Inhibition of bromodomain-containing protein 4 ameliorates oxidative stress-mediated apoptosis and cartilage matrix degeneration through activation of NF-E2-related factor 2-heme oxygenase-1 signaling in rat chondrocytes.

Abstract

During the progression of osteoarthritis, dysregulation of extracellular matrix (ECM) anabolism, abnormal generation of reactive oxygen species, and proteolytic enzymes have been shown to accelerate the degradation process of cartilage. The purpose of the current study was to investigate the functional role of bromodomain-containing protein 4 (BRD4) in hydrogen peroxide (H2 O2 )-stimulated chondrocyte injury and delineate the underlying molecular mechanisms. We observed that the expression BRD4 was markedly elevated in rat chondrocytes after H2 O2 stimulation. Additionally, inhibition of BRD4 using small interfering RNA or JQ1 (a selective potent chemical inhibitor) led to repression of H2 O2 -induced oxidative stress, as revealed by a decrease in the reactive oxygen species production accompanied by a decreased malondialdehyde content, along with increased activities of antioxidant markers superoxide dismutase, catalase, and glutathione peroxidase on exposure of chondrocytes to H2 O2 . Meanwhile, depletion of BRD4 led to repress the oxidative stress-induced apoptosis of chondrocytes triggered by H2 O2 accompanied by an increase in the expression of anti-apoptotic Bcl-2 and a decrease in the expression of pro-apoptotic Bax and caspase 3 as well as attenuated caspase 3 activity. Moreover, knockdown of BRD4 or treatment with JQ1 markedly attenuated ECM deposition, reflected in a marked upregulation of proteoglycans collagen type II and aggrecan as well as downregulation of ECM-degrading enzymes matrix metalloproteinase 13 and A disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS-5). More importantly, inhibition of BRD4-activated NF-E2-related factor 2 (Nrf2)-heme oxygenase-1 signaling. Mechanistically, the protective effect of BRD4 inhibition on H2 O2 -stimulated apoptosis and cartilage matrix degeneration was markedly abrogated by Nrf2 depletion. Altogether, we concluded that the protective effect of BRD4 inhibition against oxidative stress-mediated apoptosis and cartilage matrix degeneration occurred through Nrf2-heme oxygenase-1 signaling, implying that BRD4 inhibition may be a more effective therapeutic strategy against osteoarthritis.