Su F, Schneider R J
Department of Biochemistry and Kaplan Cancer Center, New York University School of Medicine, New York 10016, USA.
J Virol. 1996 Jul;70(7):4558-66. doi: 10.1128/JVI.70.7.4558-4566.1996.
The HBx protein is a small polypeptide encoded by mammalian hepadnaviruses that is essential for viral infectivity and is thought to play a role in development of hepatocellular carcinoma during chronic hepatitis B virus infection. HBx is a transactivator that stimulates Ras signal transduction pathways in the cytoplasm and certain transcription elements in the nucleus. To better understand the activities of HBx protein and its mechanism of action, we have explored the manner by which HBx activates the transcription factor NF-kappaB during transient expression. We show that HBx induces prolonged formation, in a Ras-dependent manner, of transcriptionally active NF-kappaB DNA-binding complexes, which make up the family of Rel-related proteins, p50, p52, RelA, and c-Rel. HBx was found to activate NF-kappaB through two distinct cytoplasmic pathways by acting on both the 37-kDa IkappaBalpha inhibitor and the 105-kappaDa NF-kappaB1 precursor inhibitor protein, known as p105. HBx induces phosphorylation of IkappaBalpha, a three- to fourfold reduction in IKBalpha stability, and concomitant nuclear accumulation of NF-kappaB DNA-binding complexes, similar to that reported for human T-cell leukemia virus type 1 Tax protein. In addition, HBx mediates a striking reduction in cytoplasmic p105 NF-kappaB1 inhibitor and p50 protein levels and release of RelA protein that was sequestered by the p105 inhibitor, concomitant with nuclear accumulation of NF-kappaB complexes. HBx mediated only a slight reduction in the cytoplasmic levels of NF-kappaB2 p100 protein, an additional precursor inhibitor of NF-kappaB, which is thought to be less efficiently processed or less responsive to release of NF-kappaB. No evidence was found for HBx activation of NF-kappaB by targeting acidic sphingomyelinase- controlled pathways. Studies also suggest that stimulation of NF-kappaB by HBx does not involve activation of Ras via the neutral sphingomyelin-ceramide pathway. Thus, HBx protein is shown to activate the NF-kappaB family of Rel-related proteins by acting on two distinct NF-kappaB cytoplasmic inhibitors.
乙肝病毒X蛋白（HBx蛋白）是一种由哺乳动物嗜肝DNA病毒编码的小多肽，对病毒感染性至关重要，并且被认为在慢性乙型肝炎病毒感染期间肝细胞癌的发展中发挥作用。HBx是一种反式激活因子，可刺激细胞质中的Ras信号转导途径以及细胞核中的某些转录元件。为了更好地理解HBx蛋白的活性及其作用机制，我们探讨了HBx在瞬时表达过程中激活转录因子核因子κB（NF-κB）的方式。我们发现，HBx以Ras依赖的方式诱导转录活性NF-κB DNA结合复合物的长时间形成，这些复合物由Rel相关蛋白家族、p50、p52、RelA和c-Rel组成。研究发现，HBx通过作用于37 kDa的κBα抑制蛋白和105 kDa的NF-κB1前体抑制蛋白（称为p105），通过两条不同的细胞质途径激活NF-κB。HBx诱导κBα的磷酸化，使IκBα稳定性降低三到四倍，并伴随NF-κB DNA结合复合物的核内积累，这与人类1型T细胞白血病病毒Tax蛋白的情况类似。此外，HBx介导细胞质中p105 NF-κB1抑制蛋白和p50蛋白水平显著降低，以及被p105抑制蛋白隔离的RelA蛋白释放，同时伴随NF-κB复合物的核内积累。HBx仅使NF-κB2 p100蛋白（NF-κB的另一种前体抑制蛋白，被认为加工效率较低或对NF-κB释放反应较弱）的细胞质水平略有降低。未发现HBx通过靶向酸性鞘磷脂酶控制的途径激活NF-κB的证据。研究还表明，HBx对NF-κB的刺激不涉及通过中性鞘磷脂-神经酰胺途径激活Ras。因此，研究表明HBx蛋白通过作用于两种不同的NF-κB细胞质抑制剂来激活Rel相关蛋白的NF-κB家族。
