CITED2-Mediated Mechanotransduction and its use for Chondroprotection

Novel prevention and therapeutic treatment for cartilage degradation is urgently called for, as cartilage degradation is a hallmark for arthritic diseases including osteoarthritis (OA) and rheumatoid arthritis (RA), and there is a high incidence of arthritis-related disability and high medical costs. While both underuse (e.g. physically inactive lifestyle) and overuse (e.g. high impact or intense repetitive joint use as seen in certain sports) are risk factors for cartilage degradation, recent studies highlight that dynamic moderate loading is associated with reduced incidence of developing OA. Exercise is prescribed in most cases at moderate levels for both OA and RA patients, and accumulating studies demonstrate appropriate exercise maintains cartilage homeostasis and may exert a role in chondroprotection, but the mechanisms underlying how mechanical signals are translated into chondroprotective actions are largely unknown. Identifying the molecules that mediate mechanotransduction and its mechano-responsive nature will not only provide a biomechanical basis for developing more effective mechanical loading-based exercises for chondroprotection, but also can provide novel targets or molecular switches to develop chemical-based modalities for disease treatment. The overall objectives of this dissertation were to determine the mechanical response of transcriptional regulator CBP/p300 Interacting Transactivator with ED-rich Tail 2 (CITED2) to various mechanical loading regimes, elucidating CITED2-mediated chondroprotective pathways, and determining the potential of CITED2 as a target for the prevention and treatment of cartilage degradation in arthritis. The global hypothesis was that CITED2 mediates a mechanical pathway of chondroprotection, CITED2 is required for cartilage integrity, and restoration or increasing levels of CITED2 exerts efficacy in the prevention and treatment of cartilage degradation. This hypothesis was tested with four specific aims: 1) To determine the gene expression response of CITED2 to various mechanical loading regimes, and the role of CITED2 in mediating mechanical regulation of matrix metalloproteinases (MMPs), 2) To determine the CITED2-mediated loading-induced pathway and test whether it is required for loading-induced downregulation of MMPs, 3) To determine whether deficiency of CITED2 is a causal factor for cartilage degradation in arthritic diseases such as OA, and 4) Test the concept of CITED2 as a target for chondroprotection. The key findings of this dissertation include: 1) CITED2 expression is induced by moderate dynamic loading in chondrocytes in an intensity- and duration-dependent manner, and the upregulation of CITED2 is sustained for at least 12 hours after loading. The induction of CITED2 is required for the downregulation of MMPs (i.e. MMP-2, -3, and -13), 2) Dynamic moderate loading induces CITED2 by activating p38δ, which in turn triggers Sp1 and HIF-1α to transactivate CITED2. CITED2 competes with MMP transactivator Ets-1 for binding to limiting amounts of co-factor p300, resulting in repression of MMP expression. 3) Deficiency of CITED2 is associated with cartilage degradation in human OA and disease progression of post-traumatic OA in mice subject to destabilization of the medial meniscus (DMM). Experimental knockdown of CITED2 caused cartilage degradation and deletion of CITED2 in adult cartilage not only resulted in an early OA phenotype, but also accelerated the disease progression of cartilage degradation in DMM mice, revealing a chondroprotective role of CITED2, which is required for cartilage integrity maintenance. 4) Restoring levels of CITED2, via gene transfer or small molecule epigallocatechin gallate (EGCG), exerts efficacy in slowing cartilage degradation in RA and OA mouse models. Together, these studies provide feasibility for developing CITED2-activation-based therapies for the prevention and treatment of cartilage degradation