Foie gras (9) Adipocyte ROS

Time to look at the Vaughan mouse weight gains and actual diet compositions. Here is a recap of the weight gains:

and here is Table 1 with the percentages of energy from linoleic acid added in red by myself:

It's clear that the saturated fat group were only fed 6% of their calories as insulin sensitising linoleic acid. In addition to this relatively low amount of LA, the stearic acid in the saturates has an high F:N ratio and will in part offset the low F:N ratio of the LA component.

If adipocytes resist insulin they stay small.

The oleic acid group were fed 7% of calories as LA but with only 12% of calories as saturated fat there is nothing to oppose the insulin sensitisation effect of LA so they gained a significant amount of weight.

The high (35% of calories) LA diet is uncoupling, produced low levels of ROS as a consequence, and so limited weight gain. Given long enough it would normalise bodyweight.

The high omega 3 diet contained an obesogenic level of LA coupled with an uncoupling level of alpha linolenic acid, though putting a number to the level of ALA needed to uncouple is difficult, but it is lower than the level of LA needed. This is another combination which I expect, given long enough, would normalise body weights. 

I feel that some explanation is needed as to why the LA/ALA 18:3n3 diet mice, and to some extent the high LA diet mice, weigh more than the saturated fat diet mice, to the point that the 18:3n3 diet looks obesogenic.

There are, initially, two effects of fatty acids with multiple double bonds. The first is the reduction in RET through complex I which itself has two effects. Under peak insulin action there is reduced negative feedback so adipocytes distend more than they should. Second is that during fasting, when fat is the primary fuel, RET should occur to limit glucose utilisation in order to spare glucose for the brain/hypothalamus and so limit hunger. With the blunted RET from LA/ALA more glucose is used by muscle etc so more food must be eaten, so breaking the fast, in order to keep glucose levels adequate to limit hunger. Much of this extra food then gets stored.

These two together put fat in to adipocytes and demand more food intake. This is the classic situation under D12492.

This is also likely to be the initial situation when using 35% LA or a mix of 11% LA with 23% ALA in the period before uncoupling becomes established.

The second effect is via uncoupling.

There will be weight loss in these latter two diets, but only once UCP-1 is activated in white adipocytes to lower delta psi and so reduce insulin signalling. At that point adipocyte FFAs can either be oxidised to release heat in situ (beiging of WAT) or transferred to BAT where high levels of UCP-1 can oxidise them to release heat in bulk.

This concept suggests that, by 14 days, the mice on the "18:2n6" diet would be in weight loss and should have low ROS generation due to uncoupling, after an initial weight gain.

The effect should be more marked in the "18:3n3" group, ie an higher initial weight gain, then incipient weight loss by 14 days.

This is why I like the Schwartz data, daily resolution of food intakes and fat mass changes allow you look at things more mechanistically. It would have been nice to have these data from the current pro-linoleic acid study but thats not what the study was all about.

Here are the mRNA data for inflammatory gene expression in adipocytes in vivo (or immediately post euthanasia!), which I am taking to be a surrogate for ROS generation:

 

From the ROS perspective the SFA adipocytes are generating ROS by RET, so are limiting insulin signalling-induced lipid droplet distension. The mice are slim. And healthy.

The 18:2n6 mice are uncoupled, have low ROS due to this and are actively losing weight after an initial gain. Same for the 18:3n3 mice.

The HF fed mice (9.6% LA, obesogenic, failing to limit insulin signalling) have low ROS because they are failing to generate enough of them via RET to limit caloric ingress, ie have "pathological" insulin sensitivity. "Healthy" insulin sensitivity, through healthy ROS, is shown by the SFA group. The HF group are simply sequestering calories in to lipid droplets without oxidising them. Here weight gain is on-going but there is no issue with high ROS because they are effective at sequestering calories. Except...

Now the HF fed mice are really interesting. They have levels of inflammatory gene mRNA expression comparable to all of the other groups, including the SFA group (p>0.05 for the comparison) but look at their MPO activity, an indicator of active inflammation. I've rearranged chart G so all columns are on the same scale:

All groups of mice have comparable levels of inflammatory gene mRNA expression (pax the SFA fed group) but only the HF group have actively inflamed adipose tissue.

Why?

We can say that generating mRNA from pro-inflammatory genes alone is not sufficient to activate the inflammatory cascade to the extent of activating the myeloperoxidase system.

I have to ask myself what, exactly, is the function of these genes we are looking at, within physiology, at the most basic level.

I would suggest that they might be to deal with normal ROS generated from normal metabolism. The SFA diet induces high levels of healthy ROS via RET. It generates a large, effective response in ROS mitigating genes. All other groups, at the 14 day mark of the study, have low levels of RET derived ROS, so low levels of mRNA from inflammatory (or rather mitigating) genes. Including the HF diet group.

What is different about the HF diet group is that they are, through Protons, unable to limit caloric ingress. As much of the excess calories as possible will be rendered in to harmless stored triglycerides but all that is needed to generate frank inflammation is the generation of a delta psi in excess of 170mV. This leads to ROS which are only in a small part derived from RET, ie are mostly pathologically derived.

I think the HF diet fed mouse adipocytes are doing this. There is tissue damage occurring and lipid peroxides are produced at levels which signal danger of serious injury and so macrophages move in to sort out the damage. Probably making incorrect assumptions about the source of the damage, leading to pathology. This appears, in this study, to be independent of the expression of what are considered, in this study, to be pro-inflammatory genes.

Activation of the myeloperoxidase system, as observed in the current study, is not a simple consequence of activating mRNA generation of inflammatory genes.

It just strikes me that expressing a gene and using its product may be greatly influenced by factors this study doesn't address.

So there are at least three descriptions possible for the state of ROS generation in the adipocytes of these mice. There are no simple linear relationships. You need some sort of framework to understand what is going on.

Protons.

Which makes me happy.

Peter