Enhancement of Intervertebral Disc Cell Senescence by WNT/β-Catenin Signaling-Induced Matrix Metalloproteinase Expression

Objective. To determine whether intervertebral disc (IVD) cells express beta-catenin and to assess the role of the WNT/beta-catenin signaling pathway in cellular senescence and aggrecan synthesis. Methods. The expression of beta-catenin messenger RNA (mRNA) and protein in rat IVD cells was assessed by using several real-time reverse transcription-polymerase chain reaction, Western blot, immunohistochemical, and immunofluorescence analyses. The effect of WNT/beta-catenin on nucleus pulposus (NP) cells was examined by transfection experiments, an MTT assay, senescence-associated beta-galactosidase staining, a cell cycle analysis, and a transforming growth factor (TGF beta)/bone morphogenetic protein (BMP) pathway-focused microarray analysis. Results. We found that beta-catenin mRNA and protein were expressed in discs in vivo and that rat NP cells exhibited increased beta-catenin mRNA and protein upon stimulation with lithium chloride, a known activator of WNT signaling. LiCl treatment inhibited the proliferation of NP cells in a time-and dose-dependent manner. In addition, there was an increased level of cellular senescence in LiCl-treated cells. Long-term treatment with LiCl induced cell cycle arrest and promoted subsequent apoptosis in NP cells. Activation of WNT/beta-catenin signaling also regulated the expression of aggrecan. We also demonstrated that WNT/beta-catenin signaling induced the expression of matrix metalloproteinases (MMPs) and TGF beta in NP cells. Conclusion. The activation of WNT/beta-catenin signaling promotes cellular senescence and may modulate MMP and TGF beta signaling in NP cells. We hypothesize that the activation of WNT/beta-catenin signaling may lead to an increased breakdown of the matrix, thereby promoting IVD degeneration.