GIRK proteins in breast cancer cells

G-Protein activated K+ channels (GIRKs) represent classical G-protein effectors, that mediate the regulation of excitability via hormones and neurotransmitters in electrically excitable tissue. Recent findings suggest that GIRKs are present in cancer cells contributing significantly to the malignancy of breast tumours. Despite of overwhelming evidence for their occurrence and a clear correlation with disease progression, insight into possible pathophysiological mechanisms, how GIRK proteins may promote cancer development does, at present, not exist. This proposal aims to test the crucial hypotheses that GIRKs promote cancer progression, either as the canonical and well known G-protein effectors, as G-protein independent K+ channels or via hitherto unidentified functions that are not related to K+ permeation at all. Experiments will be performed on non-tumourigenic, basal B-type, and malign, luminal type, tumour cells with different invasive potential. GIRK related K+ channels will be electrophysiologically characterized in benign and malign breast cancer cells, in cell lines overexpressing GIRK variants and isoforms and cell lines where GIRK genes have been specifically silenced via siRNA. The effect of environmental factors that are relevant for tumour progression (mechanical stress; hypoxia) on GIRK variant/isoform protein and mRNA levels will be studied. Analysis of simple vital cell functions that are important for tumorigenesis, such as proliferation, cell-adhesion and migration will be performed and the eventual role(s) of GIRKs variants/isoforms will be assessed by via specific overexpression/silencing. Characterization of the subcellular distribution of GIRK variants/isoforms in benign and malign breast cancer cell lines under different cancer related environmental factors, protein phosphorylation state and upon presence of signalling molecules will be performed in order to elucidate a possible role of GIRK trafficking in cancerogenesis. We believe that the results may lead to a deeper understanding of how tumour cells accomplish their malignant potential, especially in the formation of remote metastases. Such understanding could pave the way towards more efficient treatment of breast cancer, but also of other malignancies.

Worldwide, more than half a million woman die every year from breast cancer. In Austria, every 8th woman develops breast cancer during her life. Within the western hemisphere, breast cancer represents the most abundant cancer amongst women. Thanks to progress in diagnosis and therapy the five year survival rate amounts to approx. 85% in Austria. Still, there is a need for additional and also alternative insight into the cell biology of breast cancerogenesis and progression, since this insight paves the way for additional prognostic and therapeutic measures. The recently finalised research project (funded by FWF) had set the objective to investigate the, hitherto unexplored, contribution of a signal transduction protein, subunit 1 of G-protein activated K+ channels (GIRK1), to the progression of breast cancer. GIRKs represent direct effectors of G-proteins and hence are a link between electrical membrane potential and the presence of hormones and neurotransmitters in the extracellular space. During the course of the project it was shown that selected and, for the progression of tumours, crucial cellular parameters such as wound healing, chemotaxis, cellular velocity and motility and neoangiogenesis were massively affected by GIRK1 overexpression. Using electrophysiological techniques it was shown that functional GIRK ion channels exist in the plasma membrane of breast cancer cells (MCF-7), albeit at low density. A region within the GIRK1 protein (aminoacids 235 402) was identified to be crucial for the cancerogenic activity. The research project allowed important insight into hitherto unknown cellular and molecular mechanisms in breast cancer cells. The findings also explain the negative correlation between GIRK1 overexpression in the primary tumour and survival rates in patients suffering from breast cancer (this correlation was found during another research project, funded by FWF, und is currently under investigation (KLIF-182)). Further research on the molecular mechanism how GIRK1 overexpression affects cancerogenesis and progression in breast epithelial cells is inevitable and hopefully will pave the way to more efficient treatment options.