Abstract
The calcium ion (Ca2+) is a highly versatile signaling messenger involved in a diverse range of physiological processes such as gene transcription/expression, proliferation, differentiation and cell death. Intracellular Ca2+ signals are generated through a 10 000 – 20 000 fold gradient across the cell membrane and via release from the external milieu and/or internal Ca2+ stores. Cells have a unique signaling toolkit to control Ca2+ homeostasis including a selection of ion channels, pumps, exchangers and Ca2+ binding proteins.
We have reported that ouabain, an endogenous steroid hormone and ligand to the Na+,K+-ATPase, can trigger dendritic growth in cortical neurons through signal transduction. This involves a Ca2+-dependent transcriptional program regulated by CREB and CRE-mediated gene activation, primarily regulated through Ca2+/calmodulin-dependent protein kinases. The process also includes Ca2+ oscillations and phosphorylation of mitogen-activated protein kinases (ERK 1/2). These data suggest a novel role for Na+,K+-ATPase and Ca2+ in dendritic growth during development.
Previous work has shown that treatment with protein kinase C (PKC) inhibitors results in a prolonged Ca2+ increase leading to calpain activation and release of apoptosis-inducing factor (AIF). We have demonstrated that hyperpolarization- activated cyclic nucleotide-gated (HCN) channel 2 is responsible for the Ca2+ influx. The influx is regulated via dephosphorylation of a residue in the intracellular C- terminal. This data shows a novel role for HCN channel 2 in cell death and a new possible drug target.
Bladder cancer is overall one of the ten most common cancers. We have shown that treatment with Bacillus Calmette-Guerin (BCG), currently the most effective intravesical agent against bladder cancer, induces an intracellular Ca2+ increase and reduces cell proliferation in urinary bladder cancer (T24) cells. Store depletion by SERCA inhibition blocked the BCG-triggered signal, thereby suggesting a role of the endoplasmic reticulum as a Ca2+ source. This signaling event was dependent on phospholipas C since pharmacological inhibition or small interference RNA-mediated gene silencing abolished the response. Finally EdU incorporation revealed that BCG- controlled cell proliferation was mediated via a Ca2+- and PLC-dependent signaling cascade.
In summary this thesis presents three studies highlighting three different roles for Ca2+ signaling. They show that Ca2+ signaling is involved in processes critical for cell differentiation, cell proliferation, and cell death, three aspects highly coordinated with development and disease.
