Uric acid (UA) is uniquely maintained at high concentration in higher primates. Contrastingly, hyperuricemia (HU) is a risk factor for gout and associates with diverse diseases, including NAFLD, while its mechanism remains unclear. In this study, we aim to explain UA effect based on the hypothesis that UA-binding proteins function as UA sensors and regulate various physiological processes.
To find UA sensors, pull-down assay using UA-binding beads and cell homogenate was carried out and 6 proteins were found as UA binding proteins. Specific binding of UA to these proteins was confirmed by binding assay to in vitro synthesized proteins, including NDFIP1. Meanwhile, we performed a proteomic analysis of human primary hepatocytes treated with and without UA, followed by an enrichment analysis which showed NAFLD is one of the remarkable diseases. When NDFIP1 was knocked down in HepG2 cells, an increase of fat accumulation by UA exposure was attenuated, suggesting NDFIP1 plays a role in the HU-induced NAFLD. Additionally, the proteomic analysis also showed UA exposure stabilized PTEN, whose ubiquitination is promoted by NDFIP1. Furthermore, a decrease in PTEN ubiquitination and an increase in the protein level were observed after UA exposure. Since the change of PTEN expression is reported to promote insulin resistance that induces NAFLD, binding of UA to NDFIP1 is a possible mechanism of HU-induced NAFLD.
In conclusion, NDFIP1 was found as UA binding protein and was suggested as a UA sensor that regulates HU-induced NAFLD by modulating PTEN.