Gang Wu, Zhenyu Ma, Jianmin Qian, Bin LiuDepartment of General Surgery, Huashan Hospital, Fudan University, Shanghai, People’s Republic of ChinaAbstract: Hepatocellular carcinoma (HCC), as the fifth most common cancer worldwide, has become the third leading cause of cancer-related deaths. It is reported that protein phosphatase 4 (PP4) is an essential protein for nucleation, growth, and stabilization of microtubules in centrosomes/spindle bodies during cell division. Besides, previous studies have identified protein phosphatase 4 regulatory subunit 1 (PP4R1) as a constitutive interaction partner of PP4 catalytic subunit PP4C. The PP4C-PP4R1 PP4 complex plays a role in dephosphorylation, regulation of histone acetylation, and NF-κB activation. However, little is known about the pathological functions of PP4R1 in human cancers. Thus, in order to investigate how PP4R1 functions in human HCC, two common hepatocarcinogenesis HCC cell lines HepG2 and SMMC-7721 were employed, transduced with recombinant lentivirus expressing PP4R1 short hairpin RNA. Compared with the controls, the cells treated with Lv-shPP4R1 showed a significant decrease in cell proliferation and colony formation. The results of flow cytometry showed that the knockdown of PP4R1 caused HepG2 cells arrest at G2/M phase in the cell cycle. Furthermore, the transduction of Lv-shPP4R1 into HepG2 cells led to the inactivation of two major mitogen-activated protein kinase signaling cascades: p38 and c-Jun N-terminal kinase (JNK), indicating that PP4R1 could promote cell proliferation, which might be regulated by p38 and c-Jun N-terminal kinase pathways. In a word, this study highlights the crucial role of PP4R1 in promoting HCC cell growth, which might elucidate the pathological mechanism of HCC.Keywords: hepatocellular carcinoma, PP4R1, shRNA, proliferation, apoptosis