Modulation of Folate Receptor via the Estrogen Receptor

a型叶酸受体（FR）在主要类型的妇科恶性肿瘤中过表达，特别是在卵巢癌、子宫癌和宫颈癌中，而其在正常组织中的表达限于某些上皮细胞的腔表面，在那里它不能进入循环。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以及肿瘤成像剂的有希望的肿瘤靶点，预计很快将被引入临床使用。在寻找特异性调节肿瘤中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表达;雌激素效应被他莫昔芬和ICI迅速逆转182，780。我们假设，在体内，ER阳性和FR-α阳性妇科肿瘤中，FR-α表达被循环雌激素抑制，并且抗雌激素的短暂治疗将通过FR-α启动子的去抑制产生FR-α表达的显著增加。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基因有望大大提高肿瘤成像的灵敏度和FR靶向治疗的疗效，我们要求提供资金以建立强有力的实验基础，以保证在FR-α靶向的临床研究中使用ER配体。因此，本提案的主要目标是：（i）了解ER配体对FR-α基因的转录调节机制，以及（ii）通过开发几种表达ER和FR-α的小鼠肿瘤异种移植模型，研究抗雌激素在体内肿瘤中上调FR-α的潜力。此外，这项研究还有助于本实验室分离FR基因中的关键调控元件，以开发用于肿瘤特异性表达外源基因的基因治疗载体，并可能发现雌激素抑制基因的新介质。

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.