Salt-inducible kinase 1 : A novel regulator within a cell sodium-sensing network

Na+, K+-ATPase (NK) is expressed in all animal cells and plays a crucial role in cellular ion homeostasis. The NK activity is tightly regulated by specific signaling networks that must be well controlled in time and space (at the plasma membrane or in the intracellular compartments) to guarantee the specificity of the physiological response. The aim of this thesis was to identify a sodium-sensing network that is ubiquitously present in mammalian cells and senses accurately small elevations in intracellular sodium and translates this signal to the NK molecule. Furthermore, it was investigated whether this network could participate or has cell biology relevance during the development of hypertension and complications such as cardiac hypertrophy. We identified a new protein, salt-inducible kinase 1 (SIK1) that associates with the NK complex and participates in the regulation of its activity during elevations in intracellular sodium. The increases in intracellular sodium in the opossum kidney proximal tubule cell line (OK cells) are paralleled by the elevation in intracellular calcium through the Na+/Ca2+ exchanger, which is leading to the activation of SIK1 (Threonine 322 phosphorylation) via a calcium-calmodulin dependent kinase. The activation of the SIK1 triggers the phosphorylation of the protein phosphatase methylesterase-1, which dissociates from its protein phosphatase 2A (PP2A). PP2A dephosphorylates the NK alpha-subunit, which leads to its increased NK activity. We also observed that SIK1 activity (pT182 phosphorylation) was significantly elevated in both hypertensive Milan rats and OK cells carrying the hypertension linked mutation in the cytoskeleton protein -adducin when compared with the -adducin normotensive control. By blocking the SIK1 network, we prevented the increase in NK activity induced by the mutated variant of -adducin. Furthermore, we examined in cardiomyocytes whether angiotensin II (AngII) per se and independently of the rise in blood pressure may contribute the development of cardiac hypertrophy by influencing the SIK1 network. Our data demonstrated that AngII and the presence of alpha-adducin carrying the hypertension-linked mutation increase myocyte enhancer factor-2 (MEF2) and nuclear factor of activated T-cells (NFAT) activity, effects that were blocked by inhibition of SIK1. We also demonstrated that AngII increases the hypertrophic-associated cardiac genes: natriuretic peptide precursor type B, skeletal actin, and alpha-myosin heavy chain. Together, the results of this thesis reveal novel information about a cell sodium-sensing network, with SIK1 at its core, and the regulation of the active sodium transport in renal epithelial cells. Additionally, it highlights the significance of this network in hypertension and derived complications such as cardiac hypertrophy