IκB Kinase Complex Is an Intracellular Target for Endotoxic Lipopolysaccharide in Human Monocytic Cells

Abstract

Endotoxic lipopolysaccharide (LPS) is a proinflammatory agonist produced by gram-negative bacteria and a contributor to the majority of the 400,000 septic shock cases recorded annually in US hospitals. The primary target cells for LPS are monocytes and macrophages. Their response consists of massive production of proinflammatory cytokines, reactive oxygen- and nitrogen-intermediates, procoagulants, and cell adhesion molecules. In turn, expression of these LPS-responsive factors contributes to collapse of the circulatory system, to disseminated intravascular coagulation, and to a 30% mortality rate. A common intracellular mechanism responsible for the expression of septic shock genes in monocytes and macrophages involves the activation of NF-κB. This transcription factor is regulated by a family of structurally related inhibitors including IκB, IκBβ, and IκBɛ, which trap NF-κB in the cytoplasm. In this report, the investigators show that LPS derived from different gram-negative bacteria activates cytokine-responsive IκB kinases containing catalytic subunits termed IKK (IKK1) and IKKβ (IKK2). The kinetics of IKK and IKKβ activation in LPS-stimulated human monocytic cells differ from that recorded on their stimulation with tumor necrosis factor-, thereby implying a distinct activation mechanism. LPS-activated IKK complexes phosphorylate all 3 inhibitors of NF-κB: IκB, IκBβ, and IκBɛ. Moreover, LPS activates IKKβ preferentially, relative to IKK. Thus, IKK complex constitutes the main intracellular target for LPS-induced NF-κB signaling to the nucleus in human monocytic cells to activate genes responsible for septic shock.