The molecular basis for high affinity of a universal ligand for human bombesin receptor (BnR) family members

Genetic and pharmacological inhibition of GRPR protects against acute kidney injury via attenuating renal inflammation and necroptosis

Gastrin-releasing peptide (GRP) binds to its receptor (GRP receptor [GRPR]) to regulate multiple biological processes, but the function of GRP/GRPR axis in acute kidney injury (AKI) remains unknown. In the present study, GRPR is highly expressed by tubular epithelial cells (TECs) in patients or mice with AKI, while histone deacetylase 8 may lead to the transcriptional activation of GRPR. Functionally, we uncovered that GRPR was pathogenic in AKI, as genetic deletion of GRPR was able to protect mice from cisplatin- and ischemia-induced AKI. This was further confirmed by specifically deleting the GRPR gene from TECs in GRPRFlox/Flox//KspCre mice. Mechanistically, we uncovered that GRPR was able to interact with Toll-like receptor 4 to activate STAT1 that bound the promoter of MLKL and CCL2 to induce TEC necroptosis, necroinflammation, and macrophages recruitment. This was further confirmed by overexpressing STAT1 to restore renal injury in GRPRFlox/Flox/KspCre mice. Concurrently, STAT1 induced GRP synthesis to enforce the GRP/GRPR/STAT1 positive feedback loop. Importantly, targeting GRPR by lentivirus-packaged small hairpin RNA or by treatment with a novel GRPR antagonist RH-1402 was able to inhibit cisplatin-induced AKI. In conclusion, GRPR is pathogenic in AKI and mediates AKI via the STAT1-dependent mechanism. Thus, targeting GRPR may be a novel therapeutic strategy for AKI.

GRPR down-regulation inhibits spermatogenesis through Ca2+ mediated by PLCβ/IP3R signaling pathway in long-term formaldehyde-exposed rats

Formaldehyde (FA), which is known as an air pollutant, has been proven to induce male infertility. However, the underlying mechanism of FA-induced male infertility remains elusive. In this study, 24 male SD rats were exposed to different levels of FA (0, 0.5, 2.46, and 5 mg/m3) for eight consecutive weeks. Through HE staining and sperm smear, we observed that FA exposure resulted in spermatogenic injury and the sperm quality decreased in rats. The qRT-PCR and Western blot analysis further revealed that GRPR was down-regulated in testicular tissues of FA-exposed rats as well as primary spermatogenic cells. Meanwhile, ZDOCK uncovered an interaction between GRPR and PLCβ. In addition, the CCK8, Fluo 3-AM and Flow cytometry results showed that FA exposure suppressed the expression of GRPR, PLCβ and IP3R, consequently reducing the Ca2+ concentration in spermatogenic cells, inducing apoptosis and inhibiting proliferation of spermatogenic cells. Moreover, rescue experiments confirmed that promoting GRPR could improve intracellular Ca2+ concentration by upregulating PLCβ and IP3R, partially reducing the apoptosis and promoting the proliferation of FA-treated spermatogenic cells. These findings revealed that GRPR participates in spermatogenesis through Ca2+ mediated by the PLCβ/IP3R signaling pathway in FA-exposed rats.

Structures of human gastrin-releasing peptide receptors bound to antagonist and agonist for cancer and itch therapy

Gastrin releasing peptide receptor (GRPR), a member of the bombesin (BBN) G protein-coupled receptors, is aberrantly overexpressed in several malignant tumors, including those of the breast, prostate, pancreas, lung, and central nervous system. Additionally, it also mediates non-histaminergic itch and pathological itch conditions in mice. Thus, GRPR could be an attractive target for cancer and itch therapy. Here, we report the inactive state crystal structure of human GRPR in complex with the non-peptide antagonist PD176252, as well as two active state cryo-electron microscopy (cryo-EM) structures of GRPR bound to the endogenous peptide agonist gastrin-releasing peptide and the synthetic BBN analog [D-Phe⁶, β-Ala¹¹, Phe¹³, Nle¹⁴] Bn (6-14), in complex with G_q heterotrimers. These structures revealed the molecular mechanisms for the ligand binding, receptor activation, and G_q proteins signaling of GRPR, which are expected to accelerate the structure-based design of GRPR antagonists and agonists for the treatments of cancer and pruritus.

The Nonpeptide Agonist MK-5046 Functions As an Allosteric Agonist for the Bombesin Receptor Subtype-3

Allosteric ligands of various G-protein-coupled receptors are being increasingly described and are providing important advances in the development of ligands with novel selectivity and efficacy. These unusual properties allow expanded opportunities for pharmacologic studies and treatment. Unfortunately, no allosteric ligands are yet described for the bombesin receptor family (BnRs), which are proposed to be involved in numerous physiologic/pathophysiological processes in both the central nervous system and peripheral tissues. In this study, we investigate the possibility that the bombesin receptor subtype-3 (BRS-3) specific nonpeptide receptor agonist MK-5046 [(2S)-1,1,1-trifluoro-2-[4-(1H-pyrazol-1-yl)phenyl]-3-(4-[[1-(trifluoromethyl)cyclopropyl]methyl]-1H-imidazol-2-yl)propan-2-ol] functions as a BRS-3 allosteric receptor ligand. We find that in BRS-3 cells, MK-5046 only partially inhibits iodine-125 radionuclide (125I)-Bantag-1 [Boc-Phe-His-4-amino-5-cyclohexyl-2,4,5-trideoxypentonyl-Leu-(3-dimethylamino) benzylamide N-methylammonium trifluoroacetate] binding and that both peptide-1 (a universal BnR-agonist) and MK-5046 activate phospholipase C; however, the specific BRS-3 peptide antagonist Bantag-1 inhibits the action of peptide-1 competitively, whereas for MK-5046 the inhibition is noncompetitive and yields a curvilinear Schild plot. Furthermore, MK-5046 shows other allosteric behaviors, including slowing dissociation of the BRS-3 receptor ligand 125I-Bantag-1, dose-inhibition curves being markedly affected by increasing ligand concentration, and MK-5046 leftward shifting the peptide-1 agonist dose-response curve. Lastly, receptor chimeric studies and site-directed mutagenesis provide evidence that MK-5046 and Bantag-1 have different binding sites determining their receptor high affinity/selectivity. These results provide evidence that MK-5046 is functioning as an allosteric agonist at the BRS-3 receptor, which is the first allosteric ligand described for this family of receptors. SIGNIFICANCE STATEMENT: G-protein-coupled receptor allosteric ligands providing higher selectivity, selective efficacy, and safety that cannot be obtained using usual orthosteric receptor-based strategies are being increasingly described, resulting in enhanced usefulness in exploring receptor function and in treatment. No allosteric ligands exist for any of the mammalian bombesin receptor (BnR) family. Here we provide evidence for the first such example of a BnR allosteric ligand by showing that MK-5046, a nonpeptide agonist for bombesin receptor subtype-3, is functioning as an allosteric agonist.

Design, synthesis and biological evaluation of novel 1, 3, 4-oxadiazole PD176252 analogues as potential GRPR inhibitors

Background:

GRPR is over-expressed in cancer cells and is a potential drug target for the treatment of cancer. PD176252, as the most representative non-peptide inhibitor of GRPR, can inhibit the growth of cancer cells, but its low selectivity to cancer cells and normal cells limits its further application.

Objective:

The aim of this study was to design and synthesize novel GRPR inhibitor with stronger anti-cancer activity and higher affinity with GRPR than the lead compound PD176252.

Methods:

A series of 1, 3, 4-oxadiazole derivatives as PD176252 analogues (4a-4j, 6a-6q) were synthesized and investigated their cytotoxic activity on four cancer lines with high expression of GRPR (gastric (HGC-27), colon (HCT-116), prostate (PC-3), and lung (A549)) and one human cell line (gastric mucosal epithelial (GES-1)) by MTT assay. Flow cytometry analysis and Western Blot were used to determine whether the compound induced programmed apoptosis of cancer cells. Competitive binding experiment was used to verify the affinity between GRPR and the optimal compound.

Results:

Compound 6m exhibited significant growth inhibition on all tested cancer cell lines, especially gastric cancer cells (HGC-27 cellular IC50 0.37±0.04μM). Also, the selectivity of 6m to HGC-27 was much higher than that of PD176252. Flow cytometric analysis and Western Blot proved that 6m significantly promoted the apoptosis of HGC-27 cells. Moreover, competitive binding experiment confirmed the close binding of 6m with GRPR, which indicated 6m with a higher affinity than lead compound PD176252.

Conclusion:

Our results suggested that 6m, as a novel GRPR inhibitor, had a higher affinity with GRPR and potential anti-cancer effect than PD176252, which can be used as a template for further optimization.

Molecular basis for high affinity and selectivity of peptide antagonist, Bantag-1, for the orphan BB3 receptor

Bombesin-receptor-subtype-3 (BB3 receptor) is a G-protein-coupled-orphan-receptor classified in the mammalian Bombesin-family because of high homology to gastrin-releasing peptide (BB2 receptor)/neuromedin-B receptors (BB1 receptor). There is increased interest in BB3 receptor because studies primarily from knockout-mice suggest it plays roles in energy/glucose metabolism, insulin-secretion, as well as motility and tumor-growth. Investigations into its roles in physiological/pathophysiological processes are limited because of lack of selective ligands. Recently, a selective, peptide-antagonist, Bantag-1, was described. However, because BB3 receptor has low-affinity for all natural, Bn-related peptides, little is known of the molecular basis of its high-affinity/selectivity. This was systematically investigated in this study for Bantag-1 using a chimeric-approach making both Bantag-1 loss-/gain-of-affinity-chimeras, by exchanging extracellular (EC) domains of BB3/BB2 receptor, and using site-directed-mutagenesis. Receptors were transiently expressed and affinities determined by binding studies. Bantag-1 had >5000-fold selectivity for BB3 receptor over BB2/BB1 receptors and substitution of the first EC-domain (EC1) in loss-/gain-of affinity-chimeras greatly affected affinity. Mutagenesis of each amino acid difference in EC1 between BB3 receptor/BB2 receptor showed replacement of His(107) in BB3 receptor by Lys(107) (H107K-BB3 receptor-mutant) from BB2 receptor, decreased affinity 60-fold, and three replacements [H107K, E11D, G112R] decreased affinity 500-fold. Mutagenesis in EC1's surrounding transmembrane-regions (TMs) demonstrated TM2 differences were not important, but R127Q in TM3 alone decreased affinity 400-fold. Additional mutants in EC1/TM3 explored the molecular basis for these changes demonstrated in EC1, particularly important is the presence of aromatic-interactions by His(107), rather than hydrogen-bonding or charge-charge interactions, for determining Bantag-1 high affinity/selectivity. In regard to Arg(127) in TM3, both hydrogen-bonding and charge-charge interactions contribute to the high-affinity/selectivity for Bantag-1.

GRPR-targeted Protein Contrast Agents for Molecular Imaging of Receptor Expression in Cancers by MRI

Gastrin-releasing peptide receptor (GRPR) is differentially expressed on the surfaces of various diseased cells, including prostate and lung cancer. However, monitoring temporal and spatial expression of GRPR in vivo by clinical MRI is severely hampered by the lack of contrast agents with high relaxivity, targeting capability and tumor penetration. Here, we report the development of a GRPR-targeted MRI contrast agent by grafting the GRPR targeting moiety into a scaffold protein with a designed Gd³⁺ binding site (ProCA1.GRPR). In addition to its strong binding affinity for GRPR (K_d = 2.7 nM), ProCA1.GRPR has high relaxivity (r₁ = 42.0 mM⁻¹s⁻¹ at 1.5 T and 25 °C) and strong Gd³⁺ selectivity over physiological metal ions. ProCA1.GRPR enables in vivo detection of GRPR expression and spatial distribution in both PC3 and H441 tumors in mice using MRI. ProCA1.GRPR is expected to have important preclinical and clinical implications for the early detection of cancer and for monitoring treatment effects.

Drug delivery and release systems for targeted tumor therapy

Most toxic agents currently used for chemotherapy show a narrow therapeutic window, because of their inability to distinguish between healthy and cancer cells. Targeted drug delivery offers the possibility to overcome this issue by selectively addressing structures on the surface of cancer cells, therefore reducing undesired side effects. In this broad field, peptide-drug conjugates linked by intracellular cleavable structures have evolved as highly promising agents. They can specifically deliver toxophores to tumor cells by targeting distinct receptors overexpressed in cancer. In this review, we focus on these compounds and describe important factors to develop a highly efficient peptide-drug conjugate. The necessary properties of tumor-targeting peptides are described, and the different options for cleavable linkers used to connect toxic agents and peptides are discussed, and synthetic considerations for the introduction of these structures are reported. Furthermore, recent examples and current developments of peptide-drug conjugates are critically evaluated with a special focus on the applied linker structures and their future use in cancer therapy.