The integrated physiology of glucose homeostasis: regulation by extracellular and intracellular nucleotide sensors

Abstract

Physiological glucose levels are maintained by the complex integration of neuroendocrine, hormonal and nutritional signals controlled by multiple tissues in the body. A dysregulation in these mechanisms leads to increasingly prevalent conditions characterised by an inability to regulate blood glucose levels, such as diabetes. Maintaining glycaemia within a target range remains a daily challenge for individuals with both Type 1 and Type 2 diabetes and a better understanding of the pathophysiology of impaired glucose homeostasis in these conditions is still required to identify more effective and targeted therapeutic approaches. Work in this thesis focused on elucidating the mechanisms by which lipid overflow, be it from increasingly sedentary behaviour or overfeeding, leads to the development of insulin and anabolic resistance in skeletal muscle. Loss of insulin-stimulated glucose clearance by skeletal muscle is a main driver for impaired glucose disposal in Type 2 diabetes and a role for excessive lipid availability in this pathology is well established. Here, muscle cells were treated with high concentrations of a saturated fatty acid and data demonstrated that lipid overflow led to impaired anabolic sensitivity, inflammatory cytokine release and mitochondrial dysfunction. Furthermore, these experiments elucidated a novel role for adenosine tri-phosphate, acting as a signalling molecule, in the regulation of muscle glucose metabolism, identifying insulin and exercise mimetic roles of the nucleotide that could be therapeutically targetable. This work was translated into humans, where the effect of lipid overflow by high-fat overfeeding was assessed in an experimental model of inactivity-induced insulin and anabolic resistance. Data suggested that two days of disuse (by forearm immobilisation) were sufficient to cause substantial muscle insulin resistance. After 7 days, muscle strength was significantly reduced and anabolic resistance was evident due to decreased forearm balance of potent anabolic amino acids such as leucine. Contrary to the hypothesis, high-fat overfeeding did not accelerate or exacerbate these impairments, suggesting that removal of contraction represents a potent stimulus for loss of substrate demand by muscle, irrespective of energy balance. Insulin replacement therapy has been the cornerstone of treatment for Type 1 and advanced Type 2 diabetes for over 8 decades. A serious and inadvertent consequence of prolonged insulin therapy is the increased risk of hypoglycaemia. Hypoglycaemia can lead to impaired physiological defences against a decrease in blood glucose and loss of awareness of these changes. AMP-activated protein kinase activators, which are widely used (to target peripheral tissues) as anti-hyperglycaemic agents in Type 2 diabetes have demonstrated central effects that amplify the first defence against hypoglycaemia, or counterregulatory response. Data presented here demonstrated that peripheral administration of a brain permeable AMP-activated protein kinase activator amplified the counterregulatory response to hypoglycaemia by enhancing glucagon levels in healthy rats, without altering fasting blood glucose. This demonstrates important clinical implications for the pharmaceutical use of AMP-activated protein kinase activators as the central roles that regulate blood glucose may supersede the peripheral effects of these compounds, during hypoglycaemia. Work presented here highlights the complexity of the regulation of glycaemia and discusses the contribution of extracellular and intracellular nucleotides/nucleotide sensors to glucose homeostasis. It can be concluded from this work that strategies to manage or treat diabetes in future should consider the importance of tissue-specific or metabolic status specific actions of the targets of interest.