Curcumin is a common chemical ingredient of curry. It has, however, been studied in clinical trials regarding its applicability in treating patients suffering from pancreatic and colon cancer, as well as multiple myeloma (37
). In Taiwan, several doctors of traditional Chinese medicine consider curcumin to be beneficial for patients suffering from hepatitis. The results of this study demonstrate that curcumin inhibits HCV replication in cellular analysis, and its mechanism partially occurs through HO-1 induction and PI3K-AKT inhibition.
HO-1, a curcumin-induced gene, is thought to be a potential therapeutic protein for the re-establishment of homeostasis in several pathologic conditions (38
) and is also involved in inhibiting HCV replication (28
). The HO-1 products biliverdin and iron contribute to certain anti-HCV mechanisms of HO-1 (26
). In this study, HO-1 knockdown partially reversed curcumin-inhibited HCV replication, supporting the evidence for the anti-HCV effect of HO-1. Since HO-1 is induced by ROS or certain electrophiles, ROS has also been reported to inhibit HCV replication (41
). Arsenic trioxide-inhibited HCV replication is also suggested to be mediated through the induction of oxidative stress (43
). HO-1, an oxidative stress-induced gene, may be involved in the ROS-inhibited HCV replication.
As a downstream kinase of PI3K, AKT is an important molecule in regulating a wide range of signaling pathways (44
). In HCV-infected cells, the PI3K-AKT signaling pathway is involved in certain pathological mechanisms. For example, the activities of PI3K, AKT and their downstream target mTOR are increased in the HCV-replicating cells (45
). HCV NS5A binds to PI3K, while enhancing the phosphotransferase activity of the catalytic domain (46
). The HCV-activated PI3K-AKT contributes to cell survival enhancement. In addition to cell survival, AKT leads to the protein accumulation of SREBP-1, an important transcription factor regulating genes involved in fatty acid and cholesterol synthesis (47
). HCV NS4B has been found to enhance the protein expression levels of SREBPs and fatty acid synthase through PI3K activity, subsequently inducing a lipid accumulation in hepatoma cells (48
). Therefore, inhibition of the PI3K-SREBP signaling pathway should decrease the HCV-induced HCC development and the cellular fatty acid level. Curcumin has been reported to inhibit HCV replication via suppression of the AKT-SREBP-1 pathway (14
). In the present study, data also demonstrated that curcumin-inhibited PI3K-AKT was slightly involved in the anti-HCV activity of curcumin.
Activation of the MEK-ERK signal cascade enhances the replication of viruses, such as the human immunodeficiency (49
), the influenza (50
), the corona- (51
) and the herpes simplex viruses (52
). By contrast, in the case of HBV, activation of MEK-ERK signaling led to the inhibition of HBV replication (53
). In the HCV study, interleukin-1 has been reported to have the potential to effectively inhibit HCV replication and protein expression by activating the ERK signaling pathway (54
). HCV IRES-dependent protein synthesis was enhanced by MEK-ERK inhibitor PD98059 (55
). Another report also suggests that inhibition of MEK-ERK signaling leads to the upregulation of HCV replication and protein production (56
). Consistent with the results of the present study, those findings confirm that the curcumin-inhibited MEK-ERK signaling pathway contributes to the increase of HCV replication.
NF-κB, one of the major signaling transduction molecules activated in response to oxidative stress, is able to modulate the transcription of a large number of downstream genes. The HCV core protein has been shown to activate NF-κB, inducing resistance to TNF-α-induced apoptosis in hepatoma cells (57
). HCV NS2 activates the IL-8 gene expression by activating the NF-κB pathway in HepG2 cells (58
). In the infectious JFH1 model, HCV is suggested to enhance hepatic fibrosis progression through the induction of TGF-β1, mediated by a ROS-induced and NF-κB-dependent pathway (59
). These evidences indicate that the activation of NF-κB by HCV induces hepatic disease progression. In this study, the NF-κB expression is abundant in the cytoplasm of Huh7.5 cells, expressing the HCV genotype 1b subgenomic replicon (). The absence of NF-κB nuclear translocation indicates that NF-κB is not likely to participate in the mechanism of hepatocarcinogenesis in this cell line. The absense of complete HCV core and HCV NS2 sequences in the subgenomic replicon used in this study, is likely to be the reason for the absence of NF-κB nuclear translocation. Therefore, it is likely to contribute to the inability of the NF-κB inhibitor to suppress the HCV protein expression in this cell line. In fact, the genomic variation of HCV core protein generates a distinct functional regulation of NF-κB, which may inhibit or activate NF-κB activity (60
In certain reports, the inhibition of NF-κB shows anti-HCV activity: for example, the Acacia confusa
) and San-Huang-Xie-Xin-Tang extracts (62
) suppress HCV replication associated with NF-κB inhibition. In the present study, curcumin-inhibited NF-κB does not have any benefit in anti-HCV activity. Thus, the presence or absence of the inhibition of NF-κB in anti-HCV therapy is likely to depend on the activation status of NF-κB, although additional investigations are required on the subject.
In conclusion, this study proved that curcumin inhibits HCV replication through the induction of the HO-1 expression and the inhibition of the PI3K-AKT signaling pathway. However, the curcumin-inhibited MEK-ERK mechanism contributes negatively to its anti-HCV activity.