核糖体是由核糖体RNA和核糖体蛋白组成的复合体,其功能是参与蛋白质合成.SUMO化修饰的底物蛋白对核糖体的形成有重要调控作用.前期研究发现,KRAB型锌指蛋白Apak能特异地抑制p53所介导的凋亡通路.进一步研究发现,在核仁应激及癌基因激活条件下,抑癌蛋白ARF促进Apak发生SUMO化修饰并促使其移位于核仁.为了进一步探讨SUMO化修饰的Apak对核糖体RNA合成的调控功能,本研究通过Northern blot检测SUMO化修饰的Apak对核糖体RNA合成的影响,实时定量PCR检测核糖体RNA转录水平,RNA-Ch IP方法检测核糖体RNA与Apak蛋白的相互作用,结果表明,SUMO化修饰的Apak抑制47S核糖体RNA前体的合成且抑制RNA聚合酶Ⅰ介导转录的18S和5.8S r RNA的合成;在放线菌素D以及癌基因诱导下,促进Apak与18S,5.8S r RNA相互作用.本研究对理解Apak的功能和作用机制提供了新的依据,为深入研究KRAB型锌指蛋白家族分子对核糖体RNA的调控奠定了基础.
The tumor suppressor p53 locates at the key point of cell growth or apoptosis balance, and the expression level of p53 is tightly controlled by ubiquitin ligases including MDM2. Upon DNA damage stresses, p53 was accumulated and activated, leading to cell cycle arrest or apoptosis. We previously showed that Smad ubiquitylation regulatory factor 1/2 (Smurf1/2) promotes p53 degradation by interacting with and stabilizing MDM2, and consequently enhancing MDM2-mediated ubiquitylation of p53. However, it is unclear how the Smurf1-MDM2 interaction is regulated in response to DNA damage stress. Here, we show that in response to etoposide treatment Smurf1 dissociates from MDM2, resulting in MDM2 destabilization and p53 accumulation. The negative regulation of Smurf1 on apoptosis is released. Notably, this dissociation is a slow process rather than a rapid response, implicating high expression of Smurf1 might confer the resistance against p53 activation. Consistent with this notion, we observed that Smurf1/2 ligases are highly expressed in colon cancer, esophageal squamous cell carcinoma and pancreatic cancer tissues, suggesting the oncogenic tendency of Smurf1/2.
