Molecular mechanisms mediating metabolic benefits of glucagon-like peptide-1 receptor agonists

PROJECT SUMMARY
Although the ability of glucagon-like peptide-1 receptor (Glp1r) agonists to stimulate insulin secretion
and reduce caloric intake has been recognized for over two decades, surprisingly little is known about
the molecular mechanisms behind these effects. We have previously shown that activation of the
hypothalamic Glp1r reduces food intake by engaging key nutrient sensing mechanisms such as
mechanistic Target of Rapamycin Complex-1 (mTORC1). Since mTORC1 is also an important
regulatory component of -cell function, this suggests that elucidating how Glp1r agonists regulate
mTORC1 and its downstream targets will address a key knowledge gap about the mechanism of action
of an important class of diabetes and obesity drugs. We have identified a novel interaction stimulated
by the clinically relevant Glp1r agonist liraglutide (Lira) whereby the canonical target of Glp1r signaling,
cAMP-dependent protein kinase A (PKA), phosphorylates the mTORC1 regulatory protein Raptor
resulting in increased mTORC1 signaling. We have also identified the transcription factor Hypoxia-
Inducible Factor (HIF) as a target of Glp1r signaling. This is relevant since HIF stimulates glycolysis, a
mechanism necessary for the anorectic and insulinotropic effects of Glp1r agonists, and increased HIF
expression in the hypothalamus and -cells reduces food intake and stimulates insulin secretion,
respectively. We also provide preliminary data showing that Lira no longer reduces body weight or
glucose levels in novel knockin mice replacing endogenous Raptor with a PKA-resistant Raptor. Our
preliminary data, therefore, lead us to hypothesize that a Glp1r-PKA-mTORC1-HIF-glycolysis axis in
the hypothalamus and -cells mediates the ability of Lira to reduce body weight (Aim 1) and stimulate
insulin secretion (Aim 2), respectively. The clinical relevance of this is further emphasized by our
preliminary data showing that two variants of the Glp1r found in humans that are associated with
improved cardiometabolic outcomes and improved responsiveness to Lira also stimulate mTORC1
signaling to a greater degree than wild-type Glp1r. We will, therefore, use mice expressing these human
Glp1r variants to test the hypothesis that Lira promotes greater weight loss and improved glucose
tolerance in these mice via enhanced mTORC1 signaling (Aim 3). We will complete these Aims by
leveraging our extensive expertise in assessing metabolic phenotypes in mice, including real-time
measurements of energy balance parameters as well as pancreatic function in isolated islets and in
vivo using hyperglycemic clamps. We will apply these approaches to a suite of novel mouse models
that allow us to modulate or measure the expression and activity of target proteins in specific cell types.
Accomplishing these Aims will delineate specific molecular mechanisms that can be leveraged towards
either the improvement of the effectiveness of Lira or the design of more efficient weight-lowering drugs.

Grant Number: 5R01DK132852-04
NIH Institute/Center: NIH
Principal Investigator: Julio Ayala