Current Pharmaceutical Design - Volume 9, Issue 17, 2003

New treatments are currently required for the common metabolic diseases obesity and type 2 diabetes. The identification of physiological and biochemical factors that underlie the metabolic disturbances observed in obesity and type 2 diabetes is a key step in developing better therapeutic outcomes. The discovery of new genes and pathways involved in the pathogenesis of these diseases is critical to this process, however identification of genes that contribute to the risk of developing these diseases represents a significant challenge as obesity and type 2 diabetes are complex diseases with many genetic and environmental causes. A number of diverse approaches have been used to discover and validate potential new targets for obesity and diabetes. To date, DNA-based approaches using candidate gene and genome-wide linkage analysis have had limited success in identifying genomic regions or genes involved in the development of these diseases. Recent advances in the ability to evaluate linkage analysis data from large family pedigrees using variance components based linkage analysis show great promise in robustly identifying genomic regions associated with the development of obesity and diabetes. RNA-based technologies such as cDNA microarrays have identified many genes differentially expressed in tissues of healthy and diseased subjects. Using a combined approach, we are endeavouring to focus attention on differentially expressed genes located in chromosomal regions previously linked with obesity and / or diabetes. Using this strategy, we have identified Beacon as a potential new target for obesity and diabetes.

Modern societies have moved from famine to feast and obesity and its co-morbidities now sweep the world as a global epidemic. Numerous scientific laboratories and pharmaceutical companies have taken the challenge and are now exploiting novel molecular targets for treatment of obesity. The pre-proglucagon system constitutes interesting candidates as potential targets for new anti-obesity drugs. In the periphery, pre-proglucagon derived peptides, Glucagon-Like Peptide-1 (GLP-1), Glucagon-Like Peptide-2 (GLP-2) and oxyntomodulin (OXM) are involved in a wide variety of physiological functions, including glucose homeostasis, gastric emptying, intestinal growth, insulin secretion as well as the regulation of food intake. Peripheral administration of GLP-1 derivatives and analogues to both rodents and man have shown promising effects on food intake and body weight suggesting that such therapies constitute potential anti-obesity treatment. In the central nervous system, pre-proglucagon and hence GLP-1, GLP-2 and OXM are exclusively found in a small population of nerve cells in the nucleus of the solitary tract. These constitute a neural pathway linking the “viscerosensory”brainstem to hypothalamic nuclei involved in energy homeostasis. Intracerebroventricular administration of all of the three derived peptides robustly decrease food intake. It is evident that central GLP-1 agonism probably in combination with GLP-2 and / or OXM agonism constitute a potential pharmacological tool to reduce food intake and maybe also enhance energy expenditure. This and other aspects of the current state of the role of central pre-proglucagon in energy homeostasis are reviewed.

In order to develop an effective pharmacological treatment for obesity, an endogenous factor that promotes a positive energy balance by increasing appetite and decreasing fat oxidation could represent the drug target scientists have been looking for. The recently discovered gastric endocrine agent ghrelin, which appears to be the only potent hungerinducing factor to naturally circulate in our blood stream, was discovered in 1999. Since then the acylated peptide hormone ghrelin has evolved from an endogenous growth hormone secretagogue to a regulator of energy balance to a pleiotropic hormone with multiple sources, numerous target tissues and most likely several physiological functions. Although neither the exact mechanism of action by which ghrelin increases food intake and adiposity is known, nor the putatively differential effects of brain-derived and stomach-derived ghrelin on energy homeostasis have been determined, blocking or neutralizing ghrelin action still seems one of the more reasonable pharmacological approaches to reverse a chronically positive energy balance. However, based on growing experience with compounds targeting the neuroendocrine regulation of energy balance, it is quite possible that a ghrelin antagonist will either fail to cure obesity due to the existence of compensatory mechanisms or undesired effects might reveal the true biological function of ghrelin (e.g. cardiovascular mechanisms, anti-proliferative effects, reproduction).

Triacylglycerol storage in adipose tissue is mediated by a host of transporters, enzymes and binding proteins. Additionally, several hormones (both autocrine and endocrine) are known to interact with cell surface receptors and modulate triacylglycerol synthesis (such as acylation stimulating protein, ASP). The many proteins involved contribute to the robustness of the system and, in most cases, deletion of a single gene is not deleterious and adipose tissue is preserved.On the other hand, this does not mean that gene disruption is not without effect, and in fact often results in a leaner, and presumably “healthier” mouse. These insights provide valuable indications for potential drug tools to delay and/or reverse obesity.In this review we examine the potential of ASP as a candidate target. ASP deficiency in mice decreases adipose tissue mass, increases insulin sensitivity and energy expenditure even in obese ob / ob mice, suggesting that partial interference of ASP action could be advantageous. ASP interacts with a specific cell surface receptor present in adipose tissue and certain structural components, such as the tightly folded core region, are implicated in activity. We propose that interference of the ASP-receptor interaction using an antagonist offers future prospect for an anti-obesity target.

Adiponectin is an insulin-sensitizing hormone whose blood concentration is reduced in obesity and type 2 diabetes. Administration of recombinant adiponectin in rodents increases glucose uptake and increases fat oxidation in muscle, reduces fatty acid uptake and hepatic glucose production in liver, and improves whole body insulin resistance. The exact receptor and signaling systems are unknown, however, recent studies suggest adiponectin activates AMPK, a putative master metabolic regulator. Thus, excitement surrounds the potential for adiponectin, or a homologue of adiponectin, as pharamacotherapy agents for patients suffering from the metabolic syndrome and more particularly for individuals with insulin resistance and type 2 diabetes.