Tamoxifen, a drug routinely prescribed to treat breast cancer, may actually help some cancers proliferate.
It could do so by mimicking the hormone oestrogen, say David Shapiro and colleagues at the University of Illinois at Urbana-Champaign. They have discovered that in addition to inducing tumour growth, oestrogen gives cancer a helping hand by shielding tumour cells from the immune system. In cultured human cancer cells, it enhances production of a protein called PI-9. This protein blocks the body鈥檚 immune system from destroying tumour cells.
, which usually reduces the effects of oestrogen, can have the same effect, the team also found. Both the hormone and the drug bind to receptors in cell nuclei that activate the gene for PI-9. In tumour cells with an abundance of these receptors, the drug was as effective as oestrogen at providing protection against the immune system鈥檚 natural killer cells ().
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The finding might explain why Tamoxifen is notoriously less effective in women with tumours that contain high levels of oestrogen receptors.
鈥淚t鈥檚 a provocative mechanism,鈥� says Peter Kushner, a molecular biologist at the University of California, San Francisco, who studies breast cancer. He says doctors won鈥檛 stop prescribing Tamoxifen for these women until the mechanism is verified in animal studies, but if the mechanism is then confirmed it will motivate researchers to explore alternative drug options.
