Forbes Davanea, Jackman Mark, Bishop Amy, Thomas Mark, Kambadur Ravi, Sharma Mridula
Animal Genomics, AgResearch, East Street, Hamilton, New Zealand.
J Cell Physiol. 2006 Jan;206(1):264-72. doi: 10.1002/jcp.20477.
Myostatin, a secreted growth factor, is a member of the TGF-beta superfamily and an inhibitor of myogenesis. Previously, we have shown that myostatin gene expression is regulated at the level of transcription and that myostatin is a downstream target gene of MyoD. Here we show that myostatin gene expression is auto-regulated by a negative feedback mechanism. Northern blot analysis indicated that there are relatively higher levels of myostatin mRNA in the biceps femoris muscle of cattle that express a non- functional myostatin allele (Belgian Blue) as compared to normal cattle. In contrast, addition of exogenous myostatin decreases endogenous myostatin mRNA. Consistent with this result, wild type myostatin protein is able to repress myostatin promoter activity via Activin type IIb receptor (ActRIIB) and ALK5 (P < 0.001). However, non-functional myostatin (Piedmontese) failed to repress the myostatin promoter suggesting that myostatin auto-regulates its promoter by negative feedback inhibition. Auto-regulation by myostatin appears to be signaled through Smad7, since the expression of the inhibitory Smad7 is induced by myostatin and the over-expression of Smad7 in turn inhibits the myostatin promoter activity (P < 0.001). In contrast down regulation of Smad7 by siRNA results in increased myostatin mRNA indicating that Smad7 is a negative regulator of myostatin gene expression. Consistent with these results, a decrease in Smad7 mRNA and concomitant increase in myostatin expression is seen in myotubes that express non functional myostatin. In addition, interference with myostatin signaling prevents the induction of Smad7 promoter activity by myostatin. Based on these results, we propose that myostatin auto-regulates its gene expression through a Smad7 dependent mechanism in myogenic cells.
肌肉生长抑制素是一种分泌型生长因子，属于转化生长因子-β（TGF-β）超家族成员，是肌肉生成的抑制剂。此前，我们已经表明肌肉生长抑制素基因表达在转录水平受到调控，并且肌肉生长抑制素是MyoD的下游靶基因。在此我们表明，肌肉生长抑制素基因表达通过负反馈机制进行自我调节。Northern印迹分析表明，与正常牛相比，表达无功能肌肉生长抑制素等位基因（比利时蓝牛）的牛股二头肌中肌肉生长抑制素mRNA水平相对较高。相反，添加外源性肌肉生长抑制素会降低内源性肌肉生长抑制素mRNA水平。与该结果一致，野生型肌肉生长抑制素蛋白能够通过激活素IIb型受体（ActRIIB）和ALK5抑制肌肉生长抑制素启动子活性（P < 0.001）。然而，无功能的肌肉生长抑制素（皮埃蒙特牛）未能抑制肌肉生长抑制素启动子，这表明肌肉生长抑制素通过负反馈抑制对其启动子进行自我调节。肌肉生长抑制素的自我调节似乎是通过Smad7发出信号的，因为抑制性Smad7的表达由肌肉生长抑制素诱导，而Smad7的过表达反过来抑制肌肉生长抑制素启动子活性（P < 0.001）。相反，通过小干扰RNA（siRNA）下调Smad7会导致肌肉生长抑制素mRNA增加，表明Smad7是肌肉生长抑制素基因表达的负调节因子。与这些结果一致，在表达无功能肌肉生长抑制素的肌管中，可见Smad7 mRNA减少，同时肌肉生长抑制素表达增加。此外，干扰肌肉生长抑制素信号传导可阻止肌肉生长抑制素对Smad7启动子活性的诱导。基于这些结果，我们提出肌肉生长抑制素在成肌细胞中通过Smad7依赖性机制对其基因表达进行自我调节。
