Glucagon-Like Peptides: GLP-1 and GLP-2 Biology

GLP-1 (glucagon-like peptide-1) and GLP-2 (glucagon-like peptide-2) are both encoded within the proglucagon gene and are co-released from intestinal L-cells in response to nutrient ingestion. Despite their shared origin, the two peptides act through entirely different receptor systems and serve distinct physiological functions. GLP-1 has become the most therapeutically impactful peptide in metabolic medicine. GLP-2, while less widely known, is the primary endocrine regulator of intestinal mucosal growth and barrier integrity.

Proglucagon processing

The proglucagon gene encodes a 160-amino-acid precursor protein that is processed differently depending on the tissue. In pancreatic alpha cells, prohormone convertase 2 (PC2) cleaves proglucagon to release glucagon, glicentin-related pancreatic polypeptide (GRPP), and a major proglucagon fragment. In intestinal L-cells and certain brainstem neurons, prohormone convertase 1/3 (PC1/3) cleaves the same precursor differently, producing GLP-1, GLP-2, glicentin, and oxyntomodulin.

This tissue-specific processing means that the same gene simultaneously serves opposing metabolic functions: glucagon from the pancreas raises blood glucose, while GLP-1 from the gut lowers it. GLP-1 and GLP-2 are produced in equimolar amounts from the same secretory granules in L-cells, meaning every nutrient-triggered release of GLP-1 is accompanied by an equivalent release of GLP-2.

GLP-1 biology

The active forms of GLP-1 are GLP-1(7-36) amide and GLP-1(7-37), cleaved from the proglucagon precursor. Native GLP-1 has a plasma half-life of approximately 2 minutes, rapidly degraded by dipeptidyl peptidase-4 (DPP-4), which cleaves the N-terminal His-Ala dipeptide to produce inactive GLP-1(9-36).

Receptor and signaling. The GLP-1 receptor (GLP-1R) is a class B G protein-coupled receptor expressed in pancreatic beta cells, the hypothalamus, brainstem, heart, kidney, and gastrointestinal tract. Activation couples primarily to Gs, increasing intracellular cAMP. In beta cells, this potentiates glucose-stimulated insulin secretion — critically, in a glucose-dependent manner. GLP-1R activation does not cause insulin release at low blood glucose, which is why GLP-1 agonists carry a low risk of hypoglycemia compared to sulfonylureas.

Key physiological effects:

• Glucose-dependent insulin secretion — amplifies the beta cell response to elevated blood glucose
• Glucagon suppression — reduces alpha cell glucagon secretion at elevated glucose levels, further lowering hepatic glucose output
• Gastric emptying delay — slows gastric motility, reducing the rate of nutrient absorption and postprandial glucose spikes
• Appetite suppression — central GLP-1R signaling in the hypothalamus and brainstem reduces food intake and promotes satiety
• Cardiovascular effects — GLP-1R is expressed on cardiomyocytes and vascular endothelium; clinical trials of GLP-1 agonists have demonstrated reductions in major adverse cardiovascular events (MACE), possibly through anti-inflammatory and direct cardioprotective mechanisms

Therapeutic GLP-1 receptor agonists. The short half-life of native GLP-1 drove the development of DPP-4-resistant analogs:

• Liraglutide — acylated with a C16 palmitic acid chain enabling non-covalent albumin binding, extending the half-life to approximately 13 hours (once-daily injection)
• Semaglutide — acylated with a C18 fatty diacid with a spacer, achieving a half-life of approximately 7 days (once-weekly injection). An oral formulation (Rybelsus) co-formulated with the absorption enhancer SNAC was the first oral GLP-1 agonist approved
• Tirzepatide — a dual GLP-1/GIP receptor agonist based on the GIP sequence with modifications that confer GLP-1R activity. Acylated with a C20 fatty diacid for once-weekly dosing. Demonstrated superior weight loss and glycemic control compared to semaglutide in the SURMOUNT and SURPASS trials
• Retatrutide — a triple agonist targeting GLP-1, GIP, and glucagon receptors. By re-engaging the glucagon receptor, retatrutide adds energy expenditure and hepatic lipid mobilization to the incretin-based mechanisms. Phase 2 data showed up to 24% body weight reduction at 48 weeks

GLP-2 biology

GLP-2 is a 33-amino-acid peptide co-secreted with GLP-1 from L-cells. Like GLP-1, it is rapidly inactivated by DPP-4, with a native half-life of approximately 7 minutes.

Receptor and signaling. The GLP-2 receptor (GLP-2R) is a class B GPCR with a much more restricted distribution than GLP-1R. Expression is concentrated in the gastrointestinal tract — enteric neurons, subepithelial myofibroblasts, and enteroendocrine cells — with limited expression in the CNS. Activation couples to Gs/cAMP and downstream signaling that promotes intestinal epithelial proliferation, inhibits apoptosis, and increases mucosal blood flow.

Key physiological effects:

• Intestinal mucosal growth — GLP-2 increases villus height, crypt depth, and crypt cell proliferation rate in the small intestine
• Barrier function — enhances tight junction integrity and reduces intestinal permeability
• Nutrient absorption — upregulates expression of nutrient transporters (SGLT1, GLUT2) and digestive enzymes
• Anti-inflammatory effects — reduces mucosal inflammation and pro-inflammatory cytokine expression in animal models of colitis
• Inhibition of gastric acid secretion and gastric motility — complementary to GLP-1's effects on gastric emptying

Teduglutide is a DPP-4-resistant GLP-2 analog (Ala2 substituted with Gly) with a half-life of approximately 2 hours. It is the only approved GLP-2-based therapy, indicated for short bowel syndrome (SBS). In clinical trials, teduglutide significantly reduced parenteral nutrition requirements in SBS patients by promoting adaptive intestinal growth — increasing villus height and absorptive surface area.

The co-secretion relationship

Because GLP-1 and GLP-2 are released simultaneously, every meal triggers a coordinated response: GLP-1 manages the metabolic consequences of nutrient absorption (insulin, glucagon, appetite, gastric emptying) while GLP-2 maintains and repairs the intestinal epithelium that absorbs those nutrients. This pairing makes physiological sense — the gut must simultaneously process and maintain itself in response to nutrient loads.

Multi-agonist approaches

The most significant recent development in GLP-1-based therapeutics is the move toward multi-receptor agonism. Single-target GLP-1 agonists were effective; engaging additional receptors has proven substantially more so.

Tirzepatide's dual GLP-1/GIP agonism exploits the fact that GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 activate complementary but non-redundant signaling pathways in beta cells, adipose tissue, and the CNS. Retatrutide's addition of glucagon receptor agonism introduces a thermogenic and lipolytic component absent from pure incretin signaling. The clinical data suggest that incretin-based therapy is not approaching a ceiling — additional receptor engagement continues to produce larger metabolic effects. Whether quad-agonist or higher-order combinations will yield further gains is an active area of investigation.