Rada Miran, Althubiti Mohammad, Ekpenyong-Akiba Akang E, Lee Koon-Guan, Lam Kong Peng, Fedorova Olga, Barlev Nickolai A, Macip Salvador
Department of Molecular and Cell Biology, Mechanisms of Cancer and Aging Laboratory, University of Leicester, Leicester, UK.
Department of Biochemistry, Faculty of Medicine, Umm Al-Qura University, Mecca, Saudi Arabia.
Oncotarget. 2017 Nov 20;8(63):106639-106647. doi: 10.18632/oncotarget.22543. eCollection 2017 Dec 5.
p53 is a tumour suppressor that is activated in response to various types of stress. It is regulated by a complex pattern of over 50 different post-translational modifications, including ubiquitination by the E3 ligase MDM2, which leads to its proteasomal degradation. We have previously reported that expression of Bruton's Tyrosine Kinase (BTK) induces phosphorylation of p53 at the N-terminus, including Serine 15, and increases its protein levels and activity. The mechanisms involved in this process are not completely understood. Here, we show that BTK also increases MDM2 and is necessary for MDM2 upregulation after DNA damage, consistent with what we have shown for other p53 target genes. Moreover, we found that BTK binds to MDM2 on its PH domain and induces its phosphorylation. This suggested a negative regulation of MDM2 functions by BTK, supported by the fact BTK expression rescued the inhibitory effects of MDM2 on p53 transcriptional activity. Indeed, we observed that BTK mediated the loss of the ubiquitination activity of MDM2, a process that was dependent on the phosphorylation functions of BTK. Our data together shows that the kinase activity of BTK plays an important role in disrupting the MDM2-p53 negative feedback loop by acting at different levels, including binding to and inactivation of MDM2. This study provides a potential mechanism to explain how BTK modulates p53 functions.
p53是一种肿瘤抑制因子，可在应对各种类型的应激时被激活。它受50多种不同翻译后修饰的复杂模式调控，包括E3连接酶MDM2介导的泛素化，这会导致其经蛋白酶体降解。我们之前报道过，布鲁顿酪氨酸激酶（BTK）的表达会诱导p53在N端（包括丝氨酸15）磷酸化，并增加其蛋白水平和活性。这一过程所涉及的机制尚未完全明确。在此，我们表明BTK还会增加MDM2的表达，并且在DNA损伤后对MDM2的上调是必需的，这与我们在其他p53靶基因上的研究结果一致。此外，我们发现BTK在其PH结构域与MDM2结合并诱导其磷酸化。这表明BTK对MDM2的功能具有负调控作用，BTK表达挽救了MDM2对p53转录活性的抑制作用这一事实也支持了这一点。实际上，我们观察到BTK介导了MDM2泛素化活性的丧失，这一过程依赖于BTK的磷酸化功能。我们的数据共同表明，BTK的激酶活性通过在不同层面发挥作用，包括与MDM2结合并使其失活，在破坏MDM2-p53负反馈环中发挥重要作用。这项研究提供了一种潜在机制来解释BTK如何调节p53的功能。
