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

Abstract

BackgroundDuring 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 FindingsChondrocytic 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/SignificanceExperimental 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.

Gene Expression Responses to Mechanical Stimulation of Mesenchymal Stem Cells Seeded on Calcium Phosphate Cement

“…In fact, previous gene expression studies of cells subjected to large mechanical loads have reported regulation of stress and inflammatory genes. 15,35,36 In comparison, there were no signs of upregulation of inflammatory or stressrelated genes in the present study, supporting the suggestion of an appropriate level of strain being utilized. Our study also showed a markedly lower total number of changes in gene expression in response to a mechanical loading.…”

Characterization of a Model of an Arteriovenous Fistula in the Rat

“…[33][34][35] To determine the functional significance of such induction of NOS, the effect of L-NAME, an inhibitor of NOS, was assessed in this model of an AVF. Such inhibition led to an exaggeration of the histological changes observed in the venous limb along with the up-regulation of pro-inflammatory genes such as MCP-1 and CINC-1, the latter representing the rat homologue of human IL-8.…”

“…This response includes early activation of pro-inflammatory transcription factors, NF-B and AP-1, transcription factors known to be activated in endothelial cells by shear stress. [33][34][35]43,44 It is tempting to speculate that activation of these transcription factors represents a maladaptive response to hemodynamic stress in the venous segment of an AVF since this response sets the stage for ensuing inflammation and neointimal hyperplasia. The significance of hemodynamic stress in this model is underscored by the fact that systemic hypertension, as imposed by two different models, exacerbated venous neointimal hyperplasia in the AVF; the adverse effects of systemic hypertension on the longevity of AVFs merits further investigation.…”

Potential Mechanisms of PTOA: Inflammation

“…The signaling events activated in chondrocytes and other resident cells are common to both mechanotransduction and cytokine stimulation [ 20 ]. A large number of studies have addressed interleukin-1 (IL-1) as a major infl ammatory cytokine that drives the progression of the disease (see for review [ 21 , 22 ]).…”

“…The fi ndings that mechanical stimuli modulate NF-κB signaling [ 20 ] provide an explanation for why NF-κB-related gene signatures may be upregulated in mouse models of PTOA in the absence of overt signs of infl ammation such as synovitis and immune cell infi ltration. Such profi ling studies have revealed that infl ammatory signatures are present before the appearance of overt OA and, in some models, are associated with increased numbers of activated T-and B-lymphocytes in the spleens of the mice destined to develop OA [ 50 ].…”