A role for CBFß in maintaining the metastatic phenotype of breast cancer cells.

Epithelial to mesenchymal transition (EMT) is a dynamic process that drives cancer cell plasticity and is thought to play a major role in metastasis. Here we show, using MDA-MB-231 cells as a model, that the plasticity of at least some metastatic breast cancer cells is dependent on the transcriptional co-regulator CBFß. We demonstrate that CBFß is essential to maintain the mesenchymal phenotype of triple-negative breast cancer cells and that CBFß-depleted cells undergo a mesenchymal to epithelial transition (MET) and re-organise into acini-like structures, reminiscent of those formed by epithelial breast cells. We subsequently show, using an inducible CBFß system, that the MET can be reversed, thus demonstrating the plasticity of CBFß-mediated EMT. Moreover, the MET can be reversed by expression of the EMT transcription factor Slug whose expression is dependent on CBFß. Finally, we demonstrate that loss of CBFß inhibits the ability of metastatic breast cancer cells to invade bone cell cultures and suppresses their ability to form bone metastases in vivo. Together our findings demonstrate that CBFß can determine the plasticity of the metastatic cancer cell phenotype, suggesting that its regulation in different micro-environments may play a key role in the establishment of metastatic tumours.

The RUNX Transcriptional Coregulator, CBFß, Suppresses Migration of ER+ Breast Cancer Cells by Repressing ERα-Mediated Expression of the Migratory Factor TFF1.

Core binding factor ß (CBFß), the essential coregulator of RUNX transcription factors, is one of the most frequently mutated genes in estrogen receptor-positive (ER+) breast cancer. Many of these mutations are nonsense mutations and are predicted to result in loss of function, suggesting a tumor suppressor role for CBFß. However, the impact of missense mutations and the loss of CBFß in ER+ breast cancer cells have not been determined. Here we demonstrate that missense mutations in CBFß accumulate near the Runt domain-binding region. These mutations inhibit the ability of CBFß to form CBFß-Runx-DNA complexes. We further show that deletion of CBFß, using CRISPR-Cas9, in ER+ MCF7 cells results in an increase in cell migration. This increase in migration is dependent on the presence of ERα. Analysis of the potential mechanism revealed that the increase in migration is driven by the coregulation of Trefoil factor 1 (TFF1) by CBFß and ERα. RUNX1-CBFß acts to repress ERα-activated expression of TFF1. TFF1 is a motogen that stimulates migration and we show that knockdown of TFF1 in CBFß-/- cells inhibits the migratory phenotype. Our findings reveal a new mechanism by which RUNX1-CBFß and ERα combine to regulate gene expression and a new role for RUNX1-CBFß in the prevention of cell migration by suppressing the expression of the motogen TFF1. IMPLICATIONS: Mutations in CBFß contribute to the development of breast cancer by inducing a metastatic phenotype that is dependent on ER.

HIF1-alpha expressing cells induce a hypoxic-like response in neighbouring cancer cells.

BACKGROUND: Hypoxia stimulates metastasis in cancer and is linked to poor patient prognosis. In tumours, oxygen levels vary and hypoxic regions exist within a generally well-oxygenated tumour. However, whilst the heterogeneous environment is known to contribute to metastatic progression, little is known about the mechanism by which heterogeneic hypoxia contributes to cancer progression. This is largely because existing experimental models do not recapitulate the heterogeneous nature of hypoxia. The primary effector of the hypoxic response is the transcription factor Hypoxia inducible factor 1-alpha (HIF1-alpha). HIF1-alpha is stabilised in response to low oxygen levels in the cellular environment and its expression is seen in hypoxic regions throughout the tumour. METHODS: We have developed a model system in which HIF1-alpha can be induced within a sub-population of cancer cells, thus enabling us to mimic the effects of heterogeneic HIF1-alpha expression. RESULTS: We show that induction of HIF1-alpha not only recapitulates elements of the hypoxic response in the induced cells but also results in significant changes in proliferation, gene expression and mammosphere formation within the HIF1-alpha negative population. CONCLUSIONS: These findings suggest that the HIF1-alpha expressing cells found within hypoxic regions are likely to contribute to the subsequent progression of a tumour by modifying the behaviour of cells in the non-hypoxic regions of the local micro-environment.