Endocrine, Metabolic & Immune Disorders-Drug Targets (Formerly Current Drug Targets - Immune, Endocrine & Metabolic Disorders) - Volume 11, Issue 2, 2011

The phenomenon of G protein-coupled receptor (GPCR) ligand ‘bias’, the ability of an agonist to activate (or inhibit) only a part of its receptor's downstream signals, has been recognized for over 15 years [1]. ‘Bias’, or functional selectivity, can take many forms; from ‘reversal of potency’, where two ligands activate different signaling pathways with opposite potency, to outright ‘reversal of efficacy’ where a ligand that antagonizes one pathway activates another. Examples of functional selectivity extend across the full spectrum of GPCR signaling. Most of the initial work in the area focused on the ability of certain drugs to bias receptor coupling to different heterotrimeric G protein pools, e.g. favoring cAMP production or phosphatidylinositol hydrolysis, e.g. see [2]. This early work established two key points in GPCR pharmacology; that GPCRs are capable of assuming multiple ‘active’ states, and that chemically distinct ligands can activate receptors in qualitatively different ways. More recent work has explored biased ligands that dissociate the two major consequences of GPCR activation; heterotrimeric G protein activation and arrestin-mediated desensitization. With the recognition that arrestin binding not only uncouples the receptor and G protein, but also initiates signaling from receptor-arrestin ‘signalsomes’ [3], this work has come to define a new type of bias; ligands that select between two major, and mutually exclusive, GPCR signaling states. Both G protein pathwayselective, i.e. non-desensitizing, and arrestin pathway-selective, i.e. G protein-independent, biased agonists have now been described for a number of GPCRs. Such biased agonism is an emerging concept in drug design, and carries with it both the potential for new, more efficacious drugs, and the risk that incomplete characterization of drug efficacy may lead to unintended side effects. The collection of articles that follow are intended to provide a brief introduction to biased agonism and to illustrate some of the current and potential roles that arrestin pathway-selective biased agonists may play in medicine. Besides providing a detailed topic review, each paper highlights a different facet of biased signaling. The first paper by B. T. Andresen introduces the concept of biased agonism and defines some of the newer terms finding their way into the pharmaceutical lexicon. In telling the story of carvedilol (Coreg®), a β-adrenergic receptor blocker with proven survival benefit in congestive heart failure that was subsequently shown to possess arrestin pathway-selective agonism [4,5], Dr. Andresen makes the point that biased ligands have actually been in the clinic for a long time. We just did not recognize it. Traditional high throughput methods that rely on single functional readouts to categorize ligand efficacy are not designed to capture ligand bias, yet bias may be more the norm than the exception in GPCR pharmacology. Whether carvedilol, which has proven safe and effective over decades of use, owes some of its some of its clinical efficacy to arrestin pathway activation is unknown, but it certainly raises the question of whether future β blockers should be designed to incorporate, or avoid, activation of arrestin signaling. The article by D. A. Zidar discussing natural biased agonism of chemokine receptors [6,7] makes the somewhat humbling point that the phenomenon of biased agonism is not merely a product of synthetic pharmacology. It turns out that Nature has been exploiting ligand bias to control leukocyte trafficking and immune responsiveness far longer than pharmaceutical scientists have been working to discover clinically useful biased drugs. This has obvious implications for chemokine signaling where there are overlapping affinities and more than twice the number of endogenous chemokines than chemokine receptors. Does this simply reflect a need for redundancy in a critical physiological system, or is ligand bias a common solution to the need for fine regulation in complex signaling systems? How many other endogenous substances are biased agonists, and how might pharmaceuticals capitalize on the fact that biological systems may already be poised to respond selectively? D. G. Tilley raises other important points. The angiotensin AT1 receptor is the most intensively studied GPCR with respect to arrestin-dependent signaling, and recent unbiased phospho-proteomic surveys of the response to [Sar1-Ile4-Ile8]-Ang II, an arrestin pathway-selective AT1 receptor agonist [8,9], have revealed startling complexity, to the point that the G proteinindependent signaling network may ultimately prove to be as extensive as that regulated by classical G protein signaling. Dr. Tilley discusses biased AT1 receptor agonism in the cardiovascular system and evidence that arrestin pathway-selectivity may be advantageous in the treatment of cardiac disease. He also discusses a surprising ‘off-target’ effect of the peroxisome proliferator-activated receptor gamma agonist, troglitazone, an oral hypoglycemic thiazolidinedione that was removed from the market over 10 years ago, that turns out to be an arrestin-selective AT1 receptor biased agonist [10]. Ligand bias is not only commonplace in natural and synthetic pharmacology; it also turns up in the most unexpected places! Finally, B. N. Bohinc and D. Gesty-Palmer discuss biased agonism of the type 1 parathyroid hormone (PTH) receptor, and how G protein-dependent and arrestin-dependent signaling pathways contribute to the regulation of bone turnover. They describe recent work using an arrestin pathway-selective PTH1 receptor agonist that appears to dissociate the ordinarily coupled effects of PTH on bone-forming osteoblasts and bone-resorbing osteoclasts in a manner that allows for selective acceleration of bone formation [11]. Results like these offer early proof of principal that biased agonists can be used to obtain physiological responses in vivo that cannot be achieved using conventional agonist or antagonist ligands. As the new decade begins, we seem poised on the brink of a brave new world of GPCR targeted pharmaceuticals that will exploit the natural phenomenon of ligand bias to tailor drug efficacy in a manner that enhances therapeutically beneficial signals while blocking deleterious ones [12]. And the future may be nearer than we think. Biased agonism is attracting attention throughout the pharmaceutical industry, and arrestin pathway-selective compounds are beginning to enter clinical trials, with a biased AT1 receptor agonist being studied in acute heart failure and a bradykinin receptor biased agonist under development for small cell lung cancer. Meanwhile ever more sophisticated screening approaches designed specifically to identify biased agonists are coming on-line. Consequently, there is a high probability that by the time the decade is out, biased drugs will be finding their way into the clinic, this time not by accident, but by design.....

Biased agonism is one of the fastest growing topics in G protein-coupled receptor pharmacology; moreover, biased agonists are used in the clinic today: carvedilol (Coreg®) is a biased agonist of beta-adrenergic receptors. However, there is a general lack of understanding of biased agonism when compared to traditional pharmacological terminology. Therefore, this review is designed to provide a basic introduction to classical pharmacology as well as G protein-coupled receptor signal transduction in order to clearly explain biased agonism for the non-scientist clinician and pharmacist. Special emphasis is placed on biased agonists of the beta-adrenergic receptors, as these drugs are highly prescribed, and a hypothetical scenario based on current clinical practices and proposed mechanisms for treating disease is discussed in order to demonstrate the need for a more thorough understanding of biased agonism in clinical settings. Since biased agonism provides a novel mechanism for treating disease, greater emphasis is being placed to develop biased agonists; therefore, it is important for biased agonism to be understood in equal measure of traditional pharmacological concepts. This review, along with many others, can be used to teach the basic concepts of biased agonism, and this review also serves to introduce the subsequent reviews that examine, in more depth, the relevance of biased agonism towards the angiotensin type 1 receptor, parathyroid hormone receptor, and natural biased ligands towards chemokine receptors.

Angiotensin II type 1 receptor antagonists (AT1R blockers, or ARBs) are used commonly in the treatment of cardiovascular disorders such as heart failure and hypertension. Their clinical success arises from their ability to prevent deleterious Gαq protein activation downstream of AT1R, which leads to a decrease in morbidity and mortality. Recent studies have identified AT1R ligands that concurrently inhibit Gαq protein-dependent signaling and activate Gαq proteinindependent/ β-arrestin-dependent signaling downstream of AT1R, events that may actually improve cardiovascular performance more than conventional ARBs. The ability of such ligands to induce intracellular signaling events in an AT1R-β-arrestin-dependent manner while preventing AT1R-Gαq protein activity defines them as biased AT1R ligands. This mini-review will highlight recent studies that have defined biased signaling at the AT1R and discuss the possible clinical relevance of β-arrestin-biased AT1R ligands in the cardiovascular system.

Parathyroid hormone (PTH) is a principle regulator of bone and calcium metabolism and PTH analogs hold great promise as a therapy for metabolic bone diseases such as osteoporosis. PTH acts principally through the type I PTH/PTH-related peptide receptor (PTH1R), a G protein-coupled receptor (GPCR). GPCRs are a family of seven transmembrane cell surface receptors that share conserved structural, functional, and regulatory properties. Recent studies demonstrate that the complex metabolic effects induced by PTH1R stimulation are not entirely a consequence of conventional GPCR signaling. β-arrestins, in addition to their GPCR desensitizing actions, also serve as multifunctional scaffolding proteins linking the PTH1R to signaling molecules independent of the classic G protein-coupled second messengerdependent pathways. In vitro, D-Trp12,Tyr34-bPTH(7-34) (PTH- arr), a β-arrestin selective biased agonist for the PTH1R, antagonizes receptor-G protein coupling but activates arrestin-dependent signaling. In vivo, intermittent administration of, PTH-βarr to mice, induces anabolic bone formation, completely independent of classic G protein-coupled signaling mechanisms. While both PTH-βarr and the conventional agonist PTH(1-34) stimulate anabolic bone formation in mice, unlike PTH(1-34), which activates G protein coupling, PTH-βarr does not induce hypercalcemia or increase markers of bone resorption. This newly recognized ability of β-arrestins to serve as signal transducers for the PTH1R represents an innovative paradigm of receptor signaling which can be targeted to induce a subset of physiologic responses in bone. Exploitation of β-arrestin biased agonism may offer therapeutic benefit for the treatment of metabolic bone diseases such as osteoporosis.

Chemokine receptors are a group of homologous seven transmembrane receptors (7TMR) that direct cell migration. Their ligands comprise a family of proteins that share structural, biochemical, and physiological features to govern leukocyte trafficking. Multiple endogenous chemokines with overlapping function have evolved for the majority of chemokine receptors. This duplicity of ligands has traditionally been seen to confer physiologic redundancy, especially as it pertains to chemotaxis mediated through G-protein activation. Yet, several recent reports also suggest that chemokine receptors are capable of differential signaling in a ligand-specific manner. This review will explore emerging concepts related to ligand bias at chemokine receptors. Recent studies show that although the endogenous ligands of CCR7 have apparent equipotency for G-protein signaling, they differentially activate the G-protein coupled receptor kinase (GRK)/β-arrestin system to selectively control receptor desensitization. In contrast, similar studies using endogenous ligands for CCR5, a human immunodeficiency virus (HIV) co-receptor, suggest this receptor is not subject to ligand bias by its principle chemokines. Nonetheless, this receptor does appear to be capable of biased agonism by synthetic chemokine analogues. These observations provide compelling evidence that ligand bias exists both as a naturally relevant and therapeutically important phenomenon. This review will highlight the evidence for differential signaling by CCR7 and CCR5, speculate on the physiologic relevance, and discuss the rationale behind the development of biased agonists for the treatment of HIV infection.

Advanced Glycation Endproducts (AGEs) are a group of heterogeneous compounds formed by the non enzymatic reactions between aldehydic group of reducing sugars with proteins, lipids or nucleic acids. Formation and accumulation of AGEs are related with the aging process and are accelerated in diabetes. Type 2 diabetes are the most common form of diabetes, which are characterized by hyperglycemia and insulin resistance associated to a progressive deterioration of beta cell function and mass. The pathogenic role of AGEs in vascular diabetic complications is widely recognised. Recently, other aspects of the detrimental effects of AGEs in type 2 diabetes have emerged: AGEs interfere with the complex molecular pathway of insulin signalling, leading to insulin resistance; AGEs modify the insulin molecule, and consequently, its function; AGEs decrease insulin secretion and insulin content. In this article, we review the role of AGEs in type 2 diabetes, beyond their involvement in vascular complications.

Chemokines are the initial mediators of leukocyte migration across concentration gradients in vitro and to sites of inflammation in vivo. Chemokines signal via specific seven-transmembrane spanning G-protein coupled receptors (GPCRs). About 50 chemokine ligands and 18 receptors have been identified to date, and several are involved in leukocyte trafficking in human inflammation. Several chemokines signal via a single receptor, while others signal via multiple receptors. This redundancy may be necessary to mediate essential biological processes in vivo. There is evidence that specific chemokines and their receptors are expressed in the peripheral nerves or cerebrospinal fluid of patients with autoimmune neuropathies such as Guillain-Barre syndrome and Chronic Inflammatory Demyelinating Polyradiculoneuropathy and their animal models. Hematogenous leukocyte trafficking and chemokine-mediated signaling have also been implicated in the generation of neuropathic pain following peripheral nerve injury. Chemokine receptors, being GPCRs, provide an attractive drug target for modulating the harmful effects of peripheral nerve inflammation. The efficacy of antiinflammatory therapies, including treatments that restrict leukocyte migration, has been established in several inflammatory disorders such as multiple sclerosis. There are several ongoing clinical trials testing chemokine receptor antagonists as specific anti-inflammatory drugs. This review evaluates the current status of the chemokine biology of peripheral neuropathies, highlighting areas where further studies are needed and discusses potentially selective drug targets for peripheral nerve inflammation and neuropathic pain.

Human plasmacytoid dendritic cells (pDC) are crucial for the modulation of adaptive immune responses in the course of neoplastic, viral and autoimmune diseases. In several of these disorders deregulated pDC-derived interferon-α (IFN-α), a key cytokine produced by pDC, plays a central role. Apart from IFN-α, pDC can produce a variety of other mediators, which are involved in immunological cross-talk. The most recently discovered are the cytotoxic serine protease granzyme B (GrB) and indoleamine 2,3-dioxygenase, which have been described to be involved in the suppression of effector T cell responses. Here we review the regulation of pDC function by a variety of immunomodulatory agents, which may be developed as future candidates for the therapy of a variety of diseases. Moreover, we introduce the novel concept of enhancing immune responses after vaccination in poor responders by increasing pDC-derived IFN-α and simultaneously inhibiting pDC-derived GrB secretion. Finally we discuss potential approaches to abrogate pDC-mediated tolerance induction against tumors and viral infections.

Eosinophils are a subset of leukocytes the traditionally associated with Th2-related diseases and helminth infections. However, accumulating evidence suggests that eosinophils play a more prominent role in the immune response to bacterial and viral pathogens than previously realized. Specifically, eosinophils possess antimicrobial properties against a broad range of pathogens, and release specific and secondary granules as a result of pathogen recognition. Pathogen recognition is accomplished through expression of Toll-like receptors, as well as other surface and intracellular receptors expressed by the eosinophil. Interestingly, specific killing mechanisms employed by each granule protein differ based on pathogen recognition, but ultimately release of eosinophil granules leads to direct killing of many different pathogens. The precise mechanisms of killing by granule proteins and the circumstances in which specific proteins are secreted are only now being determined. Future efforts to understand these mechanisms may lead toward clinical use of granule proteins as antimicrobial agents in humans, in addition to revealing implications regarding the use of eosinophil-depleting therapies for allergic disorders. This review will summarize the literature to date regarding the role of eosinophils in non-parasitic infections.

Organ transplantation has become a major therapeutic option for patients with irreversible organ diseases. Immunosuppressive agents are usually required to prevent allograft rejection in patients who undergo an organ transplantation. Such drugs suppress both specific and nonspecific immunity, and render the recipient more susceptible to both infection and malignancy. Therefore, the development of more effective and less toxic immunosuppressive agents could improve the clinical outcomes of organ transplant recipients. Since the early days of clinical and experimental liver transplantation, it has been known that the liver is less likely to be rejected in comparison to other organs and may be tolerogenic even across a fully allogeneic MHC barrier in some specific cases. Spontaneous acceptance of liver allografts has been observed in several species. Orthotopic liver transplantation (OLT) in certain rat strains is accepted without immunosuppressive agents and serum from post-OLT recipients displays immunosuppressive activity. Attempts have been made to identify the immunosuppressive factors that are present in post-OLT serum to elucidate the mechanism of immunolo- gical tolerance and to discover novel immunosuppressive agents for potential use in organ transplantation. In this review we will focus on established and recent findings in the identification of immunosuppressive factors in a rat tolerogenic OLT model. The most recent therapeutic methods in organ transplantation and future prospects will be discussed.