These are just some of the illustrations I doodled out for the last post while thinking about insulin sensitivity/resistance in D12942 fed mice. I hope they make it clearer what Shulman was looking at in 2016 and what he moved on to look at in 2021, comparing D12942 feeding to control mice versus to his mouse model with the Thr1150 to alanine (Thr1150A) switch.
I've left various possible routes for the development of insulin resistance dashed for D12942 because that still needs a significant amount of discussion, see below. I've also added in yellow a line for the state of phosphorylation of the Thr1150A substituted mouse mutant. What the 30 minute clamp is looking at is the residual physiological insulin resistance at a time when hunger on D12942 has almost normalised and the level of phosphorylation of Thr1150 is approaching what it should be if D12942 was a physiological high fat diet.
This clamp induced change will be a generic pattern, here are all of the scenarios I imagined two posts ago, with the Thr1150A mutant added, just to reiterate the colour scheme:
and here's what I expect happens when you measure insulin sensitivity by glucose infusion rate during a mild insulin clamp at 30 minutes:
If I am correct it suggests that all of these cells are physiologically normal under normal clamp conditions. This is not "pathological" insulin resistance.
The next question is why cells under all of these conditions have differing levels of insulin resistance before the clamp started.
The obvious explanation is that the phosphorylation of Thr1150 is a surrogate for the activity of PKCε, which is a surrogate for cell membrane DAGs, which are a surrogate for free fatty acid availability. High fat diets provide high fatty acid availability, so high levels of phosphorylation of Thr1150. If you are oxidising fat it is essential that you reduce the oxidation of glucose by an appropriate amount.
The phosphorylation of Thr1150 is an enzymic mechanism of inducing insulin resistance, in insulin sensitive cells, in proportion to the availability of free fatty acids.
This is completely separate from the redox induced insulin resistance posited by the Protons hypothesis.
It has its uses. It has its limitations.
The first use is that it allows fatty acid induced insulin resistance without the need for those redox changes intrinsic in fatty acid oxidation. If you want a safety mechanism to limit ROS generation to tightly controlled levels this could be one method of achieving it. A fine tuning mechanism.
The second is as a safety net for when the redox signal from fatty acid oxidation fails, as it does under the beta oxidation of linoleic acid.
This latter is what I want to look at in more detail today.
When we change a mouse from chow to D12942 there is a sudden switch from a metabolic substrate mixture with a normal redox balance to one with reduced ability to generate ROS. Because PUFA are preferentially oxidised, the effect should occur rapidly.
The insulin secreted in response to a meal acts *too* effectively and activates both the pyruvate dehydrogenase complex and the acetyl-CoA carboxylase complex to allow the generation of malonyl-CoA in the cytoplasm, which inhibits fatty acid entry in to the mitochondria. This marked fall in FAO results in a marked fall in ROS generation and reduces ROS derived insulin resistance still further.
This is very straight forward from the ROS perspective. Insulin over-acts, fatty acid oxidation plummets, there is an hypocaloric state which cannot be corrected by fatty acid oxidation and so carbohydrate oxidation predominates. D12942 only provides 20% of calories as carbohydrate so the mice need to eat an awful lot of it to stay alive. Here's the acute ROS mediated pathological insulin sensitivity and associated hunger. The unusable fat is mostly (but not completely) dumped in to adipocytes by the same insulin sensitivity.
Over the following days the free fatty acid levels in plasma rise because they are not being oxidised (see Astrup 1998, discussed here) and this allows the progressive accumulation of DAGs in the plasma membrane where they activate PKCε, which phosphorylates Thr1150 *irrespective* of the levels of ROS being generated within a given cell. If D12942 behaved like a physiological high fat diet we should get this, just from phosphorylation of Thr1150:
But failed fatty acid oxidation derived insulin resistance has dropped the baseline from which Thr1150 insulin resistance has to start. Combining the immediate intrinsic ROS signal change with the slower rise in FFA/DAG mediated insulin resistance gives this pattern:
As Thr1150 mediated insulin resistance rises, irrespective of the failed ROS signal, it progressively limits the pathological *excess* insulin signalling and allows progressively more fatty acid oxidation to become possible. The hunger of the first day gradually reduces until by day seven, while hunger is still slightly above that of chow fed mice, it is no longer statistically significantly so.
The next situation we need to look at is the mouse with the Thr1150A engineered mutation when fed D12942 but I've written enough for today. Oh, and I think we also need to consider the situation once *genuine* pathological insulin resistance is established in control and Thr1150A mice.
Peter
