Abstract
In this review, we outline the new expertise and research progress with luteolin as an antitumor agent, and clarify the related results from the aspects of tumor proliferation, apoptosis, invasion, metastasis, sensitivity to radiotherapy and chemotherapy, angiogenesis, and immunotherapy. In recent years, with the development of medical technology, the early detection rate of tumors has increased significantly. However, the number of cancer patients remains high. Therefore, a new and reasonably effective tumor therapeutic drug is urgently demanded. Luteolin, a flavonoid and widespread in nature, attracts more and more attention due to its universal biological utility, especially in the study of antitumor activity. This article reviews the work published in the past 20 years on the role and mechanism of luteolin as an antitumor agent, showing that this compound has a variety of effects for antitumor treatment by acting on different cytokines. Although clinical studies have not yet been widely carried out, a series of basic studies have confirmed that luteolin is a reasonably effective antineoplastic agent or anticancer adjuvant. Besides, derivatives of luteolin have good application prospects.
Introduction
Cancer is a significant public health issue that endangers people's lives, and the incidence and mortality rates are increasing. In 2020, almost 10 million patients died of cancer. 1 Compared with the data of 2018, the death toll rises, which causes huge population loss, and heavy economic and medical burdens. Moreover, the climbing trend in the death toll will become more serious due to the ageing population. Although chemoradiotherapy is the main strategy of tumor treatment, chemotherapeutic drugs have extremely serious cytotoxic effects and produce drug resistance. 2 Research on traditional Chinese medicine confirmed that many herbal preparations have satisfactory clinical and therapeutic effects on tumors. Therefore, in order to better develop and design antitumor drugs, more and more research focuses on natural drugs.
Luteolin, also known as cyanidenon and flavin, is formed of yellow needle-like crystals. Its chemical name is 3’,4’,5,7-tetrahydroxyflavone and its molecular formula is C15H10O6. Luteolin was first isolated from the plant Reseda luteola, and its trivial name luteolin is derived from this species name. 3 Afterward, vegetables and fruits such as celery, parsley, broccoli, onions, carrots, peppers, cabbages, apple skins, and chrysanthemum flowers were found to be luteolin rich.4-6 Luteolin was first discovered owing to its powerful anti-inflammatory effects.7-9 With further research, scientists also found that luteolin has many other biological effects, such as antitumor function, antiviral efficacy, anti-oxidation, and enhancing immune regulation.10-12 Moreover, luteolin has low toxicity and few adverse reactions so that it has great potential for clinical applications. 13 In recent years, more attention has been paid to its powerful antitumor effect.14-17 Therefore, more and more researchers have devoted themselves to the study of the antitumor mechanism of luteolin.
This is a narrative review, which introduces the progress of research on luteolin in tumor treatment. In this review, we are committed to presenting the new expertise and research progress regarding luteolin as an antitumor agent, clarifying the related results from the aspects of tumor proliferation, apoptosis, invasion, metastasis, angiogenesis, sensitivity to radiotherapy and chemotherapy, and immunotherapy, as seen in Figure 1. For this review, we used the keywords “luteolin,” “tumor,” “proliferation,” “apoptosis,” “invasion,” “migration,” “angiogenesis,” “chemotherapy,” “radiation,” “immunotherapy,” and their combinations by searching and analyzing the literature published from 2001 to 2022 on the Web of Science and PubMed.
Antitumor mechanism of luteolin.
Inhibition of Cell Proliferation
The basic characteristic of most tumors is the rapid proliferation of tumor cells. A growing solid tumor can not only consume nutrition from the body, but also compress the nearby organs. Therefore, manipulating cell proliferation is a hot topic in cancer treatment. Luteolin can reduce tumor cell proliferation and facilitate tumor cell apoptosis, which plays an antitumor role.18,19 A number of studies has confirmed that luteolin can inhibit the growth and development of many various cancer cells, such as breast cancer, advanced liver cancer, and other types of tumors.20,21 By acting on human myeloma cell line rpmi-8226, Chen et al found that luteolin can inhibit the proliferation of tumor cells in a dose-dependent manner. 22 Further studies showed that luteolin can block the cell cycle in the G2/M phase by increasing the level of p21/cip1 protein and downregulating the level of cdc2, cdc25c, and cyclin b1. 23 In the process of exploring glioma treatment, some experts and scholars announced that according to cell counting kit (CCK)-8 experiment, luteolin can significantly inhibit the proliferation of glioma cells in a time-dependent and dose-dependent manner. By further exploration, this mechanism may be established through activating the Mitogen-activated protein kinase (MAPK) pathway. 24 Raina et al also found that luteolin may regulate Akt/mTOR and MAPK pathways to inhibit cell cycle regulatory genes, including ccnd1, 2, and 3, cdkn1a, and cdkn2b, cdk4 and cdk2, to show its antiproliferative activity. 25 Besides, through the mitochondrial pathway, luteolin can activate the programed cell death system to inhibit the proliferation of tumor cells.15,26 In addition, regulating PI3K/Akt and ERK1/2 signaling pathways is also luteolin's mechanism of inhibiting cell proliferation. 27 Therefore, it can be concluded that luteolin can achieve its antitumor effects by inhibiting the proliferation of tumor cells.
Induce Cell Apoptosis
Apoptosis is a kind of programed cell death regulated by genes. Apoptosis is strictly regulated, with caspase usually inactivated. 28 However, once the apoptotic reaction initiates, the proenzyme form of caspase is activated and the cascade reaction of apoptotic proteases begins. One strategy of treating tumors is to induce apoptosis. 29 In order to develop and seek new anticancer drugs, we should analyze in depth the mechanism and principle of drug-induced apoptosis. It has been found that luteolin can upregulate the expression of pro-apoptotic genes such as apaf1, bax, bad, bid, caspase-3, and caspase-9, and downregulate the expression of anti-apoptotic genes including naip and bcl-2.25,30 In endometriosis, it was also found that luteolin can induce apoptosis by activating pro-apoptotic factors such as caspase-3, caspase-8, and caspase-9, and inhibiting the expression of chemokines CCL2 and CCL5. 31 In lung cancer cells, luteolin can enhance the activities of caspase-2, caspase-3/7, caspase-8/10, and caspase-9 through caspase dependent or caspase independent pathways.32,33 In addition, some studies also showed that luteolin can mediate cell apoptosis by upregulating death receptor 5. 34 Based on the above experimental results, we can find that luteolin can induce apoptosis of malignant tumor cells by regulating gene expression, signal pathway, and enzymatic reaction, rather than simple drug toxicity.
Inhibiting the Invasion and Metastasis of Cells
Invasion and metastasis reflect the degree of malignancy of tumors, which often occur in patients with advanced tumors. Consequently, inhibiting invasion and metastasis is an important strategy for the treatment of tumors. 35 For glioblastomas, after treatment with luteolin, dramatic induction of cell migration is caused by cell invasion induced by spheroids and Matrigel. 36 In the transplanted tumor model of breast cancer, researchers found that the expression of mmp9, mmp2, and cxcr4 in the lung tissues of mice was decreased significantly after treatment with luteolin.37,38 Some scholars also found that luteolin adjusts the expression of mmp9 by reducing the level of h3k27ac and h3k5, which is induced by Akt/mTOR. In this manner it inhibits the reproduction and metastasis of androgen protein kinase positive triple negative breast cancer (TNBC). 38 In a study of ovarian cancer cells, luteolin effectively inhibited the phosphorylation of the ERK signaling pathway, thereby reducing the level of mmp9 and inhibiting tumor cells significantly. 39 In addition, the function of luteolin to the migration and invasion of colorectal cancer cells (CRC) was tested through wound repair and cross pore test. This study showed that luteolin inhibits cell migration and invasion by upregulating mir-384 and downregulating multifunctional proteins. 40 Researchers also evaluated the efficacy of luteolin in human glioblastoma cell lines. Studies showed that luteolin can downregulate the expression of mmp2 and mmp9 and upregulate the expression of timp-1, -2 by regulating the P-IGF-1R/I3K/AKT/mTOR pathway, so as to inhibit the invasion of tumor cells. 41 Furthermore, antimetastatic activity is developed by decreasing p-akt, hif-1 alpha, vegf-a, p-vegfr-2, mmp2, and mmp9 protein levels. 42 In summary, luteolin can regulate multiple signaling pathways and their related proteins to decrease the invasion and metastasis of cancer cells.
Inhibition of Angiogenesis
Tumor angiogenesis is the requirement to obtain sufficient nutrition and oxygen for tumor growth.43,44 Anti-angiogenesis becomes an important means for the clinical treatment of tumors.45,46 Up to now, with the development of anti-angiogenic drugs, some progress has been made in tumor angiogenesis.47,48 It was found that luteolin can downregulate the expression of vegf and mmp9, and exert its anti-angiogenic effect through the hif-1α and stat3 signaling pathways. 49 Some scholars also expressed the view that luteolin can downregulate the expression of vegf and reduce the formation of angiogenesis mimic tubes by regulating the Notch1 pathway in gastric cancer cells. 50 In addition, luteolin inhibited both microvascular germinations induced by growth arrest-specific protein 6 in the aortic ring test and angiogenesis in the chicken chorioallantoic membrane test. Luteolin's anti-angiogenic effect may be related to the regulation of the PI3K/Akt/mTOR pathway and Gas6/Axl signaling pathway. 51 Through an intervention experiment involving luteolin in pancreatic cancer, it was pointed out that the expression of vegf and n-ca in tumor cells is downregulated after luteolin intervention. 52 When speaking of exploring the treatment of infantile hemangioma, luteolin can inhibit the proliferation of stem cells and downregulate the level of vegf in a concentration-dependent manner. Moreover, it can also inhibit both angiogenesis and neovascularization in animal models, possibly by regulating the Wnt pathway. 53 In breast cancer research, luteolin was found to decrease the secretion of progesterone dependent vegf and the growth of mpa-dependent xenografts in nude mice. 54 Based on the above experimental results, we can conclude that luteolin can play an anti-angiogenic role in tumor treatment.
Improve the Efficacy of Radiotherapy and Chemotherapy
At present, radiotherapy and chemotherapy are the best choices for unresectable malignant tumors.55,56 Previous studies showed that the combination of phytochemicals and chemotherapy may improve the overall efficacy of cancer treatment and minimize toxicity. 57 Luteolin has been investigated as a potent chemopreventive agent for numerous types of cancers.15,57 It can enhance the sensitivity of ovarian cancer to cisplatin, thus improving the curative effect. 58 It also has a synergistic effect with conventional chemotherapeutic drugs such as etoposide and cisplatin, which can inhibit proliferation of human choriocarcinoma cell lines.57,59 In breast cancer, luteolin can enhance the efficacy of paclitaxel by regulating the expression of fas. 60 During the process of exploring the effects of luteolin, oxaliplatin, and drug combination on gastric adenocarcinoma cell lines, it was found that the drug combination could inhibit the proliferation of tumor cells by changing the proportions in the cell cycle. 61 In a study of oral cancer stem cells, scholars found that luteolin can significantly inhibit the proliferation rate, and acetaldehyde dehydrogenase 1 and CD44 positive activity. Combined with radiotherapy, luteolin can effectively inhibit the invasion and metastasis of oral cancer, which means that it restores the radiosensitivity of oral cancer cells. 62 Irinotecan, one of the main drugs used for the treatment of CRC, can lead to the development of intestinal mucositis. However, luteolin can reduce the occurrence of this injury in test mice. Irinotecan downregulates ppar gamma, which enhances the expression of inflammatory and oxidative genes, while luteolin has the opposite effect. When they act on tumor cells together, the inflammation caused by the chemotherapeutic drugs is significantly reduced. However, when these experiments are carried out on ppar gamma silenced tumor cells, luteolin is found to have no such effect. 63 Luteolin can also inhibit chromosomal aberrations caused by mutagenic substances in rats. This effect is more pronounced than that of quercetin. 64 In conclusion, luteolin can improve the efficacy of radiotherapy and chemotherapy and prevent cancer.
Enhance Immune Response
Immunotherapy is used to enhance the immune system of patients by improving the ability to recognize tumors or providing the function of the missing immune effect. 65 It is a treatment method that is expected to achieve a lifelong cure. Therefore, discovering drugs such as molecules widely found in plants, which help regulate the immune system, is crucial to protect our bodies from bacterial infections, inflammatory diseases, and tumor development.65,66 Luteolin is an effective immunomodulator. Studies showed that luteolin can induce splenocyte proliferation, especially T cells, making it a candidate for immunotherapy. 67 It contributes to inhibiting tumor expansion by improving antitumor immunity induced by CD4+ and CD8+ T cells.65,68 It can effectively activate B and T cells and enhance the cellular activity of natural killer cells, which can participate in the host protective immune response before adaptive T cell responses. 69 In addition, luteolin can also reduce the content of PD-L1 to improve the microenvironment of immunosuppression, thus inhibiting tumor metastasis.14,70 As a tumor suppressor ligand, CD47 can bind to receptor signal regulatory protein ɑ (SIRPɑ) on phagocytes. As a signal of “don't eat me,” it can inhibit the phagocytosis from macrophages to tumor cells, and eventually cause the immune escape of tumor cells. 71 Relevant studies showed that the targeting CD47-SIRPɑ interacting antibodies can effectively inhibit a variety of tumors.72,73 In multiple myeloma cells, luteolin attenuated the link between CD47 and SIRPɑ by reducing pyroglutamate modification of CD47. 74 At present, many studies have shown that combined therapy with immunosuppressants can achieve better antitumor efficacy than single drug therapy.30,75,76 In recent years, the efficacy of luteolin in immunotherapy has attracted extensive attention and become a research hotspot for the next step.
Results and Discussion
Fruits and vegetables have been recognized to have a preventive effect on human diseases, including cancer. There are many literature reports that have found that chemical substances found in plants may play a major role, and these plants can be used as supplementary drugs to inhibit the growth of cancer. In conclusion, luteolin has the advantages of being nature-sourced, safe, and low cost. 62 Through searching the above literature, we found that luteolin exerts a variety of antitumor effects by acting on different proteins, as seen in Table 1. Through these studies, we can confirm that luteolin has the effects of antitumor cell proliferation, promoting tumor cell apoptosis, inhibiting angiogenesis, inhibiting tumor cell invasion and migration, increasing the sensitivity to radiotherapy and chemotherapy, and improving the immune response. Studies have shown that apigenin, a flavonoid very similar to luteolin, also has very similar biological activities in in vivo models. 77 Therefore, luteolin is expected to become an important adjuvant in tumor therapy. While the current research data are basically from cells and animals, we need to acquire more pharmacokinetic parameters and clarify the toxic effects of the drug before this substance becomes a prescription drug, which may involve human subjects. In addition, the development of standardized doses and the preparation of derivatives can also be carried out in clinical trials.
Molecular Mechanism of Luteolin in Different Tumors.
Footnotes
Acknowledgements
The authors would like to thank their colleagues in the Department of General Surgery of Wenling First People's Hospital and the teachers of the Department of Basic Medicine of Taizhou University for their help and encouragement.
Authors’ Contributions
BF was involved in conception and design and administrative support. CM and YW provided study materials. ZC, ZZ, and SX collected and assembled the data. ZC, DC, and YC analyzed and interpreted the data. All authors were involved in manuscript writing and final approval of the manuscript.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article. This work was supported by The Taizhou Municipal Science and Technology Bureau, The Wenling Social Development Science and Technology Project (grant number 21ywb112, 2021S00003).
