The mystery of why breast cancer cells that express ERBB2 (HER2), a member of the epidermal growth factor receptor family, have a higher incidence of metastasis to specific organs is now one step closer to being solved. Hung and colleagues show that ERBB2 signalling increases the expression of a chemokine receptor whose ligand is highly expressed in lung, liver and bone marrow where metastases of breast cancer commonly develop.

Chemokines are a superfamily of small cytokine-like peptides involved in cell adhesion and migration. The expression of the CXC chemokine receptor 4 (CXCR4) and its natural ligand CXC chemokine ligand 12 (CXCL12) are known to be involved in breast cancer metastasis. As ERBB2 and CXCR4 can both be expressed in breast cancer cells, Hung and co-workers investigated whether there was a link between the two.

In human breast cancer cell lines, they showed that transgenic expression of ERBB2 leads to increased CXCR4 expression, most likely through an increased rate of translation of CXCR4 mRNA. Further in vitro studies demonstrated that blocking the activity of ERBB2 with the inhibitor trastuzumab (Herceptin), suppresses CXCR4 expression and blocks invasion mediated by CXCL12. Furthermore, inhibition of CXCR4 by a neutralizing antibody inhibits the migration and adhesion of ERBB2-expressing cells mediated by CXCL12. The authors also have evidence that degradation of CXCR4, once it has bound CXCL12, is reduced in ERBB2-expressing cells. This might be because ERBB2, by an unknown mechanism, reduces the ubiquitylation of CXCR4. In addition, studies in immunocompromised mice show that ERBB2-expressing breast cancer cells that no longer express significant levels of CXCR4, due to RNA interference, have a markedly reduced capacity to form lung metastases.

The human breast cancer biopsy samples that Hung and colleagues examined show that CXCR4 expression correlates with both increased ERBB2 expression and a poor prognosis. These findings indicate that targeting both the CXCR4–CXCL12 pathway and ERBB2 might improve the survival of breast cancer patients with ERBB2-positive tumours.