Cytotoxicity of dietary flavonoids on different human cancer types

Flavonoids are ubiquitous in nature. They are also in food, providing an essential link between diet and prevention of chronic diseases including cancer. Anticancer effects of these polyphenols depend on several factors: Their chemical structure and concentration, and also on the type of cancer. Malignant cells from different tissues reveal somewhat different sensitivity toward flavonoids and, therefore, the preferences of the most common dietary flavonoids to various human cancer types are analyzed in this review. While luteolin and kaempferol can be considered as promising candidate agents for treatment of gastric and ovarian cancers, respectively, apigenin, chrysin, and luteolin have good perspectives as potent antitumor agents for cervical cancer; cells from main sites of flavonoid metabolism (colon and liver) reveal rather large fluctuations in anticancer activity probably due to exposure to various metabolites with different activities. Anticancer effect of flavonoids toward blood cancer cells depend on their myeloid, lymphoid, or erythroid origin; cytotoxic effects of flavonoids on breast and prostate cancer cells are highly related to the expression of hormone receptors. Different flavonoids are often preferentially present in certain food items, and knowledge about the malignant tissue-specific anticancer effects of flavonoids could be purposely applied both in chemoprevention as well as in cancer treatment.

Chemotherapy and dietary phytochemical agents

Chemotherapy has been used for cancer treatment already for almost 70 years by targeting the proliferation potential and metastasising ability of tumour cells. Despite the progress made in the development of potent chemotherapy drugs, their toxicity to normal tissues and adverse side effects in multiple organ systems as well as drug resistance have remained the major obstacles for the successful clinical use. Cytotoxic agents decrease considerably the quality of life of cancer patients manifesting as acute complaints and impacting the life of survivors also for years after the treatment. Toxicity often limits the usefulness of anticancer agents being also the reason why many patients discontinue the treatment. The nutritional approach may be the means of helping to raise cancer therapy to a new level of success as supplementing or supporting the body with natural phytochemicals cannot only reduce adverse side effects but improve also the effectiveness of chemotherapeutics. Various plant-derived compounds improve the efficiency of cytotoxic agents, decrease their resistance, lower and alleviate toxic side effects, reduce the risk of tumour lysis syndrome, and detoxify the body of chemotherapeutics. The personalised approach using various phytochemicals provides thus a new dimension to the standard cancer therapy for improving its outcome in a complex and complementary way.

Ferulic Acid: A Natural Phenol That Inhibits Neoplastic Events through Modulation of Oncogenic Signaling

Despite the immense therapeutic advances in the field of health sciences, cancer is still to be found among the global leading causes of morbidity and mortality. Ethnomedicinally, natural bioactive compounds isolated from various plant sources have been used for the treatment of several cancer types and have gained notable attention. Ferulic acid, a natural compound derived from various seeds, nuts, leaves, and fruits, exhibits a variety of pharmacological effects in cancer, including its proapoptotic, cell-cycle-arresting, anti-metastatic, and anti-inflammatory activities. This review study presents a thorough overview of the molecular targets and cellular signaling pathways modulated by ferulic acid in diverse malignancies, showing high potential for this phenolic acid to be developed as a candidate agent for novel anticancer therapeutics. In addition, current investigations to develop promising synergistic formulations are also discussed.

Established Human Cell Lines as Models to Study Anti-leukemic Effects of Flavonoids

Despite the extensive work on pathological mechanisms and some recent advances in the treatment of different hematological malignancies, leukemia continues to present a significant challenge being frequently considered as incurable disease. Therefore, the development of novel therapeutic agents with high efficacy and low toxicity is urgently needed to improve the overall survival rate of patients. In this comprehensive review article, the current knowledge about the anticancer activities of flavonoids as plant secondary polyphenolic metabolites in the most commonly used human established leukemia cell lines (HL-60, NB4, KG1a, U937, THP-1, K562, Jurkat, CCRF- CEM, MOLT-3, and MOLT-4) is compiled, revealing clear anti-proliferative, pro-apoptotic, cell cycle arresting, and differentiation inducing effects for certain compounds. Considering the low toxicity of these substances in normal blood cells, the presented data show a great potential of flavonoids to be developed into novel anti-leukemia agents applicable also in the malignant cells resistant to the current conventional chemotherapeutic drugs.

Phloretin, as a Potent Anticancer Compound: From Chemistry to Cellular Interactions

Phloretin is a natural dihydrochalcone found in many fruits and vegetables, especially in apple tree leaves and the Manchurian apricots, exhibiting several therapeutic properties, such as antioxidant, antidiabetic, anti-inflammatory, and antitumor activities. In this review article, the diverse aspects of the anticancer potential of phloretin are addressed, presenting its antiproliferative, proapoptotic, antimetastatic, and antiangiogenic activities in many different preclinical cancer models. The fact that phloretin is a planar lipophilic polyphenol and, thus, a membrane-disrupting Pan-Assay Interference compound (PAIN) compromises the validity of the cell-based anticancer activities. Phloretin significantly reduces membrane dipole potential and, therefore, is expected to be able to activate a number of cellular signaling pathways in a non-specific way. In this way, the effects of this minor flavonoid on Bax and Bcl-2 proteins, caspases and MMPs, cytokines, and inflammatory enzymes are all analyzed in the current review. Moreover, besides the anticancer activities exerted by phloretin alone, its co-effects with conventional anticancer drugs are also under discussion. Therefore, this review presents a thorough overview of the preclinical anticancer potential of phloretin, allowing one to take the next steps in the development of novel drug candidates and move on to clinical trials.

Epidemiological Evidences on Dietary Flavonoids and Breast Cancer Risk: A Narrative Review

Epidemiological studies on associations between intake of flavonoids and breast cancer risk are highly needed to assess the actual effects of flavonoids in humans. Experimental investigations in vitro conditions cannot detect and model the real action of these phytochemicals due to the limitations to consider absorption and metabolic biotransformation as well as several complex interactions. Therefore, the data about association findings between intake of flavonoids and breast cancer risk are compiled and analyzed in the current review by evaluating both the results obtained using food composition databases as well as different biomarkers. Although several case-control studies demonstrate some reduction in breast cancer risk related to high consumption of flavones and flavonols, large-scale prospective cohort studies with follow-up times of many years do not confirm these findings. Intake of isoflavones can be associated with a decrease in breast tumorigenesis only in Asian countries where the consumption of soy foods is high but not among Western women with significantly lower ingestion amounts, suggesting the presence of so-called threshold level of effect. Besides doses, the timing of exposure to isoflavones seems also to be a significant factor as childhood and prepubertal age can be critical periods. Although women may need to consume high amounts of isoflavones typical to Asian diets to gain beneficial effects and protection against mammary carcinogenesis, it is still too early to give any specific recommendations to prevent breast tumors by diet rich in certain flavonoids.

Gut Microbiota-Assisted Synthesis, Cellular Interactions and Synergistic Perspectives of Equol as a Potent Anticancer Isoflavone

It is well known that, historically, plants have been an important resource of anticancer agents, providing several clinically approved drugs. Numerous preclinical studies have shown a strong anticancer potential of structurally different phytochemicals, including polyphenolic constituents of plants, flavonoids. In this review article, suppressing effects of equol in different carcinogenesis models are unraveled, highlighting the mechanisms involved in these anticancer activities. Among flavonoids, daidzein is a well-known isoflavone occurring in soybeans and soy products. In a certain part of population, this soy isoflavone is decomposed to equol under the action of gut microflora. Somewhat surprisingly, this degradation product has been shown to be more bioactive than its precursor daidzein, revealing a strong and multifaceted anticancer potential. In this way, it is important to bear in mind that the metabolic conversion of plant flavonoids might lead to products that are even more efficient than the parent compounds themselves, definitely deserving further studies.

Cytotoxic action of methylquercetins in human lung adenocarcinoma cells

Lung cancer is the malignant disorder associated with a high number of fatalities in women and men worldwide. Despite continuous improvements in diagnostic strategies and therapeutic modalities over the past decades, the prognosis and survival rate of patients suffering from lung cancer are still unsatisfactory and suggest the requirement for further molecular studies with different lung cancer models. In the present study, the anticancer action of two methylated metabolites of quercetin, isorhamnetin and tamarixetin, was assessed by studying their antiproliferative and apoptosis-inducing potential in human lung adenocarcinoma cell lines, A549 and HCC-44. Both methylquercetins decreased the viability of lung cancer cells at doses significantly lower than those effective for parent quercetin. The IC50 values measured for isorhamnetin were 26.6 and 15.9 µM in A549 and HCC-44 cells, respectively. For tamarixetin, the IC50 values were 19.6 and 20.3 µM in A549 and HCC-44 cells, respectively. These results were many-fold lower than the respective values for quercetin (72.2 and 107.6 µM for A549 and HCC-44 cells, respectively). Based on the activation of caspase family members, both metabolites induced apoptotic cell death in the tested cell lines, predominantly via the extrinsic pathway in A549 cells and in both intrinsic and extrinsic pathways in HCC-44 cells. As A549 and HCC-44 lines were originally established from a male and female patient, current data may suggst some gender differences in the action of quercetin derivatives. Addition of a methyl group in the 3'- or 4'-position of the B-ring of quercetin significantly increased the anticancer activity of this flavonol towards lung adenocarcinoma cells, which demonstrated that these compounds may be considered as potential novel candidates for the development of future chemotherapeutics in the fight against lung cancer.

Anticancer action of plant products: changing stereotyped attitudes

Compared to humans, plants can synthesize an extremely diverse array of chemical compounds, including phenolic acids, flavonoids, stilbenes, lignans, terpenoids, alkaloids, and many other types of secondary metabolites that have been demonstrated to exert important bioactivities and impacts on the human health. As a result of extensive and sustained efforts, some phytochemicals like vincristine, vinblastine, and paclitaxel have already been approved as anticancer drugs today, while several others are under clinical trials. However, despite this remarkable success, studies on anticancer action of plant-derived products have been and paradoxically are still in some places, mixed up with alternative approaches and thereby considered non-credible, especially in regions where the role of traditional medicine has not been historically so prevalent as in several Asian countries. As a result, only about 10% of higher plants have been explored regarding the potential therapeutic effects of their constituents. Moreover, as one function of secondary metabolites includes the protection of plants against diverse environmental stresses, the content and composition of these phytochemicals might importantly vary between different regional habitats. Therefore, the stereotyped attitudes to plant products as something related to alternative medicine must be changed to identify new lead molecules for novel anticancer drugs. It is possible that plants still harbor an important spectrum of pharmaceutically interesting, but still unidentified, chemical compounds.

GPCRs as targets for flavonoids in cancer cells: new options for intervention

For a long time, the family of receptor tyrosine kinases, including epidermal growth factor receptor and insulin-like growth factor 1 receptor, was regarded as the main players stimulating cell proliferative signaling. Today, it is increasingly clear that many G protein-coupled receptors (GPCRs) are also involved in controlling the hallmarks of cancer by activating diverse intracellular signaling networks. GPCRs can therefore be considered as promising drug targets for fighting against diverse types of human malignancies. Although plant polyphenols, flavonoids, are well known for their diverse anticancer effects inhibiting the growth, proliferation, migration, and invasion of malignant cells, involvement of GPCRs in these activities has still remained largely unelucidated. Therefore, in this review article, the current knowledge about the role of GPCRs in anticancer action of structurally varied flavonoids is compiled, highlighting the ability of these natural polyphenols to modulate the expression levels of GPCRs but also suppress the action of endogenous ligands and downstream tumor-promoting events. These data show that targeting the respective GPCRs by specific flavonoids may open new perspectives in the therapeutic intervention in human malignancies.