Sequential dynamics of inflammatory cytokine, angiogenesis inducing factor and matrix degrading enzymes during spontaneous resorption of the herniated disc

Abstract

Intervertebral disc herniation (HD) is one of the most common orthopaedic conditions. MRI analysis of HD has revealed a spontaneous resorption mechanism related with neo-vascularization. It appears that the interaction of activated macrophages with disc tissues leads to the generation of inflammatory cytokines. Moreover, inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha) is required for the induction of angiogenesis inducing factors such as vascular endothelial growth factor (VEGF) or matrix degrading enzymes such as MMP-3, MMP-7 and plasmin. We hypothesized that these molecules play a crucial role during spontaneous HD resorption. In this study, we have examined the sequential expression of these molecules using a co-culture system which is composed of the interaction of activated macrophages and disc tissues as a model of the acute response of inflammation occurred in HD. We have also considered the mechanism of activating latent MMPs during HD resorption process. Current our results indicate that upregulation of both TNF-alpha mRNA and protein expressions occur first in the inflammation induced by HD. VEGF upregulation follows the increased level of TNF-alpha expression. Both plasmin and MMP-3 are upregulated at later time points. We also demonstrate that both TNF-alpha and VEGF induce upregulated expression of urokinase-type plasminogen activator (u-PA). Our previous work has demonstrated that TNF-alpha could upregulate the expression of VEGF, MMP-3 and MMP-7 in the co-culture system. It has been reported that plasmin could affect to activate latent MMPs. Based on these findings, we suggest that TNF-alpha acts as the initiator of inflammation following contact between macrophages and disc chondrocytes and that plasmin and u-PA play a crucial role in activation of MMPs. We propose a spontaneous HD resorption cascade. Further understanding of the resorption process may provide future novel therapies for HD.