Abelmoschus esculentus (Okra) Prevents Insulin Resistance and Restores Neuron Autophagy by Regulating Dipeptidyl Peptidase-4 and Thus Improving Hippocampal Function.

Diabetes is highly linked to the occurrence of Alzheimer disease (AD), which is characterized by beta amyloid peptide (Aß) and hyperphosphorylation of tau (p-tau), and neuron damage particularly in hippocampus. Type 2 diabetes (T2D) is featured by insulin resistance, and phosphorylation of Ser307-IRS-1 is regarded as a resistance marker. Inhibitors of dipeptidyl peptidase-4 (DPP-4) are effective tools for treating T2D. Previously, we reported subfractions of Abelmoschus esculentus (AE, okra) (F1 rich in quercetin glycosides; F2 composed of polysaccharide) attenuated DPP-4 and its downstream signals of insulin resistance, thus preventing Aß-induced neuron damage. Since autophagy could be protective, we now explore if AE works to modulate neuron autophagy by regulating DPP-4 and insulin resistance and, thus, improves the hippocampal function and behavior. We demonstrated that AE subfractions attenuate Aß-induced insulin resistance and the expression of p-tau and normalize the autophagy and survival of hippocampal neurons. The action of AE may be attributed to the downregulation of DPP-4, which plays a critical role in mediating insulin resistance and hinders neuron autophagy. The in vivo findings reveal that the hippocampal insulin resistance appears to link with loss of memory, reduction of curiosity, and depression, whereas treatment with AE significantly improves the insulin sensitivity and hippocampal function. Noteworthy, even at only 5 µg/mL, F2 seems to exhibit a meaningful effect. In conclusion, we suggest that AE attenuates insulin resistance and recovers neuron autophagy which are regulated by DPP-4, thus preventing the damage to the hippocampus, improving recognition and emotion. AE may be an effective adjuvant or supplement to prevent insulin resistance-associated pathogenesis of AD if these results can be confirmed in human clinical trials.

Regulation of polyamine homeostasis through an antizyme citrullination pathway.

This study reveals an uncovered mechanism for the regulation of polyamine homeostasis through protein arginyl citrullination of antizyme (AZ), a natural inhibitor of ornithine decarboxylase (ODC). ODC is critical for the cellular production of polyamines. AZ binds to ODC dimers and promotes the degradation of ODC via the 26S proteasome. This study demonstrates the protein citrullination of AZ catalyzed by peptidylarginine deiminase type 4 (PAD4) both in vitro and in cells. Upon PAD4 activation, the AZ protein was citrullinated and accumulated, leading to higher levels of ODC proteins in the cell. In the PAD4-overexpressing and activating cells, the levels of ODC enzyme activity and the product putrescine increased with the level of citrullinated AZ proteins and PAD4 activity. Suppressing cellular PAD4 activity reduces the cellular levels of ODC and downregulates cellular polyamines. Furthermore, citrullination of AZ in the C-terminus attenuates AZ function in the inhibition, binding, and degradation of ODC. This paper provides evidence to illustrate that PAD4-mediated AZ citrullination upregulates cellular ODC and polyamines by retarding ODC degradation, thus interfering with the homeostasis of cellular polyamines, which may be an important pathway regulating AZ functions that is relevant to cancer biology.

Baicalein, 7,8-Dihydroxyflavone and Myricetin as Potent Inhibitors of Human Ornithine Decarboxylase.

BACKGROUND: Human ornithine decarboxylase (ODC) is a well-known oncogene, and the discovery of ODC enzyme inhibitors is a beneficial strategy for cancer therapy and prevention. METHODS: We examined the inhibitory effects of a variety of flavone and flavonol derivatives on ODC enzymatic activity, and performed in silico molecular docking of baicalein, 7,8-dihydroxyflavone and myricetin to the whole dimer of human ODC to investigate the possible binding site of these compounds on ODC. We also examined the cytotoxic effects of these compounds with cell-based studies. RESULTS: Baicalein, 7,8-dihydroxyflavone and myricetin exhibited significant ODC suppression activity with IC50 values of 0.88 µM, 2.54 µM, and 7.3 µM, respectively, which were much lower than that of the active-site irreversible inhibitor α-DL-difluoromethylornithine (IC50, the half maximal inhibitory concentration, of approximately 100 µM). Kinetic studies and molecular docking simulations suggested that baicalein, and 7,8-dihydroxyflavone act as noncompetitive inhibitors that are hydrogen-bonded to the region near the active site pocket in the dimer interface of the enzyme. Baicalein and myricetin suppress cell growth and induce cellular apoptosis, and both of these compounds suppress the ODC-evoked anti-apoptosis of cells. CONCLUSIONS: Therefore, we suggest that the flavone or flavonol derivatives baicalein, 7,8-dihydroxyflavone, and myricetin are potent chemopreventive and chemotherapeutic agents that target ODC.

Multifaceted interactions and regulation between antizyme and its interacting proteins cyclin D1, ornithine decarboxylase and antizyme inhibitor.

Ornithine decarboxylase (ODC), cyclin D1 (CCND1) and antizyme inhibitor (AZI) promote cell growth. ODC and CCND1 can be degraded through antizyme (AZ)-mediated 26S proteasomal degradation. This paper describes a mechanistic study of the molecular interactions between AZ and its interacting proteins. The dissociation constant (Kd) of the binary AZ-CCND1 complex and the respective binding sites of AZ and CCND1 were determined. Our data indicate that CCND1 has a 4-fold lower binding affinity for AZ than does ODC and an approximately 40-fold lower binding affinity for AZ than does AZI. The Kd values of AZ-CCND1, AZ-ODC and AZ-AZI were 0.81, 0.21 and 0.02 µM, respectively. Furthermore, the Kd values for CCND1 binding to the AZ N-terminal peptide (AZ34-124) and AZ C-terminal peptide (AZ100-228) were 0.92 and 8.97 µM, respectively, indicating that the binding site of CCND1 may reside at the N-terminus of AZ, rather than the C-terminus. Our data also show that the ODC-AZ-CCND1 ternary complex may exist in equilibrium. The Kd values of the [AZ-CCND1]-ODC and [AZ-ODC]-CCND1 complexes were 1.26 and 4.93 µM, respectively. This is the first paper to report the reciprocal regulation of CCND1 and ODC through AZ-dependent 26S proteasomal degradation.

Fumarate analogs act as allosteric inhibitors of the human mitochondrial NAD(P)+-dependent malic enzyme.

Human mitochondrial NAD(P)+-dependent malic enzyme (m-NAD(P)-ME) is allosterically activated by the four-carbon trans dicarboxylic acid, fumarate. Previous studies have suggested that the dicarboxylic acid in a trans conformation around the carbon-carbon double bond is required for the allosteric activation of the enzyme. In this paper, the allosteric effects of fumarate analogs on m-NAD(P)-ME are investigated. Two fumarate-insensitive mutants, m-NAD(P)-ME_R67A/R91A and m-NAD(P)-ME_K57S/E59N/K73E/D102S, as well as c-NADP-ME, were used as the negative controls. Among these analogs, mesaconate, trans-aconitate, monomethyl fumarate and monoethyl fumarate were allosteric activators of the enzyme, while oxaloacetate, diethyl oxalacetate, and dimethyl fumarate were found to be allosteric inhibitors of human m-NAD(P)-ME. The IC50 value for diethyl oxalacetate was approximately 2.5 mM. This paper suggests that the allosteric inhibitors may impede the conformational change from open form to closed form and therefore inhibit m-NAD(P)-ME enzyme activity.