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Purpose: Ion channel activity is involved in several basic cellular behaviors that are integral to metastasis (e.g., proliferation, motility, secretion, and invasion), although their contribution to cancer progression has largely been ignored. The purpose of this study was to investigate voltage-gated Na+ channel (VGSC) expression and its possible role in human breast cancer. Experimental Design: Functional VGSC expression was investigated in human breast cancer cell lines by patch clamp recording. The contribution of VGSC activity to directional motility, endocytosis, and invasion was evaluated by in vitro assays. Subsequent identification of the VGSC α-subunit(s) expressed in vitro was achieved using reverse transcription-PCR, immunocytochemistry, and Western blot techniques and used to investigate VGSCα expression and its association with metastasis in vivo. Results: VGSC expression was significantly up-regulated in metastatic human breast cancer cells and tissues, and VGSC activity potentiated cellular directional motility, endocytosis, and invasion. Reverse transcription-PCR revealed that Nav1.5, in its newly identified “neonatal” splice form, was specifically associated with strong metastatic potential in vitro and breast cancer progression in vivo. An antibody specific for this form confirmed up-regulation of neonatal Nav1.5 protein in breast cancer cells and tissues. Furthermore, a strong correlation was found between neonatal Nav1.5 expression and clinically assessed lymph node metastasis. Conclusions: Up-regulation of neonatal Nav1.5 occurs as an integral part of the metastatic process in human breast cancer and could serve both as a novel marker of the metastatic phenotype and a therapeutic target.

Whole-cell patch-clamp recordings showed that a sub-population (10%) of Jurkat cells, a model of human T-cells, expressed a functional voltage-gated sodium channel, which was tetrodotoxin (TTX)-resistant. Expression of voltage-gated sodium channel protein was confirmed by western blots. Semi-quantitative PCR analysis revealed that mRNAs for the alpha-subunits of multiple voltage-gated sodium channel subtypes were present but indicated that Na(v)1.5 was the predominant subtype, consistent with the TTX-resistant nature of the recorded currents. Importantly, 10 microM TTX reduced the number of Jurkat cells invading a Matrigel basement membrane by 93.0+/-5.5%. Since similar sodium channels have also been detected in normal human T-lymphocytes, it is concluded that the activity of voltage-gated sodium channels could represent a novel mechanism potentiating the invasive capacity of these cells.