Monosodium urate crystals trigger Nrf2- and heme oxygenase-1-dependent inflammation in THP-1 cells

Gouty arthritis is an inflammatory disease that is caused by an accumulation of monosodium urate (MSU) crystals in the joints. MSU is capable of activating the nucleotide-binding oligomerization domain-like receptor pyrin domain containing 3 (NLRP3) inflammasome, leading to interleukin-1β (IL-1β) secretion. Reactive oxygen species (ROS) are major mediators of the NLRP3/IL-1β interaction. Although nuclear factor E2-related factor 2 (Nrf2) is recognized as a transcription factor that is involved in the response to oxidative stress, the effect of MSU on Nrf2 and on Nrf2-mediated antioxidant enzymes remains unclear. The treatment of THP-1 monocytes using phorbol 12-myristate 13-acetate (PMA) was shown to initiate inflammatory responses. Here, we showed that THP-1 cells, following treatment with MSU crystals, significantly increased IL-1β release, NLRP3 inflammasome activation and ROS production. MSU also promoted the nuclear translocation of Nrf2 and activated lysosomal destabilization. Moreover, the levels of heme oxygenase-1 (HO-1) in gene and protein expressions were upregulated by MSU. MSU-induced IL-1β secretion and NLRP3 inflammasome activation were inhibited by the knockdown of Nrf2 and via the HO-1 inhibitor zinc (II) protoporphyrin IX (ZnPP). In addition, HO-1 inhibition increased the level of superoxide anion production and the consumption of glutathione. These findings suggest that Nrf2 and HO-1 mediate redox homeostasis and interact with pro-inflammatory factors in MSU-challenged THP-1 cells, thereby providing new insight into how MSU-induced gouty inflammation is mediated by specific mechanisms that are involved in the Nrf2/Ho-1 antioxidant signaling pathway.Cellular & Molecular Immunology advance online publication, 11 August 2014; doi:10.1038/cmi.2014.65