A gut-derived hormone regulates cholesterol metabolism.

The reciprocal coordination between cholesterol absorption in the intestine and de novo cholesterol synthesis in the liver is essential for maintaining cholesterol homeostasis, yet the mechanisms governing the opposing regulation of these processes remain poorly understood. Here, we identify a hormone, Cholesin, which is capable of inhibiting cholesterol synthesis in the liver, leading to a reduction in circulating cholesterol levels. Cholesin is encoded by a gene with a previously unknown function (C7orf50 in humans; 3110082I17Rik in mice). It is secreted from the intestine in response to cholesterol absorption and binds to GPR146, an orphan G-protein-coupled receptor, exerting antagonistic downstream effects by inhibiting PKA signaling and thereby suppressing SREBP2-controlled cholesterol synthesis in the liver. Therefore, our results demonstrate that the Cholesin-GPR146 axis mediates the inhibitory effect of intestinal cholesterol absorption on hepatic cholesterol synthesis. This discovered hormone, Cholesin, holds promise as an effective agent in combating hypercholesterolemia and atherosclerosis.

Cholesin receptor signalling is active in cardiovascular system-associated adipose tissue and correlates with SGLT2i treatment in patients with diabetes.

BACKGROUND: Recently deorphanized G protein-coupled receptor 146 (GPR146) was shown to respond to signal from a newly identified hormone-cholesin-and to play a role in hepatic lipid metabolism. However, the importance of its biological activity in human organism remains elusive, mainly due to the lack of studies on human tissues up to this point. This study aimed to identify the cholesin receptor-associated genes and clinical factors linked with their expression in cardiovascular system and associated adipose tissues. METHODS: Right cardiac auricle, aortic wall, saphenous vein, and adipose tissue (periaortic-PAT, epicardial-EAT, thymic-TAT) samples were collected during coronary artery bypass grafting. Clinical records of the study participants were assessed for the presence of diabetes, medications taken and serum cholesterol levels. GPR146 mRNA expression in all gathered tissues was assessed with qPCR, and RNA seqencing was performed in selected tissues of 20 individuals to identify pathways associated with GPR146 expression. RESULTS: We included 46 participants [37 male, 23 with type 2 diabetes, median age 68.50 (Q1-Q3: 63.00-72.00) years, BMI 28.39 (26.06-31.49) kg/m2]. GPR146 expression in adipose tissues significantly correlated with BMI, c-peptide, total cholesterol, and LDL concentrations. Selected metabolic pathways were significantly and positively enriched in GPR146-dependent manner. GPR146-coexpressed genes contained key regulators of lipid metabolism involved in such pathways as fatty acid metabolism, tricarboxilic acid cycle and peroxisomal metabolism. Those genes correlated positively with serum concentrations of LDL, HDL, and total cholesterol. SGLT2i treatment was associated with inversion of GPR146-related signature in EAT, suggesting potential impact on cholesin-GPR146 network. CONCLUSIONS: GPR146 expression is associated with serum lipids and metabolically-relevant transcriptomic changes in EAT similar to SGLT2i-associated ones.

Cholesin and GPR146 in Modulating Cholesterol Biosynthesis.

BACKGROUND: Cholesterol homeostasis in the human body is a crucial process that involves a delicate balance between dietary cholesterol absorption in the intestine and de novo cholesterol synthesis in the liver. Both pathways contribute significantly to the overall pool of cholesterol in the body, influencing plasma cholesterol levels and impacting cardiovascular health. Elevated absorption of cholesterol in the intestines has a suppressive impact on the synthesis of cholesterol in the liver, serving to preserve cholesterol balance. Nonetheless, the precise mechanisms driving this phenomenon remain largely unclear. SUMMARY: This review aimed to discuss the previously unrecognized role of cholesin and GPR146 in the regulation of cholesterol biosynthesis, providing a novel conceptual framework for understanding cholesterol homeostasis. KEY MESSAGES: The discovery of cholesin, a novel protein implicated in the regulation of cholesterol homeostasis, represents a significant advancement in our understanding of cholesterol biosynthesis and its associated pathways. The cholesin-GPR146 axis could have profound implications across various therapeutic areas concerning abnormal cholesterol metabolism, offering new hope for patients and improving overall healthcare outcomes.

Is GPR146 really the receptor for proinsulin C-peptide?

Proinsulin C-peptide has previously been proposed to interact with a G-protein coupled receptor (GPCR), specifically the orphan receptor GPR146. To investigate the potential of C-peptide in treating complications of diabetes, such as kidney damage, it is necessary to understand its mode of action. We used CHO-K1 cells expressing human GPR146 to study human and murine C-peptide in dynamic mass redistribution and GPCR ß-arrestin assays, as well as with fluorescence confocal microscopy. Neither assay revealed any significant intracellular response to C-peptide at concentrations of up to 33 µM. We observed no internalisation of C-peptide by fluorescence microscopy. Our results do not support GPR146 as the receptor for C-peptide, but suggest that further investigations of the mode of action of C-peptide should be undertaken.

Elimination of GPR146-mediated antiviral function through IRF3/HES1-signalling pathway.

As the most important host defence against viral infection, interferon (IFN) stimulates hundreds of antiviral genes (ISGs) that together establish an 'antiviral state'. However, the antiviral function of most ISGs in viral infection still need further exploration. Here, we demonstrated that the expression of G-protein-coupled receptor 146 (GPR146) is highly increased by both IFN-ß and IFN-γ in a signal transducer and activator of transcription 1-dependent signalling pathway. Most importantly, overexpression of GPR146 protects the host cells from vesicular stomatitis virus and Newcastle disease virus infection but not from infection by herpes simplex virus. In contrast, the virus-induced IFN-ß production changed little in Gpr146-knockout cells. Furthermore, the Gpr146-deficient mice showed similar susceptibility to wild-type mice with vesicular stomatitis virus infection. Interestingly, the expression of GPR146 in virus-infected cells was strikingly reduced and can partially explain why the viral infection was little influenced in Gpr146-knockout mice. Surprisingly, virus-activated IFN regulatory factor 3 (IRF3) signalling not only induces the expression of IFN but also represses GPR146 expression through HES1 (hairy and enhancer of split-1)-mediated transcriptional activity to establish a dynamic equilibrium between pro-viral and antiviral stages in host cells. Taken together, these data reveal the antiviral role of GPR146 in fighting viral infection although the GPR146-mediated protection is eliminated by IRF3/HES1-signalling, which suggests a potential therapeutic significance of both GPR146 and HES1 signalling in viral infection.

Targeting orphan G protein-coupled receptors for the treatment of diabetes and its complications: C-peptide and GPR146.

G protein-coupled receptors (GPCRs) are the most abundant receptor family encoded by the human genome and are the targets of a high percentage of drugs currently in use or in clinical trials for the treatment of diseases such as diabetes and its associated complications. Thus, orphan GPCRs, for which the ligand is unknown, represent an important untapped source of therapeutic potential for the treatment of many diseases. We have identified the previously orphan GPCR, GPR146, as the putative receptor of proinsulin C-peptide, which may prove to be an effective treatment for diabetes-associated complications. For example, we have found a potential role of C-peptide and GPR146 in regulating the function of the retinal pigment epithelium, a monolayer of cells in the retina that serves as part of the blood-retinal barrier and is disrupted in diabetic macular oedema. However, C-peptide signalling in this cell type appears to depend at least in part on extracellular glucose concentration and its interaction with insulin. In this review, we discuss the therapeutic potential of orphan GPCRs with a special focus on C-peptide and GPR146, including past and current strategies used to 'deorphanize' this diverse family of receptors, past successes and the inherent difficulties of this process.

Low O2-induced ATP release from erythrocytes of humans with type 2 diabetes is restored by physiological ratios of C-peptide and insulin.

ATP release from erythrocytes in response to reduced oxygen (O2) tension stimulates local vasodilation, enabling these cells to direct perfusion to areas in skeletal muscle in need of O2. Erythrocytes of humans with type 2 diabetes do not release ATP in response to low O2. Both C-peptide and insulin individually inhibit low O2-induced ATP release from healthy human erythrocytes, yet when coadministered at physiological concentrations and ratios, no inhibition is seen. Here, we determined: that 1) erythrocytes of healthy humans and humans with type 2 diabetes possess a C-peptide receptor (GPR146), 2) the combination of C-peptide and insulin at physiological ratios rescues low O2-induced ATP release from erythrocytes of humans with type 2 diabetes, 3) residual C-peptide levels reported in humans with type 2 diabetes are not adequate to rescue low O2-induced ATP release in the presence of 1 nM insulin, and 4) the effects of C-peptide and insulin are neither altered by increased glucose levels nor explained by changes in erythrocyte deformability. These results suggest that the addition of C-peptide to the treatment regimen for type 2 diabetes could have beneficial effects on tissue oxygenation, which would help to ameliorate the concomitant peripheral vascular disease.

Elucidating the multifaceted roles of GPR146 in non-specific orbital inflammation: a concerted analytical approach through the prisms of bioinformatics and machine learning.

Background: Non-specific Orbital Inflammation (NSOI) is a chronic idiopathic condition marked by extensive polymorphic lymphoid infiltration in the orbital area. The integration of metabolic and immune pathways suggests potential therapeutic roles for C-peptide and G protein-coupled receptor 146 (GPR146) in diabetes and its sequelae. However, the specific mechanisms through which GPR146 modulates immune responses remain poorly understood. Furthermore, the utility of GPR146 as a diagnostic or prognostic marker for NSOI has not been conclusively demonstrated. Methods: We adopted a comprehensive analytical strategy, merging differentially expressed genes (DEGs) from the Gene Expression Omnibus (GEO) datasets GSE58331 and GSE105149 with immune-related genes from the ImmPort database. Our methodology combined LASSO regression and support vector machine-recursive feature elimination (SVM-RFE) for feature selection, followed by Gene Set Enrichment Analysis (GSEA) and Gene Set Variation Analysis (GSVA) to explore gene sets co-expressed with GPR146, identifying a significant enrichment in immune-related pathways. The tumor microenvironment's immune composition was quantified using the CIBERSORT algorithm and the ESTIMATE method, which confirmed a positive correlation between GPR146 expression and immune cell infiltration. Validation of GPR146 expression was performed using the GSE58331 dataset. Results: Analysis identified 113 DEGs associated with GPR146, with a significant subset showing distinct expression patterns. Using LASSO and SVM-RFE, we pinpointed 15 key hub genes. Functionally, these genes and GPR146 were predominantly linked to receptor ligand activity, immune receptor activity, and cytokine-mediated signaling. Specific immune cells, such as memory B cells, M2 macrophages, resting mast cells, monocytes, activated NK cells, plasma cells, and CD8+ T cells, were positively associated with GPR146 expression. In contrast, M0 macrophages, naive B cells, M1 macrophages, activated mast cells, activated memory CD4+ T cells, naive CD4+ T cells, and gamma delta T cells showed inverse correlations. Notably, our findings underscore the potential diagnostic relevance of GPR146 in distinguishing NSOI. Conclusion: Our study elucidates the immunological signatures associated with GPR146 in the context of NSOI, highlighting its prognostic and diagnostic potential. These insights pave the way for GPR146 to be a novel biomarker for monitoring the progression of NSOI, providing a foundation for future therapeutic strategies targeting immune-metabolic pathways.

Hypoxia activates GPR146 which participates in pulmonary vascular remodeling by promoting pyroptosis of pulmonary artery endothelial cells.

BACKGROUND: Hypoxia is a risk factor of pulmonary hypertension (PH) and may induce pulmonary artery endothelial cells (PAECs) injury and inflammation. Pyroptosis is a form of cell death through maturation and secretion of inflammatory mediators. However, the mechanistic association of pyroptosis, PAECs injury, and inflammation remain unknown. Here, we explored in detail the effects of hypoxia on pyroptosis of PAECs. EXPERIMENTAL APPROACH: Using RNA sequencing, we screened differentially expressed genes in pulmonary artery tissue of a Sugen5416/hypoxia-induced (SuHx) rat PH model. We examined the role of the differentially expressed gene G-protein coupled receptor 146 (GPR146) in PAECs through immunohistochemistry, immunofluorescence, CCK-8 assays, western blotings, real-time PCR, detection of reactive oxygen species, and lactate dehydrogenase release experiments. KEY RESULTS: According to RNA sequencing, GPR146 was 11.64-fold increased in the SuHx-induced PH model, compared to the controls. Further, GPR146 was highly expressed in pulmonary arterial hypertension human lung tissue and SuHx-induced rat PH lung tissues. Our results suggested that the expression of pyroptosis-related proteins was markedly increased under hypoxia, both in vivo and in vitro, which was inhibited by silencing GPR146. Moreover, inhibiting NLRP3 or caspase-1 effectively suppressed cleavage of caspase-1, production of interleukin (IL)-1ß, IL-6, and IL-18 in PAECs by hypoxia and overexpression of GPR146. CONCLUSION: Our results indicated that GPR146 induced pyroptosis and inflammatory responses through the NLRP3/caspase-1 signaling axis, thus triggering endothelial injury and vascular remodeling. Hypoxia may promote PAECs pyroptosis through upregulation of GPR146 and thereby facilitate the progression of PH. Taken together, these insights may help identify a novel target for the treatment of PH.

The actions of C-peptide in HEK293 cells are dependent upon insulin and extracellular glucose concentrations.

Connecting peptide, or C-peptide, is a part of the insulin prohormone and is essential for the proper folding and processing of the mature insulin peptide. C-peptide is released from the same beta cell secretory granules as insulin in equimolar amounts. However, due to their relative stabilities in plasma, the two peptides are detected in the circulation at ratios of approximately 4:1 to 6:1 (C-peptide to insulin), depending on metabolic state. C-peptide binds specifically to human cell membranes and induces intracellular signaling cascades, likely through an interaction with the G protein coupled receptor, GPR146. C-peptide has been shown to exert protective effects against the vascular, renal, and ocular complications of diabetes. The effects of C-peptide appear to be dependent upon the presence of insulin and the absolute, extracellular concentration of glucose. In this study, we employed HEK293 cells to further examine the interactive effects of C-peptide, insulin, and glucose on cell signaling. We observed that C-peptide's cellular effects are dampened significantly when cells are exposed to physiologically relevant concentrations of both insulin and C-peptide. Likewise, the actions of C-peptide on cFos and GPR146 mRNA expressions were affected by changes in extracellular glucose concentration. In particular, C-peptide induced significant elevations in cFos expression in the setting of high (25 mmol) extracellular glucose concentration. These data indicate that future experimentation on the actions of C-peptide should control for the presence or absence of insulin and the concentration of glucose. Furthermore, these findings should be considered prior to the development of C-peptide-based therapeutics for the treatment of diabetes-associated complications.

Orphan GPR146: an alternative therapeutic pathway to achieve cholesterol homeostasis?

Atherosclerosis predisposes to myriad cardiovascular complications, including myocardial infarction and stroke. Statins have revolutionised cholesterol management but they do not work for all patients, particularly those with familial hypercholesterolaemia (FH). Genome-wide association studies have linked SNPs at orphan G protein-coupled receptor 146 (GPR146) to human atherosclerosis but how GPR146 influences serum cholesterol homeostasis was only recently described. Gpr146 deletion in mice reduces serum cholesterol and atherosclerotic plaque burden, confirming GPR146 as a potential therapeutic target for managing circulating cholesterol. Critically, this effect was independent of the low-density lipoprotein receptor. While still an orphan, the activation of GPR146 by serum suggests identification of its endogenous ligand is tantalisingly close. Herein, we discuss the evidence for GPR146 inhibition as a treatment for atherosclerosis.

Multi-Omic Meta-Analysis of Transcriptomes and the Bibliome Uncovers Novel Hypoxia-Inducible Genes.

Hypoxia is a condition in which cells, tissues, or organisms are deprived of sufficient oxygen supply. Aerobic organisms have a hypoxic response system, represented by hypoxia-inducible factor 1-α (HIF1A), to adapt to this condition. Due to publication bias, there has been little focus on genes other than well-known signature hypoxia-inducible genes. Therefore, in this study, we performed a meta-analysis to identify novel hypoxia-inducible genes. We searched publicly available transcriptome databases to obtain hypoxia-related experimental data, retrieved the metadata, and manually curated it. We selected the genes that are differentially expressed by hypoxic stimulation, and evaluated their relevance in hypoxia by performing enrichment analyses. Next, we performed a bibliometric analysis using gene2pubmed data to examine genes that have not been well studied in relation to hypoxia. Gene2pubmed data provides information about the relationship between genes and publications. We calculated and evaluated the number of reports and similarity coefficients of each gene to HIF1A, which is a representative gene in hypoxia studies. In this data-driven study, we report that several genes that were not known to be associated with hypoxia, including the G protein-coupled receptor 146 gene, are upregulated by hypoxic stimulation.

Evidence for an interaction between proinsulin C-peptide and GPR146.

Microvascular diseases, such as retinopathies, neuropathies, and nephropathies, are a devastating consequence of type 1 and type 2 diabetes. The etiology of diabetes-associated microvascular dysfunction is poorly understood, and, likewise, treatment modalities for these disorders are limited. Interestingly, proinsulin C-peptide has been shown to play a protective role against diabetes-associated complications in experimental animals and in diabetic humans and is thus an attractive therapeutic target. However, an important step in the development of C-peptide-based therapeutics is identification of the C-peptide receptor, which is likely a G protein-coupled receptor (GPCR). Using a unique Deductive Ligand-Receptor Matching Strategy, we sought to determine whether one of the known orphan GPCRs is essential for C-peptide signaling. Knockdown of GPR146, but not GPR107 or GPR160, blocked C-peptide-induced cFos expression in KATOIII cells. Furthermore, stimulation with C-peptide caused internalization of GPR146, and examples of punctate colocalization were observed between C-peptide and GPR146 on KATOIII cell membranes. These data indicate that GPR146 is likely a part of the C-peptide signaling complex and provide a platform for the elucidation of the C-peptide signalosome.