Activation of the nuclear transcription factor NF-kB by TNF-aand IL-1 plays a key role in the inflammatory process by inducing the transcription of a host of proinflammatory cytokines, chemokines, adhesion molecules, and Cox-2. Other functions include regulation of cellular apoptosis and osteoclast activation. These processes are responsible for the histopathological changes seen in inflammatory arthritis, including RA. The central role of NF-kB in all of these processes makes it a compelling target for therapeutic intervention in inflammatory arthritis. Though some medications currently used to treat inflammatory arthritis, such as sulfasalazine and leflunomide, are known to have some indirect NF-kB blocking properties, little is known about the effects of direct inhibition of NF-kB. Here, Clohisy et al (Journal of Immunology, 171:5547, 2003) examine the effect of direct inhibition of NF-kB in mice using mutant constructs of I-kB which lack phosphorylation sites required for dissociation of I-kB from NF-kB. Dissociation of I-kB from NF-kB is required for nuclear translocation of NF-kB and its subsequent ability to induce the transcription of proinflammatory genes.
Inflammatory arthritis was induced in susceptible mice by administration of arthritogenic serum from K/B x N mice. Concurrently, mice were injected daily with various mutant constructs of I-kB, lacking critical N-terminal phosphorylation sites. Serum and articular TNF and RANKL (a cytokine involved in osteoclast activation) expression was examined in treated and untreated mice. Distal joints were collected and examined for gross and histological evidence of inflammation, osteoclast activation, and bony erosions. Finally, micro-computed tomography was performed on intact mouse legs, and three-dimensional images were examined for evidence of bony erosions.
Cells retrieved from arthritic joints demonstrated abundant expression of TNF, RANKL, DNA-bound (activated) NF-kB compared with controls. Co-administration of mutant constructs of I-kB reduced joint levels of activated NF-kB and TNF-aby 90% compared to arthritic control joints (Co-administration of wild-type I-kB also reduced TNF-alevels by 90% and NF-kB levels by 75%). Mice administered I-kB proteins demonstrated significantly less gross joint redness and swelling, reduced histological evidence of osteoclast recruitment and bony erosions, and less micro-CT evidence of periarticular bony destruction compared with control arthritic mice.
In a mouse model of inflammatory arthritis, direct inhibition of NF-kB using mutant constructs of I-kB is effective in reducing/blocking many of the pathologic consequences of disease, i.e. inflammation and bony erosions.
Prior studies in vitro have provided proof-of-concept that inhibition of activation of NF-kB (by over-expressing wild-type I-kB) can reduce many pro-inflammatory effects of TNF-aand IL-1. However, this is the first demonstration of effective direct inhibition of NF-kB in vivo in inflammatory arthritis. Though the concept holds promise as a therapeutic target in RA in humans, concerns about potential toxicities may limit applicability considering that NF-kB is expressed in nearly all cell types and plays a broader role in regulating gene transcription than just those directly involved in the pathogenesis of inflammatory arthritis. In addition, the institution of NF-kB blockade concurrently with the induction of inflammatory arthritis does not parallel the reality of human inflammatory arthritis, in which disease onset often predates treatment by years. However, with such a compelling target, more extensive animal and human studies are eagerly anticipated
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