Fatty acid desaturase activity, fish oil and colorectal cancer chemoprevention

描述(申请人提供)：结直肠癌是美国癌症相关死亡的第二大原因。动物模型和观察研究表明，海洋来源的n-3多不饱和脂肪酸[PUFA]，如二十碳五酸[EPA]和二十二碳六酸[DHA]可能会降低结直肠癌的风险。此外，n-3和n-6多不饱和脂肪酸的相对比例可能最能决定鱼油的化学预防效果。这一比例很重要，因为n-6多不饱和脂肪酸，花生四烯酸(ARA)，通过环氧合酶途径转化为前列腺素E2(PGE2)，PGE2是一种炎性二十烷类化合物，在结直肠肿瘤中异常产生，而EPA则转化为抗炎前列腺素E3(PGE3)。虽然n-6和n-3多不饱和脂肪酸的比例可以通过饮食措施来控制，但遗传因素也可能影响这一比例。最近的全基因组关联和单倍型研究表明，高达28%的ARA组织水平的加性差异可以由单个基因--脂肪酸去饱和酶1(FADS1)的变异来解释。FADS1是亚油酸(最常消耗的多不饱和脂肪酸)转化为ARA的限速酶，rs174537中T等位基因(群体频率为13%，HapMap-CEU)的纯合子具有较低的脂肪酸去饱和酶活性，从而降低组织中ARA的水平。我们的假设是，与FADS1活性较高的个体相比，遗传决定的FADS1活性较低的个体将从补充鱼油中获得更大的好处，因为组织中ARA水平较低，随后n-6与n-3 PUFA的比率更有利。为了验证这一假设，我们招募了150名最近确诊为腺瘤性息肉的参与者，并进行了为期6个月的3X2因素随机双盲对照试验。我们的第一个因素将是FADS1基因(GG、GT和TT)，第二个因素将是鱼油补充剂(鱼油与安慰剂)。我们的主要结果将是通过Ki-67标记检测直肠上皮细胞增殖的变化和通过TUNEL检测直肠隐窝的凋亡。次要终点将包括直肠上皮细胞COX-2和15-PGDH的表达，直肠细胞产生PGE2和PGE3，直肠粘膜组织脂肪酸水平，以及炎症生物标志物(C-反应蛋白)、脂肪因子(瘦素、脂联素)和胰岛素敏感性标志物的变化。我们的具体目标包括：1)确定鱼油补充剂对直肠上皮细胞增殖指数和直肠隐窝细胞凋亡标志物的影响；2)确定基因决定的脂肪酸去饱和酶1活性对鱼油补充剂用于结直肠癌化学预防的影响。我们的长期目标是确定可能影响补充鱼油疗效的遗传因素，以便使用海洋来源的n-3多不饱和脂肪酸进行更明确的腺瘤复发试验。我们预计鱼油将具有抗肿瘤作用，与FADS1活性高的个体相比，FADS1活性低的个体将有更大的反应。

DESCRIPTION (provided by applicant): Colorectal cancer is the second leading cause of cancer-related mortality within the United States. Animal models and observational studies have suggested that marine-derived n-3 polyunsaturated fatty acids [PUFA] such as eicosapentanoic acid [EPA] and docosahexanoic acid [DHA] may reduce the risk of colorectal cancer. In addition, it may be the relative proportion of n-3 to n-6 PUFAs that best determines the chemopreventive effects of fish oils. This ratio is important because the n-6 PUFA, arachidonic acid (ARA), is converted via the cyclo-oxygenase pathway to prostaglandin E2 (PGE2), an inflammatory eicosanoid aberrantly produced in colorectal neoplasms while EPA is converted to the anti-inflammatory prostaglandin E3 (PGE3). While the ratio of n-6 to n-3 PUFAs can be manipulated through dietary measures, genetic factors may also influence this ratio. Recent genome-wide association and haplotype studies have demonstrated that up to 28% of the additive variance in tissue levels of ARA is explained by variants in a single gene, fatty acid desaturase 1 (FADS1). FADS1 is the rate-limiting enzyme in the conversion of linoleic acid, the most commonly consumed PUFA, to ARA, and homozygotes for the T allele (population frequency of 13%, HapMap -CEU) in rs174537 have lower fatty acid desaturase activity and subsequently lower tissue levels of ARA. Our hypothesis is that individuals with genetically determined lower activity of FADS1 will derive greater benefit from fish oil supplementation than individuals with higher FADS1 activity because of lower tissue levels of ARA and subsequently a more favorable n-6 to n-3 PUFA ratio. To test this hypothesis we will recruit 150 participants with recently identified adenomatous polyps and conduct a 6-month double blind 3 X 2 factorial randomized controlled trial. Our first factor will be FADS1 genotype (GG, GT, and TT) and our second factor will be fish oil supplementation (fish oil versus placebo). Our primary outcome will be the change in rectal epithelial cell proliferation as measured by Ki-67 labeling and rectal crypt apoptosis as measured by TUNEL. Secondary endpoints will include rectal epithelial cell expression of COX-2 and 15-PGDH, rectal cell production of PGE2 and PGE3, rectal mucosal tissue levels of fatty acids, and changes in biomarkers of inflammation (C-reactive protein), adipokines (leptin, adiponectin), and markers of insulin sensitivity. Our specific aims include: 1) to determine the efficacy of fish oil supplements on rectal epithelial cell proliferation indexes and markers of rectal crypt apoptosis, and 2) to determine the effect of genetically-determined fatty acid desaturase 1 activity on fish oil supplementation for colorectal cancer chemoprevention. Our long-term objectives are to determine genetic factors that might influence the efficacy of fish oil supplementation in order to conduct a more definitive adenoma recurrence trial using marine-derived n-3 PUFAs. We anticipate that fish oil will have anti-neoplastic effect and individuals with low FADS1 activity will have a greater response compared to individuals with high FADS1 activity.