Inflammatory Mediators Increase SUMOylation of Retinoid X Receptor a in α c-Jun N-Terminal Kinase-Dependent Manner in Human Hepatocellular Carcinoma Cells

Retinoid X receptor alpha [RXR alpha; nuclear receptor (NR)2B1] is a crucial regulator in the expression of a broad array of hepatic genes under both normal and pathologic conditions. During inflammation, RXRa undergoes rapid post-translational modifications, including c-Jun N-terminal kinase (JNK)-mediated phosphorylation, which correlates with a reduction in RXR alpha function. A small ubiquitin-like modifier (SUMO) acceptor site was recently described in human RXRa, yet the contributors, regulators, and consequences of SUMO-RXR alpha are not well understood. Inflammation and other stressors alter nuclear receptor function in liver and induce SUMOylation of several NRs as part of proinflammatory gene regulation, but linkages between these two pathways in liver, or for RXR alpha directly, remain unexplored. We sought to determine if inflammation induces SUMOylation of RXR alpha in human liver-derived (HuH-7) cells. Lipopolysaccharide, interleukin-1b, and tumor necrosis factor alpha (TNF alpha) rapidly and substantially stimulated SUMOylation of RXR alpha. Two RXR alpha ligands, 9-cis retinoic acid (9cRA) and LG268, induced SUMOylation of RXR alpha, whereas both inflammation- and ligand-induced SUMOylation of RXR alpha require the K108 residue. Pretreatment with 1,9-pyrazoloanthrone (SP600125), a potent JNK inhibitor, abrogates TNF alpha- and 9cRA-stimulated RXR alpha SUMOylation. Pretreatment with SUMOylation inhibitors markedly augmented basal expression of several RXR alpha-regulated hepatobiliary genes. These results indicate that inflammatory signaling pathways rapidly induce SUMOylation of RXR alpha, adding to the repertoire of RXR alpha molecular species in the hepatocyte that respond to inflammation. SUMOylation, a newly described post-translational modification of RXR alpha, appears to contribute to the inflammation-induced reduction of RXR alpha-regulated gene expression in the liver that affects core hepatic functions, including hepatobiliary transport.