.

Obviously excess storage, mediated via linoleic acid, wins. Otherwise there would be no linoleic acid mediated obesity. So when an adipocyte is at maximum size and there is a sudden surge in insulin/glucose/FFA availability then the adipocyte will attempt to get bigger. At some point it will fail.

Which leads me on to this D12492 mouse paper:

Adipocyte Death, Adipose Tissue Remodeling, and Obesity Complications

The photomicrographs are very pretty. The red arrows (placed by the authors) indicate "crown like structures" (CLSs). This is a simple H&E stained image of adipose tissue from a mouse after eating D12492 for eight weeks:

The CLSs appear to be lipid droplets with thickened material surrounding them which looks a lot like cytoplasm. If you go on to use immunohistochemistry to label perilipin A, which labels the protein surrounding the lipid droplet in functional adipocytes, there isn't any. The numbers indicate individual CLSs. Golden brown indicates perilipin A, clearly stained in the (un-numbered) living adipocytes:

If you stain the same section with F4/80, which picks out macrophages, you get this:

which shows that what, on H&E, looks like s thick surround of adipocyte cytoplasm, is in fact a population of macrophages surrounding the remains of a dead adipocyte. Big Eaters. They are clearing up debris

This is the image from a mouse sacrificed after 12 weeks of eating D12942:

and by week 16 we have this

There are no adipocytes visible in this image. It's all crown-like structures. The authors have not placed arrows because they would need to be everywhere. By 16 weeks of the mice eating D12942 their adipose tissue is in crisis, many adipocytes are dead and there is a marked inflammatory response which is clearing up the debris.

By 20 weeks there is significant recovery of adipose architecture, presumably from a supply of stem cells/preadipocytes, but the formation of CLSs continues, a consequence of the continued feeding of D12942. This is the view at week 20 when the study ended:

There is nothing tidy about the death of adipocytes during the formation of CLSs under D12942. The process is known as pyroptosis. I'm not sure how real pyroptosis might be, after all ferroptosis is a well recognised and well researched process which seems to be little more that linoleic acid intoxication. But assuming pyroptosis is real it is considered to be part of the innate immune system by which cells, when they have certain types of overwhelming infection, kill themselves. The process is messy.

Macrophages don't like mess. They get in there to sort it out. They also signal to the rest of the body that something is very wrong and it's time to optimise metabolic conditions to maximally enhance immune function.

Here's what it looks like if you immunostain the macrophages of CLSs for TNF-α or Il-6

Of course both of these cytokines will cause insulin resistance. Which is adaptive (another post there, you think the innate immune system does stupid things?). Not only in the surrounding adipocytes but also systemically. Which leaves a few open questions.

There are thinkers who surmise that adipose tissue inflammation is causal of insulin resistance and even that this insulin resistance, which generates hyperinsulinaemia, might be the actual cause of obesity. I know it sounds strange, but who knows? Everyone is welcome to their opinion.

Or we could hypothesis, as I do, that linoleic acid is the cause of insulin *sensitivity* which enhances insulin's storage signal (without inflammation) to the point where adipocytes die in association with over distension and there is then a massive inflammatory response as a secondary consequence.

There's a lot more to say about unhappy adipocytes and cytokines but I'll leave this post now by suggesting that the residual insulin resistance seen in IGT when FFAs are normalised by acipimox. As in here:

Overnight Lowering of Free Fatty Acids With Acipimox Improves Insulin Resistance and Glucose Tolerance in Obese Diabetic and Nondiabetic Subjects

might be mediated by TNF-α, IL-6 and their kindred signaling molecules from CLSs.

Sadly even this may not be quite as simple as it sounds.

Peter