(PQA1) Mechanism of Vitamin D Chemoprevention Against Colon Cancer

描述（由申请人提供）：本申请的目的是解决RFA-CA-12-015 PQA1：长期用于其他适应症的药物预防癌症发病率和死亡率的分子机制是什么？维生素D （VD）是一种长期广泛使用的补充剂，用于维持骨骼健康和治疗骨质疏松症，但越来越多的证据表明，VD对结肠癌具有强大的化学预防作用，其机制尚不清楚。因此，需要解决的核心问题是：维生素D预防结肠癌的分子机制是什么？在美国，结肠癌是癌症相关死亡的主要原因。低VD状态与结肠癌发病率增加有关，VD的化学预防作用已在结肠癌模型中得到证实，并且正在进行大型随机临床试验来评估VD对人类结肠癌的化学预防效果，但VD抗结肠癌活性的机制仍有待确定。次生胆汁酸（BA）是有效的结肠癌促进剂。已知VD通过调节宿主和结肠细菌BA代谢来影响结肠次级BA水平。因此，我们假设VD对次级BA水平的调节是VD对结肠癌化学预防活性的关键分子机制。此外，促肿瘤的继发性BA石胆酸（LCA）也能激活维生素D受体（VDR）， LCA-VDR信号与VD-VDR信号一样，也通过诱导Cyp3A促进继发性BA分解代谢，这反而限制了LCA的促肿瘤作用。因此，我们也假设VD或LCA激活的VDR信号限制了继发性BA水平，从而减轻BAs对肿瘤的促进作用。我们将在维生素D激素或VDR水平改变的基因工程小鼠和细菌次生BA产生改变的非生菌小鼠中使用结肠癌模型来验证这些假设。在Aim 1中，我们将确定vdr依赖性宿主和微生物调节西方饮食诱导的继发性BAs和结肠肿瘤发生的机制。我们将评估过表达上皮性VDR的WT、VDR（-/-）和转基因小鼠的继发性BA水平和结肠肿瘤发生情况，这些小鼠被喂食西方饮食或VD补充饮食。在目的2中，我们将剖析VD不依赖的LCA-VDR信号在结肠肿瘤发生中的作用。我们将比较西方饮食或饮食LCA对WT、VDR（-/-）、Cyp27b1（-/-）和VDR(-/-)/Cyp27b1（-/-）小鼠继发性BAs和结肠肿瘤发生的影响。Cyp27b1（-/-）和VDR(-/-)/Cyp27b1（-/-）小鼠不能合成VD激素，这使我们能够在不干扰内源性VD激素作用的情况下解剖LCA-VDR信号的化学预防作用。在Aim 3中，我们将评估微生物7¿-去羟化酶（7- dh）在VD化学预防中的作用。我们将比较与缺乏7-DH的嗜酸乳杆菌单一相关的无菌小鼠与与表达7-DH的scindens梭状芽孢杆菌相关的小鼠在饲喂西方饮食或添加VD的饮食中的肿瘤发生情况。这些研究将极大地促进我们对VD对结肠癌的化学预防机制的理解。

DESCRIPTION (provided by applicant): The goal of this application is to address RFA-CA-12-015 PQA1: What is the molecular mechanism by which a drug that is chronically used for other indications protects against cancer incidence and mortality? Vitamin D (VD) is a widely and chronically used supplement for maintenance of bone health and for treatment of osteoporosis, but a growing body of evidence has indicated that VD has potent chemopreventive effects against colon cancer by unknown mechanisms. Thus the central question to be addressed is: What is the molecular mechanism by which Vitamin D prevents colon cancer? Colon cancer is a leading cause of cancer- related deaths in the US. Low VD status is associated with increased colon cancer incidence, chemopreventive effects of VD have been demonstrated in colon cancer models, and large randomized clinical trials are being conducted to evaluate VD's chemopreventive efficacy against colon cancer in humans, but the mechanism underlying VD's anti-colon cancer activity remains to be defined. Secondary bile acids (BA) are potent colon cancer promoters. VD is known to affect colonic secondary BA levels by regulating host and colonic bacterial BA metabolism. Therefore we hypothesize that VD regulation of secondary BA levels is a key molecular mechanism underlying VD's chemopreventive activity against colon cancer. Moreover, the tumor-promoting secondary BA lithocholic acid (LCA) can also activate the vitamin D receptor (VDR), and LCA-VDR signals, like VD-VDR signals, also promote secondary BA catabolism by inducing Cyp3A, which can paradoxically limit LCA's tumor-promoting effects. Therefore we also hypothesize that VDR signals activated by VD or LCA limit secondary BA levels, thus mitigating tumor promotion by BAs. We will test these hypotheses using colon cancer models in genetically engineered mice with altered vitamin D hormone or VDR levels, and in gnotobiotic mice with altered bacterial secondary BA production. In Aim 1 we will determine the VDR-dependent host and microbe mechanisms that regulate Western diet-induced secondary BAs and colonic tumorigenesis. We will assess secondary BA levels and colonic tumorigenesis in WT, VDR(-/-) and transgenic mice over-expressing epithelial VDR that are fed Western diet or VD supplemented diet. In Aim 2 we will dissect the role of VD- independent LCA-VDR signals in colonic tumorigenesis. We will compare effects of Western diet or dietary LCA on secondary BAs and colonic tumorigenesis in WT, VDR(-/-), Cyp27b1(-/-) and VDR(-/-)/Cyp27b1(-/-) mice. Cyp27b1(-/-) and VDR(-/-)/Cyp27b1(-/-) mice cannot synthesize VD hormone, allowing us to dissect the chemopreventive effects of LCA-VDR signals without confounding effects of endogenous VD hormone. In Aim 3 we will assess the role of microbial 7¿-dehydroxylase (7-DH) in VD chemoprevention. We will compare tumorigenesis in gnotobiotic mice mono-associated with Lactobacillus acidophilus lacking 7-DH to mice also associated with Clostridium scindens expressing 7-DH fed Western diet or diet supplemented with VD. These studies will greatly advance our understanding of chemopreventive mechanisms of VD against colon cancer.