A Novel Principle for Partial Agonism of Liver X Receptor Ligands

Abstract

Partial, selective activation of nuclear receptors is a central issue in molecular endocrinology but only partly understood. Using LXRs as an example, we show here that purely agonistic ligands can be clearly and quantitatively differentiated from partial agonists by the cofactor interactions they induce. Although a pure agonist induces a conformation that is incompatible with the binding of repressors, partial agonists such as GW3965 induce a state where the interaction not only with coactivators, but also corepressors is clearly enhanced over the unliganded state. The activities of the natural ligand 22(R)-hydroxycholesterol and of a novel quinazolinone ligand, LN6500 can be further differentiated from GW3965 and T0901317 by their weaker induction of coactivator binding. Using biochemical and cell-based assays, we show that the natural ligand of LXR is a comparably weak partial agonist. As predicted, we find that a change in the coactivator to corepressor ratio in the cell will affect NCoR recruiting compounds more dramatically than NCoRdissociating compounds. Our data show how competitive binding of coactivators and corepressors can explain the tissue-specific behavior of partial agonists and open up new routes to a rational design of partial agonists for LXRs.Nuclear receptors are a family of transcriptional regulators whose activity can be modulated by their binding to small molecule compounds, such as hormones and metabolites. For many members of the family, this property has allowed their use as drug targets (1). In most cases, however, full activation or inhibition of the receptor is not desired. Instead, agonists are required that only partially activate the receptor. Partial agonists can display tissue-specific activation or repression of nuclear receptors, as has been shown for the estrogen receptor partial agonist raloxifen (2). However, little is known about the molecular mechanisms leading to partial versus full agonism.Another example for nuclear receptors, which, if they are to be used as drug targets, require partial agonists, are the liver X receptors ␣ (LXR␣, 2 NR1H3) and ␤ (LXR␤, NR1H2). They have been shown to play a central role in the transcriptional regulation of lipid and cholesterol homeostasis and inflammation (3-7). Activation of LXR-dependent transcription leads to increased expression of cholesterol transporters (8 -11) and has been shown to enhance the efflux of cholesterol from macrophages (10 -12), reducing the formation of atherosclerotic plaques (13)(14)(15). This observation has raised hopes that the manipulation of LXR activity would be of therapeutic value in the treatment of lipid disorders and atherosclerosis. However, activation of LXRs by agonistic compounds induces the expression of enzymes involved in the synthesis of fatty acids in liver cells (16 -19). As a consequence, agonists for LXRs cause liver steatosis and elevated serum triglyceride levels in mice (19,20). Thus, to develop LXR ligands as drugs for the treatment of atherosclerosis, partial, selective activator...

Regulation of Cholesterologenesis by the Oxysterol Receptor, LXRα

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Liver X receptor agonists ameliorate TNFα-induced insulin resistance in murine brown adipocytes by downregulating protein tyrosine phosphatase-1B gene expression

“…To activate NR1HR we tested the synthetic compounds T0901317 and GW3965 at the optimal doses of 3 and 15 μmol/l, respectively, calculated from dose-response experiments (Fig. 1b), as well as the natural ligand (22R)-hydroxycholesterol at 100 μmol/l, a dose 20 times higher than its EC50 [28], using (22S)-hydroxycholesterol (100 μmol/l) as a negative control (Fig. 1a).…”

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