Altiok S, Groner B
Friedrich Miescher Institute, Basel, Switzerland.
Mol Cell Biol. 1994 Sep;14(9):6004-12. doi: 10.1128/mcb.14.9.6004-6012.1994.
beta-Casein gene expression in mammary epithelial cells is under the control of the lactogenic hormones, glucocorticoids, insulin, and prolactin. The hormonal control affects gene transcription, and several regulatory elements in the beta-casein gene promoter between positions -80 and -221 have previously been identified. A region located in the promoter between positions -170 and -221 contains overlapping sequences for negative and positive regulatory elements. A sequence-specific single-stranded DNA-binding factor (STR), composed of two proteins with molecular masses of 35 and 54 kDa, recognizes the upper strand of this region and has a repressing role in transcription. High-level STR binding activity was observed in nuclear extracts from mammary glands of pregnant and postlactating mice and from noninduced HC11 mammary epithelial cells, cells with a low level of transcriptional activity of the beta-casein gene. STR activity is downregulated in mammary epithelial cells during lactation of the animals and after lactogenic hormone induction of HC11 cells in culture. These cells strongly transcribe the beta-casein gene. We investigated the mechanism of downregulation and found that a lactogenic-hormone-induced molecule (I-STR) inhibits STR from binding to its DNA target. I-STR is composed of RNA. STR is sequestered into the cytoplasm by I-STR after lactogenic hormone induction of mammary epithelial cells and remains present in an RNA-bound form. A high-affinity STR binding site was found in the 5' untranslated region of beta-casein mRNA. We propose that beta-casein mRNA can function as I-STR. beta-Casein mRNA may positively regulate its own transcription by translocating STR from the nucleus to the cytoplasm. The beta-casein STR binding sequence increases expression of a transfected beta-galactosidase gene when it is placed into the 5' untranslated region sequence of the mRNA. STR may have a positive role in posttranscriptional regulation.
β-酪蛋白基因在乳腺上皮细胞中的表达受泌乳激素、糖皮质激素、胰岛素和催乳素的调控。激素调控影响基因转录，此前已在β-酪蛋白基因启动子中-80至-221位之间鉴定出几个调控元件。位于启动子中-170至-221位之间的一个区域包含负性和正性调控元件的重叠序列。一种由分子量为35和54 kDa的两种蛋白质组成的序列特异性单链DNA结合因子（STR）识别该区域的上链，并在转录中起抑制作用。在怀孕和哺乳后小鼠乳腺的核提取物以及未诱导的HC11乳腺上皮细胞（β-酪蛋白基因转录活性低的细胞）中观察到高水平的STR结合活性。在动物泌乳期间以及培养的HC11细胞经泌乳激素诱导后，乳腺上皮细胞中的STR活性下调。这些细胞强烈转录β-酪蛋白基因。我们研究了下调机制，发现一种泌乳激素诱导分子（I-STR）抑制STR与其DNA靶点结合。I-STR由RNA组成。在乳腺上皮细胞经泌乳激素诱导后，STR被I-STR隔离到细胞质中，并以RNA结合形式存在。在β-酪蛋白mRNA的5'非翻译区发现了一个高亲和力的STR结合位点。我们提出β-酪蛋白mRNA可以作为I-STR发挥作用。β-酪蛋白mRNA可能通过将STR从细胞核转运到细胞质来正向调节其自身转录。当β-酪蛋白STR结合序列置于mRNA的5'非翻译区序列中时，它会增加转染的β-半乳糖苷酶基因的表达。STR可能在转录后调控中起积极作用。
