Orthosteric and allosteric modulation of human HCAR2 signaling complex

Structures of G-protein coupled receptor HCAR1 in complex with Gi1 protein reveal the mechanistic basis for ligand recognition and agonist selectivity

Hydroxycarboxylic acid receptor 1 (HCAR1), also known as lactate receptor or GPR81, is a class A G-protein-coupled receptor with key roles in regulating lipid metabolism, neuroprotection, angiogenesis, cardiovascular function, and inflammatory response in humans. HCAR1 is highly expressed in numerous types of cancer cells, where it participates in controlling cancer cell metabolism and defense mechanisms, rendering it an appealing target for cancer therapy. However, the molecular basis of HCAR1-mediated signaling remains poorly understood. Here, we report four cryo-EM structures of human HCAR1 and HCAR2 in complex with the Gi1 protein, in which HCAR1 binds to the subtype-specific agonist CHBA (3.16 Å) and apo form (3.36 Å), and HCAR2 binds to the subtype-specific agonists MK-1903 (2.68 Å) and SCH900271 (3.06 Å). Combined with mutagenesis and cellular functional assays, we elucidate the mechanisms underlying ligand recognition, receptor activation, and G protein coupling of HCAR1. More importantly, the key residues that determine ligand selectivity between HCAR1 and HCAR2 are clarified. On this basis, we further summarize the structural features of agonists that match the orthosteric pockets of HCAR1 and HCAR2. These structural insights are anticipated to greatly accelerate the development of novel HCAR1-targeted drugs, offering a promising avenue for the treatment of various diseases.

Insights into the Activation Mechanism of HCA1, HCA2, and HCA3

Hydroxy-carboxylic acid receptors HCA1, HCA2, and HCA3 can be activated by important intermediates of energy metabolism. Despite the research focusing on HCA2, its clinical application has been limited by adverse effects. Therefore, the role of HCA1 as a promising target for the treatment of lipolysis warrants further exploration. As HCAs exhibit high similarity when activated with diverse selective agonists, a conserved yet unique activation mechanism for HCAs remains undisclosed. Herein, we unveil the cryo-electron microscopy structures of the 3,5-DHBA-HCA1-Gi signaling complex, the acifran- and MK6892-bound HCA2-Gi signaling complexes, and the acifran-HCA3-Gi signaling complex. Comparative analysis across HCAs reveals key residues in HCA1 contributing to the stabilization of the ligand-binding pocket. Furthermore, chimeric complexes and mutational analyses identify residues that are pivotal for HCA2 and HCA3 selectivity. Our findings elucidate critical structural insights into the mechanisms of ligand recognition and activation within HCA1 and broaden our comprehension of ligand specificity binding across the HCA family.

Ligand Recognition and Activation Mechanism of the Alicarboxylic Acid Receptors

Endogenous ligands for alicarboxylic acid receptors are important metabolic intermediates that play a significant role in regulating body energy and maintaining homeostasis. However, the molecular mechanism of alicarboxylate ligand-mediated counterpart receptors is currently unclear. We resolve the active state structure of HCA2-niacin, and the structural analysis explains the mechanism of niacin selectivity in the alicarboxylic acid receptors family. Homology modeling, molecular dynamics simulation and mutagenesis experiments reveal different ligand recognition modes and activation mechanisms of the alicarboxylic acid receptors, analyze the flexibility of the binding pocket and elucidate the important role of disulfide bonds on receptor activation and ligand binding. These more detailed molecular mechanisms further elucidate the relevant mechanisms of human metabolism and provide key clues for subsequent drug development of alicarboxylic acid receptors.

Microbiota-derived succinic acid mediates attenuating effect of dietary tomato juice supplementation on steatohepatitis through enhancing intestinal barrier

The hepatoprotective potential of tomato juice (TJ) has been reported in chronic liver models, and its potential prebiotic properties may be key to its preventative effects. However, the mechanistic role of the gut microbiota and its derived metabolites in ameliorating nonalcoholic steatohepatitis (NASH) via TJ remains unclear. In this study, we explored how TJ regulates gut microbiota and succinic acid (SA) to restore intestinal barrier function and thus suppress NASH progression. TJ supplementation effectively reduced serum lipid concentrations, alleviated endotoxin levels, and suppressed activation of the endotoxin-TLR4-NF-κB pathway in methionine-choline-deficient (MCD) diet-induced NASH mice. TJ restored the MCD diet-induced gut microbiota dysbiosis, increased the abundance of short-chain fatty acid and SA-producing bacteria (Bifidobacterium, Ileibacterium, Odoribacter, and Parasutterella) and enhanced the expression of intestinal barrier-associated proteins (E-cadherin, Claudin-1, MUC-2, and ZO-1). The hepatoprotective and enteroprotective effects of TJ were abolished in an antibiotic-treated mouse model, underscoring the pivotal role of the gut microbiota in the beneficial effects of TJ on NASH. Fecal metabolomics demonstrated that TJ significantly upregulated the tricarboxylic acid cycle, pyruvate metabolism, and butanoate metabolism pathways, increasing levels of butyric acid (BA) and SA-metabolites associated with reduced hepatic steatosis and intestinal damage. We further found that the physiological concentration of SA, rather than BA, could reduce pro-inflammatory cytokines (TNF-α and IL-6) levels and enhance mucin proteins and tight junction markers in the LPS-induced colon cell line LS174T. This study uncovers new mechanisms by which TJ prevents NASH, highlighting the potential of TJ and SA as effective dietary supplements for patients with chronic liver diseases.

The genetically predicted causal associations between circulating 3-hydroxybutyrate levels and malignant neoplasms: A pan-cancer Mendelian randomization study

OBJECTIVE

The ketogenic diet or exogenous supplementation with 3-hydroxybutyrate (3HB) is progressively gaining recognition as a valuable therapeutic or health intervention strategy. However, the effects of 3HB on cancers have been inconsistent in previous studies. This study aimed to comprehensively investigate the causal effects of circulating 3HB levels on 120 cancer phenotypes, and explore the 3HB mediation effect between liver fat accumulation and cancers.

METHODS

Univariate Mendelian randomization (UVMR) was used in this study to investigate the causal impact of circulating 3HB levels on cancers. We conducted meta-analyses for 3HB-cancer associations sourced from different exposure data. In multivariate MR(MVMR), the body mass index, alcohol frequency and diabetes were included as covariates to investigate the independent effect of 3HB on cancer risk. Additionally, utilizing mediation MR analysis, we checked the potential mediating role of 3HB in the association between liver fat and cancer.

RESULTS

Integrating findings from UVMR and MVMR, we observed that elevated circulating 3HB levels were associated with reduced risk of developing diffuse large B-cell lymphoma(DLBCL) (OR[95%CI] = 0.28[0.14-0.57] p = 3.92e-04), biliary malignancies (OR[95%CI] = 0.30[0.15-0.60], p = 7.67e-04), hepatocellular carcinoma(HCC) (OR[95%CI] = 0.25[0.09-0.71], p = 9.33e-03), primary lymphoid and hematopoietic malignancies (OR[95%CI] = 0.76[0.58-0.99], p = 0.045). Further UVMR analysis revealed that an increase in the percent liver fat was associated with reduced 3HB levels (Beta[95%CI] = -0.073[-0.122∼-0.024], p = 0.0034) and enhanced susceptibility to HCC (OR[95%CI] = 13.9[9.76-19.79], p = 3.14e-48), biliary malignancies (OR[95%CI] = 4.04[3.22-5.07], p = 1.64e-33), nasopharyngeal cancer (OR[95%CI] = 3.26[1.10-9.67], p = 0.03), and primary lymphoid and hematopoietic malignancies (OR[95%CI] = 1.27[1.13-1.44], p = 1.04e-4). Furthermore, 3HB fully mediated the effect of liver fat on susceptibility to DLBCL (OR[95%CI] = 1.076[1.01-1.15], p = 0.034).

CONCLUSIONS

Circulating 3HB is associated with a reduced susceptibility to developing DLBCL, HCC, biliary malignancies, and primary lymphoid and hematopoietic malignancies. The impaired ketogenesis induced by metabolic-dysfunction associated fatty liver disease (MAFLD) contributes to risk of DLBCL.

Niacin-induced flushing: Mechanism, pathophysiology, and future perspectives

Flushing is a typical physiological reaction to high emotional reactions. It is characterized by cutaneous vasodilation and a feeling of warmth and skin redness, especially in the face areas. Flushing is frequently linked to social anxiety, but it can also be a sign of a number of benign and malignant medical disorders. The study focuses on niacin-induced flushing, a well-researched side effect of the niacin, a drug which increases cholesterol levels. Niacin-induced flushing occurs when the hydroxycarboxylic acid receptor 2 (HCA2 or GPR109A) is activated. This starts a signaling cascade that releases prostaglandins, especially PGD2, which causes cutaneous vasodilation. Furthermore, niacin directly interacts with the transient receptor potential (TRP) channel TRPV1, offering a different, non-prostaglandin-based explanation for flushing brought on by niacin, highlighting the intricate physiological mechanisms behind this widespread occurrence. The review delves deeper into the advantages of niacin treatment for the cardiovascular system, highlighting how it can improve lipid profiles and lower cardiovascular events when used with statins. To sum it up, this study offers a thorough understanding of flushing, including its physiological foundation, many etiologies, diagnostic difficulties, and the subtleties of flushing caused by niacin. The investigation of innovative dose forms and nanomedicine highlights the continuous endeavors to improve patient compliance and reduce side effects, laying the groundwork for further developments in flushing treatment.

Multiple recent HCAR2 structures demonstrate a highly dynamic ligand binding and G protein activation mode

A surprisingly clear picture of the allosteric mechanism connecting G protein-coupled receptor agonists with G protein binding-and back - is revealed by a puzzle of thirty novel 3D structures of the hydroxycarboxylic acid receptor 2 (HCAR2) in complex with eight different orthosteric and a single allosteric agonist. HCAR2 is a sensor of β-hydroxybutyrate, niacin and certain anti-inflammatory drugs. Surprisingly, agonists with and without on-target side effects bound very similarly and in a completely occluded orthosteric binding site. Thus, despite the many structures we are still left with a pertinent need to understand the molecular dynamics of this and similar systems.

Structure-guided engineering of biased-agonism in the human niacin receptor via single amino acid substitution

The Hydroxycarboxylic acid receptor 2 (HCA2), also known as the niacin receptor or GPR109A, is a prototypical GPCR that plays a central role in the inhibition of lipolytic and atherogenic activities. Its activation also results in vasodilation that is linked to the side-effect of flushing associated with dyslipidemia drugs such as niacin. GPR109A continues to be a target for developing potential therapeutics in dyslipidemia with minimized flushing response. Here, we present cryo-EM structures of the GPR109A in complex with dyslipidemia drugs, niacin or acipimox, non-flushing agonists, MK6892 or GSK256073, and recently approved psoriasis drug, monomethyl fumarate (MMF). These structures elucidate the binding mechanism of agonists, molecular basis of receptor activation, and insights into biased signaling elicited by some of the agonists. The structural framework also allows us to engineer receptor mutants that exhibit G-protein signaling bias, and therefore, our study may help in structure-guided drug discovery efforts targeting this receptor.