Involvement of Voltage-Gated Sodium Channels in Breast Cancer Cells Invasiveness and Regulation by Docosahexaenoic Acid

Abstract

Voltage-gated sodium channels (NaV) are expressed in highly invasive metastatic cancer cells derived from epithelial tissues. In the highly invasive breast cancer cell line MDA-MB-231, we showed that the abnormal expression of NaV1.5 was responsible for a sustained inward sodium current at the membrane potential, which promotes the invasion of the extracellular matrix. We found that NaV1.5 activity leads to a perimembrane acidification, favourable for the pH-dependent proteolytic activity of cysteine cathepsins which play a predominant role in cell invasiveness. Moreover, NaV and the sodium-proton exchanger NHE1 (a pH regulator) seem to be colocalized in specialized lipid rafts domains, caveolae, where cathepsins-B are known to be secreted. Since docosahexaenoic acid (DHA, 22:6 n-3) has been reported to reduce the aggressiveness of some breast cancers, and to be a modulator of ion channels, we investigated its role on NaV activity and cell invasiveness. Supplementation of MDA-MB-231 cells culture medium with 1 µM DHA for five days affected neither cell migration nor proliferation but significantly reduced cell invasiveness, while oleic acid (OA, 18:1 n-9), used as a control, had no effect. Moreover, the combination of DHA with tetrodotoxin (specific NaV blocker) demonstrated no further effect on invasion, suggesting a possible regulation of NaV activity by DHA. Indeed, NaV currents were reduced with DHA but not with OA supplementation. These fatty acids were inefficient to reduce sodium currents when acutely applied, suggesting a middle- or long-term regulation of NaV channels. However, no effect on NaV mRNA or protein expression and no change in NaV raft localization was observed. Thus, DHA effects could occur through a modification in cholesterol/phospholipids composition of the plasma membrane and/or a modification in factors regulating NaV channels activity and/or addressing to the plasma membrane.