Biomechanical Thresholds Regulate Inflammation through the NF-κB Pathway: Experiments and Modeling

Background: During normal physical activities cartilage experiences dynamic compressive forces that are essential to maintain cartilage integrity. However, at non-physiologic levels these signals can induce inflammation and initiate cartilage destruction. Here, by examining the pro-inflammatory signaling networks, we developed a mathematical model to show the magnitude-dependent regulation of chondrocytic responses by compressive forces. Methodology/Principal Findings: Chondrocytic cells grown in 3-D scaffolds were subjected to various magnitudes of dynamic compressive strain (DCS), and the regulation of pro-inflammatory gene expression via activation of nuclear factor-kappa B (NF-κB) signaling cascade examined. Experimental evidences provide the existence of a threshold in the magnitude of DCS that regulates the mRNA expression of nitric oxide synthase (NOS2), an inducible pro-inflammatory enzyme. Interestingly, below this threshold, DCS inhibits the interleukin-1β (IL-1β)-induced pro-inflammatory gene expression, with the degree of suppression depending on the magnitude of DCS. This suppression of NOS2 by DCS correlates with the attenuation of the NF-κB signaling pathway as measured by IL-1β-induced phosphorylation of the inhibitor of kappa B (IκB)-α, degradation of IκB-α and IκB-β, and subsequent nuclear translocation of NF-κB p65. A mathematical model developed to understand the complex dynamics of the system predicts two thresholds in the magnitudes of DCS, one for the inhibition of IL-1β-induced expression of NOS2 by DCS at low magnitudes, and second for the DCS-induced expression of NOS2 at higher magnitudes. Conclusions/Significance: Experimental and computational results indicate that biomechanical signals suppress and induce inflammation at critical thresholds through activation/suppression of the NF-κB signaling pathway. These thresholds arise due to the bistable behavior of the networks originating from the positive feedback loop between NF-κB and its target genes. These findings lay initial groundwork for the identification of the thresholds in physical activities that can differentiate its favorable actions from its unfavorable consequences on joints.