Utilization of an In Vivo Reporter for High Throughput Identification of Branched Small Molecule Regulators of Hypoxic Adaptation

SummarySmall molecules inhibiting hypoxia inducible factor (HIF) prolyl hydroxylases (PHDs) are the focus of drug development efforts directed toward the treatment of ischemia and metabolic imbalance. A cell-based reporter produced by fusing HIF-1α oxygen degradable domain (ODD) to luciferase was shown to work as a capture assay monitoring stability of the overexpressed luciferase-labeled HIF PHD substrate under conditions more physiological than in vitro test tubes. High throughput screening identified novel catechol and oxyquinoline pharmacophores with a “branching motif” immediately adjacent to a Fe-binding motif that fits selectively into the HIF PHD active site in in silico models. In accord with their structure-activity relationship in the primary screen, the best “hits” stabilize HIF1α, upregulate known HIF target genes in a human neuronal line, and exert neuroprotective effects in established model of oxidative stress in cortical neurons

Synthesis and Anti-Inflammatory Activity of 2-Amino-4,5,6,7-tetrahydrobenzo[b]thiophene-Derived NRF2 Activators

This is the first study investigating the nuclear factor (erythroid‐derived 2)‐like 2 (NRF2) activity of compounds containing a new scaffold, tetrahydrobenzo[b]thiophene. Eighteen compounds were synthesised and confirmed their NRF2 activation through NQO1 enzymatic activity and mRNA expression of NQO1 and HO‐1 in Hepa‐1c1c7 cells. The compounds disrupted the interaction between Kelch‐like ECH‐associated protein 1 (KEAP1) and NRF2 via interfering with the KEAP1’s Kelch domain. The compounds exhibited anti‐inflammatory activity in Escherichia coli Lipopolysaccharide (LPS( Ec ))‐stimulated RAW 264.7 cells. The anti‐inflammatory activity of the compounds was associated with their ability to activate NRF2. The compounds reversed the elevated levels of pro‐inflammatory cytokines (IL‐1β, IL‐6, TNF‐α, and IFN‐γ) and inflammatory mediators (PGE2, COX‐2, and NF‐κB). The compounds were metabolically stable in human, rat, and mouse liver microsomes and showed optimum half‐life (T(1/2)) and intrinsic clearance (Cl(int)). The binding mode of the compounds and physicochemical properties were predicted via in silico studies

Development of Neh2-Luciferase Reporter and Its Application for High Throughput Screening and Real-Time Monitoring of Nrf2 Activators

SummaryThe NF-E2-related factor 2 (Nrf2) is a key transcriptional regulator of antioxidant defense and detoxification. To directly monitor stabilization of Nrf2, we fused its Neh2 domain, responsible for the interaction with its nucleocytoplasmic regulator, Keap1, to firefly luciferase (Neh2-luciferase). We show that Neh2 domain is sufficient for recognition, ubiquitination, and proteasomal degradation of Neh2-luciferase fusion protein. The Neh2-luc reporter system allows direct monitoring of the adaptive response to redox stress and classification of drugs based on the time course of reporter activation. The reporter was used to screen the Spectrum library of 2000 biologically active compounds to identify activators of Nrf2. The most robust and yet nontoxic Nrf2 activators found—nordihydroguaiaretic acid, fisetin, and gedunin—induced astrocyte-dependent neuroprotection from oxidative stress via an Nrf2-dependent mechanism

pH-Dependent Substrate Preference of Pig Heart Lipoamide Dehydrogenase Varies with Oligomeric State: Response to Mitochondrial Matrix Acidification

Cycling of intracellular pH has recently been shown to play a critical role in ischemia-reperfusion injury. Ischemia-reperfusion also leads to mitochondrial matrix acidification and dysfunction. However, the mechanism by which matrix acidification contributes to mitochondrial dysfunction, oxidative stress, and the resultant cellular injury has not been elucidated. We observe pH-dependent equilibria between monomeric, dimeric, and a previously undescribed tetrameric form of pig heart lipoamide dehydrogenase (LADH), a mitochondrial matrix enzyme. Dynamic light scattering studies of native LADH in aqueous solution indicate that lowering pH favors a shift in average molecular mass from higher oligomeric states to monomer. Sedimentation velocity of LADH entrapped in reverse micelles reveals dimer and tetramer at both pH 5.8 and 7.5, but monomer was observed only at pH 5.8. Enzyme activity measurements in reverse Aerosol OT micelles in octane indicate that LADH dimer and tetramer possess lipoamide dehydrogenase and diaphorase activities at pH 7.5. Upon acidification to pH 5.8 only the LADH monomer is active and only the diaphorase activity is ob¬served. These results indicate a correlation between pH-dependent changes in the LADH reaction specificity and its oligomeric state. The acidification of mitochondrial matrix that occurs during ischemia-reperfusion injury is sufficient to alter the structure and enzymatic specificity of LADH, thereby reducing mitochondrial defenses, increasing oxidative stress, and slowing the recovery of energy metabolism. Matrix acidification may also disrupt the quaternary structure of other mitochondrial protein complexes critical for cellular homeostasis and survival

Zinc Switch in Pig Heart Lipoamide Dehydrogenase: Steady-State and Transient Kinetic Studies of the Diaphorase Reaction

Elevation of intracellular Zn2+ following ischemia contributes to cell death by affecting mitochondrial function. Zn2+ is a differential regulator of the mitochondrial enzyme lipoamide dehydrogenase (LADH) at physiological concentrations (Ka = 0.1 µM free zinc), inhibiting lipoamide and accelerating NADH dehydrogenase activities. These differential effects have been attributed to coordination of Zn2+ by LADH active-site cysteines. A detailed kinetic mechanism has now been developed for the diaphorase (NADH-dehydrogenase) reaction catalyzed by pig heart LADH using 2,6-dichlorophenol-indophenol (DCPIP) as a model quinone electron acceptor. Anaerobic stopped-flow experiments show that two-electron reduced LADH is 15-25-fold less active towards DCPIP reduction than four-electron reduced enzyme, or Zn2+-modified reduced LADH (the corresponding values of the rate constants are (6.5 ± 1.5) × 103 M-1·s-1, (9 ± 2) × 104 M-1·s-1, and (1.6 ± 0.5) × 105 M-1·s-1, respectively). Steady-state kinetic studies with different diaphorase substrates show that Zn2+ accelerates reaction rates exclusively for two-electron acceptors (duroquinone, DCPIP), but not for one-electron acceptors (benzoquinone, ubiquinone, ferricyanide). This implies that the two-electron reduced form of LADH, prevalent at low NADH levels, is a poor two-electron donor compared to the four-electron reduced or Zn2+-modified reduced LADH forms. These data suggest that zinc binding to the active-site thiols switches the enzyme from one- to two-electron donor mode. This zinc-activated switch has the potential to alter the ratio of superoxide and H2O2 generated by the LADH oxidase activity

A novel approach to distinguish between enzyme mechanisms: quasi-steady-state kinetic analysis of the prostaglandin H synthase peroxidase reaction.

A method of analysis for steady-state kinetic data has been developed that allows relationships between key partial reactions in the catalytic cycle of a functioning enzyme to be determined. The novel approach is based on a concept of scalar and vector 'kinetic connectivities' between enzyme intermediates in an arbitrary enzyme mechanism. The criterion for the agreement between experimental data and a proposed kinetic model is formulated as the kinetic connectivity of intermediate forms of the enzyme. This concept has advantages over conventional approaches and is better able to describe the complex kinetic behaviour of prostaglandin H synthase (PGHS) when catalysing the oxidation of adrenaline by H(2)O(2). To interpret the experimental data for PGHS, a generalized model for multi-substrate enzyme reactions was developed with provision for irreversible enzyme inactivation. This model showed that two enzyme intermediates must undergo inactivation during the catalytic cycle. These forms are proposed to be PGHS compound I and a compound I-adrenaline complex