Going With The Flow

The next time you happen to be in Paris and have a spare moment you might wander over to, or even up, the Eiffel Tower. The exercise will do you good, assuming you don’t have a heart attack, and you can extend your knowledge of science by scanning the names of 72 French scientists that you’ll find beneath the square thing that looks like a 1st floor balcony. Chances are you won’t recognize any of them: they really are History Boys – only two were still alive when Gustave Eiffel’s exhibit was opened for the 1889 World’s Fair.

One of the army of unknowns is a certain Michel Eugène Chevreul – and he’s a notable unknown in that he gave us the name of what is today perhaps the most familiar biological chemical – after DNA, of course. Although Chevreul came up with the name (in 1815) it was another Frenchman, François Poulletier de la Salle who, in 1769, first extracted the stuff from gallstones.

A few clues

The ‘stuff’ has turned out to be essential for all animal life. It’s present in most of the foods we eat (apart from fruit and nuts) and it’s so important that we actually make about one gram of it every day to keep up our total of some 35 grams – mostly to be found in cell membranes and particularly in the plasma membrane, the outer envelope that forms the boundary of each cell. The cell membrane protects the cell from the outside world but it also has to allow chemicals to get in and out and to permit receptor proteins to transmit signals across the barrier. For this it needs to be flexible – which why membranes are formed from two layers of lipids back-to-back. The lipid molecules have two bits: a head that likes to be in contact with water (blue blobs in picture) to which is attached two ‘tails’ ­– fatty acid chains (fatty acids are unbranched chains of carbon atoms with a methyl group (CH3–) at one end and a carboxyl group (–COOH) at the other).

Bilayer

Cholesterol_molecule_ball

A lipid bilayer                                          

De la Salle’s substance

 

The lipid ‘tails’ can waggle around, giving the plasma membrane its fluid nature and, to balance this, membranes contain roughly one molecule of ‘stuff’ for every lipid (the yellow strands in the lipid bilayer). As you can see from the model of the substance found by de la Salle, it has four carbon rings with a short, fatty acid-like tail (the red blob is an oxygen atom). This enables it to slot in between the lipid tails, strengthening the plasma membrane by making it a bit more rigid, so it’s harder for small molecules to get across unless there is a specific protein carrier.

Bilayer aThe plasma membrane. A fluid bilayer made of phospholipids and cholesterol permits proteins to diffuse within the membrane and allows flexibility in their 3D structures so that they can transport small molecules and respond to extracellular signals.


De la Salle’s ‘stuff’ has become famous because high levels have been associated with heart disease and much effort has gone into producing and promoting drugs that reduce its level in the blood. This despite the fact that numerous studies have shown that lowering the amount of ‘stuff’ in our blood has little effect on mortality. In fact, if you want to avoid cardiovascular problems it’s clear your best bet is to eat a Mediterranean diet (mostly plant-based foods) that will make no impact on your circulating levels of ‘stuff’.

By now you will have worked out that the name Chevreul came up with all those years ago is cholesterol and it will probably have occurred to you that it’s pretty obvious that our efforts to tinker with it are doomed to failure.

We’ve known for along time that if you eat lots of cholesterol it doesn’t make much difference to how much there is in your bloodstream – mainly because cholesterol in foods is poorly absorbed. What’s more, because it’s so important, any changes we try to make in cholesterol levels are compensated for by alterations the internal production system.

Given how important it is and the fact that we both eat and make cholesterol, it’s not surprising that quite complicated systems have evolved for carting it around the body and delivering it to the right places. These involve what you might think of as molecular container ships: called lipoproteins they are large complexes of lipids (including cholesterol) held together by proteins. The cholesterol they carry comes in two forms: cholesterol itself and cholesterol esters formed by adding a fatty acid chain to one end of the molecule – which makes them less soluble in water.

lipoprotein-structureChol est fig

Lipoprotein                                                               Cholesterol ester

Formed by an enzyme – ACAT –
adding a fatty acid to cholesterol.
Avasimibe blocks this step.

 

So famous has cholesterol become even its taxi service has passed into common language – almost everyone knows that high-density lipoproteins (HDLs) carry so-called ‘good cholesterol’ (back to the liver for catabolism) – low concentrations of these are associated with a higher risk of atherosclerosis. On the other hand, high concentrations of low-density lipoproteins (LDLs) go with increasing severity of cardiovascular disease – so LDLs are ‘bad cholesterol’.

What’s this got to do with cancer?

The evidence that cholesterol levels play a role in cancer is conflicting. A number of studies report an association between raised blood cholesterol level and various types of cancer, whilst others indicate no association or the converse – that low cholesterol levels are linked to cancers. However, the Cancer Genome Atlas (TCGA) that profiles DNA mutations and protein expression reveals that the activity of some genes involved in cholesterol synthesis reflect patient survival for some cancer types: increased cholesterol synthesis correlating with decreased survival. Perhaps that accounts for evidence that the class of cholesterol lowering drugs called statins can have anti-tumour effects.

In a recent development Wei Yang and colleagues from various centres in China have come up with a trick that links cholesterol metabolism to cancer immunotherapy. They used a drug (avasimibe) that blocks the activity of the enzyme that converts cholesterol to cholesterol ester (that’s acetyl-CoA acetyltransferase – ACAT1). The effect of the drug is to raise cholesterol levels in cell membranes, in particular, in killer T cells. As we’ve noted, this will make the membranes a bit more rigid and a side-effect of that is to drive T cell receptors into clusters.

One or two other things happen but the upshot is that the killer T cells interact more effectively with target tumour cells and toxin release by the T cells – and hence tumour cell killing – is more efficient. The anti-cancer immune response has been boosted.

Remarkably, it turned out that when mice were genetically modified so that their T cells lacked ACAT1, a subset of these cells (CD8+) up-regulated their cholesterol synthesis machinery. Whilst this seems a paradoxical response, it’s very handy because it is these CD8+ cells that kill tumour cells. Avasimibe has been shown to be safe for short-term use in humans but the genetic engineering experiments in mice suggest that a similar approach, knocking out ACAT1, could be used in human immunotherapy.

References

Yang, W. et al. (2016). Potentiating the antitumour response of CD8+ T cells by modulating cholesterol metabolism. Nature 531, 651–655.

Dustin, M.L. (2016). Cancer immunotherapy: Killers on sterols. Nature 531, 583–584.

 

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Invisible Army Rouses Home Guard

Writing this blog – perhaps any blog – is an odd pastime because you never really know who, if anyone, reads it or what they get out of it. Regardless of that, one person that it certainly helps is me. That is, trying to make sense of the latest cancer news is one of the best possible exercises for making you think clearly – well, as clearly as I can manage!

But over the years one other rather comforting thing has emerged: more and more often I sit down to write a story about a recent bit of science only to remember that it picks up a thread from a piece I wrote months or sometimes years ago. And that’s really cheering because it’s a kind of marker for progression – another small step forward.

Thus it was with this week’s headline news that a ‘cancer vaccine’ might be on the way. In fact this development takes up more than one strand because it’s about immunotherapy – the latest craze – that we’ve broadly explained in Self Help Part-1Gosh! Wonderful GOSH and Blowing-up Cancer and it uses artificial nanoparticles that we met in Taking a Swiss Army Knife to Cancer.

Arming the troops

What Lena Kranz and her friends from various centres in Germany described is yet another twist on the idea of giving our inbuilt defence – i.e. the immune system – a helping hand to tackle tumours. They made small sacs of lipid called nanoparticles (they’re so small you could get 300 in the width of a human hair), loaded them with bits of RNA and injected them into mice. This invisible army of fatty blobs was swept around the circulatory system whereupon two very surprising things happened. The first was that, with a little bit of fiddling (trying different proportions of lipid and RNA), the nanoparticles were taken up by two types of immune cells, with very little appearing in any other cells. This rather fortuitous result is really important because it means that the therapeutic agent (nanoparticles) don’t need to be directly targetted to a tumour cell – thus avoiding one of the perpetual problems of therapy.

The second event that was not at all a ‘gimme’ was that the immune cells (dendritic cells and macrophages) were stimulated to make interferon and they also used the RNA from the nanoparticles as if it was their own to make the encoded proteins – a set of tumour antigens (tumour antigens are proteins made by tumour cells that can be useful in identifying the cells. A large number of have now been found: one group of tumour antigens includes HER2 that we met as a drug target in Where’s That Tumour?)

The interferon was released into the tumour environment in two waves, bringing about the ‘priming’ of T lymphocytes so that, interacting via tumour antigens, they can kill target cells. By contrast with taking cells from the host and carrying out genetic engineering in the lab (Gosh! Wonderful GOSH), this approach is a sort of internal re-wiring achieved by giving a sub-set of immune system cells a bit of genetic code (in the form of RNA).

TAgs RNA Nano picNanoparticle cancer vaccine. Tiny particles (made of lipids) carry RNA into cells of the immune system (dendritic cells and macrophages) in mice. A sub-set of these cells releases a chemical signal (interferon) that promotes the activation of T lymphocytes. The imported RNA is translated into proteins (tumour antigens) – that are presented to T cells. A second wave of interferon (released from macrophages) completes T cell priming so that they are able to attack tumour cells by recognizing antigens on their surface (Kranz et al. 2016; De Vries and Figdor, 2016).

So far Kranz et al. have only tried this method in three patients with melanoma. All three made interferon and developed strong T-cell responses. As with all other immunotherapies, therefore, it is early days but the fact that widely differing strategies give a strong boost to the immune system is hugely encouraging.

Other ‘cancer vaccines’

As a footnote we might add that there are several ‘cancer vaccines’ approved by the US Food and Drug Administration (FDA). These include vaccines against hepatitis B virus and human papillomavirus, along with sipuleucel-T (for the treatment of prostate cancer), and the first oncolytic virus therapy, talimogene laherparepvec (T-VEC, or Imlygic®) for the treatment of some patients with metastatic melanoma.

How was it for you?

As we began by pointing out how good writing these pieces to clarify science is for me, the question for those dear readers who’ve made it to the end is: ‘How did I do?’

References

Kranz, L.M. et al. (2016). Systemic RNA delivery to dendritic cells exploits antiviral defence for cancer immunotherapy. Nature (2016) doi:10.1038/nature18300.

De Vries, J. and Figdor, C. (2016). Immunotherapy: Cancer vaccine triggers antiviral-type defences.Nature (2016) doi:10.1038/nature18443.