栄養障害と治療のジャーナル

概要

栄養学会議 2015: ケルセチンと緑茶の組み合わせによる前立腺がんの化学予防効果の向上 - ピウェン・ワン - チャールズ・R・ドリュー医科大学

ピウェン・ワン

緑茶（GT）緑茶ポリフェノール（GTP）の前立腺がんに対する化学予防作用は、前臨床細胞培養および動物モデルで十分に実証されています。しかし、ヒトでの研究結果は一貫していません。GTポリフェノール（GTP）の生体内におけるバイオアベイラビリティの低さと代謝の広さは、GTの抗がん活性を制限します。私たちは、メチル化阻害剤ケルセチン（Q）が生体内で前立腺腺がんの化学予防を強化するかどうかを決定しました。私たちは、ヒトの前立腺組織およびマウス異種移植前立腺腫瘍組織において、GT摂取後に約50%のGTPがメチル化された形になり、したがってメチル化によってGTPの抗がん活性が低下することを発見しました。天然のメチル化阻害剤ケルセチン（Q）とGTを混合すると、前立腺腺癌LNCaPおよびPC-3細胞中のGTPの細胞内濃度が4～10倍に増加し、GTPのメチル化が減少することが判明しました。この混合治療により、両方の細胞株において細胞増殖の阻害とアポトーシスの誘導が強化されました。次に、GTとQの併用効果をin vivoで検証するために動物実験を実施しました。重症複合免疫不全（SCID）マウスにアンドロゲン感受性LAPC-4前立腺腺癌細胞を移植し、GT、Q、GT+Q、またはコントロールで治療しました。アンドロゲン感受性LAPC-4前立腺腺癌細胞は、介入の1週間前に重症複合免疫不全（SCID）マウスの皮下注射されました。飲料として投与された淹れたてのお茶のGTP濃度は0.07％で、Qは食事に0.2％または0.4％補充されました。 6週間の介入後、腫瘍の成長は、制御と比較して16% (Q)、21% (GT)、45% (GT+Q) 阻害されました。非メチル化GTPの組織濃度は併用群で有意に増加し、カテコール-O-メチル転移および多剤耐性関連タンパク質 (MRP)-1 のタンパク質発現の減少と関連していました。併用治療は、増殖の阻害、アンドロゲン受容体 (AR) およびホスファチジルイノシトール3-キナーゼ (PI3K)/Akt シグナル伝達、およびアポトーシスの刺激の有意な増加と関連していました。腫瘍阻害に対するGT+0.4% Qの併用効果は、腫瘍接種前に介入を開始した別の実験でさらに確認されました。併用により、アンドロゲン受容体、前立腺特異抗原、血管内皮増殖因子のタンパク質発現の阻害が強化されました。

Quercetin (Q) is a flavonoid that is found in most vegetables and fruits that are edible, particularly in onions, apples, and red wine. The inhibitory effect of Q on the actions of MRPs and COMT has been documented well. Q itself has been shown to exhibit chemo preventive activities specially in prostate cancer. We were able to demonstrate in vitro that the combined use of Q with GT significantly increased the cellular concentrations of non-methylated EGCG in prostate cancer LNCaP and PC-3 cells, leading to enhanced anti-proliferative effects. The present study was designed to test the hypothesis that the combined effect of Q and GT in vivo leads to an increased anticarcinogenic effect in a xenograft prostate tumor mouse model using severe combined immune deficiency (SCID) mice and to elucidate the mechanisms of the increased anticarcinogenic effect of the combination treatment.

The effect of the combination treatment was related to the concentration of GTPs in tumor tissue, which in turn was dependent on the Q dose. The dose of GT used in this study is equivalent to the consumption of 5-6 cups of green tea per day for an adult human. This estimate is based on the observation that the consumption of 5-6 cups of tea daily achieved similar tissue concentrations in human prostate compared to tissue in mice consuming the same brewed GT. Q dose would be equivalent to 1.0g (low dose) and 2.0g (high dose) per day for an adult based on blood concentrations of Q and its metabolites as observed in the present study (data not shown) relative to that from a human study. The consumption of 1000 mg of Q per day was not associated with any adverse effects in humans. A pilot clinical trial is on-going to determine the Q concentration necessary in humans to increase the bioavailability of EGCG. Our results showed that the combination treatment decreased the protein expression of MRP1 in tumor tissues. However, no changes of mRNA expression of MRP1 were observed, indicating that post-transcriptional regulation such as microRNA (miRNA) may be responsible. Many polyphenols including GT and Q have been shown to modulate the expression of miRNA, a class of small non-coding RNAs that interact with mRNA to regulate the gene expression post-transcriptionally. Several other investigators demonstrated the inhibitory effects of Q on the activities of transport-regulating proteins such as p-glycoprotein and MRPs, leading to an increased absorption of GTPs from the intestinal tract and retention in the tissues. Although Q is extensively methylated, sulfated, or glucuronidated upon uptake it has been demonstrated that these Q metabolites, such as isorhamnetin and 7-O-glucuronosyl quercetin exhibited equal or stronger inhibition on the activities of MRPs compared to Q. Considering the importance of MRPs in the development of chemo resistance during chemotherapy, GT and Q may also be good candidates to be combined with chemotherapy drugs to reduce drug resistance and enhance therapeutic efficacy.

The important role of catechol O-methylation of GTPs in cancer prevention has been demonstrated in several studies. Due to a common polymorphism of COMT its activity can vary by 3 to 4-fold. A case control study in Asian-American women provided evidence that the risk of breast cancer was significantly reduced only among tea drinkers possessing at least one low-activity COMT allele. We found earlier that EGCG was extensively methylated in human prostate tissues obtained from prostatectomy and in mouse tissues after GT consumption. In cell culture experiments methylation significantly decreased the anticancer activities of EGCG as shown by our laboratory and other investigators. Previously we demonstrated in vitro that the combination of GT and Q significantly decreased the activity and protein expression of COMT in various cancer cell lines. This inhibition of COMT was associated with a decrease in EGCG methylation and increase in the anti proliferative activity. Similarly, Landis-Piwowar et al. demonstrated that EGCG treatment in breast cancer cells of lower COMT activity led to stronger proteasome inhibition and apoptosis induction. The present study confirmed the inhibition of COMT in vivo both in mRNA and protein expression by the combination treatment of GT and Q, which may contribute to the increased concentrations of non-methylated EGCG in tumor tissues and supports the important role of COMT in GT chemoprevention.

Cancer results from a multistage process with distinct molecular and cellular alterations. Therefore, treatments targeting many concerted processes may be advantageous in cancer prevention, therapy and reducing resistance to the treatment. Natural compounds such as GT and Q target multiple events and signaling pathways throughout the stages of carcinogenesis. In combination these compounds may increase the anticarcinogenic activity by expanding the coverage of molecular targets. The androgen receptor (AR) signaling pathway plays a critical role in prostate tumor growth and progression, thus it is an important target in prostate cancer prevention and treatment. Nevertheless, there are other signaling pathways particularly the phosphatidylinositol 3-kinases/Akt/ mammalian target of rapamycin (PI3K/Akt/mTOR) pathway that crosstalk with AR signaling and may directly regulate the expression and activation of AR. The upregulation and activation of PI3K/Akt/mTOR pathway is thought to play an important role in prostate cancer due to the decreased expression or loss of the negative regulator, tumor suppressor phosphatase and tensin homolog (PTEN). Akt is activated after phosphorylation by phosphorylated PI3K and in turn activates its substrates, one being mTOR, which leads to increased cell proliferation and survival. Therefore, an effective intervention strategy in prostate cancer may need to target both AR and PI3K/Akt/mTOR signaling pathways. Both GT and Q inhibit AR signaling through multiple mechanisms including the decrease of AR expression and its nuclear translocation. The combined use of GT and Q in the present study demonstrated an increasing ability to inhibit AR expression compared to individual treatments. In addition, the phosphorylation of Akt was significantly inhibited by the combination treatment while only a slight but non-significant decrease by the individual treatments. Further evidence of a stronger inhibition of AR and PI3K/Akt signaling was also provided through increased inhibition of AR-mediated PSA expression in tumor tissues from mice treated with GT+Q. Similar effects were demonstrated by a recent study that combined mTOR inhibition (everolimus) with an anti-androgen (bicalutamide) to block both pathways, resulting in tumor growth was statistically significantly reduced. In addition to their applications in cancer prevention, GT and Q may be ideal candidates to be combined with anti-androgens to enhance the therapeutic efficacy in a less-toxic manner in the treatment of advanced prostate cancer.

This study provides a novel regimen by combining GT and Q to enhance the chemoprevention of prostate cancer in a non-toxic manner. This was associated with an increased bioavailability of non-methylated GTPs and enhanced anti proliferative and proapoptotic effect. These results warrant future human intervention studies to confirm the combined effect of GT and Q in prostate cancer prevention and treatment.

Biography

ピウェン・ワンは、2000年に中国の山東医科大学で医学博士号、2003年に理学修士号を取得しました。2008年にテキサス工科大学で博士号を取得し、UCLA人間栄養センターで博士研究員として研究を続けました。現在はチャールズ・R・ドリュー医科大学の内科、がん研究・研修部門の助教授を務めています。定評のある学術誌に20本以上の論文を発表しており、数多くの科学誌の査読者を務めています。

注:この研究の一部は、2015 年 10 月 26 日から 28 日まで米国イリノイ州シカゴで開催された第 4 回国際栄養会議および展示会で発表されました。