The basic finding is that manipulating superoxide levels as close as possible to the ETC suggests that it is THE mediator of insulin resistance. Again, I'll skip a large amount of the extreme cleverness utilised and look at the bottom line and its implications. BTW the cleverness was very, very clever. How superoxide controls responsiveness to insulin, nobody knows (though George has some interesting ideas). But it appears to be a generic finding. They looked at steroids, they looked at TNF alpha, excess insulin (good old Somogyi) and, as you might expect, palmitic acid (as in the last post, on a background of 25mmol/l glucose). All cause insulin resistance in the models used. Also bear in mind that they are looking at myotubules and rather peculiar adipocyte-like cells. But I think they are probably correct in this basic conclusion.
Superoxide is core to insulin resistance.
It is very interesting to take this concept and look at various insulin resistance syndromes over the next few weeks.
Of course these folks are in obesity research so you have to be quite cautious when looking at their models and results. You also have to be very, very wary about their conclusions. This is the last sentence of the abstract:
"These data place mitochondrial superoxide at the nexus between intracellular metabolism [tick, agree] and the control of insulin action [tick, agree] potentially defining this as a metabolic sensor of energy excess [woaaaaah, care here]."
This is a slightly tricky sentence. It's that "excess" which bugs me. Look at section L from Fig 4 in the discussion to see how they are thinking:
Here we have a schematic of inactivity and overnutrition causing increased mitochondrial superoxide production. This clearly relates to the Denmark paper where people were paid to eat to excess while deliberately reducing their exercise. Fasting insulin spiked from 35pmol/l to 74pmol/l in 3 days. You can say that overnutrition certainly generates superoxide production. But is this what is happening in weight gain outside of paying people to over eat? That is not how most obese people become obese!
Inactivity and over nutrition are macroscopic changes and superoxide generation is a sub cellular mitochondrial effect. You have to be very careful in how you link the two features together. Superoxide may always signal insulin resistance but are there other drivers of superoxide production in addition to caloric excess?
The situation which keeps coming back to me is starvation.
There is no over nutrition during starvation. There is plenty of superoxide production. Why?
Humans have a brain which is rather dependent on glucose. Using glucose for non brain purposes during starvation would be potentially fatal. All tissues which can become insulin resistant should do so under these conditions.
Superoxide is utterly essential to the survival of starvation. Insulin resistance is a complete necessity.
It looks very much as if fat oxidation (especially palmitate) is directly set up to ensure this happens. It's the reason I was blogging about beta oxidation and FADH2 here. Fat supplies only two molecules of NADH for each of FADH2 and the beta oxidation derived FADH2 enters the electron transport chain through electron-transferring flavoprotein dehydrogenase, directly to the CoQ couple. This is a good situation to generate reverse electron transport, subsequent superoxide and trigger a specific refusal to process insulin. An overnight fasted human has total FFAs of around 0.5mmol/l and they stabilise at around 1.5mmol/l by four days of starvation. They stay there until some food, especially carbohydrate, is eaten.
This level (1.5mmol/l) should, by necessity, develop enough insulin resistance to stop GLUT4 dependent tissues from using glucose, to spare it for brain tissue.
Survival during starvation does not just necessitate using stored fat for energy. It necessitates the near complete abrogation of glucose usage for anything other than brain function. Not after that mere 14 hour fast before an oral glucose tolerance test, but certainly by four days without food. This abrogation cannot be reversed in a couple of hours during an OGTT. This is the "diabetes of starvation".
Superoxide is not always a marker of excess, though this is certainly one way of generating it. It is more accurately a marker of any situation in which insulin resistance is beneficial to survival.
Peter
And I really will get to emails some time soon (mea culpa!)
10 comments:
I appreciate this approach of thinking about IR as a natural response by the body when necessary, rather than a disorder. I have similarly seen obesity discussed in terms of a defense mechanism. Even if both these premises turn out to be false (and I doubt they will) I think it useful to "shake up" dogmatic thinking in this way and examine what has been taken as established "fact" from another angle. Perhaps this is how science should be done?
This is a bit off topic, but with all of the talk about the brain needing Glucose during starvation, I was curious if you've heard about the hypothesis that Lactate is preferentially used as a fuel in the brain?
http://en.wikipedia.org/wiki/Lactic_acid#Brain_metabolism
aelephant, Yes: Lactate to pyruvate to oxphos. Direct fuel injection, as close as you can get. Bugger glycolysis in the neurons themselves. I think it's probably correct. Now does insulin influence the PDH complex in the neurones as it does in other tissues? And does neuronal lipid accumulation affect this? So many questions.
Peter
And if the ROS => Nucleus => decreased phospholylation of Fox01 => retention in nuleus => increased activity of Fox 01 => gluconeogenesis pathway in hepatocytes (as seen in HCV and diabetes) works in the fasting state, we also have the source of that extra glucose the brain needs.
Ketoacids from gluconeogenic amino acids and lactate, and glycerol, being the main substrates.
How do the ROS get from the mitochondria to the nucleus?
Complex 1 and ROS, HCV model
http://www.ncbi.nlm.nih.gov/pubmed/16150732
"Liver mitochondria from transgenic mice expressing the HCV proteins core, E1 and E2 demonstrated oxidation of the glutathione pool and a decrease in NADPH content. In addition, there was reduced activity of electron transport complex I, and increased ROS production from complex I substrates. There were no abnormalities observed in complex II or complex III function. Incubation of control mitochondria in vitro with recombinant core protein also caused glutathione oxidation, selective complex I inhibition, and increased ROS production. Proteinase K digestion of either transgenic mitochondria or control mitochondria incubated with core protein showed that core protein associates strongly with mitochondria, remains associated with the outer membrane, and is not taken up across the outer membrane. Core protein also increased Ca(2+) uptake into isolated mitochondria. These results suggest that interaction of core protein with mitochondria and subsequent oxidation of the glutathione pool and complex I inhibition may be an important cause of the oxidative stress seen in chronic hepatitis C."
If, as it appears, this sort of thing is also done by other viruses, the idea of pathogenic factors playing a role in the obesity epidemic isn't so far out.
In my day (60s, 70s) we ate junk food, sugar, and to excess. People got very unhealthy and dropped dead of heart attacks, yet modern type of obesity was quite rare. There was only one fat kid per school.
http://ajpregu.physiology.org/content/290/1/R188.short
Viral obesity.
There's also been a correlation between the growing size of vaccination schedules and growing size of people. It would be good to see a study looking at whether total number of shots correlates with BMI or DM2 (not just vaccinated vs unvaccinated).
Here's the rest of the complex 1 ROS => Fox01 pathway:
From the HCV model of IR.
But as far as I can tell the fructose + high carb model is the same at the transcription end - and maybe the complex 1 end is substantitively the same.
In a Lustig sort of way, of course; "fructose is exactly the same metabolically as hepatitis C".
I don't think I'm there quite yet.
HCV infection induced JNK activation via increased mitochondrial ROS production, resulting in decreased FoxO1 phosphorylation, FoxO1 nuclear accumulation, and, eventually, increased glucose production. We also found that HCV NS5A mediated increased ROS production and JNK activation, which is directly linked with the FoxO1-dependent increased gluconeogenesis.
Hi Peter, great post.
“All tissues which can become insulin resistant should do so under these conditions.”
I’m guessing this is MOST important in the early stages of starvation when ketones are too low to support the brain. And you’re saying this is related to the spike in insulin resistance after only 3 days of overfeeding: both are caused by superoxide produced as a byproduct from palmitate oxidation. Right?
Thanks, Bill
Ah, perhaps the JNK is the circuit that conveys ROS complex 1 messages from the mitochonria to the nucleus.
Interestingly, this seems to be much the same circuit that cortisol uses.
I find the analogue circuitry metaphor to be the best way to get my head around transcription factors.
The nucleotides are digital memory, the proteins are analogue circuitry.
"Inflammatory signals, changes in levels of reactive oxygen species, ultraviolet radiation, protein synthesis inhibitors, and a variety of stress stimuli can activate JNK. One way this activation may occur is through disruption of the conformation of sensitive protein phosphatase enzymes; specific phosphatases normally inhibit the activity of JNK itself and the activity of proteins linked to JNK activation.
JNK, by phosphorylation, modifies the activity of numerous proteins that reside at the mitochondria or act in the nucleus." Wikipedia.
I suspect that it's not actually necessary to know any of this, and that genomics in biochemistry textbooks is just displacing the more useful clinical and physiological detail you find in older editions.
But Fox01, JNK and the gang do make for handy search terms.
This paper is a review of papers on HCV mitochondrial manipulation. Page 7 mentions reducing equivalents and ROS.
Also the paper is around PPARs and PPAR alpha is upregulated in carb restriction, inhibited by HCV core protein.
"The combination of these possible effects [Complex 1 inhibition with concomitant complex 4 inhibition by Ca+] would result in an over-load of harmful reducing equivalents throughout the respiratory chain complexes and in an extra-production of ROS with respect to their basal level [60,61]."
www.hindawi.com/journals/ppar/aip/605302.pdf
Gotta love the Italians; they always seem to be streets ahead in HCV research.
If you only read free radical and aging theory, it's easy to assume that ROS just diffuse everywhere until they hit an antioxidant, maybe a receptor, or an oxidizable protein or fat, which they damage.
It's hard to know how they move, but I think it likely that most are targetted in various ways.
e.g., Catalase is very efficient, but it cannot be everywhere. High efficiency is redundant if ROS are also going everywhere else.
A batter with a high hit rate needs a reliable pitcher.
It more likely that SOD and catalase are coupled in some way.
Similarly, GSH must be delivered to GPx; it cannot just wander over at diffusion rates.
I predict (with admittedly no evidence) that the majority of ROS are emitted in the direction of their proper targets or are guided along currents by weak (e.g. non-covalent bond type) forces.
Is this the connection between selenium supplements and increased diabetes risk (nearly doubled in this study; http://naldc.nal.usda.gov/download/46763/PDF)
Dietary Se seems to be protective if anything. Which may be a matter of timing and adaptation.
Post a Comment