Pleiotrophic roles of metallothioneins in osteoarthritis pathogenesis

Abstract

The zinc-ZIP8-MTF1 axis induces metallothionein (MT) expression and is a catabolic regulator of experimental osteoarthritis (OA) in mice. The main aim of the current study was to explore the roles and underlying molecular mechanisms of MTs in OA pathogenesis. Experimental OA in mice was induced by destabilization of the medial meniscus (DMM) or intra-articular (IA) injection of adenovirus carrying a target gene (Ad-Zip8, Ad-Mtf1, Ad-Epas1, Ad-Nampt, Ad-Mt1 or Ad-Mt2) into wild-type, Zip8fl/fl; Col2a1-Cre, Mtf1fl/fl; Col2a1-Cre and Mt1/Mt2 double knockout mice. Primary cultured mouse chondrocytes were infected with Ad-Mt1 or Ad-Mt2, and gene expression profiles analyzed via microarray and reverse transcription-polymerase chain reaction (RT-PCR). Proteins in human and mouse OA cartilage were identified via immunostaining. Chondrocyte apoptosis in OA cartilage was determined using terminal deoxynucleotidyl transferase (TdT)-mediated deoxyuridine triphosphate (dUTP) nick end labelling (TUNEL). MTs were highly expressed in human and mouse OA cartilage. Hypoxia-inducible factor 2α (HIF-2α), nicotinamide phosphoribosyltransferase (NAMPT) and several proinflammatory cytokine pathways, as well as the zinc-ZIP8-MTF1 axis were identified as upstream regulators of MT expression. Genetic deletion of Mt1 and Mt2 enhanced cartilage destruction through increasing chondrocyte apoptosis. Unexpectedly, aberrant overexpression of MT2, but not MT1, induced upregulation of matrix-degrading enzymes and downregulation of matrix molecules through nuclear factor-kappa B (NF-κB) and activator protein-1 (AP-1) activation, ultimately leading to OA. MTs play an antiapoptotic role in posttraumatic OA. However, aberrant and chronic upregulation of MT2 triggers an imbalance between chondrocyte anabolism and catabolism, consequently accelerating OA development. These findings collectively highlight pleiotropic roles of MTs as regulators of chondrocyte apoptosis as well as catabolic and anabolic pathways during OA pathogenesis.