Clocking In With Cancer Treatment

I always hesitate to say things like ‘you may recall’ as, from much undergraduate teaching, I’ve learned that blithe throwaways like ‘you’ll remember this from last Monday’s lecture’ tend to be met with blank stares and trying ‘you met this idea in the first year’ on second year classes will draw forth outright mirth blending with mutinous howls. So let’s just start by noting that three weeks ago in Our Inner Self we had a march-past of our intestinal army of bacteria and saw that it is in continuous flux, its make-up oscillating in time to our biological clock – the daily variation that governs most of our bodily functions including the sleep-wake cycle. That’s amazing stuff but a sharp bit of lateral thinking raises interesting questions. If most of the important things in our bodies tick to circadian rhythms, is cell proliferation one of them – after all, the process of cells making more of themselves is at the heart of life. Answer ‘yes.’ But, as abnormal cell proliferation – i.e. something going wrong – is a perfectly adequate three-word definition of cancer, a small step extends the question to ‘do tumours also have rhythm?’ Answer, again, ‘yes.’ A little background before we explain.

Turning back to the clock

In Twenty more winks we saw that there’s a connection between sleep (or rather lack of it) and cancer and showed how two pairs of genes (CRY/PER and CLOCK/BMAL1) lie at the core of circadian timekeeping. They control the sleep-wake cycle and much else. The proteins they make form an orchestrated feedback loop, synchronised by light-induced signalling. That is, the expression of each pair oscillates with a period of roughly 24 hours, but the pairs are out of step to the tune of about 12 hours. The proteins encoded by these genes regulate the expression of many other genes that ensure the cells and tissues of the body beat to an appropriate rhythm. Many messengers spread circadian oscillations around the body via the blood of which, in humans, cortisol (made by the adrenal glands) is perhaps the most familiar (it’s a steroid hormone: the medication dexamethasone is cortisol with two small, extra bits that make it 25 times more potent). You can fairly easily measure cortisol concentration in blood and you’d expect to find that at nine in the morning you’d have roughly double your midnight amount. In other words cortisol is part of your wake-up call. It turns on your appetite, gets you geared up for physical activity and it also activates anti-stress and anti-inflammatory signal pathways. EGFR & cortisol Cross-talk between EGFR and cortisol during the active phase (right: high cortisol) and the resting phase (left: low cortisol). (from Lauriola et al., 2014).

Getting the message across

Taking the memory-prodding risk yet again, in Mission Impossible? we described how biological signals from the outside world bind to receptors (proteins) to convey their message (I’m here, do something!) to the interior of cells. So the picture is: cells receive many signals from messengers that, one way or another, talk to the nucleus, switching on genes that drive proliferation. Most external messengers are proteins themselves – one example is a potent growth promoter called epidermal growth factor (EGF) that works by switching on the EGF receptor (EGFR). Cortisol isn’t a protein: as we’ve noted, it’s a steroid – which means it can diffuse across membranes – but, once inside a cell it works in essentially the same way, by binding to its specific receptor. The upshot of all this is that messengers transmit information from outside the cell to the nucleus – where DNA lives, the cells’ repository of genetic material – so that genes become activated to produce proteins.

Oscillating signals: cellular chattering

The picture of multiple, linear signalling pathways co-existing within cells invites the idea that their protein components might be unable to resist tapping in to their neighbours’ conversations – and so it has turned out. However, for pathways like the EGFR that signal cell growth, cross-talk with cortisol signalling is more than merely listening in. Proteins activated by the steroid hormone can actually interfere with the relays in the EGFR pathway so that EGF signals are suppressed during the active phase (day-time in us, night-time in rodents) but enhanced during the resting phase.

The meaning for life

So growth signaling is under circadian control – by and large our cells do their multiplying when we are at rest. Interesting although perhaps not unexpected. But, as The Bonzo Dog Doo-Dah Band warbled, ‘here comes the twist’ (Urban Spaceman, 1968 if you’re struggling). These pathways are the very ones that are hyper-activated (i.e. mutated) to drive cancer cells to make more of themselves and they are, accordingly, the targets for many anti-cancer drugs. However, chemotherapy is usually administered as single bursts, at daily or longer intervals, and drugs are progressively removed by metabolism thereafter. This means that much of the impact may be lost if, when the drug concentration is at its highest, the target pathway is already suppressed by high glucocorticoids . There’s evidence consistent with this idea from animals bearing EGFR-driven tumours treated with specific inhibitors that are more effective if administered in the resting phase rather than in the active phase.

It’s all in the timing

So two new messages are now making themselves heard in the world of cancer biology. The first is beginning to tell the full story of clock complexity. The second takes up this theme by pointing out that a circadian clock-based model in cancer therapy may offer improved methods for prevention and treatment.

References

Lauriola, M. et al. Diurnal suppression of EGFR signalling by glucocorticoids and implications for tumour progression and treatment. Nat. Commun. 5:5073 doi: 10.1038/ncomms6073 (2014).

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Twenty more winks

In Episode One we alerted ourselves to the large amount of evidence saying that a good night’s sleep really is essential if you wish to reduce your chances of a wide variety of medical misfortunes. But what do we know about how molecules respond to sleep disruption to produce such nasty effects?

Molecular Clocks

Life on earth depends on energy sent forth by the sun and, in synchrony with the rotation of our planet, many of the inner workings of mammals fluctuate over each period of roughly 24 hours. This pattern is called the circadian clock, its most obvious manifestation being the sleep-wake cycle. Over the years considerable evidence has accumulated that the link between shift-work and cancer is probably due to circadian rhythm disruption and suppression of nocturnal production of a hormone called melatonin. All living things make melatonin (in mammals in the pineal gland of the brain) and it signals through a variety of protein receptors on cells to regulate the sleep-wake cycle but it also plays a role in protecting DNA from damage.

Melatonin production is regulated by the circadian oscillator, itself controlled by two sets of proteins that control each other’s expression in a feedback loop. Thus one pair, CLOCK and BMAL1, activates Cryptochrome and Period. They in turn repress CLOCK and BMAL1 – the upshot being that the activities of both pairs oscillate over a day-night cycle: as one goes up the other comes down. These central regulators are encoded by evolutionarily ancient genes (two for Cryptochromes and three for Period proteins). In plants and insects CRY1 responds to light but in mammals CRY1 and CRY2 work independently of light to inhibit BMAL1-CLOCK.

Two interlocked feedback loops control clock protein expression

CRY-CLOCK

OUTCOME: ≈ 24 hour cycle expression of PER & CRY

BMAL1 & CLOCK 12 hours out of phase

Alarming the Clock

So having sounded the alarm that just one night’s sleep shortage has obvious effects, what do the genes make of it? Well, the short answer is they get upset. A recent study took blood samples from a group of normal people and found that more than 700 genes (about 3% of our total number) significantly changed their level of expression over 1 week of insufficient sleep (5.7 h) by comparison with 1 week of sufficient sleep (8.5 h). About two-thirds were reduced whilst one-third was up-regulated (made more of their protein product). Unsurprisingly, among those that went down were the major clock regulators. It’s worth noting that the sleep perturbation in this experiment was relatively mild – intended to be similar to that experienced by many individuals. The genes most strongly affected play roles in a wide range of biological processes – DNA structure (hence gene expression), metabolism, stress responses and inflammation. The responses of genes to changes in sleep patterns are not the result of mutation (i.e. changes in the sequence of DNA)  but, at least in part, they’re caused by small changes in the structure of DNA. {These are 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}. Thus there is evidence for gene silencing by hyper-methylation of CRY2 (adding methyl groups (CH3) to its DNA) and the converse effect of hypo-methylation (removing methyl groups) of CLOCK occurs in women engaged in long-term shift work and is associated with an increased risk of breast cancer.

Inflaming the Problem

The cells that mediate inflammation and immune responses also have circadian clocks – meaning that normally these processes are rhythmically controlled and clock disruption (for example by sleep loss) affects this pattern. Disabling the clock in mice (by knocking out CRY altogether) switches on the release of pro-inflammatory messengers and knocking out one of the Period genes (PER2) makes mice cancer-prone – reflecting the fact that MYC (the key proliferation driver) is directly controlled by circadian regulators and is consistently elevated in the absence of PER2.

Clock Faces

The mass that comprises a tumour is a mixture of cells – cancer cells and normal cells attracted to the locale – so it’s a quite abnormal environment and in particular there may be regions where the supply of oxygen and nutrients is limited. This is sensed as a stress by the cells, one response being to lower protein production until normal conditions are restored. If this doesn’t happen within a given time the response switches to one leading to cell suicide. One way in which overall protein output can be reduced is by activating an enzyme (IRE1α) that breaks down code-carrying messenger RNAs that direct assembly of new proteins. Remarkably, it has emerged that one of the mRNAs targetted by IRE1α is the core circadian clock gene, PER1. The degradation of PER1 mRNA means that less PER1 protein is made, which in turn disrupts the clock. However, it seems that PER1 has other roles that include helping the cell suicide response – a major anti-cancer defence. All of which suggests that disruption of the IRE1α/ PER1 balance might have serious consequences. Indeed IRE1α mutations have been found in a variety of cancers including brain tumours in which low levels of PER1 are an indicator of poor prognosis. The IRE1α mechanism coincidentally activates the transcription factor XBP1 (as well as PER1 mRNA decay) and one target of XBP1 is the gene encoding a messenger (CXCL3) that makes blood vessels sprout offshoots. Thus this master regulator suppresses cell death, activates proliferation (lowering PER1 deregulates MYC) and promotes new blood vessel formation.

A Tip for Snoozing

If you’re still wide awake it just goes to prove the utter fascination of biology – but today’s story says that you have to find ways of, if not falling asleep, at least courting insensibility (as Christopher Fry put it). If it’s a real problem for you may I make a really radical suggestion? Turn to our physicist friends and select from their recent literary avalanche. A ‘brief history of …’ something or other will do fine. It’s a knock-out! Sweet dreams!!

References

Möller-Levet, C.S., Archer, S.N., Bucca, G., Laing, E.E., Slak, A., Kabiljo, R., Lo, J.C.Y., Santhi, N., von Schantz, M., Smith, C.P. and Dijk, D.-J. (2013). Effects of insufficient sleep on circadian rhythmicity and expression amplitude of the human blood transcriptome. PNAS 110, E1132-E1141.

Fu, L.N. et al. (2002). The circadian gene Period2 plays an important role in tumor suppression and DNA damage response in vivo. Cell 111, 41-50.

Zhu, Y. et al. (2011). Epigenetic impact of long-term shiftwork: pilot evidence from circadian genes and whole-genome methylation analysis. Chronobiol Int, 28, 852–861.

Pluquet, O. et al. (2013). Posttranscriptional Regulation of PER1 Underlies the Oncogenic Function of IREα. Cancer Res., 73, 4732-4743.