ERK5 activation by Gq-coupled muscarinic receptors is independent of receptor internalization and β-arrestin recruitment

The biased M3 mAChR ligand PD 102807 mediates qualitatively distinct signaling to regulate airway smooth muscle phenotype

Airway smooth muscle (ASM) cells attain a hypercontractile phenotype during obstructive airway diseases. We recently identified a biased M3 muscarinic acetylcholine receptor (mAChR) ligand, PD 102807, that induces GRK-/arrestin-dependent AMP-activated protein kinase (AMPK) activation to inhibit transforming growth factor-β-induced hypercontractile ASM phenotype. Conversely, the balanced mAChR agonist, methacholine (MCh), activates AMPK yet does not regulate ASM phenotype. In the current study, we demonstrate that PD 102807- and MCh-induced AMPK activation both depend on Ca2+/calmodulin-dependent kinase kinases (CaMKKs). However, MCh-induced AMPK activation is calcium-dependent and mediated by CaMKK1 and CaMKK2 isoforms. In contrast, PD 102807-induced signaling is calcium-independent and mediated by the atypical subtype protein kinase C-iota and the CaMKK1 (but not CaMKK2) isoform. Both MCh- and PD 102807-induced AMPK activation involve the AMPK α1 isoform. PD 102807-induced AMPK α1 (but not AMPK α2) isoform activation mediates inhibition of the mammalian target of rapamycin complex 1 (mTORC1) in ASM cells, as demonstrated by increased Raptor (regulatory-associated protein of mTOR) phosphorylation as well as inhibition of phospho-S6 protein and serum response element-luciferase activity. The mTORC1 inhibitor rapamycin and the AMPK activator metformin both mimic the ability of PD 102807 to attenuate transforming growth factor-β-induced α-smooth muscle actin expression (a marker of hypercontractile ASM). These data indicate that PD 102807 transduces a signaling pathway (AMPK-mediated mTORC1 inhibition) qualitatively distinct from canonical M3 mAChR signaling to prevent pathogenic remodeling of ASM, thus demonstrating PD 102807 is a biased M3 mAChR ligand with therapeutic potential for the management of obstructive airway disease.

Biochemical and physiological insights into TRH receptor-mediated signaling

Thyrotropin-releasing hormone (TRH) is an important endocrine agent that regulates the function of cells in the anterior pituitary and the central and peripheral nervous systems. By controlling the synthesis and release of thyroid hormones, TRH affects many physiological functions, including energy homeostasis. This hormone exerts its effects through G protein-coupled TRH receptors, which signal primarily through G_q/11 but may also utilize other G protein classes under certain conditions. Because of the potential therapeutic benefit, considerable attention has been devoted to the synthesis of new TRH analogs that may have some advantageous properties compared with TRH. In this context, it may be interesting to consider the phenomenon of biased agonism and signaling at the TRH receptor. This possibility is supported by some recent findings. Although knowledge about the mechanisms of TRH receptor-mediated signaling has increased steadily over the past decades, there are still many unanswered questions, particularly about the molecular details of post-receptor signaling. In this review, we summarize what has been learned to date about TRH receptor-mediated signaling, including some previously undiscussed information, and point to future directions in TRH research that may offer new insights into the molecular mechanisms of TRH receptor-triggered actions and possible ways to modulate TRH receptor-mediated signaling.

OXTRHigh stroma fibroblasts control the invasion pattern of oral squamous cell carcinoma via ERK5 signaling

The Pattern Of Invasion (POI) of tumor cells into adjacent normal tissues clinically predicts postoperative tumor metastasis/recurrence of early oral squamous cell carcinoma (OSCC), but the mechanisms underlying the development of these subtypes remain unclear. Focusing on the highest score of POIs (Worst POI, WPOI) present within each tumor, we observe a disease progression-driven shift of WPOI towards the high-risk type 4/5, associated with a mesenchymal phenotype in advanced OSCC. WPOI 4-5-derived cancer-associated fibroblasts (CAFs^WPOI4-5), characterized by high oxytocin receptor expression (OXTR^High), contribute to local-regional metastasis. OXTR^High CAFs induce a desmoplastic stroma and CCL26 is required for the invasive phenotype of CCR3⁺ tumors. Mechanistically, OXTR activates nuclear ERK5 transcription signaling via Gαq and CDC37 to maintain high levels of OXTR and CCL26. ERK5 ablation reprograms the pro-invasive phenotype of OXTR^High CAFs. Therefore, targeting ERK5 signaling in OXTR^High CAFs is a potential therapeutic strategy for OSCC patients with WPOI 4-5.

Worst pattern of invasion (WPOI) is a parameter used to quantify tumor invasiveness of oral squamous cell carcinoma (OSCC). Here the authors show that a fibroblast subset characterized by the expression of the oxytocin receptor is enriched in highly invasive WPOI 4-5 OSCC tumors and can be targeted to reduce the desmoplastic stroma and tumor metastasis.

β-Arrestin2 Is Critically Involved in the Differential Regulation of Phosphosignaling Pathways by Thyrotropin-Releasing Hormone and Taltirelin

In recent years, thyrotropin-releasing hormone (TRH) and its analogs, including taltirelin (TAL), have demonstrated a range of effects on the central nervous system that represent potential therapeutic agents for the treatment of various neurological disorders, including neurodegenerative diseases. However, the molecular mechanisms of their actions remain poorly understood. In this study, we investigated phosphosignaling dynamics in pituitary GH1 cells affected by TRH and TAL and the putative role of β-arrestin2 in mediating these effects. Our results revealed widespread alterations in many phosphosignaling pathways involving signal transduction via small GTPases, MAP kinases, Ser/Thr- and Tyr-protein kinases, Wnt/β-catenin, and members of the Hippo pathway. The differential TRH- or TAL-induced phosphorylation of numerous proteins suggests that these ligands exhibit some degree of biased agonism at the TRH receptor. The different phosphorylation patterns induced by TRH or TAL in β-arrestin2-deficient cells suggest that the β-arrestin2 scaffold is a key factor determining phosphorylation events after TRH receptor activation. Our results suggest that compounds that modulate kinase and phosphatase activity can be considered as additional adjuvants to enhance the potential therapeutic value of TRH or TAL.

The MAP kinase ERK5/MAPK7 is a downstream effector of oxytocin signaling in myometrial cells

The hormone oxytocin (OT) has pleiotropic activities both in the central nervous system as well as in peripheral tissues, including uterotonic effects on the myometrium during parturition. OT effects are mediated by a single transmembrane receptor, belonging to the GPCR (G protein-coupled receptor) superfamily and coupled primarily to Gq- and Gi-containing heterotrimeric G proteins. Upon receptor stimulation, one well-studied downstream effect is activation of the ERK1/2 MAP (mitogen-activated protein) kinase, and studies have shown that induction of COX-2 by OT in the myometrium required ERK1/2 activity. Many studies investigating the role of ERK1/2 in myometrial tissue were based on the use of chemical inhibitors that, to varying degrees, also inhibited ERK5/MAPK7. Here we report that OT activates ERK5 in a human myometrial cell line in a dose- and time-dependent manner through the activation of Gi/o heterotrimers. Using complementary approaches, we demonstrate that OT-induced COX-2 induction and the concomitant release of PGF2α into the media are primarily ERK5-dependent and to a much lesser extent ERK1/2-dependent. Moreover, in contrast to ERK1/2 activation, ERK5 activation is downstream of Gi/o activation. Here, we also found that ERK5 impacted both basal and to a lesser extent, OT-mediated myometrial cell contraction in vitro. Finally, tracking both ERK1/2 and ERK5 activity during different stages of gestation in rat myometrium, we showed that they followed distinct patterns starting at the onset of labor corresponding to the highest COX-2 expression levels. Overall, our results reveal an important, hitherto unrecognized role for ERK5 in myometrial cell contraction involving induction of COX-2. This novel pathway is likely to play an important role in supporting uterine contractions during parturition.

Designing Hybrids Targeting the Cholinergic System by Modulating the Muscarinic and Nicotinic Receptors: A Concept to Treat Alzheimer's Disease

The cholinergic hypothesis has been reported first being the cause of memory dysfunction in the Alzheimer's disease. Researchers around the globe have focused their attention on understanding the mechanisms of how this complicated system contributes to processes such as learning, memory, disorientation, linguistic problems, and behavioral issues in the indicated chronic neurodegenerative disease. The present review reports recent updates in hybrid molecule design as a strategy for selectively addressing multiple target proteins involved in Alzheimer's disease (AD) and the study of their therapeutic relevance. The rationale and the design of the bifunctional compounds will be discussed in order to understand their potential as tools to investigate the role of the cholinergic system in AD.

Protein Kinase C ζ Interacts with a Novel Binding Region of Gαq to Act as a Functional Effector

Heterotrimeric G proteins play an essential role in the initiation of G protein-coupled receptor (GPCR) signaling through specific interactions with a variety of cellular effectors. We have recently reported that GPCR activation promotes a direct interaction between Gαq and protein kinase C ζ (PKCζ), leading to the stimulation of the ERK5 pathway independent of the canonical effector PLCβ. We report herein that the activation-dependent Gαq/PKCζ complex involves the basic PB1-type II domain of PKCζ and a novel interaction module in Gαq different from the classical effector-binding site. Point mutations in this Gαq region completely abrogate ERK5 phosphorylation, indicating that Gαq/PKCζ association is required for the activation of the pathway. Indeed, PKCζ was demonstrated to directly bind ERK5 thus acting as a scaffold between Gαq and ERK5 upon GPCR activation. The inhibition of these protein complexes by G protein-coupled receptor kinase 2, a known Gαq modulator, led to a complete abrogation of ERK5 stimulation. Finally, we reveal that Gαq/PKCζ complexes link Gαq to apoptotic cell death pathways. Our data suggest that the interaction between this novel region in Gαq and the effector PKCζ is a key event in Gαq signaling.

The RAF-MEK-ERK pathway: targeting ERK to overcome obstacles to effective cancer therapy

AIM

Currently, dozens of BRAF inhibitors and MEK inhibitors targeting RAF-MEK-ERK pathway have been introduced into clinical trials for cancer therapy. However, after 6-8 months of initial response, acquired drug resistance among the majority of those treated patients sharply diminished their clinical efficacy.

DISCUSSION

Important mechanisms responsible for acquired resistance of BRAF inhibitors and MEK inhibitors have been elucidated. Continually, ERK1/2 locates in the critical position and features unique characteristics, such as activating hundreds of substrates, participating in feedback regulation, being catalyzed by MEK specifically and no acquired resistant mutation.

CONCLUSION

Taking in account the inspiring outcomes of ERK inhibitors in preclinical research, ERK1/2 might be the optimal target to overcome acquired drug resistance in RAF-MEK-ERK pathway.

Biased signaling regulates the pleiotropic effects of the urotensin II receptor to modulate its cellular behaviors

Biased agonism by G-protein-coupled receptor ligands has opened up strategies for targeted physiological or therapeutic actions. We hypothesized that urotensin II (UII)-derived peptides displayed unexpected physiological effects because of such biased signaling on the UII human urotensin (hUT) receptor. We determined the coupling to G proteins and β-arrestins of the UII-activated hUT receptor expressed in HEK293 using bioluminescence resonance energy transfer (BRET) biosensors, as well as the production of IP1-3 and cAMP using homogenous time-resolved Forster resonance energy transfer (FRET) (HTRF)-based assays. The activated receptor coupled to Gi1, GoA, Gq, and G13, excluding Gs, and recruited β-arrestins 1 and 2. Integration of these pathways led to a 2-phase kinetic phosphorylation of ERK1/2 kinases. The tested peptides induced three different profiles: UII, urotensin-related peptide (URP), and UII4-11 displayed the full profile; [Orn(8)]UII and [Orn(5)]URP activated G proteins, although with pEC50s 5-10× higher, and did not or barely recruited β-arrestin; urantide also failed to recruit β-arrestin but displayed a reversed rank order for Gi and Gq vs. Go pEC50s (-8.79±0.20, -8.43±0.21, and -7.86±0.36, respectively, for urantide, -7.87±0.10, -7.23±0.27, and -8.55±0.19, respectively, for [Orn(5)]URP) and was a partial agonist of all G-protein pathways. Interestingly, the peptides differently modulated cell survival but similarly induced cell migration and adhesion. Thus, we demonstrate biased signaling between β-arrestin and G proteins, and between G-protein subtypes, which dictates the receptor's cellular responses.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.