Objective Hepatocellular carcinoma(HCC)is a highly fatal cancer,and it is the second leading cause of cancer death worldwide among men.Despite liver transplantation and surgery are available for early-stage HCC,only 25%of patients are amenable to surgery.For it heterogeneity and multiple etiologies,HCC is known as a chemo-resistance tumor.Previous studies have demonstrated that cancer stem cells(CSCs),a small subpopulation of cells bearing progenitor cell-like features,possess both self-renewal and differentiation capabilities,which are responsible for tumor relapse,metastasis and drug resistance,resulting in disease progression and mortality.Aminopeptidase N(CD13)is a novel marker for liver CSC that is responsible for therapy resistance in HCC.However,functional characterization of CD13 and therapeutic utility of targeting it in HCC remained elusive.Methods CD13 expressions were evaluated various types of clinical samples including a tissue microarray containing 403 HCC patients.The effects of CD13 modulation on proliferation,cell cycle,invasion and xenograft tumor growth were determined.CD13 regulatory mechanism was dertermined by LC-MS/MS mass spectrometry,coimmunoprecipitation and gene expression profiling.The anticancer effect of targeting CD13 by ubenimex,a clinical used CD13 inhibitor,was examined in both sorafenib sensitive and resistant HCC cells and patient-derived xenografts(PDX).Results Overexpression of the CD13 protein was significantly correlated with elevated AFP,tumor encapsulation,microvascular invasion and high TNM stage.CD13 is a powerful prognostic marker of poor outcome in patients with HCC,especially for patients with early-stage disease and normal AFP as well as those whose prognoses are very difficult to predict by conventional clinical indexes.CD13 could positively regulate HCC cell proliferation,invasiveness and sorafenib resistance.By establishing our HCC PDX platform,we show that HCC tumors with higher CD13 expression are more prone to grow tumor in the mice,suggesting CD13 play a major role in driving tumor origin and growth.Mechanistically,CD13 interacts with HDAC5 to deacetylate NF-κ B p65 subunit to prevent it from degradation,which promote HCC progression and chemotherapy resistance.Ubenimex,a CD13 inhibitor,greatly suppress the tumor growth of HCC PDX models and showed synergetic effect with Quisinostat,a novel second-generation HDAC inhibitor of HDAC that could inhibits HDAC5 activity.Surprisingly,targeting CD13 effectively restored the sensitivity of HCC cells towards sorafenib in both sorafenib-resistant cell lines and PDX models.Consistently,clinical data also indicated that high expression of CD13 correlates with a worse prognosis in HCC patients following sorafenib treatment.Conclusions Our study identified,for the first time,that CD13 as a regulator in HCC progression and sorafenib resistance by forming CD13/HDAC5 complex and regulating NF-κ B p65 acetylation.Given that the synergistic effect of CD13 and HDAC5 inhibitors,we hope that our findings will motivate more clinical trials in HCC that involves these inhibitors.More importantly,CD13 inhibitor could reverse sorafenib resistance and enhance the effect of sorafenib.Elevated CD13 in HCC predicts shorter overall survival and more rapid recurrence of resectable HCC.Thus,better understanding of CD13 mediated signaling pathways would allow us to further elucidate the mechanisms of CSCs involving in HCC tumorigenesis and chemoresistance,therefore guiding better treatments for this lethal cancer.