Scientists in the US have found two proteins that may reduce the effectiveness of breast cancer drug tamoxifen.
The team of scientists at Baylor College of Medicine in Houston, Texas, found that low levels of Rho GDI-alpha led to an increase in MTA-2. The combination of the two proteins made the treatment less effective.
The study also found that high levels of MTA-2 encouraged the spread of tumours.
The scientists, led by Professor Suzanne Fuqua, wrote in the Journal of the National Cancer Institute: "These are the first data suggesting a tight, clinically important connection between the two pathways, Rho GDI-alpha and MTA-2. Our data also suggest a possible mechanism, in which the loss of Rho GDI-alpha function promotes distant progression of breast tumours by triggering downstream molecules, such as MTA-2, with metastasis-promoting activities."
The majority of breast cancers are hormone sensitive, meaning they are fuelled by the female hormone oestrogen.
Tamoxifen works by blocking the molecular "receptors" on tumours that respond to the hormone.
The scientists compared two groups of oestrogen-sensitive tumours.
Four came from women who had not experienced cancer recurrence after taking tamoxifen. These were matched against five metastatic tumours from women taking tamoxifen whose cancers had spread while they were on the drug.