Determinants involved in subtype-specific functions of rat trace amine-associated receptors 1 and 4

Succinate receptor 1 signaling mutually depends on subcellular localization and cellular metabolism

Succinate is a pivotal tricarboxylic acid cycle metabolite but also specifically activates the Gi- and Gq-coupled succinate receptor 1 (SUCNR1). Contradictory roles of succinate and succinate-SUCNR1 signaling include reports about its anti- or pro-inflammatory effects. The link between cellular metabolism and localization-dependent SUCNR1 signaling qualifies as a potential cause for the reported conflicts. To systematically address this connection, we used a diverse set of methods, including several bioluminescence resonance energy transfer-based biosensors, dynamic mass redistribution measurements, second messenger and kinase phosphorylation assays, calcium imaging, and metabolic analyses. Different cellular metabolic states were mimicked using glucose (Glc) or glutamine (Gln) as available energy substrates to provoke differential endogenous succinate (SUC) production. We show that SUCNR1 signaling, localization, and metabolism are mutually dependent, with SUCNR1 showing distinct spatial and energy substrate-dependent Gi and Gq protein activation. We found that Gln-consumption associated with a higher rate of oxidative phosphorylation causes increased extracellular SUC concentrations, accompanied by a higher rate of SUCNR1 internalization, reduced miniGq protein recruitment to the plasma membrane, and lower Ca2+ signals. In Glc, under basal conditions, SUCNR1 causes stronger Gq than Gi protein activation, while the opposite is true upon stimulation with an agonist. In addition, SUCNR1 specifically interacts with miniG proteins in endosomal compartments. In THP-1 cells, polarized to M2-like macrophages, endogenous SUCNR1-mediated Gi signaling stimulates glycolysis, while Gq signaling inhibits the glycolytic rate. Our results suggest that the metabolic context determines spatially dependent SUCNR1 signaling, which in turn modulates cellular energy homeostasis and mediates adaptations to changes in SUC concentrations.

Protocol to characterize Gi/o and Gs protein-coupled receptors in transiently transfected cells using ELISA and cAMP measurements

Activation of Gs or Gi/o protein-coupled receptors (GPCRs) leads to changes of intracellular cyclic adenosine monophosphate (cAMP) levels. This protocol describes steps for cloning HA- and FLAG-tagged GPCRs, transient transfection of CHO-K1 or HEK293-T cells, and determination of basal and ligand-induced changes in intracellular cAMP levels. We detail enzyme-linked immunosorbent assays to determine relative GPCR plasma membrane and total expression levels. For complete details on the use and execution of this protocol, please refer to Schulze et al. (2022).1.

Combining metabolic phenotype determination with metabolomics and transcriptional analyses to reveal pathways regulated by hydroxycarboxylic acid receptor 2

BACKGROUND

The adaptation of cellular metabolism is considered a hallmark of cancer. Oncogenic signaling pathways support tumorigenesis and cancer progression through the induction of certain metabolic phenotypes associated with altered regulation of key metabolic enzymes. Hydroxycarboxylic acid receptor 2 (HCA₂) is a G protein-coupled receptor previously shown to act as a tumor suppressor. Here, we aimed to unveil the connection between cellular metabolism and HCA₂ in BT-474 cells. Moreover, we intend to clarify how well this metabolic phenotype is reflected in transcriptional changes and metabolite levels as determined by global metabolomics analyses.

METHODS

We performed both, siRNA mediated knockdown of HCA₂ and stimulation with the HCA₂-specific agonist monomethyl fumarate. Seahorse technology was used to determine the role of HCA₂ in BT-474 breast cancer cell metabolism and its potential to induce a switch in the metabolic phenotype in the presence of different energy substrates. Changes in the mRNA expression of metabolic enzymes were detected with real-time quantitative PCR (RT-qPCR). Untargeted liquid chromatography-mass spectrometry (LC-MS) metabolic profiling was used to determine changes in metabolite levels.

RESULTS

Knockdown or stimulation of HCA₂ induced changes in the metabolic phenotype of BT474 cells dependent on the availability of energy substrates. The presence of HCA₂ was associated with increased glycolytic flux with no fatty acids available. This was reflected in the increased mRNA expression of the glycolytic enzymes PFKFB4 and PKM2, which are known to promote the Warburg effect and have been described as prognostic markers in different types of cancer. With exogenous palmitate present, HCA₂ caused elevated fatty acid oxidation and likely lipolysis. The increase in lipolysis was also detectable at the transcriptional level of ATGL and the metabolite levels of palmitic and stearic acid.

CONCLUSIONS

We combined metabolic phenotype determination with metabolomics and transcriptional analyses and identified HCA₂ as a regulator of glycolytic flux and fatty acid metabolism in BT-474 breast cancer cells. Thus, HCA₂, for which agonists are already widely used to treat diseases such as psoriasis or hyperlipidemia, may prove useful as a target in combination cancer therapy.

Succinate receptor 1 inhibits mitochondrial respiration in cancer cells addicted to glutamine

Cancer cells display metabolic alterations to meet the bioenergetic demands for their high proliferation rates. Succinate is a central metabolite of the tricarboxylic acid (TCA) cycle, but was also shown to act as an oncometabolite and to specifically activate the succinate receptor 1 (SUCNR1), which is expressed in several types of cancer. However, functional studies focusing on the connection between SUCNR1 and cancer cell metabolism are still lacking. In the present study, we analyzed the role of SUCNR1 for cancer cell metabolism and survival applying different signal transduction, metabolic and imaging analyses. We chose a gastric, a lung and a pancreatic cancer cell line for which our data revealed functional expression of SUCNR1. Further, presence of glutamine (Gln) caused high respiratory rates and elevated expression of SUCNR1. Knockdown of SUCNR1 resulted in a significant increase of mitochondrial respiration and superoxide production accompanied by an increase in TCA cycle throughput and a reduction of cancer cell survival in the analyzed cancer cell lines. Combination of SUCNR1 knockdown and treatment with the chemotherapeutics cisplatin and gemcitabine further increased cancer cell death. In summary, our data implicates that SUCNR1 is crucial for Gln-addicted cancer cells by limiting TCA cycle throughput, mitochondrial respiration and the production of reactive oxygen species, highlighting its potential as a pharmacological target for cancer treatment.

Endocrine, Metabolic and Pharmacological Effects of Thyronamines (TAM), Thyroacetic Acids (TA) and Thyroid Hormone Metabolites (THM) - Evidence from in vitro, Cellular, Experimental Animal and Human Studies

Thyroid hormone metabolites (THM) with few or no iodine substituents such as 3,5-T2, the thyronamines 3-T1AM and T0AM, and their oxidation products, the thyroacetic acids (TA) formed by monoamine oxidases, have recently attracted major interest due to their metabolic actions which are in part distinct from those of the classical thyromimetic hormone T3, the major ligand of T3 receptors. This review compiles and discusses in vitro effects of 3,5-T2, TAM and TA reported for thyrocytes, pancreatic islets and hepatocytes as well as findings from in vivo studies in mouse models after single or repeated administration of pharmacological doses of these agents. Comparison of the 3,5-T2 effects on the transcriptome with not yet published proteome data in livers of obese mice on high fat diet indicate a distinct anti-steatotic effect of this THM. Furthermore, uptake, metabolism, and cellular actions via various receptors such as trace amine-associated receptors (TAAR), alpha-adrenergic, GPCR and T3 receptors are discussed. Studies on postulated pathways of biosynthesis of 3-T1AM, its effects on the HPT-axis and thyroid gland as well as insulin secretion are reviewed. 3-T1AM also acts on hepatocytes and interferes with TRPM8-dependent signaling in human cell lines related to the eye compartment. Human studies are presented which address potential biosynthesis routes of 3,5-T2 and 3-T1AM from THM precursors, especially T3. The current state of diagnostic analytics of these minor THM in human blood is portrayed comparing and critically discussing the still divergent findings based on classical immunoassay and recently developed liquid-chromatography/mass- spectrometry methods, which allow quantification of the thyronome spectrum from one single small volume serum sample. The clinical perspectives of use and potential abuse of these biologically active THM is addressed.

Metabolites of lactic acid bacteria present in fermented foods are highly potent agonists of human hydroxycarboxylic acid receptor 3

The interplay of microbiota and the human host is physiologically crucial in health and diseases. The beneficial effects of lactic acid bacteria (LAB), permanently colonizing the human intestine or transiently obtained from food, have been extensively reported. However, the molecular understanding of how LAB modulate human physiology is still limited. G protein-coupled receptors for hydroxycarboxylic acids (HCAR) are regulators of immune functions and energy homeostasis under changing metabolic and dietary conditions. Most mammals have two HCAR (HCA₁, HCA₂) but humans and other hominids contain a third member (HCA₃) in their genomes. A plausible hypothesis why HCA₃ function was advantageous in hominid evolution was lacking. Here, we used a combination of evolutionary, analytical and functional methods to unravel the role of HCA₃in vitro and in vivo. The functional studies included different pharmacological assays, analyses of human monocytes and pharmacokinetic measurements in human. We report the discovery of the interaction of D-phenyllactic acid (D-PLA) and the human host through highly potent activation of HCA₃. D-PLA is an anti-bacterial metabolite found in high concentrations in LAB-fermented food such as Sauerkraut. We demonstrate that D-PLA from such alimentary sources is well absorbed from the human gut leading to high plasma and urine levels and triggers pertussis toxin-sensitive migration of primary human monocytes in an HCA₃-dependent manner. We provide evolutionary, analytical and functional evidence supporting the hypothesis that HCA₃ was consolidated in hominids as a new signaling system for LAB-derived metabolites.

Although it has been known for 15 years that HCA₃ is present in humans and other hominids but absent in all other mammals, no study so far aimed to understand why HCA₃ was functionally preserved during evolution. Here, we take advantage of evolutionary analyses which we combine with functional assays of hominid HCA₃ orthologs. In search for a reasonable scenario explaining the accumulated amino acid changes in HCA₃ of hominids we discovered D-phenyllactic acid (D-PLA), a metabolite produced by lactic acid bacteria (LAB), as the so far most potent agonist specifically activating HCA₃. Further, oral ingestion of Sauerkraut, known to contain high levels of D-PLA, caused subsequent plasma concentrations sufficient to activate HCA₃. Our data interpreted in an evolutionary context suggests that the availability of a new food repertoire under changed ecological conditions triggered the fixation of HCA₃ which took over new functions in hominids.

These findings are particularly important because they unveiled HCA₃, which is not only expressed in various immune cells but also adipocytes, lung and skin, as a player that transfers signals of LAB-derived metabolites into a physiological response in humans. This opens up new directions towards the understanding of the versatile beneficial effects of LAB and their metabolites for humans.