PKB/SGK-dependent GSK3-phosphorylation in the regulation of LPS-induced Ca2+ increase in mouse dendritic cells

The function of dendritic cells (DCs) is modified by glycogen synthase kinase GSK3 and GSK3 inhibitors have been shown to protect against inflammatory disease. Regulators of GSK3 include the phosphoinositide 3 kinase (PI3K) pathway leading to activation of protein kinase B (PKB/Akt) and serum and glucocorticoid inducible kinase (SGK) isoforms, which in turn phosphorylate and thus inhibit GSK3. The present study explored, whether PKB/SGK-dependent inhibition of GSK3 contributes to the regulation of cytosolic Ca2+ concentration following stimulation with bacterial lipopolysaccharides (LPS). To this end DCs from mutant mice, in which PKB/SGK-dependent GSK3α,β regulation was disrupted by replacement of the serine residues in the respective SGK/PKB-phosphorylation consensus sequence by alanine (gsk3KI), were compared to DCs from respective wild type mice (gsk3WT). According to Western blotting, GSK3 phosphorylation was indeed absent in gsk3KI DCs. According to flow cytometry, expression of antigen-presenting molecule major histocompatibility complex II (MHCII) and costimulatory molecule CD86, was similar in unstimulated and LPS (1 μg/ml, 24 h)-stimulated gsk3WT and gsk3KI DCs. Moreover, production of cytokines IL-6, IL-10, IL-12 and TNFα was not significantly different in gsk3KI and gsk3WT DCs. In gsk3WT DCs, stimulation with LPS (1 μg/ml) within 10 min led to transient phosphorylation of GSK3. According to Fura2 fluorescence, LPS (1 μg/ml) increased cytosolic Ca2+ concentration, an effect significantly more pronounced in gsk3KI DCs than in gsk3WT DCs. Conversely, GSK3 inhibitor SB216763 (3-[2,4-Dichlorophenyl]-4-[1-methyl-1H-indol-3-yl]-1H-pyrrole-2,5-dione, 10 μM, 30 min) significantly blunted the increase of cytosolic Ca2+ concentration following LPS exposure. In conclusion, PKB/SGK-dependent GSK3α,β activity participates in the regulation of Ca2+ signaling in dendritic cells.