Estrogen receptor (ER)-negative
breast carcinomas are often difficult to treat as they do not respond to
hormone therapy. In an attempt to determine if expressing the human
estrogen receptor in an ectopic manner could restore the
hormone responsiveness of these cells, we have expressed the human ER in ER-negative MDA-MB 231
breast cancer cells using a recombinant adenovirus gene delivery system that allows high level expression of ER in essentially all cells. In these cells, the ER was correctly translated, had a wild type
hormone binding affinity (Kd = 0.6 nM), bound well to
estrogen response element-containing
DNA, and showed an activation pattern of
estrogen response element-reporter gene activity by
estrogen and
antiestrogens very similar to that observed in MCF-7
breast cancer cells containing endogenous ER (stimulation by
estrogen, no stimulation by the
antiestrogens trans-
hydroxytamoxifen or
ICI 164384, and blockade of
estradiol stimulation by trans-
hydroxytamoxifen or
ICI 164384). Intriguingly,
estradiol stimulation of these cells was also able to induce expression of pS2, an
estrogen regulated gene considered to be a favorable prognostic marker for endocrine
therapy in ER-positive
breast cancer cells. Expression of the ER had no effect by itself on the proliferation rate of MDA-MB 231 cells. However, treatment of the ER-containing cells with
estradiol or with the pure
antiestrogen ICI 164 384 suppressed proliferation of the cells while the
antiestrogen trans-
hydroxytamoxifen had little effect on proliferation; and cotreatment with trans-
hydroxytamoxifen reversed the
estradiol- or ICI 164 384-evoked suppression of proliferation. To understand the mechanism underlying the inhibition of proliferation by
estradiol, we examined the expression of several growth related endogenous genes. c-Myc protooncogene expression was strongly inhibited by treatment with
estradiol as was expression of BRCA1 and BRCA2 genes, which is in agreement with their mitogenic-dependent expression, while expression of
beta-actin, a housekeeping gene, was not affected by
hormone treatment. Together, these data suggest that reexpressing the human ER in
breast cancer cells that no longer express this
protein renders them sensitive to
hormone treatment. The ability of the
antiestrogen ICI 164 384 to suppress the proliferation of ER-negative
breast cancer cells that reexpress ER might be useful ultimately as an endocrine gene therapy approach for controlling the growth of ER-negative
breast cancer cells. The application of recombinant adenoviruses expressing the human ER presents interesting features which might be used as a basis for designing more powerful and effective treatments for ER-negative breast
cancers.