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Therapies that boost female hormones could cause specialised cancer cells to induce growth and to spread to other parts of the body, a study in the US showed.

Women's exposure to natural and synthetic progestins - medications that have effects similar to those of progesterone (a steroid hormone) - leads to the production of specialised cancer cells in the body that act like stem cells in humans. 

"The findings show that exposure to natural and synthetic progestins leads to the development of these cancer stem-cell like cells," said Salman Hyder, the Zalk Endowed Professor at the University of Missouri in the US. 

Both natural and synthetic progestins significantly increased protein expression of CD44 -- a molecule involved in cell proliferation, cell communication and migration. 

Additionally, the presence of progestins caused these components to behave like cancer stem cell-like cells.

These rare cells are a small population of cells that - acting like normal stem cells - are self-renewing, create identical copies of themselves and proliferate exponentially. 

Further testing showed that the rare subset of cancer cells actually were enriched by progestins, the researchers said in the paper published in the journal Breast Cancer - Targets and Therapy.

"These cells greatly increase the likelihood of resistance to therapies and the risk for metastasis. Our findings also suggest that clinicians may be able to combat the progestin-dependent tumour growth through immunotherapy," Hyder said.

The results could help scientists target these rare cells that proliferate in breast cancers and metastasise elsewhere, and may help clinicians identify immunotherapies to combat the spread of the disease.