Activation of bombesin receptor Subtype-3 by [D-Tyr6,β-Ala11,Phe13,Nle14]bombesin6-14 increased glucose uptake and lipogenesis in human and rat adipocytes

Structural insights into ligand recognition, selectivity, and activation of bombesin receptor subtype-3

Bombesin receptor subtype-3 (BRS3) is an important orphan G protein-coupled receptor that regulates energy homeostasis and insulin secretion. As a member of the bombesin receptor (BnR) family, the lack of known endogenous ligands and high-resolution structure has hindered the understanding of BRS3 signaling and function. We present two cryogenic electron microscopy (cryo-EM) structures of BRS3 in complex with the heterotrimeric Gq protein in its active states: one bound to the pan-BnR agonist BA1 and the other bound to the synthetic BRS3-specific agonist MK-5046. These structures reveal the architecture of the orthosteric ligand pocket underpinning molecular recognition and provide insights into the structural basis for BRS3's selectivity and low affinity for bombesin peptides. Examination of conserved micro-switches suggests a shared activation mechanism among BnRs. Our findings shed light on BRS3's ligand selectivity and signaling mechanisms, paving the way for exploring its therapeutic potential for diabetes, obesity, and related metabolic disorders.

Discovery of oridonin as a novel agonist for BRS-3

BACKGROUND

Bombesin Receptor Subtype-3 (BRS-3, Bombesin-like receptor, BB₃) is an orphan G-protein coupled receptor (GPCR). Recent studies have shown that BRS-3 played a vital role in glucose regulation, insulin secretion, and energy homeostasis. Therefore, discovering more novel exogenous ligands with diverse structures for BRS-3 will be of great importance for target validation and drug development.

PURPOSE

In this study, we aim to discover new agonists of BRS-3 from our natural compound libraries, providing a new probe to study the function of BRS-3.

STUDY DESIGN

Multiple cell-based assays and in vivo experiments were performed to identify the new ligand.

METHODS

BRS-3 overexpression cells were coupled with FLIPR assay, homogeneous time-resolved fluorescence (HTRF) IP-ONE assay, dynamic mass redistribution (DMR) assay, β-arrestin2 recruitment assay, and western blot to determine receptor activation and downstream signaling events. To further validate the target of BRS-3, a series of in vitro and in vivo experiences were conducted, including glucose uptake, glucose transporter type 4 (GLUT4) transportation in C2C12, and oral glucose tolerance test (OGTT) in mice.

RESULTS

We discovered and identified oridonin as a novel small molecule agonist of BRS-3, with a moderate affinity (EC₅₀ of 2.236 × 10^-7 M in calcium mobilization assay), specificity, and subtype selectivity. Further in vitro and in vivo tests demonstrated that oridonin exerted beneficial effects in glucose homeostasis through activating BRS-3.

CONCLUSIONS

Oridonin, as the discovered new ligand of BRS-3, provides a valuable tool compound to investigate BRS-3's function, especially for target validation in type 2 diabetes and obesity. Oridonin is promising as a lead compound in the treatment of metabolic disorders. Compared to the known agonists of BRS-3, we can take advantage of the multiple reported pharmacological activities of ODN as a natural product and assess whether these pharmacological activities are regulated by BRS-3. This may facilitate the discovery of novel functions of BRS-3.

Bombesin Receptor Subtype-3 in Human Diseases

This review summarizes the recent findings of the roles of bombesin receptor subtype-3 (BRS-3) in various patho-physiological conditions. Studies have demonstrated that two mammalians bombesin-like peptides, GRP and NMB, exhibit a large range of functions by binding to three receptors. Knockout studies showed that the mice BRS-3 has important effects on tumor growth, energy homeostasis, glucose regulation, satiety, and lung development (1,7). BRS-3 is an orphan receptor whose natural ligand is unknown. However, several agonists and antagonists have been synthesized which facilitate its characterization, (D-Tyr6, β-Ala11, Phe13, Nle14) Bn-(6-14) and MK-5046 are agonists, whereas ML-18 and Bantag-1 are antagonists. With the development of several selective, high-affinity BRS-3 agonists and antagonists, recent studies provided some insights into the biological effects of BRS-3 in several disease states including lung cancer, obesity, diabetes mellitus, asthma, and kidney diseases.