Inflammation: a role for NR4A orphan nuclear receptors?

Inflammation is paradoxical; it is essential for protection following biological, chemical or physical stimuli, but inappropriate or misdirected inflammation is responsible for tissue injury in a variety of inflammatory diseases. The polarization of immune cells is critical in controlling the stages of inflammatory response. The acute phase of inflammation is characterized by a T-lymphocyte:T_h2 cytokine profile and involves a co-ordinated migration of immune cells to the site of injury where production of cytokines and acute-phase proteins brings about healing. However, persistent inflammation can result in inappropriate and prolonged T-lymphocyte:T_h1 cytokine-mediated action and reaction of self-molecules, leading to a chronic phase in diseases such as RA (rheumatoid arthritis), Ps (psoriasis) and atherosclerosis. The inflammatory response is also controlled by activated macrophage cells, with classically activated (M1) cells producing a wide variety of pro-inflammatory mediators, while alternatively activated (M2) macrophages participate in anti-inflammatory response. Members of the NR4A subfamily (NR4A1/NUR77, NR4A2/NURR1 and NR4A3/NOR1) of orphan NRs (nuclear receptors) have emerged as key transcriptional regulators of cytokine and growth factor action in diseases affecting our aging population. As ligand-independent and constitutively active receptors, the activity of these transcription factors is tightly controlled at the level of expression, post-translational modification and subcellular localization. NR4A subfamily members are aberrantly expressed in inflamed human synovial tissue, psoriatic skin, atherosclerotic lesions, lung and colorectal cancer cells. Significantly, prolonged or inappropriate inflammatory responses contribute to the pathogenesis of these diseases. In activated cells, NR4A receptors are rapidly and potently induced, suggesting that these receptors may act as important transcriptional mediators of inflammatory signals. NR4A receptors may contribute to the cellular processes that control inflammation, playing a critical part in the contribution of chronic inflammation or they may have a protective role, where they may mediate pro-resolution responses. Here, we will review the contribution of the NR4A orphan NRs to integration of cytokine signalling in inflammatory disorders.