Unravelling the mechanism of ghrelin secretion and the effects of ghrelin reduction using a receptor decoy approach
The incidence of obesity, and the associated morbidities and mortality are increasing. Strategies to manage this disease hinge on the balance of caloric intake and energy expenditure. This regulation depends largely on endocrine input from the periphery. The recently discovered stomach derived hormone, ghrelin, has emerged as a key player in the regulation of appetite and energy storage. Ghrelin achieves these functions through binding the ghrelin receptor in appetite regulating neurons and in peripheral metabolic organs including the pancreas and adipose tissue. Since ghrelin acts on energy regulating metabolic organs, its secretion from the stomach is tightly coupled to energy availability. Ghrelin levels increase during periods of fasting and decrease after a meal is consumed. Under chronic energy surplus (such as obesity) ghrelin levels decrease while in chronic energy deficit (anorexia nervosa, weight loss) ghrelin levels increase. While major advances have been made in understanding both the function of ghrelin and its dysregulation in disease, little is known about the cellular regulation of ghrelin secretion. This is due to the lack of cellular models of ghrelin secretion. In this thesis, I describe the development of a novel ghrelin secreting primary rat stomach cell culture. Using this system I elucidated the roles and mechanisms of neurotransmitters, hormones (insulin and glucagon), nutrients (glucose) and anti-diabetics (metformin) in the regulation of ghrelin secretion. These findings have clearly demonstrated the ability of ghrelin cells to sense energy availability and provide important insights for ghrelin altering therapies. To evaluate both the function of ghrelin and the feasibility of reducing circulating ghrelin, I developed a novel in vivo ghrelin-reducing strategy. In vivo expression of a decoy protein based on the ligand binding domains of the ghrelin receptor was expressed in mice. Mice treated with this plasmid construct had reduced circulating levels of ghrelin. Interestingly, reduced circulating ghrelin was protective from high fat diet-induced obesity and resulted in improved glucose metabolism. This work demonstrates both the importance of ghrelin in peripheral energy storage and the feasibility of this novel ghrelin reducing approach for the treatment of obesity and insulin resistance.