Bismuth Keren, Maric Dragan, Arnheiter Heinz
Mammalian Development Section, National Institute of Neurological Disorder and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
Pigment Cell Res. 2005 Oct;18(5):349-59. doi: 10.1111/j.1600-0749.2005.00249.x.
Recent studies show that the melanocyte transcription factor MITF not only activates differentiation genes but also genes involved in the regulation of the cell cycle, suggesting that it provides a link between cell proliferation and differentiation. MITF, however, comes in a variety of splice isoforms with potentially distinct biological activities. In particular, there are two isoforms, (-) and (+) MITF, that differ in six residues located upstream of the DNA binding basic domain and show slight differences in the efficiency with which they bind to target DNA. Using in vitro BrdU incorporation assays and FACS analysis in transiently transfected cells, we show that (+) MITF has a strong inhibitory effect on DNA synthesis while (-) MITF has none or only a mild one. The strong inhibitory activity of (+) MITF is not influenced by a number of mutations that modulate MITF's transcriptional activities and is independent of the protein's carboxyl terminus but dependent on its aminoterminus. A further dissection of the molecule points to the importance of an aminoterminal serine, serine-73, which in both isoforms is phosphorylated to comparable degrees. The results suggest that one or several aminoterminal domains cooperate with the alternatively spliced hexapeptide to render MITF anti-proliferative in a way that does not depend on direct E box binding.
最近的研究表明，黑素细胞转录因子MITF不仅能激活分化基因，还能激活参与细胞周期调控的基因，这表明它在细胞增殖和分化之间建立了联系。然而，MITF有多种剪接异构体，其生物学活性可能不同。特别是，有两种异构体，(-)MITF和(+)MITF，它们在DNA结合碱性结构域上游的六个残基上存在差异，并且在与靶DNA结合的效率上显示出细微差异。通过在瞬时转染细胞中进行体外BrdU掺入试验和FACS分析，我们发现(+)MITF对DNA合成有强烈的抑制作用，而(-)MITF则没有或只有轻微的抑制作用。(+)MITF的强抑制活性不受调节MITF转录活性的一些突变的影响，并且不依赖于该蛋白的羧基末端，而是依赖于其氨基末端。对该分子的进一步剖析表明氨基末端丝氨酸（丝氨酸-73）很重要，在两种异构体中该丝氨酸的磷酸化程度相当。结果表明，一个或几个氨基末端结构域与选择性剪接的六肽协同作用，使MITF以不依赖于直接E盒结合的方式具有抗增殖作用。
