Modulation of Folate Receptor via the Estrogen Receptor

叶酸受体(FR)在主要类型的妇科恶性肿瘤中过度表达，特别是在卵巢癌、子宫癌和宫颈癌中，而在正常组织中的表达仅限于某些上皮细胞的腔表面，在那里它无法进入循环。因此，FR-α是目前处于临床前研究和临床试验中的各种治疗方法的有前景的肿瘤靶点，以及有望很快引入临床使用的肿瘤显像剂。在寻找在肿瘤中特异性调节FR-α的方法时，我们发现在HeLa(宫颈癌)细胞和BG-1(卵巢癌)细胞中，在亚纳摩尔浓度的雌激素存在的情况下，雌激素受体(ER)迅速抑制FR-α基因启动子并显著下调内源性FR-α的表达；他莫昔芬和ICI182,780迅速逆转了雌激素的作用。我们假设，在体内，在ER阳性和FR-α阳性的妇科肿瘤中，FR-α的表达受到循环雌激素的抑制，抗雌激素的短暂治疗将通过取消FR-α启动子的抑制而显著增加FR-α的表达。基于对ER突变形式的观察，我们进一步预测，某些ER配体不仅会去抑制FR-α启动子，而且会进一步激活它。由于以这种方式调节FR-α基因有望显著提高肿瘤成像的敏感性和FR靶向治疗的有效性，我们要求提供资金，以建立一个强有力的实验基础，以保证在FR-α靶向的临床研究中使用ER配体。因此，这一建议的主要目的是：(I)了解ER配体对FR-α基因转录调控的机制；(Ii)通过建立几种表达ER和FR-α的小鼠肿瘤异种移植模型，研究体内抗雌激素上调FR-α的可能性。此外，这项研究应该有助于该实验室正在进行的分离FR基因中关键调控元件的努力，以开发针对肿瘤特异性外源基因表达的基因治疗载体，并可能揭示雌激素抑制基因的新媒介。

The folate receptor (FR) type a is overexpressed in major types of gynecological malignancies, particularly in ovarian, uterine and cervical cancers whereas its expression in normal tissues is restricted to the luminal surface of certain epithelial cells where it is inaccessible to circulation. Consequently, FR-alpha is a promising tumor target for a variety of therapeutic approaches, currently in pre-clinical studies and in clinical trials, and for tumor imaging agents, which are expected to be introduced soon for clinical use. In searching for ways to specifically modulate FR-alpha in tumors we have found that in HeLa (cervical carcinoma) cells and in BG-1 (ovarian carcinoma) cells, in the presence of subnanomolar concentrations of estrogen, the estrogen receptor (ER) rapidly represses the FR-alpha gene promoter and strikingly downregulates endogenous FR-alpha expression; the estrogen effect is rapidly reversed by tamoxifen and ICI 182,780. We hypothesize that in vivo, in ER-positive and FR-alpha positive gynecological tumors, FR- alpha expression is repressed by circulating estrogen and that brief treatment with antiestrogens will produce a substantial increase in FR- alpha expression by derepression of the FR-alpha promoter. Based on observations with a mutant form of ER, we further predict that certain ER ligands will not only derepress the FR-alpha promoter but will further activate it. Since modulation of the FR-alpha gene in this manner may be expected to considerably enhance the sensitivity of tumor imaging and the efficacy of FR-targeted therapies, we request funding to establish a strong experimental basis to warrant the use of ER ligands in clinical studies of FR-alpha targeting. Accordingly, the main goals of this proposal are to: (i) understand the mechanism of transcriptional modulation of the FR-alpha gene by ER ligands and (ii) examine the potential for antiestrogens to upregulate FR-alpha in tumors in vivo by developing several ER and FR-alpha expressing mouse tumor xenograft models. This study should, in addition, help ongoing efforts in this laboratory to isolate critical regulatory elements in FR genes to develop gene therapy vectors for tumor-specific expression of exogenous genes and may also uncover novel mediators of gene repression by estrogen.