Regulation of Estrogen Receptor Alpha through novel function of HMGA1a.-Towards a novel breast cancer therapy-

HMGA 1a是一种DNA结合转录因子，被发现通过新的序列特异性RNA结合诱导选择性剪接。我们在雌激素受体α（ERα）前体mRNA中发现了HMGA 1a RNA结合位点。HMGA 1a与ERα外显子1上游33个核苷酸的RNA结合。HMGA 1a通过ERα外显子1的外显子跳跃诱导ERα46亚型mRNA表达，HMGA 1a RNA结合位点的RNA诱饵抑制MCF-7乳腺癌细胞ERα46亚型mRNA表达。ERα外显子1上的HMGA 1a RNA结合位点位于5'端假剪接位点的上游。因此，HMGA 1a将U1 snRNP捕获到该上游5'剪接位点，导致ERα外显子1的真实5'剪接位点功能障碍，从而诱导外显子跳跃，并通过选择性剪接实现ERα46亚型的表达。结果表明，表达HMGA 1a RNA结合位点的RNA诱骗物的MCF-7细胞ERα46蛋白表达量明显下降，并建立了稳定的MCF-7细胞转染细胞系。将此稳定的转染子与雌激素颗粒一起皮下植入卵巢切除的裸鼠，导致植入细胞的生长减弱。由于ERα46亚型蛋白能够抑制全长ERα的雌激素反应，因此，“HMGA 1a的RNA诱饵通过调节ERα的选择性剪接改善MCF-7细胞的雌激素反应”的研究结果将为阐明ERα阳性乳腺癌雌激素抵抗的机制提供线索。

HMGA1a, known as a DNA-binding transcription factor, was found to induce alternative splicing through novel sequence-specific RNA-binding. We found an HMGA1a RNA-binding site in Estrogen Receptor alpha(ERα) pre-mRNA. HMGA1a binds an RNA sequence 33 nucleotides upstream the 5' splice site of ERα exon 1.Interestingly, HMGA1a induces ERα46 isoform mRNA expression by exon skipping of ERα exon 1, and an RNA decoy of the HMGA1a RNA binding site inhibits ERα46 isoform mRNA expression in cultured MCF-7 mammary carcinoma cells. The HMGA1a RNA binding site in ERα exon 1 is located adjacently upstream a pseudo 5' splice site. Thus, HMGA1a traps U1 snRNP to this upstream 5' splice site and leads to dysfunction of the authentic 5' splice site of ERα exon 1.In this way, exon skipping is induced and consequent expression of ERα46 isoform is achieved through alternative splicing. Confirming the decrease of ERα46 protein expression in MCF-7 cells expressing the RNA decoy of HMGA1a RNA binding site, a stable transfectant of MCF-7 cells was established. This stable transfectant was implanted subcutaneously to ovarectomized nude mice with estrogen pellet, resulting in attenuated growth of the implanted cells. Since ERα46 isoform protein is known to inhibit the estrogen response of full length ERα, the findings shown here "an RNA decoy of HMGA1a improves estrogen response of MCF-7 cells by regulating alternative splicing of ERα" will give us a clue in deciphering the mechanism of estrogen resistance in ERα positive mammary carcinoma.