(PQA1) Mechanism of Vitamin D Chemoprevention Against Colon Cancer

描述(由申请人提供)：本申请的目标是解决RFA-CA-12-015 PQA1：一种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是一种广泛用于维持骨骼健康和治疗骨质疏松症的长期使用的补充剂，但越来越多的证据表明，维生素D对结肠癌具有有效的化学预防作用，其机制尚不清楚。因此，需要解决的中心问题是：维生素D预防结肠癌的分子机制是什么？结肠癌是美国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，大型随机临床试验正在进行中，以评估VD对人类结肠癌的化学预防效果，但VD抗结肠癌作用的机制尚不清楚。次级胆汁酸(BA)是强大的结肠癌促进剂。已知VD通过调节宿主和结肠细菌的BA代谢来影响结肠次级BA水平。因此，我们推测，VD对次级BA水平的调节是VD对结肠癌化学预防作用的关键分子机制。此外，促肿瘤的次级BA石胆酸(LCA)还可以激活维生素D受体(VDR)，而LCA-VDR信号和VD-VDR信号一样，也通过诱导细胞色素P3A(CyP3A)促进次级BA的分解代谢，这反而限制了LCA的促肿瘤作用。因此，我们还假设，由VD或LCA激活的VDR信号限制了次级BA水平，从而减轻了BAS对肿瘤的促进作用。我们将在维生素D荷尔蒙或VDR水平改变的基因工程小鼠和细菌次级BA产生改变的诺维生小鼠身上使用结肠癌模型来验证这些假说。在目标1中，我们将确定依赖VDR的宿主和微生物机制，这些机制调控西方饮食诱导的继发性BAS和结肠肿瘤的发生。我们将评估WT、VDR(-/-)和过度表达上皮VDR的转基因小鼠的继发性BA水平和结肠肿瘤的发生。在目标2中，我们将剖析非VD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和结肠癌发生的影响。Cyp27b1(-/-)和VDR(-/-)/Cyp27b1(-/-)小鼠不能合成VD激素，这使我们能够在不混淆内源性VD激素影响的情况下剖析LCA-VDR信号的化学预防作用。在目标3中，我们将评估微生物7-脱羟基酶(7-DH)在化学预防VD中的作用。我们将比较缺乏7-水解酶的嗜酸乳杆菌与表达7-水解酶的嗜酸乳杆菌相关的灵知生菌小鼠的肿瘤发生情况，并将表达7-水解酶的嗜酸乳杆菌与西方饮食或添加了VD的饮食相关联的小鼠进行比较。这些研究将极大地促进我们对VD预防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.