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  • Title: Critical role of the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase-3 signaling pathway in recovery from anthrax lethal toxin-induced cell cycle arrest and MEK cleavage in macrophages.
    Author: Ha SD, Ng D, Pelech SL, Kim SO.
    Journal: J Biol Chem; 2007 Dec 14; 282(50):36230-9. PubMed ID: 17951252.
    Abstract:
    Anthrax lethal toxin (LeTx) is a virulence factor causing immune suppression and toxic shock of Bacillus anthracis infected host. It inhibits cytokine production and cell proliferation/differentiation in various immune cells. This study showed that a brief exposure of LeTx caused a continual MEK1 cleavage and prevented tumor necrosis factor-alpha (TNF) production in response to lipopolysaccharide (LPS) in non-proliferating cells such as human peripheral blood mononuclear cells or mouse primary peritoneal macrophages. In human monocytic cell lines U-937 and THP-1, LeTx induced cell cycle arrest in G0-G1 phase by rapid down-regulation of cyclin D1/D2 and checkpoint kinase 1 through MEK1 inhibition. However, THP-1 cells adaptively adjusted to LeTx and overrode cell cycle arrest by activating the phosphatidylinositol 3-kinase/Akt signaling pathway. Inhibitory Ser-9 phosphorylation of glycogen synthase kinase 3beta (GSK3beta) by Akt prevented proteasome-mediated cyclin D1 degradation and induced cell cycle progress in LeTx-intoxicated THP-1 cells. Recovery from cell cycle arrest was required before recovering from on-going MEK1 cleavage and suppression of TNF production. Furthermore, pretreatment with LeTx or the GSK3-specific inhibitor SB-216763, or transfection with dominant active mutant Akt or degradation-defected mutant cyclin D1 protected cells from LeTx-induced cell cycle arrest, on-going MEK1 cleavage and suppression of TNF production. These results indicate that modulation of phosphatidylinositol 3-kinase/Akt/GSK3beta signaling cascades can be beneficial for protecting or facilitating recovery from cellular LeTx intoxication in cells that depend on basal MEK1 activity for proliferation.
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