Omega 3s and G-protein coupled receptors

Let's just summarise the role of omega 6 fats in Sauer's rat model of cancer:

In the lab situation rapid hepatoma tumour growth needs either arachidonic or linoleic acids. The acids must be taken up in to the hepatoma cells, they must be acted on by lipoxygenase to produce 13-hydroxyoctadecadienoic acid, better known as 13-HODE. 13-HODE appears to be the mitogen which promotes rapid cancer growth. 13-HODE looks like a repair signal gone wrong in cancer cells. Omega 3 fatty acids block omega 6 fatty acid uptake in to hepatoma cells. That's all well and good but the reason I got in to this paper was omega 3 PUFA signalling, rather than those omega 6 issues...



OK, Sauer starts to give some pointers on the function of omega 3 fatty acids in health. That's interesting, as I'm no great lover of any sort of PUFA when I view them from the Protons perspective, yet omega 3s seem to come out pretty well, certainly at low doses. You know my fall back, omega 3 PUFA don't always behave like omega 6 PUFA because they get used as signalling molecules blah blah blah. My own inability to tie the molecular structure of omega 3s to their clinical effects is very frustrating! That they probably act are sites "above" the ETC suggest that they act as what I view as 'high level signals".


Well they do.


The signalling appears to be through a G-protein linked receptor with all of the usual cAMP cascade that follows binding of a ligand to such a receptor. What I found particularly interesting was the effect produced on fat pads of normal rats when EPA (other papers from Sauer suggest all omega 3s act on whichever receptor is involved) was added to the perfusate.

OK, here is a neat little graph taken from here:

This is from fed rats. In the fed state the FFA uptake by the inguinal fat pad of a rat is about 6 mcg/min/gram, white open squares.

Adding EPA at 0.84mmol/l (a bit supraphysiological for EPA but let's let that ride) and FFA uptake by the fat pad drops to zero, or close to zero. Or, in fact, you could argue a suggestion of fatty acid relase, shown as a negative uptake value. Black circles. Fatty acids are not taken up, they end up in the venous effluent in the experiment, plus a little extra.

Whooooah, so do FFAs go through the roof when you take fish oil IRL??

Well no. That's because of this graph from here in the same paper:

Here we have the free fatty acid release from the inguinal fat pad of a healthy rat who has been starved for 48 hours. Fatty acid release is trundling along at about 3mcg/min/gram until EPA is added, again at around 0.8mmol/l. The release of FFAs, in the fasted state, is eliminated. Table 1 in the same paper shows you can get this effect of halting lipolysis in starved rats with under 0.3mmol/l EPA.

Both effects are mediated through a G-protein coupled receptor, ie high level signalling compared to electrons and superoxide in the electron transport chain.

Obviously there are a number of serious problems with this paper but, as a proof of concept, I buy it. I doubt DHA or alpha linolenic acid would work as well (or the group would have used them for this proof of point exercise!) and I think the levels of EPA used produce a very artefactual "switch-like" effect which is probably a graded response. I doubt 0.8mmol/l or even 0.3mmol/l of EPA is exactly physiological but...

Let's suggest that there is a progressive removal of the influence of adipocytes from the FFA flux in/out of plasma as the level of omega 3s in arterial blood increases. Omega 3 fatty acids render adipocytes irrelevant to free fatty acid levels in the plasma.

That is one hell of an idea.

Next we need a brief look at hepatoma cells, again the graph is provided by Sauer and it shows that omega 3 fatty acids, in a G-protein coupled receptor manner, completely turn off the uptake of ALL fatty acids in to hepatoma cells.

If, and it's quite a big "if", the same effects apply to hepatocytes as well as hepatoma cells, we then have a very straightforward mechanism for the protective effects of omega 3 fish oils on hepatic lipidosis. From my point of view this is quite real as there are pretty convincing papers showing that cats, in real life, can be largely protected against the potentially fatal hepatic lipidosis of rapid weight loss by modest doses of omega 3 fatty acids.

Soooo while omega 3s stop the release of all FFAs from adipocytes, they simultaneously stop the uptake of all fatty acids in to the two primary storage organs for fatty acids, adipocytes and liver.

Do plasma FFAs go up or down?

They do, of course, go down. A paradox? Next paper.

Health warning: This paper is so steeped in VLDL and ApoB lipophobia that it makes difficult reading. But there is so little published on FFAs and omega 3 supplementation that it's worth the ondansetron to read it. It's looking at how omega 3 supplements might lower fasting triglycerides, which are the devil incarnate for CVD risk. A huge chunk of VLDL comes from FFAs released from adipocytes and their subsequent repackaging by the liver. Apparently, and I quote from the abstract:

"FO [fish oil] counteracts intracellular lipolysis in adipocytes by suppressing adipose tissue inflammation"

A bit like insulin resistance is caused by "inflammation". Well, maybe it's that simple. They have taken the concept of high level signalling to its 2013 pedestal without looking for basic mechanisms. They have placed the G-protein coupled receptor on to macrophages in the fat pads, which subsequently control the adipocyte lipolysis using cytokines. I haven't checked how good this concept is. Sauer never looked that deeply. Looks a bit modern to me.

Personally I would guess that there are similar receptors on both adipocytes and hepatocytes, but the review does not seem to cover the ability of omega 3s to inhibit general fatty acid uptake by these two tissues. Ah well.

What they do argue is that omega 3 fatty acids upregulate lipoprotein lipase, pretty well whole body. Of course liver and adipocytes ignore this fatty acid bonanza, as above. LPL upregulation is what I needed to know from this paper.

So where do "spare" fatty acids go to? They go to muscles. Upregulated lipoprotein lipase (heralded as the saviour from elevated fasting triglycerides) allows increased lipid release from VLDL to lower those fasting triglycerides. But it's worth bearing in mind that cells do not "see" VLDL, the LPL is on the vascular endothelium and the cells behind the vessel wall only ever receive "free" fatty acids. These are not labelled as from albumin, VLDL or chylomicrons.

Slight aside for later: It seems likely that chylomicrons are going spill their lipids via that same LPL, worth remembering.

The story in the review can be sumarised as omega 3 fatty acids block the release of FFAs from adipocytes and increase the activity of lipoprotein lipase pretty well whole body. VLDL drops, FFAs drop. All is happy in the cardiovascular system. If you believe.


Various "bits" of omega 3s, especially the lipid peroxides of DHA, are signals for mitochondrial biogenesis. I had a paper which specified which lipoxide was most effective but must have missed the "save" button. Mea culpa yet again. There are hints here.


That's a very neat story, which has more than a grain of truth to it.

Why is it like this? What does it mean, physiologcally? Speculation time:


Omega 3 fats come from plants. Mostly from chloroplasts. Where do humans get their omega 3s from? Certainly not from plants. If we did then the rabid Dr Furhman would not be (correctly) recommending DHA supplementation (along with B12) to avoid brain collapse on veg*n diets. Actually, this link is quite funny when cited by Mc-Starch-Dougall:

"There is no evidence of adverse effects on health or cognitive function with lower DHA intake in vegetarians"

Well, I found it amusing. It's almost the converse of the neurological truism which states that being concerned about having a neurodegenerative disease probably means you don't actually have one.

Anyhoo. Away from the coast we have to get our DHA from animals (or buy algae derived supplements). They get it from grass. There is DHA present in adipose tissue of herbivores just as much as it is present in lipid membranes of their cells. My suspicion is that DHA is a signal to your metabolism that you have just eaten animal fat, from an animal who's food chain starts with grass [or algae]. The more fat you eat, the stronger the signal. We do not need much DHA overall for our brains as it is well protected in this site, but we might well be using it at low levels as a [G-protein coupled receptor sensed] signal to target metabolic adaptation to process fat. So is McDougall correct that veg*n "brain" tissue is OK, despite their periphery being depleted? Shrug.

Fish oil supplements? Well, using our "dietary fat is here" marker to pharmacologically modify some perceived CVD risk factor, without the appropriate change in source of metabolic fuel supply, looks to me to be of very limited value. Large intervention trials do show some benefit from omega 3s provided you do your stats well enough, you have a large enough population to pick up a very small effect and you give a high enough dose. But they do not seem to be any sort of panacea. Especially of you are avoiding dietary fat while "faking" the signal that you have eaten dietary fat...

This is not exactly surprising when you try to pick the likely physiology apart. I like the concept of DHA as an animal fat signal.

Peter

Final thought: Do we need omega 3 PUFA at anything above the most minimal levels if we are in saturated fat based ketosis? Of course I don't know. But the signal to cope with starvation is palmitic acid (physiological insulin resistance), not DHA. I live in starvation mode, not on a mixed diet with only intermittent access to healthy ruminant fat. I have long wanted to look at the selective release of FFAs from adipocytes in extended starvation. My suspicion is that in the early days after glycogen depletion palmitic acid is preferentially released over other lipids, PUFA are not needed/wanted. By a few weeks all the palmitate is gone and whatever is left then gets released. People like David Blaine suddenly start to feel weak, wobbly and are probably hypoglycaemic once they run out of palmitate and have to release less saturated fats. Two to four weeks if you carry some spare weight. Sauer's rats had only ever been fed a low fat omega 6 based diet and had no serious palmitate reserves, PUFA release came early for these.