Schreyer Sylvia, Ledwig Daniela, Rakatzi Irini, Klöting Ingrid, Eckel Jürgen
Molecular Cardiology, Department of Clinical Biochemistry and Pathobiochemistry, German Diabetes Research Institute, D-40225 Düsseldorf, Germany.
Endocrinology. 2003 Apr;144(4):1211-8. doi: 10.1210/en.2002-220723.
Insulin receptor substrate (IRS) proteins represent key elements of the insulin-signaling cascade. IRS-4 is the most recently characterized member of the IRS family with an undefined in vivo function. In contrast to IRS-1 and IRS-2, IRS-4 exhibits a limited tissue expression, and IRS-4 protein has not been detected in any mouse or primary human tissue so far. The purpose of the present study was to analyze the expression of IRS-4 in rat muscle and human skeletal muscle cells and assess involvement of IRS-4 in initial insulin signaling. Using immunoblotting and immunoprecipitation, the specific expression of IRS-4 protein could be demonstrated in rat soleus and cardiac muscle and human skeletal muscle cells, but it was not significantly detectable in quadriceps and gastrocnemius. A prominent down-regulation of IRS-4 was observed in heart and soleus muscle of WOKW rats, an animal model of the metabolic syndrome. In human skeletal muscle cells, both IRS-1 and IRS-2 are rapidly phosphorylated on tyrosine in response to insulin, whereas essentially no tyrosine phosphorylation of IRS-4 was observed in response to both insulin and IGF-I. Instead, a 2-fold increase in IRS-4 tyrosine phosphorylation was observed in myocytes subjected to osmotic stress. In conclusion, IRS-4 protein is expressed in heart and skeletal muscle in a fiber type specific fashion. Our data suggest that IRS-4 does not function as a substrate of the insulin and the IGF-I receptor in primary muscle cells but may be involved in nonreceptor tyrosine kinase signaling.
胰岛素受体底物（IRS）蛋白是胰岛素信号级联反应的关键元件。IRS-4是IRS家族中最新被鉴定的成员，其体内功能尚不明确。与IRS-1和IRS-2不同，IRS-4的组织表达有限，迄今为止在任何小鼠或原代人体组织中均未检测到IRS-4蛋白。本研究的目的是分析IRS-4在大鼠肌肉和人骨骼肌细胞中的表达，并评估IRS-4在胰岛素初始信号传导中的作用。通过免疫印迹和免疫沉淀法，可证明IRS-4蛋白在大鼠比目鱼肌、心肌和人骨骼肌细胞中有特异性表达，但在股四头肌和腓肠肌中未检测到明显表达。在代谢综合征动物模型WOKW大鼠的心脏和比目鱼肌中观察到IRS-4显著下调。在人骨骼肌细胞中，IRS-1和IRS-2在胰岛素刺激下酪氨酸迅速磷酸化，而在胰岛素和IGF-I刺激下均未观察到IRS-4的酪氨酸磷酸化。相反，在受到渗透压应激的心肌细胞中观察到IRS-4酪氨酸磷酸化增加了2倍。总之，IRS-4蛋白以纤维类型特异性方式在心脏和骨骼肌中表达。我们的数据表明，IRS-4在原代肌肉细胞中并非作为胰岛素和IGF-I受体的底物发挥作用，但可能参与非受体酪氨酸激酶信号传导。
