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

Abstract

The differentiated phenotype of chondrocyte is lost in pathological situations and after interleukin (IL)-1beta challenge. Wnt proteins and the inorganic pyrophosphate (PP(i)) transporter Ank regulate the differentiation process in many cell types. We investigated the possible contribution of Ank and/or PP(i) to the maintenance of the differentiated chondrocyte phenotype with special care to Wnt signaling. Primary articular chondrocytes lost their phenotype upon IL-1beta challenge, with cessation of type II collagen and Sox-9 expression. Ank expression and PP(i) transport were strongly reduced by IL-1beta, whereas Wnt-5a was the only Wnt protein increased. Transient overexpression of Ank counteracted most of IL-1beta 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-1beta. 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 PP(i) compensated for most effects of Ank deficiency on Type II collagen, Sox-9, and Wnt-5 expression, both in IL-1beta and Ank knock-down conditions. Phenotype changes induced by IL-1beta were also supported by activation of the JNK pathway, but this latter was not sensitive to PP(i) supplementation. Altogether our data demonstrate that the transport of PP(i) by ANK contributed to the maintenance of the differentiated phenotype of chondrocyte by controlling the canonical Wnt pathway in a Wnt-5a-dependent manner.