Cancer stem cell breakthrough

Targeting cancer stem cells could be a powerful way to beat cancer in the future (Image from the Cancer Research UK LRI EM Unit)

It’s always exciting to see headlines about scientific breakthroughs, and today is no exception – we have “Breakthrough found to kill cancer spread”, “Scientists make breakthrough in fight against deadliest cancer cells” and the ambitious “Cancer – The end?”

The headlines came after researchers in Boston, US, discovered a way to target cancer stem cells – the rogue cells that are believed to be at the heart of many cancers, which are resistant to radiotherapy and current chemotherapy drugs. Their results are published today in the journal Cell.

But what’s the story behind the headlines? And is this really ‘the end for cancer’?

What are cancer stem cells?
We’ve got a longer post here explaining more about cancer stem cells, and research into them.   Briefly, cancer stem cells are rare ‘immortal’ cells that are found in a number of different cancers, including breast, bowel and prostate cancer, and leukaemia.  They produce not only ‘normal’ cancer cells that go on to divide many times, forming the bulk of the tumour – but also make new cancer stem cells.

Although they’re elusive, many researchers think cancer stem cells are the driving force behind many cancers.  Certainly, they’re resistant to radiotherapy and chemotherapy. The theory goes that these treatments kill off the ‘normal’ tumour cells but don’t touch the stem cells. Eventually these start growing again and the cancer comes back.

Although the importance  – and in some cases existence  – of cancer stem cells is still a hotly-debated topic in research, there is a growing body of evidence to suggest that rogue stem cells may play a role in at least a proportion of cancers.

Many researchers around the world – including several funded by Cancer Research UK – are working on the challenge of cancer stem cells. And now Piyush Gupta and his colleagues in the States have made an important scientific breakthrough, throwing the door open to exciting future treatments for cancer.

What did they do?
It’s very difficult to work with ‘real life’ cancer stem cells, as they only make up a tiny fraction of cells in a given tumour, so are tricky to extract from tumour samples.

So the researchers developed a technique for growing immortal – but non-cancerous – human breast cells in the lab, and then used genetic engineering to give them the properties of cancer stem cells. This meant that they could grow stem cell-like cancer cells in thousands of tiny plastic dishes.

The next step was to expose these cells to thousands of different chemical compounds, to find ones that killed them.  To start with, the scientists studied around 16,000 compounds, including those from commercial chemical ‘libraries’ as well as extracts from natural sources. Just 32 of these killed the stem cell-like cells. And, of these, only four worked consistently.

Narrowing it down further, the researchers found that one chemical – an agricultural antibiotic called salinomycin – was particularly selective and effective at killing off the stem cells.  Further tests in the lab showed that salinomycin was over a hundred-fold more effective at killing cancer stem cells than paclitaxel – a commonly-used breast cancer drug.

Tests with mice transplanted with human breast cancer cells revealed that salinomycin was effective at reducing the levels of stem cells in the tumours, and reduced the chances of the cancer spreading around the body.

Is salinomycin the ‘cure for cancer’?
We need to stress that these were laboratory experiments, and there is no evidence yet that salinomycin can treat cancer in humans.  Salinomycin is currently used as an antibiotic for chickens and cows, and it can be toxic or even fatal to humans, causing serious muscle and heart problems.

There’s still a long way to go before salinomycin is tested in cancer patients, and we would not recommend anybody to take salinomycin as a treatment for cancer.  More research needs to be done to find out if it works on human tumours in the body, and if a safe dose can be given.

It may even be the case that salinomycin itself never makes it to the clinic. Quite often researchers need to tweak the chemical formula of a promising compound to make it safe or effective, and this can take time. Or there are serious side effects that mean that it’s unsafe for general use.

So why is this research so important?
Scientifically speaking, this is really good stuff.  This is the first time that researchers have found a compound that acts on cancer stem cells. This tells us that they are not entirely invincible after all, and there is hope for future treatments that can kill off stem cells as well as the ‘bulk’ of the tumour.

But  – perhaps more importantly – researchers have now discovered a way to test thousands of chemicals for activity against cancer stem cells. So even if salinomycin doesn’t turn out to be suitable for use in cancer patients, we now have a technique that can be used to discover further drugs that might be.

This research is unlikely to lead to a ‘magic bullet’ for all cancers. The role of stem cells in cancer is still not crystal clear, and although there’s evidence they play a role in many types of cancer, they’re not always implicated. And this research has only been done using stem cells ‘manufactured’ in the lab from breast cells, so we don’t yet know if it will apply to ‘real life’ cancer stem cells – or to other types of cancer.

Regardless of the caveats, today’s story is still very good news for the field of cancer research, and we look forward to seeing further significant moves forward in cancer stem cell research – and treatment – in the future.

Kat

Further reading

• Behind the Headlines – stem cells and cancer research
• Scientists discover stem cell clue to lung cancer
• Cancer stem cell subpopulation drives pancreatic cancer metastasis
• Melanoma appears to deft stem cell theory
• Tracking down bowel cancer stem cells
• Article on cancer stem cells in The Economist
• Cancer Research UK podcast featuring a package on stem cells

Reference:
Gupta, P. et al. (2009). Identification of Selective Inhibitors of Cancer Stem Cells by High-Throughput Screening Cell DOI: 10.1016/j.cell.2009.06.034