Rea D, Parker M G
Molecular Endocrinology Laboratory, Imperial Cancer Research Fund, London, United Kingdom.
Cancer Res. 1996 Apr 1;56(7):1556-63.
The estrogen receptor gene gives rise to variant mRNAs, generated by alternative mRNA splicing, as well as the full-length mRNA containing eight coding exons. It has been postulated that one of these, the exon 5 variant, may be important in the development of hormone-independent and anti-estrogen-resistant breast cancer since it has the potential to encode a truncated receptor that retains the N-terminal activation domain AF-1, but lacks the hormone-binding domain. We have expressed the variant using an inducible promoter in estrogen receptor-positive MCF-7 cells and analyzed the effect of the variant protein on gene expression and cell growth. Inducible expression was validated using a specific antiserum that recognized a novel epitope on the exon 5 variant. The variant was able to stimulate transcription of a reporter gene in transiently transfected chicken embryo fibroblasts in the absence of hormone but showed weak constitutive activity when it was stably expressed in MCF-7 cells. The variant had no effect on the expression of the estrogen target genes, pS2, and the progesterone receptor. Finally, we analyzed whether the proliferation of MCF-7 cells was altered by the expression of the exon 5 variant and found that the stimulatory effects of estrogen and growth inhibitory effects of tamoxifen and ICI 182780 were unchanged. We therefore conclude that expression of the variant alone is not sufficient to give rise to hormone independence or tamoxifen resistance of breast cancers.
雌激素受体基因通过可变mRNA剪接产生多种变体mRNA，以及包含八个编码外显子的全长mRNA。据推测，其中一种，即外显子5变体，可能在激素非依赖性和抗雌激素耐药性乳腺癌的发展中起重要作用，因为它有可能编码一种截短的受体，该受体保留了N端激活域AF-1，但缺乏激素结合域。我们在雌激素受体阳性的MCF-7细胞中使用诱导型启动子表达了该变体，并分析了变体蛋白对基因表达和细胞生长的影响。使用识别外显子5变体上新表位的特异性抗血清验证了诱导型表达。该变体在无激素情况下能够刺激瞬时转染的鸡胚成纤维细胞中报告基因的转录，但在MCF-7细胞中稳定表达时显示出较弱的组成型活性。该变体对雌激素靶基因pS2和孕激素受体的表达没有影响。最后，我们分析了外显子5变体的表达是否改变了MCF-7细胞的增殖，发现雌激素的刺激作用以及他莫昔芬和ICI 182780的生长抑制作用均未改变。因此，我们得出结论，单独表达该变体不足以导致乳腺癌的激素非依赖性或他莫昔芬耐药性。
