Characterization of UBO-QIC as a Gαq inhibitor in platelets

A human obesity-associated MC4R mutation with defective Gq/11α signaling leads to hyperphagia in mice

Melanocortin 4 receptor (MC4R) mutations are the most common cause of human monogenic obesity and are associated with hyperphagia and increased linear growth. While MC4R is known to activate Gsα/cAMP signaling, a substantial proportion of obesity-associated MC4R mutations do not affect MC4R/Gsα signaling. To further explore the role of specific MC4R signaling pathways in the regulation of energy balance, we examined the signaling properties of one such mutant, MC4R (F51L), as well as the metabolic consequences of MC4RF51L mutation in mice. The MC4RF51L mutation produced a specific defect in MC4R/Gq/11α signaling and led to obesity, hyperphagia, and increased linear growth in mice. The ability of a melanocortin agonist to acutely inhibit food intake when delivered to the paraventricular nucleus (PVN) was lost in MC4RF51L mice, as well as in WT mice in which a specific Gq/11α inhibitor was delivered to the PVN; this provided evidence that a Gsα-independent signaling pathway, namely Gq/11α, significantly contributes to the actions of MC4R on food intake and linear growth. These results suggest that a biased MC4R agonist that primarily activates Gq/11α may be a potential agent to treat obesity with limited untoward cardiovascular and other side effects.

Myogenic Vasoconstriction Requires Canonical Gq/11 Signaling of the Angiotensin II Type 1 Receptor

Background Blood pressure and tissue perfusion are controlled in part by the level of intrinsic (myogenic) arterial tone. However, many of the molecular determinants of this response are unknown. We previously found that mice with targeted disruption of the gene encoding the angiotensin II type 1a receptor (AT1AR) (Agtr1a), the major murine angiotensin II type 1 receptor (AT1R) isoform, showed reduced myogenic tone; however, uncontrolled genetic events (in this case, gene ablation) can lead to phenotypes that are difficult or impossible to interpret. Methods and Results We tested the mechanosensitive function of AT1R using tamoxifen-inducible smooth muscle-specific AT1aR knockout (smooth muscle-Agtr1a^-/-) mice and studied downstream signaling cascades mediated by G_q/11 and/or β-arrestins. FR900359, Sar1Ile4Ile8-angiotensin II (SII), TRV120027 and TRV120055 were used as selective G_q/11 inhibitor and biased agonists to activate noncanonical β-arrestin and canonical G_q/11 signaling of the AT1R, respectively. Myogenic and Ang II-induced constrictions were diminished in the perfused renal vasculature, mesenteric and cerebral arteries of smooth muscle-Agtr1a^-/- mice. Similar effects were observed in arteries of global mutant Agtr1a^-/- but not Agtr1b^-/- mice. FR900359 decreased myogenic tone and angiotensin II-induced constrictions whereas selective biased targeting of AT1R-β-arrestin signaling pathways had no effects. Conclusions This study demonstrates that myogenic arterial constriction requires G_q/11-dependent signaling pathways of mechanoactivated AT1R but not G protein-independent, noncanonical pathways in smooth muscle cells.

Targeting Oncogenic Gαq/11 in Uveal Melanoma

Uveal melanoma is the most common intraocular cancer in adults and arises from the transformation of melanocytes in the uveal tract. While treatment of the primary tumor is often effective, 36-50% of patients develop metastatic disease primarily to the liver. While various strategies have been used to treat the metastatic disease, there remain no effective treatments that improve survival. Significant insight has been gained into the pathways that are altered in uveal melanoma, with mutually exclusive activating mutations in the GNAQ and GNA11 genes being found in over 90% of patients. These genes encode the alpha subunits of the hetetrotrimeric G proteins, Gq and G11, and mutations result in activation of several important signaling pathways, including phospholipase C and activation of the transcription factor YAP. In this review, we discuss current efforts to target various signaling pathways in the treatment of uveal melanoma including recent efforts to target Gq and G11 in mouse models. While selective targeting of Gq and G11 provides a potential therapeutic strategy to treat uveal melanoma, it is evident that improved inhibitors and methods of delivery are needed.

Marine Bile Natural Products as Agonists of the TGR5 Receptor

Agonism of the G protein-coupled bile acid receptor "Takeda G-protein receptor 5" (TGR5) aids in attenuating cholesterol accumulation due to atherosclerotic progression. Although mammalian bile compounds can activate TGR5, they are generally weak agonists, and more effective compounds need to be identified. In this study, two marine bile compounds (5β-scymnol and its sulfate) were compared with mammalian bile compounds deoxycholic acid (DCA) and ursodeoxycholic acid (UDCA) using an in vitro model of TGR5 agonism. The response profiles of human embryonic kidney 293 cells (HEK293) transfected to overexpress TGR5 (HEK293-TGR5) and incubated with subcytotoxic concentrations of test compounds were compared to nontransfected HEK293 control cells using the specific calcium-binding fluorophore Fura-2AM to measure intracellular calcium [Ca²⁺]_i release. Scymnol and scymnol sulfate caused a sustained increase in [Ca²⁺]_i within TGR5 cells only, which was abolished by a specific inhibitor for G_αq protein (UBO-QIC). Sustained increases in [Ca²⁺]_i were seen in both cell types with DCA exposure; this was unaffected by UBO-QIC, indicating that TGR5 activation was not involved. Exposure to UDCA did not alter [Ca²⁺]_i, suggesting a lack of TGR5 bioactivity. These findings demonstrated that both scymnol and scymnol sulfate are novel agonists of TGR5 receptors, showing therapeutic potential for treating atherosclerosis.

The chromodepsins - chemistry, biology and biosynthesis of a selective Gq inhibitor natural product family

Covering: up to April 2021The bacterial cyclic depsipeptides FR900359 (FR) and YM-254890 (YM) were shown to selectively inhibit Gαq proteins with high potency and selectivity and have recently emerged as valuable pharmacological tools due to their effective mechanism of action. Here, we summarize important aspects of this small and specialized natural product family, for which we propose the name chromodepsins, starting from their discovery, producing organisms and structural variety. We then review biosynthesis, structure-activity relationships and ecological and evolutionary aspects of the chromodepsins. Lastly, we discuss their mechanism of action, potential medicinal applications and future opportunities and challenges for further use and development of these complex inhibitor molecules from nature.

An experimental strategy to probe Gq contribution to signal transduction in living cells

Heterotrimeric G protein subunits Gαq and Gα11 are inhibited by two cyclic depsipeptides, FR900359 (FR) and YM-254890 (YM), both of which are being used widely to implicate Gq/11 proteins in the regulation of diverse biological processes. An emerging major research question therefore is whether the cellular effects of both inhibitors are on-target, that is, mediated via specific inhibition of Gq/11 proteins, or off-target, that is, the result of nonspecific interactions with other proteins. Here we introduce a versatile experimental strategy to discriminate between these possibilities. We developed a Gαq variant with preserved catalytic activity, but refractory to FR/YM inhibition. A minimum of two amino acid changes were required and sufficient to achieve complete inhibitor resistance. We characterized the novel mutant in HEK293 cells depleted by CRISPR–Cas9 of endogenous Gαq and Gα11 to ensure precise control over the Gα-dependent cellular signaling route. Using a battery of cellular outcomes with known and concealed Gq contribution, we found that FR/YM specifically inhibited cellular signals after Gαq introduction via transient transfection. Conversely, both inhibitors were inert across all assays in cells expressing the drug-resistant variant. These findings eliminate the possibility that inhibition of non-Gq proteins contributes to the cellular effects of the two depsipeptides. We conclude that combined application of FR or YM along with the drug-resistant Gαq variant is a powerful in vitro strategy to discern on-target Gq against off-target non-Gq action. Consequently, it should be of high value for uncovering Gq input to complex biological processes with high accuracy and the requisite specificity.

Macrocyclic Gq Protein Inhibitors FR900359 and/or YM-254890-Fit for Translation?

Guanine nucleotide-binding proteins (G proteins) transduce extracellular signals received by G protein-coupled receptors (GPCRs) to intracellular signaling cascades. While GPCRs represent the largest class of drug targets, G protein inhibition has only recently been recognized as a novel strategy for treating complex diseases such as asthma, inflammation, and cancer. The structurally similar macrocyclic depsipeptides FR900359 (FR) and YM-254890 (YM) are potent selective inhibitors of the Gq subfamily of G proteins. FR and YM differ in two positions, FR being more lipophilic than YM. Both compounds are utilized as pharmacological tools to block Gq proteins in vitro and in vivo. However, no detailed characterization of FR and YM has been performed, which is a prerequisite for the compounds' translation into clinical application. Here, we performed a thorough study of both compounds' physicochemical, pharmacokinetic, and pharmacological properties. Chemical stability was high across a large range of pH values, with FR being somewhat more stable than YM. Oral bioavailability and brain penetration of both depsipeptides were low. FR showed lower plasma protein binding and was metabolized significantly faster than YM by human and mouse liver microsomes. FR accumulated in lung after chronic intratracheal or intraperitoneal application, while YM was more distributed to other organs. Most strikingly, the previously observed longer residence time of FR resulted in a significantly prolonged pharmacologic effect as compared to YM in a methacholine-induced bronchoconstriction mouse model. These results prove that changes within a molecule which seem marginal compared to its structural complexity can lead to crucial pharmacological differences.

Heterotrimeric Gq proteins as therapeutic targets?

Heterotrimeric G proteins are the core upstream elements that transduce and amplify the cellular signals from G protein-coupled receptors (GPCRs) to intracellular effectors. GPCRs are the largest family of membrane proteins encoded in the human genome and are the targets of about one-third of prescription medicines. However, to date, no single therapeutic agent exerts its effects via perturbing heterotrimeric G protein function, despite a plethora of evidence linking G protein malfunction to human disease. Several recent studies have brought to light that the Gq family-specific inhibitor FR900359 (FR) is unexpectedly efficacious in silencing the signaling of Gq oncoproteins, mutant Gq variants that mostly exist in the active state. These data not only raise the hope that researchers working in drug discovery may be able to potentially strike Gq oncoproteins from the list of undruggable targets, but also raise questions as to how FR achieves its therapeutic effect. Here, we place emphasis on these recent studies and explain why they expand our pharmacological armamentarium for targeting Gq protein oncogenes as well as broaden our mechanistic understanding of Gq protein oncogene function. We also highlight how this novel insight impacts the significance and utility of using G(q) proteins as targets in drug discovery efforts.

Proteinase-Activated Receptor 4 Activation Triggers Cell Membrane Blebbing through RhoA and β-Arrestin

Proteinase-activated receptors (PARs) are a four-member family of G-protein-coupled receptors that are activated via proteolysis. PAR4 is a member of this family that is cleaved and activated by serine proteinases such as thrombin, trypsin, and cathepsin-G. PAR4 is expressed in a variety of tissues and cell types, including platelets, vascular smooth muscle cells, and neuronal cells. In studying PAR4 signaling and trafficking, we observed dynamic changes in the cell membrane, with spherical membrane protrusions that resemble plasma membrane blebbing. Since nonapoptotic membrane blebbing is now recognized as an important regulator of cell migration, cancer cell invasion, and vesicular content release, we sought to elucidate the signaling pathway downstream of PAR4 activation that leads to such events. Using a combination of pharmacological inhibition and CRISPR/CRISPR-associated protein 9 (Cas9)-mediated gene editing approaches, we establish that PAR4-dependent membrane blebbing occurs independently of the Gα _q/11- and Gα _i-signaling pathways and is dependent on signaling via the β-arrestin-1/2 and Ras homolog family member A (RhoA) signaling pathways. Together these studies provide further mechanistic insight into PAR4 regulation of cellular function. SIGNIFICANCE STATEMENT: We find that the thrombin receptor PAR4 triggers cell membrane blebbing in a RhoA-and β-arrestin-dependent manner. In addition to identifying novel cellular responses mediated by PAR4, these data provide further evidence for biased signaling in PAR4 since membrane blebbing was dependent on some, but not all, signaling pathways activated by PAR4.

Cell-permeable high-affinity tracers for Gq proteins provide structural insights, reveal distinct binding kinetics and identify small molecule inhibitors

Background and Purpose

G proteins are intracellular switches that transduce and amplify extracellular signals from GPCRs. The G_q protein subtypes, which are coupled to PLC activation, can act as oncogenes, and their expression was reported to be up‐regulated in cancer and inflammatory diseases. G_q inhibition may be an efficient therapeutic strategy constituting a new level of intervention. However, diagnostic tools and therapeutic drugs for G_q proteins are lacking.

Experimental Approach

We have now developed G_q‐specific, cell‐permeable ³H‐labelled high‐affinity probes based on the macrocyclic depsipeptides FR900359 (FR) and YM‐254890 (YM). The tracers served to specifically label and quantify G_q proteins in their native conformation in cells and tissues with high accuracy.

Key Results

FR and YM displayed low nanomolar affinity for Gα_q, Gα₁₁ and Gα₁₄ expressed in CRISPR/Cas9 Gα_q‐knockout cells, but not for Gα₁₅. The two structurally very similar tracers showed strikingly different dissociation kinetics, which is predicted to result in divergent biological effects. Computational studies suggested a “dowel” effect of the pseudoirreversibly binding FR. A high‐throughput binding assay led to the discovery of novel G_q inhibitors, which inhibited G_q signalling in recombinant cells and primary murine brown adipocytes, resulting in enhanced differentiation.

Conclusions and Implications

The Gq protein inhibitors YM and FR are pharmacologically different despite similar structures. The new versatile tools and powerful assays will contribute to the advancement of the rising field of G protein research.

Rational design of a heterotrimeric G protein α subunit with artificial inhibitor sensitivity

Transmembrane signals initiated by a range of extracellular stimuli converge on members of the Gq family of heterotrimeric G proteins, which relay these signals in target cells. Gq family G proteins comprise Gq, G11, G14, and G16, which upon activation mediate their cellular effects via inositol lipid–dependent and –independent signaling to control fundamental processes in mammalian physiology. To date, highly specific inhibition of Gq/11/14 signaling can be achieved only with FR900359 (FR) and YM-254890 (YM), two naturally occurring cyclic depsipeptides. To further development of FR or YM mimics for other Gα subunits, we here set out to rationally design Gα16 proteins with artificial FR/YM sensitivity by introducing an engineered depsipeptide-binding site. Thereby we permit control of G16 function through ligands that are inactive on the WT protein. Using CRISPR/Cas9-generated Gαq/Gα11-null cells and loss- and gain-of-function mutagenesis along with label-free whole-cell biosensing, we determined the molecular coordinates for FR/YM inhibition of Gq and transplanted these to FR/YM-insensitive G16. Intriguingly, despite having close structural similarity, FR and YM yielded biologically distinct activities: it was more difficult to perturb Gq inhibition by FR and easier to install FR inhibition onto G16 than perturb or install inhibition with YM. A unique hydrophobic network utilized by FR accounted for these unexpected discrepancies. Our results suggest that non-Gq/11/14 proteins should be amenable to inhibition by FR scaffold–based inhibitors, provided that these inhibitors mimic the interaction of FR with Gα proteins harboring engineered FR-binding sites.

Effects of Oncogenic Gαq and Gα11 Inhibition by FR900359 in Uveal Melanoma

Uveal melanoma is the most common intraocular tumor in adults and often metastasizes to the liver, leaving patients with few options. Recurrent activating mutations in the G proteins, Gα_q and Gα₁₁, are observed in approximately 93% of all uveal melanomas. Although therapeutic intervention of downstream Gα_q/11 targets has been unsuccessful in treating uveal melanoma, we have found that the Gα_q/11 inhibitor, FR900359 (FR), effectively inhibits oncogenic Gα_q/11 signaling in uveal melanoma cells expressing either mutant Gα_q or Gα₁₁. Inhibition of oncogenic Gα_q/11 by FR results in cell-cycle arrest and induction of apoptosis. Furthermore, colony formation is prevented by FR treatment of uveal melanoma cells in 3D-cell culture, providing promise for future in vivo studies. This suggests direct inhibition of activating Gα_q/11 mutants may be a potential means of treating uveal melanoma. IMPLICATIONS: Oncogenic Gα_q/11 inhibition by FR900359 may be a potential treatment option for those with uveal melanoma.

Characterization of signalling and regulation of common calcitonin receptor splice variants and polymorphisms

The calcitonin receptor (CTR) is a class B G protein-coupled receptor that is a therapeutic target for the treatment of hypercalcaemia of malignancy, Paget's disease and osteoporosis. In primates, the CTR is subject to alternative splicing, with a unique, primate-specific splice variant being preferentially expressed in reproductive organs, lung and kidney. In addition, humans possess a common non-synonymous single-nucleotide polymorphism (SNP) encoding a proline/leucine substitution in the C-terminal tail. In low power studies, the leucine polymorphism has been associated with increased risk of osteoporosis in East Asian populations and, independently, with increased risk of kidney stone disease in a central Asian population. The CTR is pleiotropically coupled, though the relative physiological importance of these pathways is poorly understood. Using both COS-7 and HEK293 cells recombinantly expressing human CTR, we have characterized both splice variant and polymorphism dependent response to CTs from several species in key signalling pathways and competition binding assays. These data indicate that the naturally occurring changes to the intracellular face of CTR alter ligand affinity and signalling, in a pathway and agonist dependent manner. These results further support the potential for these primate-specific CTR variants to engender different physiological responses. In addition, we report that the CTR exhibits constitutive internalization, independent of splice variant and polymorphism and this profile is unaltered by peptide binding.

The acute glucose lowering effect of specific GPR120 activation in mice is mainly driven by glucagon-like peptide 1

The mechanism behind the glucose lowering effect occurring after specific activation of GPR120 is not completely understood. In this study, a potent and selective GPR120 agonist was developed and its pharmacological properties were compared with the previously described GPR120 agonist Metabolex-36. Effects of both compounds on signaling pathways and GLP-1 secretion were investigated in vitro. The acute glucose lowering effect was studied in lean wild-type and GPR120 null mice following oral or intravenous glucose tolerance tests. In vitro, in GPR120 overexpressing cells, both agonists signaled through Gα_q, Gα_s and the β-arrestin pathway. However, in mouse islets the signaling pathway was different since the agonists reduced cAMP production. The GPR120 agonists stimulated GLP-1 secretion both in vitro in STC-1 cells and in vivo following oral administration. In vivo GPR120 activation induced significant glucose lowering and increased insulin secretion after intravenous glucose administration in lean mice, while the agonists had no effect in GPR120 null mice. Exendin 9–39, a GLP-1 receptor antagonist, abolished the GPR120 induced effects on glucose and insulin following an intravenous glucose challenge. In conclusion, GLP-1 secretion is an important mechanism behind the acute glucose lowering effect following specific GPR120 activation.

Gaq proteins: molecular pharmacology and therapeutic potential

Seven transmembrane G protein-coupled receptors (GPCRs) have gained much interest in recent years as it is the largest class among cell surface receptors. G proteins lie in the heart of GPCRs signalling and therefore can be therapeutically targeted to overcome complexities in GPCR responses and signalling. G proteins are classified into four families (Gi, Gs, G12/13 and Gq); Gq is further subdivided into four classes. Among them Gαq and Gαq/11 isoforms are most crucial and ubiquitously expressed; these isoforms are almost 88% similar at their amino acid sequence but may exhibit functional divergences. However, uncertainties often arise about Gαq and Gαq/11 inhibitors, these G proteins might also have suitability to the invention of novel-specific inhibitors for each isoforms. YM-254890 and UBO-QIC are discovered as potent inhibitors of Gαq functions and also investigated in thrombin protease-activated receptor (PAR)-1 inhibitors and platelet aggregation inhibition. The most likely G protein involved in PAR-1 stimulates responses is one of the Gαq family isoforms. In this review, we highlight the molecular structures and pharmacological responses of Gαq family which may reflect the biochemical and molecular role of Gαq and Gαq/11. The advanced understanding of Gαq and Gαq/11 role in GPCR signalling may shed light on our understanding on cell biology, cellular physiology and pathophysiology and also lead to the development of novel therapeutic agents for a number of diseases.

Agonist-mediated activation of Bombyx mori diapause hormone receptor signals to extracellular signal-regulated kinases 1 and 2 through Gq-PLC-PKC-dependent cascade

Diapause is a developmental strategy adopted by insects to survive in challenging environments such as the low temperatures of a winter. This unique process is regulated by diapause hormone (DH), which is a neuropeptide hormone that induces egg diapause in Bombyx mori and is involved in terminating pupal diapause in heliothis moths. An G protein-coupled receptor from the silkworm, B. mori, has been identified as a specific cell surface receptor for DH. However, the detailed information on the DH-DHR system and its mechanism(s) involved in the induction of embryonic diapause remains unknown. Here, we combined functional assays with various specific inhibitors to elucidate the DHR-mediated signaling pathways. Upon activation by DH, B. mori DHR is coupled to the Gq protein, leading to a significant increase of intracellular Ca(2+) and cAMP response element-driven luciferase activity in an UBO-QIC, a specific Gq inhibitor, sensitive manner. B. mori DHR elicited ERK1/2 phosphorylation in a dose- and time-dependent manner in response to DH. This effect was almost completely inhibited by co-incubation with UBO-QIC and was also significantly suppressed by PLC inhibitor U73122, PKC inhibitors Gö6983 and the Ca(2+) chelator EGTA. Moreover, DHR-induced activation of ERK1/2 was significantly attenuated by treatment with the Gβγ specific inhibitors gallein and M119K and the PI3K specific inhibitor Wortmannin, but not by the Src specific inhibitor PP2. Our data also demonstrates that the EGFR-transactivation pathway is not involved in the DHR-mediated ERK1/2 phosphorylation. Future efforts are needed to clarify the role of the ERK1/2 signaling pathway in the DH-mediated induction of B. mori embryonic diapause.

Total synthesis and structure-activity relationship studies of a series of selective G protein inhibitors

G proteins are key mediators of G protein-coupled receptor signalling, which facilitates a plethora of important physiological processes. The cyclic depsipeptides YM-254890 and FR900359 are the only known specific inhibitors of the G_q subfamily of G proteins; however, no synthetic route has been reported previously for these complex natural products and they are not easily isolated from natural sources. Here we report the first total synthesis of YM-254890 and FR900359, as well as of two known analogues, YM-385780 and YM-385781. The versatility of the synthetic approach also enabled the design and synthesis of ten analogues, which provided the first structure-activity relationship study for this class of compounds. Pharmacological characterization of all the compounds at G_q-, G_i- and G_s-mediated signalling provided succinct information on the structural requirements for inhibition, and demonstrated that both YM-254890 and FR900359 are highly potent inhibitors of G_q signalling, with FR900359 being the most potent. These natural products and their analogues represent unique tools for explorative studies of G protein inhibition.

Strategies for Investigating G-Protein Modulation of Voltage-Gated Ca2+ Channels

G-protein-coupled receptor modulation of voltage-gated ion channels is a common means of fine-tuning the response of channels to changes in membrane potential. Such modulation impacts physiological processes such as synaptic transmission, and hence therapeutic strategies often directly or indirectly target these pathways. As an exemplar of channel modulation, we examine strategies for investigating G-protein modulation of CaV2.2 or N-type voltage-gated Ca(2+) channels. We focus on biochemical and genetic tools for defining the molecular mechanisms underlying the various forms of CaV2.2 channel modulation initiated following ligand binding to G-protein-coupled receptors.

GPR142 Agonists Stimulate Glucose-Dependent Insulin Secretion via Gq-Dependent Signaling

GPR142 is an islet-enriched G protein-coupled receptor that has been investigated as a novel therapeutic target for the treatment of type 2 diabetes by virtue of its insulin secretagogue activity. However, the signaling pathways downstream of GPR142 and whether its stimulation of insulin release is glucose-dependent remain poorly characterized. In this study, we show that both native and synthetic GPR142 agonists can activate Gq as well as Gi signaling when GPR142 is recombinantly expressed in HEK293 cells. However, in primary pancreatic islets, a native cellular system, the insulin secretagogue activity of GPR142 agonists only requires Gq activation. In addition, our results show that stimulation of insulin secretion by GPR142 in pancreatic islets is strictly glucose-dependent.

On the selectivity of the Gαq inhibitor UBO-QIC: A comparison with the Gαi inhibitor pertussis toxin

Gαq inhibitor UBO-QIC (FR900359) is becoming an important pharmacological tool, but its selectivity against other G proteins and their subunits, especially βγ, has not been well characterized. We examined UBO-QIC's effect on diverse signaling pathways mediated via various G protein-coupled receptors (GPCRs) and G protein subunits by comparison with known Gαi inhibitor pertussis toxin. As expected, UBO-QIC inhibited Gαq signaling in all assay systems examined. However, other non-Gαq-events, e.g. Gβγ-mediated intracellular calcium release and inositol phosphate production, following activation of Gi-coupled A1 adenosine and M2 muscarinic acetylcholine receptors, were also blocked by low concentrations of UBO-QIC, indicating that its effect is not limited to Gαq. Thus, UBO-QIC also inhibits Gβγ-mediated signaling similarly to pertussis toxin, although UBO-QIC does not affect Gαi-mediated inhibition or Gαs-mediated stimulation of adenylyl cyclase activity. However, the blockade by UBO-QIC of GPCR signaling, such as carbachol- or adenosine-mediated calcium or inositol phosphate increases, does not always indicate inhibition of Gαq-mediated events, as the βγ subunits released from Gi proteins following the activation of Gi-coupled receptors, e.g. M2 and A1Rs, may produce similar signaling events. Furthermore, UBO-QIC completely inhibited Akt signaling, but only partially blocked ERK1/2 activity stimulated by the Gq-coupled P2Y1R. Thus, we have revealed new aspects of the pharmacological interactions of UBO-QIC.

Interdicting Gq Activation in Airway Disease by Receptor-Dependent and Receptor-Independent Mechanisms

Gαqβγ heterotrimer (Gq), an important mediator in the pathology of airway disease, plays a central role in bronchoconstriction and airway remodeling, including airway smooth muscle growth and inflammation. Current therapeutic strategies to treat airway disease include the use of muscarinic and leukotriene receptor antagonists; however, these pharmaceuticals demonstrate a limited clinical efficacy as multiple Gq-coupled receptor subtypes contribute to these pathologies. Thus, broadly inhibiting the activation of Gq may be an advantageous therapeutic approach. Here, we investigated the effects of broadly inhibiting Gq activation in vitro and ex vivo using receptor-dependent and receptor-independent strategies. P4pal-10 is a protease activated receptor 4-derived pepducin that exhibits efficacy toward multiple Gq-coupled receptors. Mechanistic studies demonstrated that P4pal-10 selectively inhibits all G protein coupling to several Gq-coupled receptors, including protease activated receptor 1, muscarinic acetylcholine M3, and histamine H1 receptors, while demonstrating no direct effect on Gq. We also evaluated the ability of FR900359, also known as UBO-QIC, to directly inhibit Gq activation. FR900359 inhibited spontaneous Gαq nucleotide exchange, while having little effect on Gαsβγ, Gαiβγ, or Gα12/13βγ heterotrimer activity. Both P4pal-10 and FR900359 inhibited Gq-mediated intracellular signaling and primary human airway smooth muscle growth, whereas only FR900359 effectively interdicted agonist-promoted airway contraction in human precision cut lung slices. These studies serve as a proof of concept that the broad-based inhibition of Gq activation may be a useful therapeutic approach to treat multiple common pathologies of airway disease.

Differential Regulation of ERK1/2 and mTORC1 Through T1R1/T1R3 in MIN6 Cells

The MAPKs ERK1/2 respond to nutrients and other insulin secretagogues in pancreatic β-cells and mediate nutrient-dependent insulin gene transcription. Nutrients also stimulate the mechanistic target of rapamycin complex 1 (mTORC1) to regulate protein synthesis. We showed previously that activation of both ERK1/2 and mTORC1 in the MIN6 pancreatic β-cell-derived line by extracellular amino acids (AAs) is at least in part mediated by the heterodimeric T1R1/T1R3, a G protein-coupled receptor. We show here that AAs differentially activate these two signaling pathways in MIN6 cells. Pretreatment with pertussis toxin did not prevent the activation of either ERK1/2 or mTORC1 by AAs, indicating that G(I) is not central to either pathway. Although glucagon-like peptide 1, an agonist for a G(s-)coupled receptor, activated ERK1/2 well and mTORC1 to a small extent, AAs had no effect on cytosolic cAMP accumulation. Ca(2+) entry is required for ERK1/2 activation by AAs but is dispensable for AA activation of mTORC1. Pretreatment with UBO-QIC, a selective G(q) inhibitor, reduced the activation of ERK1/2 but had little effect on the activation of mTORC1 by AAs, suggesting a differential requirement for G(q). Inhibition of G(12/13) by the overexpression of the regulator of G protein signaling domain of p115 ρ-guanine nucleotide exchange factor had no effect on mTORC1 activation by AAs, suggesting that these G proteins are also not involved. We conclude that AAs regulate ERK1/2 and mTORC1 through distinct signaling pathways.