Glucagon‐like peptide‐1: The missing link in the metabolic clock?

Circadian expression of clock genes in peripheral tissues is critical to the coordinated regulation of intestinal digestive and absorptive functions, insulin secretion, and peripheral tissue nutrient deposition during periods of nutrient ingestion, thereby preventing metabolic dysregulation. As glucagon‐like peptide‐1 is a key incretin hormone that regulates glucose‐dependent insulin secretion, we hypothesized that this intestinal hormone is a player in the peripheral metabolic clock, linking nutrient ingestion to insulin secretion. We have now established that secretion of glucagon‐like peptide‐1 from the intestinal L cell shows a rhythmic pattern in rats and humans in vivo that is altered by circadian disruptors, such as constant light exposure, consumption of a Western diet and feeding at inappropriate times (i.e., during the light period in rodents). Interestingly, the alterations in the rhythm of the glucagon‐like peptide‐1 secretory responses were found to parallel the changes in the pattern of insulin responses in association with significant impairments in glucose tolerance. Furthermore, we have detected circadian clock gene expression, and showed circadian secretion of glucagon‐like peptide‐1 from both the murine and human L cell in vitro. These findings demonstrate that glucagon‐like peptide‐1 is a functional component of the peripheral metabolic clock, and suggest that altered release of glucagon‐like peptide‐1 might play a role in the metabolic perturbations that result from circadian disruption.