Regulation of UBE4B and its Role in Response to DNA Damage in Cancer
Creators: Abuetabh, Yasser H
Publisher: name: University of Alberta Library
Year: 2023

<p>The human cell is equipped with highly intricate molecular signaling mechanisms to orchestrate the cellular responses to various stresses that endanger cellular genome integrity. Thus, genome integrity is always kept under surveillance to prevent and protect the cell from passing damaged DNA that may ultimately cause serious diseases, including cancer.<br>
          The tumor suppressor p53 is well known for its fundamental role in detecting and eradicating different oncogenic insults by promoting cell cycle arrest, DNA repair, senescence, and apoptosis. Hence, the p53 role falls in the hub of the DNA damage response pathway. Unsurprisingly, the p53 protein is frequently inactivated in most human tumors, leading to increased aggressiveness of the disease and an unfavorable overall survival rate.<br>
          Ubiquitination factor E4B (UBE4B) plays a pivotal role in negatively regulating p53 during homeostasis and after DNA damage. Recently, our research group showed that phosphorylated p53 (the active form of p53) is targeted by UBE4B for degradation in an Hdm2-independent manner in response to DNA damage. Thus, UBE4B inactivated the p53 response to DNA damage, leading to cell cycle progression and apoptosis attenuation. Therefore, it is not surprising that UBE4B is frequently overexpressed in many cancers, including breast, liver, and brain cancers. However, the regulation of UBE4B in response to DNA damage in cancer is still largely unknown.<br>
          Posttranslational modifications, specifically phosphorylation, modulate the activity of many proteins involved in the DNA damage response pathway. Therefore, I aimed to determine whether UBE4B is regulated through the phosphorylation/dephosphorylation process in response to DNA damage. I used a number of molecular techniques, including coimmunoprecipitation, transfection/infection, immunoblotting, in vivo ubiquitination assays, and flow cytometry analysis.<br>
          We identified two serine residues of the UBE4B protein phosphorylated in response to DNA damage. We developed antibodies that detect phosphorylated UBE4B at these residues. Our data demonstrated that the UBE4B protein was phosphorylated in response to DNA damage dominantly through upstream ATR- and ATM-mediated signaling. ATR- and ATM mediated phosphorylation of UBE4B decreased its affinity binding with p53 and led to the accumulation of p53. Phosphorylated UBE4B was mainly localized in the nucleus. Furthermore, I identified that wild-type p53 phosphatase 1 (Wip1) reversed UBE4B phosphorylation. UBE4B dephosphorylation by Wip1 seemed to be a critical step in stabilizing the activity of the UBE4B protein in response to DNA damage. Inhibition of Wip1 led to a significant increase in UBE4B phosphorylation and p53 accumulation. These data provide the first report demonstrating that the phosphorylation/dephosphorylation process regulates UBE4B protein activity in response to DNA damage.<br>
          The phosphorylation/dephosphorylation events of UBE4B in response to DNA damaging agents in cancer cells are biologically significant. I demonstrated that inhibition of the UBE4B regulatory pathway could sensitize cancer cells to radio- and chemotherapies and lead to favorable outcomes. Ultimately, understanding how UBE4B is regulated in cancer cells in response to DNA damaging agents may lead to the development of novel therapeutic strategies that could improve the prognosis of cancer patients.</p>