Involvement of KCNQ1 K+ channels in cell volume regulation in human mammary epithelial cells

Abstract

Transepithelial ion transport across mammary epithelial cells underlies milk secretion and breast cyst formation. However, little is known about the molecular nature of the K+ channels involved. We have investigated the location and functionality of the KCNQ1 K+ channel in the mammary epithelial cell line MCF‐7. Using RT‐PCR, we determined that KCNQ1, as well as its accessory subunits KCNE1‐3, were present in MCF‐7 cells. KCNQ1 protein expression was confirmed by Western blotting and immunolocalization with confocal microscopy. When cells were cultured as a polarized monolayer, KCNQ1 was located in the apical membrane. We next investigated the potential role this channel may play in the regulatory volume decrease (RVD) of these cells. Switching from an isotonic to a hypotonic solution resulted in an initial cell swelling, followed by an RVD response. This could be inhibited by the KCNQ1 inhibitors chromonol 293B and XE991, and also when a dominant‐negative N‐terminus truncated KCNQ1 isoform was transfected into MCF‐7 cells. Since MCF‐7 cells are also known to express hIK (KCNN4) and BK (KCNMA1) channels, implicated in the RVD response in other cell types, we additionally investigated the ability of MCF‐7 cells to RVD in the presence of clotrimazole and charybdotoxin. Neither agent inhibited the RVD in these cells. Finally, whole cell recordings from baby hamster kidney cells expressing KCNQ1 and the accessory subunit KCNE3 revealed a volume‐sensitive K+ current, which was inhibited by 293B. These data suggest that KCNQ1 may play important physiological roles in the mammary epithelium, potentially mediating apical K+ secretion and well as being critically involved in the regulation of cell volume. Supported by the Canadian Breast Cancer Foundation, Atlantic Chapter.