Studies on the atypical orphan receptor SHP

Nuclear receptors (NRs) comprise the largest family of transcription factors in eukaryotes and play critical roles in nearly every aspect of vertebrate development and adult physiology. In general, they function as transcriptional regulators that when bound to DNA either activate or suppress target gene expression. The activity of approximately half of the human NRs appears to be regulated by ligands, whereas the remaining, so called orphan receptors, still lack identified ligands. Intense research during the past years has revealed that NRs function in concert with multiple transcriptional cofactors, and a large number of candidate proteins have been isolated that influence the transcriptional activity of NRs either positively (coactivators) or negatively (corepressors). Most coactivators interact in a liganddependent manner with the ligand binding domain of NRs, and in agreement with structural conservation of the coactivator interaction surface on the NRs, most of these coactivators contain short conserved LXXLL interaction motifs which are responsible for the interaction with the receptor.

When searching for novel cofactors for the ligand binding domain of NRs, an atypical orphan receptor was isolated consisting of only a putative ligand-binding domain. Based on its small size and its ability to interact with a variety of non-steroid NRs it was termed SHP (short heterodimer partner or NROB2). During the past years, much research has concentrated on elucidating the basis for the inhibitory effect of SHP on nuclear receptor signalling and recent published work has established SHP as a negative regulator of cholesterol 7a-hydroxylase (CYP7A1) gene expression, which encodes the enzyme responsible for the rate- limiting step in conversion of cholesterol to bile acids in the liver.

In addition to the interaction with non-steroid receptors, we have discovered that SHP interacts with and inhibits the ligand induced transcriptional activity of estrogen receptors (ERs). Delineation of the NR interaction domain revealed that the central part of SHP was involved, which suggested the existence of a different interaction mechanism for SHP compared with conventional receptor dimerization. Further investigation revealed that the interaction occurred via two LXXLL motifs within SHP that bound directly to the ligand-induced activation domain on ERs, an interaction mechanism that seems to be generally applicable for SHP interaction with NRs. This interaction results in a competition for binding with coactivators, which we believe is the first step in SHP inhibition, although the competition alone was not sufficient to mediate the inhibitory effect of SHP. To further elucidate the repression mechanism a yeast two-hybrid screening was performed aiming to identify putative corepressors for SHP. A novel SHP interacting protein, SCIP-1 was found, that appears to be specific for SHP ligand-binding domain. We suggest that SHP repression, in a second step, involves targeting of SCIP-1 to NRs where SCIP-1 exerts its corepressing effect by antagonizing the formation of a transactivation complex. We also describe a third aspect of SHP inhibition, namely the possibility for SHP to tether the glucocorticoid receptor to specific nuclear compartments, which might represent an additional inhibitory mechanism. The co-expression in certain tissues of SHP and ER or GR, respectively, together with the findings presented in this thesis, argues for a physiological role for SHP in both estrogen and glucocorticoid signalling in addition to the observed role of SHP in bile acid regulation. An exciting observation is the ability of SHP to antagonize PGC-1 coactivation of GR ligand induced transcriptional activity, which might have biological relevance in modulating glucocorticoid action in the liver.

In summary, the potential of SHP to silence or redirect NR signalling provides a novel and unique mechanism for an orphan receptor and places SHP structurally and functionally between NRs and their associated transcriptional cofactors.

List of scientific papers

I. Johansson L, Thomsen JS, Damdimopoulos AE, Spyrou G, Gustafsson JA, Treuter E (1999). "The orphan nuclear receptor SHP inhibits agonist-dependent transcriptional activity of estrogen receptors ERalpha and ERbeta. " J Biol Chem 274(1): 345-53
https://pubmed.ncbi.nlm.nih.gov/9867849

II. Johansson L, Bavner A, Thomsen JS, Farnegardh M, Gustafsson JA, Treuter E (2000). "The orphan nuclear receptor SHP utilizes conserved LXXLL-related motifs for interactions with ligand-activated estrogen receptors. " Mol Cell Biol 20(4): 1124-33
https://pubmed.ncbi.nlm.nih.gov/10648597

III. Zhang H, Thomsen JS, Johansson L, Gustafsson JA, Treuter E (2000). "DAX-1 functions as an LXXLL-containing corepressor for activated estrogen receptors. " J Biol Chem 275(51): 39855-9
https://pubmed.ncbi.nlm.nih.gov/11053406

IV. Bavner A, Johansson L, Toresson G, Gustafsson JA, Treuter E (2001). "A transcriptional inhibitor targeted by the atypical orphan nuclear receptor SHP." EMBO Reports (Submitted)

V. Johansson L, Figuera R, Gustafsson JA, Treuter E (2001). "Glucocorticoid receptor as a target for the atypical orphan receptor and corepressor SHP." (Manuscript)