In the present study, high throughput screening we identified EGCG as a novel inhibitor of human CBR1 with an IC50 value of 0.59 μM. Its potency against CBR1 compares

favorably with the potency of the known natural flavonoid inhibitors of the same enzyme, including quercetin, kaempferol, quercitrin, and genistein with IC50 values between 1 and 10 μM, and the synthetic inhibitor hydroxy-PP with an IC50 value of 0.79 μM.20, 24 Unlike most known inhibitors of CBR1, however, EGCG is already taken by humans through tea and other beverages, and purified EGCG and its analogues have been entered into different clinical trials for cancer chemoprevention and treatment; this is paving the way for EGCG to be evaluated for HCC in light of this study. EGCG is the most abundant and active compound with anticancer activity in tea. The mechanism for the cancer-preventive effect of EGCG is still under active investigation. Several putative binding proteins, including salivary proline-rich proteins, fibronectin,

fibrinogen, and histidine-rich glycoproteins, have been identified; more recently, proteins such as the 67-kDa laminin receptor,25 B cell lymphoma 2,26 vimentin,27 insulin-like high throughput screening assay growth factor 1 receptor,28 FYn,29 glucose-regulated protein 78,30 and zeta chain associated protein kinase 70,31 among others, have been identified. None of these putative EGCG-binding proteins, however, can account for the inhibition of reduction of DNR by EGCG, except for CBR1, which is identified in this study. It has been reported that EGCG alone can inhibit the growth of human HCC cell lines in vitro and induce apoptosis in HCC cells,32 and this is consistent with our observations (Supporting Information Figs. 4 and 7). The inhibitory effect selleck of EGCG on HCC xenografts has also been shown to be associated with inhibition of the vascular endothelial growth factor/vascular endothelial growth factor receptor axis.33 The aim of our research was to test the synergic

effect of EGCG on DNR by inhibiting CBR1. We therefore avoided using toxic EGCG concentrations and deliberately selected lower doses of EGCG that showed minimal toxicity in HCC cells. Several lines of evidence suggest that inhibition of CBR1 by EGCG is responsible for its ability to block DNR resistance and its synergy with DNR for the inhibition of HCC both in vitro and in vivo. EGCG specifically enhanced DNR-induced G2/M phase cell cycle arrest and cell apoptosis in HCC cells with higher CBR1 expression such as HepG2 cells. In contrast, knockdown of CBR1 expression in HepG2 cells by RNAi recapitulated EGCG’s effect. On the other hand, Hep3B with spontaneously lower expression of CBR1 exhibited little synergistic response to EGCG and DNR. Overexpression of CBR1 in Hep3B conferred resistance to DNR, which was overcome by EGCG.