Unravelling the Basic Calcium Phosphate crystal-dependent chondrocyte protein secretome; a role for TGF-β signaling.

ObjectiveBasic Calcium Phosphate crystals play an active role in the progression of osteoarthritis. However, the cellular consequences remain largely unknown. Therefore, we characterized for the first time the changes in the protein secretome of human OA articular chondrocytes as a result of BCP stimulation using two unbiased proteomic analysis methods.MethodIsolated human OA articular chondrocytes were stimulated with BCP crystals and examined by RT-qPCR and ELISA after twenty-four and forty-eight hours. Forty-eight hours conditioned media were analysed by label-free LC-MS/MS and an antibody array. The activity of BCP dependent TGF-β signalling was analysed by RT-qPCR and luciferase reporter assays. The molecular consequences regarding BCP-dependent TGF-β signalling on BCP-dependent IL-6 were investigated using specific pathway inhibitors.ResultsSynthesized BCP crystals induced IL-6 expression and secretion upon stimulation of human articular chondrocytes. Concomitant induction of catabolic gene expression was observed. Analysis of conditioned media revealed a complex and diverse response with a large number of proteins involved in TGF-β signalling, both in activation of latent TGF-β and TGF-β superfamily members, which were increased compared to non-stimulated OA chondrocytes. Activity of this BCP driven TGF-β signalling was confirmed by increased activity of expression of TGF-β target genes and luciferase reporters. Inhibition of BCP driven TGF-β signalling resulted in decreased IL-6 expression and secretion with a moderate effect on catabolic gene expression.ConclusionBCP crystal stimulation resulted in a complex and diverse chondrocyte protein secretome response. An important role for BCP-dependent TGF-β signaling was identified in development of a pro-inflammatory environment