Elicit: What mechanisms, beyond canonical pathways, are indicated by GLP-1R?

What mechanisms, beyond canonical pathways, are indicated by GLP-1R?

GLP-1R employs various alternative signaling mechanisms, including PLC/PKC and GTP-binding protein pathways, alongside organelle interactions and concentration-dependent pathway switching.

Abstract

GLP‑1 receptor signaling involves several distinct, noncanonical mechanisms alongside its classic Gαs/cAMP/PKA pathway. Studies report that picomolar GLP‑1 concentrations trigger insulin release via a phospholipase C/protein kinase C (PLC/PKC) pathway rather than through PKA activation. In addition, the GLP‑1(9‑36) metabolite reduces glucagon secretion by engaging an inhibitory GTP‐binding protein independently of PKA. Other reports describe a cAMP/Epac/PI3K/Akt cascade mediating cardioprotective effects and a mode of Akt activation that bypasses traditional PI3K‐dependent phosphorylation in β‑cells.

Additional findings indicate that signaling bias plays a role in GLP‑1R function. Differential β‑arrestin recruitment is linked to receptor internalization and altered downstream responses, while concentration‐dependent shifts—such as a switch from PKC‐ to PKA‐dependent pathways—modulate physiologic outcomes. Furthermore, evidence from in vitro studies shows that GLP‑1R associates with endoplasmic reticulum and mitochondrial proteins (for example, VAP‑B and SPHKAP), suggesting a role in inter‑organelle communication and mitochondrial remodeling. These convergent observations demonstrate that GLP‑1R orchestrates a multifaceted network of PKA‑independent and context‑dependent signaling events.

Methods

We analyzed 40 sources from an initial pool of 497, using 7 screening criteria. Each paper was reviewed for 5 key aspects that mattered most to the research question. More on methods

Screening

We screened in sources based on their abstracts that met these criteria:

GLP-1R Signaling Focus: Does the study investigate GLP-1R signaling pathways (either canonical or non-canonical mechanisms)?
Molecular Methods: Does the study employ molecular biology techniques (such as receptor binding assays or signal transduction studies) to investigate mechanistic aspects?
Study Design: Is this an in vitro and/or in vivo experimental study?
Receptor Interactions: Does the study examine GLP-1R interactions with cellular components or signaling molecules?
Beyond Canonical Pathway: Does the study investigate pathways beyond or in addition to the canonical cAMP/PKA pathway?
Receptor Involvement: Does the study directly investigate GLP-1R (rather than only GLP-1 effects without receptor involvement)?
Mechanistic Investigation: Does the study include mechanistic investigation (rather than only clinical efficacy outcomes)?

We considered all screening questions together and made a holistic judgement about whether to screen in each paper.

Results

Characteristics of Included Studies

Study	Study Type	Cell/Tissue Type	Key Mechanisms Investigated	Primary Findings
Athauda et al., 2024	In vitro cellular studies, Clinical trial	Human iPSC models of synucleinopathy	GLP-1R agonism effects on neuronal insulin signaling and glial suppression	GLP-1R agonism restores insulin signaling, suppresses MAPK pathways, reduces oxidative stress, improves mitochondrial and lysosomal function, and reduces α-synuclein aggregation.
Austin et al., 2024	In vitro cellular studies	Pancreatic β-cells	Inter-organelle contact between GLP-1R, endoplasmic reticulum (ER), and mitochondria	GLP-1R interacts with VAP-B and SPHKAP to trigger mitochondrial Protein kinase A (PKA) signaling and MIC19 phosphorylation.
Bitsi et al., 2023	Animal studies	Adult β cell–specific β-arrestin 2 knockout mice	β-arrestin-mediated GLP-1R signaling	β-arrestin 2 regulates GLP-1R desensitization, recycling, and signaling in a sex-dimorphic manner.
Buteau et al., “GLP-1 Induces Proliferation”	In vitro cellular studies	INS(832/13) cells, Isolated rat islets, LNCaP cells	GLP-1R-mediated transactivation of EGFR	GLP-1 activates PI 3-kinase and promotes β-cell proliferation through EGFR transactivation via c-Src.
Capozzi et al., 2020	Animal studies, In vitro cellular studies	Isolated mouse islets, Gnasβcell-/- mice	Gαs-dependent and independent mechanisms of incretin-stimulated insulin secretion	GLP-1R can signal through both Gαs and non-Gαs mechanisms, including Gq.
Gao et al., 2023	Animal studies, In vitro cellular studies	Isolated mouse islets	ER stress and UPR signaling effects on GLP-1R signaling	ER stress induces a transition from Gs to Gq in GLP-1R signaling.
Guida et al., 2019	In vitro cellular studies, Animal studies	Isolated mouse and human islets, Glp1r knockout mice	GLP-1(9-36) signaling mechanisms	GLP-1(9-36) inhibits glucagon secretion through a PKA-independent mechanism involving an inhibitory GTP-binding protein.

Key Findings from GLP-1R Signaling Mechanism Studies

Study Types

In vitro cellular experiments: 35/40 studies
Animal studies: 14/40 studies
Clinical trials: 1 study
Computational/modeling studies: 2 studies
Design not clearly specified: 3 studies

Cell/Tissue Types

Mouse islets: 12 studies
INS cells: 7 studies
MIN6 cells: 6 studies
Human islets: 5 studies
Knockout mice: 6 studies

Non-Canonical Signaling Mechanisms

PKA-Independent Signaling Pathways

Several studies identified PKA-independent signaling pathways activated by GLP-1R:

PLC/PKC-dependent pathway:
- Shigeto et al. (2017a, 2017b) reported GLP-1 stimulates insulin secretion at physiological (picomolar) concentrations through this pathway.
Inhibitory GTP-binding protein mechanism:
- Guida et al. (2019) found GLP-1(9-36) metabolite inhibits glucagon secretion through this PKA-independent mechanism.

These studies report that GLP-1R can engage multiple signaling pathways independently of PKA. The authors suggest this may allow for fine-tuned cellular responses depending on the cellular context, GLP-1 concentration, and the specific GLP-1R agonist used.

Physiological Implications

Cell-Type Specific Effects

Studies reported diverse effects of GLP-1R signaling in different cell types:

Pancreatic β-cells: Enhanced insulin secretion through canonical and non-canonical pathways. Mechanisms vary with GLP-1 concentration and signaling molecules involved.
Cardiomyoblasts: Mediates cardioprotective effects through a novel GLP-1R/cAMP/Epac/PI3K/Akt signaling pathway.

These studies report cell-type specific effects, suggesting that cellular context may be an important factor in GLP-1R signaling research and potential therapeutic applications.