激酶TTK/hMps1藉由磷酸化MDM2調控組織蛋白H2B泛素化以及氧化壓力下的DNA損傷及修復反應

Abstract

MDM2為一著名的E3泛素連接酶，而其最為著名的下游受體為抑癌蛋白p53。然而除了p53外，已經有許多MDM2的下游受體被報導，而其中有許多是參與在細胞週期調控的重要蛋白。但是關於MDM2的活性調控，尤其是本篇論文提到的遭遇氧化壓力的環境，相關的作用機制並沒有被真正的了解。在本篇論文，我們發現一著名的調控紡錘絲檢查點的激酶，hMps1，會磷酸化MDM2，進而增加MDM2對於下游受體，組蛋白子單位H2B的泛素化。透過氨基丙酸 (Ala) 突變在hMps1磷酸化的位點，會降低經由hMps1磷酸化而增加的組蛋白H2B的泛素化。在細胞內，H2B的泛素化會由於氧化壓力而增加而降低hMPs1或MDM2的表現，會降低H2B的泛素化。補回野生型 (WT) MDM2能夠回復H2B泛素化，不過如果補回的是氨基丙酸突變在Mps1磷酸化的位點的MDM2，H2B的泛素化回復的效率會變差。此實驗證明了Mps1磷酸化MDM2對於組蛋白H2B的泛素化的重要性。而本篇論文也證明了，hMps1及MDM2藉由調控H2B的泛素化，來幫助細胞在氧化壓力下的DNA損傷反應及修復。

MDM2 is an E3 ubiquitin ligase that targets proteins involved in cell cycle progression. Although many MDM2 substrates have been identified, how its activity is regulated is not fully understood, especially when cells are under oxidative stress. Here we show that upon oxidative stress, MDM2 can be phosphorylated by human Mps1 (hMps1)/TTK, a kinase known to function in the spindle assembly and DNA damage checkpoints. Consequently, MDM2-mediated H2B ubiquitination is enhanced, and Ala substitution at the hMps1/TTK phosphorylation sites compromises this effect. In cells, H2B ubiquitination was enhanced upon oxidative stress. Depletion of hMps1/TTK or MDM2 not only abrogated this induction but also resulted in reduced DNA repair, cell survival, and ATR signaling. Consistent with these observations, cells expressing a ubiquitination-deficient H2B mutant were also defective in DNA repair and ATR signaling. Complementation with WT MDM2 but not the phospho-deficient mutant rescued the H2B ubiquitination and DNA repair, suggesting the requirement for hMps1/TTK-mediated MDM2 phosphorylation. Taken together, we propose that hMps1/TTK phosphorylates MDM2 upon oxidative damage to promote H2B ubiquitination, which in turn facilitates oxidative DNA damage signaling and repair.