Molecular Biology of Hormone and Drug Receptors in Health and Disease
Summary: Robert Lefkowitz's research program is concerned with the molecular properties and regulatory mechanisms that control the function of plasma membrane receptors for hormones and drugs under normal and pathological circumstances.
Our research program is concerned with the molecular properties and regulatory mechanisms that control the function of plasma membrane receptors for hormones and drugs under normal and pathological circumstances. Receptors are the cellular macromolecules with which biologically active substances (e.g., hormones, drugs, neurotransmitters, growth factors, viruses, lipoproteins) initially interact.
As models for the study of receptors, we have used the receptors for epinephrine (adrenaline) and related compounds, which are generally termed adrenergic receptors, as well as the receptor for angiotensin 2, a powerful hormone that constricts blood vessels. Such receptors are found throughout the brain, heart, smooth muscle cells, and most other cells of the human organism. There are at least nine distinct subtypes of adrenergic receptors (α1, α2, β1, β2, etc.), which interact not only with endogenous epinephrine and norepinephrine but also with a variety of clinically important drugs.
Some years ago we isolated the genes for all of the known adrenergic receptors, as well as a number of closely related receptors, and determined their complete amino acid sequences. The structure of the receptors consists of a polypeptide chain that weaves back and forth across the plasma membrane seven times ("seven-transmembrane-spanning receptors"). Remarkably, the structures of these receptors are similar to each other, to the visual light receptor rhodopsin, to the "smell receptors" in the nose, the taste receptors, opiate receptors, and hundreds of others. There appear to be about 1,000 such receptors encoded in the human genome that regulate virtually every known physiological process. Moreover, drugs that target these many receptors, either directly or indirectly, account for a substantial fraction of all prescription drug sales worldwide.
One important insight to come from our studies of receptors is that their properties are not fixed. Rather, the properties of the receptors are influenced by the hormones and drugs with which they interact, as well as by a variety of disease states. There are important clinical implications of the ever-changing nature of the receptors. For example, this provides a basis for beginning to understand the phenomenon of drug tolerance, or desensitization, the diminishing effect of drugs over time. This phenomenon markedly compromises the therapeutic efficacy of epinephrine, opiates like morphine, and many other drugs. When drugs such as opiates combine with their receptors, they not only stimulate them but also produce changes that impair their function, thus leading to desensitization. As a result, cells are less able to respond to the drugs or hormones.
Our research has helped us to understand, in molecular terms, how the receptors become functionally desensitized. In this connection we have discovered two new families of proteins that function to desensitize the receptors. The first is a novel family of enzymes, the G protein–coupled receptor kinases (GRKs), that modify the structure of the receptors by introducing a phosphate group when the receptors are stimulated. The second is a group of proteins, the arrestins, that bind to the phosphorylated receptors and prevent them from acting. Both proteins are widely distributed, and their actions appear to be universal for the family of seven transmembrane receptors.
Recently we discovered an entirely unexpected function of the β-arrestins and GRKs. Originally discovered and named for their ability to "desensitize" some functions of the receptors, the β-arrestins are also able to serve as signaling proteins in their own right. At the same time that they shut off G protein activation by the receptor, they also initiate a second wave of signaling to other pathways. We are just beginning to understand the consequences of this signaling.
Understanding the actions of β-arrestins and GRKs may lead to the development of new drugs and new treatments for human diseases. For example, two of the most frequently used drugs for the treatment of heart and circulatory diseases are β blockers (β-adrenergic receptor blockers) and ARBs (angiotensin receptor blockers). Both types of drugs work by binding to a seven-transmembrane-spanning receptor (the β-adrenergic receptor for epinephrine or the angiotensin receptor, respectively). In so doing they prevent the potentially deleterious effects of overstimulation of these receptors, which can lead to hypertension, angina, or heart failure. However, the conventional blockers prevent all the actions of epinephrine or angiotensin, including some that may be beneficial. Until recently all the actions of these receptors were thought to be carried out by a single mechanism, the activation of a molecular switch called a G protein, but we now know that some of their actions are carried out by activation of the β-arrestins. We have found that certain of these β-arrestin-mediated actions, such as promoting cell survival and opposing cell death, are potentially beneficial in the setting of cardiovascular and other diseases.
Recently, we have found that it is possible to design drugs that block the potentially harmful effects of epinephrine or angiotensin mediated through G protein stimulation (like "conventional" blockers) while at the same time stimulating potentially beneficial pathways mediated through β-arrestins. We are testing the effects of such novel compounds in animal models of, for example, congestive heart failure, in the hope that these compounds will provide a new approach to the treatment of such illnesses.
A major focus of our current work is to establish the molecular basis for the ability of the receptors to independently stimulate signaling through either G proteins or β-arrestins. This work involves the application of a variety of biophysical techniques to the study of the conformations of the receptors, the β-arrestins, and receptor β-arrestin complexes involved in these activities. It is hoped that eventually atomic level information can be obtained from crystal structures of the different receptor and β-arrestin conformations. Such work may aid in the design of evermore selective drugs with fewer and fewer side effects.
This work is supported in part by grants from the National Institutes of Health.
As of April 09, 2012