PLK1 and HOTAIR Accelerate Proteasomal Degradation of SUZ12 and ZNF198 during Hepatitis B Virus–Induced Liver Carcinogenesis

Abstract

Elucidating mechanisms of hepatitis B virus (HBV)-mediated hepatocarcinogenesis is needed to gain insights into the etiology and treatment of liver cancer. Cells where HBV is replicating exhibit increased expression of Plk1 kinase and reduced levels of two transcription repression factors, SUZ12 and ZNF198. SUZ12 is an essential subunit of the transcription repressive complex PRC2. ZNF198 stabilizes the transcription repressive complex composed of LSD1, Co-REST and HDAC1. These two transcription repressive complexes are held together by binding the long noncoding RNA HOTAIR. In this study we linked these regulatory events mechanistically, by showing that Plk1 induces proteasomal degradation of SUZ12 and ZNF198 by site-specific phosphorylation. Plk1-dependent ubiquitination of SUZ12 and ZNF198 was enhanced by expression of HOTAIR, significantly reducing SUZ12 and ZNF198 stability. In cells expressing the HBV X protein (HBx) downregulation of SUZ12 and ZNF198 mediated global changes in histone modifications. In turn, HBx-expressing cells propagated an altered chromatin landscape after cell division, as exemplified by changes in histone modifications of the EpCAM promoter, a target of PRC2 and LSD1/Co-REST/HDAC1 complexes. Notably, liver tumors from X/cmyc bitransgenic mice exhibited downregulation of SUZ12 and ZNF198 along with elevated expression of Plk1, HOTAIR, and EpCAM. Clinically, similar effects were documented in a set of HBV-related liver tumors consistent with the likelihood that downregulation of SUZ12 and ZNF198 leads to epigenetic reprogramming of infected hepatocytes. Since both Plk1 and HOTAIR are elevated in many human cancers, we propose that their combined effects are involved in epigenetic reprogramming associated broadly with oncogenic transformation.

The role of long non-coding RNAs in hepatitis B virus-related hepatocellular carcinoma

“…Transfection of cells with plasmids that express constitutively active Plk1 decreased the half-life of ZNF198 and SUZ12, which is consistent with the notion that phosphorylation and ubiquitination destabilize the proteins (H. Zhang et al, 2015). The shortened half-life was further reduced in cells overexpressing HOTAIR, but the action of the lncRNA was not dependent on Plk1.…”

“…Since HOTAIR, SUZ12 and ZNF198 are all associated through interaction with the PRC2 or LSD1/Co-REST/HDAC1 complexes, a functional link between HOTAIR and the ubiquitination process was investigated (H. Zhang et al, 2015). Transfection of cells with plasmids that express constitutively active Plk1 decreased the half-life of ZNF198 and SUZ12, which is consistent with the notion that phosphorylation and ubiquitination destabilize the proteins (H. Zhang et al, 2015).…”

“…An inverse relationship between expression of the mitotic polo-like kinase1 (Plk1) and concentrations of SUZ12 and ZNF198 has been reported (Golsteyn et al, 1994), and this interaction has been implicated in transformation of hepatocytes that is caused by HBx (Studach et al, 2012). In a recent detailed analysis of this putative hepatocarcinogenic mechanism, Zhang et al observed that changes to the phosphorylation status of both SUZ12 and ZNF198 are mediated by the site-specific activity of Plk1 (H. Zhang et al, 2015). The phosphorylation disrupted interaction of ZNF198 with the LSD1/Co-REST/HDAC1 complex and phosphorylation of SUZ12 inhibited its binding to PRC2.…”

“…Zhang and colleagues also sought to validate their findings by quantifying expression of Plk1 and HOTAIR in liver tumors from bitransgenic mice that overexpressed c-myc and X (H. Zhang et al, 2015). Increased concentrations of both Plk1 mRNA and HOTAIR were observed in cancers from the murine samples.…”

Up-regulated HOTAIR induced by fatty acids inhibits PTEN expression and increases triglycerides accumulation in HepG2 cells

“…Although the expression of HOTAIR has not been explored in NAFLD, some studies underlined its role in other kinds of liver disease and HCC. Zhang et al [32] showed that HOTAIR accelerated carcinogenesis in hepatitis B-infected liver. Ding et al [33] showed that HOTAIR promote the migration and invasion of HCC cells, while Ishibashi et al [34] showed that HOTAIR expression is associated with HCC progression.…”