Voltage-gated Na+ channels (VGSCs) are heteromeric proteins composed of pore-forming α subunits and smaller β subunits. The β subunits are multifunctional channel modulators and are members of the immunoglobulin superfamily of cell adhesion molecules (CAMs). β1, encoded by SCN1B, is best characterized in the central nervous system (CNS), where it plays a critical role in regulating electrical excitability, neurite outgrowth and migration during development. β1 is also expressed in breast cancer (BCa) cell lines, where it regulates adhesion and migration in vitro. In the present study, we found that SCN1B mRNA/β1 protein were up-regulated in BCa specimens, compared with normal breast tissue. β1 upregulation substantially increased tumour growth and metastasis in a xenograft model of BCa. β1 over-expression also increased vascularization and reduced apoptosis in the primary tumours, and β1 over-expressing tumour cells had an elongate morphology. In vitro, β1 potentiated outgrowth of processes from BCa cells co-cultured with fibroblasts, via trans-homophilic adhesion. β1-mediated process outgrowth in BCa cells required the presence and activity of fyn kinase, and Na+ current, thus replicating the mechanism by which β1 regulates neurite outgrowth in CNS neurons. We conclude that when present in breast tumours, β1 enhances pathological growth and cellular dissemination. This study is the first demonstration of a functional role for β1 in tumour growth and metastasis in vivo. We propose that β1 warrants further study as a potential biomarker and targeting β1-mediated adhesion interactions may have value as a novel anti-cancer therapy.

Voltage-gated Na+ channels (VGSCs) are heteromeric proteins composed of pore-forming α subunits and smaller β subunits. The β subunits are multifunctional channel modulators and are members of the immunoglobulin superfamily of cell adhesion molecules (CAMs). β1, encoded by SCN1B, is best characterized in the central nervous system (CNS), where it plays a critical role in regulating electrical excitability, neurite outgrowth and migration during development. β1 is also expressed in breast cancer cell lines, where it regulates adhesion and migration in vitro. Using the Oncomine database, we found that SCN1B mRNA was up-regulated in breast cancer, compared with normal breast. Similarly, β1 protein was up-regulated in a cohort of breast cancer tissue specimens compared with surrounding normal tissue. Over-expression of β1 in MDA-MB-231 cells substantially increased tumor growth and metastasis from orthotopic xenografts in immune-deficient mice. β1 had no effect on the density of Ki67-positive cycling cells in the primary tumors. However, β1 increased the density of CD31-positive vessel structures and reduced the density of apoptotic cells expressing activated caspase-3. β1-expressing MDA-MB-231 cells had an elongate morphology in tumors in vivo and in 2D culture in vitro. β1 potentiated the outgrowth of processes from MDA-MB-231 cells and MCF-7 cells co-cultured with fibroblasts, via trans-homophilic adhesion. Both the src family tyrosine kinase inhibitor PP2 and siRNA targeting fyn kinase blocked β1-mediated process extension in breast cancer cells. Thus, β1-mediated process outgrowth in breast cancer cells requires the presence and activity of fyn kinase. This recapitulates the mechanism by which β1 regulates neurite outgrowth in CNS neurons. We conclude that when present in breast tumors, β1 drives pathological growth and cellular dissemination by recapitulating its well-defined role in CNS ontogeny. This study is the first demonstration of a functional role for β1 as a CAM in tumor growth and metastasis. We propose that targeting β1-mediated adhesion interactions may have potential as a novel anti-cancer therapy. Citation Format: Michaela Nelson, Rebecca Millican-Slater, Lorna C. Forrest, William J. Brackenbury. The sodium channel auxiliary subunit SCN1B promotes breast tumor growth and metastasis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1972. doi:10.1158/1538-7445.AM2014-1972