Abstract 6304: Decrypting EGFR signaling with BRET biosensors: A novel approach to study RTK mutations and the effects of inhibitors

Receptor tyrosine kinases (RTKs) comprise nodes at the center of complex signaling networks regulating various aspects of cell growth, differentiation and survival. Dysregulated RTK activity can alter many cellular processes, often culminating in cancer. Consequently, RTKs are prime targets for new anti-cancer agents. Yet, the efficacy of these drugs is limited by the development of drug-induced adverse events and acquired resistance. Discovery of novel therapeutics devoid of such limitations requires a deeper understanding of often overlooked determinants of therapeutic efficacy, including kinetics, subcellular localization, signaling bias and drug target mutations. In this study, we present a live-cell BRET-based biosensor platform applicable to studying such key elements, which allows for real-time spatiotemporal monitoring of RTK signaling across more than 10 effector proteins/pathways. Using EGFR and two of its ligands (EGF and Epiregulin) as a model system, we used our platform to highlight spatiotemporal signaling bias. Indeed, Epiregulin-induced engagement of effector protein Grb2 was more efficacious (25%) and displayed faster kinetics relative to that observed with EGF. Interestingly, EGF, but not Epiregulin, promoted a gradual increase in Grb2 levels at early endosomes. These data highlight opportunities for the identification of biased ligands displaying improved therapeutic efficacy. The impact of various glioblastoma-related EGFR mutations on receptor signaling were also profiled. The EGFR(vI) mutant showed constitutive activity on the PLCG1 effector pathway while the EGFR(vV) mutant was uniquely constitutive on SHIP1 and SHIP2 pathways. Further, EGFR(vIII) was devoid of constitutive activity on some effector proteins (PLCG1, Grb2) and highly constitutive on others (PI3K, SHIP1/2). The G598V mutant showed constitutive activity on all pathways except Grb2. This data shows how oncogenic mutations can rewire an RTK's signaling network. Identification and targeting of common signaling nodes exploited by various mutants could enhance drug efficacy and minimize mutation-related drug resistance. Finally, RTK biosensors were used to screen for tyrosine kinase inhibitor (TKI) activity on EGFR kinase domain mutants found in NSCLC (T790M and C797S). 1st generation TKI Gefitinib and 4th generation TKI Osimertinib reversed EGF activation of WT EGFR. However, only Osimertinib could reverse EGF-mediated activation of EGFR(T790M) but was ineffective at inhibiting EGFR(C797S). The platform could thus serve as a tool to assess anti-cancer treatment efficacy against various RTK mutants. These data position our biosensor platform as a versatile technology to study multiple facets of RTK biology and pharmacology. Access to real-time, spatiotemporal readouts across multiple signaling pathways will enable the development of novel RTK-based anti-cancer agents.

Citation Format: Florence Gross, Guilhem Dugast, Arturo D. Mancini, Stephan Schann, Xavier Leroy. Decrypting EGFR signaling with BRET biosensors: A novel approach to study RTK mutations and the effects of inhibitors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6304.

The regulator of G-protein signaling RGS16 promotes insulin secretion and β-cell proliferation in rodent and human islets

Objective

G protein-coupled receptor (GPCR) signaling regulates insulin secretion and pancreatic β cell-proliferation. While much knowledge has been gained regarding how GPCRs are activated in β cells, less is known about the mechanisms controlling their deactivation. In many cell types, termination of GPCR signaling is controlled by the family of Regulators of G-protein Signaling (RGS). RGS proteins are expressed in most eukaryotic cells and ensure a timely return to the GPCR inactive state upon removal of the stimulus. The aims of this study were i) to determine if RGS16, the most highly enriched RGS protein in β cells, regulates insulin secretion and β-cell proliferation and, if so, ii) to elucidate the mechanisms underlying such effects.

Methods

Mouse and human islets were infected with recombinant adenoviruses expressing shRNA or cDNA sequences to knock-down or overexpress RGS16, respectively. 60 h post-infection, insulin secretion and cAMP levels were measured in static incubations in the presence of glucose and various secretagogues. β-cell proliferation was measured in infected islets after 72 h in the presence of 16.7 mM glucose ± somatostatin and various inhibitors.

Results

RGS16 mRNA levels are strongly up-regulated in islets of Langerhans under hyperglycemic conditions in vivo and ex vivo. RGS16 overexpression stimulated glucose-induced insulin secretion in isolated mouse and human islets while, conversely, insulin secretion was impaired following RGS16 knock-down. Insulin secretion was no longer affected by RGS16 knock-down when islets were pre-treated with pertussis toxin to inactivate Gαi/o proteins, or in the presence of a somatostatin receptor antagonist. RGS16 overexpression increased intracellular cAMP levels, and its effects were blocked by an adenylyl cyclase inhibitor. Finally, RGS16 overexpression prevented the inhibitory effect of somatostatin on insulin secretion and β-cell proliferation.

Conclusions

Our results identify RGS16 as a novel regulator of β-cell function that coordinately controls insulin secretion and proliferation by limiting the tonic inhibitory signal exerted by δ-cell-derived somatostatin in islets.

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RGS16 is up-regulated under hyperglycemic conditions in islets.

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RGS16 is a key regulator of insulin secretion and β-cell proliferation.

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RGS16 attenuates Gαi/o protein activity downstream of δ-cell derived SST.

β-Arrestin Recruitment and Biased Agonism at Free Fatty Acid Receptor 1

FFAR1/GPR40 is a seven-transmembrane domain receptor (7TMR) expressed in pancreatic β cells and activated by FFAs. Pharmacological activation of GPR40 is a strategy under consideration to increase insulin secretion in type 2 diabetes. GPR40 is known to signal predominantly via the heterotrimeric G proteins Gq/11. However, 7TMRs can also activate functionally distinct G protein-independent signaling via β-arrestins. Further, G protein- and β-arrestin-based signaling can be differentially modulated by different ligands, thus eliciting ligand-specific responses ("biased agonism"). Whether GPR40 engages β-arrestin-dependent mechanisms and is subject to biased agonism is unknown. Using bioluminescence resonance energy transfer-based biosensors for real-time monitoring of cell signaling in living cells, we detected a ligand-induced GPR40-β-arrestin interaction, with the synthetic GPR40 agonist TAK-875 being more effective than palmitate or oleate in recruiting β-arrestins 1 and 2. Conversely, TAK-875 acted as a partial agonist of Gq/11-dependent GPR40 signaling relative to both FFAs. Pharmacological blockade of Gq activity decreased FFA-induced insulin secretion. In contrast, knockdown or genetic ablation of β-arrestin 2 in an insulin-secreting cell line and mouse pancreatic islets, respectively, uniquely attenuated the insulinotropic activity of TAK-875, thus providing functional validation of the biosensor data. Collectively, these data reveal that in addition to coupling to Gq/11, GPR40 is functionally linked to a β-arrestin 2-mediated insulinotropic signaling axis. These observations expose previously unrecognized complexity for GPR40 signal transduction and may guide the development of biased agonists showing improved clinical profile in type 2 diabetes.

GPR40 agonists for the treatment of type 2 diabetes: life after 'TAKing' a hit

The free fatty acid receptor GPR40 has been proposed as a potential target for type 2 diabetes (T2D) pharmacotherapy. This idea has been validated in both preclinical and clinical studies, in which activation of GPR40 was shown to improve glycaemic control by stimulating glucose-dependent insulin secretion; however, the recent termination of phase III clinical trials using the GPR40 agonist TAK-875 (fasiglifam) has raised important questions regarding the long-term safety and viability of targeting GPR40 and, more specifically, about our understanding of this receptor's basic biology. In the present review, we provide a summary of established and novel concepts related to GPR40's pharmacobiology and discuss the current status and future outlook for GPR40-based drug development for the treatment of T2D.

The fatty acid receptor FFA1/GPR40 a decade later: how much do we know?

Glucose homeostasis requires the highly coordinated regulation of insulin secretion by pancreatic β cells. This is primarily mediated by glucose itself, but other nutrients, including free fatty acids (FFAs), potentiate the insulinotropic capacity of glucose. A decade ago, the seven-transmembrane domain receptor (7TMR) GPR40 was demonstrated to be predominantly expressed in β cells and activated by long-chain FFAs. This discovery added a new dimension to our understanding of FFA-mediated control of glucose homeostasis. Furthermore, GPR40 has drawn considerable interest as a novel therapeutic target to enhance insulin secretion in type 2 diabetes. However, our understanding of the biology of GPR40 remains incomplete and its physiological role controversial. Here we summarize the current state of knowledge and emerging concepts regarding the role of GPR40 in regulating glucose homeostasis.

Malignant melanoma of the choroid in neurofibromatosis

A 60-year-old white woman with generalized neurofibromatosis and multiple melanocytic hamartomas of the iris developed an unusual choroidal mass, with secondary sensory retinal separation in the left eye. Ophthalmoscopically the tumor had a peculiar donut configuration that was caused by a large focus of central necrosis within a spindle B melanoma.