Regions of low
oxygen (
hypoxia) are found in >50% of breast tumours, most frequently in the more aggressive
triple negative breast cancer subtype (TNBC).
Metastasis is the cause of 90% of
breast cancer patient deaths. Regions of tumour
hypoxia tend to be more acidic and both
hypoxia and
acidosis increase tumour
metastasis. In line with this the metastatic process is dependent on pH regulatory mechanisms. We and others have previously identified increased hypoxic expression of Na+ driven
bicarbonate transporters (NDBTs) as a major mechanism of tumour pH regulation.
Hypoxia induced the expression of NDBTs in TNBC, most frequently SLC4A4 and SLC4A5. NDBT inhibition (S0859) and
shRNA knockdown suppressed migration (40% reduction) and invasion (70% reduction) in vitro. Tumour xenograft
metastasis in vivo was significantly reduced by NDBT knockdown. To investigate the mechanism by which NDBTs support
metastasis, we investigated their role in regulation of phospho-signalling, epithelial-to-mesenchymal transition (EMT) and metabolism. NDBT knockdown resulted in an attenuation in hypoxic phospho-signalling activation; most notably LYN (Y397) reduced by 75%, and LCK (Y394) by 72%. The metastatic process is associated with EMT. We showed that NDBT knockdown inhibited EMT, modulating the expression of key EMT
transcription factors and ablating the expression of
vimentin whilst increasing the expression of
E-cadherin. NDBT knockdown also altered metabolic activity reducing overall
ATP and extracellular
lactate levels. These results demonstrate that targeting
hypoxia-induced NDBT can be used as an approach to modulate phospho-signalling, EMT, and metabolic activity and reduce tumour migration, invasion, and
metastasis in vivo.