Speckmann Bodo, Walter Philippe L, Alili Lirija, Reinehr Roland, Sies Helmut, Klotz Lars-Oliver, Steinbrenner Holger
Institute for Biochemistry and Molecular Biology I, Hepatology and Infectiology, Heinrich-Heine-University, Düsseldorf, Germany.
Hepatology. 2008 Dec;48(6):1998-2006. doi: 10.1002/hep.22526.
Selenoprotein P (SeP), the major selenoprotein in plasma, is produced mainly by the liver, although SeP expression is detected in many organs. Recently, we reported stimulation of SeP promoter activity by the forkhead box transcription factor FoxO1a in hepatoma cells and its attenuation by insulin. Here, we demonstrate that this translates into fine-tuning of SeP production and secretion by insulin. Overexpression of peroxisomal proliferator activated receptor-gamma coactivator 1alpha (PGC-1alpha) enhanced the stimulatory effect of FoxO1a on SeP promoter activity. We identified a novel functional binding site for hepatocyte nuclear factor (HNF)-4alpha, termed hepatocyte nuclear factor binding element 1, in the human SeP promoter directly upstream of the FoxO-responsive element daf16-binding element 2 (DBE2). Point mutations in hepatocyte nuclear factor binding element 1 alone or together with DBE2 decreased basal activity and responsiveness of the SeP promoter to PGC-1alpha. Moreover, the PGC-1alpha-inducing glucocorticoid dexamethasone strongly enhanced SeP messenger RNA levels and protein secretion in cultured rat hepatocytes, whereas insulin suppressed the stimulation of both PGC-1alpha and SeP caused by dexamethasone treatment. In a brain-derived neuroblastoma cell line with low basal SeP expression, SeP transcription was stimulated by PGC-1alpha together with FoxO1a, and overexpression of HNF-4alpha potentiated this effect.
High-level expression of SeP in liver is ensured by concerted action of the coactivator PGC-1alpha and the transcription factors FoxO1a and HNF-4alpha. Hence, the production of SeP is regulated similarly to that of the gluconeogenic enzyme glucose-6-phosphatase. As hepatic SeP production is crucial for selenium distribution throughout the body, the present study establishes PGC-1alpha as a key regulator of selenium homeostasis.
硒蛋白P（SeP）是血浆中的主要硒蛋白，主要由肝脏产生，尽管在许多器官中都检测到了SeP的表达。最近，我们报道了叉头框转录因子FoxO1a在肝癌细胞中刺激SeP启动子活性，而胰岛素则使其减弱。在此，我们证明这转化为胰岛素对SeP产生和分泌的精细调节。过氧化物酶体增殖物激活受体γ共激活因子1α（PGC-1α）的过表达增强了FoxO1a对SeP启动子活性的刺激作用。我们在人SeP启动子中位于FoxO反应元件daf16结合元件2（DBE2）直接上游的位置鉴定出一个新的肝细胞核因子（HNF）-4α功能性结合位点，称为肝细胞核因子结合元件1。单独或与DBE2一起的肝细胞核因子结合元件1中的点突变降低了SeP启动子的基础活性以及对PGC-1α的反应性。此外，诱导PGC-1α的糖皮质激素地塞米松强烈提高了培养的大鼠肝细胞中SeP信使RNA水平和蛋白质分泌，而胰岛素抑制了地塞米松处理引起的PGC-1α和SeP的刺激。在基础SeP表达较低的脑源性神经母细胞瘤细胞系中，PGC-1α与FoxO1a一起刺激SeP转录，并且HNF-4α的过表达增强了这种作用。
共激活因子PGC-1α与转录因子FoxO1a和HNF-4α的协同作用确保了肝脏中SeP的高水平表达。因此，SeP的产生与糖异生酶葡萄糖-6-磷酸酶的产生受到类似的调节。由于肝脏SeP的产生对于硒在全身的分布至关重要，本研究确立了PGC-1α作为硒稳态的关键调节因子。
