Activin Signal Transduction in Cell Growth, Differentiation and Apoptosis

属于TGF-β超家族的激活素与包含两种跨膜丝氨酸/苏氨酸激酶（称为I型和II型）的受体复合物结合并通过其发出信号。迄今为止，已经从哺乳动物来源中克隆了两种II型（ActR-IIA，-IIB）和两种I型（ActR-IA，IB）激活素受体。在本研究中，我们试图阐明I型受体在激活素信号传导中对生长、分化和凋亡的作用，并获得了以下结果。(1)我们通过电穿孔方法建立了稳定的过表达ActR-IA或ActR-IB的HS-72转染子。ActR-IA的过表达抑制了激活素诱导的细胞周期停滞在G1期，这是由通过诱导p21（一种细胞周期蛋白依赖性激酶抑制剂）抑制视网膜母细胞瘤蛋白磷酸化和随后的凋亡引起的。相反，过表达ACtR-IB的HS-72克隆显著促进激活素诱导的细胞凋亡。这些结果表明ActR-IA和ActR-IB在激活素信号转导中彼此不同。(2)在激活素诱导的HS-72细胞凋亡中观察到细胞骨架α-胞衬蛋白的显著降解。ActR-IA在HS-72细胞中的过表达可抑制α-胞衬蛋白的降解，提示激活素的凋亡信号与α-胞衬蛋白的蛋白降解有关。(3)发现卵泡抑素（激活素结合蛋白）促进激活素与细胞表面硫酸乙酰肝素的结合。当细胞<125>在卵泡抑素存在下与β I-激活素孵育时，观察到激活素的显著降解。这种激活素降解被硫酸乙酰肝素、氯喹和溶酶体蛋白酶抑制剂消除。这些结果表明，细胞相关卵泡抑素加速激活素进入细胞的摄取，导致溶酶体酶降解增加，因此在激活素清除系统中发挥作用。

Activin belonging to the TGF-beta superfamily binds to and signals through a receptor complex comprising two transmembrane serine/threonine kinases, called type I and type II.So far, two type II (ActR-IIA,-IIB) and two type I (ActR-IA,IB) activin receptors have been cloned from mammalian sources. In this study, we attempted to elucidate the role of type I receptors in activin signaling for growth, differentiation and apoptosis, and obtained following results.(1) We established stable HS-72 transfectants overexpressing ActR-IA or ActR-IB by the electroporation procedure. Overexpression of ActR-IA suppressed activin-induced cell-cycle arrest in the G1 phase caused by inhibition of retinoblastoma protein phosphorylation through induction of p21, a cyclin-dependent kinase inhibitor, and subsequent apoptosis. In contrast, HS-72 clones that overexpressed ACtR-IB significantly facilitated activin induced apoptosis. These results indicate that ActR-IA and ActR-IB are distinct from each other in activin signal transduction.(2) Significant degradation of a cytoskeleton, alpha-fodrin was observed upon activin-induced apoptosis in HS-72 cells. Overexpression of ActR-IA in HS-72 cells caused prevention of the alpha-fodrin degradation, suggesting that the apoptotic signal of activin in relevant to the proteolytic degradation of alpha-fodrin.(3) Follistatin (activin-binding protein) was found to promote the binding of activin to cell surface heparan sulfate. When the cells were incubated with ^<125>I-activin in the presence of follistatin, significant degradation of activin was observed. This activin degradation was abolished by heparan sulfate, chloroquine, and lysosomal protease inhibitors. These results indicate that cell-associated follistatin accelerates the uptake of activin into cells, leading to increased degradation by lysosomal enzymes, and thus plays a role in the activin clearance system.