PPAR gamma With Oxidized PUFA is the Gatekeeper to Metabolic Inflexibility

In a recent post I talked about three groups of pigs with differing levels of insulin sensitivity.

Group 1 was fed a low fat diet and remained insulin sensitive.

Group 2 was given 40% of calories as a special soybean oil blend with the triply unsaturated alpha-linolenic acid (ALA) largely removed.  These pigs became what Peter from hyperlipid would call “pathologically insulin sensitive” based on the fact that they had low fasting blood glucose in the context of very low insulin levels.

Group 3 was given regular soybean oil including the oxidation prone ALA.  This group developed pathological insulin resistance as determined by a high fasting blood glucose level in the context of massively higher circulating insulin.  In this group, lipogenic genes were switched on and oleic acid – the end product of lipogenesis – replaced the linoleic acid in their plasma fat.

In a follow up article I said that pigs who remained lean on a high PUFA diet had the super power of linoleic acid accumulation.  Somehow they can accumulate linoleic acid forever without turning on lipogenic genes and the torpid metabolism.

PPAR gamma is the gatekeeper of the torpid metabolism

This 2018 paper demonstrates that it is PPAR gamma that controls the switch into a torpid metabolism^​1​.  Obesity prone mice were given either a low fat or a high fat diet – 60% of calories as a high-PUFA lard and soybean oil, with protein from purified casein with some sugar and purified starch.  This particular high fat diet is known to cause obesity and insulin resistance quite quickly in this strain of mice.

The mice were further divided into groups with normal PPAR gamma levels or mice whose PPAR gamma activity had been eliminated.  The mice on the high fat diet who lacked PPAR gamma look like the pigs  who were given the soybean oil with the ALA removed – they are very insulin sensitive!  The mice with functional PPAR gamma become pathologically insulin resistant.  Lack of PPAR gamma prevents insulin resistance.

Mice lacking PPAR gamma also gained significantly less weight than “normal” mice on the high fat diet, just like the pigs with the ALA removed soybean oil.

I’ve talked about PPAR gamma before, the “Master Regulator of Adipogenesis and Obesity”.^​2​  It is a transcription factor that turns on other genes, including the lipogenic genes such as SCD1.  It may be beneficial when it is activated in fat cells, leading to the creation of thermogenic brown and beige fat cells.  When it is active in the liver and muscle tissue you will store fat.

PPAR gamma is activated by oxidized PUFA

PPAR gamma is “ligand activated”, which means that it is only turned on when it binds to a certain target (AKA ligand).  Specifically, PPAR gamma binds to certain oxidized PUFA.  For instance, PPAR gamma is activated by 15-HETE^​3​, an oxidized linoleic acid metabolite (OXLAM)^​4​.  PPAR gamma has a non-specific ligand binding pocket, however.  It can bind to a variety of ligands, many of which have presumably not been characterized yet.

Let’s think again about the pigs given regular soybean oil.  The presence of ALA was the difference between highly insulin sensitive pigs and very insulin resistant pigs.  ALA is an omega 3 fatty acid with three double bonds that is highly prone to oxidation.  The reason that flaxseed oil is sold in a dark brown bottle is because it’s mostly ALA.   If light hits flaxseed oil it oxidizes into linseed oil: wood polish.

So the presence of ALA presumably provided an oxidized metabolite which switched on PPAR gamma in the pigs which in turn turned on the lipogenic genes and made them torpid.

Back to the mice.  ALA isn’t necessary for torpor, but it may get you there a bit quicker.  The mice given the high fat diet had plenty of linoleic acid and were able to become torpid just fine.  Until the PPAR gamma was removed.  Without PPAR gamma, insulin sensitivity was maintained.

PPAR gamma activity in the liver and muscle tissues is the gatekeeper into torpor.

PDK4 and Metabolic Inflexibility

How does PPAR gamma make a mouse insulin resistant?  The authors of the paper go on to show that PPAR gamma binds to the promoter of a gene called PDK4 and turns it on.  The torpid, insulin resistant mice have the highest levels of PDK4 and the highest blood glucose levels.

This is interesting because obese and diabetic humans have high levels of PDK4 expression in skeletal muscle compared to lean humans.^​5​

Even more interesting is the fact that a major change between torpid hibernators in fall and winter versus summer is that in winter they massively up-regulate PDK4 in skeletal muscle.^​6​  So once again, we see a parallel between a torpid hibernator, an obese human, pigs fed soybean oil and an obesity prone mouse on a high fat diet.  We don’t KNOW that the pigs have higher PDK4 levels because the authors didn’t look into it, but I would put some money on it and I don’t even enjoy betting.

Here’s what PDK4 does.  When glucose is taken up by a cell, it is converted to pyruvate in the cytosol (cell water) by a process called glycolysis.  Pyruvate enters the mitochondria and the pyruvate dehydrogenase complex (PDC) converts it to Acetyl-CoA where it is burned as fuel.  PDK4 (pyruvate dehydrogenase kinase 4) phosphorylates the PDC and turns it OFF.  When PDK4 is at high levels, the PDC is inactive and glucose cannot be efficiently burned.

This makes sense if you’re a hibernating animal planning to live on your stored fat all winter.  A hibernating bear lives off of stored body fat all winter.  There is no source of starch.  High PDK4 expression keeps the bear set to burn fat.

A healthy omnivore, however, should be able to use glucose efficiently when it is available.  One of the main differences between obese and lean humans is that obese humans burn glucose less efficiently.  They are making too much PDK4 in muscle tissue.

As quantitatively the largest organ in the body, skeletal muscle accounts for 30% to 40% of the metabolic rate in adults in the resting state. Contributing to 80% of insulin-stimulated glucose uptake, it is a major site for glucose oxidation and fatty acid metabolism. The skeletal muscle exhibits remarkable metabolic flexibility in fuel usage in response to various metabolic challenges such as energy deprivation and changes in diet composition. In lean healthy individuals, under insulin stimulation, skeletal muscle is able to switch from predominantly lipid oxidation and high rates of fatty acid uptake to the suppression of lipid catabolism and elevated glucose uptake, oxidation and storage. However, obese and T2D patients manifest greater rates of lipid oxidation in skeletal muscle and are relatively insulin resistant, resulting in metabolic inflexibility.

Zhang, et al. The pivotal role of pyruvate dehydrogenase kinases in metabolic flexibility

They are making too much PDK4 because they have oxidized PUFA that switched on PPAR gamma in their muscle tissue.  They are metabolically inflexible because of the PDK4.