Flavones: Six Selected Flavones and Their Related Signaling Pathways That Induce Apoptosis in Cancer

Cancer is a horrific disease that, to date, has no cure. It is caused by various factors and takes many lives. Apoptosis is a programmed cell death mechanism and if it does not function correctly in cancer cells, it can lead to severe disease. There are various signaling pathways for regulating apoptosis in cancer cells. Flavonoids are non-artificial natural bioactive compounds that are gaining attention as being capable of for inducing apoptosis in cancer cells. Among these, in this study, we focus on flavones. Flavones are a subclass of the numerous available flavonoids and possess several bioactive functions. Some of the most reported and well-known critical flavones, namely apigenin, acacetin, baicalein, luteolin, tangeretin, and wogonin, are discussed in depth in this review. Our main aim is to investigate the effects of the selected flavones on apoptosis and cell signaling pathways that contribute to death due to various types of cancers.


Cancer and Apoptosis
Cancer is one of the world's most deadly diseases, is expensive and complicated to treat, and has many side effects due to the chemical drugs used in its treatments. The causes of cancer are diverse and can be divided into two groups: intrinsic and extrinsic risk factors. Intrinsic risks arise from accidental errors in DNA replication and cannot be modified. Extrinsic risks include both endogenous and exogenous factors. Endogenous risk factors that can be partially modified include aging, hormones, growth factors, inflammation, and DNA repair mechanisms. Exogenous risk factors are adjustable and include radiation, chemical carcinogens, tumor-causing viruses, and smoking [1].
The hallmarks of cancer cells include the continuous maintenance of proliferation signals, avoidance of growth inhibitors, activation of invasion and metastasis, induction of angiogenesis, and the possibility of permanent proliferation. In addition, the abnormality of cellular energy metabolism, avoidance of immune destruction, tumor-promoting inflammation, genomic instability, and mutation have been suggested [2]. The malfunction of apoptosis is also a cause of cancer so cancer cells with apoptotic problems do not follow this mechanism [3].
Apoptosis, along with necrosis and autophagy, is a form of programmed cell death in cancers [4]. During the apoptosis mechanisms, there are significant morphological changes such as cell shrinkage, condensation of nuclear chromatin, DNA fragmentation, blebbing, and the formation of an apoptotic body [5,6]. Subsequently, phosphatidylserine released

An Overview of Flavonoids and Their Structure-Activity Relationship in Anticancer Effects
To effectively prevent cancers through apoptosis and treat them without side effects, flavonoids as phenolic compounds play a significant role as natural biological response modifiers (BRM) [21]. Flavonoids are the secondary metabolites made by plants for selfprotection from external factors and are the most common compounds present in plants and fungi such as vegetables and fruits. As shown in Figure 1, flavonoids are based on a 15-carbon skeleton consisting of two benzene rings (A and B rings), and then depending on the linkage through the heterocyclic pyran ring, flavonoids can be divided into six major subclasses, which include anthocyanidins, flavones, flavan-3-ols, flavanones, flavonols, and isoflavones [22][23][24]. Currently, more than 6000 flavonoids have been identified among the subclasses. There are many studies that show flavonoids induce apoptosis through various pathways and have anticancer effects [25,26]. Many researchers anticipate that these flavonoids will bring enormous benefits to the treatment of cancer [27].
As an anticancer effect, the structure-activity relationship revealed in the comparison of flavonoids showed that the adjacent di-OH at the 3 and 4 of the B ring and the C2-C3 double bonds were important in the potent antiproliferative activity [28,29]. In polymethoxylated flavones, C3-OH and C8 methoxyl groups play an important role in antiproliferation [30]. Compared to flavones, the hydroxyl group of C3 of flavonols does not show antiproliferative effects, and the high activity of myricetin compared to quercetin is explained by the presence of the third OH in the B ring adjacent to the catechol group [31,32]. In other studies, the di-OH 3 and 4 , C2-C3 double bonds, and the carbonyl group C4 were shown to play important roles in the inhibition of proliferation through cell cycle arrest [33,34]. In addition, the structure-activity relationship of anticancer action through angiogenesis inhibition shows that flavanones are more effective at inhibiting the angiogenesis factor (VEGF) than flavones or flavanols. This is because the increase in the hydroxyl group of the B ring increases the inhibitory effect. Additionally, the absence of a double bond between C2 and C3 and the level of glycosylation may play an important role in angiogenesis inhibition [31,35,36]. were shown to play important roles in the inhibition of proliferation through cell cycle arrest [33,34]. In addition, the structure-activity relationship of anticancer action through angiogenesis inhibition shows that flavanones are more effective at inhibiting the angiogenesis factor (VEGF) than flavones or flavanols. This is because the increase in the hydroxyl group of the B ring increases the inhibitory effect. Additionally, the absence of a double bond between C2 and C3 and the level of glycosylation may play an important role in angiogenesis inhibition [31,35,36].

Flavones as Excellent Anticancer Agents
Flavones play a crucial role as anticancer agents [23,41] (Table 1). This is the reason that we focused on flavones and their anticancer relationship through apoptosis. In addition to the above six flavones, numerous flavones have biological and pharmacological effects on cancer through mitosis inhibition and angiogenesis inhibition. An abnormal cell cycle causes excessive cell proliferation, leading to cancer, and generating limitless new blood vessels is an advantage for cancer cell growth [41]. Another example of the biological and pharmacological effects of flavones on cancer is the inhibition of the protein tyrosine kinase. The binding of various growth factors to their receptors is related to cancer development. The abnormal activation of protein tyrosine kinase involved in this binding is a major target for anticancer treatments. Therefore, the inhibition of protein tyrosine kinase could be a useful anticancer strategy. For example, luteolin blocks the ATP binding site on the catalytic unit of protein kinase C (PKC). Chrysin also inhibits tyrosine kinase activity [41]. Flavone-8-acetic acid is involved in ornithine decarboxylase inhibition and the related anticancer activity. Rutin and luteolin are involved in aromatase inhibition related to breast cancer. Combretastatin acts as a powerful tubulin polymerization inhibitor, resulting in anticancer activity (Table 1) [41].
Overall, six flavones with anticancer effects were selected from numerous flavonoids, and the anticancer effects through the signaling pathways inducing apoptosis were investigated. In this paper, we review the apoptosis and signaling pathways that form the anticancer effects of flavones. This paper could therefore be useful for researchers studying the relationship between the anticancer effects of flavones and their signaling pathways that induce apoptosis.

Six Selected Flavones and Their Anticancer Effects
The anticancer effects of flavones have been studied by many researchers. Flavones showing typical anticancer effects include apigenin, acacetin, baicalein, luteolin, tangeretin, and wogonin. Figure 2 shows the chemical structures of flavones. These flavones are effective in treating breast cancer, lung cancer, stomach cancer, liver cancer, skin cancer, ovarian cancer, cervical cancer, and prostate cancer, all of which have high incidences and high mortality rates worldwide [23,[75][76][77][78][79]. development. The abnormal activation of protein tyrosine kinase involved in this binding is a major target for anticancer treatments. Therefore, the inhibition of protein tyrosine kinase could be a useful anticancer strategy. For example, luteolin blocks the ATP binding site on the catalytic unit of protein kinase C (PKC). Chrysin also inhibits tyrosine kinase activity [41]. Flavone-8-acetic acid is involved in ornithine decarboxylase inhibition and the related anticancer activity. Rutin and luteolin are involved in aromatase inhibition related to breast cancer. Combretastatin acts as a powerful tubulin polymerization inhibitor, resulting in anticancer activity (Table 1) [41].
Overall, six flavones with anticancer effects were selected from numerous flavonoids, and the anticancer effects through the signaling pathways inducing apoptosis were investigated. In this paper, we review the apoptosis and signaling pathways that form the anticancer effects of flavones. This paper could therefore be useful for researchers studying the relationship between the anticancer effects of flavones and their signaling pathways that induce apoptosis.

Six Selected Flavones and Their Anticancer Effects
The anticancer effects of flavones have been studied by many researchers. Flavones showing typical anticancer effects include apigenin, acacetin, baicalein, luteolin, tangeretin, and wogonin. Figure 2 shows the chemical structures of flavones. These flavones are effective in treating breast cancer, lung cancer, stomach cancer, liver cancer, skin cancer, ovarian cancer, cervical cancer, and prostate cancer, all of which have high incidences and high mortality rates worldwide [23,[75][76][77][78][79].

Apigenin
Apigenin is called 4′,5,7-trihydroxyflavone ( Figure 2A). The molecular formula of apigenin is C15H10O5 and its molecular weight is 270.24 g/mol [80]. It is found in many plants, fruits, and beverages such as parsley, grapes, apples, chamomile tea, and red wine [80,81]. Many studies have revealed the anticancer effects of apigenin [82]. In particular, it is a flavone that is effective against breast, prostate, liver, skin, colorectal, and lung cancers [83].

Apigenin
Apigenin is called 4 ,5,7-trihydroxyflavone ( Figure 2A). The molecular formula of apigenin is C 15 H 10 O 5 and its molecular weight is 270.24 g/mol [80]. It is found in many plants, fruits, and beverages such as parsley, grapes, apples, chamomile tea, and red wine [80,81]. Many studies have revealed the anticancer effects of apigenin [82]. In particular, it is a flavone that is effective against breast, prostate, liver, skin, colorectal, and lung cancers [83].

Apoptosis Pathways
In cancer cells, apoptosis is a very complex mechanism that is indispensable. If apoptosis does not function properly, cancer cells proliferate indefinitely [3]. As shown in Figure 3, there are two main pathways in apoptosis; one is the intrinsic pathway and the other is the extrinsic pathway [10].

Apoptosis Pathways
In cancer cells, apoptosis is a very complex mechanism that is indispensable. If apoptosis does not function properly, cancer cells proliferate indefinitely [3]. As shown in Figure 3, there are two main pathways in apoptosis; one is the intrinsic pathway and the other is the extrinsic pathway [10].

Intrinsic Pathway
In the intrinsic pathway, cytochrome c is released from the mitochondria by the Bcl-2 family, which combines with apoptotic protease activation factor-1 (APAF-1) and ATP to form an apoptosome complex by binding pro-caspase-9. It activates caspase-9 and caspase-3, -6, and -7 which are the executioners of apoptosis [10,11,102].

Extrinsic Pathway
The extrinsic pathway is mediated by the death receptors including the Fas receptor, tumor necrosis factor receptor (TNF), and TNF-related apoptosis-inducing ligand receptor (TRAIL) [103]. These death receptors have death domains called the TNF receptorassociated death domain (TRADD) and the Fas-associated death domain (FADD) [3]. Death ligands (Fas L, TNF) bind to the death receptor to form a death-inducing signaling complex (DISC), which activates caspase-8 and caspase-3, the executioner of apoptosis [3,10,103,104].
Numerous signaling pathways are involved in these two major (extrinsic and intrinsic) apoptosis pathways. The regulation of apoptosis is imperative in cancer cells as well as some of the major cell signaling pathways involved in the survival and death of cancer cells.

Intrinsic Pathway
In the intrinsic pathway, cytochrome c is released from the mitochondria by the Bcl-2 family, which combines with apoptotic protease activation factor-1 (APAF-1) and ATP to form an apoptosome complex by binding pro-caspase-9. It activates caspase-9 and caspase-3, -6, and -7 which are the executioners of apoptosis [10,11,102].

Extrinsic Pathway
The extrinsic pathway is mediated by the death receptors including the Fas receptor, tumor necrosis factor receptor (TNF), and TNF-related apoptosis-inducing ligand receptor (TRAIL) [103]. These death receptors have death domains called the TNF receptorassociated death domain (TRADD) and the Fas-associated death domain (FADD) [3]. Death ligands (Fas L, TNF) bind to the death receptor to form a death-inducing signaling complex (DISC), which activates caspase-8 and caspase-3, the executioner of apoptosis [3,10,103,104].
Numerous signaling pathways are involved in these two major (extrinsic and intrinsic) apoptosis pathways. The regulation of apoptosis is imperative in cancer cells as well as some of the major cell signaling pathways involved in the survival and death of cancer cells.

Wnt Pathway
The Wnt pathway is a signaling pathway involved in cell proliferation, migration, stem cell differentiation, and various diseases including cancer. In this pathway, β-catenin belongs downstream of Wnt and plays a key role in apoptosis while triggering transcription by the Wnt ligand [114][115][116]. Wnt acts as a ligand in this pathway and its receptor is Frizzled [114]. In the absence of Wnt, destruction complexes such as Axin, glycogen synthase kinase-3 beta (GSK3β), adenomatous polyposis coli protein (APC), and casein kinase (CK) phosphorylate β-catenin. Due to proteasome protein degradation, β-catenin is degraded in the cytoplasm and cannot meet the T-cell factor (TCF) so transcription does not occur [114,117]. On the other hand, when the Wnt ligand binds to its receptor Frizzled, phosphorylation of the lipoprotein receptor-related protein (LRP), a co-receptor of Wnt, occurs, and Axin is phosphorylated and moves to the cell together with CK and GSK3β [114,115]. The β-catenin, freed from the influence of GSK3β, enters the nucleus and meets TCF to initiate β-catenin-dependent gene expression [114,117]. Inhibition of the Wnt/β-catenin signaling pathway showed anticancer effects by attenuating survival signals and inducing apoptosis [117,118]. Another mechanism is that β-catenin downregulates anti-apoptosis proteins and induces apoptosis by including transcription of p53 and c-myc, thus inducing the transcription of pro-apoptosis proteins [119].

JAK/STAT Pathway
Abnormal activation of the janus kinase (JAK)/signal transducer and activators of the transcription (STAT) pathway promotes tumorigenesis [120]. The cytokine receptors do not have tyrosine kinase activity so there is a problem in signal transduction. When cytokines, such as chemokines, interleukins, interferon, and TNF-α, a protein immunomodulatory agent secreted from immune cells, bind to a cytokine receptor without tyrosine kinase activity, JAK is gathered around the cytokine receptor to transmit a signal [121][122][123]. In this way, JAK phosphorylates STAT to form a STAT dimer and enters the nucleus to express the STAT target gene [123]. Blocking this JAK/STAT pathway in cancer cells suppresses the expression of the target gene that controls essential cellular function and prevents the avoidance of apoptosis, a characteristic of cancer cells, and an anticancer effect can be expected [124].

MAPK Pathway
The mitogen-activated protein kinase (MAPK) pathway plays an important role in cell proliferation, differentiation, angiogenesis, tumor metastasis, and apoptosis [125]. MAP kinase kinase kinase (MAPKKK) phosphorylates MAP kinase kinase (MAPKK) by external growth factors, stress, and cytokines. MAPKK phosphorylates MAPK, and MAPK phosphorylates and activates the target protein [125,126]. MAPKKK, MAPKK, and MAPK can be divided into four major signaling systems. The four-member classes in MAPK are extracellular signaling-regulated kinase1/2 (ERK1/2), ERK5, c-Jun NH-2-terminal kinase (JNK), and p38-MAPK. There are ERK1/2, ERK5, JNK1/2/3, and p38-MAPKα/β/γ/δ sub-families in each family [125][126][127]. Among them, ERK is known to have anti-apoptotic action, and JNK and p38-MAPK are likely to act in both anti-and pro-apoptosis [127]. An increase in ERK1/2/5 in cancer cells has been confirmed in the MAPK pathway, which could be a major target for anticancer effects [127][128][129]. The activation of JNK and p38-MAPK normally promotes but does not necessarily lead to apoptosis, and both JNK and p38-MAPK mediate anti-apoptosis and pro-apoptosis [130]. To explain apoptosis in the MAPK pathway, it can be anti-or pro-apoptosis depending on the response of the target protein to various stimuli or the molecular context according to the cell type [130].

p53
As a tumor suppressor, p53 is activated when DNA is damaged, causing cell cycle arrest and apoptosis. When this protein is mutated, cells that should die do not die and cancer occurs [131]. In the normal apoptosis process in the p53 pathway, when cellular stress signals occur, p53 induces the pro-apoptotic member BH-3-only proteins (BIM, NOXA, PUMA, and BAX) [132][133][134][135]. After this, BIM, NOXA, and PUMA suppress the Bcl-2 family of anti-apoptotic proteins, and apoptosis occurs through an intrinsic pathway by activated BAX and BAK [132][133][134]. Figure 4 shows the signaling pathways that are involved in apoptosis, which are interconnected. Numerous signaling pathways are involved in this apoptosis mechanism.
MAPK are extracellular signaling-regulated kinase1/2 (ERK1/2), ERK5, c-Jun NH-2nal kinase (JNK), and p38-MAPK. There are ERK1/2, ERK5, JNK1/2/3, and MAPKα/β/γ/δ sub-families in each family [125][126][127]. Among them, ERK is known to anti-apoptotic action, and JNK and p38-MAPK are likely to act in both anti-and protosis [127]. An increase in ERK1/2/5 in cancer cells has been confirmed in the MAPK way, which could be a major target for anticancer effects [127][128][129]. The activation o and p38-MAPK normally promotes but does not necessarily lead to apoptosis, and JNK and p38-MAPK mediate anti-apoptosis and pro-apoptosis [130]. To explain apo in the MAPK pathway, it can be anti-or pro-apoptosis depending on the response target protein to various stimuli or the molecular context according to the cell type

p53
As a tumor suppressor, p53 is activated when DNA is damaged, causing cell arrest and apoptosis. When this protein is mutated, cells that should die do not d cancer occurs [131]. In the normal apoptosis process in the p53 pathway, when ce stress signals occur, p53 induces the pro-apoptotic member BH-3-only proteins NOXA, PUMA, and BAX) [132][133][134][135]. After this, BIM, NOXA, and PUMA suppress th 2 family of anti-apoptotic proteins, and apoptosis occurs through an intrinsic pathw activated BAX and BAK [132][133][134]. Figure 4 shows the signaling pathways that are involved in apoptosis, which a terconnected. Numerous signaling pathways are involved in this apoptosis mechan

Apigenin
Apigenin has exhibited anticancer effects on many cancer cells such as breast, prostate, liver, colorectal, skin, and lung cancers [83]. Apigenin induced apoptosis in cancer cells by downregulating various signaling pathways such as the PI3K/AKT pathway, ERK1/2, NF-кB, JAK/STAT, and Wnt/β-catenin [81]. First, in breast cancer, there was no induction of apoptosis through the intrinsic pathway because the mitochondrial membrane potential was not reduced in human epidermal growth factor receptor 2(HER-2)-expressing breast cancer BT-474 cells without the effect of Bcl-2 or BAX. However, there was an apoptotic process leading to the cleavage of caspase-8 and PARP via an extrinsic pathway. In addition, it has been demonstrated that breast cancer cells induce apoptosis through the inhibition of STAT3 signaling [136]. Apigenin also revealed that HER-2-expressing MCF-7 cells, another type of breast cancer cell, were involved in an apoptosis process that induced caspase-8 and PARP cleavage and apoptosis through a p53-dependent pathway [137]. In the case of prostate cancer, apigenin decreased Bcl-2 and Bcl-xL and increased BAX in PC-3 and DU145 human prostate cancer cells. In addition, the apoptosis inhibitors XIAP, c-IAP1, c-IAP2, and survivin were inhibited, resulting in apoptosis [138]. As a result of treatment with apigenin in Hep G2 cells, which are liver cancer cells, apoptosis was induced through the inhibition of the PI3K/AKT/mTOR pathway [139]. In HCT-116 colorectal cancer cells, apigenin increased tumor-suppressor proteins such as p53 and p21 [140]. In the case of SW480 cells, apoptosis was induced by increasing caspase-3 and BAX and decreasing Bcl-2 [141,142]. In addition, apigenin induced apoptosis by inhibiting the phosphorylation of STAT3 in colon cancer cells and simultaneously downregulating anti-apoptotic proteins such as Bcl-xL and Mcl-1 [143]. Apigenin induced apoptosis by regulating AKT and MAPK in A375SM human melanoma cells [144]. In A549 lung cancer cells, caspase-3, -8, and -9 were induced by apigenin and promoted cytochrome c, and apoptosis-inducing factor (AIF) was promoted through the mitochondria and caspase-3 and -9 were activated [145]. TRAIL-induced apoptosis by the upregulation of death receptors 4 and 5 occurred in p53dependent NSCLC lung cancer cells. In addition, BAD and BAX were upregulated; Bcl-xL and Bcl-2 were downregulated; and NF-кB, AKT, and ERK activation were inhibited [146].

Acacetin
Acacetin induced apoptosis in cancer cells through various pathways such as mitochondria-mediated death signaling, caspase activation, the β-catenin pathway, and NF-кB/AKT signaling for breast, gastric, colorectal, and prostate cancers [78,[84][85][86][87]. In breast cancer cells, MCF-7 and acacetin decreased Bcl-2 and released cytochrome c and AIF through the loss of mitochondrial membrane potential. Stress-activated protein kinase/c-Jun NH-4 terminal kinase1/2 (SAPK/JNK1/2) and c-Jun were activated and then apoptosis was induced through the SAPK/JNK1/2-c-Jun pathway [87]. In gastric cancer AGS cells, an apoptotic pathway was formed by caspase activity through ROS generation, mitochondrial mediation, and Fas activation [78]. In SW480 and HCT-116 colorectal cancer cells, the downregulation of the β-catenin pathway and increase in the mitochondrial membrane potential depolarization increased the BAX:Bcl-2 ratio and although there was no change in caspase, apoptosis was induced by AIF [86]. In DU145 prostate cancer cells, acacetin reduced phospho-AKT in a concentration-dependent manner, and phospho-GSK-3β, downstream of AKT, decreased and p53 increased. In addition, the activity of phospho-IкB and NF-кB was decreased, and apoptosis was induced through the NF-кB/AKT pathway due to the decrease in XIAP, and Bcl-2 [84].

Luteolin
Luteolin induced apoptosis in breast, lung, gastric, liver, and cervical cancers [91][92][93]. First, in breast cancer, luteolin inhibited PI3K/AKT activation and increased FOXO3a activation, leading to cell cycle arrest and apoptosis [152]. Additionally, luteolin induced apoptosis through the downregulation of the human telomerase reverse transcriptase (hTERT) [153]. In SCLC lung cancer cells, luteolin caused cisplatin to become more susceptible to anticancer effects through JNK-activated apoptosis [154]. In NSCLC lung cancer cells, the expression of BAX, p53, p21, caspase-3, and -9 decreased and Bcl-2 increased due to microRNA-34a-5p inhibition. So, activation of microRNA-34a-5p blocked the caspase pathway and can be considered to cause apoptosis of lung cancer [155]. In BGC-823 gastric cancer cells, caspase-3, -9, and cytochrome c increased, the ratio of BAX to Bcl-2 increased, and apoptosis was induced through the inhibition of the MAPK and PI3K signaling pathways [156]. In SMMC-7721 liver cancer cells, luteolin increased caspase-8 and decreased Bcl-2 [75], and in Hep-G2 cells, it was reported that apoptosis was induced through BAX/BAK mitochondrial translocation and JNK activation [157]. In HeLa cervical cancer cells, luteolin increased the expression of various pro-apoptotic proteins, including BAX, BAD, BID, APAF1, TRADD, FAS, FADD, and caspase-3 and -9, but the expression of anti-apoptotic proteins, such as BCL-2 and MCL-1, decreased. Additionally, apoptosis occurred through the inhibition of the AKT and MAPK pathways due to the downregulation of MAPK, ERK1/2, and AKT and the upregulation of p53 at the transcriptional level [93].

Tangeretin
Tangeretin induced apoptosis in breast, gastric, prostate, and bladder cancers through various pathways [95][96][97]. The treatment of drug-resistant MDA-MB-231 breast cancer cells with tangeretin induced apoptosis by increasing BAX, caspase-3, and -8 and decreasing Bcl-2 [158]. In AGS gastric cancer cells, caspase-3, -8, and -9, as well as BAX, tBID, and p53, were upregulated and apoptosis was induced through Fas/Fas L and the p53-dependent mitochondrial pathway [95]. In PC-3 and LNCaP prostate cancer cells, caspase-3 and cleaved caspase-3 and-9 were regulated. Only PC-3, BAX, and Bcl were regulated. Apoptosis was induced by inhibiting epithelial-mesenchymal transition (EMT), a cellular program important for cancer progression in these prostate cancer cells. In DU145 prostate cancer cells, apoptosis and DNA cleavage were induced through the regulation of the androgen receptor (AR)-PI3K/AKT/mTOR-Notch signaling pathway. In BFTC-905 bladder cancer cells, apoptosis was induced by regulating the release of cytochrome c and AIF from the mitochondria and cleaved caspase-3 and -9 and pro-caspase-3 and -9 [97].

Wogonin
Wogonin induced apoptosis through various pathways in breast cancer, colorectal cancer, lung cancer, ovarian cancer, and glioma [101]. Wogonin induced apoptosis by decreasing Bcl-2 and survivin and increasing BAX, p53, and caspase-3, -8, and -9 in MCF-7 breast cancer cells. In addition, apoptosis was induced through the inhibition of the PI3K/AKT/survivin signaling pathway and ERK activation [159]. In HT-29 colorectal cancer cells, Bcl-2 expression was reduced, BAX expression increased, and apoptosis was induced through the PI3K/AKT pathway [160]. In addition, in HCT116 cells, cell cycle arrest through the inhibition of the β-catenin-dependent Wnt signaling pathway showed attenuated proliferation and apoptosis in colorectal cancer [161]. In SW48, another colorectal cancer cell, apoptosis was induced by an increase in BAX and caspase-3, -8, and -9 and the inhibition of PI3K/AKT/STAT-3 [162]. In A549 lung cancer cells, wogonin induced apoptosis associated with increases in caspase-3, -8, and -9 and PARP and decreases in XIAP, c-FLIP, cIAP-1, -2, AKT, and STAT-3 [163]. In A2780 ovarian cancer cells, apoptosis was induced due to an increase in BAX and p53 and a decrease in AKT and Bcl-2 [98]. In U251 and U87 glioblastoma cells, wogonin induced not only the upregulation of caspase-3 and-9 but also PARP expression. In addition, it induced the generation of reactive oxygen species (ROS) and activated endoplasmic reticulum (ER) stress to induce apoptosis [101].
Comprehensively, Figure 5 shows the cell signaling regulation of apoptosis by apigenin, acacetin, baicalein, luteolin, tangeretin, and wogonin. Table 3 also shows the cancer cell lines, treatment concentrations, and apoptotic regulation of cell signaling pathways associated with the six above selected flavones. cer cells, Bcl-2 expression was reduced, BAX expression increased, and apoptosis was induced through the PI3K/AKT pathway [160]. In addition, in HCT116 cells, cell cycle arrest through the inhibition of the β-catenin-dependent Wnt signaling pathway showed attenuated proliferation and apoptosis in colorectal cancer [161]. In SW48, another colorectal cancer cell, apoptosis was induced by an increase in BAX and caspase-3, -8, and -9 and the inhibition of PI3K/AKT/STAT-3 [162]. In A549 lung cancer cells, wogonin induced apoptosis associated with increases in caspase-3, -8, and -9 and PARP and decreases in XIAP, c-FLIP, cIAP-1, -2, AKT, and STAT-3 [163]. In A2780 ovarian cancer cells, apoptosis was induced due to an increase in BAX and p53 and a decrease in AKT and Bcl-2 [98]. In U251 and U87 glioblastoma cells, wogonin induced not only the upregulation of caspase-3 and-9 but also PARP expression. In addition, it induced the generation of reactive oxygen species (ROS) and activated endoplasmic reticulum (ER) stress to induce apoptosis [101].
Comprehensively, Figure 5 shows the cell signaling regulation of apoptosis by apigenin, acacetin, baicalein, luteolin, tangeretin, and wogonin. Table 3 also shows the cancer cell lines, treatment concentrations, and apoptotic regulation of cell signaling pathways associated with the six above selected flavones.

Anticancer Effects of Six Selected Flavones in an In Vivo Model
Apigenin suppressed colorectal cancer and reduced tumor volume in prostate cancer without reducing appetite when ingested in Sprague-Dawley (SD) rats. Colorectal cancer was induced by azoxymethane (AOM), which is effective for the induction of colon carcinoma in SD rats. In these SD mice, apigenin treatment had the potential to suppress colorectal cancer by reducing aberrant crypt foci (ARF), which are clusters of abnormal tube-like glands, and increasing apoptosis [141]. In addition, in transgenic adenocarcinoma of mouse prostate (TRAMP) mice, apigenin inhibited the formation of prostate carcinoma by regulating the PI3K/AKT/FOXO pathway [164,165]. However, low bioavailability and instability in the gut were present [166,167]. Amounts of 20 and 50 µg of apigenin were administered to a xenograft model in which PC-3 prostate cancer cells were implanted into athymic nude mice. This resulted in a significant decrease in protein expression of XIAP and survivin in PC-3 tumor xenografts. A dose-dependent decrease in HDAC1 expression and an increase in Bax and PARP cleavage were observed in apigenin-treated mice [138].
Acacetin inhibited tumor growth through STAT3 regulation in DU145 prostate cancer cells in a nude mouse xenograft model [168].
As a result of analyzing the gene expression of a lung cancer xenograft tumor mouse model, the genes when baicalein induce apoptosis, ITGB3 (+6.96) and TNFRSF25 (+3.4), were most significantly upregulated [77]. In another study, mTOR inhibition prevented tumor growth in a breast cancer xenograft mouse model [169]. In addition, cervical cancer cell apoptosis was induced through an increase in BAX and a decrease in Bcl-2 in an in vivo tumor model [170].
Luteolin reduced the weight and volume of gastric tumors in a rat model by the inhibition of Notch1 and β-catenin [171]. When luteolin was administered to the lung cancer xenograft tumor mouse model, tumor suppression was shown by the inhibition of mutant EGF receptors, a major oncogene that induces tumorigenesis in many types of cancer, including NSCLC. This was followed by inducing the degradation of the EGF receptors and then preventing the PI3K/AKT/mTOR signal, which resulted in apoptosis and toxicity, and weight loss did not occur [172].
Tangeretin inhibited tumor growth in the MDA-MB-231 breast cancer cell nude mouse xenograft model [94]. Tangeretin had a strong inhibitory effect on tumor growth in the SGC-7901 gastric cancer cell nude mouse xenograft model [173].
As a result of treatment with wogonin in BALB/C nude mice xenografted with A2780 ovarian cancer cells and HT-29 colorectal cancer cells, the tumor volume and weight were reduced [174]. Table 4 shows the in vivo functions of anticancer effects in various cancer types. Tumor volume and weight were reduced [174] 7. Discussion and Conclusions

The Prospects of Compounds Such as Flavones and Flavonoids for Anticancer Effects
Cancer is still a disease whose treatments face many challenges. Anticancer research on polyphenols, such as flavonoids, has been actively conducted over the past few decades [175]. Of course, among flavonoids, flavones cannot be said to have the greatest anticancer effects and their mechanisms have not been fully elucidated. However, as shown in this review, flavones are effective in treating various cancers and they have special qualities that make them more effective than other flavonoids. This provides ample evidence to support flavones as having good prospects for treating cancer in the future so they are worth studying. However, in order to more clearly understand the effects of flavones and flavonoids, further studies are needed that focus on treatment effects, diet and bioavailability, absorption, metabolism, physiologically relevant models, and structural relationships between flavones and flavonoid subclasses [176]. In addition, it is important to proceed with clinical research using these data. There have been some clinical trials involving flavone-based agents, such as flavopiridol, which inhibits cdk and induces apoptosis, but more trials are needed [177].

Noteworthy ER-Mediated Apoptosis and the Need to Study Related Flavones
In this review, the apoptosis of the external and internal pathways has been mainly investigated, but the endoplasmic reticulum (ER)-mediated apoptosis pathway is also emerging as an important apoptosis pathway. The endoplasmic reticulum is an important organelle of eukaryotic cells and is necessary for maintaining intracellular homeostasis; controlling the synthesis, secretion, and folding of membrane-bound proteins; and transporting calcium ions [178]. When a misfolded protein accumulates in the ER and leads to activation of the unfolded protein response (UPR), it is called ER stress [178,179]. Through this ER stress, the combination with the mitochondria plays an important role in regulating apoptosis [178]. The first key protein in the ER stress-mediated apoptosis pathway is the C/EBP homologous protein (CHOP). The activation of the protein kinase R-like ER kinase (PERK) phosphorylates eukaryotic initiation factor 2 (EIF2a) and the activating transcription factor 4 (ATF4) is activated to upregulate the CHOP. The expression of this CHOP is also increased by activating transcription factor 6 (ATF6). The CHOP subsequently decreases the expression of anti-apoptotic Bcl-xL and Bcl-2 and conversely increases the pro-apoptotic BAX, BAK, NOXA, BIM, and PUMA [180,181]. Another pathway is that inositol-requiring enzyme 1 (IRE1) binds to tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2) to activate caspase 12, which in turn, activates caspase-3. In addition, the IRE1α-TRAF2 complex activates apoptosis signal-regulating kinase 1 (ASK1) and induces phosphorylation of JNK to induce apoptosis through c-jun, activates BAX and BAK, and inactivates Bcl-2 to play an important role in apoptosis [182].
Therefore, this endoplasmic reticulum-mediated apoptosis is very important and a pathway in which natural compounds, such as flavones, control ER stress [178] to act as an anticancer agent, which can be an important approach in the fight against cancer. Studies have been conducted on natural compounds, such as several flavonoids, but more research is needed.

Conclusions
This review includes a basic introduction to the selected flavones and their anticancer effects. In addition, we investigated the various pathways involved in apoptosis and discussed the signaling pathways by which flavones induce apoptosis. Flavones could be promising compounds for anticancer action in the future after further research considering this review information. Lastly, we hope this review will serve as a valuable manual for anticancer-related research using flavones.

Conflicts of Interest:
The authors declare that they have no conflict of interest.