Ribonucleotide reductase (RR)M2B is an enzyme belonging to the ribonucleotide reductase enzyme family, which is essential for DNA synthesis and repair. RRM2B plays an important role in tumor progression and metastasis; however, little is known about the expression and underlying molecular mechanisms of RRM2B in hepatocellular carcinoma (HCC). In the present study, we report that down-regulation of RRM2B in HCC is negatively associated with intrahepatic metastasis, regardless of p53 status. Moreover, the ectopic overexpression of RRM2B decreased HCC cell migration and invasion in vitro, whereas silencing RRM2B expression resulted in increased migration and invasion in vitro and intrahepatic and lung metastasis in vivo. Additionally, knockdown of RRM2B by short hairpin RNA (shRNA) in HCC cells was associated with epithelialmesenchymal transition (EMT), including the down-regulation of E-cadherin, and the concomitant up-regulation of N-cadherin and slug. A further experiment showed that RRM2B inhibited cell migration and spreading through regulation of the early growth response protein 1 (Egr-1) / phosphatase and tensin homolog (PTEN) / Akt1 pathway. Consistently, we also detected a significant correlation between RRM2B and E-cadherin protein expression in HCC tissues. Furthermore, Egr-1 also directly bound to the RRM2B promoter and repressed RRM2B transcription, thereby establishing a negative regulatory feedback loop. Conclusion: These findings indicate that RRM2B suppresses cell migration and spreading by way of modulation of the Egr-1/PTEN/Akt1 pathway. (HEPATOLOGY 2014;59:1459-1470 H uman hepatocellular carcinoma (HCC) is the most common malignant tumor in the liver and third leading fatal cancer.1 The prognosis for HCC remains poor, mainly due to the propensity for metastatic progression and poor response to pharmacological treatment.2 In HCC, the liver is the major target organ of metastasis, which is known as intrahepatic metastasis. The portal vein is the main route for intrahepatic metastases of HCC cells in animal model systems and in human patients. 3,4 Therefore, the identification of metastatic factors and elucidation of the underlying molecular mechanism that are involved in the progression of metastasis become critical issues.