Pleiotropic Roles of Nrf2 as Regulators of Chondrocyte Apoptosis, Oxidative Stress, Inflammatory Response and Catabolic and Anabolic Pathways in Osteoarthritis

Abstract

Nrf2, a redox regulated transcription factor, has recently been shown to play a role in cartilage integrity but the mechanism remains largely unknown. Osteoarthritis (OA) is a multifactorial disease in which focal degradation of cartilage occurs. Here, we investigated the regulation of oxidative stress, inflammatory response, apoptosis, catabolic and anabolic factors by Nrf2 in IL-1β-stimulated human OA chondrocytes. Ectopic expression of Nrf2 in OA chondrocytes suppressed the generation of ROS while Nrf2 knockdown significantly increased the ROS levels in IL-1β-stimulated OA chondrocytes. Further, Nrf2 activation significantly inhibited the IL-1β-induced activation of extrinsic and intrinsic apoptotic pathways as determined by inhibition of DNA fragmentation, activation of Caspase-3,-8,-9, cleavage of PARP, release of cytochrome-c, suppression of mitochondrial dysfunction, suppression of pro-apoptotic proteins Bax, Bad and Bid expression and enhanced expression of anti-apoptotic proteins Bcl2, Bcl-xl and Mcl-1 in OA chondrocytes. To examine the role of Nrf2 in IL-1β-induced inflammatory response, we determined the mRNA expression and protein levels of several cytokines and chemokines using PCR array and multiplex immunoassay. Our data demonstrate that Nrf2 activation inhibited the IL-1β-mediated expression of several inflammatory mediators and Gene Ontology analysis revealed significant enrichment of MAPK signaling, TNF signaling, NF-κβ signaling and cytokine receptor signaling pathways. Additionally, Nrf2 activation inhibited the expression matrix degrading proteases-MMP-13, 3, 9, and ADAMTS-4 and enhanced the expression of OA related anabolic factors-COL2A1, ACAN, FRZB and GDF5 and SOX-5, 8 and 9 in IL-1β-stimulated OA chondrocytes. Importantly, we also discovered that Nrf2 mediated suppression of catabolic factors was dependent on ERK1/2 activation by Nrf2. Our data demonstrate that Nrf2 regulates multiple pathways involved in chondrocyte apoptosis, inflammation and matrix degradation via activation of ERK1/2/ELK1-P70S6K-P90RSK axis. Our findings collectively support the development of therapies targeting Nrf2 for the management of OA.