Introduction to Special Issue on Mitochondrial Redox Signaling in Health and Disease

A Novel Imidazopyridine Derivative Exerts Anticancer Activity by Inducing Mitochondrial Pathway-Mediated Apoptosis

Background

Cancer remains a major clinical challenge because of the lack of effective drug for its treatment. To find out novel cancer chemotherapeutic molecules, we explored the anticancer effect of novel imidazopyridine compound 9i and also investigated the underlying molecular mechanism.

Methods

Human cervical cancer cell (HeLa) viability was measured by an MTT assay after treatment with compound 9i. Clonogenicity of HeLa cells was investigated by an in vitro colony formation assay. Cell death was visualized by propidium iodide (PI) staining. Fluorescence-activated cell sorting (FACS) was used to determine apoptosis and mitochondrial membrane potential in HeLa cells. The expression level of apoptosis-related proteins was also determined by western blot.

Results

Compound 9i suppressed HeLa cell viability in a time- and dose-dependent manner. Compound 9i induced mitochondrial outer membrane permeabilization (MOMP), activated caspase cascade, and finally resulted in apoptosis.

Conclusion

Compound 9i induces mitochondrial pathway-mediated apoptosis in human cervical cancer cells, suggesting that 9i could be a potential lead compound to be developed as a cancer therapeutic molecule.

Intertwined ROS and Metabolic Signaling at the Neuron-Astrocyte Interface

Metabolism and redox signalling share critical nodes in the nervous system. In the last years, a series of major findings have challenged the current vision on how neural reactive oxygen species (ROS) are produced and handled in the nervous system. Once regarded as deleterious by-products, ROS are now shown to be essential for a metabolic and redox crosstalk. In turn, this coupling defines neural viability and function to control behaviour or leading to neurodegeneration when compromised. Findings like a different assembly of mitochondrial respiratory supercomplexes in neurons and astrocytes stands behind a divergent production of ROS in either cell type, more prominent in astrocytes. ROS levels are however tightly controlled by an antioxidant machinery in astrocytes, assumed as more efficient than that of neurons, to regulate redox signalling. By exerting this control in ROS abundance, metabolic functions are finely tuned in both neural cells. Further, a higher engagement of mitochondrial respiration and oxidative function in neurons, underpinned by redox equivalents supplied from the pentose phosphate pathway and from glia, differs from the otherwise strong glycolytic capacity of astrocytes. Here, we recapitulate major findings on how ROS and metabolism differ between neural cells but merge to define reciprocal signalling pathways, ultimately defining neural function and fate.

Important Role of Mitochondria and the Effect of Mood Stabilizers on Mitochondrial Function

Mitochondria primarily serve as source of cellular energy through the Krebs cycle and β-oxidation to generate substrates for oxidative phosphorylation. Redox reactions are used to transfer electrons through a gradient to their final acceptor, oxygen, and to pump hydrogen protons into the intermembrane space. Then, ATP synthase uses the electrochemical gradient to generate adenosine triphosphate (ATP). During these processes, reactive oxygen species (ROS) are generated. ROS are highly reactive molecules with important physiological functions in cellular signaling. Mitochondria play a crucial role in intracellular calcium homeostasis and serve as transient calcium stores. High levels of both, ROS and free cytosolic calcium, can damage mitochondrial and cellular structures and trigger apoptosis. Impaired mitochondrial function has been described in many psychiatric diseases, including mood disorders, in terms of lowered mitochondrial membrane potential, suppressed ATP formation, imbalanced Ca2+ levels and increased ROS levels. In vitro models have indicated that mood stabilizers affect mitochondrial respiratory chain complexes, ROS production, ATP formation, Ca2+ buffering and the antioxidant system. Most studies support the hypothesis that mitochondrial dysfunction is a primary feature of mood disorders. The precise mechanism of action of mood stabilizers remains unknown, but new mitochondrial targets have been proposed for use as mood stabilizers and mitochondrial biomarkers in the evaluation of therapy effectiveness.

Melatonin alleviates inflammation-induced apoptosis in human umbilical vein endothelial cells via suppression of Ca2+-XO-ROS-Drp1-mitochondrial fission axis by activation of AMPK/SERCA2a pathway

Endothelia inflammation damage is vital to the development and progression of chronic venous disease. In the present study, we explored the protective effect of melatonin on endothelia apoptosis induced by LPS, particularly focusing on the mitochondrial fission. We demonstrated that human umbilical vein endothelial cells (HUVEC) subjected to LPS for 12 h exhibited a higher apoptotic rate. However, melatonin (1-20 μM) treatment 12 h before LPS had the ability to protect HUVEC cell against LPS-mediated apoptosis in a dose-dependent manner. Furthermore, LPS induced the cytoplasmic calcium overload which was responsible for the upregulation of calcium-dependent xanthine oxidase (XO). Higher XO expression was associated with reactive oxygen species (ROS) overproduction, leading to the Drp1 phosphorylation at the Ser616 site and migration on the surface of mitochondria. Furthermore, phosphorylated Drp1 initiated the mitochondrial fission contributing to the caspase9-dependent mitochondrial apoptosis as evidenced by lower membrane potential, more cyt-c leakage into the nuclear, and higher expression of proapoptotic proteins. However, melatonin treatment could trigger the AMPK pathway, which was followed by the increased SERCA2a expression. Activation of AMPK/SERCA2a by melatonin inhibited the calcium overload, XO-mediated ROS outburst, Drp1-required mitochondrial fission, and final mitochondrial apoptosis. In summary, this study confirmed that LPS induced HUVEC apoptosis through Ca2+-XO-ROS-Drp1-mitochondrial fission axis and that melatonin reduced the apoptosis of HUVEC through activation of the AMPK/SERCA2a pathway.

Melatonin increases human cervical cancer HeLa cells apoptosis induced by cisplatin via inhibition of JNK/Parkin/mitophagy axis

Considering that chemotherapy resistance is vital to the progression of cervical carcinoma, emerging researchers are focused on developing anti-tumor drugs to assist the treatment efficiency of chemotherapy. Melatonin has anti-tumor activity via several mechanisms including its anti-proliferative and pro-apoptotic effects as well as its potent pro-oxidant action in tumor cells. Therefore, melatonin may be useful for the treatment of tumors in association with chemotherapy drugs. Here, we studied the effect and mechanism of melatonin on HeLa cells apoptosis under cisplatin (CIS) treatment, particularly focusing on the caspase-9-related apoptosis pathway and mitophagy-mediated anti-apoptotic mechanism. The result indicated that co-stimulation of HeLa cells with CIS in the presence of melatonin further increased cellular apoptosis. Furthermore, concomitant treatments with melatonin and CIS significantly enhanced the mitochondrial structure and function damage, substantially augmented the caspase-9-dependent mitochondrial apoptosis with evidenced by lower mitochondria membrane potential, higher mitochondria ROS, and more pro-apoptotic proteins compared to the treatment with CIS alone. Mechanistically, melatonin inactivated mitophagy via blockade of JNK/Parkin, leading to the inhibition of anti-apoptotic mitophagy. The mitophagy had the ability to clear and remove damaged mitochondria, impairing CIS-mediated mitochondrial apoptosis. Activation of JNK/Parkin could alleviate the lethal effect of melatonin on HeLa cells. In summary, this study confirmed that melatonin sensitizes human cervical cancer HeLa cells to CIS-induced apoptosis through inhibition of JNK/Parkin/mitophagy pathways.

IGF-II promotes neuroprotection and neuroplasticity recovery in a long-lasting model of oxidative damage induced by glucocorticoids

Insulin-like growth factor-II (IGF-II) is a naturally occurring hormone that exerts neurotrophic and neuroprotective properties in a wide range of neurodegenerative diseases and ageing. Accumulating evidence suggests that the effects of IGF-II in the brain may be explained by its binding to the specific transmembrane receptor, IGFII/M6P receptor (IGF-IIR). However, relatively little is known regarding the role of IGF-II through IGF-IIR in neuroprotection. Here, using adult cortical neuronal cultures, we investigated whether IGF-II exhibits long-term antioxidant effects and neuroprotection at the synaptic level after oxidative damage induced by high and transient levels of corticosterone (CORT). Furthermore, the involvement of the IGF-IIR was also studied to elucidate its role in the neuroprotective actions of IGF-II. We found that neurons treated with IGF-II after CORT incubation showed reduced oxidative stress damage and recovered antioxidant status (normalized total antioxidant status, lipid hydroperoxides and NAD(P) H:quinone oxidoreductase activity). Similar results were obtained when mitochondria function was analysed (cytochrome c oxidase activity, mitochondrial membrane potential and subcellular mitochondrial distribution). Furthermore, neuronal impairment and degeneration were also assessed (synaptophysin and PSD-95 expression, presynaptic function and FluoroJade B® stain). IGF-II was also able to recover the long-lasting neuronal cell damage. Finally, the effects of IGF-II were not blocked by an IGF-IR antagonist, suggesting the involvement of IGF-IIR. Altogether these results suggest that, in or model, IGF-II through IGF-IIR is able to revert the oxidative damage induced by CORT. In accordance with the neuroprotective role of the IGF-II/IGF-IIR reported in our study, pharmacotherapy approaches targeting this pathway may be useful for the treatment of diseases associated with cognitive deficits (i.e., neurodegenerative disorders, depression, etc.).

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First evidence that IGF-II reverts oxidative synaptic damage produced by corticoids.

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IGF-II recovers mitochondrial function in synapses after oxidative damage.

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IGF-II restores mitochondrial distribution in neurons after oxidative damage.

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Evidence of the involvement of IGF-II receptor in the recovery of synaptic function.

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IGF-II reverts neurodegeneration induced by oxidative damage produced by corticoids.

Mitochondrial complex I deactivation is related to superoxide production in acute hypoxia

Mitochondria use oxygen as the final acceptor of the respiratory chain, but its incomplete reduction can also produce reactive oxygen species (ROS), especially superoxide. Acute hypoxia produces a superoxide burst in different cell types, but the triggering mechanism is still unknown. Herein, we show that complex I is involved in this superoxide burst under acute hypoxia in endothelial cells. We have also studied the possible mechanisms by which complex I could be involved in this burst, discarding reverse electron transport in complex I and the implication of PTEN-induced putative kinase 1 (PINK1). We show that complex I transition from the active to ‘deactive’ form is enhanced by acute hypoxia in endothelial cells and brain tissue, and we suggest that it can trigger ROS production through its Na⁺/H⁺ antiporter activity. These results highlight the role of complex I as a key actor in redox signalling in acute hypoxia.

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Complex I is involved in the superoxide burst produced by cells in acute hypoxia.

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Complex I is deactivated in acute hypoxia.

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Deactive complex I is involved in superoxide production in acute hypoxia, probably through its Na⁺/H⁺ antiporter activity.

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Complex I deactivation occurs in brain tissue hypoxia ex vivo and in vivo.