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Milewska-Kranc A, Ćwikła JB, Kolasinska-Ćwikła A. The Role of Receptor-Ligand Interaction in Somatostatin Signaling Pathways: Implications for Neuroendocrine Tumors. Cancers (Basel) 2023; 16:116. [PMID: 38201544 PMCID: PMC10778465 DOI: 10.3390/cancers16010116] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2023] [Revised: 12/19/2023] [Accepted: 12/22/2023] [Indexed: 01/12/2024] Open
Abstract
Neuroendocrine tumors (NETs) arise from neuroendocrine cells and manifest in diverse organs. Key players in their regulation are somatostatin and its receptors (SSTR1-SSTR5). Understanding receptor-ligand interactions and signaling pathways is vital for elucidating their role in tumor development and therapeutic potential. This review highlights SSTR characteristics, localization, and expression in tissues, impacting physiological functions. Mechanisms of somatostatin and synthetic analogue binding to SSTRs, their selectivity, and their affinity were analyzed. Upon activation, somatostatin initiates intricate intracellular signaling, involving cAMP, PLC, and MAP kinases and influencing growth, differentiation, survival, and hormone secretion in NETs. This review explores SSTR expression in different tumor types, examining receptor activation effects on cancer cells. SSTRs' significance as therapeutic targets is discussed. Additionally, somatostatin and analogues' role in hormone secretion regulation, tumor growth, and survival is emphasized, presenting relevant therapeutic examples. In conclusion, this review advances the knowledge of receptor-ligand interactions and signaling pathways in somatostatin receptors, with potential for improved neuroendocrine tumor treatments.
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Affiliation(s)
| | - Jarosław B. Ćwikła
- School of Medicine, University of Warmia and Mazury, Aleja Warszawska 30, 10-082 Olsztyn, Poland
- Diagnostic Therapeutic Center–Gammed, Lelechowska 5, 02-351 Warsaw, Poland
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2
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Ampofo E, Nalbach L, Menger MD, Laschke MW. Regulatory Mechanisms of Somatostatin Expression. Int J Mol Sci 2020; 21:ijms21114170. [PMID: 32545257 PMCID: PMC7312888 DOI: 10.3390/ijms21114170] [Citation(s) in RCA: 38] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2020] [Revised: 06/07/2020] [Accepted: 06/09/2020] [Indexed: 12/17/2022] Open
Abstract
Somatostatin is a peptide hormone, which most commonly is produced by endocrine cells and the central nervous system. In mammals, somatostatin originates from pre-prosomatostatin and is processed to a shorter form, i.e., somatostatin-14, and a longer form, i.e., somatostatin-28. The two peptides repress growth hormone secretion and are involved in the regulation of glucagon and insulin synthesis in the pancreas. In recent years, the processing and secretion of somatostatin have been studied intensively. However, little attention has been paid to the regulatory mechanisms that control its expression. This review provides an up-to-date overview of these mechanisms. In particular, it focuses on the role of enhancers and silencers within the promoter region as well as on the binding of modulatory transcription factors to these elements. Moreover, it addresses extracellular factors, which trigger key signaling pathways, leading to an enhanced somatostatin expression in health and disease.
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Affiliation(s)
- Emmanuel Ampofo
- Correspondence: ; Tel.: +49-6841-162-6561; Fax: +49-6841-162-6553
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Robichaux WG, Cheng X. Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development. Physiol Rev 2018; 98:919-1053. [PMID: 29537337 PMCID: PMC6050347 DOI: 10.1152/physrev.00025.2017] [Citation(s) in RCA: 138] [Impact Index Per Article: 23.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2017] [Revised: 09/05/2017] [Accepted: 09/06/2017] [Indexed: 12/13/2022] Open
Abstract
This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.
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Affiliation(s)
- William G Robichaux
- Department of Integrative Biology and Pharmacology, Texas Therapeutics Institute, The Brown Foundation Institute of Molecular Medicine, The University of Texas Health Science Center , Houston, Texas
| | - Xiaodong Cheng
- Department of Integrative Biology and Pharmacology, Texas Therapeutics Institute, The Brown Foundation Institute of Molecular Medicine, The University of Texas Health Science Center , Houston, Texas
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Halls ML, Canals M. Genetically Encoded FRET Biosensors to Illuminate Compartmentalised GPCR Signalling. Trends Pharmacol Sci 2017; 39:148-157. [PMID: 29054309 DOI: 10.1016/j.tips.2017.09.005] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2017] [Revised: 09/07/2017] [Accepted: 09/27/2017] [Indexed: 01/17/2023]
Abstract
Genetically encoded Förster resonance energy transfer (FRET) biosensors have been instrumental to our understanding of how intracellular signalling is organised and regulated within cells. In the last decade, the toolbox, dynamic range and applications of these sensors have expanded beyond basic cell biology applications. In particular, FRET biosensors have shed light onto the mechanisms that control the intracellular organisation of G protein-coupled receptor (GPCR) signalling and have allowed the visualisation of signalling events with unprecedented temporal and spatial resolution. Here we review the use of these sensors in the GPCR field and how it has already provided invaluable advances towards our understanding of the complexity of GPCR signalling.
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Affiliation(s)
- Michelle L Halls
- Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, VIC 3052, Australia.
| | - Meritxell Canals
- Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, VIC 3052, Australia.
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Hu B, Cilz NI, Lei S. Somatostatin depresses the excitability of subicular bursting cells: Roles of inward rectifier K + channels, KCNQ channels and Epac. Hippocampus 2017; 27:971-984. [PMID: 28558129 DOI: 10.1002/hipo.22744] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2017] [Revised: 05/08/2017] [Accepted: 05/18/2017] [Indexed: 12/17/2022]
Abstract
The hippocampus is a crucial component for cognitive and emotional processing. The subiculum provides much of the output for this structure but the modulation and function of this region is surprisingly under-studied. The neuromodulator somatostatin (SST) interacts with five subtypes of SST receptors (sst1 to sst5 ) and each of these SST receptor subtypes is coupled to Gi proteins resulting in inhibition of adenylyl cyclase (AC) and decreased level of intracellular cAMP. SST modulates many physiological functions including cognition, emotion, autonomic responses and locomotion. Whereas SST has been shown to depress neuronal excitability in the subiculum, the underlying cellular and molecular mechanisms have not yet been determined. Here, we show that SST hyperpolarized two classes of subicular neurons with a calculated EC50 of 0.1 μM. Application of SST (1 μM) induced outward holding currents by primarily activating K+ channels including the G-protein-activated inwardly-rectifying potassium channels (GIRK) and KCNQ (M) channels, although inhibition of cation channels in some cells may also be implicated. SST-elicited hyperpolarization was mediated by activation of sst2 receptors and required the function of G proteins. The SST-induced hyperpolarization resulted from decreased activity of AC and reduced levels of cAMP but did not require the activity of either PKA or PKC. Inhibition of Epac2, a guanine nucleotide exchange factor, partially blocked SST-mediated hyperpolarization of subicular neurons. Furthermore, application of SST resulted in a robust depression of subicular action potential firing and the SST-induced hyperpolarization was responsible for its inhibitory action on LTP at the CA1-subicilum synapses. Our results provide a novel cellular and molecular mechanism that may explain the roles of SST in modulation of subicular function and be relevant to SST-related physiological functions.
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Affiliation(s)
- Binqi Hu
- Department of Biomedical Sciences, School of Medicine and Health Sciences, University of North Dakota, Grand Forks, North Dakota 58203
| | - Nicholas I Cilz
- Department of Biomedical Sciences, School of Medicine and Health Sciences, University of North Dakota, Grand Forks, North Dakota 58203
| | - Saobo Lei
- Department of Biomedical Sciences, School of Medicine and Health Sciences, University of North Dakota, Grand Forks, North Dakota 58203
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Paramonov VM, Mamaeva V, Sahlgren C, Rivero-Müller A. Genetically-encoded tools for cAMP probing and modulation in living systems. Front Pharmacol 2015; 6:196. [PMID: 26441653 PMCID: PMC4569861 DOI: 10.3389/fphar.2015.00196] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2015] [Accepted: 08/28/2015] [Indexed: 11/19/2022] Open
Abstract
Intracellular 3′-5′-cyclic adenosine monophosphate (cAMP) is one of the principal second messengers downstream of a manifold of signal transduction pathways, including the ones triggered by G protein-coupled receptors. Not surprisingly, biochemical assays for cAMP have been instrumental for basic research and drug discovery for decades, providing insights into cellular physiology and guiding pharmaceutical industry. However, despite impressive track record, the majority of conventional biochemical tools for cAMP probing share the same fundamental shortcoming—all the measurements require sample disruption for cAMP liberation. This common bottleneck, together with inherently low spatial resolution of measurements (as cAMP is typically analyzed in lysates of thousands of cells), underpin the ensuing limitations of the conventional cAMP assays: (1) genuine kinetic measurements of cAMP levels over time in a single given sample are unfeasible; (2) inability to obtain precise information on cAMP spatial distribution and transfer at subcellular levels, let alone the attempts to pinpoint dynamic interactions of cAMP and its effectors. At the same time, tremendous progress in synthetic biology over the recent years culminated in drastic refinement of our toolbox, allowing us not only to bypass the limitations of conventional assays, but to put intracellular cAMP life-span under tight control—something, that seemed scarcely attainable before. In this review article we discuss the main classes of modern genetically-encoded tools tailored for cAMP probing and modulation in living systems. We examine the capabilities and weaknesses of these different tools in the context of their operational characteristics and applicability to various experimental set-ups involving living cells, providing the guidance for rational selection of the best tools for particular needs.
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Affiliation(s)
- Valeriy M Paramonov
- Department of Physiology, Institute of Biomedicine, University of Turku , Turku, Finland ; Turku Center for Biotechnology, University of Turku and Åbo Akademi University , Turku, Finland
| | - Veronika Mamaeva
- Department of Clinical Science, University of Bergen , Bergen, Norway
| | - Cecilia Sahlgren
- Turku Center for Biotechnology, University of Turku and Åbo Akademi University , Turku, Finland ; Department of Biomedical Engineering, Eindhoven University of Technology , Eindhoven, Netherlands
| | - Adolfo Rivero-Müller
- Department of Physiology, Institute of Biomedicine, University of Turku , Turku, Finland ; Faculty of Natural Sciences and Technology, Åbo Akademi University , Turku, Finland ; Department of Biochemistry and Molecular Biology, Medical University of Lublin , Lublin, Poland
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Schmidt M, Dekker FJ, Maarsingh H. Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions. Pharmacol Rev 2013; 65:670-709. [PMID: 23447132 DOI: 10.1124/pr.110.003707] [Citation(s) in RCA: 203] [Impact Index Per Article: 18.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Since the discovery nearly 60 years ago, cAMP is envisioned as one of the most universal and versatile second messengers. The tremendous feature of cAMP to tightly control highly diverse physiologic processes, including calcium homeostasis, metabolism, secretion, muscle contraction, cell fate, and gene transcription, is reflected by the award of five Nobel prizes. The discovery of Epac (exchange protein directly activated by cAMP) has ignited a new surge of cAMP-related research and has depicted novel cAMP properties independent of protein kinase A and cyclic nucleotide-gated channels. The multidomain architecture of Epac determines its activity state and allows cell-type specific protein-protein and protein-lipid interactions that control fine-tuning of pivotal biologic responses through the "old" second messenger cAMP. Compartmentalization of cAMP in space and time, maintained by A-kinase anchoring proteins, phosphodiesterases, and β-arrestins, contributes to the Epac signalosome of small GTPases, phospholipases, mitogen- and lipid-activated kinases, and transcription factors. These novel cAMP sensors seem to implement certain unexpected signaling properties of cAMP and thereby to permit delicate adaptations of biologic responses. Agonists and antagonists selective for Epac are developed and will support further studies on the biologic net outcome of the activation of Epac. This will increase our current knowledge on the pathophysiology of devastating diseases, such as diabetes, cognitive impairment, renal and heart failure, (pulmonary) hypertension, asthma, and chronic obstructive pulmonary disease. Further insights into the cAMP dynamics executed by the Epac signalosome will help to optimize the pharmacological treatment of these diseases.
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Affiliation(s)
- Martina Schmidt
- Department of Molecular Pharmacology, Groningen Research Institute for Pharmacy, University of Groningen, 9713 AV Groningen, The Netherlands.
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Shpakov AO. Somatostatin receptors and signaling cascades coupled to them. J EVOL BIOCHEM PHYS+ 2012. [DOI: 10.1134/s0022093012040020] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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Calebiro D. Thyroid-stimulating hormone receptor activity after internalization. ANNALES D'ENDOCRINOLOGIE 2011; 72:64-7. [PMID: 21511242 DOI: 10.1016/j.ando.2011.03.008] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The thyroid-stimulating hormone (TSH) receptor (TSHR) belongs to the large family of G-protein-coupled receptors (GPCRs) and is predominantly coupled to G(s). Thus, the effects of TSH are largely mediated by the stimulation of adenylyl cyclase and the ensuing rise of intracellular cyclic AMP (cAMP) concentrations. Like for other GPCRs, a prolonged stimulation of the TSHR leads to its internalization into endosomes followed by its recycling to the cell surface. Until recently, GPCRs were believed to activate "classical" G-protein-dependent pathways only when located on the cell surface and to cease doing so upon agonist-induced internalization. However, our recent findings on the TSHR and similar ones on the parathyroid hormone and sphingosine receptors suggest that internalized GPCRs can continue to signal through G(s)-cAMP in an intracellular compartment. Interestingly, this type of intracellular cAMP signaling differs from that occurring on the cell surface, as it is persistent and apparently leads to specific signaling outcomes. Although further studies are needed to investigate the possible physiological and pathophysiological consequences of GPCR-cAMP signaling in the endocytic compartment, endosomes should no longer be viewed as passive carriers for receptors en route to degradation but rather as specialized intracellular platforms for GPCR signaling.
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Affiliation(s)
- D Calebiro
- Institute of Pharmacology and Toxicology and Rudolf-Virchow Center, DFG-Research center for experimental biomedicine, University of Würzburg, Würzburg, Germany.
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FRET measurements of intracellular cAMP concentrations and cAMP analog permeability in intact cells. Nat Protoc 2011; 6:427-38. [PMID: 21412271 DOI: 10.1038/nprot.2010.198] [Citation(s) in RCA: 165] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Real-time measurements of second messengers in living cells, such as cAMP, are usually performed by ratiometric fluorescence resonance energy transfer (FRET) imaging. However, correct calibration of FRET ratios, accurate calculations of absolute cAMP levels and actual permeabilities of different cAMP analogs have been challenging. Here we present a protocol that allows precise measurements of cAMP concentrations and kinetics by expressing FRET-based cAMP sensors in cells and modulating them with an inhibitor of adenylyl cyclase activity and a cell-permeable cAMP analog that fully inhibits and activates the sensors, respectively. Using this protocol, we observed different basal cAMP levels in primary mouse cardiomyocytes, thyroid cells and in 293A cells. The protocol can be generally applied for calibration of second messenger or metabolite concentrations measured by FRET, and for studying kinetics and pharmacological properties of their membrane-permeable analogs. The complete procedure, including cell preparation and FRET measurements, takes 3-6 d.
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Mehta S, Zhang J. Reporting from the field: genetically encoded fluorescent reporters uncover signaling dynamics in living biological systems. Annu Rev Biochem 2011; 80:375-401. [PMID: 21495849 PMCID: PMC4384825 DOI: 10.1146/annurev-biochem-060409-093259] [Citation(s) in RCA: 80] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Abstract
Real-time visualization of a wide range of biochemical processes in living systems is being made possible through the development and application of genetically encoded fluorescent reporters. These versatile biosensors have proven themselves tailor-made to the study of signal transduction, and in this review, we discuss some of the unique insights that they continue to provide regarding the spatial organization and dynamic regulation of intracellular signaling networks. In addition, we explore the more recent push to expand the scope of biological phenomena that can be monitored using these reporters, while also considering the potential to integrate this highly adaptable technology with a number of emerging techniques that may significantly broaden our view of how networks of biochemical processes shape larger biological phenomena.
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Affiliation(s)
- Sohum Mehta
- Department of Pharmacology and Molecular Sciences, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
| | - Jin Zhang
- Department of Pharmacology and Molecular Sciences, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
- Solomon H. Snyder Department of Neuroscience and Department of Oncology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
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