Wahedi Mastura, Wortham Aaron M, Kleven Mark D, Zhao Ningning, Jue Shall, Enns Caroline A, Zhang An-Sheng
From the Department of Cell, Developmental, and Cancer Biology, Oregon Health & Science University, Portland, Oregon 97239 and.
the Department of Nutritional Sciences, University of Arizona, Tucson, Arizona 85721.
J Biol Chem. 2017 Nov 3;292(44):18354-18371. doi: 10.1074/jbc.M117.801795. Epub 2017 Sep 18.
Systemic iron homeostasis is maintained by regulation of iron absorption in the duodenum, iron recycling from erythrocytes, and iron mobilization from the liver and is controlled by the hepatic hormone hepcidin. Hepcidin expression is induced via the bone morphogenetic protein (BMP) signaling pathway that preferentially uses two type I (ALK2 and ALK3) and two type II (ActRIIA and BMPR2) BMP receptors. Hemojuvelin (HJV), HFE, and transferrin receptor-2 (TfR2) facilitate this process presumably by forming a plasma membrane complex with BMP receptors. Matriptase-2 (MT2) is a protease and key suppressor of hepatic hepcidin expression and cleaves HJV. Previous studies have therefore suggested that MT2 exerts its inhibitory effect by inactivating HJV. Here, we report that MT2 suppresses hepcidin expression independently of HJV. In mice, increased expression of exogenous MT2 in the liver significantly reduced hepcidin expression similarly as observed in wild-type mice. Exogenous MT2 could fully correct abnormally high hepcidin expression and iron deficiency in mice. In contrast to MT2, increased Hjv expression caused no significant changes in wild-type mice, suggesting that Hjv is not a limiting factor for hepcidin expression. Further studies revealed that MT2 cleaves ALK2, ALK3, ActRIIA, Bmpr2, Hfe, and, to a lesser extent, Hjv and Tfr2. MT2-mediated Tfr2 cleavage was also observed in HepG2 cells endogenously expressing MT2 and TfR2. Moreover, iron-loaded transferrin blocked MT2-mediated Tfr2 cleavage, providing further insights into the mechanism of Tfr2's regulation by transferrin. Together, these observations indicate that MT2 suppresses hepcidin expression by cleaving multiple components of the hepcidin induction pathway.
全身铁稳态通过十二指肠铁吸收的调节、红细胞铁循环以及肝脏铁动员来维持,并由肝脏激素铁调素控制。铁调素的表达通过骨形态发生蛋白(BMP)信号通路诱导,该通路优先使用两种I型(ALK2和ALK3)和两种II型(ActRIIA和BMPR2)BMP受体。血色素沉着症相关蛋白(HJV)、遗传性血色素沉着症蛋白(HFE)和转铁蛋白受体2(TfR2)可能通过与BMP受体形成质膜复合物来促进这一过程。Matriptase-2(MT2)是一种蛋白酶,是肝脏铁调素表达的关键抑制因子,可切割HJV。因此,先前的研究表明MT2通过使HJV失活发挥其抑制作用。在此,我们报告MT2独立于HJV抑制铁调素表达。在小鼠中,肝脏中外源MT2表达的增加显著降低了铁调素表达,与野生型小鼠中观察到的情况相似。外源性MT2可以完全纠正小鼠中异常高的铁调素表达和铁缺乏。与MT2相反,Hjv表达的增加在野生型小鼠中未引起显著变化,表明Hjv不是铁调素表达的限制因素。进一步的研究表明,MT2可切割ALK2、ALK3、ActRIIA、Bmpr2、Hfe,在较小程度上还可切割Hjv和Tfr2。在内源性表达MT2和TfR2的HepG2细胞中也观察到MT2介导的Tfr2切割。此外,铁负载的转铁蛋白可阻断MT2介导的Tfr2切割,为转铁蛋白对Tfr2的调节机制提供了进一步的见解。总之,这些观察结果表明MT2通过切割铁调素诱导途径的多个组分来抑制铁调素表达。