Nrf2 deficiency promotes RANKL-induced osteoclast differentiation and bone resorption

Abstract

Nrf2 (Nuclear factor-erythroid 2-related factor 2)는 redox-sensitive transcription factor로, antioxidant-response element와 결합함으로써 다양한 항산화/해독유전자의 발현을 조절한다. 파골세포 분화를 유도하는 RANKL(Receptor activator of NF-B Ligand)은 ROS(Reactive oxygen species)을 생성한다. 본 연구는 산화적 스트레스 반응을 감소시키는 핵내 전사인자인 Nrf2를 주목하여, Nrf2 knockout 마우스의 파골 전구세포를 사용하여 파골세포 분화에 대한 Nrf2의 역할을 조사하였다. Nrf2를 결손시킨 골수에서 유래된 대식세포에서 파골세포 분화, actin ring형성, 파골세포의 골 흡수가 wild-type 세포보다 크게 촉진되었으며 RANKL 자극여부에 인해서, 결실된 Nrf2에서 몇 가지의 항산화 유전자(Peroxiredoxins I, V와 VI, Sulfiredoxin, Thioredoxin 1 and Thioredoxin reductase 1, 2)가 결여되어있었고, 세포 내 ROS수준이 증가되어있었다. 또한 항산화제인 N-acetylcysteine을 처리해주었을 때 세포 내 ROS가 줄어들어 가속화된 파골세포 분화를 크게 줄였다. 결여된 Nrf2에서 파골세포 분화에 결정적인 역할을 하는 NFATc1, c-Fos 유도와 MAP kinases (p38, ERK, JNK)가 RANKL유도에 의해 높게 활성화되는 것으로 나타났다. 이러한 결과들로부터, 항산화 유전자 발현을 조절하여 세포의 산화/환원상태를 조절하는 Nrf2는 RANKL에 의한 파골세포 분화를 저해할 것이며 이로써 골다공증과 같은 염증성 질병에 새로운 방향으로 치료, 예방책을 제시할 수 있다.;Nuclear factor-erythroid 2-related factor 2 (Nrf2) is a redox-sensitive transcription factor, which regulates the expression of a variety of antioxidant and detoxification genes through an antioxidant-response element. Given that reactive oxygen species (ROS) have thought to be involved in receptor activator of nuclear factor κB (NF-κB) ligand (RANKL)-induced osteoclast (OC) differentiation, we here sought to examine the role of Nrf2 in OC differentiation using OC precursor cells of Nrf2 knockout mice. In bone marrow derived macrophage cells deficient in Nrf2, the RANKL-induced OC differentiation, actin ring formation and osteoclatic bone resorption were substantially promoted in comparison with wild-type cells. In both un-stimulated and RANKL-stimulated conditions, loss of Nrf2 led to an increase of intracellular ROS and a defect in expression of several antioxidant genes including peroxiredoxins I, V and VI, sulfiredoxin, thioredoxin 1, thioredoxin reductase 1 and 2. Moreover, the pretreatment of antioxidant N-acetylcysteine significantly reduced the acceleration of RANKL-induced OC differentiation by Nrf2 loss through decreasing intracellular ROS. In addition, Nrf2 deficiency promoted RANKL-induced activation of mitogen-activated protein kinases including c-Jun N-terminal kinase, extracellular signal-regulated kinase and p38, and induction of c-Fos as well as consequent induction of nuclear factor of activated T cell, cytoplasmic 1 that is a pivotal determinant of OC differentiation. Our results suggest that Nrf2 might inhibit RANKL-induced OC differentiation by regulating cellular redox status in part through the control of its target antioxidant gene expression, and might provide a new strategy for treating osteoporosis and other bone diseases.