Folate and PSMA Interact to Regulate DNA Methylation and Prostate Carcinogenesis

描述(申请人提供)：叶酸，作为一个碳循环的一部分，对S-腺苷蛋氨酸的从头合成至关重要。SAM反过来为DNA甲基化提供甲基，这是表观遗传调控的关键模式。研究表明，在怀孕期间操纵叶酸水平会改变基因的表观遗传状态，成年后接受甲基供体治疗会逆转DNA甲基化的变化。鉴于叶酸水平对DNA甲基化的影响，饮食中的叶酸状态影响几种癌症的风险也就不足为奇了。叶酸对细胞增殖也是必不可少的。因此，动物研究表明，叶酸摄入的时机会调节疾病的结局：补充叶酸可以防止肿瘤的发生，而叶酸的消耗则会抑制肿瘤的生长。前列腺对叶酸的需求很高，而且似乎对DNA甲基化的变化也很敏感，这表明叶酸可能在前列腺癌中也起到了作用。我们已经开发了一个体外模型，在这个模型中，我们可以通过控制叶酸水平，在整个全球低甲基化的背景下产生同时的肿瘤抑制基因启动子超甲基化，这让人联想到前列腺癌。此外，我们还发现了一种与细胞内叶酸水平密切相关的蛋白质--前列腺特异性膜抗原(PSMA)，它与前列腺癌的发生有关。PSMA是一种独特的叶酸水解酶，可能是叶酸转运体，在前列腺癌和肿瘤新生血管内皮细胞中显著上调。此外，在低水平的叶酸存在的情况下，PSMA的表达增加了细胞侵袭力，这是一种对肿瘤形成和进展都很重要的活动。我们的假设是：前列腺中的有效叶酸水平过低，然后在较长一段时间内过多会导致癌症发生。为了验证这一假说，我们将使用以下特定目标：1)在一个新的体内模型中，确定(A)PSMA的表达是否调节细胞内叶酸水平，以及(B)叶酸水平是否调节前列腺组织中的DNA甲基化；2)确定低水平的叶酸是否增加PSMA的侵袭能力，并导致基因组低甲基化、DNA不稳定和随后发生癌症；3)确定在前列腺癌发生之前，补充叶酸是否具有保护作用，以及在启动之后，叶酸补充是否通过促进PSMA介导的前列腺系统叶酸的摄取以及通过改变表观遗传编程促进前列腺癌的生长和进展。相关性：最近的报告描述了叶酸补充剂与前列腺癌之间的显著关联。1998年，美国饮食中的叶酸强化已将人口从叶酸缺乏转变为叶酸充足。研究表明，过量的叶酸摄入会增加患乳腺癌、结直肠癌以及现在的前列腺癌的风险。我们已经证明，近200万60岁或以上的男性的血清叶酸水平超过足够的5倍。这一事实，再加上60岁年龄组男性临床前前列腺癌的高患病率，强调了研究叶酸和前列腺癌之间关系的重要性。 与公共健康相关：这些研究有可能提供有关补充叶酸对前列腺癌发生和发展的安全性和潜在保护作用的重要信息，考虑到最近美国人的饮食中强制添加叶酸，这些研究尤其及时。饮食叶酸摄入量是一个容易修改的因素；前列腺癌是一种发展缓慢的疾病-即使我们只能通过饮食叶酸摄入量略微增加进展时间，也可能导致巨大的临床影响。如果我们了解饮食中的叶酸操作如何改变DNA甲基化模式，以及叶酸的影响如何受到PSMA表达的调节，我们就可能能够检测到前列腺癌的高危男性，并为其提供营养或化学干预，从而预防或延迟疾病。

DESCRIPTION (provided by applicant): Folate, as part of the one carbon cycle, is critical for the de novo synthesis of S-adenosyl methionine (SAM). SAM, in turn, provides the methyl group for DNA methylation, a key mode of epigenetic regulation. Studies have shown that manipulation of folate levels during gestation alters the epigenetic status of genes, and that treatment with methyl donors in adulthood reverses DNA methylation changes. Given the effect of folate levels on DNA methylation, it is not surprising that dietary folate status influences the risk for several cancers. Folate is also essential for cell proliferation. Accordingly, animal studies show that the timing of folate intake modulates disease outcome: supplementation protects against tumor initiation while folate depletion inhibits tumor growth. The prostate has a high requirement for folate, and also seems susceptible to alterations in DNA methylation, suggesting that folate might also play a role in prostate cancer. We have developed an in vitro model, in which we can produce simultaneous tumor suppressor gene promoter hypermethylation in an overall context of global hypomethylation reminiscent of prostate cancer, by manipulating folic acid levels. In addition, we have shown that a protein intimately related to intracellular folate levels, Prostate-Specific Membrane Antigen (PSMA), contributes to prostate carcinogenesis. PSMA, a unique folate hydrolase, and possible folate transporter, undergoes significant up-regulation in prostate cancer and in the endothelial cells of tumor neovasculature. Moreover, in the presence of low levels of folate, PSMA expression increases cell invasiveness, an activity important for both tumor formation and progression. Our hypothesis is: Low, followed by excess levels of available folates in the prostate over an extended period of time leads to carcinogenesis. To test this hypothesis we will use the following specific aims; 1) Determine if (a) PSMA expression regulates intracellular folate levels, and (b) if folate levels regulate DNA methylation in prostate tissues in a novel in vivo model, 2) Ascertain if low levels of folates increase the invasive capacity of PSMA, and lead to genomic hypomethylation, DNA instability and subsequently carcinogenesis and 3) Establish if, prior to initiation of prostate carcinogenesis folate supplementation is protective, and if, following initiation, folate supplementation promotes prostate tumor growth and progression by enhanced uptake of systemic folate in the prostate mediated by PSMA, and by altering epigenetic programming. Relevance: Recent reports describe a significant association between folate supplementation and prostate cancer. Folic acid fortification of the U.S.diet in 1998 has converted the population from a largely folate-deficient to folate-replete. Studies have shown that excess folic acid intake increases the risk of breast, colorectal, and now, prostate cancer. We have shown that nearly 2 million men aged 60 or above have serum folate levels greater than 5 fold adequate. This fact, in combination with the high prevalence of preclinical prostate cancer in men of the >60 age group, underscores the importance of investigating the relationship between folate and prostate cancer. PUBLIC HEALTH RELEVANCE: These studies have the potential to yield important information regarding both the safety of, and the potentially protective effects, of folic acid supplementation on prostate carcinogenesis and progression, and are particularly timely given the recent mandatory fortification of the American diet with folic acid. Dietary folate intake is an easily modifiable factor; prostate cancer is a slow growing disease - even if we can effect only a modest increase in time to progression via dietary folate intake, it could result in a large clinical impact. If we understand how dietary folate manipulation can alter DNA methylation patterns, and how the effect of folate may be modulated by PSMA expression, we may be able to detect and provide nutritional or chemo- intervention to men at-risk for prostate cancer, preventing or delaying the disease.