p16INK4a and its regulator miR-24 link senescence and chondrocyte terminal differentiation-associated matrix remodeling in osteoarthritis

Introduction: Recent evidence suggests that tissue accumulation of senescent p16(INK4a)-positive cells during the life span would be deleterious for tissue functions and could be the consequence of inherent age-associated disorders. Osteoarthritis (OA) is characterized by the accumulation of chondrocytes expressing p16(INK4a) and markers of the senescence-associated secretory phenotype (SASP), including the matrix remodeling metalloproteases MMP1/MMP13 and pro-inflammatory cytokines interleukin-8 (IL-8) and IL-6. Here, we evaluated the role of p16(INK4a) in the OA-induced SASP and its regulation by microRNAs (miRs).Methods: We used IL-1-beta-treated primary OA chondrocytes cultured in three-dimensional setting or mesenchymal stem cells differentiated into chondrocyte to follow p16(INK4a) expression. By transient transfection experiments and the use of knockout mice, we validate p16(INK4a) function in chondrocytes and its regulation by one miR identified by means of a genome-wide miR-array analysis.Results: p16(INK4a) is induced upon IL-1-beta treatment and also during in vitro chondrogenesis. In the mouse model, Ink4a locus favors in vivo the proportion of terminally differentiated chondrocytes. When overexpressed in chondrocytes, p16(INK4a) is sufficient to induce the production of the two matrix remodeling enzymes, MMP1 and MMP13, thus linking senescence with OA pathogenesis and bone development. We identified miR-24 as a negative regulator of p16(INK4a). Accordingly, p16(INK4a) expression increased while miR-24 level was repressed upon IL-1-beta addition, in OA cartilage and during in vitro terminal chondrogenesis.Conclusions: We disclosed herein a new role of the senescence marker p16(INK4a) and its regulation by miR-24 during OA and terminal chondrogenesis.