The common C-terminal sequences of substance P and neurokinin A contact the same region of the NK-1 receptor

The Repurposing of Non-Peptide Neurokinin-1 Receptor Antagonists as Antitumor Drugs: An Urgent Challenge for Aprepitant

The substance P (SP)/neurokinin-1 receptor (NK-1R) system is involved in cancer progression. NK-1R, activated by SP, promotes tumor cell proliferation and migration, angiogenesis, the Warburg effect, and the prevention of apoptosis. Tumor cells overexpress NK-1R, which influences their viability. A typical specific anticancer strategy using NK-1R antagonists, irrespective of the tumor type, is possible because these antagonists block all the effects mentioned above mediated by SP on cancer cells. This review will update the information regarding using NK-1R antagonists, particularly Aprepitant, as an anticancer drug. Aprepitant shows a broad-spectrum anticancer effect against many tumor types. Aprepitant alone or in combination therapy with radiotherapy or chemotherapy could reduce the sequelae and increase the cure rate and quality of life of patients with cancer. Current data open the door to new cancer research aimed at antitumor therapeutic strategies using Aprepitant. To achieve this goal, reprofiling the antiemetic Aprepitant as an anticancer drug is urgently needed.

Neurokinin-1 Receptor (NK-1R) Antagonists as a New Strategy to Overcome Cancer Resistance

Nowadays, the identification of new therapeutic targets that allow for the development of treatments, which as monotherapy, or in combination with other existing treatments can contribute to improve response rates, prognosis and survival of oncologic patients, is a priority to optimize healthcare within sustainable health systems. Recent studies have demonstrated the role of Substance P (SP) and its preferred receptor, Neurokinin 1 Receptor (NK-1R), in human cancer and the potential antitumor activity of NK-1R antagonists as an anticancer treatment. In this review, we outline the relevant studies published to date regarding the SP/NK-1R complex as a key player in human cancer and also evaluate if the repurposing of already marketed NK-1R antagonists may be useful in the development of new treatment strategies to overcome cancer resistance.

C3-heteroaroyl cannabinoids as photolabeling ligands for the CB2 cannabinoid receptor

A series of tricyclic cannabinoids incorporating a heteroaroyl group at C3 were prepared as probes to explore the binding site(s) of the CB1 and CB2 receptors. This relatively unexplored structural motif is shown to be CB2 selective with K(i) values at low nanomolar concentrations when the heteroaromatic group is 3-benzothiophenyl (41) or 3-indolyl (50). When photoactivated, the lead compound 41 was shown to successfully label the CB2 receptor through covalent attachment at the active site while 50 failed to label. The benzothiophenone moiety may be a photoactivatable moiety suitable for selective labeling.

Neurokinin A engages neurokinin-1 receptor to induce NF-κB-dependent gene expression in murine macrophages: implications of ERK1/2 and PI 3-kinase/Akt pathways

Neurokinin A (NKA) belongs to the tachykinin neuropeptide family. Its biological functions are primarily mediated by the neurokinin (NK)-2 receptor. NKA has been implicated in several inflammatory conditions. However, there are limited data about the mechanism of its pathogenetic action. Here, we investigated proinflammatory effects of NKA on peripheral immune cells using the mouse macrophage/monocyte cell line RAW 264.7 and primary peritoneal macrophages. The signaling mechanistic pathways involved were also studied. In mouse macrophages with no detectable NK-2 receptors, NKA induces the upregulation of NK-1 but not NK-2 receptor expression. Furthermore, NKA engages this NK-1 receptor, resulting in inflammatory-like responses involving activation of the transcription factor nuclear factor (NF)-κB and induction of NF-κB-responsive proinflammatory chemokine expression. NKA activates NF-κB as evidenced by induced phosphorylation (leading to degradation) of its inhibitory protein IκBα, increased cellular levels of the transactivation-active phospho(Ser276)-p65 and its nuclear translocation, as well as enhanced DNA-binding activity of NF-κB. These responses are specifically inhibited by selective NK-1 receptor antagonists but not NK-2 receptor antagonists, thereby excluding the role of NK-2 receptor. Further investigation on the upstream signaling mechanisms suggests that two NF-κB-activating pathways (extracellular signal-regulated kinase 1/2 and phosphatidylinositol 3-kinase/protein kinase B) are activated by NKA. Specific inhibitors of the two pathways block NF-κB-dependent chemokine expression. The inhibitory effects are mediated through regulation of nuclear translocation, DNA-binding activity, and/or transactivation activity of NF-κB. Together, we provide novel evidence that NKA engages NK-1 receptors on mouse macrophages to elicit NF-κB-dependent cellular responses. The findings reveal cellular mechanisms that may underlie NKA-mediated inflammatory and immunological conditions.

Functional consequences of alteration of N-linked glycosylation sites on the neurokinin 1 receptor

The neurokinin 1 receptor (NK1R), a G protein-coupled receptor involved in diverse functions including pain and inflammation, has two putative N-linked glycosylation sites, Asn-14 and Asn-18. We studied the role of N-linked glycosylation in the functioning of the NK1R by constructing three receptor mutants: two single mutants (Asn --> Gln-14 and Asn --> Gln-18) and a double mutant, lacking both glycosylation sites. Using a lentiviral transfection system, the mutants were stably transfected into NCM 460 cells, a nontransformed human colonic epithelial cell line. We observed that the magnitude of glycosylation as estimated by changes in gel migration depends on the number of glycosylation sites available, with the wild-type receptor containing the greatest amount of glycosylation. All mutant receptors were able to bind to substance P and neurokinin A ligand with similar affinities; however, the double mutant, nonglycosylated NK1R showed only half the B(max) of the wild-type NK1R. In terms of receptor function, the ablation of both N-linked glycosylation sites did not have a profound effect on the receptors' abilities to activate the MAP kinase families (p42/p44, JNK, and p38), but did affect SP-induced IL-8 secretion. All mutants were able to internalize, but the kinetics of internalization of the double mutant receptor was more rapid, when compared with wild-type NK1R. Therefore, glycosylation of NK1R may stabilize the receptor in the plasma membrane. These results contribute to the ongoing elucidation of the role of glycosylation in G protein-coupled receptors and the study of the neurokinin receptors in particular.

Topological analysis of the complex formed between neurokinin A and the NK2 tachykinin receptor

Neurokinin A stimulates physiological responses in the peripheral and central nervous systems upon interacting primarily with the tachykinin NK2 receptor (NK2R). In this study, the structure of NKA bound to the NK2R is characterised by use of fluorescence resonance energy transfer. Four fluorescent NKA analogues with Texas red introduced at amino acid positions 1, 4, 7 and 10 were prepared. When bound to a NK2R carrying enhanced green fluorescent protein at the N-terminus, all peptides reduce green fluorescent protein fluorescence from 10% to 50% due to energy transfer. The derived donor-acceptor distances are 46, 55, 59 and 69 A for the fluorophore linked to positions 1-10, respectively. The monotonic increase in distance clearly indicates that the peptide adopts an extended structure when bound to its receptor. The present data are used, in combination with rhodopsin structure, fluorescence studies, photoaffinity labelling and site-directed mutagenesis data to design a computer model of the NKA-NK2R complex. We propose that the N-terminus of NKA is exposed and accessible to the extracellular medium. Subsequent amino acids of the NKA peptide become progressively more buried residues up to approximately one-third of the transmembrane-spanning domain.

The tachykinin tale: molecular recognition in a historical perspective

Crystallography, mutational mapping and crosslinking are but a few of the experimental techniques that have helped to elucidate the underlying principles of molecular recognition between macromolecules and to improve our understanding of the evolution of the structure-activity relationship (SAR). While this development has been particularly successful for small and rigid ligands and substrates that bind to larger hydrophilic biomolecules, our understanding of membrane-embedded proteins is still rather limited. This review uses the example of the neuropeptide family of tachykinins and their G-protein coupled receptors (GPCR) to present how complementary experimental strategies over the past decades have nourished and modified conceptual models of the structural requisites of molecular recognition and function. Given the little we know, the pertinent question is how we proceed from here.

The two NK-1 binding sites correspond to distinct, independent, and non-interconvertible receptor conformational states as confirmed by plasmon-waveguide resonance spectroscopy

Two nonstoichiometric ligand binding sites have been previously reported for the NK-1 receptor, with the use of classical methods (radioligand binding and second messenger assays). The most populated (major, NK-1M) binding site binds substance P (SP) and is related to the adenylyl cyclase pathway. The less populated (minor, NK-1m) binding site binds substance P, C-terminal hexa- and heptapeptide analogues of SP, and the NK-2 endogenous ligand, neurokinin A, and is coupled to the phospholipase C pathway. Here, we have examined these two binding sites with plasmon-waveguide resonance (PWR) spectroscopy that allows the thermodynamics and kinetics of ligand-receptor binding processes and the accompanying structural changes of the receptor to be monitored, through measurements of the anisotropic optical properties of lipid bilayers into which the receptor is incorporated. The binding of the three peptides, substance P, neurokinin A, and propionyl[Met(O(2))(11)]SP(7-11), to the partially purified NK-1 receptor has been analyzed by this method. Substance P and neurokinin A bind to the reconstituted receptor in a biphasic manner with two affinities (K(d1) = 0.14 +/- 0.02 nM and K(d2) = 1.4 +/- 0.18 nM, and K(d1) = 5.5 +/- 0.7 nM and K(d2) = 620 +/- 117 nM, respectively), whereas only one binding affinity (K(d) = 5.5 +/- 0.4 nM) could be observed for propionyl[Met(O(2))(11)]SP(7-11). Moreover, binding experiments in which one ligand was added after another one has been bound to the receptor have shown that the binding of these ligands to each binding site was unaffected by the fact that the other site was already occupied. These data strongly suggest that these two binding sites are independent and non-interconvertible on the time scale of these experiments (1-2 h).

Substance P selectively decreases paired pulse depression in the rat hippocampal slice

Background

Although being widespread in the hippocampus, the role tachykinins play in synaptic transmission is unclear. The effect of substance P on field potentials evoked by stimulation of the Schaffer collateral-commissural fibres and recorded from the CA1 region of the rat hippocampal slice were studied.

Results

Perfusion of substance P (8 μM) had no effect on the fEPSP or population spike. Substance P did however cause a selective reduction in the paired pulse depression of population spikes evoked by paired stimulation at interpulse intervals of 20–80 msec. A comparison of the actions of other tachykinin receptor agonists gave an order of potency of substance P > [β-Ala⁸]-neurokinin A (4–10) > senktide. The effect of substance P was reduced by the neurokinin-1 receptor antagonist SR140333, but not by the neurokinin-2 or neurokinin-3 receptor antagonists, MDL 29,913 or [Trp⁷, β-Ala⁸]-neurokinin A (4–10).

Conclusion

The order of potency of the agonists, and the effects of the antagonists, both indicate that the effect of substance P on paired pulse depression is mediated by neurokinin-1 receptors.

Photolabelling the rat urotensin II/GPR14 receptor identifies a ligand-binding site in the fourth transmembrane domain

A urotensin II (U-II) peptide analogue containing the photoreactive p -benzoyl-L-phenylalanine (Bz-Phe) in the sixth position was used to identify ligand-binding sites of the rat U-II receptor, also known as GPR14. [Bz-Phe(6)]U-II bound the receptor expressed in COS-7 cells with high affinity (IC(50) 0.7 nM) and was as potent as U-II in the agonist-induced production of inositol phosphate. Photolabelling of the U-II receptor with (125)I-[Bz-Phe(6)]U-II resulted in the specific formation of a glycosylated (125)I-[Bz-Phe(6)]U-II-U-II receptor complex of 60 kDa. Digestion of the 60 kDa complex with endoproteinase Glu-C generated a fragment of 17 kDa circumscribing the labelled fragment to residues 148-286. Digestion of the ligand-receptor complex with endoproteinase Arg-C produced a short peptide of 4 kDa corresponding to fragments 125-148, 167-192 or 210-233. CNBr treatment of the endoproteinase-Glu-C and -Arg-C fragments yielded 2 kDa fragments, defining the labelling site to methionine residues 184/185 of the fourth transmembrane domain. Photolabelling of two mutant receptors, M184L/M185L and M184A/M185A, led to a significant decrease in the overall yield of covalent labelling. Taken together, our results indicate that position 6 of U-II normally occupied by phenylalanine would interact with Met(184) and/or Met(185) of the fourth transmembrane domain of the U-II receptor. This information should be of significant value in the study of the interactions between U-II and its cognate receptor.

Mutations in the extracellular amino-terminal domain of the NK2 neurokinin receptor abolish cAMP signaling but preserve intracellular calcium responses

By combining real time measurements of agonist binding, by fluorescence resonance energy transfer, and of subsequent responses, we proposed previously that the neurokinin NK2 receptor preexists in equilibrium between three states: inactive, calcium-triggering, and cAMP-producing. Thr(24) and Phe(26) of the NK2 receptor extracellular domain are considered to interact with neuropeptide agonists based on the reduction of affinity when they are substituted by alanine. Using fluorescence resonance energy transfer, we now quantify the binding kinetics of two Texas Red-modified neurokinin A agonists to the fluorescent wild-type (Y-NK2wt) and the mutant (Y-NK2mut) receptor carrying Thr(24) --> Ala and Phe(26) --> Ala mutations. TR1-neurokinin A binds with a fast component and a slow component to the Y-NK2wt receptor and triggers both a calcium and a cAMP response. In contrast, on the mutant receptor, it binds in a single fast step with a lower apparent affinity and activates only the calcium response. Another agonist, TRC4-neurokinin A, binds to both wild-type and mutant receptors in a single fast step, with similar affinities and kinetics and promotes only calcium signaling. Kinetic modeling of ligand binding and receptor interconversions is carried out to analyze phenotypic changes in terms of binding alterations or changes in the transitions between conformational states. We show that the binding and response properties of the Y-NK2mut receptor are best described according to a phenotype where a reduction of the transition between the inactive and the active states occurs.

Involvement of the second extracellular loop (E2) of the neurokinin-1 receptor in the binding of substance P. Photoaffinity labeling and modeling studies

Substance P (SP) interacts with the neurokinin-1 (NK-1) G-protein-coupled receptor, which has been cloned in several species. In the present study, the domains of the NK-1 receptor involved in the binding of SP and SP-(7-11) C-terminal fragment have been analyzed using two peptide analogs containing the photoreactive amino acid para-benzoylphenylalanine ((p-Bz)Phe) in position 8 of their sequence. This study was carried out with [BAPA-Lys(6),(p-Bz)Phe(8),Pro(9),Met(O(2))(11)]SP-(7-11) and [BAPA(0),(p-Bz)Phe(8)]SP on both rat and human NK-1 receptors expressed in CHO cells. Combined trypsin and endo-GluC enzymatic complete digestions and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis led to the identification of the same domain of covalent interaction, (173)TMPSR(177), for the two photoactivatable peptides. Further digestion of this fragment with carboxypeptidase Y led to the identification of (173)TMP(175) in the second extracellular loop (E2) of the NK-1 receptor as the site of covalent attachment. Models of the conformation of this E2 loop in the human NK-1 receptor were generated using two different strategies, one based on homology with bovine rhodopsin and the other based on the solution conformation preferences of a synthetic peptide corresponding to the E2 loop.

Direct evidence for the interaction of neurokinin A with the tachykinin NK(1) receptor in tissue

Neurokinin A (NKA) is a tachykinin peptide that binds with high affinity to the tachykinin NK(2) receptor. Recent homologous binding studies, however, have shown that neurokinin A is also a high-affinity ligand for the tachykinin NK(1) receptor. In this report, we demonstrate that a photoreactive neurokinin A analogue specifically labels the NK(1) receptor in rat submandibular gland membranes and show via bioassay that neurokinin A is a potent stimulator of salivary secretion. Through the use of specific non-peptide antagonists in both photolabeling and functional assays, we unequivocally demonstrate that neurokinin A can specifically interact with the NK(1) receptor in vivo and elicit NK(1) receptor-mediated physiological responses.

Molecular characterization of the substance P*neurokinin-1 receptor complex: development of an experimentally based model

Molecular models for the interaction of substance P (SP) with its G protein-coupled receptor, the neurokinin-1 receptor (NK-1R), have been developed. The ligand.receptor complex is based on experimental data from a series of photoaffinity labeling experiments and spectroscopic structural studies of extracellular domains of the NK-1R. Using the ligand/receptor contact points derived from incorporation of photolabile probes (p-benzoylphenylalanine (Bpa)) into SP at positions 3, 4, and 8 and molecular dynamics simulations, the topological arrangement of SP within the NK-1R is explored. The model incorporates the structural features, determined by high resolution NMR studies, of the second extracellular loop (EC2), containing contact points Met(174) and Met(181), providing important experimentally based conformational preferences for the simulations. Extensive molecular dynamics simulations were carried out to probe the nature of the two contact points identified for the Bpa(3)SP analogue (Bremer, A. A., Leeman, S. E., and Boyd, N. D. (2001) J. Biol. Chem. 276, 22857-22861), examining modes of ligand binding in which the contact points are fulfilled sequentially or simultaneously. The resulting ligand.receptor complex has the N terminus of SP projecting toward transmembrane helix (TM) 1 and TM2, exposed to the solvent. The C terminus of SP is located in proximity to TM5 and TM6, deeper into the central core of the receptor. The central portion of the ligand, adopting a helical loop conformation, is found to align with the helices of the central regions EC2 and EC3, forming important interactions with both of these extracellular domains. The model developed here allows for atomic insight into the biochemical data currently available and guides targeting of future experiments to probe specific ligand/receptor interactions and thereby furthers our understanding of the functioning of this important neuropeptide system.