Nuclear Factor Kappa B (NF-kB)

Nuclear factor kappa B is a family of dimeric transcription factors, most often the p65 and p50 pair, held in the cytoplasm by inhibitory IkB proteins. Activation is a controlled release rather than a synthesis: a signal activates the IkB kinase complex, which phosphorylates IkB and marks it for destruction, freeing the dimer to enter the nucleus. There it drives transcription of TNF-alpha, IL-1, IL-6, chemokines, cyclooxygenase-2 and adhesion molecules. TNF-alpha is both inducer and product, giving a feed-forward loop, while IkB alpha is itself a target gene, supplying negative feedback.

Nearly every inflammatory receptor converges here: Toll-like receptors through MyD88, the IL-1 receptor, the TNF receptors and the antigen receptors of T and B cells. Part of the anti-inflammatory action of glucocorticoids is interference with the pathway. No approved direct inhibitor exists, for an instructive reason: mice lacking the beta subunit of the IkB kinase die in utero from hepatocyte apoptosis, because the pathway that drives inflammation also keeps cells alive.

This makes the pathway a weak selling point: saying a compound inhibits nuclear factor kappa B narrows the mechanism almost not at all, since it sits downstream of every inflammatory input and upstream of most outputs. The useful questions are which receptor was engaged, at what concentration, in which cell, and whether anything downstream changed in an animal.

The specific failure is the reporter assay. A luciferase construct in a transfected cell line, inhibited at tens or hundreds of micromolar, is a screening result routinely reported as anti-inflammatory activity. Concentrations that high are unreachable in plasma, cytotoxicity suppresses reporter signal in the same range, and the assay measures transcriptional output in one engineered cell, not inflammation in a tissue.