Gossypetin Prevents the Progression of Nonalcoholic Steatohepatitis by Regulating Oxidative Stress and AMP-Activated Protein Kinase

Nonalcoholic steatohepatitis (NASH) is a severe liver metabolic disorder, however, there are still no effective and safe drugs for its treatment. Previous clinical trials used various therapeutic approaches to target individual pathologic mechanisms, but these approaches were unsuccessful because of the complex pathologic causes of NASH. Combinatory therapy in which two or more drugs are administered simultaneously to patients with NASH, however, carries the risk of side effects associated with each individual drug. To solve this problem, we identified gossypetin as an effective dual-targeting agent that activates AMP-activated protein kinase (AMPK) and decreases oxidative stress. Administration of gossypetin decreased hepatic steatosis, lobular inflammation and liver fibrosis in the liver tissue of mice with choline-deficient high-fat diet and methionine-choline deficient diet (MCD) diet-induced NASH. Gossypetin functioned directly as an antioxidant agent, decreasing hydrogen peroxide and palmitate-induced oxidative stress in the AML12 cells and liver tissue of MCD diet-fed mice without regulating the antioxidant response factors. In addition, gossypetin acted as a novel AMPK activator by binding to the allosteric drug and metabolite site, which stabilizes the activated structure of AMPK. Our findings demonstrate that gossypetin has the potential to serve as a novel therapeutic agent for nonalcoholic fatty liver disease /NASH. SIGNIFICANCE STATEMENT: This study demonstrates that gossypetin has preventive effect to progression of nonalcoholic steatohepatitis (NASH) as a novel AMP-activated protein kinase (AMPK) activator and antioxidants. Our findings indicate that simultaneous activation of AMPK and oxidative stress using gossypetin has the potential to serve as a novel therapeutic approach for nonalcoholic fatty liver disease /NASH patients.

Phospholipid-dependent interaction between dibromothymoquinone and iron-sulfur protein in mitochondrial ubiquinol-cytochrome c reductase

Dibromothymoquinone (DBMIB) inhibits antimycin A-sensitive ubiquinol-cytochrome c reductase activity; the maximal inhibition is 90%. DBMIB alters the EPR spectra of reduced iron-sulfur protein in intact ubiquinol-cytochrome c reductase. The maximal spectral change occurs with 60 mol inhibitor per mol cytochrome c1 in the reductase. DBMIB causes little alteration in the EPR characteristics of iron-sulfur protein when ubiquinol-cytochrome c reductase is delipidated. When delipidated ubiquinol-cytochrome c reductase is replenished with phospholipid, the effect of DBMIB reappears. However, when DBMIB is added to delipidated protein prior to replenishment with phospholipid, very little spectral alteration is observed. DBMIB does not alter the EPR spectra of purified iron-sulfur protein, with or without phospholipid in the preparation. Reduced DBMIB does not alter the EPR characteristics of iron-sulfur protein in intact or delipidated ubiquinol-cytochrome c reductase. Cysteine and other thiol compounds can reverse the spectral alternation caused by DBMIB. This reversal probably results from the reduction of DBMIB.

Spin-label electron paramagnetic resonance and differential scanning calorimetry studies of the interaction between mitochondrial succinate-ubiquinone and ubiquinol-cytochrome c reductases

The interaction between succinate-ubiquinone and ubiquinol-cytochrome c reductases in the purified, dispersed state and in embedded phospholipid vesicles was studied by differential scanning calorimetry and by electron paramagnetic resonance (EPR). When the purified, detergent-dispersed succinate-ubiquinone reductase, ubiquinol-cytochrome c reductase, and cytochrome c oxidase undergo thermodenaturation, they show an endothermic transition. However, when these isolated electron-transfer complexes are embedded in phospholipid vesicles, they undergo exothermodenaturation. The energy released could result from the collapse of the strained interaction between unsaturated fatty acyl groups of phospholipids and an exposed area of the complex formed by removal of interacting proteins. The exothermic enthalpy change of thermodenaturation of a protein-phospholipid vesicle containing both succinate-ubiquinone and ubiquinol-cytochrome c reductases was smaller than that of a mixture of protein-phospholipid vesicles formed from the individual electron-transfer complexes. This suggests specific interaction between succinate-ubiquinone reductase and ubiquinol-cytochrome c reductase in the membrane. This idea is supported by saturation transfer EPR studies showing that the rotational correlation time of spin-labeled ubiquinol-cytochrome c reductase is increased when mixed with succinate-ubiquinone reductase prior to embedding in phospholipid vesicles. These results indicate that succinate-ubiquinone reductase and ubiquinol-cytochrome c reductase are indeed present in the membrane as a supermacromolecular complex. No such supermacromolecular complex is detected between NADH-ubiquinone and ubiquinol-cytochrome c reductases or between succinate-ubiquinone and NADH-uniquinone reductases.

Studies of protein-phospholipid interaction in isolated mitochondrial ubiquinone-cytochrome c reductase

The interaction between phospholipids, ubiquinone and highly purified ubiquinol-cytochrome c reductase was studied using differential scanning calorimetry. The enzyme complex and its delipidated forms undergo thermodenaturation at 337.3 and 322.7 K, respectively. The reduced reductase is more stable toward thermodenaturation than is the oxidized enzyme. While phospholipids restored enzymatic activity to the delipidated enzyme complex and stabilized the enzyme toward thermodenaturation, ubiquinone showed little effect on the thermostability of ubiquinol-cytochrome c reductase. The effect of phospholipids on the thermotropic properties of ubiquinol-cytochrome c reductase is dependent upon the molecular properties of the phospholipid. When ubiquinol-cytochrome c reductase was embedded in closed asolectin vesicles, an exothermic transition peak was observed upon thermodenaturation. When the asolectin concentration in the reconstituted preparation was less than 0.3 mg/mg protein, an amorphous structure was observed in the electron micrograph and the preparation showed an endothermic transition upon thermodenaturation. The thermotropic properties of the enzyme-phospholipid vesicles were affected by the phospholipid head groups as well as the fatty-acyl chains, with those phospholipids having the most highly unsaturated fatty-acyl chains having the greatest effect. The energy for the exothermic transition may be derived from the collapse, upon thermodenaturation, of a strained interaction between the unsaturated fatty-acyl groups of phospholipids and protein molecules resulting from vesicle formation. The exothermic transition of the enzyme-phospholipid vesicle was abolished when cholesterol was included in the vesicles and when reductase was treated with a proteolytic enzyme prior to incorporation into the phospholipid vesicles.

Studies on protein-lipid interactions in cytochrome c oxidase by differential scanning calorimetry

The interaction between cytochrome c oxidase and phospholipids was studied by differential scanning calorimetry. The active, lipid-sufficient cytochrome c oxidase undergoes thermodenaturation at 336 K with a relatively broad and concentration dependent endothermic transition. The delipidated enzyme shows an endothermic denaturation temperature at 331.3 K. When the delipidated cytochrome c oxidase was treated with chymotrypsin, a lowered thermodenaturation temperature was observed. When the delipidated cytochrome c oxidase was reconstituted with asolectin to form a functionally active enzyme complex, the thermodenaturation shifted to a higher temperature, with a sharper transition thermogram. The increase in thermotransition temperature and enthalpy change of thermodenaturation of the asolectin-reconstituted enzyme is directly proportionate to the amount of asolectin used, up to 0.5 mg asolectin per mg protein. The thermotransition temperature and enthalpy changes of thermodenaturation for the phospholipid-reconstituted cytochrome c oxidase are affected by the phospholipid headgroup and the fatty acyl groups. Among phospholipids with the same acyl moiety but different head groups, phosphatidylethanolamine was found to be more effective than phosphatidylcholine in protecting cytochrome c oxidase from thermodenaturation. An exothermic transition thermogram was observed for delipidated cytochrome c oxidase embedded in phospholipid vesicles formed with phospholipids containing unsaturated fatty acyl groups. The increase in exothermic transition temperature and exothermic enthalpy change of thermodenaturation of the oxidase-cytochrome c-cytochrome c oxidase complex destabilized cytochrome c but not cytochrome c oxidase toward thermodenaturation.