Abstract:
Breast cancer (BC) is the primary cause of cancer-related deaths in women. Ion channels and in particular K+ channels drive tumorigenesis in various types of cancer including BC, considering these channels as potential targets for new anti-cancer strategies. Among these, K+ channels are important regulators of cell migration and proliferation, involved in cellular processes such as membrane potential, cell volume and Ca2+ signaling. As such, voltage- and Ca2+-activated K+ channels with large conductance (BKCa) are aberrantly expressed in BC, promoting tumor growth and progression, and are associated with high tumor grade and poor outcome. Expression of BKCa channels were found to be linked with worse outcome of patients receiving tamoxifen (TAM) as endocrine therapy, and in vitro low concentrations of the active metabolites of TAM (TAM+M) promoted proliferation of human and murine BKCa-proficient BC cells. This thesis aims to better understand the molecular mechanism of TAM+M-induced BKCa-dependent proliferation in BC cells by applying gene expression analysis, whole cell patch clamp recordings and FRET-based K+ imaging as well as molecular docking studies. TAM+M induced K+ currents and dynamics in a BKCa-dependent manner and irrespective of the ER expression level. Genetic and pharmacological ablation of BKCa activity confirmed the channel-stimulating effects of TAM+M in murine and human BC cells. Mechanistically, the data suggest that the gain of BKCa activity upon TAM+M stimulation increased the driving force of Ca2+ from multiple stores. Elevation of the intracellular Ca2+ concentration leads to a depolarized membrane potential that activates the BKCa channels in a voltage-dependent manner. Additionally, the increased cytosolic Ca2+ levels may represent a feed forward mechanism for BKCa channel activation.
Regarding the role of auxiliary BKCa subunits, increased KCNMB1 expression in BC patients was linked with better event-free survival and lower risk of recurrence. In vitro the TAM+M-induced intracellular K+ dynamics were inhibited by β1-subunit expression in human BKCa-proficient BC cells. Altogether, the data suggest that TAM+M modulates BKCa activity causing alterations in K+ and Ca2+ homeostasis. These alterations likely influence malignant BC cell behaviors and contribute to the outcome of BC patients receiving endocrine therapy.
Breast cancer