In Australia, 1 woman in 8 will develop breast cancer. Although the survival rate of patients with early stage breast cancer is improving, only 26.3% of patients with metastasis can survive beyond 5 years following diagnosis1, since currently no effective treatment for metastatic breast cancer is available.
As a potential therapeutic target, BMP4 (bone morphogenetic protein 4) has been shown to either suppress or promote metastasis in different types of cancer2. Canonical BMP4 signalling is transduced by SMAD4 (mothers against decapentaplegic homolog 4), which is a putative tumour suppressor. Non-canonical BMP4 signalling can be transduced via the MAPK, PI3K/AKT and NF-κB pathways independent of SMAD4, and may thereby promote cancer progression. We hypothesise that BMP4 inhibits metastasis through canonical signalling, and when SMAD4 is non-functional, BMP4 promotes metastasis through non-canonical signalling.
SMAD4 expression was reduced in the human breast cancer MDA-MB-231-HM cells by short-hairpin RNA. These cells, the non-silencing control cells, and the SMAD4-negative human breast cancer MDA-MB-468 cells were treated with BMP4 for varying times. By western blotting, we found that the feedback inhibition of phosphorylated SMAD1/5/8 in canonical signalling was reduced when SMAD4 is reduced or absent, consistent with a lower induction of the feedback inhibitor Smad7. Agreeing our hypothesis, when cells were cultured in vitro, BMP4 slightly inhibited the proliferation of SMAD4-expressing cells (P < 0.01), but promoted that of the SMAD4-reduced cells (P < 0.05). However, in the absence of BMP4, a reduction of SMAD4 inhibited the proliferation of cells (P < 0.0001). In a xenograft model, growth of the SMAD4-reduced primary tumours was slower compared to the control (P < 0.05) and metastatic burden following primary tumour growth or tail vein injection of tumour cells was also attenuated (P < 0.01). However, this was in an environment with no source of human BMP4. These findings suggest that SMAD4 may be required for the growth and metastasis of breast tumours in the absence of human BMP4.