Induction of the Estrogen Specific Mitogenic Response of MCF-7 Cells by Selected Analogues of Estradiol-17β: A 3D QSAR Study

Analogues of estradiol-17β (E2) have been evaluated
for estrogen receptor (ER) binding affinity
and mitogenic potential in the human breast cancer cell line MCF-7.
These 42 compounds
represent subtle modifications of the natural estrogen structure
through the placement of
hydroxyl, amino, nitro, or iodo groups around the ring system in
addition to, or as replacement
of, the 3- and 17β-hydroxyls of E2. The mitogenic
activity of the analogues was found to be
related to ER binding only to a limited extent. In order to
elucidate structural features that
are uniquely responsible for receptor binding affinity or mitogen
potential of estrogens, the
three-dimensional quantitative structure−activity (QSAR) method
Comparative Molecular Field
Analysis (CoMFA) was employed. Separate CoMFA models for receptor
binding and cell growth
stimulation were optimized through the use of various alignment rules
and region step size.
Whereas the CoMFA contour plots did outline the shared structural
requirements for the two
measured biological properties, specific topological features in this
set of estrogens were
delineated that distinguish mitogenic potential from ER binding
ability. In particular, steric
interference zones which affected growth extend in a band from above
the A-ring to position 4
and below, whereas the ER binding steric interference zones are limited
to isolated polyhedra
in the 1,2 and 4 positions and the α face of the B-ring. In
addition, electronegative features
located around the A-, B-, or C-rings contribute to receptor affinity.
However, growth is
dependent only on electronegative and electropositive properties near
the 3-position. In a final
QSAR model for the mitogenic response, the value of ER binding was
included along with
structural features as a descriptor in CoMFA. The resulting
3D-QSAR has the most predictive
potential of the models in this study and can be considered a prototype
model for the general
evaluation of a steroidal estrogen's growth stimulating ability in
MCF-7 cells. For example,
the location of D-ring contours illustrate the model's preference for
17β-hydroxy steroids over
the less mitogenic 17α- and 16α-hydroxy compounds. In
addition, the enhanced mitogenic
effect of steric bulk in the 11α-position is also evident. The
QSAR studies in this report illustrate
the fact that while ER binding may be a required factor of the estrogen
dependent growth
response in MCF-7 cells, particular structural characteristics, in
addition to those responsible
for tight receptor binding, must be present to induce an optimal
mitogenic response. Therefore,
this report demonstrates that the CoMFA QSAR method can be utilized to
characterize
structural features of test compounds that account for different types
of estrogenic responses.

本研究对雌二醇-17β（estradiol-17β，E2）的类似物展开评估，检测其在人乳腺癌细胞系MCF-7中的雌激素受体（estrogen receptor，ER）结合亲和力与促有丝分裂活性。本次纳入的42种化合物，通过在环系上引入羟基、氨基、硝基或碘基（既可额外修饰，也可替代E2的3位与17β位羟基），对天然雌激素的结构进行了精细修饰。研究发现，此类类似物的促有丝分裂活性仅在有限程度上与ER结合亲和力相关。为阐明决定雌激素受体结合亲和力或促有丝分裂活性的专属结构特征，本研究采用了三维定量构效关系（three-dimensional quantitative structure-activity relationship，QSAR）方法——比较分子场分析（Comparative Molecular Field Analysis，CoMFA）。通过调整不同的对齐规则与区域步长，分别针对受体结合活性与细胞增殖刺激活性优化了CoMFA模型。尽管CoMFA等高线图确实勾勒出了两种检测生物学特性共有的结构需求，但本组雌激素的特定拓扑特征仍可被区分，用于区分促有丝分裂活性与ER结合能力。具体而言，影响细胞增殖的空间位阻干扰区域呈带状分布，从A环上方延伸至4位及其下方区域；而ER结合相关的空间位阻干扰区域仅局限于1、2、4位以及B环α面的孤立多面体区域。此外，位于A、B或C环附近的电负性特征有助于提升受体结合亲和力。不过，细胞增殖活性仅依赖于3位附近的电负性与电正性性质。在最终的促有丝分裂响应QSAR模型中，我们将ER结合活性数值与结构特征一同作为CoMFA的描述符纳入模型。由此得到的三维QSAR模型为本研究中预测性能最优的模型，可作为通用评估甾体雌激素在MCF-7细胞中促生长能力的原型模型。例如，D环等高线的位置体现了该模型对17β-羟基甾体的偏好，其活性优于促有丝分裂活性较弱的17α-与16α-羟基化合物。此外，11α位空间位阻提升所带来的促有丝分裂效应增强也得到了印证。本研究中的QSAR分析表明，尽管ER结合可能是MCF-7细胞中雌激素依赖型增殖响应的必要条件，但除了与紧密受体结合相关的结构特征外，还需具备特定的结构特性，才能诱导最优的促有丝分裂响应。因此，本研究证实，CoMFA QSAR方法可用于表征受试化合物中导致不同类型雌激素响应的结构特征。