Identification of a Liver-Specific Uridine Phosphorylase that Is Regulated by Multiple Lipid-Sensing Nuclear Receptors

In this work, we report the characterization of a novel liver-specific gene (L-UrdPase), whose expression is regulated by a number of hepatic nuclear receptors (including liver X receptors, peroxisome proliferator-activated receptor alpha, farnesoid X receptor, and hepatic nuclear factor-4alpha), which have been shown to be involved in lipid metabolism. L-UrdPase encodes a previously uncharacterized protein with similarity to an intestine-specific uridine phosphorylase. Enzymatic assays confirmed that L-UrdPase has uridine phosphorylase activity. However, L-UrdPase has a highly restricted, nonoverlapping pattern of expression with its intestinal counterpart and is regulated in a distinct manner by several different nuclear receptors. The identification of the liver uridine phosphorylase and its characterization as a target of lipid-sensing nuclear receptors implies the existence of a previously unknown nuclear receptor signaling pathway that links lipid and uridine metabolism.