Pinocembrin And Its Effect On Mitochondria

Pinocembrin, which is also known as 5,7-dihydroxyflavone (C₁₅H₁₂O₄), has been obtained from plants such as Eucalyptus and Pinus heartwood. Pinocembrin is found in propolis, which is widely consumed by humans due to its medicinal properties. Actually, pinocembrin is a major component of propolis. It has been reported that pinocembrin induces cytoprotective effects in human cells.

Pinocembrin is an antioxidant and anti-inflammatory molecule, meaning that it acts against the deleterious effects elicited by reactive species/free radicals and attenuates inflammation. Some researchers also described an anti-tumor action of pinocembrin experimentally. Recently, our research group published a paper showing that pinocembrin is able to cause mitochondrial protection in the SH-SY5Y human neuroblastoma cell line.

These cells are utilized experimentally in order to investigate whether selected molecules would exert neuroprotection, for example. Additionally, the SH-SY5Y cells are used in Neurotoxicology in order to analyze the mechanisms underlying the deleterious effects of toxicants (pollutants, drugs, xenobiotics, others).

Mitochondria are organelles containing double membrane and are responsible for the production of adenosine triphosphate (ATP) in both animal and vegetal cells. The electron transfer chain is found in the inner mitochondrial membrane (IMM) and presents the protein complexes called complex I (NADH dehydrogenase), complex II (succinate dehydrogenase), complex III (coenzyme Q: cytochrome c-oxidoreductase), and complex IV (cytochrome c oxidase). Besides, coenzyme Q and cytochrome c are mobile components responsible for the electron transfer between the protein complexes of the electron transfer chain.

This system utilizes the energy released from the electron flux between these complexes to transport protons (H⁺) across the IMM. The concentration of the protons increases in the intermembrane space, generating a difference in charge and pH between the intermembrane space and the mitochondrial matrix. This is experimentally measured and is called mitochondrial membrane potential. The electrons and protons are obtained from substrates utilized by the cells such as fuels (glucose, fructose, fatty acids, amino acids, ketone bodies, others).

The energy obtained from these fuels is conserved and utilized to produce ATP in the mitochondria. Complex V (ATP synthase/ATPase) utilizes the proton gradient generated by the electron transfer chain to synthesize ATP from ADP and Pi. Oxygen gas (O₂) is the final acceptor of electrons in the electron transfer chain. This O₂ is obtained by the animals during breathing. The major role of O₂ in animal cells is to accept electrons in the electron transfer chain. Therefore, we breathe to amplify the production of ATP in order to maintain the complexity of our cells! After being released from the mitochondria, ATP is used as a source of energy in different physiological events in the cells (muscle contraction, transport of ions across the plasma membrane, detoxification of ammonia, synthesis of proteins, synthesis of lipids, others).

Even though the electron transfer chain is crucial in the oxidative phosphorylation process, this is a major source of free radicals, which are able to cause damage to the cells due to their high reactivity. There is evidence showing a role for free radicals in neurodegeneration (Parkinson’s disease, Alzheimer’s disease, Huntington’s disease), schizophrenia, bipolar disorder, major depression, type I and type II diabetes mellitus, and cardiovascular disease, among others. The production of free radicals is also increased during infection and inflammation. Importantly, free radicals are not produced only in the mitochondria. However, they are a major source of free radicals and mitochondrial dysfunction, which is also observed in the disorders mentioned above, enhances free radicals generation.

Moreover, mitochondria are a target of several toxicants, such as cocaine, ecstasy, cyanide, heavy metals, ethanol, among others. Some drugs utilized pharmacologically may affect mitochondrial function, such as fluoxetine. Excessive intake of vitamins may also cause disruption in the mitochondrial function, leading to decreased cell viability, as observed experimentally. Mitochondrial also control cell death by the intrinsic apoptotic pathway, i.e. a coordinated event responsible for the controlled death of cells. Apoptosis is a physiological event that occurs during the human development and continues into the adulthood. Nonetheless, increased rates of cell death by apoptosis may result in tissue dysfunction, causing diseases in humans.

Therefore, the mitochondrial homeostasis is crucial to maintaining the normal function of the different cell types humans present in their body (with exception of the red blood cells, which do not present mitochondria). In this context, we have utilized pinocembrin in the SH-SY5Y cells aiming to reveal whether this natural compound would promote mitochondrial protection during the exposure of these cells to paraquat (an agrochemical that generates free radicals and induces cell death by different mechanisms experimentally). The cells were treated with pinocembrin at 1 – 25 µM for 4 h prior to the administration of paraquat for additional 24 h.

Pinocembrin Pretreatment’s Impact On Mitochondrial Function

We have found that pinocembrin pretreatment at 25 µM decreased the impact of paraquat on the mitochondrial function, as assessed through the analyses of the activities of the complexes of the electron transfer chain. Furthermore, pinocembrin reduced the production of free radicals by the mitochondria in the cells that were challenged with paraquat. Consequently, the levels of ATP were found in a normal range of the cells that were pretreated with pinocembrin and exposed to paraquat posteriorly. Pinocembrin decreased the levels of the molecular markers of redox disturbance (oxidative and nitrosative stress) in the membranes of the mitochondria isolated from the SH-SY5Y cells.

Interestingly, pinocembrin stimulated the production of antioxidant enzymes, which are responsible for the metabolism of free radicals and reactive species in the cells. The upregulation of these enzymes occurred through the activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) transcription factor. This is a protein responsible for the control of the expression of several genes, whose products (other proteins and enzymes) are involved in both antioxidant defense and detoxification in mammalian cells. When the Nrf2-dependent signaling was blocked, the mitochondria-related protective effects induced by pinocembrin were suppressed. Thus, pinocembrin is very likely to cause mitochondrial protection by a mechanism dependent on the activation of this transcription factor.

Future research would be necessary to evaluate whether pinocembrin is a cytoprotective agent also in vivo regarding mitochondrial function and dynamics. This is particularly important because the metabolism (biotransformation) of xenobiotics (and pinocembrin is a xenobiotic to the human body) modifies the molecules we ingest. It is necessary to excrete xenobiotics, decreasing their time of permanence in the body. Biotransformation may be viewed as a series of enzymatic reactions necessary to protect human cells against strange molecules.

Unfortunately, there are cases in which biotransformation generates toxic agents after modifying xenobiotics. Importantly, the investigation about the possible toxic effects that may result from the exposure to pinocembrin should be performed in order to ensure a secure concentration range that may be utilized by humans (and other animals) without affecting their health. Indeed, it is not recommended to use purified pinocembrin as a supplement, such as vitamin supplements are inadvertently utilized, for example. However, pinocembrin presents a pharmacological potential that needs to be investigated carefully.

These findings are described in the article entitled Pinocembrin Provides Mitochondrial Protection by the Activation of the Erk1/2-Nrf2 Signaling Pathway in SH-SY5Y Neuroblastoma Cells Exposed to Paraquat, published in the journal Molecular Neurobiology. This work was led by Marcos de Oliveira from the Federal University of Mato Grosso, Cuiaba, Brazil.