Dangerous changes in cancer cells which allow them to spread around the body could be triggered by the body's own stem cells, say US scientists.
Stem cells may allow breast cancer cells to spread
A Whitehead Institute team found human breast cancers in mice are more likely to spread if mixed with stem cells from the bone marrow.
They believe these changes could be blocked or reversed - making the cancer less deadly.
UK experts said the Nature study could point to future treatments.
When an original cancer spreads to form new tumours in other parts of the body such as the lung or liver, this is called metastasis, and often means that the patient is far less likely to be cured of the disease.
Doctors hope that by understanding how and why a tumour suddenly changes its behaviour, a treatment could be found to stop this happening, and keep the cancer fixed in one part of the body.
The latest research has linked the arrival of a particular type of "stem cell" to metatasis in breast cancer cells.
Mesenchymal stem cells are found in the bone marrow, and are a "master cell" used by the body to help generate new bone, fat, cartilage and muscle.
They were already a suspect in cancer spread after it was noticed that they naturally migrate in large numbers to tumour sites.
When the scientists mixed human breast cancer tumours in mice with these cells, there was seven times more cancer spread to the lungs compared with breast cancer tumours left to their own devices.
They are suggesting that the presence of the stem cells produces changes in the way that the cancer cell genes work that make them metastasise - but once the cells spread, these changes are reversed.
This, the researchers say, not only makes these key genes hard to spot, but means that dangerous changes in cancer cells are potentially reversible.
HIV treatment hope
The research has also highlighted a potential treatment to block the changes.
A chemical called cytokine CCL5, produced by the stem cells, had an effect on breast cancer cells in the laboratory.
Medication that blocks the action of this is already used to help patients with HIV - and they suggested it should be tried on patients with spreading cancer.
Dr Kat Arney, from Cancer Research UK, said: "It's becoming increasingly clear that many cancers aren't just made up of cancer cells, but they are rogue tissues that also contain many other types of cells.
"This is a very interesting paper, showing that mesenchymal stem cells may play a part in helping breast cancer to spread.
"Although these results don't tell us if exactly the same situation is present in cancers within humans - as they have been done using mice - it's a good indicator that these stem cells may play a role in breast cancer, and could point towards targets for future treatments."