Nuclear Factor-κB Signaling Regulates the Nociceptin Receptor but Not Nociceptin Itself

The nociceptin receptor (NOP) and nociceptin are involved in the pathways of pain and inflammation. The potent role of nuclear factor-κB (NFκB) in the modulation of tumor necrosis factor-α (TNF-α) and interleukin (IL)-1β on the nociceptin system in human THP-1 cells under inflammatory conditions were investigated. Cells were stimulated without/with phorbol-myristate-acetate (PMA), TNF-α, IL-1β, or PMA combined with individual cytokines. To examine NFκB's contribution to the regulation of the nociceptin system, PMA-stimulated cells were treated with NFκB inhibitor BAY 11-7082, JSH-23, or anacardic acid before culturing with TNF-α or IL-1β. NOP and prepronociceptin (ppNOC) mRNA were quantified by RT-qPCR; cell membrane NOP and intracellular nociceptin protein levels were measured by flow cytometry. Phosphorylation and localization of NFκB/p65 were determined using ImageStream. PMA + TNF-α decreased NOP mRNA compared to stimulation with PMA alone, while PMA + IL-1β did not. BAY 11-7082 and JSH-23 reversed the repression of NOP by PMA + TNF-α. TNF-α and IL-1β attenuated PMA's upregulating effects on ppNOC. None of the inhibitors preserved the upregulation of ppNOC in PMA + TNF-α and PMA + IL-1β cultures. TNF-α strongly mediated the nuclear translocation of NFκB/p65 in PMA-treated cells, while IL-1β did not. Proinflammatory cytokines suppressed NOP and ppNOC mRNA in PMA-induced human THP-1 cells. NFκB signaling seems to be an important regulator controlling the transcription of NOP. These findings suggest that the nociceptin system may play an anti-inflammatory role during immune responses.