Flavonoids have been reported to inhibit tumor growth by causing cell cycle arrest. However, little is known about the direct targets of flavonoids in tumor growth inhibition. In the present study, we developed a novel method using magnetic FG beads to purify flavonoid-binding proteins, and identified ribosomal protein S9 (RPS9) as a binding partner of the flavonoid apigenin. Similar to treatment with apigenin, knockdown of RPS9 inhibited the growth of human colon cancer cells at the G2/M phase by downregulating cyclin-dependent kinase 1 (CDK1) expression at the promoter level. Furthermore, knockdown of RPS9 suppressed G2/M arrest caused by apigenin. These results suggest that apigenin induces G2/M arrest at least partially by directly binding and inhibiting RPS9 which enhances CDK1 expression. We therefore raise the possibility that identification of the direct targets of flavonoids may contribute to the discovery of novel molecular mechanisms governing tumor growth.
Apo2 ligand (Apo2L)/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising cancer therapeutic agent. Recombinant human Apo2L/TRAIL has been under clinical trials, whereas various kinds of malignant tumors have resistance to Apo2L/TRAIL. We and others have shown that several anticancer agents and flavonoids overcome resistance to Apo2L/TRAIL by upregulating death receptor 5 (DR5) in malignant tumor cells. However, the mechanisms by which these compounds induce DR5 expression remain unknown. Here we show that the dietary flavonoid apigenin binds and inhibits adenine nucleotide translocase-2 (ANT2), resulting in enhancement of Apo2L/TRAIL-induced apoptosis by upregulation of DR5. Apigenin and genistein, which are major flavonoids, enhanced Apo2L/TRAIL-induced apoptosis in cancer cells. Apigenin induced DR5 expression, but genistein did not. Using our method identifying the direct targets of flavonoids, we compared the binding proteins of apigenin with those of genistein. We discovered that ANT2 was a target of apigenin, but not genistein. Similarly to apigenin, knockdown of ANT2 enhanced Apo2L/TRAIL-induced apoptosis by upregulating DR5 expression at the post-transcriptional level. Moreover, silencing of ANT2 attenuated the enhancement of Apo2L/TRAIL-induced apoptosis by apigenin. These results suggest that apigenin upregulates DR5 and enhances Apo2L/TRAIL-induced apoptosis by binding and inhibiting ANT2. We propose that ANT2 inhibitors may contribute to Apo2L/TRAIL therapy.
There are several reports of clinical trials of aspirin in sporadic colon cancer. However, only one double-blind trial of aspirin in patients with familial adenomatous polyposis (FAP) has been reported to date. This double-blind, randomized, placebo-controlled clinical trial was therefore performed to evaluate the influence of low-dose aspirin enteric-coated tablets (100 mg/day for 6–10 months) in 34 subjects with FAP (17 each in the aspirin and placebo groups). The increase in mean diameter of colorectal polyps tended to be greater in the placebo group compared with the aspirin group, which showed a response ratio, that is, aspirin response rate (number of subjects with reduced polyps/total)/placebo response rate (number of subjects with reduced polyps/total), of 2.33 (95% confidence interval: 0.72–7.55). Subgroup analysis revealed that the number of subjects with a mean baseline polyp diameter of ≤2 mm, and the diameter and number of polyps after intervention showed a significant reduction in the aspirin group. Adverse effects of aspirin, such as anastomotic ulcer, aphtha in the large intestine, and progression of anemia, occurred in three subjects. Moreover, none of the subjects developed colorectal cancer. The results thus indicated a potential for aspirin to reduce colorectal adenoma development in patients with FAP, but careful follow-up is needed to avoid or rapidly counter severe adverse effects.
Adenoma; chemoprevention; colorectum; FAP; low-dose aspirin
A small molecule compound, JTP-74057/GSK1120212/trametinib, had been discovered as a very potent antiproliferative agent able to induce the accumulation of CDK inhibitor p15INK4b. To conduct its drug development rationally as an anticancer agent, molecular targets of this compound were identified as MEK1/2 using compound-affinity chromatography. It was shown that JTP-74057 directly bound to MEK1 and MEK2 and allosterically inhibited their kinase activities, and that its inhibitory characteristics were similar to those of the known and different chemotype of MEK inhibitors PD0325901 and U0126. It was further shown that JTP-74057 induced rapid and sustained dephosphorylation of phosphorylated MEK in HT-29 colon and other cancer cell lines, while this decrease in phosphorylated MEK was not observed in PD0325901-treated cancer cells. Physicochemical analyses revealed that JTP-74057 preferentially binds to unphosphorylated MEK (u-MEK) in unique characteristics of both high affinity based on extremely low dissociation rates and ability stabilizing u-MEK with high thermal shift, which were markedly different from PD0325901. These findings indicate that JTP-74057 is a novel MEK inhibitor able to sustain MEK to be an unphosphorylated form resulting in pronounced suppression of the downstream signaling pathways involved in cellular proliferation.
JTP-74057/GSK1120212/trametinib; chemical probe-affinity chromatography; novel MEK inhibitor; unphosphorylated MEK (u-MEK)
While p73 overexpression has been associated with increased apoptosis in cancer tissues, p73 overexpressing tumors appear to be of high grade malignancy. Why this putative tumor suppressor is overexpressed in cancer cells and what the function of overexpressed p73 is in breast cancers are critical questions to be addressed. By investigating the effect of p53 inactivation on p73 expression, we found that both protein and mRNA levels of TAp73 were increased in MCF-7/p53siRNA cells, MCF-7/p53mt135 cells and HCT-116/p53−/− cells, as compared to wild type control, suggesting that p53 inactivation by various forms upregulates p73. We showed that p53 knockdown induced p73 was mainly regulated at the transcriptional level. However, although p53 has a putative binding site in the TAp73 promoter, deletion of this binding site did not affect p53 knockdown mediated activation of TAp73 promoter. Chromatin immuno-precipitation (ChIP) data demonstrated that loss of p53 results in enhanced occupancy of E2F-1 in the TAp73 promoter. The responsive sequence of p53 inactivation mediated p73 upregulation was mapped to the proximal promoter region of the TAp73 gene. To test the role of E2F-1 in p53 inactivation mediated regulation of p73 transcription, we found that p53 knockdown enhanced E2F-1 dependent p73 transcription, and mutations in E2F-1 binding sites in the TAp73 promoter abrogated p53 knockdown mediated activation of TAp73 promoter. Moreover, we demonstrated that p21 is a mediator of p53-E2F crosstalk in the regulation of p73 transcription. We concluded that p53 knockdown/inactivation may upregulate TAp73 expression through E2F-1 mediated transcriptional regulation. p53 inactivation mediated upregulation of p73 suggests an intrinsic rescuing mechanism in response to p53 mutation/inactivation. These findings support further analysis of the correlation between p53 status and p73 expression and its prognostic/predictive significance in human cancers.
Bacteraemia caused by Streptococcus bovis is often associated with colorectal tumours. Also, experimental studies have been proposed that S bovis acts as a promoter of tumours. We report the case of a man with colon adenoma who had a high proportion of S bovis in his faecal flora. He was treated with a Lactobacillus casei preparation (BLP) and the effect on the faecal flora was examined. L casei reduced the proportion of S bovis (from 43% down to 9%), and the effect continued after the administration of BLP was stopped. Our data indicate that BLP can repress the excessive colonisation of S bovis.
Malignant tumors carry a high risk of death, and the prevention of malignant tumors is a crucial issue in preventive medicine. To this end, many chemopreventive agents have been tested, but the effects of single agents have been found to be insufficient to justify clinical trials. We have therefore hypothesized that combinations of different chemopreventive agents may synergistically enhance the preventive effect of chemopreventive agents used singly. To provide the treating physician with some guideline by which to choose the most effective agents to be combined, we propose a strategy which we have termed the “combination-oriented molecular-targeting prevention” of cancer. As the molecular target of our model, we focused on tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), which specifically causes apoptosis in malignant tumor cells. Many of these agents were found to up-regulate the expression of death receptor 5, a TRAIL receptor. They were also found to synergistically induce apoptosis in malignant tumor cells when combined with TRAIL. Here, we strongly advocate that the strategy of “combination-oriented molecular-targeting prevention” of cancer will be a practical approach for chemoprevention against human malignant tumors.
Apoptosis; Cancer; Combination; DR5; Prevention; TRAIL
Isoliquiritigenin is a chalcone derivative with potential in cancer chemoprevention. Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anti-cancer agent, some cancer cells are resistant to TRAIL treatment. Current studies have tried to overcome TRAIL-resistant cancer cells. Here, we show for the first time that isoliquiritigenin overcomes TRAIL resistance in colon cancer HT29 cells.
HT29 cells were treated with isoliquiritigenin and/or TRAIL, and apoptosis induction was detected by flow cytometry and fluorescence microscopy. Protein expression relating to the TRAIL pathway was analyzed by Western blotting.
A single treatment with isoliquiritigenin scarcely induced apoptosis in HT29 cells. Combined treatment with suboptimal concentrations of isoliquiritigenin and TRAIL markedly induced apoptosis, however. The effect was blocked by a pan-caspase inhibitor and a caspase-3, 8, 9, or 10 inhibitor, suggesting that the combination facilitates caspase-dependent apoptosis. Furthermore, the apoptosis induced by isoliquiritigenin and TRAIL was blocked by a dominant negative form of the TRAIL receptor. This result indicates that the combined effect is caused by specific interaction between TRAIL and its receptors. Isoliquiritigenin increased the amount of DR5 protein among TRAIL receptors. Isoliquiritigenin did not significantly increase levels of the Bcl-2 family proteins Bcl-2, Bcl-xL, and BAX.
Our results suggest that isoliquiritigenin has the potential to overcome resistance to TRAIL in cancer cells and its chemopreventive effects may depend on TRAIL function.
Isoliquiritigenin; Chalcone; DR5; TRAIL; Apoptosis
Paclitaxel is used widely in the treatment of breast cancer. Not all tumors respond to this drug, however, and the characteristics that distinguish resistant tumors from sensitive tumors are not well defined. Activation of the spindle assembly checkpoint is required for paclitaxel-induced cell death. We hypothesized that cyclin-dependent kinase (CDK) 1 activity and CDK2 activity in cancer cells, which reflect the activation state of the spindle assembly checkpoint and the growth state, respectively, predict sensitivity to paclitaxel.
Cell viability assays and DNA and chromatin morphology analyses were performed in human breast cancer cell lines to evaluate sensitivity to paclitaxel and the cell cycle response to paclitaxel. We then examined the specific activities of CDK1 and CDK2 in these cell lines and in xenograft models of human breast cancer before and after paclitaxel treatment. Protein expression and kinase activity of CDKs and cyclins were analyzed using a newly developed assay system.
In the cell lines, biological response to paclitaxel in vitro did not accurately predict sensitivity to paclitaxel in vivo. Among the breast cancer xenograft tumors, however, tumors with significantly increased CDK1 specific activity after paclitaxel treatment were sensitive to paclitaxel in vivo, whereas tumors without such an increase were resistant to paclitaxel in vivo. Baseline CDK2 specific activity was higher in tumors that were sensitive to paclitaxel than in tumors that were resistant to paclitaxel.
The change in CDK1 specific activity of xenograft tumors after paclitaxel treatment and the CDK2 specific activity before paclitaxel treatment are both associated with the drug sensitivity in vivo. Analysis of cyclin-dependent kinase activity in the clinical setting could be a powerful approach for predicting paclitaxel sensitivity.
Smenospongine, a sesquiterpene aminoquinone isolated from the marine sponge Dactylospongia elegans, was previously reported by us to induce erythroid differentiation and G1 phase arrest of K562 chronic myelogenous leukemia cells. In this study, we investigated the effect of smenospongine on the cell cycles of other leukemia cells, including HL60 human acute promyelocytic leukemia cells and U937 human histiocytic lymphoma cells by flow cytometric analysis. Smenospongine induced apoptosis dose-dependently in HL60 and U937 cells. The smenospongine treatment increased expression of p21 and inhibited phosphorylation of Rb in K562 cells, suggesting the p21-Rb pathway play an important role in G1 arrest in K562 cells. However, the p21 promoter was not activated by the smenospongine treatment based on a luciferase assay using the transfected K562 cells. Smenospongine might induce p21 expression via another mechanism than transactivation of p21 promoter.
Smenospongine; G1 arrest; apoptosis; leukemia cells; p21; Rb
Inactivation of the p16INK4a gene is one of the most frequent defects that contribute to oncogenesis in human cancer, since it is a tumor-suppressor gene. Therefore, functional restoration of p16INK4a is one of the most effective methods for cancer prevention. We proposed the concept of ‘gene-regulating chemoprevention’ and ‘molecular-targeting prevention’ of cancer, which assumes that transcriptional regulation by drugs on tumor-suppressor genes or functionally similar genes to the tumor-suppressor genes contributes to the prevention of human malignancies. The p16INK4a homologs p15INK4b, p18INK4c and p19INK4d have been recently identified, and these four members constitute the INK4 family of proteins. All directly bind to cyclin D-cyclin dependent kinase (CDK) 4/6 and are therefore specific inhibitors of these complexes. We recently showed that histone deacetylase (HDAC) inhibitors, promising chemopreventive and chemotherapeutical agents, induce p15INK4b and p19INK4d gene expression and cause growth arrest, suggesting that both genes are important molecular targets for HDAC inhibitors. Furthermore, we found that 12-O-tetradecanoylphorbol-13-acetate (TPA), which is widely used as a tumor promoter and protein kinase C activator, promotes human cancer cell growth through the down-regulation of p18INK4c gene expression. This suggests that a mouse two-stage carcinogenesis model using TPA might partially represent the most common human carcinogenesis pathway related to RB. Our results suggest that the INK4 family consists of attractive and promising molecular targets for the ‘gene-regulating chemoprevention’ and ‘molecular-targeting prevention’ of cancer.
INK4 family; p15INK4b; p16INK4a; p18INK4c; p19INK4d; histone deacetylase inhibitors; TPA; protein kinase C; prevention
A transcriptional regulator, Hes1, plays crucial roles in the control of differentiation and proliferation of neuronal, endocrine, and T-lymphocyte progenitors during development. Mechanisms for the regulation of cell proliferation by Hes1, however, remain to be verified. In embryonic carcinoma cells, endogenous Hes1 expression was repressed by retinoic acid in concord with enhanced p27Kip1 expression and cell cycle arrest. Conversely, conditional expression of a moderate but not maximal level of Hes1 in HeLa cells by a tetracycline-inducible system resulted in reduced p27Kip1 expression, which was attributed to decreased basal transcript rather than enhanced proteasomal degradation, with concomitant increases in the growth rate and saturation density. Hes1 induction repressed the promoter activity of a 5′ flanking basal enhancer region of p27Kip1 gene in a manner dependent on Hes1 expression levels, and this was mediated by its binding to class C sites in the promoter region. Finally, hypoplastic fetal thymi, as well as livers and brains of Hes1-deficient mice, showed significantly increased p27Kip1 transcripts compared with those of control littermates. These results have suggested that Hes1 directly contributes to the promotion of progenitor cell proliferation through transcriptional repression of a cyclin-dependent kinase inhibitor, p27Kip1.
One of the best approaches against cancer is prevention. Inactivation of the p53 or p16INK4a genes has been extensively reported in most human cancer cells. Both p53 and p16INK4a function as tumor suppressors. Therefore, functional restoration of these molecules is considered to be one of the most useful methods for cancer prevention and therapy. We have proposed a concept termed ‘gene-regulating chemoprevention and chemotherapy’ regarding the above pathway. This concept assumes that transcriptional regulation by drugs on tumor-suppressor genes, downstream target genes or functionally similar genes (for example, family genes) of the tumor-suppressor genes would contribute to the prevention of human malignancies. Histone deacetylase (HDAC) inhibitors have been shown to be potent inducers of growth arrest, differentiation and apoptotic cell death. Previously, we demonstrated that HDAC inhibitors, such as sodium butyrate and trichostatin A (TSA), transcriptionally induce the cyclin-dependent kinase inhibitor p21WAF1/Cip1, a downstream target gene of p53, in a p53-independent manner. Furthermore, we have recently shown that HDAC inhibitors activate Gadd45, another downstream target gene of p53, and p19INK4d, a gene functionally similar to p16INK4a. Our results, taken together with previous findings, suggest that HDAC inhibitors may be one of the most attractive and promising agents for ‘gene-regulating chemoprevention’ and ‘molecular-targeting prevention’ of cancer.
histone deacetylase inhibitors; prevention; p53; p16INK4a; p21WAF1/Cip1; Gadd45; p19INK4d
p73 is a member of the p53 family often overexpressed in human cancer. Its regulation, particularly following DNA damage, is different from that of p53. Following DNA damage, we found induction of p73 at both the protein and mRNA levels. Furthermore, by using different p73 promoter fragments, we found a role for E2F1 in mediating transcription of p73. However, this observation alone does not account for the observed DNA damage-induced activation of p73 in the cells used in these experiments. By analyzing the p73 promoter sequence, we revealed a new mechanism of p73 induction associated with the removal of transcriptional repression from the p73 promoter. We found, in fact, that treatment of cells with DNA damaging agents induced nuclear export of the transcription factor C-EBPα and blockage of this export abolished drug-induced p73 activation. We also show that C-EBPα has a direct repressive activity on transfactor E2F1, and for this repression the binding of C-EBPα to its consensus sequence in the DNA is required. These data suggest that in normal conditions a repressor complex involving C-EBPα, E2F1 and perhaps other proteins is present on the p73 promoter. This repressor complex is destroyed following damage by removal of C-EBPα from nuclei.
Quercetin, a flavonoid, widely distributed in many fruits and vegetables, is well known to have an antitumor effect despite its mutagenicity. In this study, we examined the effect of dietary quercetin on duodenum-tumorigenicity of mice induced by a chemical carcinogen, N-ethyl-N′-nitro-N-nitrosoguanidine (ENNG). Eight-week-old male C57BL/6 mice were divided into 4 groups; ENNG without quercetin (group A), ENNG with 0.2% quercetin (group B), ENNG with 2% quercetin (group C), and 2% quercetin without ENNG (group D). ENNG was given in drinking water for the first 4 weeks, and thereafter quercetin was given in a mixed diet. At week 20, the average number of duodenal tumors per mouse was significantly higher in group C (mean±SE, 7.26±1.75, p<0.05) than in group A (2.32±0.31). The size of the duodenal tumors increased significantly in group B (1.79±0.09 mm, p<0.001) compared with group A (1.43±0.09 mm). In contrast, no duodenal tumor was induced in group D. The present findings suggest that excessive intake of quercetin occasionally is a risk factor for carcinogenesis of some specific organs such as the upper intestine.
flavonoids; quercetin; mice; duodenal tumor; ENNG