Novitch B G, Spicer D B, Kim P S, Cheung W L, Lassar A B
Department of Biological Chemistry and Molecular Pharmacology Harvard Medical School 240 Longwood Avenue, Boston, Massachusetts, 02115, USA.
Curr Biol. 1999 May 6;9(9):449-59. doi: 10.1016/s0960-9822(99)80210-3.
The onset of differentiation-specific gene expression in skeletal muscle is coupled to permanent withdrawal from the cell cycle. The retinoblastoma tumor-suppressor protein (pRb) is a critical regulator of this process, required for both cell-cycle arrest in G0 phase and high-level expression of late muscle-differentiation markers. Although the cell-cycle defects that are seen in pRb-deficient myocytes can be explained by the well-described function of pRb as a negative regulator of the transition from G1 to S phase, it remains unclear how pRb positively affects late muscle-gene expression.
Here, we show that the myogenic defect in Rb-/- cells corresponds to a deficiency in the activity of the transcription factor MEF2. Without pRb, MyoD induces the accumulation of nuclear-localized MEF2 that is competent to bind DNA yet transcriptionally inert. When pRb is present, MyoD stimulates the function of the MEF2C transcriptional activation domain and the activity of endogenous MEF2-type factors. Co-transfection of MyoD together with an activated form of MEF2C containing the Herpesvirus VP16 transcriptional activation domain partially bypasses the requirement for pRb and induces late muscle-gene expression in replicating cells. This ectopic myogenesis is nevertheless significantly augmented by co-expression of an E2F1-pRb chimeric protein that blocks the cell cycle.
These findings indicate that pRb promotes the expression of late-stage muscle-differentiation markers by both inhibiting cell-cycle progression and cooperating with MyoD to promote the transcriptional activation activity of MEF2.
骨骼肌中分化特异性基因表达的起始与细胞周期的永久退出相关联。视网膜母细胞瘤肿瘤抑制蛋白（pRb）是这一过程的关键调节因子，对于G0期的细胞周期停滞和晚期肌肉分化标志物的高水平表达均是必需的。尽管在pRb缺陷型肌细胞中观察到的细胞周期缺陷可以通过pRb作为从G1期到S期转变的负调节因子这一已充分描述的功能来解释，但pRb如何正向影响晚期肌肉基因表达仍不清楚。
在这里，我们表明Rb-/-细胞中的成肌缺陷对应于转录因子MEF2活性的缺乏。没有pRb时，MyoD诱导核定位的MEF2积累，其能够结合DNA但转录无活性。当存在pRb时，MyoD刺激MEF2C转录激活结构域的功能和内源性MEF2型因子的活性。将MyoD与含有疱疹病毒VP16转录激活结构域的激活形式的MEF2C共转染，部分绕过了对pRb的需求，并在复制细胞中诱导晚期肌肉基因表达。然而，通过共表达阻断细胞周期的E2F1-pRb嵌合蛋白，这种异位成肌作用显著增强。
这些发现表明，pRb通过抑制细胞周期进程以及与MyoD协同促进MEF2的转录激活活性，从而促进晚期肌肉分化标志物的表达。
