Down-regulation of p57 (KIP2) cyclin-dependent kinase inhibitors accelerates the growth and invasion of hepatocellular carcinoma (HCC), suggesting that p57 may play an important role in liver carcinogenesis. However, the mechanism or oncogenic signal leading to p57 down-regulation in HCC remains to be determined. Herein, we demonstrated that Jab1/Csn5 expression is negatively correlated with p57 levels in HCC tissues. Kaplan-Meier analysis of tumor samples revealed that high Jab1/Csn5 expression with concurrent low p57 expression is associated with poor overall survival. The inverse pattern of Jab1 and p57 expression was also observed during carcinogenesis in a chemically induced rat HCC model. We also found that mechanistically, Jab1-mediated p57 proteolysis in HCC cells is dependent on 26S-proteasome inhibitors. We further demonstrated that direct physical interaction between Jab1 and p57 triggers p57 down-regulation, independently of Skp2 and Akt pathways, in HCC cells. These data suggest that Jab1 is an important upstream negative regulator of p57 and that aberrant expression of Jab1 in HCC could lead to a significant decrease in p57 levels and contribute to tumor cell growth. Furthermore, restoration of p57 levels induced by loss of Jab1 inhibited tumor cell growth and further increased cell apoptosis in HCC cells. Moreover, silencing Jab1 expression further enhanced the antitumor effects of cisplatin-induced apoptosis in HCC cells. Conclusion: Jab1-p57 pathway confers resistance to chemotherapy and may represent a potential target for investigational therapy in HCC. (HEPATOLOGY 2016;63:898-913) H epatocellular carcinoma (HCC) is the sixth most prevalent cancer in the world and is the third leading cause of cancer-related mortality. (1) Only 10% to 20% of HCC tumors can be removed completely by surgery in patients with HCC, and the reported cumulative 5-year recurrence rate after curative hepatectomy averages above 70% in both Eastern and Western medical centers. (2) At present, there is not only a lack of predictive and prognostic biomarkers in HCC patients but also a lack of effective therapeutic targets. Systemic treatment approaches are still the main option for patients with advanced HCC. Cisplatin-based regimens result in higher objective response rates than do non-cisplatin-based regimens in advanced HCC. However, the 15% response rate significantly limits the overall therapeutic benefit. (3) Abbreviations: CDK, cyclin-dependent kinase; HCC, hepatocellular carcinoma; siRNA, small interfering RNA; TCGA, the Cancer Genome Atlas; TGF-b, transforming growth factor b.