In-depth proteomic characterization of endogenous nuclear receptors in mouse liver

Nuclear receptors (NRs) are a superfamily of transcription factors that, upon binding to ligands, bind specific DNA sequences and regulate a transcriptional program governing cell proliferation, differentiation, and metabolism. In the liver, by sensing lipid-soluble hormones and dietary lipids and governing the expression of key liver metabolic genes, NR proteins direct a large array of key hepatic functions that include lipid and glucose metabolism, bile secretion, and bile acid homeostasis. Although much has been learned about the physiology of NRs, little is known about their protein expression and DNA binding activity in the liver because of their low abundance and the lack of high-throughput methods for detection at the protein level. Here we report a method for profiling the DNA binding activity of the NR transcription factor superfamily in mouse liver. We use DNA constructs of hormone response elements (HREs) as affinity reagents to enrich NR proteins from nuclear extracts of mouse liver and then identify them using mass spectrometry. We evaluated 20 DNA constructs containing various combinations of HREs for their ability to enrich endogenous NR proteins and found that two different HREs are sufficient to achieve isolation and identification of nearly all endogenous NR proteins from one mouse liver. We have detected proteins for 35 members of the NR family out of 41 that are expressed in mouse liver at mRNA level. Thus, this method allows coverage of most of the whole NR proteome and establishes a practical assay for the investigation of NR actions in mouse liver. We anticipate that this method will find widespread use in future investigations of NR actions in liver biology and pathology. Furthermore, this workflow is a useful tool for NR biologists interested in measuring NR expression, DNA binding, post-translational modifications, cellular localization, and other functional aspects of NRs in organs under normal physiological and pathological conditions, as well as during pharmacological intervention.