G Protein-Coupled Receptors in Pancreatic β-Cells: From Trafficking and Localization to Insulin Secretion and Diabetes

Abstract

G protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors and are critical regulators of β-cell physiology. Nearly 300 GPCRs are expressed in human islets, where they integrate metabolic, hormonal, neuronal, and inflammatory cues to control insulin secretion, proliferation, and survival. Altered GPCR signaling contributes to β-cell dysfunction and the pathogenesis of both type 1 and type 2 diabetes. This review provides an overview of GPCR functions in β-cell biology, highlighting receptors that stimulate or inhibit glucose-stimulated insulin secretion, as well as those influencing β-cell fate. We also examine GPCR biosynthesis, trafficking, and subcellular localization—processes that shape receptor availability and signaling specificity. Aberrant folding, retention, or misrouting of GPCRs can disrupt β-cell function and contribute to metabolic disease. Thus, beyond receptor pharmacology, understanding the molecular mechanisms governing GPCR biogenesis and spatial distribution is essential for designing targeted strategies to preserve β-cell function and improve glucose homeostasis.

1. Background

Diabetes mellitus is defined by chronic hyperglycemia resulting from impaired insulin production, secretion, or action [1]. Type 1 diabetes (T1D) is an autoimmune disorder in which T cell–mediated destruction of pancreatic β-cells reduces β-cell mass and causes insulin deficiency [2,3,4]. Type 2 diabetes (T2D), often associated with obesity, arises from impaired insulin secretion and insulin resistance, leading to defective glucose metabolism [2,3,5,6].

Glucose is the primary insulin secretagogue. After a meal, glucose enters β-cells via glucose transporter type 2 (GLUT2), increasing the ATP/ADP ratio and closing ATP-sensitive K⁺ channels, which depolarizes the membrane. This activates voltage-gated Ca²⁺ channels, allowing Ca²⁺ influx that triggers insulin granule fusion and exocytosis [2,6,7,8].

Although glucose is the primary regulator, amino acids, fatty acids, hormones, and incretins also modulate insulin release through diverse pathways [1,2,4]. Among these, G protein–coupled receptors (GPCRs) play a central role in islet signaling. Approximately 300 GPCRs are expressed in human pancreatic islets and regulate hormone secretion. Many of these receptors are orphan GPCRs, with unknown ligands and functions. Consequently, many β-cell GPCRs remain poorly characterized, particularly those expressed at low levels.

The GLP-1 receptor (GLP-1R) is one of the best-characterized GPCRs in pancreatic β-cells, where it potentiates insulin secretion and forms the basis of GLP-1R agonist therapies for type 2 diabetes (T2D). Additional receptors with therapeutic potential include FFAR1, GPR119, GPRC5B, and GPRC5C [9]. Beyond these, more than 30 GPCRs have been associated with β-cell dysfunction, insulin resistance, and T2D progression [10]. Several orphan GPCRs are increasingly suggested to influence β-islet biology, although their precise physiological and pathological roles remain incompletely defined. These include GPR183 [11], members of the SREB receptor family (GPR27, GPR85, and GPR173) [12], GPR3, GPR6, and GPR12 [13], and GPRC5C [14].

This review focuses on the role of GPCRs in β-cell biology, highlighting their intracellular trafficking from the endoplasmic reticulum to the plasma membrane and their contributions to proliferation, apoptosis, and the regulation of insulin secretion.

2. Synthesis, Trafficking and Localisation of GPCRs

GPCRs constitute the largest and most diverse family of membrane receptors [15,16]. Structurally, GPCRs are defined by seven transmembrane domains with an extracellular N-terminus and an intracellular C-terminus. Based on sequence and structure, GPCRs are divided into five families—Rhodopsin (Class A), Adhesion (Class B2), Secretin (Class B1), Glutamate (Class C), and Frizzled/Taste2 (Class F). Many receptors, however, remain orphans with unknown ligands and functions [17].

Classical GPCR signaling is mediated through heterotrimeric G proteins composed of α and βγ subunits. In their inactive state, GDP-bound α associates with βγ. Ligand binding induces a conformational change that promotes GDP–GTP exchange on the α subunit, causing α and βγ dissociation and downstream signaling [18,19]. Gα subunits fall into four main families. Gαs stimulates adenylyl cyclase (AC) and increases cAMP, whereas Gαi inhibits AC. Gαq activates phospholipase Cβ (PLCβ) to generate IP3 and diacylglycerol (DAG), and Gα12/13 regulates RhoGEFs and cytoskeletal dynamics [18]. Non-canonical GPCR signaling can deviate from the classical G protein/β-arrestin model and may occur within intracellular compartments. These mechanisms offer new opportunities for drug development beyond conventional GPCR ligands. One such mechanism is sustained endosomal signaling, in which GPCRs and their associated G proteins continue to signal after internalization into endosomes. This has been described for several receptors, including the parathyroid hormone receptor (PTHR) [20], the thyroid-stimulating hormone receptor (TSHR) [21], and the vasopressin V2 receptor (V2R) [22].

Although β-arrestins often terminate GPCR signaling at the plasma membrane, they can also initiate distinct intracellular signaling pathways. For example, β-arrestins can couple to Gαi proteins and form signaling scaffolds with downstream effectors such as ERK, thereby regulating cellular processes including cell migration [23]. Additional non-canonical mechanisms include activation of heterotrimeric G proteins (e.g., Gαi) by non-GPCR proteins such as GIV/Girdin, which triggers key signaling pathways in cancer [24], and inhibition of protein kinase A (PKA) by the GPCR Smoothened (SMO) within the Hedgehog pathway [25].

Non-canonical signaling has also been described for Gαq subunits localized to mitochondria, where they regulate cellular energy production [26], and for Gβγ dimers present in intracellular compartments such as endosomes, mitochondria, the endoplasmic reticulum, and the Golgi apparatus. In intracellular compartments, Gβγ subunits can activate kinases, reshape the cytoskeleton and modulate vesicular trafficking. Collectively, these findings have stimulated interest in the development of cell-permeant ligands capable of modulating GPCR activity within intracellular compartments [27].

By virtue of their signaling diversity, GPCRs translate extracellular cues into finely tuned intracellular responses. While cell biology research has traditionally emphasized internalization-driven signaling, GPCR biogenesis and trafficking to the plasma membrane are equally important. These processes shape receptor abundance, functional versatility, and regulatory potential.

2.1. Trafficking from Endoplasmic Reticulum (ER)/Golgi to Plasma Membrane

Most GPCRs traffic through the classical secretory pathway, moving from the endoplasmic reticulum (ER) to the Golgi apparatus and ultimately to the plasma membrane. Following activation, many receptors internalize via clathrin- and dynamin-dependent endocytosis into early endosomes. Rab GTPases then regulate sorting toward recycling endosomes or lysosomal degradation. This dynamic trafficking controls receptor turnover and signaling and also contributes to the establishment and maintenance of cell polarity by directing GPCRs to specific membrane domains.

In pancreatic ductal and islet epithelial cells, receptors are selectively targeted to apical or basolateral surfaces to ensure spatially appropriate signaling [28,29]. As in other tissues, these cells exhibit pronounced apical–basolateral polarity. The apical membrane faces the lumen, whereas the basolateral membrane is oriented toward the bloodstream and is separated by tight junctions. Consequently, accurate GPCR localization depends on precise trafficking signals.

During biosynthesis, GPCRs are synthesized in the ER and transported through the Golgi and trans-Golgi network (TGN), where they are sorted into distinct vesicular carriers for delivery to the appropriate plasma membrane domain [30] (Figure 1). In the ER, GPCRs undergo folding, oligomerization, interactions with molecular chaperones, and multiple post-translational modifications, including glycosylation, ubiquitination, and palmitoylation. N-linked glycosylation of the extracellular N-terminus is essential for proper folding, stability, and quality control. In contrast, palmitoylation of cysteine residues within the C-terminus or intracellular loops promotes membrane anchoring and facilitates efficient trafficking to the plasma membrane [31,32]. Ubiquitination can serve as a signal for ER-associated degradation (ERAD) of misfolded GPCRs [33].

Correctly folded GPCRs must pass ER quality-control checkpoints before exiting. Molecular chaperones such as calnexin, calreticulin, binding immunoglobulin protein (BiP/HSPA5), and endoplasmic reticulum protein 57 (ERp57) assist in folding and prevent aggregation. Accessory proteins, including receptor activity–modifying proteins (RAMPs) and receptor transport proteins (RTPs), particularly for olfactory receptors, facilitate trafficking of specific GPCRs to the plasma membrane [34,35]. For example, RAMPs are essential for the surface expression and ligand specificity of the calcitonin receptor-like receptor (CLR) [36].

GPCRs then traverse the ER–Golgi intermediate compartment (ERGIC) and the Golgi apparatus, where additional modifications such as O-glycosylation and complex N-glycosylation occur. The Golgi also acts as a sorting hub, directing GPCRs either to the plasma membrane or toward degradation pathways. Quality control occurs in these compartments. Correctly folded receptors proceed to downstream compartments, whereas misfolded receptors are retrotranslocated into the cytosol and targeted for ubiquitination and proteasomal degradation through the ERAD pathway [33,37,38].

Export from the ER and trafficking through the Golgi rely on specific sequence motifs within GPCRs. ER export signals include diacidic (DXE), dihydrophobic (FF, LL), and basic motifs that promote packaging into COPII vesicles [39,40]. Conversely, ER retention motifs such as RXR sequences can prevent premature exit of incompletely folded receptors [41]. Oligomerization, including both homo- and heterodimerization, may also be required for proper trafficking. For example, γ-aminobutyric acid type B (GABAB) receptor subunits must form heterodimers to reach the plasma membrane; otherwise, they are retained in the ER [42]. Similar requirements have been described for certain class C GPCRs, taste receptors, and metabotropic glutamate receptors (mGluRs).

Disruption of GPCR trafficking pathways can have pathophysiological consequences. For instance, mutations in rhodopsin that impair folding or ER export are a major cause of retinitis pigmentosa [43], whereas misfolding of the vasopressin V2 receptor (V2R) leads to nephrogenic diabetes insipidus [44]. Pharmacological chaperones—small molecules that stabilize partially folded GPCRs—have therefore been explored as therapeutic strategies to rescue the surface expression of these mutant receptors [34].

2.2. Internalization of GPCRs Through Clathrin-Coated Pits (CCPs)

At the plasma membrane, ligand binding activates GPCRs and initiates downstream signaling cascades. Receptor internalization occurs mainly via clathrin-coated pits (CCPs), leading to desensitization of G-protein signaling; however, some GPCRs continue signaling from endosomes.

The classical model involves GPCR phosphorylation by GPCR kinases (GRKs) or second-messenger kinases (PKA/PKC), followed by β-arrestin recruitment to activated receptors, which uncouples them from their G proteins. However, the binding of β-arrestin to GPCRs can lead to the activation of non-G protein signaling pathways [45]. β-arrestin promotes CCP entry by binding both the receptor and endocytic proteins (e.g., AP2 and clathrin) [46,47,48,49]. The endocytic classes (“class A” and “class B”) differ in arrestin engagement. These are distinct from the five major GPCR families, although Class A often overlaps with the rhodopsin-like family and Class B with the secretin family.

Class A of GPCRs prefers to recruit arrestin-2 and presents a short connection at CCPs and the plasma membrane, whereas class B recruits both arrestins equally and these co-internalizes with their receptors [15,37,50]. GPCRs that do not internalize with β-arrestin are generally dephosphorylated and recycled to the plasma membrane. In contrast, GPCRs that internalize in complex with β-arrestin are recycled improperly to the plasma membrane or not recycled at all [15]. In addition to the classical pathway of endocytosis by CCPs, there are other pathways by which GPCRs are internalized, independent of clathrin but dependent on dynamin, such as fast endophilin-mediated endocytosis (FEME). This pathway is mediated by the protein endophilin-A, which binds to activated GPCRs and to the endocytic molecules dynamin and synaptojanin, co-trafficking with its cargo into FEME vesicles [17,48]. Also, some GPCRs can be endocytosed through specialized membrane microdomains such as caveolae and lipid rafts [46,47].

2.3. Trafficking at the Endosomal Compartment

Once internalized, GPCRs follow three main trafficking pathways. They may be targeted for lysosomal degradation, resulting in signal termination; recycled to the plasma membrane to enable receptor resensitization; or retained in endosomes, where they subsequently enter slower degradation or recycling routes [51].

A group of small GTPases known as Rab proteins plays a central role in GPCR trafficking to endosomes. Following endocytosis, apical and basolateral cargo is delivered to Rab5-positive apical or basolateral early endosomes (AEE or BEE). From there, cargo follows several alternative routes. It may be recycled directly to the plasma membrane via Rab4-positive recycling endosomes. Alternatively, it can be transported to multivesicular bodies and then to Rab7-positive late endosomes, which contain cargo destined for lysosomal degradation. Cargo may also be routed to Rab8/10-positive common recycling endosomes (CREs), where proteins are resorted and returned to the cell surface. Before reaching the membrane, both apical and basolateral cargo can transit through Rab11a-positive apical recycling endosomes (AREs). In addition to the basolateral-to-apical route, an apical-to-basolateral pathway involving Rab25 has been described, although it remains poorly understood [30,52] (Figure 1).

The mechanisms that sort GPCRs into these trafficking pathways remain incompletely understood. Nevertheless, receptors contain sorting sequences within their intracellular domains that guide them toward specific routes. For example, GPCRs destined for recycling often possess cis-acting motifs, such as type 1 PDZ ligands, in their intracellular C-terminal tails [17,51]. Lysosomal sorting involves ubiquitination and deubiquitination mediated by E3 ligases and deubiquitinating enzymes (DUBs), as well as ESCRT-associated machinery, GPCR-associated sorting protein-1 (GASP-1), and the autophagy protein Beclin-2 [17,48,49,53,54].

2.4. Subcellular Localisation of GPCRs

GPCRs were initially thought to localize exclusively to the plasma membrane and to be activated only by extracellular ligands. However, recent studies demonstrate that many GPCRs also localize to intracellular compartments and signal from the Golgi membranes, endosomes (where they colocalize with β-arrestin and ERK1/2), mitochondria, cell division compartments, and the nuclear membrane, where they can regulate gene expression [18,55].

Trafficking of GPCRs to these intracellular sites requires specific sorting sequences located at the N-terminus or, in some cases, the C-terminus. During receptor insertion into the ER membrane, signal peptides are cleaved by signal peptidases. This early processing promotes proper folding and efficient transport through the secretory pathway.

Sorting signals for apical targeting can reside in cytoplasmic, transmembrane, or extracellular domains. Apical delivery often depends on association with lipid rafts or non-raft carriers and is frequently mediated by lectins that recognize N- and O-glycans and facilitate direct transport from the trans-Golgi network (TGN) to the apical membrane [52]. In the pancreas, examples of ciliary GPCRs include FFAR4 (free fatty acid receptor 4) and PTGER4 (prostaglandin E receptor 4), which localize to the primary cilia of α- and β-cells and regulate insulin and glucagon secretion in response to fatty acids or PGE₂ [56]. In contrast, basolateral sorting signals typically reside in cytoplasmic domains and consist of short peptide motifs, such as tyrosine-based (YxxØ) or dileucine-based sequences. These motifs are recognized by heterotetrameric adaptor protein complexes (AP-1, AP-3, and AP-4), which interact with clathrin to mediate trafficking [57].

Most GPCRs lack classical signal sequences; therefore, the mechanisms governing their trafficking to the plasma membrane or to intracellular compartments remain incompletely defined [37]. In general, GPCRs located in the ER, Golgi, endosomes, and outer nuclear membranes orient their N-terminus toward the lumen and their C-terminus toward the cytoplasm. In mitochondria, GPCRs are found in both the outer and inner membranes, with signaling domains oriented toward the matrix or intermembrane space [18,58].

3. Functional Roles in Pancreatic β-Cells

Pancreatic β-cells express numerous GPCRs that regulate key processes such as β-cell proliferation, apoptosis, and insulin secretion, thereby contributing to the maintenance of glucose homeostasis. Consequently, GPCRs represent important therapeutic targets for type 2 diabetes (T2D).

3.1. Regulation of β-Cell Survival, Proliferation, and Homeostasis

In pancreatic β-cells, GPCRs integrate hormonal, metabolic, and inflammatory cues to fine-tune signaling networks that regulate cell survival, proliferation, and functional homeostasis. Among these receptors, the leucine-rich repeat–containing receptor LGR4 (GPR48) supports β-cell survival, proliferation, and maturation. Knockdown reduces cell viability, whereas overexpression protects against cytokine-induced apoptosis through inhibition of the RANK–TRAF6/NF-κB pathway. Notably, β-cell–specific deletion impairs proliferation in females, while global knockout improves glucose tolerance, suggesting sex- or inflammation-dependent effects [59,60].

The GLP-1 receptor (GLP-1R) enhances glucose-stimulated insulin secretion (GSIS) and promotes proliferation through PI3K/PDX-1 signaling. Consistent with this protective function, activation of peroxisome proliferator-activated receptor β/δ (PPARβ/δ) increases GLP-1R expression, and its agonist GW501516 rescues lipotoxic β-cells via Akt/Bcl-2 pathways [61,62,63]. Similarly, GPR39, a zinc-sensing receptor, preserves β-cell mass and survival by regulating pancreatic and duodenal homeobox 1 (Pdx1) and hepatocyte nuclear factor 1α (HNF-1α). Knockout mice exhibit impaired insulin secretion and glucose tolerance, whereas Zn²⁺ activation protects against β-cell stress [64].

The short-chain fatty acid receptor FFAR2 also influences β-cell mass. Genetic deletion reduces islet size and increases apoptosis, whereas Gq-mediated signaling promotes proliferation, although reported effects vary [65]. In addition, GPR56 (ADGRG1), activated by collagen III, promotes survival through cAMP/PKA signaling and inhibition of RhoA/caspase-3 activity; receptor deletion increases cytokine-induced apoptosis [66,67]. Consistently, loss of GPRC6A decreases islet size, GSIS, and insulin content, and systemic knockout further induces insulin resistance [68,69]. Orphan receptors GPRC5B and GPRC5C display dual effects, as their downregulation reduces apoptosis but also limits proliferation [9,70]. The estrogen-sensitive receptor GPR30 (GPER1) likewise protects β-cells by preventing cytokine-induced apoptosis through estradiol or G-1 signaling, independently of nuclear estrogen receptors [71,72].

In contrast, some GPCRs exert detrimental effects. Activation of C-C motif chemokine receptor 9 (CCR9) by its ligand CCL25 does not alter basal caspase-3/7 activity but enhances cytokine-induced apoptosis, whereas the antagonist vercirnon blocks this response [73]. Prostaglandin E₂ receptors also show opposing roles. EP3 inhibition with the antagonist DG-041 enhances human β-cell proliferation, primarily via phospholipase Cγ1 phosphorylation, and EP3 signaling may suppress AKT/mTOR activity to limit growth. EP4 modulation alone has little effect; however, EP4 activation combined with EP3 blockade increases proliferation and improves β-cell survival under cytokine stress, likely through PKA-mediated signaling [74].

Collectively, receptors such as LGR4, GLP-1R, EP4, GPR39, FFAR2, GPR56, GPRC6A, GPRC5B/C, and GPR30 promote β-cell survival and proliferation, whereas EP3 and CCR9 compromise β-cell integrity by promoting apoptosis or inhibiting growth. These opposing actions highlight the central role of GPCR signaling in regulating β-cell metabolism and underscore its importance for maintaining β-cell mass and glucose homeostasis in diabetes (Figure 2).

3.2. Regulation of Insulin Secretion

By integrating hormonal, neuronal, and nutritional cues, several GPCRs directly or indirectly regulate β-cell function and insulin secretion. Their activation influences intracellular pathways such as cAMP or Ca²⁺ signaling, while also modulating incretin release from enteroendocrine cells.

Several GPCRs function as positive regulators of insulin secretion and contribute to the maintenance of glucose homeostasis. For example, GPR116, which is expressed in δ-cells, promotes somatostatin release through Gq/11–Ca²⁺ signaling. Whole-body deficiency reduces β-cell mass, insulin content, and insulin secretion, resulting in hyperglycemia [75]. GPR119, present in both β-cells and enteroendocrine L-cells, responds to lipids such as oleoylethanolamide (OEA) and to synthetic agonists by increasing cAMP levels, insulin secretion, and GLP-1 release in rodents [76,77,78]. However, β-cell–specific knockout models and human clinical studies suggest that most of its benefits arise from incretin-mediated effects rather than direct β-cell signaling.

Similarly, GPR120 (FFAR4), activated by free fatty acids or selective agonists, enhances GLP-1 secretion and directly stimulates insulin release through Ca²⁺-dependent pathways. Knockout mice develop hyperglycemia and impaired glucose tolerance, underscoring its physiological importance [78,79,80]. β₂-adrenergic receptors (β₂ARs) also support β-cell function and protect against age-related declines in insulin secretion. Receptor deletion reduces insulin release and causes glucose intolerance, whereas overexpression restores insulin secretion and improves glucose tolerance in aged animals [81].

Conversely, several GPCRs negatively regulate insulin secretion by integrating metabolic, hormonal, and inflammatory signals that suppress β-cell output and contribute to β-cell dysfunction. Olfactory receptor 109 (Olfr109), which recognizes insulin-derived peptides, reduces insulin secretion via Gi/cAMP signaling and promotes intra-islet inflammation through β-arrestin-1–STAT3–CCL2 pathways. Antagonism of this receptor improves glucose homeostasis in diabetic mice [82]. Brain angiogenesis inhibitor-3 (Bai3), activated by complement 1q-like-3 (C1ql3), similarly suppresses insulin release by inhibiting cAMP signaling. Knockdown enhances insulin secretion in both rodent and human islets, and genetic variants have been linked to type 2 diabetes (T2D) risk [83].

The orphan receptor GPR21 also contributes to insulin resistance and inflammation; its deletion improves glucose tolerance and reduces adipose and hepatic inflammation in high-fat diet–fed mice [84,85,86]. In addition, classical inhibitory GPCRs such as neuropeptide Y receptor type 1 (Y1R) and somatostatin receptors 1 and 5 (SSTR1/5) suppress insulin secretion through Gi-dependent signaling. Neuropeptide Y reduces cAMP production, whereas somatostatin limits Ca²⁺ influx by activating KATP channels [87].

Beyond these well-characterized receptors, several GPCRs exert context-dependent or modulatory effects. FFAR2 and FFAR3 can either stimulate or inhibit insulin secretion depending on whether signaling occurs through Gq or Gi pathways, with dose-dependent effects observed in human islets [88,89,90]. The α₂A-adrenergic receptor (ADRA2A) mediates adrenergic suppression of insulin release via Gi/o signaling, and genetic variants associated with receptor overexpression correlate with impaired secretion and increased T2D risk [91,92]. The A2B adenosine receptor (A2BAR) regulates β-cell–immune interactions: pharmacological antagonism enhances insulin release, whereas global knockout worsens insulin sensitivity by increasing macrophage-driven inflammation [93,94]. Additional inhibitory GPCRs include GPR14 (urotensin II receptor), which suppresses insulin release in a glucose-dependent manner [95,96]; GPR109A, which inhibits cAMP signaling under inflammatory stress [97]; and the melatonin receptors MT1 and MT2, where elevated MTNR1B expression reduces insulin secretion and is associated with increased T2D risk [98,99].

Together, these findings highlight the dual roles of GPCRs in β-cell physiology. Stimulatory receptors, including GPR119, GPR120, and β₂AR, enhance insulin secretion and support metabolic homeostasis, whereas inhibitory receptors such as Olfr109, Bai3, ADRA2A, and MTNR1B restrict insulin output and may promote glucose intolerance. The context-dependent activity of receptors such as FFAR2/3 further illustrates the complexity of GPCR signaling within islets. Selective activation of stimulatory pathways or blockade of inhibitory GPCRs may therefore represent promising therapeutic strategies to preserve β-cell function and improve glucose control (Figure 2).

3.3. GPCR Regulation of Glucose-Stimulated Insulin Secretion (GSIS)

While insulin secretion encompasses responses to multiple nutrients and modulators, GSIS describes the insulin release that is uniquely dependent on glucose levels. Several GPCRs regulate GSIS and β-cell function through distinct signaling pathways. For example, GPR56, the most abundant in human islets, enhances GSIS and survival via collagen III–cAMP/PKA signaling, though its expression is reduced in T2D [66,100]. Similarly, succinate receptor 1 (SUCNR1), activated by succinate under hyperglycemia, stimulates GSIS through G_q-PKC, and its β-cell deletion aggravates glucose intolerance in HFD-fed mice [101,102]. Thus, succinate functions as a hormone-like metabolite, potentiating insulin secretion under hyperglycemic conditions similar to the incretins GLP-1 and GIP. Although the mechanisn is still under investigation, the muscarinic receptor subtype 3 (M3R) also potentiates GSIS at high glucose, with β-cell–specific deletion impairing secretion [103,104,105]. In INS-1 and MIN6 cells, trace amine-associated receptor 1 (TAAR1) stimulates GSIS via Gs coupling. This increases insulin secretion through PKA/Epac signaling, activates MAPK pathways, induces CREB–Irs-2, and promotes β-cell proliferation. Activation of TAAR1 triggers calcium influx from the outside and intracellular calcium release, necessary for the activation of ERK1/2 [106]. By contrast, cannabinoid receptors 1 and 2 (CB1/CB2) inhibit GSIS through suppression of Ca²⁺ influx [107,108]. GPRC6A is activated by L-arginine, an insulin secretagogue, and contributes to pancreatic β-cell function. L-arginine administration led to decreased ERK activation in vivo, and a reduction in cAMP accumulation and insulin secretion ex vivo, in islets isolated from Gprc6a^-/-mice. These findings suggest that L-arginine may mediate GSIS through GPRC6A-dependent cAMP signaling. Down-regulation of GPRC5B/GPRC5C was associated with a slight decrease in GSIS and cAMP levels in mouse pancreatic islets. Down-regulation of GPRC5C also attenuated the GSIS-stimulating and cAMP-increasing effects of all-trans retinoic acid (ATRA) [69,70,109,110,111,112].

Incretin receptors remain central to β-cell physiology. GLP-1 enhances (GSIS) and maintains glucose homeostasis through GLP-1R activation. In intact islets, this effect is mediated primarily by Gs-dependent cAMP signaling rather than Ca²⁺-dependent pathways. In intact islets, this effect is primarily mediated by Gs-dependent cAMP signaling rather than Ca²⁺-dependent pathways. In mouse β-cells, physiological GLP-1 levels recruit β-arrestin 2 (ARRB2), which attenuates insulin secretion by uncoupling GLP-1R from cAMP/PKA signaling, whereas pharmacological GLP-1 concentrations require ARRB2 for ERK/CREB activation. By contrast, GIP receptor–mediated potentiation of insulin secretion depends on ARRB2, which promotes cytoskeletal remodeling and F-actin depolymerization, thereby facilitating insulin granule fusion with the plasma membrane [113,114,115,116]. In INS-1 and MIN6 cells, (TAAR1) stimulates GSIS via Gs coupling. This increases insulin secretion through PKA/Epac signaling, activates MAPK pathways, induces CREB–Irs-2, and promotes β-cell proliferation [106]. The proposed signaling pathway involves GPR40–PLC/PKC–TRPC3. Binding of free fatty acids (FFAs) to GPR40 activates PLC and increases nonselective cation channel (NSCC) currents by opening TRPC3 channels, leading to plasma membrane depolarization and potentiation of GSIS in pancreatic β-cells [117]. In addition, GPR40 activation enhances the effects of FFAs on glucose and lipid metabolic remodeling, promoting the generation of TCA cycle and lipid-derived intermediates that stimulate insulin secretion [118]. GPR55 enhances Ca²⁺-dependent GSIS and supports β-cell survival [119,120,121], while the 5-HT2B receptor promotes GSIS via Ca²⁺ signaling and mitochondrial activity [122]. GPR142 activation increases cAMP and GSIS, whereas its knockdown in mouse islets induces inflammatory responses, upregulates genes linked to glucotoxicity and β-cell dysfunction, and reduces expression of key transcription factors (Pdx1, Pax6) [123,124,125].

The GPR30 agonist G-1 enhances insulin secretion and suppresses glucagon and somatostatin release, mimicking 17β-estradiol. Female GPR30⁻/⁻ mice show impaired glucose- and estrogen-stimulated insulin secretion, yet are protected from diet-induced obesity and insulin resistance, whereas male knockouts develop obesity, dyslipidemia, insulin resistance, and inflammation [51,72,126,127]. Thus, GPR30 regulates β-cell function and metabolism in a sex-dependent manner, and its interaction with ERα/ERβ remains unclear.

GPR75 is expressed in β-cells and may signal through G_q, prompting studies on its role in insulin secretion. C-C motif chemokine ligand 5 (CCL5), a putative ligand, stimulated insulin release in mouse and human islets with species-specific differences: in mice, CCL5 enhanced insulin secretion at high glucose, while in humans, the effect was only transient. In vivo, CCL5 improved glucose tolerance in ob/ob and lean mice by increasing insulin secretion without altering insulin sensitivity. However, since CCL5 is not a confirmed GPR75 ligand, these effects cannot be attributed to GPR75 [128]. Moreover, Akbari et al. [129] reported higher plasma insulin in WT versus Gpr75 KO mice after 14 weeks of HFD, leaving the role of GPR75 in insulin secretion unresolved. Other receptors, such as GPR105 [130,131], GPR27 [132,133], and GPR54 [115,134,135,136], show variable or context-dependent effects with sometimes conflicting results. These findings highlight GPCRs as key regulators of insulin secretion and β-cell survival, suggesting that selective activation of stimulatory pathways or inhibition of suppressive signaling may help preserve β-cell function and improve glucose homeostasis (Figure 2).

Genome-wide association studies (GWAS) have implicated GPCRs in diabetes by identifying numerous susceptibility loci near GPCR genes. These findings highlight GPCRs as important therapeutic targets for type 2 diabetes (T2D), given their roles in β-cell dysfunction, obesity-associated inflammation, and insulin resistance.

GWAS in T1D have also identified risk loci involving GPCR-related genes that participate in immune pathways and may contribute to autoimmune destruction of pancreatic β-cells. For example, specific alleles of the olfactory receptor OR14J1C have been associated with increased T1D risk, potentially acting independently or in combination with HLA genes [137]. In addition, GPR26 has been linked to protection against hyperglycemia, although its expression is reduced under high-glucose conditions, which may exacerbate inflammation in diabetic patients [138]. GWAS in T2D have further identified GPCR single nucleotide polymorphisms (SNPs) that influence disease risk, progression, and drug response. Examples include variants in GIPR, GLP-1R, MC4R, and FFARs [139,140], as well as MTNR1B [141] and GPR26 [138]. These genetic variants affect insulin secretion, appetite regulation, β-cell function, glucose homeostasis, and inflammation, underscoring the importance of GPCRs for personalized T2D treatment strategies.

Among the GPCRs discussed in this review, several have emerged as established therapeutic targets for T2D. A prominent example is GLP-1R. The GLP-1R agonist exenatide was approved in 2005 for T2D treatment, followed by additional peptide-based agonists such as liraglutide and dulaglutide. More recently, several small-molecule GLP-1R agonists have entered clinical development, including danuglipron and orforglipron, both of which completed phase II clinical trials in 2023, while another candidate, compound 29 (DA-302168S), is expected to enter clinical development in 2025 [142]. Small-molecule agonists targeting GPR119 have also been widely investigated; compounds such as MBX-2982, APD668, and HG043 demonstrated efficacy in preclinical models, and others, including DS-8500a and GSK1292263, advanced to human trials, where they improved GLP-1 secretion and glycemic control [143]. Similarly, several potent and selective GPR120 agonists, including compounds 5g, 11b, 18f, and LXT34, have shown antidiabetic activity in animal studies [144]. The selective GPR40 (FFAR1) agonist fasiglifam (TAK-875) progressed to phase III trials but was discontinued due to hepatotoxicity concerns. Ongoing research aims to clarify the mechanisms underlying this toxicity and to develop safer GPR40-targeted agents, as this receptor remains one of the most promising GPCR targets for T2D therapy [145]. Beyond these examples, additional druggable GPCRs—including GPR55, GPR142, and cannabinoid receptors CB1 and CB2—are under active investigation, with agonists or antagonists in various stages of preclinical and clinical development. Many of the GPCRs summarized in

Table 1. Functional roles of GPCRs in human pancreatic β-cells.

Receptor Name	Class	Transduction Mechanism	Ligand	Function	Cell/Animal Model	Study Method	Reference
Glucagon-like peptide 1 receptor (GLP-1R)	B	Gs Gq/G11	Glucagon-like peptide-1 (GLP1), Glucagon	Support β-cell survival and proliferation	- INS-1E cells - mouse β-cells - rat, mouse and human islets	- q-PCR - Western Blot	[61,63,116,117]
GPR39	A	Gq/G11 Gs G12/G13	Zn2+		- pancreatic acinar cell line AR42J - mouse islets - mice	- q-PCR - immunofluorescence	[64]
GPR56/ADGRG1	Adhesion	Gq/G11 G12/G13	Collagen III		- INS-1 cells - MIN6 cells - human and mouse islets - mice	- RNA sequencing - Western Blot - q-PCR - immunofluorescence	[66,67,100]
GPCR family C group 6 member A (GPRC6A)	C	Gq/G11	Basic amino acids, divalent and trivalent cations, the bone-derived peptide osteocalcin, and the steroid hormone testosterone		- MIN6 cells - βTC-6 cells - mouse islets - mice	- q-PCR - Western Blot	[68,69,111,112]
G protein-coupled receptor class C group 5 member B(GPRC5B)	C	Gi ?	Unknown		- MIN6 cells - human islets - mice	- q-PCR - Western Blot - immunofluorescence	[70,110]
G protein-coupled receptor class C group 5 member C (GPRC5C)	C	Gs ?	Unknown		- MIN6 cells - mouse and human islets	- RNA sequencing - Western Blot - q-PCR - immunofluorescence	[9]
Estrogen-sensitive GPR30 (GPER1)	A	Gi/Go Gs	17β-estradiol		- human islets - mice	- q-PCR - Western Blot - immunofluorescence - microarray	[71,72]
C-C motif chemokine receptor 9 (CCR9)	A	Gi/Go	C-C motif chemokine ligand 25 (CCL25)	Compromise β-cell integrity by promoting apoptosis or inhibiting growth	- human and mouse islets	- q-PCR	[73]
Prostaglandin E2-subtypes (EP3, EP4)	A	Gi/Go	Prostaglandin E2 (PGE2)		- mouse and human islets	- q-PCR	[74]
GPR116	Adhesion	Gq/G11	Peptides derived from the Stachel sequence: TSFSILMSPDSPD	Stimulates insulin secretion/ Promote insulin secretion and metabolic balance	- mouse islets - mice	- RNA sequencing - q-PCR - in situ hybridization combined with immunofluorescence	[75]
GPR119	A	Gs	Oleoylethanolamide, monooleoylglycerol		- BRIN-BD11 cells - insulinoma cell lines HIT-T15 (hamster), NIT-1 (mouse), and RIN-5F (rat) - mouse islets - mice	- immunofluorescence - Northern Blot	[76,77]
GPR120 (FFAR4)	A	Gq/G11	Long-chain fatty acids		- BRIN-BD11 cells - INS-1 cells - rat and mouse islets - mice	- q-PCR - in situ hybridization - immunofluorescence - Western Blot	[78,79,80]
β2-adrenergic receptors (β2ARs)	A	Gs Gi/Go	Adrenaline Noradrenaline		- INS-1E cells - mouse islets - mice	- q-PCR	[81]
Olfactory receptor 109 (Olfr109)	A	Gi/Go	Pancreatic peptide insB: 9–23	Inhibits insulin secretion/ Restrict insulin output, often contributing to glucose intolerance and T2D	- human islets - mice	- RNAscope - in situ hybridization - q-PCR - Western Blot - immunofluorescence - RNA sequencing	[82]
Brain angiogenesis inhibitor-3 (BAI3)	Adhesion	Gi/Go ?	Complement 1q-like-3 (C1ql3)		- INS1(832/13) β-cells - mouse and human islets	- q-PCR - immunofluorescence	[83]
GPR21	A	Gq/G11 ?	Unknown		- mice	- q-PCR	[84,85,86]
Neuropeptide Y receptor type 1 (Y1R) Somatostatin receptors 1/5 (SSTR1/5)	A	Gi/Go	Neuropeptide Y Somatostatin		- mouse islets	-	[87]
Free fatty acid receptor 2/3 (FFAR2/3)	A	Gq/G11 Gi/Go	Short chain fatty acids (acetate)	Exert context-dependent or modulatory effects (positive/negative regulators of insulin secretion)	- MIN6 cells - INS-1E cells - mouse and human islets - mice	- immunofluorescence - q-PCR	[65,88,89,90]
Alpha-2A adrenergic receptor (ADRA2A)	A	Gi/Go Gs	Adrenaline Noradrenaline		- mouse β cells - mouse and rat islets - mice - rats	- q-PCR - Western Blot	[91,92]
A2B adenosine receptor (A2BAR)	A	Gs Gq/G11	Adenosine		- MIN6 cells - mouse islets - mice	- q-PCR - Western Blot	[93,94]
Urotensin II receptor (GPR14)	A	Gq/G11	Urotensin II		- rats	-	[95,96]
GPR109A	A	Gi/Go	Nicotinic acid		- mouse islets - mice	- q-PCR - in situ hybridization - immunofluorescence	[97]

and

Table 2. Modulation of glucose-stimulated insulin secretion (GSIS) by human GPCRs.

Receptor Name	Class	Transduction Mechanism	Ligand	Function	Cell/Animal Model	Study Method	Reference
GPR56/ADGRG1	Adhesion	Gq/G11 G12/G13	Collagen III	Stimulate GSIS	- INS-1 cells - MIN6 cells - human and mouse islets - mice	- RNA sequencing - Western Blot - q-PCR - immunofluorescence	[66,67,100]
Succinate receptor 1 (SUCNR1/GPR91)	A	Gi/Go Gq/G11	Succinate		- EndoC-βH1 cells - MIN6 cells - mice	- q-PCR - Western Blot - immunohistochemistry	[101,102]
Muscarinic receptor subtype 3 (M3R)	A	Gq/G11	Acetylcholine		- mouse islets - mice	- q-PCR	[103,104,105]
Trace-amine associated receptor 1 (TAAR1)	A	Gs Gq/G11	Trace amines, monoamine neurotransmitters, thyronamine 3-iodothyronamine		- INS-1 cells - MIN6 cells	- q-PCR	[106]
Glucagon-like peptide 1 receptor (GLP-1R)	B	Gs Gq/G11	Glucagon-like peptide-1 (GLP1), Glucagon		- INS-1E cells - mouse β-cells - rat, mouse and human islets	- q-PCR - Western Blot	[61,63,96,116]
Glucose-dependent insulinotropic polypeptide receptor (GIP-R)	B	Gs	Gastric inhibitory polypeptide (GIP)		- mouse β-cells - mouse and human islets	- q-PCR	[116]
GPR40 (FFAR1)	A	Gq/G11	Long-chain fatty acids		- INS-1 832/3 cells - mouse β-cells - rat islets - rats and mice	-	[117,118]
GPR55	A	Gq/G11 G12/G13	Cannabinoid endogenous ligands (endocannabinoids) and other non-cannabinoid lipid transmitters		- BRIN-BD11 cells - MIN6 β-cells - mouse and human islets - mice	- Western Blot - immunofluorescence	[119,120,121]
5-hydroxytryptamine 2B receptor (5-HT2B)	A	Gq/G11	Serotonin		- INS-1(832/13) cells - human islets	- q-PCR - immunohistochemistry - Western Blot - RNA sequencing	[122]
GPR142	A	Gs Gq/G11	L-tryptophan		- INS-1 cell culture - mouse and human islets - mice	- q-PCR - immunofluorescence - in situ hybridization	[123,124,125]
Estrogen-sensitive GPR30 (GPER1)	A	Gi/Go Gs	17β-estradiol		- human islets - mice	- q-PCR - Western Blot - immunofluorescence - microarray	[71,72]
Cannabinoid receptors 1 and 2 (CB1/CB2)	A	-Gi/Go -Gs	Cannabinoids	Inhibit GSIS	- mouse β-cells - mouse islets - mice	- q-PCR - immunofluorescence	[107,108]
GPR75	A	Gq/G11 ?	Unknown	Variable or context-dependent effects with sometimes conflicting results	- MIN6 cells - mouse islets - mice	- q-PCR - immunohistochemistry - immunofluorescence - Western Blot - RNA sequencing	[128,129]
GPR105	A	Gi/Go	UDP and UDP sugars		- mouse islets - mice	- q-PCR	[130,131]
GPR27	A	Gq/G11 ?	Unknown		- MIN6 cells - mouse islets - mice	- q-PCR	[132,133]
GPR54	A	Gq/G11	Kisspeptins		- MIN6 cells - mouse and human islets - rats	- q-PCR - immunohistochemistry	[134,135,136]

represent potential future therapeutic targets, given their established roles in glucose metabolism and insulin secretion (Table 1 and Table 2).

4. Conclusions

GPCRs play multifaceted roles in pancreatic β-cell biology, regulating insulin secretion, proliferation, and apoptosis through diverse G-protein–dependent and –independent pathways. Stimulatory receptors enhance β-cell output and resilience, while inhibitory receptors suppress secretion and often aggravate metabolic dysfunction. Many GPCRs display context-specific or species-dependent actions, complicating their translation into clinical therapies. Importantly, receptor synthesis, trafficking, and subcellular localization critically determine GPCR availability, signaling specificity, and functional outcomes in β-cells. Dysregulation can impair insulin secretion or compromise β-cell survival, underscoring the need to consider receptor cell biology alongside pharmacology. While incretin-based therapies targeting GLP-1R demonstrate the therapeutic potential of GPCR modulation, future research should focus on clarifying orphan receptor functions, defining signaling bias, and unraveling the molecular mechanisms governing GPCR biosynthesis and spatial distribution. Such insights will be essential for the development of next-generation GPCR-targeted strategies to preserve β-cell function and improve glucose homeostasis in diabetes.