Protein disulfide isomerase plays a crucial role in mediating chemically-induced, glutathione depletion-associated hepatocyte injury in vitro and in vivo.

Recently we have shown that protein disulfide isomerase (PDI or PDIA1) is involved in mediating chemically-induced, glutathione (GSH) depletion-associated ferroptotic cell death through NOS activation (dimerization) and NO accumulation. The present study aims to determine the role of PDI in mediating chemically-induced hepatocyte injury in vitro and in vivo and whether PDI inhibitors can effectively protect against chemically-induced hepatocyte injury. We show that during the development of erastin-induced ferroptotic cell death, accumulation of cellular NO, ROS and lipid-ROS follows a sequential order, i.e., cellular NO accumulation first, followed by accumulation of cellular ROS, and lastly cellular lipid-ROS. Cellular NO, ROS and lipid-ROS each play a crucial role in mediating erastin-induced ferroptosis in cultured hepatocytes. In addition, it is shown that PDI is an important upstream mediator of erastin-induced ferroptosis through PDI-mediated conversion of NOS monomer to its dimer, which then leads to accumulation of cellular NO, ROS and lipid-ROS, and ultimately ferroptotic cell death. Genetic manipulation of PDI expression or pharmacological inhibition of PDI function each can effectively abrogate erastin-induced ferroptosis. Lastly, evidence is presented to show that PDI is also involved in mediating acetaminophen-induced liver injury in vivo using both wild-type C57BL/6J mice and hepatocyte-specific PDI conditional knockout (PDIfl/fl Alb-cre) mice. Together, our work demonstrates that PDI is an important upstream mediator of chemically-induced, GSH depletion-associated hepatocyte ferroptosis, and inhibition of PDI can effectively prevent this injury.

Role of mitochondrial reactive oxygen species in chemically-induced ferroptosis.

Ferroptosis is a form of iron-dependent regulated cell death which is different from apoptosis. Chemically-induced ferroptosis is characterized by an accumulation of lipid reactive oxygen species (ROS) in the cells. A number of earlier studies have suggested the involvement of mitochondrial ROS in ferroptosis, and the present study seeks to further investigate the role of mitochondrial ROS in the induction of chemically-induced ferroptotic cell death. We find that during erastin-induced, glutathione depletion-associated ferroptosis, mitochondrial ROS accumulation is an important late event, which likely is involved in the final execution of ferroptotic cell death. The mitochondrion-originated ROS is found to accumulate in large quantities inside the nuclei during the late phases of erastin-induced ferroptosis. Completion of the late-phase accumulation of mitochondrion-produced ROS inside the nucleus of a cell likely marks an irreversible point in the cell death process. Similarly, accumulation of large amounts of mitochondrion-produced ROS inside the nucleus is also observed in the late phases of RSL3-induced ferroptosis. The results of this study indicate that the mitochondrial ROS play an important role in the final steps of both erastin- and RSL3-induced ferroptotic cell death.

Strong Protection by 4-Hydroxyestrone against Erastin-Induced Ferroptotic Cell Death in Estrogen Receptor-Negative Human Breast Cancer Cells: Evidence for Protein Disulfide Isomerase as a Mechanistic Target for Protection.

Ferroptosis is a recently identified form of regulated cell death, characterized by excessive iron-dependent lipid peroxidation. Recent studies have demonstrated that protein disulfide isomerase (PDI) is an important mediator of chemically induced ferroptosis and also a new target for protection against ferroptosis-associated cell death. In the present study, we identified that 4-hydroxyestrone (4-OH-E1), a metabolic derivative of endogenous estrogen, is a potent small-molecule inhibitor of PDI, and can strongly protect against chemically induced ferroptotic cell death in the estrogen receptor-negative MDA-MB-231 human breast cancer cells. Pull-down and CETSA assays demonstrated that 4-OH-E1 can directly bind to PDI both in vitro and in intact cells. Computational modeling analysis revealed that 4-OH-E1 forms two hydrogen bonds with PDI His256, which is essential for its binding interaction and thus inhibition of PDI's catalytic activity. Additionally, PDI knockdown attenuates the protective effect of 4-OH-E1 as well as cystamine (a known PDI inhibitor) against chemically induced ferroptosis in human breast cancer cells. Importantly, inhibition of PDI by 4-OH-E1 and cystamine or PDI knockdown by siRNAs each markedly reduces iNOS activity and NO accumulation, which has recently been demonstrated to play an important role in erastin-induced ferroptosis. In conclusion, this study demonstrates that 4-OH-E1 is a novel inhibitor of PDI and can strongly inhibit ferroptosis in human breast cancer cells in an estrogen receptor-independent manner. The mechanistic understanding gained from the present study may also aid in understanding the estrogen receptor-independent cytoprotective actions of endogenous estrogen metabolites in many noncancer cell types.

Attenuation of amyloid-ß-induced mitochondrial dysfunction by active components of anthocyanins in HT22 neuronal cells.

Alzheimer's disease (AD) is a common form of neurodegenerative disease in the elderly. Amyloid-ß (Aß)-associated neurotoxicity is an important component of the neurodegenerative change in AD. Recent studies have revealed a beneficial effect of anthocyanins in improving learning and memory in AD animal models. Using cultured HT22 mouse hippocampal neuronal cells as an in vitro model, we examined in this study the protective effect of ten pure components of anthocyanins against Aß 42-induced cytotoxicity and also investigated the mechanism of their protective effects. We found that treatment of HT22 cells with the pure components of anthocyanins dose-dependently rescued Aß 42-induced cytotoxicity, with slightly different potencies. Using petunidin as a representative compound, we found that it enhanced mitochondrial homeostasis and function in Aß 42-treated HT22 cells. Mechanistically, petunidin facilitated ß-catenin nuclear translocation and enhanced the interaction between ß-catenin and TCF7, which subsequently upregulated mitochondrial homeostasis-related protein Mfn2, thereby promoting restoration of mitochondrial homeostasis and function in Aß 42-treated HT22 cells. Together, these results reveal that the pure components of anthocyanins have a strong protective effect in HT22 cells against Aß 42-induced cytotoxicity by ameliorating mitochondrial homeostasis and function in a ß-catenin/TCF-dependent manner.

Biochemical mechanism of erastin-induced ferroptotic cell death in neuronal cells.

Ferroptosis is a new form of nonapoptotic cell death closely associated with glutathione (GSH) peroxidase 4 inhibition and/or GSH depletion, resulting in the accumulation of cellular iron and lipid peroxides. The exact mechanism by which GSH depletion causes the accumulation of reactive oxygen species (ROS) and lipid-ROS and subsequent ferroptotic cell death in neuronal cells remains unclear. In the present study, using immortalized HT22 mouse hippocampal neuronal cells as a model, we show that nitric oxide (NO) accumulation via protein disulfide isomerase (PDI)-mediated neuronal nitric oxide synthase (nNOS) activation plays a critical role in chemically-induced ferroptosis. Mechanistically, we find that erastin-induced GSH depletion leads to activation of PDI, which then mediates ferroptosis by catalyzing nNOS dimerization, followed by accumulation of cellular NO, ROS and lipid ROS and ultimately ferroptotic cell death. Pharmacological inhibition of PDI enzymatic activity or selective PDI knockdown can effectively abrogate erastin-induced ferroptosis in HT22 cells. The results of this study reveal an important role of PDI in mediating chemically induced ferroptosis in a neuronal cell model, and PDI may serve as a potential drug target for protection against GSH depletion-associated ferroptotic neuronal cell death.

AREA-AFFINITY: A Web Server for Machine Learning-Based Prediction of Protein-Protein and Antibody-Protein Antigen Binding Affinities.

Protein-Protein binding affinity reflects the binding strength between the binding partners. The prediction of protein-protein binding affinity is important for elucidating protein functions and also for designing protein-based therapeutics. The geometric characteristics such as area (both interface and surface areas) in the structure of a protein-protein complex play an important role in determining protein-protein interactions and their binding affinity. Here, we present a free web server for academic use, AREA-AFFINITY, for prediction of protein-protein or antibody-protein antigen binding affinity based on interface and surface areas in the structure of a protein-protein complex. AREA-AFFINITY implements 60 effective area-based protein-protein affinity predictive models and 37 effective area-based models specific for antibody-protein antigen binding affinity prediction developed in our recent studies. These models take into consideration the roles of interface and surface areas in binding affinity by using areas classified according to different amino acid types with different biophysical nature. The models with the best performances integrate machine learning methods such as neural network or random forest. These newly developed models have superior or comparable performance compared to the commonly used existing methods. AREA-AFFINITY is available for free at: https://affinity.cuhk.edu.cn/.

Binding affinity prediction for antibody-protein antigen complexes: A machine learning analysis based on interface and surface areas.

Specific antibodies can bind to protein antigens with high affinity and specificity, and this property makes them one of the best protein-based therapeutics. Accurate prediction of antibody‒protein antigen binding affinity is crucial for designing effective antibodies. The current predictive methods for protein‒protein binding affinity usually fail to predict the binding affinity of an antibody‒protein antigen complex with a comparable level of accuracy. Here, new models specific for antibody‒antigen binding affinity prediction are developed according to the different types of interface and surface areas present in antibody‒antigen complex. The contacts-based descriptors are also employed to construct or train different models specific for antibody‒protein antigen binding affinity prediction. The results of this study show that (i) the area-based descriptors are slightly better than the contacts-based descriptors in terms of the predictive power; (ii) the new models specific for antibody‒protein antigen binding affinity prediction are superior to the previously-used general models for predicting the protein‒protein binding affinities; (iii) the performances of the best area-based and contacts-based models developed in this work are better than the performances of a recently-developed graph-based model (i.e., CSM-AB) specific for antibody‒protein antigen binding affinity prediction. The new models developed in this work would not only help understand the mechanisms underlying antibody‒protein antigen interactions, but would also be of some applicable utility in the design and virtual screening of antibody-based therapeutics.

Biochemical mechanism underlying the pathogenesis of diabetic retinopathy and other diabetic complications in humans: the methanol-formaldehyde-formic acid hypothesis.

Hyperglycemia in diabetic patients is associated with abnormally-elevated cellular glucose levels. It is hypothesized that increased cellular glucose will lead to increased formation of endogenous methanol and/or formaldehyde, both of which are then metabolically converted to formic acid. These one-carbon metabolites are known to be present naturally in humans, and their levels are increased under diabetic conditions. Mechanistically, while formaldehyde is a cross-linking agent capable of causing extensive cytotoxicity, formic acid is an inhibitor of mitochondrial cytochrome oxidase, capable of inducing histotoxic hypoxia, ATP deficiency and cytotoxicity. Chronic increase in the production and accumulation of these toxic one-carbon metabolites in diabetic patients can drive the pathogenesis of ocular as well as other diabetic complications. This hypothesis is supported by a large body of experimental and clinical observations scattered in the literature. For instance, methanol is known to have organ- and species-selective toxicities, including the characteristic ocular lesions commonly seen in humans and non-human primates, but not in rodents. Similarly, some of the diabetic complications (such as ocular lesions) also have a characteristic species-selective pattern, closely resembling methanol intoxication. Moreover, while alcohol consumption or combined use of folic acid plus vitamin B is beneficial for mitigating acute methanol toxicity in humans, their use also improves the outcomes of diabetic complications. In addition, there is also a large body of evidence from biochemical and cellular studies. Together, there is considerable experimental support for the proposed hypothesis that increased metabolic formation of toxic one-carbon metabolites in diabetic patients contributes importantly to the development of various clinical complications.

Mechanism of Erastin-Induced Ferroptosis in MDA-MB-231 Human Breast Cancer Cells: Evidence for a Critical Role of Protein Disulfide Isomerase.

Ferroptosis is a form of regulated cell death resulting predominantly from catastrophic accumulation of lipid reactive oxygen species (ROS). While the antioxidant systems that counter ferroptosis have been well characterized, the mechanism underlying ferroptosis-associated accumulation of lipid ROS remains unclear. In this study, we demonstrated that protein disulfide isomerase (PDI) is a novel mediator of ferroptosis, which is responsible for the accumulation of lipid ROS and ultimately ferroptosis in MDA-MB-231 human breast cancer cells. Treatment with erastin led to a significant increase in inducible nitric oxide synthase (iNOS)-mediated nitric oxide production, which contributes to the accumulation of the death-inducing cellular lipid ROS. Small interfering RNA (siRNA)-mediated PDI knockdown or pharmacological inhibition of PDI's isomerase activity with cystamine strongly suppressed iNOS dimerization and its catalytic activation, subsequently prevented lipid ROS accumulation, and conferred strong protection against erastin-induced ferroptosis. Remarkably, PDI knockdown in MDA-MB-231 cells also largely abrogated the protective effect of cystamine against erastin-induced ferroptotic cell death. Together, these experimental observations demonstrate a noncanonical role of PDI in ferroptosis, which may serve as a potential therapeutic target for ferroptosis-related diseases.

Protection against glutathione depletion-associated oxidative neuronal death by neurotransmitters norepinephrine and dopamine: Protein disulfide isomerase as a mechanistic target for neuroprotection.

Oxidative stress is extensively involved in neurodegeneration. Clinical evidence shows that keeping the mind active through mentally-stimulating physical activities can effectively slow down the progression of neurodegeneration. With increased physical activities, more neurotransmitters would be released in the brain. In the present study, we investigated whether some of the released neurotransmitters might have a beneficial effect against oxidative neurodegeneration in vitro. Glutamate-induced, glutathione depletion-associated oxidative cytotoxicity in HT22 mouse hippocampal neuronal cells was used as an experimental model. We showed that norepinephrine (NE, 50 µM) or dopamine (DA, 50 µM) exerted potent protective effect against glutamate-induced cytotoxicity, but this effect was not observed when other neurotransmitters such as histamine, γ-aminobutyric acid, serotonin, glycine and acetylcholine were tested. In glutamate-treated HT22 cells, both NE and DA significantly suppressed glutathione depletion-associated mitochondrial dysfunction including mitochondrial superoxide accumulation, ATP depletion and mitochondrial AIF release. Moreover, both NE and DA inhibited glutathione depletion-associated MAPKs activation, p53 phosphorylation and GADD45α activation. Molecular docking analysis revealed that NE and DA could bind to protein disulfide isomerase (PDI). In biochemical enzymatic assay in vitro, NE and DA dose-dependently inhibited the reductive activity of PDI. We further revealed that the protective effect of NE and DA against glutamate-induced oxidative cytotoxicity was mediated through inhibition of PDI-catalyzed dimerization of the neuronal nitric oxide synthase. Collectively, the results of this study suggest that NE and DA may have a protective effect against oxidative neurodegeneration through inhibition of protein disulfide isomerase and the subsequent activation of the MAPKs‒p53‒GADD45α oxidative cascade.

Pathogenic Mechanism of Autoimmune Diabetes Mellitus in Humans: Potential Role of Streptozotocin-Induced Selective Autoimmunity against Human Islet ß-Cells.

Human type 1 diabetes mellitus is a chronic autoimmune disease characterized by the selective loss of insulin-producing ß-cells in pancreatic islets of genetically susceptible individuals. In this communication, a new hypothesis is postulated which is based on the observations that streptozotocin (STZ), a chemically reactive and cytotoxic compound produced by certain gram-positive bacteria, can be preferentially taken up into islet ß-cells and induce cytotoxicity and autoimmunity. It is hypothesized that humans might be occasionally exposed to STZ through opportunistic infections with the STZ-producing bacteria and/or through ingestion of certain food products that contain STZ. In addition, the potential presence of the STZ-producing bacteria in the gut microbiota of some individuals might be another source of long-term STZ exposure. Because of the high chemical reactivity of STZ and its breakdown products, these chemicals can covalently modify certain cellular macromolecules (e.g., DNA and proteins), and the covalently modified cellular components would serve as new antigens, potentially capable of inducing both humoral and cellular autoimmune responses in the islets of certain individuals. In addition to STZ exposure, the eventual development of autoimmunity against STZ-exposed islet ß-cells also depends critically on the genetic predisposition of the susceptible individuals plus the opportunistic presence of a conducive, strong environmental trigger, which often is presented as severe febrile viral infections subsequently inducing strong aberrant reactions of the body's immune system. The proposed pathogenic hypothesis is supported by a considerable body of direct and indirect evidence from laboratory animal studies and clinical observations. Certainly, more experimental and clinical studies are needed to carefully further examine each of the key components of the proposed pathogenic hypothesis.

Importance of interface and surface areas in protein-protein binding affinity prediction: A machine learning analysis based on linear regression and artificial neural network.

Protein-protein interaction plays an important role in all biological systems. The binding affinity between two protein binding partners reflects the strength of their association, which is crucial to the elucidation of the biological functions of these proteins and also to the design of protein-based therapeutic agents. In recent years, many studies have been conducted in an effort to improve the ability to predict the binding affinity of a protein-protein complex. Different sequence and structural features have been adopted in the prediction, but the surface or interface areas of the protein-protein complex were often not given adequate consideration. In the present study, different types of interface and surface areas in the protein-protein complex were used to construct or train linear, nonlinear or mixed models using linear regression and artificial neural network to predict the binding affinity of protein-protein interactions. The relative importance of the different types of areas in the selected models for affinity prediction was analyzed using variable-controlling approach. In terms of performance, the best area-based binding affinity predictors appeared to be superior or at least comparable to the widely-used predictors PRODIGY (a contacts-based predictor) and LISA (Local Interaction Signal Analysis). This work highlights the importance of interface and surface areas in protein-protein binding interactions. It also sheds light on the more suitable computational approaches that may aid in solving some of the scientific and technical issues associated with protein-protein binding affinity prediction. SIGNIFICANCE: Protein-protein interactions are ubiquitous in living systems. Protein-protein binding affinity is a metric that estimates the binding strength between two protein binding partners. Reliable information on their binding affinity is of great value in understanding complex biological processes as well as in designing protein-based therapeutics. In this work, the interface and surface areas in protein-protein interaction are explored with respect to their relative importance in better predicting the protein-protein binding affinity. The results from this study showed that different types of areas contribute importantly to protein-protein interactions and thus should be jointly considered in an explicit manner to improve affinity predictions. In addition, the effective application of interface and surface areas may also facilitate the simulation of the protein folding and binding processes.

Clinicopathological features and resistance mechanisms in HIP1-ALK-rearranged lung cancer: A multicenter study.

Anaplastic lymphoma kinase (ALK)-rearranged non-small cell lung cancer (NSCLC) respond well to ALK tyrosine kinase inhibitors (TKIs), and echinoderm microtubule-associated protein-like 4 (EML4)-ALK-rearranged NSCLC accounts for the majority of those patients. However, few studies have evaluated ALK-TKIs treatment for patients with huntingtin-interacting protein 1 (HIP1)-ALK fusions. This retrospective study evaluated the clinicopathological characteristics, genomic features, response to ALK-TKIs, and resistance mechanisms in 11 cases with HIP1-ALK fusions from five Chinese centers. Patients who received crizotinib at the Chinese centers had an objective response rate of 90% [9/10 cases, 95% confident index (CI): 54.1%-99.5%], median progression-free survival of 17.9 months (95% CI: 5.8-NA months), and median overall survival of 58.8 months (95% CI: 24.7-NA months). One patient who received first-line lorlatinib treatment achieved partial response for > 26.5 months. Despite the small sample size, HIP1-ALK (H21:A20) variant was the most common variant (four of 11 cases, 36.4%) and associated with better outcomes. Among the 11 cases, there were eight patients having available specimens for genetic testing before ALK-TKIs treatment and four patients undergoing biopsy after ALK-TKIs failure. The most common coexisting gene was TP53 among 11 patients and two of four patients after crizotinib failure harbored acquired ALK mutations (e.g., L1152V/Q1146K and L1196M). Brigatinib treatment appeared to be effective for a patient who failed crizotinib treatment because of the L1152V/Q1146K mutations, which might be related to increased binding affinity to these mutants. Although HIP1-ALK-rearranged NSCLC appears to initially respond well to ALK-TKIs, crizotinib resistance may be correlated with the AKAP9-BRAF fusion, ALK compound mutations (L1152V/Q1146K), and the ALK L1196M mutation. Larger studies are needed to evaluate the significance of HIP1-ALK-rearranged NSCLC.

17ß-Estradiol-Induced Conformational Changes of Human Microsomal Triglyceride Transfer Protein: A Computational Molecular Modelling Study.

Human microsomal triglyceride transfer protein (hMTP) plays an essential role in the assembly of apoB-containing lipoproteins, and has become an important drug target for the treatment of several disease states, such as abetalipoproteinemia, fat malabsorption and familial hypercholesterolemia. hMTP is a heterodimer composed of a larger hMTPα subunit and a smaller hMTPß subunit (namely, protein disulfide isomerase, hPDI). hPDI can interact with 17ß-estradiol (E2), an endogenous female sex hormone. It has been reported that E2 can significantly reduce the blood levels of low-density lipoprotein, cholesterol and triglyceride, and modulate liver lipid metabolism in vivo. However, some of the estrogen's actions on lipid metabolism are not associated with estrogen receptors (ER), and the exact mechanism underlying estrogen's ER-independent lipid-modulating action is still not clear at present. In this study, the potential influence of E2 on the stability of the hMTP complex is investigated by jointly using multiple molecular dynamics analyses based on available experimental structures. The molecular dynamics analyses indicate that the hMTP complex in the presence of E2 has reduced interface contacts and surface areas. A steered molecular dynamics analysis shows that the forces required to separate the two subunits (namely, hPDI and hMTPα subunit) of the hMTP complex in the absence of E2 are significantly higher than the forces required to separate the complex in which its hPDI is already bound with E2. E2 makes the interface between hMTPα and hPDI subunits more flexible and less stable. The results of this study suggest that E2-induced conformational changes of the hMTP complex might be a novel mechanism partly accounting for the ER-independent lipid-modulating effect of E2.

The Use of Chinese Yang/Qi-Invigorating Tonic Botanical Drugs/Herbal Formulations in Ameliorating Chronic Kidney Disease by Enhancing Mitochondrial Function.

Background: Chronic kidney disease (CKD) is a leading cause of morbidity and mortality. Mitochondrial dysfunction has been implicated as a key factor in the development of CKD. According to traditional Chinese medicine (TCM) theory, many Chinese Yang/Qi-invigorating botanical drugs/herbal formulations have been shown to produce promising outcomes in the clinical management of CKD. Experimental studies have indicated that the health-promoting action of Yang/Qi invigoration in TCM is related to the up-regulation of mitochondrial energy generation and antioxidant status. Objective: In this review, we aim to test whether Chinese Yang/Qi-invigorating tonic botanical drugs/herbal formulations can provide medical benefits in CKD and its complications. And we also explore the possible involvement of mitochondrial-associated signaling pathway underlying the beneficial effects of Yang/Qi invigoration in TCM. Methods: A systematic search of "PubMed", "China National Knowledge Infrastructure (CNKI)" and "Google Scholar" was carried out to collect all the available articles in English or Chinese related to Chinese Yang/Qi-invigorating tonic botanical drugs/herbal formulations and their effects on mitochondrial function and chronic kidney disease. Result and Discussion: The relationship between the progression of CKD and mitochondrial function is discussed. The effects of Chinese Yang/Qi-invigorating tonic botanical drugs/herbal formulations and their active ingredients, including phytosterols/triterpenes, flavonoids, and dibenzocyclooctadiene lignans, on CKD and related alterations in mitochondrial signaling pathways are also presented in this review. In the future, exploration of the possible beneficial effects and clinical studies of more Yang- and Qi-invigorating botanical drugs/herbal formulations in the prevention and/or/treatment of CKD and the molecular mechanisms relating to the enhancement of mitochondrial functions warrants further investigation. Conclusion: Given the critical role of mitochondrial function in safeguarding renal functional integrity, the enhancement of mitochondrial energy metabolism and antioxidant status in kidney tissue is likely involved in renal protection. Future studies on the biochemical and chemical basis underlying the effects of Chinese Yang/Qi-invigorating tonic botanical drugs/herbal formulations from a mitochondrial perspective will hopefully provide novel insights into the rational development of new drugs for the prevention and/or treatment of CKD.

Mechanism of reactivation of the peroxidase catalytic activity of human cyclooxygenases by reducing cosubstrate quercetin.

Our earlier studies show that the peroxidase activity of cyclooxygenase 1 and 2 (COX-1 and COX-2) can be reactivated in vitro and in vivo by the presence of certain naturally-occurring flavonoids such as quercetin and myricetin, which serve as reducing cosubstrates. These compounds can activate COX at nanomolar concentrations. In the present study, quercetin is used as a representative model compound to investigate the chemical mechanism by which the peroxidase activity of human COX-1 and COX-2 is reactivated after each catalytic cycle. Molecular docking and quantum mechanics calculations are carried out to probe the interactions of quercetin with the peroxidase sites of COX-1/2 and the reactivation mechanism. It is found that some of the partially-ionized states of quercetin can bind tightly and closely inside the peroxidase active sites of the COX enzymes and directly interact with heme Fe ion. While quercetin contains several phenolic hydroxyl groups, it is found that only the C-3'-OH group can effectively donate an electron for the reduction of heme because it not only can bind closely and tightly inside the peroxidase sites of COX-1/2, but it can also facilely donate an electron to heme Fe ion. This investigation provides a mechanistic explanation for the chemical process by which quercetin reactivates COX-1/2 peroxidases. This knowledge would aid in the rational design of drugs that can selectively target the peroxidase sites of COX-1/2 either as activators or inhibitors.

Insulin-induced conformational changes in the full-length insulin receptor: structural insights gained from molecular modeling analyses.

Insulin receptor plays an important role in the regulation of energy metabolism. Dysfunction of insulin receptor (IR) can lead to many disease states, such as diabetes mellitus. Deciphering the complex dynamic structures of human IR and its mechanism of activation would greatly aid in understanding IR-mediated signaling pathways and also in designing new drugs (including nonpeptidal insulin analogs) to treat diabetes mellitus. Experimental evidence about IR structures has been gradually obtained by biologists over the past three decades. Based on available experimental structures of IR in different states, here we employ molecular modeling approach to construct the full-length IR structures in different states and model its structural and conformational changes during insulin-induced IR activation. Several key possible intermediate states are constructed based on structural alignment, rotation, and computational modeling. Based on the structures of the full-length IR in different states, it appears that there are two possible conformational transition pathways: one is symmetric and the other one is asymmetric. Structural changes and motions of different domains of the full-length IR along the pathways are analyzed. The role of insulin binding to IR in facilitating the conformational transition of the receptor is analyzed. Information and insights derived from our present structural modeling analyses may aid in understanding the complex dynamic, structural, and conformational changes during the process of IR activation.

Mechanism underlying resveratrol's attenuation of paclitaxel cytotoxicity in human breast cancer cells: Role of the SIRT1-FOXO1-HER3 signaling pathway.

Resveratrol (RES), a dietary phenolic compound, was reported to have cancer chemoprotective and chemotherapeutic effects. Earlier we unexpectedly observed that RES has a growth-enhancing effect in some breast cancer cells and can diminish the susceptibility of MDA-MB-231 and SKBR-3 cells to paclitaxel-induced cell death, but this phenomenon is not observed in MCF-7 cells. The present study seeks to determine the mechanism underlying RES's attenuation of paclitaxel cytotoxicity in cancer cells. It is found that RES reduces the anticancer action of paclitaxel only in the human breast cancer cells that express HER3 protein. Treatment of SKBR-3 cells with RES increases HER3 expression in a dose-dependent manner. The induction of HER3 expression by RES confers resistance of breast cancer cells against paclitaxel cytotoxicity. Furthermore, it is observed that the SIRT1-FOXO1 signaling pathway plays an important role in mediating RES-induced upregulation of HER3 expression. In conclusion, the present study reveals the mechanism for RES-induced resistance against paclitaxel in some human breast cancer cells, and it is suggested that the combined use of RES and paclitaxel is not suitable for treating human breast cancer that expresses HER3 protein.

Inhibition of cyclooxygenase by blocking the reducing cosubstrate at the peroxidase site: Discovery of galangin as a novel cyclooxygenase inhibitor.

Earlier we have shown that certain flavonoids (e.g., quercetin) are high-affinity reducing cosubstrates for cyclooxygenase (COX) 1 and 2. These compounds can bind inside the peroxidase active sites of COXs and donate an electron from one of their B-ring hydroxyl groups to hematin. Based on these earlier findings, it is postulated that some of the natural flavonoids such as galangin that are structural analogs of quercetin but lack the proper B-ring hydroxyl groups might function as novel inhibitors of COXs by blocking the effect of the reducing cosubstrates. This idea is tested in the present study. Computational docking analysis together with quantum chemistry calculation shows that galangin can bind inside the peroxidase active sites of COX-1 and COX-2 in a similar manner as quercetin, but it has little ability to effectively donate its electrons, thereby blocking the effect of the reducing cosubstrates like quercetin. Further experimental studies confirm that galangin can inhibit, both in vitro and in vivo, quercetin-mediated activation of the peroxidase activity of the COX-1/2 enzymes. The results of the present study demonstrate that galangin is a novel naturally-occurring inhibitor of COX-1 and COX-2, acting by blocking the function of the reducing cosubstrates at the peroxidase sites.

Modulation of Enzyme-Catalyzed Synthesis of Prostaglandins by Components Contained in Kidney Microsomal Preparations.

In the kidney, prostaglandins formed by cyclooxygenase 1 and 2 (COX-1 and COX-2) play an important role in regulating renal blood flow. In the present study, we report our observations regarding a unique modulatory effect of renal microsomal preparation on COX-1/2-mediated formation of major prostaglandin (PG) products in vitro. We found that microsomes prepared from pig and rat kidneys had a dual stimulatory-inhibitory effect on the formation of certain PG products catalyzed by COX-1 and COX-2. At lower concentrations, kidney microsomes stimulated the formation of certain PG products, whereas at higher concentrations, their presence inhibited the formation. Presence of kidney microsomes consistently increased the Km values of the COX-1/2-mediated reactions, while the Vmax might be increased or decreased depending on stimulation or inhibition observed. Experimental evidence was presented to show that a protein component present in the pig kidney microsomes was primarily responsible for the activation of the enzyme-catalyzed arachidonic acid metabolism leading to the formation of certain PG products.