The Inorganic Pyrophosphate Transporter ANK Preserves the Differentiated Phenotype of Articular Chondrocyte

The differentiated phenotype of chondrocyte is lost in pathological situations and after interleukin (IL)-1 beta challenge. Wnt proteins and the inorganic pyrophosphate (PPi) transporter Ank regulate the differentiation process in many cell types. We investigated the possible contribution of Ank and/or PPi to the maintenance of the differentiated chondrocyte phenotype with special care to Wnt signaling. Primary articular chondrocytes lost their phenotype upon IL-1 beta challenge, with cessation of type II collagen and Sox-9 expression. Ank expression and PPi transport were strongly reduced by IL-1 beta, whereas Wnt-5a was the only Wnt protein increased. Transient overexpression of Ank counteracted most of IL-1 beta effects on Type II collagen, Sox-9, and Wnt-5a expression. When resting chondrocytes were transfected with a siRNA against Ank, this reproduced the phenotype induced by IL-1 beta. In both cases, no markers for hypertrophic chondrocytes were detected. The conditioned supernatant from chondrocytes knocked-down for Ank contained Wnt-5a, which activated Tcf/Lef reporter plasmids and promoted translocation of beta-catenin into the nucleus without activating the c-Jun N-terminal kinase (JNK) pathway. Supplementation with PPi compensated for most effects of Ank deficiency on Type II collagen, Sox-9, and Wnt-5 expression, both in IL-1 beta and Ank knock-down conditions. Phenotype changes induced by IL-1 beta were also supported by activation of the JNK pathway, but this latter was not sensitive to PPi supplementation. Altogether our data demonstrate that the transport of PPi by ANK contributed to the maintenance of the differentiated phenotype of chondrocyte by controlling the canonical Wnt pathway in a Wnt-5a-dependent manner.