MECHANISMS IN ENDOCRINOLOGY: FXR signalling: a novel target in metabolic diseases.

During the last decades, it has become clear that the gastrointestinal tract plays a pivotal role in the regulation of glucose homeostasis. More than 40 hormones originate from the gastrointestinal tract and several of these impact glucose metabolism and appetite regulation. An astonishing example of the gut's integrative role in glucose metabolism originates from investigations into bile acid biology. From primary animal studies, it has become clear that bile acids should no longer be labelled as simple detergents necessary for lipid digestion and absorption but should also be recognised as metabolic regulators implicated in lipid, glucose and energy metabolism. The nuclear farnesoid X receptor (FXR) is a part of an exquisite bile acid-sensing system that among other things ensures the optimal size of the bile acid pool. In addition, intestinal and hepatic FXR also impact the regulation of several metabolic processes such as glucose and lipid metabolism. Accordingly, natural and synthetic FXR agonists and certain FXR-regulated factors (i.e. fibroblast growth factor 19 (FGF19)) are increasingly being evaluated as treatments for metabolic diseases such as type 2 diabetes and non-alcoholic fatty liver disease (and its inflammatory version, non-alcoholic steatohepatitis). Interestingly, decreased FXR activation also benefits glucose metabolism. This can be obtained by reducing bile acid absorption using bile acid sequestering agents (approved for the treatment of type 2 diabetes) or inhibitors of intestinal bile acid transporters,that is the apical sodium-dependent bile acid transporter (ASBT). This article discusses recent clinical trials that provide insights about the role of FXR-FGF19-targetted therapy for the treatment of metabolic diseases.