TNF-α induced Nrf2 activation in monocytes

Background: Oxidative stress has been implicated in a variety of inflammatory diseases including Rheumatoid Arthritis (RA). The transcription factor Nuclear factor-erythroid 2 (NF-E2)-related factor 2 (Nrf2) is known to maintain the cellular defence against oxidative stress via binding to the antioxidant response element (ARE) within the regulatory regions of antioxidative and detoxifying genes. Nrf2 has also been suggested as critical regulator of the innate immune response. Our aim was to investigate if TNF-α plays a role in Nrf2 activation in monocytes. Methods: The Nrf2-activity was measured using an ARE-luciferase mouse treated with antibody-induced arthritis AIA (a RA inflammation model) in the IVIS imaging-system. For in vitro studies, Nrf2/ARE activation was analyzed in luciferase assays with the human monocytic cell line THP-1 stable transfected with pGL4-ARE. We used various kinase inhibitors and antioxidants to elucidate the involved signal transduction. Gene expression of Nrf2 target genes HO-1 and NQO1, was studied by qRT-PCR. ROS production and accompanying oxidative stress damage in TNF α stimulated monocytes and murine macrophages was investigated using H2DCF-DA and oxyblot technique. Results: IVIS imaging-system data illustrate a time-dependent Nrf2 activation in the course of AIA. Furthermore, we show that TNF-α stimulation of THP-1 cells results in Nrf2 activation and subsequent upregulation of HO-1 and NQO1. This Nrf2 activation depends on ERK and p38 kinase activity and NADPH Oxidase activation. Discussion: These data demonstrate that TNF-α stimulation leads to Nrf2 activation in phagocytotic cells. This may lead to increased protection against oxidative stress in the course of phagocytosis-associated ROS production.