Grimm S L, Nordeen S K
Program in Molecular Biology, University of Colorado Health Sciences Center, Denver, Colorado 80262, USA.
J Biol Chem. 1999 Apr 30;274(18):12790-6. doi: 10.1074/jbc.274.18.12790.
Mouse mammary tumor virus (MMTV) expression is restricted primarily to mammary epithelial cells. Sequences responsible for both the mammary-specific expression of MMTV and the activation of cellular oncogenes are located within two enhancer elements at the 5'-end of the long terminal repeat. Whereas the Ban2 enhancer (-1075 to -978) has been well characterized, the mammary-specific enhancer of MMTV from -956 to -862 has only recently been recognized as a key determinant of mammary-specific oncogene activation by MMTV. The present study identifies and characterizes three binding sites located within this element. Transient transfection of deletion and mutation constructs shows that all three sites contribute to the basal expression of MMTV in mammary cells. One of the binding activities (footprint I) is restricted to mammary cells, whereas the other two sites bind factors found in both mammary and nonmammary cells. The multimerized mammary-specific enhancer of MMTV on its own can enhance a minimal promoter in a mammary-specific fashion. However, the FpI binding site alone cannot mediate this effect. Thus, it is the binding of multiple factors in a combinatorial fashion that mediates the mammary-restricted expression of MMTV.
小鼠乳腺肿瘤病毒（MMTV）的表达主要局限于乳腺上皮细胞。负责MMTV乳腺特异性表达和细胞癌基因激活的序列位于长末端重复序列5'端的两个增强子元件内。虽然Ban2增强子（-1075至-978）已得到充分表征，但MMTV从-956至-862的乳腺特异性增强子直到最近才被认为是MMTV激活乳腺特异性癌基因的关键决定因素。本研究鉴定并表征了该元件内的三个结合位点。缺失和突变构建体的瞬时转染表明，所有这三个位点都有助于MMTV在乳腺细胞中的基础表达。其中一种结合活性（足迹I）仅限于乳腺细胞，而其他两个位点则结合在乳腺和非乳腺细胞中均发现的因子。MMTV多聚化的乳腺特异性增强子自身能够以乳腺特异性方式增强最小启动子。然而，单独的FpI结合位点不能介导这种效应。因此，是多种因子以组合方式的结合介导了MMTV的乳腺限制性表达。
