Beyer Kim S, Beauchamp Roberta L, Lee Ming-Fen, Gusella James F, Näär Anders M, Ramesh Vijaya
Molecular Neurogenetics Unit, Center for Human Genetic Research, Massachusetts General Hospital, Boston 02114, and Department of Cell Biology, Harvard Medical School, Charlestown, Massachusetts 02129, USA.
J Biol Chem. 2007 Nov 2;282(44):32152-7. doi: 10.1074/jbc.M706592200. Epub 2007 Sep 11.
Magicin, a protein that we isolated earlier as an interactor of the neurofibromatosis 2 protein merlin, was independently identified as MED28, a subunit of the mammalian Mediator complex. Mediator complex is an evolutionarily conserved transcriptional cofactor, which plays an essential role in positive and negative gene regulation. Distinct Mediator subunit composition is thought to contribute to gene regulation specificity based on the interaction of specific subunits with subsets of transcription factors. Here we report that down-regulation of Med28 expression in NIH3T3 cells results in a significant induction of several genes associated with smooth muscle cell (SMC) differentiation. Conversely, overexpression of MED28 represses expression of SMC genes, in concordance with our knockdown data. More importantly, multipotent mesenchymal-derived murine precursors can transdifferentiate into SMCs when Med28 is down-regulated. Our data also show that Med28 functions as a negative regulator of SMC differentiation in concert with other Mediator subunits including Med6, Med8, and Med18 within the Mediator head module. Our results provide strong evidence that MED28 may function as a scaffolding protein by maintaining the stability of a submodule within the head module and that components of this submodule act together in a gene regulatory program to suppress SMC differentiation. The results presented here demonstrate for the first time that the mammalian Mediator subunit MED28 functions as a repressor of SMC differentiation, which could have implications for disorders associated with abnormalities in SMC growth and differentiation, including atherosclerosis, asthma, hypertension, and smooth muscle tumors.
Magicin是我们先前分离出的一种蛋白质，作为神经纤维瘤病2蛋白merlin的相互作用分子，它后来被独立鉴定为MED28，即哺乳动物中介体复合物的一个亚基。中介体复合物是一种进化上保守的转录辅因子，在基因的正负调控中起关键作用。基于特定亚基与转录因子子集的相互作用，不同的中介体亚基组成被认为有助于基因调控的特异性。在此我们报告，NIH3T3细胞中Med28表达的下调导致与平滑肌细胞（SMC）分化相关的几个基因显著诱导。相反，MED28的过表达抑制SMC基因的表达，这与我们的敲低数据一致。更重要的是，当Med28下调时，多能间充质来源的小鼠前体细胞可转分化为SMC。我们的数据还表明，Med28与中介体头部模块中的其他中介体亚基（包括Med6、Med8和Med18）协同作用，作为SMC分化的负调节因子。我们的结果提供了强有力的证据，表明MED28可能通过维持头部模块内一个子模块的稳定性而作为一种支架蛋白发挥作用，并且该子模块的成分在一个基因调控程序中共同发挥作用以抑制SMC分化。此处呈现的结果首次证明，哺乳动物中介体亚基MED28作为SMC分化的抑制因子发挥作用，这可能对与SMC生长和分化异常相关的疾病有影响，包括动脉粥样硬化、哮喘、高血压和平滑肌瘤。
