Abstract
Despite the potent antiinflammatory effects of pharmacologically induced adenosine 5'-monophosphate kinase (AMPK) activation on Toll-like receptor 4 (TLR4)-induced cellular activation, there is little evidence that AMPK is activated during inflammatory conditions. In the present studies, we examined mechanisms by which TLR4 engagement may affect the ability of AMPK to become activated in neutrophils and macrophages under in vitro conditions and in the lungs during lipopolysaccharide (LPS)-induced acute lung injury. We found that incubation of neutrophils or macrophages with LPS diminished the ability of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) or hydrogen peroxide (H(2)O(2)) to activate AMPK. Although ratios of AMP to adenosine 5'-triphosphate (ATP) were increased in LPS-treated neutrophils and in the lungs of LPS exposed mice, a condition that should result in AMPK activation, no activation of AMPK was found. Immunocytochemistry and Western blot analysis revealed that nuclear to cytosolic translocation of the proinflammatory mediator high mobility group box 1 protein (HMGB1) correlated with inhibition of AMPK activation in LPS-stimulated macrophages. Moreover, while induced overexpression of HMGB1 resulted in inhibition of AMPK activation, Small interfering RNA (siRNA)-induced knockdown of HMGB1 was associated with enhanced activation of AMPK in macrophages incubated with AICAR. Increased interaction between liver kinase B1 (LKB1), an upstream activator of AMPK, and HMGB1 was found in LPS-stimulated macrophages and in the lungs of mice exposed to LPS. These results suggest that nuclear to cytoplasmic translocation of HMGB1 in TLR4-activated cells potentiates inflammatory responses by binding to LKB1, thereby inhibiting the antiinflammatory effects of AMPK activation.
Highlights
Adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK) is a heterotrimeric serine/threonine kinase consisting of a catalytic α subunit and β and γ regulatory subunits
There is little evidence that adenosine 5′-monophosphate kinase (AMPK) is activated in inflammatory states, such as acute lung injury, despite the presence of conditions including increased release of reactive oxygen species and diminished generation of adenosine 5′-triphosphate (ATP), which would be expected to result in AMPK activation [9,17,18,19]
There was no significant difference between the viability of control neutrophils and neutrophils treated with the combination of LPS and AICAR or LPS and H2O2
Summary
Adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK) is a heterotrimeric serine/threonine kinase consisting of a catalytic α subunit and β and γ regulatory subunits. Recent studies have shown that exposure of cells to reactive oxygen species or glycogen can induce AMPK activation independently of changes in cellular ATP-to-AMP ratios [9,10]. AMPK has primarily been characterized as a major regulator of TLR4 ENGAGEMENT SUPPRESSES AMPK ACTIVITY metabolism, recent studies have shown that AMPK activation has potent antiinflammatory effects in multiple cell populations, including neutrophils, macrophages and endothelial cells [11,12,13,14,15]. There is little evidence that AMPK is activated in inflammatory states, such as acute lung injury, despite the presence of conditions including increased release of reactive oxygen species and diminished generation of ATP, which would be expected to result in AMPK activation [9,17,18,19]. A potentially important, and presently unanswered question, relates to the mechanisms that may prevent activation of AMPK in such conditions
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