Rejon Carlis A, Hancock Mark A, Li Yining N, Thompson Thomas B, Hébert Terence E, Bernard Daniel J
Department of Pharmacology and Therapeutics, McGill University, Montréal, Québec, Canada.
Cell Signal. 2013 Dec;25(12):2717-26. doi: 10.1016/j.cellsig.2013.09.002. Epub 2013 Sep 7.
TGFβ superfamily ligands greatly outnumber their receptors. Thus, receptors are shared between ligands and individual ligands can bind multiple receptors. Bone morphogenetic proteins (BMPs) bind and signal via both BMP type II (BMPR2) and activin type II (ACVR2) receptors. We hypothesized that, in addition to its canonical receptor ACVR2, activin A might similarly bind and signal via BMPR2. First, using surface plasmon resonance, we showed that activin A binds to the BMPR2 extracellular domain (ECD), though with lower affinity compared to the ACVR2-ECD. We confirmed these results in cells, where radiolabeled activin A bound to ACVR2 and BMPR2, but not to other type II receptors (AMHR2 or TGFBR2). Using homology modeling and site-directed mutagenesis, we identified key residues in BMPR2 that mediate its interaction with activin A. The soluble ECDs of ACVR2 or BMPR2 dose-dependently inhibited activin A-, but not TGFβ-induced signaling in cells, suggesting that activin binding to BMPR2 could have functional consequences. To address this idea, we altered BMPR2 expression levels in immortalized murine gonadotrope-like cells, LβT2, in which activins potently stimulate follicle-stimulating hormone β (Fshb) subunit transcription. BMPR2 expression potentiated activin A responses whereas depletion of endogenous BMPR2 with short interfering RNAs attenuated activin A-stimulated Fshb transcription. Additional data suggest, for the first time, that BMPR2 may form functional complexes with the canonical activin type I receptor, activin receptor-like kinase 4. Collectively, our data show that BMPR2, along with ACVR2, functions as a bona fide activin type II receptor in gonadotrope-like cells, thereby broadening our understanding of mechanisms of activin action.
转化生长因子β（TGFβ）超家族配体的数量远多于其受体。因此，受体是配体共享的，且单个配体可以结合多种受体。骨形态发生蛋白（BMPs）通过BMP II型受体（BMPR2）和激活素II型受体（ACVR2）进行结合并传递信号。我们推测，除了其经典受体ACVR2外，激活素A可能同样通过BMPR2进行结合并传递信号。首先，利用表面等离子体共振技术，我们发现激活素A能与BMPR2细胞外结构域（ECD）结合，不过与ACVR2-ECD相比，其亲和力较低。我们在细胞中证实了这些结果，放射性标记的激活素A能与ACVR2和BMPR2结合，但不与其他II型受体（AMHR2或TGFBR2）结合。通过同源建模和定点诱变，我们确定了BMPR2中介导其与激活素A相互作用的关键残基。ACVR2或BMPR2的可溶性ECD能剂量依赖性地抑制激活素A诱导的细胞信号传导，但不能抑制TGFβ诱导的信号传导，这表明激活素与BMPR2的结合可能具有功能后果。为了验证这一观点，我们改变了永生化小鼠促性腺激素样细胞LβT2中BMPR2的表达水平，在这些细胞中激活素能有效刺激促卵泡激素β（Fshb）亚基转录。BMPR2的表达增强了激活素A的反应，而用小干扰RNA耗尽内源性BMPR2则减弱了激活素A刺激的Fshb转录。其他数据首次表明，BMPR2可能与经典的激活素I型受体——激活素受体样激酶4形成功能复合物。总的来说，我们的数据表明，BMPR2与ACVR2一起，在促性腺激素样细胞中作为真正的激活素II型受体发挥作用，从而拓宽了我们对激活素作用机制的理解。
