Protective potential of thymoquinone against cadmium, arsenic, and lead toxicity: A short review with emphasis on oxidative pathways

Heavy metals are among the most important environmental pollutions used in various industries. Their extensive use has increased human susceptibility to different chronic diseases. Toxic metal exposure, especially cadmium, arsenic, and lead, causes oxidative damages, mitochondrial dysfunction, and genetic and epigenetic modifications. Meanwhile, thymoquinone (TQ) is an effective component of Nigella sativa oil that plays an important role in preventing the destructive effects of heavy metals. The present review discusses how TQ can protect various tissues against oxidative damage of heavy metals. This review is based on the research reported about the protective effects of TQ in the toxicity of heavy metals, approximately the last 10 years (2010-2021). Scientific databases, including Scopus, Web of Science, and PubMed, were searched using the following keywords either alone or in combination: cadmium, arsenic, lead, TQ, and oxidative stress. TQ, as a potent antioxidant, can distribute to cellular compartments and prevent oxidative damage of toxic metals. However, depending on the type of toxic metal and the carrier system used to release TQ in biological systems, its therapeutic dosage range may be varied.

Hepatic Response to the Interaction Between Thymoquinone and Iron-Dextran: an In Vitro and In Vivo Study

Iron is one of the most important essential elements for cell function. However, iron overload can exert destructive effects on various tissues, especially the liver. The present study was designed to evaluate the effect of thymoquinone (TQ) on hepatotoxicity induced by iron-overload in in vitro and mouse model. After in vitro studies, thirty mice were divided into five groups, six each. Group 1 received normal saline. Group 2 received five doses of iron dextran (i.p; 100 mg/kg, one dose every 2 days). Group 3 received TQ (orally, 2 mg/kg/day). Groups 4 and 5 were administrated iron dextran saline (i.p; 100 mg/kg, one dose every 2 days) following treatment with 0.5 and 2 mg/kg/day of TQ, respectively. Based on the findings of the DPPH experiment, although TQ has significant anti-radical potential, at a safe dose of 15 × 10⁺³ nM, it reduced the IC₅₀ of iron dextran on HepG2 cells by about 25%, in in vitro. Following administration of low-dose TQ (0.5 mg/kg), a significant improvement was observed in serum hepatic enzymes activity and hepatic lipid peroxidation compared to iron dextran. However, administration of TQ-high dose (2 mg/kg) led to decrease antioxidant defense alongside increased serum hepatic enzymes and pathological damages in iron dextran-treated animals. Due to the different efficacy of TQ in treatment groups, it seems that the TQ therapeutic index is low and does not have significant safety in the iron overload status.

Response of Skin-Derived and Metastatic Human Malignant Melanoma Cell Lines to Thymoquinone and Thymoquinone-Loaded Liposomes

Thymoquinone has been proved to be effective against neoplasms, including skin cancer. Its high lipophilicity, however, may limit its potential use as a drug. Melanoma remains the deadliest of all skin cancers worldwide, due to its high heterogeneity, depending on the stage of the disease. Our goal was to compare the anti-cancer activity of free thymoquinone and thymoquinone-loaded liposomes on two melanoma cell lines that originated from different stages of this cancer: skin-derived A375 and metastatic WM9. We evaluated the proapoptotic effects of free thymoquinone by flow cytometry and Western blot, and its mitotoxicity by means of JC-1 assay. Additionally, we compared the cytotoxicity of free thymoquinone and thymoquinone in liposomes by WST-1 assay. Our results revealed a higher antiproliferative effect of TQ in WM9 cells, whereas its higher proapoptotic activity was observed in the A375 cell line. Moreover, the thymoquinone-loaded liposome was proved to exert stronger cytotoxic effect on both cell lines studied than free thymoquinone. Differences in the response of melanoma cells derived from different stages of the disease to thymoquinone, as well as their different responses to free and carrier-delivered thymoquinone, are essential for the development of new anti-melanoma therapies. However, further research is required to fully understand them.

Thymoquinone, as a Novel Therapeutic Candidate of Cancers

To date, natural products are widely used as pharmaceutical agents for many human diseases and cancers. One of the most popular natural products that have been studied for anticancer properties is thymoquinone (TQ). As a bioactive compound of Nigella sativa, TQ has shown anticancer activities through the inhibition of cell proliferation, migration, and invasion. The anticancer efficacy of TQ is being investigated in several human cancers such as pancreatic cancer, breast cancer, colon cancer, hepatic cancer, cervical cancer, and leukemia. Even though TQ induces apoptosis by regulating the expression of pro- apoptotic and anti-apoptotic genes in many cancers, the TQ effect mechanism on such cancers is not yet fully understood. Therefore, the present review has highlighted the TQ effect mechanisms on several signaling pathways and expression of tumor suppressor genes (TSG). Data from relevant published experimental articles on TQ from 2015 to June 2020 were selected by using Google Scholar and PubMed search engines. The present study investigated the effectiveness of TQ alone or in combination with other anticancer therapeutic agents, such as tyrosine kinase inhibitors on cancers, as a future anticancer therapy nominee by using nanotechnology.