Chen S L, Wang S C, Hosking B, Muscat G E
University of Queensland Centre for Molecular and Cellular Biology Institute for Molecular Bioscience St. Lucia, 4072 Queensland, Australia.
Mol Endocrinol. 2001 May;15(5):783-96. doi: 10.1210/mend.15.5.0637.
Skeletal muscle differentiation and the activation of muscle-specific gene expression are dependent on the concerted action of the MyoD family and the MADS protein, MEF2, which function in a cooperative manner. The steroid receptor coactivator SRC-2/GRIP-1/TIF-2, is necessary for skeletal muscle differentiation, and functions as a cofactor for the transcription factor, MEF2. SRC-2 belongs to the SRC family of transcriptional coactivators/cofactors that also includes SRC-1 and SRC-3/RAC-3/ACTR/AIB-1. In this study we demonstrate that SRC-2 is essentially localized in the nucleus of proliferating myoblasts; however, weak (but notable) expression is observed in the cytoplasm. Differentiation induces a predominant localization of SRC-2 to the nucleus; furthermore, the nuclear staining is progressively more localized to dot-like structures or nuclear bodies. MEF2 is primarily expressed in the nucleus, although we observed a mosaic or variegated expression pattern in myoblasts; however, in myotubes all nuclei express MEF2. GRIP-1 and MEF2 are coexpressed in the nucleus during skeletal muscle differentiation, consistent with the direct interaction of these proteins. Rhabdomyosarcoma (RMS) cells derived from malignant skeletal muscle tumors have been proposed to be deficient in cofactors. Alveolar RMS cells very weakly express the steroid receptor coactivator, SRC-2, in a diffuse nucleocytoplasmic staining pattern. MEF2 and the cofactors, SRC-1 and SRC-3 are abundantly expressed in alveolar and embryonal RMS cells; however, the staining is not localized to the nucleus. Furthermore, the subcellular localization and transcriptional activity of MEF2C and a MEF2-dependent reporter are compromised in alveolar RMS cells. In contrast, embryonal RMS cells express SRC-2 in the nucleus, and MEF2 shuttles from the cytoplasm to the nucleus after serum withdrawal. In conclusion, this study suggests that the steroid receptor coactivator SRC-2 and MEF2 are localized to the nucleus during the differentiation process. In contrast, RMS cells display aberrant transcription factor SRC localization and expression, which may underlie certain features of the RMS phenotype.
骨骼肌分化以及肌肉特异性基因表达的激活依赖于MyoD家族和MADS蛋白MEF2的协同作用，它们以合作的方式发挥功能。类固醇受体辅激活因子SRC-2/GRIP-1/TIF-2是骨骼肌分化所必需的，并且作为转录因子MEF2的辅因子发挥作用。SRC-2属于转录辅激活因子/辅因子的SRC家族，该家族还包括SRC-1和SRC-3/RAC-3/ACTR/AIB-1。在本研究中，我们证明SRC-2主要定位于增殖的成肌细胞的细胞核中；然而，在细胞质中观察到微弱（但显著）的表达。分化诱导SRC-2主要定位于细胞核；此外，核染色逐渐更定位于点状结构或核体。MEF2主要在细胞核中表达，尽管我们在成肌细胞中观察到镶嵌或斑驳的表达模式；然而，在肌管中所有细胞核都表达MEF2。在骨骼肌分化过程中，GRIP-1和MEF2在细胞核中共表达，这与这些蛋白的直接相互作用一致。源自恶性骨骼肌肿瘤的横纹肌肉瘤（RMS）细胞被认为缺乏辅因子。肺泡型RMS细胞以弥漫性核质染色模式非常微弱地表达类固醇受体辅激活因子SRC-2。MEF2以及辅因子SRC-1和SRC-3在肺泡型和胚胎型RMS细胞中大量表达；然而，染色并不定位于细胞核。此外，MEF2C和MEF2依赖性报告基因的亚细胞定位和转录活性在肺泡型RMS细胞中受损。相比之下，胚胎型RMS细胞在细胞核中表达SRC-2，并且血清撤除后MEF2从细胞质穿梭到细胞核。总之，本研究表明类固醇受体辅激活因子SRC-2和MEF2在分化过程中定位于细胞核。相比之下，RMS细胞表现出异常的转录因子SRC定位和表达，这可能是RMS表型某些特征的基础。
