Ketosis IV: Cellular Regulation

As we have seen already the control of ketosis is controlled through a variety of mechanisms in our bodies. And, most importantly, many of these mechanisms are intricately tied to our bodies’ own energy sensing pathways. We have seen ketosis regulation at a variety of levels throughout our bodies.  It can be controlled through conscious action, by eating a high-fat diet or fasting.  It can be controlled throughout our internal environment by hormones and other signaling molecules like insulin and glucagon.  And now we will go down a bit farther even, and look at how ketosis is controlled at the cellular level.  Again this one will be a technical article, so as always if you have questions post them to comments!

PPARa

PPARa is a receptor found in many tissues of the body, especially liver and adipose, that is a major factor in lipid metabolism, and stimulating ketogenesis (1).  PPARa is activated by certain fatty acids inside our body, but is also activated by fibrate drugs, and is used as a molecular target for people with lipid disorders (2). 

Now in the usual setting it is tough to assess the role of PPARa in ketogenesis or lipid metabolism, because it is usually expressed a fair amount in most tissues.  However, we can use these fibrate drugs to activate PPARa when we want to, and study the effects of increased PPARa activation.  And that is exactly what the authors of this study did (3).   By activating PPARa with bezafibrate the authors were able to find out some amazing actions of PPARa.  The first graph from the paper we will look at is this one showing fatty acid metabolism.

In this graph the black squares represent the patients treated with bezafibrate (and who would have greater PPARa activation) while the white ones are the controls.  They are measuring the % recovery of CO2, which is very complicated way of measuring how much b-oxidation is happening.  As we can see the black squares have significantly more b-oxidation happening, and another graph from the same paper also shows a lower level of circulating fatty acids which is another way increased fatty acid metabolism can be seen.

Now as we know increasing the amount of fatty acids we breakdown should lead to greater levels of acetyl-CoA, and then to greater levels of ketone bodies.  While the authors didn’t measure acetyl-CoA levels, they did measure serum levels of BHB (b-hydroxybutyrate, the main ketone body in humans).  This next graph shows the levels of BHB expressed as a % increase from the baseline.  We can see that in the patients treated with bezafibrate (again the black squares) by hour 6 they have had almost a 5-fold increase in BHB!  Since we have such a large increase in circulating ketones, we can indeed say the PPARa is responsible for the activation of ketogenesis.

I’d like to discuss one more figure from this very interesting paper, and that is this one showing the triglyceride levels of the patients treated with bezafibrate. 

While this one doesn’t have to do directly with ketosis, I think it is very interesting.  As you can see the group with greater PPARa activation also has significantly lower triglycerides.  Through researching this topic this was one thing I saw fairly frequently and I think is a huge benefit of ketosis.  When we are in ketosis we are not making and packaging lipids into triglycerides, or even LDL particles.  While personally I think the triglycerides are the most important thing measured on a standard lipid panel as they signal the metabolic health of the individual, I think ketosis through PPARa activation can improve all numbers on that panel. 

There are many other things PPARa activation does for us, as it is involved in many other pathways.  It has also been shown to help prevent obesity, in addition to lowering triglycerides (4).  However, the next main action of PPARa I would like to discuss is its activation of FGF21.

FGF21

To discuss the actions of FGF21 we will cover much of this article, as it is a very comprehensive experiment covering many of the actions of FGF21 (5).  I think the first thing to ask is does PPARa really activate FGF21? Well, yes, and we can see in this graph that in human liver cells activation of PPARa with a drug (GW) we do indeed get a huge increase FGF21 expression.

Now, here is where things get a bit interesting.  We know the PPARa activation activates ketogenesis from the graphs above, we also know that PPARa activation activates FGF21, so the next natural question is how do we know the effects we saw in the previous study on PPARa aren’t actually coming from FGF21?  Well these authors asked that same question and designed a pretty cool experiment to test it. 

Several groups of genetically engineered mice were used to see whether FGF21 is primarily responsible for the induction of ketosis and lowering triglycerides.  Here is the figure of their results so we know what we are talking about but let’s break it down since it has so much information on it.

We will start at the outermost terms, fed and fasted.  These are pretty self-explanatory and just reflect the state the mice were in at the time of the study.  As we can imagine, the fasted group was included, as they would be expected to have a higher rate of ketogenesis. 

Next we have the designations WT and PPARa-/-.  This where the real cool genetic engineering happened.  WT just means “Wild Type” and is simply a control mouse with no engineering.  The PPARa-/- is the engineered mice.  These mice have been created to have NO PPARa!  They don’t express it at all!  If they don’t express PPARa they are the ideal subjects to see whether or not PPARa is responsible for the effect we can see, or whether it is all from FGF21. 

Well if PPARa activates FGF21 and we made mice with no PPARa shouldn’t they also not have any activation of FGF21?  Well that is taken care of in the innermost terms.  The “Veh” term just means vehicle and is a control.  The “FGF” term is a sign for an injection of FGF21.  So, while these mice might not activate express FGF21 naturally through PPARa stimulation, they do express it through getting an injection of it into their bodies. 

Now that we know what we are looking at, what does the figure tell us?  Well, all we really care about are the PPARa-/- sections that I circled here.

We see the same pattern in both the fed and fasted state, and that is for higher BHB levels and lower triglycerides in the presence of FGF21 and absence of PPARa.  So we still have ketogenesis, and we still have lowered TGs, all without PPARa!  This means that the FGF21 is real stimulator of ketogenesis!  However, it is important to keep in mind that we do still need PPARa to start all this under normal physiological conditions.

FGF21 stimulates ketogenesis and can lower TG levels in the liver, but does it have actions in other parts of the body?  The answer to that is yes!  If we want to stimulate ketogenesis where, other than the liver, would you expect it to act?  That’s right, our adipose tissue.

The authors noticed throughout all this that these engineered mice had significantly smaller fat cells, so they decided to see what FGF21 was doing there.  Well, it appears it was stimulating lipolysis, or fat breakdown!  You all remember our old friend hormone-sensitive lipase (HSL) from the first article on regulation?  Well, as we can see in this graph, FGF21 increases HSL expression!

This effect even happens in the mice lacking PPARa, showing this is a true effect of FGF21.

So through with this article in mind I think we can see that FGF21 is the primary molecular activator of ketogenesis.  It does this both by activating lipolysis in adipose tissue and by stimulating ketogenesis in the liver.  While FGF21 expression is driven by PPARa under normal physiological conditions, PPARa is merely the upstream activator of ketogenesis, not the actual activator itself.

Summary

Wow! What a lot of information we have covered in the past three articles.  We have gone from the ways you can consciously induce ketogenesis through your actions like fasting and high-fat diets, all the way down to the specific molecules that activate ketogenesis on the cellular level.  In this article we covered:

• PPARa is a primary activator of ketogenesis through its activation of FGF21.
• FGF21 acts on the liver to stimulate ketogenesis.
• FGF21 also acts on the adipose tissue to stimulate lipolysis through activation of hormone-sensitive lipase.
• Activation of FGF21 and PPARa can lower triglyceride levels while increasing ketone body synthesis.  This is a major benefit of ketosis.
• Together PPARa and FGF21 are the main cellular pathway for ketosis.

Well, I think that should cover all that I wanted to get through in terms of how ketogenesis is regulated!  I know it was a lot of information, and some of it was not that important to you if you just want to know how ketosis can benefit you, but I always think an understanding of the science is important.

Now that we have covered all this science we can start to look at the benefits of ketosis.  We will cover some of the medical applications of ketogenic diets, as well as some other ways ketosis benefits us.  However, I think next time we will look at a topic near and dear to my heart: medium-chain triglycerides!  Stay tuned!

P.S. If you want to get a head start on the next article check out this fantastic review of MCT’s that will be a primary part of the article!