Nuclear Receptors: A Family Profile
One pathway used by multicellular organisms to process extracellular signals into a transcriptional response within a target cell is via a nuclear receptor. These receptors comprise a superfamily of transcription factors that become active upon binding to steroid hormones, vitamins A and D, and other bioactive lipids (e.g., oxysterols, fatty acids, bile acids). Nuclear receptors are also among the most successful targets for therapeutic drugs.
Because of their high pharmacological potential, we are exploring the use of nuclear receptor expression profiling in translational research. For example, in individuals with diseases such as lung cancer and metabolic syndrome, transcriptional profiling of nuclear receptors may be employed as a diagnostic tool to provide patient-tailored therapeutic information. (Information on nuclear receptor profiling and its potential for studying physiology and disease can be found online at www.nursa.org.)
Of Worms and Men
In addition to their prominent role in vertebrates, nuclear receptors are also found in invertebrate animals. However, the ligands for many of these receptors and their biological roles have, thus far, remained a mystery. We discovered the hormonal ligands for DAF-12, a nuclear receptor that exists in the free-living nematode Caenorhabditis elegans. Like all nematode worms, C. elegans go through several larval stages before they reach reproductive maturity. In unfavorable conditions, such as overcrowding, low food supply, or changes in temperature, the C. elegans worm stops its normal larval development and goes into a long-lived period of dormancy, called "dauer." When conditions are favorable, a signaling cascade in the worm causes it to exit dauer and reinitiate normal development into a reproductive adult. This entire process is regulated by the production of the newly discovered hormones, called dafachronic acids, which bind and activate the DAF-12 nuclear receptor. Remarkably, the components of this endocrine pathway, including the hormones, the enzyme that makes them, and their receptor are conserved from worms to humans.
The study of DAF-12 in C. elegans, a free-living worm, has led to the discovery of a potential new therapeutic target to treat a myriad of debilitating human, animal, and plant diseases caused by parasitic nematodes. We have found that the infectious stage of parasitic nematodes, which is similar to the dauer stage in C. elegans, is dependent on the presence of DAF-12. Only when the parasites are in the favorable environment of their host do they reactivate their developmental program and become reproductive adults, a process that is governed by hormonal activation of the parasites' DAF-12 receptor. Importantly, by pharmacologically activating DAF-12 in parasites, we have been able to interrupt the life cycle, prevent infection, and even kill parasites. Our current work is focused on developing agonists and antagonists that target the DAF-12 signaling pathway in parasites as novel anthelmintic drugs.
FGF19: Hitting the Digestion Reset Button
Within the nuclear receptor superfamily there exist several receptors that are regulated by ligands derived from dietary lipids. Our discovery of one of these receptors, called FXR (the bile acid receptor), has revealed the existence of an intricate signaling network that is regulated by feeding and governs bile acid, carbohydrate, and lipid metabolism.
After a meal, bile acids are released from the gallbladder into the proximal intestine to act as detergents to help digest food. Once digestion is finished, the bile acids are taken up in the distal intestine and recycled back to the liver. During this process, the uptake of bile acids is detected in the intestine by their receptor, FXR, which functions as a postprandial sensor and initiates an endocrine-feedback signaling pathway that governs postprandial metabolism. Activation of FXR leads to the synthesis of several regulatory proteins, including an endocrine hormone called fibroblast growth factor 19 (FGF19, which is referred to as FGF15 in rodents). Immediately after its synthesis in the intestine, FGF19 circulates to the liver, where it causes repression of bile acid synthesis through a signal transduction pathway that requires an orphan nuclear receptor called SHP (small heterodimer partner). FGF19 also has insulin-like effects in the liver that promote protein synthesis and the storage of glucose as glycogen. This coordinated postprandial mechanism helps to reset the entire enterohepatic metabolic network for the next meal and thereby maintain normal homeostasis of glucose, cholesterol, triglycerides, and bile acids. Disturbances in the equilibrium of this network can cause diseases such as fatty liver, cholesterol gallstones, cholestasis, and inflammatory bowel disease. For this reason, new drugs that target FXR and FGF19 are being considered as agents for treating or preventing these diseases.
FGF21: Burning the Midnight Oil
In contrast to the fed state, in which the body stores nutrients as fat, during fasting and starvation, the body switches to mobilizing fat as a source of energy. This metabolic pathway is governed by PPARalpha, a nuclear receptor that is activated in the liver during fasting and is the molecular target of hyperlipidemic drugs known as fibrates. We found that a key mediator of PPARalpha action is another endocrine FGF called FGF21. FGF21 is a direct target gene of PPARalpha, and in mice FGF21 promotes several of the body's adaptive responses to starvation, including the mobilization of stored fat and its conversion into ketone bodies, suppression of growth and female reproduction, and alterations in sleep/wake cycles. Work from several laboratories has shown that pharmacological administration of FGF21 to obese animals and humans results in weight loss; lowering of serum insulin, triglycerides, and cholesterol; and (as we've shown in mice) a marked increase in life span. Our work has found that in addition to having direct effects in adipose tissue (e.g., to increase glucose uptake and mobilize fatty acids), the effects of FGF21 in the brain are required for causing weight loss. Remarkably, in both rodents and primates FGF21 also governs nutrient preference. Pharmacological administration of FGF21 to mice suppresses the desire to eat sweets and consume alcohol. Together, these findings suggest that FGF21 might be used therapeutically to treat many of the clinical features associated with metabolic syndrome. However, FGF21 also has unwanted pharmacological effects, such as bone loss and elevated glucocorticoids. Understanding the basis for these different effects and their sites of action is an active area of study. At present, it appears that a majority of FGF21's actions are mediated through effects on the brain, thus defining a novel neuroendocrine circuit that governs nutrient metabolism.
As of April 8, 2016