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.