Enhancement of intervertebral disc cell senescence by WNT/β‐catenin signaling–induced matrix metalloproteinase expression

Objective To determine whether intervertebral disc (IVD) cells express β‐catenin and to assess the role of the WNT/β‐catenin signaling pathway in cellular senescence and aggrecan synthesis. Methods The expression of β‐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/β‐catenin on nucleus pulposus (NP) cells was examined by transfection experiments, an MTT assay, senescence‐associated β‐galactosidase staining, a cell cycle analysis, and a transforming growth factor (TGFβ)/bone morphogenetic protein (BMP) pathway–focused microarray analysis. Results We found that β‐catenin mRNA and protein were expressed in discs in vivo and that rat NP cells exhibited increased β‐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/β‐catenin signaling also regulated the expression of aggrecan. We also demonstrated that WNT/β‐catenin signaling induced the expression of matrix metalloproteinases (MMPs) and TGFβ in NP cells. Conclusion The activation of WNT/β‐catenin signaling promotes cellular senescence and may modulate MMP and TGFβ signaling in NP cells. We hypothesize that the activation of WNT/β‐catenin signaling may lead to an increased breakdown of the matrix, thereby promoting IVD degeneration.