Our own worst enemies? Why resistance is not futile, and what that means for cancer research

Sarah Byrne, Imperial College London

When the new millennium dawned, it felt like the future was finally here.

“Is this the breakthrough we’ve been waiting for?” the May 2001 cover of TIME magazine asked. Gleevec pills, golden and bullet-shaped, shone bright against a dark background. The imagery was clear: was this the magic bullet that would cure cancer once and for all?

“I think there is no question that the war on cancer is winnable,” said the director of the Memorial Sloan-Kettering Cancer Center, quoted in the same article.

Gleevec was a new drug to treat chronic myeloid leukaemia (CML), a fatal blood cancer affecting hundreds of people per year in the UK and several thousand in the US. It was also the first of a new generation of ‘targeted therapies’, smart drugs that would precisely target cancer cells. These were to be more effective than traditional chemotherapy, especially for hard-to-treat cancers such as CML, and with fewer side-effects as well.

But problems started to appear. Some patients who were initially responding well started to relapse: their cancer was developing resistance to the new drug. In the following years, several alternatives to Gleevec were developed to treat the drug-resistant cases. And again they initially seemed to work, but eventually the same problem arose. A decade later, that problem remains unsolved.

Resistance has in fact plagued most attempts to develop targeted therapies for cancer. It seems to be an inherent problem of the approach: its greatest strength — the precision targeting of a single gene or protein — is also its weakness. Only a small change or mutation in the cancer cell is necessary to stop it working.

But hasn’t all this happened before? The same rhetoric — ‘magic bullet’, ‘miracle drug’ — heralded the arrival of penicillin. And look at how that turned out.

Resistance is now a well-known problem in bacterial infections. These include the infamous MRSA ‘superbug’ which can now evade most commonly-used antibiotics; including, of course, penicillins. It’s a similar story with viral infections, including HIV: resistance is an increasing concern. Resistance to anti-fungal pesticides is a major issue for agriculture.

It’s not just the tiny things, either. When the disease mxyomatosis was introduced to control the rabbit population in Australia and Europe, it ended up producing a resistant population (‘superbunnies’, maybe?) and numbers began to increase again. It isn’t even strictly limited to living things. Resistance has been observed in prions — the abnormal protein molecules involved in neurological diseases BSE and CJD — which few would define as ‘alive’, though perhaps that definition is becoming less certain.

We do know that resistance is universal; inescapable. Whenever you apply a selective pressure to a population — anything that kills or impairs a large proportion of that population — you favour the survival of those who can resist it. Before long, they become the population.

Cancer cells are no different. They want to survive, to live as long as possible: forever, if they can. They want to be individuals, do their own thing, spread and migrate and colonise, build infrastructure to support themselves; heedless of the damage they cause to the body as a whole. Blind to the fact that they might be killing the host that supports them.

Wait, does that sound familiar?

We often refer to cancer cells as ‘abnormal’, because of the changes in their characteristics and behaviour compared to ‘normal’ healthy body cells. But think of the ancestry of a cell. Once, in a world long before we or any complex animals existed, unicellular organisms — tiny beings each consisting of a single cell — were the norm.

Their descendants are the ‘normal’ cells that make up our bodies. But they’re different now. Obedient and well-behaved; staying quietly in their assigned place in the body. Not taking more resources than are allotted to them. Following orders even to the point of sacrificing themselves willingly for the greater good: the needs of the many outweigh the needs of the few.

Not many of us would relish the chance to live in a society like that. It seems to go against every natural instinct. We want the freedom to travel where we will, to have as many or as few children as we choose, to consume what we want: survive and thrive and pass on our genes. Even if it harms the biosphere that supports us all. That’s our nature, the same as most living things.

So when you think about it, which cells are really the abnormal ones?

And right here is the problem we have come up against. If we didn’t have that drive to live and survive, we probably wouldn’t be trying to cure cancer in the first place. But we can’t have it both ways. If we are to have the imperative to survive, so must other forms of life — our common evolutionary history makes sure of that — and sometimes their needs come in conflict with our own. Usually, of course, we win. But when the conflict comes from within our own bodies, from our own oppressed cells turning freedom-fighter against us? The irony is particularly cruel, and particularly difficult to overcome.

None of this should detract from the advances that have been made. Gleevec was essentially a success story, as was penicillin in its time. For all the problems, Gleevec and its successors have dramatically improved the life expectancy of people living with CML, a report released in December 2012 showed .  Every extra year a patient gets to spend with their loved ones, to live their lives as they choose, must count as a win.

But the recurring resistance problem highlights a paradox at the heart of medicine: the strong instinctive compulsion to survive that keeps us fighting disease and death, may ultimately be the same force that keeps us from succeeding. At times, we are quite literally our own worst enemies.

The Logistics of Cellular Traffic

David Gershlick, University of Leeds

In every cell proteins are continuously crafted and assimilated into the cells of intricate organisms. After synthesis the proteins get directed by a complex concert of cellular machinery in order to assume their appropriate role. The eukaryotic cell can be roughly divided into several different sub-sections. Surrounded by a liquid membrane, analogous to the rubber of a balloon, cells contain a watery molecular soup, which is further accompanied by membranous sub-compartments (referred to as organelles). These organelles form specialised environments with distinct roles and characteristics. Proteins fulfil functions within the membranes, at the membrane periphery or in the liquid throughout the cell. Organising these processes is a multifaceted task with numerous components needing to deliver particular proteins to specific destinations.

Scientists choosing the task of unravelling these phenomena had a complex assignment. Early techniques allowed cells to be sliced into thin sections and imaged or split open and separated. It was the 1974 Nobel laureate George Palade who coupled the observed structures with particular functions. Palade, faced with an unnerving array of cross sections of cells, wanted to understand how proteins leave the cell. The approach involved adding a radioactive element, which was incorporated into freshly made proteins, then checking where the radioactive proteins were at different time points. Essentially, the route a protein took through the cell was tracked. These observations set the stage for the introduction of the secretory pathway as a functional system of organelles.

In the following years many conceptual breakthroughs were made. One such event was the isolation and characterisation of the ‘coated vesicle’. Vesicles are small spherical compartments shuttling from one organelle to another. Filled with proteins and other components they provide a mechanism for the transport of proteins, without having to cross a membrane. Vesicle budding/fusion events were characterised by in vitro reconstitution from isolated organelles. The rate at which these vesicles bud, migrate and fuse is unexpectedly high. It was once calculated that in mammalian cells there are approximately 155 of one particular type of vesicle budding per second. Cells are alive with hundreds of independent vesicles, sometimes travelling the length of the cell to specifically deliver their valuable contents.

In the late seventies researchers realised that there was a plethora of functional proteins waiting to be discovered. Yeast was the perfect organism for this work. They are single cells, with genomes simpler than mammalian or plant, but a seemingly as complex cellular architecture. Yeast geneticists led the way over the following 20 years with several key studies all with a shared principle. The genomes of whole populations of yeast were randomly mutated and screened to look for protein sorting defects, and the responsible mutation isolated. Although there was a degree of overlap in the studies, often new essential proteins were discovered. These methods identified a large array of effectors, allowing the mechanisms of specific processes to be elucidated.

It was understood that if a protein needs to reach a particular cellular destination to fulfil its role then it cannot do so passively. They need to be directed somehow to fill the place reserved for them in the relevant vesicle or compartment. Thus, to differentiate proteins from one another, they have specific signals. This led to the distinction of ‘cargo’ that is transported, from ‘receptors’ that mediate transport steps. In a further layer of complexity, these receptors must continuously recycle to pick up another round of cargo, much like a postman returning to pick up more letters to deliver. Often receptors pass through multiple compartments to deliver cargo. These mechanisms and protein interactions occupy scientists (including myself) to this day.

With the discovery of fluorescent proteins the study of protein trafficking had a technical revolution. Fluorescent proteins light up in a distinctive manner, in a background of effectively invisible peers. Making fused chimeras consisting of a protein of interest attached to a fluorescent protein has become commonplace. Fluorescent microscopy allows observation of the location of a protein within the context of a three-dimensional cellular environment avoiding having to slice the cell into sections. Impressive recent advances allow single molecules to be observed, as well as the imaging of vesicles in living cells.

By complementing technical developments with scientific progress the conserved mechanisms that marshal a very complex system are being exposed. Associated with defects of the pathways there are various human disorders, where understanding membrane trafficking holds hope for effective therapies. A range of microorganisms are known to hijack these pathways obtaining access to the protected inner cell, a better understanding of these perturbations not only sheds light on the processes mediating homeostasis in healthy cells but would also drive medical innovations. In my field of plant biology we study these processes to not only to gain an understanding of cell biology but also to work towards global issues. Comprehending the secretory pathway allows us to progress to the goals of creating storage compartments in plant cells for industrial and pharmaceutical proteins, to generate extra-nutritious food and even to produce biofuel in a more sustainable and yet profitable manner.

The revolutionary breakthroughs described in this article seem to have occurred every 5-10 years, and perhaps it would be prudent to anticipate another such progression. However, I believe, such predictions are misplaced. Science funding seems to have changed impetus from the so called ‘blue-sky’ research to an applied focus. Each of the advances discussed above are a direct result of curiosity driven blue-sky work, as any major novel innovations would also likely be.  That is not to say that applied research is not valuable, but with the majority of researchers having to focus on foreseeable impact in order to justify funding, it would seem obvious that the likelihood of major unexpected breakthroughs decreases. However, if the historical advances have taught us anything, it is that progress can happen in unexpected places at unexpected times and it is an exciting time to observe the subtle mysteries of the cell being gradually disentangled.

What makes us tick?

John Ankers, University of Liverpool

From the changing seasons to our daily sleeping patterns or the beating of our hearts, biological cycles are all around us. What we now know is that some of these very different natural cycles work together like cogs or gears in a giant clock. Understanding how these clocks work (and how they can go wrong) might bring new hope for treating diseases such as cancer.

In the northern hemisphere, the passage of the Earth around the sun gives us cold winters and warm summers. These, together with daily tidal patterns controlled by the moon’s orbit and a day-to-night cycle driven by a constantly spinning planet, put considerable strain on life on earth. How have plants and animals adapted to deal with such dramatic changes?

Imagine a huge, but invisible, clock. Environmental changes brought about by the sun and the moon turn cogs in this clock. The seasonal cog moves around once per year, the day-to-night cog is smaller and turns every 24 hours. These cogs are linked: as the earth moves around the sun, not only do the seasons change but the days get longer or shorter. Plants and animals have their own invisible clockwork. Each turn of the Earth’s light/dark cycle creates “circadian” (Latin meaning “about a day”) patterns of daily growth in plants and sleep in humans. The mechanics of the circadian cycle are complex, having evolved over millions of years, and sensitive to unexpected changes in the environment. This allows, for example, humans to react to abnormal periods of light or darkness (which may be experienced as jet lag) or plants to change their metabolism to compensate for a particularly harsh winter.

Many of the cells in the human body are repaired or replaced continuously (skin cells replace roughly every day, whereas nerve cells are never replicated). Fortunately for our clocks, this “cell cycle” is governed by a number of failsafes, ensuring that the correct cells are replicated in the correct way, and that this doesn’t happen too slowly – leading to problems in early development, or too quickly – leading to the possibility of tumour formation. There has been much work done (and a Nobel prize awarded) on the discovery of proteins that regulate the pace of cell division. The cell cycle is mechanically connected to our daily circadian cycle (their cogs are linked), suggesting that the human body might be able to renew itself more efficiently at different times of the day or night. Recently, the cell cycle has been shown to interact with an intriguing group of different cogs. These much smaller, faster cogs (cycling from 100 minutes up to 6 hours) can be “hooked up” to the clockwork when they receive emergency signals – such as the need to respond to an infection, or if DNA inside the cell is damaged – freezing the cell cycle of a faulty cell before it can be replicated so that threats may be averted or any damage repaired.

All of this makes our clock incredibly complex. Nevertheless, many of these cogs are turning now, in every cell in your body. Some are accelerating, stopping and restarting whilst some are checking, repairing, destroying and rebuilding, and it is the links between these cogs that have dramatic effects on the overall clockwork, and hence the health of our cells and tissues. So what happens to the clock in diseases like Alzheimer’s or cancer? And how do we, as observers, even contemplate fixing a faulty clock?

One of the reasons cancer is so difficult to treat is that a faulty cog in our clock (such as a cell cycle that is cycling too quickly) may be joined to many other “healthy” cogs such as those driving DNA replication and repair or the response to infection. Chemotherapeutic agents are used to destroy cells in the body with faulty cell cycles, but by doing so may also interfere with some other healthy cogs leading to severe side effects. Think of the challenge in repairing one faulty cog in the workings of a grandfather clock without disturbing the rest. Now imagine doing that whilst all the cogs are moving!

But it isn’t all bad news. Such a complex problem is being tackled in new and fairly unorthodox ways. My field, that of “Systems Biology”, is devoted to reconstructing the mechanisms that beat, pulse and oscillate inside the human body as computer models, just as an engineer might build a computer model of a plane or a skyscraper to identify and correct problems in its design. These models allow us to “virtually” unpick the clockwork of the human cell to see what makes it tick, and then probe and prod various parts in a way that might be too costly in the laboratory or unfeasible during surgery.

As we begin to understand more about how the clockwork of the cell is connected – how large, slow cogs wheels such as changing seasons, the circadian clock or cell cycle turn with smaller cogs like those driving emergency responses (or even smaller cogs cycling every few seconds like those driving electrical pulses in the heart or nervous system) – we may be able to design combination treatments that exploit the links between them. Slowing the cell cycle temporarily, for example, might allow a second drug to prompt a response to infection, which would otherwise have been blocked. Although there is still much to learn about our internal clockwork – exactly how connected are all of these different cogs? Are there any we have yet to discover? – the ability to help a diseased cell to repair (or re-set) itself no doubt offers an exciting prospect, and is surely only a matter of time.

Inducing Apoptosis- Countdown to Self-Destruction

By Susan Turrell, University of Leeds

From very early on in its life, a human cell is destined towards a particular fate. This job could be conducting electrical signals along a neural circuit, travelling through the body’s system of blood vessels on the lookout for harmful pathogens, or sensing light that has been focussed onto the retina of the eye, allowing us to visualise the world around us. But what happens if this pre-established plan goes wrong? What if this cell becomes infected with a malicious micro-organism, or if a vital signalling pathway becomes erratic and unstable?

Like an ageing car, if the cell is too damaged or dangerous to mend, it’s seen as a write-off and needs to be scrapped. Fortunately, every cell in our bodies has instructions for a self-destruct program maintained within its DNA.  If it can’t be mended, events are set in motion that culminate in the termination of that cell. This process is called apoptosis.

Apoptosis is a neat and precise method of eradicating cells in a multicellular organism. It involves the systematic shutdown of the cell, and occurs in an ordered sequence. First, the material in the nucleus, called chromatin, condenses and the cell shrinks and contracts. Second, the nucleus disintegrates and the structures inside the cell fragment. Finally, small enveloped pieces of the cell break off in a process called blebbing. The cell has essentially been packaged up into parcels called ‘apoptotic bodies’ for immune cells to engulf and dispose of.

However, apoptosis is not just a method of ‘clearing up’ damaged cells. The cells in our bodies are multiplying and dividing all the time, and yet we don’t just keep on getting bigger and bigger. Programmed cell death balances out this growth so that the number of cells in our bodies stays relatively constant. Apoptosis is also a fundamental part of development in the foetus. It is essential for sculpting individual digits by removing the webbed tissue between our fingers and toes. It’s also important in the developing nervous system. When they’re growing, several nerve cells all strive to form a connection to a corresponding nerve or muscle cell. Those that make contact can transmit electrical impulses to stimulate movement or sensation, while those that fail to reach are eliminated.

So what’s the mechanism behind this process? This countdown to controlled self-destruction is triggered in two ways. The cell can receive an external signal from other cells, or the process can be kick-started from within.  For example, receptors on the cell surface await a signal from immune cells which are like sentries, patrolling for potentially dangerous fugitives lurking inside cells. When they recognise that a cell is harbouring pathogens such as viruses or bacteria, the immune cells release factors which cause the infected cell to commence its ‘suicide program’. As well as this system, sensors inside the cell such as the protein p53 act as wardens for irreparable cell damage. p53 effectively performs an M.O.T on the cell by inspecting the DNA contained in nucleus. Depending on the level of any damage found, it either directs the repair of the affected DNA strand, or activates the self-destruct program. It does this so that any damaged DNA is not copied and passed on when the cell divides. Once these cell sensors are activated they start a cascade that amplifies the ‘death signal’ so that it cannot be switched off. The signal gets passed along to different proteins like a baton in a relay race, but each protein has several batons and so each handover involves more and more runners. In this way the cell becomes committed to the death program and can’t recover.

The final runners in this relay are a group of proteins called caspases. Caspases are expressed as inactive enzymes and have evolved to chop up other proteins when they get switched on. The termination signal is passed from ‘initiator’ caspases down to ‘executioner’ caspases, which are the bulldozers of apoptosis. These enzymes set about dismantling the structural components of the cell and this deconstruction leads to the breakdown of the cell contents.

When apoptosis stops working it can have disastrous results. One consequence is the uncontrolled growth of cells, leading to cancer. Cancer is caused by multiple mutations in different types of genes, and one of the most common proteins affected is the guardian of the genome, p53. If defective, this protein can’t activate apoptosis, and therefore cells that already have damaged and mutated DNA are allowed to multiply. Lots of cancer cells also have mutations in proteins involved in the apoptotic signalling cascade, so they can grow even when the cell is instructed to commit suicide.

I’m currently developing a gene therapy vector to treat cancer. This involves transporting a gene for a protein called TRAIL into cancer cells. TRAIL recognises cells that are carcinogenic and binds to cell surface death receptors. This activates the apoptotic signalling cascade from the outside.  If successful, this therapy would be specific to cancer cells, so would have fewer side effects than conventional cancer therapies. However, as some cancers have damaged apoptotic pathways, this treatment won’t be useful for all cancer types. I like the idea behind this potential therapy because we are using the body’s own defence system to kill the cancerous cells, we just give it a little extra ammunition.

Apoptosis is one of the mechanisms that maintains the balance between growth and stasis, health and disease. This balancing act is vital, as a problem in a tiny element of this pathway can have a massive detrimental effect. The body has evolved a way to sacrifice defective parts for the benefit of the whole organism. For this reason, each individual cell holds the seed to its own destruction.