Deficiency in Embryonic Stem Cell Marker Reduced Expression 1 Activates Mitogen‐Activated Protein Kinase Kinase 6–Dependent p38 Mitogen‐Activated Protein Kinase Signaling to Drive Hepatocarcinogenesis

Background and Aims Embryonic stem‐cell–related transcription factors are central to the establishment and maintenance of stemness and pluripotency, and their altered expression plays key roles in tumors, including hepatocellular carcinoma (HCC), a malignancy with no effective treatment. Here, we report on the embryonic stem cell marker, reduced expression 1 (REX1; also known as zinc finger protein 42), to be selectively down‐regulated in HCC tumors. Approach and Results Deficiency of REX1 in HCC was attributed to a combination of hypermethylation at its promoter as well as histone modification by methylation and acetylation. Clinically, hypermethylation of REX1 was closely associated with neoplastic transition and advanced tumor stage in humans. Functionally, silencing of REX1 potentiated the tumor‐initiating and metastasis potential of HCC cell lines and xenografted tumors. Lentivirus‐mediated Rex1 ablation in liver of male immunocompetent mice with HCC, induced by hydrodynamic tail vein injection of proto‐oncogenes, enhanced HCC development. Transcriptome profiling studies revealed REX1 deficiency in HCC cells to be enriched with genes implicated in focal adhesion and mitogen‐activated protein kinase (MAPK) signaling. From this lead, we subsequently found REX1 to bind to the promoter region of mitogen‐activated protein kinase kinase 6 (MKK6), thereby obstructing its transcription, resulting in altered p38 MAPK signaling. Conclusions Our work describes a critical repressive function of REX1 in maintenance of HCC cells by regulating MKK6 binding and p38 MAPK signaling. REX1 deficiency induced enhancement of p38 MAPK signaling, leading to F‐actin reorganization and activation of nuclear factor erythroid 2‐related factor 2–mediated oxidative stress response, which collectively contributed to enhanced stemness and metastatic capabilities of HCC cells.