PPM1H is Down-Regulated by ATF6 and Dephosphorylates p-RPS6KB1 to Inhibit Progression of Hepatocellular Carcinoma

Abstract Background: Our previous studies found that polymorphism of activating transcription factor 6 (ATF6) was associated with susceptibility to hepatocellular carcinoma (HCC), and ATF6 promoted HCC progression. So genes down-regulated by ATF6 might play a tumor suppressing role. In present study, we aimed to identify downstream genes down-regulated by ATF6, and clarified its role in HCC progression. Methods: To determine the role of ATF6 on downstream gene expression, qRT-PCR and western blot were performed in HCC cells and liver specifically Atf6 -knocked out mice. Tumor tissues from 134 HCC patients were analyzed by immunohistochemistry. Then, MTT, transwell and clone formation assays were performed in hepatoma cells. DEN/CCl 4 induced HCC mouse model and nude mice tumorigenicity assay were conducted to investigate the effect of PPM1H on HCC tumorigenesis and development. 3D model of PPM1H was obtained by homology multi-template modeling, the potential substrates of PPM1H were screened by Rosetta and verified through co-immunoprecipitation, western blot and in vitro phosphatase assays. Results: The expression of Protein phosphatase magnesium or manganous dependent 1H (PPM1H) mRNA and protein can be inhibited by ATF6. PPM1H inhibited proliferation, migration, invasion and clone formation of cancer cells. PPM1H inhibited induced HCC nodules formation and tumor xenograft growth in two mouse models. The protein docking energy of RPS6KB1 with PPM1H in the BMP/TGFβ pathway is the lowest, and PPM1H directly dephosphorylated p-RPS6KB1. PPM1H exhibited higher expression level in adjacent para-cancer tissues than in HCC tissues. Patients with higher expression of PPM1H indicated a better prognosis. Conclusions: We first discovered RPS6KB1 as a new PPM1H dephosphorylation substrate. PPM1H exhibited suppressive effect on the progression of HCC through dephosphorylating p-RPS6KB1. This might contribute to finding novel target for therapy of HCC.