The Orphan Nuclear Receptor SHP Inhibits Hepatocyte Nuclear Factor 4 and Retinoid X Receptor Transactivation: Two Mechanisms for Repression

Abstract

The orphan nuclear hormone receptor SHP interacts with a number of other nuclear hormone receptors and inhibits their transcriptional activity. Several mechanisms have been suggested to account for this inhibition. Here we show that SHP inhibits transactivation by the orphan receptor hepatocyte nuclear factor 4 (HNF-4) and the retinoid X receptor (RXR) by at least two mechanisms. SHP interacts with the same HNF-4 surface recognized by transcriptional coactivators and competes with them for binding in vivo. The minimal SHP sequences previously found to be required for interaction with other receptors are sufficient for interaction with HNF-4, although deletion results indicate that additional C-terminal sequences are necessary for full binding and coactivator competition. These additional sequences include those associated with direct transcriptional repressor activity of SHP. SHP also competes with coactivators for binding to ligand-activated RXR, and based on the ligand-dependent interaction with other nuclear receptors, it is likely that coactivator competition is a general feature of SHP-mediated repression. The minimal receptor interaction domain of SHP is sufficient for full interaction with RXR, as previously described. This domain is also sufficient for full coactivator competition. Functionally, however, full inhibition of RXR transactivation requires the presence of the C-terminal repressor domain, with only weak inhibition associated with this receptor interaction domain. Overall, these results suggest that SHP represses nuclear hormone receptor-mediated transactivation via two separate steps: first by competition with coactivators and then by direct effects of its transcriptional repressor function.Nuclear hormone receptors are transcription factors whose activities are regulated by direct binding of small lipophilic molecules such as steroids, thyroid hormone, retinoids, and vitamin D 3 (24). These receptors have been studied extensively due to their important roles in the control of development and cellular homeostasis. The nuclear hormone receptor superfamily also includes numerous orphan receptors, which do not have identified ligands (10, 23). The structural similarities shared by superfamily members reside in two distinct functional domains. The first and most highly conserved is the DNA binding domain, also called the C domain, which targets receptors to specific DNA sequences known as hormone response elements. The second is the ligand binding domain (LBD), which is also called the E domain. It is located in the C-terminal region and is involved in several functions in addition to ligand binding, including dimerization and ligand-dependent transcriptional activation.Crystallographic studies (e.g., references 30, 43, and 46) have revealed that ligand binding provokes a conformational change in the receptors that allows them to bind a diverse group of proteins termed coactivators (reviewed in references 14, 37, and 47). These coactivators bind to a common surface of the receptors formed by sever...

Loss of orphan receptor small heterodimer partner sensitizes mice to liver injury from obstructive cholestasis

“…The primary basis for this effect is the induction of SHP by FXR in the presence of elevated bile acid levels, inhibiting transactivation by the nuclear receptors liver receptor homolog-1 (LRH-1) and possibly hepatocyte nuclear factor 4 alpha (HNF4␣) and repressing expression of Cyp7A1, the rate-limiting enzyme of bile acid biosynthesis, as well as a number of other bile acid biosynthetic enzymes. 7,8,[23][24][25] This function of SHP has been strongly supported by results with Shp Ϫ/Ϫ mice, which reveal defective repression in response to synthetic FXR agonists 10,11 or very short term dietary bile acid treatment. 26 However, such studies have also demonstrated the importance of SHP-independent pathways that can efficiently repress Cyp7A1 and other negative targets in response to longer term treatments with dietary bile acids.…”

“…20 It is a transcriptional repressor and exerts its regulatory functions through protein-protein interactions with other nuclear hormone receptors and possibly other transcription factors, which can inhibit or even reverse their transactivation. [20][21][22][23][24][25] Regulation of bile acid metabolism is the most prominent of a number of physiological roles proposed for SHP. The primary basis for this effect is the induction of SHP by FXR in the presence of elevated bile acid levels, inhibiting transactivation by the nuclear receptors liver receptor homolog-1 (LRH-1) and possibly hepatocyte nuclear factor 4 alpha (HNF4␣) and repressing expression of Cyp7A1, the rate-limiting enzyme of bile acid biosynthesis, as well as a number of other bile acid biosynthetic enzymes.…”

Molecular characterization of the role of orphan receptor small heterodimer partner in development of fatty liver

“…In addition, we did not observe changes of HNF4␣ mRNA in SHP Ϫ/Ϫ and OB/SHP Ϫ/Ϫ livers as compared with their littermates, although SHP was reported to be a negative regulator for HNF4␣. 37 These observations suggest that HNF4␣ is unlikely the intermediate factor by which SHP regulates MTP transcription. We found that the orphan receptor LRH-1 can also bind to the MTP promoter and increase MTP expression in hepatocytes, and that this transactivation is repressed by SHP.…”

The Bile Acid Sensor Farnesoid X Receptor Is a Modulator of Liver Immunity in a Rodent Model of Acute Hepatitis

“…SHP activation by FXR ligands in hepatocytes leads to the repression of cholesterol 7␣-hydroxylase expression, the rate-limiting enzyme in the neutral pathway that leads to bile acid production from cholesterol (33). SHP is also a known target for other nuclear receptors, including the estrogen receptors and PPAR␥ (65,66), and is essential in the regulation of the expression/activity of hepatocyte NF4 and retinoid X receptor (66). A body of evidence suggest that SHP represents an important mediator of FXR activity and acts as a corepressor of FXR target genes in different physiological contexts (33,36,46,47).…”