Stein B, Yang M X
Signal Pharmaceuticals, San Diego, California 92121, USA.
Mol Cell Biol. 1995 Sep;15(9):4971-9. doi: 10.1128/MCB.15.9.4971.
Bone metabolism is regulated by a balance between bone resorption caused by osteoclasts and bone formation caused by osteoblasts. This balance is disturbed in postmenopausal women as a result of lower serum estrogen levels. Estrogen, which is used in hormone replacement therapy to prevent postmenopausal osteoporosis, downregulates expression of the interleukin 6 (IL-6) gene in osteoblasts and bone marrow stromal cells. IL-6 is directly involved in bone resorption by activating immature osteoclasts. We show here that NF-kappa B and C/EBP beta are important regulators of IL-6 gene expression in human osteoblasts. Importantly, the IL-6 promoter is inhibited by estrogen in the absence of a functional estrogen receptor (ER) binding site. This inhibition is mediated by the transcription factors NF-kappa B and C/EBP beta. Evidence is presented for a direct interaction between these two factors and ER. We characterized the protein sequence requirements for this association in vitro and in vivo. The physical and functional interaction depends in part on the DNA binding domain and region D of ER and on the Rel homology domain of NF-kappa B and the bZIP region of C/EBP beta. The cross-coupling between ER, NF-kappa B, and C/EBP beta also results in reduced activity of promoters with ER binding sites. We further show that the mechanism of IL-6 gene repression by estrogen is clearly different from that of activation of promoters with ER binding sites. Therefore, drugs that separate the transactivation and transrepression functions of ER will be very helpful for treatment of osteoporosis without causing undesirable side effects.
骨代谢由破骨细胞引起的骨吸收和成骨细胞引起的骨形成之间的平衡来调节。由于血清雌激素水平降低，绝经后女性的这种平衡会受到干扰。用于激素替代疗法以预防绝经后骨质疏松症的雌激素，会下调成骨细胞和骨髓基质细胞中白细胞介素6（IL-6）基因的表达。IL-6通过激活未成熟破骨细胞直接参与骨吸收。我们在此表明，NF-κB和C/EBPβ是人类成骨细胞中IL-6基因表达的重要调节因子。重要的是，在没有功能性雌激素受体（ER）结合位点的情况下，雌激素会抑制IL-6启动子。这种抑制是由转录因子NF-κB和C/EBPβ介导的。有证据表明这两种因子与ER之间存在直接相互作用。我们在体外和体内对这种关联的蛋白质序列要求进行了表征。物理和功能相互作用部分取决于ER的DNA结合结构域和区域D以及NF-κB的Rel同源结构域和C/EBPβ的bZIP区域。ER、NF-κB和C/EBPβ之间的交叉偶联也会导致具有ER结合位点的启动子活性降低。我们进一步表明，雌激素对IL-6基因的抑制机制与具有ER结合位点的启动子的激活机制明显不同。因此，能够区分ER的反式激活和反式抑制功能的药物对于治疗骨质疏松症将非常有帮助，且不会引起不良副作用。
