Sertad1 Antagonizes iASPP Function By Hindering Its Entrance into Nuclei to Interact with P53 in Leukemic Cells
Authors: Qiu Shaowei , Yu Jing , Yu Tengteng , Xing Haiyan , An Na , Jia Yujiao , Liu Shuang , Tang Kejing , Tian Zheng , Rao Qing , Mi Yingchang , Wang Min , Wang Jianxiang 
Published: October 2019

Abstract
Introduction: As the important suprressor of P53, iASPP was found to be overexpressed in leukemia, and functioned as oncogene that inhibited apoptosis of leukemia cells. Sertad1 is identified as one of the proteins that can bind with iASPP in our previous study by two-hybrid screen. Sertad1 is highly expressed in carcinomas from pancreatic, lung and ovarian tissues, which considered Sertad1 as an oncoprotein. In this study, our findings revealed that Sertad1 could interact with iASPP in the cytoplasm near nuclear membrane, which could block iASPP to enter into nucleus to interact with P53, and inhibited the function of iASPP eventually.
Methods: Co-immunoprecipitation and fluorescence confocal microscopic imaging were used to confirm the interaction between iASPP and Sertad1, the exact binding domains and the subcellular colocalization.The plasmids of iASPP and Sertad1 were transfected alone or co-transfected into K562 cells, the stable subclones that highly expressed iASPP, Sertad1 or both of them were then established by limiting dilution and named as K562-iASPPhi, K562-Sertad1hi, and K562-Douhi, respectively. The cell proliferation, cell cycle and apoptosis of above subclones were investigated by flow cytometry. Further, silence of the above two proteins was performed to confirm their functions. Immunoblotting analysis and immunofluorescence were performed to explore the possible mechanisms of difference between the biological functions of the above subclones.
Results: Sertad1 expression level varied in leukemic cell lines and AML patients irrespectively of iASPP and P53. Interaction between iASPP and Sertad1 did exist in 293 cell and leukemic cells, both iASPP and Sertad1 scattered in the cytoplasm and nucleus, and their colocalizations were mainly in the cytoplasm, which encircled the nucleus. iASPP binds directly to Sertad1 through its PHD-bromo domain, C-terminal domain and Cyclin-A domain in a reduced order, and Serta domain failed to bind to iASPP.
Overexpression of iASPP in K562 cells (iASPPhi) could result in the increased cell proliferation, cell cycle arrest in G2/M phase and resistance to apoptosis induced by chemotherapy drugs. While overexpression of iASPP and Sertad1 at the same time (Douhi) could slow down the cell proliferation, lead the cells more vulnerable to the chemotherapy drugs. As figure showed, in K562-Douhi cells, both iASPP and Sertad1 were obviously located in the cytoplasm, which encircled the nuclei, the subcellular colocalization was nearly outside the nuclei. The immunoblotting analysis further supported the conclusions. The resistance of iASPP to chemotherapeutic drug was accompanied by Puma protein expression in a p53-independent manner. By knocking down the expersssion of iASPP and Sertad separately, we found that iASPP is dispensable for maintenance of anti-apoptotic function and Sertad1 is indispensable for cell cycle in leukemic cells.
Conclusions: In normal situation, the protein iASPP and Sertad1 scatter in the nucleus and cytoplasm, mainly in the cytoplasm. As convinced by our study, iASPP was overexpressed in the leukemia cell lines and primary AML patients, it could function as oncogene through its binding with P53 protein in the nucleus, inhibit the function of P53. When iASPPhi cells were exposed to apoptosis stimuli, Puma protein could play an important role in this process, irrespective of the expression level of P53. But when iASPP and Sertad1 were both overexpressed in the leukemic cells, Sertad1 could tether iASPP outside the nucleus mainly through its PHD-bromo domain, prevent it from inhibiting P53 function, suppress the leukemic cell growth and stimulate cell apoptosis by rescuing the P53 eventually. Our data provided a new insight to overcome iASPP protein, namely through its binding partners, when the similar proteins or drugs that can tether iASPP outside the nucleus such as Sertad1 are transfected into the leukemic cells, it may restore p53 function to eliminate the leukemic cells.
Figure 1 Figure 1.

Disclosures
Wang: Novartis: Consultancy; Bristol Myers Squibb: Consultancy.