Mitochondrial Protein HSG (Mfn-2) Is A Primary Mediator of Oxidative Apoptosis

程序性细胞死亡或凋亡对于大多数器官的发育以及成体组织的稳态和重塑都是重要的。 然而，终末分化的心肌细胞的不可阻挡的损失是心力衰竭的关键原因，心力衰竭是目前发达国家的主要死亡原因。在这里，我们证明，HSG（也称为mitofusin-2），线粒体蛋白，是氧化应激介导的心肌细胞凋亡的主要决定因素。心肌梗死通过活性氧依赖性机制显著提高内源性HSG表达和心肌细胞凋亡。同样，氧化应激与H2 O2同时增加HSG的表达和细胞凋亡在培养的大鼠心肌细胞。此外，腺病毒基因转移介导的HSG过表达足以抑制基础和血清刺激的Akt激活，并引发强大的心肌细胞凋亡。HSG诱导的细胞凋亡通过抑制半胱天冬酶-9而不是半胱天冬酶-8以及通过表达组成型活性PI 3 K突变体以激活Akt或线粒体抗细胞凋亡蛋白Bcl-xL而完全消除，表明HSG通过抑制PI 3 K-Akt细胞存活信号传导和由此产生的初级线粒体细胞凋亡途径的激活来促进心肌细胞凋亡。重要的是，siRNA介导的HSG沉默保护细胞免受氧化应激诱导的凋亡。目前的研究结果表明，心脏HSG功能作为心肌细胞凋亡的主要决定因素，在缺血和氧化应激反应，通过抑制主要的细胞存活信号，Akt，导致线粒体细胞死亡途径的激活。重要的是，增加的心脏HSG表达对于氧化应激诱导的心肌细胞凋亡是必要的和足够的，表明HSG失调可能是心力衰竭的关键致病因素和有希望的治疗靶点（Shen et al.，282：23354-23361，2007） 此外，在血管平滑肌细胞（VSMC）中，HSG作为强有力的细胞增殖抑制剂发挥作用（Chen等人，Nature Cell Biology，6：872-883，2004）。当过表达时，HSG触发VSMC凋亡（Guo等人，Circ. Res. 101：1113-22，2007）。 首先，多种死亡诱导刺激，包括氧化应激与H2 O2，抑制PKC与staurosporine，激活PKA与forskolin，和血清剥夺，同时升高Mfn-2的表达和诱导VSMC凋亡。第二，HSG的过表达触发了培养物和球囊损伤大鼠颈动脉中VSMC的凋亡，从而有助于HSG介导的血管成形术后新生内膜形成的预防，而HSG沉默保护VSMC免受H2 O2或HSG过表达诱导的凋亡，表明HSG的上调对于氧化应激介导的VSMC凋亡是必要且充分的。最后，HSG的促凋亡作用是独立的线粒体融合的作用，但介导的线粒体凋亡途径的激活。同时抗增殖和促凋亡作用标志着HSG作为治疗心血管增殖性疾病的潜在治疗靶点，因为VSMCs的过度生长是动脉粥样硬化和血管成形术后再狭窄发展的关键病因因素。

Programmed cell death, or apoptosis, is important for the development of most organs also for adult tissue homeostasis and remodeling. However, an inexorable loss of terminally differentiated heart muscle cells is a crucial causal factor for heart failure, the current leading cause of death in developed countries. Here, we demonstrate that HSG (also named mitofusin-2), a mitochondria protein, is a major determinant of oxidative stress-mediated cardiomyocyte apoptosis. Myocardial infarction profoundly elevates endogenous HSG expression and myocyte apoptosis in vivo via a reactive oxygen species (ROS)-dependent mechanism. Similarly, oxidative stress with H2O2 concurrently increases HSG expression and apoptosis in cultured rat cardiomyocytes. Furthermore, adenoviral gene transfer-mediated overexpression of HSG is sufficient to suppress both basal and serum-stimulated Akt activation, and triggers robust cardiomyocyte apoptosis. The HSG-induced apoptosis is fully abrogated by inhibition of caspase-9 but not caspase-8, and by expression of a constitutively active PI3K mutant to activate Akt or a mitochondrial antiapoptotic protein, Bcl-xL, indicating that HSG promotes cardiomyocytes apoptosis via inhibition of the PI3K-Akt cell survival signaling and the resultant activation of the primary mitochondrial apoptotic pathway. Importantly, siRNA-mediated HSG silencing protects cells against oxidative stress-induced apoptosis. The present results indicate that cardiac HSG functions as a major determinant of heart muscle cell apoptosis in response to ischemia and oxidative stress, via inhibiting the primary cell survival signal, Akt, resulting in activation of the mitochondrial cell death pathway. Importantly, increased cardiac HSG expression is both necessary and sufficient for oxidative stress-induced heart muscle cell apoptosis, suggesting that HSG deregulation may be a crucial pathogenic element and a promising therapeutic target for heart failure (Shen et al., J. Biol. Chem. 282:23354-23361, 2007) Additionally, in vascular smooth muscle cells (VSMCs), HSG functions as a powerful cell proliferation suppressor (Chen et al. Nature Cell Biology, 6:872-883, 2004). When overexpressed, HSG triggers VSMC apoptosis (Guo et al., Circ. Res. 101:1113-22, 2007). First, multiple death-inducing stimuli, including oxidative stress with H2O2, inhibition of PKC with staurosporine, activation of PKA with forskolin, and serum deprivation, concurrently elevated Mfn-2 expression and induced VSMC apoptosis. Second, overexpression of HSG triggered apoptosis of VSMCs in culture and in balloon-injured rat carotid arteries as well, thus contributing to HSG-mediated prevention of neointima formation after angioplasty, while HSG silencing protected VSMCs against H2O2 or HSG overexpression induced apoptosis, indicating upregulation of HSG is necessary and sufficient for oxidative stress mediated VSMC apoptosis. Finally, HSG proapoptotic effect was independent of its role in mitochondrial fusion, but mediated by activation of the mitochondrial apoptotic pathway. The concurrent antiproliferative and proapoptotic effects mark HSG as a potential therapeutic target in treating cardiovascular proliferative disorders, because overgrowth of VSMCs is a pivotal etiological factor in the development of atherosclerosis and restenosis after angioplasty.