Cells that provide tumours with support may also defend against the cancer's spread, so chemotherapies that target these cells could inadvertently fuel metastasis.

Pericytes are cells that provide structural support to blood vessels, including those that feed tumours. Raghu Kalluri at Harvard Medical School in Boston, Massachusetts, and his group found that patients with invasive breast cancer tended to fare poorly if they had few pericytes covering the blood vessels in their tumours. Selectively killing pericytes in tumour-bearing mice slowed tumour growth but increased metastasis and the expression of a cancer-promoting gene called Met.

Two chemotherapies that target pericytes — imatinib and sunitinib — produced similar effects in mice, but their effects on metastasis were suppressed when a Met inhibitor was given simultaneously. The authors suggest that pericyte loss makes tumour blood vessels leaky, which may set in motion events that promote metastasis.

Cancer Cell 21, 66–81 (2012)