DNA polymerase POLD1 promotes proliferation and metastasis of bladder cancer by stabilizing MYC

Hitherto, most studies on DNA polymerase d (POLD1) have mainly focused on the effect of POLD1 inactivation mutation in tumors. Nonetheless, the mechanism underlying high POLD1 expression in tumorigenesis remains elusive. Herein, we substantiated the pro-carcinogenic role of POLD1 in bladder cancer (BLCA) and found that POLD1 expression is related to malignancy and prognosis of BLCA. Next, we demonstrated that POLD1 could promote proliferation and metastasis of BLCA via MYC in vivo and in vitro. Furthermore, POLD1 and MYC were colocalized in the nucleus and co-expressed during the cell cycle. Mechanistically, we validated that POLD1 could interact directly with MYC, and POLD1 could stabilize MYC by counteracting GSK3ß-mediated phosphorylation of threonine 58 and blocking the interaction between E3 ubiquitin ligase FBXW7 and MYC. Moreover, MYC could transcriptionally activate POLD1, forming a POLD1-MYC positive feedback loop to enhance the pro-carcinogenic effect of POLD1-MYC on BLCA. Overall, our study identified a novel MYC-driven oncogene POLD1, providing a potential diagnostic and therapeutic target for BLCA. Overall design: To investigate the downstream mechanism of POLD1 regulation of bladder cancer, we performed RNA sequencing on three repeated siNC and siPOLD1-treated 5637 cells.

迄今为止，针对DNA聚合酶d（DNA polymerase d, POLD1）的多数研究主要聚焦于其失活突变在肿瘤中的作用。然而，POLD1高表达在肿瘤发生过程中的潜在分子机制仍尚不明确。本研究中，我们证实了POLD1在膀胱癌（bladder cancer, BLCA）中的促癌作用，并发现POLD1的表达水平与膀胱癌的恶性程度及预后密切相关。后续研究证实，POLD1可通过MYC在体内及体外促进膀胱癌的增殖与转移。此外，POLD1与MYC可在细胞核中共定位，并在细胞周期进程中共表达。从机制层面而言，我们验证了POLD1可直接与MYC相互结合；同时POLD1能够通过拮抗糖原合成激酶3β（GSK3β）介导的苏氨酸58位点磷酸化，并阻断E3泛素连接酶FBXW7与MYC的相互作用，从而稳定MYC蛋白。此外，MYC可在转录层面激活POLD1的表达，由此形成POLD1-MYC正向反馈环路，以强化二者对膀胱癌的促癌作用。综上，本研究鉴定出一个新的由MYC驱动的癌基因POLD1，为膀胱癌的临床诊断与治疗提供了潜在靶点。实验设计：为探究POLD1调控膀胱癌的下游分子机制，我们对3次生物学重复的经阴性对照小干扰RNA（siNC）及靶向POLD1的小干扰RNA（siPOLD1）处理的5637细胞进行了RNA测序（RNA-seq）。