Current Medicinal Chemistry - Immunology, Endocrine & Metabolic Agents - Volume 2, Issue 1, 2002

Disease phenotype and manifestation can largely be attributed to aberrant patterns of gene expression. This is becoming increasingly evident as genomics and bioinformatics tools provide us with new information on how gene expression patterns change in the pathophysiological state. Transcriptional control mechanisms influence these genetic networks and some success has been attained by manipulating the action of transcription factors toward a more favorable therapeutic profile. From endocrine-mimics that modulate nuclear receptor driven gene expression to indirectly altering homeostatic regulation of lipoprotein metabolism, transcription factor pharmacotherapy is a current reality and will increasingly contribute to the treatment of human disease.

Steroidal compounds such as prednisone and dexamethasone are full agonists of the glucocorticoid receptor (GR). They are enormously valuable in the treatment of a wide variety of inflammatory diseases. However, the value of these glucocorticoids is greatly limited by their side effects. Our increasing understanding of the molecular mechanisms involved in the activation of GR has provided new opportunities to discover and develop novel compounds that maintain the efficacy of currently used steroids but avoid some of the dose limiting side effects.

This article will review both the binding and the functional cellular assays that can be used to characterize nuclear receptor ligands in vitro. The discussion will be broken into two sections, concentrating on cell-free and cell-based systems. Each section will provide an overview of available assays moving from generic screening assays to more specific technologies that can be used to discriminate the pharmacology of receptor / ligand interactions.

A novel class of antidiabetic agents, the thiazolidinediones, was developed in the 70s and 80s by screening newly synthesized compounds for their ability to lower blood glucose in diabetic rodents. Three molecules from this class, troglitazone, rosiglitazone and pioglitazone, were ultimately approved for the treatment of patients with type II diabetes. Although these compounds were developed without an understanding of their molecular mechanism of action, by the early 90s evidence began to accumulate linking the thiazolidinediones the nuclear receptor PPARγ (NR1C3). It was ultimately demonstrated that these molecules were high affinity ligands of PPARγ and that they increased the transcriptional activity of the receptor. Although many questions remain, multiple lines of evidence now indicate that the antidiabetic activities of the thiazolidinediones are mediated by their direct interaction with the receptor and the subsequent modulation of PPARγ target gene expression. The knowledge that PPARγ ligands can improve insulin resistance in diabetics, coupled with the availability of rapid assays for the identification and characterization of nuclear receptor ligands, has led to a virtual explosion in the number of new PPARγ ligands that are under development as antidiabetic agents. In this article we will briefly review the biology of PPARγ, and then provide an update of new synthetic PPARγ ligands that are under investigation or in development as antidiabetic drugs.

The Liver X receptors (LXRs), members of the nuclear receptor superfamily, play an important role in controlling lipid homeostasis by activating several genes involved in reverse cholesterol transport. These include members of the ATP binding cassette (ABC) superfamily of transporter proteins ABCA1 and ABCG1, surface constituents of plasma lipoproteins like apoE, and cholesterol ester transport protein (CETP). LXRs also play an important role in fatty acid metabolism by activating the sterol regulatory element-binding protein 1c gene (SREBP1c). hLXRα itself is an autoinducible gene, and ”auto-induction“ in response to LXR ligands is observed in multiple human cell-types including macrophages. Based on their ability to induce reverse cholesterol transport LXRs appear to be useful and novel targets for the treatment of atherosclerosis, one of the most fatal diseases in the western world. In this article, we review the biological functions of LXRs and discuss the possibility of identifying LXR ligands as drugs for the treatment of atherosclerosis.

Cholesterol and bile acid metabolism is tightly controlled by nuclear receptors. The liver X receptor (LXR), an oxysterol-activated nuclear receptor, limits cholesterol accumulation in the body. LXR achieves this effect : 1) by enhancing reverse cholesterol transport to the liver : 2) by the stimulation of cholesterol excretion through its conversion to bile acids : and 3) by the inhibition of intestinal cholesterol absorption. Whereas LXR is a master controller of cholesterol metabolism, the farnesol X receptor (FXR), a bile acid-activated receptor, coordinates bile acid homeostasis. FXR stimulates the re-uptake of bile acids from the intestine via a process termed entero-hepatic circulation. Activated FXR also protects the liver against the toxic effects of excessive bile acid concentrations, through an indirect mechanism. In fact, FXR induces the small heterodimer partner (SHP), an atypical nuclear receptor, that attenuates further bile acid synthesis and bile acid import into the liver by inhibiting the action of nuclear receptors, such as the liver receptor homolog-1, the hepatic nuclear factor 4α, and the retinoid X receptor RXR). Finally, steroid and xenobiotic receptor / pregnane X receptor (SXR / PXR) exerts a hepatoprotective function by favoring the catabolism of toxic compounds such as secondary bile acids and xenobiotics. The complexity of these nuclear receptor-controlled regulatory circuits is only being recognized and further study is definitively required to understand the cross-regulation between these nuclear receptors and other transcription factors.