Triple-negative breast cancer (TNBC) is highly aggressive and is particularly challenging to treat compared to estrogen receptor-positive and HER2-positive breast cancer. In TNBC, the Nav1.5 α subunit of voltage-gated Na+ channels (VGSCs), encoded by SCN5A, is up-regulated and promotes metastasis. β1, a VGSC subunit and a cell adhesion molecule (CAM), encoded by SCN1B, controls Nav1.5 expression and activity. TNBC also often develops intratumoral hypoxia. The cellular hypoxic response is regulated transcriptionally by hypoxia-inducible factors (HIFs). Given that hypoxia causes an increase in intracellular [Na+] in breast cancer and up-regulates Nav1.5 activity via SUMOylation in the ischaemic heart, we sought to investigate whether hypoxia and/or HIFs regulate Nav1.5 in TNBC.
In silico analysis of putative HIF binding sites identified hypoxia response elements (HREs) in the SCN5A and SCN1B promoters. In MDA-MB-231 cells, dimethyloxalylglycine (DMOG), a HIF stabiliser and α-ketoglutarate analogue, decreased Na+ currents but did not affect the gating properties of VGSCs. Similarly, exposure of MDA-MB-231 cells to chronic hypoxia for 48 hours decreased Na+ currents in the absence and presence of GN44028, a HIF-1α inhibitor. Acute hypoxia significantly increased persistent Na+ current through VGSCs, and the effect was suppressed by SENP1, a deSUMOylase. DMOG upregulated SCN1B after 48 hours, whereas SCN5A expression was unaffected. Both DMOG and chronic hypoxia pre-incubation decreased ATP levels, synthesis of Nav1.5 and its surface expression. Cell adhesion and elongation increased in cells treated with DMOG. Overall, this study suggests that HIF stabilisation using DMOG in MDA-MB-231 cells increases SCN1B expression, cell adhesion, and cell elongation, but HIFs may not have a direct effect on SCN5A expression or VGSC activity. The decrease in Na+ currents in cells treated with DMOG or chronic hypoxia was due to decreased surface expression of Nav1.5 caused by a reduction in channel synthesis and/or ATP-dependent membrane trafficking. On the other hand, acute hypoxia may increase persistent Na+ current via SUMOylation of Nav1.5 in TNBC cells. These findings may inform the development of therapeutic strategies targeting hypoxia-associated signalling and VGSC regulation in TNBC.
