Age and oxidative stress regulate Nrf2 homeostasis in human articular chondrocytes

Objective: The purpose of this study was to investigate the effect of age and oxidative stress on regulation of nuclear factor erythroid-2-related factor 2 (Nrf2) in young, old, and osteoarthritic (OA) human articular chondrocytes. Design: Levels of Nrf2 in primary human chondrocytes isolated from young, old, and OA donors were measured by immunoblotting, qPCR, and immunohistochemistry. Effects on levels of Nrf2, antioxidant proteins regulated by Nrf2, as well as p65, and the anabolic response to insulin-like growth factor-1 (IGF-1) were evaluated after induction of oxidative stress with menadione, Nrf2 knockdown with siRNA, and/or Nrf2 activation with RTA-408. Results: Nrf2 protein levels were significantly lower in older adult chondrocytes (∼0.59 fold; p = 0.034) and OA chondrocytes (∼0.50 fold; p = 0.016) compared to younger cells. Menadione significantly increased Nrf2 protein levels in young chondrocytes by just under four-fold without changes in old chondrocytes. Nrf2 knockdown and activation differentially regulated levels of anti-oxidant proteins including sulfiredoxin and NAD(P)H quinone dehydrogenase 1. Nrf2 activation with RTA-408 also decreased basal p65 phosphorylation, increased aggrecan and type II collagen gene expression, and increased production of proteoglycans in OA chondrocytes treated with IGF-1. Conclusions: Targeted therapeutic strategies aimed at maintaining Nrf2 activity could be useful in maintaining chondrocyte homeostasis through maintenance of intracellular antioxidant function and redox balance.