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

Papers identified with Elicit search

n = 497

Papers screened using:

• 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

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

Cellular/Biological Model

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

Key Mechanisms

• General GLP-1R signaling: 4 studies
• β-arrestin-mediated signaling: 2 studies
• Biased agonism: 2 studies
• Non-canonical Akt activation: 2 studies
• PLC/PKC-dependent pathways: 2 studies

Other Mechanisms

• EGFR transactivation
• Gαs-dependent and independent mechanisms
• ER stress/UPR effects
• Various kinase cascades (e.g., MAPK, PI3K/Akt)

The included studies predominantly used in vitro and animal models to investigate various signaling pathways and mechanisms related to GLP-1R, with many focusing on β-cell function and insulin secretion.

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
• GLP-1R/cAMP/Epac/PI3K/Akt signaling pathway:
  • Nuamnaichati et al. (2020) identified this novel pathway mediating cardioprotective effects
  • Epac acts as a key regulator instead of PKA
• Non-canonical Akt activation:
  • Widenmaier et al. (2009) and Widenmaier “Non-canonical Activation” discovered GLP-1 can activate Akt in β-cells without requiring PI3K or typical Akt activation site phosphorylation

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.

Concentration-Dependent Alternative Pathways

Several studies reported concentration-dependent activation of alternative signaling pathways by GLP-1R:

• PKC vs PKA-dependent pathways:
  • Shigeto and Kaku (2014) found:
    • Picomolar GLP-1: PKC-dependent pathway for insulin secretion
    • Nanomolar GLP-1: Canonical PKA-dependent pathway
• Receptor affinity and internalization:
  • Jones et al. (2018) observed:
    • Low-affinity agonists: Less receptor internalization, greater insulin secretion
    • High-affinity agonists: More receptor internalization, less insulin secretion
• β-arrestin 2 roles:
  • Zaïmia et al. (2023) demonstrated:
    • Physiological doses: β-arrestin 2 dampens insulin secretion by partially uncoupling cAMP/PKA signaling
    • Pharmacological doses: β-arrestin 2 required for ERK/CREB pathway activation

These studies report concentration-dependent effects, suggesting that the dose of GLP-1 or GLP-1R agonists may be an important factor in experimental design and interpretation.

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
  • Studies: Shigeto et al., 2017a; Shigeto et al., 2017b; Zaïmia et al., 2023
• Cardiomyoblasts:
  • Novel GLP-1R/cAMP/Epac/PI3K/Akt signaling pathway
  • Mediates cardioprotective effects
  • Study: Nuamnaichati et al. (2020)
• Gut intraepithelial lymphocytes (IELs):
  • Modulates inflammation and affects gut microbiota composition
  • Expands known functions of GLP-1 to include immunomodulation
  • Study: Wong et al. (2022)
• Neurons:
  • Neuroprotective effects in Parkinson’s disease models
  • Involves improvements in mitochondrial and lysosomal function
  • Study: Athauda et al. (2024)

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.

Integration with Known Pathways

Studies revealed complex interactions between GLP-1R signaling and other established signaling pathways:

• EGFR transactivation:
  • Buteau et al. (2003) and Buteau et al. “GLP-1 Induces Proliferation” showed GLP-1R can transactivate EGFR
  • Leads to activation of PI3-kinase and promotion of β-cell proliferation
• mTORC1 signaling:
  • Le et al. (2021) and Le et al. “GLP-1 Receptor Activation” demonstrated GLP-1R activation can stimulate mTORC1 signaling
  • Involves a novel PKA-dependent mechanism and phosphorylation of Raptor
• RhoA-ROCK signaling:
  • Kong et al. (2014) revealed GLP-1 can enhance insulin secretion by inhibiting the RhoA-ROCK pathway
  • Involves PKA-mediated phosphorylation of RhoA
• Kinase networks:
  • Xiao et al. (2023) used proteomic profiling to identify a wide range of kinases affected by GLP-1R activation
  • Includes MAPKs, PKCs, and LKB1

These studies report that GLP-1R signaling interacts with numerous other cellular signaling networks, potentially allowing for regulation of diverse physiological processes.