Related flavonoids cause cooperative inhibition of the sarcoplasmic reticulum Ca²⁺ ATPase by multimode mechanisms

Mur ligase F as a new target for the flavonoids quercitrin, myricetin, and (-)-epicatechin

MurC, D, E, and F are ATP-dependent ligases involved in the stepwise assembly of the tetrapeptide stem of forming peptidoglycan. As highly conserved targets found exclusively in bacterial cells, they are of significant interest for antibacterial drug discovery. In this study, we employed a computer-aided molecular design approach to identify potential inhibitors of MurF. A biochemical inhibition assay was conducted, screening twenty-four flavonoids and related compounds against MurC-F, resulting in the identification of quercitrin, myricetin, and (-)-epicatechin as MurF inhibitors with IC50 values of 143 µM, 139 µM, and 92 µM, respectively. Notably, (-)-epicatechin demonstrated mixed type inhibition with ATP and uncompetitive inhibition with D-Ala-D-Ala dipeptide and UM3DAP substrates. Furthermore, in silico analysis using Sitemap and subsequent docking analysis using Glide revealed two plausible binding sites for (-)-epicatechin. The study also investigated the crucial structural features required for activity, with a particular focus on the substitution pattern and hydroxyl group positions, which were found to be important for the activity. The study highlights the significance of computational approaches in targeting essential enzymes involved in bacterial peptidoglycan synthesis.

Inhibition of MurA Enzyme from Escherichia coli by Flavonoids and Their Synthetic Analogues

MurA catalyzes the first step of peptidoglycan (PG) biosynthesis and is a validated target for the development of new antimicrobial agents. In this study, a library of 49 plant flavonoids and their synthetic derivatives were evaluated for their inhibitory properties against MurA fromEscherichia coli. The compounds were tested with and without preincubation and with the addition of DTT to understand the mechanism of inhibition. Thirteen compounds were identified as reversible, time-dependent inhibitors, with inhibition levels ranging from 480 nM to 57 μM, and ampelopsin as the most potent compound. To investigate the major pharmacophore elements responsible for the activity, 2D-QSAR and docking analyzes were performed. The results showed that the catechol moiety and an additional aromatic system were the most important features contributing to the activity of the compounds. However, most of the compounds did not show antibacterial activity againstE. coli andStaphylococcus aureusstrains, suggesting that their inhibitory activity against MurA may not be strong enough to induce antibacterial effects. Nevertheless, our results suggest that flavonoids are a promising starting point to develop new inhibitors of MurA and show great potential for further steps in drug development.

Oligomeric Proanthocyanidins Confer Cold Tolerance in Rice through Maintaining Energy Homeostasis

Oligomeric proanthocyanidins (OPCs) are abundant polyphenols found in foods and botanicals that benefit human health, but our understanding of the functions of OPCs in rice plants is limited, particularly under cold stress. Two rice genotypes, named Zhongzao39 (ZZ39) and its recombinant inbred line RIL82, were subjected to cold stress. More damage was caused to RIL82 by cold stress than to ZZ39 plants. Transcriptome analysis suggested that OPCs were involved in regulating cold tolerance in the two genotypes. A greater increase in OPCs content was detected in ZZ39 than in RIL82 plants under cold stress compared to their respective controls. Exogenous OPCs alleviated cold damage of rice plants by increasing antioxidant capacity. ATPase activity was higher and poly (ADP-ribose) polymerase (PARP) activity was lower under cold stress in ZZ39 than in RIL82 plants. Importantly, improvements in cold tolerance were observed in plants treated with the OPCs and 3-aminobenzamide (PARP inhibitor, 3ab) combination compared to the seedling plants treated with H₂O, OPCs, or 3ab alone. Therefore, OPCs increased ATPase activity and inhibited PARP activity to provide sufficient energy for rice seedling plants to develop antioxidant capacity against cold stress.

Natural Polyphenols as SERCA Activators: Role in the Endoplasmic Reticulum Stress-Related Diseases

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) is a key protein responsible for transporting Ca²⁺ ions from the cytosol into the lumen of the sarco/endoplasmic reticulum (SR/ER), thus maintaining Ca²⁺ homeostasis within cells. Accumulating evidence suggests that impaired SERCA function is associated with disruption of intracellular Ca²⁺ homeostasis and induction of ER stress, leading to different chronic pathological conditions. Therefore, appropriate strategies to control Ca²⁺ homeostasis via modulation of either SERCA pump activity/expression or relevant signaling pathways may represent a useful approach to combat pathological states associated with ER stress. Natural dietary polyphenolic compounds, such as resveratrol, gingerol, ellagic acid, luteolin, or green tea polyphenols, with a number of health-promoting properties, have been described either to increase SERCA activity/expression directly or to affect Ca²⁺ signaling pathways. In this review, potential Ca²⁺-mediated effects of the most studied polyphenols on SERCA pumps or related Ca²⁺ signaling pathways are summarized, and relevant mechanisms of their action on Ca²⁺ regulation with respect to various ER stress-related states are depicted. All data were collected using scientific search tools (i.e., Science Direct, PubMed, Scopus, and Google Scholar).

Inhibition of the human secretory pathway Ca2+, Mn2+-ATPase1a by 1,3-thiazole derivatives

The human Golgi/secretory pathway Ca²⁺,Mn²⁺-ATPase 1 (hSPCA1) transports Ca²⁺ and Mn²⁺ into the Golgi lumen. Studies of the biological functions of hSPCA1 are limited by a lack of selective pharmacological tools for SPCA1 inhibition. The aim of this study was therefore to identify compounds that specifically inhibit hSPCA1 activity. We found that five 1,3-thiazole derivatives exhibited inhibitory action towards the ATP hydrolysis activity of hSPCA1a in a concentration-dependent manner. Among the derivatives tested, compound 1 was the most potent, completely inhibiting hSPCA1a activity with a half-maximal inhibition (IC₅₀) value of 0.8 μM. Compound 1 also partially inhibited the activity of another Ca²⁺,Mn²⁺-ATPase (hSPCA2) and Ca²⁺-ATPase (rSERCA1a), but had no effect on Na⁺,K⁺-ATPase or H⁺,K⁺-ATPase. Treatment of HeLa cells with compound 1 led to fragmentation of the Golgi ribbon into smaller stacks. In addition, compound 1 mobilized intracellular Ca²⁺ in HeLa cells that had been pre-treated with thapsigargin. Therefore, based on its selectivity and potency, compound 1 may be a valuable tool with which to further explore the role of SPCA1 in cellular processes.

Estrogen inhibits endoplasmic reticulum stress and ameliorates myocardial ischemia/reperfusion injury in rats by upregulating SERCA2a

Background

The incidence of coronary heart disease (CHD) in premenopausal women is significantly lower than that of men of the same age, suggesting protective roles of estrogen for the cardiovascular system against CHD. This study aimed to confirm the protective effect of estrogen on myocardium during myocardial ischemia/reperfusion (MI/R) injury and explore the underlying mechanisms.

Methods

Neonatal rat cardiomyocytes and Sprague–Dawley rats were used in this study. Different groups were treated by bilateral ovariectomy, 17β-estradiol (E2), adenoviral infection, or siRNA transfection. The expression of sarcoplasmic reticulum Ca²⁺ ATPase pump (SERCA2a) and endoplasmic reticulum (ER) stress-related proteins were measured in each group to examine the effect of different E2 levels and determine the relationship between SERCA2a and ER stress. The cell apoptosis, myocardial infarction size, levels of apoptosis and serum cardiac troponin I, ejection fraction, calcium transient, and morphology changes of the myocardium and ER were examined to verify the effects of E2 on the myocardium.

Results

Bilateral ovariectomy resulted in reduced SERCA2a levels and more severe MI/R injury. E2 treatment increased SERCA2a expression. Both E2 treatment and exogenous SERCA2a overexpression decreased levels of ER stress-related proteins and alleviated myocardial damage. In contrast, SERCA2a knockdown exacerbated ER stress and myocardial damage. Addition of E2 after SERCA2a knockdown did not effectively inhibit ER stress or reduce myocardial injury.

Conclusions

Our data demonstrate that estrogen inhibits ER stress and attenuates MI/R injury by upregulating SERCA2a. These results provide a new potential target for therapeutic intervention and drug discovery in CHD.

Sarco/Endoplasmic Reticulum Calcium ATPase Inhibitors: Beyond Anticancer Perspective

The sarco/endoplasmic reticulum calcium ATPase (SERCA), which plays a key role in the maintenance of Ca²⁺ ion homeostasis, is an extensively studied enzyme, the inhibition of which has a considerable impact on cell life and death decision. To date, several SERCA inhibitors have been thoroughly studied and the most notable one, a derivative of the sesquiterpene lactone thapsigargin, is gradually approaching a clinical application. Meanwhile, new compounds with SERCA-inhibiting properties of natural, synthetic, or semisynthetic origin are being discovered and/or developed; some of these might also be suitable for the development of new drugs with improved performance. This review brings an up-to-date comprehensive overview of recently discovered compounds with the potential of SERCA inhibition, discusses their mechanism of action, and highlights their potential clinical applications, such as cancer treatment.

Enhancement of Quercetin-Induced Apoptosis by Cotreatment with Autophagy Inhibitor Is Associated with Augmentation of BAK-Dependent Mitochondrial Pathway in Jurkat T Cells

A flavonoid antioxidant quercetin promotes dose-dependent activation of the ATM-CHK-p53 pathway, downregulation of antiapoptotic survivin, and upregulation of proapoptotic NOXA in human T cell acute lymphoblastic leukemia Jurkat clones (J/Neo and J/BCL-XL). However, the downregulation of antiapoptotic BAG3 and MCL-1 occurred in J/Neo cells but not in J/BCL-XL cells overexpressing BCL-XL. Additionally, several BCL-XL-sensitive intrinsic mitochondrial apoptotic events including apoptotic sub-G₁ cell accumulation, TUNEL-positive DNA fragmentation, BAK activation, mitochondrial membrane potential (Δψm) loss, caspase-9/caspase-8/caspase-3 activation, and PARP cleavage were induced only in J/Neo cells. Both cytosolic and mitochondrial ROS levels were elevated in quercetin-treated J/Neo cells; however, the ROS elevations were almost completely abrogated in J/BCL-XL cells, suggesting the ROS elevations were downstream of BCL-XL-sensitive mitochondrial damage and dysfunction. Wild-type A3, FADD-deficient I2.1, and caspase-8-deficient I9.2 Jurkat clones exhibited similar susceptibilities to the cytotoxicity of quercetin, excluding an involvement of extrinsic pathway in triggering the apoptosis. The autophagic events such as attenuation of AKT-mTOR pathway, formation of acridine orange-stainable acidic vesicular organelles, conversion of microtubule-associated protein 1 light chain 3-I (LC3-I) to LC3-II, and downregulation of p62/SQSTM1 level were detected in quercetin-treated J/Neo and J/BCL-XL cells, regardless of BCL-XL overexpression. Cotreatment with the autophagy inhibitor (3-methyladenine, LY294002, or chloroquine) resulted in a significant enhancement of quercetin-induced BAK activation and subsequently the mitochondrial damage-mediated apoptosis pathway by augmenting the downregulation of BAG3 and MCL-1 levels in J/Neo cells. These results demonstrated that quercetin induces intrinsic apoptosis and cytoprotective autophagy, and autophagy inhibition can potentiate BAK-dependent apoptotic activity of quercetin in Jurkat T cells.

Natural flavonoids inhibit the plasma membrane Ca2+-ATPase

Research on flavonoids from plant sources has recently sparked increasing interest because of their beneficial health properties. Different studies have shown that flavonoids change the intracellular Ca²⁺ homeostasis linked to alterations in the function of mitochondria, Ca²⁺ channels and Ca²⁺ pumps. These findings hint at plasma membrane Ca²⁺-ATPase (PMCA) involvement, as it transports Ca²⁺ actively to the extracellular medium coupled to ATP hydrolysis, thus maintaining ion cellular homeostasis. The present study aims to investigate the effect of several natural flavonoids on PMCA both in isolated protein systems and in living cells, and to establish the relationship between flavonoid structure and inhibitory activity on PMCA. Our results show that natural flavonoids inhibited purified and membranous PMCA with different effectiveness: quercetin and gossypin were the most potent and their inhibition mechanisms seem to be different, as quercetin does not prevent ATP binding whereas gossypin does. Moreover, PMCA activity was inhibited in human embryonic kidney cells which transiently overexpress PMCA, suggesting that the effects observed on isolated systems could occur in a complex structure like a living cell. In conclusion, this work reveals a novel molecular mechanism through which flavonoids inhibit PMCA, which leads to Ca²⁺ homeostasis and signaling alterations in the cell.

Mechanism of Fructus Aurantii Flavonoids Promoting Gastrointestinal Motility: From Organic and Inorganic Endogenous Substances Combination Point of View

BACKGROUND

Fructus Aurantii (FA) derived from the dried, and unripe fruit of Citrus aurantium L. is one of the commonly used traditional Chinese medicines to treat gastrointestinal motility dysfunction diseases. According to the literature research, FA flavonoids (FAF) are important active ingredients of FA promoting gastrointestinal motility, but the exact material basis and mechanism of action are still not very clear.

OBJECTIVE

This experiment was designed to illustrate the material basis of FAF promoting gastrointestinal motility and explore the mechanism of action from an organic and inorganic combination point of view.

MATERIALS AND METHODS

In this experiment, high-performance liquid chromatography (HPLC) method was used to analyze the composition and content of FAF. Based on the prominent prokinetic effect of FAF on mice, the mechanism of action was speculated through a combination of HPLC coupled with quadrupole time-of-flight mass spectrometry (HPLC-QTOF-MS) and inductively coupled plasma mass spectrometry (ICP-MS).

RESULTS

With the method of HPLC, ten dominating components of FAF including neoeriocitrin, narirutin, rhoifolin, naringin, hesperidin, neohesperidin, neoponcirin, naringenin, hesperetin, and nobiletin accounting for more than 86% of FAF were identified. Combined HPLC-QTOF-MS with ICP-MS, the endogenous substances with difference in the blood of mice were analyzed, in which 4-dimethylallyltryptophan, corticosterone, phytosphingosine, sphinganine, LysoPC (20:4(5Z, 8Z, 11Z, 14Z)), LysoPC(18:2 (9Z, 12Z)), and Ca²⁺, Mg²⁺, Zn²⁺ metal ions had significant changes, involving tryptophan metabolism, corticosterone metabolism, sphingolipid metabolism, and other pathways.

CONCLUSION

The results preliminarily elaborated the mechanism of FAF promoting gastrointestinal motility from an organic and inorganic point of view, which provide valuable information for researching and developing new multi-component Chinese medicine curing gastrointestinal underpower associated diseases.

SUMMARY

Fructus Aurantii flavonoids are one of the main components of Fructus Aurantii that possess prominent gastrointestinal motility promoting efficacyThe mainly material basis of Fructus Aurantii flavonoids promoting gastrointestinal motility were neoeriocitrin, narirutin, rhoifolin, naringin, hesperidin, neohesperidin, neoponcirin, naringenin, hesperetin, and nobiletinFructus Aurantii flavonoids can regulate the content of 4-dimethylallyltryptophan, corticosterone, phytosphingosine, sphinganine, LysoPC (20:4(5Z, 8Z, 11Z, 14Z)), LysoPC.(18:2(9Z, 12Z)) and Ca²⁺, Mg²⁺, Zn²⁺-metal ions, through tryptophan metabolism, corticosterone metabolism, sphingolipid metabolism, and other pathways to present its gastrointestinal motility promoting efficacy. Abbreviations used: FA: Fructus Aurantii; FAF: Fructus Aurantii flavonoids; HPLC: High performance liquid chromatography; HPLC-QTOF-MS: High performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry; ICP-MS: Inductively coupled plasma mass spectrometry; PCA: Principal components analysis; CG: Control group; FAFLG: Low-dosage group of Fructus Aurantii flavonoids; FAFMG: Middle-dosage group of Fructus Aurantii flavonoids; FAFHG: High-dosage group of Fructus Aurantii flavonoids; DPG: Domperidone group.

Inhibitory action of linoleamide and oleamide toward sarco/endoplasmic reticulum Ca2+-ATPase

BACKGROUND

SERCA maintains intracellular Ca²⁺ homeostasis by sequestering cytosolic Ca²⁺ into SR/ER stores. Two primary fatty acid amides (PFAAs), oleamide (18:1^9-cis) and linoleamide (18:2^9,12-cis), induce an increase in intracellular Ca²⁺ levels, which might be caused by their inhibition of SERCA.

METHODS

Three major SERCA isoforms, rSERCA1a, hSERCA2b, and hSERCA3a, were individually overexpressed in COS-1 cells, and the inhibitory action of PFAAs on Ca²⁺-ATPase activity of SERCA was examined.

RESULTS

The Ca²⁺-ATPase activity of each SERCA was inhibited in a concentration-dependent manner strongly by linoleamide (IC₅₀ 15-53μM) and partially by oleamide (IC₅₀ 8.3-34μM). Inhibition by other PFAAs, such as stearamide (18:0) and elaidamide (18:1^9-trans), was hardly or slightly observed. With increasing dose, linoleamide decreased the apparent affinity for Ca²⁺ and the apparent maximum velocity of Ca²⁺-ATPase activity of all SERCAs tested. Oleamide also lowered these values for hSERCA3a. Meanwhile, oleamide uniquely reduced the apparent Ca²⁺ affinity of rSERCA1a and hSERCA2b: the reduction was considerably attenuated above certain concentrations of oleamide. The dissociation constants for SERCA interaction varied from 6 to 45μM in linoleamide and from 1.6 to 55μM in oleamide depending on the isoform.

CONCLUSIONS

Linoleamide and oleamide inhibit SERCA activity in the micromolar concentration range, and in a different manner. Both amides mainly suppress SERCA activity by lowering the Ca²⁺ affinity of the enzyme.

GENERAL SIGNIFICANCE

Our findings imply a novel role of these PFAAs as modulators of intracellular Ca²⁺ homeostasis via regulation of SERCA activity.

At the interface of antioxidant signalling and cellular function: Key polyphenol effects

The hypothesis that dietary (poly)phenols promote well‐being by improving chronic disease‐risk biomarkers, such as endothelial dysfunction, chronic inflammation and plasma uric acid, is the subject of intense current research, involving human interventions studies, animal models and in vitro mechanistic work. The original claim that benefits were due to the direct antioxidant properties of (poly)phenols has been mostly superseded by detailed mechanistic studies on specific molecular targets. Nevertheless, many proposed mechanisms in vivo and in vitro are due to modulation of oxidative processes, often involving binding to specific proteins and effects on cell signalling. We review the molecular mechanisms for 3 actions of (poly)phenols on oxidative processes where there is evidence in vivo from human intervention or animal studies. (1) Effects of (poly) phenols on pathways of chronic inflammation leading to prevention of some of the damaging effects associated with the metabolic syndrome. (2) Interaction of (poly)phenols with endothelial cells and smooth muscle cells, leading to effects on blood pressure and endothelial dysfunction, and consequent reduction in cardiovascular disease risk. (3) The inhibition of xanthine oxidoreductase leading to modulation of intracellular superoxide and plasma uric acid, a risk factor for developing type 2 diabetes.

Distinct action of flavonoids, myricetin and quercetin, on epithelial Cl⁻ secretion: useful tools as regulators of Cl⁻ secretion

Epithelial Cl⁻ secretion plays important roles in water secretion preventing bacterial/viral infection and regulation of body fluid. We previously suggested that quercetin would be a useful compound for maintaining epithelial Cl⁻ secretion at a moderate level irrespective of cAMP-induced stimulation. However, we need a compound that stimulates epithelial Cl⁻ secretion even under cAMP-stimulated conditions, since in some cases epithelial Cl⁻ secretion is not large enough even under cAMP-stimulated conditions. We demonstrated that quercetin and myricetin, flavonoids, stimulated epithelial Cl⁻ secretion under basal conditions in epithelial A6 cells. We used forskolin, which activates adenylyl cyclase increasing cytosolic cAMP concentrations, to study the effects of quercetin and myricetin on cAMP-stimulated epithelial Cl⁻ secretion. In the presence of forskolin, quercetin diminished epithelial Cl⁻ secretion to a level similar to that with quercetin alone without forskolin. Conversely, myricetin further stimulated epithelial Cl⁻ secretion even under forskolin-stimulated conditions. This suggests that the action of myricetin is via a cAMP-independent pathway. Therefore, myricetin may be a potentially useful compound to increase epithelial Cl⁻ secretion under cAMP-stimulated conditions. In conclusion, myricetin would be a useful compound for prevention from bacterial/viral infection even under conditions that the amount of water secretion driven by cAMP-stimulated epithelial Cl⁻ secretion is insufficient.