Summary The nuclear bile acid receptor FXR is critical for regulation of lipid and glucose metabolism. Here we report that FXR is a target of SIRT1, a deacetylase that mediates nutritional and hormonal modulation of hepatic metabolism. Lysine 217 of FXR is the major acetylation site targeted by p300 and SIRT1. Acetylation of FXR increases its stability but inhibits heterodimerization with RXRα, DNA binding, and transactivation activity. Down-regulation of hepatic SIRT1 increased FXR acetylation with deleterious metabolic outcomes. Surprisingly, in mouse models of metabolic disease, FXR interaction with SIRT1 and p300 was dramatically altered, FXR acetylation levels were elevated, and overexpression of SIRT1 or resveratrol treatment reduced acetylated FXR levels. Our data demonstrate that FXR acetylation is normally dynamically regulated by p300 and SIRT1 but is constitutively elevated in metabolic disease states. Small molecules that inhibit FXR acetylation by targeting SIRT1 or p300 may be promising therapeutic agents for metabolic disorders.

Small Heterodimer Partner (SHP) inhibits activities of numerous transcription factors involved in diverse biological pathways. As an important metabolic regulator, SHP plays a key role in maintaining cholesterol and bile acid homeostasis by inhibiting cholesterol conversion to bile acids. While SHP gene induction by increased bile acids is well established, whether SHP activity is also modulated remains unknown. Here, we report surprising findings that SHP is a rapidly degraded protein via the ubiquitin-proteasomal pathway and that bile acids or bile acid-induced intestinal fibroblast growth factor 19 (FGF19) increases stability of hepatic SHP by inhibiting proteasomal degradation in an extracellular signal-regulated kinase (ERK)-dependent manner. SHP was ubiquitinated at Lys122 and Lys123, and mutation of these sites altered its stability and repression activity. Tandem mass spectrometry revealed that upon bile acid treatment, SHP was phosphorylated at Ser26, within an ERK motif in SHP, and mutation of this site dramatically abolished SHP stability. Surprisingly, SHP stability was abnormally elevated in ob/ob mice and diet-induced obese mice. These results demonstrate an important role for regulation of SHP stability in bile acid signaling in normal conditions, and that abnormal stabilization of SHP may be associated with metabolic disorders, including obesity and diabetes.[Keywords: ERK; FGF19; SHP; bile acids; proteasome; protein stability] Supplemental material is available at http://www.genesdev.org.

Background: Small heterodimer partner (SHP) is a key mediator of bile acid signaling. Results: Bile acid signal-induced phosphorylation at Thr-55 by protein kinase C is important for SHP-mediated recruitment of chromatin modifiers and histone modifications. Conclusion: Thr-55 phosphorylation is critical for epigenomic regulation of liver metabolic genes. Significance: SHP Thr-55 phosphorylation may potentially provide a therapeutic target for bile acid-related diseases.

Angptl4 (Angiopoietin-like 4) is a circulating protein secreted by white and brown adipose tissues and the liver. Structurally, Angptl4 contains an N-terminal coiled-coil domain (CCD) connected to a C-terminal fibrinogen-like domain (FLD) via a cleavable linker, and both full-length Angptl4 and its individual domains circulate in the bloodstream. Angptl4 inhibits extracellular lipoprotein lipase (LPL) activity and stimulates the lipolysis of triacylglycerol stored by adipocytes in the white adipose tissue (WAT). The former activity is furnished by the CCD, but the Angptl4 domain responsible for stimulating adipocyte lipolysis is unknown. We show here that the purified FLD of Angptl4 is sufficient to stimulate lipolysis in mouse primary adipocytes and that increasing circulating FLD levels in mice through adenovirus-mediated overexpression (Ad-FLD) not only induces WAT lipolysis but also reduces diet-induced obesity without affecting LPL activity. Intriguingly, reduced adiposity in Ad-FLD mice was associated with increased oxygen consumption, fat utilization, and the expression of thermogenic genes ( and ) in subcutaneous WAT. Moreover, Ad-FLD mice exhibited increased glucose tolerance. Chronically enhancing WAT lipolysis could produce ectopic steatosis because of an overflow of lipids from the WAT to peripheral tissues; however, this did not occur when Ad-FLD mice were fed a high-fat diet. Rather, these mice had reductions in both circulating triacylglycerol levels and the mRNA levels of lipogenic genes in the liver and skeletal muscle. We conclude that separating the FLD from the CCD-mediated LPL-inhibitory activity of full-length Angptl4 reveals lipolytic and thermogenic properties with therapeutic relevance to obesity and diabetes.

Small Heterodimer Partner (SHP) inhibits numerous transcription factors that are involved in diverse biological processes, including lipid and glucose metabolism. In response to increased hepatic bile acids, SHP gene expression is induced and the SHP protein is stabilized. We now show that the activity of SHP is also increased by posttranslational methylation at Arg-57 by protein arginine methyltransferase 5 (PRMT5). Adenovirus-mediated hepatic depletion of PRMT5 decreased SHP methylation and reversed the suppression of metabolic genes by SHP. Mutation of Arg-57 decreased SHP interaction with its known cofactors, Brm, mSin3A, and histone deacetylase 1 (HDAC1), but not with G9a, and decreased their recruitment to SHP target genes in mice. Hepatic overexpression of SHP inhibited metabolic target genes, decreased bile acid and hepatic triglyceride levels, and increased glucose tolerance. In contrast, mutation of Arg-57 selectively reversed the inhibition of SHP target genes and metabolic outcomes. The importance of Arg-57 methylation for the repression activity of SHP provides a molecular basis for the observation that a natural mutation of Arg-57 in humans is associated with the metabolic syndrome. Targeting posttranslational modifications of SHP may be an effective therapeutic strategy by controlling selected groups of genes to treat SHP-related human diseases, such as metabolic syndrome, cancer, and infertility.Small Heterodimer Partner (SHP) (NR0B2) was discovered as a unique member of the nuclear receptor superfamily that lacks a DNA binding domain but contains a putative ligand binding domain (32). SHP forms nonfunctional heterodimers with DNA binding transcriptional factors, including nuclear receptors, and thereby acts as a transcriptional corepressor in diverse biological processes, including metabolism, cell proliferation, apoptosis, and sexual maturation (1,3,11,35,36,39). Well-studied hepatic functions of SHP are the inhibition of bile acid biosynthesis, fatty acid synthesis, and glucose production in response to bile acid signaling (1,3,4,12,19,22,37,38). We previously showed that SHP inhibits the expression of a key bile acid biosynthetic gene, the CYP7A1 (cholesterol 7␣ hydroxylase) gene, by coordinately recruiting chromatin-modifying repressive cofactors, mSin3A/histone deacetylase 1 (HDAC1), NCoR/ HDAC3, methyltransferase G9a, and the Swi/Snf-Brm remodeling complex, to the CYP7A1 gene promoter (9,16,25). GPS2, a subunit of the NCoR corepressor complex, was recently shown to act as a SHP cofactor and participates in differential regulation of bile acid biosynthetic genes, the CYP7A1 and CYP8B1 (sterol 12␣ hydroxylase) genes (31).Consistent with its important functions in metabolic pathways, naturally occurring heterozygous mutations in the SHP gene have been associated with human metabolic disorders (7,8,27). About 30% of these reported mutations occur at arginine residues, implying that functionally relevant posttranslational modification (PTM) at these sites may be important for SHP function. In r...