Food Fix For Pharma Failure

 

If you held a global quiz, Question: “Which biological molecules can you name?” I guess, setting aside ‘DNA‘, the top two would be insulin and glucose. Why might that be? Well, the World Health Organization reckons diabetes is the seventh leading cause of death in the world. The number of people with diabetes has quadrupled in the last 30 years to over 420 million and, together with high levels of blood glucose (sugar), it kills nearly four million a year.

There are two forms of diabetes: in both the level of glucose in the blood is too high. That’s normally regulated by the hormone insulin, made in the pancreas. In Type 1 diabetes insulin isn’t made at all. In Type 2 insulin is made but doesn’t work properly.

When insulin is released into the bloodstream it can ‘talk’ to cells by binding to protein receptors that span cell membranes. Insulin sticks to the outside, the receptor changes shape and that switches on signalling pathways inside the cell. One of these causes transporter molecules to move into the cell membrane so that they can carry glucose from the blood into the cell. When insulin doesn’t work it is this circuit that’s disrupted.

Insulin signalling. Insulin binds to its receptor which transmits a signal across the cell membrane, leading to the activation of the enzyme PIK3. This leads indirectly to the movement of glucose transporter proteins to the cell membrane and influx of glucose.

So the key thing is that, under normal conditions, when the level of blood glucose rises (after eating) insulin is released from the pancreas. Its action (via insulin receptors on target tissues e.g., liver, muscle and fat) promotes glucose uptake and restores normal blood glucose levels. In diabetes, one way or another, this control is compromised.

Global expansion

Across most of the world the incidence of diabetes, obesity and cancer are rising in parallel. In the developed world most people are aware of the link between diabetes and weight: about 90% of adults with diabetes are overweight or obese. Over 2 billion adults (about one third of the world population) are overweight and nearly one third of these (31%) are obese — more than the number who are underweight. The cause and effect here is that obesity promotes long-term inflammation and insulin resistance — leading to Type 2 diabetes.

Including cancer

The first person who seems to have spotted a possible connection between diabetes and cancer was the 19th-century French surgeon Theodore Tuffier. He was a pioneer of lung and heart surgery and of spinal anaesthesia and he’s also a footnote in the history of art by virtue of having once owned A Young Girl Reading, one of the more famous oil paintings produced by the prolific 18th-century artist Jean-Honoré Fragonard (if you want to see it head for the National Gallery of Art in Washington DC). Tuffier noticed that having type 2 diabetes increased the chances of patients getting some forms of cancer and pondered whether there was a relationship between diabetes and cancer.

It was a good question then but it’s an even better one now when this duo have become dominant causes of morbidity and mortality worldwide.

We now know that being overweight increases the risk of a wide range of cancers including two of the most common types — breast and bowel cancers. Unsurprisingly, the evidence is also clear that diabetes (primarily type 2) is associated with increased risk for some cancers (liver, pancreas, endometrium, colon and rectum, breast, bladder).

With all this inter-connecting it’s perhaps not surprising that the pathway by which insulin regulates glucose also talks to signalling cascades involved in cell survival, growth and proliferation — in other words, potential cancer initiators. The central player in all this is a protein called PIK3 (it’s an enzyme that adds phosphate groups (so it’s a ‘kinase’) to a lipid called phosphatidylinositol bisphosphate, an oily, water-soluble component of the plasma membrane). It’s turned out that PIK3 is one of the most commonly mutated genes in human cancers — e.g., PIK3 mutations occur in 25–40% of all human breast cancers.

Signalling pathways switched on by mutant PIK3. A critical upshot is the activation of cell survival and growth that leads to cancer.

Accordingly, much effort has gone into producing drugs to block the action of PIK3 (or other steps in this signal pathway). The problem is that these have worked as cancer treatments either very variably or not at all.

The difficulty arises from the inter-connectivity of signalling that we’ve just described: a drug blocking insulin signalling causes the liver to release glucose and prevents muscle and fats cells from taking up glucose. Result: blood sugar levels rise (hyperglycaemia). This effect is usually transient as the pancreas makes more insulin that restores normal glucose levels.

Blockade of mutant PIK3 by an inhibitor. This blocks the route to cancer but glucose levels rise in the circulation (hyperglycaemia) promoting the release of insulin (top). Insulin can now signal through the normal pathway (bottom), overcoming the effect of the anti-cancer drug. Note that the cell has two copies of the PIK3 gene/protein, one of which is mutated, the other remaining normal.

Is our journey really necessary?

By now you might be wondering whether there is anything that makes grappling with insulin signaling worth the bother. Well, there is — and here it is. It’s a recent piece of work by Benjamin Hopkins, Lewis Cantley and colleagues at Weill Cornell Medicine, New York who looked at ways of getting round the insulin feedback response so that the effect of PIK3 inhibitors could be boosted.

Sketch showing the effect of diet on the potency of an anti-cancer drug in mice. The red line represents normal tumour growth. The black line shows the effect of PIK3 blockade when the mice are on a ketogenic diet: tumour growth is suppressed. On a normal diet the drug alone has only a slight effect on tumour growth. Similar results were obtained in a variety of model tumours (Hopkins et al., 2018).

They first showed that, in a range of model tumours in mice, insulin feedback caused by blockade of PIK3 was sufficient to switch on signalling even in the continued presence of anti-PIK3 drugs. The really brilliant result was that changing the diet of the mice could offset this effect. Switching the mice to a high-fat, adequate-protein, low-carbohydrate (sugar) diet essentially stopped the growth of tumours driven by mutant PIK3 treated with PIK3 blockers. This is a ketogenic (or keto) diet, the idea being to deplete the store of glucose in the liver and hence limit the rise in blood glucose following PIK3 blockade.

Giving the mice insulin after the drug drastically reduces the effect of the PIK3 inhibitor, supporting the idea that that a keto diet improves responses to PIK3 inhibitors by reducing blood insulin and hence its capacity to switch on signalling in tumour cells.

A few weeks prior to the publication of the PIK3 results another piece of work showed that adding the amino acid histidine to the diet of mice can increase the effectiveness of the drug methotrexate against leukemia. Methotrexate was one of the first anti-cancer agents to be made and has been in use for 70 years.

These are really remarkable results — as far as I know the first time diet has been shown to influence the efficacy of anti-cancer drugs. It doesn’t mean that all tumours with mutations in PIK3 have suddenly become curable or that the long-serving methotrexate is going to turn out to be a panacea after all — but it does suggest a way of improving the treatment of many types of tumour.

References

Hopkins, B.D. et al. (2018). Suppression of insulin feedback enhances the efficacy of PI3K inhibitors. Nature 560, 499-503.

Kanarek, N. et al. (2018). Histidine catabolism is a major determinant of methotrexate sensitivity. Nature 559, 632–636.

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Holiday Reading (2) – Poking the Blancmange

An evolutionary hiccup

It’s well known that tracing our family tree back 400 million years reveals a fishy past. This history is enshrined in our DNA in the pattern of nerves that control breathing. From time to time that control throws a wobbly in the form of involuntary spasms of the diaphragm manifested as a fit of hiccups – what the medics call singultus, which in Latin means sobbing – readily brought on by contemplating a comprehensive map of intracellular signalling pathways. Hiccups, however, are caused, as Neil Shubin, in his wonderful book Your Inner Fish has explained, by a mis-firing neuron in our brain stems that produces the type of electric signals that control the regular motion of amphibian gills. A genetic recipe hoarded in the nuclear loft is inadvertently recalled. For the most part this result of DIY evolution is no more than mildly embarrassing, although the poor fellow who made the Guinness Book of Records by hiccupping for 68 years may have used a stronger term.

As we’re really talking about cancer, we should mention that persistent spasms of hiccups and difficultly swallowing may be indicators of esophageal cancer, where a tumour in the gullet (the tube connecting the back of the mouth to the stomach) grows into the trachea and flips the hiccup switch by mechanical pressure.

Je pense, donc je suis un blanc-manger

Whilst the key feature of all these pathways is that they connect the outside world to the nucleus of a cell, it’s become clear that each pathway does not exist in isolation. Individual pathways can talk to each other – sometimes called cross-talk – individual domino runs intersecting, if you like. So evolution has cooked up thousands of proteins floating around in our cells that can be mapped into discrete signal pathways but, in the molecular jostle of the cell, each may affect any of the others – if not directly then via just a few intermediates. To avoid the Tokyo subway syndrome it’s easiest to think of the cell as a blancmange: poke it anywhere and the whole thing wobbles.

NetworkBlancmange

The complex network of signalling pathways in cells.

Left: the dots represent proteins that inter-communicate (lines) – best thought of as a blancmange.

Why is grasping this picture of what seems like a molecular madhouse important? Well, one thing we should bear in mind is that the set-up may look chaotic to us but our cells somehow make perfect sense of it all because they take clear decisions as to what to do. But the reason for grappling with it at all, other than to be humbled by our ignorance, is that these signal systems are a major target for anti-cancer drugs. To be more precise, it’s disruptions in these proliferation-controlling pathways, caused by mutations, at which we take aim with the contents of our drug cocktail cabinet.

What goes wrong in cancer?

If you want a three word definition of cancer ‘cells behaving badly’ will do fine. If you insist on being scientific ‘abnormal cell proliferation’ covers it nicely, meaning that control of cell replication has been overcome to the extent that cells reproduce more rapidly than they should or at an inappropriate time or in the wrong place. Underlying this abnormal behaviour is damage to DNA, that is, mutations. This remains true even if the initial cause does not directly affect DNA. It’s estimated that about 20% of the global cancer burden comes from infections, mainly in contaminated drinking water. These can cause chronic inflammation that eventually leads to mutations and thence to cancer. Other factors, for example, tobacco smoke and radiation, can directly damage DNA and about 10% of cancers are set off by what you might call a taxing inheritance – mutations already present in DNA at birth.

The capacity for high-throughput sequencing of complete human genomes has spawned ambitious projects that include Genomics England’s sequencing of 100,000 genomes by 2017 and The Cancer Genome Atlas that aims to provide a mutation data base for all the major cancers. One of the most mind-boggling facts that has already emerged from this revolution is the extent of disruption that can occur in the genomes of cancer cells: as many as one hundred thousand mutations within one cell. For the sake of completeness we should note that, cancer being cancer, the mutational spectrum is astonishing and, at the other end of the scale, there’s a childhood leukemia that results from just one change to DNA and there’s a type of central nervous system tumour that appears to develop without any mutations at all. For the most part, however, cancer cells carry a mind-boggling number of mutations and the assumption, nay hope, is that the vast majority of these changes are ‘passenger’ mutations that do not affect cellular behaviour: they’re a by-product of the genetic mayhem characterizing cancer cells. The ones that count are ‘driver’ mutations that can arise in any of several hundred of our 20,000 or so genes, changing the activity of the proteins they encode to contribute to cancer development. Only a small number (half a dozen or so), of these drivers, acting together, is required for cancer to emerge. Thus, although only a relatively small group of ‘drivers’ is needed, almost limitless combinations can arise.

The accumulation of mutations takes time, which is why cancers are largely diseases of old age: two thirds of them only appear in people over the age of sixty. The estimate is that if we lived to 140 everyone would get cancer but, pending that happy day, when or whether the disease manifests itself in an individual is indeed a matter of genetic roulette – genetic evolution within cancer cells. So wonderful has the technology become we can now inspect individual cells in tumours to reveal that driver mutations occur in single cells that can expand to form groups of cells, called clones. These multiple clones can modulate their mutational profile independently and, as a result, proliferate at different rates. So you can picture tumours as a complex patchwork of genetically related, competing clones. In other words, as we’ve suspected all along, cancers are a form of dynamic Darwinism.

The critical point is that key mutations drive cancer and they do so by upsetting the normal working of signal pathways that control whether cells proliferate or not. You could say it’s Nature poking the blancmange but these are delicately selected pokes – the product of the evolution of a cancer’s genetic signature – that just tweak signalling mechanisms enough to make cells a little more likely to multiply. In coming up with drugs that target specific mutations we’re giving the blancmange another poke – the aim being, of course, to prod it back to normality.

An obvious question

Having mentioned that, albeit very rarely, cancers emerge that don’t seem to be driven by changes in the sequence of DNA – how do they do that? The answer lies in epigenetic modifications – any modification of DNA, other than in the sequence of bases, that affects how an organism develops or functions. They’re brought about by tacking small chemical groups either on to some of the bases in DNA itself or on to the proteins (histones) that act like cotton reels around which DNA wraps itself. In effect this makes the DNA more difficult to get at for the molecular machines that turn the information in genes into proteins. So these small chemical additions act as a kind of ‘super switch’ that can, for example, block genes that act as brakes on cell proliferation – hence promoting cancer.

Reference

Neil Shubin Your Inner Fish, Random House, 2008.