Muñoz-Cánoves P, Miralles F, Baiget M, Félez J
Department de Receptors Cel, lulars, Institut de Recerca Oncològica (IRO), Barcelona, Spain.
Thromb Haemost. 1997 Mar;77(3):526-34.
Urokinase-type plasminogen activator (uPA) is one of the components of blood's fibrinolytic cascade. uPA acts as a broad spectrum proteolytic enzyme involved in different physio-pathological processes including cellular fibrinolysis, adhesion, migration, invasion and remodeling. Here, we present evidence that uPA participates in myogenesis, a process which requires drastic cell membrane reorganization, leading to the plurinucleated myotube from the progenitor myoblast. We have dissected the expression of uPA throughout the different myogenic compartments and found an increase in uPA enzymatic activity associated with myotube formation in C2C12 myoblast cells, with uPA mRNA increasing prior the onset of fusion and differentiation. When both fusion and differentiation were blocked by specific inhibitors (DMSO, cytochalasin B) the levels of uPA were strongly downregulated. This process was reversible and specific: the removal of the inhibitors immediately restored the levels of uPA mRNA while the specific inhibition of uPA enzymatic activity by an anti-uPA antibody resulted in a 50% reduction of the extent of fusion and in the abrogation of muscle-specific gene products, such as alpha-actin and MyoD. Moreover, the conversion of fibroblasts to muscle-like cells upon acquisition of MyoD resulted in a dramatic increase of uPA mRNA, which was partially due to transcriptional activation of the uPA gene. These results indicate that the increase in uPA expression prior to fusion and differentiation occurs via a MyoD-mediated mechanism whereas the normal MyoD expression requires the plasminogen activation-dependent activity of this protease. Therefore, these studies extend the sphere of influence of myogenic factors to fibrinolysis, an intrinsic component of the hematological system. Taken together, one mechanism used by the myoblast cell to become a differentiated myotube, involving the inductive extracellular proteolysis of urokinase, is proposed.
尿激酶型纤溶酶原激活剂（uPA）是血液纤维蛋白溶解级联反应的组成成分之一。uPA作为一种广谱蛋白水解酶，参与不同的生理病理过程，包括细胞纤维蛋白溶解、黏附、迁移、侵袭和重塑。在此，我们提供证据表明uPA参与了肌生成过程，这一过程需要剧烈的细胞膜重组，从而使祖代成肌细胞形成多核肌管。我们剖析了uPA在不同肌生成区室中的表达情况，发现在C2C12成肌细胞中，与肌管形成相关的uPA酶活性增加，在融合和分化开始之前uPA mRNA就有所增加。当融合和分化被特异性抑制剂（二甲基亚砜、细胞松弛素B）阻断时，uPA水平会大幅下调。这个过程是可逆且特异的：去除抑制剂后，uPA mRNA水平立即恢复，而抗uPA抗体对uPA酶活性的特异性抑制导致融合程度降低50%，并使肌肉特异性基因产物（如α-肌动蛋白和MyoD）消失。此外，成纤维细胞在获得MyoD后转化为类肌细胞，导致uPA mRNA显著增加，这部分归因于uPA基因的转录激活。这些结果表明，在融合和分化之前uPA表达的增加是通过MyoD介导的机制发生的，而正常的MyoD表达需要这种蛋白酶的纤溶酶原激活依赖性活性。因此，这些研究将肌生成因子的影响范围扩展到了纤维蛋白溶解，这是血液系统的一个内在组成部分。综上所述，提出了成肌细胞成为分化肌管所采用的一种机制，即涉及尿激酶诱导的细胞外蛋白水解作用。
