Protective role of frizzled-related protein B on matrix metalloproteinase induction in mouse chondrocytes

Introduction: Our objective was to investigate whether a lack of frizzled-related protein B (FrzB), an extracellular antagonist of the Wnt signaling pathways, could enhance cartilage degradation by facilitating the expression, release and activation of matrix metalloproteinases (MMPs) by chondrocytes in response to tissue-damaging stimuli. Methods: Cartilage explants from FrzB(-/-) and wild-type mice were challenged by excessive dynamic compression (0.5 Hz and 1 MPa for 6 hours). Load-induced glycosaminoglycan (GAG) release and MMP enzymatic activity were assessed. Interleukin-1 beta (IL-1 beta) (10, 100 and 1000 pg/mL for 24 hours) was used to stimulate primary cultures of articular chondrocytes from FrzB(-/-) and wild-type mice. The expression and release of MMP-3 and -13 were determined by RT-PCR, western blot and ELISA. The accumulation of beta-catenin was assessed by RT-PCR and western blot. Results: Cartilage degradation, as revealed by a significant increase in GAG release (2.8-fold, P = 0.014) and MMP activity (4.5-fold, P = 0.014) by explants, was induced by an excessive load. Load-induced MMP activity appeared to be enhanced in FrzB(-/-) cartilage explants compared to wild-type (P = 0.17). IL-1 beta dose-dependently induced Mmp-13 and -3 gene expression and protein release by cultured chondrocytes. IL-1 beta-mediated increase in MMP-13 and -3 was slightly enhanced in FrzB(-/-) chondrocytes compared to wild-type (P = 0.05 and P = 0.10 at gene level, P = 0.17 and P = 0.10 at protein level, respectively). Analysis of Ctnn1b and Lef1 gene expression and beta-catenin accumulation at protein level suggests that the enhanced catabolic response of FrzB(-/-) chondrocytes to IL-1 beta and load may be associated with an over-stimulation of the canonical Wnt/beta-catenin pathway. Conclusions: Our results suggest that FrzB may have a protective role on cartilage degradation and MMP induction in mouse chondrocytes by attenuating deleterious effects of the activation of the canonical Wnt/beta-catenin pathway.