Low Cellular NAD⁺ Compromises Lipopolysaccharide-Induced Inflammatory Responses via Inhibiting TLR4 Signal...

Area of Science:

• Immunology
• Metabolic pathways
• Cellular signaling

Background:

• Nicotinamide adenine dinucleotide (NAD+) is crucial for metabolism and implicated in inflammatory diseases.
• Limited data exists on NAD+'s role in human monocyte pro-inflammatory responses.
• Key innate immune responses like TLR4 and NLRP3 inflammasome activation are central to inflammation.

Purpose of the Study:

• To investigate the effect of NAD+ levels on Toll-like receptor 4 (TLR4) and NOD-like receptor thermal protein domain associated protein 3 (NLRP3) inflammasome activation in human primary monocytes.
• To elucidate the signaling pathways influenced by NAD+ in monocytes during inflammation.

Main Methods:

• Human primary monocytes were isolated and treated to manipulate intracellular NAD+ levels using nicotinamide riboside and FK866.
• Cells were stimulated with lipopolysaccharide (LPS) and subsequently with ATP or cholesterol crystals to activate NLRP3.
• Cytokine release, TLR4 signaling pathway activation (NF-κB), and protein phosphorylation were analyzed using Western blot and proteomic analysis.

Main Results:

• Elevated NAD+ enhanced interleukin-1 beta (IL-1β) release in LPS-primed monocytes stimulated with ATP.
• NAD+ depletion using FK866 inhibited both NLRP3-dependent and -independent inflammatory responses.
• FK866-mediated NAD+ depletion suppressed phosphorylation of key signaling proteins in the TLR4 pathway, including TAK1, IKKβ, IkBα, MEK1/2, ERK1/2, and p38.
• Inhibition was not due to sirtuin or PARP1 suppression.

Conclusions:

• NAD+ plays a significant role in modulating TLR4 signal transduction in human monocytes.
• NAD+ depletion inhibits key inflammatory responses, suggesting a complex interaction with inflammation.
• The findings question the therapeutic application of nicotinamide riboside in inflammatory conditions.