Carnosic acid (CA, C20H28O4) is characterized as a phenolic diterpene and has been isolated from the Rosmarinus officinalis L. plant. This is a natural compound presenting several beneficial effects to human cells. Carnosic acid is an antioxidant, meaning that it may decrease the impact of reactive species/free radicals in biological systems. Moreover, carnosic acid exhibits the ability to modulate the immune response, acting as an anti-inflammatory agent.
The ability of carnosic acid in exerting antioxidant effects is not associated only with its chemical structure, i.e. carnosic acid is not just a scavenger of reactive species. Otherwise, carnosic acid is a potent activator of the nuclear factor erythroid 2-related factor 2 (Nrf2) protein, a transcription factor found in an inactive form in the cytosol bound to Kelch-like ECH-associated protein 1 (Keap1).
Keap1 may be viewed as the regulator of Nrf2, and reactive species/free radicals and electrophiles react with the Nrf2-Keap1 complex causing the release of Nrf2, which migrates to the cell nucleus and triggers the expression of selected genes whose products, i.e. proteins with or without enzymatic activity, participate in the antioxidant defense and in the metabolism of xenobiotics in mammals. In the nucleus, Nrf2 forms protein complexes with other small proteins and binds to the antioxidant response element (ARE) region of the genes, activating the transcription of proteins such as superoxide dismutase (SOD), catalase (CAT), glutamate-cysteine ligase (GCL), glutathione peroxidase (GPx), glutathione reductase (GR), glutathione-S-transferase (GST), thioredoxin, thioredoxin reductase (TR), and heme oxygenase-1 (HO-1), to cite a few. The enzymes convert reactive species in non-toxic molecules and also participate in the process of detoxification (which facilitates the biotransformation and excretion of “strange” molecules to the cells).
The production of reactive species/free radicals is a physiological process; however, during the exposure to toxicants (such as pollutants and natural toxins) or to xenobiotics (such as the components of plants: polyphenols, terpenes, several other molecules found only in vegetal; other agents commonly ingested by the humans, such as drugs and food additives), the production of reactive species/free radicals may increase, leading to oxidative or nitrosative stress, depending on the specific reactive species are generated.
The production of reactive species/free radicals is enhanced during pathological conditions, such as in neurodegenerative diseases and cardiovascular diseases, among several others (including metabolic disturbances, cancer, and infection). Therefore, the components presented in the vegetal diet cause several effects in the human cells not just for their chemical structure, but by the fact that they are not produced in the human body. Carnosic acid, for example, induces several benefits to the human health, but it is not produced in the body. This is a xenobiotic.
Human Health Benefits Of Carnosic Acid
The ingestion of moderated amounts of carnosic acid has been viewed as secure. However, like any other molecule, the ingestion (or administration of carnosic acid by other ways) of carnosic acid at high amounts may cause intoxication. Thus, carnosic acid presents beneficial effects when administrated at low doses and may be toxic at high doses. When administrated to some mammalian cells at the concentration of 1 µM, carnosic acid protects the cellular components (i.e. lipids, proteins, DNA, RNA) against chemical stressors such as hydrogen peroxide (H2O2), paraquat (an agrochemical), 6-hydroxydopamine (which is used to induce Parkinson’s like disease experimentally), among other toxicants. The increase in the concentration of carnosic acid to 5 µM may lead to cytotoxicity, depending on the cell type studied. At low concentrations, carnosic acid is detected by the cells as a xenobiotic and the Nrf2 transcription factor is activated, leading to an increase in the expression of proteins involved in cytoprotection, as mentioned above. These proteins would act not only in the biotransformation and posterior excretion of carnosic acid.
They will modulate biotransformation of other xenobiotics present in the cell at that specific moment. Importantly, these proteins will defend the cells against reactive species/free radicals that are produced by several sources, including those originated as part of the mechanism of action of toxicants (pollutants, agrochemicals, other). Overall, carnosic acid exposure caused an adaptive response in the cells, which become more resistant than in the absence of carnosic acid. This a basic response to several benefit xenobiotics presented in the human diet. However, carnosic acid exerts such effects at very low concentrations when compared to others, such as resveratrol, sulforaphane, pinocembrin, and naringenin (when comparing the effects in the same experimental model).
We have found, for example, that a pretreatment with carnosic acid at 1µM protected SH-SY5Y cells (a neuroblastoma cell line obtained from the human brain) exposed to paraquat, an agrochemical that has been viewed as a potential inducer of Parkinson’s disease in humans). In more details, carnosic acid prevented (since we used an experimental design of pretreatment) the oxidation of lipids and proteins found in the membranes of the organelles called mitochondria. The mitochondria are the main site of ATP production in mammalian cells. ATP is a source of energy for the cells work. Therefore, the protection of mitochondria leads to benefits on the maintenance of the cellular bioenergetics state in such cells.
Furthermore, oxidative (or nitrosative) damage to mitochondrial membranes favors cell death, since these organelles coordinate the intrinsic apoptotic pathway, a process by which the mammalian cells die (this event is either physiological or pathological, depending on the circumstance). Besides, mitochondria are a major source of reactive species/free radicals in the human cells. Mitochondrial dysfunction causes impaired utilization of O2, decreased production of ATP, release of pro-apoptotic factors (i.e. proteins located in the mitochondria that would trigger cell death when present in the cytosol), and increased production of free radicals.
Altogether, these factors cause cell death in several ways, including necrosis, leading to inflammation. Additionally, mitochondrial impairment affects synaptic plasticity, a process necessary to learning and memory, for example. In this context, mitochondrial homeostasis is crucial to the maintenance of the cellular quality and viability.
The study of the effects of carnosic acid on humans is limited, but clinical trials are necessary to reveal whether carnosic acid would be effective in causing benefits also in the humans. Experimentally, carnosic acid may be characterized as an excellent cytoprotective agent. Nonetheless, the heterogeneity observed in humans may lead to different effects, including intoxication. Thus, caution is needed when using diet components (or even supplements) containing carnosic acid (or any other isolated molecule). It is not recommended to ingest purified carnosic acid, for example. However, carnosic acid is a promising pharmacological agent regarding its ability to reduce the impact of chemical stressors on human cells.
These findings are described in the recently published article entitled Carnosic Acid Protects Mitochondria of Human Neuroblastoma SH-SY5Y Cells Exposed to Paraquat Through Activation of the Nrf2/HO-1Axis, in the journal Molecular Neurobiology. This work was led by Marcos Roberto de Oliveira from the Federal University of Mato Grosso.